Seismotectonics and rates of active crustal deformation in the Burmese arc and adjacent regions

Abstract

The close vicinity of the Burmese subduction zone to the Himalayan collision zone across northeast India produces complex tectonics giving rise to a high level of seismicity. Using the hypocentral data of shallow earthquakes ( h≤70 km) for the period 1897–1995, a large number of focal mechanism solutions and other geophysical data in correlation with major morphotectonic features in the Burmese arc and the adjoining areas, we identified 12 broad seismogenic zones of relatively homogeneous deformation. Crustal deformation rates have been determined for each one of these sources based on summation of moment tensors. The results indicate that along the Kopili–Bomdila fault zone in eastern Himalaya, the deformation is taken up as a compression of 0.12±0.01 mm/yr along N16° and an extension of 0.05±0.004 mm/yr along N104° direction. The deformation velocities show a NS compression of 18.9±2.5 mm/yr and an EW extension of 17.1±2.2 mm/yr in the Shillong Plateau region, while a compression of 5.4±2.8 mm/yr along N33° is observed in the Tripura fold belt and the Bengal basin region. The vertical component in the Shillong Plateau shows crustal thickening of 2.4±0.3 mm/yr. The deformation velocities in Indo–Burman ranges show a compression of 0.19±0.02 mm/yr along N11° and an extension of 0.17±0.01 mm/yr along N101° in the Naga hills region, a compression of 3.3±0.4 mm/yr along N20° and an extension of 3.1±0.36 mm/yr along N110° in the Chin hills region and a compression of 0.21±0.3 mm/yr in N20° and an extension of 0.18±0.03 mm/yr along N110° in the Arakan–Yoma region. The dominance of strike-slip motions with the P axis oriented on an average along N17° indicate that the Burma platelet may be getting dragged along with the Indian plate and the motion of these two together is accommodated along the Sagaing fault. The velocities estimated along Sagaing transform fault in the back-arc region suggest that the deformation is taken up as an extension of 29.5±4.7 mm/yr along N344° and a compression of 12.4±1.9 mm/yr along N74° in the northern part of the fault zone, and a compression of 17.4±2.3 mm/yr along N71° and an extension of 59.8±8.0 mm/yr along N341° in the southern part of the fault zone. The average shear motion of about 13.7 mm/yr is observed along the Sagaing fault. The deformation observed in the southern part of the syntaxis zone along the Mishmi thrust indicate a compression of 0.63±0.08 mm/yr in N58° and an extension of 0.6±0.07 mm/yr in N328° direction. The region of Shan Plateau, west of Red River fault, shows a compression of 17.7±2.6 mm/yr along N36° and an extension of 16.1±2.4 mm/yr along N126°.