HR: 0800h
AN: S41C-1036    [Abstracts]
TI: Slowness Anomalies of PKP Phases Recorded at the Seismic Array in Eielson, Alaska (ILAR)
AU: * Koper, K D
EM: koper@eas.slu.edu
AF: Saint Louis University, Dept. of EAS 3507 Laclede Ave., St. Louis, MO 63103 United States
AU: Parker, V
EM: parkerv@eas.slu.edu
AF: Saint Louis University, Dept. of EAS 3507 Laclede Ave., St. Louis, MO 63103 United States
AB: The Eielson, Alaska seismic array (ILAR) is well situated to record PKP_DF waves from earthquakes occurring in the South Sandwich Islands (SSI) region. Such ray paths are nearly aligned with Earth's rotation axis and are useful for constraining models of inner core anisotropy. The many previous studies of PKP_DF waves traversing the SSI-Alaska corridor generally find waves that arrive several seconds faster than expected, with highly attenuated and often complicated shapes. Simple radially or cylindrically symmetric Earth models cannot explain these observations, and it may be the case that mantle heterogeneities are biasing the SSI-Alaska PKP_DF waves. In this study, we take advantage of the small aperture of ILAR to make independent measurements of differential PKP_DF-PKP_BC travel times and differential PKP_DF-PKP_BC horizontal slowness vectors for 37 SSI earthquakes that occurred from 1996-2004. Anomalies in slowness (ray parameter and backazimuth) of a phase reflect heterogeneous Earth structure in a manner complementary to travel time anomalies. At a reference distance of 152°, we find a mean differential travel time residual of 3.1 ± 0.1~s, a mean differential ray parameter of 2.9±0.2~s/deg, and that PKP_DF waves arrive from a backazimuth rotated approximately 10° counterclockwise relative to corresponding PKP_BC waves. Joint modeling of the differential travel times and differential ray parameters indicates that (1) lower mantle heterogeneities are not responsible for the properties of PKP_DF from SSI-ILAR, (2) the lower several hundred kilometers of the outer core has a slightly lower velocity, and/or velocity gradient, than current reference models, and (3) there is a strong, radial velocity gradient within the inner core at a radius of 600-900~km. However, the differential slowness anomalies cannot be fully explained by variations in deep Earth structure, implying that local site effects at ILAR are somewhat different for PKP_DF and PKP_BC phases.
DE: 1212 Earth's interior: composition and state (7207, 7208, 8105, 8124)
DE: 7203 Body waves
DE: 7207 Core (1212, 1213, 8124)
SC: Seismology [S]
MN: Fall Meeting 2005