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 It is well known that CU, the historical reference site in Mexico City, and other sites to the southwest, present amplifications, whereas the northern ones appear to be free of such effects.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

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We present an analysis of the information collected by the ¿Sintió un sismo? (SUS) web‐based system. One of the most devastating events in central Mexico in the past 35 yr struck near the Mexican states of Puebla and Morelos on 19 September 2017. At the moment of the event, several programs and projects were in place to monitor and perform quick assessments of the magnitude of the earthquake and the severity of its effects on the population and infrastructure. The SUS platform gathers questionnaires designed in Spanish to estimate macroseismic intensities. The availability of such a system in the dominant language of the country permits a broad reach, only limited by the disparity of the services and internet access. By analyzing residuals of the median attenuation intensity of the event, we confirm previous observations on the site and regional effects in Central Mexico such as the strong influence of the Trans‐Mexican volcanic belt on the ground‐motion amplification. In addition, we obtained correlations between peak parameters and macroseismic intensities that reveal the character of the affected structures’ responses. We emphasize the potential usability of systems similar to the SUS at the regional level and their impact on the decision‐making process and support for further research using all available datasets.

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Abstract. The world experienced the beginning of the COVID-19 pandemic by the end of 2019 to the beginning of 2020. Governments implemented strategies to contain it, most based on lockdowns. Mexico was no exception. The lockdown was initiated in March 2020, and with it, a reduction in the seismic noise level was witnessed by the seismic stations of the national and Valley of Mexico networks. Stations located in municipalities with more than 50 000 people usually experience larger seismic noise levels at frequencies between 1 and 5 Hz, associated with human activity. The largest noise levels are recorded in Mexico City, which has the largest population in the country. The largest drop was observed in Hermosillo, Sonora; however, it was also the city with the fastest return to activities, which seems to correlate with a quick increase in confirmed COVID-19 cases. Mexico initiated a traffic-light system to modulate the re-opening of economic activities for each state. Therefore, since 1 June, noise levels have generally reflected the colour of the state traffic light. Furthermore, the reduction in the noise level at seismic stations has allowed identification of smaller earthquakes without signal processing. Also, people in cities have perceived smaller or more distant quakes.

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