Residents of the St. George region have a 30 percent chance of experiencing an aftershock of 5.0 on the Richter scale or larger in the next few weeks, according to the senior staff seismologist at the University of Utah seismograph station.
Sue Nava made that prediction Wednesday afternoon, while the staff at the seismograph station waited for the aftershock.Aftershocks can hit the region for several months following an earthquake. "The probability of having one or more aftershocks at the magnitude 5.0 or larger is 20 percent during the first day," she said.
That didn't happen, but it's even more likely that a shock will hit in the next few weeks. The U. was studying one possible shake that hit Thursday morning. A rumble of 5.0 could cause damage.
The seismograph station boasts a wall of slowly revolving steel drums, where earth movements are shown; wall maps pinpointing seismograph stations; charts; reference books and computer links.
The station is on the seventh floor of the U's William Browning Building - which seems a strange place to study ground movement. But Nava explained that the seismometer that senses the earth trembling is in the basement and that the office is a kind of nerve center for a network of 80 stations scattered throughout Utah.
"We have a seismometer down there that measures the ground motion. It amplifies the signal 2,800 times so we can see it, basically," she said.
Nava showed a chart that recorded Wednesday's quake, which the station estimated at 5.9 on the Richter scale. The chart shows a flat line until some small movements, then the pen jiggled wildly for more than a minute before tapering off.
The network of 80 remote seismograph stations also records earth movements. These unattended instruments send data to the Browning building by radio, microwave and telephone.
The U.S. Geological Survey estimated Wednesday's temblor at 5.5, but Nava defends the U.'s higher figure. The USGS calculation was made by the National Earthquake Information Center based in Boulder, Colo.
"Our stations are closer, and we feel our magnitude will be more accurate," she said.
People felt Wednesday's quake in far-flung locations, Nava said, "west to Las Vegas, south to Flagstaff, east to the Escalante-Boulder area, north to Richfield."
The university dispatched two crews, of two persons each, to southwestern Utah on Wednesday. "Their purpose is to put out portable seismometers to help us locate the depth" of any aftershock, she said.
An earthquake can be centered anywhere from half a mile to four miles beneath the surface. Knowing the depth is important in pinpointing which fault was involved, because the faults angle down unpredictably.
"A shallow earthquake will cause more local damage" than a deeper movement of the same intensity.
Knowing much about Wednesday's quake is difficult "because we have inadequate seismograph stations in southern Utah." The nearest permanent station to the epicenter is in Cedar City, 55 miles to the north.