Tsunamis, Storm Surge Demonstrate Power of Moving Water
12/5/2024 – Tsunamis are huge waves caused by earthquakes. Storm surge is a huge wave caused by hurricane winds. Both have unimaginable destructive power.
West Coast Tsunami Warning This Morning
This morning, a magnitude 7.3 earthquake struck Northern California. That caused the National Weather Service to issue tsunami warnings from Oregon down to San Francisco and beyond.
About three hours later, the warning was cancelled without explanation. Since then, significant aftershocks ranging from 3.0 to 5.3 magnitude have occurred. We will learn more in coming days.
Different Types of Earthquakes Trigger Different Tsunami Threat Levels
Massive earthquakes don’t always trigger massive tsunamis. The magnitude 7.9 quake that destroyed much of San Francisco in 1906 reportedly produced a wave height of only three inches. Small localized, underwater landslides created 1-2 foot waves, but their destruction was confined to small areas.
Geology determines, in large part, the height of a tsunami. Imagine two different types of geologic faults.
In a thrust fault, one tectonic plate is forced under another, pushing it up and causing a vertical displacement of the seafloor. This type of fault commonly generates a tsunami.
However, a strike-slip fault causes a lateral displacement. Land along a fault moves in opposite directions without causing elevation changes. Strike-slip faults typically do not generate tsunamis because they generally do not displace large volumes of water. (Contrast the animations in the two links above.)
Other factors may also affect tsunami formation. They include:
- Depth: Shallow earthquakes (occurring at depths less than 70 km) are more likely to cause tsunamis than deep earthquakes, as the energy has a more direct impact on the seafloor.
- Seafloor Topography: The configuration of the ocean floor, including ridges and trenches, can amplify or reduce tsunami formation.
- Underwater Landslides or Underwater Volcanic Activity triggered by the earthquake can also contribute to tsunami generation.
In summary, while massive offshore earthquakes increase the risk of tsunamis, they do not guarantee one will occur.
In this case, officials issued the tsunami warning out of an abundance of caution when seismographs first detected the earthquake. Later, they cancelled the warning when it became clear no massive waves threatened. They made the right call initially. Here’s why.
Record Waves, Record Destruction
Some massive earthquakes, like the magnitude 9.0, 2011 Tōhoku earthquake in Japan, did produce devastating tsunamis. One wave reached more than six miles inland. A 48-foot high tsunami wiped out a nuclear power plant at Fukushima. And the death toll ultimately reached almost 20,000 people.
So you can imagine why California officials immediately issued warnings today for residents to evacuate to higher ground.
During the great Galveston hurricane of 1900, a giant wave of water caused by storm surge, not a tsunami, inundated the island with 8- to 12-feet of water. It killed an estimated 6,000 to 12,000 people.
One cubic foot of seawater weighs 64 pounds. Imagine getting stuck by tens of thousands of cubic feet. That’s why storm surge is the leading cause of death associated with tropical events, according to the National Weather Service.
Don’t take either storm surge or tsunamis lightly.
Elevation: Key to Survival
The National Weather Service’s Tsunami Warning Center offers an excellent FAQ page concerning tsunami warnings.
Interestingly, shortly after NWS issued the tsunami warning for the San Francisco area this afternoon, I had a huge spike in traffic on ReduceFlooding.com. Everyone was going to a post with the headline “Easy way to find the elevation of your home and the slopes around it.“
I notice the same spike in traffic every time floods threaten. People want to know whether they live higher than the expected crest of the flood, storm surge or, in this case, tsunami.
Published in 2020, the post has received more than 150,000 page views already this year. The biggest peaks were during the May storm, Beryl, Helene and Milton.
Ironically, when I wrote that post about elevation, tsunamis were the furthest thing from my mind.
Even more ironic, shortly before the warning, someone emailed me asking whether I thought a development near the San Jacinto West Fork was safe from flooding. I replied to her that if she had a concern about flood risk, she should buy on the highest ground she could afford, as far from the water as she could get.
Posted by Bob Rehak on 12/5/2024
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