Tumbling rocks an unpredictable reality in Hawai'i

By Mike Gordon
Advertiser Staff Writer

The five-ton boulder that killed Dara Rei Onishi as she slept in her home early yesterday was caused by the same geologic forces that produced a deadly landslide at Sacred Falls in 1999, the Kamehameha Highway road closure at Waimea Bay in 2000 and a landslide in February that dumped tons of rocks onto the base of Manoa Falls.

Such incidents illustrate a simple fact: Falling rocks are part of the natural erosion of the Islands, and there is no way to predict when or where they will plummet.

"It's a common occurrence in areas where there aren't any people," said state geologist Glenn Bauer, of the Department of Land and Natural Resources. "It happens. The island is old."

But geologists also say boulders rarely plow through homes.

"Any individual house is probably not at any big risk," said professor Greg Moore of the University of Hawai'i's Department of Geology and Geophysics. "The chance of a boulder striking an individual house is pretty small."

No one is quite sure how many O'ahu homes could be considered too close to a valley wall. Moore estimated anywhere from 10,000 to 20,000 homes, but called those figures a guess.

Moore said some kind of debris hits or comes close to a home about once a year. But those odds aren't good enough for him.

"Being a geologist and knowing that this sort of things happens, I bought a house at the top of a hill and not at the bottom of a hill," he said.

The Islands, created by giant volcanoes, are made of basalt, which weathers and cracks over time.

"The reason we have the valleys is because of this whole process of erosion," Moore said. "The rocks have simply been falling off the sides of cliffs for a million years."

State geologist Bauer said tree roots often grow between the cracks, placing pressure on ancient lava flows.

Debris tumbles loose without warning; he's had rocks come whizzing by him as he has hiked O'ahu's trails.

"They can expand and contract depending on the conditions," Bauer said. "It was probably a combination of things that caused this rock to get dislodged."

But without seeing it, Bauer can't say for sure what caused this boulder to fall.

"It might have been hanging there for years," he said.

Because the boulder came from private property, the state would not be investigating what caused it to fall, Bauer said.

However, he thinks the landowner should hire an engineering geologist to do an assessment.

"Someone should hike up there and determine if there is anything else that would come down," Bauer said. "I don't know what was left behind. Sometimes you have a rock come off and there are other rocks left behind that could still come down."

Moore said it is difficult to stabilize crumbling cliffs and hillsides. The solutions are extreme: huge steel nets or, as in Japan, a coating of concrete.

"It looks pretty ugly, but it stabilizes the hillside for tens of years," Moore said.

Reach Mike Gordon at mgordon@honoluluadvertiser.com or 525-8012.