More than 85 aftershocks follow M4.0 earthquake northwest of The Geysers, California

More than 85 aftershocks follow M4.0 earthquake northwest of The Geysers, California

Teo Blašković

A M4.0 earthquake struck 6 km (3.7 miles) northwest of The Geysers geothermal field, California, at 12:48 UTC on August 14, at a depth of 3.4 km (2.1 miles). The event was followed by more than 85 aftershocks through 20:18 UTC with magnitudes ranging from 0.3–2.9.

The strongest aftershocks occurred within minutes of the mainshock, including M2.9, M2.8, and M2.6 events at depths of 2.7–3.4 km (1.7–2.1 miles). Most aftershocks were concentrated between 2 km and 4 km (1.2–2.5 miles) depth, consistent with the known geothermal reservoir beneath The Geysers. One deeper event was recorded at 9.6 km (6 miles).

The sequence displayed a classic mainshock–aftershock pattern, with the majority of seismicity occurring in the first hour, followed by a gradual decline in frequency and magnitude through the afternoon. Shaking was reported as weak to light in parts of Lake and Sonoma counties.

The Geysers is the world’s largest geothermal field, located about 115 km (72 miles) north of San Francisco, comprising a complex of power plants. In 2018, the field supplied about 50 % of California’s geothermal generation, according to the operator. The area covers approximately 78–116 km2 (30–45 mi2).

The field is known for frequent swarms of small to moderate earthquakes, most of which are linked to geothermal energy production.

Studies by the USGS, California Geological Survey (CGS), and Calpine Corporation, the field’s operator, have established that steam extraction cools and contracts subsurface rock, while water reinjection increases pore pressure and thermal stress — processes that can trigger small earthquakes by causing existing faults in the reservoir to slip.

More than 99 % of earthquakes at The Geysers are small, registering M3.0 or less, while the strongest in over 35 years was M5.0 in December 2016. This earthquake had particularly notable shallow hypocentral depth of 1.5 km (0.9 miles), which is of similar depth to the injection wells in the field. The event was recorded at nearby continuous GPS stations, indicating horizontal displacements of over 1 cm (0.4 inches).