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Roger Weller, geology instructor
by Carolyn Harris
Cascadia’s Coming Calamity
comes to the subject of earthquakes in America, we automatically think of
Northridge and other quakes that have afflicted the state of California in the
past. We hear a lot of talk about the San Andreas Fault and the impending “big
one” that is bound to occur in the future and, indeed, there is no doubt that
when it does occur it will cause tremendous devastation at a local or regional
level. However, there is an even greater, more ominous threat quietly looming
over us in the not so distant future that very few people are even aware of; an
event that could wreak havoc on a global scale, affecting not only North America
but also millions of others who live in its danger zone. The source of this
impending calamity lies beneath the Pacific Ocean approximately sixty miles off
North America’s west coastline—the Cascadia fault.
A massive earthquake in this fault or subduction zone would inflict mayhem over a huge geographical area, causing widespread destruction of villages all along the Pacific Northwest coastline, damaging critical infrastructure in many nearby cities to include Seattle and Portland, and sending a train of tsunamis racing across the Pacific Ocean at over 400 miles per hour toward Japan, Indonesia and Australia. The Cascadia fault is a long, sloping crack in the earth’s crust, approximately 680 miles in length, that stretches from northern Vancouver Island and runs southward all the way to northern California. It is a part of the notorious circum-Pacific seismic belt or “Ring of Fire” in which most of the world’s disastrous earthquakes and volcanoes take place.
Subduction Zone Earthquakes
zones are formed due to plate tectonics. The earth’s crust, or lithosphere, is
fragmented into several large, rigid plates that slowly move in different
directions independently of each other. When a plate that is oceanic
lithosphere collides or converges with a continental plate, a subduction zone
will form. The oceanic lithosphere is denser and thinner than the continental
lithosphere and and is forced downward or subducted into the earth’s mantle to
be recycled and melted by the regions high temperatures (around 150-300 km below
the surface). As noted in the diagram below, there are three significant
characteristics associated with subduction zones: A deep ocean trench, a
volcanic arc, and a plane of concentrated earthquake activity.
all, a deep ocean trench occurs where the oceanic plate bends downward for its
descent into the mantle. The angle of subduction is largely related to the
temperature and density of the subducting slab. The older and colder the slab,
the steeper the angle of subduction and the faster it sinks, speeding up the
rate of convergence. The younger and warmer the oceanic slab, the angle of
subduction will be more shallow. There is also a greater potential for the
plates to become “locked” or stick together and, as a result, build up intense
elastic energy over time, sometimes for centuries. Therefore, shallow angle
subduction zones tend to be more violent in their seismic activity when they
become “unlocked”. Cascadia is a shallow angle Subduction Zone that is formed
by the Juan de Fuca oceanic plate subducting underneath the westward moving
North American continental plate at the rate of 40 mm a year.
Secondly, chains of volcanic mountains (a volcanic arc) form on the overriding plate and run parallel to the deep ocean trench. The melting and recycling of the subducted ocean crust allows for this molten material to eventually force its way back up to the earth’s surface via volcanic activity. The range of volcanoes between Mt. Lassen in Northern California to Mt. Garibaldi in Canada (The Cascade Mountains) are a result of this phenomenon. Cascadia’s volcanoes, like most subduction zones with volcanic arcs, form a line that is roughly parallel to the coast about one hundred miles inland with the cones spaced about forty-five miles apart.
subduction zones create a plane of concentrated earthquake activity, more
shallow near the fault line but becoming deeper as they occur further inland
away from the trench. Three kinds of earthquakes are produced: those in the
upper plate (crustal faults), those in the lower plate (deep faults), and those
between the plates. The first two category earthquakes occur every several
decades and could produce seismic events up to magnitude 7 (M7), comparable to
that of Northridge in 1994. However, it is the third category that concerns
officials the most. These subduction zone quakes, produce M8 and M9 events that
release hundreds of times more energy and are by far the largest, most violent
earthquakes in the world (megathrust earthquakes). To put this in perspective,
an M6 earthquake has 1 megaton of seismic energy, M8 earthquakes have 1,000
megatons of energy and M9’s have an unbelievable 32,000 megatons of seismic
energy. Magnitude 9 quakes differ from smaller ones because they last longer
(several minutes) and have more low-frequency energy, thus causing more
destruction. The low frequencies are responsible for triggering landslides,
damaging large structures and creating devastating tsunamis. Earthquake size is
also proportional to fault area and the Cascadia fault is long. It could easily
produce an earthquake magnitude 9.0 or greater if the rupture were to occur over
the entire area.
of the world’s earthquakes occur along the shores of the Pacific Ocean, which
coincides largely with the edges of the huge Pacific tectonic plate. (It is
known as the “Ring of Fire” because 75% of the world’s volcanoes are also
located here). Major seismic events have taken place on the three corners
of the Pacific plate in the last three years—Chile in the southeast (M8.8) on
February 27, 2010; New Zealand in the southwest on February 22, 2011; and Japan
in the northwest (M9) on March 11, 2011. Only the northeast corner remains
ominously “quiet” for the time being.
