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A destructive intraslab earthquake occurred in Mexico City on September 19, 2017 (Mw 7.1), causing significant damage and hundreds of human losses not only in the epicentral area, but also in the States of Morelos, Puebla, Mexico and in Mexico City. Only in Mexico City itself, around 230 people died, and more than 40 buildings collapsed. The intensities recorded in some lakebed areas of the city, especially in zones with soil periods around 1.5 s, were relatively high, even surpassing spectral values of 1.0 g; the vertical component, due to the proximity of the earthquake, was unusually high for Mexico City. The 2017 earthquake raised questions critical to understanding the city’s seismic vulnerability and resilience, and they are partly answered in this article. Using 77 accelerometric stations, the amplification pattern of the seismic intensities is characterized as well as the correlations of buildings structural characteristics with the site effects. A comprehensive statistical analysis of the damages is shown to analyze and understand the structural behavior of damaged buildings. It is including not only the structural types and the year of construction, but also the main structural problems identified (structural pathologies), such as irregularities, both in elevation and plan, soft story, and corner effect. The building damage database was constructed with 2125 reports of buildings carried out by universities and engineering associations after the earthquake, of which 543 had severe damage. It is also included the information of all buildings with no damage in the city thanks to the cadastral information provided by the Mexico City government, and post-earthquake inspections and visual inspections using Google Street View. A full study of selected neighborhoods, which compares similar buildings with and without damage, is included, yielding relevant statistical information on which pathologies cause more damage and even collapses.

espanolEl terremoto intraplaca del 19 de septiembre de 2017 (Mw7.1), cuyo epicentro se localizo cerca de los limites entre los estados de Puebla y Morelos, aproximadamente a 120 km de Ciudad de Mexico, causo danos severos en estas regiones. Este articulo analiza los cocientes espectrales de estaciones acelerometricas en el lecho lacustre de la Ciudad de Mexico con respecto al espectro promedio de Fourier en sitios de la zona de suelo firme para estudiar y comparar los cambios, en el tiempo, con el comportamiento de los efectos de sitio y su relacion con el dano presentado durante este terremoto; estas cocientes tambien exhiben el problema de asentamiento en algunos lugares de la ciudad debido a la sobreexplotacion del acuifero para el suministro de agua, en donde se ha visto cambios en el periodo del suelo, inclusos cercanos al 40 por ciento. Finalmente, se presentan mapas de pseudoaceleracion y energia histeretica para la Ciudad de Mexico con una discusion de una posible correlacion con los danos reportados.Palabras clave: cocientes espectrales, pseudoaceleracion, energia histeretica, espectro de Fourier, terremoto de Mexico, amplificaciones espectrales EnglishThe September 19, 2017, intraslab earthquake (Mw7.1), whose epicenter was located near the limits betweenthe states of Puebla and Morelos at approximately 120km from Mexico City, caused severe damage in these regions. This article analyzes the spectral ratios of accelerometric stations in the lake-bed of Mexico City with respect to the average Fourier spectra at hill zone sites in order to study and compare over time the changes in the behavior of local effects and their relationship with the damage presented during this earthquake. These ratios also exhibit the settlement problem in some places in the city due to overexploiting the aquifer for water supply purposes, where changes in soil periods, even larger than 40%, are seen. Finally, pseudoacceleration and hysteretic energy maps for Mexico City with a discussion with a possible correlation with reported damages are presented. Key words: Spectra ratios, pseudoacceleration demands, hysteretic energy demands, Fourier spectra, Mexico earthquake, spectral amplifications.

Landslides caused by a low magnitude earthquake swarm (2.8 ≤ M ≤ 3.6) in 2012 were documented at the Santa Rosa Canyon in northeastern Mexico. Disrupted landslides from falls and slides, in both rocks and soils, were identified based on fieldwork and high-resolution satellite imagery along stream banks from natural cliffs and along the road cut in the epicentral area. Most of the landslides occurred on slopes greater than 40°, where geological features played a key role in triggering slope instabilities. The maximum distance limit for disrupted slides from the epicentral area was 7 km. The area affected by landslides during the early stage of the seismic sequence (July through August 2012) was 90 km2. Landslide identification was limited in some areas by the resolution of the satellite imagery and dense cloud coverage. Both the epicentral distance and the area affected by landslides are above the global bounds reported in literature. The final landslide inventory is the first documented case of earthquake-induced landslides in northeastern Mexico.

