WAVE-FORM INVERSION FOR POINT-SOURCE MOMENT TENSOR RETRIEVAL WITH VARIABLE HYPOCENTRAL DEPTH AND STRUCTURAL MODEL

In the inversion scheme of high-frequency seismograms, recorded by a local network, we have developed the unconstrained moment tensor description to study weak events which are believed to reflect small-scale complexities of the tectonic structure. In order to allow the source mechanism to change with time each moment tensor component has its own time history. In the inversion, the measure of the similarity between the synthetic seismograms and the observed records is obtained by minimizing the norm L2 of their difference. The initial synthetic seismograms are constructed by summing a set of time-dependent functions, named base functions. These base functions are computed by normal mode summation for a discrete set of source depths and for two structural models, considered to represent acceptable extremes for the region under study. In the course of the inversion, using a linear interpolation, new base functions are computed for intermediate values of the starting depths and structures. Thereafter new synthetic seismograms are computed and compared with the observed records until the convergence is reached, within some pre-assigned threshold for the norm L2. The method has been tested by treating, as 'observed' records, synthetic seismograms (both displacement and velocity) generated for instantaneous and finite duration sources. The 'observed' records have been computed for two structures: one belonging to the structural range within which the base functions were computed, the other lying outside this range. Random noise has been superimposed on the records. When inverting synthetic records computed for a structure contained within the range of interpolation for the base functions, the full moment tensor is retrieved satisfactorily (provided that the noise level does not exceed about 20 per cent). For records computed for a structure lying outside this range a spurious volumetric part of the moment tensor appears. However, if the volumetric part of the moment tensor is removed, the source time function can be reconstructed satisfactorily, and the source mechanism can be determined from the deviatoric part of the moment tensor. The procedure has been applied to vertical component seismograms recorded in the Friuli (NE Italy) area for the M(L) = 2.9 event of 1987 December 27. The retrieved source mechanism is in agreement with the distribution of the few first arrival polarities available and the source time function indicates a possible multiple rupture process.