Both Seattle and Mexico City are built on top of soil that's surrounded by rocks. Think of it as a bowl that can trap earthquake energy in the form of waves, even making the earthquake worse for buildings and people living on those soils.
There are similarities and big differences between the bowls that the respective cities sit on. Mexico City was built on a shallow bowl, an old dry lake bed with loose sedimentary soils. Seattle's bowl is deeper, made up of glacial leftovers, called “till." But it is harder, packed down by the weight of massive glaciers, which covered the land the city is built on.
"Their lake-bed sediments are soft clays that are weaker than the glacial soils that underlay most of downtown Seattle," writes Art Frankel of the U.S. Geological Survey in response to an email.
Frankel is also an associate professor in the Department of Earth and Space Sciences at the University of Washington.
“So the Seattle basin will amplify ground shaking in the Seattle and Bellevue areas relative to rock sites outside of the basin,” adds Frankel. “This is especially important for tall buildings (10 stories or more). The amplification will be even larger for sites on fill or alluvium in SoDo, and obviously impact URM’s there.”
URM stands for Unreinforced Masonry Buildings. The City of Seattle tracks how many of those buildings there are and where they’re located. The good news is that many of them have been retrofitted, required when they are remodeled. But city records show many of them are yet untouched.
Alluvium are the loose soils, such as river deposits and fill. In the magnitude 6.8 Nisqually earthquake in 2001, the combination of more vulnerable buildings on weaker soil, such as in Pioneer Square, saw some of the worst damage.
In the magnitude 8 earthquake which hit Mexico City in 1985, killing thousands, many of the older unreinforced buildings collapsed or were so severely damaged, they were torn down. They’ve now been mostly replaced with newer buildings built to modern codes.
Yet, some of the older buildings which remained came down in this quake, which, while smaller, was centered much closer to the capital. The quake energy also came from a different direction than in 1985.
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