Salvador Ceja
GSC-350
Nov. 2007

September 19, 1985

8.1 on the Richter scale; Mexico City's grand earthquake.


In the following article I will tell my personal story regarding my experience of the 1985 Mexico City earthquake. I will also research the impact the quake had on such a dense urban environment. I will research the geologic conditions that led to the mass destruction experienced in the Mexico city area and concentrate on the Taxcoco Lake area where most of the destruction was evident. I will explain the liquefaction effect that was experienced in the basin. I will explore the conditions buildings were exposed to and why these conditions lead to their collapse.



My Experience-

I will never forget this day! I was exactly 5years and 19 days old. To this day I clearly remember that morning and the incident that was about to take place. I was on vacation at my grandparent’s ranch in Sahuayo, Michoacan in Mexico. I had just woken up and was getting ready to eat my grandmother’s delicious breakfast. My cousin was also visiting and we were playing out in the central courtyard located between the kitchen and our bedrooms. We were playing with my new toys recently given to me for my birthday. I was out in the open in the middle of the courtyard pushing my orange imitation dukes of hazard plastic Dodge charger along the garden soil. I was pretending to outrun my cousin who was chasing my toy car with his police car. We joyfully pushed our cars along the dirt. I remember the silence, the only sound in the remote ranch was that of the wind, the sound coming from the kitchen and my cousins car chase sounds. Seconds later all I can recall is the shear panic, massive shaking and rustling of fruit trees and breaking of pottery. I was five, I was scared and panicked. I did not understand that on that day I was experiencing one of the most devastating earthquakes Mexico has ever experienced. The ground shook for about three minutes but to a young impressionable mind the shaking lasted an eternity. My mother and grandmother ran out onto the courtyard and hugged my cousin and I. We did the only thing anyone can do under such circumstances…. Out in the open we prayed.
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Original image from USGS, edited by Salvador Ceja



The Event-

Early in the morning a few minutes after 7am a devastating earthquake of 8.1 magnitude on the Richter scale struck of the western coast of Mexico on September 19th, 1985. One day later it was followed by a magnitude 7 aftershock. The combination of these seismic events caused severe damage primarily in the Mexico City region. The earthquakes epicenter was located 50 km off the western coast of Mexico. Extreme seismic activity is prominent in the area due to the subduction of the “Cocos” plate underneath the Mexican mainland. This earthquake claimed the lives of over 5,000 people and injured tens of thousands in the Mexico City basin. I experienced the earthquake first hand and survived unharmed. My grandparent’s ranch was located at about the halfway between the earthquake epicenter and Mexico city and survived with nothing more than a thin crack on the front wall.

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1985 Mexico City

Devestation during and after the quake (Unknown photographer)


Preface-

In the following article I will research the impact the quake had on such a dense urban environment. I will research the geologic conditions that led to the mass destruction experienced in the Mexico city area and concentrate on the Taxcoco Lake area where most of the destruction was evident. I will explain the liquefaction effect that was experienced in the basin. I will explore the conditions buildings were exposed to and why these conditions lead to their collapse. As an architecture major I am primarily interested in the natural frequency phenomena and the tendency for buildings to resonate. Building resonance was not a consideration when most of Mexico City’s buildings were designed and built. I will explore the construction techniques and the technology used pre-quake and post-quake in the Mexico city region. I will perform a case study of one major building collapse and explore what factors in addition to the ground motion lead to the buildings destruction. I will perform a case study about the newest and most technologically advanced building in Mexico City and how the Architects and Engineers plan to keep the building standing if another earthquake similar to the 1985 quake takes place. I will discuss new technology and equipment in use today, designed to save lives and prevent damage.

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photo by: CIRES (Images of damage visible after the earthquake)



The Facts


Shaking

It is not surprising that most areas around the Mexico City basin survived the 1985 earthquake relatively unharmed. Sahuayo for example was only shook up and with the exception of limited collapses and mostly cracked or damaged buildings human casualties were minimal. Mexico City, far from the epicenter was damaged because it rests on an ancient lake bed. Its geology is dangerous and reacts heavily to the slightest soil vibrations. Its silt and volcanic clays as well as the mixture of water are highly susceptible to earthquakes and react in the form of liquefaction (vibrating of sandy or clay soil types resulting in shifting and moving soil causing amplifications in the input force or “S waves”).
Liquefaction caused the most damage and along with thousands of lives also claimed hundreds of man made structures. During the earthquake and immediately after due to after shakes over four hundred buildings collapsed and seriously damaged over three thousand buildings. The soil conditions in the Mexico City area also led to landslides, rockslides and ground rupture but the most damaging was the liquefaction. The as is visible in image-01 above the epicenter was located in an oceanic fault and therefore caused water to shift as the plates gave way. This sudden movement led to the creation of large tsunami wave that was visible and cause minor damage as far as the Galapagos Islands of the coast of South America.

An interesting feature about the damage pattern in buildings was that the most effected were those between 8 and 18 stories in height leading to the conclusion that the height and material properties were most conducive to building resonance at those height in combination to the “S-waves” and their two-second period of horizontal ground accelerations experienced that day. The earthquake acceleration (waves) felt that day led to major structural failures and finally total or partial building collapse.

