An Improved Method for the Recognition of Seismic Families: Application to the Garfagnana-Lunigiana Area, Italy

摘要

With the aim to find a more objective way to detect seismic families,we applied a series of successive steps to constrain the results of a waveform similarityanalysis. The evaluation of similarity was carried out on the waveforms recordedin the period 1999–2003 by the stations operating in the Garfagnana area,located in northern Tuscany (Italy). The algorithm is based on the cross-correlationtechnique applied in a process that overcomes the limit of one order of magnitudebetween events to be compared through a bridging technique. In practice, if twocouples of events (A, B) and (B, C), each exceeding the correlation threshold, sharea common quake (B), then all three events are attributed to the same family even ifthe match between A and C is below a value chosen as a reference for similarity.To avoid any subjective choice of threshold for cross-correlation values, the resultsfrom the computation algorithm are submitted to a routine that gives increasingreliability to them if they are confirmed by the three components of the seismogramand if the number of families detected by each station is confirmed by more recordings.This latter constraint is made possible by the geometry of the recording network,with interdistances between stations of the order of 40–50 km. The process finallyleads to the recognition of 27 families detected and confirmed by, on average, 3stations that represent 40% of the recording capabilities. Since the performances ofthe recording network have been very odd in the past, especially in the early yearsof operation, the reliability of the detection is much higher, as in most cases thestations that detected the families were the only ones to be effectively recording.The methodology proved to be more efficient than other methods applied in thepast; moreover, the results could be probably improved even more if, instead of doinga one-run process, it would be borne as a trial-and-error approach.