GREAT EARTHQUAKE PRODUCTION SENSITIVITY TO DOWNDIP LIMIT OF SEISMOGENIC ZONE DETERMINATION USING 2.5D FINITE DIFFERENCE COMPUTER MODELING

The occurrence of great earthquakes, larger than magnitude 8.5 Mw, do not occur at every subduction zone. In this study, we set out to determine if one cause of this is due to the downdip limit of some seismogenic zones not being sufficiently deep to allow for rupture large enough for a great earthquake. We built a model of a megathrust seismogenic zone in Matlab and simulated earthquakes on the interplate boundary. Several versions of the model were run to test our hypothesis that a shallower downdip limit would suppress larger earthquakes than a deeper downdip limit. We also tested versions simulating the subduction of seafloor topography such as seamounts with differing values of friction. The results from the modeling show that a shallower downdip limit, and therefore a narrower seismogenic zone, likely does in fact limit the largest magnitude earthquake possible at a subduction zone. Findings from the variable friction models show that regions of high friction tend to stifle rupture propagation while regions of low friction can either inhibit or promote further rupture propagation. These conclusions help explain several observations at specific subduction zones, such as the observed lack of large events at Tonga and the interaction of rupture and subducted topography in Java