Richland’s Chamna Preserve earthquake unusual for this area. Here’s how it was different

The Friday night earthquake that lightly rattled southern areas of the Tri-Cities was a bit of an outlier for the area.

Earthquakes already are not common in the Tri-Cities area, but most recent ones have been either much shallower or much deeper.

At about 9:20 p.m. Friday a 2.8 magnitude earthquake was registered in the Richland area by the Pacific Northwest Seismic Network.

The LIGO Hanford observatory, where several seismometers are located, said the epicenter was just across the Yakima River from the Chamna Natural Preserve in Richland.

Seismologist Stephen Malone, a professor emeritus and the University of Washington and a former director of the Pacific Northwest Seismic Network, said that looking back as early as the 1960s earthquakes in the area typically occurred in swarms, with one quake triggering another and then another sometimes over weeks.

The hypothesis is that swarms likely took place in the Columbia River basalts created from sheets of molten lava 12 million to 20 million years ago, Malone said.

Some Tri-Cities area earthquakes have been quite shallow like the 2011 earthquakes along the Columbia River just north of Richland that were at depths of just over a mile or less.

There also have been deeper earthquakes near the Horse Heaven Hills Fault Zone and Rattlesnake Hills Fault Zone in previous decades. They likely took place beneath the basalt in older crustal rock, Malone said.

As recently as 2008, a 3.7 magnitude earthquake occurred about 12 miles beneath the ground near the Horse Heaven Hills.

The Friday earthquake, at about five miles deep, seems to fall right between those deeper and shallower earthquakes, which is not common, Malone said.

There is a possibility that the Friday earthquake was at the bottom of the basalt, he said.

But unlike swarms, where some of the movement is usually vertical, the recent earthquake had an “almost pure strike slip” with blocks moving horizontally, Malone said.

It is difficult to say if it was related to the Rattlesnake Hills Fault Zone, but it is possible that it could have taken place on a small piece of the fault or an offshoot from it, he said.

Every year there are more than 1,000 earthquakes with magnitude 1 or greater in Washington and Oregon. But typically just about two dozen of these are large enough to be felt.

The Richland earthquake was felt by some Tri-Cities area residents, particularly those in south Richland.

A few items fell off shelves and some pictures were left askew. Some people reported in social media posts that they felt a slight shaking and initially thought it was the wind, nearby street racing or the washing machine shaking.

Although there was speculation on social media that the small Friday earthquake might benefit the region by preventing a bigger one.

That’s a popular misconception, according to the Pacific Northwest Seismic Network.

In fact, an increase in small earthquakes in a region may very slightly increase the likelihood of a larger event, it said.