Many studies have found that ground-motion duration has a considerable influence on damage to conventional civil structures. Historically, Mexico City has been severely affected by long-duration distant subduction earthquakes because of the heterogeneous local site conditions underlying the subsoil of the city, which significantly amplify the soil response. Using a reliable database of earthquakes that occurred in the subduction zone of the Pacific Coast of Mexico, recorded in the hill zone of the Valley of Mexico from 1985 to 2020, predictive models are proposed in this study from existing definitions of strong-motion duration. These models were developed using a mixed-effects approach, which is a statistical method that can consider the correlation between data recorded from the same event. Such models have valuable applications in seismic and structural engineering because they can accurately define the dimension and randomness of strong-motion duration.

Current seismic regulations do not assess the seismic performance of structures during a given seismic event, and therefore do not require an estimation of their residual displacement or level of structural damage. Nevertheless, this information is relevant after intense ground motions to assess the possibility of re-occupying a building. This paper develops simple expressions to estimate the maximum displacement demand from the residual displacement of single-degree-of-freedom systems subjected to long-duration narrow-banded ground motions generated in soft soil sites (particularly, those located in Mexico City). To overcome the limited scope of previous studies, more than 35,000 seismic simulations were performed. The influence of the lateral strength of the single-degree-of-freedom systems is considered. A simplified relation between the residual and maximum displacement demands is established.

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.

We present regional maps of earthquake-induced liquefaction hazard in Mexico considering the lateral or horizontal spreading displacement, as index, D LL . The methodology to prepare the liquefaction hazard maps consists of five steps: (1) identifying zones with soil deposits that are more susceptible to displaying liquefaction based on geologic available information at a 1:1,000,000 scale; (2) characterizing the seismic hazard as a set of stochastic events that collectively describe the hazard, compatible with the distribution of location, depth, frequencies, magnitudes and attenuation of the seismic strong ground motion; (3) employing a parametric method, based on empirical data, to estimate the demand of permanent ground deformation expected due to liquefaction (in this study, the lateral spreading displacement of the ground, D LL ) by event and for the site required; (4) characterizing the earthquake-induced liquefaction hazard as a set of stochastic events that describe the spatial distribution demand of liquefaction for each event; and (5) performing a probabilistic analysis of liquefaction hazard. The results of liquefaction hazard associated with return periods of 150 and 500 years are shown on maps of Mexico. Those maps are compared qualitatively with historical information collected from sites where, based on descriptions, the phenomenon of earthquake-induced liquefaction is seen to have occurred from the year 1593 to 2010. The results obtained in this study provide a first approximation to the liquefaction hazard zones in the country, in accordance with sites where historical evidence of liquefaction has been reported. In addition, the application could be important in land-use planning and urban development, particularly in regions with a historical certainty of earthquake-induced liquefaction, but with little or no geotechnical and/or geophysical studies. These maps can be used to locate zones where more in-depth studies are required to estimate, with less uncertainty, the potential for earthquake-induced liquefaction.

A seismic-event-based methodology to generate earthquake-induced translational landslide maps using Newmark method is proposed. The steps are: (1) to construct a GIS-based geotechnical database; (2) to identify those areas that are susceptible to the occurrence of translational landslides based on available geological information; (3) to compute a static safety factor; (4) to compute the critical acceleration that defines the threshold acceleration required to cause a displacement; (5) to characterize the seismic hazard as a set of stochastic events, collectively exhaustive and mutually exclusive, that fully describes the hazard spatial distribution and annual frequency of occurrence (in accordance with the earthquake location, depth and magnitude) with the appropriate ground-motion prediction equations; (6) to compute the Newmark displacement; and finally, (7) to carry out a probabilistic translational landslide hazard analysis to estimate an exceedance rate of a given displacement. This methodology is applied to Mexico, and maps for return periods of 150 and 500 years are presented. Results shown in maps are estimations of where translational landslides may occur and should be useful to carry out local studies to elaborate recommendations of site specific hazard reduction plans as well as to calculate insurance rates. In addition, these results are useful to identify civil protection actions, risk management at regional and local level, and land use planning, as well as for promoting more detailed vulnerability and risk studies at different scales.

