The magnitude 8.1 earthquake and tsunami that killed 192 people last year in Samoa, American Samoa and Tonga triggered another two earthquakes.
The massive 8.1 earthquake that hit the South Pacific region on September 29, 2009 triggered another two major quakes of magnitudes 7.8 which, when combined, “represent the energy release of another magnitude-8 quake,” says Keith Koper, director of the University of Utah Seismograph Stations and co-author of a study to be published in the journal Nature.
“At first, we thought it was one earthquake,” Koper said. “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 by the first quake ended up being responsible for some of the damage and tsunami waves.”
The University of Utah writes;
The most important scientific aspect of the quakes was their unprecedented pattern, Koper says. In technical terms, it is the first known case of a large “normal” fault earthquake (the 8.1) occurring on a plate of Earth’s crust beneath the ocean, and then triggering major “thrust” quakes (the 7.8s) in the “subduction zone,” where the oceanic plate is diving or “subducting” beneath a continental plate of Earth’s crust.
Usually the opposite occurs: big “megathrust” quakes on the subduction zone boundary between two plates trigger other quakes on the oceanic plate that is diving or “subducting” under the continental plate.
More can be read at the University of Utah website, which follows on explaining what actually happened that day, something that will no doubt help those hoping to predict earthquakes and their destructive force in the future.
Image Credit: Keith Koper, University of Utah Seismograph Stations