Lessons Learned from Two Earthquakes That Afflicted Mexico City—After a Pause of 32 Years

As history would have it, the recent Mw7.1 Puebla-Morelos intraplate event occurred on September 19, 2017, coincidental to the 32nd anniversary of the 1985 Mw8.0 Michoacan interplate event. The former had less impact both in affected areas and in severity than the latter which had profound impacts on the nation. Factors affecting the respective impacts include: ground motion input, site setting of Mexico City with its thick lake-sediment deposits, and structural types of affected building stocks. The interplay of these factors at the two events shared general similarities with specific nuances, and the author participated in the Canadian reconnaissance teams after both events. This paper discusses the vulnerability of Mexico City to the two events that are representative of two types of source mechanisms: the recent event being moderate yet closer, while the earlier one being major but further away. In addition to the difference in earthquake magnitude and epicentral distance, they also generate ground motions of different frequency contents. Thus, site amplification differs in the two events depending on thickness of lake sediments in various seismic zones within the city, and the paper first presents an overview of the physical and seismo-tectonic setting of Mexico City. The earthquake and damage data for this pair of events are summarized in a table. Four aspects of the general earthquake impacts are then discussed: ground failure, building performance, building retrofit, and lifelines performance. Moreover, the earthquake early warning system, having its origin because of the 1985 event, has been fully operative since 1993. Up to the 2017 event, the system had recorded 6,896 earthquakes and issued 158 early warnings. Finally, a succinct summary highlights key lessons learned from this pair of seismic events.