AN ENHANCED IMAGE OF THE PAMIR HINDU-KUSH SEISMIC ZONE FROM RELOCATEDEARTHQUAKE HYPOCENTERS

We determine the shape of the seismic zone in the Pamir-Hindu Kush reg ion defined by [30-42 degrees N, 68-78 degrees E] by obtaining improve d hypocentral locations with 90 per cent confidence limits of less tha n 30 km (the depth error bar for most of the earthquakes) of about 600 0 shallow and intermediate-depth earthquakes. Available S and depth-ph ase arrival times are also used together with the P-wave arrival times in the joint hypocentre determination technique. To obtain the best p ossible hypocentral locations, the study region is divided into three depth ranges, 0-60, 60-160 and >160 km, The 0-60 km depth zone is then subdivided laterally into 19 blocks, with the deeper regions divided into two blocks each. The improved delineation of the seismic zone obt ained by using the relocated hypocentres implies that the intermediate -depth seismicity in the Pamir-Hindu Kush region is most simply explai ned by a single S-shaped seismic zone, 700 km long and no more than 30 km wide and with most activity concentrated at 100-300 km depth. The main features observed are: (1) the eastward steepening of the north-d ipping Hindu Kush seismic zone through to its overturning at its easte rn end beneath the Pamirs, where it dips to the southeast; (2) the cur vature and forking of the subducting slab at depths greater than 200 k m within the eastern part of the Hindu Kush seismic zone; (3) the very abrupt cut-off in intermediate-depth seismicity at 90-110 km depth wi th no extension to shallower depths under the Pamirs, and with a persi stent gap between the intermediate and shallow seismicity in the north ern Pamirs; and (4) the unusual horizontal T-axes for intermediate-dep th earthquakes of the Pamir seismic zone, which align with its curvatu re. This study shows that the seismic zone under the Hindu Kush has st ress axes which follow the classical pattern for subducting slabs cont rolled by gravity, whereas the Pamir region has horizontal T-axes that follow the trend of the contorted seismic zone. This suggests that th e Pamir seismic zone is a slab deformed due to flow in the upper mantl e.