Test of source-parameter inversion of intensity data

We demonstrate that the approximate source kinematics of the San Fernando, 1971 earthquake can be back-predicted by analysing its macroseismic intensi ty data set (felt reports) objectively and quantitatively. This is done by inverting either the data set of the intensity values observed in all sites , or the intensities tessellated with the Voronoi polygons technique. It is shown that the kinematic characteristics found following our method (epice ntral coordinates, source depth, seismic moment, rupture length, Mach numbe r, fault plane solution) match those determined by other authors, via instr umental measurements, rather well. The prerequisite for obtaining these res ults is that local amplification must not affect groups of neighboring site s. It was possible to invert the U.S.G.S. "felt reports'' for the source be cause this data set is sufficiently uncontaminated by local site responses, and retains relevant regional traces of source effects. Isoseismal maps ca nnot be safely used for this task, because qualitative drawing criteria giv e subjective results. Isoseismals, based on incomplete space frequency samp lings, give rise to spurious effects, whereas the Voronoi polygons produce easy-to-grasp, quantitative and objective, representations of macroseismic intensity data. The tests performed, up to now on a series of earthquakes, suggest that the combined use of tessellation and of our KF model is promis ing mostly for inverting intensities of preinstrumental earthquakes.