Dozens of aftershocks pulsed through a four-state region following Thursday morning's 5.5-Richter earthquake on the Idaho-Wyoming border. One of them, a 5.2 shake at 7:42 p.m. Thursday, was felt in Salt Lake City.
The largest quake hit at 2:05 a.m., damaging the Wyoming state fish hatchery just on the Wyoming side of the state line, near Auburn, Wyo. It is estimated at 5.5 on the Richter Scale, the scale most commonly cited in earthquake measurements.University of Utah Seismograph Stations personnel say the strength is between 5.8 and 5.9 on another type of calibration, called the "moment magnitude scale." Walter J. Arabasz, director of the stations, said the moment scale better measures the total seismic energy radiated from an earthquake.
The aftershocks were centered in the same region as the main earthquake, said Sue Nava, senior staff seismologist at the stations. The epicenter was calculated at 24 miles east-northeast of Soda Springs, Idaho, and 12 miles west of Afton, Wyo.
As of 3 p.m. Thursday, more than 27 aftershocks of 3.5 or greater on the Richter scale were recorded by the stations. In addition, "there were quite a few last night," said Nava.
She said seismologists expect the shaking to continue, "although we hope that they will diminish in frequency and magnitude."