Giovanna Laurenzano

Seismic response in Catania by different methodologies

The prediction of the seismic response and assessment of the amplification factor of the surface soils is a topic of maximum interest in engineering seismology and the final goal of any microzonation study. Two very different numerical techniques are described in this paper. One is a 2-D method, the Spectral Element Method (SPEM-2D), which solves the propogation of the seismic field through complex geological structures. The other is a 1-D method (by the GEODIN code), commonly used in engineering practice, which takes into account the detailed shear waves soil profile or superficial layers, including soil non-linearity. The seismic response of the surface by 1-D code is evaluated, using as input motion at the conventional bedrock sealed recorded accelerograms and synthetic accelerograms given by the 2-D code at a given depth. A comparison between soil response at the surface, given by the 2-D method, and by the 1-D method is presented in the paper. In particular the effects of the seismic input, of the shear waves soil profile and of the soil non-linearity, are also analyzed and discussed.

Automatic Zonation of Urban Areas Based on the Similarity of h/v Spectral Ratios

Abstract In this article we propose an automatic procedure that performs thezonation of urban areas based on a set of horizontal-to-vertical ( H/V ) spectral ratiosfrom ambient noise recordings collected at many measurement points. The techniquesearches for the connected areas where the similarity among the spectral ratios ismaximized. The problem is posed as one of the optimal partitioning of the Delaunaytriangulation of the available measurement points. The technique explores and triesto partition some random variations of a Euclidean minimum spanning tree of thetriangulation. The optimization is performed using a genetic algorithm.Thetechniqueis applied to the zonation of the town of Vittorio Veneto in northeastern Italy and toa synthetic data set that tries to simulate the effects of some typical geological con-ditions.Introduction Seismic microzonation concerns the identification andmapping at local or site scales of areas having different po-tentials of hazardous earthquake effects, such as ground-shaking intensity, liquefaction, or landslide potential (Akiand Lee, 2003). It is commonly performed by experts takinginto account geophysical and geological information of dif-ferent natures. For ground-shaking intensity, a useful sup-port comes from the computation of horizontal-to-vertical(

Seismic Response from Microtremors in Catania (Sicily, Italy)

A survey of environmental seismic noise (microtremors) was carried out within the Catania municipal area in May 1999. The aim of the data acquisition was to improve our prediction of the seismic ground motion locally, using Nakamuras approach. The seismic noise was recorded at 39 different sites. These sites were chosen according to the following criteria: 1) significance in terms of geological, geotechnical, or local characteristics; 2) presence of a lithological transition (e.g.: the transition from lava to sediments); 3) presence of other geotechnical or geophysical measurements; 4) alignment with some of the transects along which the ground motion had been simulated numerically. The data were acquired independently by two teams, who used different instrumentation. Measurements overlapped at seven locations. We have found that more than one half of the sites exhibit weak or no amplification. These sites are located either on lava (e.g., sites in downtown Catania and in the northern part of the municipal area) or on well-consolidated sedimentary soils (Western districts of the city). The only sites which bear evidence of some amplification are located either on soil fill lying over lava, or on the fine alluvial deposits of the Catania Plain. In the first case the H/V amplitude is usually moderate and the fundamental frequency is between 5 and 12 Hz, while the sites of the Plain have a much lower natural frequency (about 1.3 Hz) and larger H/V amplitude. Some sites displayed anomalous, though relatively small in most cases, H/V peaks, in the low frequency band 0.2-1 Hz. Our analysis, performed at some sites, comparing the measured H/V spectral ratios (HVSR) to those simulated numerically for an earthquake, supports the interpretation raised recently by other authors of an effect of the deep structure. Finally, the numerical simulations also predict an appreciable lateral variability of the HVSR, where the underlying structure is not a plane layer model, rather usual in Catania. This implies that seismic noise measurements made at single, isolated points should be used with care in the interpretation.

Numerical Modeling of the 13 December 1990 M 5.8 East Sicily Earthquake at the Catania Accelerometric Station

The record of Catania enea–enel accelerometric station, with its anomalously high amplitude, represents an outlier for the dataset of seismograms recorded by the Italian accelerometric network during the 13 December 1990 M 5.8 East Sicily earthquake. Using numerical modeling tools, we corroborate the interpretation of this anomaly as an effect of the crustal structure or site response, rather than as a finite source effect. In the first part of this study, we compare the recorded seismograms to those computed by two numerical methods—the 2D Spectral Element Method (spem), a technique which solves the 2D full-wave propagation through a complex geological structure; and the Wavenumber Integration Method (wim), which solves the 3D full-wave propagation in a horizontally-layered structure. The comparison shows that the recorded waveforms are reproduced accurately using a laterally heterogeneous structure, whereas none of the representations in terms of horizontal plane layers, which simplify the same structure, provides satisfactory results. Furthermore, we compare the horizontal-to-vertical spectral ratios (hvsr) obtained from (1) the earthquake record, (2) the seismogram simulated by the spem, and (3) the seismic noise recorded at the same site. Again, the overall agreement is very good. This study has several outcomes. Firstly, that the numerical modeling approach based on the spem, and used in a previous study to simulate ground shaking for a destructive scenario earthquake, provides reliable results. Secondly, that the 2D model used to represent the crustal structure beneath this area is realistic. Indeed, simplified 1D models may not be an adequate means to reproduce realistic seismograms and predict the ground motion in the frequency band of interest for seismic hazard (i.e., 0.5–10 Hz). Finally, the high amplitude displayed by the Catania station during the 1990 earthquake is explained as a combined effect of site and structure-path, while finite source models appear unnecessary. In general, this study emphasizes the importance of methods that accurately model the wavefield propagation through realistic geologic structures for predicting ground motion.