Miguel Angel Jaimes

Seismograms from a seismic station installed in the city of Guadalajara during the intense and unusual seismic swarm that occurred from 8 May to late September were recently recovered in the national historical seismograms archive of the Mexican Seismological Service. These seismograms show earthquakes recorded in August and early September 1912 by two Wiechert mechanical instruments installed with the purpose of observing this unusual sequence and as one of the permanent seismic stations of the budding Mexican Seismic Network. The earthquakes recorded in these seismograms range from barely visible signals to larger magnitude earthquakes that were reported as having been felt as moderate or strong. In some cases, it is possible to identify the arrival of the Pand S phases. The difference in arrival times suggests that the epicenter for some of these recorded events was approximately 8–12 km from the site where the Wiechert instruments were installed. This epicentral distance from the seismic station is consistent with the spatial distribution of the more intense shaking observed during the swarm. The shape and duration of the seismic traces also confirm that these events correspond to shallow earthquakes located near the seismic station. A coda-wave duration relation calibrated for crustal earthquakes in the Mexico City basin was used to estimate the magnitude of selected events. The largest estimated magnitude, interpreted as a conservative lower-bound value, was Mw 4.1, for the largest event recorded on 31 August 1912. In a classification of the intensity of shaking for earthquakes of the swarm reported instrumentally, this earthquake is classified as strong. Thus, those events classified as very strong and causing the more intense shaking most probably have a larger magnitude, compatible with the lower bound of the magnitude range inferred from macroseismic data for the largest earthquakes of the sequence.

Aftershock sequences of large earthquakes underscore the need to understand their potential implications for seismic hazard and risk estimation. Even though large Mexican earthquakes typically show a relatively small number of aftershocks, they may cause significant damage to infrastructure and buildings. This study focuses on the behavior and modeling of aftershocks that follow moderate-to-large interplate and intraslab earthquakes in Mexico. Initially, the epidemic-type aftershock sequence model is used to describe the short-term behavior of aftershock occurrences. However, this model appears to fall short of reproducing the actual number of aftershocks. To address this, we applied an alternative approach based on Yeo and Cornell’s modified Omori law to evaluate aftershock productivity, specifically recalculating the productivity parameters to achieve a better fit to the observed data. We analyze the aftershock sequences with magnitudes mb≥4, ≥4.5, and ≥5 occurring within the next 30 days after moderate-to-large subduction and intraslab mainshocks of magnitude Mw≥6, examining their temporal decay and spatial distribution. Our findings reveal consistent patterns in the decay of the number of aftershocks over time and distance from the hypocenter. Subduction interplate earthquakes tend to generate more aftershocks than intraslab earthquakes within the subducted Cocos plate. A notable exception, considered an outlier, is the Mw 8.2 Chiapas earthquake of 19 September 2017, which was followed by an unusually large number of aftershocks. Our results can be instrumental in forecasting the adequate number of aftershocks and understanding their spatial and magnitude–frequency distributions. In addition, the results of this study could inform simulation-based mainshock–aftershock hazard assessments as part of a comprehensive approach to seismic hazard analysis.

The recent seismic swarms in the Mexico City basin underscore the need to evaluate their potential implications for seismic hazard and risk in the capital city. Despite their small magnitude (Mc<3.3), these earthquakes caused unusually large ground accelerations, surpassing previous destructive earthquakes. This work aims to forecast seismic damage scenarios resulting from earthquakes in the Mexico City basin. It is assumed that earthquakes as large as Mw 5.5 may occur. Three scenarios were investigated where previous events took place. A deterministic approach estimates ground-motion intensity measures like peak ground acceleration and/or spectral pseudoacceleration, utilizing ground-motion models from comparable tectonic regions. Local site effects were based on the geotechnical zonation of the city. To estimate the damage, vulnerability curves were calculated for residential buildings in the cadastral record of Mexico City. In scenario 1, La Magdalena, ∼13% of 1–2-story buildings would be affected. In scenario 2, San Juan de Aragón, up to 15% of similar structures may be impacted. Scenario 3, Milpa Alta, shows about 5% damage for 1–2-story buildings, primarily concentrated to the southeast. In the three scenarios, 1- and 2-story buildings suffer damage, independent of the geotechnical region. For 3–5-story buildings, damage is concentrated in the lake zone of the basin, where soft sediments underlie the city. For buildings 6–10 stories high, damage lies preferentially in the transition zone between the soft sediments and the highlands. No appreciable damage is observed for buildings taller than 11 stories in any scenario. Local crustal earthquakes, as considered here, represent a significant risk to Mexico City. Outlining damage scenarios may help decision-makers and insurance entities implement mitigation measures to reduce potential impacts on residential structures.

