Thanks to Linode, a top-rated cloud computing company with over a
million customers, for supporting this episode of SciShow.
Head to linode.com/scishow to learn more and get a
$100 60-day credit on a new Linode account.
It is widely predicted that the U.S. West Coast will sooner or later get a
massive shakeup.
As one tectonic plate slides beneath another just off the coast, slabs of
rock will strain against each other and eventually slip.
That will trigger an epic earthquake and tsunami that will rock
the whole Pacific Northwest.
Now this has not happened since the establishment of the United States,
so officially, the United States has never actually had an earthquake like
this in its history, but we know it’s coming—because it’s happened every
few hundred years since ancient times.
In places like this, knowing about prehistoric earthquakes can be a huge
lifesaver.
If scientists know when and where massive earthquakes have
happened in the past, they can tell where they’re likely to happen again
and prepare for that.
But it’s not always straightforward to figure out when and where the earth shook hundreds
or thousands of years ago.
So, uncovering ancient earthquakes takes some creativity.
Here are four ways scientists do it.
Today, we know that the last colossal earthquake in the Pacific Northwest
happened in the year 1700.
It’s not always possible to pin down the exact year of a prehistoric
earthquake.
Originally, geologists only had a rough idea that the last
major earthquake in this region happened about three centuries ago.
But in 1997, scientists were actually able to narrow the earthquake down
to a season, using a process called dendroseismology.
And if you know the dendro- prefix, you will know that this technique
involves trees.
Indeed, studying old earthquakes using information from
tree rings.
So trees are super sensitive to their environments, and they grow faster
or slower depending on what’s happening around them.
Generally, if it's warm and rainy, they’ll grow more than if
it’s cold and dry.
And if they’re stressed out by traumatic events like droughts or floods,
there will likely be signs of that in the rings.
But no matter what, each year, a tree trunk gets a new ring, which you
can see in a cut-open stump.
And based on that ring, you can get an idea whether the tree had a good or bad year.
Scientists often use this information for piecing together the climate of the
past.
But it can also be useful for studying earthquakes.
When researchers looked at coastal trees in Oregon and Washington in
the mid-’90s, they found a few things.
For one, they found some forests of dead trees.
And they used a technique called cross dating to see when
these trees had died.
Cross dating involves comparing the rings of one tree whose age you
don’t know to another tree whose age you do know to assign each ring a
year.
Since ring width is related to environmental conditions, nearby trees tend
to have the same pattern of thick and thin rings.
For instance, you could cut down a tree today, and since you know its
outer ring is from this year, you can count backward and assign a year to
every ring.
Then, if you want to know when some other tree died, you can line up its
rings with the newly cut tree to figure that out, by matching those patterns
of good and bad years.
So, scientists used this method on a bunch of remains of cedar trees from
these dead forests, and they found that they had all stopped growing after
the same year: 1699.
But some trees had survived whatever happened that season.
And when researchers looked back through their rings, they found signs
that many of the surviving trees had gone through some major upheaval
right around 1700.
Some trees had what are called traumatic resin canals—basically tubes
of sap that form when cells get disturbed by events like flooding and
waterlogging.
Others grew what’s known as reaction wood, which is thick-celled wood
that makes wide rings, often in response to things like tilting and flooding.
Various trees also had dramatic increases or decreases in ring width,
Indicating some sudden change in their environment.
The researchers could be pretty certain that they were looking at the
survivors of an earthquake that happened in late 1699 or early 1700.
And by comparing this tree tring data with historical tsunami records in
Japan, they were even able to pinpoint the actual day of the earthquake:
January 26, 1700.
Tree ring data can be extremely precise.
But you don’t always have an old forest hanging onto earthquake data for
you.
Especially if you want to look really far back in time.
Fortunately, one common method for dating geological events, known as
radiometric dating, can take you back tens or even hundreds of
thousands of years.
Radiometric dating is a way of estimating a material’s age based on the
amount of a given radioactive element in it, such as carbon-14.
Radioactive elements like this break down on a characteristic time scale,
so the less there is, the more time has passed since the material formed.
And scientists can estimate just how much time has passed based on
how much of the radioactive element is left.
In a 2020 study, researchers studying a fault in Scandinavia applied this
method to tiny crystals that grow in the high-pressure, high-temperature
environment along the fault plane.
They were interested in connecting the age of the crystals with another
key piece of information that the crystals hold.
See, as the two sides of the fault slide past each other, lines get
scratched onto the crystals, showing how they are moving relative to each
other.
These lines, called slickensides, are essentially a map of the crystals’
movement.
So scientists can connect the crystals in a fracture with a certain rock
movement.
By putting that information together with the age of the crystal, they can
piece together how a region moved and fractured over time.
For instance, in Scandinavia, there’s a web of fractures in the bedrock
that has been forming since prehistoric times.
And this method could make it possible for scientists to link each fracture
to a date and show how seismic activity gradually created the web of
fractures that exists today.
But rock fractures and dead trees aren’t the only clues to ancient
earthquakes.
In certain places, the earth holds records of quakes that
turned the ground to liquid.
This phenomenon is called liquefaction.
And it tends to happen in places with loose soil that’s saturated with water.
