Samoa Tsunami caused by two quakes, the second an 8.0 quake East in the Tonga Trench
Seismologists missed thesecond earthquake last year after recording the 8.1 quake
â€œIt is sort of shocking,â€ said Thorne Lay, a professor of earth and planetary sciences at the University of California, Santa Cruz, who led a team that uncovered the seismic fingerprints months later. â€œIf we hadnâ€™t done the careful analysis, we would have missed it.â€
Magnitude-8 earthquakes are known as great earthquakes for their ferocious destructive power, and networks of seismometers can pinpoint much smaller earthquakes.
In this case, however, the great earthquake was hidden by a greater quake.
The greater earthquake, of magnitude 8.1, struck Sept. 29, 2009, off the Samoa Islands in the Pacific. That in itself was a surprise to seismologists, because there have not been any known magnitude-8 earthquakes around the Tonga trench, where the Pacific plate slips beneath the Australian plate, leading seismologists to believe that the fault there is well lubricated and does not build up enough stress to generate great earthquakes.
Still, the two tectonic plates are speeding toward each other there faster than anywhere else in the world, some 10 inches a year.
The resulting tsunami, which swept into Samoa, American Samoa and Tonga, killing more than 100 people, also puzzled scientists.
The magnitude-8.1 earthquake did not occur at the plate boundary, but to the east where a fault within the Pacific plate pulled apart.
That kind of earthquake pushes up the seafloor and could readily generate a tsunami, but the initial wave would have been a trough as the seafloor dropped downward. Instead, some deep-ocean tsunami sensors detected the initial wave as a crest.
Examining data from global positioning satellite markers, John Beavan, a geophysicist at GNS Science in New Zealand, a government-owned research institute, and his colleagues found that Niuatoputapu, a small Tonga island, had jumped about 16 inches to the east instead of the expected movement of less than an inch to the southwest.
The position of Niuatoputapu
â€” and the initial tsunami crest â€” could be explained if there was an almost simultaneous quake of magnitude 8.0 caused by an upward-thrusting fault in the Tonga trench.
Meanwhile, Dr. Lay was trying to figure out how to explain the seismic data. â€œYou had observations of shaking in different directions where there shouldnâ€™t have been anything,â€ he said.
Only when his team factored in a second earthquake did the story start to make sense. In addition, their analysis indicated that the second earthquake consisted of two subevents, each of magnitude 7.8.
â€œIt took us 4.5 months to figure it out,â€ Dr. Lay said. â€œIt was very difficult to tease apart the event.â€ By contrast, they were able to solve within a few hours the dynamics of the magnitude-8.8 quake off Chile in February.
Separate papers by Dr. Beavanâ€™s team and Dr. Layâ€™s team appear in the current issue of the journal Nature.
Dr. Lay said the seismic data indicates the larger earthquake occurred first, and the hidden earthquake followed less than 45 seconds later. He surmises that the shaking of the first quake loosened the second fault and set it off.
Noticing a Magnitude-8.0 Earthquake Months Later
By KENNETH CHANG
Published: August 23, 2010