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
JSS006 Poster presentation 1945
Rupture characteristics and deformation partitioning in the Himalayan
front of NW Himalaya: evidence from a post 1335 AD earthquake
Dr. Anand Pandey
National Geophysical Research Institute Scientist
Prabha Pandey, G.D. Singh, G.V. Ravi Prasad, K. Dutta, D. K. Ray, Madhukar Milki
The Himalayan Frontal Thrust (HFT) is the southernmost terrain defining intracrustal fault marked by
the underthrusting of the Indo-Gangetic alluvium under the Siwaliks of the Sub-Himalaya. The HFT is
physiographically recognized as an abrupt topographic break separating the Siwalik range from the
Piedmont/alluvial plain. The Sub-Himalayan belt in the hangingwall of the HFT has attained a height of
~900 m above MSL (or ~600 m relative relief with reference to the HFT outcrops) during Late-
Quaternary period. Invariably, the Siwalik rocks are observed in anticlinal disposition close to HFT zone
and the folding has been attributed to fault-bend folding. This led to the growth and upliftment of
different levels of strath terraces in response to the fold growth related to activity on HFT. Some recent
active tectonic studies and paleoseismic investigations have argued for the growth related to episodic
seismic slip on the HFT. The surface ruptures of some past earthquakes have also been documented in
the HFT zone, which is recognized as the locus of primary earthquake rupture front of the active
Himalayan mountain belt.We explored the mountain front and HFT zone on the same line in parts of
NW Himalaya. Three levels of strath terraces and shear zone patterns in the growing Dhanaura anticline
have been mapped along transverse stream sections in Himachal Sub-Himalaya. A surface rupture scarp
(5-7 m high) with considerable lateral extension has been identified in HFT zone. A trenching
experiment across the scarp in lower T1 terrace near village Kathgarh has been carried out. The trench
lie between two previously explored trenches (Kumar et al., 2006) at 1 and 5 km on either side in the
region. In the trench section, two imbricate splays of surface rupture fault are observed, thrusting the
Middle Siwalik mudstone/sandstone over the Holocene deposits. The Holocene terrace deposits have
been folded by fault-propagation mechanism with thinning and truncation of the overturned limb by the
fault. The charcoal samples within 50cm of the pre-earthquake surface in the footwall, marked by soil
development, have yielded calibrated 14C ages older than AD 133585 (2σ) suggesting that the event
postdates the deposit containing charcoal. However, a distinctly contrasting structural expression in the
trench has been observed with reference to the previous trenches by Kumar et al., 2006. Analyzing the
deformation characteristics of rupture in the trenches and in the outcrops within the HFT zone, it is
obvious to infer that the rupture is partitioned into various splay faults near the surface giving rise to
variability in their structural expressions. This also suggests that the rupture front behaves more like en
echelon fault system in the Himalayan front rather than a single discreet fault.
Keywords: paleoseismology, rupture, himalaya