00:00Before an earthquake shakes the ground, something sneaky might happen first.
00:05A slow, quiet movement with no shaking at all.
00:08No one is aware that right under their feet, the land is preparing to tremble violently,
00:14wreaking havoc, bringing destruction and devastation.
00:17A new study shows that such a slow creep might be how earthquakes begin.
00:23To figure this out, scientists didn't start with giant pieces of the Earth.
00:26Instead, they went into a lab and used clear plastic sheets called plexiglass.
00:32They pushed two sheets of plexiglass sideways against each other,
00:36just like how tectonic plates, huge chunks of Earth's crust, push against each other underground.
00:42As they pushed, the plastic cracked, kind of like what the Earth does during an earthquake.
00:48And even though it was plastic and not rock, the physics, the way things break and move, was the same.
00:55Earthquakes happen when two tectonic plates try to slide past each other, but they get stuck.
01:02The edges between them don't move easily because of something called friction.
01:06It's kind of like when your shoes grip the ground.
01:08Over time, the plates keep trying to move, and stress builds up.
01:13Think of it like bending a stick.
01:15At first, it doesn't break.
01:16But if you keep pushing, it will eventually snap.
01:19The spot where the plates are stuck is called a fault.
01:23And there's a thin, brittle part in that fault.
01:26By brittle, I mean that it doesn't bend.
01:29It just breaks.
01:31Here's how it happens.
01:33First, a tiny crack forms.
01:35It's small and moves slowly.
01:36There's no shaking yet.
01:38But something is already happening under the surface.
01:41The crack spreads, building up more energy.
01:44When it finally reaches the edge of the brittle zone, boom, it speeds up, suddenly racing forward really fast, almost as fast as sound.
01:53And that's when the Earth starts to shake.
01:56So scientists decided to find out how that very first crack starts.
02:01They conducted their experiment, whose setup was similar to a type of fault in the real world called a strike-slip fault, like the San Andreas fault in California.
02:10Even though the materials, plastic versus rock, are different, the way cracks form and move is exactly the same.
02:19The experiment has helped scientists understand that earthquakes don't always start suddenly.
02:24Sometimes there's a slow, quiet start, like a warning sign before the big quake.
02:30This early stage of an earthquake is called a nucleation front.
02:34It's like a seed of a crack, gently creeping through the material long before any actual rupture happens.
02:41This quiet and slow movement doesn't release energy into the surroundings.
02:46At first, scientists thought of cracks as simple one-dimensional lines, like a straight tear in paper.
02:52But something didn't add up.
02:53The slow-moving nucleation front wasn't behaving like a normal, fast-moving crack.
02:59And it wasn't clear why or how it suddenly sped up and became an earthquake.
03:04The answer came when scientists realized they had to think in two dimensions instead of one.
03:10Instead of imagining the crack as just a fine line, they began to think of it as a patch, like a growing circle,
03:17that starts at the surface where two materials touch.
03:20As this patch spreads out, more material along its edge has to break.
03:25The important thing here is that the energy needed to break the material is related to how long the patch's edge, or perimeter, becomes.
03:34The bigger the perimeter, the more energy it takes to keep breaking.
03:37That's why this patch moves slowly at first.
03:40It doesn't yet cause the violent shaking we associate with an earthquake.
03:44Because it moves without sending shaking waves, this stage is called aseismic.
03:50Eventually, the growing crack patch spreads beyond the special brittle zone, where the materials are stuck.
03:57Outside this zone, the energy needed to keep breaking the material doesn't increase anymore.
04:03Instead, there's now extra energy building up, more than what's needed to keep the crack growing slowly.
04:09This extra energy doesn't stay quiet.
04:12It suddenly powers the crack to move much faster, turning it into a full-speed rupture.
04:17That's the moment when the earthquake begins, and the ground starts to shake.
04:22These findings help explain how earthquakes might start off slow and silent before quickly turning dangerous.
