IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS014 Poster presentation 2294
Seismotectonic model for Kangra-Chamba Region of NW Himalaya
Mr. Naresh Kumar
Department of Geophysics
Jyoti Sharma, S.Mukhopadhyaya
A homogeneous earthquake catalogue with minimum detection threshold of M=4.3 for a period 1975 to
2004 shows that the rupture area of devastating Kangra earthquake of 1905 is still more active than the
adjoining regions of NW Himalaya. Since 2004, a campaign mode seismic array is in operation in this
region for constraining the seismotectonic model. With the improved detection threshold, local 174
earthquakes of magnitude ranging between 1 and 5 are inverted through Local Earthquake Tomography
(LET) to simultaneously determining the average 1D velocity model. Further the data were inverted for
3 dimensional velocity variations that further reduced the location error. Mostly the seismic events are
located in the upper crust above the detachment (decollment) plane, the surface separating the
underthrusting Indian plate from over riding sedimentary wedge of Himalaya. In the depth section, the
foci of large numbers of earthquakes show clear clustering along linear planes seen as sub-surface
extension of the Main Boundary Thrust (MBT) and Punjal Thrust (PT). The first motion P-wave polarity
of 22 well located events of magnitude more than 2.5 recorded at more than 12 stations are used for
fault plane solutions to observe the sub-surface tectonic movement of the region. The clusters of other
small magnitude events around these earthquakes are also used to obtain the composite fault plane
solution for observing the slip movement along tectonic discontinuity. The majority of the fault
mechanisms are of oblique types showing a combination of strike slip component with reverse and
normal fault mechanisms. The north trending upper crustal fault movement is of reverse-faulting with
right-lateral strike-slip motion whose dip angle goes on decreasing with depth. The locus of these fault
plane solutions coupled with plane defined by sharp cut-off depth of crustal seismicity helps to trace the
decolment beneath large part of the Kanra-Chamba of frontal Himalaya. However, in the eastern part of
the region flanking the western part of the Punjab reentrant, a close clustering of epicenters is seen and
in this section a few earthquakes below detachment zone are having normal movement. The deduced
3-D velocity model shows presence of NE-SW trending low velocity zone bordering the Punjab. It is
inferred that the interaction of two thrust faults with perpendicular movement to each other in the
deeper section of the eastern part of the study area is the source of stress concentration to generate
the normal fault movement with left-lateral strike-slip motion in NW-SE direction. The seismic pattern,
velocity structure, fault plane mechanism as well as emerging picture of stress distribution are
integrated to constrain the seismotectonic model of the one of most active section of the NW Himalaya.
Keywords: himalaya, seismotectonics, velocity structure