The Generation of Tsunamis

Abstract

Abstract Tsunamis are gravity‐driven water waves. Most are generated by vertical displacement of the seabed that propagates through the water column to the sea surface. The resulting elevated surface wave collapses through gravity and then propagates outward from the source. Dispersion of the initial wave generates a multiple wave train. Tsunamis are mainly (∼80%) generated by earthquakes, but other mechanisms include subaerial and submarine landslides and volcanic collapse and eruption. Other, less frequent, tsunami mechanisms include bolide (asteroid) impact and weather events (meteotsunamis), but these are generated at the water surface, respectively, from external impact and from wind friction. The magnitude of a tsunamigenic earthquake has the main control on the tsunami, although “tsunami” earthquakes generate tsunamis much larger than expected from their earthquake source magnitude. Tsunamigenic earthquake mechanisms include interplate boundary rupture, splay faulting, and intraplate rupture. Landslide tsunami mechanisms include slumps and translational failures that may be initiated from either the bottom or the top. Landslide volume, water depth, and initial acceleration are the main controls on tsunami magnitude, although other factors such as the failure mechanism and the location of initiation are influential. There are three main aspects of a tsunami; (i) initial wave generation, (ii) propagation, and (iii) onland run‐up. Initial wave generation from earthquakes is mainly from seabed vertical displacement, and a rule of thumb suggests that in most instances the maximum initial wave elevation is up to twice this. The maximum initial wave elevation from a landslide tsunami is theoretically determined by the ocean depth and thus could be thousands of meters. Local tsunami run‐up elevations vary with source mechanism and vary considerably. Although dependent on local bathymetry and topography, these are likely to be more elevated and focused from submarine landslides than from earthquakes. The different mechanisms generate different tsunami wave frequencies; these determine travel distances, with the low frequency tsunamis from earthquakes traveling much farther than tsunamis from landslides, which are much higher frequency. Final onland run‐up is mainly dependent on source mechanism as well as local offshore bathymetry and coastal topography.