Volcano Watch — Strong quake may be due

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Eleven years ago on November 16, residents of Hawaii were awakened by a strong 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 Kaoiki Fault Zone is also a location with recurring earthquakes. Five earthquakes occurred in this area between 1941 and 1983, with an average recurrence interval of 10.5, plus or minus 1.5 years.

There is also a pattern in the magnitudes of the earthquakes through time, with every other earthquake having a magnitude greater than 6 and the intervening ones having magnitudes between 5 and 5.5. If this pattern continues, we may have a technique to actually forecast future earthquakes in this region.

The 1983 Kaoiki earthquake caused widespread damage from Volcano to Hilo. Several types of damage were observed near the epicenter, including structural damage to buildings, damage to water tanks, extensive breakage inside houses, and disruption of electrical and telephone service. The earthquake also triggered numerous landslides in Hawaii Volcanoes National Park, along the Hamakua coast highway (37 miles from the epicenter), and in Kealakakua Bay on the Kona coast (31 miles from the epicenter).

In the National Park, parts of Crater Rim Drive collapsed into the adjacent caldera. Hilo also suffered some damage, including several houses that slid off their post-and-pier foundations. Structural damage occurred at Hilo Hospital, and several buildings in the old part of Hilo suffered damage.

We recently completed some measurements of ground movement in the Kaoiki Fault Zone using the Global Positioning System (GPS). The GPS uses an array of satellites to locate points on the ground within a few millimeters horizontally and a few centimeters vertically. The data indicate that the region is now moving east-southeast at several centimeters per year. This is a major change from the very slow movement rates measured during previous years. These new measurements also show that the Kaoiki fault zone is compressing; such compression will increase the strain in the region and eventually lead to the next earthquake.

Since 1833, nine earthquakes in Hawaii had magnitudes larger than 6.5, suggesting an average recurrence interval of 15 to 17 years. Based on this historical record, it is clear that Hawaii will experience future earthquakes at least as large as the 1983 Kaoiki earthquake and perhaps as large as the 1868 earthquake, which had an estimated magnitude of 7.9. Such future earthquakes will cause widespread damage and disruption of essential facilities and infrastructure.

The extent of such damage and disruption can be minimized by adoption and enforcement of appropriate building codes. Last year, Hawaii County adopted the 1991 Unified Building Code and took an important step in mitigating the effects of future earthquakes.