Just how common are mass earth shakeups like the Montana-Yellowstone
Earthquake, anyhow?
Geologists tell us they're frequent, with a dozen or more major quakes,
and thousands of minor tremors happening each year.
Earthquakes are the natural outcome of the fact that the earth, while
seeming substantial and changeless, is constantly, if most gradually, in
the process of change. Mountains are thrust up. Glaciers carve them
down. Volcanoes pour out their molten rock. Rivers and floods scour
their erosive paths. Sediments slide and settle.
The enormous masses which great internal earth forces have raised up to
mountain height, create counter stresses. These forces build up for
years, sometimes for centuries or longer.
Eventually something has to give. When this happens on a grand, or
spectacular scale, we call it an earthquake.
[Illustration: Stress, caused by the quake, resulting from slump of
surficial material caused the attractive curving of this fence along
Highway 287 east of Hebgen Dam. (U. S. Geological Survey)]
Whether you're a connoisseur, expert, or not, the spectacular 1959
Montana-Yellowstone Earthquake was a beaut. Geologists call it a
"textbook earthquake," because it included nearly all of the classic
actions, or results which quakes are likely to cause.
[Illustration: Damkeeper George Hungerford points out how much the
tidal waves, or seiches, overtopped Hebgen Dam at the time of the
quake. Hungerford is standing on earthfill washed down from dam-top
level by the gigantic waves. The quake also cracked the dam's
concrete core as shown. (Montana Power Co.)]
It ranked right along with San Francisco's 1906 shakedown as among the
severest earthquakes on the North American continent.
In seismic measurements, it rated 7.8 on the Richter Scale, as compared
with San Francisco's 8.2.
It set up so called tidal waves, or seiches, on Hebgen Lake. There were
at least three of these huge waves--20-ft. high--which overtopped the
entire 721-ft. length of the dam by four feet. Eyewitness statements
relate that the velocity of the tidal wave was so great that it caused
the water literally to leap over the top of the dam. It filled the small
generating plant with a 2 to 4 ft. deep layer of rocks.
Although the dam stood, the quake caused several fractures in the core
wall, one of which showed a 3- to 4-inch separation, and shattered the
dam's concrete spillway.
The earthquake created three major faults, with displacement on the Red
Canyon Fault running as much as 20 ft., which stacks up impressively
alongside the 26-ft. maximum displacement resulting from San Francisco's
quake. (These two earthquakes differed, however, in that the Montana
displacement was vertical, while San Francisco's was horizontal.)
[Illustration: _BEFORE_]
[Illustration: _AFTER_]
According to the Society of Military Engineers, surveys from benchmarks
outside the earthquake-affected areas show that the earth in the Hebgen
Dam Quake area near Hebgen Dam has settled between eighteen and nineteen
feet from its level before the quake.
It wasn't uniform, though. The quake caused tilting, which showed up in
the way the north side of Hebgen Lake had sunk eight feet, while the
south side of the lake, docks, boats, etc., were sticking eight feet out
of the water.
The quake also caused many sink, or more properly, blow holes. These
phenomena are also known as sandspouts. Water, compressed and forced up
and out by quake action washes out layers of sand sub strata. The
overhead surface areas naturally drop into the hole, leaving a puzzling
hunk of slumped ground--separate from the normal scarps--as big as 15 x
50 ft. in area.
The Montana-Yellowstone quake sent seismographs jiggling as far away as
New Zealand. It caused fluctuation of water level in wells as much as
ten feet in nearby Idaho, a tenth of a foot in Hawaii, 3,200 miles away,
and .01 ft. in Puerto Rico.
The huge concrete Hungry Horse Dam, near Columbia Falls, Montana, 250
miles NW of the quake area, showed measurable displacement as a result
of the quake. In remote Seattle, the diminished tremors were still
strong enough to break loose the floating amphitheatre in Lake
Washington.
[Illustration: Rock Cr. Camp]
[Illustration: Collapse of Dolomite Buttress]
[Illustration: Camp as buried by landslide]
But by far the most spectacular effect of Montana's Earthquake was the
huge landslide at the mouth of the Madison Canyon.
At the site of the slide, a relatively strong and nearly vertical layer
of dolomite rock supported a huge bank, or mountain, of comparatively
unstable schist and kept it from sliding into the valley in the same way
that a retaining wall keeps a hillside terrace from slipping downhill.
The tremendous shock waves of the earthquake fractured this dolomite
buttress, and some 43 million cubic yards, or 80 million tons of rock,
timber, and other mountainside debris cascaded off the slope, hurtled
into the canyon, and surged up the opposite side, carrying huge trees
and house-sized boulders as if they were weightless, hollow toys.
