19.5: Beyond the San Andreas - Earthquakes of the North Coast

Jan 26, 1700

The evening of Jan 26 felt like any other evening as the Yurok people settled down for the night in what is now northern California, but in the darkness of the night, they were roused with violent shaking. Living in earthquake country, the Yurok were familiar with the sensation of the earth shifting beneath them, but this was different; it was much stronger and lasted much longer. “The earth would quake and quake again and quake again,” they said in the stories they passed on to their grandchildren. The people went to the top of a hill to dance a dance they believed would send the earthquake away. Though their dancing did not end the earthquake, the tradition did save the dancers from what came next. Below, waters rushed in, flooding the coast and devastating the village they’d left behind. It was the strongest earthquake any of these people had felt in their lives or would ever feel again, but there were stories of this kind of earthquake and tsunami from many generations past. The Tolowa people, just north of the Yurok, tell a story of a boy and a girl who were the only ones to make it to high ground. On their return to their coastal village, they found not a single living person.

People all along the coast of the Pacific Northwest, from the Yurok and Tolowa to the people of what is now British Columbia, Canada, told stories of how they survived a megathrust earthquake and tsunami (see Earthquake Mechanics). However, in the next few centuries, many of these oral histories were lost, and the rest nearly forgotten until they were recovered in the 1980s.

Oral History as Geological Evidence

By the 1980s, geologists were well aware that the Pacific Northwest was home to a subduction zone (Figure \(\PageIndex{1}\)), but they were somewhat perplexed by the lack of historical earthquakes. Other subduction zones, like those of Japan and Sumatra had a well-documented history of earthquakes. The two largest earthquakes in history, in Chile in 1960 and Alaska in 1964 respectively, had occurred on subduction zones, yet the Cacadia subduction zone remained quiet. Some geologists figured that the Juan de Fuca plate must slide smoothly past the overriding North American plate without locking, much like the creeping sections of the San Andreas Fault in the San Francisco Bay Area, but when the government wanted to put new nuclear power plants in Oregon and Washington, the geologists consulted decided it would be wise to take a deeper look into the earthquake hazards of the Cascadia subduction zone. Perhaps Cascadia was not aseismically creeping; perhaps instead, it had been locked for all of recorded history. If that were true, it would mean that Cascadia was capable of very large earthquakes. The first step to find this out was to ask the people who have been here the longest. An interdisciplinary effort, led by geologist Ruth Ludwin, turned up many oral histories that told of great earthquakes accompanied by flooding waters. The geologists decided to treat the stories they uncovered, not as myths, but as historical evidence.

Figure \(\PageIndex{1}\): Map of the Cascadia subduction zone. "Cascadia Map" by the Federal Emergency Management Agency is in the public domain. Access a detailed description.

The Three Layer Cake

Though the oral histories provided compelling evidence, geologists wanted other evidence to support their case as well. One form of evidence is found in the stratigraphic record found in tidal marshes. A distinctive sequence that geologists affectionately refer to as the “three layer cake” is a clear indicator of an earthquake and tsunami. When a subduction zone is locked, strain accumulates, pushing the coastal land upwards. Muddy tidal flats become marshes as they are lifted higher in the intertidal zone. The soil or marsh peat developed in this ecosystem becomes the first layer of the “cake” (Figure \(\PageIndex{2}\)).

During an earthquake, the strain is released and the land drops suddenly back down to the lower part of the intertidal zone, where the marsh plants won’t survive. Within minutes, the first tsunami wave rolls in, followed by others for several hours. The tsunami waves bring in sand from nearby beaches and dunes, depositing the second layer, as shown in the second panel of figure \(\PageIndex{2}\). Even after the tsunami has retreated, the land is still lower than it was before the earthquake. The marsh has now returned to mud flat, and the third layer, a layer of tidal silt (mud), finishes out the three layer sequence, as shown in the final panel of figure \(\PageIndex{2}\). Gradually, as the land lifts again, the marsh returns and the sequence repeats.

The “three layer cake” has been found throughout the Pacific Northwest coast, including several sites in northern California, and has also been found at other subduction zones around the world. In some places along the Cascadia subduction zone, the three layer sequence repeats itself multiple times, suggesting that the 1700 earthquake was only the most recent of many such events.

