THE HIMALAYAN FRONTAL THRUST (HFT) IS NOT BLIND

The collision of Indian subcontinent with Eurasia has resulted in three major thrust earthquakes in, 1905, 1934, and 1950 along the Himalayan Frontal Thrust (HFT) zone. None are reported to have produced primary surface ruptures. Lack of primary surface rupture has led previous workers to attribute their occurrence to a blind thrust, or more specifically, a fault plane that does not reach the surface. Our observations and review of existing data along a 600 km length of the Indian HFT do not support this contention. Seismic-reflection profiles, exploratory well data, occurrence of discontinuous fault scarp segments, and the exposure of the fault in shallow trenches at sites along the HFT show the fault to reach the surface. The lack of a systematic and continuous fault scarp in Quaternary alluvial sediments is probably due to erosional and depositional processes caused by heavy monsoonal rain along the HFT. Given that we observe clear evidence of surface rupture earthquakes, the question arises why surface rupture has not been recorded in the major historical events. It seems unlikely but possible that studies of these major historical earthquakes missed evidence of surface rupture. If the historical earthquakes did not rupture the surface, we are left with prospects that either (1) the surface ruptures we do observe represent yet greater earthquakes than observed historically or (2) that sufficient strain may be stored on the shallower portions of the thrust outboard of the historical rupture zones to ultimately produce surface rupture earthquakes or (3) that the major historical earthquakes did not occur on the HFT.