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Himalayan Geology, Vol. 30 (2), 2009, pp.139-145 Printed in India
Recent Seismicity and Stress
pattern in NW Himalaya
DILIP K. YADAV, NARESH KUMAR, AJAY
PAUL
Wadia Institute of Himalayan
Geology, Dehra Dun-248001, India
Abstract:
Recent micro-seismicity and
fault plane solutions (FPSs) of earthquakes show variable tectonic
deformation in the India-Eurasia continent-continent collision Zone. The
seismicity data from 2004-2007 containing earthquakes in the magnitude range
1.0-4.9 was divided into three parts to observe the tectonic behavior along
major thrusts such as Main Central Thrust (MCT), Main Boundary Thrust (MBT)
and Himalayan Frontal Thrust (HFT) at different places in NW Himalaya. The
recent and earlier seismicity shows three clusters of events along these
major faults and small lineaments, first cluster is located in Kangra-Chamba
region (epicenter zone of great M8.0 Kangra earthquake of 1905), second
cluster is in the Garhwal region, and the third cluster is in Kumaon -
Dharchula region of NW Himalaya. FPSs are determined for the earthquakes
having at least 10 P-wave first motion polarities and accurate station
azimuthal coverage. The P-T axes obtained from these FPSs are plotted on a
focal sphere to observe the P-T axes distribution. Most of the P-axes of
cluster-I are concentrated near the periphery indicating thrust faulting
with majority of P-axes trending NNE. In cluster-II, FPSs of 14 events
indicates that most of the deformation was caused due to thrust and
strike-slip movements, where P-axes are concentrated near periphery.
The Stress tensor inversion of
these fault plane solutions (FPSs) indicate the stress regime prevailing in
these sectors. Cluster-I indicate dominant NNE directed compressive stress
regime which may be correlated with the prevailing stress condition due
northward movement of Indian Plate. However in cluster-II the maximum
compressive stress (Paxis) is directed along NW direction, which may be due
to two compressive regimes in this region. In the western part of this
cluster, one of these two compressive regimes is developed due to the MBT
and Panjal Thrust (PT) that exerts force to NNE direction while the other
regime is due to Rampur Window (RW) and HHC (Higher Himalayan Crystalline)
that are exerting force towards SW. As a result whole of this study indicate
that the resultant compressive stress is towards NW direction in the region
between Shimla and Dehradun. |
