Recurrent transient deformation in NW Sulawesi

The Celebes Sea subducts beneath the North Arm of Sulawesi, Indonesia, at the Minahassa trench. Over the past three decades, only a few Mw>7 earthquakes ruptured this plate interface, despite a 40 mm/yr convergence rate. Delineating the extent of the overriding plate at the western termination of the Minahassa subduction zone, the left-lateral Palu-Koro fault did host a Mw7.5 earthquake in September 2018. Observations of post-seismic surface motion following the 2018 event were interpreted in a previous study to result from afterslip that extended underneath the co-seismic rupture plane. A mismatch between observed post-seismic surface motions and predictions from afterslip distributions remained at the North Arm of Sulawesi. In this study we revisit the GNSS monument campaign sites in NW Sulawesi, and combine this with a reprocessing of the available continuous GNSS sites. We analyse the post-2018 time series to determine whether the post-seismic signal can be ascribed to a single source. This is not the case, as we detect another, yet smaller amplitude signal. We take a Bayesian approach and find that this smaller magnitude signal corresponds to slow slip on the Minahassa subduction interface, thus accommodating some of the accumulated slip rate deficit. This delayed-triggered slow slip event that thus modifies the post-2018 transient signal occurred near the western termination of the Minahassa subduction interface but not directly adjacent to the Palu-Koro fault. There was no clear rise in macro-seismic activity in this time period.
Further analysis of the 20-year long time series reveals additional periods of transient surface motion. Three of these periods are likely the result of distinct slow slip events and one is a post-seismic signal from the 2008 subduction Mw7.4 earthquake. The presumed slow slip events generally take more than 300 days to quiet down again. The next event is likely to start within the next five years.