Abstract
The Indo-Australian plate is undergoing distributed internal deformation caused by the lateral transition along its northern boundary—from an environment of continental collision to an island arc subduction zone1,2. On 11 April 2012, one of the largest strike-slip earthquakes ever recorded (seismic moment magnitude Mw 8.7) occurred about 100–200 kilometres southwest of the Sumatra subduction zone. Occurrence of great intraplate strike-slip faulting located seaward of a subduction zone is unusual. It results from northwest–southeast compression within the plate caused by the India–Eurasia continental collision to the northwest, together with northeast–southwest extension associated with slab pull stresses as the plate underthrusts Sumatra to the northeast. Here we use seismic wave analyses to reveal that the 11 April 2012 event had an extraordinarily complex four-fault rupture lasting about 160 seconds, and was followed approximately two hours later by a great (Mw 8.2) aftershock. The mainshock rupture initially expanded bilaterally with large slip (20–30 metres) on a right-lateral strike-slip fault trending west-northwest to east-southeast (WNW–ESE), and then bilateral rupture was triggered on an orthogonal left-lateral strike-slip fault trending north-northeast to south-southwest (NNE–SSW) that crosses the first fault. This was followed by westward rupture on a second WNW–ESE strike-slip fault offset about 150 kilometres towards the southwest from the first fault. Finally, rupture was triggered on another en échelon WNW–ESE fault about 330 kilometres west of the epicentre crossing the Ninetyeast ridge. The great aftershock, with an epicentre located 185 kilometres to the SSW of the mainshock epicentre, ruptured bilaterally on a NNE–SSW fault. The complex faulting limits our resolution of the slip distribution. These great ruptures on a lattice of strike-slip faults that extend through the crust and a further 30–40 kilometres into the upper mantle represent large lithospheric deformation that may eventually lead to a localized boundary between the Indian and Australian plates.
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Acknowledgements
We thank H. Kanamori, Z. Duputel and G. Hayes for discussions and exchanges of information about this event. A. Hutko provided early short-period back-projection results. We thank R. Abercrombie for comments on this paper. 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), the European ORFEUS Data Center and the NIED F-net Data Centre were used to access the data. This work was supported by NSF grant EAR0635570 (T.L.) and EAR0951558 (K.D.K.).
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H.Y. contributed to the surface wave back-projections and finite fault modelling; K.D.K. performed the short-period back-projections; and T.L. performed finite-fault inversions and guided the synthesis.
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Supplementary Information
This file contains Supplementary Figures 1-7 and full legends for Supplementary Movies 1 -2. (PDF 2244 kb)
Supplementary Movie 1
This movie shows animations of the short-period back-projections from European and Japanese (F-net) stations for the Mw 8.7 event - see Supplementary Information file for full legend. (MOV 4839 kb)
Supplementary Movie 2
This movie shows animations of the short-period back-projections from European and Japanese (F-net) stations for the Mw 8.2 event - see Supplementary Information file for full legend. (MOV 3512 kb)
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Yue, H., Lay, T. & Koper, K. En échelon and orthogonal fault ruptures of the 11 April 2012 great intraplate earthquakes. Nature 490, 245–249 (2012). https://doi.org/10.1038/nature11492
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DOI: https://doi.org/10.1038/nature11492
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