1. * Partial support for this work came from the National Science Foundation of China under grants 00410785, 40521002 。 Fig.4 Fault plane solutions of intermediate-depth earthquakes. Blackened quadrants of lower hemisphere projections are for compressional first motions. Intermediate-depth Earthquakes in the Pamir-Hindu Kush Region ： Evidences for Collision between two Opposing Subduction Zones * Xiaoting LOU and Jieyuan NING Institute of Theoretical and Applied Geophysics, School of Earth and Space Sciences, Peking University, Beijing 100871, China LOCATED at the western Himalayan syntaxis, the Pamir-Hindu Kush region is one of the most active regions of intermediate-depth earthquakes. The accurate spatial distribution of earthquakes and their focal mechanism solutions can provide constraints on the plate interactions and the tectonic evolution in this region. THERE is an S-shaped intermediate seismic zone in this region. It includes two parts with different strikes and dip directions: Hindu Kush and Pamir. The formation of the two intermediate- depth seismic zones is still under debate. So far, there are two major geodynamic models. One model suggests that there are originally two subduction slabs in opposing polarities. The other one favors a single northward-dipping subduction zone, of which the Pamir part overturns later and forms the current two parts with opposite dip directions. WITH the help of forty years high quality data of International Seismological Center from 1964 to 2003 and a robust relocation technique (Waldhauser and Ellsworth, 2000), we reexamine the configuration of the intermediate-depth earthquakes beneath Pamir-Hindu Kush region. A distinct spatial feature of those intermediate-depth earthquakes is that one W-B zone overlaps another beneath Hindu Kush region. ENLIGHTENED by this picture, we propose that there having been collision between two opposing subduction zones, which gets support from Focal Mechanism Solutions and can match with regional tectonic features. Fig.1 Map view of Intermediate-depth Earthquake distribution. Fig.2 Vertical Profiles of Intermediate-depth Earthquake distribution. DEDUCTION Double-difference relocation of the intermediate-depth earthquakes in Pamir-Hindu Kush region shows that the double-layer structure mentioned by Geoscientists in east Hindu-Kush region is that one W-B zone overlaps another. Abrupt changes in spatial earthquake distribution proclaims that they occurred on opposing slabs. Further support comes from the fault plane solutions. The horizontal compression prevails in the contact region of the opposing W-B zones. In Pamir region, from southwest to northeast, along-strike horizontal axis changes from P to B and then T; P axes gradually change to be perpendicular to the strike of the W-B zone. Geological features also do not support the contortion model. Especially, convex structure of the west syntaxis of the Himalayas means that there is less material from India plate could go down to the mantle beneath Pamir region. Fig. 3 A cartoon of of the tectonic model for Parmir-Hindu Kush region.