Observation of a Mid-Mantle Discontinuity beneath Northeast China from S to P Converted Waves Recorded by the USArray Stations

Observation of a Mid-Mantle Discontinuity beneath Northeast China from S to P Converted Waves Recorded by the USArray Stations (a) Geographic locations of the two deep earthquakes near the border of China, Russia, and North Korea. White dashed lines indicate the Wadati-Benioff zone. (b) A portion of the USArray records of the 05/19/2008 deep earthquake. Theoretical travel times of the iasp91 model for the major expected body wave arrivals are indicated by straight lines. Note a clear phase between 40 and 45 s after the direct P arrival (indicated by red arrows) can be seen in most of the individual seismograms. The phase has a slightly negative slowness compared to the P wave. (c) same as (b) for the 02/18/2010 event. Not that in this case the unknown phase arrives at 35-40s after the direct P arrival, a few seconds earlier than those observed from the 05/19/2008 event. (d) Geographic distribution of the S to P conversion points from the 05/19/2008 (black squares) and the 02/18/2010 (blue circles). S-wave velocity perturbations at ~1000 km depth from Grand (2002) is also shown.
Strong and localized seismic discontinuity and reflectors have been observed in the lower mantle at various depths beneath western Pacific subduction zones [Niu and Kawakatsu, 1997; Kaneshima and Helffrich, 1999; Castle and Creager, 1999; Niu et al., 2003]. The lateral extension of these anomalous structures, however, is not well constrained. The information may hold the key to the understanding of the nature of these seismic structures as well as the related mantle processes. The USArray opens a new window for “viewing” and mapping these seismic anomalies. We found a clear later phase ~35-42 s after the direct P wave at most of the USArray recordings of two deep earthquakes that occurred near the border between east Russia and northeast China. The measured incident angle and arriving direction of this arrival suggest that it is an S to P wave converted at ~1000 km below Earth’s surface. The mid-mantle discontinuity has a lateral dimension of greater than 200 km and 50 km along the EW and NS direction, respectively. It dips toward the east by ~12 degrees. The discontinuity is located in a region with a slightly higher P- and S-wave velocity, suggesting that the discontinuity is likely related to the subducted Pacific slab. One possible explanation is the breakdown of hydrous magnesium silicate phases, which is observed to be stable in cold environment at upper-most lower mantle pressure condition.
</p><p>Castle, J. C., and K. C. Creager (1999), A steeply dipping discontinuity in the lower mantle beneath Izu-Bonin, J. Geophys. Res., 104, 7279 – 7292.
</p><p>Kaneshima, S., and G. Helffrich (1999), Dipping lower-velocity layer in the mid-lower mantle: Evidence for geochemical heterogeneity,Science, 283, 1888–1891, 1999.
</p><p>Niu, F., and H. Kawakatsu (1997), Depth variation of the mid-mantle seismic discontinuity, Geophys. Res. Lett., 24, 429– 432.
</p><p>Niu, F., H. Kawakatsu, and Y. Fukao (2003), Seismic evidence for a chemical heterogeneity in the mid-mantle: A strong and slightly dipping seismic reflector beneath the Mariana subduction zone, J. Geophys. Res., 108(B9), 2419, doi:10.1029/2002JB002384,
</p><p>Acknowledgements: Acknowledgments. We thank the EarthScope and IRIS for supplying the USArray data. This work is supported by NSF and the Department of Earth Science, Rice University</p>


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