Figure \(\PageIndex{2}\): Cartoon showing the three layer sequence that indicates that an earthquake and tsunami has occurred. "Diagram showing land subsidence and sand deposit following tsunami" by Coastal and Marine Hazards and Resources Program is in the public domain / A derivative from the original work by Atwater et. al, 2005. Access a detailed description.

Ghost Forests

When a forest suddenly drops below the high-tide line and becomes a salt marsh, the trees living there can no longer survive. Eventually only the stumps remain, in some places fully buried in the salt marsh and in others exposed at low tide (Figure \(\PageIndex{3}\)). These are called ghost forests and they are another piece of evidence in the Cascadia earthquake story.

Figure \(\PageIndex{3}\): Neskowin Ghost Forest in Oregon. "Neskowin Ghost Forest" by RocketSams is licensed under CC BY-SA 4.0.

The Orphan Tsunami

Radiocarbon dating of the ghost forests helped geologists get an idea of when the most recent earthquake along the Cascadia Subduction Zone occurred, but radiocarbon dating is not precise enough to narrow down to a particular year or even a particular decade, and that posed a potential problem because geologist were not yet sure whether the oral histories and geologic evidence came from one single huge earthquake or multiple smaller earthquakes within a few years of one another. Understanding that, of course, is key to understanding the potential future hazard; when Cascadia goes again, will it be in one big one, or two smaller ones? To help solve this mystery, geologists once again needed some interdisciplinary help; this time from Japanese earthquake historians.

In fact, the Japanese historians were facing a mystery of their own. Japan has a well documented history of both earthquakes and tsunamis. Even the accepted international term, tsunami, is Japanese. Usually, a historical record of a tsunami is accompanied by a record of an earthquake preceding it, but on Jan 27 and 28, 1700, a series of tsunamis hit harbors along the Japanese coast with no known earthquake preceding it. It was called an “orphan tsunami” because it had no parent earthquake.

While even the largest earthquakes are not usually felt outside the region where they occur, tsunamis can be felt all around the world (or all around one ocean basin anyway). For example, the 2011 Tohoku, Japan earthquake produced a tsunami that reached California, though it was relatively small by the time it got here. In 1700, this scenario played in reverse, with the earthquake occurring in North America and the tsunami reaching Japan.

By carefully piecing together many historical records, the historians and geologists together were able to put together a picture of what the 1700 tsunami looked like when it arrived in Japan. The records precisely matched what would be expected for a tsunami generated in Cascadia, and the timeframe fit within the range expected based on the radiocarbon dating. But the Japanese records didn’t just confirm what geologists already knew, they added a new level of detail. By calculating the time it would take the tsunami to arrive in Japan, the team could now establish an exact time for the earthquake, which we now know occurred on Jan 26 at 9:00 pm. They could also determine the size of earthquake that would be required to produce the tsunami that reached Japan. The scientists determined that the earthquake must have been one single, large earthquake that ruptured the entire length of the Cascadia Subduction Zone from Cape Mendocino to British Columbia, causing an M9 earthquake.

Video \(\PageIndex{1}\): Tsunami Forecast Model Animation: Cascadia 1700

This animation shows a model of how the 1700 Cascadia tsunami propagated across the Pacific Ocean and reached Japan. The video contains no audio. Access a detailed description.

Query \(\PageIndex{1}\)

The Future of Cascadia Earthquakes

The 1700 Cascadia earthquake was the most recent Cascadia subduction zone earthquake and ruptured the entire length of the subduction zone from northern California to British Columbia. But the 1700 earthquake was just the most recent of many earthquakes. Though the older earthquakes don’t have detailed tsunami records from japan, they are preserved in the geologic record. In the past 5000 years, there have been 10 subduction zone earthquakes on the northern section of the Cascadia subduction zone. Geologists estimate the average recurrence interval for full rupture of the 1,100 km (700 mile) subduction zone is 500 to 530 years. If recurrence intervals could be relied on like bus schedules, we would expect an earthquake of near M9 and tsunami to hit the North American coast sometime between 2200 and 2230; plenty of time still to prepare. But earthquakes don’t operate on a schedule. Some of the individual intervals between earthquakes have been as short as a few decades, while others have been as long as a millennium. The next megathrust earthquake could happen tomorrow or it might not happen until the Space Needle is an archaeological relic.

