Project Abstract
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There is an unresolved debate about the way in which the rupture areas of large crustal earthquakes scale with seismic moment. The SCEC Broadband Platform (BBP) provides the opportunity to study in detail the impact of using two different M-A scaling relations in simulations: Leonard (2010), which the BBP Phase 1 validation project used as a guideline for selecting fault areas (Dreger et al., 2013 and Goulet et al., 2015), and Hanks and Bakun (2008), which results in smaller fault rupture areas than Leonard (2010) for about M>6.7. In our evaluation we use the already-implemented simulation methods and rupture generators to study both previously validated events (Type A) and a suite of selected event scenarios (Type B). For Type A validations, we utilize the simulated waveforms computed from the BBP Phase 1 validation project (Dreger et al., 2013.) and results are evaluated using the bias of simulated RotD50 with respect to observations (termed goodness of fit, or GOF). We re-compute the events of interest using the HB08 scaling relation to define the fault rupture area. For Type B validations, we study four event scenarios, repeating each using both M-A relations to define fault geometry. The results are dependent on the magnitude of the scenario (as expected) and vary between simulation methods. For Type A events, both the EXSIM and UCSB methods appear largely unaffected by the decrease in fault width associated with Hanks and Bakun (2008) scaling. For UCSB with the Type B scenarios, an increase in the average level of simulations is observed for the smaller fault areas. The SDSU and Graves and Pitarka methods behave similarly, which is to be expected at long periods. For Graves and Pitarka at short periods (<1 sec) the change to smaller fault area results in a slight decrease in the level of simulated motions. For Graves and Pitarka and SDSU at long periods (>1 sec) the change to smaller fault area results in an increase (up to about 30% for Landers and 20% for the M 7.0 scenario events) in the level of simulated motions. Based on communications with the modelers, in general this behavior is as expected. We hope that quantifying the impact of two M-A scaling relations on four BBP simulation methods will provide guidance to the modelers for the simulation of future earthquake scenarios, in Phase 2 of the Validation effort, and in other forward simulations. |
