Afterslip in the ETS Zone at Nankai Trough, SW Japan

Postseismic and interseismic deformation at the Nankai subduction zone in southwestern Japan following 1944 M8.1 Tonankai and 1946 M8.4 Nankai earthquakes was recorded with GPS, leveling and tide gauge data. Using this data, we have estimated the vertical velocity field of southwestern Japan from immediately after the 1944/46 events until near present day. Analysis of these data with 2D and 3D postseismic and interseismic deformation models show that observed inland postseismic subsidence pattern is diagnostic of about 50 years of viscoelastic mantle flow uplift along the southern coastline is diagnostic of afterslip on the subduction interface. Furthermore, both kinematic inversions and forward models with rate-strengthening afterslip show that afterslip overlaps the zone of Episodic Tremor and Slip (ETS) on the subduction interface. This gives us the opportunity to probe physical conditions within the ETS zone that will permit creep manifesting as both afterslip and spontaneous episodic slow slip events. Nearly all models for ETS require very low effective normal stress of order 1-10 MPa and rate-weakening friction conditions, however our rate-strengthening afterslip simulations as well as kinematic afterslip inversions suggest much higher effective normal stresses of order 100-300 MPa at Nankai. In this study, we are examining several hypotheses for reconciling this apparent discrepancy in physical conditions using a suite of 3D postseismic deformation models with afterslip coupled to viscous mantle flow. We examine (1) afterslip with rate-dependent velocity-weakening/strengthening friction parameters (increase in stability with sliding rate), (2) dilatant strengthening in a fluid-saturated shear zone, and (3) viscous stabilization at higher slip rates. We will present our results examining the physical conditions in the ETS zone that allow for both afterslip and spontaneous episodic slip events.