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Changes in stress state and seismicity pattern responsible for the 2019 Ridgecrest, California, earthquakes
  • Kazuyoshi Nanjo
Kazuyoshi Nanjo
University of Shizuoka

Corresponding Author:nanjo@u-shizuoka-ken.ac.jp

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Monitoring stress state in the Earth’s crust plays a crucial role in our understanding of an earthquake’s mechanism, especially how earthquake ruptures nucleate, as well as in calculating the distribution of hazards. Crustal deformation due to the 2019 Ridgecrest earthquakes, which occurred near the town of Ridgecrest, California, that culminated in a preceding earthquake of magnitude (M) 6.4 and a subsequent M7.1 event, caused stress perturbation in nearby regions. However, implications of future seismic activity are still unclear. Here we analyze the occurrence of small earthquakes compared to larger ones—the b-values, showing how the nucleation area for both the M6.4 and M7.1 earthquakes had low b-values before these events occurred, and mid-to-high b-values thereafter. The slip distribution of the M7.1 event is also well correlated with the b-value map. Additionally, the time and local-dependent variations in b-values of the Ridgecrest earthquakes are linked with estimates of changes to Coulomb stress. The main conclusion is that the b-value mapping provide insight into the stress state in the fault zone, which is likely closely related to the nucleation and evolution of earthquakes in the sequence. The combined approach of stress-change and b-value analyses to the post-M7.1-quake sequence shows an area that is currently being stressed near the Garlock fault that hosted past large earthquakes. The b-values are not as low as those immediately before the M6.4 and M7.1 events, but contribute the most recent values in a decreasing trend of the b-value. Together with geodetic and seismological observations, monitoring the spatial and temporal distribution of b-value would contribute to seismic hazards in the Eastern California Shear Zone.