French Polynesia tsunami warning center (CPPT)

The Geophysical Laboratory, which is also the French Polynesia Tsunami Warning Center (Centre Polynesien de Prevention des Tsunamis - CPPT) disposes of the data recorded by the Polynesian Seismic Network which includes 21 short-period stations, 4 broad-band three component long period stations and 2 tide gauge stations. These stations are, for the most, telemetred toward the CPPT in Tahiti which is equipped with data processing means. The data acquisition is performed on optic discs, tape drive recordings and graphic recordings. In the CPPT, the Tsunami Warning is based on the measurements of the Seismic Moment through the mantle magnitude Mm and the proportionality of observed tsunami height to this seismic moment. The new mantle magnitude scale, Mm used the measurement of the mantle Rayleigh and Love wave energy in the 50-300 s. period range and is directly related to the seismic moment through Mm = log Mo - 20. The knowledge of the seismic moment allows computation of an estimate of the high-seas amplitude of a range of expectable tsunami heights. In establishing seismic thresholds for tsunami warning, we assume that tsunami risk is substantial when the upper level found on the amplitude predicted at PPT reach 1 m. On this basis, the risk levels have been identified as a function of the magnitude Mm. For the Polynesian Islands the destructive tsunami danger would subsequently exist for Mm ≥ 8.7 (Mo ≥ 5 x 1028 dyn-cm), in the case of epicenters in Samoa, Tonga, Kermadec and Mm ≥ 9.0 (Mo ≥ 1029 dyn-cm) for other epicenters. This procedure is fully automatic: One computer detects, locates and estimates the seismic moment through the Mm magnitude and, in terms of moment, gives an amplitude window of the expected tsunami. These different operations are executed in real time. In addition, the operator can use the historic references and, if necessary, the acoustic T waves. 13 refs, 11 figs