The Danger that Lurks Beneath the Sea
The surface of the earth is a bit like a cracked egg. The Lithosphere, which forms the crust and upper mantle of the earth, is broken up into various sized pieces called Tectonic Plates. There are considered to be eight major plates around the earth, and many more minor ones.
Each major plate varies between about 50 and 250 miles (80 and 400 km) in thickness. Although we are normally unaware of it on the surface, these plates constantly stretch, move, slide and collide with one another. In doing so, faults can develop along the boundaries of these plates.
There are different types of faults, depending on the direction of the movement of these plates. Plates move as a result of a process called Continental Drift, which is based on the concept of seafloor spreading that was accepted by the geo-scientific community in the late 1950’s and early 1960’s.
It is the movement of these tectonic plates that leads to such geological phenomena as ocean trench formation along plate boundaries, as well as volcanic activity and mountain-building. But by far the most destructive feature of plate tectonics is the manifestation of earthquakes.
There are four different types of earthquakes. They are explosion earthquakes, collapse earthquakes, volcanic earthquakes and tectonic earthquakes. Explosion earthquakes are those that are set off by the detonation of nuclear or other types of explosives.
Collapse earthquakes occur in underground caverns and mines as a result of falls and other movements of the underlying rock. Volcanic earthquakes are those that are generated by volcanic activity. But by far the most destructive earthquakes are those that are caused by the movements of tectonic plates.
Ring of Fire
Although it is commonly referred to as a “Ring of Fire”, this region of the Pacific Ocean is actually shaped more like a horseshoe. It is the most seismically active region on the planet, and it accounts for about ninety percent of all the world’s earthquakes in any given year.
This Ring of Fire extends over a distance of some 25,000 miles (40,000 kms), and is associated with a nearly continuous series of ocean trenches, volcanic arcs and volcanic belts, as can be seen from the diagram below.
The Pacific Ring of Fire
This belt of fire flanks all the countries that border on the Pacific Ocean, and contains 452 active and dormant volcanoes. These volcanoes are the direct result of plate tectonics, as a result of the movements and collisions of these different plates.
Subduction Zones
Subduction zones occur wherever two tectonic plates collide with one another, in a way which causes one plate to ride over the top of the other. What happens is that the younger plate rides over the older plate because it is less dense.
The older, heavier plate then bends and plunges steeply into the earth. As it does so, it forms a trench that can be as much as 70 miles wide, over a thousand miles long, and several miles deep.
An example of this is the Marianas Trench near the Marianas Islands, which is the deepest sea floor in the world, descending to 36,000 feet below sea level. It is caused by the Pacific Plate moving underneath the leading edge of the Eurasian Plate. But this is not the only example.
Off the Pacific coast of South America, the Nazca Plate is being subducted beneath the westward moving South American Plate, while the Cocos Plate is being subducted beneath the Caribbean Plate in Central America.
Farther to the north, a portion of the Pacific Plate, together with the small Juan de Fuca Plate is being subducted beneath the North American Plate. And to the west the northwest-moving Pacific Plate is being subducted beneath the Aleutian Islands.
Finally, off the eastern coastline of Japan, the Pacific Plate is being subducted beneath the Okhotsk Plate. The most significant feature of these various subduction zones is that they generate Megathrust Earthquakes, which are the most destructive kind.
Megathrust Earthquakes
As the name suggests, megathrust earthquakes are capable of generating gigantic forces within the surface of the earth, which is the reason for their destructive power. Since the year 1900, there have been six megathrust earthquakes, and all have been rated above 9.0 on the Richter scale.
While a seismograph is a device used by scientists to detect movements in the earth, the relative magnitude of these movement is measured on a sliding scale, called the Richter scale. Because this scale is open ended, there is no upper limit to the size of earthquakes.
The Richter scale is a measure of the energy released by an earthquake, and is measured in increments of ten. What this means in practice is that an earthquake measuring 4 on the Richter scale is ten times greater than one rated as a 3, while an 8 is ten times greater than a 7.
