Space time distribution of afterslip following the 2003 Tokachi-oki earthquake: Implications for variations in fault zone frictional properties

GPS time series following the September 25, 2003 (UT) Tokachi-oki earthquake (M(W)similar to8.0) reveal significant deformation. We use the Network Inversion Filter to invert for the spatial and temporal evolution of afterslip in the 30 days following the earthquake. Afterslip is concentrated adjacent to the coseismic rupture zone, between the 2003 rupture and the inferred source regions of the 1968 Tokachi-oki and the 1973 Nemuro-oki earthquakes. The inversion shows a rapid decay of slip-rate in the 2003 rupture zone, with slower deceleration in the surrounding regions. The stress-velocity paths for the afterslip regions approximately follow dtau/d ln(v)similar to0.6 MPa, suggestive of steady-state velocity strengthening friction. The complementary spatial pattern of coseismic rupture and afterslip may indicate along strike spatial variations of frictional properties, although other interpretations can not be ruled out.