Joint Modeling of Velocity Structure and Hypocentral Locations in the Seismically Active Kachchh, Saurashtra, and Narmada Regions of Western India: An Active Intraplate Region

The Kachchh, Saurashtra, and Narmada regions, located in the western part of India, are seismically active intraplate regions. After the setup of the dense network by the Institute of Seismological Research (ISR), a large number of local earthquakes were recorded. Availability of such a large data set has enabled us to obtain 1D velocity models for the three regions separately, using the earthquake data recorded at digital broadband seismic stations operating during 2006-2015. A total of 570, 114, and 225 well-recorded earthquakes with azimuth gap of < 180 degrees (each of which had a minimum of five P-phase and five S-phase readings) that occurred in the three regions, respectively, are used in the analysis. The P-and S-wave velocities of the resulting seven-layer model for Kachchh (up to the depth of 34 km) vary from 3.01 to 7: 88 km/s and 1.74 to 4: 55 km/s; of the four-layer model for Saurashtra (up to the depth of 18 km) vary from 4.61 to 6: 73 km/s and 2.66 to 3: 89 km/s; and of the six-layer model for Narmada (up to the depth of 30 km) vary from 3.22 to 7: 09 km/s and 2.08 to 3: 92 km/s, respectively. The obtained optimum 1D velocity models in this study are further used to relocate all the earthquakes recorded from 2006 to 2015. We observe that the depths of the earthquakes are better constrained although the horizontal difference in earthquake locations is small. We also find that in the Kachchh rift both the upper and lower crusts are seismically active, with earthquakes occurring up to Moho. In Saurashtra, Narmada, and Cambay regions, the upper crust is seismically active, whereas the lower crust is aseismic. These region-specific 1D velocity models can also be used as reliable starting models to obtain 3D velocity structure of these regions in seismic tomography studies.