GNGTS 2013 - Atti del 32° Convegno Nazionale

Santamaría-Gómez, A., Bouin, M.N., Collilieux, X., Wöppelmann, G., 2011. Correlated errors in GPS position time series: Implications for velocity estimates, J. Geophys. Res. 116, B01405, doi:10.1029/2010JB007701. Scargle J. D., 1982: Studies in Astronomical Time Series Analysis II. Statistical Aspects of Spectral Analysis of Unevenly Sampled Data. Astrophysical Journal, 263, 835-853. Shen Z.K., Jackson D.D. and Ge B.X., 1996: Crustal deformation across and beyond the Los Angeles basin from geodetic measurements. J. Geophys. Res. 101, 27957-27980. Wessel P. and Smith W.H.F.; 1998: New, improved version of the Generic Mapping Tools released . Eos Trans. AGU, 79, 579. Williams, S.D.P., Bock, Y., Fang, P., Jamason, P., Nikolaidis, R-M., Prawirodirdjo, L., Miller, M., Johnson J., 2004. Error analysis of continuous GPS position time series, J. Geophys. Res. 109, B03412, doi:10.1029/2003JB002741. Williams, S.D.P., 2008. CATS: GPS coordinate time series analysis software, GPS Solutions 12(2), 147–153, doi:10.1007/ s10291-007-0086-4. Local vs. Duration magnitude relationship for the Mt. Etna seismic catalogue S. D’Amico, E. Giampiccolo, T. Tuvè Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo, Catania, Italy Introduction and state of the art. Studies on seismicity at Mt. Etna are of extreme importance for the high seismic and volcanic risk which characterizes the area. In this area, seismic events are in part located at less than 5 km b.s.l. depth, producing arrivals with medium- to low-frequency content and/or complicated signatures at stations just a few kilometers distant from the epicentral area (Patanè and Giampiccolo, 2004); on the other hand, earthquakes which present high frequency content and sharp arrivals, similar to those of typical earthquakes of tectonic areas, are mainly located between 5 and 20 km. Seismicity mainly occurs in the form of swarms, whereas foreshock-mainshock-aftershock sequences are rarely recorded, and seldom exceeds magnitude 4.0 (Ferrucci and Patanè, 1993). Notwithstanding, the estimation of magnitude is considered as an important parameter to be routinely measured with minimum uncertainty. In fact, an increase of the magnitude level of the local events, accompanied by migration of the foci and/or a change in the focal volume, is considered as an indicator of a change in the volcano-dynamics (Del Pezzo and Petrosino, 2001). Moreover, the occurrence of severe damage during small magnitude events is not new in the Mt. Etna area where previous earthquakes (in the years 1865, 1911, 1914, and 1952) attained epicentral intensities as large as IX-X EMS (Grünthal, 1998), in spite of macroseismic magnitudes M M < 5 (Azzaro, 2004). Since the 80’s, the earthquake magnitude at Mt. Etna has been routinely estimated by the duration of the seismic event ( M D ), by using Caltabiano et al. (1986) relationship: M D = - 1.367 + 2.068 log τ + 0.212 log Δ (1) where τ is the duration time of the event in seconds calculated at a reference station and Δ is the hypocentre distance in km. Duration magnitude is determined visually on the drum recorders, from the onset of the first arrival up to the time when the amplitude of the coda is not distinguishable from the background noise. After Caltabiano et al. (1986) a new formula was introduced by Cardaci and Privitera (1996), based on the methodology proposed by Real and Teng (1973), which takes into account the stations response and the medium characteristics at each station. However, in order to ensure a homogenous database of magnitude observations, for monitoring purpose the Cardaci and Privitera (1996) relationship had never been adopted and the Caltabiano et al. (1986) formula is still in use at INGV- OE. In volcanic areas the calculation of the local magnitude M L is more objective than that of M D because the measurement of the signal amplitude is less ambiguous with respect to the decay of 34 GNGTS 2013 S essione 2.1