Francisco J. Chávez-García

An Alternative Approach to the spac Analysis of Microtremors: Exploiting Stationarity of Noise

The spac (Spatial autocorrelation) method to analyze ambient vibration records was introduced by Aki (1957). Currently, this method is being used for the analysis of microtremor data from an array of stations: crosscorrelation functions are computed between pairs of stations, and then averaged for different station pairs, at the same interstation distance but with different orientation. In this article we propose the idea of exploiting recordings of microtremors over long times as a substitute for spatial averaging, as was suggested in Aki (1957). This idea has several advantages. The two most important are, first, that it is not required to obtain simultaneous recordings using an array of stations, whose locations must obey a very rigid scheme; and second, the ability to obtain results for a large number of closely-spaced distance intervals. Our proposal is tested using data from the Parkway, Wainuiomata, temporary array. These data are supplemented with additional measurements performed during February 2003, to resolve an uncertainty regarding the low-frequency part of our results. Given the irregular distribution of our array, we are able to obtain results for many different station pairs. The phase-velocity dispersion curves we derive from our measurements, interpreted with the spac method, are compared with previous results in this sedimentary basin. Our results suggest that the spac method is more general than appears in recently published papers.

Site effects at Euroseistest—I. Determination of the valley structure and confrontation of observations with 1D analysis

This paper describes the process of construction of the 2D model of Volvis geological structure and results of empirical and theoretical approaches to the evaluation of site response at Euroseistest. The construction of the 2D model is based on a re-interpretation of the available geophysical and geotechnical data in an effort to improve the definition of the subsoil structure at Euroseistest in terms of the most important parameters needed to model site response. The results of this re-interpretation are compared with a previous published 2D model of the same alluvial valley. Different analysis of the measurements and different criteria in the synthesis of data have led to a different model, even if both studies had access to the same field measurements. This underscores the fact that a model results of an interpretation and is not uniquely determined by the data, no matter how detailed they are. The well known subsoil structure opened the possibility to correlate the geometry and the dynamic properties of the 2D model with the results of site response determined from a detailed analysis of two events in frequency and time domains and 1D numerical modeling. The study of site response shows the important effect of the lateral variations on the ground motion and suggests that the contribution of locally generated surface waves to the resonant peak may be important. In the case of Volvis graben, the limitations of the 1D approximation to simulate ground motion under complex soil conditions in both frequency and time domains are also shown. This paper lays the ground for a companion article dealing with 2D site effects in this basin.

Site effects in Mexico City eight years after the September 1985 Michoacan earthquakes

Abstract The purpose of this paper is to take a comprehensive look at site effects in Mexico City for the 1985 Michoacan earthquake. We examine, successively, 1D and 2D models. For the latter, we consider in detail both large scale and small scale heterogeneities, using extensively the Aki-Larner wave propagation method, in the version given by Bard and Gariel. In particular, we make a critical review of the different explanations proposed for the large duration of strong ground motion in the lake zone. Our purpose is two-sided. We first outline the difference between what is well established and what remains still unexplained regarding the seismic response of Mexico City basin. On the other hand, we wish to make explicit the conditions that the proposed models require to explain strong motion duration. Our results allow us to qualify the models proposed to date and to point out what could be the experiments and the new data required to find a truly satisfactory explanation of strong ground motion at Mexico City.

Site effects at euroseistest : II. Results from 2D numerical modeling and comparison with observations

This paper presents results of numerical modeling of site response for Euroseistest. Ground motion across a very detailed model of the subsoil of this valley has been simulated for vertically incident SH waves. The predominance of locally generated surface waves is very clear in the synthetic seismograms. These results are then compared with published studies of observed site effects at this basin and with a detailed analysis of two events in the time domain. It is discussed in which sense it is possible to obtain a good fit between observations and 1D models, even though the real behavior involves locally generated Love waves. For this reason, it can be misleading to rely on an incomplete observation such as empirical transfer functions. Finally, it is stressed that in order to predict ground motion in alluvial valleys the information contained in the phase cannot be neglected.

Subsoil Structure Using spac Measurements along a Line

The spac (SPatial AutoCorrelation) method was proposed almost 50 years ago by Aki (1957). This method allows a phase-velocity dispersion curve to be obtained from microtremor measurements using an array of stations arranged in a circle. The subsoil structure is subsequently derived from the inversion of that dispersion curve. In this article we show that it is possible to get similar results using microtremors recorded along a line. We use microtremor records obtained by using four broadband seismographs disposed along a line, with different interstation spacings (5, 10, 20, and 40 m). Our data are precessed by using the standard spac procedure, with the exception of the azimuthal average. The final subsoil structure as determined from the inversion of the phase-velocity dispersion curve shows excellent agreement with previous results at the site of our measurements. Our results suggest that the use of the spac method is not restricted to a particular geometry of the array, provided that the basic requirement of stationarity is fulfilled.

