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Remote triggering of deep earthquakes in the 2002 Tonga sequences


It is well established that an earthquake in the Earth's crust can trigger subsequent earthquakes, but such triggering has not been documented for deeper earthquakes. Models for shallow fault interactions suggest that static (permanent) stress changes can trigger nearby earthquakes, within a few fault lengths from the causative earthquake1,2,3, whereas dynamic (transient) stresses carried by seismic waves may trigger earthquakes both nearby and at remote distances4,5,6,7,8. Here we present a detailed analysis of the 19 August 2002 Tonga deep earthquake sequences and show evidence for both static and dynamic triggering. Seven minutes after a magnitude 7.6 earthquake occurred at a depth of 598 km, a magnitude 7.7 earthquake (664 km depth) occurred 300 km away, in a previously aseismic region. We found that nearby aftershocks of the first mainshock are preferentially located in regions where static stresses are predicted to have been enhanced by the mainshock. But the second mainshock and other triggered events are located at larger distances where static stress increases should be negligible, thus suggesting dynamic triggering. The origin times of the triggered events do not correspond to arrival times of the main seismic waves from the mainshocks and the dynamically triggered earthquakes frequently occur in aseismic regions below or adjacent to the seismic zone. We propose that these events are triggered by transient effects in regions near criticality, but where earthquakes have difficulty nucleating without external influences.

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Figure 1: Epicentral locations of the two 2002 Tonga deep earthquake sequences.
Figure 2: Vertical cross-section perpendicular to the deep Tonga slab showing the 95% confidence ellipsoid for the location of the triggered 2002 Tonga sequence.
Figure 3: Map showing the location of the first sequence in the 2002 Tonga deep earthquakes, and the calculated static Coulomb stress changes at 585 km (a) and 610 km depth (b) associated with the mainshock (see Methods).
Figure 4: The 1986 Tonga sequence.


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We thank D. Suetsugu and the operators of the SPANET network for their efforts, R. Stein for making his code available, and J. Conder and N. Kagotho for reading the manuscript. Waveforms data were supplied by the Incorporated Research Institutions for Seismology (IRIS), Geoscope, and GeoForschungsNetz (GEOFON). Arrival time data were obtained from the ISC and PDE (US Geological Survey). This research was supported by the National Science Foundation.

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Tibi, R., Wiens, D. & Inoue, H. Remote triggering of deep earthquakes in the 2002 Tonga sequences. Nature 424, 921–925 (2003).

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