Earthquake Mechanism and Seafloor Deformation for Tsunami Generation

Abstract

Tsunamis are generated in the ocean by rapidly displacing the entire water column over a significant area. The potential energy resulting from this disturbance is balanced with the kinetic energy of the waves during propagation. Only a handful of submarine geologic phenomena can generate tsunamis: large-magnitude earthquakes, large landslides, and volcanic processes. Asteroid and subaerial landslide impacts can generate tsunami waves from above the water. Earthquakes are by far the most common generator of tsunamis. Generally, earthquakes greater than magnitude (M) 6.5–7 can generate tsunamis if they occur beneath an ocean and if they result in predominantly vertical displacement. One of the greatest uncertainties in both deterministic and probabilistic hazard assessments of tsunamis is computing seafloor deformation for earthquakes of a given magnitude. This entry reviews past methodologies and current developments of seismogenic tsunami generation models. Subduction zones are the tectonic plate boundaries where most seismogenic tsunamis are sourced. Three factors combine to explain this observation: (1) most subduction zones occur beneath oceans, (2) most of the world’s largest earthquakes occur along subduction zones, (3) the dominant mechanism of subduction zone earthquakes is thrusting, resulting in significant vertical displacement of the seafloor. Normal-faulting earthquakes in the outer rise and outer trench slope of subduction zones and thrust faulting in the back arc can also generate tsunamis. Other tectonic environments where seismogenic tsunamis can occur are oceanic convergence boundaries as distinguished from subduction zones by Bird (2003) and local, submarine dip-slip fault zones in intraplate settings. To simulate the tsunami generation process, elastic deformation models are needed to determine the magnitude and pattern of seafloor displacement from earthquake source parameters. To calculate the initial tsunami amplitude distribution on the ocean surface, a 1/cosh(kh) low-pass filter is applied to the seafloor displacement field, where k is the wave number and h is the water depth (Kajiura 1963). This filter primarily has an effect for small earthquakes and surface-rupturing earthquakes that generate a scarp or short-wavelength displacement. Otherwise, the initial wave field of a tsunami is almost identical to the vertical seafloor displacement field. Horizontal displacements also contribute to tsunami generation in regions of steep bathymetry (Tanioka and Satake 1996). Although the rupture process of large subduction zone earthquakes is extremely complex, simple mechanical