As a guide, earthquakes rated below the number 5 are generally considered to be minor. Those between 5 and 7 are considered to be moderate, while earthquakes measuring between 7 and 8 are regarded as major, and those above 8 are considered to be catastrophic.
All megathrust earthquakes occur within subduction zones, where one tectonic plate is forced underneath another. As can be imagined, the subduction process can generate colossal forces within the earth, particularly when two colliding plates become locked together.
When this happens, instead of moving smoothly past one another, the overhanging plate begins to buckle as the pressure of their respective movements builds. Finally, when the pressure becomes too great, the plate boundary ruptures and a megathrust earthquake occurs.
The effect of this sudden release of pressure beneath the sea bed is to release a series of seismic waves that radiate outwards from the fault in all directions. When these pressure waves reach the surface, they shake the ground, and it is these convulsive movements of the earth that cause all the damage above the ground.
Tsunamis
When these megathrust earthquakes occur beneath the sea, the sudden release of energy distorts the sea bed itself. The pressure waves then generated by this release of energy cause a succession of massive ocean waves called Tsunamis, which then fan out in all directions from the source.
The word Tsunami is a Japanese term which literally means “Harbour wave”. It gained this meaning because it is seldom noticed by fishermen operating out at sea, but only becomes evident once it approaches the land – hence the use of the term “Harbour wave”.
A Tsunami is a series of ocean waves generated by the pressure waves of an undersea earthquake. When they strike the land they can cause widespread destruction of coastal communities. About 80% of all Tsunamis occur within the Pacific Ocean’s “Ring of Fire”.
A Tsunami can travel at the speed of a modern airliner, or about 500 miles (800 kms) an hour. At that speed it can race across the entire Pacific Ocean in less than a day. While in deep ocean Tsunami waves may be hardly noticeable, but as they approach the land they grow in energy and height.
As a Tsunami approaches the coast, the shallow waters cause the waves to slow down in speed to about 50 miles (80 kms) an hour. However, this also causes them to grow in size, where they have been known to reach up to hundreds of feet in height.
Because a Tsunami’s trough, or low point between waves, can sometimes reach the shore first, this produces the well-known vacuum effect, where coastal water is sucked out to sea, exposing the sea floor. This can often be a vital warning sign before the arrival of the wave itself.
Residents of coastal communities need to know that Tsunamis are usually composed of a series of waves, so the danger does not end once the initial wave strikes. In fact successive waves often grow in destructive power, making it even more important for people in the area to escape to higher ground.
The Great Chilean Earthquake of 1960
The greatest earthquake ever recorded took place on a Sunday afternoon on May 22, 1960, off the coast of Chile, and measured 9.5 on the Richter scale. It has come to be known as the Great Chilean earthquake, or the Valdivia earthquake, as Valdivia was the city that experienced the most damage.
This megathrust earthquake was caused by a rupture under the sea of the Nazca plate subducting beneath the South American plate. The shaking continued for almost ten minutes in places, and was so violent that people were unable to stand, and many experienced severe motion sickness.
The undersea rupture at the boundary of these two plates occurred at a depth of 20 miles (33 kms), and was between 530 and 625 miles (850 and 1,000 kms) in length. The width of the undersea rupture was 125 miles (200 kms) wide.
It was this sudden release of energy that generated the main Tsunami which then radiated outwards in all directions, as well as a series of localised Tsunamis which battered the Chilean coast with waves of up to 82 feet (25 metres) in height.
The main Tsunami raced across the Pacific Ocean and devastated the town of Hilo in Hawaii, killing 61 people. Waves as high as 35 feet (11 metres) were recorded 6,200 miles (10,000 kms) away, affecting countries as far away as Hong Kong, the Philippines, Japan, New Zealand and Australia.
The earthquake also triggered numerous landslides in coastal Chile, particularly in the glacial valleys of the southern Andes. In the city of Valdivia, it was estimated that about 40% of the houses were destroyed, leaving some 20,000 people homeless.
