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Characteristics of Fracture Networks Regularly Activated by Geothermal Production at the Coso Geothermal Field

Description: 

The long-term producing Coso Geothermal Field (CGF) in California targets a reservoir characterized by an extensive fracture network and complex fluid pathways, as well as regular natural and induced seismic activity. Located in a transtensional setting, the reservoir is surrounded by active fault systems, allowing for a direct comparison between the characteristics of faults that are routinely reactivated by geothermal operations with those in the surrounding region that are not directly influenced by injection and production. In this study, we analyze more than 97,000 earthquakes (Mw -0.4 to 4.2) occurring within 12 km of the CGF between 1996 and 2010, including approximately 56,000 events from within the field and 18,000 events from the nearby seismically active Rose Valley, to identify and compare seismically active structures.

We first relocate the earthquakes using waveform-correlated differential times and compute focal mechanisms. We find that the region is dominated by the expected strike-slip and normal faulting, with the CGF reservoir exhibiting a larger variation in focal mechanisms than the surrounding area – highlighting the structural complexity and heterogeneity of the fracture network within the field. Moreover, we combine the focal mechanisms, earthquake relocations, and waveform similarity analysis to infer fault planes. By improving the characterization of active structures within and around the CGF, we can gain a clearer understanding of how operational parameters influence the local stress field and seismic response of the reservoir.

Session: From Drilling to Ground Shaking: Mechanisms, Monitoring and Mitigation of Induced Earthquakes - I

Type: Oral

Date: 4/17/2026

Presentation Time: 08:15 AM (local time)

Presenting Author: Joanna M. Holmgren

Student Presenter: No

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