Eduardo Reinoso

Spectral ratios for Mexico City from free-field recordings

Spectral ratios obtained for lakebed sites with respect to the average Fourier spectra of the Southwest hill-zone sites in the Mexico City Valley are presented. The amount and quality of data used allowed us to measure relevant amplification characteristics. For all earthquakes, the ratios are relatively constant regardless the magnitude, epicentral distance and azimuth of subduction and normal faulting earthquakes. With data recorded at Station Central de Abastos, it is shown that these ratios are a much better approach than horizontal to vertical ratios, which yield only a rough estimation of site amplification. Also for Central de Abastos, where little nonlinear behavior has been observed, an empirical approximation is used to account for these effects. We also built maps of equal amplification that exhibit those zones that have experienced damage during earthquakes and those at the highest risk. The ratios presented here are useful and reliable to accurately predict the amplification of motion.

Three-dimensional scattering of seismic waves from topographical structures

Abstract A direct boundary element method for calculating the three-dimensional scattering of seismic waves from irregular topographies and buried valleys due to incident P-, S- and Rayleigh waves is presented. It has been formulated with isoparametric quadratic boundary elements and the comparison with other results show that the method is accurate and efficient. The study of the behaviour of two types of mountains for different incidences is also shown. For some incidences, factors of vertical amplification can reach up to 20 times the incident motion and factors of horizontal amplification could be as high as four times the free-field motion. The largest amplifications have been found in mountains with vertical walls while mountains with smooth slopes exhibit little amplification with factors smaller than four. Results in the time domain show how the duration of motion could be incremented compared with the free-field motion and illustrate the great amplification of the incident wavelet at some sites of the mountains.

Influence of Frequency Content and Peak Intensities in the Rocking Seismic Response of Rigid Bodies

The rocking response of a family of bodies given by its width and height due to synthetic and recorded strong ground motions is presented; this allow us to identify the main parameters that govern their response: peak acceleration and velocity, dominant frequency, and acceleration time-history. Recorded strong ground motions were scaled to the same peak acceleration and also to the same peak velocity to analyze the effect of both parameters in the response of rigid bodies. These three parameters were used to build an equation to obtain the height/width overturning plots. The proposed equation was tested for many well-known strong ground motions and the results were compared to other methods shown to be more accurate. This parametric equation does not need iterations or numerical approximations to be solved and can provide engineers, in a very practical way, minimum design requirements or particular specifications to protect non structural elements.

A Virtual Reference Site for the Valley of Mexico

We propose the use of the average spectra of northern strong-motion stations located at the hill zone (e.g., MD, TX, ES, 64) as the reference for Mexico Citys ground motion. This virtual-site proposal is based on the analysis of recent data from the Mexico City Acelerometric Network. The northern stations show amplitudes, both in time and frequency, that are consistently smaller than those of hill-zone stations located south and west of the city (e.g., CU). It is well known that CU, the historical reference site in Mexico City, and other sites to the southwest, present amplifications, whereas the northern ones appear to be free of such effects. The spectral ratio of the averages of the stations located in the south and west with respect to the northern stations shows a relatively constant amplification of up to 3 times in the 0.7-10.0-Hz-frequency band. This amplification is a very unusual feature that should be explained. The geologic conditions at the hill zone show that older, Miocene-age deposits are located north of the city. Considering that northern sites represent the basement, we assume that the configuration along the hill zone in the N-S direction can be approximated by a simple, dipping, homogeneous layer. We computed the antiplane seismic response for this model and averaged and compared it with the spectral ratio obtained from strong ground motion data. The agreement is good and suggests how a smooth, large-scale feature could amplify seismic ground motion in a broad frequency band. Manuscript received 12 October 2001

COMPARISON OF METHODS TO PREDICT RESPONSE SPECTRA AT INSTRUMENTED SITES GIVEN THE MAGNITUDE AND DISTANCE OF AN EARTHQUAKE

We compare four methods, all based on Bayesian regressions, to predict response spectra at instrumented sites, located at the hill and lakebed zones in Mexico City, that have recorded several recent earthquakes. The regressions are built as functions of magnitude and closest distance to the rupture area of the earthquake, using more than 20 subduction earthquakes (thrust-faulting mechanism) recorded since 1964. The four methods are: (1) a regression on observed response spectra; (2) a regression on observed Fourier amplitude spectra and use of random vibration theory to estimate the response spectra, (3) a regression on observed response spectra at the reference hill-zone station multiplied by response spectral ratios; and (4) a regression on Fourier amplitude spectra at the reference hill zone, multiplied by Fourier spectral ratios and use of the random vibration theory to estimate response spectra.

