HR: 0800h
AN: U11B-0836    [Abstracts]
TI: Time Reversal of Seismic Waves and the Great Sumatra Earthquake
AU: * Montagner, J
EM: jpm@ipgp.jussieu.fr
AF: Dept.Seismology, IPG, 4 Place Jussieu, Paris, 75252 France
AU: Larmat, C
EM: carene@gps.caltech.edu
AF: Seismological Laboratory, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA CA91125 United States
AU: Fink, M
EM: mathias.fink@espci.fr
AF: Laboratoire Ondes et Acoustique, ESPCI, rue Vauquelin, Paris, 75005 France
AU: Capdeville, Y
EM: capdevil@ipgp.jussieu.fr
AF: Dept.Seismology, IPG, 4 Place Jussieu, Paris, 75252 France
AU: Clevede, E
EM:
AF: Dept.Seismology, IPG, 4 Place Jussieu, Paris, 75252 France
AU: Tourin, A
EM:
AF: Laboratoire Ondes et Acoustique, ESPCI, rue Vauquelin, Paris, 75005 France
AB: The occurrence of the disastrous Sumatra-Andaman earthquake on dec. 26, 2004 makes it necessary to develop innovative techniques for studying the complex spatio-temporal rupture mechanism of this giant earthquake. We present here the first application of time-reversal to seismic data. This concept was previously successfully applied for acoustic waves in many fields such as medical imaging, oceanography and non destructive testing. Time-reversal experiments are using the time reversal invariance and the spatial reciprocity of the wave equation. Their primary goal is the refocusing of energy at the location and the time of an acoustic source by time-reversing the recorded signals. As in acoustics, seismic waves are also potentially reversible. In order to achieve a time reversal experiment within the three-dimensional Earth, we take advantage of the increasing power of numerical methods (such as spectral elements methods), which enable to simulate more and more accurately the propagation of seismic waves in heterogeneous media. In addition, at very long period (longer than 200s), the available earth models are sufficiently good to enable the backpropagation of 3-component seismic waves of real data. For the first time, we have performed several synthetic and real data time-reversal experiments for seismic waves by a purely numerical technique. The seismograms of the FDSN Global network used as an active network, seismic waves are backpropagated by numerical calculation in reversed time. The time reversal technique is applied to real data at very long period (T>200s). For a point source,the coordinates of the earthquake and the strike of the fault plane are correctly retrieved, and the focusing in space and time is sharp. When time-reversing seismic data of the recent Great Sumatra- Andaman earthquake (Dec 26, 2004, Magnitude 9.3), in spite of the large extension of the source in space and time, it is shown that we still refocus at the location and time of the centroid and that the migration of the rupture from the south towards the north is visible. Therefore, the technique of time reversal enables to gain insight into the spatio-temporal history of complex earthquakes. So far, our time-reversal technique is limited to the simplest cases in order to be able to perform many tests in a reasonable computing time. However, it will be possible in the future to improve these results by broadening the frequency range of seismic data in heterogeneous earth models and the technique can be used to investigate any kind of seismic sources.
DE: 3285 Wave propagation (0689, 2487, 4275, 4455, 6934)
DE: 7209 Earthquake dynamics (1242)
DE: 7290 Computational seismology
SC: Union [U]
MN: Fall Meeting 2005