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Large Earthquakes Are Hazards to be Reckoned With in Hawaii; Could Affect Building Codes, Accoding to USGS Researcher
Released: 4/8/1997

Contact Information:
U.S. Department of the Interior, U.S. Geological Survey
Office of Communication
119 National Center
Reston, VA 20192
Pat Jorgenson 1-click interview
Phone: 415-329-4000

The term "geologic hazards" in Hawaii generally means volcanic eruptions and lava flows. A hazard that might not come to mind is the possibility of earthquakes, as large as magnitude-eight, under the flanks of the active volcanoes, according to Fred Klein, a seismologist with the U.S. Geological Survey in Menlo Park, Calif.

"The earthquake shaking hazard in some areas of Hawaii is comparable to that near the major faults of California, Klein said Wednesday, April 9, at the annual meeting of the Seismological Society of America (SSA), in Honolulu.

Klein demonstrated how large Hawaiian earthquakes occur as the flanks of volcanoes are pushed sideways by a growing rift zone, and the six-mile-thick flank slips on its base, causing an earthquake. "The force can be demonstrated by pushing a brick sideways as it rests on a rough table; your hand is the rift zone, the brick is the flank, and its sudden slip is the earthquake."

The largest earthquake hazard in Hawaii, according to Klein, is on the south and west mobile flanks of Kilauea and Mauna Loa volcanoes. In south Hawaii the average recurrence intervals, or times between earthquakes, are 29-44 years for a magnitude-7 or larger, and 120-190 years for a magnitude-8 or larger. The largest earthquakes in Hawaii’s recorded history were the April 2, 1868 magnitude-7.9; and the November 29, 1975, magnitude-7.2 earthquakes. In the Kona and Hualalai areas of west Hawaii, the average recurrence intervals for magnitude-5.5 earthquakes is 3-10 years and 265-300 years for magnitude-7 events. The largest historic Kona earthquake was a magnitude-6.9, on August, 21, 1951.

Klein presented a new map that delineates the amount of "probabilistic ground shaking," (PGA) where the calculated shaking has a 10 percent chance of occurring during a 50 year period. The degree of shaking is not only useful for identifying the hazardous zones, but also for telling engineers how much shaking buildings may receive. "The probabilistic 0.5 g (half the force of gravity) peak shaking calculated for Hilo means that a building designed to withstand 0.5 g has only a 10% chance of being damaged in a 50 year period by an earthquake anywhere on the island," Klein said.

Klein said Hawaiian PGAs at lava sites are calculated from a relation where shaking increases with magnitude and decreases with distance. Amplification of sites on volcanic ash or unconsolidated soil are about two times those of hard lava sites.

On a map for a 50-year exposure time with a 90 percent probability of not being exceeded, the PGA is 1.0-1.3 g on Kilauea’s and Mauna Loa’s mobile south flanks; 0.7 g in the central Kona area of Mauna Loa’s west flank; and 0.3 g at the north tip of the island. Klein said the new seismic hazard map supports an argument to raise Hawaii Island from the Universal Building Code’s seismic zone three, to zone four. Zone four is presently the highest level of hazard zone, and is assigned to much of California.

Editors: Dr. Klein may be contacted for interviews by calling the SSA newsroom at the Hawaiian Regent Hotel in Honolulu, at 808-921-5036.

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