Anatomy of a tsunami warning

A December 2024 earthquake that occurred off California’s coast highlights concerns surrounding how tsunami risk is communicated.
 

By Rebecca Owen, Science Writer, (@beccapox.bsky.social)
 

Citation: Owen, R., 2025, Anatomy of a tsunami warning, Temblor, http://doi.org/10.32858/temblor.358
 

On December 5th, 2024 at 10:44 a.m. local time, a strong earthquake struck off the coast of Northern California, rattling buildings and disrupting electricity to coastal communities like Ferndale and Fortuna. The event was initially reported as magnitude 7.3, but was eventually downgraded to magnitude 7.0. Because the quake originated underwater near a confluence of faults, the threat of a destructive tsunami seemed plausibile.
 

 

In the minutes after the December 2024 quake, nearly 5 million people from the Bay Area to southern Oregon received a tsunami warning. That emergency communication advised them to evacuate and move away from the water toward higher ground. One confusing and worrisome hour later, the warning was cancelled. With the threat of a tsunami over and no major damage or injuries from the earthquake reported, questions emerged about the efficacy and accuracy of the tsunami warning system in the context of its wide geographical reach. These questions highlight the public’s need for more information about the seismic circumstances that can create a tsunami, as well as more accessible, straightforward communication from emergency officials.

“If we’re talking about a place that has correct and frequent tsunami warnings, well-marked tsunami evacuation routes, areas where schoolchildren have practiced [evacuating] — those are places where it’s reasonable to expect that people understand what tsunami warnings are all about. This was not one of those cases,” says Jackie Caplan-Auerbach, a seismologist at Western Washington University. “I think in some respects this [tsunami warning] was a really, really ideal test case.”
 

A tsunami that never was

The December 2024 earthquake occurred offshore, 60 miles west of Ferndale, California. It originated from an area known as the Mendocino Triple Junction, where the Pacific, North America, and Gorda section of the Juan de Fuca plates all converge. This region encompasses the southern tip of the Cascadia Subduction Zone, the northern end of the San Andreas Fault, and the eastern arm of the Mendocino Fault. This tangle of faults is one of the most seismically active regions in the United States and it has hosted other large earthquakes in the recent past. However, its history of tsunami generation follows a different pattern.

“Tsunamis come from displacement of the seafloor; you have to move the water. To move the water, you move the bottom of the ocean up or down,” says Harold Tobin, seismologist at the University of Washington.

When a thrust fault like the Cascadia Subduction Zone slips, Tobin says, it pushes the sea-floor up vertically: “That is the kind of earthquake fault that we really see as most likely to create a large tsunami hazard.”

The December 2024 quake originated from the Mendocino Fault where the Gorda plate moved eastward relative to the Pacific plate. The Mendocino Fault is a strike-slip fault. This means that the two sides of the earth slide past each other horizontally. Although this type of earthquake can cause significant shaking on land, it generally doesn’t produce tsunamis.

“Yes, strike-slip faults are less likely to create a tsunami,” Tobin says. “But you can’t discount them 100 percent.” Strike-slip faults have generated tsunamis before. For instance, sometimes the sea-floor is sloped so that even if the fault is moving side to side, there is still a large volume of water being displaced. An example of this is the 2018 Palu earthquake, where a magnitude 7.5 event on a strike-slip fault generated underwater landslides and propagated tsunami waves through a narrow bay, causing a devastating disaster in Indonesia.

Although Bay Area residents may be aware of the dangers of a large earthquake, the idea of a destructive tsunami pushing onto San Francisco’s beaches or Berkeley and Oakland’s marinas is harder to imagine. In the Bay Area, the tsunami hazard is lower than in northern California counties. “Once you pass Cape Mendocino, everything north of there is Cascadia. That’s a true geological difference,” says Tobin. And an earthquake occurring on the San Andreas or Hayward faults is also not likely to generate an inland tsunami, he says.

