Carlo G. Lai

Simultaneous measurement and inversion of surface wave dispersion and attenuation curves

Surface wave tests are non-invasive seismic techniques that have traditionally been used to determine the shear wave velocity (i.e. shear modulus) profile of soil deposits and pavement systems. Recently, Rix et al. [J. Geotech. Geoenviron. Engng 126 (2000) 472] developed a procedure to obtain near-surface values of material damping ratio from measurements of the spatial attenuation of Rayleigh waves. To date, however, the shear wave velocity and shear damping ratio profiles have been determined separately. This practice neglects the coupling between surface wave phase velocity and attenuation that arises from material dispersion in dissipative media. This paper presents a procedure to measure and invert surface wave dispersion and attenuation data simultaneously and, thus, account for the close coupling between the two quantities. The methodology also introduces consistency between phase velocity and attenuation measurements by using the same experimental configuration for both. The new approach has been applied at a site in Memphis, TN and the results obtained are compared with independent measurements.

Propagation of Data Uncertainty in Surface Wave Inversion

Although in recent years surface wave methods have undergone significant development that has greatly enhanced their capabilities, little effort has been spent to determine the uncertainty associated with surface wave measurements. The objective of this study is to determine how the uncertainty of the experimental data is mapped into the uncertainty of the shear wave velocity profile via the inversion algorithm. The methodology developed in this study for estimating the uncertainty of the shear wave velocity profile from surface wave measurements is based on the assumption that the experimental data are normally distributed. The validity of this hypothesis was experimentally verified using data gathered at two sites in Italy where surface wave tests were performed using linear arrays of multiple receivers. The experimental dispersion curve measured at the site was subsequently inverted to obtain the expected shear wave velocity profile together with an estimate of the associated standard deviation. The final results show that uncorrelated noise has a very little influence on multistation surface wave tests, confirming their robustness for applications in noisy environments.

Simultaneous Measurement of Surface Wave Dispersion and Attenuation Curves

In existing surface wave test procedures, experimental dispersion and attenuation curves are determined separately (i.e., uncoupled) using different source-receiver configurations and different interpretation methods. A new procedure based on displacement transfer functions is proposed in which dispersion and attenuation data are derived simultaneously (i.e., coupled) from a single set of measurements using the same source-receiver array. The new approach is motivated by the recognition that in dissipative media, Rayleigh phase velocity and attenuation are not independent as a result of material dispersion. Therefore, a coupled analysis of dispersion and attenuation is a more robust, fundamentally correct approach. The new approach is also more consistent with coupled inversion techniques to obtain the shear wave velocity and shear damping ratio profiles. The proposed approach is illustrated using data measured at a site in Atlanta, Georgia.

Solution of the Rayleigh Eigenproblem in Viscoelastic Media

We present a technique for the solution of the complex-valued eigenproblem associated with the propagation of surface waves in general linear viscoelastic media. The new technique permits simultaneous determination of the Rayleigh dispersion and attenuation curves and the displacement and stress eigenfunctions for vertically heterogeneous, linear viscoelastic media with arbitrary values of material damping ratio. The technique is based on the Cauchy residue theorem of complex analysis that takes full advantage of the holomorphic properties of the Rayleigh secular function, which is viewed as an analytic mapping of the complex-valued Rayleigh phase velocity. Because the eigenvalue problem is solved directly in the complex domain with no simplifying assumptions, the algorithm implicitly accounts for the inherent coupling between phase velocity and attenuation of seismic waves as a result of material dispersion. The technique overcomes the limitations of previous algorithms that often break up the complex structure of the problem and/or require a priori information about the number of eigenvalues and their approximate value. The algorithm is validated via comparisons with closed-form solutions for a uniform half-space. Examples are also used to compare solutions obtained with the proposed technique and one based on the assumption of weak dissipation in strongly and weakly dissipative layered media.

ASCONA: Automated Selection of COmpatible Natural Accelerograms

This paper describes an automated procedure for selecting and scaling real spectrum-compatible records. The methodology allows one to choose from a predefined database, assembled from accredited strong-motion accelerometric data banks, real records satisfying properly defined seismological constraints with the additional requirement of spectrum-compatibility with a reference spectrum in a specified period range. Among the different sets of records satisfying these constraints, the user can specify the desired one, based on additional requirements (e.g., limited scaling factors). The proposed algorithm allows one to select records compatible with either an acceleration or a displacement response spectrum.

Mesozonation of the Italian territory for the definition of real spectrum-compatible accelerograms

The Italian building code defines the seismic action in terms of elastic acceleration response spectra derived from the results of a probabilistic seismic hazard study performed for the whole national territory. This representation of the seismic input is insufficient for several situations (e.g. analysis of geotechnical systems or time-history analyses of structures), for which the seismic input needs to be specified in terms of accelerograms. This work illustrates a methodology for the seismic mesozonation of the Italian territory, with the aim of defining suites of 7 real accelerograms recorded at outcropping rock sites with flat topographic conditions and, most importantly, compatible with the elastic acceleration response spectrum defined by the Italian building code at any location in Italy. These accelerograms do not require any correction and can be directly used for nonlinear dynamic analyses of structures and geotechnical systems. The mesozonation is based on identification of groups of spectra with similar characteristics and shape. For each of these groups, a parent spectrum is defined and used for selecting real spectrum-compatible records. Limited linear scaling is then applied to these accelerograms to make them compatible with all the response spectra of the group. The results of this work for the 475-years return period are accessible through the SEISM-HOME Web-GIS (www.eucentre.it/seismhome.html) providing, for any site in Italy, a suite of 7 real accelerograms spectrum-compatible, on average, with the acceleration response spectrum prescribed by the Italian building code. SEISM-HOME is a useful tool for practitioners needing ready-to-use time-histories for seismic analyses.

