Abstract

Within the ”International Bathymetric Chart of the Mediterranean (IBCM)” project of the Intergovernmental Oceanographic Commission (IOC) of UNESCO, thematic maps have been constructed; one theme selected is the seismicity within and around the Mediterranean Sea. To construct a map of seismicity, one has to report, on a map with a geographic background, the symbols representing occurrences of earthquakes. Current practice is to assign ”attributes” to the epicenter as a function of the earthquake&�146; s parameters, such as focal depth, magnitude, etc., and to draw a symbol centered on the epicenter’ s projection of each earthquake onto the map; then the reported symbol itself varies (in size, shape, ornaments, color, etc.) according to the epicenter’ s attributes. The natural choice for background map was the bathymetric base map of the IBCM project (30° - 46° N, 6° W -36.5° E + an insert for the Black Sea: 40° - 47.5° N , 26.5° - 42.5° E). Ideally the dataset reported on the map, should have been worked out for this purpose. It should have included one record per physical seismic event, with the parameters estimated/computed through a homogeneous procedure; this was clearly out of reach, as no general earth quake database has yet been established for the Mediterranean and surroundings areas. The decision to rely on datasets easily accessible to users was taken; this ruled out non-instrumental seismicity data (most often called ”historical seismicity”). The European-Mediterranean Seismological Centre (EMSC) has been computing and disseminating earthquake locations (epicenters and focal depths) in a homogeneous manner, since 1976; although EMSC has not determined magnitudes, but has simply reported estimations by other institutions, it was considered that, because earthquake location is the predominant parameter here, the EMSC sub-dataset’ s homogeneity overpasses the other defects. For the instrumental period before 1976, the sub-dataset chosen to start is the ”earthquake catalogue” published by the International Seismological Centre (ISQ; this ”catalogue” actually often reports several determinations of earthquake parameters issued by different institutions, together with the ISC’ s proper determination when it has been computed (from 1964 on). A careful analysis of the ISC’ s sub- dataset has been conducted to unravel its intimate characteristics. A rationale has been developed to select the determination of parameters to adopt, so as to end up with a final sub-dataset showing one parameter determination for each physical seismic event. The details of the selection procedure are set down together with the statistical properties on which they are based. This procedure is not expected to be the only reasonable one to construct a map of seismicity; it is simply one way of solving the problems posed. Users will find the basic information necessary to check the procedure and its implementation, or even to design their own way of handling the datasets. Limitations on the reasoning are also stressed; they have, in turn, an influence on how to display the final results graphically.

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