HR: 1340h
AN: S43C-1021    [Abstracts]
TI: The Sources of Destructive Earthquakes Retrieved From Their Regional Intensity Patterns by a new Inversion Technique
AU: * Sirovich, L
EM: sirovich@ogs.trieste.it
AF: O.G.S., Borgo Grotta Gigante, 42C, Sgonico, TS 34010 Italy
AU: Pettenati, F
EM: fpettenati@ogs.trieste.it
AF: O.G.S., Borgo Grotta Gigante, 42C, Sgonico, TS 34010 Italy
AB: On the J.G.R. (2004, Vol. 109, in press) and the B.S.S.A. (2004, Vol. 94, 5, in press) we demonstrated that it is possible to retrieve geometric and kinematic information on the sources of some destructive earthquakes of the past by inverting their regional macroseismic intensity patterns. In fact, in the study cases, the inversion results agree with the seismological instrumental measurements and/or with neotectonic evidence and/or with authoritative tectonic interpretation, independent from our work. The intensity patterns of the following earthquakes were inverted more or less successfully. First, direct validations were feasible in the case of these recent and well-recorded earthquakes: 1987 Whittier Narrows, 1990 Sierra Madre, 1994 Northridge, California; and 1936 Cansiglio, NE Italy, 1990 Santa Lucia, Sicily, Italy; and, less confidently, three small-magnitude events in 1983, 1989 and 2000 in SW Norway. Then, in the case of the earthquakes of the preinstrumental era, which follow, only the compatibility with neotectonic evidence and/or with authoritative seismotectonic interpretation, and the orientation of the maximum horizontal geodynamical compressive stress was demonstrable: we refer to the strongest earthquake that has ever struck the Mediterranean (in the XVII Century, in SE Sicily), and its strong foreshock of Jan. 9, 1693; the 1904 Oslo, Norway, the 1873 Belluno NE Italy, and the 1741 Fabriano, Central Italy, earthquakes. This series of validations, or of plausible results, leads to the hope that more knowledge about pre-instrumental events can be obtained from intensity data: a key towards improving the calculation of seismic hazard, mostly in the Old World. Our technique is able to retrieve 11 geometric and kinematic source parameters that are: the three nucleation coordinates, the fault-plane solution, the seismic Moment, the rupture velocities and rupture lengths along-strike and antistrike, the shear wave velocity in the half-space. We do this by an automatic nonlinear inversion with a Niching Genetic Algorithm (NGA), and by using our simplified KF formula for body-waves that radiate from a linear source (B.S.S.A., Vol. 86, 1019-1027; B.S.S.A., Vol. 89, 1203-1213; B.S.S.A., Vol. 93, 47-60). The NGA inversion finds the minima on the hypersurface of the minimum residuals (calculated-minus-observed intensity at all sites) in the multi-parameter model space. For almost pure dip-slip mechanisms, two minimum-variance source models are found, which resemble the two auxiliary planes of the same theoretical fault-plane solution. In other words, in certain conditions, the problem is bimodal. Some tests suggest that the retrieved fault-plane solutions remain rather stable either in coastal areas or in the presence of asymmetric distributions of the sites surveyed in the field.
DE: 7230 Seismicity and seismotectonics
DE: 7260 Theory and modeling
DE: 8123 Dynamics, seismotectonics
DE: 3260 Inverse theory
SC: Seismology [S]
MN: 2004 AGU Fall Meeting