The 2009 Samoa–Tonga great earthquake triggered doublet

• Published in: Nature (London) Vol. 466; no. 7309; pp. 964 - 968
• Main Authors: Lay, Thorne, Ammon, Charles J., Kanamori, Hiroo, Rivera, Luis, Koper, Keith D., Hutko, Alexander R.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 19-08-2010; Nature Publishing Group
• Subjects: 704/2151/210; 704/2151/213; 704/2151/508; American Samoa; Boundaries; Earth Sciences; Earthquakes; Faults (Geology); Humanities and Social Sciences; letter; Lithosphere; multidisciplinary; Plate tectonics; Rock; Samoa (Nation); Science; Science (multidisciplinary); Sciences of the Universe; Seismic activity; Seismic engineering; Seismic hazard; Seismic phenomena; Seismic waves; Seismology; Tectonics; Thrust; Tonga; Tsunamis; Wave analysis; Weather hazards; Samoa (Nation); Tonga
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature09214

Double strike: two linked earthquakes caused 2009 South Pacific tsunami
The tsunami that struck the Samoan and northern Tongan islands in September 2009 was preceded by a magnitude-8 earthquake on the outer slope of the oceanic trench, where the Pacific plate bends as it enters the subduction zone. This was initially thought to be the sole source of the tsunami, but a more complex picture is emerging. Two groups report the occurrence of two earthquakes, a 'triggered doublet', at nearly the same time and place. What is not clear is which triggered the other. Beavan
et al
. use an analysis of Global Positioning System station displacements and tsunami models to show that the outer-rise earthquake was accompanied — possibly triggered by — a near-simultaneous mega-thrust earthquake in the adjacent Tonga subduction zone. Lay
et al
. analyse the available seismic data, and their model suggests that the outer-rise event triggered megathrust faulting. Either way, this dual strike suggests a mechanism for the occasional large tsunamis generated at the Tonga subduction zone.
On 29 September 2009, a tsunami devastated the Samoan and northern Tongan islands. Here, an unusual earthquake sequence that preceded this tsunami is analysed. A magnitude-8.1 intraplate faulting event in the outer trench-slope at the northern end of the Tongan subduction zone was followed by extensive interplate faulting, with total moment equivalent to that of a magnitude-8.0 earthquake. Overlap of the seismic signals had obscured the fact that distinct faults had ruptured with different geometries.
Great earthquakes (having seismic magnitudes of at least 8) usually involve abrupt sliding of rock masses at a boundary between tectonic plates. Such interplate ruptures produce dynamic and static stress changes that can activate nearby intraplate aftershocks, as is commonly observed in the trench-slope region seaward of a great subduction zone thrust event
1
,
2
,
3
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4
. The earthquake sequence addressed here involves a rare instance in which a great trench-slope intraplate earthquake triggered extensive interplate faulting, reversing the typical pattern and broadly expanding the seismic and tsunami hazard. On 29 September 2009, within two minutes of the initiation of a normal faulting event with moment magnitude 8.1 in the outer trench-slope at the northern end of the Tonga subduction zone, two major interplate underthrusting subevents (both with moment magnitude 7.8), with total moment equal to a second great earthquake of moment magnitude 8.0, ruptured the nearby subduction zone megathrust. The collective faulting produced tsunami waves with localized regions of about 12 metres run-up that claimed 192 lives in Samoa, American Samoa and Tonga. Overlap of the seismic signals obscured the fact that distinct faults separated by more than 50 km had ruptured with different geometries, with the triggered thrust faulting only being revealed by detailed seismic wave analyses. Extensive interplate and intraplate aftershock activity was activated over a large region of the northern Tonga subduction zone.