Recurrent seismicity in Rajasthan State in the tectonically stable NW Indian Craton

Indian peninsular region comprises several Archean cratonic blocks (Dharwar, Bastar, Singhbhum, Aravalli – Bundelkhand), bordered by Proterozoic mobile belts. Therefore, this region is considered as tectonically stable and designated as the least vulnerable region to earthquake hazard except the still active Central Indian Tectonic Zone (CITZ). The latter is a major suture between southern and northern Indian blocks. Seismicity in India is common along its northern and northeastern (Himalayan) region defining the Indian Plate margin, in collision with the Eurasian (Tibetan) Plate. Being tectonically active, this region has a documented record of frequent earthquakes including some high magnitude and devastating ones. The northwestern Indian block is amongst the relatively stable Precambrian regions of India and categorized under Zone – II by the Indian Meteorology Department, one of the seismically least vulnerable regions. However, the region has been a site of recurring low to moderate magnitude earthquakes. Compilation of the earthquake data from the Rajasthan State in NW India documents at least 45 earthquakes in this region during the last one and a half decades. Several of these have remained unnoticed because of their low magnitude. The NE –SW trending Aravalli Mountain Region (AMR), running across the eastern half of Rajasthan State represents the most significant tectonomorphic feature of NW India. The AMR is an ensemble of Proterozoic age Aravalli and Delhi Supergroup rocks (metasediments, volcanics and intrusives) that overlie an Archean basement (Banded Gneissic Complex – BGC). These Proterozoic mobile belts have evolved through several episodes of deformation that have shaped its present day geomorphology. The western part of the State, the Marwar Block, is relatively younger in age and was accreted to the AMR during the 1 Ga subduction event. The western margin of AMR, also called as the Western Margin Fault, represents a major suture between the two terranes. However, the entire region was cratonized by end Proterozoic and has remained tectonically stable during the Phanerozoic Eon. The Archean basement and overlying Proterozoic cover rocks are infested with several major and minor faults and shear zones. The most prominent ones are the NE-SW trending major lineaments corresponding with the regional tectonic grain and several minor ones across. The earthquake epicenter distribution pattern shows a close spatial association with these lineaments. In the absence of any significant tectonic activity in the region and rise in pore pressure either due to magmatism or excessive rainfall, we attribute recurrence of earthquakes in the region to reactivation of such old sutures/weaker zones as a response to stress build-up along the northern margin of the Indian Plate resulting from ongoing northward convergence of the Indian Plate. Some of the faults in western part are traceable into the Cambay Basin active faults in the south that may have triggered seismic activity in western Rajasthan.