June 28, 2006
By Rod Thompson
HILO Researchers at the University of Hawaii have discovered that huge slumping of the sides of Kilauea volcano, covering as much as 100 square miles, has taken place regularly without being noticed by Big Island residents or Kilauea geologists.
Photo: Kilauea volcano (file photo)
Previously called slumps, these events are now called "slow earthquakes" by geologists at the university's School of Ocean and Earth Science and Technology.
Scientists don't know if quiet slumping prevents damaging earthquakes or sets up conditions for major quakes to occur, said research team leader Ben Brooks.
"We're not saying anything about hazards," he said.
But he's excited. "This is a big thing at SOEST," he said.
To understand the picture, an observer has to go back to Nov. 8, 2000, when an array of global positioning system instruments revealed that a piece of Kilauea 3 miles wide, 9 miles long and 2.5 miles deep moved seaward about 1 inch on the surface and more than 3 inches underground.
If it had happened in five seconds, it would have produced a 5.7 magnitude earthquake. But it happened over 36 hours and no one felt anything.
Now Brooks' team, analyzing GPS data that was collected but not studied, has learned that the 2000 event was one of four since 1998.
The events were on Sept. 20, 1998; Nov. 8-9, 2000; Dec. 16, 2002; and Jan. 26, 2005. The events are remarkably evenly spaced, taking place exactly 25.5 months apart, give or take seven days.
In each case, the movement at any given spot is small. The whole area, known as the Hilina Slump, is moving seaward at 2.4 to 4 inches per year.
But since the movement is over such a wide area, if the events took place as fast earthquakes, they would have ranged from 5.5 to 5.8 in magnitude.
While Brooks can't assess hazards to human, it's a fact that the Hilina Slump is the site of the 7.2 magnitude Kalapana earthquake of 1975 that killed two people and caused $1 million in damage.
It's possible that the slump and the 1975 quake have the same origin, a table-like fault zone 4 to 7 miles deep, the team's research paper in the June 30 edition of Earth and Planetary Science Letters says.
Since the late 1980s, geologists have known of truly enormous slumps and earthquakes around Hawaii, such as the quake over a million years ago that broke Oahu in half and created the Nuuanu Pali.
In the 1980s, the slumps seemed different from the quakes. Now Brooks isn't sure.