Site-City Seismic Interaction in Mexico City–Like Environments: An Analytical Study

Recent destructive earthquakes have confirmed the importance of hazard and vulnerability studies to predict and prevent the impact of large seisms. Most seismic risk analysis considers the buildings as a passive constituent integrated into the vulnerability analysis. This approach neglects the possible contribution of the building vibration to the free field. This article describes an analysis of site-city effects, that is, the seismic interaction of the city with soft soil layers. An analytic method derived from soil-structure interaction studies is described and applied to the Roma Norte zone of Mexico City. The building parameters are derived from detailed studies of the JA and PC buildings. The soil-city system is subjected to the Mexican ( M w 7.3) 14 September 1995 earthquake. The simulated ground surface motions, which incorporate the wave field radiated from the buildings of Roma Norte district, are compared to seismic records from this zone. Analytical procedures reproduce the long time duration and beating of the observed ground motion in Mexico City. Parametric analysis are also performed to identify the predominant factors (e.g., urbanization density, soil-to-city stiffness ratio) that favor the site-city interaction effects. A simple relation is proposed to estimate the expected efficiency of the site-city interaction effects for any city.

Complex site effects and building codes: Making the leap

The engineering community is aware of the importance of site effects, but it lags behind seismological studies when it comes to incorporating site effect considerations in design spectra for seismic norms. This lag is reflected in the conspicuous fact that current building codes make allowance for 1D site effects but ignore complex site effects. The purpose of this paper is to explore a way for including complex site effects in a building code environment. We take as example Eurocode 8, which is a modern code that exemplifies the current approach to site effect consideration. We examine the restrictions that we have imposed to make the problem of a feasible size and discuss the approach we have taken. We propose a strategy to incorporate a class of complex site effects in a design elastic spectrum.

Site effect evaluation at Mexico City: Dominant period and relative amplification from strong motion and microtremor records

Abstract Site effects in Mexico City are discussed in terms of simple 1D, one-layer, linear models. The analysis is focussed on two parameters: dominant period and maximum amplification relative to a firm site within the city. The data used is a compilation of strong motion data and microtremor measurements. Strong motion data consist of digital acceleration records for nine events recorded by the Accelerographic Network of Mexico City. The authors analyzed spectral ratios of horizontal components of soft soil sites relative to an average of firm site observations for this data set. Dominant period, maximum relative amplification and an estimate of material damping were computed from the empirical transfer functions thus obtained. Microtremor data were compiled from measurement of different groups during the period 1985–1992. In all, 409 measurement points were analyzed. Values of dominant period obtained from microtremor measurements are in excellent agreement with those obtained from empirical transfer functions for strong motion data. The synthesis of results allows us to draw a detailed and robust map of dominant period for Mexico City. Based on this map, the authors propose some modifications to the current microzonation of Mexico City and evaluate a proposed model to account for site effects in this city.

Site Effects and Design Provisions: The Case of Euroseistest

Modern seismic codes usually include provisions for site effects by considering different coefficients chosen on the basis of soil properties at the surface and an estimate of the depth of bedrock. However, complex local geology may generate site amplification on soft soils significantly larger than what would be expected if we assume that the subsoil consists of plane soil layers overlaying a homogeneous half-space. This paper takes advantage of the large number of previous studies of site effects done at Euroseistest (northern Greece). Those studies have supplied a very detailed knowledge of the geometry and properties of the materials filling this shallow valley. In this paper we discuss the differences between site effects evaluated at the surface using simple 1-D computations and those evaluated using a very detailed 2-D model of the subsoil structure. The 2-D model produces an additional amplification in response spectra that cannot be accounted for without reference to the lateral heterogeneity of the valley structure. Our numerical results are extensively compared with observations, which show that the additional amplification computed from the 2-D model is real and affects by a significant factor response spectra, and thus suggests that some kind of aggravation factor due to the complexity of local geology is worthy of consideration in microzonation studies and seismic codes.

The contribution of the built environment to the ‘free-field’ ground motion in Mexico City

Abstract Soil–structure interaction (SSI) effects on building dynamic behaviour have been studied extensively. In comparison, the radiation of waves away from the soil–foundation interface has received little attention. Recent studies point out that SSI in an urban environment can modify the ground motion recorded in the free-field. These modifications will be important when two conditions are met: structures founded on soft soils and coincidence between the vibration periods of the structure and those of the superficial layers. Both conditions are met in Mexico City lake zone. In this study, we investigate SSI effects on ‘free-field’ motion. The data we use consist of microtremors recorded on soft soils in Mexico City, a densely built environment. Our objective was to identify the modifications to free-field ground motion caused by neighbouring structures. Data were analysed using H/V spectral ratios. Large variations in the level of amplification and resonant frequency were determined from microtremors in very closely spaced stations. Our results suggest consistently that free-field ground motion is significantly affected by the presence of neighbouring structures.