Abstract
Abstract. The volcanic region of Mt. Etna (Sicily, Italy) represents a perfect lab for testing innovative approaches to seismic hazard assessment. This is largely due to the long record of historical and recent observations of seismic and tectonic phenomena, the high quality of various geophysical monitoring and particularly the rapid geodynamics clearly demonstrate some seismotectonic processes. We present here the model components and the procedures adopted for defining seismic sources to be used in a new generation of probabilistic seismic hazard assessment (PSHA), the first results and maps of which are presented in a companion paper, Peruzza et al. (2017). The sources include, with increasing complexity, seismic zones, individual faults and gridded point sources that are obtained by integrating geological field data with long and short earthquake datasets (the historical macroseismic catalogue, which covers about 3 centuries, and a high-quality instrumental location database for the last decades). The analysis of the frequency–magnitude distribution identifies two main fault systems within the volcanic complex featuring different seismic rates that are controlled essentially by volcano-tectonic processes. We discuss the variability of the mean occurrence times of major earthquakes along the main Etnean faults by using an historical approach and a purely geologic method. We derive a magnitude–size scaling relationship specifically for this volcanic area, which has been implemented into a recently developed software tool – FiSH (Pace et al., 2016) – that we use to calculate the characteristic magnitudes and the related mean recurrence times expected for each fault. Results suggest that for the Mt. Etna area, the traditional assumptions of uniform and Poissonian seismicity can be relaxed; a time-dependent fault-based modeling, joined with a 3-D imaging of volcano-tectonic sources depicted by the recent instrumental seismicity, can therefore be implemented in PSHA maps. They can be relevant for the retrofitting of the existing building stock and for driving risk reduction interventions. These analyses do not account for regional M > 6 seismogenic sources which dominate the hazard over long return times (≥ 500 years).
Highlights
The most severe effect of this tectonic activity is the intense seismicity shaking the urbanized areas of the volcano, with obvious implications arising in terms of seismic hazard
The role of a different, wider stress field acting in the Timpe area – a structurally homogeneous domain characterized by a general east–west extension (Bousquet and Lanzafame, 2004) – is proved by the analyses of long time series of geodetic and seismic data (Bruno et al, 2012; Solaro et al, 2010; Bozzano et al, 2013; Palano, 2016, and references therein), highlighting the influence of large-scale instability processes where the strain is released by a steady process on decennial timescale (Bonforte et al, 2011)
A relatively minor variation of b values (0.9– 1.1) is evident at intermediate crustal levels in the range of 10–16 km, while at depths higher than 20 km low b values (≤ 0.9) prevail again. This overall picture shows analogies with the pattern found by Murru et al (1999, 2007) on a temporally different earthquake dataset (1999-2005), highlighting two areas characterized by higher b values than other surrounding areas: (i) beneath the central craters and (ii) in the eastern flank, at a www.nat-hazards-earth-syst-sci.net/17/1981/2017/
Summary
The role of a different, wider stress field acting in the Timpe area – a structurally homogeneous domain characterized by a general east–west extension (Bousquet and Lanzafame, 2004) – is proved by the analyses of long time series of geodetic and seismic data (Bruno et al, 2012; Solaro et al, 2010; Bozzano et al, 2013; Palano, 2016, and references therein), highlighting the influence of large-scale instability processes where the strain is released by a steady process on decennial timescale (Bonforte et al, 2011) In this scenario we assume that modeled faults are constantly (on average) loaded in time as expected in a typical tectonic process. We consider the role of eruptive activity controlling the occurrence of low-magnitude earthquakes in the central-summit area of the volcano to be negligible for seismic hazard purposes, since this is an uninhabited zone and the risk is very low
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