Broadband Synthetic Seismograms for Magnitude 9 Earthquakes on the Cascadia Megathrust Based on 3D Simulations and Stochastic Synthetics, Part 2: Rupture Parameters and Variability

We used a combination of 3D finite-difference simulations (< 1 Hz) and 1D stochastic synthetics (> 1 Hz) to generate broadband (0-10 Hz) synthetic seismograms for numerous M-w 9 earthquake rupture scenarios on the Cascadia megathrust. Slip consists of multiple high-stress-drop subevents (M-w 8) with short rise times on the deeper portions of the fault, superimposed on a background slip distribution with longer rise times. We identify key rupture parameters that control the intensity of ground shaking and resulting seismic hazard; these include the hypocenter location, the down-dip limit of slip, the average rupture velocity, and the character (i. e., location, magnitude, and stress drop) of subevents. Extending the down-dip limit of rupture to the top of the nonvolcanic tremor zone results in localized regions with a factor of 5-10 increase in spectral acceleration (SA) for periods < 5 s, compared to a rupture that is completely offshore. This is primarily due to the closer proximity of highstress- drop subevents to inland locations when the rupture is allowed to extend deeper. Similarly, we find that the hypocenter location can result in a variation in the intensity of ground motions of a factor of > 10, due to the effects of rupture directivity (i.e., SA at periods > 1 s). We also observe a coupling between rupture directivity and basin amplification. The intensity of ground motions is also strongly affected by the magnitude, stress drop, and location of high-stress-drop subevents, which are poorly constrained. Overall, our results quantify the effect of kinematic rupture parameters on ground motions for an M-w 9 earthquake in Cascadia and emphasize the need for further constraints on these parameters to improve seismic hazard estimates in the Pacific Northwest.