S U M M A R YSeveral years of broad-band teleseismic data from the GRSN stations have been analysed for crustal structure using P -t o 4 converted waves at the crustal discontinuities. An inversion technique was developed which applies the ThomsonHaskell formalism for plane waves without slowness integration. The main phases observed are Moho conversions, their multiples in the crust, and conversions at the base of the sediments. The crustal thickness derived from these data is in good agreement with results from other studies. For the Grafenberg stations, we have made a more detailed comparison of our model with a previously published model obtained from refraction seismic experiments. The refraction seismic model contains boundaries with strong velocity contrasts and a significant low-velocity zone, resulting in teleseismic waveforms that are too complicated as compared to the observed simple waveforms. The comparison suggests that a significant low-velocity zone is not required and that internal crustal boundaries are rather smooth.

We determined the structure of the crust beneath the central and western Tien Shan by analyzing broadband (1-20 s) analog seismograms of converted P-SV phases generated by earthquakes at teleseismic distances and recorded by 14 seismograph stations. The one-dimensional structures that best explain the waveforms reveal pronounced differences in crustal velocities east and west of the Talasso-Fergana fault. Specifically, the transition zone between the crust and the mantle east of the fault is about 2 times broader than that west of the fault. Also, velocities at depths between 10 and 35 km are about 10% lower in the east, although the depth range of these lower velocities is not well resolved. The Talasso-Fergana fault is an important boundary for other observables; in addition to the structural discontinuities observed at the surface, the areas east of the Talasso-Fergana fault are associated with abnormally low mantle velocities, outcrops of basalt, low Q, and short-wavelength variations in anisotropy. Integrating these observations, we interpret the broad mantle gradient as being due to vertical intrusions of manfie material into the lower crust. Likewise, the low velocities in the midcrust could be thermally induced or due to the introduction of magmatically derived fluids. The gross contrast in structure east and west of the Talasso-Fergana fault could reflect a contrast in the dynamics of mountain building in these two regions. We postulate that while crustal shortening is the dominant mechanism controlling topography west of the Talasso-Fergana fault, vertical uplift caused by a mass deficiency in the upper mantle may contribute significantly to the generation and maintenance of the high elevations east of the fault. 2000-3000 m elevation. The Tien Shah is bordered to the north and west by the Kazakh shield, the Turanian platform, and the Dzungarian basin and to the south and east by the Tarim massif and the Trans-Tien Shan depression. Based on distinct deposition and accretion histories, the Tien Shan can be divided into three fault bounded units (north, middle, and south [Kravchenko, 1979]). All three units consist largely of sedimentary rocks that formed during the late Proterozoic (in the north) to Cambrian (in the south). These units were accreted onto Eurasia beginning in the early Paleozoic for the northern unit to the late Carboniferous for the southern unit [Burtman, 1975; Krestnikov, 1962]. The whole area appears to have been stable throughout the Sciences, Moscow, 1962. Krestnikov, V.N., I.L. Nersesov, and D.V. Stange, The relationship between the deep structure and Quaternary tectonics of the Pamir and Tien Shan, Tectonophysics, 104, 67-83, 1984. Langston, C.A., Corvalis, Oregon, crustal and upper mantle receiver structure from teleseismic P and S waves, Bull. Wave residuals at stations in Nepal: Evidence for a high velocity region beneath the Karakorum, Geophys. Res. Lett., 18, 1909-1912, 1991. Petersen, N.V., and L.P. Vinnik, Detection of waves convened from P to SV in the crust by multichanne...

Seismic estimates of sharpness of the mantle discontinuities are important for constraining models of composition and temperature in the deep Earth but these data are difficult to obtain. We explore a possibility to determine sharpness of the major mantle discontinuities (those at depths around 400 and 650 km) from the broad‐band records of phases converted from P to S underneath the receiver. Our estimates are obtained from a comparative analysis of waveforms of the converted phases and those of the P waves in the teleseismic records of BRV (Kazakhstan), GRF (Germany), NRE0 (Norway) and YKW (Canada). The data indicate that the shape of the pulse converted at the 400‐km discontinuity is close to that of the P wave whereas the pulse of the 650‐km conversion is substantially broader. We infer from this comparison that the 400‐km discontinuity is sharp (width of the transition zone is less than 5–7 km). The 650‐km transition is modelled by a linear gradient zone 20–30 km thick.

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Mantle Pds (Fig. 1) converted phases are detected in the records of 11 seismograph stations in easternmost Russia and China. These data reveal neither a strong depression on the 660‐km discontinuity nor a layer of partial melting atop the 410‐km discontinuity that were found previously beneath this region in studies using long‐period underside SH reflections and multiple ScS reverberations. Apparently, a significant deepening of the 660‐km discontinuity occurs only if the subducted plate penetrates the discontinuity, which is not the case in the study region. Most estimates of thickness of the mantle transition zone (MTZ) in Eurasia based on the Pds data are a few kilometres larger than the standard value (250 km), and suggest that the average thickness of the MTZ beneath Eurasia is about 10 kilometres more than beneath the surrounding oceans.