Big Island holds potential for catastrophic quakes

By Jan TenBruggencate
Advertiser Science Writer

The threat of a severe earthquake is as great in parts of the Big Island as it is anywhere in the country, including California, which has a reputation as a quake danger zone.

Seismologists say the Big Island is at high risk of large quakes, such as the one in Punalu'u in 1975. Advertiser library photo • Nov. 30, 1975

"The scenario that would affect the rest of the chain would be something along the Kona side, the southwest side of the Big Island," said Charles "Chip" McCreery, geophysicist in charge at the Pacific Tsunami Warning Center.

A team that includes Fred Klein, U.S. Geological Survey researcher, and Paul Okubo, Hawaiian Volcano Observatory scientist, recently published a report on the situation, in which they associate the most dangerous quakes with shifting of flanks of volcanoes. Such areas tend to have an excessive number of very large earthquakes, they said.

"We were surprised by how high the hazard is ... The key point is that the hazard is as high as — or higher than — more notorious places such as Los Angeles and San Francisco," said Ivan Wong, a seismologist with URS Corp., an architecture and engineering firm in Oakland, Calif.

"This information is not new to the earthquake community, but it hasn't gotten out to the public," said Brian Yanagi, earthquake and tsunami program manager for the state Civil Defense office.

"There are sections of the Big Island that present a larger ground acceleration hazard than many areas of the Mainland. The southeast part of the Big Island is seismically the most active area in the state," he said.

While many Mainland fault zones involve vertical faults, the volcanic flank faults are more horizontal. That means the rifts may never reach the surface and are very difficult to measure.

"These tend to cause the largest earthquakes on the Big Island," Yanagi said.

The largest earthquake known to have occurred in Hawai'i gave a sample of possible devastation. The 1868 temblor destroyed 100 structures and killed 77.

The magnitude-7.9 quake, centered near the southern part of the Big Island, created fissures miles long. A tsunami destroyed coastal villages and the quake was felt as far away as Ni'ihau.

From 1973 to 1989, earthquakes ranging from 6.2 to 7.2 magnitude caused nearly $17 million damage on the Big Island.

Emergency officials are struggling to find ways to help protect Hawai'i from such catastrophic quakes.

The Klein group describes the possibility of earthquakes more serious than the 1868 quake, a scenario that frightens McCreery.

"We haven't had anything like that in the whole historical record. Huge events happen, but on a geologic time scale. We just don't know when they might happen, and there's actually the possibility that those events won't occur," at least in the lifetime of anyone today, he said.

Tsunami officials are working on ways to assess the hazard and techniques for warning people about the destructive waves that may be associated with them, he said.

Civil Defense authorities are hoping that the new hazard information can lead to improved building standards.

Yanagi said the seismic hazard ranking for the Big Island is Zone 4, the highest earthquake risk category. By contrast, the other islands have low risk rankings, with Maui County at 2B, O'ahu at 2A and Kaua'i at 1. Those rankings, built into building codes for the counties, are probably insufficient because they are so broad, he said.

State officials and seismologists are working to replace the Uniform Building Code, the U.S. building standard, with a new International Building Code that will attempt to refine actual risk areas.

Instead of broad islandwide zones, the new code will calculate rankings based on the amount of ground movement anticipated in the worst likely earthquake in specific areas, Yanagi said.

For some areas in the Islands with extremely high earthquake risks, it may mean higher construction costs to meet safety requirements.

Advertiser news services contributed to this report. Reach Jan TenBruggencate at (808) 245-3074 or jant@honoluluadvertiser.com.