Prediction of railway-induced ground vibrations in tunnels

The authors of this paper present the results of a study concerned with the assessment of the vibrational impact induced by the passage of commuter trains running in a tunnel placed underground the city of Rome. Since the railway line is not yet operational, it was not possible to make a direct measurement of the ground vibrations induced by the railway traffic and the only way to make predictions was by means of numerical simulations. The numerical model developed for the analyses was calibrated using the results of a vibration measurement campaign purposely performed at the site using as a vibration source a sinusoidal vibration exciter operating in a frequency-controlled mode. The problem of modeling the vibrational impact induced by the passage of a train moving in a tunnel is rather complex because it requires the solution of a boundary value problem of three-dimensional elastodynamics in a generally heterogeneous, nonsimply connected continuum with a moving source. The subject is further complicated by the difficulties of modeling the source mechanism, which constitutes itself a challenge even in the case of railway lines running at the surface. At last, the assessment of the vibrational impact at a receiver placed inside a building (e.g., a human individual or a sensitive instrument) requires an evaluation of the role played by the structure in modifying the computed free-field ground motion. So far, few attempts have been made to model the whole vibration chain (from the source to the receiver) of railway-induced ground vibrations, with results that have been only moderately successful. The numerical simulations performed in this study were made by using a simplified numerical model aimed to capture the essence of the physical phenomena involved in the above vibration chain including the influence of the structural response as well as the dependence of the predicted vibration spectra on the train speed.

Probabilistic Seismic Hazard Assessment at the Eastern Caribbean Islands

A probabilistic seismic hazard analysis has been performed to compute probabilistic seismic hazard maps for the eastern Caribbean region (10° N-19° N, 59° W-64° W), which includes in the north the Leeward Islands (from Anguilla to Dominica) and in the south the Windward Islands (from Martinique to Grenada), Barbados, Trinidad, and Tobago. The analysis has been conducted using a standard logic-tree approach that allowed systematically taking into account the model-based (i.e., epistemic) uncertainty and its influence on the computed ground motion param- eters. Hazard computations have been performed using a grid of sites with a space resolution of 0.025 degrees covering the territory of the considered islands. Two different computation methodologies have been adopted: the standard Cornell-McGuire approach (Cornell, 1968; McGuire, 1976) based on the definition of appropriate seismogenic zones (SZ), and the zone-free approach developed by Woo (1996), which overcomes the ambiguities related with the definition of seis- mic sources. The interplay and complexities between shallow crustal, intraplate, and interface subduction seismicity of the Caribbean region have been thoroughly investigated. By merging all available databases, a comprehensive and updated earthquake catalog for the region has been compiled. Also, a thorough investigation has been undertaken to identify the most suitable ground motion prediction equa- tions to be used in the analyses. Uniform hazard spectra have been calculated for the horizontal component of ground motion (rock and level site conditions), 4 return periods (RP) (95-, 475-, 975-, and 2475-yr), and 22 spectral accelerations (SA) with structural periods ranging from 0 to 3 s. SAs at 0.2 and 1.0 s for 2475-yr RP have been calculated to allow the definition of seismic hazard in the region of study according to the International Building Code (IBC, International Code Council (ICC), 2009).

SEISMIC HAZARD ASSESSMENT OF THE HISTORICAL SITE OF JAM IN AFGHANISTAN AND STABILITY ANALYSIS OF THE MINARET

The Minaret of Jam in Afghanistan was recently included in UNESCOs List of Worlds Endangered Monuments. The minaret is the worlds second tallest (∼60 m) after the Qutub Minar in New Delhi and it is also one of the oldest (∼800 years). It is situated at the centre of the Hindukush range in the Ghor Province at the junction of Rivers Hari-Rud and Jam-Rud, at an elevation of 1900 m. The Minaret of Jam is in danger of collapse due to a 3.4° northward inclination inducing high stresses in its deteriorated brick masonry. The precarious conditions of the monument render it highly vulnerable to earthquakes. Therefore prior to any intervention to protect it, an assessment of the seismic hazard at the site, followed by an evaluation of the towers seismic vulnerability, is of foremost importance. This article illustrates the results of the seismic hazard assessment of the archaeological site of Jam using both the probabilistic explanation (PSHA) and the deterministic explanation (DSHA) approaches. Uncertainty in PSHA has been handled within a logic-tree framework. Uniform hazard response spectra have been computed for return periods of 72, 224, 475 and 975 years. The earthquake input defined by the seismic hazard assessment of Jam has then been used to perform the dynamic analysis of the minaret. The latter has been carried out using a lumped mass approach and a 3D finite element model, accounting in both cases for dynamic soil-structure interaction.

Some Examples of 1D, Fully Stochastic Site Response Analyses of Soil Deposits

It is well known that the seismic amplification characteristics of ground motion are strongly influenced by the geotechnical properties of soil deposits and the associated uncertainties. Also the variability of the seismic input significantly affects the results of the ground response analyses. A single deterministic site response analysis, using the mean values of mechanical and physical properties of soil layers and a single recorded time history, does not allow assessing the uncertainty associated to the computed ground motion, due to the aleatory nature of model parameters and the variability of seismic input. Even a series of parametric analyses may not be sufficient, since some critical combinations of geotechnical parameters and seismic input may induce significant ground amplification that is completely overlooked by such an analyses. The uncertainty and reliability of results obtained from ground response analysis may only be achieved via fully stochastic-based procedures. This work aims at illustrating the results of 1D linear equivalent, fully stochastic site response analysis at two sites, one in Southern India, the other in Northern Italy, that have been recently carried out using Monte Carlo simulations associated with the Latino Hypercube sampling technique.