Magnetotelluric investigation in the swarm prone intraplate Talala region of Saurashtra, Gujarat, western India

The Talala region of Saurashtra has experienced three moderate seismic events of magnitudes Mw5.0, Mw4.8 (on 6th November 2007), and Mw5.1 (on 20th October 2011) accompanied by swarms. The local earthquake data suggested two epicentral trends (ENE- WSW and NNW-SSE). The focal mechanism of all three earthquakes exhibit strike-slip mechanism with nodal planes sympathetic to these two trends. Based on low gravity values and ambiguous seismic reflections, the ENE-WSW trending fault was inferred and named as Girnar fault. The Magnetotelluric (MT) investigation has been carried out (i) to locate and characterize the Girnar fault and (ii) to ascertain the cause of seismicity in the region. The MT data is acquired along the similar to 62 km long NW-SE oriented profile, starting from the Sendarda village (NW) to the Hadmatiya village (SE) in the Junagadh district (Saurashtra), India. The regional strike of N19 degrees E is obtained from the MT data. The data is subjected to 2D modeling and two conductors are identified along the profile, the first conductor is located between Ajeb and Aambalgadh village in the northern part of the profile with a resistivity range from 20 to 3500 Ohm.m and the second conductor is similar to 15 km south of the first conductor near Hirenvel village with a resistivity range from 50 to 3500 Ohm.m. The second conductor continues vertically down to 12-15 km depth and inferred as the Girnar fault. The shallow clustered seismicity (similar to 5 km) near the second conductor advocates that it may be related to stresses induced by intense monsoon on the critically stressed Girnar fault, which has fractured the basement. The basement got fractured either from the local stress accumulation near the structural boundary or from the movement of the fluids to the less permeable crust.