The Cahuilla earthquake swarm's earthquakes initially began about five miles underground after fluid entered a fault system and altered pressure within the rocks.
Notes on verification
Confirmed by peer-reviewed Science paper (Ross et al. 2020), Caltech press release, ScienceDaily, and ScienceAlert, all corroborating the fluid-injection mechanism and ~5 mile/8 km depth.
Sources
- A strange earthquake swarm lasted for years. Scientists finally know ... (nationalgeographic.com)
- https://www.caltech.edu/about/news/natural-fluid-injections-triggered-cahuilla-earthquake-swarm (caltech.edu)
- https://www.science.org/doi/10.1126/science.abb0779 (science.org)
- https://www.sciencealert.com/scientists-think-they-ve-explained-a-four-year-swarm-of-earthquakes-in-california (sciencealert.com)