HR: 15:05h
AN: S43C-06 INVITED    [Abstracts]
TI: Speeding up seismic tsunami warning using W phase
AU: * Kanamori, H
EM: hiroo@gps.caltech.edu
AF: Caltech Seismological Laboratory, 1200 E. California Blvd., Pasadena, CA 91125,
AU: Rivera, L
EM: luis@sismo.u-strasbg.fr
AF: Universite Louis Pasteur, EOST-IPGS, Strasbourg, F67084, France
AB: W phase is a long period phase arriving before S wave. It can be interpreted as superposition of the fundamental, 1st, 2nd, and 3rd overtones of Rayleigh waves and has a group velocity of 8 km/s at 1000 s and 8.6 km/s at 100 sec. At a distance of 50° the W-phase energy is mainly contained within a time window of 700 s after the P arrival. Since the amplitude of long period waves better represents the tsunami potential of an earthquake, the use of W phase has a merit in assessing the tsunami potential at an earliest possible time. To use W phase we need to solve two problems. First, many STS-1 records at short distances are clipped at or before the surface wave arrival. Thus, with the traditional frequency domain deconvolution, the wraparound effect from the end of the record makes the beginning part unusable, even if W phase is on scale before the surface wave arrival. Second, no systematic analysis has been made to use W phase for source studies. We solved the first problem by using a time-domain recursive deconvolution method described by Zhu with some modification. We investigated the second problem by inverting the displacement seismograms windowed over a short duration after the P arrival. The duration is given by 15*Δ (in degree) s. Thus, at a distance of 50°, we use the record only up to 23 min after the origin time which is the distinct advantage of using W phase for tsunami warning purposes. If many stations are available at shorter distances, the time can be considerably shortened. The bandwidth of W phase is approximately from 0.001 to 0.01 Hz, and we band-pass filter the data from 0.0005 to 0.005 Hz in most cases. Having extracted W phase from the vertical component records, we concatenate them in time in the order of distance to obtain a single W-phase time series. This time series constitutes the data column vector. We synthesize similar concatenated W-phase time series by mode summation for each of the moment tensor elements. These time series constitute the column vectors of the matrix which maps the unknown moment tensor to the data. We performed a linear inversion using a point source for several large earthquakes including the 2004 Sumatra-Andaman earthquake, the 2005 Nias earthquake, the 2006 Kuril Is. earthquake, and the 2007 Peruvian earthquake. The results are satisfactory, and give promise of the use of W phase for rapid assessment of tsunami potential. A practical procedure for operational use of W phase for tsunami warning purposes will be discussed.
DE: 7215 Earthquake source observations (1240)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 7255 Surface waves and free oscillations
DE: 7260 Theory
DE: 7299 General or miscellaneous
SC: Seismology [S]
MN: 2007 Fall Meeting