A Lesson in Risk Mitigation from Mexico's 1985 and 2012 Earthquakes
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.