Stress Regime of the Nankai Trough Megathrust: A Stress Analysis Incorporating Geodetic and Seismic Fault Slip

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

The stress field in different tectonic regimes provides insight into the mechanics of faulting for the considered region. Crustal stress fields are routinely determined using inversion methods that solve for a stress tensor best describing the distribution of slip vectors from earthquake focal mechanisms. Slow fault slip is generally not represented in focal mechanism catalogs, and is therefore excluded from stress inversions. In the Nankai trough region of Japan, a substantial portion of fault slip along the subducting Philippine Sea plate is accommodated by geodetic episodes of slow fault slip with durations of days to years, or slow slip events (SSEs), so standard earthquake focal mechanism catalogs are not a comprehensive representation of local crustal deformation. We utilize focal mechanisms of deep short-term SSEs determined from inversions of GNSS data to include slow fault slip in an analysis of the stress regime of the Nankai trough megathrust. We invert SSE focal mechanisms in conjunction with a declustered standard focal mechanism catalog to estimate the best-fitting spatially variable stress field. Previous studies have revealed principal stress orientations in the Nankai trough that are misoriented for megathrust faulting. The area of apparent principal stress misorientation coincides with the location of SSEs. Our results suggest that crustal principal stress orientations of the Nankai trough are well oriented for megathrust faulting when slip due to SSEs is considered. Stress inversions assume that the input (focal mechanisms of fault slip) samples all sources of slip, therefore regional principal stress analyses of the subduction zone interface should include all substantial sources of megathrust slip. Our results emphasize the importance of including all major sources of slip in stress analyses, and suggest that the stress state of the Nankai Trough is less unique than previously thought, compared to other subduction zones.

Presenting Author: Tyler J. Newton

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