Identification of seismogenic zones from GNSS data: an example of Kyushu Island, Japan

To identify a seismogenic zone from GNSS (Global Navigation Satellite System)-based surface deformation data, we analyzed velocity fields of Kyushu Island, Japan before and during the 2016 Mw 7.0 Kumamoto earthquake. Many active faults exist along the seismogenic MTL (Median Tectonic Line) in Kyushu Island. For the analyses of the surface deformation from GNSS data, we used LSC (Least Square Collocation) and triangulation methods. During the pre-seismic period, there is a significant jump in rotation rate across MTL with frequent seismic activity along the high shear-strain rate zone. In southern Kyushu, another high shear-strain rate exists along a WNW-trending shear zone with little seismic activity. It indicates that high shear-strain rate zones are not necessarily seismogenic zones. Then we divided Kyushu into two areas to the north and south of MTL since these two areas show velocity fields remarkably different from each other. The flow pattern in the northern and southern areas is general shearing (pure shearing combined with simple shearing) and pulsating or super-simple shearing (simple shearing combined with rigid-body rotating), respectively. The velues of W_k (kinematic vorticity number) calculated from average velocity gradient tensor in the northern and southern areas are 0.16 and 1.08, respectively. It indicates that incoherent boundaries with high shear-strain rate and jump in W_k coincide with seismogenic zones. To test our findings, we also analyzed the surface deformation along and around the San Andreas Fault (SAF) using GNSS data. In contrast to MTL in Kyushu, there is no rotation-rate jump across SAF, although a high rotation rate and high shear-strain rate exist along SAF as in MTL. The flow pattern is a general shearing both in NE and SW areas to SAF, but W_k of the NE area (North American Plate) is 0.13 which is significantly different from that (W_k = 0.83) of the SW area (Pacific Plate). Our result from SAF also indicates that seismogenic zones are incoherent boundaries in velocity field.