The Tonga subduction zone is approximately beneath the Fiji Islands in the Pacific Ocean. The two earthquakes were 300 km apart from each other and the difference in their depth was 65 km. Their magnitudes were 7.6 and 7.7.
In the commentary, Green sheds light on a paper by Tibi et al. also appearing in the Aug. 21 issue of Nature. Tibi et al. argue that the second large earthquake was triggered by the passage of seismic waves generated by the first earthquake.
"Tibi and colleagues' observations are a major advance in understanding deep earthquakes," Green said. "They might provide a new constraint on the mechanism by which these earthquakes begin. Their work provides a major piece of information as to how earthquakes get started, which may in the long run contribute to the prediction of damaging earthquakes that threaten people in California and elsewhere."
The authors demonstrated for the first time remote triggering of one deep earthquake by another. "The Tonga earthquakes occurred only 7 minutes apart," said Green. "Equally interesting, the triggered earthquake occurred in a place where no earthquakes have ever been recorded before."
Green explained that the observations by Tibi et al. have significant implications for the physical process that initiates deep earthquakes. In the commentary, he outlines the three principal physical mechanisms that have been proposed by geophysicists to explain the initiation of deep earthquakes. "One of these observations can be ruled out by Tibi et al.'s observations," said Green. "Moreover, we can use the recent seismological work of other researchers to show that one of the remaining two mechanisms is the most probable cause." That mechanism is 'phase-transformation-induced faulting,' which Green and a graduate student discovered in 1989.
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Journal
Nature