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Long-lived shallow slow-slip events on the Sunda megathrust

Abstract

During most of the time between large earthquakes at tectonic plate boundaries, surface displacement time series are generally observed to be linear. This linear trend is interpreted as a result of steady stress accumulation at frictionally locked asperities on the fault interface. However, due to the short geodetic record, it is still unknown whether all interseismic periods show similar rates, and whether frictionally locked asperities remain stationary. Here we show that two consecutive interseismic periods at Simeulue Island, Indonesia experienced significantly different displacement rates, which cannot be explained by a sudden reorganization of locked and unlocked regions. Rather, these observations necessitate the occurrence of a 32-year slow-slip event on a shallow, frictionally stable area of the megathrust. We develop a self-consistent numerical model of such events driven by pore-fluid migration during the earthquake cycle. The resulting slow-slip events appear as abrupt velocity changes in geodetic time series. Due to their long-lived nature, we may be missing or mis-modelling these transient phenomena in a number of settings globally; we highlight one such ongoing example at Enggano Island, Indonesia. We provide a method for detecting these slow-slip events that will enable a substantial revision to the earthquake and tsunami hazard and risk for populations living close to these faults.

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Fig. 1: Tectonic setting of the study (inset map).
Fig. 2: Observations and modelling results for the eighteenth- and nineteenth-century coral record at Simeulue projected along the cross-section shown in Fig. 1a.
Fig. 3: Numerical model of an SSE on a velocity-strengthening fault.
Fig. 4: Illustration of the pore-fluid-driven SSE model presented in this paper.
Fig. 5: A possible ongoing long-lived SSE near Enggano Island.

Data availability

The coral data used in this paper are from ref. 20 (https://doi.org/10.1016/j.quascirev.2015.06.003), also available at https://doi.org/10.21979/N9/5QCLZX. The daily RINEX files for the GNSS station MLKN are available for public download at ftp://ftp.earthobservatory.sg/SugarData. The processed time series is provided at https://doi.org/10.21979/N9/LMK36Z. Topography and bathymetry plotted in Figs. 1 and 5 are from the ETOPO1 dataset available at https://doi.org/10.7289/V5C8276M. The figures in this paper were made using MATLAB and Generic Mapping Tools70.

Code availability

All computations in this study were carried out using MATLAB; code is available at https://researchdata.ntu.edu.sg/dataverse/longlivedsse/.

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Acknowledgements

This research was supported by the National Research Foundation Singapore under a Singapore NRF Investigatorship to E.M.H. (proposal ID NRF-NRFI05–2019–0009), Singapore NRF Fellowship to A.J.M. (NRF-NRFF11–2019–0008), the Earth Observatory of Singapore (EOS), the National Research Foundation of Singapore, and the Singapore Ministry of Education under the Research Centers of Excellence initiative. Data for the Enggano GPS station MLKN were taken from the SuGAr network maintained by EOS and the Indonesian Institute of Sciences (LIPI). This is EOS contribution number 335.

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R.M., A.J.M., L.L.H.T. and E.M.H. designed the study. R.M. and E.O.L. developed the inverse method. R.M. conducted the data analysis and developed the numerical models for the study. L.F. processed the GPS data and provided the time series for MLKN. All authors jointly wrote the paper.

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Correspondence to Rishav Mallick.

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The authors declare no competing interests.

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Peer review information Nature Geoscience thanks Daniel Melnick and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: Stefan Lachowycz.

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Extended data

Extended Data Fig. 1 Testing multiple hypotheses to explain the trends in the coral time series.

Testing multiple hypotheses to explain the trends in the coral time series. a, Assuming a piecewise linear fit for the time series with a common timing for the velocity change, Tchange, we evaluate the misfit (with Gaussian error statistics - thick blue line). b, We evaluate the reduced χ2 and the significance of error reduction using the F-test for the following hypotheses: piecewise linear fits with an abrupt velocity change at Tchange (blue line - this is the preferred model), continuous acceleration (red line) and piecewise linear fits with an abrupt velocity change whose timing varies for each station (red diamonds).

