Regional and Depth Variations in Source-Scaling Properties for Subduction Earthquakes

ABSTRACT Scaling relationships for earthquake source parameters provide fundamental information for characterizing source faults and simulating ground motions and/or tsunamis for scenario earthquakes. For subduction earthquakes, although variations in the physical properties of subducting oceanic plates, such as temperature, subduction velocity, and seismogenic zone depth, have been identified between subduction zones with different tectonic environments, whether variations in source-scaling relationships exist is still under debate. We investigated the regional variations in source parameters for subduction earthquakes by extracting these parameters from heterogeneous slip models for many events (Mw 6.7–9.1) in nine regions, based on source inversion analyses conducted by the U.S. Geological Survey. Regional differences were observed in the mean residuals of the extracted rupture areas with respect to the scaling relationship, although these differences were not substantial enough to reach statistical significance. Meanwhile, by investigating the depth of the source fault for each event, we found a depth-related variation indicating that the size of the rupture area correlates with the top depth of the source fault. This correlation leads to a magnitude-dependent scatter in rupture area and complicates the self-similar scaling model for events with Mw less than 8.4. For small magnitude (Mw<7.5), the event for which rupture propagates upward and extends to the shallow part of the subduction zone has a larger rupture area, whereas the event propagating only downward has a smaller rupture area. The coexistence of these two event types for Mw less than 7.5 increases the scatter in the scaling relationship of rupture area. For large magnitude (Mw 7.5–8.4), most events extend ruptures to the shallow part of the subduction zone and have larger rupture areas, and thus the scatter is smaller than that for small Mw. The association between rupture propagation direction and rupture area may be useful for updating seismic hazard assessment for subduction earthquakes.