Far-field simulation of tsunami propagation in the Pacific Ocean: impact on the Marquesas Islands (French Polynesia)

The islands in the Marquesas archipelago are most exposed to far-field tsun amis in French Polynesia. Four recent earthquakes located at the Pacific ri m (Kurile 1994, Chile 1995, Mexico 1995 and Peru 1996 earthquakes) generate d trans-Pacific tsunamis and caused contrasting inundations in several inha bited Marquesian bays. The aim of this study is to better understand the am plification phenomena observed in the Marquesas Islands and to improve the risk assessment. We present here numerical simulations of these four tsunam is by means of a finite difference model solving the equations of tsunami p ropagation at different scales. The water heights computed in the bays are in excellent agreement with the available observations and thus validate ou r numerical method. These numerical results allow us to determine (1)which propagation azimuths are dangerous for the whole Marquesas archipelago and (2) the behavior of each studied bay (amplitude, frequency) in response to tsunami waves. We observe that the seismogenic subduction zone along South America is likely to define one of the most dangerous tsunami generation ar eas for the Marquesas archipelago, owing to the source geometries and to th e trapping of the tsunami energy by bathymetric features in the southern Pa cific Ocean (fracture zones, seamount chains).