The Role of Frontal Thrusts in Tsunami Earthquake Generation

The frontal sections of subduction zones are the source of a poorly understood hazard: tsunami earthquakes, which generate larger-than-expected tsunamis given their seismic shaking. Slip on frontal thrusts is considered to be the cause of increased wave heights in these earthquakes, but the impact of this mechanism has thus far not been quantified. Here, we explore how frontal thrust slip can contribute to tsunami wave generation by modeling the resulting seafloor deformation using fault-bend folding theory. We then quantify wave heights in 2D and expected tsunami energies in 3D for both thrust splays (using fault-bend folding) and down-dip decollement ruptures (modeled as elastic). We present an analytical solution for the damping effect of the water column and show that, because the narrow band of seafloor uplift produced by frontal thrust slip is damped, initial tsunami heights and resulting energies are relatively low. Although the geometry of the thrust can modify seafloor deformation, water damping reduces these differences; tsunami energy is generally insensitive to thrust ramp parameters, such as fault dip, geological evolution, sedimentation, and erosion. Tsunami energy depends primarily on three features: decollement depth below the seafloor, water depth, and coseismic slip. Because frontal ruptures of subduction zones include slip on both the frontal thrust and the down-dip decollement, we compare their tsunami energies. We find that thrust ramps generate significantly lower energies than the paired slip on the decollement. Using a case study of the 25 October 2010 M-w 7.8 Mentawai tsunami earthquake, we show that although slip on the decollement and frontal thrust together can generate the required tsunami energy, <10% was contributed by the frontal thrust. Overall, our results demonstrate that the wider, lower amplitude uplift produced by decollement slip must play a dominant role in the tsunami generation process for tsunami earthquakes.

Automated summary

This study reveals the contribution of frontal thrust slip to tsunami generation by modeling seafloor deformation using fault-bend folding theory. The results show that frontal thrust slip generates lower tsunami energies compared to slip on the down-dip decollement, indicating that slip on the down-dip decollement plays a dominant role in the tsunami generation process of tsunami earthquakes.