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THE SEISMICITY AND THE RECENT CRUSTAL MOVEMENT IN THE HIMALAYA REGION

Ye Hong (Yeh Hung), Zhang Wenyu, Yu Zhishui and Xiu Qin (Institute of Geology, National Bureau of Seismology)  
In this paper three aspects of analysis are made for the recent crustal movement in the Himalayas on the basis of earthquake data recorded from 1897-1976.1. The characteristics of the present active faults in this region were studied by the fault plane solutions.2. The vectors of the tectonic movement in the lithosphere blocks were calculated by the null vector method.3. The rate of the crustal slip, was estimated from the seismic moment data.In light of the analysis mentioned above, a model of recent crustal movement in the Himalayas is tentatively presented as follows.1. According to the results calculated by the null vector method, the direction of the tectonic movement of the lower lithosphere in the Himalayas is NNE, and its plunge is nearly horizontal, indicating that convergence of the NNE-ward drifting Indian plate and the Eurasian plate was mainly a sort of collision in nature with intensive horizontal compression and shortening rather than underthrusting.2. The regional trend of the western part of the Himalayan arc is nearly perpendicular to the direction of the tectonic movement of the Indian plate. During the process of intensive horizontal shortening of the lithosphere. its upper part upthrusts with a rather low angle over the Ganges Plain, and now the type of movement is still going on chiefly along the main boundary fault and the Ganges Plain boundary fault. The strike of the eastern part of the Himalayas is nearly parallel to the direction of the tectonic movement of the Indian plate. Along this fault belt the Indian plate is moving NNE-ward and has a left-lateral strike slip relative to the Eurasian plate with some upthrust component.3. As a result of the intensive push of the Indian plate applied to the Himalayas, shortening of the latter at a rate of about 3.4 cm/y. was estimated. In the eastern part of the Himalayas the strain energy seems to be released chiefly by earthquake dislocations, but in the western part mainly through aseismic creeps. In the Qinghai-Xizang Plateau north of River Yarlungzangbo, the rate of shortening is probably less than that of the Himalayas. The north-south shortening of this Plateau causes the thickening of the crust and the eastward flow of the crustal materials, and consequently the intensive right-lateral strike slip along the Miju and Red River faults on the south side of this plateau.
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