Jan. Feb. March April May June July Aug. Sep. Oct. Nov. Dec.
152 211 293 328 313 314 337 300 258 235 165 132
There is thus a conspicuous minimum at mid-winter, and but little
difference between the monthly means from April to August. This is
closely analogous to what is seen in the daily range of the magnetic
elements in similar latitudes (see MAGNETISM, TERRESTRIAL). There is
also considerable resemblance between the curve whose ordinates
represent the diurnal inequality in the current passing from north to
south, and the curve showing the hourly change in the westerly component
of the horizontal magnetic force in similar European latitudes.
8. _Relations with Sun-spots, Auroras and Magnetic Storms._--Weinstein
gives curves representing the mean diurnal inequality for separate
years. In both lines the diurnal amplitudes were notably smaller in the
later years which were near sun-spot minimum. This raises a presumption
that the regular diurnal earth currents, like the ranges of the magnetic
elements, follow the 11-year sun-spot period. When we pass to the large
and irregular earth currents, which are of practical interest in
telegraphy, there is every reason to suppose that the sun-spot period
applies. These currents are always accompanied by magnetic disturbances,
and when specially striking by brilliant aurora. One most conspicuous
example of this occurred in the end of August and beginning of September
1859. The magnetic disturbances recorded were of almost unexampled size
and rapidity, the accompanying aurora was extraordinarily brilliant, and
E.M.F.'s of 700 and 800 volts are said to have been reached on telegraph
lines 500 to 600 km. long. It is doubtful whether the disturbances of
1859 have been equalled since, but earth current voltages of the order
of 0.5 volts per mile have been recorded by various authorities, e.g.
Sir W.H. Preece (10).
It was the practice for several years to publish in the _Ann. du bureau
central meteorologique_ synchronous magnetic and earth current curves
from Parc Saint Maur corresponding to the chief disturbances of the
year. In most cases there is a marked similarity between the curve of
magnetic declination and that of the north-south earth current. At times
there is also a distinct resemblance between the horizontal force
magnetic curve and that of the east-west earth current, but exceptions
to this are not infrequent. Similar phenomena appear in synchronous
Greenwich records published by Airy in 1868; these show a close
accordance between the horizontal force curves and those of the currents
from magnetic east to west. Originally it was supposed by Airy that
whilst rapid movements in the declination and north-south current curves
sometimes occurred simultaneously, there was a distinct tendency for
the latter to precede the former. More recent examinations of the
Greenwich records by W. Ellis (11), and of the Parc St Maur curves by
Moureaux, have not confirmed this result, and it is now believed that
the two phenomena are practically simultaneous.
There has also been a conflict of views as to the connexion between
magnetic and earth current disturbances. Airy's observations tended to
suggest that the earth current was the primary cause, and the magnetic
disturbance in considerable part at least its effect. Others, on the
contrary, have supposed earth currents to be a direct effect of changes
in the earth's magnetic field. The prevailing view now is that both the
magnetic and the earth current disturbances are due to electric currents
in the upper atmosphere, these upper currents becoming visible at times
as aurora.
9. There seems some evidence that earth currents can be called into
existence by purely local causes, notably difference of level. Thus K.A.
Brander (12) has observed a current flowing constantly for a good many
days from Airolo (height 1160 metres) to the Hospice St Gotthard (height
2094 metres). In an 8-km. line from Resina to the top of Vesuvius L.
Palmieri (13)--observing in 1889 at three-hour intervals from 9 A.M. to
9 P.M.--always found a current running uphill so long as the mountain
was quiet. On a long line from Vienna to Graz A. Baumgartner (14) found
that the current generally flowed from both ends towards intervening
higher ground during the day, but in the opposite directions at night.
During a fortnight in September and October 1885 hourly readings were
taken of the current in the telegraph cable from Fort-William to Ben
Nevis Observatory, and the results were discussed by H.N. Dickson (15),
who found a marked preponderance of currents up the line to the summit.
The recorded mean data, otherwise regarded, represent a "constant"
current, equal to 29 in the arbitrary units employed by Dickson, flowing
up the line, together with the following diurnal inequality, + denoting
current towards Fort-William (i.e. down the hill, and nearly east to
west).
Hour | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
| | | | | | | | | | | | |
A.M. | -21 | -41 | +13 | +23 | +55 | -3 | +25 | -32 | -59 | -62 | -46 | +6 |
P.M. | +24 | +18 |+115 | +18 | +75 | -5 | +50 | -9 | -56 | -37 | -28 | -34 |
There is thus a diurnal inequality, which is by no means very irregular
considering the limited number of days, and it bears at least a general
resemblance to that shown by Weinstein's figures for an east-west line
in Germany. This will serve to illustrate the uncertainties affecting
these and analogous observations. A constant current in one direction
may arise in whole or part from plate E.M.F.'s; a current showing a
diurnal inequality will naturally arise between _any_ two places some
distance apart whether they be at different levels or not. Finally, when
records are taken only for a short time, doubts must arise as to the
generality of the results. During the Ben Nevis observations, for
instance, we are told that the summit was almost constantly enveloped in
fog or mist. By having three earth plates in the same vertical plane,
one at the top of a mountain, the others at opposite sides of it, and
then observing the currents between the summit and each of the base
stations, as well as directly between the base stations--during an
adequate number of days representative of different seasons of the year
and different climatic conditions--many uncertainties would soon be
removed.
10. _Artificial Currents._--The great extension in the applications of
electricity to lighting, traction and power transmission, characteristic
of the end of the 19th century, has led to the existence of large
artificial earth currents, which exert a disturbing influence on
galvanometers and magnetic instruments, and also tend to destroy metal
pipes. In the former case, whilst the disturbance is generally loosely
assigned to stray or "vagabond" earth currents, this is only partly
correct. The currents used for traction are large, and even if there
were a perfectly insulated return there would be a considerable
resultant magnetic field at distances from the track which were not
largely in excess of the distance apart of the direct and return
currents (16). At a distance of half a mile or more from an electric
tram line the disturbance is usually largest in magnetographs recording
the vertical component of the earth's field. The magnets are slightly
displaced from the position they would occupy if undisturbed, and are
kept in continuous oscillation whilst the trams are running (17). The
extent of the oscillation depends on the damping of the magnets.
The distance from an electric tram line where the disturbance ceases to
be felt varies with the system adopted. It also depends on the length of
the line and its subdivision into sections, on the strength of the
currents supplied, the amount of leakage, the absence or presence of
"boosters," and finally on the sensitiveness of the magnetic
instruments. At the U.S. Coast and Geodetic Survey's observatory at
Cheltenham the effect of the Washington electric trams has been detected
by highly sensitive magnetographs, though the nearest point of the line
is 12 m. away (18). Amongst the magnetic observatories which have
suffered severely from this cause are those at Toronto, Washington
(Naval Observatory), Kew, Paris (Parc St Maur), Perpignan, Nice, Lisbon,
Vienna, Rome, Bombay (Colaba) and Batavia. In some cases magnetic
observations have been wholly suspended, in others new observatories
have been built on more remote sites.
As regards damage to underground pipes, mainly gas and water pipes,
numerous observations have been made, especially in Germany and the
United States. When electric tramways have uninsulated returns, and the
potential of the rails is allowed to differ considerably from that of
the earth, very considerable currents are found in neighbouring pipes.
Under these conditions, if the joints between contiguous pipes forming a
main present appreciable resistance, whilst the surrounding earth
through moisture or any other cause is a fair conductor, current passes
locally from the pipes to the earth causing electrolytic corrosion of
the pipes. Owing to the diversity of interests concerned, the extent of
the damage thus caused has been very variously estimated. In some
instances it has been so considerable as to be the alleged cause of the
ultimate failure of water pipes to stand the pressure they are exposed
to.
BIBLIOGRAPHY.--See Svante August Arrhenius, _Lehrbuch der kosmischen
Physik_ (Leipzig, 1903), pp. 984-990. For lists of references see J.E.
Burbank, _Terrestrial Magnetism_, vol. 10 (1905), p. 23, and P.
Bachmetjew (8). For papers descriptive of corrosion of pipes, &c., by
artificial currents see _Science Abstracts_ (in recent years in the
volumes devoted to engineering) under the heading "Traction, Electric;
Electrolysis." The following are the references in the text:--(1)
_Phil. Trans. R.S._ for 1849, pt. i. p. 61; (2) _Phil. Trans. R.S._
vol. 151 (1861), p. 89, and vol. 152 (1862), p. 203; (3) _Etude des
courants telluriques_ (Paris, 1884); (4) _Die Erdstroeme im deutschen
Reichstelegraphengebiet_ (Braunschweig, 1900); (5) _Phil. Trans. R.S._
vol. 158 (1868), p. 465, and vol. 160 (1870), p. 215; (6) _Mem. de
l'Academie St-Petersbourg_, t. 31, No. 12 (1883); (7) T. Moureaux,
_Ann. du Bureau Central Met._ (Annee 1893), 1 Mem. p. B 23; (8) P.
