Local tsunamis and distributed slip at the source

Variations in the local tsunami wave field are examined in relation to hete rogeneous slip distributions that are characteristic of many shallow subduc tion zone earthquakes. Assumptions inherent in calculating the coseismic ve rtical displacement held that defines the initial condition for tsunami pro pagation are examined. By comparing the seafloor displacement from uniform slip to that from an ideal static crack, we demonstrate that dip-directed s lip variations significantly affect the initial cross-sectional wave profil e. Because of the hydrodynamic stability of tsunami wave forms, these effec ts directly impact estimates of maximum runup from the local tsunami. In mo st cases, an assumption of uniform slip in the dip direction significantly underestimates the maximum amplitude and leading wave steepness of the loca l tsunami. Whereas dip-directed slip variations affect the initial wave pro file, strike-directed slip variations result in wavefront-parallel changes in amplitude that are largely preserved during propagation from the source region toward shore, owing to the effects of refraction. Tests of discretiz ing slip distributions indicate that small fault surface elements of dimens ions similar to the source depth can acceptably approximate the vertical di splacement field in comparison to continuous slip distributions. Crack mode ls for tsunamis generated by shallow subduction zone earthquakes indicate t hat a rupture intersecting the free surface results in approximately twice the average slip. Therefore, the observation of higher slip associated with tsunami earthquakes relative to typical subduction zone earthquakes of the same magnitude suggests that tsunami earthquakes involve rupture of the se afloor, whereas rupture of deeper subduction zone earthquakes may be imbedd ed and not reach the seafloor.