Multi-Hazard Mitigation Plan: 07. Earthquakes

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Chapter 7:Hazard Analysis—Earthquakes The largest earthquake on the island during the 20`h century occurred on the south flank of Kilauea in 1975. This earthquake had a magnitude of 7.2 and caused coastal subsidence at Kalapana, generated a tsunami that killed 2 people in the Hawaii Volcanoes National Park, destroyed houses in the Ka'u district, sank fishing boats in Keauhou Bay within the North Kona district, and damaged boats and piers in Hilo,within the South Hilo district.30 A large earthquake, unrelated to volcanic activity, was located 25 miles beneath Honomu in the South Hilo district in 1973. This earthquake had a magnitude of 6.2 and caused$5.6 mil- lion worth of damage and injured 11 people.31 7.2.1 Kiholo Bay Earthquake The most recent major earthquakes in the State of Hawaii were the Magnitude 6.7 Kiholo Bay and Magnitude 6.0 Mahukona earthquakes that occurred on October 15, 2006 at 7:07am and 7:14 am respectively (Robertson et al, 2006; SERI, 2006; EERI et al, 2006). Both earthquakes were centered neat- the Kona coastline of Hawaii. A map of ground shaking intensity for the island is shown in Figure 7-7 (Adapted from USGS Shakemap downloads). The maps show that largest ground shaking for this earthquake was at the northern end of the island, but did not directly coincide with the epicenter of the earthquake. The largest ground motions were recorded at the towns of Waimea and Hawi. These areas had amplified ground motion due to softer soil conditions at these locations. The most heavily damaged buildings were concentrated in the Waimea and Hawi areas with some damage also in the Honokaa and Kona areas. There was very little damage at the south end of the island. The main October 15 Kiholo Bay earthquake probably reflected the long-term accumulation and release of lithospheric flexural stresses. The long-term stresses consist in part of stresses generated in the crust and mantle by the weight of the volcanic rock that composes the islands. Such deeper mantle earthquakes at approximately 30 to 40 km depth result from flexural fracture of the underlying lithosphere in long-term geologic response to the load of the island mass. This is one of the seismotectonic mechanisms for damaging (but not the largest) earthquakes in the Hawaiian islands. Past examples of such "mantle" earthquakes include the 1973 M6.2 Honomu (on the northeast coast of the island), the 1938 M7 Maui, and the 1871 M7 Lana`i earthquakes. 30 Heliker,C."Volcanic and Seismic Hazards on the Island of Hawaii",U.S.Geological Survey, 1990. 31 Ibid. 7-7 Hawaii County Multi-Hazard Mitigation Plan