AGU Abstract

T54B-06. Velocity Field in the NW Himalayan Syntaxis: Implications for Future Seismicity
Trans Amer. Geophys. Un. Dec 2011.

VELOCITY FIELD IN THE NW HIMALAYAN SYNTAXIS: IMPLICATIONS FOR FUTURE SEISMICITY
PRESENTATION TYPE:

Roger G Bilham, Walter Szeliga, Bikram Singh Bali, Asif Khan, Abdul Wahab, Faisal Khan

For the past eight years we have monitored crustal deformation in Ladakh, the
Karakoram, Kohistan, Zanskar, Salt Range and Pir Pinjal, using a combination of fixed and campaign GPS measurements, to provide quantitative constraints on the rates of convergence in the NW syntaxis of the Himalaya. We find a 13-17 mm convergence rate with maximum SSW velocity gradients NE of the Kashmir Valley beneath the Zanskar range, and maximum SSE directed gradients NW of the Peshawar basin beneath the Kohistan range. The inferred locking line appear to follow the 3.5 km contour as it does elsewhere in the Himalaya, however, this results in a 200 km wide décollement, twice the width of the central Himalaya. The SSE velocity of the Potwar Plateau is 3 mm/yr, significantly slower than the 6-12 mm/yr inferred from geological offsets along the Kalabagh fault in the past 10 My, and hence an inferred slip deficit exists between the Kohistan Range and the Salt Range. No great earthquakes are known in this area, and it is unclear whether the deficit is annulled
by accelerated salt-decollement creep or by seismic rupture. A brief period of accelerated creep followed the 2005 Kashmir earthquake.

Velocities across the Kashmir Valley and Pir Pinjal are suggestive of a locked décollement (no creep) implying possible SE translation of the entire Kashmir Valley in great earthquakes. Were the segment of the Himalaya between the 2005 Kashmir Earthquake, and the Mw7.8 Kangra earthquake to slip 20 m in a single earthquake, it could do so in a Mw=9. No earthquake of this severity is known, although the historical record includes several earthquakes that may account for partial slip of the decollement. Slip on the Reasi fault north
of the frontal Pir Pinjal range front can account for less than half the observed convergence at this longitude, and although no surface slip has been detected in the ranges fronting the Punjab plains, we deduce that the frontal folds, and associated blind thrusts, may absorb convergence of 5-9 mm/yr.