Bachmetjew, _Mem. de l'Academie St-Petersbourg_, vol. 12, No. 3
(1901); (9) _Terrestrial Magnetism_, vol. 3 (1898), p. 130; (10)
_Journal Tel. Engineers_ (1881); (11) _Proc. R.S._ vol. 52 (1892), p.
191; (12) _Akad. Abhandlung_ (Helsingfors, 1888); (13) _Acad. Napoli
Rend._ (1890), and _Atti_ (1894, 1895); (14) _Pogg. Ann._ vol. 76, p.
135; (15) _Proc. R.S.E._ vol. 13, p. 530; (16) A. Ruecker, _Phil. Mag._
1 (1901), p. 423, and R.T. Glazebrook, ibid. p. 432; (17) J. Edler,
_Elektrotech. Zeit._ vol. 20 (1899); (18) L.A. Bauer, _Terrestrial
Magnetism_, vol. 11 (1906), p. 53. (C. Ch.)
EARTH-NUT, the English name for a plant known botanically as _Conopodium
denudatum_ (or _Bunium flexuosum_), a member of the natural order
Umbelliferae, which has a brown tuber-like root-stock the size of a
chestnut. It grows in woods and fields, has a slender flexuous smooth
stem 2 to 3 ft. high, much-divided leaves, and small white flowers in
many-rayed terminal compound umbels. Boswell Syme, in _English Botany_,
iv. 114, says: "The common names of this plant in England are various.
It is known as earth-nut, pig-nut, ar-nut, kipper-nut, hawk-nut,
jar-nut, earth-chestnut and ground-nut. Though really excellent in taste
and unobjectionable as food, it is disregarded in England by all but
pigs and children, both of whom appreciate it and seek eagerly for it."
Dr Withering describes the roots as little inferior to chestnuts. In
Holland and elsewhere on the continent of Europe they are more
generally eaten.
EARTH PILLAR, a pillar of soft rock, or earth, capped by some harder
material that has protected it from denudation. The "bad lands" of
western North America furnish numerous examples. Here "the formations
are often beds of sandstone or shale alternating with unindurated beds
of clay. A semi-arid climate where the precipitation is much
concentrated seems to be most favourable to the development of this type
of formation." The country round the Dead Sea, where loose friable sandy
clay is capped by harder rock, produces "bad-land" topography. The cap
of hard rock gives way at the joints, and the water making its way
downwards washes away the softer material directly under the cracks,
which become wider, leaving isolated columns of clay capped with hard
sandstone or limestone. These become smaller and fewer as denudation
proceeds, the pillars standing a great height at times, until finally
they all disappear.
EARTHQUAKE. Although the terrible effects which often accompany
earthquakes have in all ages forced themselves upon the attention of
man, the exact investigation of seismic phenomena dates only from the
middle of the 19th century. A new science has been thus established
under the name of _seismology_ (Gr. [Greek: seismos], an earthquake).
_History._--Accounts of earthquakes are to be found scattered through
the writings of many ancient authors, but they are, for the most part,
of little value to the seismologist. There is a natural tendency to
exaggeration in describing such phenomena, sometimes indeed to the
extent of importing a supernatural element into the description. It is
true that attempts were made by some ancient writers on natural
philosophy to offer a rational explanation of earthquake phenomena, but
the hypotheses which their explanations involved are, as a rule, too
fanciful to be worth reproducing at the present day. It is therefore
unnecessary to dwell upon the references to seismic phenomena which have
come down to us in the writings of such historians and philosophers as
Thucydides, Aristotle and Strabo, Seneca, Livy and Pliny. Nor is much to
be gleaned from the pages of medieval and later writers on earthquakes,
of whom the most notable are Fromondi (1527), Maggio (1571) and
Travagini (1679). In England, the earliest work worthy of mention is
Robert Hooke's _Discourse on Earthquakes_, written in 1668, and read at
a later date before the Royal Society. This discourse, though containing
many passages of considerable merit, tended but little to a correct
interpretation of the phenomena in question. Equally unsatisfactory were
the attempts of Joseph Priestley and some other scientific writers of
the 18th century to connect the cause of earthquakes with electrical
phenomena. The great earthquake of Lisbon in 1755 led the Rev. John
Michell, professor of mineralogy at Cambridge, to turn his attention to
the subject; and in 1760 he published in the _Philosophical
Transactions_ a remarkable essay on the Cause and Phenomena of
Earthquakes. A suggestion of much scientific interest was made by Thomas
Young, when in his _Lectures on Natural Philosophy_, published in 1807,
he remarked that an earthquake "is probably propagated through the earth
nearly in the same manner as a noise is conveyed through the air." The
recognition of the fact that the seismologist has to deal with the
investigation of wave-motion in solids lies at the very base of his
science. In 1846 Robert Mallet communicated to the Royal Irish Academy
his first paper "On the Dynamics of Earthquakes"; and in the following
year W. Hopkins, of Cambridge, presented to the British Association a
valuable report in which earthquake phenomena were discussed in some
detail. Mallet's labours were continued for many years chiefly in the
form of Reports to the British Association, and culminated in his great
work on the Neapolitan earthquake of 1857. An entirely new impetus,
however, was given to the study of earthquakes by an energetic body of
observers in Japan, who commenced their investigations about the year
1880, mainly through the influence of Prof. John Milne, then of Tokyo.
Their work, carried on by means of new instruments of precision, and
since taken up by observers in many parts of the world, has so extended
our knowledge of earthquake-motion that seismology has now become
practically a new department of physical science.
It is hardly too much to say, however, that the earliest systematic
application of scientific principles to the study of the effects of an
earthquake was made by Mallet in his investigation of the Neapolitan
earthquake mentioned above. It is true, the great Calabrian earthquake
of 1783 had been the subject of careful inquiry by the Royal Academy of
Naples, as also by Deodat Dolomieu and some other scientific
authorities; but in consequence of the misconception which at that time
prevailed with regard to the nature of seismic activity, the results of
the inquiry, though in many ways interesting, were of very limited
scientific value. It was reserved for Mallet to undertake for the first
time an extensive series of systematic observations in an area of great
seismic disturbance, with the view of explaining the phenomena by the
application of the laws of wave-motion.
Neapolitan earthquake, 1857.
The "Great Neapolitan Earthquake," by which more than 12,300 lives were
lost, was felt in greater or less degree over all Italy south of the
parallel of 42 deg., and has been regarded as ranking third in order of
severity among the recorded earthquakes of Europe. The principal shock
occurred at about 10 P.M. on the 16th of December 1857; but, as is
usually the case, it had been preceded by minor disturbances and was
followed by numerous after-shocks which continued for many months. Early
in 1858, aided by a grant from the Royal Society, Mallet visited the
devastated districts, and spent more than two months in studying the
effects of the catastrophe, especially examining, with the eye of an
engineer, the cracks and ruins of the buildings. His voluminous report
was published in 1862, and though his methods of research and his
deductions have in many cases been superseded by the advance of
knowledge, the report still remains a memorable work in the history of
seismology.
Much of Mallet's labour was directed to the determination of the
position and magnitude of the subterranean source from which the
vibratory impulses originated. This is known variously as the _seismic
centre_, _centrum_, _hypocentre_, _origin_ or _focus_. It is often
convenient to regard this centre theoretically as a point, but
practically it must be a locus or space of three dimensions, which in
different cases varies much in size and shape, and may be of great
magnitude. That part of the surface of the earth which is vertically
above the centre is called the _epicentre_; or, if of considerable area,
the epicentral or epifocal tract. A vertical line joining the epicentre
and the focus was termed by Mallet the _seismic vertical_. He calculated
that in the case of the Neapolitan earthquake the focal cavity was a
curved lamelliform fissure, having a length of about 10 m. and a height
of about 31/2 m., whilst its width was inconsiderable. The central point
of this fissure, the theoretical seismic centre, he estimated to have
been at a depth of about 61/2 m. from the surface. Dr C. Davison, in
discussing Mallet's data, was led to the conclusion that there were two
distinct foci, possibly situated on a fault, or plane of dislocation,
running in a north-west and south-east direction. Mallet located his
epicentre near the village of Caggiano, not far from Polla, while the
other seems to have been in the neighbourhood of Montemurro, about 25 m.
to the south-east.
