Scientists studying the massive earthquake that struck the South Pacific on September 29, 2009, have found that it actually involved two great earthquakes: an initial one with magnitude 8.1, which then triggered another magnitude 8 earthquake seconds later on a different fault. The details of this rare event, called a “triggered doublet,” are unlike anything seismologists have seen before.

“We know of no precedent for the Samoa triggered doublet,” said Thorne Lay, professor of Earth and planetary sciences at the University of California, Santa Cruz, who led a seismological analysis of the event published in the August 19 issue of Nature.

The earthquakes unleashed devastating tsunami waves that swept onto the islands of Samoa, American Samoa, and Tonga, killing 192 people. It took months, however, for seismologists to make sense of the confusing seismic data and figure out exactly what happened in the Earth’s crust to cause this disaster.

Most great earthquakes (earthquakes of magnitude 8 or greater) occur in subduction zones, where one plate of the Earth’s crust dives beneath another plate. The Tonga subduction zone in the South Pacific marks the boundary where the Pacific plate is sinking under the Australian plate.

In the sequence of events on September 29, the first earthquake actually occurred not at the subduction zone, but within the Pacific plate at a site 50 to 100 kilometers (30 to 60 miles) east of the plate boundary. The rupture occurred along an extensional or “pull-apart” fault in the middle of the plate. Such large extensional faulting near a subduction zone is rare, and this is the third largest such event recorded in the 110-year history of seismological monitoring.