Two-dimensional scattering of P, SV and Rayleigh waves : Preliminary results for the Valley of Mexico

A direct boundary element method for calculating the two-dimensional scattering of seismic waves from irregular topographies and buried valleys due to incident P-, SV- and Rayleigh waves is employed to model a section of the Mexico City Valley. The method has been formulated with isoparametric quadratic boundary elements and contains, with respect to previous works in the field, some improvements that are briefly presented. Because the Mexico City Valley is relatively flat and shallow and the contrast of S-waves between the clays and the basement rock is very high, it is believed that the one-dimensional theory is enough to explain the amplification patterns. Although this is true for most sites, results from recent accelerometric data suggest that two- and three-dimensional models are needed to explain the amplification behaviour at some places. In this work, two accelerometric sites have been chosen: Site 84 to probe that the one-dimensional model works well for most sites, and Site TB, as an example of irregular response. The two-dimensional method presented here was used to model a section of the valley where site TB is located, showing that this method yields results closer to the observations than the one-dimensional approach.

Event-based approach for probabilistic flood risk assessment

ABSTRACT An event-based approach for probabilistic flood risk assessment for financial and civil protection applications is proposed. The risk parameters are evaluated in an event-based probabilistic framework for a set of hazard events and then all these results are probabilistically integrated, including, in a formal way, all uncertainties related to each and every part of the process. The hazard is defined as a stochastic or historic set of events, collectively exhaustive and mutually exclusive, that describes the spatial distribution, the annual frequency, and the randomness of the hazard intensity. The risk is expressed in different economic terms: the average annual loss, the pure risk premium, and the loss exceedance curve. As an illustrative example, this approach is applied to Acapulco city located on the Pacific coast of Mexico. These metrics are of particular importance for risk retention (financing) schemes or risk transfer instruments, and, therefore, they will be a particularly valuable contribution to further studies in defining strategies for financial protection against disasters. With these results, further results may be obtained for civil protection and land use applications.

Seismic Vulnerability of an Inventory of Overturning Objects

A methodology is presented for assessing the probability of overturning under the action of ground motions of given intensities, and the expected values and standard deviations of damage produced by overturning of objects in a group or inventory exposed to the same seismic event. We apply this methodology to one example of the typical contents located on the base (i.e., free-field) of a middle-class house or apartment. A detailed inventory was gathered, and recent well-recorded accelerograms at the site were used to compute the rocking response of every object. Vulnerability functions for the whole inventory computed at four different sites in terms of epicentral distance and site effects show large differences between them.

Event-based approach for probabilistic agricultural drought risk assessment under rainfed conditions

An event-based approach for the probabilistic risk assessment of agricultural drought under rainfed conditions to estimate the economic impact is proposed. The risk parameters are evaluated in an event-based probabilistic framework for a set of hazard events; these results are probabilistically integrated including, in a formal way, all uncertainties related to every part of the process. The hazard is defined as a stochastic or historic set of events, collectively exhaustive and mutually exclusive, that describes the spatial distribution, the annual frequency, and the randomness of the hazard intensity. The risk is expressed in different economic terms: the average annual loss (or pure risk premium) and the loss exceedance curve; these metrics are of particular importance for risk retention (financing) schemes or risk transfer instruments. As an illustrative example, this approach is applied to probabilistic drought risk assessment of maize under rainfed conditions in Mexico. These results are the base of further studies in defining strategies for financial protection against agricultural losses and disasters.

Ground-Motion Prediction Model From Intermediate-Depth Intraslab Earthquakes at the Hill and Lake-Bed Zones of Mexico City

We present ground-motion prediction equations, based on Bayesian regressions, to predict response spectra at sites located within the hill and lake-bed zones of Mexico City for intermediate-depth, normal-faulting intraslab earthquakes with magnitude ranges 5.2 ≤ Mw ≤ 7.4; distances from sites to the fault surface 103 ≤ R ≤ 464 km; and focal depths of the events 40 ≤ H ≤ 128 km, all at sites located in hill and lake-bed zones in Mexico City. The equations are built as functions of magnitude and distance to the fault surface, using more than 20 intraslab earthquakes. We compare our results with previous attenuation models for intraslab (hill zone) and interplate earthquakes (hill and lake-bed zones) in Mexico City. The results also show that the uncertainty for intraslab earthquakes at sites located within the hill zone is larger than that for sites located in lake-bed zone.