HR: 0830h
AN: S21E-0379    [PDF]
TI: Anomalous top layer in the inner core beneath the eastern hemisphere
AU: * Yu, W
EM: yu@mantle.geo.sunysb.edu
AF: Dept. of Geosciences, SUNY at Stony Brook, Stony Brook, NY 11794-2100 United States
AU: Wen, L
EM: Lianxing.Wen@sunysb.edu
AF: Dept. of Geosciences, SUNY at Stony Brook, Stony Brook, NY 11794-2100 United States
AU: Niu, F
EM: niu@rice.edu
AF: Dept. of Earth Science, Rice University, 6100 Main Street, Houston, TX 77005 United States
AB: Recent studies reported hemispheric variations in seismic velocity and attenuation in the top of the inner core. It, however, remains unclear how the inner core hemisphericity extends deep in the inner core. Here, we analyze PKPbc-PKIKP and PKiKP-PKIKP waveforms collected from the Global Seismographic Network (GSN), regional recordings from the German Regional Seismic Network (GRSN) and Graefenberg (GRF) sampling along the equatorial path (the ray path whose ray angle is larger than 35$^{\circ}$ from the Earth's rotation axis). The observed global and regional PKPbc-PKIKP differential traveltimes and PKIKP/PKPbc amplitude ratios suggest a simple W2 model (Wen/Niu:2002) in the western hemisphere with a constant velocity gradient of 0.049(km/sec)/100km and a Q value of 600 in the top 400 km of the inner core. In the eastern hemisphere, the data require a change of velocity gradient and Q value at about 235 km below the inner core boundary (ICB). Based on forward modeling, we construct radial velocity and attenuation models in the eastern hemisphere which can explain both the PKiKP-PKIKP and PKPbc-PKIKP observations. The inner core in the eastern hemisphere has a flat velocity gradient extending to about 235 km below the ICB. We test two solutions for the velocity models in the deeper portion of the inner core, with one having a first-order discontinuity at 235 km below the ICB with a velocity jump of 0.07(km/sec) followed by the PREM gradient, and the other having a gradual velocity transition with 0.1(km/sec)/100km gradient extended from 235 km to 375 km below the ICB followed by the PREM gradient. The observed traveltimes exclude the sharp discontinuity velocity model, as it predicts a kink in differential traveltimes at distance of 151$^{\circ}$-152$^{\circ}$ which is not observed in the global and regional datasets. The observed PKIKP/PKPbc amplitude ratios can be best explained by a step function of attenuation with a Q value of 250 at the top 300 km and a Q value of 600 at 300-400 km below the ICB. The top portion of the inner core in the eastern hemisphere is anomalous compared to the rest of the inner core, in having a flat velocity gradient, higher velocities and higher attenuation.
DE: 7200 SEISMOLOGY
DE: 7207 Core and mantle
DE: 7260 Theory and modeling
SC: Seismology [S]
MN: 2003 Fall Meeting