On This Date in History: Mexico City has an interesting history. It is built on the ancient Aztec City of Tenochtitilan which was built as a water based society really on top of Lake Texcoco through the use of chinampas or water gardens. When the Spanish showed up in the early 16th century, they eventually conquored the Aztecs and decided that this big lake just would not do. Lake Texcoco was an enclosed lake and tended to flood the city and beside that it impaired their use of horses. So, they began projects to drain the lake. Well, in 1519 the city had 250, 000 inhabitants which was a lot of folks when one considers that London had about 200,000 people and Madrid about 64,000 around the same time. By the turn of the 20th century, Mexico City still had about the same population that it had in 1519 but by 1953, the city’s population had mushroomed to over 3 million and the area went from 29 square miles to about 150. Much of the expanded city was on the dry lake bed of Lake Texcoco. By 1996, Mexico City covered 870 square miles and was home for about 17 million people.
Now, with the lake gone and the population rising, water became scarce so they started drilling wells on the old lake bed for water. The result of the extraction of groundwater has been some serious subsidence. Beyond that, an old lake bed is not a very stable geological formation on which to build a city. It has the ability to jiggle alot when shaken. That makes Mexico City one of the most vulnerable cities in the world when it comes to earthquakes. On this date in 1985 an earthquake registering 8.0 on the richter scale shook Mexico City and the surrounding region. The earthquake struck at about 7:17:47 AM on September 19, 1985 releasing about 5.61 x 1024 ergs (10 to the power of 24) or about 19 kilotons of TNT which was about twice as much energy than the Hiroshima atomic bomb. It was the greatest earthquake event in Mexico since the Great Jalisco Earthquake of 1932. The geology of the former lake bed is suspected to have contributed to great resonance effects with dominant two second horizontal ground accelerations that were recorded in the area. The epicenter of the quake was relatively shallow but was 300 km away from Mexico City which is pretty far away in relation to the damage. Evidence supporting the assertion that the physical characteristics of the earth around Mexico City contributed to the extreme effects is the fact that a large percent of the building damaged or destroyed were between 8 and 18 stories high. Think of it has buiding a house on jello.
The earthquake was felt as far north as Houston, Texas and as far northwest as El Paso, Texas and all the way in Tuscon, AZ a swimming pool at the University of Arizona lost water due to the sloshing from the vibration. To the west, a tsunami was generated that was about 3 meters high at Zihuatenejo and 1.4 meters at Acapulco. That tsunami resonated across the Pacific to Hawaii and even a 5 cm rise was recorded at Tahiti. An aftershock of magnitude 7.5 on Sept 21 caused a second tsunami, though some geologists suspect that this was a separate earthquake rather than an aftershock. That is something for academics to argue about because it really makes little difference. The damage was horrific. Obviously, severe damage was experienced in Mexico City and the death toll has been pegged in excess of 9500 with some estimates of fatalities approaching 35, 000. At the time, Mexico City probably had about 15 million people so, when you think about it, the percentage of fatalities vs the total population was pretty small…but when you are talking about such huge numbers. small percentages can translate into large numbers of people. Like the fatality totals, total casualties may never be known for certain but injuries numbered at least 30,000 and some 100,000 people were left homeless. 412 buildings collapsed and 3124 were severely damaged in Mexico City. The earthquake affected about 825,000 square miles. Damage was caused across Central Mexico from Lazaro Cardenas on the Pacific Coast to Vera Cruz on the Gulf Coast.
An interesting aspect of this event was that, initially, the political power of the country said it didn’t need any help. The Revolutionary Party had been in power for so long that it was considered by many to be an institution. But, many political analysts suggest that the governmental early rejection of foreign aid led to the downfall of the Revolutionary Party. A number of rather somber ceremonies were held today to commemorate what happened in Mexico City 25 years ago with civic memorial services as well as Roman Catholic Masses conducted to remember the dead. At the location of the Hotel Regis, patrol cars and ambulances lined the streets and turned on their sirens to mark the time of the quake. That was a particularly imfamous building collapse as shortly following the hotel’s demise, a gas leak fed a fire that made it quite difficult to try to rescue anyone who may have survived.
The really sad thing is that this type of catostrophe will probably happen again. When the Spanish first conquored the city they considered moving it but chose not to do so. The center of the city is really pretty close to the center of the old lake bed and that lake bed has a relatively high water content. Hence, when there is considerable shaking then liquification of the sediment takes place making it unsuitable to support large structures. The same thing happened in the landfill areas around San Francisco Bay in the 1991 earthquake there. In California, there is the San Andreas Fault which makes that part of the US susceptable to earthquakes but Mexico is extremely vulnerable as there’s a lot of constant friction between the North American Plate, the Cocos Plate and the Pacific Plate. It’s one of the more seismically active regions of the world with some 90 temblors of maginitude 4.o of more recorded on average each year. Not only is the region active and the dry lakebed not overly stable, the lakebed also has a natural harmonic pitch of about 2.5 which means everything on the lakebed shakes at the same frequency and it just happens that 2.5 is a very common frequency of shallow earthquakes. Mexico City is kinda like New Orleans, or Los Angeles or Pompeii or Seattle or any of a number of large, urban environments that are extremely vulnerable to natural disasters that have come in the past and will come again in the future. It’s just part of the risk of living on an active planet.