The 1700 Cascadia Megathrust Earthquake and the Future of Cascadia Margin

Cascadia earthquakes are triggered by the movement of North American and Juan de Fuca Plates and to a lesser degree the Pacific Plate. The motions cause shallow and deep earthquakes, some that have epicenters on the land and do not cause tsunami, and others that have epicenters in the ocean. These oceanic earthquakes, like the 1700 Megathrust Earthquake, have the potential to trigger large tsunami that arrive at the coast of Oregon and Washington in 15-20 minutes. This quake provides a warning for the future.

The map shows that there were crustal earthquakes in Washington in 900AD and 1982, deep earthquakes in 1949, 1965, and 2001, and a subduction zone earthquake in 1700. The following chart appears below the map:

Cascadia Earthquakes

Source	Affected Area	Max. Size	Recurrence
Subduction Zone	W. WA, OR, CA	M9	500-600 yr
Deep Juan de Fuca plate	W. WA, OR	M7+	30-50 yr
Crustal faults	WA, OR, CA	M7+	hundreds of yrs?

PRESENTER: On January 27th, 1700, Japanese chroniclers reported a mysterious wave, which struck the island of Honshu and caused widespread damage. The records referred to it as an orphan wave, because unlike most tsunamis on record, it had struck seemingly out of nowhere without being preceded by a detectable earthquake.

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About 12 hours earlier and more than 7,000 kilometers away on Vancouver Island, several villages of the Hesquiaht First Nation were completely destroyed while the inhabitants were sleeping. And 300 years later in 1986, paleontologists excavating along the Washington coast discovered a ghost forest of hundreds of trees that had all been killed by being suddenly submerged in saltwater. The orphan wave, the lost villages, and the ghost forest all had the same cause, a massive earthquake along the Cascadia subduction zone where the Juan de Fuca tectonic plate is being forced under the North American plate.

Further research has shown that there have been seven major earthquakes in the Cascadia subduction zone in the last 3,500 years with the time between major quakes ranging from 200 to 900 years. More than 300 years have passed since the quake of 1700, and the region has grown into an economic center home to more than 10 million people. We need to ask, what would happen if the next Cascadia earthquake happens tomorrow?

This is a worst-case scenario, and the odds of it happening in the near future are still quite low. It's been estimated at around a 12% chance in the next 50 years. But it's a real possibility. So let's imagine what would happen in the worst-case scenario for the next great Cascadia earthquake.

For this scenario, we're going to look at the impact of a magnitude 9.0 earthquake occurring on a weekday morning in late spring. Throughout the region, people are beginning their days. Students are arriving at school. And in the major cities of Vancouver, Victoria, Seattle, and Portland, businesses are gearing up for another normal workday. The major seaports and airports in the region will handle more than 200,000 passengers and $600 million worth of cargo by the end of the day.

At around 10:00 AM, after more than two centuries of building pressure, the Cascadia fault line finally slips and a 1,000-kilometer-long section of the North American plate moves 20 meters to the west, causing one of the largest earthquakes to strike the continent in more than half a century. Seismic waves race outward from the epicenter at 5 kilometers per second and reach the coast before moving inland. This means that people living far enough inland will start seeing posts about the earthquake on social media seconds before they feel the shaking start.

In order to understand the effect that the earthquake will have, we need to understand two different measurements-- magnitude and intensity. Magnitude is the measure of how much energy is released by an earthquake. A magnitude 9 earthquake like this one will release more than 2 exajoules of energy, equal to almost half a billion tons of TNT.

Intensity is a measure of how violent the shaking is at any given point. And here we actually get some good news. Since the epicenter of the quake is more than 100 kilometers off the coast, most cities will experience less violent shaking than other recent earthquakes. Here's an intensity map for the Northridge earthquake, which struck California in 1994, compared to an intensity map for a hypothetical magnitude 9.0 earthquake. The two big differences are that the Cascadia earthquake will be felt over a much wider area and it will last for much longer. The Northridge earthquake lasted for 20 seconds. The Cascadia earthquake will last for more than four minutes.

To get an idea of the impact that the quake will have, let's take a closer look at one of the largest cities in the area, Seattle, Washington, with a population of around 700,000. The intensity of the quake in this area is between a 6 and a 7 on the Mercalli intensity scale, which is associated with mild to moderate damage. Most of the large and recently built buildings in the city sustained fairly little damage.

Of greater concern is the 1,100 unreinforced masonry buildings in the city with more than 30,000 occupants. These buildings pre-date the current codes, and most of them haven't been retrofitted. Many of these buildings are prone to collapse. And those that don't collapse might cause injuries as a result of falling debris. Other risks facing the city include soil liquefaction, where areas of wet soil begin to behave like a liquid during an earthquake and cause damage to roads and foundations.

Thousands of landslides occur across the city, blocking roads and damaging buildings. Landslides, building collapses, and soil liquefaction occur throughout the Pacific Northwest. By the time the shaking stops, water, power, and cell service have been lost across most of the region. Nearly 1,000 highway bridges and overpasses have collapsed or are rendered unusable, cutting off-road access for dozens of communities. And the worst is still yet to come.

When the fault line slipped, it displaced billions of liters of water, creating a wave that begins to radiate outwards at 800 kilometers per hour. People living along the coast will have around 20 minutes to react. For those that have limited mobility or are cut off by damaged roads and bridges, options will be very limited. The height of a tsunami depends on the shape of the coastline. Because of the steep coasts in the region, the tsunami won't be as high as some other recent ones. However, at 10 to 15 meters, it's still high enough to completely inundate low-lying coastal communities and cause flooding up rivers more than 50 kilometers from the coast.

Over the next several hours, the wave will radiate outwards across the Pacific and cause damage as far away as Alaska, Hawaii, and Japan. Aftershocks continue possibly for months after the initial quake. Relief efforts are almost immediately mobilized from all levels of government, but they'll have their work cut out for them. It's immediately apparent that this has been one of the worst disasters to strike the continent in recent history.

Hundreds of thousands of buildings have been destroyed or damaged beyond repair. More than 100,000 people are left homeless, with many more being displaced. Up to 10,000 people might be killed. Around 70% of the deaths and injuries are a result of the tsunami, with the remainder being caused by collapsing buildings and falling debris. This would be the deadliest disaster to hit the United States or Canada in more than a century.

The cost is difficult to estimate, but the immediate cost could exceed $150 billion, making it one of the most expensive natural disasters in recorded history. The long-term economic impact could easily double that as the ports, airports, and corporations based in the area have their services interrupted for months.

Recovery is a long process. Efforts are focused on restoring essential services to population centers first. Seattle and Portland have their electricity and water fully restored after about six weeks. Meanwhile, the hardest-hit areas along the coast and isolated communities inland may be left without road access, utilities, and health care for up to three years after the quake. Thousands of the people who have been displaced may never return home.

The odds of this scenario occurring in the near future remain fairly low. But earthquakes are an inevitability, whether they happen a year from now or not for another century. So what's being done to prepare for this scenario? Funding has been allocated to bring thousands of vulnerable buildings, highways, and bridges up to code. But the process will take years or decades. Improvements have also been made to early warning systems.

With efforts from all levels of government and improvements to disaster preparedness and education, we have the best possible chance of preparing the Pacific Northwest for the next major earthquake. I've included links and resources in the description as well as links to the studies which were used to make this video. If you have ideas for future videos, leave a comment. And if you'd like to see more videos like this, click here to subscribe.