Multi-Hazard Mitigation Plan: 07. Earthquakes

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Chapter 7:Hazard Analysis—Earthquakes 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 sites. 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 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 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. As demonstrated in the 2006 earthquake, where some strong motion stations recorded peak horizontal accelerations close to lg, site response effects can be significant on the Big Island. As part of FEMA-supported studies following the earthquake, a new 1:100,000-scale map of site conditions on the Big Island of Hawaii was produced. The mapping makes use of about 25 new SASW measurements (Wong et al., 2008) and 1:100,000-scale geologic mapping by Sherrod et al. (2007). An earlier 2006 site class map portrayed nearly all of the island as NEHRP site class B; however, based on about 20 SASW measurements in areas mapped as basalt, it is believed that most of the island should be mapped as NEHRP C or D. Vs30 estimates for these basalt sites ranged from 844 to 1,812 ft/sec, spanning 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 data include alluvium, ash/tephra, and artificial fill. These were assigned to map units NEHRP site class D, C to E, and C to E, respectively. Geologic deposits for which there is no quantitative velocity data and preliminary site class assignments have been made are sand dunes (D), landslide deposits (D), and glacial deposits (D). Other earthquake-induced ground failure hazards include liquefaction and landslide. Liquefaction occurs when loose granular soils below the water table temporarily lose strength due to excess pore water pressure build-up during prolonged strong earthquake ground shaking. Accordingly, higher potential would tend to occur at sites with these subsurface characteristics in regions of higher seismicity, since events of Richter magnitude 6 or greater with EPGA of greater than O.l Og are generally necessary to begin to induce liquefaction. 7-16 Hawaii Countv Multi-Hazard Mitigation Plan