Evidence of Amplification Effects in Fault Zone Related to Rock Mass Jointing

Abstract

Results of geological, geomechanical and seismometric investigations aiming at the analysis of the seismic response in a carbonate ridge of the Nera River valley (Central Apennines -Italy) are discussed. Geological and geomechanical surveys were aimed at defining the stratigraphic and structural setting of the outcropping formations and the jointing conditions of the rock mass. Velocimetric records of both ambient noise and small-magnitude earthquakes were analysed in order to identify amplification conditions. The analysis was carried out in the time domain, through directional energy evaluation, and in the frequency domain, through H/V spectral ratios and spectral ratios with respect to a reference station. A local amplification factor was estimated from Housner intensity. The study revealed a significant seismic amplification in a fault zone. This effect was observed in intensely jointed and mylonitic rock masses, located inside moderately jointed rock masses, and is the result of specific geometries and significant impedance contrasts. A map of fault zones prone to amplification of ground motion was constructed, taking into account the jointing conditions of the rock masses and the structural setting of the investigated ridge. The study relied on an integrated methodological approach, which combined the available data under union and intersection criteria.

The role of tectonic damage and brittle rock fracture in the development of large rock slope failures

“…23 illustrates for the Canadian landslides described in this paper how the rock mass structure and discontinuity surface conditions are degraded as the zones associated with tectonic damage move downward and to the right in the GSI table. Recent work by Martino et al (2006) also found that rock mass quality as quantified by the number of discontinuities per unit volume increased and the average size of the rock block decreased near faults. They also found the seismic amplification can occur in fault zones if there is a sharp contrast in rock mass quality between the background rock mass and the fault zone (Martino et al, 2006).…”

Numerical and experimental evaluation of site effects at ridges characterized by complex geological setting

“…valley effects (Semblat et al 2005;Pagliaroli et al 2014 among others), a satisfactory understanding of topography effects, especially from a quantitative point of view, has not yet been achieved (Pagliaroli et al 2011). This is essentially because of the intertwining of effects related to local morphology with those associated to stratigraphic features (e.g., soft debris aprons resting on rocks), near surface weathering (LeBrun et al 1999) and complex structural heterogeneities (e.g., pervasive fracturing) generally due to the presence of fault zones (Rovelli et al 2002;Martino et al 2006). The latter aspect has been specifically investigated by Marzorati et al (2011), who highlighted the primary influence of rock mass condition (orientation and opening of fractures and joints) on ground motion amplification along rocky ridges, where site effects are generally supposed to be related to ''canonical'' morphological/topographic issues.…”

Detection of directivity in seismic site response from microtremor spectral analysis

“…In the last two decades some workers Vidale et al, 1991;Spudich et al, 1996;Xu et al, 1996;Martino et al, 2006) have reported cases of notable directional variations of seismic site response under different geological and topographical conditions. Our recent studies provided new evidence of this phenomenon (Del Gaudio and Wasowski, 2007).…”