Of the three California areas that the California Volcano Observatory counts every week, the Geysers geothermal field south of the Clear Lake Volcanic Field is the one that dominates the tally. In the week of October 19 to 25, the field shook 87 times above magnitude 1, and the largest of those earthquakes reached magnitude 2.5. The Sierra Nevada block south of Mammoth Mountain and Long Valley recorded 10, and the Coso Volcanic Field recorded 5.
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All California volcanoes stayed at the lowest alert level. The reason The Geysers produces so many small quakes while its neighbors stay quiet lies largely in the plumbing of the field.
A weekly tally and what it counts
The figures come from the California Volcano Observatory’s weekly update, issued on October 27 at 1:39 p.m. Pacific time and flagged as preliminary. The count covers earthquakes greater than magnitude 1, which sets a floor: the network records many smaller events, and the weekly number would be several times larger with a lower threshold. The update classifies the activity as typical for the field and lists every monitored California volcano at NORMAL, with aviation color code GREEN.
The largest shock, magnitude 2.5, is in the range that people standing near the epicentre may notice as a short jolt and that carries no structural consequence. Across the seven days it stood as the top figure for the field, with the other two areas topping out at 2.9 for the Sierra Nevada block and 1.9 for Coso. By magnitude, the Sierra Nevada block actually produced the stronger single event; by count, The Geysers outran it more than eight to one. The update does not discuss why the field is so active, and it does not mention geothermal injection. The explanation comes from peer-reviewed work on the field’s catalog.
Injection and the seismic clock
Christopher Johnson, Elizabeth Totten and Roland Bürgmann, studying 360,307 earthquakes of magnitude 0 and above recorded between 2005 and 2014, reported in Geophysical Research Letters that “water injection is the main cause of seismicity as fluids penetrate into the reservoir.” Operators pump treated wastewater and recycled water into the hot rock to replenish the steam that drives the turbines, and the team found that seismicity follows that pumping on an annual cycle. Injection peaks in January in both the northwest and southeast parts of the field; earthquake rates peak afterward, with a delay that stretches from about zero to six months as events migrate deeper, to depths of roughly 4 to 5 kilometers.
Production of steam, by contrast, showed no correlation with seismicity in their data. The authors estimated hydraulic diffusivities of 0.1 to 1.0 square meters per second and concluded that steeply dipping, conductive faults let fluid travel a few kilometers below where it is injected. Lawrence Berkeley National Laboratory researchers Jonny Rutqvist and Curtis Oldenburg offered a different emphasis in their modeling study of the field. Using coupled thermal, hydrological and mechanical simulations, they concluded that cooling of the hot rock by the injected water, and the shrinkage that cooling causes, is the most important factor, because it lowers the stress holding critically stressed fractures in place and lets them slip. Their model also reproduced a time lag between peak seasonal injection and peak seismicity, the same signature seen in the earthquake catalog.
A rate that rose after the pipeline
The Santa Rosa pipeline is a landmark in the field’s history. A 2004 report by Lawrence Berkeley National Laboratory, Calpine and the Northern California Power Agency, which installed a 23-station microearthquake array to watch the effects, recorded that Santa Rosa wastewater injection began in December 2003 and that operators were evaluating a 50 percent increase over the initial rate. The same report states that without injection, reservoir pressures and steam flow would decline quickly, and that injection-related seismicity increased around the injection wells once the Santa Rosa water arrived. It also admitted that “to date there are no conclusive scientific results on the overall effect of the increased water injections.” The largest historical event it lists is a magnitude 4.6 in 1982.
The later study, led by Daniel Trugman and Peter Shearer and published in the Journal of Geophysical Research with Adrian Borsa and Yuri Fialko, compared The Geysers with the Salton Sea and Coso geothermal fields. At The Geysers, the background rate of earthquakes tracked monthly fluid injection, with a correlation coefficient of 0.61. After the Santa Rosa pipeline came online in 2003, the mean background rate climbed about 50 percent, from roughly 0.7 to 1.0 earthquakes per day in the catalog analyzed, and the winter peak in seismicity sharpened.
The same paper found that the b-value, which describes the ratio of small to large earthquakes, rose from 1.13 in 1998 to 2004 to 1.26 in 2005 to 2011. The authors tied the shift to induced seismicity and noted that the rate of larger events, those of magnitude 3.5 and above, increased in recent years as well, partly through activity in the deeper northwest sector. At Salton Sea and Coso, by comparison, the background rate had been relatively steady since about 1990.
No cause claimed for this particular week
None of the published analysis is a statement about this particular week; the observatory made no claim about cause, and no injection data for October accompanied the update. The week of October 19 to 25 also falls in autumn, ahead of the January injection peak that Johnson and colleagues describe, so a count of 87 above magnitude 1 reads best as ordinary background for the field rather than as a spike.
The open question for the winter is whether the field’s tally climbs on schedule. If the delay of up to six months in the northwest holds, the weekly CVO numbers published between now and spring should show whether this year follows the annual cycle that the two papers documented.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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