The Stress-Similarity Triggering Model for Aftershocks Applied to the Ridgecrest, California Earthquake Sequence

Session: Crustal Stress and Strain and Implications for Fault Interaction and Slip [Poster]
Type: Poster
Date: 4/28/2020
Time: 08:00 AM
Room: Ballroom
Description: 

The stress-similarity triggering model hypothesizes that more aftershocks occur in areas where the mainshock stress change is similar in orientation to the background stress. An example is the normal faulting regions of onshore Japan that were activated by extensional stress changes from the M_W9.0 Tohoku-Oki earthquake. Conceptually, adding two perfectly-aligned stress tensors is equivalent to multiplying the deviatoric part of either tensor by a scalar. The shear stresses on planes of all orientations are consequently also multiplied by that scalar, and the combined shear and normal stress change moves faults of most orientations towards failure. For similar but not identical stress tensors, the deviatoric stress change similarly promotes failure for many fault orientations. The stress-similarity model avoids the problem of choosing the geometry of the target faults and allows investigation of how spatial variations in background stress may affect aftershock triggering.

I determined the background stress field in the Ridgecrest area from the inversion of earthquake focal mechanisms, with the spatial resolution adapted to the density of earthquakes. Local variations in stress orientation and faulting style are observed. The M_W6.4 and M_W7.1 earthquakes triggered clusters of off-fault aftershocks primarily in locations with high stress similarity. The spatial density of the first 3 months of M≥2.0 aftershocks increases as a function of stress similarity, with a factor of 15 difference in aftershock density between high-stress-similarity and low-stress-similarity areas. This result is robust with respect to the choice of mainshock slip model and the uncertainty of the background stress field. The aftershock density varies substantially inside the high-stress-similarity lobes, however, suggesting that other variable background conditions also play a role.

Presenting Author: Jeanne Hardebeck

Authors