The intensity, or violence, of an earthquake is greatest in or near the
epicentre, whence it decreases in all directions. A line drawn through
points of equal intensity forms a curve round the epicentre known as an
_isoseist_, an _isoseismal_ or an _isoseismic line_. If the intensity
declined equally in all directions the isoseismals would be circles, but
as this is rarely if ever the case in nature they usually become
ellipses and other closed curves. The tract which is most violently
shaken was termed by Mallet the _meizoseismic area_, whilst the line of
maximum destruction is known as the _meizoseismic line_. That isoseismal
along which the decline of energy is most rapid was called by K. von
Seebach a _pleistoseist_.
In order to determine the position of the seismic centre, Mallet made
much use of the cracks in damaged buildings, especially in walls of
masonry, holding that the direction of such fractures must generally be
at right angles to that in which the normal earthquake-wave reached
them. In this way he obtained the "angle of emergence" of the wave. He
also assumed that free-falling bodies would be overthrown and projected
in the direction of propagation of the wave, so that the epicentre might
immediately be found from the intersection of such directions. These
data are, however, subject to much error, especially through want of
homogeneity in the rocks, but Mallet's work was still of great value.
Charleston earthquake, 1886.
A different method of ascertaining the depth of the focus was adopted by
Major C.E. Dutton in his investigation of the Charleston earthquake of
the 31st of August 1886 for the U.S. Geological Survey. This catastrophe
was heralded by shocks of greater or less severity a few days previously
at Summerville, a village 22 m. north-west of Charleston. The great
earthquake occurred at 9.51 P.M., standard time of the 75th meridian,
and in about 70 seconds almost every building in Charleston was more or
less seriously damaged, while many lives were lost. The epicentral tract
was mainly a forest region with but few buildings, and the principal
records of seismological value were afforded by the lines of railway
which traversed the disturbed area. In many places these rails were
flexured and dislocated. Numerous fissures opened in the ground, and
many of these discharged water, mixed sometimes with sand and silt,
which was thrown up in jets rising in some cases to a height of 20 ft.
Two epicentres were recognized--one near Woodstock station on the South
Carolina railway, and the other, being the centre of a much smaller
tract, about 14 m. south-west of the first and near the station of
Rantowles on the Charleston and Savannah line. Around these centres and
far away isoseismal lines were drawn, the relative intensity at
different places being roughly estimated by the effects of the
catastrophe on various structures and natural objects, or, where visible
records were wanting, by personal evidence, which is often vague and
variable. The Rossi-Forel scale was adopted. This is an arbitrary scale
formulated by Professor M.S. de Rossi, of Rome, and Dr F.A. Forel, of
Geneva, based mostly on the ordinary phenomena observed during an
earthquake, and consisting of ten degrees, of which the lowest is the
feeblest, viz. I. Microseismic shock; II. Extremely feeble shock; III.
Very feeble shock; IV. Feeble; V. Shock of moderate intensity; VI.
Fairly strong shock; VII. Strong shock; VIII. Very strong shock; IX.
Extremely strong shock; X. Shock of extreme intensity. Other
conventional scales, some being less detailed, have been drawn up by
observers in such earthquake-shaken countries as Italy and Japan. A
curve, or theoretical isoseismal, drawn through certain points where the
decline of intensity on receding from the epicentre seems to be greatest
was called by Dutton an "index-circle"; and it can be shown that the
radius of such a circle multiplied by the square root of 3 gives the
focal depth theoretically. In this way it was computed that in the
Charleston earthquake the origin under Woodstock must have had a depth
of about 12 m. and that near Rantowles a depth of nearly 8 m. The
determination of the index-circle presents much difficulty, and the
conclusions must be regarded as only approximate.
It is probable, according to R.D. Oldham, that local earthquakes may
originate in the "outer skin" of the earth, whilst a large world-shaking
earthquake takes its origin in the deeper part of the "crust," whence
such a disturbance is termed a _bathyseism_. Large earthquakes may have
very extended origins, with no definite centre, or with several foci.
Great Indian earthquake, 1897.
The gigantic disaster known as the "Great Indian Earthquake," which
occurred on the 12th of June 1897, was the subject of careful
investigation by the Geological Survey of India and was described in
detail by the superintendent, R.D. Oldham. It is sometimes termed the
Assam earthquake, since it was in that province that the effects were
most severe, but the shocks were felt over a large part of India, and
indeed far beyond its boundaries. Much of the area which suffered most
disturbance was a wild country, sparsely populated, with but few
buildings of brick or stone from which the violence of the shocks could
be estimated. The epicentral tract was of great size, having an
estimated area of about 6000 sq. m., but the mischief was most severe in
the neighbourhood of Shillong, where the stonework of bridges, churches
and other buildings was absolutely levelled to the ground. After the
main disturbance, shocks of greater or less severity continued at
intervals for many weeks. It is supposed that this earthquake was
connected with movement of subterranean rock-masses of enormous
magnitude along a great thrust-plane, or series of such planes, having a
length of about 200 m. and a maximum breadth of not less than 50 m. It
is pointed out by Oldham that this may be compared for size with the
great Faille du Midi in Belgium, which is known to extend for a distance
of 120 m. The depth of the principal focus, though not actually capable
of determination, was probably less than 5 m. from the surface. From the
focus many secondary faults and fractures proceeded, some reaching the
surface of the ground. Enormous landslips accompanied the earthquake,
and as an indirect effect of these slides the form of the water-courses
became in certain cases modified. Permanent changes of level were also
observed.
Kangra earthquake, 1905.
Eight years after the great Assam earthquake India was visited by
another earthquake, which, though less intense, resulted in the loss of
about 20,000 lives. This catastrophe is known as the Kangra earthquake,
since its centre seems to have been located in the Kangra valley, in the
north-west Himalaya. It occurred on the 4th of April 1905, and the first
great shocks were felt in the chief epifocal district at about 6.9 a.m.,
Madras time. Although the tract chiefly affected was around Kangra and
Dharmsala, there was a subordinate epifocal tract in Dehra Dun and the
neighbourhood of Mussoorie, whilst the effects of the earthquake
extended in slight measure to Lahore and other cities of the plain. It
is estimated that the earthquake was felt over an area of about
1,625,000 m. Immediately after the calamity a scientific examination of
its effects was made by the Geological Survey of India, and a report was
drawn up by the superintendent, C.S. Middlemiss.
California earthquake, 1906.
The great earthquake, which, with the subsequent fire, wrought such
terrible destruction in and around San Francisco on the 18th of April
1906, was the most disastrous ever recorded in California. It occurred
between 10 and 15 minutes after 5 A.M., standard time of the 120th
meridian. The moment at which the disaster began and the duration of the
shock varied at different localities in the great area over which the
earthquake was felt. At San Francisco the main shock lasted rather more
than one minute.
According to the official Report, the earthquake was due to rupture and
movement along the plane of the San Andreas fault, one of a series which
runs for several hundred miles approximately in a N.W. and S.E.
direction near the coast line. Evidence of fresh movement along this
plane of dislocation was traced for a distance of 190 m. from San Juan
on the south to Point Arena on the north. There the trace of the fault
is lost beneath the sea, but either the same fault or another appears 75
m. to the north at Point Delgada. The belt of disturbed country is
notoriously unstable, and part of the fault had been known as the
"earthquake crack." The direction is marked by lines of straight cliffs,
long ponds and narrow depressions, forming a Rift, or old line of
seismic disturbance. According to Dr G.K. Gilbert the earthquake zone
has a length of 300 or 400 m. The principal displacement of rock, in
1906, was horizontal, amounting generally to about 10 ft. (maximum 21
ft.), but there was also locally a slight vertical movement, which
towards the north end of the fault reached 3 ft. Movement was traced for
a distance of about 270 m., and it is estimated that at least 175,000
sq. m. of country must have been disturbed. In estimating the intensity
of the earthquake in San Francisco a new scale was introduced by H.O.
Wood. The greatest structural damage occurred on soft alluvial soil and
"made ground." Most of the loss of property in San Francisco was due to
the terrible fire which followed the earthquake and was beyond control
owing to the destruction of the system of water-supply.
Immediately after the catastrophe a California Earthquake Investigation
Committee was appointed by the governor of the state; and the American
Association for the Advancement of Science afterwards instituted a
Seismological Committee. The elaborate Report of the State Investigation
Committee, by the chairman, Professor A.C. Lawson, was published in
1908.
On the 17th of August 1906 a disastrous earthquake occurred at
Valparaiso, and the year 1906 was marked generally by exceptional
seismic activity.
