Study of the seismic sequence of Porto S. Giorgio (Marche)-3 july 1987

Abstract

In this paper we present some remarks about the Porto S. Giorgio seismic sequence, occurred in central Italy on 3 July 1987 with a main shock of Md=4.9. The sequence, occurred in a low seismicity zone, was characterized by absence of foreshocks. Focal mechanisms of the four greatest shocks, derived from P-first arrivals, show compressive solution with negligible strike components; it seems to confirm the tectonic trend of thrust faults remarked by some authors. Some characteristics of the sequence underline the coupling between the cumulative frequency vs. time trend and the modality of a Kelvin's solid strain release which has a time relaxation of about 5 days. Furthermore we analyse b-value and Gutemberg relation to evaluate the energy release. Finally macroseismic analysis related to the felt area is shown with isoseismal map