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

Abstract

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 first 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 classified 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