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Roger Weller, geology instructor
by Jose Guerrero
The Mexico City Earthquake
It was 7:18 in the morning on September 19, 1985 when many people were woken up by an earthquake with a magnitude of 8.1 in Mexico City. It is said that about 20 million people felt the trembling. The earthquake which was the strongest ever to hit the city caused significant damage leaving 10,000 people dead, about 30,000 people injured, and thousands of people homeless. It is estimated that it caused about 4 billion U. S. dollars worth of damage. Many buildings were damaged including many hospitals which led to a lack of care for the thousands that were injured. There was a loss of electricity which put a stop to public transportation and to traffic lights. The telephone system was down causing a lack of communication. At first President Miguel de la Madrid refused to call a full national emergency plan and refused international help but quickly changed his mind.
Photo courtesy of quebuentrip.com
Although the earthquake was centered about 250 miles west of the city and because of the unstable ground, there was some serious trembling which lasted almost three minutes. The earthquake actually occurred in the Pacific Ocean off the coast of the state of Michoacán which is a distance of more than 350 kilometers from the city.
How can an earthquake
cause so much damage over 200 miles from its origin? What happened in Mexico in
1985 compares to a quake that happened along the San Andreas Fault near San
Francisco in 1952 which left Bakersfield in ruins. To answer this question, we
have to go back in history almost 700 years. In 1325 the Aztecs, one of the high
civilizations of Mesoamerica, founded their capital Tenochtitlan. They build it
on an artificial island in a shallow lake in Mexico's central altiplano.
Although the old capital was flooded again and again, the Spanish did not
abandon the site in what they called Lago de Texcoco, but enlarged it instead.
After Mexico's independence the settlement became the capital of the newly
founded country. During the last century, the lake was completely drained, to
make room for the housing needs of the ever-growing population of Mexico City.
The earthquake's epicenter was actually just off the west coast of Mexico,
several hundred miles from Mexico City, but the geography of the region made the
city particularly susceptible to the shaking.
Figure 1 (photos courtesy of Mehmet Celebi, USGS) Figure 2
Mexico City sits on a plateau that is surrounded by mountains and volcanoes. This area was once covered by lakes in ancient times. When the aquifer below the city slowly drained, it was discovered that the city was sitting on top of a combination of sand and dirt that was less stable than bedrock and can be very volatile during an earthquake. A lake bed in a basin is the worst place to construct a building especially if they are tall. Hard rock just shakes with the same frequency and amplitude as a seismic wave but the sediments of an ancient lake bed are different. The shaking is amplified and can lose their consistency becoming a liquid. This amplification and liquefaction cause weak foundations of tall buildings to collapse.
The geologic reason
why Mexico and mainly Mexico City are so vulnerable to earthquake damage is
because of the Cocos Plate which produces a high seismic zone, a subduction zone
which is specifically in a section of the Michoacan seismic gap fault line with
coordinates of 18.190 N and 102.533 W. The Cocos Plate pushes and slides under
the North American Plate, mainly along the states of Michoacan and Guerrero.
Below is an image of a subducting slab.
Photo Courtesy of www.tectonics.caltech.edu/outreach/highlights/mase/
The volatile trenches
along the Cocos Plate usually have had seismic events from 30 to 70 years before
the earthquake of 1985. This zone outside the gap has been the cause of 42
earthquakes with a magnitude of 7.0 or more during the 20th century
before the 1985 earthquake. Below is an image of the deep trench off the
southern coast of Mexico.
Photo Courtesy of www.tectonics.caltech.edu/outreach/highlights/mase/
Shockwaves hit the coast of the mouth of the Rio Balsas at 7:17 a.m. and two minutes later hit Mexico City which is 350 kilometers away. The earthquake was a multiple event with two epicenters and the second shaking 26 seconds after the first one. Since there are multiple breaks in the fault line, the quake lasted longer. The ground shook for about five minutes and the city of Mexico shook for about three minutes. It is estimated that movement along the fault line was about three meters. The energy released during the main event was equivalent to approximately 1,114 Nuclear weapons exploding. The earthquake was felt over 825,000 square kilometers, as far away as Los Angeles and Houston in the United States.
While the fault line was located just off the Pacific coast of Mexico, there was relatively little effect on the sea itself. The earthquake did produce a number of tsunamis but they were small, ranging between one and three meters in height. Ecuador reported the highest waves of 60 centimeters.
disasters occurred after as a result of the earthquake. Landslides, rockslides,
sandblows, and even a tsunami were generated. Rockslides were reported along the
highways. Sandblows and cracks in the road were seen in the Lazaro Cardenas
area. In that area a tsunami caused great damage. Waves three meters high were
reported at Zihuatenejo and made it as far as Hawaii.
Photo courtesy of USGS
damage was assessed after the earthquake, it was discovered that about 3,000
buildings in Mexico City had been demolished and 100,000 more had serious
damage. Many hotels such as the Regis Hotel shown below crumbled to the ground.
Photo courtesy of www.rugusavay.com
One building at the National College of Professional Education fell and trapped hundreds of students who were attending morning classes. Many factories also crumbled because of the shoddy materials they were made with. The earthquake caused gas lines to break which in turn caused fires and explosions throughout the city.
During the six months after the Mexico City earthquake, 152 damaged buildings had to be demolished and more than a million people still remained without electricity. In the first three days after the earthquake only 38% of all utility services (gas, electricity, and water) had been restored. Among the damages there were approximately 1300 transformers, 5 transmission lines, 8 substations, and 600 lamp posts. Thirty-two metro stations were affected but the majority resumed normal services within a few days. Services for making exterior phone calls were seriously damaged and not completely restored until 6 months later. The estimated number of jobs lost was between 150,000 and 200,000.
Photo Courtesy of
Mexico’s president, Miguel de la Madrid was criticized for his weak response to this natural disaster. At first, he rejected offers of international aid and tried to play down the damage caused by the earthquake. So the Mexican citizens decided to organize their own rescue brigades. Later on, an early-alert earthquake warning system was established in Mexico City and other safety measures were enacted. One was the Sistema de Alerta Sísmica-SAS (the Seismic Alert System), which sends early-warning messages electronically from sensors along the coastal subduction zone in Guerrero. An alarm will go off if an earthquake of over 6.0 magnitude is detected. Below is an image of the President.
Photo Courtesy of: http://mexicosearthquake.blogspot.com/