Accelerating Creep in Northern Japan due to Erosion of Locked Asperities in the Decade Prior to the M9 Tohoku Earthquake
Abstract
The conventional "asperity model" posits that faults are partitioned into fixed velocity-weakening (VW) patches (asperities) that are locked interseismically and velocity-strengthening (VS) regions that creep stably without accumulating stress. However, studies of GPS-derived deformation in northern Japan have shown that interseismic strain in the Tohoku region did not accumulate at a constant rate (as expected) but gradually decreased from 1996 to 2011. This change in strain rate is consistent with locked asperities shrinking by ∼75% in area during this period and associated accelerations in creep rate of up to 3-4 mm/yr^2. A mechanism for the erosion of asperities over time has been explored previously using quasi-dynamic numerical simulations of asperity rupture in which thermal pressurization allows rupture to extend well beyond the VW patch. Thermal pressurization causes shear stress during rapid slip to decrease to very low levels. During the interseismic period, stress gradually recovers to steady state friction at the plate rate, at which point stable creep initiates. The creep front propagates inward, effectively eroding the locked asperity. In this study we exhaustively explore the rate of asperity erosion that is required to explain the geodetic accelerations through MCMC inversions and quasi-mechanical models of asperity erosion. From the earthquake catalog, we identify 67 seismic asperities on the plate interface from earthquakes with magnitude M6.5 and larger. In the forward model, asperities are assigned a rupture size and an erosion rate. Acceleration of creep surrounding eroding asperities is computed assuming creep at constant stress. The MCMC inversion solves for the size of asperities given the rupture size, erosion rate, and time of last rupture. The result is posterior probability density functions of erosion rate of all 67 asperities. Results will be compared with theoretical and numerical predictions.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T51J0316B
- Keywords:
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- 5112 Microstructure;
- PHYSICAL PROPERTIES OF ROCKSDE: 7230 Seismicity and tectonics;
- SEISMOLOGYDE: 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICSDE: 8163 Rheology and friction of fault zones;
- TECTONOPHYSICS