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The 2009 Samoa–Tonga great earthquake triggered doublet


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 event1,2,3,4. 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.

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Figure 1: The Samoa–Tonga great earthquake doublet region.
Figure 2: Observed and modelled R1 STFs.
Figure 3: P-wave back-projection using the Japanese F-Net stations.
Figure 4: Surface wave modelling for simple and composite models.


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This work made use of GMT and SAC software and Federation of Digital Seismic Networks (FDSN) seismic data. The Incorporated Research Institutions for Seismology (IRIS) Data Management System (DMS) and the F-Net and Hi-Net data centres were used to access the data. C. Ji kindly shared details of his model. Z. Duputel wrote the W-Phase software version used in this study. We thank H. Savage and E. Brodsky for discussions of frictional conditional stability. This work was supported by NSF grant EAR0635570 and USGS Award Number 05HQGR0174.

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All authors contributed equally to the analysis and preparation of this paper.

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Correspondence to Thorne Lay.

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Supplementary information

Supplementary Information

This file contains Supplementary Methods Applications, Supplementary Figures S1-16 and legends for Supplementary Movies 1 and 2. (PDF 3126 kb)

Supplementary Movie 1

Animation of P wave back-projections from 6 regional networks. (MOV 1735 kb)

Supplementary Movie 2

Animation of the USGS located aftershock sequence. (MOV 4085 kb)

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Lay, T., Ammon, C., Kanamori, H. et al. The 2009 Samoa–Tonga great earthquake triggered doublet. Nature 466, 964–968 (2010).

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