Multi-Hazard Mitigation Plan: Executive Summary

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Executive Summafv As demonstrated in the 2006 earthquake, where some strong motion stations recorded peak horizontal accelerations close to Ig, 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). • Landslide Hazard Maps A conceptual level slope risk map was prepared for the Hawaii HAZUS Conversion Project, using an adaptation of the slope hazard methodology given in the FEMA 2007 HAZUS-MH MR3 Technical Manual. The URS approach involved the interactions of three primary slope hazard input criteria simplified to low,medium and high hazard susceptibility The current HAZUS mapping for the Island of Hawaii is an initial pilot study allowing a scientific basis for incorporating slope hazards into island wide scenario loss estimates. This approach provides a simplified method where high resolution topography and rainfall data can be used with existing geologic maps in digital format to develop slope hazard criteria for rural,remote and developing areas where detailed slope hazard input data is not available. Plan Update and Maintenance Rather than just describe the need for plan maintenance and general tasks, this chapter was revised to provide specific actions and summary of specific ongoing hazard mitigation projects that will impact the next adoption of the mitigation plan. Thus, this provides a checklist of the resources expected for the plan maintenance taskings. This should ensure continuity and connectivity with ongoing and future work that should improve the next plan. This mitigation plan will be reviewed annually with input from an interagency Hazard Mitigation Planning Committee and an organized network of community groups in each district. The annual review will result in revised work-plans; budget requests to the County CIP, State Civil Defense, and other funding sources; suggested amendments to codes and plans; and proposed revisions to the text of this plan. xxi Hawaii County Multi-Hazard Mitigation Plan