GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 1.1 95 Fig. 3 compares the distribution of the events in relation to the magnitude estimations for the old and new catalogues: the numbers of earthquakes increase strongly in the microseismicity range and also some events with negative magnitudes are located (the lowest values are -0.8). This issue evidences that our procedure can detect a larger number of seismic events in the microseismicity field improving the catalogue completeness and the knowledge of the seismicity for the studied area; further this procedure helps strongly the operators that manually recognize the seismic events for the bulletin purposes. Clearly the spatial density of the recordings stations and the related level of background noise influence strongly the sensibility of the detections and the capacity of locating also the microseismicity both in Antelope ® real-time system and in the bulletin production. Conclusions. ��� ��� �� ���� ����� �� ��� ����������� �� � ��������� ��� ��������� �������� The aim of this study is the development of a procedure for automatic off-line locations in NE Italy; these locations are performed by Antelope ® software that detects and locates automatically all seismic events, also the weakest ones concerning the microseismicity field. It is worth to evidence that a final manual revision is anyway necessary to check and to validate the automatic results, deleting possible false events; further we are discussing about automatic (and not manually revised) locations so it is possible that in some cases the goodness of the results can seem not very satisfactory. In the automatic location we associate both P and S phases in case of local earthquakes; however the S phases can be more difficult to pick with acceptable uncertainties and their propagation in the Po Plain is very complex (Bragato et al., 2011b) to be model by simple 1D velocity model. The detection parameters are very important to recognize the seismic events and to discriminate between local, regional and teleseismic earthquakes; in particular the frequency range for filtering the signals in the detection procedure and the grids definition are strictly connected and the selection of appropriate parameters are a fundamental step for all following procedures. Further the system must be capable to detect as many events as possible also in case of earthquakes that occur few seconds from each other; the SNR ratio can be not very efficient in this case because the earthquakes can cover each other in the STA/LTA computation. In this case the configuration of parameters ‘det_tmin’ and ‘det_tmax’ seems to work satisfactory as demonstrated in cases of multiple events correctly detected (for both P and S phases) and located by Antelope ® . Fig. 3 – Comparison of the distribution of events for magnitude for old and new catalogues.