Beginning to Place Limits on the Timing, Size and Spatial Extent of Great Earthquakes along the Himalayan Frontal Thrust with Paleoseismology

The ongoing convergence between India and Eurasia has resulted in the globes's highest mountain range and the largest recorded intracontinental earthquakes on record. The largest of these events have occurred along the Himalayan frontal thrust. Towards ultimately understanding the relationship of strain accumulation to strain release and the role of this process in the Himalayan mountain building process, we have embarked upon a quest to define the timing, size, and lateral extent of earthquake ruptures along the length of the 2500 km long Himalayan frontal thrust. A dozen trench sites along some ~1800 km of the sytem are beginning to reveal the spatial, temporal, and size characteristics of the most recent earthquakes along the arc. Our most recent work at 3 sites in India located along a 500 km stretch of the thrust extending east from Nepal will be discussed in the context of prior studies of ours and others along the arc. General aspects of our findings are that 1) earthquake displacements observed in trenches point to the past and potential occurrence of earthquake ruptures greater than M8 earthquakes that have occurred on the structure historically, 2) that the locus of strain release represented in these earthquake displacements is well separated from the locus of strain accumulation beneath the Main Central thrust to the north as defined by geodecists, 3) the HFT takes up 10-20 mm/yr of the total ~50 mm/yr convergence across the India-Eurasia plate boundary, and 4) that the eastern Nepal border marks relatively closely the boundary between two of the most recent surface rupture events along the HFT.