Este estudio presenta un procedimiento de diseño simplificado para aisladores sísmicos aplicados a equipos rígidos simplemente apoyados en la Ciudad de México. Se evalúa la respuesta sísmica de estos equipos no anclados, enfocándose en criterios de estabilidad como la prevención del balanceo, volcamiento y deslizamiento bajo cargas sísmicas. La metodología propuesta permite calcular las dimensiones y características iniciales de los aisladores, asegurando que se cumplan los requisitos de estabilidad. Mediante las ecuaciones dinámicas gobernantes, se verifica la efectividad del diseño simplificado de los aisladores, demostrando que el equipo permanece estable sin iniciar movimientos de balanceo ni deslizamiento. Este enfoque ofrece un marco confiable para el diseño de sistemas de aislamiento sísmico de equipos que cumplan con el ambiente sísmico de la Ciudad de México.

Este estudio evalúa la fragilidad sísmica de estructuras de concreto reforzado (CR) de primer piso débil rehabilitadas con amortiguadores de inercia rotacional con embrague (Clutching Inerter Dampers, CID) a nivel del suelo cuando se someten a excitaciones sísmicas de banda angosta. Las principales ventajas que aportan los CID a nivel del suelo son las reducciones en las demandas sísmicas (e.g., distorsiones de entrepiso, aceleraciones de piso y cortantes de entrepiso). Este estudio muestra que las estructuras con CID llegan a ser sistemas confiables en términos de distorsiones máximas de entrepisos y aceleraciones de piso para moderados y grandes movimientos sísmicos del suelo. La tasa de daño de estructuras con amortiguadores de inercia rotacional con embrague a nivel del suelo es en general menor para edificaciones bajo intensidades sísmicas asociadas con estados límite de colapso incipiente, especialmente para edificaciones de baja altura. Los hallazgos de este estudio podrían guiar a los ingenieros de la práctica a usar amortiguadores de inercia rotacional con embrague en la rehabilitación de estructuras dúctiles de marcos de CR de primer piso débil sujetos a excitaciones sísmicas de banda angosta en zonas como la Ciudad de México.

Este estudio numérico evalúa la efectividad de los amortiguadores de partículas (AP) en edificios, los cuales son capaces de reducir la respuesta lateral del sistema estructural primario cuando se someten a excitaciones sísmicas de banda estrecha. Primero, se estudió la respuesta sísmica de un sistema de un grado de libertad (1GDL) para tres configuraciones: 1) un sistema de 1GDL simple; 2) un sistema de 1GDL con amortiguador de masa resonante (AMR) y 3) un sistema de 1GDL con AP. Luego, se analiza la respuesta de marcos lineales de 11 y 17 pisos con AP, AMR y sin ellos bajo movimientos sísmicos de banda estrecha. Los resultados indican que la principal ventaja de un AP es la reducción significativa de las aceleraciones dinámicas de piso y desplazamientos laterales inducidos por sismos cuando la relación entre la frecuencia predominante de la excitación sísmica y la frecuencia predominante del edificio está cerca de la unidad y el primer modo de la edificación es dominante. Es fundamental llevar a cabo una validación y contraste experimental de estos hallazgos, con el fin de que los ingenieros puedan considerar la viabilidad de implementar los AP en estructuras expuestas a excitaciones sísmicas de banda estrecha, como ocurre en la Ciudad de México.