The tsunami excitation zone at global subduction zones

Abstract:

Great and giant subduction earthquakes generate tsunamis that cause local or ocean-wide devastation. What controls the tsunami potential of megathrust earthquakes, whether their source characteristics or tectonic setting, remains poorly understood. Several mechanisms have been proposed to explain giant, trench-breaking ruptures along a sub-horizontal décollement, but an efficient mechanism for seafloor uplift is still missing. Here, we compile geological structural data from bathymetry and seismic profiles at global subduction zones to identify the tsunami excitation zone. We show that tsunamigenic potential is controlled by width of the outer wedge, the frontal region of the accretionary prism with recent internal deformation consisting of parallel and conjugate thrusts and folds. The high-angle thrusts, back-thrusts, and folds in the outer wedge provide an efficient mechanism for seafloor uplift and tsunami excitation. Tsunami earthquakes, i.e. seismic events that generate larger tsunami than is expected for their magnitude, typically break a large section of the outer wedge. The tsunami potential of giant earthquakes is also controlled by the rupture propagation into the outer wedge, such that large, great, and giant earthquakes that rupture the same fraction of the outer wedge resulting similar median and maximum tsunami runup. The tsunami excitation zone can be unambiguously identified along major subduction zones, including at well-identified seismic gaps, based on geomorphological analysis. Using the relationship between the outer-wedge width and tsunami runup and size, we predict the expected earthquake magnitude, tsunami runup at all major subduction zones. The tsunamigenic potential is the highest at northern Sunda (Indonesia), Hikurangi (New Zealand), western Makran (Iran), and the Lesser Antilles and the Chilean Valdivia subduction zones ranks among the lowest. The structural control on tsunami size explains why the trench-breaking Mw 9.2 Sumatra-Andaman rupture generated some of the largest tsunami on records, while the 1960 Mw 9.5 Valdivia megaquake generated only moderate tsunamis. The tsunami excitation zone is important to characterize the rupture style and tsunami magnitude of future seismicity, offering crucial information for seismic and tsunami hazard preparedness