Using Spatial Variation of kappa to Develop Site-Specific Attenuation Model for Improved Broadband Simulations

Ramin Motamed, Swasti Saxena, & John G. Anderson

Submitted August 11, 2017, SCEC Contribution #7495, 2017 SCEC Annual Meeting Poster #236 (PDF)

Reliable synthetics are an alternative for use in engineering practice for dynamic time history analysis of geotechnical and structural systems, and this study aims to improve upon Broadband simulations to account for site-specific surface response to an earthquake. We have developed velocity and attenuation profiles for Wildlife Liquefaction Array (WLA) in Southern California for this purpose, which are later incorporated into source-to-site Broadband simulations. The WLA site is carefully chosen for the significant number of medium to strong ground motions recorded there, and using them the profiles are determined in the seismic waveguide and shallow crustal layers.

Previous studies have shown that the rapid decay of a Fourier spectrum of an accelerogram at high frequencies is characteristic of the ground attenuation, or Q-value profile at the site. Assuming a Brune source model (1970), an attenuation parameter, kappa (Anderson et. al., 1984), is identified beyond a corner frequency, fmax (Hanks, 1982). The spatial variation of kappa is determined separately for compressional and shear waves, and used to develop Qp and Qs profiles. We also compare spectral ratios of as many as 22 earthquakes recorded at WLA with the response of a stack of horizontal layers to vertically incident SH waves to determine the velocity profile in the upper crust below the site.

The final attenuation and velocity profiles, along with a shallow Moho, present at about 22 km in the Imperial Valley basin (Magistrale et al., 2000), are used to model a layered continuum for the path of wave propagation from source to the site. WLA observes very low seismic velocities near the surface and allows for a nearly vertical incidence of direct S-waves, therefore, the results of this study are comparable to other approaches that assume only vertically-incident S-waves. Simulations in this research are generated in conformance with the Composite Source Model (Anderson, 2015) and validated against earthquakes recorded at WLA. This model delivers full wave propagation at high frequencies and includes compressional, shear and surface waves that together produce realistic-looking synthetic ground motions.

Finally, these grounds motions are compared with those obtained with a generic Q-model, the differences noted and results discussed.

Key Words
Broadband Simulations; Kappa, Q-model

Motamed, R., Saxena, S., & Anderson, J. G. (2017, 08). Using Spatial Variation of kappa to Develop Site-Specific Attenuation Model for Improved Broadband Simulations. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Engineering Implementation Interface (EEII)