Chris Milliner, Roland Bürgmann, Asaf Inbal, Teng Wang, Cunren Liang

Continuous measurements of postseismic surface deformation provide insight into variations of the frictional strength of faults and the rheology of the lower crust and upper mantle as stresses following rupture are dissipated. However, due to the difficulty of capturing the earliest phase of afterslip, most analyses have focused on understanding postseismic processes over timescales of weeks-to-years. Here we investigate the kinematics, moment release and frictional properties of the earliest phase of afterslip within the first hours following the 2016 Mw 7.1 Kumamoto earthquake using a network of five-minute sampled continuous GPS stations. Using independent component analysis to filter the GPS data we find that (1) early afterslip contributes only ~1% of total moment release within the first hour, and 8% after 24 hours. This suggests that the slip model of the mainshock, which we estimate using standard geodetic datasets (e.g., InSAR, GPS and pixel offsets), and which span the first four days of the postseismic period, is largely reflective of the dynamic rupture process and we can rule out contamination of moment release by early afterslip. (2) Early afterslip shows no evidence of a delayed nucleation or acceleration phase, where instead fault patches transition to immediate deceleration following rupture that is consistent with frictional relaxation under steady-state conditions with dependence only on the sliding velocity. (3) There is a close correlation between the near-field aftershocks and afterslip within the first hours following rupture, suggesting afterslip may still be an important possible triggering mechanism during the earliest postseismic period.