Why Mexico City Gets Such Strong Earthquakes

Mexico City is known for being incredibly susceptible to earthquakes. The recent 8.1 earthquake off Mexico’s coast was able to damage the city, despite happening 600 miles away. And the 1985 Michoacan earthquake did absolutely catastrophic damage to the city, despite occurring more than 200 miles away.

Mexico Earthquake 1985
Damage from the 1985 Mexico City earthquake

Why does Mexico City experience such strong earthquakes, when it’s not near any major fault line?

The reason is due to the ground the city is built on. Mexico City sits on an ancient lake bed, a thin layer of extremely soft soil on top of a layer of bedrock. Soft soils such as these can amplify seismic waves, which makes anything built on top of them especially vulnerable.

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Seismic waves moving from rock to soil

Earthquakes travel through the ground in the form of seismic waves. These waves travel very fast through stiff, solid materials like bedrock, and very slow through soft soils. Because the energy flux of the waves remains constant as they move between mediums, the amplitude increases as the waves slow down moving from stiff rock to soft soil.

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1-D model of soil amplification effects

When seismic waves encounter a discontinuity in the material they’re moving through, such as hitting a layer of soil on top of a layer of rock, they’re simultaneously reflected and refracted, like light bouncing off the surface of water. The refracted waves will then be reflected back and forth between the upper and lower surface of the soil. When the waves in the top layer have a higher amplitude (due to a lower velocity) than the waves in the bottom layer, they’ll be amplified due to resonance with the arriving waves. This is known as “soil amplification”, and it significantly increases the ground motion (and thus the seismic forces) at certain seismic wave frequencies.

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Amplification at certain frequencies

This mechanism becomes especially catastrophic if the amplified frequencies in the soil are also resonating (i.e.: matching the periods) of the buildings sitting on it. In Mexico City, the amplified periods were approximately 2-2.5 seconds, which occurs with buildings around 6-15 stories tall. Buildings at this height sustained FAR more damage than both shorter and taller buildings.

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This process of soil amplification is well known, and US building codes reflect it. Every building built is assigned a site class, which is essentially a measure of how soft and crummy your soil is – the softer your soil, the larger the seismic forces you have to design for. Plenty of places in the US have soils like this: parts of San Francisco, for example, or the entire city of Charleston (which has such terrible soil that anything not built on piles is slowly sinking).

But the building code provisions are a simplification of an already-simplified model. They don’t factor in the height of the soft soil layer (a critical variable in determining the amplification factor), or the fact that certain building periods will be especially vulnerable. They also don’t take into account that certain properties of soft soils, such as their tendency towards inelastic deformation), can also serve to reduce seismic forces. So it wouldn’t surprise me if these provisions change as we experience more earthquakes, and our knowledge of them increases.

Sources:

ASCE 7-10 Minimum Design Loads for Buildings
Amplification of Seismic Body Waves by Low-Velocity Surface Layers

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