Asperity Erosion Controls on Creep Acceleration in the Decade Prior to the 2011 M9 Tohoku Earthquake

The northern Japan subduction zone has traditionally been modeled as several constant sized velocity-weakening (VW) interseismically locked areas (asperities) surrounded by freely-sliding velocity-strengthening (VS) interface. When rupture occurs, the extents were thought to be limited to the bounds of the locked region; rupture being unable to propagate into the stably subducting fault interface. However, the 2011 M9 rupture spanned large areas of stably creeping fault interface, showing that locking and rupture must not be stationary and constant with time. Prior to this 2011 M9 event, 1998-2009, there was a significant acceleration in rates of deformation (reaching 3-4mm/yr^2) recorded by inland GPS data (provided by the GEONET project). Studies have linked this acceleration with the approximate 75% erosion of locking on the fault interface during this time period. Prior quasi-dynamic numerical simulations assign thermal pressurization during rupture as the culprit of rupture extending beyond the VW area, and these simulations reveal a linear erosion trend in the locked area with time as creep recovers in the VS region. This linear erosion rate appears to be inconsistent with geodetic observations that require more rapid erosion during the 1998-2009 time period. Yet, there is limited information available on how the variations of rate-state friction parameters and differing asperity sizes impacts the erosion behavior of asperities. For this project, we leverage quasi-dynamic numerical simulations, in order to reveal how different rate-state parameters and asperity sizes may change the erosion pattern. Guided by our simulations, we employ MCMC inversions of the 1998-2009 GPS data to estimate changing locking areas of large asperities along the fault (asperities which resulted in ruptures greater than M6.5from the earthquake catalog). The MCMC inversions will find the most likely changes in asperity sizesprovided time since last rupture and magnitude of ruptureas well as find the allowable ranges of various rate-state fiction parameters along the northern Japan trench. This study will result in a more complete picture of the different asperity erosion controls and possible asperity erosion patterns, as well as produce a map along the northern Japan subduction zone of allowable ranges of various rate-state friction parameters.