Mild but Unusually Deep Quakes Strike Sierra, Northern Coast

The Sierra Nevada and California’s far northern coast were sites of mild but unusually deep earthquakes this month, the U.S. Geological Survey said.

The recent quakes were centered 17 to 21 miles underground, where rock usually is too warm and flexible to produce temblors. Most California earthquakes are six to nine miles deep.

The quakes, too small and deep to be felt, originated beneath the North Coast’s Klamath Mountains and were caused by the Gorda Plate, a large chunk of Earth’s crust mostly offshore that is slowly diving under mainland Northern California, the Geological Survey said Friday.

The North Coast quakes included one Oct. 9 that was magnitude 2.5 and was centered 21 miles underground and 32 miles southeast of Crescent City.

It was followed by two 17-mile-deep quakes east of Eureka: a 2.1 jolt last Sunday and a 2.7 temblor Wednesday.

Beneath the central Sierra, a magnitude 2.0 quake occurred Oct. 9 about 19 miles deep and about 27 miles north-northeast of Madera. It was followed by a 2.4 jolt Monday 17 miles deep and 13 miles south-southeast of Merced.

Five other deep quakes have occurred since Oct. 1 in an area within 30 miles of Yosemite Valley.

Scientists do not know what triggered the Sierra’s deep quakes or why an unusual number of them happened in a short period, geophysicist Dave Hill said.

But rock in the Sierra is colder and more brittle to greater depths, allowing deep quakes in that region even if the immediate cause is not known, he added.