S. Hailemikael

From Non-invasive Site Characterization to Site Amplification: Recent Advances in the Use of Ambient Vibration Measurements.

A series of investigations has been carried out over the last decade in Europe aimed at deriving quantitative information on site amplification from non-invasive techniques, based principally on surface wave interpretations of ambient noise measurements. The present paper focuses on their key outcomes regarding three main topics. First, methodological, hardware and software developments focusing on the acquisition and the processing of both single point and array microtremor measurements, led to an efficient tool with in situ control and processing, giving rise to robust and reproducible results. A special attention has been devoted to the derivation and use of the Rayleigh wave ellipticity. Second, the reliability of these new tools has been assessed through a thorough comparison with borehole measurements for a representative – though limited – set of sites located in Southern Europe, spanning from stiff to soft, and shallow to thick. Finally, correlations between the site parameters available from such non-invasive techniques, and the actual site amplification factors as measured with standard techniques, are derived from a comprehensive analysis of the Japanese KIKNET data. This allows to propose alternative, simple site characterization providing an improved variance reduction compared with the “classical” VS30 classification. While these results could pave the road for the next generation of building codes, they can also be used now for regulatory site classification and microzonation studies, in view of improved mapping and estimation of site amplification factors, and for the characterization of existing strong motion sites.

The Seismic Microzonation of San Gregorio Through a Multidisciplinary Approach. Seismic Amplification in a Stiff Site

The village of San Gregorio (L’Aquila, Italy) was severely damaged by the April 6, 2009 earthquake. San Gregorio is situated at the base of a carbonate relief bounded by the Aterno river alluvial plain. The geological features of the area are very complex: jointed carbonate bedrock, cemented gravels and alluvial fan crop out in the village. Co-seismic ground fractures were seen along SW dipping active fault segments crossing San Gregorio. We integrated the microzonation studies with new geological, geotechnical and geophysical data for supporting the reconstruction planning of the village. Noise measurements show strong and polarized peaks in the horizontal-to-vertical spectral ratios (H/V) in the 3–7 Hz frequency band. Interestingly, the alluvial fan and the outcropping rock masses show both H/V peaks. To understand the influence of rock mass joint condition on site effect, we performed structural surveys on carbonate bedrock to look for a possible correlation between rock fracturing and ground-motion amplification.

The Seismic Site Characterization of Palazzo Centi in L’Aquila City Centre: The Case Study of a Historical Building Damaged by the April 6th 2009 Earthquake

An extensive geological, geotechnical and geophysical investigation was performed in L’Aquila city centre to restore Palazzo Centi, a historical building, damaged by the April 6, 2009 L’Aquila earthquake. This site investigation consisted of punctual and linear tests that allowed to define a detailed 3D model of the subsoil, irregularly affected by some peculiar conditions and characterized by low and variable values of the shear wave velocity V S in the near surface volume. In particular, the variable thickness of the upper fine-grained residual soils probably determined different ground motion amplifications during the main shock.

Parametric numerical study of observed amplification effects on the colle di roio limestone ridge (Central italy)

Following the seismic microzonation studies of the L’Aquila area, Central Italy, hit by the Mw 6.3 L’Aquila earthquake of 6th April 2009, we observed ground motion amplification at the hilltop of the Colle di Roio limestone ridge. In particular, horizontal-to-vertical spectral ratio amplitude of weak motion waveforms showed average peak level above 5 in the frequency range 4–5 Hz. In order to evaluate the contribution of topography and subsoil heterogeneity on the observed response, we performed 2D numerical simulations of wave propagation through the ridge using a finite difference approach. Different hypothesis of model configuration, consistent with the available geological data, were tested. A homogeneous model was considered to evaluate the effect of surface topography on ground motion amplification, while several heterogeneous models were considered in order to examine the interaction between topography and heterogeneity on the seismic response. Results were analyzed in terms of synthetic horizontal-to-vertical (H/V) spectral ratios and the latter were compared to the observed ones. The results showed that the observed ground motion amplification level cannot be explained only by surface topography effect and that the assumption of an heterogeneous model with complex inner geometry may fairly reproduce the observations.

The Experience of Seismic Microzonation in Lazio Region (Italy) Mountain Municipalities

Within the activities of seismic microzonation promoted and co-funded by Lazio Regional Administration (LRA) and the Department of Civil Protection (DPC), following the issues of the National rule OPCM 3907/2010, seismic microzonation studies were carried out in a set of municipalities, already attributed to the seismic Zone 1 (i.e. with the most severe level of seismic hazard and located in mountain areas). These municipalities show similar geologic and geomorphologic features as they are characterized by a local bedrock constituted of limestone and marly-limestone and fluvial-lacustrine deposits filling intra-mountain alluvial plains. The engineering-geological model of the subsoil as well as the jointing conditions of the outcropping rock masses were defined on the basis of available technical data from public reports, as well as from original geophysical surveys. Different types of microzones (i.e. stable, stable susceptible to seismic amplification and unstable areas) were identified according to the technical rules (ICMS 2008). Nevertheless, the experienced approach highlighted some critical features in the microzonation methodology that encourage future studies devoted to obtain the refinement of the operative procedures.

Analysis of ground motion along a topographic relief: The cerreto di spoleto case-history (Central Italy)

5th ECCOMAS Thematic Conference on Computational Methods in Structural Dynamics and Earthquake Engineering, COMPDYN 2015

Studies of seismic microzonation in Latium region (Italy) by experiencing the application of official guidelines in intra-mountain plains

In 2010, the Latium Regional Administration promoted a systematic study for the seismic microzonation of its Municipalities, according to the official national and regional guidelines. This activity was co-funded by the National Department of Civil Protection (DPC) following the issue of the National rule OPCM 3907/2010. A Working Group including researchers from Latium Regional Administration, ENEA and the Department of Earth Sciences of the ‘Sapienza’ University of Rome performed the studies in some municipalities characterised by the most severe level of seismic hazard. The activity was carried out on the basis of both archive geotechnical data and original geophysical surveys. The whole municipal territory was divided in stable areas, where no further analysis is required, and areas prone to stratigraphic or topographic amplification, as well as to earthquake-induced phenomena such as landslides, sinkholes, liquefaction and seiche. The experienced approach highlighted some critical features in the microzonation methodology that encourage the refinement of the operative procedures.