Teleseismic residuals at the Lasa-USGS Extended Array and their interpretation in terms of crust and upper-mantle structure

P-wave traveltime data for nine teleseismic events recorded by the central instruments of the Lasa (Large Aperture Seismic Array) subarrays and 20 portable instruments set up by the U.S. Geological Survey (USGS) to augment the E and F rings have been used to study the pattern of Lasa residuals. The traveltime data for each event were fitted by least squares to a straight line, and the residual was defined as the difference between the observed traveltime and the traveltime predicted by the least-squares line. Residuals were also calculated by using the apparent velocities predicted by Herrin traveltime tables as reference. Single-layer three-dimensional crustal models with a P velocity of 6.5 km/sec were evolved by using the averaged residuals at each instrument from the nine events. The models show a synclinal M discontinuity structure with an E-NE strike. The crust is 56 km toward the middle of Lasa and thins to about 40 km to the north and to the south. Although 45% of the observed residuals (first-order residuals) have magnitudes greater than 0.2 sec, it was found that, after correction for the crustal thickness variations, only 19% of the remaining residuals (second-order residuals) had magnitudes greater than 0.2 sec. The second-order residuals may be associated with the fluctuations in thickness of the low-velocity layer in the upper mantle.