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1985 MEXICO CITY QUAKES
BY: GABRIELA MUÑOZ
THE SEISMIC EVENT
Occurred in the Pacific Ocean, off the coast of the Mexican state of Michoacán. More than 350
km (217 mi) from the city. This section of gap is known as the Michoacán seismic gap. The
Volatile trenches along the Cocos plate generally have had seismic events 30-70 years before
1985. There have been 42 earthquakes of magnitude 7.8 or stronger in the 20th century prior
to the one in 1985. Shock waves hit the mouth of Rio Balsas on the coast and hit Mexico City at
7:19 am. The September 19th quake was a multiple event with two epicenters, the second
movement occurred 26 secs after the first. It was estimated that movement along the fault was a
bout 3 meters. It was said that the energy released during the earthquake was equivalent to
approximately 1,114 nuclear weapons. The ‘85 quake was felt as far away as Los Angeles and
Houston in the United States.
SEISMIC EVENT
MEXICO CITY’S VULNERABILITY TO EARTHQUAKES
• Mexico’s volcanic & seismic activity stems from the movement of the North American plate
against the Cocos and Pacific plates & it’s one of the most active trenches in the world. More
than 90 temblors above 4.0 magnitude are recorded in this zone alone. One of the main
reasons for this is that the geology of the area, especially the downtown area grew from an
island in the middle of Lake Texcoco outward, the lake was eventually drained. It is classified
into 3 sections: the old lake bed which is soft clay from volcanic ash with high water content, a
piedmont area, mucho of which is capped by 5-30 meters of lava less that 2,500 years old,
and an old river delta area. Prevailing silt and volcanic clay sediments amplify seismic
shaking, the downtown area lies on this bed which is between 7-37 meters deep and have a
high water content. With the high water content the lakebed is easily moved or compressed.
The lakebed has a natural “pitch” of one cycle every 2.5 seconds making everything built on
the bed vibrate at the same frequency. This unfortunately is the same “pitch” as a number of
shallow earthquake waves.
MEXICO’S VULNERABILITY TO EARTHQUAKES
LOCALIZATION OF THE DAMAGE
• 80% of the quake damage was confined to 4 places, Venustian Carranza,
Cuauhtémoc, Benito Juarez and Gustavo A. Madero.
• Damaged areas were located on the western part of the lake zone.
• Nearly all the buildings that collapsed that day were located in the lake zone
that extended from Tlatelolco, a short distance from the Zocalo or main plaza.
LOCALIZATION OF DAMAGE
BUILDING DAMAGE IN THE CITY
• Cuauhtémoc has the most historic downtown buildings suffered most of the
damage. 258 buildings crumbled, 143 partially collapsed and 181 were
seriously damaged. Next affected area was Venustiano Carranza, 83
buildings collapsed, 128 partially collapsed and 2000 seriously damaged.
• Over 720,000 tons of debris removed during the first 6 weeks. Federal
District reported 2831 buildings damaged for the whole city: 880 completely
ruined, 1581 were recoverable with minor repairs.
• In total 30,000 housing units destroyed and 68,000 units damaged.
BUILDING DAMAGE
HUMAN TOLL IN THE CITY
• To this day the death toll is still in dispute. 5000 bodies were recovered and
only represent the total legal certified deaths, does not include missing or
never recovered bodies.
• Number of dead is anywhere from 5-30,000 (claimed by the citizens’ groups)
to 45,000 claimed by Mexican National Seismological Service.
• Death toll was so great that they had to use the IMSS baseball field as a
morgue and use ice to conserve bodies for identification.
• Approximately 250,000 people lost their homes.
HUMAN TOLL
GEOLOGICAL & STRUCTURAL ENGINEERING ISSUES
• Earthquake damage was only to buildings, two reasons are the resonance in
the lakebed sediments and the long duration of the shaking. Most of the
damaged buildings were from 6-15 stories in height.
• Interesting characteristic is that many buildings had their upper floors collapse,
leaving the lower floors undamaged. With so much shaking, buildings with
columns eventually gave way. In other cases, floors of buildings were designed
as parking garages, open lobbies or shopping areas, which are known as
“soft spots”.
• Foundations that involve piles driven into clay and held in place by friction,
turned out to be weak. Some of these piles were pulled entirely out of the
ground.
WHAT HAS BEEN LEARNED FROM THIS
EARTHQUAKE?
•
Mexico has created new building designs to withstand the seismic activity from future
and current earthquakes. The city also has provided emergency preparedness events
to help their citizens prepare for another earthquake and funding was also obtained
from the government to design, operate and build a seismic alert system. Mexico still
has a long way to go but at the least they have prepared for a seismic event by
providing: better awareness in communities, natural disaster fund, natural disaster
preventive fund, seismic alarm system, and updated and regionalized building codes.
•
Through this process Mexico has been able to grow and become stronger as a
country by uniting in trying times like they did when the earthquake destroyed much
of the city.
WORKS CITED
• CBSNEWS.COM
• WIKIPEDIA.COM
• MEXICO GOVERNMENT PAGE
• https://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=&url=http%3A%2F%2Fwww.geocritical.co
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