The Effect of Forearc Mantle Serpentinization on Ground Motions From Thrust and Intra-slab Events in the Cascadia Subduction Zone

Previous modelling of great thrust earthquakes in Cascadia (Cohee et al., 1991) has indicated that ground motions at major urban centers (Portland, Seattle, Vancouver) could be dominated by post-critical reflections from the oceanic Moho of the subducting plate. This inference was based on a simple model of the forearc mantle wedge. Here we reexamine large-scale structural controls on ground motions from both thrust and intraplate events in light of recent evidence favouring a hydrated and serpentinized forearc mantle. We employ 2-D, P-Sv pseudo-spectral synthetic seismograms, double-couple line-sources and a structural model for central Oregon based on the study of Bostock et al. (2002) that includes both serpentinized forearc mantle and eclogitized oceanic crust. In our modelling of thrust earthquakes, we find that the presence or absence of serpentinized forearc mantle governs whether maximum ground motions along the Pacific Northwest urban corridor result from post-critical reflection at the oceanic Moho or continental Moho, respectively, as well as their range dependence. Moreover, serpentinization reduces the efficiency of the subducting oceanic crust as a waveguide by allowing energy to escape through the overlying wedge. This effect is particularly important for intraslab events as it allows focussing of energy toward the Earth's surface that would otherwise be redirected downward. As a result ground motions can be up to 5 times greater than those modelled in the absence of a serpentinized mantle forearc.