A giant earthquake that triggered a deadly southwest Pacific tsunami was actually two great temblors, finds a pair of new studies in the Aug. 19 Nature. These results uncover an unusual sequence of geological events that is the first of its kind to be observed by scientists, the study authors say.
The earthquakes, which likely struck within two minutes of each other on September 29, 2009, spawned a tsunami that killed nearly 200 people in Samoa, American Samoa and Tonga. Scientists assumed that a single quake under the ocean floor had caused the devastation, but the pattern of far-flung aftershocks, aberrant tsunami waves and the inexplicable movement of a Tongan island cast doubt on that simple explanation.
“We knew right off the bat that something was weird about this earthquake,” says geophysicist Eric Geist of the U.S. Geological Survey in Menlo Park, Calif. Geist wasn’t involved in the current studies but has puzzled over the anomalous signs produced by the quake. “This is a very complicated event, and these studies, for me, really helped explain a lot.”
The earthquake that everybody knew about was a whopper: a magnitude-8.1 quake in which the ground was pulled apart along a fault. The hidden quake was a different type. It happened about 70 kilometers from its predecessor on a thrust fault where the west-moving Pacific plate dives under the east-bound Tonga block of the Australia plate, an event called subduction. These two plates scrape past each other 24 centimeters each year, “the fastest plate tectonics on the planet,” says study coauthor John Beavan of GNS Science in Lower Hutt, New Zealand.
The two research teams separately uncovered the presence of a second earthquake using different data. Beavan and his colleagues found that tsunami gauges on the ocean floor measured a big positive pressure wave, a suspicious sign since a ground-extending earthquake would cause a drop in pressure.
Another big clue came from GPS stations on the northern Tongan island of Niuatoputapu, which has been steadily moving east. On its own, the first earthquake would have sent the island slightly back to the west. But instead, the island jumped about 40 centimeters to the east. “It was completely out of kilter,” Beavan says. The best explanation was that a second thrusting fault earthquake had caused the motion.
The other research group, led by seismologist Thorne Lay of the University of California, Santa Cruz, spotted abnormalities in seismic data that led them to dig deeper into the seismic records.
“We were able to pull together a self-consistent story of the triggered thrust earthquake, and clearly it was as big as the first event,” Lay says. “It was a magnitude-8 hidden earthquake. And you would think, ‘Well, aren’t seismologists a bunch of idiots. They can’t even find a magnitude-8 earthquake,’ but it was obscured by the strong shaking from the first one.”
Lay and his team built a model in which the normal fault earthquake happened first, triggering the hidden thrust-fault quake, Lay says. Beavan’s team’s study didn’t have the resolution to parse the timing of the earthquakes, but he says that he suspects Lay and his colleagues’ timeline is correct.
Both studies peg the second earthquake at a magnitude 8.0. Lay and his colleagues were able to distinguish two distinct but nearly simultaneous energy releases from the second earthquake, which they call subevents. Each of these subevents, they estimate, was magnitude 7.8, which combined to hit the 8.0 mark.
“We pretty well understand what’s going on with these earthquakes,” Beavan says. “The fact that these two studies, which use completely different techniques, both come up with the same answer is really nice.”
Understanding all of the forces involved may be useful in building better models of how earthquakes can trigger one another and tsunamis. This is the first example of a normal fault earthquake setting off a thrust-fault quake, Geist says, which “will set off a lot of interesting research on the mechanics of subduction zones and how they behave.”