The Jamaica earthquake of the 14th of January 1907 appears to have
accompanied movement of rock along an east and west fracture or series
of fractures under the sea a few miles from the city of Kingston. The
statue of Queen Victoria at Kingston was turned upon its pedestal the
eighth of a revolution.
Messina earthquake, 1908.
A terrible earthquake occurred in Calabria and Sicily on December 28,
1908, practically destroying Messina and Reggio. According to the
official returns the total loss of life was 77,283. Whilst the principal
centre seems to have been in the Strait of Messina, whence the
disturbance is generally known as the Messina earthquake, there were
independent centres in the Calabrian peninsula, a country which had been
visited by severe earthquakes not long previously, namely on September
8, 1905, and October 23, 1907. The principal shock of the great Messina
earthquake of 1908 occurred at 5.21 A.M. (4.21 Greenwich time), and had
a duration of from 30 to 40 seconds. Neither during nor immediately
before the catastrophe was there any special volcanic disturbance at
Etna or at Stromboli, but it is believed that there must have been
movement along a great plane of weakness in the neighbourhood of the
Strait of Messina, which has been studied by E. Cortese. The sea-floor
in the strait probably suffered great disturbance, resulting in the
remarkable movement of water observed on the coast. At first the sea
retired, and then a great wave rolled in, followed by others generally
of decreasing amplitude, though at Catania the second was said to have
been greater than the first. At Messina the height of the great wave was
2.70 metres, whilst at Ali and Giardini it reached 8.40 metres and at
San Alessio as much as 11.7 metres. At Malta the tide-gauge recorded a
wave of 0.91 metre. The depth of the chief earthquake-centre was
estimated by Dr E. Oddone at about 9 kilometres. The earthquake and
accompanying phenomena were studied also by Professor A. Ricco, Dr M.
Baratta and Professor G. Platania and by Dr F. Omori of Tokyo. After the
great disturbance, shocks continued to affect the region intermittently
for several months. In certain respects the earthquake of 1908 presented
much resemblance to the great Calabrian catastrophe of 1783.
It has been proposed by R.D. Oldham that the disturbance which causes
the fracture and permanent displacement of the rocks during an
earthquake should be called an "earthshake," leaving the term earthquake
especially for the vibratory motion. The movement of the earthquake is
molecular, whilst that of the earthshake is molar. Subsequently he
suggested the terms _mochleusis_ and _orchesis_ ([Greek: mochleuo], I
heave; [Greek: orcheomai], I dance), to denote respectively the molar
and the molecular movement, retaining the word earthquake for use in its
ordinary sense.
In most earthquakes the proximate cause is generally regarded as the
fracture and sudden movement of underground rock-masses. Disturbances of
this type are known as "tectonic" earthquakes, since they are connected
with the folding and faulting of the rocks of the earth's crust. They
indicate a relief of the strain to which the rock-masses are subjected
by mountain-making and other crustal movements, and they are
consequently apt to occur along the steep face of a table-land or the
margin of a continent with a great slope from land to sea. In many cases
the immediate seat of the originating impulse is located beneath the
sea, giving rise to submarine disturbances which have been called
"seaquakes." Much attention has been given to these suboceanic
disturbances by Professor E. Rudolph.
Professor J.H. Jeans has pointed out that the regions of the earth's
crust most affected by earthquakes lie on a great circle corresponding
with the equator of the slightly pear-shaped figure that he assigns to
the earth. This would represent a belt of weakness, subject to crushing,
from the tendency of the pear to pass into a spherical or spheroidal
form under the action of internal stresses. According to the comte de
Montessus de Ballore, the regions of maximum seismic instability appear
to be arranged on two great circles, inclined to each other at about
67 deg.. These are the Circumpacific and Mediterranean zones.
Maps of the world, showing the origins of large earthquakes each year,
accompany the Annual Reports of the Seismological Committee of the
British Association, drawn up by Professor Milne. It is important to
note that Professor Milne has shown a relationship between
earthquake-frequency and the wandering of the earth's pole from its mean
position. Earthquakes seem to have been most frequent when the
displacement of the pole has been comparatively great, or when the
change in the direction of movement has been marked. Valuable earthquake
catalogues have been compiled at various times by Alexis Perrey, R. and
J.W. Mallet, John Milne, T. Oldham, C.W.C. Fuchs, F. de Montessus de
Ballore and others.
British earthquakes.
Such earthquakes as are felt from time to time in Great Britain may
generally be traced to the formation of faults, or rather to incidents
in the growth of old faults. The East Anglian earthquake of the 22nd of
April 1884--the most disastrous that had occurred in the British Isles
for centuries--was investigated by Prof. R. Meldola and W. White on
behalf of the Essex Field Club. The shocks probably proceeded from two
foci--one near the villages of Peldon and Abberton, the other near
Wivenhoe and Rowhedge, in N.E. Essex. It is believed that the
superficial disturbance resulted from rupture of rocks along a deep
fault. An attempt has been made by H. Darwin, for the Seismological
Committee of the British Association, to detect and measure any gradual
movement of the strata along a fault, by observation at the Ridgeway
fault, near Upway, in Dorsetshire. Dr C. Davison in studying the
earthquakes which have originated in Britain since 1889 finds that
several have been "twins." A twin earthquake has two maxima of intensity
proceeding from two foci, whereas a double earthquake has its successive
impulses from what is practically a single focus. The Hereford
earthquake of December 1896, which resulted in great structural damage,
was a twin, having one epicentre near Hereford and the other near Ross.
Davison refers it to a slip along a fault-plane between the anticlinal
areas of Woolhope and May Hill; and according to the same authority the
Inverness earthquake of the 18th of September 1901 was referable to
movement along a fault between Loch Ness and Inverness. The South Wales
earthquake of June 27, 1906, was probably due to movement connected with
the Armorican system of folds, striking in an east and west direction.
It may be noted that when a slip occurs along a fault, the displacement
underground may be but slight and may die out before reaching the
surface, so that no scarp is formed. In connexion, however, with a
seismic disturbance of the first magnitude the superficial features may
be markedly affected. Thus, the great Japan earthquake of October
1891--known often as the Mino-Owari earthquake--was connected with the
formation or development of a fault which, according to Professor B.
Koto, was traced on the surface for a distance of nearly 50 m. and
presented in places a scarp with a vertical throw of as much as 20 ft.,
while probably the maximum displacement underground was very much
greater.
Although most earthquakes seem to be of tectonic type, there are some
which are evidently connected, directly or indirectly, with volcanic
activity (see VOLCANO). Such, it is commonly believed, were the
earthquakes which disturbed the Isle of Ischia in 1881 and 1883, and
were studied by Professor J. Johnston-Lavis and G. Mercalli. In addition
to the tectonic and volcanic types, there are occasional earthquakes of
minor importance which may be referred to the collapse of the roof of
caverns, or other falls of rock in underground cavities at no great
depth. According to Prof. T.J.J. See most earthquakes are due, directly
or indirectly, to the explosive action of ste by the leakage of
sea-water through the ocean floor.
Earthquake waves.
Whatever the nature of the impulse which originates the earthquake, it
gives rise to a series of waves which are propagated through the earth's
substance and also superficially. In one kind, known as normal or
condensational waves, or waves of elastic compression, the particles
vibrate to and from the centre of disturbance, moving in the direction
in which the wave travels, and therefore in a way analogous to the
movement of air in a sound-wave. Associated with this type are other
waves termed transverse waves, or waves of elastic distortion, in which
the particles vibrate across or around the direction in which the wave
is propagated. The normal waves result from a temporary change of volume
in the medium; the transverse from a change of shape. The distance
through which an earth-particle moves from its mean position of rest,
whether radially or transversely, is called the amplitude of the wave;
whilst the double amplitude, or total distance of movement, to and fro
or up and down, like the distance from crest to trough of a water wave,
may be regarded as the range of the wave. The period of a wave is the
time required for the vibrating particle to complete an oscillation. As
the rocks of the earth's crust are very heterogeneous, the
earthquake-waves suffer refraction and reflection as they pass from one
rock to another differing in density and elasticity. In this way the
waves break up and become much modified in course of transmission, thus
introducing great complexity into the phenomena. It is known that the
normal waves travel more rapidly than the transverse.
Measurements of the surface speed at which earthquake-waves travel
require very accurate time-measurers, and these are not generally
available in earthquake-shaken regions. Observations during the
Charleston earthquake of 1886 were at that time of exceptional value,
since they were made over a large area where standard time was kept.
Lines drawn through places around the epicentre at which the shock
arrives at the same moment are called coseismal lines. The motion of the
wave is to be distinguished from the movement of the vibrating
particles. The velocity of the earth-particle is its rate of movement,
but this is constantly changing during the vibration, and the rate at
which the velocity changes is technically called the acceleration of the
particle.
