Thursday morning at 4:34 a.m. the Pacific Northwest Seismic Network (PNSN), based at the University of Washington, reported a magnitude 4.9 earthquake centered 22 miles northeast of Enterprise, with a depth of 7 kilometers (about 3.5 miles). The U.S. Geological Survey (USGS) picked up that information, and also reported the quake.
Around Wallowa County, those of us who have earthquake-reporting apps on our cell phones were also notified of the temblor.
However, the location, about halfway between Finley Buttes and Buckhorn Overlook, and shallow depth were somewhat out of character for quakes that might be expected here. Our quakes most commonly occur along the Wallowa Fault, the large structure that runs along the north side of the Wallowa Mountains.
Although other faults, including one that may follow parts of Hells Canyon, are known or suspected, few are active—except the Wallowa Fault, which commonly unleashes a magnitude 2 or magnitude 3 quake every few years.
Within an hour, Thursday’s Zumwalt Prairie quake was deleted from the quake listings and maps on both the PNSN and also the U.S. Geological Survey.
It was a quake that never happened.
The explanation, said USGS seismologist Paul Caruso, lies in the confluence of an actual earthquake in Idaho, and some degree of confusion by the PNWSN computers.
At the exact time the phantom Zumwalt Prairie quake was reported, there was a real, magnitude 4.4 quake 30 miles east of Cascade, Idaho.
“That’s a series of quakes and seismicity we have been monitoring for quite awhile,” Caruso said, “ever since the magnitude 7 in March. It’s pretty active. But this was a larger quake than the M3 and M2 quakes that happen there frequently now.”
Caruso said that the phantom Zumwalt Prairie quake was likely “recorded” because a M3.3 quake occurred in Nevada at virtually the same time as the Idaho quake, and the times of arrival of seismic waves at the PNWSN seismographs were virtually simultaneous, causing a miscalculation of location and magnitude. The problem was corrected and quakes assigned their real magnitudes and locations within 30 minutes.
No one was available for comment at the PNSN seismic lab.
Zumwalt Prairie quake or not, was a good reminder that Wallowa County is located in an area of active faults and seismicity.