Juan C. Montalvo-Arrieta

A hybrid indirect boundary element—discrete wave number method applied to simulate the seismic response of stratified alluvial valleys

Abstract A hybrid indirect boundary element – discrete wavenumber method is presented and applied to model the ground motion on stratified alluvial valleys under incident plane SH waves from an elastic half-space. The method is based on the single-layer integral representation for diffracted waves. Refracted waves in the horizontally stratified region can be expressed as a linear superposition of solutions for a set of discrete wavenumbers. These solutions are obtained in terms of the Thomson–Haskell propagators formalism. Boundary conditions of continuity of displacements and tractions along the common boundary between the half-space and the stratified region lead to a system of equations for the sources strengths and the coefficients of the plane wave expansion. Although the regions share the boundary, the discretization schemes are different for both sides: for the exterior region, it is based on the numerical and analytical integration of exact Greens functions for displacements and tractions whereas for the layered part, a collocation approach is used. In order to validate this approach results are compared for well-known cases studied in the literature. A homogeneous trapezoidal valley and a parabolic stratified valley were studied and excellent agreement with previous computations was found. An example is given for a stratified inclusion model of an alluvial deposit with an irregular interface with the half-space. Results are displayed in both frequency and time domains. These results show the significant influence of lateral heterogeneity and the emergence of locally generated surface waves in the seismic response of alluvial valleys.

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 It is well known that CU, the historical reference site in Mexico City, and other sites to the southwest, present amplifications, whereas the northern ones appear to be free of such effects.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

Relationship between MMI Data and Ground Shaking in the State of Nuevo León, Northeastern Mexico

In this article, we implement a web questionnaire (available in Spanish at the [www.fct.uanl.mx][1], last accessed March 2014, as Encuesta sismo ) to gather citizens’ observations on felt earthquakes in an area of northeastern Mexico. We generated a seismic intensity map due to the fact that some events from the October 2013 to March 2014 seismic sequence (1.9≤ M c≤4.5) were recorded and felt in the region. This sequence occurred in the central area of the state of Nuevo Leon. The data obtained from this questionnaire and by interviewing people living near the epicentral area are useful for a rapid postearthquake evaluation. This is important because the maximum intensity values were of V–VI near the epicentral area. Finally, the attenuation curves obtained from our study proved to be similar to those for central and eastern United States. [1]: http://www.fct.uanl.mx

Regional landslide hazard assessment from seismically induced displacements in Monterrey Metropolitan area, Northeastern Mexico

The first regional seismic landslide hazard assessment for Northeastern Mexico is presented. Regional maps predicting the expected areal limits for potential earthquake-induced landslides in terms of coseismic displacements for the 2010 San Pedro earthquake (M 4.0) and two postulated M 5.5 and M 6.5 scenario earthquakes are shown. Permanent displacements were obtained from a global empirical regression based on a critical acceleration ratio (ac/PGA) estimation. Results for the M 4.0 San Pedro event show low cumulative displacements concentrated around the epicentral zone. For this event, occurrence of a few small landslides as disrupted slides and falls were likely. For M 5.5 and M 6.5 scenarios the coseismic displacements depict a high potential for widespread and complex slope failures mainly in the Sierra Madre Oriental hilly areas, although foothills inside the Monterrey Metropolitan Area would be affected. The worst scenario we have postulated is a M 6.5 earthquake during the hurricane season. Hypothetical full-saturated and strong seismic shaking conditions would trigger several rock and soil avalanches as well as other complex landslides. Related damage for this event would be very catastrophic to civil infrastructure and substantial injuries and loss of life are expected mainly in the southwestern portion of the Monterrey Metropolitan Area. Results constitute a useful tool in order to mitigate potential earthquake-induced damage related to regional slope failures.

Seismic site effects in the central zone of Monterrey Metropolitan Area (northeast Mexico) from a geotechnical multidisciplinary assessment

Prediction of the ground shaking response requires data expressed in terms of soil dynamic properties. Based on the analysis of surficial geology, geotechnical borehole data, seismic refraction (VS) and microtremors (H/V) surveys, we have developed a qualitative evaluation of local ground response to earthquakes in the Monterrey Metropolitan Area (MMA), the main urban and economic hub in northern Mexico. The results provide a detailed geotechnical model for the central zone of the MMA. Although such information is not yet complete for the MMA region, an initial approach has been developed in order to estimate the distribution of damage associated with expected moderate earthquakes. An empirical correlation is introduced to obtain VS as a function of standard penetration test blow counts (SPTN). According to the results, the central-southern part of the study area might experience higher amplifications of ground shaking produced by moderate seismicity because the larger thicknesses of alluvial sediments are deposited there. The 2D geotechnical model proposed would be very useful in making decisions regarding planning or land use, deployment of protocols of rapid response and for producing detailed microzonation maps for those zones with similar geological features as the MMA in northeast Mexico.