for National Geographic News
A deadly earthquake and tsunami that swept the Solomon Islands last year gained part of its power by breaking through what should have been a major geologic barrier, scientists say.
The April 1, 2007, quake struck in a section of the South Pacific where two tectonic plates, the Australian plate and the Woodlark plate, slide underneath a third, the Pacific plate.
The two plates that are sliding under, or subducting, are moving at slightly different rates and in slightly different directions from each other.
This means there's a fault between them that such a rupture would not ordinarily be expected to cross, said Fred Taylor, a geologist at the University of Texas at Austin and lead author of a new study of the earthquake.
Patterns in the 2007 event's aftershocks and the way the tsunami hit suggested that the rupture had indeed crossed the fault.
In a new study, Taylor's team used coral reefs to confirm the find, marking the first time that an earthquake has been directly observed to propagate across a plate boundary.
High and Dry
Vertical motions in earthquakes can raise coral reefs above sea level, where they dry out and die.
Taylor's team found such high-and-dry reefs on both sides of the Australia-Woodlark plate boundary, near the tsunami-hit islands of Simbo, Mono, and Ranongga (Ganongga). (See photos of the Solomons' reefs after the quake.)
This proved that both plates had been involved in the Solomons earthquake.
The power of an earthquake is related to the length of the rupture zone. The fault between the plates should have stopped the quake when it would have been smaller and presumably less deadly.
But crossing to the second plate allowed the quake to build up power and reach magnitude 8.1, creating a huge tsunami that killed at least 52 people.
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