Delhi-Haridwar Ridge – from Foreland Basin to the Himalayan – an insight through Passive and Active Seismic study

The convergence of Delhi-Haridwar Ridge (DHR) plays a vital role in understanding the Delhi-Rohtak seismicity and plate segmentation along the Himalaya. The study focuses three segments adjacent to: (i)  Indo-Tibetan Suture Zone (ITSZ), (ii) Mohand anticline, (iii) Rohtak in Haryana. Near ITSZ, we have estimated Lithospheric shear-wave velocity (Vs) structure by jointly inverting receiver function, computed using earthquake data from 30 stations of Y2 network, with high-resolution group velocity dispersion data computed using ambient noise and earthquake tomography. A profile along the DHR shows the presence of high velocity material (Vs ~3.6 km/s) at ~38 km depth, with relatively steeper Main Himalayan Thrust (MHT), providing the preliminary impression of the remnants of the ridge. Downwrapping of Moho along the eastern margin of DHR provides insights on the possible segmentation in the region. For Mohand anticline, we recorded seismic ambient noise using a three-component portable seismograph (Tromino) with a natural frequency of 0.1 Hz. We conduct an HVSR (horizontal-to-vertical spectral ratio) study on the recorded data using the Nakamura Method, a technique for estimating the resonance frequency and site amplification caused by different stratigraphic units underlain by the top of the bedrock. Using nine measuring points, variable resonance frequency has been identified in the range of 0.42 to 4.8 Hz, which indicates this region is prone to site amplification as overlain by Doon fan deposits. We further invert the P-velocity (Vp), S-velocity (Vs), and density (ρ) by using Monte Carlo inversion method and identify three different stratigraphic units. The top has a thickness of 3 m with a mean Vs, Vp, and ρ of 218 m/s, 385 m/s, and 1.17 g/cm³, respectively. The second layer has a thickness of 6 m with a mean Vs, Vp, and ρ of 406 m/s, 725 m/s, and 1.7 g/cm³, respectively. The bedrock depth in this region is 127 m with a mean Vs, Vp, and ρ of 582 m/s, 1238 m/s, and 1.8 g/cm³, respectively. Further south in the Rohtak region, we have conducted an active seismic study along five profiles over the DHR with a cumulative length of ~34 km. We have applied the conventional seismic processing techniques to produce the migrated image, in which we observe the presence of structural discontinuities associated with the buried faults. The findings from this study will be essential for seismic hazard assessment and able to explain the seismicity observed in the Delhi-Rohtak region.