Soil and soil history

As mention earlier Mexico City is built on a filled in lake and the soil is not stable. The original Aztec natives, Pre- Columbian inhabitants, began filling in the lake over two thousand years ago in order to create fertile crop land. The lake was filled in with plant matter as well as soft sands and clays from the surrounding valley. During the time of the Spanish conquest of Mexico the area was mostly drained by releasing the water through channels and tunnels to a nearby river. The surface water is gone but the soil is soft and moist since the water table is far below the city surface. The area was dramatically altered and the consequences were severe. The area is now dry and unfertile. The city relies on pumping water out from underneath the city. This too causes issues and even the city surface level is dropping and therefore the city is sinking.
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Mexico City area before it was drained. (Aztec City)

The following is an excerpt from Wikipedia on a description of Soil Liquefaction:
Soil liquefaction describes the behavior of loose saturated cohesionless soils, i.e. loose sands, which go from a solid state to have the consistency of a heavy liquid, or reach a liquefied state as a consequence of increasing porewater pressures, and thus decreasing effective stress, induced by their tendency to decrease in volume when subjected to cyclic undrained loading (e.g. earthquake loading). Liquefaction is more likely to occur in loose to moderate granular soils with poor drainage, such as silty sands or sands and gravels capped or containing seams of impermeable sediments [1]. Deposits most susceptible to liquefaction are young (Holocene-age, deposited within the last 10,000 years) sands and silts of similar grain size (well-sorted), in beds at least several feet thick, and saturated with water. Such deposits are often found along riverbeds, beaches, dunes, and areas where windblown silt (loess) and sand have accumulated. Some examples of liquefaction include quicksand, quick clay, turbidity currents, and earthquake liquefaction.


New Technology (Architects and Engineers)


The damaged caused during the 1985 Mexico City earthquake was tremendous. An incredible amount of money, property, structures and primarily lives were lost. Days after the event took place research commenced on why the buildings collapsed and what could be done to prevent future building collapse of such magnitude. Not necessarily just in Mexico City but everywhere in the world. Researchers, Engineers, and Architects volunteered to look into the reasons for the intense building resonance and the effects the soil produced on the buildings. The general realization was that the building construction type, height, materials and soil conditions are evident in multiple areas around the world. By our human intuition and nature we tend to build our densest urban communities in such places similar to Mexico City. As a result the architects and engineers quickly began researching methods for improving building’s stability and minimizing their vulnerability to resonance and immanent collapse. As a result multiple new methods were invented as well as new products were produced. Here you can see some of the types of innovations currently being used around the world.

The following is a short film I edited, enjoy.










1. Nonstructural vibration Isolation (uses springs to limit the amount of force transferred from the ground to the building)04.jpg


2. Giant shock absorbers (silicone filled, dampen the resonance effect and reduce swaying)
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(unknown photographer)


3. Base isolators (Giant rubber pads allow building to stay in place while the ground below moves back and forth, not good on resisting “P” pressure waves).
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4. Combinations shock absorber and base isolators including cross braced frame.
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5. Torre Mayor (The tallest most expensive building in Latin America. New building designed and Locate directly over the most heavily effected area of Mexico City during the 1985 earthquake. Incorporates braced frame and silicone shock dampeners)

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Based on on the USGS the earthquake was felt by 20 million people also they state, " According to some sources, the death toll from this earthquake may be as high as 35,000. It is estimated that the quake seriously affected an area of approximately 825,000 square kilometers, caused between 3 and 4 billion U.S. dollars of damage." This earthquake also produced a tsunami which based on the USGS this tsunami caused damage in Lazaro Cardenas. Also there were very high tide waves which were in Acapulco, Mexico. The tsunami was a major cause of the earthquake in Mexico. The tsunami affected many in the Pacific coast of Mexico. The tsunami although did not caused many damages it was a very important concept to the Mexico earthquake.

Reference:
USGS
http://earthquake.usgs.gov/regional/world/events/1985_09_19.php



Sources;

Booth, E.D. “The Mexican earthquake of 19th September 1985.” London : Society for Earthquakes and Civil Engineering Dynamics, c1986. Cassaro, Michael A. “The Mexico earthquakes, 1985.” New York, N.Y. : American Society of Civil Engineers, c1987. CIRES Mexico's earthquake authority http://www.cires.org.mx/ Mcgaving, Gary. Professor of Structural engineering, Cal Poly Pomona, Architectural. Mitchell, James K. “Crucibles of hazard : mega-cities and disasters in transition.” New York : United Nations University Press, 1999 Poniatowska, Elena. “Nothing, nobody : the voices of the Mexico City earthquake” Philadelphia, PA : Temple University Press, 1995 Torre Mayor; http://www.torremayor.com.mx/ (Information on “Torre Mayor,” Latin Americas tallest building) USGS at: http://neic.usgs.gov// (geographic data and plate boundaries) Wikipedia http://en.wikipedia.org/wiki/Soil_liquefaction (even though I don’t like to use it as a source, here it provides the best explanation on soil liquefaction)