Modelling the tsunami free oscillations in the Marquesas (French Polynesia)

The tsunami resonance inside basins (closed or semi-enclosed) depends on the period of the incident waves, reflection and energy dissipation, characteristics of the boundary and the geometry of the basin. When waves continuously enter the basin, they cause abnormal water level fluctuations and produced damage if their periods are close to the periods of free oscillation of the basin. These resonant oscillations inside harbours, bays, or other semi-enclosed or closed basins can have a direct influence on the management of harbours, shipping and coastal uses. So, it is important to determine these free oscillations. These resonant characteristics have been observed in the Marquesas, an archipelago prone to tsunami amplification, during the last three tsunamis (Samoa 2009, Maule 2010, Tohoku 2011). These events were recorded by the two tide gauges located in the Marquesas. In this archipelago, there are two monitored bays : the first one is located in Hiva Oa Island (Tahauku Bay) and the second in Nuku Hiva Island (Taihoae Bay). For all these tsunamis, more than 3 d of water tidal resonance were recorded. In this work, we make a free oscillation analysis of the Marquesas Archipelago using real tide gauge data, simulated tsunami data and theoretical computations. During the last century, this archipelago was hit by several trans-Pacific tsunamis. It is interesting to note that, following different tsunami reports the bays responded differently depending on the earthquake source region. For example, Tahauku and Atuona, two bays in Hiva Oa Island only 1 km apart, have different responses depending on the tsunami, as it was observed in 1946 (Aleutian earthquake) and 1960 (Chile earthquake). For this reason, we study the azimuthal dependence of the excitation of the free oscillation modes, and we show that some azimuths enhance tsunami amplification.