Multi-Hazard Mitigation Plan: Executive Summary

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Executive Summafv • Soil NEHRP Classification Maps To be able to utilize the strong motion data recorded by the USGS Hawaiian strong motion network, knowledge of the subsurface site conditions beneath the USGS stations was required. The subsurface geology and, more important, the shear-wave velocity (Vs) structure beneath the USGS stations has been unknown to date. The information is invaluable to verify the appropriateness of the empirical ground motion attenuation models being used in the state hazard maps produced by USGS and in site-specific hazard analyses for engineering design. To obtain Vs information beneath the USGS strong motion sites, Spectral Analysis of Surface Waves (SASW) surveys were performed by the University of Texas, Austin, and URS Corporation in January 2008 (Wong et al. 2008). The SASW technique has been used to obtain Vs profiles at other USGS strong motion sites (e.g., Seattle, the Imperial Valley, and Los Angeles), and this technique has been well validated against other approaches, such as down-hole surveys (e.g., Wong and Silva 2006). The technique has been particularly useful in volcanic regimes where interbedded volcanic sequences can result in low-velocity zones (e.g.,Yucca Mountain and Los Alamos). The SASW methodology is a non-destructive and non-intrusive seismic method. It utilizes the dispersive nature of Rayleigh-type surface waves propagating through a layered material to estimate the shear-wave velocity profile of the material (Stokoe et al. 1994; Joh 1996). In this context, dispersion arises when surface-wave velocity varies with wavelength or frequency. Dispersion in surface-wave velocity arises from the changing stiffness properties of the soil and rock layers with depth. Spectral analysis is used to separate the waves by frequency and wavelength to determine the experimental ("field") dispersion curve for the site. An analytical procedure is then used to theoretically match the field dispersion curve with a one-dimensional layered system of varying layer stiffness's and thicknesses. The one- dimensional Vs profile that generates a dispersion curve that matches the field dispersion curve is presented as the profile at the site. 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. 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 January 7 to 17, 2008 at 22 USGS strong motion. Several surveys were also performed at Kawaihae Harbor. The high PGA's 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. Based on the survey results, all of the 22 USGS strong motion sites are "soil" sites with Vs'0 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 Q. 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. xx Hawaii County Multi-Hazard Mitigation Plan