KENNEWICK — More than 100 small earthquakes have been recorded since Saturday about 30 miles northwest of the Tri-Cities on the edge of the Hanford Reach National Monument, according to the Pacific Northwest Seismic Network.
The quakes were at the western edge of the McGee Ranch of the monument, which is Hanford site land used as a security perimeter around the production portion of the nuclear reservation during the years it produced plutonium for the nation’s nuclear weapons program.
“This is a completely natural phenomenon; although this swarm happens just outside the Hanford site, is has nothing to do with the radioactive waste stored there,” said the seismic network, a University of Washington and University of Oregon program to monitor earthquake and volcanic activity across the Pacific Northwest, in a blog post.
As of Tuesday, the seismic network put the number of earthquakes in recent days at about 100, but with some of the smallest likely missed and others not analyzed yet. On Wednesday, Doug Gibbons, a research engineer for the network, put the number at 115.
No reports of the earthquakes being felt had been reported to the network as of Wednesday morning.
That is to be expected, both because of the remote location of the earthquakes and because the quakes were a little too small to be felt widely, Gibbons said.
According to data on a map posted by the network, most were below a magnitude 2.0. The largest may have been a magnitude 2.9 on the north end of the swarm at 8:22 p.m. Sunday.
To be widely felt earthquakes have to be at least about magnitude 3.5, Gibbons said.
The earthquakes were relatively shallow, he said.
Earthquakes in Western Washington may be 30 to 40 miles deep. But these were mostly around 5 miles deep.
Gibbons called them “shallow crustal earthquakes.”
Scientists consider them a swarm, because there was no clear mainshock and only a short time between events, according to a Wednesday blog post by Renate Hartog, manager of the Pacific Northwest Seismic Network.
The earthquakes appear to be routine tectonic activity associated with the Yakima Fold and Thrust Belt and on a fault line associated with the Umtanum Ridge, Gibbons said.
Hartog looked into seismic activity in the region since limited cataloging started in 1969.
There are many swarms and small earthquakes in Eastern Washington, but fewer distinct swarm areas are active, she found.
In particularly the many swarms east and north of the Hanford site near the Saddle Mountains have been quiet in recent decades, she said.
Some researchers have further analyzed and described some of the Eastern Washington swarms in scientific papers, she said.
“But most have just been popping off unseen and unnoticed by anyone but our analysts,” she said in her blog post.
The swarm of earthquakes come just before the start of October, Earthquake Preparedness Month, Gibbons pointed out.
It is a good reminder that the Tri-Cities region does have earthquakes, he said.
