Seismicity Pattern, Reference Velocity Model, and Earthquake Mechanics of South India

Abstract

We present a comprehensive study of seismicity and associated phenomena in south India based on analysis of local earthquakes recorded at 54 broadband seismic stations operated in three phases during February 2009 and April 2012. South India is a primarily stable continent and consists of Precambrian terrains such as the eastern Dharwar craton (EDC), the western Dharwar craton (WDC), the southern Granulite terrain (SGT), and the Cuddapah basin intruded with large dikes, shear zones, and rifted margins. A 1D reference velocity model is derived using 405 local earthquakes, each with a minimum seven P‐ and six S‐phase readings and an azimuth gap <180°⁠. Finally, all the recorded 780 local earthquakes (⁠1.2≤ML≤4.6⁠) are relocated using this reference velocity model. The seismicity is more in the EDC and northeastern part of the SGT, compared to the WDC. The earthquakes are generally confined to the upper 20 km depth. However, we also observe a few lower‐crustal earthquakes in the EDC that may be associated with the composition of rocks and geothermal gradient along with deep‐seated faults. We calculate fault‐plane solutions of 18 well‐distributed small earthquakes using moment tensor inversion and 3 earthquakes from the first motion of P polarity. The obtained focal mechanism solutions show dominance of a strike‐slip mode of deformation in south India, except for the Koyna–Warna region, where normal‐type solutions also exist. The P‐axis trend is north–south and northeast direction in the maximum part of south India, correlating with the northward movement of the Indian plate. In the easternmost part of the SGT region, we observe a rotation of the P‐axis trend in a northwest direction, which may be due to a local perturbation of the regional stress field. The stress‐tensor inversion of the fault‐plane solutions indicates that the maximum compressive stress is north–south directed.