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Can Fault-Zone Complexity Explain Spatiotemporal Variation in Observed Stress Drops?

Christine J. Ruhl, Rachel E. Abercrombie, & Peter M. Shearer

Published August 16, 2021, SCEC Contribution #11566, 2021 SCEC Annual Meeting Poster #230

High-resolution studies of earthquake stress drops have yielded significant scatter, often with orders of magnitude of variation seen on very small spatial and temporal scales. Understanding whether stress drop depends on hypocenter depth, fault properties, temporal position, or other factors requires detailed analysis. Here, we independently compute 4373 stress drops using a spectral decomposition method with depth-varying correction functions to investigate the robustness of spatiotemporal patterns observed for the well-recorded 2008 Mogul, NV earthquake sequence. Two months of unusually shallow and energetic swarm-like activity leading up to the Mw4.9 mainshock revealed complex, but organized, four-dimensional patterns of activated fault structures. Previous work using empirical Green’s function (EGF) analysis to determine stress drops of 148 events indicated spatiotemporal variations along the mainshock fault zone and off-fault structures. With our new results, we confirm that stress drops along this complex fault zone vary significantly within a small volume. The highest stress-drop foreshocks occur within the mainshock fault zone, nucleating at the edges of seismicity voids and concentrating near complex regions in the fault zone (i.e., changes in strike/dip). The highest stress-drop foreshock region is not re-ruptured by aftershocks, however low stress-drop areas are consistently low during both the foreshock and aftershock periods, implying that stress drop depends strongly on inherent individual fault properties as well as potential areas of coseismic and/or aseismic slip (i.e., the seismicity void) rather than temporal position within the sequence. These new measurements confirm that during the diffusive swarm-like phase of the foreshock sequence, stress drops increase with increasing distance from the initiation point and peak along the mainshock fault zone. Finally, we confirm that off-fault-aftershocks are lower stress drop on average than those along the mainshock fault zone. We show changes in fault geometry through time and with depth. We interpret these robust variations, even among parallel structures, to be physically real, suggesting dependence on individual fault properties that may be enigmatic to observe.

Key Words
stress drop, fault complexity, foreshocks

Ruhl, C. J., Abercrombie, R. E., & Shearer, P. M. (2021, 08). Can Fault-Zone Complexity Explain Spatiotemporal Variation in Observed Stress Drops?. Poster Presentation at 2021 SCEC Annual Meeting.

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