Crustal and Upper Mantle Structures of the High Himalaya

The crustal and mantle structures in the areas around the high Himalayan ranges are still enigmatic. The broadband data recorded at 28 stations in eastern Nepal and southern Tibet in 2001-2002 provide the basis for resolving some of the details. We used two approaches in the present study. First, the two-station differential surface wave phase velocity dispersion technique of Tanimoto and Prindle-Sheldrake (2002) and Prindle-Sheldrake and Tanimoto (submitted) is used to develop phase velocity maps for different station pairs (only those with separations greater than 50 km are selected), within a narrow range of the back-azimuths - usually less than 3.0 degrees. The assumption is that the differences in such waveforms are attributed to the path effects between the two stations. Under such conditions 151 events were recovered with good signals within specific frequency bands. Phase velocities between station pairs are inverted to generate phase velocity maps for an area discretized into 0.20 by 0.25 degree blocks (approximately 20 km) in latitude and longitude. Using a technique that approximately takes into account the finite frequency effect (Tanimoto, 2003; Prindle-Sheldrake and Tanimoto, submitted) a set of maps have been obtained for frequencies 0.01- 0.03 Hz. Variance reductions for the phase velocity maps are above 90 % in all cases. Current results indicate that at higher frequencies the main feature of higher velocities in the southern part of the region and lower velocities in the north dominates. At the lowest frequency (0.01 Hz) a regional low velocity feature under the high Himalaya is found. This last feature seems may imply the existence of a low velocity feature at depth below the high Himalaya, thus coming into conflict with results of a receiver function analysis (Schulte-Pelkum, personal communication) indicating the existence of a transitional crust. Inversion of the phase velocity maps into shear velocity with removal of elevation effect will be necessary. Secondly we are attempting to provide additional constraints by using teleseismic/local earthquake joint inversion (Roecker, ) to obtain the crustal and upper mantle velocities. Preliminary results show the differences between what is most probably the Indian plate lithosphere and that of the Eurasian plate. To resolve this question the spatial resolution for the joint version will have to be successively improved.