There is a risk, however, that waves from a tsunami originating in a different location could travel to the Bay Area. For instance, if an earthquake occurs in Japan or the Aleutian Islands, tsunami waves could race across the Pacific Ocean, explains Tobin.

Of the December 2024 earthquake, Caplan-Auerbach says, “Was it the type of earthquake that would generate a tsunami? Probably not. Is that grounds to say, ‘Oh, don’t worry. You’re not going to need to evacuate’? No.” The San Francisco Bay Area is highly populated, she notes. “These are places that are going to need time.”

This earthquake and its tsunami warning was like a fire drill, Tobin says. “It showed a whole bunch of the genuine weaknesses and flaws in how ready we are for a big tsunami,” he says.
 

Warning dispatch

Despite all of the factors that diminished the likelihood that this was a destructive tsunami event hitting the Northern California and Oregon coasts, warnings still went out to millions of residents. Many of those individuals were uncertain about what to do with a message that asked them to evacuate, find higher ground, and avoid the water. This was especially true in the Bay Area, where many people who received the highest level of tsunami alert expressed fear, confusion, or indifference. Many searched online for more guidance and weren’t able to find the resources they needed to understand the tsunami threat.

George Loew of San Francisco was at work when the earthquake occurred. For him and others in that community, the hour between when the tsunami warning was issued and when it was cancelled made for a strange, unsettling time. “Some people took it seriously. Many did not. It was unclear what we were supposed to do, other than worry,” he says. “My office is one block from the bay. We were unsure where to go.”

NOAA’s National Tsunami Warning Center is mandated to issue an alert within five minutes of an earthquake, says Dave Snider, Tsunami Warning Coordinator with the organization. “Because of this, our first messages are largely procedural to help as many people as possible move away from danger as quickly as possible,” he explains. “As more scientific information is analyzed and becomes known in the next 30 to 60 minutes, NTWC adjusts the level and coverage of a tsunami alert.”

Although the initial messages potentially gave millions of people some moments of confusion — especially in locations where tsunami risk seemed improbable — the alerts were broadcast to certain geographical regions based on National Weather Service forecast zones. The system is designed to deliver weather information, not information about geohazards. That’s why people far inland from a potential tsunami’s damaging waves also received the text alerts on their phones.

Plus, consider that in the first five minutes after an earthquake nucleates, many details of the earthquake aside from location and magnitude cannot be calculated with confidence. This is especially true offshore, especially in a region where so many faults crisscross one another. “Faulting mechanisms and related information may not be known for as much as twenty minutes after an earthquake’s end,” says Snider. “Consider that great quakes may not have completely ruptured in that first five-minute period.” For example, the magnitude 9.1 Sumatra earthquake, which triggered devastating tsunamis across the Indian Ocean region, took nearly 10 minutes to rupture.
 

The way forward

One step in helping West Coast residents better interpret their tsunami risk and make an evacuation plan comes from updating the websites and systems providing information. During the December 2024 event, people complained about being unable to locate online inundation or evacuation maps in San Francisco and wondering when the tsunami waves might reach them in a particular location.

The National Weather Service is working to improve its Tsunami Warning System website, says Snider. “The current site is cumbersome, hard to navigate, and doesn’t support a critical need to confirm an active alert where you are along the coast. Without confirmation, residents mill [about] and critical evacuation time is lost — exactly the opposite of our aim.” The updated website, he explains, may be ready in Fall 2025. However, job and budget cuts at NOAA and other federal agencies may complicate or delay when or if this resource will be available.

Similarly, several different organizations — the NTWC, the Pacific Tsunami Warning Center, and the International Tsunami Information Centre — can better coordinate their messaging and updates in the future to help people find the information they need in those critical minutes after a large earthquake occurs.

Because it hit an area that has not had recent tsunami warnings, my hope is that what we’ll learn from this is the need for enhanced or slightly different communication around events like this one,” Caplan-Auerbach says.
 

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