by David R. Cox
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Article in National Geographic about the 1986 Earthquake
National Geographic explanation of how earthquakes damage buildings
Slide show of 1986 Earthquake (25 photos)
Mexico City is famous for its earthquakes. Mexico City is high up in the mountains (over a mile high) in a mountain valley, which was once a lake. Now this lake is mostly dried up with a much smaller lake in the eastern part of the city, and some canals in the south. The early Aztecs choose this island in the upper mountain lake because it was easily defendable.
When earthquakes hit, the mild movement which should happen gets amplified by the soft mud underlying the entire valley part of Mexico City to do great damage to buildings and other structures. Mexico City is ringed by moderately high hills. The people live from the valley portion all the way up into the hills and over the other sides in many directions.
Mexico City earthquake, 1985
About 30,000 were injured, more than 100,000 people were left homeless, and the quake was felt by almost 20 million people. Severe damage was caused in parts of Mexico City and in several states of central Mexico. 412 buildings collapsed and another 3,124 were seriously damaged in Mexico City. A large percentage of the buildings which were damaged in Mexico City were between 8 and 18 stories high, indicating resonance effects around 2 Hz due to the soft soils under the city.
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September 19, 1985, at 7:17 A.M.Mexico City itself lies on unconsolidated lake-bed sediments. These soft sedimentary clay deposits amplified the seismic waves, or they subsided carrying buildings down with them. Double resonance coupling between the earthquake waves, the subsoils, and the buildings caused intensity IX shaking in some areas, lasting up to three minutes.
Some types of structures failed more frequently than others. In the highest damage category were buildings with six or more floors. Resonance frequencies of these buildings were similar to the resonance frequencies of the subsoil. Because of the "inverted pendulum effect" and unusual flexibility of Mexico City structures, upper floors swayed as much as one meter and frequently collapsed. Differential movements of adjacent buildings also resulted in damage. A flexible building often failed if it was held by adjacent, more rigid lower buildings. Damage or failure often occurred where two swaying buildings came in contact. Corner buildings were also vulnerable to damage.
Part of the problem in Mexico City is that when this soft subsoil begins to liquefy, it is totally unstable and unable to support the weight of a building over it. As such, the building nicely sinks into the soil sometimes like in the picture, 3 feet or more.
At times from the forces of the earthquake (see National Geographic explanation) a singular, entire floor of a building can collapse but leaving the rest of the building more or less intact.
http://www.gcn.ou.edu/~jahern/v&e/subduct/subduct_quakes/mexico_city.html
This is an example of how the vibration works its way up a building amplifying the shock waves until the top floors cannot absorb the stress and fail. (see National Geographic explanation)
The bottom line is that anytime the earth moves under your feet, the roof and walls swing as if they will break, and tons of concrete fall on your head, you get rattled!
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Last update of this page was on July 11, 2003
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