Time-varying interseismic strain rates and similar seismic ruptures on the Nias–Simeulue patch of the Sunda megathrust
Fossil coral microatolls from fringing reefs above the great (M_W 8.6) megathrust rupture of 2005 record uplift during the historically reported great earthquake of 1861. Such evidence spans nearly the entire 400-km strike length of the 2005 rupture, which was previously shown to be bounded by two persistent barriers to seismic rupture. Moreover, at sites where we have constrained the 1861 uplift amplitude, it is comparable to uplift in 2005. Thus the 1861 and 2005 ruptures appear to be similar in both extent and magnitude. At one site an uplift around AD 1422 also appears to mimic the amount of uplift in 2005. The high degree of similarity among certain ruptures of this Nias–Simeulue section of the Sunda megathrust contrasts with the substantial disparities amongst ruptures along other sections of the Sumatran portion of the Sunda megathrust. At a site on the northwestern tip of Nias, reefs also rose during an earthquake in AD 1843, known historically for its damaging tsunami along the eastern coast of the island. The coral microatolls also record interseismic vertical deformation, at annual to decadal resolution, spanning decades to more than a century before each earthquake. The corals demonstrate significant changes over time in the rates of interseismic deformation. On southern Simeulue, interseismic subsidence rates were low between 1740 and 1820 but abruptly increased by a factor of 4–10, two to four decades before the 1861 rupture. This may indicate that full coupling or deep locking of the megathrust began only a few decades before the great earthquake. In the Banyak Islands, near the pivot line separating coseismic uplift from subsidence in 2005, ongoing interseismic subsidence switched to steady uplift from 1966 until 1981, suggesting a 15-year-long slow slip event, with slip velocities at more than 120% of the plate convergence rate.
Additional Information© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license. Received 11 March 2015; Received in revised form 22 May 2015; Accepted 4 June 2015; Available online 17 June 2015. We thank D. Prayudi, I. Suprihanto, and J. Galetzka for field support; S. Martin and C. Vita-Finzi for help locating historical documents; and J. Freymueller, F. Taylor, R. Witter, and an anonymous reviewer for careful reviews and thoughtful suggestions that substantially improved this manuscript. This work has been supported by NSF grant EAR-0538333 (to K.S.); by MOST grants 102-2116-M-002-016 and 103-2119-M-002-022 and NTU grant 101R7625 (to C.C.S.); by NRF Fellowship NRF-NRFF2010-064 (to E.M.H.); by LIPI (Indonesian Institute of Science) and RUTI (International Joint Research Program of the Indonesian Ministry of Research and Technology); by the Gordon and Betty Moore Foundation; and by the Earth Observatory of Singapore and the National Research Foundation (NRF) Singapore and the Singapore Ministry of Education (MOE) under the Research Centres of Excellence initiative. This is Earth Observatory of Singapore contribution 66. Full-resolution coral slab x-ray mosaics for all of the slabs in this paper are available from the corresponding author. We dedicate this paper to the memory of Adi Rahman Putra, who gave his life in the pursuit of knowledge that might someday save the lives of others.
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