INTERSEISMIC STRAIN ACCUMULATION ON THE HIMALAYAN CRUSTAL RAMP (NEPAL)

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.