A study on physical property of crustal material and seismogenic environment in northeastern Pamir

Abstract

2-D crustal structure and velocity ratio are obtained by processing S-wave data from two wide-angle reflection/refraction profiles in and around Jiashi in northeastern Pamir, with the result of P-wave data taken into consideration. The result shows that: 1) Average crustal velocity ratio is obviously higher in Tarim block than in West Kunlun Mts. and Tianshan fold zone, which reflects its crustal physical property of “hardness” and stability. The relatively low but normal velocity ratio (Poisson’s ratio) of the lower crust indicates that the “downward thrusting” of Tarim basin is the main feature of crustal movement in this area. 2) The rock layer in the upper crust of Tianshan fold zone is relatively “soft”, which makes it prone to rupture and stress energy release. This is the primary tectonic factor for the concentration of small earthquakes in this area. 3) Jiashi is located right over the apex or the inflection point of the updoming lower crustal interface C and the crust-mantle boundary, which is the deep structural background for the occurrence of strong earthquakes. The alternate variation of vp/vS near the block boundaries and the complicated configuration of the interfaces in the upper and middle part of the upper crust form a particular structural environment for the Jiashi strong earthquake swarm. vp/vs is comparatively high and shear modulus is low at the focal region, which may be the main reason for the low stress drop of the Jiashi strong earthquake swarm.