South Pacific

The tsunami that hit the Tonga islands of Samoa and the North in September 2009 was preceded by an earthquake of magnitude 8 on the external side of the trench where the Pacific plate is bent as it enters the subduction zone. This was initially thought that the only source of the tsunami, but a more complex picture is emerging. Two groups report the occurrence of two earthquakes, a "double shot, almost at the same time and place.
What is clear is that caused the other. Beavan et al. analysis using a Global Positioning System station displacements and tsunami models to demonstrate that the outer-rise earthquake was accompanied - possibly triggered by one - mega thrust earthquake in nearly simultaneous adjacent Tonga subduction zone. Place et al. analysis of available seismic data and model suggests that the high external event triggered megathrust faults. Either way, this double strike suggests a mechanism for occasional large tsunamis generated in the Tonga subduction zone.
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