HR: 0800h
AN: S11B-0560    [Abstracts]
TI: An Analysis of Small-scale Heterogeneity in the Mantle with PKP Precursors Recorded at IMS Arrays using a seismic phonon method
AU: * Zou, Z
EM: zoukathy@gmail.com
AF: Saint Louis University, 3642 Lindell blvd, St Louis, MO 63108, United States
AU: Koper, K D
EM: koper@eas.slu.edu
AF: Saint Louis University, 3642 Lindell blvd, St Louis, MO 63108, United States
AU: Shearer, P M
EM: pshearer@ucsd.edu
AF: U.C. San Diego, IGPP 0225 U.C. San Diego, La Jolla, CA 92093, United States
AB: It has been known that PKP precursors are direct evidence for the existence of small-scale, non-radially symmetric structure in the Earth's deep mantle since their re-interpretation in the 1970s. However, the scattering strength and depth distribution of the small-scale heterogeneities are still not in good agreement. There are two possible reasons for the discrepancy. One is the data used in different studies. Due to uneven data sampling and different data processing schemes, different authors may obtain different PKP precursor envelopes. The other reason is differences in forward modeling theory. For the same data set, single-scattering theory may yield different results for heterogeneity strength than a multiple-scattering or diffusion based theory. In this study, we take advantage of the globally distributed international monitoring system (IMS) seismic arrays to assemble a large, geographically diverse data set of PKP precursor envelopes. An advantage of using IMS arrays over single stations is that the recordings of all elements at one array can be coherently stacked to suppress noise, and thus get high signal-to-noise (SNR) ratio PKP precursors, even for relatively small earthquakes. We sorted out more than 13,000 events recorded at over a dozen IMS arrays, and selected 2,218 events with high SNR and clear PKIKP arrivals, regardless of precursor strength. The average distance dependence and timing of the precursor envelopes is consistent with previous studies, and there is evidence of coherent large-scale lateral variations in scattering strength. For example, beneath offshore South America the lower mantle has generally weak scattering. However, there are also many small-scale variations in heterogeneity strength that may be artifacts of the source-receiver side scattering ambiguity. We have begun modeling our data set with a multiple-scattering, phonon based approach and initial results indicate that (1) confining the small-scale heterogeneities in the bottom 200~km of the mantle does not fit the data well, and (2) the best scattering models tend to have root-mean-square velocity perturbations less than 0.5% throughout the lower mantle (with a scale length of 8~km).
DE: 1025 Composition of the mantle
DE: 1212 Earth's interior: composition and state (7207, 7208, 8105, 8124)
DE: 1213 Earth's interior: dynamics (1507, 7207, 7208, 8115, 8120)
DE: 3621 Mantle processes (1038)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Seismology [S]
MN: 2007 Fall Meeting