Contrasts in regional seismic wave propagation to station WMQ in central Asia

The varying character of regional seismic phase propagation in central Asia is well illustrated by the character of the seismograms at the Urumqi station (WMQ) in Xinjiang, China. Three different classes of behaviour are seen for propagation across the relatively stable platforms to the north, along the Tien Shan chain and across the Tarim basin from Tibet. These paths demonstrate the influences of the variation in crustal structure on the regional phases particularly the Tibetan plateau and its north and south boundaries, but also the influence of the Pamirs on propagation along the Tien Shan. As an interference phase, Lg is sensitive to changes in crustal structure and can be used to study crustal heterogeneity. Lg can propagate along the Tien Shan chain trapped by the gradients in structure on either side, but with a progressive loss in amplitude and frequency. However, once the northern boundary of Tibet is crossed the high frequencies in the Lg train are lost and there is little recognizable arrival. The amplitude ratios of the different crustal phases Pn, Sn and Lg can be used to characterize the behaviour and the different classes of propagation can be further separated when account is made of the frequency content in the Lg window. The influence of the transitions in crustal structure at the northern edge of the Tibetan plateau on propagation to WMQ is investigated using numerical modelling with the pseudospectral method for models of crustal variations, this demonstrates how the crustal pinch at the northern boundary operates to modify the fairly efficient propagation inside Tibet with substantial transfer of energy into the mantle.