"We've had two people die in earthquakes in historic times in Humboldt County. That's since 1850. I've run the hazard models, even with the Cascadia earthquake, the most I can kill is maybe 40. I don't mean to be facetious about that but it turns out it's really hard to kill people in earthquakes in Humboldt county. It's much more dangerous to go driving down the road here. "
It was early fall in 1996, and I was about to get my hair cut at the place I'd been going to for at least five years, just off Forbes Avenue near the University of Pittsburgh in the Oakland section of Pittsburgh, PA. It was an emotional time, since I was giving up my Pittsburgh life and an apartment I loved, moving away from my local friends and favorite places, to a new life on the North Coast of California. So there were crosscurrents of contrary emotions, sad and exciting at the same time.
Sunshine was pouring into the room as I picked up a magazine to read while I waited. It was Rolling Stone, and inside was an article concerning the area of California that was more endangered by earthquakes than the more famous vicinity of the fabled San Andreas fault. It turned out to be the North Coast. Exactly where I was going. It was the first inkling I had that I was committing everything to a place expecting a powerful earthquake, and one of the most seismically active landscapes in the world.
One of the people prominently quoted in Rolling Stone was named Lori Dengler, in her fourth year or so of teaching at Humboldt State University. Although I haven't seen the article in years, what I most recall is a scene that had Dengler standing near route 101 in Eureka and talking about how after the earthquake the highway and the mall would be swept with a tsunami.
After the recent Community Forum at HSU, prompted by the devastating December 26, 2004 earthquake and tsunamis in the Indian Ocean, I told this story to one of the presenters, geophysicist Lori Dengler. In my eight mostly earthquake-free years here, I had seen her name often in connection with earthquake preparedness, so I knew she talked to a lot of North Coast citizens about earthquakes. I asked her if she ran into a reaction I had, at least for a moment, after reading the Rolling Stone article: if I'd known about this, I might have decided not to come here.
"Not very often," Lori said. " I have talked to a handful of people who just cannot deal with earthquakes. When that shaking starts, there's just something that just terrifies them. Sort of a primeval response. . I know people who have moved away---who have moved to Florida, they can deal with hurricanes, or moved to Minnesota, they can deal with winter storms. We haven't had earthquakes in quite awhile. That is going to change and there will be people who cannot deal with earthquakes. There's something in their basic psyche that tends to flip them out. But I'd say it's relatively rare."
For them, she added, understanding the science doesn't help. "I'd say, look, it's much more dangerous to go driving down the road here. We've had two people die in earthquakes in historic times in Humboldt County. That's since 1850. I've run the hazard models, even with the Cascadia earthquake, the most I can kill is maybe 40. I don't mean to be facetious about that but it turns out it's really hard to kill people in earthquakes in Humboldt county."
"We just don't have the kind of construction, and the kind of dense population, that leaves itself to catastrophic loss of life in an earthquake. Whereas the number of smoking related deaths in the county is probably thousands every year. I know there are more murders every year, and I know that driving on the highways is much more hazardous. And most people when you point that out say oh, okay, I can deal with that."
But there are a lot of people who choose to live here despite the earthquakes. I asked Lori what stands out in how people here deal with the situation?
"I would say that this is a more self-reliant population. I think that people in general don't look to the government to solve all their problems. The issue of preparedness is really a personal responsibility, and we see much higher levels of preparedness here than elsewhere in the state, and it's been very consistent for a long period of time. It's partly because every winter people lose their power, so they know how to cope. We also have a fair number of people who have lived here a long time, and they remember getting through earthquakes in the past."
A major earthquake off the North Coast resulting in tsunamis can happen at any time: today, tomorrow or two hundred years from now.
With a quake as strong as the one in the Indian Ocean in December 2004, tsunami waves will begin to hit the North Coast before the ground stops shaking. Tsunami waves travel at upwards of 500 mph in the open sea, and reach shore at 40 mph with 40 foot waves or higher.
People right on the coast at that time should head on foot to higher ground immediately. A height of 100 feet should be safe.
Most of Eureka and a lot of Arcata won't be inundated, but travel between them, and between Arcata and McKinleyville, will probably be impossible for an indeterminate time.
A magnitude 9 earthquake off the North Coast will be felt up the coast to Canada and down to southern California, resulting in damage in San Francisco and Sacramento.
For more information on earthquakes, maps on areas of concern and especially on preparing to survive earthquakes and tsunamis, check links through the Humboldt State University geology department page. Here's a link to get started:
On Monday, January 24, four members of Humboldt State University Geology Department hosted a Community Forum on the December earthquake and tsunami in Indonesia, and how it relates to what has occurred on the North Coast in the past, and is likely to happen again.
A capacity crowd in the Kate Buchanan room on the HSU campus gathered at 5 p.m. The lights remained dim for much of the next two hours, as the quietly attentive audience saw slides of maps and diagrams, photographs and even some video concerning the December 26 earthquake and resulting tsunami, as well as similar events in Hawaii, Alaska and here on the North Coast.
After thanking several scholars and organizations(including the National Oceanic and Atmospheric Administration and NASA) for their contributions to the presentations, Professor of Geology and HSU Geology Department chair Sue Cashman began by outlining what happened in South Asia.
The epicenter of the December 26 earthquake was off the west coast of Sumatra, on the east side of the Indian Island basin. At magnitude 9.0, it was the fourth largest earthquake recorded since instruments were first used to measure seismic events about a century ago. Nearly 300,000 people are now believed to have perished.
"This was an area known to have earthquakes," Cashman said. "It was so deadly because it generated a tsunami that radiated out in all directions across the Indian Ocean." But this was the only seismically active area in the Indian Ocean, so people at some distance from it may not have been aware of it. This may have contributed to the high number of fatalities. People did not know what an earthquake in the ocean could mean.
She gave a quick course in earthquake science. About a dozen huge plates ride the earth's mantle, under all the land and water on the planet. These plates have been moving slowly but inexorably for millennia, and they are still moving. Earthquakes and volcanoes occur in a zone where two or more plates meet.
Plates that are moving apart from each other cause seismic motion called "spreading." Plates sliding past each other create "transform" faults, like the famous San Andreas. One plate moving under another creates "subduction" zones. About 75% of the world's earthquakes occur from subduction zones, and they generate about 90% of the seismic energy released worldwide.
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The earthquake in south Asia, along what's called the Sumatra Margin, was a subduction zone quake. That's the first relevant resemblance: there is a subduction zone just off the North Coast, called the Cascadia Margin. At the Sumatra Margin, the Indian/Australian plate is subducting beneath the Eurasian plate. Off the North Coast, the eastward moving Juan de Fuca plate subducts the westward moving North American plate. In both places, earthquakes resulting from movement of the plates displace ocean water, creating tsunamis.
Geologist Mark Hemphill-Haley took the story from there. He illustrated how, in a subduction zone, one of the plates is always moving under the other, creating wrinkles in the landscape. Here on the North Coast "we see vertical uplift right now, between earthquakes."
He described the dynamics of how earthquakes and resulting tsunamis are created. "What happens in an earthquake is that the upper plate that has been storing the strain suddenly leaps oceanward, and as it does, the stuck area ruptures: you have an earthquake. But you also have a large amount of crust that moves oceanward, and it displaces a lot of water quite rapidly. The water is moved upward and then it has no place to go but outward."
The tsunami usually radiates in all directions, at speeds calculated at over 500 mph. "Once they come into the coast they tend to slow down and stack up, but they still move in at approximately 40 mph. Some of the fastest river flood waters have been clocked at only 10 mph. So we're talking about something that is quite a bit faster than any flood you've ever witnessed."
He showed some video clips from the Sumatra tsunami, taken from the third floor of a house a mile and a half inland. The water almost reached the terrified people there, fifty feet above the ground.
