On earthquakes, lakes and sensors

I live in Mexico City, one of the largest cities in the world. As all large cities, it has its ghastly demons. I am willing to argue that the one lying below it makes it unique in the world — We live atop what remains of a lake. Of course, it is not visible at plain sight (I would not surround it in this mystery setting otherwise): Our city is not Amsterdam or Venice, embracing the water, but we seem to hide it as if we were ashamed of it — there is a small surviving fraction of the once great Texcoco lake in the city’s South East, in the touristic Xochimilco borough, plus a wider region in the North East that just refused to go away, but has been historically shunned by everybody.

The lake, however invisible, is far from dry: Although it is mostly mud, it does contain some water pockets that are harvested to help our 22 million people have running water. That, of course, together with an impressive set of aqueducts bringing water from over 400Km away.

This would make mostly for nice trivia… But there is more to it: Mexico is a seismically active country. It sits at the meeting point of four major tectonic plates; along our Pacific coastline, the Cocos plate is struggling to get under the North America and Caribbean plates. Further North, the Pacific plate is sliding North, dragging the Baja California peninsula along with it. This means that Mexico’s longest coastline (particularly the Southern belly of the country).

Just how much does seismically active mean? According to the National Seismologic Service, we had over 15,000 earthquakes in 2016. Of course, the vast majority are so weak they can only be felt locally, if at all; usually, very little danger is posed by local earthquakes under magnitude 6. Only six earthquakes over said magnitude were registered in Mexico during 2016. And, not to downplay them (they meant real danger and harm to people and property, but the point I am to get to is a different one!), only one of them happened within 500Km of Mexico City.

This year, in two weeks time, we had two major earthquakes: On September 7, we had a magnitude 8.2 earthquake in the South East of the country, close to Chiapas, Oaxaca and Guatemala. This was the strongest earthquake in the region in over a century; the first reports were quite calm, but slowly we learnt about the devastation it brought to our poorest states. Less than two weeks later, in the anniversary of the September 19, 1985 earthquake that truly devastated Mexico City, we lived a rather atypical earthquake: A magnitude 7.1 earthquake with an inland epicenter.

All in all, the inhabitants of Mexico City are as well prepared as we can be for this kind of threat. After the terrible death toll of the 1985 earthquake, an early seismic alert network was established. Given that most often the epicenters are at the meeting point of the tectonic plates, sensors along the coast would be able to alert the population with enough anticipation to reach a safe place. This system, operated by the Seismic Instrumentation and Recording Center was started in 1989, with 12 stations along the coast directly South of Mexico City; today it has 97 monitoring stations along the Pacific coastline. In Mexico and Oaxaca Cities, the system is connected to alarm towers giving a relatively thorough coverage; other cities sound the alarm over all radio stations, but do require a radio to be turned on to be effective. For an earthquake hitting close to midnight as we had on September 7, I don’t expect many people outside the capital to have reacted to it on time. There are plans to deploy alarm towers on other cities that can be strongly affected, but work on it has not started yet. The September 19 earthquake’s epicenter was too far North for this early warning system to be of use — I remember hearing the alarms while we were already rushing out the stairs.

So, going back to the beast underneath Mexico City: The reason I started by explaining the city is built over a lake is because it acts as seismic wave amplifier; the New York Times produced a very clear graphic simulation of this effect. Seismic waves quickly lose strength as they travel away from their epicenter, (do note that this simulation does not correspond to any of the recent earthquakes) but our jelly-like soil can increase and lengthen their effect, reflecting wavefronts that create spikes in the movements. Completely in consonance with what the simulation shows – The latest earthquake hit when I was at the university, South-West from the lake. We felt this as a relatively short movement, between 30 and 40 seconds long. People in the center of the city reported it to be well over a minute – Because of the waves reflecting and bouncing until they eventually became quiet.

I prepared the following map showing the buildings that collapsed, superimposing the list of coordinates registered in a collaborative mapping document, processed using OpenStreetMap’s umap tool, with a map of the lake in 1519, available in Wikipedia. Most of them are almost perfectly aligned with the Western edge of the central part of the lake, where the waves broke against the mountain ridge that separates Mexico Valley from Toluca:

Map of the collapsed buildings of the September 19, 2017 earthquake, over a map of the approximate lake boundaries in 1519

Map of the collapsed buildings of the September 19, 2017 earthquake, over a map of the approximate lake boundaries in 1519

Living atop a monster requires us to constantly pay attention, to be aware of the particularities of our land. I find early warning systems one of the greatest contributions our country has done; for over a decade, it was the only one in the world, and I now see that similar systems have been deployed nationwide in Japan and Taiwan, and in regions of Canada, the United States and Romania.

Hundreds of millions of people live in seismically active regions. There is a lot we, as computer professionals, can do to help in this regard. Japan’s implementation includes EEW broadcasts), which together with mandated modifications to FM radio and mobile phone systems so they receive the warning and alert users before the emergency potentially damages infrastructure saturates communications and alerts people without requiring wide infrastructure deployment. The sensor network performs real-time signal analysis to quickly alert the population and give as much information as possible to understand the phenomenon’s origin and to be able to know how to react. Those areas are well within our occupation field, and should constitute an invitation for us to work in.

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About Gunnar Wolf

Gunnar Wolf is pursuing a MSc degree in Information Security at IPN (Escuela Superior de Ingeniería Mecánica y Eléctrica), and teaches at the undergraduate degree of UNAM (Facultad de Ingeniería). He is a free software advocate, and a developer in the Debian project. Systems and network administrator for the Economics Research Institute (UNAM), has had the opportunity to do research on topics that include the social aspects of free software projects.

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