HR: 08:45h
AN: S31B-04    [PDF]
TI: Near-antipodal PKPdf observations from the Trans-Antarctic Mountains Seismic Experiment (TAMSEIS): Constraints on inner-core anisotropy from a seismic deployment in Antarctica
AU: * Wiens, D A
EM: doug@seismo.wustl.edu
AF: Dept of Earth and Planetary Sci., Washington University, 1 Brookings Drive, St. Louis, MO 63130
AU: Nyblade, A
EM: andy@essc.psu.edu
AF: Dept of Geosciences, Penn State University, University Park, PA 16802
AU: Anandakrishnan, S
EM: sak@essc.psu.edu
AF: Dept of Geosciences, Penn State University, University Park, PA 16802
AU: Shore, P J
EM: patrick@wups.wustl.edu
AF: Dept of Earth and Planetary Sci., Washington University, 1 Brookings Drive, St. Louis, MO 63130
AB: The Trans-Antarctic Mountains Seismic Experiment (TAMSEIS), a temporary deployment of broadband seismographs in Antarctica, offers a unique opportunity to observe nearly antipodal PKPdf phases propagating sub-parallel to the axis of inner core anisotropy. Such phases are highly important for understanding inner core anisotropy but are rare in the global dataset. The 43 seismographs extend from the Ross Island region (-75, 163) to the East Antarctic Plateau (-82, 102) and, together with the South Pole station, provide a wide region of high latitude coverage. The seismographs were deployed in November, 2001 and will be recovered in December, 2003. Noise characteristics of the stations are good, as indicated by high quality recording of small events from the Arctic ridge, and reliability is generally excellent from November through March. An Arctic Ridge earthquake (Dec 8, 2001, Mw 5.2) was well observed at distances of 169 to 176 degrees. PKPdf anomalies relative to IASPEI91, determined by waveform correlation, range from about -4.3 s to -6.0 s, and PKPdf-ab anomalies range from -4.0 to -6.4 s. PKPdf-ab anomalies show a stronger correlation with df anomalies rather than ab anomalies, suggesting that the signal in the differential anomalies is largely accumulated along the df path. Both the PKPdf and the PKPdf-ab anomalies show a trend towards larger anomalies for smaller ray angles (relative to the earth's spin axis) in the inner core. This trend suggests strong anisotropy near the center of the inner core and is inconsistent with recent models for inner core anisotropy that contain an innermost inner core of reduced anisotropy or different fast axis orientation. Alternatively, the observed travel time variations could be caused by strong inner core heterogeneity. Several other events are well observed at distances of 148-156 degrees. These waveforms show no evidence of a secondary df phase arrival previously observed on different polar paths and used to infer a discontinuity in the uppermost inner core. Instead, PKPdf arrivals at all distances seem to show high intrinsic attenuation.
DE: 7203 Body wave propagation
DE: 7207 Core and mantle
DE: 8115 Core processes (1507)
SC: Seismology [S]
MN: 2003 Fall Meeting