Master student YE Lingling and her teacher LI Juan report new results from modelling of the regional SH wavefield from a dense seismic array to constrain the MTZ structure beneath northeast China west of the Japan subduction zone. By modelling triplication waveforms we show that a 130-km-thick layer of increased seismic velocities is overlying a broad slightly depressed 660 km discontinuity.

This anomaly can be associated with the westward extension of the stagnant Pacific slab that is deflected and still cold. They interpret that the fast eastward retreat of the Japan Trench facilitated the slab flattening when the subducting oceanic lithosphere hit the bottom of the transition zone, and the leading edge of the slab is currently trapped in a water-bearing mantle beneath northeast China.

The resolved shear wave velocity structure might give new clues to compositional and/or thermal anomaly identification, and for an understanding of the dynamic processes beneath the westward portion of the subducting Pacific slab.

Figure 1. A schematic cross-section illustrating the eastward retreat scenario of subducting Pacific slab with its downgoing, bottoming and flattening parts. (Image by YE)

Ye et al. A stagnant slab in a water-bearing mantle transition zone beneath northeast China: implications from regional SH waveform modeling. Geophysical Journal International. 2011, 186(2): 706-710