Acoustic emission and seismicity: the case histories of Colfiorito, Molise, Cephalonia, with particular reference to the l’ Aquila earthquake G. Paparo 1,2, G.P. Gregori1, M. Poscolieri1, C. Rafanelli1, S. De Simone1, V. Gallo1, G. Ventrice3 1 CNR–IDAC, Gruppo ICES, via del Fosso del Cavaliere, 100, Roma, Italia 2 Ambasciata d’Italia a Buenos Aires, Calle Billinghurst, 2577 1425 Buenos Aires, Argentina 3 P.M.E. Engineering, Via Tromello,32, 00135 Roma, Italia Correspondence to: M. Poscolieri ([email protected]) Abstract: Acoustic emission (AE) displays violent paroxysms preceding strong earthquakes, observed within some large area (several hundred kilometres wide) around the epicentre. We call them “storms of crustal stress” or, briefly “crustal storms”. A few case histories are discussed, all dealing with the Italian peninsula, and with the different behaviour shown by the AE records in the Cephalonia island (Greece), characterized by a different tectonic setting. AE is an effective tool for diagnosing the state of some wide slab of the Earth’s crust, and for monitoring its evolution, by means of AE at different frequencies. The same effect ought to be detected being time-delayed, when referring to progressively lower frequencies. This results into an effective check for validating the physical interpretation. A “crustal storm” typically involves some large slab of lithosphere and crust, but it cannot be easily reckoned to any specific seismic event. In fact, an earthquake responds to strictly local rheological features of the crust, which are eventually activated, and become crucial, on the occasion of a “crustal storm”, which in contrast involves a much wider region. A “crustal storm” lasts typically a few years, eventually involving several destructive earthquakes that hit at different times at different sites that lithospheric slab. The results of the present study, concerning AE time-series collected at 25 kHz (hereafter LF AE) and 150 kHz (hereafter HF AE), haves been submitted to Natural Hazards and Earth System Sciences. With regard to the case histories here discussed, one slab deals with the Italian peninsula. During 1996-1997 a “crustal storm” was on, maybe elapsing until 2002 (we lack information for the period 1998-2001). Fig 1: AE recording sites, and epicenters of the earthquakes mentioned in the text Then, a quiet period occurred from 2002 until 26 May 2008, when a new “crustal storm” started, and by the end of October 2009 it is still on. During the 19961997 “storm” two strong earthquakes occurred (Potenza and Colfiorito) and (maybe) in 2002 also the Molise earthquake can be reckoned to this “storm”. During the “storm” started in 2008 the l’Aquila earthquake occurred. Some basic additional logical analysis envisages the possibility of distinguishing some kind of “elementary” constituents of a ”crustal storm”, which can be briefly called “crustal substorms”. The concept of “storm” and “substorm” is a common logical aspect shared by several phenomena, depending on their lognormality and fractality. Compared to a “crustal storm”, a “crustal substorm” is likely to be reckoned to some specific seismic event. Owing to brevity purposes, however, the discussion of “substorms” is given in Gregori et al. (2009). AE is an effective tool for monitoring these phenomena, and the processes that are ongoing inside the crust. Eventually they result to be precursors of some more or less violent earthquake. It should be stressed, however, that the target of AE monitoring is diagnosing the Earth’s crust. In contrast, earthquake prediction implies a much different perspective, which makes sense only by making reference to some more detailed multiparametric monitoring. An AE array can provide real physical information only about the processes that are objectively ongoing inside different large slabs of the crust. Fig. 2 Figure 2 shows the weighted moving average values of the raw LF AE data at Orchi site (see fig. 1) for the 2002-2009 time lag, superposed with an offset, in order to distinguish the different years. One unique large peak appears, just a “crustal impulse” lasting 12 hours, displaying an intensity impressively larger than the signal implied by a “crustal storm”. It preceded the l’Aquila earthquake by 32 days. After this event the signal stabilized its intensity on a higher value (presumably because the acoustic impedance of the “natural probe” had changed). Figure 3 shows a detail of this peak from the raw AE LF data. It exhibits some internal structure, being the likely evidence of some lines of the tidal spectrum and/or by some free Fig. 3 oscillation of the Earth (this item is to be investigated by means of the outlier series). Figure 4 shows the original database, with no rejection of outliers, and no smoothing, while the ordinate scale is limited to the range 0-0.1. The anomalous signals associated with the outliers can be recognized. These data were the objects of several papers (Gregori et al., 2005, 2007; Paparo et al., 2006; Poscolieri et al., 2006a). The vertical yellow rectangle indentifies the aforementioned “crustal impulse” (see figs. 2 and 3). The central vertical orange arrow in figure 4 indicates the beginning of an increasing trend, which occurred since June 2008, it elapsed until the end of 2008 (right vertical orange arrow), and it continued through 2009 (left vertical grey arrow). This is a “crustal storm”, as it better shown in figure 5 (raw input data), where the storm is monitored in HF AE and LF AE both at Orchi and at Valsinni. The storm appears to have started shortly before 26 May 2008, when the Valsinni station began to be operated. All four records, either HF-AE or LFAE, either at Orchi or at Valsinni, seem to increase almost simultaneously . Figure 4 also displays a gentle seasonal modulation, which is much better evidenced in the HF AE (figure 6). Another peculiarity of figure 6 is concerned with the MFE’s (minor fracture events, i.e. abrupt discontinuities of the acoustic impedance of the natural probe underground). Whenever needed the data analysis was sometimes carried out separately over three subsets, upon selecting the data set referring to positive MFE’s, to negative MFE, and to “normal” AE records, respectively. References Fig. 4 Fig. 5 Fig. 6 Chiappini M., Gregori G.P., Paparo G., Bellecci C., Crisci G.M., De Natale G., Favali P., Marson I., Meloni A., Zolesi B., Boschi E.; 2002: Stromboli. A natural laboratory of environmental science. 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