Recurrent episodes of transient deformation in NW Sulawesi, Indonesia

The Celebes Sea subducts beneath the North Arm of Sulawesi, Indonesia, at the Minahassa trench. Over the past three decades, only a few M_w>7 earthquakes ruptured this plate interface, despite a 40 mm/yr convergence rate. The left-lateral Palu-Koro fault delineates the extent of the overriding plate at the western termination of the Minahassa subduction zone and hosted a M_w7.5 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 and reprocess the GNSS observations in NW Sulawesi. 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. This delayed-triggered, (apparently aseismic) slow slip event occurred just east of the 1996 M_w7.9 megathrust rupture.

The 20-year long time series is characterized by four 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 M_w7.4 earthquake. The presumed slow slip events generally take more than 300 days to quiet down again with a recurrence interval of about five years.