A large number of active faults have been mapped in the Nepal Himalaya. They are most commonly expressed in the Main Frontal Thrust (MFT) area. Active fault trenching began only very recently in the Himalaya and only six successful trenches have been opened so far in Nepal out of which four were opened up by the present authors.
Trenching results in eastern Nepal show that timing of the most recent surface rupture along a strike slip fault at Tokla Tea garden was between 1000 and 1200 A.D. In the same area on the bank of Birin Khola, the active fault belonging to the MFT system showed a displacement of at least 4 m at about 1200 A.D. In central Nepal, trenching in Aurahi River showed the latest event to have occurred between 896 BC -1439 A.D., thus showing no evidence of faulting with surface rupture at least during the last 560 years.
Recent trenching in mid western Nepal near Butwal, showed the rupture with an 8 m net slip to date between 1630 AD and 790 BC. Similarly trenching along the MFT in east central Nepal at Marha Khola by others showed the fault with an apparent vertical offset of 7.2 m and displacement of about 17 m. and having an age between A.D. 1020 and 1160. All these ages bracket around ∼1100 AD, so far historically an unknown earthquake event in the region.
Trenching in western Nepal near Mohana Khola by another team revealed an apparent vertical throw of ∼8 m across the fault zone with a displacement of about ∼20 m and age ranges between A.D.1410 and 1470. Therefore it may record surface rupture associates with the historically known great Himalayan earthquake of 1505.
These ruptures with such large displacements of 15-20 m may have produced mega earthquakes in the Himalaya which were larger than the post 18th century great earthquakes with Mw 7.8 to 8.4. The active faults that ruptured in about A.D. ∼1100 and A.D. 1505 in eastern and western Nepal respectively may now have become sufficiently matured for a renewed movement to produce mega earthquakes in the region.
