The SDSU Broadband Ground Motion Generation Module Version 1.5

SCEC has completed Phase 1 of its Broadband Platform (BBP) ground motion simulation results, evaluating the potential applications for engineering of the resulting PSAs generated by 5 different methods. The evaluation included part A, where the methods were evaluated based on the bias of simulation results to observations for 7 well-recorded historical earthquakes with source-station distances between 1 and 193 km, and part B, where simulation results for Mw 6.2 and Mw 6.6 strike-slip and reverse-slip scenarios were evaluated at 20 km and 50 km from the fault. The methods were assessed based on the bias of the median PSA for the 7 events in part A, and on a specified acceptance criterion compared to NGA-West2 GMPEs in part B. One of the 5 methods evaluated was BBtoolbox, a hybrid method combining deterministic low-frequency (LF) synthetics with high-frequency (HF) scatterograms (Mai et al., 2010; Mena et al., 2010; V1.4). In the validation exercise, the LFs are generated using 1-D Green’s Functions and 50 source realizations from the kinematic source generator module by Graves and Pitarka (2010, ‘GP’). However, the results from BBtoolbox V1.4 did not pass the SCEC validation phase 1. In order to obtain more accurate BB synthetics, we generated BBtoolbox V1.5 which scales the HFs to a theoretical spectral level at the merging frequency (GP, Eq. 10) fixed at 1Hz, rather than the level of the LFs in V1.4. This modification has generated much improved spectral levels at higher frequencies, while we have seen little evidence of artifacts from this technique. In addition, V1.5 introduced a new source time function with rise-time scaled as a function of moment. With these modifications, BBtoolbox V.15 became one of three methods passing the SCEC validation exercise Phase I. Here, we describe the details of BBtoolbox V1.5, and show comparisons between BBtoolbox V1.5 synthetics and observations from the valiation exercise.