Abstract
Ground motions for earthquakes of M7.5 to 9.0 on the Cascadia subduction interface are simulated based on a stochastic finite-fault model, and used to estimate average response spectra for reference firm soil conditions. The simulations are first validated by modeling the wealth of ground-motion data from the the 2011 M9.0 Tohoku earthquake of Japan. Adjustments to the calibrated model are then made to consider average source, attenuation and site parameters for the Cascadia region. This includes an evaluation of the likely variability in stress drop for large interface earthquakes, and an assessment of regional attenuation and site effects. We perform best-estimate simulations for a preferred set of input parameters. Typical results suggest mean values of 5%-damped pseudo-acceleration in the range from about 100 to 200 cm/s2, at frequencies from 1 to 4 Hz, for firm-ground conditions in Vancouver. Uncertainty in most-likely value of the parameter representing stress drop causes variability in simulated response spectra of about ±50%. Uncertainties in the attenuation model produce even larger variability in response spectral amplitudes – a factor of about two at a closest distance to the rupture plane (Rcd) of 100 km, becoming even larger at greater distances. It is thus important to establish the regional attenuation model for ground-motion simulations, and to bound the source properties controlling radiation of ground motion. We calculate theoretical 1D spectral amplification estimates for four selected Fraser River Delta sites to show how the presence of softer sediments in the region may alter the predicted ground motions. The amplification functions are largely consistent with observed spectral amplification at Fraser River delta sites, suggesting amplification by factors of 2.5 to 5 at the peak frequency of the site; we note that deep sites in the delta have a low peak frequency, ~0.3 Hz. This work will aid in seismic hazard assessment and mitigation efforts in the active Cascadia region of southwestern B.C. An important consideration is that the uncertainties are large and present a dominant unknown when assessing seismic risk. We find that variability in the expected motions exceeds a factor of two even on rock-like sites, with uncertainty
Highlights
Megathrust interplate earthquakes in a subduction zone cause catastrophic damage and loss to modern society
This study focuses on estimation of ground motions from a Cascadia megathrust event and their uncertainty, using a stochastic finite fault algorithm known as EXSIM (Motazedian and Atkinson, 2005; Atkinson et al, 2009; Boore, 2009)
All ground-motion prediction equations (GMPEs) are plotted for B/C site conditions, for a typical Cascadia soil profile
Summary
Megathrust interplate earthquakes in a subduction zone cause catastrophic damage and loss to modern society. Such complex features were highlighted during the 2011 Tohoku earthquake in Japan by observations of multiple phases of seismic wave arrivals due to local asperities, referred to as strong-motion generation areas (SMGAs) (Kurahashi and Irikura, 2011; Goda et al, 2012) Another important aspect of the Tohoku event was its remarkable site effects, leading to significant site amplification at high frequency (with little soil non-linearity), as pointed out by Ghofrani et al (2012). The path and site parameters, such as kappa, attenuation models, duration, and site amplification factors (both crustal and near-surface) are obtained from empirical investigations (Ghofrani and Atkinson, 2011; Ghofrani et al, 2012) We consider both single-rupture and multiple-rupture models, assuming a random slip distribution (within a fault plane) for both cases (see Ghofrani et al (2013) for more results). The Slip distribution is assumed to be random; the hypocenter is located at the center of the fault
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