The Effect of Forearc Mantle Serpentinization on Ground Motions from Megathrust and Intraslab Events in the Cascadia Subduction Zone

We re-examine large-scale structural controls on ground motions from megathrust and intraslab events in the Cascadia subduction zone in light of recent evidence pointing to a highly hydrated and serpentinized forearc mantle. We employ 2D P-SV pseudospectral synthetic seismograms, double-couple line sources, and a structural model for Cascadia based on the study of Bostock et al. (2002) that includes both serpentinized forearc mantle and eclogitized oceanic crust. As in a previous study (Cohee et al., 1991), we note that ground motions at greater epicentral distances are dominated by postcritical reflections from the Moho. However, in our modeling of megathrust earthquakes, we find that the presence or absence of serpentinized forearc mantle dictates whether maximum ground motions in the Pacific Northwest urban corridor (Portland, Seattle, Vancouver, Victoria) result from postcritical reflection at the oceanic Moho or continental Moho, respectively, as well as their range dependence. Moreover, serpentinization reduces the efficacy of the subducting oceanic crust as a wave guide by allowing energy to escape through the overlying wedge. This effect is particularly important for intraslab (Wadati-Benioff) events, as it allows focusing of upward-propagating energy toward the surface that would otherwise be redirected downward. Consequently, ground motions from intraslab sources at lower frequencies (0-2 Hz) can be up to 5 times greater than those modeled in the absence of a serpentinized forearc mantle. At higher frequencies, the lower Q of serpentine may act to reduce this amplification.