Residents along the West Coast were shaken last week when a 7.0 magnitude earthquake hit the region, triggering a tsunami warning and leaving more than 5 million people on edge for several hours.
Although the tsunami warning was lifted shortly after evacuations and transportation were halted, San Francisco forecasters confirmed that a small wave had been detected off California’s coastline.
“A non-destructive tsunami wave was observed earlier today near Point Arena,” the Bay Area’s National Weather Service office shared 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, measured at just nine centimeters, was detected near Arena Cove at 11:46 a.m. PST last Thursday. Arena Cove, located in Mendocino County along the northern coast, is a three-hour drive north of Ferndale in Humboldt County, which was closer to the earthquake’s epicenter.
While the earthquake’s wave didn’t lead to disaster, officials cautioned against assuming that future events will be the same.
“If you evacuated, you did the right thing. Tsunamis are rare, but they can be extremely deadly,” the National Weather Service’s Bay Area office wrote on social media. “For perspective, roughly 230,000 people lost their lives in the 2004 Indian Ocean Tsunami, the deadliest natural disaster of the 21st century.”
The earthquake was classified as a strike-slip type, where two tectonic plates slide past one another, according to geologists. The Earth’s tectonic plates are in constant motion.
“These earthquakes are less likely to generate tsunamis because their movement is mainly horizontal with minimal vertical ocean floor movement,” the US Geological Survey explained.
However, recent studies from the California Institute of Technology suggest that strike-slip faults can indeed produce large tsunamis.
“Strike-slip faults are capable of generating powerful tsunamis, as we’ve seen just six years ago,” Dr. Ahmed Elbanna, associate professor of civil and environmental engineering at The University of Illinois Urbana-Champaign and lead author of the study, told The Independent.
“When earthquake faults slip, the surrounding Earth deforms like the elastic of a waistband. This movement can increase or decrease the stress on nearby faults, potentially triggering more earthquakes,” Dr. Jason R. Patton and Dr. Wendy Bohon, scientists at the California Geological Survey, explained to The Independent via email. “We can’t predict when this will happen, but it is a possibility.”
At present, an aftershock forecast predicts less than a one percent chance of another 7.0 magnitude earthquake within the next year. Experts expect smaller aftershocks near the epicenter, but there’s no indication of increased strike-slip activity elsewhere.
If strong enough and near the ocean floor, the energy from such an earthquake can cause the ocean floor to rise or fall, according to the US Geological Survey. As the ocean floor shifts, so does the water above it. While earthquakes of magnitude 6.5 or smaller rarely cause tsunamis, those between 6.5 and 7.5 typically don’t produce destructive ones. Larger earthquakes, however, could lead to more severe outcomes.
Significant tsunamis have impacted the U.S. before and will again. A magnitude 9.2 earthquake in the Gulf of Alaska caused widespread damage and loss of life along the West Coast in 1964. Since 1800, over 150 tsunamis have struck California’s coastline. The 2011 tsunami that hit Japan also caused $100 million in damages to California’s ports and harbors.
The best way to prepare is by understanding the potential tsunami risks in your area. Interactive hazard maps, created using computer modeling, can show which regions fall within critical zones.
Looking ahead, improvements are possible. Tsunami alerts are currently based on data from floating buoys in deep ocean waters. The California Geological Survey suggests that adding more buoys in key locations could help.
“These buoys face harsh conditions at sea, so having additional ones will help compensate if some malfunction,” Patton and Bohon said.
Furthermore, advancements in GPS technology could enhance earthquake modeling, improving initial tsunami size estimates from the National Tsunami Warning Center.
While technology may evolve, guidance on actions during a tsunami threat remains largely unchanged.
When a tsunami is approaching, there are several signs to watch for. The ocean could appear as a massive wave or as a rising flood. It might also drain suddenly, exposing fish and the ocean floor. A loud roar from the ocean could also be heard. However, natural tsunami warnings aren’t always visible, so it’s crucial to stay alert for official warnings, such as broadcasts, wireless emergency alerts, and messages from officials. Sirens may also sound, signaling a risk.
For those in tsunami hazard zones or low-lying coastal areas, especially after a strong or prolonged earthquake or seeing natural warning signs, a tsunami could arrive within minutes. Experts recommend moving as far inland or as high as possible. Those who can should head to a safe location immediately, without waiting for official instructions, and avoid driving due to potential traffic delays.
People should also be cautious of earthquake damage, such as fallen power lines, and avoid buildings, bridges, and piers, as heavy objects may fall during aftershocks. Residents should stay out of tsunami hazard or evacuation zones until officials declare it safe. The first wave may not be the largest or last, and danger can persist for hours or even days.
Planning evacuation routes now can be lifesaving in the future. Emergency plans should include identified evacuation routes, tsunami hazard zones, and multiple safe locations in case of road closures or other complications. Parents should check their school’s evacuation plans. The National Weather Service encourages people to practice evacuation routes, including at night and in poor weather conditions.
Tsunami hazard areas in California extend from Crescent City to San Diego. Hazard zones, shown in yellow on maps by the California Geological Survey, indicate areas at risk. Recent updates show a growing hazard in central California’s Monterey area, with parts of northern California’s Alameda County facing potential flooding up to 18 feet.
In San Francisco, a tsunami triggered by a large Alaska earthquake would take around five hours to reach the city, home to over 800,000 people, according to 2021 maps.
“Where steep cliffs surround the coast, tsunami inundation won’t extend as far inland as flatter, low-lying areas like bays or river mouths. Areas like the entrance to San Francisco Bay can restrict tsunami flow, making the wave smaller,” Patton and Bohon explained.
In Oregon, different hazard zone maps demarcate local evacuation areas from distant ones. Green areas are outside hazard zones.
To the north, Washington has hazard maps for various tsunami scenarios. Seattle, home to more than 730,000 people, faces risk from both a Seattle Fault earthquake and a Cascadia magnitude 9.0 event. Simulations of these scenarios show the potential for widespread destruction along the Pacific Coast.
While most inland areas are safe, safety depends on the earthquake’s size and impact.
“Washington ranks second in earthquake risk in the U.S., so the tsunami threat is significant,” said Daniel Eungard, a tsunami hazards geologist with the Washington Geological Survey.
Thursday’s earthquake occurred in “earthquake country,” where three tectonic plates converge. Over the past century, nearly 40 earthquakes of magnitude 6.0 or higher have occurred within 150 miles of the epicenter, with Thursday’s event being the strongest since a 7.1-magnitude quake in Ridgecrest in 2019.
