Yellowstone earthquakes revealed

YELLOWSTONE PARK FOUNDATION

Have you ever experienced an earthquake in Yellowstone? You might say no, but chances are at least one small quake occurred during your visit.

On average, approximately 1,000 to 3,000 earthquakes take place each year in the Yellowstone area. Most are too small to be felt, but a few—like the famous quake of 1959—are exceptions.

Yellowstone is one of the most seismically active areas in the U.S. The park is contained within the Intermountain Seismic Belt, a zone of earthquake activity that runs from Montana through Wyoming, Idaho, Utah, Nevada and Arizona. Yellowstone is also an active volcano, and surface features such as its famous geysers and hot springs are direct results of the region’s underlying volcanism.

The combination of tectonic and volcanic activity often results in earthquakes, during which rock fractures along fault lines that have become stressed beyond their breaking point. Energy is then released as shock waves, or seismic waves, that reverberate and travel at high speeds throughout the surrounding rock.

The Aug. 17, 1959 earthquake that struck west of Yellowstone National Park damaged highway 287 significantly.

Earthquakes do offer some benefits; if it wasn’t for the many small quakes, Old Faithful and other geysers and hot springs could become dormant. In Yellowstone, earthquakes help to maintain hydrothermal activity by keeping the “plumbing” system open. Without periodic shaking from earthquakes, the small fractures and conduits that supply hot water to geysers and hot springs might be sealed by mineral deposits—perhaps permanently.

People do not usually feel earthquakes less than a magnitude 3.0. It typically takes an earthquake of magnitude 4.0 or greater to cause structural damage, and a magnitude 6.5 earthquake to cause the surface of the ground to rupture. Though earthquakes greater than magnitude 6 are uncommon in Yellowstone, some have occurred.

The most famous, and most tragic, earthquake in the Yellowstone area happened 57 years ago. The 7.3-magnitude quake, which caused 18 miles of surface faulting and was felt across 600,000 square miles, decimated a popular campground at the height of tourist season.

On the evening of Aug. 17, 1959, approximately 250 visitors slept throughout the Madison River canyon near Hebgen Lake, just west of Yellowstone. Some bedded down in private cabins and resorts, others at Forest Service campgrounds or makeshift campsites along the roadsides.

Just before midnight, an earthquake jolted the travelers from their sleep. A short time later a wall of water hurdled over Hebgen Dam and raged down river. And then, the unthinkable happened: an enormous section of canyon wall broke loose and crashed down just below Rock Creek Campground.

According to a U.S. Forest Service interpretive exhibit at the Earthquake Lake Visitor Center:
“It was a night of terror and chaos. Scattered families fled through the darkness. Those who could, scrambled for high ground as the newly-formed lake rose to swallow their campsites and cars. Others, trapped by the rising water, screamed for help. It would be many long, dark, and terrifying hours before dawn—and it would be many days before it was known just who had lived, and who was lost.”

Ultimately, it was learned that 28 people were killed, 19 of whom are still entombed within the landslide debris.

A visit to “Quake Lake,” as it is now known, is an easy side trip during a visit to Yellowstone. The Earthquake Lake Visitor Center, open from late May through late September, sits just above the lake formed by the Hebgen Lake earthquake. It offers interpretive displays, a working seismograph, walking path to a memorial boulder, scheduled movies and talks, and a Forest Service store operated in partnership with the Yellowstone Association.

While mostly invisible to the casual visitor, an extensive geological monitoring system is in place within Yellowstone. The park’s earthquake activity is tracked around the clock by the University of Utah Seismograph Stations, as part of the Yellowstone Volcano Observatory, a cooperative effort of the National Park Service, U.S. Geological Survey and the university.
According to seismologists from the University of Utah, which operates 26 seismograph stations throughout the park, earthquake swarms are relatively common in Yellowstone. A swarm is a series of small earthquakes in a localized area—sometimes more than 100 in a single day.

The largest known swarm occurred in 1985, with more than 3,000 earthquakes recorded during three months along the West Entrance road. More recently, during the winter of 2010, nearly 2,000 quakes were recorded between Old Faithful and West Yellowstone.
This comprehensive monitoring system helps scientists map and create a better understanding of the sub-surface geology around and beneath Yellowstone.

A version of this story was first published in the August 2016 issue of The Yellowstone Steward eNews. Visit ypf.org to learn more.