Ground-Motion Attenuation Relationships for Cascadia Subduction Zone Megathrust Earthquakes Based on a Stochastic Finite-Fault Model

Abstract

The number of strong ground motion recordings available for regression analysis in developing empirical attenuation relationships has rapidly grown in the last 10 years. However, the dearth of strong-motion data from the Cascadia subduction zone has limited this development of relationships for the Cascadia subduction zone megathrust, which can be used in the calculation of design spectra for engineered structures. A stochastic finite-fault ground-motion model has been used to simulate ground motions for moment magnitude (M) 8.0, 8.5, and 9.0 megathrust earthquakes along the Cascadia subduction zone for both rock-and soil-site conditions. The stochastic finite-fault model was validated against the 1985 M 8.0 Michoacan, Mexico, and the 1985 M 8.0 Valpariso, Chile, earthquakes. These two subduction zone megathrust earthquakes were recorded at several rock sites located near the fault rupture. For the Cascadia megathrust earthquakes, three different rupture geometries were used to model the M 8.0, 8.5, and 9.0 events. The geometries only differ in their respective fault lengths. A fault dip of 9Њ to the east with a rupture width of 90 km was selected to represent average properties of the Cascadia subduction zone geometry. A regional crustal damping and velocity model was used with the stochastic finite-fault model simulations. Ground motions were computed for 16 site locations. The parametric uncertainties associated with the variation in source, path, and site effects were included in the development of the ground motions. A functional form was fit to the ground-motion model simulations to develop regionspecific attenuation relationships for the Cascadia megathrust rupture zone for both rock and soil site conditions. The total uncertainty was based on a combination of the modeling and parametric uncertainties (sigmas). These newly developed attenuation relationships for Cascadia subduction zone megathrust earthquakes can be used in both the probabilistic and deterministic seismic-hazard studies for engineering design for the Pacific Northwest.

Generation of Synthetic Ground Motion

“…Other attenuation relationships proposed by Gregor et al [3] and Kanno et al [4] can also be used when suitable weightage is given; (5) From above results, it can be concluded that a simple mathematical expression for these three attenuation relationships can be:…”

“…where, C is the combined spectral acceleration values, Y, G and K are the spectral acceleration values for Youngs et al [2], Gregor et al [3] and Kanno et al [4] respectively.…”

“…Response spectral values as well as the time histories values given by Kanno et al [4] are in slightly higher range. Hence more weightage is given by the Youngs et al [2] and Gregor et al [3]. Brief description of attenuation laws for horizontal response spectral acceleration (5% damping) followed in this research is highlighted below.…”

Bridge Seismic Retrofit Measures Considering Subduction Zone Earthquakes

“…The database was used to develop attenuation relationships using a maximum likelihood regression method. A previous attenuation relationship used a stochastic finite-fault ground motion model and a less extensive record database to produce relationships specific to the Cascadia subduction zone (Gregor et al 2002). This model was validated using the 1985 Miochoacan, Mexico (M 8.0) and 1985 Valpariso, Chile (M 8.0) earthquakes.…”

“…Extending this research to large magnitude long duration subduction zone earthquakes has been limited by the availability of recorded ground motions as such earthquakes occur with less frequency than crustal earthquakes. Studies have been conducted with the few available ground motions (Stapleton et al 2005), and several attenuation relationships have been developed (Atkinson and Boore 2003), (Gregor, et al 2002) in order to study the effects of such earthquakes. Each approach has limitations affecting the broader applicability of the results.…”