HR: 16:30h
AN: S14A-03    [Abstracts]
TI: Seismic velocity anomalies in the D" layer beneath Hawaii
AU: * TO, A
EM: ato@jamstec.go.jp
AF: JAMSTEC, 2-15 Natsushima, Yokosuka, 237-0061, Japan
AU: Fukao, Y
EM: fukao@jamstec.go.jp
AF: JAMSTEC, 2-15 Natsushima, Yokosuka, 237-0061, Japan
AB: Global shear velocity tomographic models show two large-scale low velocity structures, so called superplumes, in the lower mantle, under Africa and under the mid-Pacific. Sharp lateral velocity changes have been documented at some borders of the superplumes. Here, we report the prominent postcursors to the Sdiff waves which sample the D" layer beneath Hawaii. The tomographic models show that this region corresponds to the northern boundary of the Pacific superplume. The events and stations are located in Papua New Guinia and in southern U.S, respectively. We first measured travel time anomalies of S and Sdiff phases with respect to PREM. They correlate well with the Vs anomaly distribution of the D" layer shown by the tomographic models. Depending on whether the paths mostly sample the D" layer within or outside of the superplume, the anomalies change by about 10 seconds within a 20 degrees change of azimuth. The result indicates strong lateral Vs heterogeneities in this region, such as the ones reported for other borders of the superplumes. The postcursors show the following features: 1) they arrive 20 to 40 seconds after the Sdiff waves 2) the arrival times strongly depend on station azimuths, rather than epicentral distances. The later phase arrives later for northern stations at smaller azimuths and earlier for southern stations at greater azimuths. This feature is in contrast to the main Sdiff phase which progressively delays to the south. Consequently the first and second arrivals come closer along southern paths than along northern paths. We applied beamforming in order to find the incident angle and direction of the second arrival. The incident angle is almost the same between the first and second arrivals but the incident azimuth is 7 degrees southward relative to the azimuth of the first arrival. There are similar and dissimilar points between this postcursor and the previously reported Sdiff splitting which sample the southern borders of the Pacific and African superplumes (Wen, 2001, To et al 2005). The newly observed postcursor is well separated from the main Sdiff phase and its amplitude is as large as the main phase, whereas the postcursor from either southern Pacific or Africa has smaller amplitude and overlaps in time with the main phase. By conducting waveform modeling for several simple structures, we discuss the origin of the later phase from the D" layer beneath Hawaii.
DE: 1213 Earth's interior: dynamics (1507, 7207, 7208, 8115, 8120)
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
SC: Seismology [S]
MN: 2007 Fall Meeting