Strong earthquake near Mexico CityAbout an hour ago now a strong Earthquake struck about 150 kilomet

Strong earthquake near Mexico CityAbout an hour ago now a strong Earthquake struck about 150 kilometers away Mexico City, Mexico. Thanks to the real-time information sharing available on social media, there are already pictures and videos emerging showing clouds of dust hanging over the city reminiscent of the earthquakes in Christchurch, New Zealand, when hundreds of masonry buildings collapsed. I’m sharing some of them over at our twitter feed: https://twitter.com/TheEarthStoryFor large earthquakes, the US Geological Survey produces an automated estimate based on a system known as PAGER that takes into account the strength of the earthquake, the local geology, the population, and the building codes to produce an early estimate of damage and casualties. This program estimates the likely casualties to be in the range of 100-1000, with a 40% chance of casualties higher than 1000, and likely damage totals in the range of hundreds of millions to billions of dollars.Mexico City is a particularly dangerous place for Earthquakes. Mexico City sits in a rift basin, surrounded by faults and volcanoes. That rift basin has gradually dropped the land downwards, allowing lake sediments to accumulate when the area floods. There are still lakes inside the city today. Lake sediments are some of the most dangerous materials during an earthquake. Seismic waves travel rapidly through hard rock, but once they enter lake sediments they will change their speed and even reverberate through the basin. This behavior amplifies the shaking, doing more damage.Lake sediments are also at risk for liquefaction during earthquakes. You can often stand and put weight on loose sediments, you can even build structures on them, but when they are shaken the sediments will loose all their strength and anything on top of them will shift. Shifting land beneath concrete or masonry structures puts them at risk of collapse.The “Moment Magnitude” scale of measuring earthquakes is the current standard for stating how much energy is released by a quake – the current magnitude estimate for this quake is 7.1 and that’s the number you’ll hear most press reports repeating. However, that number often doesn’t help in assessing the effects on people or property – a high magnitude quake that is very deep or hits certain rock types will often do little damage. Instead, the Mercalli intensity scale measures how much shaking actually is felt by structures and how much damage can be expected.On that scale, the area around Mexico City received a strong to very strong quake – shown by the yellow to orange on this map. Under those conditions, damage is expected to be limited in well-built structures in countries with strong earthquake codes, but masonry and concrete buildings that were built without reinforcement or without enforcing building codes could be in danger of collapse. We may well already be seeing that in the available images and videos.Mexico is a complicated plate tectonic setting. The Cocos Plate is subducting beneath that country, but it is a young plate that is somewhat flat. Parts of the country are being compressed; parts of it are being pulled apart due to this complicated collision. The early estimated depth for this quake is 50 kilometers – that is in the lower crust, a short distance above the boundary with the Cocos plate. This earthquake took place on a normal fault, consistent with it happening on one of the rifts where Mexico is slowly being pulled apart east to west.A few weeks ago there was another large quake in Mexico closer to the subduction zone off its coast. That quake was much less devastating than this one will be as it was farther from population centers. After an earthquake like that one, aftershocks typically occur close to the area where the fault broke. Today’s earthquake took place far outside that area. However, it is also known that seismic waves can trigger earthquakes much farther away if a fault is already stressed and basically ready to break. This quake took a couple weeks to occur, but still may have felt at most a slight influence from the previous quake earlier this month. They aren’t directly connected, this quake isn’t an aftershock of the other, but they probably aren’t totally unrelated either.One of the things we’re already seeing in videos is people running out of skyscrapers during the quake. Notably, this is a bad idea – skyscrapers can lose large sheets of glass during earthquake shaking and you don’t want to be beneath one of those. If you’re inside this type of structure, the best advice still remains: drop, cover, and hold on. Get yourself beneath the heaviest desk or table you can, hold on until the shaking stops, and then evacuate as quickly as you can. If you’re in a brick or masonry structure, different advice may be necessary as those could be at greater danger of total collapse.-JBBImage credit: USGShttps://on.doi.gov/2xujxB6Christchurch quake image: http://bit.ly/2jIsem4References:http://bit.ly/2yof8NVhttp://cnn.it/2xcLfmghttp://go.nature.com/2fjnjXf -- source link
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