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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 water logging. 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 ring 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 tool set, 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.

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