Vol. 40, n.2, June 1999 pp. 79-146
Investigating instrumental seismicity data sources for the Mediterranean region (IBCM-S)
Y. Legros and J. Bonnin
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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