HR: 09:45h
AN: T31B-08    [PDF]
TI: Physical property of a chemical heterogeneity in the mid-mantle: seismic evidence for an oceanic crust in the mid-mantle
AU: * Niu, F
EM: niu@rice.edu
AF: Department of Earth Science, Rice University, 6100 Main Street, Houston, 770 77005 United States
AU: Kawakatsu, H
EM: hitosi@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 114 Japan
AU: Fukao, Y
EM: fukao@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 114 Japan
AB: A clear later phase approximately 80 s after the direct P-wave is observed in most of individual seismograms recorded by a short-period seismometer network in Japan (J-array) from a cluster of deep earthquakes that occurred at the northern Mariana subduction zone. This phase 1) shows a P-wave particle motion; 2) arrives later from earthquakes with shallower focal depths; 3) has a steeper incident angle than that of P wave; and 4) shows a deviation of a few degrees in the arrival azimuth from that of P wave. We interpret it as an S to P converted wave which takes off downward from the source and is reflected at a velocity discontinuity (reflector) below the earthquakes. Applying an inversion technique to the data set shows that the seismic reflector dips toward southwest by about 20 at 24.25$^{\sim}$N 144.75$^{\sim}$E, and at a depth of ~1115 km with a lateral extension at least 100 x 100 km. The location corresponds to the lower edge of a high-velocity anomaly in global tomographic models. Amplitude and waveform analyses suggest a decrease of S-wave velocity by 2-6% and an increase of density by 2-9% within the reflector. There is almost no difference in P-wave velocity ($<$1%) between the reflector and the surrounding mantle. The estimated thickness of the reflector is about 12 km. These observations indicate that the observed seismic structure is more likely to be a chemical reservoir rather than a purely thermal anomaly. The seismic reflector might be a piece of subducted oceanic crust, as suggested by a previous study. It also could be related to the break down of the D-phase of dense hydrous magnesium silicates (DHMS) at mid-mantle pressure condition reported by recent mineral physics studies. Both scenarios imply that either mechanical or chemical segregation might occur within the subducted slab at mid-mantle condition.
DE: 7203 Body wave propagation
DE: 7207 Core and mantle
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting