#megathrust

Subducting a rise The Nazca plate is mostly formed at the East Pacific Rise oceanic spreading center and consumed as it is subducted beneath South America, forming the modern-day Andes mountain range. At one point on the East Pacific Rise, a hotspot sits at the same place as the ridge, creating much larger underwater volcanoes than elsewhere on the plate. Some of those volcanoes even reach the surface, including Easter island.

Planning for a tsunami

Off the coast of Oregon and Washington, a piece of ocean crust known as the Juan de Fuca plate is being pushed down into Earth’s mantle at a gigantic fault known as the Cascadia subduction zone. That fault is a megathrust, the same type of fault that produced the 2004 Indonesia tsunami and the 2011 Japan tsunami.

That fault has a long record of producing giant earthquakes and tsunami, even though there aren’t written records from many of them except possibly for the most recent one about 300 years ago. Some of those tsunami are testified to by sand layers on shore, others by parts of the land which were submerged and then re-exposed as the crust bends (http://tmblr.co/Zyv2Js1dbFv2t).

This fault has produced massive earthquakes and tsunami before and it will do so again. With the recent tsunami disasters still in mind, governments continue funding projects to better understand the hazards of tsunami waves to populated lands.

Estuaries – the open bays at the end of many large rivers – can concentrate tsunami energy. The waves get inside them and can be focused, driving water levels upward and damaging harbors and cities throughout.

This image shows a model created by scientists at Oregon State University of a tsunami wave hitting the Columbia River. You can see many places where tributary rivers that enter the estuary are at risk from flooding – as much as 4 meters along coastlines inside the estuary.

These models are extremely useful in helping planning for future tsunami impacts. The areas at greatest risk for flooding are areas that should have building codes that could allow structures to survive a large tsunami – possibly even elevation above possible wave heights. These areas also should have active evacuation and warning plans in place, in addition to drills to get people used to hearing and responding to the warnings.

These models also are useful for showing the conditions that are most hazardous. Surprisingly, the high flows on the river in the spring don’t really impact tsunami heights that much on this river. On the other hand, daily tides do. If a tsunami hits at high tide, its impact is much worse than a tsunami that hits at low tide. That information can also be used in planning responses at the time an earthquake hits and in knowing what the maximum reach of the waves could be.

-JBB

Image credit: OSU http://oregonstate.edu/ua/ncs/categories/college-engineering

Recurrence

See the layers of sediments in the first of these 2 photos? They were deposited on a memorably bad day. These sediments come from a cave found on the edge of the island of Sumatra, and all of that sand and silt above the scouring layer was deposited on December 26, 2004; the date of the great Sumatran Earthquake and tsunami.

Those sediments were deposited by the tsunami, but they’re not the only tsunami remnants in this cave. This cave sits very near the shoreline and until recent uplift it was actually underwater, so it has taken multiple hits from tsunami waves over the last few thousand years. This cave was discovered a few years ago by scientists from Nanyang Technological University in Singapore and using results from this cave they have constrained the behavior of the Sumatran megathrust going back nearly 10,000 years.

The modern tsunami wave has an erosional scour at its base and the sediments just below it are 2900 years old – probably the date the cave was uplifted out of the water. The tsunami waves in 2004 entered the cave in pulses and dropped sand in layers on the surface. After trenching at this site, the scientists found 11 similar sand deposits in the sequence at this site. Each of them has thin layers of fine marine silt and clay in-between, showing that the sequence was a rapid deposition of sand followed by a long period of slow sedimentation.

Each of these 11 sand deposits represents a tsunami wave. The scientists carbon dated the layer at the bottom of their trench and found it to be 7400 years old, giving a recurrence interval of 450 years between 7400 and 2900 years ago.

That average would seem to be a statement of how often the fault breaks, but the scientists looking at the layers also found that average number to be almost meaningless – the fault doesn’t care about the average. They found that several thin layers of sand were packed close together, with as many as 3 smaller tsunamis within a 100 year period and as long as 2200 years of no sand deposits after one of the largest tsunami waves.

There are other geologic records around the Indian Ocean like this showing repeating tsunami waves, but none has as many waves recorded as this site and none of them are detailed enough to show the clustering. This site indicates that the Sumatran fault sometimes breaks in small earthquakes that trigger small tsunami waves and then occasionally breaks in a rupture like 2004 where it produces a major wave. Although there are only a couple examples in this cave, it is after big events like the 2004 quake where the fault is quiet for over a thousand years; so one possible interpretation is that the Sumatran fault may take centuries or millennia to produce major waves again.

This behavior is similar to the behavior observed at the Cascadia Megathrust off of Oregon, Washington, and British Columbia. Evidence from landslides and turbidites off the coast suggests that the fault there may break in a rupture that triggers a magnitude 8 earthquake and tsunami waves, but then occasionally the entire fault breaks, triggering a massive magnitude 9+ quake and much larger wave (https://tmblr.co/Zyv2Js29RKOmF).

There are often press reports saying a fault is “overdue” based on analyses of how often the fault moves on average, but these types of studies show that type of calculation just doesn’t express the way major faults move. The size of the quakes and size of the fault that breaks changes over time, and the exact future behavior is extremely difficult to predict.

The Sumatran fault is guaranteed to produce another large quake and tsunami. It could be thousands of years, or it could literally be less than 100 years. The best guess from this cave is that the huge 2004 rupture means it will be millennia before it goes again, but that's not the lesson of this cave. There are areas in the cave with 3 quakes in less than 100 years, and we don’t know for certain there won’t be another one soon.

Rather than measuring a recurrence interval, the lesson of this cave is that these megathrusts are very complicated and we’ve only seen the tiniest window of their history, so if the fault decided to release another quake and tsunami 20 years from now or 2000 years from now, humans in the area can’t be surprised. That lesson isn’t one for just this site either; it’s a lesson for faults around the world.

-JBB

Orphan Tsunami

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.

-JBB

Video credit: PTWC http://bit.ly/2kyuk7y

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