The Cascadia Subduction Zone: The Very Real Threat

                                       

The "crumpled fender" zone marking the

location of the Cascadia Subduction Zone

Copyright © 2020

by Ralph F. Couey

Within the past 35 years or so, a new seismological and oceanographic threat was first proposed, and finally proven. This sleeping giant is the Cascadia Subduction Zone which lies beneath thousands of feet of ocean 60 miles off the coast of northwestern North America. The scale of catastrophe that will result from this fault cannot be underestimated. But the process that led to this discovery is a detective tale that is hard to beat. 

In 1960, a massive earthquake tore the earth apart off the coast of Chile. It was measured at magnitude 9.5 on the Richter Scale, and still remains the most powerful temblor ever recorded. Four years later, another large quake occurred off southern Alaska, that wrecked not only the capitol city of Anchorage, but dozens of smaller villages and ports along the coast. The similarity of the two events helped lead to the movement of plate tectonics from theory to fact.

Basically, the Earth’s crust is a 40 to 60-mile-thick layer of rock which floats on the semi-liquid of the next layer down, called the mantle. The crust is broken up into massive pieces, called plates, which float upon the very hot mantle.  Within the mantle, huge convection currents are generated which propel the plates around.  

Looking at a world map, it seems obvious that South America and Africa fit together like puzzle pieces. Scientists for years had seen other continents and islands that also had common edges. When brought to acceptance, plate tectonics proved that the continents and the plates beneath them had been in motion for hundreds of millions of years. The current configuration is the result of the breakup of the last great supercontinent, Pangaea which began about 175 million years ago. Now, scientists know that the motion of these separate plates create seismic zones where they crash together. These collisions have been responsible for the creation of most mountain ranges on this planet. Those boundaries became the objects of intense research, which led to the identification of subduction zones.

A subduction zone, or convergent fault, is where an oceanic plate is diving, or subducting under a continental plate. They are located all throughout the Pacific ring of fire, and nearly all of them have been responsible for the largest seismic events in history. What scientists discovered was the disquieting fact that when subduction zones rupture, they only generate earthquakes north of magnitude 8. Recently, two events, the Indonesian Boxing Day Tsunami and the Tohoku Tsunami were both born of magnitude 9 earthquakes generated by subduction zones. 

In a perfect world, the subducting plate dives smoothly beneath the continental plate at a frenetic pace of 2 to 4 inches per year. In some cases, however, the two plates stick together and stress begins to build. As that happens, the lip of the continental plate begins to curl under itself which forces the land behind it to rise and tilt. When the rock’s elasticity reaches it’s limits, the lip thrusts back forward.  Two things then happen.  A massive earthquake is generated, one that will inflict catastrophic damage on any communities along the coast.  Also, the land behind the fault subsides, or drops, as much as six feet.  At the moment when the continental plate snaps forward, the action causes a column of water to be lifted.  In the case of Cascadia, that is a column about 8,000 feet tall and about 600 miles long. That’s the tsunami. The column, split by gravity, generates two waves, one that heads shoreward, the other begins a dash across the ocean. 

A tsunami consists of not just one wave, but several, sometimes a train of as many as 9 or more which arrive from minutes to hours. The word wave is actually a misnomer, since its more properly described as a large shelf of water that when it reaches the coast, flows like a river over land, structures…and people.

Once the 800-mile-long Cascadia fault was acknowledged to exist, there was still the debate on how dangerous it was. There had never been a recorded event along the fault for the length of time that European Americans had arrived on the west coast. But that history only went back less than 200 years. Some scientists believed that the fault was aseismic, that is, the subduction occurring smoothly and without incident. Others believed that the subduction had stopped, having exhausted itself.  The first clue that something was amiss came about in 1984 when a Canadian scientist, ironically named John Adams, discovered through the use of lasers that the coastal mountains of Washington and Oregon were being bent and tilted to the east. Another clue was revealed during the very detailed research driven by the proposal to establish a string of nuclear power stations in the northwest. As the scientists dug deeper, they discovered more information that indicated that stress was being built up along the subduction zone. Those discoveries eventually led to the cancellation of the plants. In the 1970s, the Washington State Department of Transportation undertook a new survey of existing roads and were astonished to find out that the roads, and the land around them, had uplifted significantly. Also, land and islands in the Puget Sound area were found to be compressed, the islands moving closer together.

One of the most significant discoveries involved the examination of core samples from the ocean floor. Earlier, a limited number of core samples had been pulled from the ocean floor.  Analysis revealed 13 separate events where silt and sediment ejected by the Columbia and other smaller rivers onto the continental shelf spilled down into the canyons for hundreds of miles in titanic undersea landslides. The first study hypothesized that only a large earthquake could shake loose so much sediment over such a large area. An oceanographer, Chris Goldfinger, was adamant that such a thing could not happen, So, he undertook his own investigation, drawing cores from all along the suspected fault’s length. 

What he found changed his view. Analysis of all of the sediment cores, called turbidites, showed that same 13 events. The events were dated from a known event, the eruption of Mt. Mazama, now called Crater Lake, a titanic blast 7,700 years ago that deposited volcanic debris as far away as Nebraska. Identifying that layer made the dating of the subsequent landslides easier. The sediment landslides also answered a persistent question among skeptics as to the missing trench. All the other subduction zones had deep oceanic trenches where the oceanic plates dove deep into the mantle. What the core studies revealed is that the Cascadia trench was covered by the vast amount of material that slid into the fault during and after the previous earthquakes.  That explained why Cascadia did not show the deep trench that other subduction faults have.  That lack of a trench slowed the wider acceptance of the danger that the fault represented.

In later years, more refined studies identified 41 Cascadia earthquakes, 19 of which were full margin ruptures (where the entire 800 miles of the fault unzipped at once.) All of them were megathrust quakes rated at magnitude 8.0 and above. For the first time, scientists, and a growing body of the public faced the very real danger of the monster that lurked just off the coast. 

Determining the date of the last megathrust rupture came from the marriage of several different scientific disciplines.

Paleogeologist Brian Atwater had been digging in the salt marshes along the Copalis River in Washington when he discovered two very important things. First, he noted the presence of a layer of sand in the soil below the surface. It became apparent that the layer had been laid down quickly because there were intact arrowroot plants below the sand, indicating that the sand had been laid down so quickly that it sealed the lower layer from the air, thus preserving the plants. Also, he found the existence of so-called ghost forests, stands of red cedar that died when exposed to salt water.  Atwater surmised that these two events were linked, the result of a sudden and rapid lowering of the land.

These two discoveries agreed with the idea that in a subduction zone earthquake, the land behind the fault subsided, or fell, thus making it vulnerable to seawater intrusion. Atwater then enlisted the help of David Yamaguchi, an expert in dendrochronology, the study of tree rings. The ghost trees above the marsh had rotted, so the rings from them were unusable. But nearby stands of living red cedars could give the date range they were looking for. Based on those rings, they postulated that the quake had occurred somewhere between 1690 and 1720. What narrowed the focus was the idea of burrowing beneath the water and hauling up the roots of the dead trees. There, preserved in the mud, were the right set of rings. After very careful work, Yamaguchi concluded that the last ring in those trees had been laid down in the summer of 1699.

Meanwhile, anthropologists had been collecting generational legends from the Native American coastal tribes, and the First Nation tribes in Canada. All of them had a common tale, a winter’s night when the earth shook and the ocean flowed inland like a river. One tribe described the event as a battle between a whale and a Thunderbird. Some of those tribes were completely erased from the land, and researchers working in those areas began to discover the remains of cooking fires at the soil levels that matched Brian Atwater's sand layers.  It was difficult to exactly date the event, given the vagueness of the chronologies, but it was clear that about three centuries earlier, a massive catastrophic event had occurred. Then, came the final piece of the puzzle.

Kenji Satake had been working on the Cascadia problem for several years. But in a meeting with Gary Carver, in which the narrowing range of dates were discussed, the year 1700 for some reason stuck in Satake’s mind. He went back to Japan and started going through historical records.

Japan is an ancient nation, and people have been keeping meticulous records since about 500 CE. In those records, Satake found the record of what was called an orphan tsunami -- so called because there was no accompanying earthquake – that occurred on the night of January 26, 1700. On that night, a 600-mile long wall of water crashed ashore along the central and northern coasts of Japan, measuring about 16 feet high. Large storms could not generate waves that large or that long. The source had to be a massive earthquake somewhere in the Pacific basin. There were earthquakes in South America in 1687 and 1730, both tsunamis recorded by the Japanese. Alaska was eliminated for the same reason, no accompanying evidence of a large shock on the day in question. By process of elimination, Cascadia had to be the source of the wave. The travel time for a tsunami, which travels at the same speed as a 747 across open water, showed that the wave’s transit time was about 10 hours. Thus with this evidence, scientists could now conclude that the last full-margin rupture of the Cascadia Subduction Zone occurred at 9:00 pm local time on January 26, 1700.

The good news was that an epic scientific investigation had been successfully concluded. The bad news was that this fault has been accumulating strain for the past 320 years. The time between quakes varies with the studies from 246 to 500 years, but what is painfully and ominously obvious is that this fault poses a dire threat to the west coast of North America, and almost every island in the Pacific Ocean.

This video is the wave model for the 1700 Cascadia tsunami.

Note the complexity of the waves as they spread across the Pacific.

(I had a hard time embedding the video in this post, so here is the URL:)

Tsunami Forecast Model Animation: Cascadia 1700

The prevailing question now is not the events of Cascadia's past, but instead what is very likely to come in the future.  This question currently occupies the minds of civil and emergency management officials.  The last time Cascadia ruptured, there were no cities on the west coast, and none in any of the island nations in the Pacific rim.  Now, there are major cities within range of both the seismic and wave effects.  Vancouver BC, Seattle, Portland, and Sacramento are cities full of high-rise buildings as well as unreinforced masonry structures.  The shaking would last for about 5 minutes or so, as the fault unzipped at a rate of 3 miles per second.  It is not known if  high rise structures in those cities would be able to withstand that kind of movement.  What experts anticipate is that the shaking will devastate those downtown areas, and wreak catastrophic damage throughout the urban and suburban areas.  Road, rail, air, and marine transportation systems and infrastructure would be wrecked.  That's disaster enough, if one were to stop there.  But about 15 to 20 minutes after the shaking, massive tsunami wave trains, perhaps 60 to 100 feet in height, begin to crash ashore.  Don't forget that the action of the rupture subsides, or lowers the land to the east of the fault.  That makes those areas even more vulnerable to inundation.  In Japan, one city had constructed a wave wall across the mouth of the harbor.  When the 2011 9.0 quake stopped, they closed the gate.  But subsidence dropped the land between three and six feet.  Thus the wave overtopped the now much-lower wall.  In a future Cascadia rupture, the damage and death toll along the coast would be unimaginably huge.  Whoever survived the quake might drown in the wave.  The resulting damage might isolate communities, even some of the cities from outside help for at least days, if not longer.  One Emergency Manager warned people that for the first few days after the disaster, they would be on their own.  The broken terrain would bar responders from getting through to the affected communities.

More good news.  Cascadia is tied into two triple junction fault systems, the Queen Charlotte complex off Vancouver Island at its north end, and the well-known San Andreas fault which connects at the Mendocino triple junction off the coast of northern California.  Scientists now say that historical events on the San Andreas were preceded by activity on the Cascadia.  Under certain circumstances, its possible that the rupture to the north would cause significant movement in the north and central sections of the San Andreas.  This expands the zone of damage to include much of the California coast, including the cities of San Francisco and Los Angeles.  It's quite possible that the entire west coast of North America would become a smoking rubble in the aftermath of such an event.

Also, scattered across the northwest, in a chain roughly parallel to to the fault are around 20 major active volcanoes which are directly fed by the subducting material, which melts, and then rises through the crust.  One of which you might remember, Mt. St. Helens erupted horizontally and devastated a very wide area.  Multiply the effects of St. Helens by 20, and the danger can be understood.  Now, there's no indication that a Cascadia event would produce multiple volcanic eruptions, and that's a good thing.  But it cannot be ignored that one or two could blow and the result would push the collective disaster even further. Imagine Seattle after the quake and tsunami, utterly devastated, deaths in the tens of thousands, many more trapped and injured.  And then suddenly Mt. Ranier explodes.  Lava and pyroclastic flows roll towards the remains of Seattle, while ash, smoke, and sulfur aerosols  spread through the atmosphere both locally and eventually across the rest of the western United States, initiating a volcanic winter.  It really sounds like an end-of-the-world scenario.

Even if the coast only had to deal with just the quake and tsunami, the devastation would be so widespread that emergency agencies, even FEMA, would be overwhelmed.  It would take years, perhaps decades for the western parts of British Columbia, Washington, Oregon, and Northern California to recover.  The cost?  Maybe in the hundreds of billions of dollars.

So far, we've only dealt with the effects on the landward side of the fault.  But there will also be another wave that will travel west.  A lot of island nations lie in the probable path of the tsunami.  Of special interest to this writer, the Hawai'ian Islands.  The NOAA model shows the wave trains striking the northeast shores of all islands, and then bending around the southern shores, striking the west-facing shores.  The exact wave height is hard to judge, but it seems to indicate wave heights of at least ten to 30 feet.  Again, the last time this happened, there were so few people living here, that there seems to be no record --written or oral -- of such an event occurring three centuries ago.  Recently, a local scientist found what appears to the fingerprints of a tsunami strike from about 500 years ago, but apparently no other such fingerprints have been discovered.

Scientists are now working closely with civil and emergency management officials to encourage building owners to reinforce their structures.  The sense of urgency is increasing, but there still seems to be the general attitude among the public that there is still time.  Predicting earthquakes isn't even in the infancy stage; more like fetal.  As one Canadian scientist said, "The only thing I can say for certain is that the next Cascadia earthquake is one day closer today than yesterday."