Fluid-rock interaction in the intraplate active seismic zone: Boon or bane?

Abstract. The Koyna-Warna Seismogenic Region of western India has been recognized as one of the hotspots for reservoir-triggered seismicity (RTS) since 1967. The current study investigates the fluid's interaction with the severely fractured granitoid basement of this area and its potential contribution to the recurring seismicity. The presence of several secondary minerals, such as chlorite, epidote, calcite, illite, etc., along the pre-existing faults and fractures, is revealed by detailed petrologic investigation at mesoscopic and microscopic scales along with XRD analysis. This indicates the fluid-rock interaction along these mechanically weak planes and subsequent propylitic grade of hydrothermal alteration under acidic to neutral conditions (pH 5.5–7) and the temperature of above 200–220 °C up to about 350 °C. Additionally, the transformation of biotite into chlorite due to fluid interaction has been inferred from the microscopic appearance of biotitic remnant within neoformed chlorite which is further supported by the mass loss of K₂O and concurrent gain of MgO and FeO, demonstrating the replacement of potassium (K) interlayer sheet by brucite-like [Mg (OH)₂] layer during biotite chloritization. However, this released K₂O further assists in the formation of illite resulting in the mass gain of K₂O at a few certain depths, whereas the dissolution of plagioclase justifies the formation of albite and calcite as evidenced by the gain of Na₂O and CaO. The present study also highlights that the recurring nature of the seismicity in this area may be related to clay mineralization along the faults and fractures due to fluid-rock interaction, such as chlorite, illite, etc., in addition to the existing fault geometry and stress build-up due to reservoir impoundment. At increasing stress condition, the anisotropic and weakly bonded, layered crystal structure of chlorite forming ripplocations may develop kink bands and increases the yield strength proportionally with rising pressure up to dehydration temperature. Such visco-elastic behaviour of chlorite may promote aseismic creep in the faults. On the other hand, epidote noticed at certain depths has a contrasting behaviour; it tends to wear at the micron or submicron‐scale asperity contacts and produce fine particles which generate unstable sliding. However, the relatively higher abundance of chlorite in the faults and fractures disrupts the epidote‐epidote contact asperities and prevents such wearing of epidote grains into fine particles. Thus, biotite chloritization in conjunction with relatively less production of epidote along pre-existing faults and fractures helps to release the accumulated stress through a series of small-scale earthquakes and results in the steady fault creep observed in this region during the past 50 years. In this context, fluid-rock interaction along the pre-existing faults and fractures at shallow depth has acted as a blessing for the Koyna-Warna Seismogenic region shielding it from relatively large magnitude earthquakes – a boon for the region.