A mixed automatic-manual seismic catalog for Central-Eastern Italy: analysis of homogeneity

A comparison between pickings and locations obtained by automatic
and manual procedures in the analysis of the seismicity of Central-Eastern
Italy is presented. In a ?rst step we compared automatic and manual
pickings, demonstrating that in many cases the adopted algorithm,
after some tuning, is able to reproduce both the timing and the weight
assignment of a human operator. The comparison of automatic and manual
locations allowed to demonstrate that, when the automatic procedure
is able to reach a solution stable from the statistical point of view,
these locations are comparable with the manual ones within the estimated
error limits. Once established these reliability criteria, we began
to produce a mixed automatic-manual catalog: the events located by the
automatic procedure with estimated errors below the selected thresholds
(2 km in horizontal and 3 km in vertical) were directly introduced in
the catalog, other events were revised by a human operator. In this way
more than 64% of the events did not need human intervention, allowing
to correctly manage also a period of increased seismicity, characterized
by more than 4000 events per month: in total 121894 events were located
with good accuracy in a time period of less than 7 years (August 2009
- April 2016). In a last step, a further control of the reliability of the whole
procedure was performed, by manually analyzing all the events occurred
in the last month of the analyzed period and classi?ed as reliable by the
automatic procedure: two expert seismologists interpreted these events,
and the comparison demonstrated that the differences between the automatic
and manual pickings and locations are slightly larger, but comparable
with the differences between two human operators. As further
checks, an analysis of the distribution of the depth estimates on the whole
catalog demonstrated that data from the manual or the automatic part
are nearly indistinguishable for the central, better monitored area; furthermore
the automatic system demonstrated to be able to correctly locate
also quarry blasts, with a reasonable estimate of the depth of these
very critical events. Finally, a quick look at the geographical and depth
distribution of the seismicity summarized in the catalog is presented;
also in this case the main result is the good overlap of automatic and
manual locations, at least for the well-monitored areas.