An aftershock large enough for some residents to feel made the earth quiver Thursday morning near St. George. But it was far smaller than the 5.9 jolt that started the landslide that ruined three homes in Springdale, Washington County, on Sept. 2.
"There was an aftershock of magnitude 2.7," Sue Nava, senior staff seismologist at the University of Utah seismograph station, said Friday. It hit at 12:42 a.m.The Hurricane and Washington faults, which run through southwestern St. George, have long concerned scientists because each is believed capable of generating a whopper of 6.5 magnitude, according to the Utah Geological Survey. That would be many times larger than the Sept. 2 shake.
The Hurricane Fault extends from Cedar City to the Grand Canyon and is thought to be one of the most active faults in southern Utah.
A handful of smaller jolts, barely strong enough to make the seismograph pens wiggle, also were recorded in the week and a half since the quake.
Dozens of aftershocks of 3.0 or greater magnitude on the Richter scale would ordinarily have shaken the region by now. Strangely, they haven't, and earth scientists don't know what this means.
"U. seismologists are continuing to pay close attention to the St. George earthquake sequence and will continue to research behavior in the Utah region," Nava said.
A report prepared by Walter J. Arabasz, James C. Pechmann and Nava, all of the U. Seismograph Station, said that preliminary data indicate the earthquake originated at a depth of nine miles "and was caused by dominantly normal faulting on a north-south tending fault, possibly a subsurface part of the Hurricane Fault."
The Utah Geological Survey listed two other unusual aspects of the earthquake:
- "The Springdale landslide is one of the largest ever recorded to have resulted from an earthquake of this magnitude, according to geologists of the U.S. Geological Survey, and it is farther from the epicenter than landslides typically occur in a magnitude 5.9 earthquake.
- "Besides the landslide, the major geologic effects of the earthquake involved only scattered rockfalls near and particularly east of the epicenter and liquefaction along the Virgin River evidenced by small sand boils and ground cracks."