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We present a summary of measurements of azimuthal anisotropy in the continental mantle based on the SKS technique and performed mostly with the active participation of the authors. The directions of polarization of the fast quasi-shear wave and the time delays between the quasi-shear waves are obtained at nearly 70 locations in all continents, except Antarctica. These data are interpreted in terms of lattice-preferred orientation of olivine which is caused by deformations in the mantle. The depth interval responsible for anisotropy is unknown but the data suggest that it may reach at least 300 km. The fast directions in SKS do not show clear correlation with the fast directions of the teleseismic P at the same seismograph stations.In the regions of present-day convergence the fast direction of anisotropy usually aligns with the plate boundary. This correlation implies that the direction of shortening is the same in the crust and the upper mantle. In the regions of rifting, the inferred direction of mantle flow usually aligns with the direction of extension in the crust.Outside the regions of recent tectonic activity we, most likely, observe a combined effect of frozen anisotropy in the subcrustal lithosphere and of recently formed anisotropy in the asthenosphere. On a global scale, in these regions there is a positive correlation between the absolute plate velocity directions and the fast directions of anisotropy. The correlation is especially strong in central and eastern parts of North America. A clear absence of any evidence of large-scale azimuthal anisotropy in the data of long-range refraction profiling for the upper 100 km of the mantle of that region implies that the effect in SKS is generated mainly at greater depths, in the asthenosphere. Orientation of olivine at these depths reflects recent and present-day flow in the mantle rather than processes of a distant geologic past.

Summary Seismic stratification of the upper mantle is investigated by applying two complementary techniques to the records of the Graefenberg array in southern Germany. The anisotropic P receiver function technique (Kosarev et al. 1984; Vinnik & Montagner 1996) is modified by using summary seismic events instead of individual events and different weighting functions instead of the same function for the harmonic angular analysis of the SV and T components of the Pds phases. The summary events provide better separation of the second azimuthal harmonic than the individual events. The parameters of the second harmonics of SV and T thus evaluated should be similar if they reflect the effects of azimuthal anisotropy. This can be used as a criterion to identify the anisotropy. To detect the Sdp phases and their azimuthal variations caused by azimuthal anisotropy we have developed a stacking technique, which can be termed the S receiver function technique It includes axis rotation to separate interfering P and S arrivals, determination of the principal (M) component of the S‐wave motion, deconvolution of the P components of many recordings by their respective M components and stacking of the deconvolved P components with weights depending on the level of noise and the angle between the M direction and the backazimuth of the event. Both techniques yield consistent results for the Graefenberg array. As indicated by the P receiver functions, the upper layer of the mantle between the Moho and 80 km depth is anisotropic with dVs/Vs around 0.03 and the fast direction close to 20° clockwise from north. The fast direction of anisotropy below this layer is around 110°, The boundary between the upper and the lower anisotropic layers is manifested by the detectable Pds and Sdp converted phases. Shear wave splitting in SKS is strongly dominated by azimuthal anisotropy in the lower layer (asthenosphere).

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Arrival times of compressional (P) and shear (S) waves generated by earthquakes at local and teleseismic distances and recorded by seismographs located in the westem and central Tien Shan are used to determine one-and three-dimensional elastic wave velocity structures of the crust and upper mantle beneath the mountain belt. The best fit one-dimensional structures suggest that the average depth of the Mohorovicic discontinuity in this area is 50 km. The three-dimensional structure of the upper crust reveals thick sediments within each of the major depressions in the region. A 7 km-thick wedge of sediment beneath the Chu Depression is outlined at depth by a south dipping plane of seismic activity, suggesting the presence of an active decollemont. These low velocities extend continuously to the southeast toward Issyk-Kul, suggesting a structural relationship between the two. However, rather than being consumed, it appears that Issyk-Kul is overthrusting the surrounding ranges. The low-velocity sediments in the Fergana basin reach depths of 10 km and are bounded on three sides by amorphous bands of seismicity. Velocities at midcrustal depths generally are lower beneath the central Tien Shan than beneath the western Tien Shan. This pattern becomes more evident in the uppermost mantle, with P velocity contrasts of as much as 10% across a boundary that corresponds roughly to the geographical position of the Talasso-Fergana fault. The low velocities beneath the central Tien Shan exceed 150 km depth but do not appear to be deeper than 300 km depth. There is no evidence for a lithospheric root beneath this part of the range; rather, the low velocities imply the presence of a positive buoyancy force uplifting the mountains. Evidence that this low-velocity region existed before the collision suggests that the Tien Shan may not owe its rejuvenation simply to its location at the northern edge of a strong Tarim basin but rather to an anomalous upper mantle that was easier to deform than the surrounding lithosphere. the Tien Shan can be divided into three fault bounded units [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, 1987; Krestnikov, 1962]. The whole area appears to have been stable throughout the Mesozoic, with very little relief in the late Cretaceous. Tectonic activity resumed in the Oligocene, presumably as a consequence of the collision of India with Eurasia and has continued to the present day. The present high level of activity is evidenced in northsouth shortening accommodated along both east-west trending thrust faults and northwest-southeast trending strike-slip faults such as the Talasso-Fergana fault (Figure 1). The region is seismically very active; several earthquakes with M s > 8.0 have occurred in this area since 1900. Analyses of large earthquak...

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S U M M A R Y The paper gives a detailed description of the technique for measuring azimuthal anisotropy which was suggested earlier by Vinnik, Kosarev & Makeyeva (1984) and used by Kind et ai. (1985). The technique is based on the observations of long-period converted phases like SKS. Additionally, we describe a generalization of this approach which makes use of long-period S-waves of arbitrary polarization. Both modifications were applied to records of the GRF array in southern Germany. The results of this analysis, if combined with the data on P,, velocities (Bamford 1977), suggest that the direction of the fast velocity in the lithosphere of the region varies with depth.