Three Tremors Caused the Samoa Tsunami

According to new seismologic data published in the latest issue of the top-rated scientific journal Nature, it would appear that the 2009 tsunami which struck Samoa and Tonga was produced by not one or two, but three consecutive earthquakes.

As we were also telling you yesterday, it was only recently that experts at the United States Geological Survey (USGS) found evidence of more than one tremor taking place on September 29, 2009.

The largest of the tremors was a 8.1 magnitude temblor, which carefully concealed shock waves produced by two other smaller tremors, each of magnitude 7.8.

The new data became obvious to researchers only after rigorous studies of the fault lines, the tsunami direction, speed and intensity, and the displacement of island around the subduction zone.

All of these factors disagreed with the hypothesis that the 2009 tsunami had been caused by a single large earthquake, expert reveal. Rather, it would appear that a triple-whammy was responsible.

“At first, we thought it was one earthquake. 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,” says Keith Koper.

“The two quakes that were hidden were responsible for some of the damage and tsunami waves,” adds the expert, who is the director of the University of Utah Seismograph Stations, and a coauthor of the new research paper.

The study was funded by the USGS and the US National Science Foundation (NSF). The work was led by University of California in Santa Cruz (UCSC) seismologist Thorne Lay.

“Detailed analyses of the seismic waves showed that this was an unusual set of events,” explains NSF Division of Earth Sciences (DES) program director Benjamin Phillips.

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

According to the latest data, it would appear that all three earthquakes originated at a depth of between 9 and 12 miles. “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,” the NSF reports.