The issue is further complicated by the fact that the Cascadia subduction zone doesn’t always rupture along the full length. Geologists have found that there are three common modes of rupture: (1) full-ruptures along the entire length or nearly the entire length of the subduction zone, producing an earthquake of around M9, as happened in 1700; (2) ruptures of only the southern half or so of the subduction zone, producing an earthquake in the M8 range; and (3) ruptures of only the southern margin (the part in California), producing smaller earthquakes and smaller tsunamis. The average recurrence interval for the third, smallest type of earthquake is estimated at only 240 years, making northern California “overdue” for a subduction zone earthquake.

The Worst Natural Disaster in U.S. History?

If a full rupture were to occur tomorrow, the earthquake would be felt throughout the Pacific Northwest and shaking would last for four to six minutes. In some places the shaking would be strong enough that it would be difficult to stand. During the earthquake the coast would subside by about 2 meters (6 feet), resulting in a sudden increase in relative sea level. In 15 to 30 minutes after the shaking, the first tsunami would arrive. The height of the tsunami will vary along the coast depending on local bathymetry and topography. It may be as high as 9 to 12 meters (30 to 40 feet) in some places. Flooding and dangerous currents may last for days. Additional hazards, like liquefaction, landslides, fires, dam failures, and hazardous material spills are also likely.

Millions of people would be affected as well as property, infrastructure, and the environment. According to a scenario put together by Cascadia Region Earthquake Workgroup (CREW), the number of deaths could exceed 10,000, more than 30,000 people could be injured, and economic losses could approximate $70 billion for Washington, Oregon, and California alone.

Table \(\PageIndex{1}\): Comparison of select natural disasters and the CREW Cascadia disaster scenario

Name	Fatalities	Year	Type	Location	Notes
Sumatra, Indonesia	227,899	2004	Earthquake and tsunami	Indian Ocean	Deadliest earthquake in 21st century
Tohoku, Japan	19,759	2011	Earthquake and tsunami	Japan
CREW Cascadia Scenario	10,000	Future Scenario	Earthquake and tsunami	U.S., Canada
1900 Galveston Hurricane	6,000-12,000	1900	Hurricane	Texas	Deadliest natural disaster in U.S. history
1906 San Francisco Earthquake	3,000+	1906	Earthquake	California	Deadliest earthquake in U.S. history

Of course, it is important to remember that the scenario described above is the worst-case full rupture scenario, which is also the least likely. Although this may not be as reassuring to northern Californians as might be to Washingtonians, it is also important to remember that a smaller rupture produces a smaller earthquake and also a smaller tsunami. Additionally, in the smaller scenario, California cities like Eureka, Arcata and Crescent City will not have to compete with the likes of Portland and Seattle when it comes to disaster relief resources.

Box \(\PageIndex{1}\): Tsunami Hazard Maps Change With Updated Science

As scientists gain a better understanding of past Cascadia earthquakes and tsunamis, they are better able to forecast a future tsunami scenario. In some cases this has required updating tsunami hazard zones to reflect current scientific understanding. In the activity below, you can compare two tsunami hazard maps for Crescent City California. The 2021 hazard map is an update to the prior map, created in 2010. Tsunami inundation is now expected to be more extensive in Crescent City than previously thought. For example, Sutter Coast Hospital was previously located about a half mile from the designated hazard zone, but now sits right at the edge.

Query Box \(\PageIndex{1}\)

Access accessibility guidance for this activity.

Although there is no subduction zone to produce a local tsunami off the coast of southern and central California, a tsunami generated by the Cascadia Subduction Zone or the Aleutian Subduction Zone in Alaska could inundate these coastal regions as well. To view maps of tsunami hazard zones along the entire California Coast see California Geological Survey Information Warehouse: Tsunami Hazard Area Map.

Video \(\PageIndex{2}\): What's the ONE THING You Can Do To Survive A Tsunami?

