# Rapid hydrothermal triggering of induced seismicity at the Coso geothermal field

## TL;DR

This study shows that injecting cold fluids into a geothermal field can quickly trigger distant earthquakes, likely due to thermal effects and reservoir structure.

## Contribution

The study reveals that thermal effects and structural anisotropy rapidly trigger seismicity during cold fluid injection in geothermal reservoirs.

## Key findings

- Seismicity rate increases correlate with seasonal variations in reinjected fluid volume and temperature.
- Cold fluid injection triggers rapid seismic responses up to 2 km away, indicating thermal effects and structural anisotropy.
- Temperature decreases during stable injection periods also correlate with seismic activity.

## Abstract

The long-term producing Coso Geothermal Field (CGF) in California operates over 100 wells tapping into a reservoir characterized by an extensive fracture network, complex fluid pathways, and regular seismic activity. Understanding the interaction between seismicity and injection can shed important light on the hydrothermal characteristics of the field. Here, we analyze 15 years of local seismic and daily operational data from the CGF, identifying a strong correlation between short-term increase in seismicity rate and seasonal volumetric and temperature variations in the reinjected fluid. Furthermore, the seismic footprint during peak injection of colder fluids reveals a near-instantaneous response up to 2 km away from the injection well, too rapid for pore pressure diffusion alone. This short-term and distant response is observed to have directional preference, indicating structural or permeability anisotropy within the reservoir. Additionally, the seismic response correlates with the initial volumetric increase of colder fluids, but also with temperature decrease during stable injection periods, suggesting thermal effects alone can play an important role in triggering distant seismicity.

The online version contains supplementary material available at 10.1038/s41598-026-38146-x.