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.

Durante algunos eventos sísmicos recientes en México, se ha observado que, a pesar de haber un adecuado comportamiento del sistema estructural, no siempre existe un adecuado comportamiento de los sistemas no estructurales. En esta investigación se llevan a cabo análisis no lineales paso a paso para estudiar el comportamiento de fachadas prefabricadas de concreto del tipo “piso a piso”, las cuales han presentado fallas y desprendimientos aun cuando la estructura no ha sufrido daño. Para alcanzar este objetivo, se evalúan los desplazamientos y aceleraciones de cada uno de los pisos de un edificio de 15 niveles y 4 crujías, donde se calcula el nivel de daño de diferentes paneles. Para obtener estas respuestas, se diseña el sistema estructural mencionado utilizando el Apéndice A de las Normas Técnicas Complementarias para Diseño por Sismo del Reglamento de Construcciones del Distrito Federal del 2004 y se realiza un análisis dinámico incremental utilizando el registro sísmico del sismo de Michoacán del 19 de septiembre de 1985, obtenido en la estación de la Secretaría de Comunicaciones y Transportes. Se observa que al reducir las fuerzas sísmicas por ductilidad y sobrerresistencia, se podrían ignorar las fuerzas reales durante un sismo que son mayores a las de diseño, lo que podría provocar daño en las fachadas estudiadas.

We present a scheme to modify empirical Green's functions by attenuation considering: (1) geometrical spreading; (2) decay in high frequency; (3) regional attenuation; and (4) phase of the signal. The accelerograms computed with the proposed simulation method are compared, in time and frequency domains, with strong ground motions from subduction and intermediate-depth earthquakes recorded in Mexico. It is shown that this simple empirical Green's functions technique can synthesize both the shape and amplitude of the response spectra in the site, considering a postulated seismic source located at different distances from the original one.

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.

SUMMARY The purpose of this note is to contribute to the understanding of seismic ground motion in Mexico City. To this end, we (1) compute theoretical transfer functions for various models for the deep structure and the shallow clay layers, (2) study the seismic responses of various sites within the city using spectral ratios (of horizontal motion relative to the motion of nearby rock sites and relative to the vertical motion at the same site) and (3) discuss some characteristics of surface waves for models of the deep structure and the shallow clay layers in 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 City's 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

A study of the duration of strong ground motion using accelerometric data of subduction and normal-faulting Mexican earthquakes is presented. Duration is obtained based on the time between 2.5 and 97.5 per cent of the Arias intensity. An expression to predict this duration in terms of the magnitude, distance to the rupture area and site period is proposed and compared with predictions available in the literature. The effect of large duration for very distant sites and the contribution of soft soils to the duration of strong ground motion are widely discussed. We have found that large magnitude not only yields long duration at the source, but also proportionally longer duration with distance and with dominant site period compared to small magnitude. The duration obtained from the regression is used as a parameter to obtain input and hysteretic energy and on the use of damage models available in the literature. Finally, duration is used together with the random vibration theory to predict response spectra. Copyright © 2001 John Wiley & Sons, Ltd.

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.

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.

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.

Ordaz, M., Rosenblueth, E. and Reinoso, E., 1993. Future ground motions in Mexico City. In: F. Lund (Editor), New Horizons in Strong Motion: Seismic Studies and Engineering Practice. Tectonophysics, 218: 141-155. Future ground motions at a soft site of the Valley of Mexico are estimated for a postulated M 8.2 earthquake with epicentral distance of 280 km. Three techniques were used: (a) a semi-empirical estimation of Fourier acceleration spectra at a hard (reference) site plus empirical determination of site effects through empirical transfer functions; (b) semi-empirical computation of Fourier spectra at the reference site, with theoretical calculation of site response using one-dimensional horizontal S wave propagation corrected to account for surface waves; (c) use of an M 6.9 recording as the Green’s function of the postulated event. The three techniques use competing approaches, and we illustrate a rational way for combining them to obtain a single estimate that includes information from the three approaches.

A numerical method is used for calculating the two-dimensional scattering of incident SH waves to try to explain some of the amplification patterns observed from recent data of the Mexico City's accelerometric array. The method is briefly presented and its efficiency is tested against analytical and other numerical solutions for canyons and alluvial valleys. Spectral ratios computed for transition and lake-bed zones of the Mexico City valley with respect to the average motion at hill-zone sites are also presented. The one-dimensional model is used to explain the amplifications observed at a site where the valley is relatively shallow, while the two-dimensional approach is employed at another site at the centre of the valley where irregular amplification patterns have been observed. Results in the time domain are also shown.

This chapter presents direct formulations of the boundary element method for the two-dimensional scattering of seismic waves from irregular topographies and buried valleys. The BEM models were formulated with isoparametric, quadratic boundary elements, and were employed to simulate a section of the Mexico City valley. 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 sufficient to explain the amplification patterns. Although this is true for many sites, results from accelerometric data suggest that two-and three-dimensional models are needed to explain the amplification behaviour at other sites, particularly near the borders of the valley.

Se hace un análisis de la energía obtenida a partir de acelerogramas de algunos sismos fuertes obtenidos en la zona epicentral y en el valle de México. Primero se comparan los espectros de energía de entrada de algunos registros cercanos al epicentro con los espectros de respuesta de aceleración y de velocidad y se correlacionan cualitativamente con el daño. Se analiza la ecuación de energía, las definiciones de energía absoluta y relativa y la variación de éstas en el tiempo. Para sitios instrumentados en zona de lago del valle de México se comentan brevemente las características de atenuación y la variación de energía con la magnitud y dirección del movimiento. Además, se comenta la variación de los espectros de energía de entrada y de histéresis con la resistencia y con la demanda de ductilidad. Por último, se presentan los espectros elásticos de energía de entrada y de energía histerética normalizada para el sismo del 25 de abril de 1989 y los correspondientes mapas con curvas de igual energía para cuatro diferentes periodos estructurales. También se presentan curvas de igual energía y periodo estructural de dos segundos para cuatro sismos que tuvieron una intensidad importante en el valle de México.

We estimate pseudo acceleration response spectra with 5% damping (Sa) for the great Michoacán earthquake at 22 lake bed sites (in addition to five sites where the event was well recorded) in the valley of Mexico. The method used consists of (a) estimation of Fourier acceleration amplitude spectra (FAS) at the sites via observed FAS at Ciudad Universitaria (CU) and compiled spectral ratios of these sites with respect to CU obtained from recorded earthquakes, (b) estimation of the duration of strong ground motion during the Michoacán earthquake at the sites, and (c) application of random vibration theory to predict Sa. A constant duration of 71 sec at all sites is found to be satisfactory. The main uncertainties in estimated Sa come from (a) possible nonlinear behaviour of the clay during the great earthquake (the spectral ratios used in the computation are mostly for moderate earthquakes, when the clay behaves linearly), and (b) variations in site's spectral ratios due to randomness and/or its dependence on earthquake characteristics which are presently unknown. Assuming that the nonlinear behaviour was neglegible and the spectral ratios estimated from one or few recordings are representative, we find (a) Sa ≥ 600 gals over a wide area of the lake bed zone, (b) Sa ≥ 1g at 14 out of the 27 sites, (c) regions of high Sa in the center of the city and east of CU, and (d) a good correlation between severe damage and regions with Sa ≥ 800 gals. Although in general our results agree with reported effects of the earthquake, because of the uncertainties mentioned above direct use of the computed response spectra for code or design purposes is discouraged.