Wallace L. Ulrich

Acting Director & State Geologist
Wyoming State Geological Survey
P.O. Box 1347
Laramie, WY 82073

Wyoming State Geological Survey

Additional earthquakes felt in western Wyoming
August 6, 2010

Three additional earthquakes occurred on Aug. 5 and 6 in the same area as the two initial Aug. 4 earthquakes in eastern Teton County. They were felt in much of western Wyoming and portions of eastern Idaho, and there were no immediate reports of injuries or damage. Rockfalls were observed and reported on a guest ranch near the epicenter.

The seismic events are being monitored by the Wyoming State Geological Survey (WSGS) in partnership with the U.S. Geological Survey (USGS).

The first tremor, a light earthquake that occurred at 8:59 a.m. Aug. 5, was felt in Teton, Fremont and Sublette counties as well as Yellowstone National Park, according to information received by the WSGS.

The epicenter of the magnitude 4.2 earthquake was near that of the magnitude 4.8 event of Aug. 4, 20 miles east-northeast of Jackson and 40 miles west-northwest of Dubois.

The earthquake, which occurred 3.1 miles below the surface, was identified as intensity III near the epicenter and was felt in Kelly, Lander, Jackson, Moose, Dubois, Alta, Rock Springs and Yellowstone National Park as intensity II. During an intensity III event, ground motion is noticed by people indoors, standing motor cars may rock slightly and vibrations similar to a truck passing by can be felt.

The strength of an earthquake at its epicenter is called its magnitude, as measured on the Richter scale. Light earthquakes – those between magnitude 4.0 and 4.9 – do not usually cause structural damage.

The effect of an earthquake on the surface is called its intensity, as measured on the Modified Mercalli Intensity Scale. Intensities range from I (imperceptible shaking) to XII (catastrophic destruction).

At 11:45 a.m. Aug. 5, a magnitude 3.2 event occurred slightly west of the previous events. It was 15 miles east-northeast of Jackson and 45 miles west-northwest of Dubois at a depth of 4.3 miles. It was felt as intensity II in Jackson.

A third event, at 9:34 a.m. Aug. 6, was felt as intensity II in Kelly, Moose and Jackson. The earthquake was 15 miles northeast of Jackson and 45 miles west of Dubois at a depth of 3.1 miles.

The strongest of the five earthquakes to date is the Aug. 4 magnitude 4.8 event, which was felt as intensity IV in Kelly and Moose. During an intensity IV event, hanging objects swing, vehicles that are not moving rock, vibrations are felt similar to that of a passing truck, windows and dishes rattle and glasses clink.

A 3.0 magnitude earthquake occurred in the same area Aug. 4 and preceded the larger event by a few seconds.

Similar activity occurred in the area Jan. 7, 2004, when eight earthquakes were recorded within a six-hour period, with magnitudes ranging from 2.9 to 5.0.

WSGS personnel are preparing to install a strong-motion accelerometer in the area to obtain additional information. The instrument will measure ground acceleration associated with future seismic events.

Over the last three years, the WSGS and USGS, in collaboration with local partners including the Geologists of Jackson Hole and Grand Teton National Park, have helped upgrade the Teton Seismograph Network with broadband seismic stations. The station to be deployed near the epicenter of the most recent earthquakes will provide additional data for scientists and the public to better understand the seismicity in and around Jackson Hole and Teton County.

Keeping the public informed of seismic events in Wyoming and providing information pertaining to regional seismicity across the state are among the primary goals of the WSGS.

Individuals with information about ground motions or possible damage are encouraged to participate in the USGS “Did You Feel It” program by visiting http://earthquake.usgs.gov/earthquakes/dyfi/ to fill out a short questionnaire to record data about the earthquake. Additionally, those with information about an earthquake can contact WSGS Acting Director and State Geologist Wallace Ulrich at wallace.ulrich@wyo.gov or WSGS geologist Seth Wittke at seth.wittke@wyo.gov.

The WSGS has compiled seismological characterizations of all Wyoming counties, which include analyses of historic seismicity. See Earthquake Database for more information. These reports provide historic seismicity and information about long- and short-term seismic hazard analysis.