Multi-Hazard Mitigation Plan: 07. Earthquakes

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Chapter 7:Hazard Analysis—Earthquakes Table 7-1. Conversion of MMI to PGA Values S ecitic to Ilawaii32 Perceived Shaking Not Felt Weak Light Moderate Strong Very Severe Violent Extreme S trong Potential Damage None None None Very Light Light Moderate Moder- Heavy Very ate/Heavy Heavy Peak Acceleration V.2 3.2-8.1 8.1-13 13-20 20-32 32-51 51-80 80-128 >128 (%g) Peak Velocity(%g) 1.9 1.9-6.4 6.4-11 11-18 18-28 28-47 47-74 74-120 <120 Modified Mercalli r 11-111 IV V VI Vii Viii IX X Intensity(MMI) Documentation for current hazard maps is given in Seismic Hazard in Hawaii: High Rate of Large Earthquakes and Probabilistic Ground-Motion Maps, by Fred W. Klein, Arthur D. Frankel, Charles S. Mueller, Robert L. Wesson, and Paul G. Okubo, Bulletin of the Seismological Society of America, Vol. 91, No. 3, pp. 479-498. June, 2001; USGS report 2724 published at http://Uubs.usgs.gov/imgp/2000/i-2724/. 7.3.1.2 Soil Conditions The seismic ground motion at a particular site can be significantly increased by weaker or "softer" soil conditions. Rock and soil conditions are categorized in the IBC by Site Classes A through F, sometimes referred to as Soil Types. Weaker soil indicates areas of greater potential hazard therefore Site Class should also be considered in individual building assessments. 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 32 Based on Wyss&Koyanagi 1992 7-15 Hawaii Countv Multi-Hazard Mitigation Plan