HR: 0800h
AN: S11B-0557    [Abstracts]
TI: Array Observations of Short Period Pdiff Coda Waves
AU: * Xu, Y
EM: yanxuzh@gmail.com
AF: Saint Louis University, Dept. of EAS 3642 Lindell Blvd., St. Louis, MO 63108, United States
AU: Koper, K D
EM: koper@eas.slu.edu
AF: Saint Louis University, Dept. of EAS 3642 Lindell Blvd., St. Louis, MO 63108, United States
AB: There have been several reports of short-period, coherent coda waves following P_diff at distances of approximately 95° -120°. Early interpretations favored topographical scattering at the core-mantle boundary or strong volumetric scattering in the lowermost mantle (D"). However, a recent study suggested these observations can be explained by relatively mild, uniform scattering throughout the lower mantle. In this study, we present a new data set that can be used to test the competing hypotheses for the generation of short-period P_diff coda waves. We downloaded data for 824 earthquakes with magnitudes of 5.7 m_b - 7.9 m_b, depths shallower than 100~km, and at distances of 50° - 120° from the Yellowknife array (YKA) in Western Canada. YKA is a medium-aperture (20~km) array, with 19 short-period, vertical-component seismometers arranged in a cross. By using an array we can increase signal-to-noise ratio and infer the directions from which the coda waves are arriving. We included earthquakes at distances smaller than 90° so that we can test the vertical change in heterogeneity strength as waves approach D". The earthquakes are clustered in three regions: Asia, South America, and Tonga-Fiji. For each event, we used a sliding window slowness analysis technique to estimate the beam power as a function of time. Then we normalized each beam envelope by PP amplitude and corrected the amplitudes for radiation pattern effects, eliminating those data in which P, P_diff, or PP was close to a nodal plane. We then stacked the data in 2° bins, and fit a log-linear relationship between energy and time to determine the coda decay rate (CDR). The CDRs show a clear distance dependence. Asia and South America have the same pattern above D": high CDR from 50° to 70°, low CDR in the range 72°-80°, followed by a CDR increase. At distances after 90° the coda decay rates in the three clusters show significant variations, with events from Asia having the highest CDR. We are currently experimenting with various methods for simulating P_diff coda waves including a frequency-wavenumber integration technique for 1D stochastic models and a single-scattering ray theory based technique, modified to account for diffraction induced amplitude decay.
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Seismology [S]
MN: 2007 Fall Meeting