Tsunami detection by satellite altimetry

We present what is to our knowledge the first direct observation of the def ormation of the surface of the ocean upon passage of a tsunami wave, on the high seas, far from the influence of shorelines and continental shelves. W e use satellite altimetry data from the ERS-1 and TOPEX/POSEIDON programs, complemented by spectrogram techniques and synthetic maregrams to examine t he case of seven recent tsunamigenic earthquakes. We make a positive identi fication of the tsunami wave field in the case of the 1992 Nicaraguan tsuna mi, which we detect at 15 degrees S, 106 degrees W, five hours after origin time. We model the observed spectrogram by injecting a synthetic of variab le amplitude into the signal of a repetitive cycle of the satellite along t he same track, concluding that the Nicaraguan tsunami had a zero-to-peak am plitude of 8 cm in that region. In the case of the 1995 Chilean tsunami, a large scatter in the spectral properties of the reference tracks renders th e detection tentative. We fail to detect the tsunamis of five other large e vents, including the 1996 Biak and 1996 Peru earthquakes, primarily on acco unt of unfavorable source directivity in the geometry of existing satellite tracks, and of the strong and incoherent noise produced by large current s ystems, such as the Kuroshio in the Northwest Pacific.