Linearized waveform inversion of local and near‐regional events for source mechanism and rupturing processes

Abstract

We have developed an algorithm which can, given an initial trial solution, determine simultaneously earthquake‐source mechanism, hypocentre and source‐time function from the inversion of broad‐band waveform data of local and near‐regional events (epicentral distances less than 10°). The forward modelling of the seismic ground motion is computed for frequencies up to 10 Hz by multimodal summation in layered anelastic media. The non‐linear damped least‐squares technique is used to minimize the difference between the theoretical and the observed seismograms in the time domain. The use of a weighting matrix and variable damping factor in each iteration avoids the problem of the ill‐conditioned matrix inverse and guarantees the stability of the inverse solution. To reduce the influence of the earth model error on the inversion, for each source‐receiver path the most appropriate layered earth model can be used. The algorithm has been tested against synthetic data to investigate the inversion convergence and the dependence of the solution on the fact that the assumed earth model is never equal to the true one. The synthetic tests indicate that if the earth model(s) is (are) adequate, the inversion converges rapidly to the ‘true’ solution. The sensitivity of the method to the starting model is found to be frequency dependent. The initial location can be in error by approximately the P‐ or S‐wave wavelength and the initial source mechanism parameters can differ considerably from ‘true’model. The focal depth and source‐time function depend strongly on the crustal model. Subsequently, the algorithm has been successfully applied to two events: a near‐regional and a local event. The first is the 1974 March 22 event which occured in Albania, was recorded at Bari and Trieste LP stations (like WWSSN), and has been analysed using two differnt structural models based upon surface‐wave dispersion measurements. The second is the Friuli event of 1987 December 27, which was recorded at five digital SP local stations, and has been analysed using a local crustal model obtained from the inversion of P‐ and S‐wave arrival times. Copyright © 1994, Wiley Blackwell. All rights reserved