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Physical Properties of the Crust Influence Aftershock Locations

Jeanne L. Hardebeck

Submitted September 10, 2023, SCEC Contribution #12999, 2023 SCEC Annual Meeting Poster #154

Aftershocks do not uniformly surround a mainshock, and instead occur in spatial clusters. Spatially-variable physical properties of the crust may influence the spatial distribution of aftershocks. I study four aftershock sequences in Southern California (1992 Landers, 1999 Hector Mine, 2010 El Mayor-Cucapah, and 2019 Ridgecrest) to investigate which physical properties are spatially correlated with aftershock occurrence. The tested properties include those compiled by the SCEC Community Models, in particular the Community Stress, Velocity, Geodetic, Thermal, and Rheology Models, along with the Rheology Model’s geologic framework. Other data comes from the SCSN earthquake catalog and the USGS Quaternary Fault Database.

I find that aftershock spatial density exhibits an order of magnitude or more variation as a function of several properties. There is a higher spatial density of aftershocks near mapped faults, in areas of higher background earthquake rate, and in areas of higher long-term geodetic shear strain rate. Aftershocks preferentially occur where surface heat flow is moderate, and P-wave velocity and density have mid-range values. Static stress changes from the mainshock are also highly correlated with aftershock spatial density, confirming prior studies. I determine simple empirical relations between each of the properties and the aftershock spatial density, and use these relations to construct new spatial models that describe aftershock locations. Retrospective tests show that the new spatial models are a significant improvement over a simple model of decaying aftershock rate with distance from the mainshock. However, the new models still do not fully capture the dense spatial clustering of aftershocks.

Numerous spatially-varying physical properties exhibit no (or poor) correlation with aftershock spatial density, including temperature at seismogenic depths, rock composition, and rheological properties that might be expected to control aftershock occurrence. This lack of correlation may reflect the difficulty of constraining these physical properties at seismogenic depths with high enough resolution to compare with the spatial distributions of aftershocks. The possible correlation of these properties with aftershock occurrence should be investigated again in the future when higher resolution models have been developed.

Key Words
aftershocks, triggering

Hardebeck, J. L. (2023, 09). Physical Properties of the Crust Influence Aftershock Locations. Poster Presentation at 2023 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Forecasting and Predictability (EFP)