Predicted Ground Motions for Great Interface Earthquakes in the Cascadia Subduction Zone

Abstract

Ground motions for earthquakes of moment magnitude ( M ) 7.5–9.0 on the Cascadia subduction zone interface are simulated based on a stochastic finite-fault model and used to estimate average response spectra for firm-site conditions in Vancouver, Victoria, and Seattle. We also express the response spectra as ground-motion prediction equations (GMPEs) for Cascadia events. The simulations are calibrated by modeling the wealth of ground-motion data from the M 8.1 Tokachi-Oki earthquake sequence of Japan. Adjustments to the calibrated model are made to consider average source, attenuation, and site parameters for the Cascadia region. We perform best estimate simulations for a preferred set of input parameters. Typical results suggest mean values of 5%-damped pseudoacceleration in the range from about 100 to 200 cm/sec2, at frequencies from 1 to 4 Hz, for firm-ground conditions in Vancouver, Victoria, and Seattle. Uncertainty in stress drop causes uncertainty in simulated response spectra of about ±50%. Uncertainties in the attenuation model produce even larger uncertainties in response spectral amplitudes—a factor of about 2 at 100 km, becoming even larger at greater distances. It is thus important to establish the regional attenuation model for ground-motion simulations. Furthermore, combining data from regions with different attenuation characteristics—in particular Japan and Mexico—into a global subduction zone database for development of global empirical GMPEs may not be a sound practice. Time histories of acceleration for the stochastically simulated motions are provided for reference sites in Vancouver, Victoria, and Seattle. An alternative set of motions, based on lightly modifying real recordings from the Tokachi-Oki earthquake to match expected conditions for Cascadia cities, are also provided. These alternative records have similar spectral content to the simulated motions but contain additional complexity and more realistic phasing. The provision of alternative record sets allows users to conduct studies to determine the importance of these effects for structural response.