Unfelt movements of the ground are registered in the earthquake records,
or seismograms, obtained by the delicate instruments used by modern
seismologists. From the study of the records of a great earthquake from
a distant source, sometimes termed a teleseismic disturbance, some
interesting inferences have been drawn with respect to the constitution
of the interior of the earth. The complete record shows two phases of
"preliminary tremors" preceding the principal waves. It is believed that
while the preliminary tremors pass through the body of the earth, the
principal waves travel along or parallel to the surface. Probably the
first phase represents condensational, and the second phase
distortional, waves. Professor Milne concludes from the speed of the
waves at different depths that materials having similar physical
properties to those at the surface may extend to a depth of about 30 m.,
below which they pass into a fairly homogeneous nucleus. From the
different rates of propagation of the precursors it has been inferred by
R.D. Oldham that below the outer crust, which is probably not everywhere
of the same thickness, the earth is of practically uniform character to
a depth of about six-tenths of the radius, but the remaining four-tenths
may represent a core differing physically and perhaps chemically from
the outer part. Oldham also suggests, from his study of oceanic and
continental wave-paths, that there is probably a difference in the
constitution of the earth beneath oceans and beneath continents.
The surface waves, which are waves of great length and long period and
are propagated to great distances with practically a constant velocity,
have been regarded as quasi-elastic gravitational waves. Further, in a
great earthquake the surface of the ground is sometimes visibly agitated
in the epifocal district by undulations which may be responsible for
severe superficial damage. (See also for elastic waves ELASTICITY, Sec.
89.)
An old classification of earthquake-shocks, traces of which still linger
in popular nomenclature, described them as "undulatory," when the
movement of the ground was mainly in a horizontal direction;
"subsultory," when the motion was vertical, like the effect of a normal
wave at the epicentre; and "vorticose," when the movement was rotatory,
apparently due to successive impulses in varying directions.
The sounds which are associated with seismic phenomena, often described
as subterranean rumbling and roaring, are not without scientific
interest, and have been carefully studied by Davison. "Isacoustic lines"
are curves drawn through places where the sound is heard by the same
percentage of observers. The sound is always low and often inaudible to
many.
The refined instruments which are now used by seismologists for
determining the elements of earthquake motion and for recording
earthquakes from distant origins are described in the article
SEISMOMETER. These instruments were developed as a consequence of the
attention given in modern times to the study of earthquakes in the Far
East. (F. W. R.*)
Seismology in Japan.
Strange as it may appear, the advances that have been made in the study
of earthquakes and the world-wide interest shown in their phenomena were
initiated in work commenced in Japan. When the Japanese government,
desiring to adopt Western knowledge, invited to its shores bodies of men
to act as its instructors, the attention of the newcomers was naturally
attracted to the frequent shakings of the ground. Interest in these
phenomena increased more rapidly than their frequency, and at length it
was felt that something should be done for their systematic study. At
midnight on the 22nd of February 1880 movements more violent than usual
occurred; chimneys were shattered or rotated, tiles slid down from
roofs, and in the morning it was seen that Yokohama had the appearance
of a city that had suffered a bombardment. The excitement was intense,
and before the ruins had been removed a meeting was convened and the
Seismological Society of Japan established. The twenty volumes of
original papers published by this body summarize to a large extent the
results of the later study of seismology.[1]
The attention of the students of earthquakes in Japan was at first
directed almost entirely to seismometry or earthquake measurement. Forms
of apparatus which then existed, as for example the seismographs,
seismometers and seismoscopes of Mallet, Palmieri and others, were
subjected to trial; but inasmuch as they did little more than indicate
that an earthquake had taken place--the more elaborate forms recording
also the time of its occurrence--they were rapidly discarded, and
instruments were constructed to _measure_ earthquake motion. Slightly
modified types of the new instruments devised in Japan were adopted
throughout the Italian peninsula, and it is fair to say that the
seismometry developed in Japan revolutionized the seismometry of the
world. The records obtained from the new instruments increased our
knowledge of the character of earthquake motion, and the engineer and
the architect were placed in a position to construct so that the effects
of known movements could be minimized. It was no doubt the marked
success, both practical and scientific, attending these investigations
that led the Japanese government to establish a chair of seismology at
its university, to organize a system of nearly 1000 observing stations
throughout the country, and in 1893 to appoint a committee of scientific
and practical men to carry out investigations which might palliate the
effects of seismic disturbances. In the first year this committee
received a grant of L5000, and as liberal sums for the same purpose
appear from time to time in the parliamentary estimates, it may be
assumed that the work has been fraught with good results. In their
publications we find not only records of experiences and experiments in
Japan, but descriptions and comments upon earthquake effects in other
countries. In two of the volumes there are long and extremely well
illustrated accounts of the earthquake which on the 12th of June 1897
devastated Assam, to which country two members of the above-mentioned
committee were despatched to gather such information as might be of
value to the architect and builder in earthquake-shaken districts.
Seismological research.
A great impetus to seismological investigation in Europe and America was
no doubt given by the realization of the fact that a large earthquake
originating in any one part of the world may be recorded in almost any
other. Italy for many years past has had its observatories for recording
earthquakes which can be felt, and which are of local origin, but at the
present time at all its first-class stations we find instruments to
record the unfelt movements due to earthquakes originating at great
distances, and as much attention is now paid to the large earthquakes of
the world as to the smaller ones originating within Italian
territory.[2] The _Kaiserliche Akademie der Wissenschaften_ of Vienna
established earthquake observatories in Austria,[3] and the Central
Observatorium of St Petersburg has carried out similar work in Russia.
Germany attached a seismological observatory to its university at
Strassburg, whilst provision has been made for a professorship of Earth
Physics (_Geophysik_) at Goettingen.[4] In accordance with the
recommendation of the British Association, seismographs of a similar
character have been installed at stations all over the world.[5] The
principal objects of this extended and still extending system of
stations are to determine the velocity with which motion is propagated
over the surface and through the interior of the earth, to locate the
positions of sub-oceanic earthquake origins, and generally to extend our
knowledge respecting the physical nature of the planet on which we live.
Frequency of earthquakes.
We now know that earthquakes are many times more frequent than was
previously supposed. In Japan, for example, between 1885 and 1892 no
fewer than 8331 were recorded--that is to say, on the average there were
during that time more than 1000 disturbances per year. Although many of
these did not cause a sensible shaking over areas exceeding a few
hundred square miles, many of them were sufficiently intense to
propagate vibrations round and through the globe. If we pick out the
well-marked earthquake districts of the world, and give to each of them
a seismicity or earthquake frequency per unit area one-third of that in
Japan, the conclusion arrived at is that considerable areas of our
planet are on the average shaken every half-hour.
Volcanoes and earthquakes.
The knowledge which we now possess respecting the localities where
earthquakes are frequent and the forms of the foci from which they have
spread, enables us to speak definitely respecting the originating causes
of many of these phenomena. It is found, for example, that although in
many countries there may be displays of volcanic and seismic activity
taking place almost side by side, it is only rarely that there is direct
relationship between the two. Now and then, however, before a volcano
breaks into eruption there may be a few ineffectual efforts to form a
vent, each of which is accompanied by no more than a slight local
shaking of the ground. This is true even for the largest and most
violent eruptions, when mountains have with practically a single effort
blown off their heads and shoulders. Thus the earthquake which
accompanied the eruption of Bandaisan, in central Japan, in 1888 was
felt only over a radius of 25 m. The analyses of the seismic registers
of Japan clearly indicate that comparatively few shakings originate near
to the volcanoes of the country, the majority of them, like those of
many other countries, coming from regions where volcanic rocks are
absent. The greatest number spread inland from the Pacific seaboard, the
movement becoming more and more feeble as it approaches the backbone of
the country, which is drilled with numerous volcanic vents. What is true
for Japan is generally true for the western coasts of North and South
America.
Origin of earthquakes.
Speaking broadly, earthquakes are most frequent along the steeper
flexures in the earth's surface, and in those regions where there is
geological evidence to show that slow secular movements in the earth's
crust are possibly yet in progress. With a unit distance of 2 degrees,
or 120 geographical m., we find that the slopes running eastwards from
the highlands of Japan and westwards from the Andean ridges down into
the Pacific vary from 1 in 20 to 1 in 30, and it is on the faces or near
to the bottom of these slopes that seismic efforts are frequent. The
slopes running from Australia, eastern America and western Europe into
the neighbouring oceans vary between 1 in 70 and 1 in 250, and in these
regions earthquakes are of rare occurrence. The seismic activity met
with in the Himalayas and the Alps finds its best explanation in the
fact that these mountains are geologically recent, and there are no
reasons to doubt that the forces which brought their folds into
existence are yet in action.
