Tsunami Historical Series: Cascadia - 1700

Tsunami Historical Series: Cascadia - 1700 thumbnail


Just before midnight on January 27, 1700 a tsunami struck the coasts of Japan without warning since no one in Japan felt the earthquake that must have caused it. Nearly 300 years later scientists and historians in Japan and the United States solved the mystery of what caused this "orphan tsunami" through careful analysis of historical records in Japan as well as oral histories of Native Americans, sediment deposits, and ghost forests of drowned trees in the Pacific Northwest of North America, a region also known as Cascadia. They learned that this geologically active region, the Cascadia Subduction Zone, not only hosts erupting volcanoes but also produces megathrust earthquakes capable of generating devastating, ocean-crossing tsunamis. By comparing the tree rings of dead trees with those still living they could tell when the last of these great earthquakes struck the region. The trees all died in the winter of 1699 - 1700 when the coasts of northern California, Oregon, and Washington suddenly dropped 12 m (36 ft.), flooding them with seawater. That much motion over such a large area requires a very large earthquake to explain it - perhaps as large as 9.2 magnitude, comparable to the Great Alaska Earthquake of 1964. Such an earthquake would have ruptured the earth along the entire length of the 1000 km (600 mi) long fault of the Cascadia Subduction Zone and severe shaking could have lasted for 5 minutes or longer. Its tsunami would cross the Pacific Ocean and reach Japan in about 9 hours, so the earthquake must have occurred around 9:00 at night in Cascadia on January 26, 1700 (05:00 January 27 UTC).

The Pacific Tsunami Warning Center (PTWC) can create an animation of a historical tsunami like this one using the same tool, the Real-Time Forecasting of Tsunamis (RIFT) forecast model, that they use for determining tsunami hazard in real time for any tsunami today. The RIFT model takes earthquake information as input and calculates how the waves move through the world's oceans, predicting their speed, wavelength, and amplitude. This animation shows these values through the simulated motion of the waves, and as they race around the globe one can also see the distance between successive wave crests (wavelength) as well as their height (half-amplitude) indicated by their color. More importantly, the model also shows what happens when these tsunami waves strike land, the very information that PTWC needs to issue tsunami hazard guidance for impacted coastlines. From the beginning, the animation shows all coastlines covered by colored points. These are initially a blue color like the undisturbed ocean to indicate normal sea level, but as the tsunami waves reach them they will change color to represent the height of the waves coming ashore, and often these values are higher than they were in the deeper waters offshore. The color scheme is based on PTWC's warning criteria, with blue-to-green representing no hazard (< 30 cm or ~1 ft.), yellow-to-orange indicating low hazard with a stay-off-the-beach recommendation (30 - 100 cm or ~1 - 3 ft.), light red-to-bright red indicating significant hazard requiring evacuation (1 - 3 m or ~3 - 10 ft.), and dark red indicating a severe hazard possibly requiring a second-tier evacuation (> 3 m or > ~10 ft.).

Toward the end of this simulated 48 hours of activity, the wave animation will transition to the "energy map" of a mathematical surface representing the maximum rise in sea level on the open ocean caused by the tsunami, a pattern that indicates that the kinetic energy of the tsunami was not distributed evenly across the oceans but instead forms a highly directional "beam" such that the tsunami was far more severe in the middle of the "beam" of energy than on its sides. This pattern also generally correlates to the coastal impacts; note how those coastlines directly in the "beam" have a much higher impact than those to either side of it.

The full report about the Orphan Tsunami of 1700 can be found here.

For a YouTube version of this animation, please see: https://youtu.be/4W2iUl0VB8c


Notable Features

  • Tsunami generated by a ~9.2 magnitude Cascadia earthquake in 1700 along a 600 mile long fault from Vancouver Island, British Columbia to northern California.
  • A tsunami is a series of waves, not just a single wave.
  • In the open ocean, tsunami waves can travel at speeds up to 500 miles an hour, as fast as a jet plane.
  • Tsunami waves may be small in the open ocean, but wave heights can increase substantially as they approach the shore (indicated by the colored dots).
  • A majority of coastal locations in the Pacific experienced significant hazards (indicated by the red dots) due to this tsunami.
  • The wave "energy map" of maximum wave heights show that those coastlines directly in the energy "beam" of red/yellow had a much higher impact than those to either side of it.

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