Earthquake swarm rocks active California volcanic region as scientists watch for eruption signs

A swarm of earthquakes rattled an active volcanic region in California on Friday.

The US Geological Survey (USGS) recorded seven tremors near Coso Junction by Friday afternoon, with the strongest, a magnitude 3.4, striking at 10.26am PT (1.26pm ET).

The Coso volcanic field, located about 160 km (100 mi) northeast of Bakersfield, is not a single, isolated mountain vent, but is a 150-square-mile cluster containing dozens of individual volcanic features powered by a broad underlying zone of magma.

The region last erupted about 39,000 to 40,000 years ago. 

Over the past quarter-million years, the field has experienced roughly 40 distinct individual eruptive events and episodes. 

The volcanic field is actively monitored for eruption and unrest by the USGS California Volcano Observatory in cooperation with the US Navy. 

Its current alert level is Normal with a Green aviation color code. 

The USGS noted that Friday’s seismic activity did not establish that the earthquakes were volcanic or indicate an eruption is approaching.

Only one person reported shaking following the 3.3 magnitude earthquake.

The other six tremors detected were less than 2.5 magnitude, with the first recorded at 7.50am PT.

Two other earthquakes greater than magnitude 2.5 have rattled the region in the past week. 

The strongest, a magnitude 3.0, struck the same area before the latest quake, while a magnitude 2.8 was recorded on August 15. 

Most of the field lies within the Naval Air Weapons Station China Lake, between the Sierra Nevada, Owens Valley and the Garlock Fault.

The USGS classifies Coso as a moderate-threat volcanic field and has detected a zone of partially molten magma beneath its center.

That underground heat fuels hot springs, steam vents, boiling mud pots and one of the largest geothermal power-producing regions in the US, generating enough electricity for approximately 270,000 homes.

Small to moderate earthquakes are common at Coso, and the USGS says some are associated with its geothermal system.

The US Navy monitors earthquake and geothermal activity across the field, while the USGS operates a nearby seismometer and three GPS stations that track movement of the ground.

Volcanic earthquakes are typically shallow, relatively small and clustered in swarms, but those characteristics alone do not prove that magma is moving toward the surface.

In its August 18 update, the California Volcano Observatory reported 13 earthquakes greater than magnitude 1.0 at Coso during the previous week, with the largest measuring magnitude 2.8.

Despite that activity, the agency said California’s monitored volcanoes remained at normal background levels of seismicity and deformation, with Coso listed at a Normal alert level and Green aviation code.

The USGS believes Coso will erupt again at some point, but that does not mean an eruption is imminent.

Scientists expect weeks to months of escalating earthquakes and ground deformation before any future eruption, which would likely begin with a series of smaller steam explosions.

A more explosive eruption could send an ash column several thousand feet into the sky and carry ash more than 10 miles downwind, while another possible scenario could produce lava fountains, cinder and slow-moving lava flows.

The USGS believes the western half of the volcanic field is the most likely location for a future eruption.