This peculiar association of earthquakes with pronounced topographical
configuration and certain geological conditions evidently indicates that
the origin of many of them is connected with rock folding. Inasmuch as
certain large earthquakes have been accompanied by rock fracture, as for
example in 1891, when in central Japan a fault some 50 m. in length was
created, whilst the origins of others have been distinctly traced to the
line of an existing fault or its continuation, we may conclude that the
majority of earthquakes are spasmodic accelerations in the secular
movements which are creating (and in some instances possibly
obliterating) the more prominent features of the earth's surface. These
secular movements, which include upheavals, subsidences, horizontal
displacements--all of which are explained on the assumption of a crust
seeking support on a nucleus gradually contracting by loss of heat, are
collectively referred to as bradyseismical ([Greek: bradys], slow)
movements. To these may be added movements directly attributable to the
influence of gravity. Sub-oceanic districts in a state of seismic strain
may be so far loaded by the accumulation of sediments that gentle
bending may be accompanied by sudden yieldings. This possibly accounts
for the frequency of earthquakes off the mouth of the Tonegawa on the
eastern side of Japan. The distortions so frequently observed in fossils
and pebbles, the varying thickness of contorted strata, and the "creep"
in coal-mines, together with other phenomena, indicate that rocks may
flow. Observations of this nature lead to the supposition that high
plateau-like regions may be gradually subsiding under the influence of
their own weight, and that the process of settlement may from time to
time be spasmodic in its character. Whether the earthquakes which
originate round the submerged basal frontiers of the continents bounding
the Pacific are ever attributable to such activities, it is impossible
to say. All that we know with certainty is that they are sometimes
accompanied by such a vast displacement of material that the ocean has
been set into a state of oscillation for periods of 24 hours, that in
some instances there have been marked changes in depth, and that
enormous sub-oceanic landslips have occurred. These phenomena are,
however, equally well explained on the assumption of sudden faulting
accompanied by violent shaking, which would dislodge steeply inclined
beds of material beneath the ocean as it does upon the land.
Two types of earthquake motion.
Although the proximate cause of earthquake motion is traced to sudden
yieldings in the crust of the earth brought about by some form of
bradyseismical action, the existence of at least two distinct types of
seismic motion indicates that the mechanical conditions accompanying the
fracturing of rocks are not always identical. 90 or 95% of the
earthquakes which can be recorded consist of elastic or quasi-elastic
vibrations. The remainder, including the large earthquakes, not only
exhibit the elastic movements, but are accompanied by surface
undulations which are propagated most certainly for some hundreds of
miles round their origin, and then as horizontal movements sweep over
the whole surface of the globe. The former of these may accompany the
formation of a new fault or the sudden renewal of movement along an old
one; they are cracking or rending effects, without any great
displacement. The latter are probably fracturings accompanied by
vertical and horizontal displacements of masses of the earth's crust
sufficiently great to set up the observed surface undulations. These
shocks are so frequently followed a few minutes later by disturbances,
which from their similarity to the movements which have preceded them
may be called earthquake echoes, that we are led to the speculation that
we are here dealing with the caving-in of ill-supported portions of the
earth's crust, the waves from which are radiated to boundaries and then
returned to their origin to coalesce and give rise to a second impulse
not unlike the primary. Succeeding the first repetition of motion
recorded by the seismograph there is often a rhythmical repetition of
similar wave groups, suggesting the existence within our earth of
phenomena akin to multiple echoes.
Character of earthquake motion.
The introduction of new methods into seismometry quickly revolutionized
our ideas respecting the character of earthquake motion. Although an
earthquake may be strongly felt within a distance of 50 m. from its
origin, and although the movements in the upper storeys of buildings
within the shaken area may be large, the actual range of the horizontal
motion of the ground is usually less than {1/10} of an inch. With such
earthquakes ordinary seismographs for recording vertical motion do not
show any disturbance. When the movement reaches 1/2 in. it becomes
dangerous, and a back-and-forth movement of an inch is usually
accompanied by destructive effects. In this latter case the amplitude of
the vertical record which indicates the existence of surface waves will
vary between 1/2 and {1/100} of an inch. In the earthquake which
devastated central Japan on the 26th of October 1891, nearly every
building within the epifocal district fell, the ground was fissured,
forests slipped down from mountain sides to dam up valleys, whilst the
valleys themselves were permanently compressed. The horizontal movements
seem to have reached 9 in. or 1 ft., and the surface undulations were
visible to the eye.
Period and duration.
The rapidity with which the movements are performed varies throughout a
disturbance. A typical earthquake usually commences with minute elastic
vibrations, the periods of which vary between {1/5} and {1/20} of a
second. These are recorded by seismographs, and are noticed by certain
of the lower animals like pheasants, which before the occurrence of
movement perceptible to human beings scream as if alarmed. When an
earthquake is preceded by a sound we have evidence of preliminary
tremors even more rapid than those recorded by seismographs. Following
these precursors there is a shock or shocks, the period of which will be
1 or 2 seconds. From this climax the movements, although irregular in
character, become slower and smaller until finally they are
imperceptible. The duration of a small earthquake usually varies from a
few seconds to a minute, but large earthquakes, which are accompanied by
surface undulations, may be felt for 2 or 3 minutes, whilst an ordinary
seismograph indicates a duration of from 6 to 12 minutes. A free
horizontal pendulum tells us that with severe earthquakes the ground
comes to rest by a series of more or less rhythmical surgings,
continuing over 1 or 2 hours. Although the maximum displacement has a
definite direction, the successive vibrations are frequently performed
in many different azimuths. The predominating direction at a given
station in certain instances is apparently at right angles to the strike
of the neighbouring strata, this being the direction of easiest
yielding.
Velocity.
Earthquake motion as recorded at stations several thousands of miles
distant from its origin exhibits characteristics strikingly different
from those just described. The precursors now show periods of from 1 to
5 seconds, whilst the largest movements corresponding to the shocks may
have periods of from 20 to 40 seconds. The interval of time by which the
first tremors have outraced the maximum movement has also become
greater. Within a few hundreds of miles from an origin this interval
increases steadily, the velocity of propagation of the first movements
being about 2 km. per second, whilst that of the latter may be taken at
about 1.6 km. per second. Beyond this distance the velocity of
transmission of the first movements rapidly increases, and for great
distances, as for example from Japan to England, it is higher than we
should expect for waves of compression passing through steel or glass.
This observation precludes the idea that these preliminary tremors have
travelled through the heterogeneous crust of the earth, and since the
average velocity of their transmission increases with the length of the
path along which they have travelled, and we but rarely obtain certain
evidence that a seismograph has been disturbed by waves which have
reached it by travelling in opposite directions round the world, we are
led to the conclusion that earthquake precursors pass through our earth
and not round its surface. The following table relating to earthquakes,
which originated off the coast of Borneo on the 20th and 27th of
September 1897, is illustrative of the velocities here considered:--
+---------------------------+-----------+-------------+---------------+
| | | | _______ |
| | Distance | Velocity | /Average |
| Localities | from | in kms. | /depth of |
| | origin | per sec. if | \ / chord in |
| |in degrees.| on chord. | 1/4 V kms. |
+---------------------------+-----------+-------------+---------------+
| Nicolaieff | 81 deg. | 8.1 | 8.0 |
| Potsdam | 92 deg. | 8.4 | 9.1 |
| Catania, Ischia, Rocca di | | | |
| Papa, Rome | 96 deg. | 9.0 | 9.5 |
| Isle of Wight | 103 deg. | 9.8 | 10.2 |
+---------------------------+-----------+-------------+---------------+
The chords referred to here are those joining the earthquake origins and
distant observing stations, and it will be noted that one-quarter of the
square root of the average depths at which these run closely corresponds
to observed average velocities if wave paths followed chords. This
increase of velocity with average depth shows that the paths followed
through the earth must be curved with their convexity towards the centre
of the earth. These observations do not directly tell us to what extent
a true wave path is deflected from the direction of a chord, but they
suggest as an extremely plausible assumption that the square of the
speed is a linear function of the depth below the surface of the earth.
With this assumption Dr C.G. Knott shows that the square of the speed
(v squared) can be expressed linearly in terms of the average depth of the
chord d, thus: v squared = 2.9 + .026 d, the units being miles and seconds. The
formula applies with fair accuracy to moderate and high values of d, but
it gives too high a value for short chords. It follows that the square
of the speed increases 0.9% per mile of descent in the earth. The
conclusion we arrive at is that the preliminary tremors which pass
through the earth do so in the vicinity of their origin at the rate of
almost 2.3 km. per second. This velocity increases as the wave path
plunges downwards, attaining in the central regions a velocity of 16 to
17 kms., whilst the highest average velocity which is across a diameter
lies between 10 and 12 kms. per second.