The earthquake was also believed to have led to the eruption of the Cordón Caulle volcano some thirty-six hours later. Powerful aftershocks continued to be experienced throughout the region for many weeks after the initial earthquake.
The Alaska Earthquake of 1964
The Great Alaskan Earthquake of 1964 is also referred to as the Good Friday earthquake, as it struck the city of Anchorage, Alaska, at about 5:30pm on Good Friday, March 27, 1964. It was the second most powerful earthquake ever recorded.
Like the one that had occurred four years earlier in Chile, this was a subduction zone megathrust earthquake caused by the Pacific Plate being pushed beneath the North American Plate. It measured 9.2 on the Richter scale, and the ground continued shaking for four and a half minutes.
The undersea rupture occurred at a depth of 15 miles (25 kms) below sea level, and affected an area between 500 and 530 miles (800 and 850 kms) in length. This rupture was even greater than that of the Chile earthquake, as it had a slip width of 125 miles (250 kms).
The main undersea rupture point generated Tsunami waves that were reported from over 20 countries, including other parts of Alaska, British Columbia, Washington, Oregon and California, as well as Peru, New Zealand, Papua New Guinea, Japan and Antarctica.
106 people died as a result of Tsunami waves in Alaska, five in Oregon, and 13 in California. The largest Tsunami wave was recorded in Shoup Bay, Alaska, where the wave reached a height of 220 feet (67 metres). Many coastal villages along the Alaskan coast were swept away.
In the city of Anchorage itself there was enormous damage, particularly in the downtown area. Houses, buildings and other infrastructure (streets, sidewalks, water and sewer mains, electrical systems, etc), were virtually torn apart by the violent shaking of the ground, and the aftershocks that followed.
In the first 24 hours after the initial rupture, eleven major aftershocks were recorded measuring 6.2 or more on the Richter scale, and nine more of similar magnitudes over the next three weeks. In fact, diminishing aftershocks continued at sporadic intervals for almost a year afterwards.
The Japanese Earthquake of 2011
When it comes to the damage caused by Tsunamis generated by megathrust earthquakes, the images that immediately come to mind for most people are the Boxing Day event off the coast of Sumatra in 2004, and the destruction caused by the Japanese earthquake in 2011.
The megathrust undersea earthquake that struck off the Japanese Island of Honshu, took place at 2:46pm local time on Friday, March 11, 2011, and measured 9.0 on the Richter scale. It was the most powerful earthquake to have hit Japan since modern record-keeping began in 1900.
The undersea rupture occurred at a point where the Pacific Plate was being pushed beneath the Okhotsk Plate, about 43 miles (70 kms) east of the Oshika Peninsula, at a depth of about 19 miles (30 kms). The rupture affected an area 310 miles (500 kms) long and 125 miles (200 kms) wide.
In many areas, the ground continued to shake for more than six minutes. The earthquake triggered Tsunamis with waves that reached heights of up to 130 feet (40 metres) in places. Some of these waves travelled up to 6 miles (10 kms) inland.
The resulting Tsunamis devastated large areas of the Pacific coastline of Japan. Thousands of lives were lost when entire towns were swamped. The waves reached as far as the west coast of both North and South America, but due to prior warning by the authorities, relatively little damage was done.
It was a different story in Japan, where the size of the waves exceeded the retaining walls built to protect citizens in many places, causing immense destruction to buildings and infrastructure, and loss of life. In total, almost 16,000 people died in the disaster, with another 9,000 injured or missing.
According to subsequent analysis, some five hundred thousand buildings were either partially or completely destroyed, along with extensive structural damage to roads and railways. Numerous fires continued to burn for days afterward, and a dam collapsed as a result of the damage sustained.
Worst of all was the meltdown of three reactors at the Fukushima Daiichi nuclear power plant, and the escaping radiation led to the imposition of an exclusion zone of 12 miles (20 kms) around the plant, as well as the evacuation of over 200,000 people.
Estimates of the total damage caused by both the earthquake and the resulting Tsunamis have been estimated by the World Bank to be in the region of $235 billion, making it the costliest natural disaster in world history up to that time.
