SEISMOTECTONIC DOMAINS OF NORTHEASTERN INDIA AND ADJACENT AREAS D.R. NANDY and SUJIT DASGUPTA Special Investigation Division, Geological Survey of India, CALCUTTA-700 016 (INDIA) Publication and Information Division, Geological Survey of India, CALCUTTA-700 016 (INDIA) ABSTRACT Eastern Himalaya, Meghalaya plateau, upper Assam valley, northern part of the Indo-Burmese arc and the Mishmi block constitute major tectonostratigraphic domains in northeastern India. Many lineaments of faults, both parallel and oblique to the Himalayan trend, are already known. Interpretation of satellite images combined with surface geological studies suggest that many such oblique lineaments transgress the boundary of individual tectonic domains and some continue from the Tethyan Himalaya to the foredeep or cut across both Himalayan and Burmese arcs. The entire area is highly seismic; seismicity pattern, focal mechanism solutions, geological set up and fault/lineament fabric when studied together clearly defines several seismotectonic domains. In east Nepal-Sikkim, the northward push of India is accommodated through conjugate shear failure wherein seismic strike-slip movement occurs mostly along NE faults. Further east NW/WNW Kopili-Bomdila faults are associated with many large earthquakes and lateral motion along them allows a bulk southeastward movement of this segment of Himalaya towards the Burmese arc. The Mishmi block, structurally oblique to both the Himalayan and Burmese arcs, also indicates a net southeast tectonic transportation. The upper Assam valley is aseismic and arguably does not represent an area of seismic gap. Seismicity in both Meghalaya plateau and Sylhet plains is unrelated to movements along the Dauki fault. Keywords: Seismotectonic domains; thrusts; strike-slip faults; neotectonic activity; Northeast India; Indo-Burmese arc. INTRODUCTION Unparallel relief, complex geological setting, anomalous crustal structure (Hirn et at., 1984; Chun and Mc Evilly, 1986) and intense seismicity along the Himalayan front are interpreted as a direct consequence of collision between India and Asia. Contrary to the observation in previous studies (Fitch, 1970; Molnar et al., 1973; Molnar and Tapponnier, 1978; Chandra, 1978; Verma et al., 1977; Ni and Barazan- gi, 1984) which suggeest that the Himalayan earthquakes located between the Main Boundary Thrust (MBT) and the Main Central Thrust (MCT), are predominantly thrust events, the recent studies (Nandy et al., 1985; Dasgupta et al., 1987; Mukhopadhyay, 1984) from central and eastern portion of the Himalaya have shown that the majority of seismic events display strike-slip motion along subvertical plane aligned oblique to the Himalayan trend. Slip along crustal con- 371