Friday Afternoon Update: Earthquake Expert Discusses 200 Aftershocks and What Comes Next After M7 Earthquake

December 6, 2024

Thursday morning, a magnitude 7.0 earthquake struck approximately 60
miles off of Humboldt coast. The earthquake originated on a strike slip fault near the
Mendocino triple junction, which caused horizontal movement of the sea floor and prompted a
tsunami warning across coastal areas. Over 15,000 people reported shaking from Oregon to the
Bay Area and east from Chico to reading there were instances of power outages and some
minor damage, but thankfully, no catastrophic impacts. The National Weather Service issued a
warning due to potential for a significant tsunami, residents in low lying areas were advised to
evacuate. However, by early afternoon, the warnings were downgraded and all evacuation
orders were lifted when no substantial tsunami threat materialized. Seismic activity has been
continuing over the last 24 hours.

Lauren Schmitt (KMUD):
And for today’s community safety and awareness report, we check back in with our local earthquake expert, Dr. Lori Dengler, Emeritus Professor of Geology at Cal Poly Humboldt.

Lori Dengler (1:12):
Well, Lauren, back again—day two of our exciting post-Mendocino Fault earthquake. So where are we at right now? Fortunately, we don’t have to worry about tsunamis today. The tsunami warning stayed in place a little over an hour after yesterday’s earthquake, but it’s all over. Only very modest signals were observed—roughly 4 to 6 inches at Arena Cove, similar size in Crescent City.

We’re still looking at some unusual, longer-period oscillations in Humboldt Bay. There are some interesting, very low amplitude, long-period, like 55-minute swells in Humboldt Bay. We’re trying to see if that was linked to the earthquake. Sometimes, large earthquakes—just the surface waves—can excite something called a seiche, which is a periodic oscillation in a bay of water. Possibly, that’s what happened.

Certainly, a number of people observed the water slowly going down and then slowly coming back. We’re talking about very, very slow changes in the water level. So we’re following that up. We don’t have the answer yet. The good news is it was not something that was going to cause any damage. We’re talking about a really slow kind of oscillation.

There have been nearly 200 aftershocks as I speak. Maybe we’re up to 200 by now because I haven’t looked in the last 15 minutes. Most recently, a magnitude 4 occurred at noon. Folks in the Cape Mendocino area and Eel River Valley are probably feeling fairly frequent, modest earthquakes. Again, they’re located offshore. They don’t seem to have a lot of high-frequency content, so they’re not sharp, like a lot of the aftershocks from the 2022 earthquake were.

Lots of discussion about why such a large area was put into the tsunami warning yesterday, and I’m sure that’s something that will continue for weeks or perhaps months.

Lauren Schmitt (4:27):
Going to the aftershocks, can you explain what we can expect as aftershocks could continue for weeks and even months? And also, can you comment on whether they are “relieving tension,” which is a question we’ve been hearing a lot from members of the public?

Lori Dengler (4:45):
Yeah, well, right now the largest magnitude aftershock recorded was a 4.7, and it was about two minutes after the magnitude 7. Now, what happens during this period of very frequent aftershocks is that it takes a while to sort out their size and location. So we might see these magnitudes change.

There are so many of them, and only so many scientists at the USGS to examine the actual record. It’ll probably take months to really sort out the exact magnitude, but I think we’ll continue to see those sorts of numbers. Most of the aftershocks have been in the magnitude 1 and 2 range, and definitely too small for people to feel.

Every day, the USGS updates their aftershock forecast. Whenever an earthquake of magnitude 5 or larger occurs, the USGS does a statistical analysis of how likely we are to have aftershocks of certain sizes. If you go to their aftershock estimate today, you’ll find basically a 100% likelihood that we’re going to have more threes and fours.

When we talk about a magnitude 5, last I looked, there was about a 33% likelihood that we would have something as large as a 5. Yesterday, when I talked to you, I thought we’d had a 5, but then they revised the magnitude of that earthquake to a lower value.

There’s still a very small but real chance that we could have a magnitude 6—that’s a 4% likelihood right now—and less than 1% that we could have something as big or bigger. Of course, we have many faults, and there’s always a chance that one of the others could go quite independently of what we’re experiencing right now.

But the bottom line is, we’re certainly likely to continue to see this very elevated rate of earthquake activity on the Mendocino Fault and in the vicinity of the Triple Junction for certainly weeks and quite possibly months or longer.

Lauren Schmitt (7:25):
We spent the morning interviewing people around the community about their earthquake experience, and many said they ran outside when the ground started to shake. It’s my understanding that running outside is actually one of the most dangerous things a person can do during an earthquake. Can you comment and share proper safety procedures?

Lori Dengler (7:49):
You know, it’s unfortunate that this, in a way, was a very gentle earthquake. Even friends of mine who are very frightened of earthquakes described it as a large, gentle earthquake. It was very rolling, and so, yeah, it was quite possible for people to run outside, and there was no damage to speak of in this earthquake. There were some things that fell in a few stores, a few cracks in walls, but basically, the ground shaking was so mild that, yeah, you could safely do anything.

