---

Sub-Basalt Velocity Image of the Main Seismogenic Fault at Koyna, an Intraplate Active Region in Western India, Using Borehole Seismic Network.

Shashidhar D, Mahesh P, Gupta S, Mallika K.

SEISMOLOGICAL RESEARCH LETTERS

https://doi.org/10.1785/0220250319 

The authors have carried out high-resolution 3-D local earthquake tomography of the Donachiwada Fault (DF) beneath the Deccan basalt in the Koyna region, using borehole seismic data from 5325 local earthquakes recorded by a network of eight deep borehole seismometers. The resulting VP and Vp/Vs images reveal strong lateral and vertical heterogeneity within the granitic basement, with distinct velocity anomalies across the DF and correlate closely with the seismicity distribution along the DF, the plausible source of the 10 December 1967 (M 6.3) earthquake - the largest reservoir-triggered event documented globally. The study suggests that the DF extends to a depth of ~9 km and the seismicity at 5-9 km depth along the DF is related to the presence of a fluid-rich or altered zone within the brittle upper crust. By resolving the sub-basalt crustal architecture down to ~9 km depth, this study fills a long-standing observational gap left by earlier surface-based seismic investigations.

Fig: Spatial distribution of earthquakes in Koyna-Warna region. A network of eight borehole seismic stations are also plotted. (ii) Anomalies beneath the Donachiwada fault (DF) from the vertical tomograms of (a) P-wave velocity and (b) VP/VS derived from the tomographic inversion of the study region shows the seismic velocity image up to seismogenic depths of the main fault.

---