In soil that’s not saturated, vibrations from an earthquake don’t cause
major changes to the soil structure.
Typically the grains of soil will just contract a bit, and the ground may sink.
It’s kind of like if you pour flour into a measuring cup and then give it a
shake.
You’ll see the flour settle a bit as the grains of flour get closer
together.
But if soil is saturated, it cannot contract, because the water in between
soil particles can’t be condensed much.
So as the soil tries to contract, it squeezes the water, and the pressure increases
as the water pushes back on the soil.
When the pressure gets to a certain point, grains of soil lose contact with
each other and get suspended in the water.
In this state, soil particles act like a liquid instead of a solid.
They can slide past each other and roll downhill.
And this has lots of consequences.
For one, anything sitting on this soil can sink, as if it’s on quicksand.
Objects that are already in the soil tend to get pushed upward by the pressure.
And liquefaction often produces some telltale geological features that
seismologists can look for in the soil record to identify past earthquakes.
For instance, if soil liquefies below the surface, it can create fissures in
the ground and so-called sand-blow craters, as high pressure blasts
liquefied soil to the surface.
The study of these features is called paleoliquefaction, and it can be
especially useful in places that don’t have a lot of other evidence for
earthquakes.
For example, in 1886, there was a 7.1-magnitude earthquake around
Charleston, South Carolina, that caused massive damage to the area.
And it caught people off guard because there wasn’t any historical record
of major earthquakes around this area.
There also weren’t any obvious fault lines or fractures in the ground that
would point to seismic activity.
But scientists wanted to know if events like this had happened in the
distant past, and how often, so they could get a sense of the earthquake
risk in this region.
Studies in the 1980s documented a bunch of liquefaction features that
seemed to be from the 1886 quake.
And they also found some sand blows that seemed to be from much older, prehistoric
events.
The older sand blows weren’t visible on the surface.
They had to be identified through excavation, and were found
more than a meter-and-a-half deep.
Once they found these features, scientists used radiometric dating to
estimate how long ago they had formed.
And they were able to estimate that in recent times, major earthquakes
struck this region about every six centuries.
Which tells us that while we might not think of the Eastern seaboard as a
seismic region, it would be a mistake to write off the 1886 earthquake as a one-off event.
The clues to ancient earthquakes are often well underground at this point.
But we don’t always have to dig up the earth to find them.
Sometimes evidence of earthquakes gets preserved in
the rock formations of caves.
The study of this evidence is known as speleoseismology.
On cave floors, thin mounds of sediments called stalagmites grow when
water drips onto a specific spot and leaves some minerals behind.
Over time, those minerals accumulate into a mound,
kind of like a drip castle you might make at the beach if you took many
hundreds of years to do it.
Stalagmites grow directly upward.
But if this cave gets struck by an earthquake and the ground shifts, the thing
that’s directly above you also shifts.
So a stalagmite that was pointing up now might point slightly to the side.
Then, as the drips continue to fall from the cave ceiling, they might land
on the same stalagmite, but they will be slightly off-kilter, like a scoop of
ice cream that’s about to slide off the cone.
A feature like this is called a growth anomaly, and it can point to a past
earthquake.
Stalagmites can also stop growing or start growing as shifting rocks
change the flow of water that feeds the drips.
So researchers can use radiometric dating on different parts of cave
deposits to figure out how long ago one of these changes happened.
That can give them a rough idea when a past earthquake
struck.
For example, scientists analyzed stalagmites in Donnehue’s Cave in
Indiana, and they found an 1800-year-old stalagmite whose axis had
shifted a few times.
And the first two shifts appeared to line up with known earthquakes
around the years 660 and 900 CE.
This evidence isn’t always definitive, though, because similar changes
can be caused by climate events, like flooding.
So, at least for now, speleoseismology is best used alongside other
methods, rather than on its own, to explore past earthquakes.
The plus side is that since caves can remain largely unchanged for
enormous periods of time, this method could potentially be accurate for
dating earthquakes up to half a million years ago.
None of these methods are completely foolproof or even useful in every
place that’s ever been struck by an earthquake.
But put together and used as a toolset, these creative ways of exploring
our planet’s past can help us understand what could be coming our way
in the future.
They can help us figure out if we should be preparing for a Big One,
wherever we live, and what it might look like if and when it comes.
Right now, as you watch SciShow, you’re probably using cloud computing
technology.
And this video is sponsored by Linode, a cloud computing company that
strives to make this stuff simple, affordable, and accessible to everyone.
Cloud computing stores data, hosts websites, and accesses software online so you don’t
have to worry about finding the right equipment or
servers for your home or office.
To get started with Linode, head to Linode.com/scishow and click the
green sign-up button in the top right side of the screen.
Even without signing up, you can explore Linode’s products
and solutions or compare prices with other providers.
If you want to make sure that Linode will work for you in a remote area,
you can run a speed test by connecting with one of Linode’s international
facilities, all before any commitment.
And if you’re curious about costs, check out their cloud calculator and simple
pricing page so you’ll know exactly what you’ll be paying for your specific
needs You can see all that Linode has to offer by
clicking the link in the description or heading to linode.com/SciShow.
That link gives you a $100 60-day credit on a new Linode account.
Thanks to Linode for sponsoring this video, and thank you for watching.