GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 1.1 99 Picker is based on an iterative procedure for the automatic identification of phase arrival times by calculatingAIC functions (Akaike Information Criterion; Akaike, 1974). Iterations consist of different steps, separately performed for P- and S-phases, where pick identification is checked and refined based on computed locations. NonLinLoc software (Lomax et al., 2000) has been used for locating events, using a 1D model calibrated for the area (De Luca et al. , 2009). The picking analysis is still in progress; two computers are working in parallel, one analysing the sequence from the beginning and the other one starting on 1 January 2017. Fig.1 compares the number of rather-good locations obtained for each day by the automatic picker as compared with the data reported by the ISIDe database (ISIDe working group, 2016). With rather-good quality we intend locations obtained with at least 8 phase readings and computed horizontal location error less then 5 km. As a mean term, the number of automatically located events is, for each day, from 4 to 5 times that of the ISIDe database; overall, taking into account just the overlapping time windows, ISIDe reports 47815 events, while the automatic system obtained 229674 locations (corresponding to a ratio of 4.8). Fig. 1 - Daily distribution of the number of events of the ISIDe catalogue (black line) and of the new automatic picking catalogue (blue line), when available. Preliminary quality analysis. A first comparison of the automatic locations has been performed with respect to the locations of the ISIDe database (Fig. 2a). As regards the epicentral estimate, the two datasets show a very good coherence, with about 93% of epicentral differences lying within 2 km. On the contrary, the depth differences are larger, and show a significant offset (the automatic locations are shallower): this could be also due to the different propagation model used in the two procedures. This comparison is not complete, because it is relevant just to the events reported by the national monitoring of ISIDe, thus limited to higher magnitude events. In order to check the accuracy of location of low-magnitude events, we hand-picked as set events, detected by the automatic procedures within a few hours long time window with a mean-term rate of seismicity and a mean-term percentage of working stations. We selected the first hours of 9 January 2017. The comparison of locations, limited to the best solutions, is reported in Fig. 2b: nearly all the events are located within 2 km of epicentral differences, while about 92% of the events show a difference below 1 km. As regards the depth difference, it seems that for some events