Hi, everyone. I’m glad that I can be with you all virtually.
Thank you to the session organizers for inviting me to give this talk
on some of my Mendenhall postdoc work using tree rings
as a proxy and deriving different types of records
from trees to get precise dates of past Cascadia seismicity.
So paleoseismology studies really do characterize the seismic hazards
in Cascadia because we have not had a Cascadia subduction zone event
during the instrumental record. So, when we’ve had seismometers
out there, we actually have not had an event.
And, because of this, we are really dependent on paleoseismic studies
for our understanding of earthquake recurrence as well as their associated
hazards. And the next logical step in Cascadia subduction zone
paleoseismology is to improve the geochronology.
And this is really a key step because we are starting to ask questions about
how we are tying our terrestrial paleoseismology records with our
offshore paleoseismology records to better understand any potential
clustering we’re seeing in the event beds as well as the real continuity along
the Cascadia subduction zone and along the Cascadia coast.
So do we see a full extent in 1700? Is 1700 a full rip event, if you will?
Or what might we expect sequential large-magnitude 8 earthquakes
instead of one large magnitude 9? So, by improving the geochronology,
we can start to ask some of these really important questions
about Cascadia subduction zone seismicity.
So utilizing trees as a proxy for time in the Cascadia subduction zone
environment is not novel. Back in the 1980s and 1990s,
Gordon Jacoby, David Yamaguchi, Brian Atwater, they used tree ring
width records from trees that were killed in the 1700 earthquake event
to obtain a very precise age of the 1700 earthquake.
They utilized the tree ring width record, and they compared that tree ring width
record, which is like a barcode, to the master chronology for the area.
So they created a master chronology from living trees, and they determined
that the final year of the tree’s life was 1699. And then, combining that
piece of evidence with the stratigraphy from the marshes as well as the written
records from Japan across the Pacific ocean and the oral – and the oral
histories of the people who were living in the region, they came up
with the January 26, 1700, date. And what you’re seeing in this figure
here, and why we continue to return to trees in Cascadia are some of the
age probability functions you might get from a radiocarbon age from
the other types of paleoseismic proxies in the Cascadia region.
And so the length of the gray bar, the paleo – or, excuse me,
the probability age distribution. You can see that, in turbidites,
we have quite a large age probability. We can get slightly improved ages
in our terrestrial cores. Even better ages if we have
bracketing – radiocarbon bracketed the event bed –
the tsunami bed, for example. But really the gold standard here is to be
able to get to the year and to the season. And one of the only ways you can do
that in this environment is to utilize tree rings themselves.
So why are trees such an incredible geochronometer for Cascadia?
Well, it’s an abundant resource. Trees are found along most of
the entire Cascadia margin – the coastline offshore.
Or, excuse me, the coastline or the Cascadia subduction zone is offshore.
And they stay in one place their entire lives and record the climate
and ecology that they feel in their annual growth ring.
So, each and every year, a tree puts on an annual growth ring that is that
transition from the early wood, late – that light-colored wood to the dark
wood, that dark-colored wood. And within the physical and chemical
characteristics of these trees, we can derive records.
And the patterns of those records can be matched with master records that
will then precisely date that tree in time. And so what’s been used in Cascadia
primarily is tree ring width records. So the actual width of one year to
another, this is dictated by climate. So it makes for good paleoclimate
records. But, by utilizing the width record, you can match a tree you
might find in a paleoseismic site with the local master chronology.
So it is dependent on having that master chronology.
Now, we can also look at stable isotope records that we can derive from trees.
I’m not going to talk about this in this talk, but I think it’s worth mentioning.
Because it is a real frontier for utilizing trees. They found that there
was a large jump in carbon isotopes after the tsunami in Japan.
The trees actually have water stress, and that is reflected in their carbon isotopes.
So this is a potential frontier to see the extent of tsunamis as well as thinking
about the ecological impacts of large flooding events.
And another type of record we can derive from the annual growth
rings are radiogenic isotopes. And I am going to talk about that in
this talk, where we can actually pull radiogenic isotopes like carbon-14
from the rings themselves and get this annually resolved radiocarbon record.
This greatly reduces the uncertainty in the age if you don’t have a master
chronology, but we can also utilize that annual radiocarbon record to
get even uncertainties of zero. And I’ll talk about that in a few slides.
So the over-arching message here is, we can extract multiple types of
records from trees that all ultimately assist in their dating.
and the reason why we want to think about multiple ways to use trees here
is because, as I mentioned, dendrochronology and using trees
really does become the gold standard in Cascadia.
So we have radiocarbon as a potential to date back to a few – 50,000 years
or so if you really want to push it. And dendrochronology has the ability
to date events for thousands of years. So the potential is there.
Unfortunately, the length of the local master chronologies in Cascadia really
don’t extend much further back than 500 years or so.
But the precision on dendrochronology is zero uncertainty,
and so that’s really what we want to go for.
Now, the way that I’ve been utilizing radiocarbon and trees to get a precise
age is I’ve been utilizing these global radiocarbon excursion events.
And so this allows us to exactly date a tree ring record without
a master chronology. And what they are are large jumps
in radiocarbon in the atmosphere in both the northern and southern
hemisphere that occur at a very specific time. And so this an example.
This is the large jump between the year 774 and 775.
This jump is about 20 times more than we might expect to attribute to
any normal solar modulation. And so this large jump in delta-14-C,
if you capture it in your annual tree rings records, you have then precisely
dated that sample in time without the use of a master chronology.
This is very important if we want to go back further in time than just 1700.
And so I’m going to now show an example of how we utilize this
multi-proxy approach at paleoseismic sites in the Puget Lowlands.
So the Puget Lowlands are up in the northern part of the
Cascadia subduction zone. I’m going to zoom in here.
And all of the different sites that we are working with for this study
are these small black dots that are highlighted in yellow.
So there are multiple sites – paleoseismic sites across the
Puget Lowlands that all dated within radiocarbon error of each other.
So they all dated to about 1,000 years ago or so.
And the real question was, were all these events triggered
by the same earthquake? Were they potentially a clustering
of earthquakes that happened within a few years of each other?
And what are the really underlying fault mechanics of this if it was
one large event or even just a series of events?
And so the first step was to take the trees that were killed at these
paleoseismic sites and to look at the tree ring width records to
see if they dated with each other’s. And what we found – so this is
an example from three of the sites that had some of the longest tree rings
records in the Puget Lowlands – is that the trees themselves were coeval.
And not only were they coeval, but they also all died at the same time.
So they all died in the same year. Now, once again, our master
chronologies didn’t go back further enough to actually say exactly which
year, but we knew that they all dated within the same year.
And so this is where we turn to that annual radiocarbon measurement now.
So, sure enough, we found and captured, within our annual
radiocarbon measurements, this large jump from 774 to 775.
That exactly placed these tree ring records in time.
And we can now say that all of these trees died between
the years 923 and 924. So they had a full complete growing season.
They went dormant in the fall of 923. They did not start growing again in the
year 924, so they were killed by the landslide, the tsunami, or actually the
drowning from a fault that dammed up a creek in the same season.
So, if we now go back to that map, all of these sites that I have circled
in red, they are all sites that have trees in them that died in that season
in that year at paleoseismic sites, so sites that have clear other indications
that this was an earthquake event. So this is pretty dramatic.
These are events that span across the entire Puget Lowlands area
across multiple different faults. And, importantly, we also found that
there are sites, including the one that’s circled in blue here, that do not date
to that same – that same 923 to 924. So we are able – we have to now think
of other ways to – other underlying mechanisms to invoke the subsidence
that we saw at Little Skookum Inlet down here.
So thinking about other types of fault dynamics for the region.
So that’s one example of the power of dendrochronology – power of tree rings
and radiocarbon and how we can utilize them together to get very, very precise
ages, down to the season, and then thinking more broadly about what this
means for interactions of faults and the potential hazards they might pose.
So where can these techniques be applied?
Well, obviously the low-hanging fruit is to continue work along the Cascadia
subduction zone along the coast, looking again at 1700 with a closer look
at the more southern sites outside of western Washington, but also older
Cascadia subduction zone events. And I’ll go down to northern California
in honor of this particular workshop to talk about some of the projects
we’re doing in northern California to get at this question of, can we use
trees to date earthquakes in northern California?
And specifically, can we get at the penultimate Cascadia subduction
zone earthquake event? So this is a figure from Erin’s paper
that shows that the shaking that we might expect from a magnitude 9
Cascadia subduction zone earthquake in the area that I will be focusing in on
in Humboldt County in northern California is still expected to be
very strong and severe. And so the implications of very
strong and severe shaking in this landscape of these – on these
mountainous landscapes and these coastal mountains could potentially
trigger massive landslide and debris avalanches.