When this huge mass whomped down onto the river bed, it forced out the
water and air trapped underneath at hurricane velocity. The huge slide
spurted mud, air, and water with such force as to send two-ton cars
sailing through the air, and to grind others to suitcase thickness
against the rocks.
All this happened in seconds.
It would take eight seconds for the mass at the top of the mountain to
fall to the valley floor 1,200 ft. below. At the time it reached this
point, the mass would be travelling 174 miles per hour. The time it took
to zoom half-a-mile across the valley, up the opposite canyon wall, then
split and flow three-quarters-of-a-mile up and down the valley (the
slide lies one-and-a-half-miles-long in the valley), was less than
thirty seconds!
The fact that timber from the face of the mountain is spread in
relatively uniform fashion over the entire surface of the slide is
interpreted to mean that there was little tumbling action--that the
slide moved as a single, if shattered, mass.
One important scientific controversy has emerged from the earthquake. It
relates to the time relationship, or sequence between the initial shock,
the tidal waves, or seiches, how fast the huge quantities of water which
overtopped the dam moved down the valley, and whether these slugs of
water had rushed through the canyon in time to reach the site of the
slide before the mountain fell.
The stretch of the Madison running through the canyon is fresh, fast
water, but normally it takes up to two hours for an object to run the
sparkling, seven-mile, trout-rich stretch from Hebgen Dam to the mouth
of the canyon. The big surges of water--the seiches overtopping the
dam--would make it a lot faster.
There are two big, related questions.
Could the big surges of water reach the point of the slide soon enough?
And just how soon after the first shock did the mountain fall?
[Illustration: Ruined building on the fault line.]
For the first couple of days after the quake, the theory persisted that
the slide must have happened quite some time after the first shock--as
late as 5:00 A. M., according to some theorists.
But, as the facts, and the testimony of folks trapped near the
slide--the Osts, Fredericks, Smiths, and Mrs. Bennett--became available,
it was apparent that the slide must have closely followed the initial
shock. Even if you discount the disrupted time sense of people under
stress--when a minute can seem like an hour, and vice versa--it's
difficult to imagine that more than 20 minutes elapsed between the first
shock and the slide.
According to one set of calculations, big waves could have swept from
the dam to the slide site in 18 minutes or so.
[Illustration: A man stands in the gap left by the earthquake-caused
simple fault scarp. (U. S. Geological Survey)]
[Illustration: Something slipped here! Before the quake caused the
ground drop, creating this magnificent scarp, the ground surface
shown here was continuous. (U. S. Forest Service)]
[Illustration: Surface elevation after quake.]
Hebgen Lake
Hebgen Fault
Red Canyon Fault
Although the quake caused much settling of the earth packed against the
downstream side of Hebgen Dam's concrete core, the relatively slight
displacement of the sod cover is interpreted to mean that all three
tidal waves passed over the dam before this earth subsided and separated
from the core. Thus the water would have begun its race down the valley
before the heavy earth-settling shocks hit the dam area.
Those who support the high-water-at-the-moment-of-the-slide theory point
to the great volume of water damage way below the slide.
If the slide had come first, it would have dammed off the tidal waves,
and prevented such damage. They feel there just wasn't enough water in
the river bed's normal content to cause the water damage done both
upstream and downstream by the slide. And they argue, the mud and dust
in the composition of the slide would have taken up most of the water
normally found in the reach of the river buried under the slide.
There's further evidence in the numerous fish found high and dry on the
flat along the river bank several feet higher than the streambed. Most
of them were small, catfish-like chubs. There were numerous trout, and
one 18-inch carp. There is no place in the river below the pool at the
toe of the dam where carp would likely be found.
Also, there was further confirmation in the fact that three of the
especially made 11-inch squared timbers, eight and a half feet long,
with notched ends and two U-bolts used as stop-logs in the Hebgen Dam
spillway were found below the slide. Some shadow was cast on this as
absolute confirmation by the Montana Power Co.'s explanation that
stop-logs have been lost from time to time before the quake.
Those who, in spite of such evidence, oppose the theory that the high
water reached the slide area first just don't feel that the water could
have made it all the way down the canyon in so short a time. They feel
that it would have taken at least 40 minutes for the big waves to
traverse the seven miles. They have some support in L. D. Smith's
testimony that in driving down from Beaver Creek to Rock Creek right
after the shocks, he saw no such waves.
At any rate, this is one argument that geologists and hydrologists will
be batting around for a long time.
[Illustration: This view of the Madison Canyon slide gives the
feeling of the up-canyon and down-canyon flow of the 80-million-ton
mass of rock and debris. (Montana Highway Commission)]