Extended Data Fig. 2 PDFs for spatial extent of locked and creeping regions on the megathrust.

PDFs for spatial extent of locked and creeping regions on the megathrust. a, Geometric setup and terminology used to describe the 4 segments of the megathrust: shallow creeping, frictionally locked, unlocked/transition zone and freely sliding plate boundary. b, PDFs of the spatial extent of ζup for two time periods (1738–1829 and 1829–1861) assuming steady frictional locking and creep governs the evolution of slip on the megathrust. Maximum aposteriori probability (MAP) of ζup for 1738–1829 is shown as green thick line (95 % confidence interval - thin green lines); MAP of ζup for 1829–1861 is shown as red thick line (95 % confidence interval - thin red lines). To explain the 1829–1861 coral observations with only steady interseismic processes, the upper limit of the locked zone ζup would have to migrate down-dip by 50–100 km while the (c) deeper transition zone ζdown would have to migrate to a depth of 50–60 km. d, The transition from ζdown to ζfree (W) would have to narrow to infinitesimal widths (pdf is maximum at W=0) making this model unphysical.

Extended Data Fig. 3 Observations and modelling results for the 18th-19th century coral record in Simeulue.

Observations and modelling results for the 18th-19th century coral record in Simeulue. a, Subsidence rates for the two time periods, 1738–1829 (grey) and 1829-1861 (red). We assume the velocities from each epoch collectively show the average response of southern Simeulue Island to tectonic changes (filled error bar). The individual site vertical velocities are plotted with error bars, while the model predictions are shown as polygons (67% confidence level, with darker colours showing regions closer to the median). b, Estimated slip rate for 1738-1829 is shown as a grey polygon (67% confidence level). The slip rate for the period 1829-1861 is estimated using two different models: (1) steady interseismic processes with a change in locked/unlocked regions (blue polygon), (2) SSE (orange polygon) superimposed on the existing locking from 1738-1829.

Extended Data Fig. 4 1-d and 2-d marginal PDFs of the spatial parameters.

1-d and 2-d marginal PDFs of the spatial parameters (ζup−dipdown−dipfree in Extended Data Fig. 2a) describing (a) frictional domains for a model where we assume the 1738-1829 velocity field is attributed to steady frictional behaviour on the Sunda megathrust followed by (b) the occurrence of a long-lived transient slip event (SSE) from 1829-1861. The SSE is estimated to occur between ζup and ζbot with an average slip rate of Vtrans (mm/yr) (we normalize this by the plate rate Vpl). (See Methods for a more detailed description of all parameters). c, We show the joint distribution of SSE slip rate and the along-dip location where this slip occurred. The darker colours show a higher value of the PDF; the red line in the 1-d marginal PDFs is the maximum aposteriori estimate.

Extended Data Fig. 5 Snapshots through time of a frictional instability on a velocity strengthening fault.

Snapshots through time of a frictional instability on a velocity strengthening fault. A steady-state creeping fault was perturbed early in the simulation (t = 50 yrs) by a pore-fluid expulsion event and allowed to evolve. In this simulation we do not allow additional weakening from pore-pressure recovery on the fault. The colors represent different time periods (t = 190 to 230 yrs) - from the initial acceleration of the SSE (blue) until the instability is arrested and the fault resumes creeping at its steady rate velocity (yellow). The SSE nucleates at the 12–14 km position on the simulated fault, as the fault is trying to recover to its steady state creep rate. However, an overshoot of slip rates occurs leading to a pulse of high slip rate at the center of the simulated fault. The transition from creep at below 10−10 m/s to the accelerated slip pulse of 10−8 m/s occurs in a short period of time (1-2 yrs). This instability is then damped out and is smeared over the available fault area over 10–15 years.

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Mallick, R., Meltzner, A.J., Tsang, L.L.H. et al. Long-lived shallow slow-slip events on the Sunda megathrust. Nat. Geosci. 14, 327–333 (2021). https://doi.org/10.1038/s41561-021-00727-y

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