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1985 Mexico City earthquake

1985 Mexico City earthquake


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The powerful earthquake killed more than 10,000 and left another 30,000 others injured and as many as a quarter of a million people homeless. At around 7:19 a.m. on September 19, 1985, Mexico City, one of the world’s largest urban areas, was jolted by a magnitude 8.1 earthquake, one of the strongest to ever hit the area. The quake was centered off the Pacific coast of Michoacán, more than 200 miles west of Mexico City, the nation’s capital. However, much of the damage was in Mexico City, which was constructed on an ancient lake bed whose soft sediments amplify seismic waves.

Mexico City Earthquake: September 19, 1985

More than 10,000 people died as a result of the quake, some 30,000 others were injured and an estimated 250,000 people were left homeless. More than 400 buildings collapsed and thousands more were damaged. (The disaster exposed the fact that government corruption had allowed for lax enforcement of building codes.) Making matters worse, on the evening of September 20, a magnitude 7.5 aftershock shook the region.

1985 Mexico City Earthquake: Slow Government Response

Mexico’s president, Miguel de la Madrid (1934-2012), was criticized for his government’s weak response to the disaster. At first, the president rejected offers of international aid and played down the damage caused by the quake. In response, citizens organized their own rescue brigades.

In the aftermath of the 1985 earthquake, an early-alert earthquake warning system was established in Mexico City and other safety measures were enacted.


Seismic History: The Deadly 1985 Mexico City Earthquake

On this day in 1985, a magnitude 8.0 earthquake rocked Mexico City and its surrounding environs at 9:17 a.m. EDT (7:17 a.m. local time).

The quake was felt as far away as Guatemala City, Guatemala and Houston, Texas, over an area of about 319,000 square miles (825,000 square kilometers), but the most intense shaking occurred in Mexico City, Ciudad Guzman and the Pacific Coast towns of Lazaro Cardenas, Ixtapa and La Union, according to the U.S. Geological Survey (USGS).

The earthquake caused landslides, rockslides, and sandblows, opened cracks in the ground and damaged or destroyed buildings. In Mexico City, 412 buildings collapsed and another 3,124 were seriously damaged. About 60 percent of the buildings were destroyed at Ciudad Guzman, Jalisco.

The damage killed at least 9,500 people according to USGS figures, with another 30,000 people injured and more than 100,000 left homeless. Between $3 million - $4 million in damage was caused by the quake. [Top 10 Deadliest Natural Disasters in History]

A tsunami was generated which caused some damage at Lazaro Cardenas, Zihuatenejo and Manzanillo. Estimated wave heights were about 10 feet (3 meters) at Zihuatenejo and 9 feet (2.8 m) at Lazaro Cardenas.

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. The city lies in a drained lake bed, so large portions of the ground are made up of a silt and clay mixture that has a high water content and acts to amplify the shaking. This liquid-rich soil is also susceptible to liquefaction , which causes it to essentially act like a liquid, taking away the support of buildings and other structures.

The earthquakes waves also caused a resonance with the natural "pitch" of the area that amplified the shaking to certain tall buildings.


Earthquake In Mexico City, 1985

On September 15th of 1985 Mexico City had one of the worst earthquakes in history. This earthquake had a magnitude of 8.1, one of the strongest earthquakes that ever hit this area. This is an event that isn’t spoken about a lot because of the government’s involvement with the aftermath which was little to none.

With this incident, there were about 10,000 deaths, 30,000 people were injured, and around 250,000 people were left homeless. There were also more than 400 buildings that collapsed and many were damaged. The crazy part is that the day right after there was an aftershock with a magnitude of 7.5 almost the same as the original shake.

Many people were outraged with this event because the government did little to nothing to aid its citizens. The government was often saying that they would help their citizens but in this event, they lied and left people to suffer for days without water or electricity. Mexicos president at the time was Miguel de la Madrid who was under fire because of his lack of responsibility in terms of helping his people. In the beginning, he was rejecting help from other countries and instead said that Mexico didn’t need help from outside sources because the damage wasn’t that bad. This led to many citizens having to find solutions for themselves and rebuild their communities at their own pace. The government was also paying more attention to the areas where tourists would be visiting which would cause conflict with the peripheral zones because they weren’t receiving any help.

This event goes to show the way in which the government is corrupt. Not only does this show that they only care about the places that would generate wealth for the country but also that they don’t care about the citizens who live in other zones because they don’t offer them the same opportunities.


Mexico loses artist who ‘played with dolls’ after 1985 earthquake

“I guess at my age, death is around the corner, but it doesn’t worry me. When it comes, it comes,” said Spanish-Mexican artist Vicente Rojo at an event to honor his 89th birthday. Little did he know he would die only two days later on March 17, 2021.

The Mexican press is rightfully paying homage to this artist’s life and contributions to the country’s culture. He is considered one of the greats among the “Breakaway Generation,” artists that came of age in the 1950s and 1960s, rebelling against the nationalism and political focus of Mexico’s famous muralism movement.

Rojo’s life and art reflect many of the major events of 20th-century Mexico. Rojo was born in 1932 in Barcelona, Spain, to a family opposed to dictator Francisco Franco. When Rojo was 10, his father had to flee to Mexico, one of many Spanish Republicans who did so. Mexico offered asylum due to its own opposition to Franco’s fascism, and in return these Spanish refugees contributed greatly to the country’s literature, arts and publishing.

Rojo followed his father seven years later in 1949, part of a second wave of exiles fleeing repression. He not only managed to find his father on this side of the Atlantic, young Vincente discovered that he had a love and talent for art here as well.

Rojo and his generation succeeded in introducing international artistic trends into Mexico, but it was not easy. Muralists such as David Siqueiros objected that steering away from Mexico’s home-grown artistic movement invited imperialism from the United States. Rojo’s greatest contributions were in graphic arts, working with Mexico’s growing public and private publishing houses, but he was also a sculptor, creating a number of monumental public works.

It could be argued that Rojo’s contributions equal many of the artists of his generation, including José Luis Cuevas, Manuel Felguérez and Gilberto Aceves Navarro, who are far better known. But Rojo was also a designer, and in this capacity, made a contribution that none of these did.

One of the turning points in Mexico’s modern history was the 1985 Mexico City earthquake and its aftermath. The scale of destruction would challenge any government, but the city had made blunders before, during and after that shook the people’s confidence in their government. It is cited as one of the key factors in the eventual downfall of the PRI in 2000. Much of the death and destruction in 1985 was due to poor building codes and the lack of enforcement of the codes that did exist. This was very true for a section of the city dedicated to garment manufacturing just southeast of the main square. Workers here reported early for the day shift, were often shut into factories to prevent theft and worked on floors overcrowded with heavy machinery. This meant that when the 8.1 earthquake struck on September 19, factories collapsed, and many of the dead were the “seamstresses,” poor rural women who had migrated into the city to find work.

In addition, as many as 40,000 of their coworkers found themselves suddenly unemployed with literally no job to go back to. The government was too slow to react, so in the weeks afterwards, grassroots efforts arose to help these women.

One of these was to create a program to make and sell dolls using the sewing skills the women already had.

As a designer, Vicente Rojo was central to this effort. Many artists offered to help, but the designs for the dolls needed to be practical — easy and quick to make and easy to sell. After many of the women were organized, Rojo offered six themes for the dolls, on which they voted. The result was to focus on two dolls named Lucha (Struggle) and Victoria (Victory).

Thin, straight-haired Lucha represented the state the women found themselves in. Victory represented overcoming the catastrophe sometime in the future.

Rojo, despite his expertise, worked as a partner, not as a boss. He commented in a 1988 magazine interview that “… it gives me pleasure to collaborate with people who have been hit so hard by life … I made several drawings and let the seamstresses interpret them freely, using their own imagination. Fortunately, I feel this enriched and gave much life to their idea.”

The result was various interpretations of Lucha and Victoria during the years that the project was active. Rojo himself reinterpreted the idea three times. The Lucha and Victoria idea resonated with many sympathizers, drawing in additional support from individuals and institutions.

Rojo also donated an abstract doll design meant to represent multiple seamstresses hugging each other. The doll was made but was misinterpreted as a “donut” or “lifesaver.” In 1987–1988, he also donated a series of designs for cat figures with names like Blue-tailed Cat, Red-hearted Cat and Two-tailed Cat.

The success of the doll program led to an exhibit at one of Mexico City’s avant-garde museums, Carrillo Gill. Named “One called Victoria …” it consisted of 27 dolls by 20 artists working with various seamstresses. The women accepted doing the exhibit because they felt it would bring attention to their continued plight as Mexico City was slowly getting back on its feet. New versions of the exhibit were held annually from 1986 to 1990. There were even exhibitions of the dolls in other parts of Mexico, the U.S. and Europe.

However, by 1990, it was clear that the doll project was winding down as women and the country moved on. The project was never meant to be long term.

A number of Mexico’s newspapers are quoting writer Juan García Ponce (1932–2013) when Rojo would ask him about his health: “Don’t worry, Vicente, we are eternal.” Perhaps part of Rojo’s eternity will be in the memory of the women he helped, along with their descendants.

Leigh Thelmadatter arrived in Mexico 17 years ago and fell in love with the land and the culture. She publishes a blog called Creative Hands of Mexico and her first book, Mexican Cartonería: Paper, Paste and Fiesta, was published last year. Her culture blog appears regularly on Mexico News Daily.

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Among the millions of Mexicans affected economically by the coronavirus are the country’s artisans. Dependent on tourism for their livelihood, they have been forced to look for alternative means of selling their creations. One option is online sales. With that in mind, Mexico News Daily is supporting efforts by the Feria Maestros del Arte, a non-profit organization in Chapala, Jalisco, to help artisans sell their products online by donating 10% of the revenues from annual subscriptions to the Feria.

Another element of the campaign is a series of stories called Artisan Spotlight that will highlight some of Mexico's talented artisans.

We ask for your support for the Artisans Online project by purchasing or renewing a one-year subscription for US $29.99, of which $3 will help artisans reap the benefits of e-commerce. Please click here for more information about Artisans Online.

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I Survived the Mexico City Earthquake. This Is My Story

I was at home in the Roma neighborhood when the 7.1 magnitude earthquake hit Mexico City on September 19, exactly 32 years after the 1985 tragedy that killed thousands. There was no warning that the quake was coming, despite the fact that the capital boasts one of the most advanced earthquake alarms in the world.

The first tremble from beneath the earth was minor, and I thought it was a truck passing or the metro slithering underneath — normal things that make the streets rumble in this megalopolis. But the second shock hit with a force that launched me to my feet. The house swayed and twisted as if it were made of paper, and the city erupted into a cacophony of sirens and sounds of destruction as I stumbled out the front door. Suddenly, I was shirtless and shoeless in the street with my neighbors, being thrown back and forth by the ground that seemed to turn to liquid.

As we all reeled in the street, the smell of gas seeped out of the front gate of our vecindad, a condo-style building with small houses sharing a courtyard and a gated entrance. “Shut the gas and turn off the electricity, there’s a leak!” shouted one neighbor.

I hurried back in and tried to open the door to the back patio to reach our tank, but it was blocked. I could smell the gas leaking into the air. I ran back outside to the courtyard, into the neighbors’ house and up the spiral staircase to the shared roof so that I could get to our patio from above. The gas tank and some miscellaneous construction material had fallen across the door. I twisted the knob to close the tank and lifted it to an upright position as two neighbors emerged from the roof coughing and gagging from the toxic fumes.

The chaos and noise only kept growing as the city descended into total gridlock.

I grabbed a shirt, some flip-flops and my roommate’s dog, and left on foot fearing an explosion. I did a slow lap around the block and started to realize the extent of the damage. Around the corner, near the Centro Medico metro station, an apartment building teetered, close to collapsing, as glass and pieces of concrete rained down on the sidewalk. Residents of the building scattered on the street below, their gazes fixed on their precarious homes. Smoke could be seen rising above rooftops, and the streets filled with more and more panicked chilangos — the common name for Mexico City residents. I tried to call my family and friends but the networks were overwhelmed, and I couldn’t get through to anyone.


1. Early Warning Systems Save Lives

From Prevention Web

“Five years after the devastating 1985 quake, Mexico equipped itself with one of the world’s most effective early warning systems for earthquakes. SASMEX: the Seismic Alert System of Mexico comprises more than 8200 seismic sensors located in the most active earthquake zone that runs between Jalisco, Michoacán, Guerrero, Oaxaca and Mexico City.

An essential part of the system, the sensors detect the first earth tremors and SASMEX calculates the intensity of the earthquake. If the estimated magnitude of the tremor is greater than 5.5 on the Richter scale, warning notifications are immediately sent out to state and local officials and emergency focal points in all areas at risk. Mass warnings are then issued through sirens, AM and FM radios, and television broadcasts, so populations and communities at risk have time to prepare and save their lives. The system is very efficient and has already contributed to saving many lives. In April 2014, an earthquake of magnitude 7.2 hit the west coast of Mexico close to Acapulco. ‘A warning was issued in less than 10 seconds to seven main Mexican cities at risk and no death was reported,’ said Mr. Luis Felipe Puente, Head of Mexico’s Civil Protection.” https://www.preventionweb.net/news/view/52762

From ShakeAlert ® An Earthquake Early Warning System for the West Coast of the United States

“This earthquake early warning (EEW) system detects significant earthquakes so quickly that alerts can reach many people before shaking arrives. ShakeAlert is not earthquake prediction, rather a ShakeAlert message indicates that an earthquake has begun and shaking is imminent.

The U.S. Geological Survey (USGS) along with a coalition of State and university partners are now implementing Phase 3 of operations of the ShakeAlert Earthquake Early Warning System for the West Coast of the United States. Many partnerships to utilize ShakeAlert in authentic environments such as utilities, hospitals, transportation systems, and educational environments are active today and more are being developed. In 2020, the USGS and its partners will continue to expand these applications in coordination with state agencies in Washington, Oregon and California.” https://www.shakealert.org/


Earthquake Mexico 1985

On September 19, 1985, at 7:18 in the morning, the residents of Mexico City were jolted awake by an 8.1-magnitude earthquake, one of the strongest to ever hit the area. The effects of the quake were particularly devastating because of the type of ground upon which the city sits. Mexico City is on a plateau surrounded by mountains and volcanoes. The plateau region was covered by lakes in ancient times. As the aquifer under the city has slowly drained, it has been discovered that the city sits atop a combination of dirt and sand that is much less stable than bedrock and can be quite volatile during an earthquake.

The quake on September 19 was centered 250 miles west of the city but, due to the relatively unstable ground underneath the city, serious shaking lasted for nearly 3 minutes. The prolonged ground movement caused several old hotels, including the Regis, Versailles and Romano, to crumble. A building at the National College of Professional Education fell, trapping hundreds of students who were attending early-morning classes. Many factories in the city, built with shoddy materials, also could not stand. Further, the tremors caused gas mains to break, causing fires and explosions throughout the city. When the damage was finally assessed, 3,000 buildings in Mexico City were demolished and another 100,000 suffered serious damage. 10,000 people lost their lives, 30,000 were injured and thousands more were left homeless.


1985 Mexico City earthquake - HISTORY

Dr. George Pararas-Carayannis

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THE GREAT EARTHQUAKE OF 19 SEPTEMBER 1985 AND THE MAJOR EARTHQUAKE OF 21 SEPTEMBER 1985 IN MEXICO - TSUNAMI SOURCE MECHANISMS

George Pararas-Carayannis

(Excerpts from Report submitted to the UNESCO-Intergovernmental Oceanographic Commission and to the International Coordination Group for the Tsunami Warning System in the Pacific(ICG/ITSU), based on a survey of the stricken area by the suthor and from subsequent analysis of findings)

Summary


The major earthquake measuring 8.1 on the Richter scale which struck the Western Coast of Mexico on Thursday, 19 September 1985, generated a small tsunami. A major earthquake (aftershock or separate event?) on 21 September, 1985 with magnitude 7.5 generated also a small tsunami. Both tsunamis propagated across the Pacific and were recorded by several tide stations in Central America, Colombia, Ecuador, French Polynesia, Samoa, and Hawaii. No reports of damage were received from any distant locations, and only minor damage due to the first tsunami was reported in the source region along the west coast of Mexico.

A survey was undertaken by the author for the International Tsunami Information Center (ITIC) of the coastal area from Manzanillo to Zihuatanejo. Tsunami runup measurements were taken and interviews with local residents in the coastal areas were conducted. Subsequently, a survey of earthquake damage was undertaken in Mexico City.

A source mechanism study of the tsunamis was subsequently conducted using seismic and geologic data and empirical relationships. Earthquake and tsunami energies were estimated and the tsunami generation areas defined.

The earthquake energies were estimated to be 5.61 x 1024 ergs (10 raised to the 24 power) for the 19 September event while that of the 21 September event at 9.9 x 1023 ergs (10 raised to the 23 power). Tsunami energies were estimated to be 0.7 x 1020 ergs (10 raised to the 20 power) for the first event and 0.56 x 1020 ergs (10 raised to the 20 power) for the second event. The source area of the first tsunami was determined to be approximately one-half of the earthquake source area, or approximately 7,500 sq, km while the source area of the second tsunami was estimated to be equal to the earthquake area.

The relatively small tsunamis generated by these large earthquakes are attributed to the shallow angle of subduction of the Cocos plate underneath the North American plate for this particular region, and to the small vertical component of crustal displacements. However, the angle of subduction increases further south and local earthquakes from that area have the potential of producing large tsunamis on the West Coast of Mexico.

Downtown Mexico City

Destruction of buildings in Mexico City (Photo: G. Pararas-Carayannis)

Downtown Mexico City (Photo by G. Pararas-Carayannis)


EARTHQUAKE SOURCE PARAMETERS

Date and Time of Occurrence - The great earthquake occurred on 19 September 1985 at 13:17:47 UTC

Epicenter Location - Distances - The epicenter of the great earthquake of 19 September 1985 was at 18.2 N, 102.5 W., about 50 km (appr. 31 miles) off the coast of Mexico

Magnitude - The great earthquake 19 September 1985 earthquake had a magnitude of 8.1 (Ms). It was the largest event that had occurred in Mexico since the Great Jalisco earthquake of 1932. The magnitude was later revised by the USGS to 8.0

Epicenters and Aftershock Distribution of the 19 and 21 September 1985 Earthquakes - Tsunami Generation Area

Earthquake of 21 September 1985 - The major earthquake which occurred 36 hours later on 21 September 1985 (the evening of Friday, September 20 local time), had a Richter magnitude of 7.5. Its epicenter was at sea, approximately 100 Km SE of the epicenter of the 19 September great event, also along the Michoacan gap. It is believed that this was a separate earthquake rather than an aftershock.

Focal Depth - Both earthquakes had very shallow focal depths.

Aftershocks - There were many aftershocks after the main quake.

Extensive damage of the twelve-story high ,reinforced concrete building of the Ministry of Communications and Transport resulted in the near total collapse of long-distance communications between Mexico City and the rest of the world - thus complicating coordination of international rescue efforts.

Earthquake Death Toll, Injuries and Damage - There was severe damage in several states of Central Mexico and in parts of Mexico City. According to official estimates, 10,000 people were killed, 50,000 were injured, and 250,000 people were left homeless. It is believed that the death toll was underestimated and that as many as 40,000 to 50,000 people may have lost their lives.

In Mexico City alone 412 buildings collapsed and 3,124 others were heavily damaged. There was extensive destruction at Ciudad Guzman in the State of Jalsico, where approximaely 60% of all the buildings were either destroyed or heavily damaged. Extensive damage was also reported from other parts of the State of Jalisco as well as in the States of Michoacan, Vercruz and Morelos. A total of about 6,000 buildings were either destroyed or so heavily damaged that needed to be demolished.

There were also reports of damage from landslides at Atenquique, in Jalisco, at Jala in Colima, as wells as along the coastal roads near Ixtapa. Total damage by the earthquake (and tsunami ) was estimated to be between 3- 4 billion U.S (1985) dollars. Most of the damage was caused by the earthquake.


Ground Movements - Intensities and Accelerations - Liquefaction Effects

The Great Earthquake of 19 September 1985 caused strong ground motions which lasted for about three to four minutes - a rather unusual duration even for a great earthquake . Strong shaking was felt over an area of about 825,000 square kilometers. Seismic intensities and accelerations differed from point to point depending on geologic conditions. The quake was felt by about 20 million people in Mazatlan, in the State of Sinaloa to Tuxtla Gutierrez in the State of Chiapas, and as far away as Corpus Christi, Brownsville, McAllen, Ingram and El Paso and Houston, in Texas and even in Guatemala City.

Mexico City - Although the epicenter was more than 300 Km away, the valley of Mexico experienced surface seismic waves with ccelerations up to 17% g. with peaks concentrated at 2 sec. period (Quaas, et al, 1985). The maximum estimate of the Modified Mercalli intensity was IX.

Considerable liquefaction and damage to new buildings occurred in Mexico City. The extreme damage was attributed to the monochromatic type of seismic wave with this predominant period causing 11 harmonic resonant oscillations of buildings in downtown Mexico City which caused many buildings to collapse. These ground accelerations were enhanced within a layer of 30 ft. of unconsolidated sediments (of silt and volcanic clay) underneath downtown Mexico City, which had been the site of the historic Lake Texcocoa in the 15th Century. It is interesting to note that most of the buildings that were destroyed - or heavily damaged - were those that ranged between 8 and 18 stories in height - which perhaps suggests a resonance effect of the horizontal ground accelerations due to the short period ( 2-second ) seismic, surface wave.

Playa Azul-Lazaro Cardenas - Ixtapa - La Union - Maximum registered intensity was IX in the region of Playa Azul-Lazaro Cardenas (Ortega, et al, 1985) and at Ixtapa and La Union. Ground fissures had predominant orientation in NW-SE direction. Liquefaction effects and sand craters were observed in the coastal region. Extensive ground fissures and sand craters were also observed in the coastal area of Ixtapa.

Crustal Displacements and Rupture Lengths - The Great Earthquake of 19 September 1985 had two major ruptures. The horizontal displacements were estimated to be approximately 2.5 meters. A vertical displacement of 80 cm was measured in hard rock north of the city of Lazaro Cardenas. Measurements of crustal displacements were not available for the major earthquake of 21 September 1985.

Earthquake Source Area - The 19 September 1985 earthquake affected an area of 185 x 75 Km2 or approximately 13, 875 Km2. The 21 September 1985 earthquake affected an area having approximate dimensions of 75 x 70 Km2 or roughly 5,250 Km2. A total of 63 aftershocks were roughly recorded ranging in magnitude from less than 3 to more than 5. All of them had depths of 60 Km or less.

Earthquake Focal Mechanism - The focal mechanism of this earthquake corresponded to reverse faulting and was poorly controlled. The seismic history and seismic potential of this Michoacan gap have been uncertain and controversial in the past, until this event. Data from first motion instruments indicated that the main event was on a very shallow plane, which is typical with the direction of subduction of the North American plate. According to Cal Tech, long period, P-wave data, the depth was 17 Km for the hypocenter of the main event, while the depth of the major aftershock was 22 Km. Also, long period seismometers indicated that the main event resulted from two separate sub-events separated by a time lapse of 27 seconds. The overall duration was much longer than that of past events.

Earthquake Energy - Energy flux calculations (Anderson, et al, 1985), based on strong motion records of the main earthquake, indicated a low dynamic stress drop, and when observed heat flow is taken into consideration, a low absolute interplate stress.

An approximation of the energy of an earthquake can be obtained from empirically derived relationships. Earthquake energy is related to earthquake magnitude (M) by:

(1) Log10 E = 1.5 M + 11.8 (Gutenberg and Richter, 1954),

and earthquake magnitude is related to rupture length (l) by:

(2) M = 6.27 + 0.63 log10 l (Iida, 1958)

where l is measured in Kms, and M in Richter magnitude, for rupture velocities of 3-3.5 Km/sec. If we combine (1) and (2) we get

(3) Log10 E = log10 l + 22.53 , or E = 3.3 x 1022 l in ergs.

Based on these empirically derived relationships, and using the estimated earthquake source areas of both events, estimates of the earthquake energy were obtained. For the main earthquake of 19 September 1985 (M = 8.1 and l approximately 170 Km), the energy of the earthquake (E1) can be approximated to be E1 = 5.61 x 1024 ergs. For the second earthquake (M = 7.5 and l = 30 Km), the approximate energy was estimated to be: E2 = 9.9 x 1023 ergs

Tectonic Setting

The Middle America Trench defines the boundary between the Pacific, Cocos, and Nazca plates as they subduct beneath the North American and Caribbean plates. The Trench has been formed by an active subduction process and stretches from central Mexico to Costa Rica for about 1700 miles (2,750 km). The segment of the trench (fronting the city of Acapulco) is known as the Guerrero seismic gap. The last earthquake along this particular gap was in 1911, so this area had and continues to have, a high probability of recurring large earthquakes (Anderson et al., 1985). The 19 September 1985 and the 21 September, 1985 earthquakes occurred along a segment that is characterized by a low angle of subduction.


Recent Earthquakes - In the 20th century, Mexico had about 42 earthquakes with magnitudes greater than 7.


THE TSUNAMIS OF 19 AND 21 SEPTEMBER 1985 IN MEXICO - SOURCE MECHANISMS


Both earthquakes had their epicenters at sea. The first earthquake of 19 September, in spite of its large magnitude, produced a rather small tsunami. Movement in the vertical plane (0.8m) was relatively small, the angle of subduction was shallow , and volumetric displacement of the crustal block underneath the ocean was relatively small.

The major aftershock (or separate earthquake) of 21 September 1985 had its rupture and its crustal displacements further out to sea. Its subduction angle may have been somewhat steeper underneath the North American plate, thus having a larger vertical component. Although it affected a smaller area and had displacements for only 50 Km, in terms of tsunami generation, it may have been more efficient. This is illustrated by the Acapulco tide gauge record in which both tsunamis of 19 and 21 September registered almost equally, in spite of the large difference in earthquake magnitudes. However, the source area for this second event was closer to Acapulco.

Survey of Tsunami Runup

The survey of the coastal area affected by the tsunamis of the 19 and 21 September 1985 earthquakes covered the west coast of Mexico from Manzanillo to Zihuatanejo. Runup measurements were made and interviews with local residents in the coastal areas were conducted. The tsunami measured from 1 meter to approximately 3.0 meters from Manzanillo to Acapulco. The tsunami caused some damage at Lazaro Cardenas, at Zihuatenejo and at Manzanillo. Tide stations recorded maximum wave heights (peak-to-trough) of 1.4 meters at Acapulco, Mexico. There were some reports that some ships off the Pacific coast of Mexico observed unusually heavy seas near the time of the earthquake. However these waves may have been caused by local storms.

Manzanillo - A tsunami height in the order of 1 meter was reported.

Zihuatanejo - Maximum wave heights of approximately 3.0 m were measured .

Lazaro Cardenas - This was the town closest to the epicenter where the maximum tsunami height was estimated at approximately 2.8 meters, with inland inundation of up to 180 feet. Tsunami damage to coastal structures, due to the effects of flooding and erosion, was relatively minor.

Tsunami Tide Recordings at Distant Locations

La Libertad, Ecuador - 60 cm

Acajutla, El Salvador - 58 cm 24 cm

Kahului, Hawaii and at Pago Pago, American Samoa 22 cm Hilo, Hawaii

Baltra Island, Galapagos - 21cm

Apia, Western Samoa - 14 cm

Rikitea, Gambier Islands - 7 cm

Papeete, Tahiti - 5 cm


Tsunami Generating Area

As indicated previously (see diagram of estimated tsunami generating areas), the total area affected by the 19 September earthquake had approximate dimensions of 185 x 75 Km2, or 14,000 Km2. Approximately one half of this area was in the ocean, so effectively the tsunami generating area was only 7,500 Km2, which is only a small fraction of the ocean floor area usually affected by large earthquakes. For example, the Great Alaskan earthquake (M=8.5) affected a total area of approximately 215,000 Km2 and the tsunami generating area was in the order of 175,000 Km2 (Pararas-Carayannis, 1972), or approximately more than twenty times greater than the area affected by the 19 September Mexican earthquake. This partly explains the relatively small tsunami generated by this large Mexican earthquake.

The 21 September earthquake, although much smaller in magnitude (M=7.5), affected a smaller area estimated at 75 x 70 Km2 or approximately 5,000 Km2. However, all of the affected area was in the ocean, so the tsunami generating area was also approximately 5,000 Km2. The records of the tsunamis from the two events as recorded in Acapulco, show that the second event produced a tsunami which was very similar in size to the one generated by the larger event. This indicates that the efficiency of tsunami generation of the smaller event may have been greater than that of the larger event.


Tsunami Energy

The energy transfer of the earthquake to tsunami energy cannot be calculated directly because there were not extensive measurements of the crustal displacements associated with either the 19 September, or the 21 September earthquakes. The crustal measurements given have been inferred from first motion instruments. For the major quake, it is assumed that horizontal movement was 2.5 m, and vertical displacement was 0.8 meters. Based on these quantities and on the geometry of faulting, an estimate of the tsunami energy was obtained as follows.

Assuming that the total tsunami energy (Et) was equal to the potential energy (Ep), of the uplifted volume of water in the tsunami generating area, then this total tsunami energy can be estimated to be:

Et = 1/6 rgh2 ·A = 1/6 (1.03) (.980) (103) (104) (0.82) (1.85 x 107) (7.5 x 106) =

Et = Ep = Total Energy in the submerged portion of the earthquake area

h = Height of vertical displacement = 0.8 m

A = Tsunami Generating Area, 7,500 Km2

For the 21 September tsunami, the energy is roughly estimated to be:

Et = 5.6 x 1019 ergs or 0.56 x 10 raised to 20 power - ergs

This is based on the assumption that the vertical displacement for the second earthquake was also 0.8 m. However, inspection of the Acapulco record shows that both tsunamis were of the same approximate height. Therefore, the second earthquake must have been more efficient or had an angle of subduction that was greater, so the vertical component of the crustal movement could have been more than 0.8 m, and the tsunami energy proportionately greater.

The perception that tsunamis do not pose a threat in Western Mexico is erroneous. The historic record shows that about 15 destructive local tsunamis were generated in the last three centuries, (SoLoviev and Go, 1975) from earthquakes along the Middie America Trench. The wave heights of these tsunamis has ranged from 2 meters to a maximum of 9 meters.


REFERENCES AND ADDITIONAL BIBLIOGRAPHY


Singh, K.S., Ponce, L. and Nishenko, P.S. "The Great Jalisco, Mexico, Earthquake of 1932: Subduction of the Rivera Plate." , "Bulletin of Seism. Soc. Amer., Vol. 75, No. 5, pp. 1301-1313, October 1985."

Anderson, J., Bodin P., Brune, J., Masters, G., Vernon, F., Almora, D., Mena, E., Onate, M., Prince, J., Quaas, R., and Singh, K. "Strong Ground Motion and Source Mechanism of the Mexico Earthquake of September 19, 1985," Proceedings "Simposio El Temblor De Michoacan 1985 y Sus Efectos" Oaxaca, Un. Geof. Mexicana - Instituto Technologico De Oaxaca, 10-16 November 1985.

Astiz, L., Eissler, M. and Kanamori, H. "Source Parameters of the September 1985, Mexico Earthquakes" Seismological Laboratory, Cal Tech, Pasadena, Proceedings "Simposia El Temblor De Michoacan 1985 y Sus Efectos" Oaxaca, Un. Geof. Mexicana - Instituto Technologico De Oaxaca, 10-16 Nov 1985.

Bodin, P. and T. Klinger. Observations of coasta I uplift associated with the 1985 Mexican subduction earthquakes (abstract), American Geophysical Union, Fall Meeting, San Francisco, California, 1985.


How Mexico City’s Unique Geology Makes Deadly Earthquakes Even Worse

Each year, Mexico City commemorates the anniversary of its devastating 1985 temblor by holding a series of evacuation tests. This annual rite both honors the 10,000 people who lost their lives in that disaster and prepares the city’s current residents for the next natural disaster. But yesterday, soon after business had resumed, central Mexico was rocked by a real—and deadly𔃏.1-magnitude earthquake.

As buildings began to sway, crowds poured into the streets. In videos posted to Youtube and Twitter, many structures seemed to disintegrate under the vibrations. At least 200 people died, according to the Associated Press and other news outlets.

Unfortunately, Tuesday’s temblor is just the latest chapter in Mexico’s long and tragic history of earthquakes. Two weeks ago, an 8.1 magnitude earthquake—the strongest in a century—jolted southern Mexico, killing nearly 100 people. What many don’t realize is that there’s a simple reason behind this region’s propensity for cataclysms: The geology of Mexico—and particularly that of Mexico City—makes it a perfect storm for seismic catastrophe.

These latest quakes were caused by the movement of tectonic plates, the pieces of Earth’s crust that move and jostle against each other. Mexico sits atop a complicated juncture of tectonic plates, which have been engaged in a slow-motion collision for over a million years. As these plates scrape against one another, tension builds until they reach a breaking point—which is when an earthquake strikes. The sudden release of energy causes seismic waves to radiate out from the epicenter.

According to the U.S. Geological Survey, who tracks earthquake activity worldwide, over the last century there have been roughly 19 other earthquakes over 6.5 magnitude within just 155 miles of the epicenter of the latest quake. Hundreds more have shaken the thousands of miles that make up the country's coastline, many topping eight on the equivalent Richter scale.

This latest quake was centered on a region where the Cocos tectonic plate, which sits beneath the Pacific Ocean, is slowly being shoved beneath the continental North American plate. This movement is causing extreme tensions as the slab is rammed into the Earth.

Though the epicenter for the 1985 earthquake was over 200 miles away from Mexico City, the disaster nearly flattened the capital. (U.S. Geological Survey Department of the Interior/USGS/I.D. Celebi)

It gets worse. Mexico City, the country’s densely-populated capital, is even more susceptible to earthquakes than the rest of the country. This holds true even if the epicenter of the quake is positioned far from the city’s boundaries, which was the case for both this latest earthquake (which originated nearly 100 miles southeast of Mexico City in the state of Puebla), and the 1985 earthquake (whose epicenter was some 200 miles from the capital).

Though the fact that these quakes occurred on the same day 32 years apart is purely coincidence, their dramatic impact on the capital is not. The reason: Ancient sediments that underlie the city trap and magnify the vibrations that ripple through the region.

Mexico City sits atop an ancient shallow lake, with soils made of sediments that washed in from the surrounding mountains thousands of years ago. In the early 1300s, attracted by those fertile soils, the Aztecs selected an island in the lake on which to build their capital city, Tenochtitlan, which eventually became Mexico City. Though the Spanish later drained the surrounding waters to prevent frequent flooding, the effects of that decision can still be felt today.

When earthquake tremors hit solid rock, the rock simply shakes. But when they roll into the soft sediments of a basin, the vibrations can become trapped, reverberating back and forth through the material, explains Susan Hough, a seismologist with the USGS. "It's almost like a bathtub, the [seismic] waves will slosh back and forth," she says. Other seismologists have likened these lakebed dynamics to a bowl of Jello.

This reverberation doesn’t just carry these waves further—it can actually amplify them. “A basin will have natural frequencies, which depend on its shape and size, as well as the material properties of the sediments inside,” explains Jascha Polet, a geophysicist at California State Polytechnic University, Pomona, via e-mail. “When seismic waves make a basin shake at one of these natural frequencies, significant amplification may occur.”

Depending on the frequency of the seismic waves, the movement of the ground can feed energy into buildings of a certain height. This, as yesterday’s quake shows, causes them to sway—and eventually topple.

“Think of pushing a kid on a swing,” Hough adds. “If you start pushing every 5 seconds, it will just mess things up.” That is, the pushes won’t have a cumulative effect. But if you push at a consistent frequency, each push will send the child higher into the sky.

This map shows the location of all the earthquakes measuring over 7.0 magnitude that have been recorded in North America. Though many are scattered across America's west coast, note the high concentration of quakes in central and southern Mexico. (USGS)

While it’s long been known that sediments can magnify tremors, researchers didn’t learn exactly how dramatic the effects could be until 1985. The temblor nearly flattened the distant Mexico City, yet left many cities close to the epicenter nearly unscathed. "This [earthquake] taught us that soft soils can magnify motion to a degree never thought possible," University of California at Berkeley engineer Vitelmo Berto told the LA Times in 1986, a year after the disaster.

The seismic waves that were taking down buildings were five times greater than waves outside the city, according to measurements taken during that event, reported the LA Times. "No one expected the intensities of motion that were recorded in Mexico City,” Berto said. “No one had designed for it, and that is why so many buildings failed.”

Mexico City’s lakebed geology also make it prone to an even more dramatic disaster: liquefaction.

When soils are saturated with water, intense shaking can cause them to lose their solid structures and begin acting like a liquid—to the point that the ground can swallow up cars like quick sand. Liquefaction worsened the impacts of the 1985 earthquake, undermining the foundation of many buildings. While it is not yet known if this is a factor for the latest quake, “it would not be surprising,” says Polet.


Mexico City and earthquakes: a retrospective on Disaster Relief

The 1985 earthquake in Mexico City, 32 years ago to the day this week, registered an 8.1 magnitude and resulted in the deaths of more than 10,000 people. Baptist disaster relief teams were onsite within weeks. JOE WESTBURY/Index

Those were my thoughts just 48 hours ago as the news alerts scrolled across my iPhone, reporting another devastating earthquake in Mexico City. The city has been living on borrowed time since its founding on an ancient lakebed.

In hindsight, no one would build a city on such shaky ground. But cities are not built overnight, like Rome was not built in a day.

First there were just a few huts as the ancient Aztecs chose the location around 1325 and slowly filled in the lakebed with rubble. Then as word of its popularity grew, there were more huts and the jungle paths that would become roads. Traders of beads and corn became merchants trade routes were established.

And now, 692 years later, the site and its 21-million residents has become the world’s fifth largest metropolitan area.

Why is all of that important? Because that massive lakebed, which slowly filled in with silt and mud and debris through the centuries, is fine for huts but does not provide a foundation for skyscrapers and hospitals and tall apartment buildings. When the earth shakes the soil liquifies like Jell-O and magnifies the vibration throughout the structures.

There is a sermon illustration in there, but I will allow the pastors and Sunday School teachers to expound on that.

On September 19 a devastating earthquake registering 7.1 shook the city to its already shallow foundations. At least 230 are now reported dead and the toll will surely rise.

But exactly 32 years ago to the day, an earthquake registering 8.1 struck and brought even more pain and heartbreak to its residents. The official government death toll was around 10,000 but more realistic estimates ranged as high as 20,000.

Rescue works worked to free trapped citizens deep in the rubble of collapsed buildings in the Mexico City earthquake of 1985. JOE WESTBURY/Index

As an editor for the Memphis, TN-based Baptist Brotherhood Commission in 1985, I was one of Southern Baptists’ first journalists to report from the scene of the earthquake. My job, writing for the men’s laymen’s organization as well as Baptist Press, was to tell the story of how Southern Baptists were providing physical needs wrapped in spiritual counsel.

The focus was on the ministry being provided by the denomination’s army of bright yellow-shirted volunteers emblazoned with the name of their state under the prominent wording “Southern Baptist Disaster Relief.”

These volunteers, certified through extensive training in a variety of skills, are the proverbial hands and feet, arms and face of Christ. With their bright shirts they stand out in crowds of other workers and are frequently sought out by locals who have lost everything.

Southern Baptist disaster relief volunteers are never first responders. That role is restricted to government agencies who conduct search and rescue and then search and recovery. A byproduct of that effort results in identifying the greatest needs where relief teams can be most effective.

I remember my first encounter with those yellow shirts 32 years ago this Fall … in Mexico City. I flew from Memphis to San Antonio where a layman picked me up at the airport and drove me to a rest stop along I-35. That’s where I rendezvoused with an 18-wheeler and a van packed with Texas Baptist Men from the Baptist General Convention of Texas.

From there we drove all the way to Mexico City, nearly a thousand miles spread over two days. We were ferrying blankets to guard against the Fall chill, food, diapers and medicine … along with tons of precious drinking water.

I was not prepared for the horror to come.

As a member of the Southern Baptist agency which coordinated disaster relief efforts among the state conventions, I was used to seeing skyscrapers with windows sucked out by hurricane winds, homes blown away by tornadoes and businesses ruined by flooding.

None of those natural disasters, if compiled and thrown together into one gigantic nightmare, could compare to the destruction of that beautiful city.

A mother and child were among the 250,000 residents who became homeless after the quake … and benefitted from the ministry of Southern Baptist disaster relief. JOE WESTBURY/Index

The damage was not limited to one section of the sprawling metropolis but pockmarked the landscape. It was as if Godzilla had taken a morning stroll across the city and had sat here, laid down there, and left footprints a block long wherever he walked.

The scenes were repeated everywhere you looked and where the disaster relief teams set up their feeding and counseling centers. There would be no destruction for blocks until you turned a corner and were hit with the reality of the suffering and heartache that accompanies such a disaster.

Buildings were sprawled across streets, while rescue workers dug frantically for survivors. Interiors of banks, apartments and hotels had been laid bare to the wonderment of passersby because their outer walls had melted away like ice cream on a hot summer day.

The simple pine coffins stacked five deep under shade trees reminded you that the nightmare was continuing for countless relatives who lost entire families the morning the earth moved.

More than 13,000 residents were immediately transformed into street people without food to eat, water to drink, or home to return to. Children needed food and diapers. People of all ages needed their daily medications. They all needed hope.

Thousands lay buried in premature tombs as hundreds of skyscrapers remained in various stages of collapse.

When the damage assessment began to roll in the government opened the door to international aid for the first time in its history. Southern Baptists were among the first to walk in with the Good News for Modern Man translation of the Bible and life-saving resources.

In a news story I wrote for The Jackson (TN) Sun newspaper in 1985, i recounted the story of Baptist disaster relief workers who brought healing and the gospel to those in need. JOE WESTBURY/Index

Today the denomination’s disaster relief efforts are coordinated through the North American Mission Board. It works through states who train and raise funds for their teams and respond where and when needed.

During the past 20 years with The Christian Index I have experienced first-hand the ministry of Georgia Baptist disaster relief ministry. In one weekend I traveled to Albany to cover teams helping residents pull up carpet and mudout their homes after major flooding. Just 24-hours later I was onsite near Gainesville, far to the north, as chainsaw crews cleared trees from homes following a major tornado.

The disasters are too numerous to list but the commitment of the men and women make all Georgia Baptists proud. They don’t do it for the glory or the attention, but for Christ.

As I write this, there are 1,500 trained and credentialed Georgia Baptist volunteers standing at the ready. As many as 500 collegians may be trained this weekend during their annual Conclave gathering.

Hurricane Irma left its calling card last week and citizens are already being assisted with storm damage cleanup. This morning, Georgia Baptists can be proud of the men and women, with their bright yellow shirts, who are onsite in Cornelia, Kingsland, Brunswick, and in Liberty and MacIntosh counties. By Monday teams may be dispatched to Clay County, FL.

Not everyone can take the time to serve as volunteer in this ministry, but they can participate through financially supporting the work of those who are willing.

Stuart Lang, who coordinates Georgia Baptist Disaster Relief efforts, is urging individuals to respond through prayer, creating Buckets of Care, and/or donating directly to disaster relief efforts. For more information on Georgia Baptist Disaster Relief’s response, visit its website or Facebook page.


Watch the video: Σεισμός στο Μεξικό: Η στιγμή που ο εγκέλαδος σπέρνει τον πανικό (June 2022).