And so this is actually a Lidar image of two debris avalanches that are very
close to each other that we visited this past summer and took samples from.
There’s the Red Lassic debris avalanche that we are attributing
to the penultimate Cascadia subduction zone event.
The radiocarbon ages so far are tantalizingly close to other dates that
we have for the penultimate event. And as well as the Mule Slide debris
avalanche, which is a much younger event that we are thinking might
actually be attributed to the 1906 earthquake.
So not Cascadia subduction zone, but still potentially seismically triggered.
So what do these two landslides, or debris avalanches look like?
Mule Slide – this is a photo taken from far away. It’s not re-vegetated.
This is another reason why we think it might be a younger event
because there isn’t vegetation across the slide.
It almost looks like entire blocks from the land slid down the slope.
And you can actually see trees – relatively old living trees –
and I can tell you this because I sampled them, that are intact on these
blocks that have slid down the slope. And so these are the little trees
you see right here. It looks like a block landslide.
And so the idea was, can we utilize the stress indicators of the trees to get
a precise age on potentially when this block landslide occurred?
And so there’s a couple ways we went at this.
We took some samples from dead trees with our chainsaw.
And what you’re seeing here is a sample of a tree that clearly grew on a slope.
This will illustrate to you the point that I’m trying to get at.
And you can see the tree is putting more wood on the
downslope side to right itself up. This is something that conifers do.
Hardwood trees actually pull themselves up – put tension wood
on the uphill slope side. But soft wood, conifers, they’ll put
more wood on the downslope side. So the idea is you can potentially
start dating when this slide occurred by dating when you start to see
that reaction wood in the tree.
you can also look at the trees that are surviving in the landslide environment
and look at the indicators to see if there’s any other types of clues
we can derive potentially when this slide occurred.
And so, for example, this is a tree we named The Ship, where the
tree itself clearly is not righted. But it has two large trunks, stems,
growing out of it that are in proper position.
So they are growing towards the sun, righted themselves.
And we can date those stems. And those stems date to the 1930s.
So we know that the slide has to have pre-dated 1930s.
We can actually take cores of the tree itself and also start to zoom in on the
stress and the reaction wood of the trees to see if we can get a clearer date.
And not all the trees are agreeing at this point, but that particular tree that
I showed you, The Ship, we will see that these numbers are pretty small,
that this is the year 1900. This is the year 1920.
We see quite a bit of suppression around the year 1906 and 1907.
And so this does agree with the idea that this landslide could have been
triggered by the 1906 earthquake.
The tree, then, is severely stressed for the next five or so years.
So we’re going at this question with multiple types of evidence.
Now, the other landslide we’re looking at is the Red Lassic slide.
And this is a slide that we are thinking might be attributed to the penultimate
quake because we have radiocarbon dates from one of the trees that was
buried in this avalanche debris that dates to around the years
of the penultimate. And so we went back,
and we found some of these bark- bearing trees in this avalanche debris.
This is a picture of Harvey and Steve taking a cross-cutting saw to it.
And we – nice samples from these trees. So there’s quite a few rings in
the samples we were able to take. And so the next step now is to start to
do the carbon-14 wiggle-matching process to see if we can potentially
capture one of the jumps and exactly date this tree that might give us a target
for the penultimate earthquake event. And so we’re still waiting on
these radiocarbon dates, but we’re really excited to see
what comes out of this particular study.
So, in closing, dendrochronology can address longstanding
paleoseismic questions in Cascadia.
There are multiple proxies we can derive from tree rings records, and they
can answer a wide variety of questions – everything from when did the event
occur, but also potential tsunami inundation and ecological resilience
in the face of some of these large landscape changes.
High-precision radiocarbon, combined with dendrochronology,
will definitely improve ages on other earthquakes happening in the Cascadia
region including the penultimate Cascadia subduction zone earthquake.
And the preliminary dates we’re getting from northern California avalanche
debris indicate that earthquakes might be responsible for the largest slides
we see in the area and some of the smaller slides we might be able to
attribute to large weather events. But these very, very large,
deep-seated avalanches – or, excuse me, debris avalanches seem to be tied
to large earthquake events based upon the dendrochronology so far.
So, with that, I would like to thank you all very much, and I [audio cuts out]