Open AccessAn enhanced image of the Pamir–Hindu Kush seismic zone from relocated earthquake hypocentres
Author(s) -
Pegler G.,
Das S.
Publication year - 1998
Publication title -
geophysical journal international
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0956-540X
DOI - 10.1046/j.1365-246x.1998.00582.x
Subject(s) - geology , seismology , induced seismicity , seismic zone , epicenter
Summary We determine the shape of the seismic zone in the Pamir–Hindu Kush region defined by [30–42° N, 68–78° E] by obtaining improved hypocentral locations with 90 per cent confidence limits of less than 30 km (the depth error for¯ most of the earthquakes) of about 6000 shallow and intermediate‐depth earthquakes. Available S and depth‐phase arrival times are also used together with the P ‐wave arrival times in the joint hypocentre determination technique. To obtain the best possible 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 obtained by using the relocated hypocentres implies that the intermediate‐depth seismicity in the Pamir–Hindu Kush region is most simply explained 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‐dipping Hindu Kush seismic zone through to its overturning at its eastern end beneath the Pamirs, where it dips to the southeast; (2) the curvature and forking of the subducting slab at depths greater than 200 km 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 with no extension to shallower depths under the Pamirs, and with a persistent gap between the intermediate and shallow seismicity in the northern Pamirs; and (4) the unusual horizontal T‐axes for intermediate‐depth earthquakes of the Pamir seismic zone, which align with its curvature. This study shows that the seismic zone under the Hindu Kush has stress axes which follow the classical pattern for subducting slabs controlled by gravity, whereas the Pamir region has horizontal T‐axes that follow the trend of the contorted seismic zone. This suggests that the Pamir seismic zone is a slab deformed due to flow in the upper mantle.