The diagram above bears out the fact that, in the last 50 years, no megathrust quakes have occurred in the Cascadia fault. In fact, for all of recorded history, the Pacific Northwest has not had the massive earthquakes that so commonly occur in subduction zones around the Ring of Fire. For many years, the scientific community held to the belief that Cascadia was not a significant earthquake threat. They thought that Cascadia was “the exception to the rule.” There were technical papers published that pointed to the lack of a deep trench in the sea floor where the two plates converged. Some experts claimed that there was no sloping zone of earthquakes from a seafloor slab making its slow descent under the coast. Somehow the subduction process had simply run out of energy and stalled, so they thought. However, with the eruption of Mt. St. Helens in 1980, a major shift in scientific opinion began to appear. It was obvious to a handful of scientists that the subduction process in Cascadia was alive and well and there was a significant risk for future disasters. These men set out to prove their theories with scientific data.
Convinced that the
landscape of the Pacific northwest was being deformed by active subduction of
the Juan de Fuca plate, meticulous surveying and releveling of highway survey
markers was repeatedly performed over a period of years by seismologist John
Adams and other scientists. Several other types of geodetic measurements,
including GPS, were methodically used as well. The data seemed to indicate that
the coastline was lifting and the entire block of coastal mountains was tilting
to the east. After years of repeated, precise measuring, it was revealed that
convergence was occurring at about 40 mm/year, however, the
city of Victoria was moving landward at a rate of only 7 mm/yr with respect to the stable North American continent. city of Victoria was moving landward at a rate of only 7 mm/yr with respect to the stable North American continent.
At least part of the subduction fault appeared to be locked, causing shortening and buckling of the coastal region. The top diagram above illustrates uplift and shortening. If the fault is locked, ongoing convergence drags down the seaward edge of the continent and produces an upward bulge farther inland causing a buildup of elastic stress that bends and buckles the continental crust. The bottom diagram illustrates subsidence, extension, and rupture. At the time of a great earthquake, there is an abrupt slip of the locked plate and the stored elastic energy radiates as earthquake waves. The edge of the continent springs back up and the bulge collapses. This abrupt uplift of the outer continental shelf is responsible for the great tsunamis while the collapse of the bulge farther inland causes sudden coastal subsidence, which eventually shapes coastal marshes. The fault will then relock and the cycle begins all over again. The longer the intervals between cycles, the larger the stress buildup and the larger the earthquake slip.
Cedars and Peat in Coastal Tidal Marshes
Footprints of catastrophic earthquakes and tsunamis are also evident
in hundreds of coastal tidal marshes throughout the Pacific Northwest. Dozens
of geologists, armed with shovels and chain saws have sliced into the peat to
look for clues to the past. Excavation of these marshes reveals peat layers of
former marsh vegetation that were submerged by salt water due to
subsidence at the time of past great earthquakes. After the earthquakes, mud
accumulated on the drowned marsh, eventually building the level back up to
mid-tide levels where the marsh vegetation again became re-established. Evidence
also shows that many of the buried marshes were entombed by sand layers—sand
that was carried in by the waves of great tsunamis that rushed into the subsided
coastal region. Salt water marshes were formed killing plants and trees that
could not live in the salty environment, as evidenced by the ghosts
forests of cedars and other dead trees we find today. Geological records
indicate that there was not one unique event, but repeated cycles at intervals
over thousands of years. It is believed that at least 13 or more massive
earthquakes have occurred in that region in the last 6000 years. Radiocarbon
dating placed the most recent event during the late 1600’s or early 1700’s.
Series of Diagrams Showing Submergence of a Tidal Marsh and Forest During a Cascadia Earthquake
The succession of sediments produced by these events reveals the geological signature of a great earthquake followed by a tsunami. Source: Clague, J.J., Yorath, C.J.J., Franklin, R., and Turner, R.J.W. 2006. At Risk: Earthquakes and Tsunamis on the West Coast. Tricouni Press, Vancouver, 200 p.
Turbidite Core Samples
Turbidite core sampling also presented evidence for periodic sea flow landslides caused by catastrophic events. In 1971, graduate students of Oregon State University took core samples of ocean floor sediments off the Oregon Coast and found that the turbidite layers showed periodic widespread deep sea landslides that appeared to occur simultaneously far apart along the continental coastline, carrying huge amounts of sediment from the continental slope to the deep sea floor. The only logical explanation seemed to be evidence of very large prehistoric quakes in which the seismic shocks caused the widespread phenomenon. The chronology of seismic events in the core samples seemed to be in line with the data that was obtained from the research going on in the coastal marshes. The most recent turbidite event found in the cores was about 300 years ago. The intervals between the last 13 events ranged from 300 to 900 years.
With scientific evidence mounting, John Adams decided to present the data to a convention of geologists and scientists meeting in 1987. Amazingly, there was no opposition…no argument. The presentation of scientific data was so solid that most of the scientists went away with a fundamental change of thought—Cascadia was definitely a force to be reckoned with. It was no longer a question of “if” but “when” the next great event would take place. Scientists felt that if they could pinpoint the exact year of the last event, it could help narrow down predictions for the time frame of a future event.
