Scientists have identified three new "slow" earthquake events on Kilauea's southeast flank that could further understanding of how catastrophic landslides occur.

Each of the earthquakes, which occurred periodically every 25 1/2 months since 1998, took place over several hours, and were too slow to generate much shaking or to be felt by Big Island residents, according to researcher Ben Brooks of the University of Hawai'i's School of Ocean and Earth Science and Technology. But the amount of permanent ground displacement caused by the three events and a fourth, previously reported "slow" earthquake was equivalent to a sudden jolt of 5.5 to 5.8 magnitude, he said.

"It's an indication to us of the way the volcano is sliding or failing gravitationally under its own weight," he said. "The volcano's flanks are sliding down into the ocean, and it can happen in a gradual way or in a sudden way."

Brooks' research team used surface motion data from the global positioning system network to identify the earthquakes at an area known as the Hilina Slump, a massive submarine landslide that creeps outward at an average rate of 2 1/2 to 4 inches per year. It's not clear yet whether the gradual slipping makes a massive landslide more or less likely, he said.

The new observations also could help scientists determine tsunami hazards in Hawai'i and across the Pacific, since landslides in which steep volcano flanks drop into the ocean are a potential source of tsunami.

To answer additional questions about the depth at which "slow" earthquakes occur and how onshore deformations translate to the offshore portion of the slump, Brooks and colleagues from the Woods Hole Oceanographic Institute will attempt to measure surface motions of the sea floor, a much more technologically challenging task, he said.

A report on the research appears in the June 30 issue of Earth and Planetary Science Letters. Other researchers involved in the study are James Foster, Neil Frazer and Cecily Wolfe of UH, Michael Bevis of Ohio State University, and Mark Behn of the Woods Hole institute.

Reach Christie Wilson at cwilson@honoluluadvertiser.com.