NEES-2008-0523: SASW Measurements at USGS Hawaiian Strong Motion Network
| PIs | Kenneth H. Stokoe Ivan Wong Brady Cox |
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| | Organizations | United States Geological Survey (USGS) , |
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| | Start Date | 2008-01-07T00:00:00 |
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Description:
The SASW technique has been used to obtain VS profiles at USGS strong motion sites in the U.S. (e.g., Seattle, the Imperial Valley, and Los Angeles), and this technique has been well validated against other approaches, such as down-hole surveys. The technique has been particularly useful in volcanic regimes where interbedded volcanic sequences can result in low-velocity zones.
An active seismic source is required for the SASW surveys. In these surveys, one of the NSF's Network for Earthquake Engineering Simulation (NEES) mobile vibrators, known as "Thumper," was used (in far background in Figure 2). Thumper has been designed to be a moderate- to high-frequency vibrator for use in seismic reflection and surface wave projects.
The surveys took place from 7 to 17 January 2008. The 22 USGS strong motion sites surveyed are shown on Figure 1. Several surveys were also performed at Kawaihae Harbor. One USGS strong motion site was located within Hawaii Volcanic National Park and this station at the Hawaiian Volcano Observatory (HVO) was surveyed on 10 January 2008 (Figure 2). The VS profile for HVO is shown on Figure 3.
The high PGAs recorded at the Waimea Station and the North Kohala Police Station are probably due to thin soil site amplification where a strong velocity contrast exists between the soil and underlying basalt (Figure 3). Based on the survey results, all of the 22 USGS strong motion sites are "soil" sites with VS30 values ranging from 442 ft/sec at the USDA Laboratory in Hilo (National Earthquake Hazards Reduction Program [NEHRP] site class E) to 1,812 ft/sec at the South Kohala Fire Station (NEHRP C) (Table 1). Surprisingly, none of the strong motion sites had rock-like VS30 values, even sites where basalt outcropped at the surface, such as at the University of Hawaii at Hilo.
As part of another FEMA-supported study to calibrate and improve the HAZUS software use for Hawaii, we have produced a new 1:100,000-scale map of NEHRP site class map for the Big Island based on the SASW measurements and 1:100,000-scale geologic mapping by Sherrod et al. (2007) (Knudsen et al., 2008). An earlier 2006 site class map portrayed nearly the entire island as NEHRP site class B; however, based on about 20 SASW measurements in areas mapped as basalt, we believe that most of the island should be mapped as NEHRP C or D. VS30 estimates for these basalt sites spanned NEHRP classes C and D. The median value for these VS30 estimates is 1,304 ft/sec, with a log mean of 1,274 ft/sec and a standard deviation of 274 ft/sec. The sites cover a range of basaltic rock conditions as depicted on the geologic map, including lava flows, scoria cones, littoral deposits, spatter or tuff cones, cinder cones, and lava domes. Other geologic map unit groups for which only a few VS30 estimates were made from SASW seat include alluvium, ash/tephra, and artificial fill. We assigned to these map units, NEHRP site class D?, C to E, and C to E, respectively. Geologic deposits for which we do not have quantitative velocity data and have made preliminary site class assignments are sand dunes (D?), landslide deposits (D?), and glacial deposits (D?). We also attempted to relate VS30 estimates to mapped pedogenic soil units, ages of mapped basalt units, and source volcanoes for basalt units, but found little basis for making these correlations.
