36 tiny earthquakes in 6 days: North State trembles over Labor Day weekend

USGS map of fault lines  from from Mt. Shasta to Burney.

Three clusters of tiny earthquakes that rattled the North State between Thursday and Tuesday likely have nothing to do with volcanic activity.

Instead, one scientist believes said the small shakers are probably linked to movements in the Earth's crust.

The small quakes were centered as far south as Chester and as far north as McCloud.

According to the U.S. Geological Survey (USGS), some 20 earthquakes rumbled between 11 p.m. Sunday and 6:30 a.m. Tuesday. All were centered 8 to 8.7 miles north of McCloud. Their magnitude measured low on the Richter scale: between 1.5 to 2.7.

The McCloud quakes followed on the heels of two similar clusters over the Labor Day weekend.

Seven tiny quakes hit Thursday about 14 miles southeast of Burney. The strongest of those quakes reached 2.5 on the Richter scale. 

A 2.1 earthquake shook 10 miles northwest of Chester on Friday followed by eight little quakes in the same area. All measured 0.6 to 1.4 on the Richter scale.

While the earthquakes occurred close to Lassen Peak and Mt. Shasta, it’s unlikely they’re related to volcanic activity, said Jessica Ball, volcanologist at the USGS California Volcano Observatory in Menlo Park.

This screenshot of a USGS Earthquake map shows 20 small earthquakes within about 8.5 miles of McCloud from Sunday night to Tuesday morning.

The quakes in the McCloud area are “likely tectonic (relating to movement in earth’s crust) since they are located in the vicinity of a known normal fault in the Mud Creek area,” Ball said. But volcanologists are “keeping a close eye” on the active volcanoes and watching for more seismic activity.

The ones near Burney are probably not related to the quakes near Mt. Shasta. The Burney area does have active faults of its own" Ball said. "These are basin and range faults — a place where the plates are stretching." The crust is being pulled apart.

What makes these quakes unusual isn't their strength. Most earthquakes of a 2.5 magnitude or less aren’t felt by people in the area, according to Michigan Technical University’s Earthquake Magnitude Scale. There are about 900,000 earthquakes that are 2.5 or less per year in the world.

Nor are the quakes unusual. About 10 earthquakes rumble per year between five and 10 miles southeast of the summit, according to the USGS.

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What makes these quakes unusual is that there are so many clustered together over a short period of time.

That hasn't happened in the Mud Creek area since a similar “tectonic swarm” occured between Feb. 19 and April 3, 2015, near Mt. Shasta, Ball said. At that time, 20 earthquake “events” measured 1.1 to 2.1 on the Richter scale, all about five miles southeast of Mt. Shasta’s summit near the Clear Creek Trailhead. 

Read more about the 2015 incident on the USGS California Volcano Observatory page at https://on.doi.gov/2ksERme.

Jessica Skropanic is features reporter for the Record Searchlight/USA Today Network. She covers lifestyle and entertainment stories, and weekly arts feature d.a.t.e.  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.