GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 2.1 311 for the SD method and 58% for the NN method. By representing the identified clusters as topological trees, we note that the trees obtained from SD generally display a more complex internal structure than the trees obtained from NN. The greater complexity might reflect the basic features of the SD method, in particular the multilevel triggering property of the ETAS model, according to which each event is able to generate offsprings. Average leaf depth and average node depth of the trees are used to quantify and systematically analyze the internal complexity of the clusters. To exemplify the features of the two different approaches, we consider in some detail the sequence associated with the earthquake occurred on April 12 th , 1998, M5.6. Fig.1 shows the subset of earthquakes occurred in the years 1998-1999 and the results from the application of the two declustering algorithms. In particular, Fig.1-b shows the events that are associated with the background activity by SD method (top) and by NN method (bottom). Fig.1-c represents the most significant cluster identified by each method, which, in both cases, is triggered by the M5.6 earthquake on 12 April 1998. The cluster has 757 and 720 events according to the SD and the NN methods, respectively, and exactly 682 events are consistently associated to the cluster by both methods. Nonetheless, the tree obtained from SD shows a more complex internal structure than the tree obtained from NN, as also confirmed by the higher values of average node depth and average leaf depth: 6.20 and 6.18, respectively, as for the SD method, and 1.58 and 1.50 as for the NN method. This difference can be explained by a more complex, multilevel triggering model associated with the SD method; this model tends to identify longer sequences, eventually composed by multiple sub-clusters. Fig. 1 - (a) Magnitude versus occurrence times of all the earthquakes occurred in the years 1998-1999; (b) Results from SD method [top] and NN method [bottom] as for the identified background events occurred in 1998-1999. (c) The largest earthquake cluster identified by SD method [top] and NN method [bottom] in 1998-1999; the initiating background earthquake is denoted by a black dot, the subsequent triggered events identified by both methods are in blue, and the triggered events identified by only one method are in red.