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A 15 Year Slow-Slip Event on the Sunda Megathrust Offshore Sumatra

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A 15 Year Slow-Slip Event on the Sunda Megathrust Offshore Sumatra Geophysical Research Letters RESEARCH LETTER A 15 year slow-slip event on the Sunda 10.1002/2015GL064928 megathrust offshore Sumatra Key Points: 1 1 2, 3 1 4 • Corals offshore Sumatra reveal Louisa L. H. Tsang , Aron J. Meltzner , Belle Philibosian , Emma M. Hill , Jeffrey T. Freymueller , a 15 year-long interseismic and Kerry Sieh1 displacement reversal from 1966 to 1981 1Earth Observatory of Singapore, Nanyang Technological University, Singapore, 2Tectonics Observatory, Division of • A 15 year-long SSE is the most likely 3 explanation for the coral observations Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA, Columbia University, 4 • Our results suggest that the SSE New York, New York, USA, Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Alaska, USA occurred under the Banyak Islands Abstract In the Banyak Islands of Sumatra, coral microatoll records reveal a 15 year-long reversal of Supporting Information: interseismic vertical displacement from subsidence to uplift between 1966 and 1981. To explain these • Texts S1–S5 • Figures S1–S14 coral observations, we test four hypotheses, including regional sea level changes and various tectonic • Tables S1–S6 mechanisms. Our results show that the coral observations likely reflect a 15 year-long slow-slip event (SSE) on the Sunda megathrust. This long-duration SSE exceeds the duration of previously reported SSEs Correspondence to: and demonstrates the importance of multidecade geodetic records in illuminating the full spectrum of E. M. Hill, megathrust slip behavior at subduction zones. [email protected] Citation: Tsang, L. L. H., A. J. Meltzner, B. Philibosian, E. M. Hill, 1. Introduction J. T. Freymueller, and K. Sieh (2015), A 15 year slow-slip “Silent earthquakes,” or slow-slip events (SSEs), release strain on tectonic faults at slip rates faster than plate event on the Sunda megathrust rates but slowly enough that seismic shaking is not generated. They are manifested along subduction zone offshore Sumatra, Geophys. megathrusts either as preseismic slip prior to large earthquakes, afterslip following large earthquakes, or slip Res. Lett., 42, 6630–6638, doi:10.1002/2015GL064928. during the interseismic period of the earthquake cycle [Schwartz and Rokosky, 2007]. In this study, we use the term SSE to refer only to the last of these three manifestations. Received 19 JUN 2015 SSEs are visible in geodetic time series as reversals of displacements associated with a locked fault inter- Accepted 24 JUL 2015 Accepted article online 31 JUL 2015 face. SSEs have been observed to typically last from a few days [e.g., Douglas et al., 2005] to a few months Published online 22 AUG 2015 [e.g., Hirose et al., 1999], but longer-duration SSEs lasting several years have also been detected: ∼2–3 years Corrected 5 OCT 2015 [Ohta et al., 2006], ∼4 years [Fu and Freymueller, 2013], ∼5 years [Miyazaki et al., 2006; Ochi and Kato, 2013; Ohta et al., 2004; Ozawa et al., 2002], and ∼10 years [Li et al., 2014]. In order to detect SSEs of longer dura- This article was corrected on 5 OCT tion and thus illuminate the full temporal spectrum of megathrust slip behavior, multidecade geodetic 2015. See the end of the full text for details. records are required. However, existing geodetic networks have not been operating long enough to detect such events. One way to extend our geodetic observations is to take advantage of paleogeodetic data preserved in the geological record. For example, coral microatolls along the Sumatran subduction zone provide high-resolution records of vertical deformation over the past several centuries [e.g., Zachariasen et al., 2000; Meltzner et al., 2010; Philibosian et al., 2014]. Using these data, Meltzner et al. [2015] reconstructed interseismic uplift and subsidence at multiple sites over the past three centuries. At one site on Bangkaru Island, in the Banyak Islands (site PBK, Figure 1), coral records show an abruptly reversed displacement trend from inter- seismic subsidence to interseismic uplift between 1966 and 1981 (Figures 1b–1d); an SSE on the underlying megathrust is a potential explanation. ©2015. The Authors. This is an open access article under the In this paper we extend the work of Meltzner et al. [2015] to examine the potential physical processes that can terms of the Creative Commons explain the coral observations. We show that indeed, they likely reflect a very long duration SSE on the Sunda Attribution-NonCommercial-NoDerivs License, which permits use and megathrust, where, to date, no SSEs other than afterslip following major earthquakes have been recorded. We distribution in any medium, provided include previously unpublished coral observations for the Simeulue-Nias section of the megathrust, including the original work is properly cited, the use is non-commercial and no additional sites in the Banyak Islands