04:28They show that small, slow cracks can suddenly transform into powerful, fast-moving ruptures if the conditions are right.
04:37Scientists believe that they can learn to detect the aseismic stage before a crack speeds up.
04:42It might one day be possible to predict earthquakes, or at least understand the warning signs better.
04:48Now, even though earthquakes are only the third most common type of natural disaster, they cause the highest number of fatalities.
04:56NASA plays an important role in studying earthquakes by using satellites that orbit the Earth.
05:01These satellites collect data and images that show how the ground changes after an earthquake.
05:07For example, they can detect when the land shifts, rises, sinks, or cracks.
05:13These surface changes help scientists learn more about the strength and impact of earthquakes.
05:18NASA's satellites can also track changes in nighttime lights.
05:22If a city goes dark after a quake, that's a clear sign that the area may have lost power and could need help.
05:28Now, how about we talk about the most common earthquake myths, like the one about mega-quakes?
05:34Some people really worry about super-huge earthquakes, but there's a limit to how big they can get.
05:41The size of an earthquake depends on the size of the fault where it happens.
05:45A longer and deeper fault can cause a bigger earthquake.
05:49For example, the San Andreas Fault in California is long, about 800 miles, but not very deep, around 10 to 12 miles.
05:57This makes earthquakes bigger than magnitude 8.3 on that fault very unlikely.
06:03The biggest earthquake ever recorded happened in Chile in 1960.
06:08It had a magnitude of 9.5 and occurred on a huge fault that was almost 1,000 miles long and 150 miles wide.
06:17Technically, there's no set limit on the magnitude scale.
06:20But an earthquake bigger than magnitude 12 would need a fault larger than Earth, which just isn't possible.
06:27Now, earthquakes can happen near the surface or deep underground.
06:31Most occur in Earth's crust or upper mantle, down to about 500 miles deep.
06:36But the deepest ones happen only at subduction zones, places where one part of Earth's crust slides under another.
06:43In California, almost all earthquakes happen in the top 15 miles of the crust.
06:49One exception is the Cascadia subduction zone in Northern California, which continues up through Oregon, Washington, and Canada.
06:57Another misconception is that the ground can open during an earthquake.
07:01But usually, that only happens in movies.
07:04A giant crack opening and swallowing things up during an earthquake is just fiction.
07:09In real life, the ground on either side of a fault slides past each other.
07:14It doesn't pull apart or open wide.
07:16Small cracks or holes can happen during landslides or ground failures, but not along the actual fault line itself.
07:24Faults don't open up, because if they did, there'd be no friction.
07:27And without friction, there wouldn't be an earthquake at all.
07:31Now, some people think California could break off and sink into the ocean during an earthquake.
07:36But that's not how it happens.
07:38The ocean floor is just lower land with water above it.
07:41California can't fall in.
07:44What's really happening is that southwestern California is slowly sliding north toward Alaska along the San Andreas Fault.
07:51The Pacific Plate, which California sits on, is moving about 2 inches per year, about as fast as your fingernails grow.
07:58So, in about 15 million years, Los Angeles and San Francisco could be neighbors.
08:05And in 70 million years, Los Angeles might be sitting near Alaska.
08:09I can't wait.
08:11People often wonder whether an aftershock can be bigger than the earthquake itself.
08:16Aftershocks are smaller earthquakes that happen in the same area after a larger one.
08:21They're like our planet's way of settling down after the main shock.
08:25But if an aftershock turns out to be bigger than the first earthquake, then we call the bigger one the main shock, and the earlier one becomes a foreshock.
08:34About 5 to 10% of earthquakes in California are followed by a stronger one within a week.
08:40It's also possible for two quakes in the same area to be about the same size.
08:45However, very large earthquakes are rare.
08:48So, it's even rarer to see two huge ones happen close together.
08:53That's it for today.
08:54So, hey, if you pacified your curiosity, then give the video a like and share it with your friends.
08:59Or, if you want more, just click on these videos and stay on the bright side!
09:03So, let's get started!
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