Mapping the edge of subducted slabs in the lower mantle beneath southern Asia

Abstract

SUMMARY We investigate the presence of seismic structures in the Earth's mantle by searching for seismic signals that travel off the great circle path direction and are reflected or scattered off structures in the lower mantle. We focus on areas of current and past subduction beneath Eurasia by using events from Indonesia and Japan recorded at the broad-band stations in Germany, Morocco and Namibia. Applying seismic array techniques, we measure the direction and traveltime of the out-of-plane arrivals and backtrace them to their location of reflection/scattering. We backtrace the signals as P-to-P and S-to-P waves and extend the methodology to P-to-S waves. There seems to be a low number of reflection points in the regions beneath Eurasia in our study. Investigating possible causes, we find that the focal mechanism influences the presence of out-of-plane reflected waves. However, the potential coverage with our data set is large and should allow detection, but there may potentially be few seismically visible structures in the region. Most of our backtraced reflectors are located beneath southern Asia and are found shallower than 1500 km depth. They correlate well with the edges of prominent high velocity anomalies in tomographic inversions beneath southern Asia, which have been interpreted as remnants of fossil slabs of the subduction of the Tethys Oceans. We also observe few reflectors deeper than 1600 km that are located away from subducting regions and their positions coincide with the eastern edge of the African low velocity anomaly. These observations suggest that the presence of reflectors in the mid-lower mantle is not exclusively related to current or past subducting regions, but widespread throughout the mantle.