HR: 09:05h
AN: S11C-05    [Abstracts]
TI: Swell Activity in the Southern Pacific From Seismic and Infrasonic Data
AU: Reymond, D
EM: reymond.d@labogeo.pf
AF: Lab. Geophysique, CEA, Pamatai, Faaa, 98702 French Polynesia
AU: * Barruol, G
EM: barruol@upf.pf
AF: Lab. Terre Ocean, Univ. French polynesia, Faaa, 98702 French Polynesia
AU: * Barruol, G
EM: barruol@upf.pf
AF: Lab. Tectonophysique, CNRS, Univ. Montpellier II, Montpellier, 34095 France
AU: Fontaine, F
EM: fontaine@dstu.univ-montp2.fr
AF: Lab. Tectonophysique, CNRS, Univ. Montpellier II, Montpellier, 34095 France
AU: Hyvernaud, O
EM: hyvernaud@labogeo.pf
AF: Lab. Geophysique, CEA, Pamatai, Faaa, 98702 French Polynesia
AU: Maurer, V
EM: vincent\_maurer@yahoo.fr
AF: Lab. Geophysique, CEA, Pamatai, Faaa, 98702 French Polynesia
AU: Maamaatuaiahutapu, K
EM: Keitapu.Maamaatuaiahutapu@upf.pf
AF: Lab. Terre Ocean, Univ. French polynesia, Faaa, 98702 French Polynesia
AB: A temporary network of 10 broad band seismic stations has been deployed in French Polynesia for the Polynesian Lithosphere and Upper Mantle Experiment (PLUME). The stations are installed either on volcanic islands or on atolls of the various archipelagos of French Polynesia to complement the geographic coverage provided by the permanent stations. At all sites, the proximity of the ocean generates a high microseismic noise level. The power spectral density of the seismic data show clear peaks in the 0.05 to 0.09 Hz frequency range (11 to 20 s period), corresponding to typical swell frequencies. In this single frequency peak, the swell-related seismic signal is elliptically polarized and is contained within the horizontal plane. We measure hourly its amplitude and azimuth and demonstrate that in this frequency range, the amplitude of the microseismic "noise" shows very similar variations from station to station and is strongly correlated with the swell amplitude predicted by the NOAA, wind-derived, "WaveWatch" models. Deducing the swell direction from the ground particle motion has to be done with care since the island ground motion can be strongly controlled by local swell refraction processes. We find cases, however, such as in Tahiti or on roughly circular atolls, for which the azimuth of the incoming swell may be well deduced from the seismic data. This therefore demonstrates that the swell-related seismic signal observed in French Polynesia in the single frequency peak can reliably be used as a proxy for swell amplitude and azimuth. The presence of an infrasonic array installed in Tahiti also provides the opportunity to use microbarometric signal to characterize the swell activity. For a period of low wind (which is a strong noise generator) and high swell, we evidenced a clear correlation between the microseismic and infrasonic noise amplitude, together with the predicted and the locally observed swell amplitudes, suggesting that such infrasonic data can be used reliably during quiet wind conditions, to retrieve the swell amplitude but not its azimuth.
DE: 7200 SEISMOLOGY
DE: 4572 Upper ocean processes
DE: 3025 Marine seismics (0935)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting