A ‘non-destructive’ tsunami was detected after the California earthquake. Simulations show the dangers of a real one

Residents along the West Coast were shaken when a magnitude 7.0 earthquake hit the region last week, prompting a tsunami warning and a few scary hours for more than 5 million people.

While the tsunami warning was canceled shortly after evacuations and transportation were shut down, San Francisco forecasters said a small wave had been seen off the California coastline.

“A non-destructive tsunami wave was observed earlier today near Point Arena,” the Bay Area’s National Weather Service office posted on social media. “This serves as a reminder that we do indeed have a fault capable of producing tsunamis in our vicinity. We don’t deal with this often, but when we do, it pays to know your zones!”

The wave was only nine centimeters high and was measured around Arena Cove at 11:46 a.m. PST last Thursday. The cove is located in Mendocino County on the state’s northern shores. It’s a three-hour drive north to the Humboldt County town of Ferndale, which was closer to the epicenter.

Although the wave generated by this earthquake did not result in a disaster, officials have warned people not to assume that the next event will play out the same way.

“If you evacuated, you did the right thing. Tsunamis are rare but can be extremely deadly,” the National Weather Service’s Bay Area office said in a post on social media. “For perspective, about 230,000 people lost their lives in the 2004 Indian Ocean tsunami, the deadliest natural disaster of the 21st century.”

The earthquake was what’s known as a strike-slip, geologists said, meaning two of Earth’s tectonic plates slid past each other. The Earth is made of these plates and they are in constant motion.

“These types of earthquakes are less likely to cause tsunamis because their motion is primarily horizontal with minimal vertical movement of the ocean floor,” explained the US Geological Survey.

Newer though research from academics at the California Institute of Technology say strike-slip faults can generate large tsunamis.

“Strike slip faults are capable of generating powerful tsunamis that we have seen only six years ago,” said Dr. Ahmed Elbanna, associate professor of civil and environmental engineering at the University of Illinois Urbana Champaign and the work’s lead author. The independent.

“When earthquake faults slip, the surrounding earth moves and deforms like the elastic of a waistband. This movement can cause the stress on nearby faults to either increase or decrease, making further earthquakes more likely in some areas and less likely in others,” said Dr. Jason R. Patton and Dr. Wendy Bohon, scientists at the California Geological Survey. The independent in an email. “If the stress on a nearby fault increases enough, it can trigger an earthquake. However, we can’t tell if this will happen until it happens.”

In this case, a aftershock forecast indicates that there is less than a one percent chance of another 7.0 magnitude earthquake in the coming year. Scientists say the likelihood of smaller events near the site of 7.0 can be expected in the near term, but that experts do not indicate an expected increase in strike-slip activity elsewhere.

If it’s big enough and close enough to the sea floor, the energy from such an earthquake can cause the sea floor to suddenly rise or fall, according to the US Geological Survey. As the sea floor rises and falls, the sea water above it also rises. While earthquakes as large as 6.5 or below are very unlikely to trigger a tsunami, and those between 6.5 and 7.5 usually do not produce destructive tsunamis, larger earthquakes can result in much worse results.

Large tsunamis have occurred in the United States and will again. An earthquake with a magnitude of 9.2 in the Gulf of Alaska caused damage and loss of life along the West Coast in 1964. More than 150 tsunamis have struck the California coast since 1800. The 2011 tsunami that struck Japan also caused $100 million in damages to the state’s ports and harbors.

The best way to be prepared is to be aware risk that a tsunami can pose to the immediate environment. Interactive hazard maps made using computer modeling can show which areas are in critical zones.

In the future, there may still be improvements. The modeling that underlies tsunami warnings is determined using observations from floating buoys in the deep sea. The California Geological Survey says placing more of these buoys in key locations could help.

“These buoys are exposed to rough conditions at sea, so having additional buoys in the ocean will help when some buoys don’t work,” Patton and Bohon said.

Also, research focused on using GPS to help model the earthquakes that generate tsunamis could improve initial tsunami size estimates from the National Tsunami Warning Center, they said.

But guidance on what to do during a tsunami threat is largely unchanged.

When the tsunami is about to hit, there are several signs to look out for. For example, the ocean could look like a wall of water or a rising flood. It could also suddenly drain and expose fish and the seabed. A loud roaring sound can be emitted by the sea. But people may not see natural tsunami warnings and should be on the lookout for official tsunami warnings, such as broadcasts, wireless emergency alerts and messages from officials. Sirens can go off, indicating that an area is in danger, in those who have them.

For people in tsunami danger and evacuation zones or low-lying coastal areas who feel a strong or long earthquake or see natural warnings, a tsunami can arrive within minutes. The weather authorities recommend going as high or as far inland as possible. Those who can should move to a safe place and not wait for official warnings or instructions. And they should do it on foot because driving can cause traffic that can prevent people from evacuating.

They should also avoid earthquake damage such as downed power lines and stay away from buildings, bridges and piers. Heavy objects can fall from them during an aftershock. Residents should stay out of the tsunami danger or evacuation zone until officials say it is safe to do so. The first wave may not be the last or the biggest, and the danger may last for hours or days.

Planning an evacuation now can be lifesaving in the future. Emergency plans should include evacuation routes and tsunami danger zones. Plans should also include multiple identified locations that are safe for people to access in case of logistical complications such as road closures and damage. Those with children should find out about their school’s evacuation plans. The National Weather Service advises people to practice evacuation routes, including at night and in inclement weather.

In California, tsunami hazard areas stretching from Crescent City to San Diego. The current maps are drawn with boundaries from the California Geological Survey. The danger zones are highlighted in yellow. Recent changes have shown rises in central California’s Monterey, and the state said significant areas in northern California’s Alameda County could be flooded up to 18 feet.

In San Francisco, a tsunami triggered by a major earthquake in Alaska would take about five hours to reach the city of more than 800,000, the 2021 maps showed.

“Where the coast is surrounded by steep cliffs, the inundation will not extend as far inland as places that are flatter and low-lying (like bays or estuaries). Also, places like the entrance to San Francisco Bay can limit the tsunami flow, causing the tsunami to be smaller (in height and amount of land flooding),” Patton and Bohon said.

In neighboring Oregon, card colors are differentdelineates distant tsunami evacuation zones from local zones. What is shown in green is outside a danger zone.

Up the coast, Washington has created hazard maps for several tsunami scenarios. The metropolitan city of Seattle – home to more than 730,000 – is at risk in both a Seattle Fault earthquake scenario and a Cascadia magnitude 9.0 scenario. Simulations of that scenario show its destructive power throughout the Pacific and along the coast.

Most inland areas are safe, but how safe depends on the magnitude of the earthquake and its results.

“Washington ranks as the second highest earthquake risk in the United States and correspondingly a significant tsunami risk,” said Daniel Eungard, a tsunami hazard geologist at the Washington Geological Survey.

Thursday’s tremors occurred in what is known as “earthquake country” because three tectonic plates meet there. There have been just under 40 earthquakes of magnitude 6.0 or greater in the past century within 150 miles of Thursday’s quake, which was the strongest to rattle the state since a 7.1 quake struck Ridgecrest in 2019.

With reporting from the Associated Press