Tsunami waves move in and also move out in waves of debris, scouring everything in their path. The first waves were preceded by a huge withdrawal, or drawdown, of ocean from the beach. This should be a warning, but unfortunately people went down to the suddenly enlarged beach to collect shells, not realizing what was coming.
The Sumatra rupture zone is roughly the same size as the Cascadia zone, and Cascadia has generated earthquakes of the same 9.0 magnitude. The difference is that the suduction zone is closer to the North Coast, so the tsunami would reach shore much more quickly. "We suspect that in a magnitude 9 event, we'd have three to four minutes of strong shaking, and before the shaking is over, the tsunami would arrive. Our entire coastline will feel this almost instantaneously."
A tsunami is not a single "tidal wave," but a group of waves, sometimes separated by as much as half an hour.
HSU adjunct professor Harvey Kelsey then talked about another effect of a suduction quake: subsidence. "As the upper plate leaps forward during the earthquake, it stretches the upper plate and you get a subsidence. It occurs inland but right near the coast." He described the efforts of geologists to understand how earthquakes work from literally unearthing evidence of what happened in past quakes.
Kelsey recounted what we know about the last major quake in the Cascadia zone, which was in 1700. There are no written records here, but there are in Japan, which sustained considerable damage from an "orphan tsunami" which has no local shaking preceding it. Scientists now believe this tsunami resulted from the January 26, 1700 Cascadia earthquake. Other evidence for this quake comes from tribal stories of North Coast Native peoples who witnessed it, and from trees that went underground as a result of subsidence. Kelsey also described evidence of tsuamis resulting from this quake, found in deposits of beach sand swept into freshwater coastal lakes in Washington state.
By the time that geophysicist and seismologist Lori Dengler walked to the podium, the room was completely silent. Her task was to describe what the next Cascadia earthquake probably will be like, but "our point here is not to terrify you. In fact, I fully expect to survive the next Cascadia earthquake, although I would rather that it waits another hundred or two hundred years. There's absolutely no reason why we can't live with subduction zone earthquakes. We actually have our subduction zone to thank for our absolutely gorgeous scenery here."
The rugged landscape has not been smoothed by millennia of wind and rain, because it was formed relatively recently, in geological time, and it is still being formed by active plate movement. "The shape of the land here is a result of the fact that we are a techtonically youthful area."
She noted that stories of Native North Coast peoples recognize the benefits as well as the destruction caused by techtonic action. "They talk about the characters of Earthquake and Thunder who not only scared them, but also provided uplift or down-drop of the land," which added new places to fish and new sources of food.
Dengler then described the results of the 1995 earthquake planning scenario. Because it was financed by the state of California, she mentioned, the scenario modeled a magnitude 8.4 Cascadia subduction zone earthquake, which doesn't have the same impact on Oregon and Washington as a 9.0 quake would. "An 8.4 extends from Cape Mendicino up to the Oregon border. A magnitude 9 takes you all the way up to Vancouver Island... But in terms of the impacts locally, certainly the ground-shaking impacts, they are not likely to be significantly different."
She noted that for all the emphasis on the tsunami in Asia, "for most of us, it's the earthquake that we are going to need to be concerned with. Most of us don't live in a tsunami hazardous zone."
Larger magnitude earthquakes don't necessarily shake more strongly, she noted, but they shake longer. Five minutes of strong shaking is enough to topple poorly built structures, and significantly damage some better-built structures.
Dengler referred to a map of the North Coast that shows intensity zones: the higher the intensity, the more damage. Most of Eureka and much of Arcata are in intensity 8 areas, which is pretty intense, but "most well-built buildings should survive with maybe cosmetic damage."
Most of the North Coast is in the intensity 7 area, "which is enough to knock down chimneys and totally trash your kitchen." Probably the most severe effects will be in damage to bridges and roadways. "It will probably be impossible to get across the Mad River...For a couple of days we probably won't be able to drive safely between Arcata and McKinleyville."
A magnitude 9 earthquake will cause ground-shaking all the way up to Vancouver Island. Dengler plotted shaking effects based on the Alaska quake of 1992, which was 9.2. In her model of a Cascadia 9, the intensity 7 area extends all the way to San Francisco. "Items fall off the shelves in Santa Barbara, there's minor damage in Bakersfield, and Sacramento gets absolutely nailed. This earthquake is going to affect the entire state, as far as ground-shaking is concerned."
The next big Cascadia quake will result in tsunamis. "A magnitude 9 event does not respect political boundaries," Dengler said. "It will impact the entire Pacific Ocean."
The first waves of the tsunami will reach San Francisco in an hour. In two hours it will hit Santa Barbara, in three hours northern Canada, and in five hours, Hawaii. Nine hours later, it reaches Japan. "Japan is going to get damage from our next Cascadia earthquake, as it did in 1700."
On the screen behind her was one of the color-coded North Coast tsunami maps, of the Humboldt Bay area. Dengler described the projected problem areas, which included all margins of the Bay. Indian Island would be flooded, as well as some of the Eureka waterfront. "But the vast majority of Eureka is not at risk." Elevation as well as distance from the coast is a factor, so even in Samoa the high dunes are likely to be safe.
To see these North Coast tsunami maps now, click here.
Dengler then talked about preparation. Referring to the Indian Ocean tsunami she said, "Many people died needlessly, of ignorance, because they did not understand nature's warning." Some of those in the quake area did not realize what the shaking could portend, and those farther away didn't understand that the sudden disappearance of waves and water from the beach was a warning.
Because the North Coast is so close to the area where the tsunami will be generated, "the earthquake is your warning," Dengler said. "You have time to do something. First, protect yourself from the earthquake. If you're inside, duck, cover and hold. If you are inland, move to higher ground. Go on foot because the roads aren't going to be passable. Stay away from the coast for a really long time, until you have some kind of official notification that its safe to go back. We may be talking about days. The period of unusual wave activity may be 12 hours. The radio is your best source of information. In particular, get yourself a NOAA weather radio."
After these presentations, the lights came up and audience members asked questions. The geologists talked about the complexities of the effects on the landscape of a big Cascadia quake, and about the effects of past quakes, such as the 1700 earthquake which enlarged Humboldt Bay to its present size. "In the journals of the Josiah Gregg party, they noted in conversations with the Wiyot, their story was that the bay was created in a single day in an earthquake. If you imagine a marshy swampy bay that suddenly dropped down on the order of a meter it would certainly look like it was created in a single day."
There were several questions seeking more information on the North Coast tsunami. How high would the waves be? "What we've seen in a great tsunami here is an average height of 40 to 45 feet, with isolated instances of 75 to 100 feet. But we have no evidence on our coast of tsunami waves of 100 feet. So if you get yourself above a hundred feet on the open coast, you'll be okay."
Tsunamis are a series of waves. Because of the effects waves have on each other, "the first wave is never the biggest. It's always the second or third or fourth or the fifth." The bigger the tsunami, the longer the peri0od of wave activity. The Indian Ocean tsunami lasted 8 to 10 hours.
In correspondence later, Lori Dengler clarified aspects of the North Coast tsunami maps. "A Cascadia earthquake will likely isolate McKinleyville from Arcata, Aracat from Eureka, Ferndale, Fortuna, etc. Every major bridge is likely to be unpassable in the first few days due to differential settling from the ground shaking. The isolation has nothing to do with the tsunami -it's caused by the ground shaking. The current State bridge retrofit program has already completed the strengthening of many of our bridges and hopefully most will still be structurally sound. If this is the case the bridges might be made passable again relatively quickly. If there is structural damage, it could be months before the bridges work."
"Bridges aren't the only problem. Liquefaction will cause damage to roadways in the vicinity of the bay and rivers. The airport will be ok as long as the McKinleyville fault that runs beneath it doesn't rupture in the earthquake. If it does, the airport will be out of commission indefinitely. Helicopters will be very important in the response and relief periods."