(wildlife sounding) - [Narrator] A chainsaw is not an ordinary obsession for a scientist.

But Jessie Pearl loves them.

- This poor little saw.

Normal wear and tear on a chainsaw is not cutting through barnacles or mussels, but for us, that is kind of our every day.

When I first started my PhD, my advisor said, "Yeah, you know, I'm looking for someone who's willing "to throw a chainsaw in the back of the truck "and just drive up every time there's a storm."

And I was like, that's me, science is great.

And it's a lot of this.

(chainsaw buzzing) You just start to get a really deep love for a tool that's gonna get you your sample quick.

(blade sticking) That's our signal to clean it.

(gentle scraping) We got some bark.

That's what we wanted.

- [Narrator] This muddy, waterlogged wedge of Douglas fir is a key Jessie will use to help unlock the seismic history of the South Puget Sound.

- We're looking for evidence of past earthquake events, and we're looking for evidence that is preserved in the landscape.

- [Narrator] Earthquakes may seem like random events, but they happen at relatively regular intervals.

Depending on where you are, it could be tens, hundreds, or even thousands of years in between.

Piecing together the locations and timelines of past earthquakes can reveal those patterns, giving emergency planners a better idea of what to expect when the next earthquake hits the region.

- [Jessie] I'm always game for more samples.

You know, there's so much we can do with wood.

- [Narrator] The trees Jessie and geologist West Johns are interested in are near these marsh islands, where the mud and sediment are being eaten away by the tides.

These aren't just any dead trees though.

These root ball spiderwebs didn't wash down a stream or float in on the tide.

Instead, Jessie believes they died here 900 years ago.

- So, the evidence that I'm gathering, specifically, is old trees.

So trees that were an ancient forest that was submerged very quickly because of the earthquake, so the land level actually dropped, and now it's what I like to call a ghost forest.

- [Narrator] If Jessie can figure out when these trees died, and can confirm that they all died at the same time, she'll know what year the earthquake hit.

- I'd love to be able to sample.

See how those guys are like, still standing up?

- [Narrator] The answers are in the tree rings.

- Cannonball.

Let me see if I can find something hard to stand on.

Whoa.

(chainsaw engine running) Problem with chainsawing is, it really works you into the mud.

(chainsaw buzzing) Ah!

(Jessie laughs) Uh oh, did it fall?

There it is.

- There we go.

- Huzzah!

- Nice.

- [Narrator] Trees are incredible windows into the past.

Dendrochronology is the science of using their growth rings to date things.

- [Jessie] They're really wonderful in the sense that they stay in one place their entire life, and they record within the chemical and physical composition of their wood everything that happens to them, whether it's a big storm, whether it's a decade of drought, whether it's a big, severe fire that came through.

And so true ring scientists, like myself, can use all these different clues that are within the trees to tell a story about what has happened to this landscape in the past.

- [Narrator] Tree rings don't grown uniformly from year to year.

When the weather's good and there's lots of water, trees grow fast, putting on wide rings.

When things are bad, they grow slowly.

Because climate conditions are regional, every tree of the same species in the same area puts on common patterns of growth.

If you know what year your first tree died, you can count back in time on its rings.

Find a slightly older piece of wood, and you can lock in where the pattern overlaps and go even further back in time.

Find more old trees, you can go back, and back, and back.

Scientists can use the tree ring timeline, or chronology, to understand changes in climate, wildfire frequency, and even to date earthquakes.

In western Washington, the reliable Douglas fir chronology goes back less than 1,000 years.

But what do you do if the earthquake you're trying to date is even older than that?

You head to a lake at the edge of the Olympic Mountains.

- These trees, at this site, are a critical piece of the puzzle, where we're trying to piece together the timing and extent of a major earthquake that happened about 1,100 years ago.

And, here at Price Lake, we have a fault that impounded a stream and flooded the forest and killed the trees.

- [Jessie] Most of these trees are very much still underwater.

It is an underwater forest.

I'm gonna try and go on the upper.

- [Narrator] Collecting samples from an underwater forest requires underwater gear, like a beastly hydraulic motor, (motor starting) that will power an underwater chainsaw.

(chainsaw buzzing) (cheering) Out on the water, USGS divers, Pete Dal Ferro and Jenny White McKee, are scouting for trees that still have bark on them.

Bark is key for accurately dating when a tree died, because it guarantees the outermost rings haven't just rotted away during its 1,000 year slumber.

- [Jenny] Actually, there might be bark right here on top.

Look at this.

- [Bryan] I think that is bark, yeah.

I think it's orangeish, right?

- Do the jig.

The bark dance.

- [Bryan] Ready for the main event?

- Yep.

(chainsaw buzzing) (motor running) - [Woman] You're really having to push on it, huh?

- Yeah, yeah, it's pretty hard.

I gotta sorta lever way harder than you do in real life.

I'm also swimming, so, that part's hard.

I, you know, I'll try to get all the rings.

(chainsaw buzzing) (applauding) - Woohoo!

- That's enormous.

- Holy crap.

- [Jenny] Oh, Pete, that looked really hard, are you okay?

- That was super hard.

- That thing is so beautiful.

- [Jenny] It is, you know, I feel a little bit bad for the tree.

- I do too.

- [Narrator] Jessie and Bryan believe this tree is so old, they won't be able to rely on the known Douglas fir chronology for the region.

Instead, they'll have to use a far newer trick of the dendrochronology trade.

Back in the year 774 A.D., there was a massive solar storm that hit the Earth's atmosphere and created a huge pulse of radioactive particles.

Scientists call it radiocarbon, or Carbon-14.

- [Bryan] And so, all the trees around the world, we've noticed, have this big pulse in radiocarbon that's specific to the year 774 through 775.

- [Narrator] So instead of counting tree rings backwards from modern days, the researchers will look at the ring with the radiocarbon spike and they'll count forward until they hit the bark.

Using this technique for multiple samples, they'll be able to figure out the exact six-month period more than 1,000 years ago that this submerged forest died.

But there's a problem with the first slice of tree.

- [Bryan] No bark, rotten wood.

- [Narrator] There isn't actually any bark remaining.

So the divers go to another tree nearby to try their luck there.

- We followed the tree down, and there was a ledge of what felt like another outer layer.

Could be more tree, but coulda been bark.

That layer's this far into the mud.

- [Jenny] I think the bark is only surviving subsurface.

- [Jessie] Yeah, that would make sense.

- [Narrator] The mud and sediment make this cut far more difficult.

Visibility is zero.

The thick mud coats Pete's mask, and clogs his regulator, making it harder to breathe.

- Oh, it's (bleep) terrible.

The next time you're chainsawing, close your eyes, and try to put the saw back in the cut.

- [Narrator] And bits of wood and debris repeatedly bind up the saw.

Finally, after hours of work, the wedge surfaces.

- Thank you, Pete.

- Yeah.

- Well, this is plenty to deal with.

This will lock in.

I think that's gonna be plenty of rings.

- That was a fighter.

- [Narrator] During the cut, the bark separated from the wood.

But there's evidence the final ring was preserved.

- See that thin red?

That's for Doug Fir.

- That's cambium?

- That's cambium.

Yep, for sure.

- And it's still thin and red after 1,000 years.

This last ring, right here, is the year the tree died, and that's gonna tell us the year of the earthquake.

Unbelievable.

This lake was holding the secrets, but, this is the piece that'll help crack it.

- [Narrator] And while uncovering the Pacific Northwest's seismic past is the immediate aim of Jessie's research, she's also working towards a far grander goal: finding the trees to fill in the gaps in the Douglas fir tree ring chronology.

- If at the end of this project we come up with a 2,000 year long tree ring record, which, you could utilize for any other trees you might find at other ghost forest sites to figure out when they died, I mean, that would be something that will be used for generations of scientists to come.