HR: 1340h
AN: T43A-1311    [Abstracts]
TI: Regional variation of PKP C-diff slowness observed by high-dense network -implication for lateral velocity variation of the lowermost outer core
AU: * Ohtaki, T
EM: t-ohtaki@aist.go.jp
AF: Geological Survey of Japan/AIST, AIST Tsukuba Central 7, Tsukuba, 305-8567 Japan
AU: Kawakatsu, H
EM: hitosi@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AB: Clear core phases are observed at a recently deployed Japanese seismic network (Hi-net) for two intermediate depth events which occurred beneath S. Bolivia on 2001/06/29 and beneath Argentina on 2002/09/24. The epicentral distances are between 144 and 161 degrees, and between 150 and 164 degrees, respectively. The differential travel times of PKP(BC) or PKP(Cdiff) minus PKP(DF) for the S. Bolivia event suit calculated results using by the previous model of Kaneshima et al. (1994). The differential travel time residuals for the Argentina event show positive value, and become larger as the epicentral distance becomes large. Although the above results suggest that there is anomalous structure in the inner or outer core along the ray paths for the Argentina event, it is unclear which T(DF) or T(BC or Cdiff) is anomalous. Hi-net is a dense short-period seismic network which consists of about 700 stations in almost whole extent of Japan. Close look at core phases can be carried out using data from Hi-net. We checked the slowness of PKP(DF) and PKP(Cdiff) for the events. We select data whose epicentral distance is greater than 153 degree. The slowness of PKP(DF) and PKP(Cdiff) for the S. Bolivia event, and that of PKP(DF) for the Argentina event are almost same as those of PREM. However, the slowness of PKP(Cdiff) for the Argentina event is 7 percent larger than that of PREM. Thus we can conclude that the large slowness of PKP(Cdiff) is a main cause of the large differential residuals. This result suggests that a low velocity anomaly exists in the lowermost outer core which the PKP(Cdiff) for the Argentina event samples, and that the velocity structure in the inner core along the rays for the two events and in the outer core along the rays for the Argentina event sample is close to the normal, although the influence of the heterogeneous structure beneath stations and D_h cannot be ruled out yet. The central points of the diffracted rays on the inner core boundary are located in between 7 and 21N, S. of those for the S. Bolivia event. These results may imply that the velocity of the lowermost outer core is slower in the equatorial area than in the mid-latitude.
DE: 8115 Core processes (1507)
DE: 7207 Core and mantle
DE: 1213 Earth's interior--dynamics (8115, 8120)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting