**5. Results and discussions**

Large negative travel time residuals associated with the central Kachchh rift zone are detected (marked by a black dotted elliptical area shown in **Figure 2b**), suggesting relatively thin lithosphere underlying the central KRZ while surrounding unrifted parts show positive travel time residuals suggesting relatively thick lithosphere as also observed in the estimates of lithospheric thicknesses [18]. Large negative travel time residuals are also found in the Cambay rift zone (CRZ) (**Figure 2b**) suggesting relatively thin lithosphere underlying the CRZ as also modeled by an S-RF imaging and surface wave dispersion studies [19, 31]. Note that the estimated directional means of relative P-residuals show a good spatial correlation with the major tectonic regional features. The most negative values (though with large standard deviations) follow the track of the Cambay rift or Deccan plume and basaltic region of Kachchh. This observation shows the robustness of our modeling results.

The horizontal slices of dVp tomograms at different depths (50, 90, 130, and 170 km) suggest that a prominent low-velocity zone (~−2 to −4%) is present below the central Kachchh rift zone extending from 50 to 170 km depths (**Figure 3a**–**d**). At 50 km depth, a high-velocity structure is also noticed in the region north-east of the 2001 mainshock location (**Figure 3a**), which vanishes at deeper depths. But, the low-velocity zone in the KRZ becomes a bit subdued (~−1 to −3%) between 150 and 170 km depths (**Figure 3c** and **d**). However, this low-velocity zone also exists at 210 km depth, which might be an artifact due to the poor sampling of rays for the deeper zone. Tomograms also clearly bring out the fact that in the upper mantle (at 70–170 km depth) the P-wave velocity is found to be higher in the western part in comparison to that on the eastern part of the Kachchh region. In the shallower depths (at 50 km), tomogram suggests that the central KRZ, which is characterized by the crustal as well as lithospheric thinning and negative traveltime residuals (**Figure 2b**; [18]), is found to be associated with the low dVp anomaly. This lowvelocity zone associated with the central KRZ also exists between 90 and 170 km depths (**Figure 3b**–**d**), which could be attributed to the presence of carbonatite/ partial melts related to the Deccan volcanism of 65 Ma [19, 31]. Note that the presence of carbonatite melts in the shallow upper mantle depths below the KRZ has also been proposed by an isotopic ratio study of xenoliths [7] and the modeling of SKS splitting [11]. Thus, the volatile CO2 (emanating from the crystallization of carbonatite melts in the asthenosphere), can reach lower crustal depths, through deep-seated faults, which might be facilitating the deeper circulation of metamorphic fluids/volatile CO2, thereby, the generation of lower crustal earthquakes occurring in the Kachchh rift zone [18].

The most robust outcome of the tomography, which does not vary with regularization or parameterization applied, is that low-velocity perturbations prevail in the Kachchh rift zone (as shown by dotted elliptical area in **Figure 2b**) down to ~170 km depth (see **Figure 3a**–**d**). Similarly, earlier studies [8, 10] indicated that the upper mantle beneath the entire KRZ is characterized by lower velocities, about the surrounding area down to these depths. These studies do not exploit only teleseismic data but also include data from regional events. Moreover, regional and global teleseismic tomography studies [8, 32] show the upper mantle beneath the Kachchh region as a part of an extensive low-velocity heterogeneity located in western India, extending down to about 700 km. The regional teleseismic tomography from the western India indicates relatively small velocity variations (±1%), both in size and in amplitude. Such details are below resolution level of global or regional studies which invert data of permanent observatories only. Thus, our results can be viewed as being in agreement with these larger-scale investigations.
