Site Response in the Port of Tacoma, Washington
Abstract
Following the Nisqually earthquake (Feb. 28, 2001, M ~6.8), we deployed 9 portable digital seismographs in a linear array to study the seismic hazard for the Port of Tacoma (23.4 km from the epicenter). The Port of Tacoma, situated across the mouth of the Puyallup River and one of the more active ports along the west coast, suffered little damage from the earthquake. Each site used a 24-bit recorder from Refraction Technology with a Mark Products L-22 2Hz three-component geophone. Two sites at the west end of the array were in the city of Tacoma; the rest were along a line that crosses the mouth of the Puyallup River. We recorded 2 local, 2 regional, and 4 deep focus events. The event with the highest signal-to-noise ratio occurred on June 10, had a magnitude of 5, and was located in the southern Puget Sound region at an epicentral distance of 80 km and a depth of 40.7 km. Site response was estimated from this event using spectral ratios of the horizontal components at each site relative to a reference site on a hill in the city of Tacoma, and by taking the ratio of the shear wave spectra of the horizontal to vertical components (H/V technique). Seismic response using spectral ratios at sites in the valley are elevated by factors of between 2 and 6 in the .5 to 10 Hz frequency band compared to the reference site. The other site near the city of Tacoma has a lower relative response. Peaks, which may identify resonances, are also found on these responses, but their character may be dependent on slowness. Site responses estimated from the H/V technique show a similar pattern, with the two sites in Tacoma having smaller responses than those stations in the river valley, again in the frequency band .5 to 10 Hz. These responses have amplifications about a factor of 8 or higher at the principal peaks and typically look like amplified versions of the shear wave responses computed using a reference site. Apparently, the shear wave response at the reference site on a horizontal component is stronger than the typical SV response. Site responses from ratios of the teleseismic P-waves, which needed to be corrected for local site noise, are elevated at valley sites in the 0.5 to 1 Hz frequency band, with some peaks at higher frequency, but with less amplification than the responses determined using the local event.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFM.S42C0674F
- Keywords:
-
- 7212 Earthquake ground motions and engineering;
- 7223 Seismic hazard assessment and prediction