The Cascadia Subduction Zone
As we have seen, the Ring of Fire surrounding the Pacific Ocean has been the source of some of the most powerful earthquakes ever recorded. These undersea earthquakes have in turn generated Tsunamis that have devastated entire cities and caused immense damage to the areas affected.
All of these earthquakes have been caused by convulsive ruptures at the boundaries between various tectonic plates. Yet of all the subduction zones that have ruptured in recent years, none has been as potentially catastrophic as the zone that lies off the west coast of North America.
This zone is referred to as the Cascadia Subduction Zone. This megathrust fault is a long dipping fault where the Juan de Fuca Plate is being pushed beneath the North American Plate. It extends over a distance of 625 miles (1,000 kms), stretching from Northern Vancouver Island off the coast of British Columbia, Canada, down to Cape Mendocino in California.
The width of the Cascadia Subduction Zone varies along its length, depending on the temperature of the subducted oceanic plate, which heats up as it is pushed deeper beneath the continent. As it becomes hotter and more molten, it eventually loses the ability to store mechanical stress, which then manifests as a megathrust earthquake.
Because there had been no record of any powerful earthquake in the region ever since the first Western navigators had begun to explore the Pacific Northwest almost 300 years ago, it was assumed that there was no subterranean threat, and that the undersea plates moved smoothly over one another.
However, when geologists discovered that red cedar trees had been killed by the submergence of coastal forests into the tidal zone, they began to suspect that this might have been the result of a Tsunami generated by a large undersea earthquake in the distant past.
Further analysis of their tree rings indicated that the trees had stopped growing in the year 1699. This was then matched with an ancient Japanese record that showed that an “orphan” Tsunami (a Tsunami without an accompanying earthquake), had struck the coast of Japan on January 26, 1700.
It was only then that scientists realised that the Cascadia Subduction Zone had the potential to cause vastly more damage than all of the other vulnerable areas of the Ring of Fire combined.
That is because, in the period of time since the last Cascadia megaquake took place, the entire length of the fault zone had become the focus of enormous population growth and industrial activity. So all of this would now be at risk when the next megaquake occurred.
Potential Damage
Analysis of the subsurface soil along the coast of the Pacific Northwest, reveals that there have been six mega-quakes along the Cascadia Subduction Zone over the last 2,500 years. These are estimated to have occurred in 1700 AD, 1310 AD, 810 AD, 400 AD, 170 BC and 600 BC.
Seismologists now believe that when the Cascadia Subduction Zone next ruptures, there will be a subsea earthquake measuring 9.0 or greater on the Richter scale, that will generate mega-Tsunamis that will strike the coast of Northern British Columbia all the way down to California.
Although they claim that the major coastal cities of Vancouver, Seattle, Tacoma and Portland are located on inland waterways that would be sheltered from the full brunt of these Tsunamis, the same would not be true for Victoria on the Southern tip of Vancouver Island, where the Tsunamis could reach heights of 100 feet (30 metres) or more.
However, all of these coastal cities do have many vulnerable structures such as bridges and unreinforced brick buildings. Most of the damage would be the result of the earthquake itself, especially as the ground shaking is predicted to last for a full six minutes or more.
What these seismic ground waves would do to a modern skyscraper for that length of time is still an open question. While many are no doubt capable of withstanding such lateral forces, many more are not, and these would contribute to the heavy overall damage and loss of life.
What is certain is that the regional transportation routes would be wrecked. Highway 101 along the Washington, Oregon and California coasts would be destroyed by the Tsunamis, while Interstate 5 would be inoperable due to collapsed bridges and overpasses all along the route.
So when will the Cascadia Subduction Zone next rupture?
Seismologists have as yet no way of predicting this. All they can do is speak of percentages and probabilities. However, all are agreed that it might not happen for another hundred years or more. Or it may happen tomorrow.
Either way, residents of the Pacific Northwest would do well to take the necessary precautions now, in the knowledge that certain disaster awaits them at some point in the future. It lurks beneath the waves that wash along their shores.
