Qualitative assessment of tectonic lineaments over the coastal and innershelf of Kakinada and Kalingapatnam, central east coast of India

Abstract

Earth tremors of low to moderate intensity (3.0 to 5.5) recurring over last century in and around coastal and shelf regions of Visakhapatnam has prompted us to acquire and study the geophysical data between coastal regions of Kakinada and Kalingapatnam in western part of Central Bay of Bengal (17°N - 18°30'N) for understanding the recent seismicity of the region. Preliminary analysis of magnetic and shallow seismic data reveals the presence of some prominent land-Ocean tectonic lineaments, reactivation of which is considered to be responsible for tremors in the region. Total field magnetic anomaly map of the study area shows two major anomaly lineations such as: (a) N-S trending medium amplitude (~250 nT) lineation off Visakhapatnam which is considered to be offshore fragment of converging point of two onshore lineaments. Also the lineament is associated with five earth tremors of magnitude 3.7 to 4.3 at 40 m water depth. Impact of tremors is also observed in the overlying sedimentary strata in the form of vertical faults, (b) A NE-SW high amplitude anomaly zone (~820 nT) off Kalingapattiam trending NE-SW, which is in close proximity with the epicentral location of earthtremors of 5.5 magnitude. This lineament can be spatially correlated to onshore NW-SE trending structural lineament indicating the probable reactivation of offshore segment resulting in the earth tremors. Off Vizianagaram, reactivation of preexisting structural lineaments associated with the NW-SE trending Kandivalasa river fault may be the source of tremors at 20 m water depth. Magnetic basement contour pattern changes from NE-SW to NW-SE off Visakhapatnam. It may be inferred that the basement is bounded by two fault controlled lineaments passing through epicenters of earthtremors off Visakhapatnam and Vizianagaram. Basement structure shows down faulted basement (2 to 3 km) underlying the epicentral location along the lineaments trending NW-SE in the innershelf of visakhapatnam. Shallow seismic data also indicate steeply dipping sub bottom fault in the study area of Visakhapatnam. South of Visakhapatnam, shallow seismic records indicate vertical faults and shearing reflectors in the lower sedimentary layers. From the deformed lower sedimentary layers. It may be considered that the region might have undergone seismic activity in the recent past, but presently there are no reported tremors in this region. Even undeformed top sedimentary layers suggest lack of recent tectonic activity between Visakhapatnam and Kakinada in coastal and shelf regions. It may be summarized from the preliminary analysis of the geophysical data that the earth tremors observed north of Visakhapatnam are caused mainly due to reactivation of tectonic lineaments whereas south of Visakhapatnam it is relatively free from any seismicity. Based on the felt reports, tremors are attributed to shallow focus origin. Detailed investigations from multidisciplinary approach like geological, geomorphological, geophysical and remote sensing observations coupled with seismic information in the study area will throw more light on the micro seismic activity and related recent tectonics of this coastal region of east coast of India.