Evidence for an elevated 410 km discontinuity below the Luzon, Philippines region and transition zone properties using seismic stations in Taiwan and earthquake sources to the south
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences. 2007
Received: 22 May 2006
Accepted: 12 March 2007
Published: 27 June 2007
P waves from earthquakes south of Taiwan, recorded by seismic stations within and around Taiwan, were used to define two average wavespeed models WPSP01 and WPSP02 for the upper mantle and transition zone below the Luzon region. Wavespeeds are characteristic of oceanic upper mantle. The 410-km discontinuity, however, appears to be elevated to about 325 km depth, based on clear identification of the travel-time branch produced by refraction within the transition zone, and estimates of its first and second derivatives with respect to distance. A plausible explanation is low temperatures within the subducted South China Sea plate. The data also imply relatively low wavespeed jumps of 0.6–1.0% and 1.1–1.5% respectively across the elevated 410-km discontinuity and a lower discontinuity at 676 km depth, and high wavespeed gradients in the transition zone. Phase-weighted stacking on a cluster of short-period seismograms with first arrival energy from within the transition zone provides independent support for the validity of the models; later arrivals are detected close to the predicted times and slownesses for energy emerging from the lowermost upper mantle and the top of the lower mantle. An additional arrival on the stacks may be caused by a localized discontinuity near 530 km depth.