In 1964, a major earthquake struck the southern coast of Alaska. This quake triggered a tsunami that traveled through the Pacific Ocean, doing major damage in places including Hawaii and the US Pacific Northwest – places that would not have felt the earthquake. This quake followed a similar but much larger tsunami-generating earthquake in Chile in 1960, and it occurred at a key time for geoscience – the 1960s were the time when geologists were first beginning to understand how plate tectonics worked.
After the discovery of mid-ocean ridges, geologists began to realize that great quakes like the Chilean earthquake represented the counterpart – ocean crust is created at mid-ocean ridges, sinks back into the Earth at subduction zones, and occasionally these subduction zones trigger great earthquakes and major tsunami waves.
Once these processes were understood, scientists began working around the world to understand the parts of faults that can generate major tsunami waves. One site that began to be targeted in the U.S. was the Pacific Northwest, where a small piece of crust called the Juan de Fuca plate sinks beneath the continent and causes the formation of a volcanic arc.
After the Alaska quake, features along the shoreline characteristic of these earthquakes were characterized. When the quake happens, some portions of the land sink, often submerging and killing trees that have been growing for decades. Furthermore, large sand deposits that contain shells of ocean-dwelling organisms can be washed on shore by the waves.
In the 1980s scientists recognized that the faults off the coast of Washington, Oregon, northern California, and Canada could produce earthquakes, but it took the discovery of similar features to recognize that in recent history the Cascadia subduction zone has produced major earthquakes and tsunami waves. Along the coastlines there are dead trees slowly popping out of the ocean, and the bays contain similar sand deposits. By the 1990s, scientists using carbon dating methods were making it clear there had been a major earthquake and tsunami generated by this fault, but carbon dating is only precise to within a few decades.
Carbon dating on those dead trees and other sedimentary layers put the age of that earthquake in the range of the years 1695-1720. To get an exact date, scientists turned to a different type of record – Japanese history. Records from Japan recorded a tsunami wave that was considered an “orphan” – coming with no known earthquake, arriving on January 27, 1700 – 317 years ago today (the quake would have struck on January 26th and arrived the next calendar day in Japan). That tsunami age matches up perfectly with the age that those trees were killed; making it highly likely that tsunami started off the US Pacific Coast and traversed the ocean.
This simulation produced by the Pacific Tsunami Warning Center shows that tsunami wave, starting off the coast of Washington and Oregon as the Cascadia fault ruptures. It traveled up the bays and coastlines of these areas, inundating locations that are today home to thousands of people. In fact, Oregon State University recently built a marine science center in the zone that would be flooded by a similar tsunami if it were generated today, so this hazard is a major issue right now.
Ongoing research, including looking at much older sediments, has produced more detailed constraints about the behavior of this fault. When quakes happen they don’t always break the whole fault – sometimes a smaller section of it breaks, triggering a smaller tsunami, and sometimes larger sections break as seems to have happened in 1700.
The time in-between quakes also varies a lot; sometimes the fault will wait only a few hundred years before producing a major quake, sometimes it will wait thousands of years. It could be 500+ years before this fault ruptures again, or it could rupture again tomorrow. This is one of those cases where disaster preparation is important – you don’t know when it will happen, but planning for it is a must.
Scientists are still trying to better understand the behavior of large subduction faults like Cascadia. In 2011 a major quake and tsunami struck the Tohoku Coastline in Japan, in an area where there wasn’t yet agreement that a major quake was possible. Building codes and sea walls were not up to the task of holding back the waters, leading to enormous damage. In fact, researchers were just beginning to piece together evidence of potentially large, tsunami-generating quakes on that portion of the fault when the 2011 quake hit. Scientists worldwide are now working hard to study the dynamics of these faults to better predict which areas can produce devastating earthquakes like these, but there is much more work to be done before the mechanics of these faults are understood.
Understanding this orphan tsunami is a small piece in telling the story of this fault. It’s story isn’t done yet – it will rupture again – but when and how that rupture will happen is a tale that can’t be told.
Video credit: PTWC http://bit.ly/2kyuk7y
Major set of papers on this quake: https://pubs.er.usgs.gov/publica…/pp1707 Oregon construction: http://bit.ly/2k8Lh8a 2011 tsunami retrospective: http://bit.ly/2kyuI5Y Oregon dead trees: https://tmblr.co/Zyv2Js1dbFv2t Tsunami flood model: https://tmblr.co/Zyv2Js1eNhLSq