Earthquake Hazards: Seiche
One of the many additional hazards an earthquake may cause is a seiche.
A seiche is a standing wave of water that moves back and forth across a semi- or fully-enclosed body of water. On a small scale, the sloshing movement of waves across a pool or bathtub is a seiche. These same wave movements can be found on a larger scale in bays and lakes.
When I experienced a 7.9 earthquake (from 65 miles away), I witnessed these waves in our pool. The pool was below ground and there was a small ledge about 12-14 inches high around the pool. The water was always at or below ground level, so there was more than 12 inches from the top of the ledge to the water.
When we ran out the back door of your house and under the grape arbor (which was dropping grapes) I looked towards our pool. What I saw was about 3-foot waves cresting and sloshing around the pool. I'm not sure if anyone else in the family even saw it, but the memory is vivid. I even wondered if what I remembered was real, until I saw videos of pools in earthquakes showing exactly what I saw.
Typically, seiches are caused by strong winds and rapid air pressure changes that push water from one end to the other, which then rebounds back to the first side. This oscillation can continue for hours or days. The time period between the high and low parts of the seiche can be several hours apart, which can make some people mistake the seiche on a large body of water for a tidal flow.
This is a typical wind-caused seiche, such as one might experience in the Great Lakes region. Strong winds and rapid atmospheric pressure changes cause the water to rise and drop. With the rise of water, when the winds stop the water will return to the other side. According to the National Park Service, it can take eight hours for a seiche to cross Lake Superior and return, with three-foot water-level changes. Lake Erie has had seiches measuring up to 8 feet. The following graphic is from NOAA on the National Park Service page, https://www.nps.gov/articles/coastal-geohazards-seiches.htm
Earthquakes can cause seiches thousands of miles away. Thankfully our pool in Chile was far enough away from our house it didn’t cause any flooding. However, a pool sloshing its water out could certainly cause some water damage, particularly if it's close to a home and the water gets into a basement.
There's geological evidence that seismically generated seiches of up to 32 feet struck the shores around Lake Tahoe in California/Nevada in prehistoric times.
If you live in an earthquake risk area and are near a large body of water, flooding from a seiche may be a hazard you haven’t considered, but should prepare for.
And even if you don't live in a high-earthquake risk area, if you live near a body of water , you need to be aware that a large earthquake hundreds (or even thousands) of miles away could cause waves to form and threaten your home. If the earthquake causing the seiche is hundreds of miles (or more) away, you may not realize the hazard. If you want to live near water, you need to include the possibility of flooding in your preparations.