Faults and fractures of the Salton Sea geothermal field revealed by interferometry of an earthquake swarm

Eric Matzel, Dennise C. Templeton, & Christina Morency

Submitted September 7, 2025, SCEC Contribution #14583, 2025 SCEC Annual Meeting Poster #TBD

Fiber optic sensors enable dense (meter-scale) sampling of the seismic wavefield over tens of kilometers. When an earthquake is recorded by such an array, we see detailed ground motion, including arrivals scattered by heterogeneities along the entire path between the source and each channel along the length of the fiber. Given two earthquakes along similar paths, we can use interferometry to isolate the part of the wavefield between them. This results in a estimate of the Green function between the pair, modified by the two moment tensors. When applied to aftershocks or earthquake swarms, this technique is very powerful at imaging the tectonically active region.

The Virtual Seismometer method of interferometry (VSM) involves correlating the records of pairs of events recorded at individual channels to obtain an estimate of the Green's function between the sources. By effectively replacing each earthquake with a "virtual seismometer" recording all the others, VSM isolates the portion of the data that is sensitive to the source region and significantly improves our ability to see into tectonically active features. Using this technique, we are able to recover the Green's function between pairs of earthquakes, and image the structure along the path between them.

Here, we present results from the Imperial Valley Dark Fiber Experiment (Ajo-Franklin et al, 2022), which recorded thousands of local earthquakes during its deployment, including a large swarm near the Salton Sea geothermal fields. Using VSM we calculated over 1200 Green functions for a 10x10 km section of the Brawley Seismic Zone. We inverted those waveforms to measure the seismic velocity and attenuation of the active region, allowing us to identify large structural heterogeneities and estimate the local faulting and fracture patterns with a lateral resolution of about 200 m. The geothermal fields have distinctly high Vp/Vs and Qs/Qp ratios relative to their surroundings.

This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344

Citation
Matzel, E., Templeton, D. C., & Morency, C. (2025, 09). Faults and fractures of the Salton Sea geothermal field revealed by interferometry of an earthquake swarm. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology