- This is a graveyard.

These trees mark the site of a massively destructive natural disaster, one that this region is due for again soon.

(tense music) Long ago here in the Pacific Northwest, a giant earthquake struck the Cascadia fault, a seismically active tectonic plate boundary off the coast.

Almost instantly, the landscape dropped by five or six feet, unleashing a powerful tsunami.

This wall of water and sediment covered huge swaths of land leaving behind these drowned dead trees as a mark of its earth-shattering power.

This was hundreds of years ago, but it might surprise you that we know exactly when this happened.

9:00 PM Pacific Standard time on January 26th, 1700.

It was a Friday.

This is a story about the clues buried in the trees, the soil, and even records kept by Japanese monks that scientists put together to solve the mystery of what happened here and when.

How were they able to reconstruct one of the most violent disasters to ever hit North America?

Knowing this answer is important because this will happen again, but will we be ready?

(tense music) (waves lapping) (gentle music) - [David] If you're in a small boat and you go out at low tide, you can look up at buried landscapes below the modern high tide.

When these great earthquakes happened, they either raise the landscape or they drop it suddenly.

- [Joe] That's David Yamaguchi.

He was one of a handful of scientists who helped pinpoint the magnitude and date of the 1700 earthquake.

- So this is the pre-1700 buried soil.

Again, I don't know if you can see it on film, but I could feel it and I can see it by eye.

- [Joe] Ocean tsunami waves are not just salt water.

They carry with them marine and coastal sediment that can both erode a landscape and lay down a new layer of soil.

- Here, we could probably see the individual sand layers from individual tsunami waves coming in over this subtidal landscape.

(gentle music continues) It's so obvious to visitors that something dramatic happened there to kill all those trees.

The Copalis River Ghost Forest is the very best example that we have been able to find on the Washington coast of a landscape that has been lowered into the sea after a huge earthquake.

(gentle music continues) - This area is a highly active tectonic region.

Here along the Pacific Ring of Fire, the Juan de Fuca plate slips under and against the North American plate, squeezing and building up stress along the roughly 700-mile-long Cascadia subduction zone.

Every several centuries, that energy is released in the form of large, violent earthquakes.

Until the end of the 20th century, the only evidence of past catastrophic earthquakes and tsunamis in the region were oral accounts from the local tribes passed down through the generations.

In the 1980s, though, researchers discovered some tantalizing clues indicating a massive earthquake had struck here in recent history.

(gentle music) One of those pioneers was this guy, geologist Brian Atwater.

He began digging down into the layers of earth along the coast of Washington, searching for evidence of past earthquakes and tsunamis in the soil deposits that David was showing us earlier.

What he saw was evidence that the land had dropped a lot and in an alarmingly short time.

He had a hunch something massive had happened here, but when?

And more importantly, when might the next big one happen?

(gentle music) Brian pulled in an unlikely ally in David Yamaguchi.

David is a dendrochronologist, an expert in analyzing tree rings to reconstruct ancient events.

Brian had seen evidence of a colossal quake and tsunami in the soil.

Could tree rings help narrow down when it happened?

- [David] If you have a modern tree and you know what the outermost ring date, you could determine when trees were killed by matching their ring width variation patterns against those of old living trees.

- [Joe] Brian found some nearby old-growth red cedar which weren't killed in the tsunami that David could compare to the rings of the dead trees in the Ghost Forest.

- The outermost ring here is 1986, and it goes back through the centuries, 1800, 1700, 1600, 1500, about 1430 on this end.

- [Joe] After comparing tree ring patterns in living trees with those in the Ghost Forest, along with radiocarbon dating, David closed in on when this Cascadia megaquake happened.

- Fairly quickly, I knew that probably we're looking at an event that was fairly soon after the 1680s or '90s.

(gentle music) (waves lapping) - [Joe] Tribes throughout the Pacific Northwest have passed down stories about catastrophic events for generations, but because written language only arrived with European settlers in the late 1700s, there were no precise accounts or dates for these major events.

Their next clue would come from an unexpected place, thousands of miles across the ocean.

(gentle music) - [David] In 1993, Kenji Satake, who was a Japanese guy, learned about our work, and he said, "If these Americans really did have a big coastal earthquake around AD 1700, it should have produced a tsunami wave that would've crossed the Pacific Ocean and hit Japan."

- [Joe] Coastal communities in Japan who felt those waves had detailed recorded history from that time and a calendar system that could be used to pinpoint exact times and dates.

Kenji surfaced records of precisely when the tsunami hit Japan, then back-calculated how long it took to get there and pinpointed the exact timing of the event.

