Directional Variations in Travel-Time Residuals of Teleseismic P Waves in the Crust

Abstract We study the directional variation in travel-time residuals using 13,820 P-wave arrivals from 1,998 teleseismic events (15 D 98, 4.1 mb 7.3) recorded in 1991–1997 by the Kyrgyz Digital Seismic Network (KNET). Based on a modified version of the iasp91 model that accounts for the Kyrgyz crustal thickness beneath KNET, we convert P-wave travel times to travel-time residuals dt. The de-pendence of dt on backazimuth is modeled as one-, two-, and four-lobed variations in a horizontal plane (Backus, 1965). A least-squares fit of the azimuthal variation of dt indicates that the crust in the northern Tien Shan is about 11–15 km thicker than it is in the Kazakh Shield and the Chu Depression. From nine KNET stations, the one-lobe model estimates that the slowest P-wave travel-time direction is 5.0 4.8 (almost directly north) and the magnitude of variation is 1.71 0.13 sec. This result is consistent with an upwelling lower mantle plume. For the two-lobe model, the slowest P-wave travel-time directions (anisotropy term) are 89.7 and 269.7 4.7 (i.e., trending east– west). We find P-wave velocity anisotropy of 2.0%–2.9 % associated with a laye