No abstract

No abstract

SUMMARY In Northeast India, three major plates interact along two convergent boundaries: the Himalayas and the Indo–Burma Ranges, which meet at the Assam Syntaxis. To clarify this tectonic interaction and the underlying dynamics, we determine the regional seismotectonic stress from the stress inversion of 285 double couple focal mechanism solutions of earthquakes with an average magnitude of 5. We then compare the reconstructed stress regimes with the available information about geodetically determined relative displacements. North–south compression, in a direction consistent with India–Eurasia convergence, prevails in the whole area from the Eastern Himalayas to the Bengal Basin, through the Shillong–Mikir Massif and the Upper Assam Valley. E–W extension in Tibet is related to this N–S India–Eurasia convergence. Not only does the major N–S compression affect the outer segments of the Indo–Burma Ranges, it also extends into the descending slab of Indian lithosphere below these ranges, although stresses at depth are controlled by bending of the slab beneath the Burmese arc. The existence of widespread N–S compression in the Bengal Basin, far away from the Himalayan front, is compatible with the previously proposed convergence between a Shillong–Mikir–Assam Valley block and the Indian craton. E–W compression inside this block supports the hypothesis of a component of eastward extrusion. Stress inversion of focal mechanism solutions in the Indo–Burma Ranges reveals a complex stress pattern. The Burmese arc and its underlying lithosphere experience nearly arc‐perpendicular extension with ESE–WNW trends in the northernmost, NE‐trending segment and ENE–WSW trends in the main N–S arc segment. Such extensional stress, documented from many arcs, is likely a response to pull from and bending of the subducting plate. At the same time, the Indo–Burma Ranges are under compression as a result of oblique convergence between the Sunda and Indian plates. The maximum compressive stress rotates from NE–SW across the inner and northern arc to E–W near the Bengal Basin. This rotation is consistent with the deformation partitioning reflected in the rotation of relative displacement vectors, from a SSW‐directed Sunda–Burma motion to a WSW‐directed Burma–India motion. As a consequence of this partitioning, the major belt‐parallel fault zones show a variety of movements across the main N–S arc segment, from right‐lateral slip in the inner ranges to oblique reverse‐dextral slip in the outer ranges and pure thrusting in the westernmost foreland belt.

No abstract

No abstract

No abstract

The Northeastern region (NER) of India is a seismically active region which lies in the most active zone V (BMTPC, 2003) (Figure 1a) of seismic zones of India. The region jawed in between the Himalayan collision arc to the north and northeast and the Indo Burmese subduction arc to the south and southeast has produced two great earthquakes (1897 and 1950) and the whole region experienced 20 large earthquakes (M > 7.0) during the last 200 years [Angelier and Baruah, 2009]. This region is tectonically divided into several mosaics by deep-rooted faults/thrust along which episodic block/thrust/strike-slip movements are reported [Nandy, 1986, 2001]. The seismicity pattern of NER shows relatively higher concentration of events in the Shillong and Mikir Plateaus, Arunachal Himalaya and Indo-Myanmar subduction zone. Amongst a number of active faults in NER, the Kopili and the Bomdila Faults are very active, which is evidenced from the occurrence of 24 large magnitude (M w ≥5.5) seismic events in recent past (Figure 1b). Concentration of events along both the Kopili and Bomdila Faults are considered to be an intraplate seismotectonic domain. These intraplate seismic activities are fairly intense in this region due to its complex stress regime [Nandy, 2001, Angelier and Baruah, 2009]. The Kopili Valley region has produced two large earthquakes Article history

No abstract