(waves lapping) (tense music) - [David] And so he published a paper, saying, "I believe that the Americans had a big earthquake, and it happened on January 26th, 1700, at nine o'clock at night."

- [Joe] The soil, the trees, and the written stories across 5,000 miles and 300 years all pointed at the same theory, that a catastrophic quake was possible in the Pacific Northwest.

And that threat still looms large for people that live here today.

(tense music continues) A modern quake along the Cascadia subduction zone will be catastrophic.

Magnitude eight or nine shaking will level buildings and infrastructure.

Major cities from Northern California to British Columbia will shake.

Millions will lose power and access to clean water and roads and bridges will become impassable.

Tens of thousands of people along the coast will have as little as 15 minutes warning to get to high ground before a wall of water as high as 20 meters inundates the land, destroying everything in its path.

The waves will keep coming for hours.

How do we know this?

Using past evidence, researchers have built models for what we can expect when the next one comes.

- Surviving an event like that involves getting out of the path of the tsunami.

- [Joe] This is Carrie Garrison-Laney.

She's a paleoseismologist and coastal hazards specialist who studies deposits from past tsunamis.

This helps us understand modern tsunami hazards and supports the work of tsunami modelers like this guy, Christopher Moore from the Center for Tsunami Research at the Pacific Marine Laboratory.

(gentle music) - There's a really great relationship between doing muddy paleoseismology work and work that's done modeling those processes.

The NOAA Center for Tsunami Research helped to do the modeling that was used for the Washington State tsunami hazard maps and that show the upper limit of tsunami inundation.

(gentle music continues) - This is a simulation of the event that happened way back in 1700, this Cascadia subduction zone event.

We weren't around to study it back then, so your work tells us whether the tsunami initial uplift is accurate or not, because you can look at those waves as they come along the shore and say, "Here there's a large wave along the coastline, but we don't see any tsunami deposit there."

So then we can go back and say, "Oh, how do we modify our initial condition based on these measurements that you take?"

- [Joe] Models like these are crucial for helping communities plan for tsunami evacuations and decide where to build critical infrastructure like hospitals, schools, and fire stations.

- You got multiple waves coming in.

Even though it gets all the way into Puget Sound and it's lost some of its energy, it's not just one wave, it's wave after wave.

- [Christopher] There is that big wave at the leading edge, but there's even a secondary wave and a third wave that comes several hours afterwards.

We try our best to alert the public about the danger of going into the inundation zone before the waves have passed.

- I mean, six hours after the event, a lot of people would think it's over.

- Right?

- [Joe] It's possible that the 1700 quake was just one in a cluster of many earthquakes, so understanding the dynamics of how these events play out is crucial for preparing for future disasters.

- We're gradually refining our models and they're getting more and more powerful.

All of these things help us to understand what is coming.

(birds fluttering) - Engineers and architects are starting to think of new ways to get tsunami escape towers up faster.

The question is, do we have enough time to protect the coastal communities before the next tsunami hits?

Oh, there it is.

Look at that.

Wow.

- [Joe] More than 30 years after shedding light on the Cascadia subduction zone's potential for destruction, David Yamaguchi is visiting the nation's first vertical evacuation tower in Tokeland, Washington.

The Shoalwater Bay Tribe's Auntie Lee Tower stands 50 feet tall and is anchored into the ground with 55-foot-long support piers.

Shoalwater Bay Tribal Chairman Quintin Swanson has welcomed people from all over the world who, like David, are anxious to see progress being made to protect coastal residents.

- [Quintin] We'll start by getting a look up top.

(gentle music) So the idea is if you crammed everyone in like sardines, you'd be able to accommodate about 400 people.

This serves as a beacon, if you will, of survival for everyone on the southern part of the peninsula.

- [David] Everyone including tribal members and?

- And non-tribal members as well.

We have water, propane, electricity, heaters, as well as tarps to be able to run around the perimeter so that people can stay warm.

- Well, for me, it's just really cool to see this in person.

I'm impressed.

- [Joe] The Shoalwater Bay Tribe is in the process of moving their village uphill to a tsunami-safe zone.

They've acquired more than 4,000 acres of land and the momentum is building, not just for this tribe but for those who are witnessing their endeavors.

- I've watched the community band together and want to help protect our village to protect our people.

It was the start of a movement.

It made it feasible for people to start looking toward these alternatives to protect their communities.

- In the meantime, this tower will be a lifeboat in a potential wave of destruction and hopefully others along the coast can build more like it soon.

(gentle music) This is an incredible story about how geology, and biology, and history all came together to solve this mystery about a violent event in our Earth's past, and an incredible reminder of how many secret stories of Earth's history are buried under our feet.

(gentle music)