Abstract
Ground motion scenarios for Mt. Etna are created using synthetic simulations with the program EXSIM. A large data set of weak motion records is exploited to identify important input parameters which govern the modeling of wave propagation effects, such as Q-values, high frequency cut-off and geometrical spreading. These parameters are used in the simulation of ground motion for earthquakes causing severe damage in the area. Two seismotectonic regimes are distinguished. Volcano-tectonic events, though being of limited magnitude (Mmax ca. 5), cause strong ground shaking for their shallow foci. Being rather frequent, these events represent a considerable threat to cities and villages on the flanks of the volcano. A second regime is related to earthquakes with foci in the crust, at depths of 10–30 km, and magnitudes ranging from 6 to 7. In our synthetic scenarios, we chose two examples of volcano-tectonic events, i.e. the October 29, 2002, Bongiardo event (I = VIII) and the May 8, 1914, Linera earthquake (I = IX–X). A further scenario regards the February 20, 1818 event, considered representative for stronger earthquakes with foci in the crust. We were able to reproduce the essential features of the macroseismic field, in particular accounting for the possibility of strong site effects. We learned that stress drop estimated for weak motion events is probably too low to explain the intensity of ground motion during stronger earthquakes. This corresponds to findings reported in the literature claiming an increase of stress drop with earthquake size.
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Acknowledgments
This work has been carried out within the project “Urban Disaster Prevention Strategies Using MAcroseismic Fields and Fault Source” co-financed by the UE—Civil Protection Financial Instrument, Grant Agreement No. 230301/2011/613486/SUB/A5. G.T. benefited from funding provided by the MED-SUV project and SIGMA PON01_00683 project. The MED-SUV project has received funding from the European Union Seventh Framework Programme (FP7) under Grant Agreement No. 308665. The SIGMA PON01_00683 project is co-funded by FESR—Fondo Europeo di Sviluppo Regionale. We very sincerely thank the editor Atilla Ansal and the two anonymous reviewers who helped improve the quality of the manuscript. Our work benefited from the support by Stephen Convay for the English language correction.
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Appendix: Parameter study
Appendix: Parameter study
1.1 Shallow events
In order to identify suitable input parameters for the simulations with the EXSIM code (Motazedian and Atkinson 2005), we tested a number of sets with the aim of matching the empirical GMPEs published by Tusa and Langer (2015) for data recorded on Mt. Etna. For a shallow event we have tested the parameters given in Table 6.
In Giampiccolo et al. (2007) a frequency dependent Q value is reported with Q0 = 30 (at 1 Hz) and an increase f0.5 for higher frequencies. F max of 15 Hz is a standard value that is widely used in stochastic simulation (see for instance Boore 1983, 2009) and is commonly used for sources whose foci are situated in a crystalline-type basement. It is still debatable whether this high frequency cut-off F max is an effect of wave propagation or a characteristic of the source. In any case, the ω2-source model, which is at the basis of the EXSIM code, requires this band limitation for the law of energy conservation.
As mentioned in the main text, the use of the Giampiccolo et al. (2007) attenuation law together with F max = 15 Hz PGA and PGV are somewhat overestimated at close distances, whereas an underestimation of both PGA and PGV is encountered at distances >20–30 km. Greater distances were not considered by these authors for the lack of available data. Using a higher Q0 value (90 instead of 30, and applying the same exponent for frequency dependence), we find a considerable overestimation of PGA and PGV, particularly at distances less than ca. 30 km. One way to resolve the problem is the choice of a lower value for F max . From a physical viewpoint, we may justify a low F max by the fact that the foci of shallow events on Mt. Etna are hosted by layers of sedimentary material—often shales or marnes. This may limit the radiation of high frequencies from such sources. Strong heterogeneities such as faults that are present on the flanks of the volcano, may further limit the propagation of high frequency energy. Choosing F max = 5 Hz the simulated PGA and PGV fit values obtained from Tusa and Langer (see their Table 3) fairly well (Fig. 16).
The simulations carried out for the parameter couple Q0 = 90/Fmax = 15, and Q0 = 30/Fmax = 5 show that—considering the whole range of distances—a tradeoff between Q and F max can be ruled out. The choice of F max affects the peak GMPs (PGA and PGV) at close distances (say less than ca. 20–30 km), whereas the choice of Q is critical for GMPs at greater distances.
As the effects of F max are most evident for peak ground motion values at small epicenter distance, we examine whether there may be the risk of confusion with other simulation parameters, in particular the stress parameter. In a further sequence of simulations, we used the parameter combination shown in Table 7.
It can be seen from Fig. 17a, b that more or less all combinations yield a reasonable fit of the empirical relations by Tusa and Langer (2015). The combination F max = 5 Hz, stress = 5 bar best fits PGA near the source, whereas the combination F max = 3 Hz, stress = 10 bar performs slightly better than the others for PGA at greater distances. The empirical relation for PGV is matched best by the choice F max = 10 Hz, stress = 3 bar. Note that the stress drop reported by Giampiccolo et al. (2007), is found at ca. 5–10 bars. In the end, we adopted the combination stress = 5 bar, Fmax = 5 Hz as a suitable compromise between the various options in our parameter study and the results of Giampiccolo et al. (2007).
1.2 Deep events
For deep events we have used F max = 15 Hz, which is the standard value frequently used in stochastic ground motion simulation. Since the sources are situated in the crystalline-type basement, we have no specific reason to use a different value to the standard. The role of the stress parameter is discussed in the framework of the simulation of the 1818 earthquake (Sect. 5.2). Here we focus on the choice of Q, comparing values of 50, 90 and 150 for Q0, and an exponent of 0.5 for the frequency dependent increase (see Fig. 18). Compared to the empirical relations of Tusa and Langer (2015), the choice Q0 = 90 yields a good match of PGA for small and intermediate distances (<50 km), and a somewhat greater discrepancy at larger distances. PGV is slightly overestimated for small distances and fits better at larger distances. The choice Q0 = 150 tends to overestimate all values at all distances, whereas using Q0 = 50 peak the simulated ground motion values underestimate predictions of Tusa and Langer (2015). This holds specifically for intermediate and greater distances. The choice with Q0 = 90 and frequency dependent increase (f0.5) proves a reasonable compromise for fitting PGA and PGV in the distance range of interest.
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Langer, H., Tusa, G., Scarfì, L. et al. Ground-motion scenarios on Mt. Etna inferred from empirical relations and synthetic simulations. Bull Earthquake Eng 14, 1917–1943 (2016). https://doi.org/10.1007/s10518-015-9823-1
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DOI: https://doi.org/10.1007/s10518-015-9823-1