Geometry and Kinematics of the Main Frontal Thrust, Himalaya

The highest mountains on Earth, the Himalaya, are the product of 50 million years of collision between India and Eurasia (Yin and Harrison, 2000). The Main Frontal Thrust (MFT) lies at the southern margin of the Himalayan collision zone, and appears to accommodate 50-100% of the shortening across the Himalaya (Lavé et al., 2005).

Three thrust earthquakes of Mw 7.8-8.5 have occurred within the Himalayan orogen during the last century, but no matching surface ruptures have been identified (Lavé et al., 2005). Paleoseismic studies indicate that an earthquake of magnitude up to 8.8 occurred ~1100 AD on the MFT, and did reach the surface (Lavé et al., 2005). Moreover, recent research by Paul Tapponnier and his team at the Earth Observatory of Singapore has identified the surface rupture of the 1934 M8.1 earthquake, suggesting that geological records of surface ruptures exist, but have simply been obscured. This indicates that a rich record of past earthquakes on this fault system may be preserved in the subsurface geology. Deciphering this record, by defining the potential magnitudes, rupture patterns, and recurrence intervals of great earthquakes in this region, is critically important: because of the large populations that now live along the southern margin of the Himalaya, an earthquake of M8.2 or greater is projected to kill over half a million people.

Because of the inconsistent surface record of earthquakes, much of the research regarding this fault zone has been based on deformed terraces (e.g., Lavé and Avouac, 2000). These terraces can provide snapshots of the deformation at discrete intervals. However, interpreting these data requires an understanding of the subsurface fault geometries and kinematics, as blind faults may produce up to twice as much uplift per unit shortening than faults that have broken through to the surface.

I propose to study the MFT and its associated fault splays by acquiring a densely-spaced set of high-resolution seismic reflection profiles across the fault tip in order to assess the geometry and kinematics of the fault. This work will be done in collaboration with Prof. Paul Tapponnier and his team at EOS, who have been working for many years on the surface record of deformation in this region (e.g., Le Dain et al., 1984; Armijo et al., 1989). His team has acquired an initial high-resolution seismic line using explosives across the Bardibas thrust, a splay of the MFT system, which shows excellent quality reflections, with a clear dipping panel at the tip of the fault. Imaging with the portable Vibroseis equipment should allow me to acquire deeper images, hopefully to 2 km depth, with much faster acquisition time. Together, we hope to develop 3D models of these faults and to understand the magnitudes and recurrence intervals of earthquakes they produce.