# Volcano Watch — Ten years since last big quake; more are sure to come

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Ten years ago on November 16, residents of Hawaii were awakened by an earthquake at 6:13 a.m. The earthquake had a magnitude of 6.6 and was located beneath the Kaoiki Fault Zone between the summits of Mauna Loa and Kīlauea Volcanoes.

Ten years ago on November 16, residents of Hawaii were awakened by an earthquake at 6:13 a.m. The earthquake had a magnitude of 6.6 and was located beneath the Kaoiki Fault Zone between the summits of Mauna Loa and Kīlauea Volcanoes. This earthquake caused damage estimated at about \$7 million, nearly twice that of the 1975 Kalapana earthquake, which had a magnitude of 7.2.

The historical record for larger earthquakes in Hawaii is probably complete for earthquakes larger than magnitude 6.5. Since 1833, there have been nine such earthquakes, suggesting an average recurrence interval of 15 to 17 years.

The Kaoiki fault zone is also a location with recurring earthquakes. Dr. Max Wyss, a seismologist at the University of Alaska, has noted that, from 1941 to 1983, five earthquakes occurred in this area. The average recurrence interval is 10.5, plus or minus 1.5 years. Regression of the data indicates that, at a 95% confidence interval, the next event will occur between July 1993 and October 1996. There is also a pattern in the magnitudes of the earthquakes through time, with every other earthquake having magnitude greater than 6 and the intervening earthquakes having magnitudes between 5 and 5.5. If this pattern continues, then the next Kaoiki earthquake should have a magnitude between 5 and 5.5 and take place within the rather wide window identified above. However, because of the short sequence of events used to identify the pattern, we only have a model that will be tested when the next Kaoiki earthquake takes place, rather than a prediction or forecast of the next earthquake in this area.

The 1983 Kaoiki earthquake caused widespread damage from Volcano to Hilo. There were several types of damage near the epicenter, including structural damage to buildings, damage water tanks, extensive breakage inside houses, and disruption of electrical and telephone service. Most areas had restored telephone and electrical service within hours, but water supplies were affected for many people due to damage to catchment water tanks and connecting pipes. At the nearby Hawaiian Volcano Observatory, bookshelves and equipment racks toppled, the cement slab beneath the building was cracked, and some seismic and tilt monitoring equipment was disabled.

The earthquake also triggered numerous landslides, including many inside Hawaii Volcanoes National Park, along the Hamakua coast highway (37 miles from the epicenter), and in Kealakekua Bay on the Kona coast (31 miles from the epicenter). In the National Park, parts of Crater Rim Drive collapsed into the adjacent caldera. Small rock falls, extensional cracks in the ground, and collapsed lava tubes occurred many miles away from the epicenter.

Hilo also suffered some damage, including several houses that slid off their post-and-pier foundations and moved downslope. Structural damage also occurred at Hilo Hospital, and several buildings in the old part of Hilo suffered damage. The two areas that had the most damage are areas where there are thick ash deposits. Such thick soils are known to amplify the ground shaking during large earthquakes. In some parts of Hilo and the Volcano Golf and Country Club Subdivision, ash layers are beneath thin, surficial lava flows, but strong ground shaking nonetheless reflects amplification in the underlying ash.

Based on the historical record, it is clear that Hawaii will experience future earthquakes at least as large as the 1983 Kaoiki earthquake and perhaps as large at the 1868 earthquake, which had an estimated magnitude of 7.9. Such future earthquakes will certainly cause widespread damage and disruption of essential facilities and infrastructure. However, the extent of such damage and disruption can be minimized, particularly by adoption and enforcement of appropriate building codes. The recent adoption of the 1991 Unified Building Code by the County Council is an important step in mitigating the effects of future earthquakes.