Mexico

Mexico occupies one of the most seismically active stretches of the Western Hemisphere, facing simultaneous threats from oceanic [[subduction-zone]] megathrust events, crustal strike-slip faults, and active volcanic systems. The dominant tectonic engine is the Middle America Trench, where the Cocos Plate and the Rivera Plate dive beneath the North American and Caribbean plates at rates of 6 to 7 centimetres per year. This [[subduction-zone]] has generated most of Mexico's deadliest earthquakes and is responsible for the nearly continuous low-level seismicity that rattles Oaxaca, Guerrero, Michoacán, and Jalisco. Inland, the Trans-Mexican Volcanic Belt marks where subducting slab fluids rise and melt the overlying mantle wedge, producing active stratovolcanoes including Popocatépetl and Colima. The Guerrero Seismic Gap — a 200-kilometre segment of the Middle America Trench that has not produced a major rupture since the 1910s — is considered one of the most serious unresolved hazards in the hemisphere. In northern Mexico, the Gulf of California Rift extends the East Pacific Rise spreading center northward, generating moderate earthquakes along extensional faults.

Mexico City's vulnerability to seismic damage is shaped as much by local geology as by the distance to [[epicenter]]s. The former lakebed of Lake Texcoco, now occupied by much of the urban core, consists of extraordinarily soft clay deposits up to 60 metres thick that dramatically amplify [[seismic-wave]]s and extend shaking duration far beyond what hard-rock sites experience. This amplification effect has repeatedly turned distant moderate earthquakes into locally catastrophic events. The 19 September 1985 Michoacán earthquake (magnitude 8.1) killed between 5,000 and 40,000 people in Mexico City despite an [[epicenter]] 350 kilometres away on the Pacific coast; the soft-lake clay amplified long-period waves that matched the resonance frequencies of 8 to 20 storey buildings, causing hundreds to collapse. Exactly 32 years later, on 19 September 2017, a magnitude 7.1 earthquake struck the Puebla-Morelos region, killing 369 people and collapsing over 40 buildings in Mexico City. The temporal coincidence with the anniversary of the 1985 quake — during which the country was conducting its annual earthquake drill — added an eerie dimension to the disaster. In 1995, a magnitude 8.0 Manzanillo earthquake generated significant [[tsunami]] waves along the Colima and Jalisco coasts. The Oaxaca region has been struck repeatedly, including a magnitude 7.4 event in 2020 and a magnitude 7.5 in 2021 that extended damage across rural communities with limited infrastructure.

The tectonic complexity underlying Mexico reflects the interaction of the North American, Caribbean, Cocos, Rivera, and Pacific plates within a relatively compact geographic area. The subducted Cocos Plate is not a uniform slab; beneath Mexico it flattens to near-horizontal geometry beneath the Trans-Mexican Volcanic Belt in a phenomenon called flat-slab subduction, which transmits seismic stress deep into the continental interior. This geometry explains why earthquakes originating from the [[subduction-zone]] can severely damage cities 300 to 400 kilometres from the coast. [[aftershock]] sequences in Mexico can be prolonged and damaging; the 2017 Puebla earthquake was followed by thousands of [[aftershock]]s over the following months. Mexico operates SASMEX, the Seismic Alert System of Mexico, which uses coastal sensor arrays near the Middle America Trench to broadcast radio warnings to Mexico City and other cities, providing up to 60 seconds of advance notice before strong shaking from offshore events.