ALBERT

Description: Amplification effects on topographic irregularities have been thoroughly studied in the last decades. Although the number of observations is significantly increased, many aspects are still controversial. The main difficulty is to reproduce the level of amplification through theoretical models as the dependence of ground motion on the topography geometry and shape is not systematic. Moreover, observed amplification often realizes along a site-specific azimuth with a not clear theoretical explanation in terms of topographic features. Recent studies performed by the NERA-JRA1 working group have found that amplification at rock-sites on topographic irregularities is rather controlled by the subsurface velocity structure, high levels occurring at non-A sites of the Eurocode-8 classification. We present the results of seven study cases in central Apennines (Nocera Umbra, Narni, Cerreto Laziale, Introdacqua, Rovere, Campo Imperatore, Monte Ocre) trying to deepen the dualism between morphology and rock properties based on similarity and diversity of effects among them. Ground motion properties are investigated through earthquake and ambient noise records and nearsurface velocity profiles are assessed at some sites. In particular we focus on the role of rock fractures on topography. Fractures play a double role in decreasing the near-surface Vs values because of weathering and generating anisotropy in the rock compliance. As a matter of fact, the occurrence of strong directional amplification is common to all the investigated sites, and we ascribe this effect to the oriented fracture field of the sites. Results from structural geological surveys confirms the strict dependence of directional amplification on rock fractures. Since topographic irregularities and rock fractures coexist in the Apennines, we conclude that this is a key point to interpret amplification at sites with pronounced topography.

Description: Unpublished

Description: Vienna

Description: 3T. Pericolosità sismica e contributo alla definizione del rischio

Description: open

Description: The topographic amplification of seismic waves has received an increasing interest in the last four decades following observations of large amplification on mountain tops (e.g. Davis and West, 1973; Griffiths and Bollinger, 1979; Çelebi, 1987; Umeda et al., 1987; Kawase and Aki, 1990; Ponti and Wells, 1991; Hartzell et al., 1994; Pedersen et al., 1994a; Chavez-Garcia et al., 1996). The recurrence and consistency of these observations has motivated much work both in terms of theoretical investigations and numerical simulations of the diffraction of seismic waves caused by the topography (e.g. Bouchon, 1973: Bard and Tucker, 1985; Géli et al., 1988; Anooshehpoor and Brune, 1989; Gaffet and Bouchon, 1989; Sanchez-Sesma and Campillo, 1991; Pedersen et al., 1994b; Le Brun et al., 1999; Paolucci, 2002). The simulations and the observations are often in qualitative agreement with the amplification at the topography top and for wavelengths comparable to the mountain width. The disagreement concerns the calculated amplification level that tends to underestimates observations. This discrepancy has suggested that other effects could be responsible of the amplification effect, as the geological setting, complicated incident wave field, more complex topography, etc (Bard and Chaljub, 2009). Beside strong amplification, topographic irregularities have been recognized to produce directional effect of resonance; the scattered wave field is polarized in a site-characteristic direction. Spudich et al. (1996) found that directional amplification occurs transversally to the hill major axis, as subsequently assessed by several other authors (e.g., Del Gaudio and Wasowski, 2007; Massa et al., 2010; Pischiutta et al., 2010). In the framework of a statistical study performed using stations of the Italian seismic network to check the recurrence of directional amplification effects, Pischiutta et al. (2010 and 2011) and Rovelli et al. (2011) investigated the relation between the direction of maximum amplification and the hill elongation at around 40 selected stations of the Italian seismic network. They found that only the 25% of stations showed an angular relation between directional amplification and the hill elongation ranging from 80 and 90 degrees. The same conclusions were reached by Burjanek et al. (2014a and 2014b) who investigated the relation between the S-wave velocity profiles and the amplification occurrence at 25-instrumented sites with complex topography in Switzerland and Japan. They stressed that the amplification was controlled primarily by the sub-surface velocity structure and they did not identify any link between the surface topography and the observed response at the studied 25 sites. Thus recent findings have suggested that large systematic amplifications at topographic sites cannot be explained by surface geometry only, and that although the effect of geometry is present, it cannot be simply decoupled from the site response. We think that directional amplification observed at sites with pronounced topography are often correlated with rock fractures. This feature has not considered adequately so far. Here we propose a model that could explain directional amplification. Similar effects have been recently observed (Marzorati et al., 2011) and associated to gravitational instabilities (Burjanek et al., 2010) as well as to fault damage zones (Falsaperla et al., 2010; Pischiutta et al., 2012 and 2013; Di Giulio et al., 2013). Pischiutta et al. (2014 and 2014) interpreted the strong polarization in terms of fracture fields that make the rock more compliant in the strike-transverse direction (Pischiutta et al., 2012 and 2014).

Description: Unpublished

Description: Bologna

Description: 3T. Pericolosità sismica e contributo alla definizione del rischio

Description: restricted