The large surface waves radiating from an origin to a distant place have
velocities lying between 1.6 and 4 kms. per second, and it has been
observed that when the higher velocity has been noted this refers to an
observation at a station very remote from the origin. One explanation of
this is the assumption that only very large waves indicating a large
initial disturbance are capable of travelling to great distances, and as
pointed out by R.D. Oldham, large waves under the influence of gravity
will travel faster than small waves. These waves (which may be
gravitational or distortional) are recorded as slow tiltings of the
ground measured by angles of 0.5 to 10 or 15 seconds of arc, or as
horizontal displacements of 0.5 or several millimetres. Their calculated
lengths have reached 50 kms. (31 m.).
Frequency.
In the section of this article relating to the cause of earthquakes a
little has been said about their frequency or the number of times these
phenomena are repeated during a given interval of time. It has been
shown that all countries are very often moved by earthquakes which have
originated at great distances. Great Britain, for example, is crossed
about 100 times a year by earthquake waves having durations of from 3
minutes to 3 hours, whilst the vibratory motions which originate in that
country are not only small but of rare occurrence. In the earlier stages
of the world's history, because the contraction of its nucleus was more
rapid than it is at present, it is commonly inferred that phenomena
accompanying bradyseismical activity must have been more pronounced and
have shown themselves upon a grander scale than they do at the present
time. Now, although the records of our rocks only carry us back over a
certain portion of this history, they certainly represent an interval of
time sufficiently long to furnish some evidence of such enfeeblement if
it ever existed. So far from this being the case, however, we meet with
distinct evidences in the later chapters of geological history of
plutonic awakenings much more violent than those recorded at its
commencement. During Palaeozoic times many mountain ranges were formed,
and accompanying these orogenic processes there was marked volcanic
activity. In the succeeding Secondary period plutonic forces were
quiescent, but during the formation of the early Tertiaries, when some
of the largest mountain ranges were created, they awoke with a vigour
greater than had ever been previously exhibited. At this period it is
not improbable that Scotland was as remarkable for its volcanoes and its
earthquakes as Japan is at the present day. If the statement relating to
the general decrease in bradyseismical changes referred merely to their
frequency, and omitted reference to their magnitude, the views of the
geologist and physicist might harmonize. One explanation for this
divergence of opinion may rest on the fact that too little attention has
been directed to all the conditions which accompany the adaptation of
the earth's crust to its shrinking nucleus. As the latter grows smaller
the puckerings and foldings of the former should grow larger. Each
succeeding geological epoch should be characterized by mountain
formations more stupendous than those which preceded them, whilst the
fracturing, dislocation, caving-in of ill-supported regions, and
creation of lines of freedom for the exhibition of volcanic activity
which would accompany these changes, would grow in magnitude. The
written records of many countries reflect but on a smaller scale the
crystallized records in their hills. In 1844, at Comrie, in Perthshire,
as many as twelve earthquakes were recorded in a single month, whilst
now there are but one or two per year. Earthquake frequency varies with
time. A district under the influence of hypogenic activities reaches a
condition of seismic strain which usually is relieved rapidly at first,
but subsequently more slowly.
The small shocks which follow an initial large disturbance are known as
after-shocks. The first shock which in 1891 devastated central Japan was
accompanied by the formation of a large fault, and the 3364 small shocks
which succeeded this during the following two years are regarded as due
to intermittent settlements of disjointed material. The decreasing
frequency with which after-shocks occur may be represented by a curve.
Dr F. Omori points out that the continuation of such a curve gives the
means of determining the length of time which will probably elapse
before the region to which it refers will return to the same seismic
quiescence that it had prior to the initial disturbance.
Periodicity.
The positive results that we have respecting the periodicity of
earthquakes are but few. Generally earthquakes are somewhat more
frequent during winter than during summer, and this applies to both the
northern and southern hemispheres. The annual periodicity, which,
however, does not show itself if only destructive earthquakes are
considered, finds an explanation, according to Dr Knott, in the annual
periodicity of long-continued stresses, as for example those due to the
accumulation of snow and to barometric gradients. For certain earthquake
regions there appears to be a distinct semi-annual period for which no
satisfactory explanation has yet been adduced. Although the elaborate
registers of Japan, which have enabled us to group earthquakes according
to their respective origins and varying intensities, and to separate
after-shocks from initial disturbances, have been subjected by Dr Knott
to most careful analysis, with the object of discovering periodicities
connected with the ebb and flow of the tides, the lunar day or lunar
months, nothing of marked character has been found. Certainly there is
slight evidence of a periodicity connected with the times of conjunction
and opposition of the sun and moon, and a maximum frequency near the
time of perigee, but the effect of lunar stresses is comparatively
insignificant. Ordinary earthquakes, and especially after-shocks, show a
diurnal period, but we cannot say that there are more earthquakes during
the night than during the day.
Magnetic phenomena.
Many experiments and investigations have been made to determine a
possible relationship between earthquakes and electrical phenomena, but
beyond drawing attention to the fact that luminous appearances may
accompany the friction of moving masses of rock, and that a temporary
current may be established in a line by the disturbance of an
earth-plate, these inquiries have yielded but little of importance. The
inquiries respecting a possible relationship between adjustments so
frequently taking place within and beneath that region called the crust
of the earth and magnetic phenomena are, however, of a more promising
nature. We have seen that at or near the origin of earthquakes which for
several hours disturb continents, and occasionally cause oceans to
oscillate for longer periods, we sometimes have direct evidence of the
bodily displacement of many cubic miles of material. When this material
is volcanic it is almost invariably magnetic, and we perceive in its
sudden rearrangement causes which should produce magnetic effects within
an epifocal district. In Japan, where attention is being directed to
phenomena of this description, not only have such effects been observed,
but unusual magnetic disturbances have been noted prior to the
occurrence of large earthquakes. These may, of course, be regarded as
mere coincidences, but when we consider volcanic and seismic activities
as evidences of physical and chemical changes, together with mechanical
displacements of a magnetic magma, it is reasonable to suppose that they
should have at least a local influence upon magnetic needles. Another
form of disturbance to which magnetic needles are subjected is that
which accompanies the passage of large earth-waves beneath certain
observatories situated at great distances from earthquake origins. At
Utrecht, Potsdam and Wilhelmshaven the magnetographs are frequently
disturbed by seismic waves, whilst at many other European observatories
such effects are absent or only barely appreciable. To explain these
marked differences in the behaviour of magnetic needles at different
stations we are at present only in a position to formulate hypotheses.
They may be due to the fact that different needles have different
periodic times of oscillation; it is possible that at one observatory
the mechanical movements of the ground are much greater than at others;
we may speculate on the existence of materials beneath and around
various observatories which are different in their magnetic characters;
and, lastly, we may picture a crust of varying thickness, which from
time to time is caused to rise and fall upon a magnetic magma, the
places nearest to this being the most disturbed.
Effects on the human mind.
A subject to which but little attention has been directed is the effect
which displays of seismic and volcanic activities have had upon the
human mind. The effects are distinctly dual and opposite in character.
In countries like England, where earthquakes are seldom experienced, the
prevailing idea is that they are associated with all that is baneful.
For certain earthquakes, which fortunately are less than 1% of those
which are annually recorded, this is partially true. A disastrous shock
may unnerve a whole community. Effects of this nature, however, differ
in a marked manner with different nationalities. After the shock of
1891, when Japan lost 9960 of its inhabitants, amongst the wounded
indications of mental excitement were shown in spinal and other trouble.
Notwithstanding the lightheartedness of this particular nation, it is
difficult to imagine that the long series of seismic effects chronicled
in Japanese history, which culminated in 1896 in the loss of 29,000
lives by sea-waves, has been without some effect upon its mental and
moral character. Several earthquakes are annually commemorated by
special services at temples. In bygone times governments have recognized
earthquakes as visitations of an angry deity, whom they have endeavoured
to appease by repealing stringent laws and taxes. In other countries the
sermons which have been preached to show that the tremblings of the
world were visitations consequent on impiety, and the prayers which have
been formulated to ward off disasters in the future, far exceed in
number the earthquakes which gave rise to them. In 1755 many of the
English clergy held the view that Lisbon was destroyed because its
inhabitants were Catholics, whilst the survivors from that disaster
attributed their misfortune to the fact that they had tolerated a few
Protestant heretics in their midst. To avoid a recurrence of disaster
certain of these were baptized by force. In the myths relating to
underground monsters and personages that are said to be the cause of
earthquakes we see the direct effects which exhibitions of seismic and
volcanic activity have produced upon the imagination. The beliefs, or
more properly, perhaps, the poetical fancies, thus engendered have
exhibited themselves in various forms. Beneath Japan there is said to be
a catfish, which in other countries is replaced by a mole, a hog, an
elephant or other living creature, which when it is restless shakes the
globe. The Kamchadales picture a subterranean deity called Tuil, who in
Scandinavian mythology is represented by the evil genius Loki. We have
only to think of the reference in the Decalogue forbidding the making of
graven images of that which is in the earth beneath, to see in early
Biblical history evidence of a subterranean mythology; and it seems
probable that the same causes which led to the creation of Pluto, Vulcan
and Poseidon gave rise to practices condemned by Moses.
