CDMX's Seismic Sediments

On September 19, a 7.1 magnitude earthquake struck central Mexico, leaving over 300 people dead, injuring thousands, and causing billions in damages. The epicenter was in the State of Puebla, but the quake caused buildings to collapse 75 miles away in Mexico City. In the chaos that followed, chilangos rallied to provide supplies and human support to rescue workers at dozens of rescue sites within the city and surrounding areas. 

The city itself is built on lake sediments. Lake Texcoco, over which the Aztecs built Tenochtitlán in 1325, covered over 2,000 sq km of what is now the Valley of Mexico (above map). After the Spanish conquest and establishment of Mexico City in 1585, a number of projects were developed to drain the lake due to constant flooding which posed a significant health hazard. A large flood in 1607 prompted the Desagüe, a project which first started as a proposal to drain Lake Mexico through a tunnel that went through Huehuetoca and Nochistongo via canal. This initial project was unsuccessful due to poor upkeep, so Mexico City continued to experience major floods and implement Desagüe projects over the next few centuries, slowly draining the lake to its current (non-existent) state. A fantastic visualization of this progression can be seen here. This geologic history combined with demographic pressures has caused a number of problems for modern-day Mexico City, including flooding, water scarcity, subsidence, and, the focus of this post, increased susceptibility to seismic damage. 

The sediment below the majority of Mexico City are lacustrine sediments, that is, lake sediments consisting of poorly consolidated silt, sand, and clay, in many places up to 80 meters deep. These lacustrine sediments lie above thick deposits of alluvial fill, also poorly consolidated sands and silts. When earthquake waves propagate through this loose material, they slow down causing an increase in wave amplitude and damage.  

The damage in the Capital is highly concentrated in the parts of the city that sit on Texcoco lacustrine sediments, such as Roma Norte, Condesa, and Coyoacán, as opposed to the volcanic bedrock in neighborhoods like Lomas de Chapultepec and Bosques de Las Lomas. Below is a map I threw together using data from a crowd-sourced damage map of damages in CDMX and geologic data from the  Sistema Nacional de Información Estadística y Geográfica.

The darkest grey, encompassing the delegations of Benito Juarex, Iztapalapa, and Cuauhtémoc, among others, represents a lightly consolidated Quaternary sediment. To both the east and west of this sediment are volcanic formations. To get a different look, I mapped the damage data with soil data from the Comisión Nacional para el Conocimiento y Uso de la Biodiversidad:

The soil data shows an area of calcium carbonate rich soil corresponding to the Quaternary sediment noted in the geology layer, while the areas to the west consist of primarily andosols derived from volcanic rocks. The geology shown in these maps roughly corresponds with the high-damage areas, and, while they do not show the depth of the unconsolidated sediments, it is likely that they would correspond more clearly with the areas with high damage if I were to map thickness. 

Mexico is a seismically active country. Our experience last week, on the 32nd anniversary of the 1985 Mexico City Earthquake, was a wake-up call. The fatalities, while horrendous, were few when compared to the 10,000 fatalities in 1985, due to an effective early warning system and more resilient building codes; however, mistakes were made. For example, only a small portion of the Rébsamen school fell, a portion that looks like an addition. It's too early to tell if this was a case where building code was not properly enforced or contractors cut corners, but, in a city where these actions can easily have deadly consequences, it is vital that any individual that does not comply with safety regulations be held accountable. Earthquakes will always be a part of living here, and, with a huge portion of the City especially vulnerable to damage, it is essential that the government enforce code and have a reliable logistics and communications plan in place.