GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 3.1 45 this kind of signals, the ones recorded at Madonna del Sasso have the highest frequency content in the band 20-30 Hz. The lower frequency may be related to bigger source dimension, since it is known that the frequency content of the signal emitted by the fracture mechanism is a scale- dependent parameter (Amitrano et al. , 2010). On a series of 5-month monitoring data (over 2000 events) only 93 signals show these features (~4.5%). Regional earthquakes (Fig. 2b) show lemon-shape envelope and long duration (30-60 s). Most of them are characterized by quite well separated impulsive P- and S-wave onsets. Their kurtosis values are always lower than 10. The maximum energy content is in the frequency range between 1 and 20 Hz, depending on the epicentral distance. Comparing the date and time of earthquake national and international catalogues, we surely identify 25 signals relative to this source mechanism. Noise transients generated by electrical disturbances (Fig. 2c) are impulsive spike-like signals, with very short duration and high energy content (50-100 Hz). They are probably caused by atmospheric electrical discharges and other electrical disturbances due to the length of the connecting cables between the stations and the acquisition instrument, according with what observed by Spillman et al. (2007). Not by chance, most of this type of signals affect only ST1 and, in smaller amounts, ST2 which are located on the cliff walls with connection cables of more than 50-meters length. A high number of these signals was detected (~100). At the present stage of this study, we grouped all the other events in an unclassified class (Fig. 2d). It may include rockfalls, quarry blasts and other sources of natural or anthropic Fig. 2 – First classification of different types of signal: a) a microseismic event; b) an earthquake (Northern Italy, M L =3.8, 2013-11-21 10:36:49.0 UTC, 44.91N 9.03E); c) electrical transient and d) still unclassified events.