and neighboring islands of Simeulue and Nias. These added observations modifications or adaptations are made. allow us to model the possible locations and sizes of this SSE, both with improved constraints. TSANG ET AL. A 15 YEAR SSE ON THE SUNDA MEGATHRUST 6630 Geophysical Research Letters 10.1002/2015GL064928 Figure 1. (a) The Banyak Islands section was estimated to have experienced less coseismic slip than elsewhere along strike during the 2005 Mw 8.6 Nias rupture [Konca et al., 2007]. Part of this section ruptured during the 2010 Mw 7.8 Banyak Islands earthquake (source model from the U.S. Geological Survey: http://earthquake.usgs.gov/earthquakes/ eventpage/usp000hat0#scientific_finitefault). Mw 6+ earthquakes from the National Earthquake Information Center catalog are plotted. (b–d) Spatiotemporal distribution of vertical displacement rates recorded by coral microatolls in the Simeulue-Nias section. From 1966 to 1981, coral microatolls on the Banyak Islands reversed their sense of displacement from interseismic subsidence to interseismic uplift. Black lines: slab contours at 20 km depth intervals from Slab 1.0 [Hayes et al., 2012]. 2. Displacement Rate Changes Recorded by Corals We reconstructed interseismic uplift histories from coral microatolls at six sites along the Simeulue-Nias section of the Sunda megathrust (Figure 1). Microatolls are coral colonies that grow upward to a limit near mean low water springs, and their upper surfaces can track changes in relative sea level (RSL) over time [Scoffin et al., 1978; Taylor et al., 1987; Zachariasen et al., 2000; Meltzner et al., 2010]. Microatoll morphologies form because prolonged subaerial exposure at times of extreme low water limits the highest level to which the coral colonies can grow [Briggs et al., 2006; Meltzner et al., 2010; Meltzner and Woodroffe, 2015]. Flat-topped microatolls record RSL stability; colonies that rise radially outward toward their perimeter reflect rising RSL during their decades of growth, and colonies with progressively lower diedowns as they grow outward reflect gradually falling RSL. Changes in RSL are themselves the combined effect of changes in land level and changes in local absolute sea level. As islands and their fringing reefs subside or rise in response to tectonic elastic strain accumulation and release, microatolls record changes in RSL. Because these corals’ skeletons have annual growth bands, we can precisely calculate rates of change in RSL, when those changes are gradual. The coral microatolls were slabbed, X-rayed, and analyzed following the methods described by Meltzner et al., [2010, 2012] and Meltzner and Woodroffe [2015]. The interpreted coral cross sections and corresponding time series for the relevant corals are presented in Figures S1–S6 and Table S1 in the supporting information. The displacement rates at each site (and their 2 level of uncertainties) over various time periods are shown in Figures 1b–1d. Three sites (PBK, PLM, and PUB) define a trench-normal transect through the Banyak Islands, and three additional sites are located near the southeastern tip of Simeulue (LBJ) and the northern tip of Nias (MZL and SBA). TSANG ET AL. A 15 YEAR SSE ON THE SUNDA MEGATHRUST 6631 Geophysical Research Letters 10.1002/2015GL064928 The RSL history of site PBK is particularly well resolved as a result of various idiosyncrasies (as discussed by Meltzner et al. [2015]). Modern microatoll PBK-4 (Figure S1) started growing in the 1950s and first recorded a diedown in 1956. The first few diedowns were successively higher on the coral, but around 1966 there was an abrupt change, and for ∼15 years successive diedowns were lower and lower. Around 1981, the trend reversed again, and successive diedowns were higher. After correcting for an assumed eustatic sea level rise of 2 mm/yr, Meltzner et al. [2015] estimated tectonic deformation rates at PBK-4 of −2.7±3.4 mm/yr (subsidence) from the beginning of the record in 1956 until 1966, +7.8±2.1 mm/yr (uplift) from 1966 to 1981, and −2.4±0.8 mm/yr (subsidence) from 1981 until coseismic uplift in 2005 (Table S1). Although rates of sea level change are not well known in the eastern Indian Ocean in the decades prior to 1992, there is no compelling evidence that they differed markedly from the global average of +2 ± 1 mm/yr. In this paper, we adopt the rates (and correction) of Meltzner et al. [2015], but we explore alternatives in the supporting information (Text S2). The abruptness of each rate change at site PBK is well constrained. Meltzner et al. [2015] estimate that they can resolve the timing of rate changes at PBK-4 to within ±2 years of 1966 and 1981, respectively. Hence, these changes must have been rapid; each transition from the initial rate to the subsequent rate occurred over no more than 2 to 4 years, but it may have occurred over a period of a few months or even less. Because of a combination of greater erosion and less frequent diedowns on all the other corals, the rates at the other sites are not as well constrained.
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