The Department of Mines and Geology has been monitoring the seismicity
of the Central Himalayas of Nepal since 1985. Intense microseismicity
and frequent medium-size earthquakes (mL<4) tend to cluster beneath t
he topographic front of the Higher Himalaya. This 10-20km deep seismic
ity also correlates with a zone of localized uplift that has been evid
enced from geodetic data. Both microseismic and geodetic data indicate
strain accumulation on a mid-crustal ramp that had been previously in
ferred from geological and geophysical evidence. This ramp connects a
flat decollement under the Lesser and Sub-Himalaya with a deeper decol
lement under the Higher Himalaya, and probably acts as a geometric asp
erity where strain and stress build up during the interseismic period.
The large Himalayan earthquakes could nucleate there and probably act
ivate the whole flat-and-ramp system up to the blind thrusts of the Su
b-Himalaya.