Scattering of Moment Tensors During Aftershock Sequences at Global and Local Scales

John Wilding, & Zachary E. Ross

Published September 11, 2022, SCEC Contribution #12145, 2022 SCEC Annual Meeting Poster #023

Observations of regional stress field rotations in the wake of large earthquakes have been used to argue for a weak seismogenic crust. Identifying these stress rotations requires high-quality foreshock and aftershock source mechanisms, which has largely restricted robust observations to large earthquakes along subduction zones or strike-slip plate boundaries. Whether coseismic stress rotations are general features of seismicity, or are limited to a handful of well-studied events in specific tectonic environments, is an open question. If stress rotations are ubiquitous, they could serve as useful data to study crustal strength and discriminate between models of coseismic stress rotations.

To evaluate the generality of coseismic stress rotations, we compare foreshock and aftershock moment tensors to those of mainshocks using two earthquake catalogs. We use a nearest-neighbor clustering algorithm to identify earthquake sequences in the global ISC-GEM catalog and the regional Southern California catalog. Using an inner-product-based pairwise measure of moment tensor similarity, we demonstrate that, in both catalogs, aftershocks are less similar to their respective mainshocks than foreshocks are. We show that this effect, which we call moment tensor scattering, is generally observable for earthquakes as small as M 3.0. We further demonstrate that mainshock-aftershock similarity is lowest immediately following a mainshock and find evidence that mainshock-aftershock similarity recovers logarithmically to pre-mainshock levels on decadal timescales. We conclude that moment tensor scattering is a generally observable feature of seismic sequences which may be useful in future work to discriminate between models of crustal strength.

Citation
Wilding, J., & Ross, Z. E. (2022, 09). Scattering of Moment Tensors During Aftershock Sequences at Global and Local Scales. Poster Presentation at 2022 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology