Why was building damage so extensive in the 1985 mexico city earthquake?

I had an interesting conversation this week with Chris Poland, chairman and senior principal at Degenkolb.  Degenkolb is an engineering firm that provides a wide spectrum of structural engineering services to architects, Fortune 500 companies, healthcare institutions, universities, and government entities. Importantly, Degenkolb has a specialty in earthquake design and retrofitting. 

Chris and I were discussing the 7.4-magnitude earthquake that struck Mexico last month, comparing it to the 8.0-magnitude earthquake that Mexico experienced in 1985. Last month the earthquake damaged or destroyed 30,000 homes (mostly in small towns in the states of Guerrero and Oaxaca) and killed two people. 

While this is a significant event, it pales in comparison to the 1985 earthquake that caused the deaths of at least 10,000 people and damaged or destroyed almost 100,000 housing units. 

At first glance, it is easy to assume that Mexico City sustained very little damage during 2012 because the epicenter of the earthquake was approximately 500 kilometers away. This certainly is a big factor, but it is not the whole story. 

In researching the 1985 earthquake, I found out that the epicenter of that “Mexico City” earthquake was actually 350 kilometers away from the city, and yet it still caused extensive damage. Even though the 2012 earthquake was weaker (7.4 vs. 8.0) and further away from the city (500 km vs. 350 km), I don’t think this can adequately explain the major difference in loss of life between the two events (two people vs. at least 10,000 people). 

So what other factors were involved? Why did Mexico City have such significant damage in 1985 and very little in 2012?

In 1985, the majority of the buildings that collapsed or sustained major damage were in an area of Mexico City called the Lake Zone. The city was originally built by the Aztecs on an island in the middle of Lake Texcoco. In 1521, the Spanish captured the city, drained the lake, and built the city on top of the soft soil of the lakebed. Millions of people today live on top of this former lake, which has very soft soil and is therefore more susceptible to earthquakes. I have heard anecdotal stories about people who were in Mexico City in 1985 (not the lakebed), and were shocked to hear about the extent of the damage based on the amount of shaking they felt. The lakebed amplified the earthquake’s effects to a huge degree.

So, does that explain why 2012 was such a different event? 

I imagine that the soil characteristics of the lakebed are still the same in 2012 as they were in 1985. Even though the earthquake epicenter was further away in 2012, it would stand to reason that the lakebed area in Mexico City would still experience significant shaking today, as it did in 1985. But in 2012, no major damage was reported in Mexico City. 

Something else must be contributing to the situation.

Before 1957, Mexico City did not have building codes for earthquake resistance. Some regulations were passed in 1957 after an earthquake in the city, but more stringent codes were enacted in in 1976 after another, stronger earthquake shook the city. Most of the seriously damaged buildings in the 1985 earthquake were built between 1957 and 1976, when the city was starting to build upwards, in the six-to-15 floor range, but without stringent building codes. Next came the buildings that were built before 1957 (which were typically smaller than six stories). Buildings from 1976 to 1985 suffered the least damage.

It seems that the reason Mexico City didn’t experience more damage in the 2012 earthquake was (at least in part) because of stronger, more reliable buildings. The buildings that collapsed in the 1985 earthquake were replaced with structures with better technology, and better able to withstand earthquakes. 

Chris Poland told me that after every major earthquake, structural engineers study the damage to determine where the weak points are and what they can do to build safer, more reliable homes and buildings in the future. He is part of a coalition in San Francisco to make the city truly resilient from earthquake risk.

The lessons from the Mexico City earthquakes and the work going on in San Francisco are good reminders that businesses are on the forefront of helping communities, not just through philanthropy and giving, but through their core operations like structural engineering. Chris and Degenkolb are working every day to make the places where we live and work safer. They should feel proud of what they do. 

More information about Degenkolb.

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.

Nearly two-thirds of the buildings that collapsed in Mexico City's monstrous earthquake last month were built using a construction method that is now forbidden in seismic hotspots in the United States, Chile, and New Zealand, according to new data compiled by a team of structural engineers at Stanford University.

The suspect building technique called flat slab – in which floors are supported only by concrete columns – caused 61 percent of the building collapses in last month's earthquake with a magnitude of 7.1, which killed 369 people and blanketed tree-lined avenues in rubble.

