Broadband converted phases from midmantle discontinuities

A technique for detecting intermediate-period (6-12 s) SdP phases converted from S to P at a depth d in the source region is described. Previously, th ese phases were detected in short-period array recordings of deep events. T he main idea of our technique is to deconvolve the vertical component of a single record by the S waveform, and to stack the deconvolved components of a number of records, with appropriate time-shift corrections accounting fo r the difference of epicentral distance. Using this technique, the phases c onverted from discontinuities at around 660 km, 860 km, 1070 km, and 1170 k m depths beneath Sunda are are detected at seismograph stations in central and eastern Asia. Our data on '1070 km' discontinuity are very consistent w ith those inferred from short-period recordings of the same events at the J -array in Japan (Niu and Kawakatsu, 1997), but favour a few different disco ntinuities in the midmantle, rather than one with a strongly variable depth . When compared with a tomographic model of the mantle for the same region, our data suggest that '1070 km' discontinuity may act as a barrier for the downgoing lithospheric slabs.