Raised Potential Earthquake and Tsunami Hazards at the North Sulawesi Subduction Zone after a Flurry of Major Seismicity

The North Sulawesi subduction zone (NSSZ) was ruptured by a series of large earthquakes (Mw >7) since 1990s, generally triggered small to moderate tsunamis in the surrounding seas and ocean. But the 2018 Sulawesi Mw 7.5 earthquake ruptured the west subduction zone and induced a large tsunami that displaced hundreds of people. These large earthquakes, especially the deeper thrust events, generated stress loading to the shallow megathrust that could rupture to excite exceptional tsunami hazard as observed in Sumatra, Japan and the other subduction zones. Whether the stress loading from downdip events can trigger future failure of a shallow tsunami earthquake and its ensuring tsunami hazard impact remains elusive. Here we investigate the potential of earthquake and tsunami by analyzing the historical earthquake characteristics, calculating Coulomb stress changes and simulating hundreds of hypothetical earthquake ruptures to assess the plausible tsunami hazard in the NSSZ. Our results show that a series of Mw 7+ downdip megathrust earthquakes have loaded most of the megathrust, especially the shallow portion (<10 km), with increased stress >10 kPa, implicating a high potential of future large earthquakes and ensuring outsize tsunamis. Our modeled tsunami wave heights vary between >0 and 43 m along the North Sulawesi coastlines. One intriguing fact is that the end point of the tsunami wave energy dissipation path gains a comparable tsunami impact as the region in the rupture zone for all the magnitude considered, highlighting a dual-pattern threatened regions in the Celebes Sea. Importantly, the thrust and fold belt structure in the wide outer wedge of the accretionary prism and the strong seafloor bathymetry variation offshore Sulawesi Island could serve an efficient wave-amplification tool that need to be considered in future hazard assessment. Our findings alert that the earthquake and tsunami hazard potential are largely raised by these downdip major earthquakes.