Abstract
A technique for detecting intermediate-period (6–12 s) Sd P phases converted from S to P at a depth d in the source region is described. Previously, these phases were detected in short-period array recordings of deep events. The 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 for the difference of epicentral distance. Using this technique, the phases converted from discontinuities at around 660 km, 860 km, 1070 km, and ‘1170’ km depths beneath Sunda arc are detected at seismograph stations in central and eastern Asia. Our data on ‘1070 km’ discontinuity are very consistent with 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 discontinuities 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.
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