Alejandro Ramírez-Gaytán

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.

Numerical Seismic Response and Failure Modes of Old URM Walls Under Lateral Loading

Most of existing constructions were built of unreinforced masonry. The performance of this material is different from the masonry units and mortar joints. This behavior is governed by the physical and mechanical characteristics of the composite, self-weight, transmitted vertical loading and earthquake shaking. The heterogeneity of masonry is also reflected in the anisotropic behavior of this material, which presents different performance and failure modes when subjected to lateral seismic forces. The main objective of the present paper is the simulation of typical failure mechanisms of compact URM walls without transmitted vertical loading, only taking into account their self-weight. The numerical behavior and failure mechanisms at the masonry walls (with solid section and perforated) was captured by the used material model and the pushover method. The obtained failure modes by bed-joint sliding at the solid section wall and stepped shear cracks at the wall with window are similar to the observed in post-earthquake investigations on URM walls with light cover reported in literature. The low tensile strength of masonry and the lack of vertical loading on the studied unreinforced masonry walls induced different failure mechanisms and behavior. In brief, the identification of in-plane performance and collapse is difficult to predict and strongly depends on the physical and mechanical properties of masonry, vertical loading and seismic forces.

Analysis of the unusual earthquake of 13 August 2006 in Michoacán, México

The moderate earthquake of 13 August 2006 which occurred in the coastal area of Michoacán, México, offered the first opportunity to study an earthquake that has a focal mechanism oriented practically perpendicular to the vast majority of the earthquakes occurring along the subduction zone of the Mexican Pacific continental margin. The location and focal mechanism estimated in this study are in close agreement with those estimated by the Global Centroid Moment Tensor (CMT) project and the US Geological Survey, National Earthquake Information Center (NEIC) and place the earthquake in a complex tectonic region where 3 lithospheric plates converge. Our review shows that for the most severe historical earthquakes in the area the seismic recurrence period has expired, consequently the seismic hazard of this region is high and the analysis of the unusual event must be considered important. The main purposes of this study are (i) re-estimate the location and focal mechanism of the unusual event by using available seismic records close to the source, (ii) conduct a tectonic analysis of the area in relation with the previous fault plane estimated, (iii) evaluate the peak ground accelerations generated for this particular thrust event relative to those occurring during the more common events and (iv) generate the isoseismal map. The analysis of the intensities of this event together with a tectonic analysis of the area where this event occurred, attest to an unexpected behavior of this event in this region.

Evaluation of the recorded ground motions for the unusual earthquake of 13 August 2006 ( M w 5.3) in Michoacán México

The focal mechanism of the moderate earthquake of 13 August 2006 Mw = 5.3, which occurred in the border coastal area between Michoacán and Colima, México, is unusual. As shown by the Global Centroid Moment Tensor (CMT) project and the Servicio Sismológico Nacional de Mexico (SSN), the thrust mechanism is striking almost perpendicularly to the majority of earthquakes occurring along the subduction zone of the Mexican Pacific continental margin which commonly strike nearly parallel to the trench. The purpose of this study is to analyze the observed ground motions of this particular event relative to those of the common events. First, we apply the H/V technique to verify that the stations involved in this study are nearly free of site effects. Then, we compare the observed ground motions with (i) three empirical ground motion prediction equations (GMPEs) appropriate for the region, (ii) ground motions of four real earthquakes with the common mechanism, and (iii) the Fourier spectrum of a selected common event.