Response of Seismicity of Northern California Geothermal Fields to Sharp Variations of the Geomagnetic Field

In order to clarify the role of fluids in the process of electromagnetic triggering of earthquakes, by analogy with the dynamic triggering effect on the earthquake preparation zone, a statistical analysis of seismicity of three geothermal fields of Northern California (Long Valley, Coso, and Geysers) is carried out during 24 sharp variations in the X-component of the geomagnetic field Вх (|dВх/dt| ≥ 50 nT/min) recorded at the Fresno observatory in 1991‒2024. A statistically significant increase in seismicity of the geothermal fields with a delay of 2‒10 days after the geomagnetic variation is shown, when the daily number of earthquakes exceeds their average number determined for the previous 15 days, plus triple standard deviation σ. At the same time, a weaker increase in seismic activity after sharp variations in the geomagnetic field is observed for the entire territory of Northern California. This effect indicates not only the presence of a triggering potential for steep-edge geomagnetic field variations in certain sensitive zones, but also the possible role of fluids in the electromagnetic triggering of earthquakes.