Earthquakes occur frequently in areas of active faulting and volcanism;
they are caused by sudden movements between adjacent blocks of the
earth’s crust as the crust adjusts to new conditions and pressures.
Because of its volcanic history and the fact that very recent fault
movements have occurred there, it is not surprising that Yellowstone is
an especially active earthquake area. Sensitive instruments
(_seismographs_) record an average of about five earth tremors daily in
and around the Park, and on rare occasions they may record 100 tremors
or more in a single day. Nearly all these tremors are so slight that
they cannot be felt by man, but at times, perhaps only once in a human
lifetime, one is triggered with high enough intensity to sharply draw
our attention to the very real earthquake potential that exists
constantly in this geologically active area. Such a high-intensity quake
occurred in the Yellowstone region near midnight on August 17, 1959.
The Hebgen Lake Earthquake, as it is known, was centered in the Madison
Valley along the west boundary of Yellowstone National Park about 12
miles north of the town of West Yellowstone, Montana (fig. 1). As a
result of the quake, a 200-square-mile area, occupied in part by the
Hebgen Lake reservoir, subsided a foot or more; maximum subsidence was
20 feet. Movements of several feet along old faults in the highlands
along the north side of the valley produced fresh scars several miles
long (fig. 54). Moreover, the severe vibrations that rocked the
surrounding countryside caused the loose silt, sand, and gravel of the
valley floor to slip and become “faulted” in many places. By far the
most drastic result was the shaking loose of a huge landslide in the
vicinity of the Rock Creek campground about 25 miles downstream on the
Madison River from the west boundary of the Park.
[Illustration: EARTHQUAKE DAMAGE. Severe damage caused by
reactivation of a fault during the Hebgen Lake Earthquake of August
17, 1959. The building is on the Blarneystone Ranch, about 10 miles
north of West Yellowstone, Montana, and 1½ miles west of the west
boundary of Yellowstone National Park. (Fig. 54)]
Within Yellowstone National Park, the quake caused only slight damage to
buildings at Old Faithful, Mammoth Hot Springs, and a few other places.
Small landslides also occurred in various places, for example, at Tuff
Cliff near Madison Junction. The earthquake affected many thermal
features, particularly those in the main geyser basins near the west
side of the Park. In several places the intensity of the thermal
activity increased markedly, in fewer places the activity decreased.
Some geysers, long dormant, erupted immediately after the earthquake;
others erupted with much greater force and frequency than usual; still
others became dormant and have remained so. A general, widespread effect
was a noticeable increase in the muddiness of many pools and springs, as
if the quake had produced a giant surge of water coursing through the
underground channels of the geyser basins. Of immediate concern to
everyone, of course, was the earthquake’s effect on Old Faithful.
Fortunately, the only measurable effect was a slight lengthening of time
between eruptions. After several months the time interval stabilized at
about 65 minutes.
Detailed scientific studies bearing on the Hebgen Lake Earthquake in the
weeks that followed showed that it was felt over more than 600,000
square miles of the Western United States, and that it was the strongest
shock ever recorded in this part of the Rocky Mountains. In 12 years’
time many traces of the quake have disappeared, but its frightful
aspects will not soon be forgotten. It serves as a vivid reminder, once
again, of the great restlessness that through the ages has been, and
continues to be, the very special trademark of the Yellowstone country.
_The Park and Man_
A hundred years ago another powerful force entered the Yellowstone
scene. Man, arriving in ever increasing numbers, came armed with the
power to choose between preserving or destroying the wonders that nature
has taken more than 2.5 billion years to create. Sensing this grave
responsibility, he took the necessary steps to insure that these
irreplaceable natural features would be preserved and protected. Today,
Yellowstone National Park indeed exists “for the benefit and enjoyment
of the people,” a fitting and lasting symbol of a great national
heritage that now includes more than 275 places of natural and
historical interest. On the eve of the 100th anniversary of our first
National Park, we are again reminded of that continuing responsibility
we all share in preserving these unique places for the benefit and
enjoyment of all future generations of visitors.
_Acknowledgments_
The subject matter of this bulletin is based chiefly on the results of a
systematic program of geological investigations in Yellowstone National
Park, conducted by the U.S. Geological Survey during the years 1965-71.
The program, ably organized and directed by A. B. Campbell, required the
special skills and knowledge of many individuals to make a comprehensive
study of all the varied and complex features of the Park area. Without
their invaluable cooperation, assistance, and interest, this endeavor to
summarize the geologic story of Yellowstone would not have been
possible. I therefore express my sincere thanks to the colleagues listed
below, all of whom furnished unpublished information bearing on
different aspects of that story: R. L. Christiansen and H. R. Blank, Jr.
(Quaternary volcanism); H. W. Smedes and H. J. Prostka (Absaroka
volcanism); D. E. White, L. J. P. Muffler, R. O. Fournier, and A. H.
Truesdell (thermal activity); G. M. Richmond, K. L. Pierce, and H. A.
Waldrop (glaciation); E. T. Ruppel and J. D. Love (sedimentary rocks and
geologic structure); J. D. Obradovich and Meyer Rubin (radiometric
dating). W. L. Newman provided many helpful suggestions regarding the
preparation of the manuscript.
The geological studies in Yellowstone received the full support and
cooperation of former Park Superintendent J. S. McLaughlin,
Superintendent J. K. Anderson, and other personnel of the U.S. National
Park Service. In particular, the helpful advice, interest, and
enthusiasm of the entire Park Naturalist staff, especially J. M. Good
and W. W. Dunmire, former and present Chief Park Naturalists,
respectively, greatly facilitated the work in all phases of the program.