Now, several prominent engineers say some of those structure failures could have been prevented and lives could have been saved had Mexico City officials only gone forward with a proposal to forbid that type of construction when they toughened building codes after the 1985 earthquake in Mexico's capital.

"We have known for 30 years that this system killed lots of people, so why are we still using it?" asked Eduardo Miranda, a professor of civil and environmental engineering at Stanford and global expert on earthquake-resistant design who compiled the data obtained by The Associated Press. "The right decision after '85 would have been to completely ban this kind of construction. We could have saved lives."

Anahi Abadia and her husband were among the fortunate: they were in Home Depot when the earthquake hit, shaking the store so fiercely the structure screeched. Minutes later, a text came in from their neighbor: The elegant apartment they had purchased only six months earlier had collapsed, rendering their new home a pile of crushed concrete.

Two women working in Ms. Abadia's trendy apartment building died on Sept. 19 when the structure collapsed after a corner column failed, and the flat-slab structure pancaked, Professor Miranda said.

The concrete slabs used to build floors and ceilings can be cast to include some rebar for reinforcement, and give builders greater flexibility in room layout and allow for higher ceilings.

But in an earthquake, without reinforced concrete walls or lateral bracing to resist forces pushing structures sideways, buildings with that design can move too much. The columns, and connections between the slabs and columns, can easily break, prompting collapse, as was the case at a school where 26 people died, most of them children.

But at Abadia's building, the construction method was only the start of the problems: The units were designed by an architect whose license lapsed, and approved in a borough where auditors previously found illegal construction occurred unchecked.

Experts concur that the devastation caused by last month's earthquake in the city of 8.9 million people could have been much worse had the building codes not been so strong, but it also has forced an uncomfortable conversation about their shortcomings. Now, as experts race to toughen standards to retrofit hundreds of damaged buildings, they are grappling with the reality that corruption has allowed hundreds of structures to be built outside the rules atop the soft soils of Mexico City's ancient lakebed.

In the crisis following the 1985 quake, a group of academics, building officials, and engineers drafted emergency recommendations to strengthen Mexico City's seismic codes, which were swiftly passed into law.

Some architects and builders were opposed to an outright ban on flat slab construction, said Miranda, who wrote reports that informed the committee.

"There were lots of builders and owners who were not going to be happy that you just stopped their construction," said Miranda, who later served on Mexico's code committee in the 1990s, and on committees funded by the US Federal Emergency Management Agency. "These things start as technical conversations but then you run into policy and politics."

The new codes allowed flat slab construction if developers designed the building to be seismically stronger than structures with beams or concrete walls.

Authorities did not pay enough attention to evaluating if existing flat slab structures needed a seismic retrofit, Miranda said.

"It would have been better to have insisted very much about not using flat slab," said Roberto Meli, a renowned structural engineer who served on the code committee in 1985, and went on to head the federal Center for Disaster Protection founded after that quake.

The new codes gave more responsibility to a network of private engineers who are hired and paid by developers, and who submit structural plans to borough authorities. In practice, that means private engineers – not government experts – vet projects' structural safety, and corruption can intervene.

"Corruption can come in many forms, from the moment someone accepts a bribe to when someone falsifies documents, or fails to present the right structural plans to borough authorities," said Renato Berron, head of the Institute for the Security of Constructions of Mexico City, a city agency.

In recent decades, middle-class enclaves close to the city's center have experienced vertiginous growth, and many unpermitted helipads and illegal, multi-floor garages have sprouted up between historic homes. In the past 15 years, residents of Abadia's Benito Juarez borough and two others nearby have sent in the highest number of complaints about land-use violations to a city watchdog agency.

Now, those same boroughs are home to dozens of damaged buildings, according to data from the Mexican Society of Structural Engineering. Miranda, whose team compiled the data on the 44 collapses through in-person visits and detailed structural analysis, estimates that hundreds of damaged buildings were built with flat slab systems, including some built relatively recently.

Mr. Meli said he and the committee will seek to strengthen retrofitting standards first, and by early next year may reexamine flat slab, although he is not convinced prohibiting the method outright would have worked.

Abadia said she hopes shoddy builders will be held responsible.

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"A new building should not fall down," she said.

This story was reported by The Associated Press.