The Period-Dependent Effects on the Amplification of Observed Ground Motions Within the Seattle Basin

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

Previous studies have shown the Seattle basin’s potential to amplify observed ground motions. We compile a database of ground motions from earthquakes in the Pacific Northwest and compute amplifications from all records relative to ground motion prediction equations (GMPEs) for Vs30=760 m/s site conditions. We compile a catalog for 159 earthquakes in the region with M ≥ 3.0 and four recent events (M ≥ 5.2) that occured SW of Vancouver Island, Canada. Each event is classified by tectonic regime (crustal, interface, intraslab, or undetermined) using the Slab 2.0 model. For each event, waveform data from stations located within 3.0 degrees from the epicenter are requested from IRIS. We develop an automated processing workflow (ShakeMap-Amp-Tools) that follows the standard procedures: baseline correction, removal of clipped waveforms, instrument response correction, removal of waveforms with invalid amplitudes, high-pass and low-pass filtering with dynamically-selected corner frequencies, and polynomial baseline correction. For each processed waveform, the pseudo-spectral accelerations (SA) are calculated at 9 different periods (0.1, 0.2, 0.3, 0.5, 1, 2, 3, 5, and 10 s) for combined horizontal-components (RotD50, RotD100) and for radial-, transverse-, and vertical-components. The observed RotD50 ground motions for crustal earthquakes are compared with predicted ground motions using an average of the NGA-West-2 GMPEs. Comparison with predicted ground motions helps identify issues in the database and provides a feedback for assessing station quality. We compute ground motion residuals from the final database, relative to average-predictions from the GMPE suite, and assess the spatial variations of the resulting amplifications. Initial analysis of these amplification factors indicates higher spatial variability among the shorter period amplifications, while longer periods show amplifications that correlate more closely with the basin structure.

Presenting Author: John Rekoske

Authors