21
August 2003
Deep earthquakes: Chain reaction
Shallow earthquakes — that is those that occur in the Earth's crust — are known to be capable of triggering subsequent earthquakes. Now the first evidence of a similar effect for deep earthquakes is reported. On 19 August last year, two large earthquakes occurred beneath the Pacific Ocean in the Tonga subduction zone. What was unusual was their near coincidence in both time and space at the extreme lower depth limit of global earthquakes. Both were big (magnitudes 7.6 and 7.7), close to each other (just 300 km apart), deep (at 578 km and 694 km, respectively, about as deep as earthquakes get), and the second occurred 7 minutes after the first. In between these two was a smaller magnitude 5.9 shock, 2 minutes after the first big earthquake. Tibi et al. have analysed the seismic data and find evidence linking the three events, suggesting that both static and dynamic triggering can occur in such regions. These observations also provide important clues about the mechanics of deep earthquakes which are still poorly understood.
Remote triggering of deep earthquakes in the
2002 Tonga sequences
RIGOBERT TIBI, DOUGLAS A. WIENS & HIROSHI INOUE
Nature 424, 921–925 (2003); doi:10.1038/nature01903
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Earth science: Tiny triggers deep down
HARRY W. GREEN II
The documentation and characterization of remotely triggered earthquakes
deep within the Earth is an achievement that provides insight into the
mechanisms that initiate such events.
Nature 424, 893–894 (2003); doi:10.1038/424893a
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