If you’d like to recap what you’ve learned in this section and learn more about what is being done to prepare, as well as what you can do to prepare if you live in or travel to the tsunami hazard zone, watch this PBS video.

The Mendocino Triple Junction: An Earthquake Buffet

While a Cascadia Subduction Zone earthquake is perhaps the most ominous of earthquake scenarios for the north coast of California, it is not the only one. There has not been a subduction zone earthquake in over 300 years, but talk to any resident of Humboldt County, and they’re sure to have an earthquake story or two to tell you. Most of these stories don’t involve significant damage (though a few do), but earthquakes large enough to be felt are a pretty regular part of life near the Mendocino triple junction (Figure \(\PageIndex{4}\)). A triple junction is a place where three plate boundaries meet, and each of these plate boundaries, of course, has the potential to cause earthquakes. The San Andreas fault system and the Cascadia subduction zone, make up two of the three arms of the triple junction and are discussed earlier in this section and elsewhere in this chapter. But it is the third arm of triple junction that most frequently awakens the people of Eureka in the night or throws groceries from shelves in Ferndale; that is the Mendocino fracture zone, the transform plate boundary between the two plates of oceanic lithosphere off the coast of California.

Figure \(\PageIndex{4}\): Earthquakes of Magnitude 4.5 or greater between July 1, 2014 and July 1, 2024 in and near California. "California M4.5 Earthquakes" by Emily Wright was made using the Earthquake Catalog by the USGS, which is in the public domain. An is licensed under CC BY-NC 4.0. Access a detailed description.

In the chapter Cascade Range and Modoc Plateau and elsewhere in this text, we have generally referred to the last remnants of the Farallon plate collectively as the Juan de Fuca plate. This is helpful when discussing broad concepts because the three plates generally move together. Some geologists, however, consider the remnant Farallon plate as three separate plates, the Explorer plate offshore of Vancouver Island, the central Juan de Fuca Plate, and the Gorda plate, a small plate off the coast of northern California (Figure \(\PageIndex{5}\)). The Mendocino fracture zone, therefore, is the boundary between the Gorda plate and the Pacific plate (labeled "MFS" for Mendocino Fracture System in figure \(\PageIndex{5}\).

Figure \(\PageIndex{5}\): Plate tectonic map of the Pacific Northwest. MFS, Mendocino Fracture System; SAF, San Andreas Fault; Triangles: Cascade volcanoes; Red dots: earthquakes deeper than 25 km (16 miles). "Earthquakes in the Juan de Fuca Plate" by Robert S. Yeats is licensed under CC BY-NC 4.0. Access a detailed description.

Besides rupture of the three plate boundaries themselves, the triple junction region is also susceptible to intraplate earthquakes within the Gorda plate and the North American plate. There are many crustal faults in the northern coast ranges, which pose challenges to development in the region on account of the Alquist Priolo (AP) Act (see 19.2: The hazard of place). Earthquakes along these faults, however, are relatively rare compared with intraplate earthquakes in the Gorda plate. The Gorda plate could really rather be called a zone of deformation, as it behaves far less like a rigid plate than the main Juan de Fuca plate to the north. As the Pacific Plate presses northward on the Juan de Fuca plate, the little Gorda plate is caught and squeezed between the two, producing many small faults, and many earthquakes, within the plate. Many of the intraplate earthquakes in the Gorda plate are shallow, offshore earthquakes, but others are deep earthquakes with inland epicenters. The later are generated by intraplate motions in the subducting Gorda slab beneath the North American plate. These are the earthquakes shown in figure \(\PageIndex{5}\).

There have been a few larger earthquakes in this region. The Gorda Plate west of Arcata, California, sustained an earthquake of M7.3-7.6 on January 31, 1922, that was felt in Oregon and Nevada, and as far south as San Jose, California. Another earthquake of M_w 6.9-7.4 thirty miles west of Trinidad, California, on November 8, 1980, destroyed a bridge, liquefied the sand bar at Big Lagoon, and caused six injuries and $1.75 million in damage. Then, on April 25 and 26, 1992, a sequence of earthquakes, beginning with an M_w7.2 mainshock struck Cape Mendocino, injuring 356 people and destroying or damaging 1,108 structures at a cost of $61 million dollars.

Though the Mendocino fracture zone and the Gorda plate do not have the potential to produce the M8 or M9 earthquake that this region anticipates from the Cascadia subduction zone, each smaller quake can serve as a reminder of what could be yet to come. Each time the earth shakes and one takes cover under a desk or buried beneath the pillows in bed, one must wonder, “is this the big one? Will this be the time that the shaking does not stop, but ‘quakes and quakes again and quakes again’?”

Query \(\PageIndex{2}\)

References

1. Atwater, B. F., Musumi-Rokkaku, S., Satake, K., Tsuji, Y., Ueda, K., & Yamaguchi, D. K. (2005). The orphan tsunami of 1700—Japanese clues to a parent earthquake in North America. Professional Paper, Article 1707. https://doi.org/10.3133/pp1707
2. Brochure_crescent_city_2021.pdf. (n.d.). Retrieved July 2, 2024, from https://rctwg.humboldt.edu/sites/default/files/brochure_crescent_city_2021.pdf
3. Cape Mendocino Earthquakes 1992 | Redwood Coast Tsunami Work Group. (n.d.). Retrieved July 3, 2024, from https://rctwg.humboldt.edu/capemendo92
4. Finkbeiner, A. (2015, September 14). The Great Quake and the Great Drowning. Hakai Magazine. https://hakaimagazine.com/features/great-quake-and-great-drowning/
5. Goldfinger, C., Nelson, C. H., Morey, A. E., Johnson, J. E., Patton, J. R., Karabanov, E. B., Gutierrez-Pastor, J., Eriksson, A. T., Gracia, E., Dunhill, G., Enkin, R. J., Dallimore, A., & Vallier, T. (2012). Turbidite event history—Methods and implications for Holocene paleoseismicity of the Cascadia subduction zone. In Professional Paper (1661-F). U.S. Geological Survey. https://doi.org/10.3133/pp1661F
6. List of disasters in the United States by death toll. (2024). In Wikipedia. https://en.Wikipedia.org/w/index.php?title=List_of_disasters_in_the_United_States_by_death_toll&oldid=1227232457
7. Lists of 21st-century earthquakes. (2024). In Wikipedia. https://en.Wikipedia.org/w/index.php?title=Lists_of_21st-century_earthquakes&oldid=1229419266
8. Tsunami_Preparedness_Crescent_City.pdf. (n.d.). Retrieved July 2, 2024, from https://www.crescentcity.org/media/Emergency%20Preparedness/Tsunami_Preparedness_Crescent_City.pdf
9. Walton, M. A. L., Staisch, L. M., Dura, T., Pearl, J. K., Sherrod, B., Gomberg, J., Engelhart, S., Tréhu, A., Watt, J., Perkins, J., Witter, R. C., Bartlow, N., Goldfinger, C., Kelsey, H., Morey, A. E., Sahakian, V. J., Tobin, H., Wang, K., Wells, R., & Wirth, E. (2021a). Toward an Integrative Geological and Geophysical View of Cascadia Subduction Zone Earthquakes. Annual Review of Earth and Planetary Sciences, 49(Volume 49, 2021), 367–398. https://doi.org/10.1146/annurev-earth-071620-065605
10. Walton, M. A. L., Staisch, L. M., Dura, T., Pearl, J. K., Sherrod, B., Gomberg, J., Engelhart, S., Tréhu, A., Watt, J., Perkins, J., Witter, R. C., Bartlow, N., Goldfinger, C., Kelsey, H., Morey, A. E., Sahakian, V. J., Tobin, H., Wang, K., Wells, R., & Wirth, E. (2021b). Toward an Integrative Geological and Geophysical View of Cascadia Subduction Zone Earthquakes. Annual Review of Earth and Planetary Sciences, 49(Volume 49, 2021), 367–398. https://doi.org/10.1146/annurev-earth-071620-065605
11. Yeats, R. S. (2018). Earthquakes in the Juan de Fuca Plate. https://open.oregonstate.education/earthquakes/chapter/earthquakes-in-the-juan-de-fuca-plate/