Walking back to parking lot three at Santa Monica State Beach, after spending a sunny day on the sand, you may have noticed one of the blue and white “Tsunami Hazard Zone” signs, without giving it much thought.
In California, more than 150 tsunamis have hit the coast since 1880. According to the National Oceanic and Atmospheric Administration, 42 of the 150 occurred in Los Angeles County.
Considering that California is affected by almost one tsunami a year, it’s time to pay more attention to the danger zone and evacuation route signs.
If you’re wondering why you haven’t heard of all of these tsunamis hitting California, it’s because most of the recorded ones were barely noticeable, and few caused significant deaths or damage, said Nick Graehl, an engineering geologist with the California Geological Survey.
The last recorded tsunami here occurred on July 29. It was generated by an 8.2 earthquake off the Alaska Peninsula, but it brought less than a foot of water to our shores. The most recent damaging tsunami occurred in 2011, when an earthquake and tsunami that devastated Japan crossed the Pacific Ocean, causing $100 million in damage to California ports and docks.
A Californian who lives or works near the coast, or who is planning a summer day at the beach, should have a plan in case a large tsunami strikes.
How to know if a tsunami is approaching?
Natural warning signs can include feeling a strong or prolonged earthquake. Also, if you see a sudden rise or fall in the ocean level, or hear a loud roar from the sea, it’s time to get off shore.
The National Weather Service is a government agency that operates two tsunami warning centers, with the aim of monitoring such phenomena and the earthquakes that may cause them, and sends warnings of them. Sign up for text message alerts from your local government, get a NOAA battery-powered weather radio, or listen to automated TV, radio, or phone announcements.
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Not all earthquakes cause a tsunami. But if you feel any natural warning, you should immediately move to higher ground and further from the coast, Graehl said.
What do I do during a tsunami?
Evacuate the area on foot and reach higher ground. Don’t get in your car and try to drive away from the dangerous area; evacuating by car could create a traffic jam.
If you feel the earthquake: drop to the ground, cover yourself and hold on until the shaking stops.
If you were evacuated from a coastal area, stay away until authorities allow you to return.
Don’t go to shore to see a tsunami. These phenomena move faster than a person can run.
Don’t try to surf during a tsunami. Regular waves flow in a circle, without flooding higher areas. Tsunami waves are unpredictable and flood the earth like a wall of water.
How does a tsunami happen?
Most are caused by large earthquakes under the ocean floor, but can also be caused by landslides, volcanic activity, and certain types of weather. The collision of an asteroid or a comet with the Earth could also originate them.
In a tsunami caused by an earthquake, the underwater tectonic plates shift, causing the earthquake and displacing the water. Waves are generated and move in all directions across the ocean, some traveling at speeds of up to 600 mph. As they enter shallower water, they compress, their speed decreases and they increase in height, pushing the water towards the shore. What we experience on land would not be the common misperception of a massive wave that a surfer could ride up the 405 Freeway. It is more like a surge of water rushing inland, threatening anyone and anything that gets in the way. Tsunamis are also not a single wave; they can last for hours and cause devastation miles inland, just as Japan experienced in 2011.
There are two types of tsunamis: one of local origin and one of distant origin.
A tsunami of distant origin is generated by a distant earthquake. It can take anywhere from five to 13 hours to get here, depending on where you originate from. In that case, there would be time to issue a warning and for people to evacuate.
Distant tsunamis usually come from Japan, Chile, or Alaska. Graehl said the worst case scenario for Southern California is one originating from the eastern Aleutian Islands in Alaska. It would take five to six hours to arrive, which provides some time for notification and evacuation. However, on its arrival it could cause significant chaos.
A local, or near-field, tsunami can be generated by the sudden movement of offshore faults or underwater landslides. Because it is generated from a nearby source, it could arrive in minutes.
According to Graehl, Northern California could experience a significant local tsunami created by a large earthquake at the Cascadia Subduction Zone Fault, a 700-mile underwater boundary where tectonic plates are colliding, stretching from north of the Vancouver Island to Cape Mendocino in California. This type of event could cause at least six minutes of shaking, giving people just 10 minutes before surges of water up to 50 feet high hit Crescent City, which has the highest tsunami risk in the state.
In southern California, Graehl added, a tsunami of local or distant origin could be seen as swirling currents in the bay or a wall of water up to 10 to 25 feet.
How to prepare for a tsunami
- Get your local tsunami evacuation map. The California Department of Conservation released interactive hazard maps this year, a tool to help plan a safe evacuation route. The new map includes a feature that allows users to search for addresses to see if that location is in a designated tsunami hazard zone (highlighted in yellow). Find your specific map at conservation.ca.gov/cgs/tsunami/maps.
- Practice your evacuation route.
- Prepare or purchase an emergency “go bag” containing at least a 72-hour supply.
- Boaters should contact the harbormaster or local officials for tsunami safety protocols for their port.
In Southern California in particular, a small tsunami is more likely than a large one. “A smaller tsunami might not inundate sands that don’t normally flood at high tide, but it might cause unusual strong currents on beaches, harbors and bays,” Graehl said.
According to Don Drysdale, a spokesman for the California Geological Survey, you shouldn’t be on high alert at all times, but it’s something to think about when visiting a crowded beach like on Labor Day.
The spokesperson understands that people in California already have a lot to consider when it comes to safety: wildfires and earthquakes, to name a few. “But this is something that we also need to be aware of,” Drysdale said.
To read this note in English, click here.