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The SDSU Broadband Ground Motion Generation Module BBtoolbox Version 1.5

Kim B. Olsen, & Rumi Takedatsu

Published February 2014, SCEC Contribution #1985

The Southern California Earthquake Center (SCEC) has completed Phase 1 of its Broadband Platform (BBP) ground motion simulation results, evaluating the potential applications for engineering of the resulting 0.01-10 s Pseudo-Spectral Accelerations (PSAs) generated by 5 different methods. The exercise included part A, where the methods were evaluated based on the bias of simulation results to observations for 12 well-recorded historical earthquakes: 7 in western U.S., 2 in Japan, and 3 in eastern US/Canada. In addition, part B evaluated simulation results for Mw5.5, Mw6.2 and Mw6.6 scenarios at 20 km and 50 km from the fault. The methods were assessed based on the bias of the median PSA for the 12 events in part A, and on a specified acceptance criterion compared to NGA-West Ground Motion Prediction Equations (GMPEs) in part B. The results were evaluated by the bias of mean PSA from simulations using 1D velocity models with Vsmin=863 m/s (see electronic supplement of Goulet and Abrahamson, 2014, this issue) with respect to recorded data corrected for site effects. The part A events and part B scenarios are described in Tables S1 and S2, respectively, in the electronic supplement to this article.

One of the 5 methods evaluated was the Broadband Synthetics Generator Module BBtoolbox, a hybrid method combining deterministic low-frequency (LF) synthetics with high-frequency (HF) scatterograms (Mai et al., 2010, Mena et al., 2010). The LFs may be computed using deterministic or dynamic descriptions in 1D or 3D media. The HF scatterograms are generated for each component of motion based on the theory for multiple S-to-S scattering by Zeng et al. (1991, 1993). The scatterograms are based on user-specified site-scattering parameters and are partly based on the site-specific velocity structure. The seismic scattering wave energy is realized to appear after the direct P wave arrival time, which is found from 3D ray tracing (Hole, 1992). Finally, the scatterograms are convolved with an appropriate source-time function. It is assumed that the scattering operators and moment release originate throughout the fault, but starts at the hypocenter. The hybrid broadband seismograms are calculated in the frequency domain using a simultaneous amplitude and phase matching algorithm (Mai and Beroza, 2003). In the validation exercise, the LFs are generated using 50 source realizations from the kinematic source generator module by Graves and Pitarka (2014) on the SCEC BBP, through the Standard Rupture Format (SRF), identical to those used by the Graves and Pitarka method.

BBtoolbox V1.4 (as described in Mai et al., 2010; Mena et al., 2010) merges LFs and HFs by adjusting the level of the HF acceleration spectra to the corresponding LF spectral value at a specified merging frequency. This procedure was introduced in part to ensure continuity in the broadband synthetics at the merging frequency. While this approach tends to work well for LFs calculated in well-constrained 3D structural models, the results for simplified 1D velocity models averaged over a region was found to generate, at times, strongly biased HF PSA levels. In addition, the scaling of the HFs to the LFs carries any directivity effects from the LFs to arbitrarily high frequencies, also not supported by data. In addition, the shape of the existing source time functions often times did not capture the shape of the PSA trends. The bias introduced by these issues did not permit BBtoolbox V1.4 to pass the SCEC validation phase 1.

In order to obtain more accurate broadband synthetics, as defined by the SCEC validation Phase 1 targets and respective 1D Green’s Functions (GFs), we generated BBtoolbox V1.5. This article describes the changes that were made to BBtoolbox V1.4 to obtain V1.5, and presents the improved results that enabled BBtoolbox V1.5 to pass the validation.

Olsen, K. B., & Takedatsu, R. (2014). The SDSU Broadband Ground Motion Generation Module BBtoolbox Version 1.5 . Seismological Research Letters, 86(1), 81-88. doi: 10.1785/0220140102. http://srl.geoscienceworld.org/content/86/1/81.full.pdf+html

Related Projects & Working Groups
Ground-Motion Prediction