Remote Triggering of Deep Earthquakes: Insight from the 2002 Tonga Sequences - Figure 1

Remote Triggering of Deep Earthquakes: Insight from the 2002 Tonga Sequences - Figure 1 Epicentral locations of the two 2002 Tonga deep earthquake sequences. Dots represent background seismicity. Lines are contours of deep seismicity (Gudmundsson and Sambridge, 1998), with the numbers indicating the depth in km to the seismogenic zone. A) Star indicates the epicenter (S1) of the initial earthquake (Mw 7.6, depth 598 km), triangles its second (S2) and third (S3) episodes of rupture. Cross represents a foreshock that occurred 8 days before the main event. Circles are aftershocks. B) Locations of the triggered earthquake (star) (Mw 7.7, depth 664 km), and its aftershocks (circles). Cross represents a foreshock that occurred about five minutes before the main event.
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It is well established that an earthquake in the Earth’s crust can trigger subsequent ruptures, 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 event, whereas dynamic (transient) stresses carried by seismic waves may trigger earthquakes both nearby and at remote distances. Detailed analysis of the August 19, 2002 Tonga deep earthquake sequences shows 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 event (664 km depth) occurred 300 km away, in a previously aseismic region. 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 the 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 incorporating nonlinear short-term delay mechanisms in regions where high stress may predominate, but where earthquakes have difficulty nucleating without external influences.
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Tibi, R., Wiens, D.A, and Inoue, H., Remote triggering of deep earthquakes in the 2002 Tonga sequences, Nature, 424, 921-925, 2003.

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