Fracture-Mesh Faulting in the Swarm-Like 2020 Maacama Sequence Revealed by High-Precision Earthquake Detection, Location, and Focal Mechanisms

In August of 2020, an earthquake sequence initiated within the Maacama fault zone in northern California, raising questions about its relationship with the larger-scale fault. To investigate the faulting geometry and its implications for physical processes driving seismicity, we applied an integrated, multi-faceted seismic analysis including waveform-correlation-based event detection, relative relocation, and high-precision focal mechanisms. To determine mechanisms for a large population of very small earthquakes (predominantly M < 1), we combined correlation-derived polarity analysis with a recently developed technique to determine S/P amplitude ratios from single seismic components. Finally, we applied an iterative stress inversion to distinguish between the likely fault and auxiliary planes. This analysis reveals that although the sequence initiated on a right-lateral fault, plausibly a strand of the Maacama Fault, it also activated numerous en echelon left-lateral conjugate faults. Together, these interlocking faults form a fracture mesh, consistent with structures associated elsewhere with fluid-induced faulting and earthquake swarms.

Automated summary

In August 2020, a seismic sequence began in the Maacama fault zone in northern California, leading to questions about its connection with a larger fault. Through various seismic analysis techniques, including event detection, relocation, and focal mechanisms, the researchers investigated the faulting geometry and its impact on seismic activity. They discovered that the sequence originated from a right-lateral fault, likely a section of the Maacama Fault, but also activated numerous left-lateral conjugate faults, forming a fracture mesh consistent with fluid-induced faulting and earthquake swarms seen elsewhere.