Enhanced earthquake locations and highlighting the seismotectonic features in Delhi-NCR: insights from a new 1-D velocity model

The Delhi-NCR region in India is highly susceptible to seismic activity due to the dense human population and the presence of thick sediments that amplify seismic waves. To better understand the region's seismic behaviour, we created a 1-D velocity model using data from local earthquakes recorded at 17 broadband seismographs between 1998 and 2022. Out of the approximately 1100 local earthquakes recorded with permissible azimuthal gaps, we used 554 events for velocity modelling based on the RMS value and number of well-recorded phases. In total, over 4822 phases were utilized for simultaneous 1-D velocity inversion, with 2455 P and 2367 S phases. The new 1-D wave velocity model is developed with three layers up to 43 km depth, and station correction was also taken into account, which was found to be well-correlated with the geological features of the area. The station corrections indicate heterogeneity in near-surface structures due to the transition between alluvium and quartzite formations. This 1D velocity model, based on long-span earthquake data and improved earthquake depth and location, clearly delineated the highly seismogenic transition zone along the Delhi-Haridwar ridge and MDDSF. It also highlights some unique seismotectonic features, such as the geometry of earthquake distribution and the seismogenesis depth of the region, and enable us to perform waveform inversion of the earthquake M < 3.0. The velocity model can provide more reliable earthquake monitoring, therefore offers the nodal agency to evaluate seismic risk more precisely. Furthermore, the reliable location data can be used as an initial model for 3D velocity structure.