GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 1.1 93 We test different Antelope® configuration parameters on the signals recorded during 2011 and the results are compared with the locations retrieved from the NEI bulletin produced by OGS; in this comparison we compute the percentage ratio of the earthquakes recognized by our system and these parameter guides us in the selection of the best configuration model. Further we check that Antelope® loses no earthquake of the NEI bulletin with M L ≥1.0; this value is chosen as minimum threshold for the completeness of our new catalogue. It is worth to evidence that we are interested in detecting the large number possible of earthquakes without considering contingent false events that will be deleted by a later manual revision. Results. As previously described Antelope ® is a modular software that performs the automatic through three steps in sequence, the first referred to the detection procedure, the second to grid associator and the last to GENLOC inversion. The detection parameters are mainly related to the selection of different frequency ranges whereby the SNR ratio is computed and the association with the related grid is done; hence the detection is strongly correlated with the definition of the grids for the association that are set considering the geographical distribution of the recording stations. Three different grids are selected for local, regional and teleseismic events; the local grid concerns a more extended NEI area, the regional grid overlays Europa and Mediterranean areas while the teleseismic grid refers to global (other/teleseimic) earthquakes. The travel times are computed for each path between the recording stations and the grid nodes adopting standard IASPEI velocity model (Kennett, 1991); the local grid nodes have an horizontal spacing length of 4.5 km (comparable with the maximum uncertainty in the NEI bulletin) while the vertical spacing length is variable and it increase in the deeper layers. The time windows for the ratio of the short and long terms average are defined separately for the four different detections (‘Local 1’, ‘Local 2’, ‘Regional’ and ‘Teleseismic’) while the trigger-on and trigger-off SNR thresholds are fixed differently for each grid (Tab. 1). The definition of minimum and maximum time when the detection is active is very important to recognize the complete sequence of multiple events very close each other on the time axis because Antelope ® does not allow the opening of a new detection (hence the recognition of a new earthquake) if an old detection is not still closed. Since we are interested in recognizing as more earthquakes as possible on local (and partially regional) scale these parameters are set in different way for each considered grid (Tab. 1). The parameters for regional and teleseismic events are the same of the real-time configuration because we are interested in discriminating efficaciously between local and not local earthquakes without analysing in detail the regional/global seismicity. Also in the local detection we select the same parameters of the real-time system (column ‘Local 1’ in Tab. 1) but we add a new detection (column ‘Local 2’ in Tab. 1) with lower SNR threshold, therefore more sensitive to the microseismicity that the parameters ‘Local 1’ do not recognize efficiently. Further the S phases are detected by fixing the parameters proposed by Garbin and Priolo (2013) for the Trentino area. After fixing the parameters for the detection procedure, the second step concerns the association on grid; also in this step we keep the parameters of the real-time system for the regional/teleseismic events with a minimum number of phases fixed to 10 for regional events and 14 for teleseismics. The local events are located with two different configurations with the priority of ‘Local 1’ higher than ‘Local 2’ (Tab. 2). Both configurations adopt the minimum phases depending from the maximum source-receiver distance in degrees for defining associations; this is used to define low numbers-of-stations solutions only for situations where the available stations are relatively close to the event. Any solution with number of stations less than the first entry in the table is dropped while any solution with number of stations that is between two of the table entries will use the distance value from the previous (lower) entry value. In the first case the minimum number of phases is very low (4 P phases within � ��� ≈ ��� 130 km) because the distribution of the stations in Veneto is not very dense so the recognition of the microseismicity is more difficult and a considerable number of false events are detected. The application of weights depending from the source-receiver distance on the P phases reduces