3 questions answered: Why Thursday’s earthquake near Truckee could happen here

Jessica Skropanic
Siskiyou Daily News

Since mid-April, a series of small earthquakes jiggled in a jagged area from Tennant in Siskiyou County down to Truckee in Nevada County, passing through Shasta County north of Shasta Lake and near Burney.

Most of the more than 100 earthquakes that hit the North State over the past 30 days were a magnitude of 2.0 or less on the Richter Scale — meaning they are too small to be felt, according to the U.S. Geological Survey (USGS). 

But one of the latest quakes, 4.7 in magnitude, hit the Truckee area on Thursday night, the USGS reported. At that magnitude, it could be felt by a lot people in the area and miles away.

This USGS map shows earthquakes that shook the North State since mid-April through May 7, 2021.

A number of things make the Shasta Cascade region — an area that includes Shasta and Siskiyou counties — vulnerable to earthquakes, said Alex Hatem, research geologist for the USGS.

Here are three questions answered about earthquakes in the North State.

1. What happened Thursday night near Truckee?

The temblor was centered in an area about 12-13 miles northwest of Truckee, Hatem said.

It originated about 10 kilometers below the surface of the ground. “That’s about average for an earthquake in that part of the world,” she said.

Image from the USGS where a 4.7 earthquake hit  north west of Truckee just after 9:35 p.m.

That earthquake was not part of the seismic activity that happens on California's Coast.

Instead it was part of an active area surrounding the Great Basin, one that includes the Shasta Cascade region.

That area of California — from the middle of Northern California to Nevada, and to the Oregon border — is prone to earthquakes, Hatem said.

2. What makes Shasta and Siskiyou counties vulnerable to earthquakes?

There are two things at play that cause earthquakes in Northern California, Hatem said. Siskiyou and Shasta counties are at the nexus of these two processes.

First, the Pacific tectonic plate is moving east and squeezing under the North American plate. That’s what makes California's coast north of Mendocino shake. It also creates volcanoes — including Mt. Shasta — along the Ring of Fire which loops through Shasta and Siskiyou counties.

Second, there’s an extension of the North American plate happening within the Great Basin. This is a separate phenomenon from what’s happening on the coast. Yreka, Mount Shasta and Redding — everything west of the Sierra Nevada — is moving farther west. Everything east of the Wasatch Mountain Range near Salt Lake City is moving farther east.

The area between the Sierra Nevada and the Wasatch Mountains is splitting. That split area is called the Great Basin and range.

“Think of it as pulling apart Silly Putty,” Hatem said.

  • On the upper left side of the putty, where the tip of your left thumb is holding it, is Yreka. Below it is Mount Shasta and Redding.
  • Your right hand, on the right edge of the putty, is where Utah would be.
  • The middle is the Great Basin, including Nevada, Utah and Eastern California.

Now pull that putty and watch the Great Basin stretch.

The earthquake near Truckee is part of the shift in the Great Basin — a process always going on, she said.

3. How likely is it a big earthquake will hit Siskiyou and Shasta counties?

It’s hard to give that estimate, Hatem said, but the area is classified by the USGS as a moderate hazard.

That means we don’t have frequent earthquakes, but we do get shaken up once in awhile.

“Most earthquakes are small, but there can be larger ones, too — like a 6.0,” Hatem said.

A 6.0 is felt by almost everyone, and can break windows, collapse chimneys and knock plaster off the walls, according to the USGS. 

To sign up for the USGS’ shake alerts, go to https://on.doi.gov/3b9ZVVX or follow on Twitter at https://bit.ly/33w2Ofj.

To see an interactive programmable map of earthquake activity, go to the USGS Earthquake Map website at https://on.doi.gov/3tCcBeN.

Jessica Skropanic is a features reporter for the Record Searchlight/USA Today Network. She covers science, arts, social issues and entertainment stories. Follow her on Twitter @RS_JSkropanic and on Facebook. Join Jessica in the Get Out! Nor Cal recreation Facebook group. To support and sustain this work, please subscribe today. Thank you.