HR: 09:15h
AN: S31B-06    [PDF]
TI: New Evidence for Inner Core Scattering from Observations of PKiKP Coda
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: Franks, J M
EM: franksjm@slu.edu
AF: Saint Louis University, Dept. of EAS 3507 Laclede Ave., St. Louis, MO 63103 United States
AU: Dombrovskaya, M
EM: dombrom@slu.edu
AF: Saint Louis University, Dept. of EAS 3507 Laclede Ave., St. Louis, MO 63103 United States
AB: The existence of small-wavelength scatterers in Earth's inner core has been suggested based on the frequency dependence of inner core Q, and as a partial explanation for PKP and PKKP precursors. However, recent observations of anomalously large PKiKP coda waves have provided the first direct seismic evidence of inner core scatterering (ICS). There is no consensus on the physical nature of the proposed scatterers, though possibilities include the presence of partial melt and variations in the strength and orientation of anisotropy. We have recently begun a search for evidence of ICS using PKiKP data from the small aperture array stations of the International Monitoring System. These stations generally have poorer slowness resolution than LASA (the now defunct array at which the original ICS observations were made), but are small enough that high frequency energy remains coherent across the array and can be substantially enhanced through beamforming. We selected events having favorable focal mechanisms and occurring in the distance range of 50-90 degrees, where the theoretical PKiKP amplitude is very small. We processed the data in four overlapping frequency bands using a sliding window, time domain, beamforming algorithm that uses a coherency measure as a weighting factor. Out of 260 viable event/receiver combinations, 52 were positive for an anomalously large PKiKP coda, an additional 26 were inconclusive, and the remainder were negative. The majority of the positive observations had a maximum coda amplitude quite close to the theoretical PKiKP arrival time. In these cases it is unclear if the coda is being created by scatterers within the inner core, or by scatterers within the crust and mantle acting on an unusually large PKiKP phase. However, in approximately 15 cases the maximum of the PKiKP coda occurred 20-60 s after the predicted PKiKP arrival, strongly suggesting an origin in the inner core.
UR: http://mnw.eas.slu.edu/People/KKoper/CORE/
DE: 5430 Interiors (8147)
DE: 7203 Body wave propagation
DE: 7207 Core and mantle
DE: 8115 Core processes (1507)
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Seismology [S]
MN: 2003 Fall Meeting