A swarm of seven earthquakes struck the active Coso volcanic field near Coso Junction in California on Friday, with the strongest tremor measuring magnitude 3.4.
The US Geological Survey recorded the largest quake at 10.26am PT (1.26pm ET), following an initial tremor at 7.50am PT. The six other shocks detected during the day measured below magnitude 2.5.
Only one person reported feeling shaking following a magnitude 3.3 earthquake during the sequence. Geologists confirmed that the seismic activity does not indicate an eruption is approaching or establish that the quakes were volcanic in origin.
The US Geological Survey is the federal scientific agency responsible for studying natural hazards, landscape changes and ground movement across the country. Its California Volcano Observatory monitors active volcanic systems throughout the state in cooperation with the US Navy.
Seismic activity and geothermal energy
The Coso volcanic field covers a 150-square-mile cluster located about 160 km (100 mi) northeast of Bakersfield. Rather than a single mountain vent, the region contains dozens of individual volcanic features powered by a broad underlying zone of partially molten magma.
Most of the field sits within the Naval Air Weapons Station China Lake, a major military testing facility positioned between the Sierra Nevada mountain range, Owens Valley and the Garlock Fault.
The intense underground heat fuels hot springs, steam vents and boiling mud pots across the area. It also powers one of the largest geothermal energy-producing regions in the United States, generating enough electricity for approximately 270,000 homes. Geothermal power harnesses underground thermal energy to spin turbines and produce electricity.
Small to moderate earthquakes occur frequently at Coso, and the US Geological Survey notes that some tremors are directly associated with its active geothermal operations.
Eruption risk and monitoring
Two other earthquakes greater than magnitude 2.5 rattled the region during the previous week. A magnitude 3.0 quake struck the same area before Friday's swarm, while a magnitude 2.8 earthquake was recorded on August 15.
In an update issued on August 18, the California Volcano Observatory reported 13 earthquakes above magnitude 1.0 at Coso during the preceding week. Despite the activity, agency officials stated that monitored volcanoes in California remain at normal background levels of seismicity and deformation, with Coso assigned a Normal alert level and Green aviation color code.
The USGS Volcano Hazards Program uses alert levels such as Normal, Advisory, Watch and Warning to signal threat levels, while aviation color codes inform pilots of airborne volcanic ash risks. Normal and Green indicate typical background status.
Monitoring across the field is shared between military and civilian agencies. The US Navy monitors seismic and geothermal shifts across the property, while the US Geological Survey operates a nearby seismometer and three GPS stations tracking ground deformation.
Eruption history and future outlook
The Coso volcanic field last erupted between 39,000 and 40,000 years ago. Over the past quarter-million years, the site has experienced roughly 40 distinct eruptive events and episodes.
Geologists note that volcanic earthquakes are typically shallow, relatively small and clustered in swarms. However, those characteristics alone do not prove that magma is actively moving toward the surface.
Although scientists believe Coso will erupt again eventually, the US Geological Survey emphasized that an eruption is not imminent. Researchers expect weeks to months of escalating earthquakes and ground deformation before any future volcanic event.
Any future activity would likely begin with smaller steam explosions. A more explosive event could produce an ash column reaching several thousand feet into the air and scattering ash more than 10 miles downwind, while alternative scenarios include lava fountains, cinder production and slow-moving lava flows. The US Geological Survey identifies the western half of the volcanic field as the most likely location for a future eruption.
