Spectral seismograms, magnitude spectra and source parameters of a selection of Indian plate earthquakes

Spectral seismograms of 17 earthquakes in and around the Indian sub-continent as recorded by GRA1 station of the Graefenberg array are presented and their individual and common characteristics discussed. P-wave magnitude spectra for all these earthquakes have been computed. The corner frequency, which corresponds to the frequency at the maximum value of the magnitude spectrum, is found to shift, in general, to a higher value than observed at the recording station. These magnitude spectra are used to estimate model-independent and -dependent source parameters such as seismic moment, apparent stress, stress drop and fault length. For the sake of uniform comparison, one simple omega-square type of source model has been used. No clear classification of events on the basis of source parameters is found possible. However, the model-independent apparent stress appears to have a direct proportionality with the model-dependent stress drop estimates. Also, in conformity with known observations, the highest stress drop is found for the intra-plate Latur earthquake and the lowest, for the Nicobar island arc events.