Building to withstand earthquakes.
Perhaps the greatest practical benefits derived from seismological
investigations relate to important changes and new principles which have
been introduced into the arts of the engineer and builder when
constructing in earthquake countries. The new rules and formulae, rather
than being theoretical deductions from hypotheses, are the outcome of
observation and experiment. True measures of earthquake motion have been
given to us by modern seismometers, with the result that seismic
destructivity can be accurately expressed in mechanical units. From
observation we now know the greatest acceleration and maximum velocity
of an earth particle likely to be encountered; and these are measures of
the destructivity. The engineer is therefore dealing with known forces,
and he has to bear in mind that these are chiefly applied in a
horizontal direction. A formula connecting the acceleration requisite to
overturn bodies of different dimensions has been given. The acceleration
which will fracture or shatter a column firmly fixed at its foundation
to the moving earth may be expressed as follows:--
1 gFAB
a = -- ----,
6 fw
where
a = the acceleration per sec. per sec.
F = the force of cohesion, or force per unit surface, which when
gradually applied produces fracture.
A = area of base fractured.
B = thickness of the column.
f = height of centre of gravity of column above the fractured base.
w = the weight of the portion broken off.
With this formula and its derivatives we are enabled to state the height
to which a wall, for example, may be built capable of resisting any
assumed acceleration. Experience has shown that yielding first shows
itself at the base of a pier, a wall or a building, and it is therefore
clear that the lower portion of such structures should be of greater
dimensions or stronger than that above. Piers having these increased
dimensions below, and tapering upwards in a proper manner, so that every
horizontal section is sufficiently strong to resist the effects of the
inertia of its superstructure, are employed to carry railways in Japan.
In that country cast-iron piers are things of the past, whilst piers of
masonry, together with their foundations, no longer follow the rules of
ordinary engineering practice.
After flood, fire, earthquake, or when opportunity presents itself,
changes are introduced in the construction of ordinary buildings. In a
so-called earthquake-proof house, although externally it is similar to
other dwellings, we find rafters running from the ridge pole to the
floor sills, an exceedingly light roof, iron straps and sockets
replacing mortices and tenons, and many other departures from ordinary
rules. Masonry arches for bridges or arched openings in walls (unless
protected by lintels), heavy gables, ornamental copings, cappings for
chimneys, have by their repeated failure shown that they are undesirable
features for construction in earthquake countries. As sites for
buildings it is well to avoid soft ground, on which the movement is
always greater than on hard ground. Excessive movement also takes place
along the face of unsupported openings, and for this reason the edges of
scarps, bluffs, cuttings and river-banks are localities to be avoided.
In short, the rules and precautions which have to be recognized so as to
avoid or mitigate the effects of earthquake movement are so numerous
that students of engineering and architecture in Japan receive a special
course of lectures on this subject. When it is remembered that a large
earthquake may entail a loss of life greater than that which takes place
in many wars, and that for the reconstruction of ordinary buildings,
factories and public works an expenditure of several million pounds
sterling is required, the importance of these studies cannot be
overrated. Severe earthquakes are fortunately unknown in the British
Isles, but we have simply to turn our eyes to earthquake-shaken colonies
and lands in close commercial touch with Great Britain to realize the
importance of mitigating such disasters as much as possible, and any
endeavour to obviate the wholesale destruction of life should appeal to
the civilized communities of the world.
Applications of seismometry.
An unexpected application of seismometry has been to record the
vibration of railway trains, bridges and steamships. An instrument of
suitable construction will give records of the more or less violent
jolting and vibratory movements of a train, and so localize
irregularities due to changes in the character of ballast and sleepers,
to variation in gauge, &c. An instrument placed on a locomotive throws
considerable light upon the effects due to the methods of balancing the
wheels, and by alterations in this respect a saving of fuel of from 1 to
5 lb. of coal per mile per locomotive has sometimes been effected.
By mapping the centres from which earthquakes originate off the coast of
Japan, we have not only determined districts where geological activity
is pronounced, but have placed before the cable engineer well-defined
localities which it is advisable to avoid; and in the records of unfelt
earthquakes which originate far from land similar information is being
collected for the deeper parts of the oceans. Occasionally these records
have almost immediately made clear the cause of a cable failure. From
lack of such information in 1888, when the cables connecting Australia
with the outer world were simultaneously broken, the sudden isolation
was regarded as a possible operation of war, and the colonists called
out their naval and military reserves. Records of earthquakes
originating at great distances have also frequently enabled us to
anticipate, to correct, to extend, or to disprove telegraphic accounts
of the disasters. Whatever information a seismogram may give is certain,
whilst the information gathered from telegrams may in the process of
transit become exaggerated or minimized. Otherwise unaccountable
disturbances in records from magnetographs, barographs and other
instruments employed in observatories are frequently explained by
reference to the traces yielded by seismometers. Perhaps the greatest
triumph in seismological investigation has been the determination of the
varying rates at which motion is propagated through the world. These
measurements have already thrown new light upon its effective rigidity,
and if we assume that the density of the earth increases uniformly from
its surface towards its centre, so that its mean density is 5.5, then,
according to Knott, the coefficient of elasticity which governs the
transmission of preliminary tremors of an earthquake increases at a rate
of nearly 1.2% per mile of descent. (J. Mi.)
AUTHORITIES.--J. Milne, _Seismology_ (London, 1898), _Earthquakes_
(London, 1898), Bakerian Lecture, "Recent Advances in Seismology,"
_Proc. Roy. Soc._, 1906, 77, p. 365; J.A. Ewing, _Memoir on Earthquake
Measurement_ (Tokyo, 1883); C.E. Dutton, _Earthquakes in the Light of
the New Seismology_ (London, 1904); "The Charleston Earthquake of Aug.
31, 1886," Ninth Annual _Report_ of the United States Geological
Survey, 1889; W.H. Hobbs, _Earthquakes, an Introduction to Seismic
Geology_ (London, 1908), "The San Francisco Earthquake and Fire,
1906," _Bull. U.S. Geol. Surv._ No. 324; "The California Earthquake of
Ap. 18, 1906," _Rep. State Earthq. Com._ (Washington, D.C., 1908);
R.D. Oldham, "Report on the Great Earthquake of 12 June 1897," _Mem.
Geol. Surv. India_, xxix. 1899, "On the Propagation of Earthquake
Motion to great Distances," _Phil. Trans._, 1900, A, vol. 194, p. 135,
"The Constitution of the Interior of the Earth as revealed by
Earthquakes," _Quar. Jour. Geol. Soc._, 1906, 62, p. 456; 1907, 63, p.
344; C. Davison, _A Study of Recent Earthquakes_ (London, 1905); _The
Hereford Earthquake of December 17, 1896_ (Birmingham, 1899), "The
Investigation of Earthquakes," _Beitraege z. Geophysik_, Bd. ix., 1908,
p. 201, and papers on British earthquakes in _Quart. Jour. Geol.
Soc._; T.J.J. See, "The Cause of Earthquakes, Mountain Formation and
Kindred Phenomena connected with the Physics of the Earth," _Proc.
Amer. Phil. Soc._, 1906, 45, p. 273; F. Frech, "Erdbeben und
Gebirgsbau," _Petermann's Mitteilungen_, Bd. 53, 1907, p. 245 (with
maps); C.G. Knott, _The Physics of Earthquake Phenomena_ (Oxford,
1908); Comte F. de Montessus de Ballore, _Les Tremblements de terre:
geographie seismologique_ (Paris, 1906), _La Science seismologique_
(1907); _Transactions of the Seismological Society of Japan;
Seismological Journal_ (Yokohama); _Bollettino della Societa
Sismologica Italiana_ (Rome); _Reports of the British Association_,
containing the annual reports of the Committee for Seismological
Investigations; papers in the _Beitraege zur Geophysik_ and the
_Ergaenzungsbaende_.