A tectono-geomorphological perspective of micro-earthquake swarm activity prone areas around Bhavnagar and Jamnagar in Saurashtra, western India

Abstract

Areas around Bhavnagar and Jamnagar in eastern and northern parts of Saurashtra respectively, are identified previously as prone to recurrent earthquake swarm activity. Sparse seismotectonic studies so far have not revealed any linkage with known faults in Bhavnagar area while no structures are known to occur in the area around Jamnagar. We carried out detailed tectono-geomorphic studies with contrasting geomorphic settings with a view to provide an insight into the swarm activity from a geomorphological perspective. The study is based on extensive field studies and quantitative geomorphic analysis of landscape and drainage. The area around Bhavnagar is divisible into two tectono-geomorphic domains – the E-W trending Sihor Fault zone located to the west of Bhavnagar (domain-I) and the terrain to its south which shows undulating landscape controlled by N–S trending structural trends (domain-II). The quadrant in the SW between these two domains consists of southward titled trappean hills which form source area for drainages flowing northward through Sihor Fault zone and those flowing eastward through domain-II. Late Quaternary, possibly Holocene tectonic activity along the Sihor Fault is indicated by the northward tilt of alluvial plain surface, knickpoint in rivers and small scale minor faults along the fault zone, prominent decrease in depth of fluvial incision away from the fault zone and youthful landscape indicated by drainage basin analysis. Domain-II comprises the largely erosional pediplain developed over trappean and Tertiary rocks to the south of Bhavnagar. The drainages show an early stage of landscape development due to ongoing tectonic activity. The area around Jamnagar is dominantly a peneplain formed over subhorizontal trappean lava flows. The peneplain is a manifestation of the prolonged period of non-erosion and deposition during the Cenozoic. The rivers show youthful landscape in the central part of the peneplain which includes, large knickpoints, rapids, anomalous river bends, steepening of channel gradient and large potholes. This ENE-WSW trending zone of anomalous fluvial geomorphic anomalies points to the existence of a fault or fold in the subsurface. The structure appears to be active as indicated by the denser cluster of earthquake swarm along this zone. We infer that the generation of earthquake swarm activity comprising low to moderate magnitude shocks is caused by the sheared rocks along the Sihor Fault near Bhavnagar and the ENE-WSW trending subsurface structure near Lalpur which provide avenues for downward percolation of rainwater to deeper levels causing loss of frictional resistance along weak planes.