Local Magnitude Scale and 1-D Velocity Model for Central Northern India

Abstract

Northern India's seismic monitoring has advanced significantly in the past with the establishment of a dense and well-distributed network of seismometers. This has greatly enhanced the ability to detect and analyze seismic events in the region and provided a high-quality dataset within the study region.  We use a 20-year dataset of 413 earthquakes comprising 3,191 P-arrivals and 2,986 S-arrivals to develop a one-dimensional velocity model for the region. The dataset is meticulously curated based on the azimuthal gap, minimum station requirements, and root mean square travel-time residual. A collection of preliminary models taken from previous studies conducted in and around the central north Indian region is subjected to random perturbations and utilized in a coupled hypocenter and one-dimensional seismic velocity inversion. The model exhibiting a reduced travel time residual in comparison to its predecessors is adopted as the final 1-D velocity model. This final five-layered model up to a depth of 100 km reveals a sediment thickness of 3.5 km with a P-wave velocity of 3.9 km/s and an upper crustal layer down to 20 km with a P-wave velocity of 6.01 km/s, and Moho depth of 42 km with P-wave and S-wave velocities at the Moho of 8.18 km/s and 4.71 km/s, respectively.  In defining the local magnitude scale for Central Northern India, we analyzed 166 earthquakes, ensuring each event had at least three synthetic Wood-Anderson amplitudes, with a data set comprised of 1404 maximum amplitudes of S- and Lg waves.  The derived ML scale, expressed as =log A (nm) + 0.752   log R (km) + 0.00129   R (km) -1.315+S, has been validated for earthquakes with magnitudes up to 4.6(Mw) over hypocentral distances of up to 1000 km.