Stress From Plate Bending in the Nankai Trough

Date: 4/26/2019

Time: 02:30 PM

Room: Vashon

Earthquakes in the crust of the downgoing plate of a subduction zone can be destructive, for example the 2018 M7.0 Anchorage, 2017 M7.1 Puebla, and 2001 M6.8 Nisqually earthquakes. These intraplate events often have normal-faulting mechanisms associated with stresses from bending of the downgoing plate. Prior studies have inverted earthquake moment tensors for stress orientations in the downgoing plate, and interpreted the stress orientations in terms of plate bending or unbending. However, this approach can’t separate the plate bending stresses from the stresses associated with the loading of the coupled plate interface. In this study, we isolate the intraplate bending stress by modeling the total stress as a combination of the intraplate stress and the stress due to loading. We compute the loading stress from a geodetically-determined model of plate coupling. We then invert for the intraplate stress such that the total stress best fits the moment tensors of earthquakes occurring in the subducting crust. The inversion constrains the orientations and the sign of the principal stress axes of the intraplate stress. The absolute stress amplitudes can be estimated relative to the amplitude of the modeled loading stress, given some assumptions. We focus on the Nankai Trough because of the abundance of well-recorded earthquakes in the crust of the downgoing plate, and the existence of dense geodetic data to constrain the interface coupling. We find that most of the downgoing plate is in along-strike tension, consistent with prior results. The down-dip stress, in contrast, shows alternating zones of tension and compression. We find a statistically significant correlation between the intraplate absolute stress and the plate curvature, in both the along-strike and down-dip directions. The sign of the correlation is consistent with the intraplate stress being controlled by the bending of an initially-planar plate to its current curvature. These results suggest that plate geometry can be used to infer the stresses driving intraplate earthquakes.

Presenting Author: Jeanne Hardebeck

Authors