Deccan Trap configuration in and around Koyna-Warna Seismic Zone, India: A magnetotelluric approach

Abstract

The spatial variation of Deccan Traps thickness in and around the Koyna-Warna Seismic Zone (KWSZ), India, is crucial for the study of reservoir triggered seismicity in the region. Due to the increased trap thickness, the increment of pressure on the basement directly affects the stress accumulation within the basement fault system. Such a stressed fault system within the pressurized basement is prone to the release of strain due to the fluid infiltration. Moreover, the knowledge of the trap thickness variation aids in selecting a proper place for exploratory borehole drilling and install a seismometer to effectively eliminate the noise generated by the overlying trap. The dimensionality analysis of the magnetotelluric (MT) data acquired in KWSZ in the grid shows one dimensional (1D) characteristics in the shorter period (0.001 s – 1 s), which signifies that the layered Deccan basalts on the top is followed by the granitic basement in KWSZ. Therefore, 1D inversion is applied to delineate the trap thickness and create the detailed configuration of Deccan Traps in and around KWSZ. The result shows that the trap thickness mainly varies along E-W direction with an average thickness of ∼800 m. The base of the trap is almost flat and dipping (∼1 deg) towards the west. The flat bottom and the varying thickness signify that the trap thickness is controlled by the topographic variation, providing information for future drilling of exploratory boreholes. The high trap thickness regions, associated with a higher average overburden pressure of 25.6 MPa, coincide with seismicity distribution in KWSZ, which provides the critical inputs for pressure-seismicity relation.