Oral History of Native American Peoples
Unfortunately, there was no written history to help scientists in their search for answers. However teams of mud, marsh and sand diggers had found evidence of people living along the coast when the rupture zone last tore apart. They found fire pits full of charcoal and woven mats and clam shells. Upon examining the oral history of the Native American peoples that lived in the area at the time we do find myths and legends relating an epic struggle between Thunderbird and Whale (supernatural beings), a “shaking” and “jumping” of the earth—“a rolling back of the waters”. Other stories relate that in the period not long before European contact, a strong earthquake occurred on a winter’s night. It was followed by a large tsunami that destroyed the village at the head of Pachena Bay on the west coast of Vancouver Island. There were no survivors. Only a neighboring village up the hill survived to pass the tale on to future generations. In another account, the “canoes came down in the trees.” The Cowichan people on Vancouver Island experienced shaking that collapsed their houses and caused numerous landslides. The shaking was so violent that people could not stand and so prolonged that it made them seasick. These events are recorded in the oral traditions of the First Nations people on Vancouver Island. Although these stories tell us a time of day (night) and season of the year (Winter), they did not bring scientists any closer to a specific date for the earthquake.
At this point, the search led scientists across the ocean to Japan. Kenji Satake had received a doctorate from Caltech where he had studied Cascadia’s quake and tsunami. He was on a trip to Japan in 1994 when he had a “eureka” moment. If the Cascadia tsunami swept across the Pacific ocean, perhaps there would be a record of it somewhere in Japan. After all, the Japanese had been keeping meticulous records of earthquakes and tsunamis as far back as the 7th century. Satake found what he was looking for. There were records of an “orphan tsunami” on January 27th of the year 1700 with wave heights of 2 to 3 meters documented for five sites along the coast of Japan. (Recent computer simulations put the wave heights up to 6 meters). Japanese accounts relate houses being destroyed and fires breaking out. The earthquake did not appear to be caused by any local disturbance. Satake looked into possible sources other than Cascadia, but through the process of elimination, eventually ruled them all out. After taking into account the time it would take for a tsunami to travel across the Pacific Ocean and correcting for time zone difference, he concluded that the source of the tsunami, the great Cascadia earthquake, must have occurred along the North American coast on January 26 at about 9 p.m. The time of night and season of the year were in agreement with Native American folklore.
Tree Ring Studies
In an attempt to make the data conclusive with corroborating evidence, further studies were performed on the ghost forests of cedar trees submerged in the tidal marshes of the Pacific Coast. More recent studies of the tree ring datings suggested that they grew their final layer of wood between August of 1699 and May 1700. This finding lended credence to the Cascadia earthquake occurring in January of 1700 as well.
All the paleoseismic research for Cascadia shows that earthquakes occur in clusters of up to five events, with an average time interval of 300 years between quakes. The two most recent quakes on this fault occurred in the year 1700 (a magnitude 9 event) and approximately the year 1500. It has now been 312 years since the last event. We don’t know if the current cluster of earthquake activity is over or if it has another event left in it. If the latter is true, we are living on borrowed time. The question is whether it will be a full length rupture as it was in 1700. We also need to ask ourselves if we are prepared for such a quake.
Cascadia is virtually identical to the offshore fault that devastated Sumatra
in 2004. The Sunda Trench is almost the same length, the same width and with the
same tectonic forces at work. More than 230,000 people in countries all around
the Indian Ocean died or disappeared in that quake. In some places the tsunami
swells topped more than a hundred feet. Jerry Thompson, in his book,
“Cascadia’s Fault,” makes the claim that the Cascadia fault can and will
generate the same kind of earthquake that Sumatra experienced: magnitude 9 or
higher. It will slam 5 cities at once: Vancouver, Victoria, Seattle, Portland,
and Sacramento. It will cause physical damage as far south as San Francisco.
It will cripple or destroy smaller towns and coastal villages all along the west
coast from Vancouver Island to Eureka in northern California. Not only that, he
claims that there is evidence from mud-core samples to suggest that an
earthquake along this subduction zone could transfer enough stress to trigger
the San Andreas fault at the point where the two faults connect, about three
hundred miles north of San Francisco. Cascadia tsunamis will slam the beaches of
the west coast of North America as well as Alaska and Hawaii (xix). He goes on
to say that, In 1982, the National Oceanic and Atmospheric Administration (NOAA)
estimated that 900,000 people would be a risk from a fifty-foot Cascadia tsunami
striking our western seaboard (xx). (More recent computer simulations have
projected that tsunami waves could even be as high as seventy to ninety feet).
One can only imagine the death and destruction that will also occur around the
Pacific rim nations as the tsunami makes its way across the ocean.
We don’t know how long we have until this “sleeping giant” awakens. Hopefully, we can become more aware and informed of this potential threat. Hopefully, we will learn from past experiences with earthquake disasters. And most of all, hopefully we will make plans ahead of time how we will deal with disaster when it when it does strike.
Thompson, Jerry. Cascadia’s Fault: The Coming Earthquake and Tsunami That Could Devastate North America. Counterpoint. Berkeley. 2011. Print.