Deadly Tonga earthquake revealed as 3 big quakes

August 19, 2010

A magnitude-8.1 earthquake and tsunami that killed 192 people last year in Samoa, American Samoa and Tonga was in fact a triple-whammy.

The 8.1 "great earthquake" concealed and triggered two major quakes of magnitude 7.8, seismologists report in a paper in this week's issue of the journal Nature.

The National Science Foundation (NSF) and the U.S. Geological Survey funded the research, which was led by seismologist Thorne Lay of the University of California at Santa Cruz.

"At first, we thought it was one earthquake," says paper co-author Keith Koper, director of the University of Utah Seismograph Stations.

"When we looked at the data, it turned out it wasn't just one great earthquake, but three large earthquakes that happened within two minutes of one another. The two quakes that were hidden were responsible for some of the damage and tsunami waves."

The two magnitude-7.8 quakes combined "represent the energy release of another magnitude-8 quake," says Koper.

The quake on September 29, 2009, generated tsunami waves that varied in height depending upon where they struck. In some places the water reached more than 49 feet above sea level.

The disaster killed at least 149 people in Samoa, 34 people in American Samoa and nine on Niuatoputapu, an island in the northern part of Tonga.

It is the first known case of a large "normal" fault earthquake (the 8.1 quake) occurring on a sea-floor tectonic plate, then triggering major "thrust" quakes (the 7.8 quakes) in the "subduction zone" where the oceanic plate is diving or "subducting" beneath a continental plate of Earth's crust.

"Detailed analyses of the seismic waves showed that this was an unusual set of events," says Benjamin Phillips, program director in NSF's Division of Earth Sciences, which funded the research.

Usually, big 'megathrust' quakes that happen on the subduction zone boundary between two plates trigger other quakes on the oceanic plate that is diving or 'subducting' under the continental plate.

Thrust quakes are those in which ground is pushed together along a fault, forcing the ground on one side of the fault either under or over ground on the other side.

In the southwest Pacific Ocean, the Pacific Plate is moving westward and is being thrust under the Tonga block, a "microplate" on the northeast edge of the Australian plate.

During normal quakes, ground is pulled apart along a fault. The magnitude-8.1 quake occurred when the Pacific plate broke apart at the "outer rise" where it begins to dive westward beneath the Tonga block.

"This is the first time a large normal-faulting quake has been shown to trigger large thrust-faulting earthquakes," says Koper.

"This study will affect the way earthquake and tsunami hazards are calculated, not just in this region but potentially in other places around the world."

All three quakes began at depths from nine to 12 miles beneath the surface.

The magnitude-8.1 quake lasted 60 seconds. The first magnitude-7.8 quake started sometime between 49 and 89 seconds after the 8.1 quake. The second 7.8 began 90 to 130 seconds after the first quake started.

Scientists know of only three previous cases of great earthquakes--those measuring magnitude 8 or more--that happened due to pull-apart or normal faulting within a diving seafloor plate.

They were the 1933 Sanriku, Japan, quake (about magnitude 8.4), which killed more than 3,000 people; the 1977 Sumba, Indonesia quake (8.3), which claimed 189 lives; and the 2007 Kuril Islands, Russia quake (magnitude 8.1).

"We've had seismometers only 100 years and good observations only the last 50 years, so not enough earthquake cycles have been observed to see this before."
In addition to Utah's Koper, other co-authors of the Nature paper are seismologists Charles Ammon of Pennsylvania State University, Hiroo Kanamori of the California Institute of Technology, Luis Rivera of the University of Strasbourg in France and Alexander Hutko of the Incorporated Research Institutions for Seismology's Seattle data center.

National Science Foundation

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