Hunt for Slow Slip Events Along the Sumatran Subduction Zone in a Decade of Continuous GPS Data

Abstract:

Slow slip events (SSEs) have been observed in GPS time series for many subduction zones worldwide, but they have not been documented in the decade-long time series from the Sumatran GPS Array (SuGAr) for the Sumatran subduction zone. An outstanding question has been whether the Sunda megathrust has simply not exhibited this behavior, or whether the signals have been obscured by the prodigious number of earthquakes (and their ensuing postseismic deformation) that have occurred within the time of geodetic observation. In this study, we remove all known tectonic signals from the SuGAr time series to search for evidence of SSEs. We find the residual position time series to be essentially flat at the centimeter scale. To search for signals at the millimeter scale we tested various filtering and visualization techniques. Despite these efforts, we conclude that it is difficult to confirm that SSEs exist at this scale using the current data, although we do see a few suspicious signals. For example, in the recent data since 2012, we find some signals that coincidently appeared at two nearby stations KTET and SMGY, which sit on Sipora and North Pagai islands, respectively, in the Mentawai seismic gap. These anomalous signals might suggest an SSE in the Mentawai seismic gap; however, we cannot confirm this hypothesis based on only two stations. The lack of evidence for SSEs could be the result of them occurring at a magnitude, location, or time scale that is undetectable with the current resolution of the SuGAr. Alternatively, the lack of observed events within those bounds could reflect this being a very active period in the earthquake cycle, or that the mechanical properties of this megathrust are not conducive to such events.