Conjugate Rupture Patterns and Role of Fluids in the 2020 Maacama Fault, California Earthquake Swarm

David R. Shelly, Robert J. Skoumal, & Jeanne L. Hardebeck

Published August 4, 2021, SCEC Contribution #11168, 2021 SCEC Annual Meeting Poster #086

In August of 2020, an earthquake swarm initiated near the Maacama Fault and the town of Willits, California, with magnitudes up to Mw 4.2. Although generally decreasing in intensity, the swarm persisted episodically at least into December 2020. The Maacama Fault accommodates primarily right-lateral motion, as part of the broader San Andreas system in Northern California, representing a northern extension of the Hayward/Calaveras fault zones.

To gain insight into the associated faulting physics, we applied matched filter event detection and relative relocation, using ~400 routinely cataloged earthquakes as waveform templates. After processing, we detected and precisely relocated ~4000 total events, transforming a diffuse cloud of initial catalog locations into illuminated fault structures. We find that the swarm initiated at ~8 km depth along a structure striking NNW, dipping steeply to ENE, which is plausibly part of the Maacama fault system. Following its initiation, larger en echelon, cross-cutting structures became activated, striking toward the NE, in an apparently conjugate orientation, extending both updip to ~6 km depth and laterally from the earlier activity.

These patterns exhibit notable similarities with other swarms inferred to be driven by fluid pressure transients, including examples in tectonic regions (e.g. an extensional fault stepover near San Ramon in the eastern San Francisco Bay Area) and volcanic regions (e.g. Long Valley and Yellowstone Calderas). Together with pronounced spatial-temporal migration, a high degree of faulting complexity, including conjugate, en echelon structures, may be diagnostic of fluid-mediated faulting. In fact, this may reflect the active symbiosis between crustal fluid transport and strain accommodation in the crust, whereby fluids trigger faulting and faulting accelerates fluid diffusion. Natural swarms, such as this, may reflect similar source physics to some induced earthquake sequences, despite the different source of fluids.

Key Words
earthquake swarms, fluids, faulting complexity

Shelly, D. R., Skoumal, R. J., & Hardebeck, J. L. (2021, 08). Conjugate Rupture Patterns and Role of Fluids in the 2020 Maacama Fault, California Earthquake Swarm. Poster Presentation at 2021 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)