What Physical Properties Control Aftershock Locations?

Jeanne L. Hardebeck

Published August 12, 2021, SCEC Contribution #11264, 2021 SCEC Annual Meeting Poster #255

Aftershocks do not uniformly surround a mainshock, and instead occur in spatial clusters. Some of this spatial clustering may be due to secondary aftershock triggering, although only about half of aftershock are secondary, so this can’t be the only cause of spatial clustering. Stress changes from the mainshock are widely thought to control aftershock locations, but can’t capture the full spatial complexity of aftershocks that cluster on small length scales. The spatial distribution of aftershocks may also be related to spatially-variable physical properties of the crust. It has been shown that properties such as heat flow, seismic velocity, topography, and proximity to major faults are correlated with earthquake locations (e.g. Hauksson, GJI 2011). I investigate whether they are also correlated with the spatial patterns of aftershock sequences.

I look for spatial correlations between aftershocks of the Landers, Hector Mine, El Mayor, and Ridgecrest earthquakes and various physical properties of the crust. I study four aftershock sequences to ensure that I don’t over-interpret the features of a single sequence. I investigate multiple properties related to stress, structure, kinematics, heat, and rheology. Many of the crustal properties of interest have already been compiled for southern California, due to the long-standing interest in its seismic hazard, the availability of data from the SCSN, and the compilation efforts of the SCEC Community Models. For the properties that are significantly correlated with aftershock locations for all four sequences, I determine empirical relations between these properties and aftershock rate.

Preliminary results show several correlated properties, listed in descending order of correlation. Multiple measures of mainshock stress change, including Coulomb stress change and stress similarity, are equally well correlated with aftershock locations. The aftershock rate decays with distance to the nearest fault. It scales with the background seismicity rate as r^0.85, in contrast with rate-and-state models that predict it to scale with r. Aftershocks preferentially occur in locations with mid-range values of heat flow and seismic velocity, suggesting rocks at the extremes are either too strong to fail often or too weak to accumulate much strain. Aftershock rates also increase with increasing geodetic strain rate. These empirical relationships can be used to develop new spatial kernels for aftershock occurrence.

Key Words
aftershocks, stress, material properties

Hardebeck, J. L. (2021, 08). What Physical Properties Control Aftershock Locations?. Poster Presentation at 2021 SCEC Annual Meeting.

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