But it’s a really bad habit to get into. And I always like to go back to the December 2022 earthquake, where the ground shaking happened when almost everyone was in bed, and the shaking was so strong that no one could get out of bed. For an earthquake of that shaking strength, there were no injuries, basically, and that’s because people stayed put.

This earthquake didn’t cause damage, didn’t cause injuries—it wasn’t strong enough. It was certainly large enough in magnitude. Had it been closer to the coast and had the rupture been more rapid…

But this is very typical of big earthquakes on the Mendocino Fault. We had a 6.6 in 2016, we had a 7.1 in 1994—neither one of them caused damage, because, again, these earthquakes seem to rupture a little more slowly.

It’s just a really poor idea to run outside. What you’re doing is making it more likely that you’ll get injured, that you’ll need medical care, and that it will divert resources—or maybe medical care won’t even get to you.

The last two people to die in a California earthquake due to earthquake shaking was, I believe, in 2003—I might be off a year—in Paso Robles. It was a mid-6 magnitude earthquake. There were a group of people inside an ice cream parlor, and two of them chose to run out the door. They were crushed by debris coming down from the roof. The people that opted to stay in the store and crouched near the counter—they didn’t have a scratch.

So again, if you value your life and think your loved ones need you, please don’t run while the ground is shaking. Drop, cover, and hold on.

Lauren Schmitt (10:54):
There have been three major earthquakes in December over the past four years. Is this a coincidence, or is there some connection? And is there such a thing as “earthquake weather”?

Lori Dengler (11:09):
There is no such thing as “earthquake weather.” Hot, dry, humid, rainy, windy, snowy, frigid—all weather is “earthquake weather.”

Earthquakes occur miles beneath the Earth’s surface, where the daily fluctuation of temperatures never reaches. So we can completely throw out the idea that rainy days or dry days make any difference as far as when we have earthquakes.

Yes, we’ve had a number of earthquakes in December: December 21, 1954, an earthquake near Fickle Hill; the two December earthquakes in 2021 and 2022; and, of course, yesterday’s. But we’ve had many more earthquakes that weren’t in December.

If we look back at the record, the largest earthquake in the last 400 years occurred in January. The Cape Mendocino earthquakes occurred in April. One of our deadliest earthquakes happened in June 1932.

It’s just statistics—randomness. It has nothing to do with physical properties.

Lauren Schmitt (14:38):
I know we didn’t experience any life-threatening tsunami activity, but did we see any surges generated by the earthquake in the ocean?

Lori Dengler (14:51):
We recorded a tsunami—it shows up on tide gauges. At Arena Cove, the signal was 4 to 6 inches. At Crescent City, you can kind of see it on the gauge. In Humboldt Bay, we’ve seen oscillations at the North Spit.

I wouldn’t call it a tsunami—it’s more likely very periodic oscillations. We’re looking into what might have triggered it. My working hypothesis is that the surface waves from the earthquake, which can lift water up and down, triggered what we call a seiche, an oscillation in the bay.

We know that earthquakes do this. For example, the 2002 Denali earthquake in Alaska triggered oscillations at Lake Union in Washington State. That wasn’t a tsunami, but the surface waves excited the waters in Lake Union enough to cause damage to boats.

In 1964, surface waves caused damage in the Gulf of Mexico. So we know surface waves can trigger oscillations in bodies of water. In this case, the changes in water elevation were very modest—nothing dangerous.

Lauren Schmitt (16:50):
Something new I learned yesterday is that if you’re in a boat out in the ocean, you’re actually out of harm’s way as long as you’re in water around 30 fathoms deep, or about 100 feet. Can you explain why?

Lori Dengler (17:04):
Yes, the further offshore you go, the less of a problem tsunamis are. In the open ocean, a tsunami has very low amplitude. For example, during the Japan tsunami in 2011, where waves onshore exceeded 100 feet, the tsunami in deep ocean water was only about 4 feet.

As a tsunami approaches the shore, it slows down and interacts with the seafloor, which causes the wave to build up. But in deep water, it doesn’t have that interaction—it’s just a very long-period wave with little disturbance to the water itself.

If you’re in a boat in deep water, you’re much safer. However, in places like Humboldt Bay, it’s not practical to try to get your boat out into deep water in time. It’s better to stay put and avoid the water entirely. Crescent City is different; they have a more organized plan for their fishing fleet to head out during tsunami warnings.

Lauren Schmitt (21:36):
And Lori, of course, after every major incident, there’s always an after-action review. I’m hoping you can share yours and highlight the takeaways and lessons learned from yesterday’s earthquake event.

Lori Dengler (21:52):
The biggest takeaway is reviewing the tsunami warning. Why was it issued? What was the appropriate level of alert? How well was the warning information transmitted?

This was the first time the Wireless Emergency Alert (WEA) system was used for a tsunami warning. Some people in areas like Alameda County, far from any tsunami risk, received alerts saying, “Tsunami warning—go to high ground.” This caused confusion because there was no hazard in those areas.

The current system can’t provide precise hazard alerts for specific parts of the coast—it’s too broad. Improving that will be a key focus moving forward.

For now, relax, enjoy your holidays, but think about earthquake safety. Reduce your hazards and have a plan in place.