Neotectonism – An offshore evidence from eastern continental shelf off Visakhapatnam

A minor tremor measuring 4.5 on the Richter scale occurred around 4.58 p.m. on 18 December 1995, with its epicentre located in the Bay of Bengal, nearly 30 km east of Vizianagaram on the Andhra Pradesh coast. The location of the epicentre is around 18°N and 83°42'E. It is significant that the epicentre falls in the innershelf off Vizianagaram, where a suite of rocks (Santapalle Rocks) outcrop at a water depth of around 30 m. Bathymetry and magnetic data were collected over this region along closely spaced, coast parallel profiles covering the shelf regions. Observed magnetic anomalies are of short wavelength and high amplitude of the order of 600 to 700 nT. Two-dimensional modelling of these anomalies with conventional as well as inversion methods indicates a severely faulted and folded basement of the order of 1 to 6 km. Three closely spaced river channels (Nagavali, Vamsadhara and Nellimarla) trending NW–SE are associated with the faulted and folded basement. Faulting associated with the Nellimarla river channel extending into the offshore zone may be the possible cause of the tremor. Strong magnetic anomalies, complex basement structure, exposed rock outcrops in the innershelf (20 m) in the vicinity of the epicentre of Vizianagaram earth tremor provide evidence of Neo-tectonic activity in this region. The epicentral region falls in a shallow marine environment ideal for generating a geophysical database for stable continental region earthquakes.