LINEARIZED WAVE-FORM INVERSION OF LOCAL AND NEAR-REGIONAL EVENTS FOR SOURCE MECHANISM AND RUPTURING PROCESSES

We have developed an algorithm which can, given an initial trial solut ion, determine simultaneously earthquake-source mechanism, hypocentre and source-time function from the inversion of broad-band waveform dat a of local and near-regional events (epicentral distances less than 10 -degrees). The forward modelling of the seismic ground motion is compu ted for frequencies up to 10 Hz by multimodal summation in layered ane lastic media. The non-linear damped least-squares technique is used to minimize the difference between the theoretical and the observed seis mograms in the time domain. The use of a weighting matrix and variable damping factor in each iteration avoids the problem of the ill-condit ioned matrix inverse and guarantees the stability of the inverse solut ion. To reduce the influence of the earth model error on the inversion , for each source-receiver path the most appropriate layered earth mod el can be used. The algorithm has been tested against synthetic data t o investigate the inversion convergence and the dependence of the solu tion on the fact that the assumed earth model is never equal to the tr ue one. The synthetic tests indicate that if the earth model(s) is (ar e) adequate, the inversion converges rapidly to the 'true' solution. T he sensitivity of the method to the starting model is found to be freq uency dependent. The initial location can be in error by approximately the P- or S-wave wavelength and the initial source mechanism paramete rs can differ considerably from 'true' model. The focal depth and sour ce-time function depend strongly on the crustal model. Subsequently, t he algorithm has been successfully applied to two events: a near-regio nal and a local event. The first is the 1974 March 22 event which occu red in Albania, was recorded at Bari and Trieste LP stations (like WWS SN), and has been analysed using two different structural models based upon surface-wave dispersion measurements. The second is the Friuli e vent of 1987 December 27, which was recorded at five digital SP local stations, and has been analysed using a local crustal model obtained f rom the inversion of P- and S-wave arrival times.