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Ask 2 Weather: What would Houston look like if we had an earthquake?

What would happen if an earthquake hit Houston?
What would happen if an earthquake hit Houston? (iStock/doguhakan)

At KPRC 2, we’re dedicated to keeping Houstonians informed. As part of our new Ask 2 series, the newsroom will answer your questions about all things Houston.

The question: What would Houston look like if we had an earthquake? I know it’s clay and sand, would that help or hurt?

The answer: This Channel 2 viewer is obviously looking for an answer to a hypothetical question. The first thing for anyone reading this to understand is that earthquakes are exceedingly rare statewide, much less in southeast Texas. Very minor earthquakes have been recorded in the region, but no major ones. Even the small ones are almost non-existent.

The bottom line is that we have enough to worry about with hurricanes here, so please do not lose sleep over earthquakes!

For the sake of our curious viewer, though, let’s dive a little deeper into the hypothetical. In a small earthquake, one that we could barely feel, Houston would be absolutely fine. A major earthquake, however, would be a catastrophe. Because of the silty, sandy soil in Houston, a major earthquake would cause the ground to take on liquid properties -- a process called liquefaction. Buildings would likely lose their footing, break apart and begin to sink. Elevated roads would also suffer major damage or collapse completely.

The science behind the phenomenon is interesting. It comes down to the types of waves that earthquakes generate:

P Waves: These are “primary” waves because they are the fastest and the first to be felt in an earthquake. P waves are compression-tension waves that act horizontally, and shake the ground from side to side.

P waves are compression waves that shake the earth horizontally.  Image courtesy of the USGS
P waves are compression waves that shake the earth horizontally. Image courtesy of the USGS

S Waves: These are called “secondary” waves because they move less quickly. They are felt after the P waves. S waves shake the ground vertically and resemble waves on the ocean. They have crests and troughs.

S waves shake the ground vertically.  These waves are responsible for liquefaction of soft soil in an earthquake.  Image courtesy of the USGS
S waves shake the ground vertically. These waves are responsible for liquefaction of soft soil in an earthquake. Image courtesy of the USGS

Both P waves and S waves cause damage in an earthquake, but S waves often cause more damage. They are responsible for liquefaction, which turns granular, soft soil (clay, silt, sand) into something like quicksand.

Liquefaction is most dangerous in soil that is saturated with water. The S waves cause the grains in the soil to separate, which breaks the stabilizing frictional force between the grains. Water fills the space between the grains and allows the soil to become free-flowing, acting more like a viscous liquid than a solid.

This video illustrating liquefaction from the Incorporated Research Institutions for Seismology shows how the process works.

Unlike soft, granular soil, bedrock is impervious to liquefaction because it is solid. It moves as a singular unit during an earthquake and provides more stability for structures built on it.

Houston soil is swampy and silty, so you can imagine what would happen if we experienced a major earthquake here. Buildings would very likely start to break apart and sink into the ground. Elevated roads would collapse.

San Francisco, which is highly prone to big earthquakes, is built mostly of bedrock but has areas of more granular soil, too. The City’s Marina District is one such area and fares very badly in earthquakes. It would be analogous to Houston in terms of stability.

In the San Francisco earthquake of 1906 and in the 1989 Loma Prieta earthquake that hit the Bay Area, buildings in the Marina District were destroyed by liquefaction.

The great earthquake of 1906 caused major damage in San Francisco.  These buildings, built on soft soil, were destroyed from liquefaction.  Image courtesy of the USGS.
The great earthquake of 1906 caused major damage in San Francisco. These buildings, built on soft soil, were destroyed from liquefaction. Image courtesy of the USGS.
The Marina District in San Francisco suffered major damage during the 1989 Loma Prieta earthqake.  Image courtesy of the USGS.
The Marina District in San Francisco suffered major damage during the 1989 Loma Prieta earthqake. Image courtesy of the USGS.

During the Loma Prieta earthquake, a portion of the approach to the Bay Bridge, which was built on soft, sandy soil, collapsed and killed 42 people.

This elevated highway, part of the approach to the San Francisco Bay Bridge,  collapsed during the 1989 Loma Prieta earthquake.  42 people died as a result of the collapse.  Image courtesy of the USGS.
This elevated highway, part of the approach to the San Francisco Bay Bridge, collapsed during the 1989 Loma Prieta earthquake. 42 people died as a result of the collapse. Image courtesy of the USGS.

So, if Houston were to experience a major earthquake -- which, again, it will NOT -- damage similar to that illustrated above would be possible. That’s hypothetically speaking, of course.

Fortunately, we live far away from any earthquake hot-spots!

Do you have a burning H-Town-related question? Send it our way, and we will try to hunt down an answer.

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