HR: 0800h
AN: T51C-0683    [Abstracts]
TI: Seismic imaging of scatterer migration using waveform data of repeating earthquakes
AU: * Cheng, X
EM: xcheng@rice.edu
AF: Department of Earth Science, Rice University, 6100 Main St, Houston, TX 77005, United States
AU: Niu, F
EM: niu@rice.edu
AF: Department of Earth Science, Rice University, 6100 Main St, Houston, TX 77005, United States
AU: Silver, P G
EM: silver@dtm.ciw.edu
AF: Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, Washington, DC 20015, United States
AU: Nadeau, R M
EM: nadeau@seismo.berkeley.edu
AF: Berkeley Seismological Lab., 207 McCone Hall, Univ. of California, Berkeley, CA 94720, United States
AB: Time-lapse seismic imaging (4D) is a new and rapidly-evolving technology. It has attracted wide attention in recent years, because accurate imaging of the evolving subsurface structure has significant applications in resource exploration and environmental monitoring. To apply this technology to monitor the time-varying stress field associated with earthquakes, one needs highly-repeatable powerful sources that can propagate through tens of kilometers to generate similar waveform data that allows for investigating temporal changes in the crustal velocity structure at seismogenic depth. One natural source is repeating earthquakes, which are believed to occur at nearly the same location with the same source mechanism. As such, they produce virtually identical seismograms at a given station. Yet, there are subtle differences in these records that can result, in principle, from either temporal changes in the medium, or from slight differences in source parameters, or both, assuming the signal to noise ratio is high. Approaches such as using differential seismograms or making differential seismic images potentially suffer from the systematic bias introduced by changes in source parameters. For example, variations in source location can be significant when natural sources are used since changes in the medium are typically very small. In order to minimize this bias, we adopt the so-called coda wave interferometry technique. To quantify the difference between seismograms in a repeating earthquake cluster, we compute the cross correlation between the first seismogram and each subsequent seismogram within a moving time window. The lag time τ(t) is obtained when the maximum cross correlation, C_m(t), is reached, and a decorrelation index D(t) is defined as 1- C_m(t). We find that temporal changes in source location, background velocity and the scattered wave field have very different influences on the two functions, and can thus be separated on this basis. Uniform changes in the background velocity, for example, results in a monotonic increase or decrease of lag time as a function of elapsed time on the seismogram, and consequently these changes can be estimated from the slope of lag time function τ(t). Our finite difference synthetic simulations demonstrated that these various influences are distinguishable by migrating the decorrelation index D(t). We are able to image localized changes in scattering field from waveform data that substantial changes in location and the background velocity are presented. Our technique of imaging scatterer migration thus can be applied to broad regions where relatively loosely defined clusters are available.
DE: 7205 Continental crust (1219)
DE: 7209 Earthquake dynamics (1242)
DE: 7215 Earthquake source observations (1240)
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting