Le cycle sismique en Himalaya

We discuss the seismic cycle in the Himalayas and its relation to mountain building on the basis of geodetic, seismological and geological data collected in the Himalaya of Nepal. On average over several seismic cycles, localized slip on a major thrust fault, the Main Himalayan Thrust fault, MHT, accommodates the ~ 21 mm·yr^-1 convergence rate between southern Tibet and India. The geodetic data show that the MHT is presently locked from the sub-Himalayas to beneath the front of the high range where it roots into a sub-horizontal ductile shear zone under southern Tibet. Aseismic slip during the interseismic period induces stress accumulation at the southern edge of this shear zone triggering intense microseismic activity and elastic straining of the upper crust at the front of the high range. This deformation is released, on the long term, by major earthquakes on the MHT. Such an event is the M_w 8.4-1934-earthquake that ruptured a 250-300-km long segment. The major seismic events along the Himalayas since the 19th century have released more than 70 % of the crustal strain accumulated over that period, suggesting that, if any, aseismic slip on the MHT cannot account for more than 30 % of the total slip.