Earthquake rupture facts for kids
An earthquake rupture is when the ground breaks and moves during an earthquake. It's like a crack spreading in the Earth's crust. Earthquakes happen for different reasons. Sometimes, it's from landslides or magma moving inside a volcano. Most often, though, it's when two sides of an existing fault suddenly slip past each other.
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How Earthquakes Start
Every earthquake starts at a specific spot deep underground. This starting point is called the 'nucleation' zone. Scientists are still learning how big this starting spot is. It could be smaller than a football field or much larger.
Sometimes, smaller earthquakes called foreshocks happen before a big one. These foreshocks can be a sign that a larger earthquake is coming. However, some big earthquakes happen without any foreshocks at all.
Once an earthquake begins, the break quickly starts to spread. It's hard for scientists to study this process up close. This is because the ground shakes so much near where it starts.
How Earthquakes Spread
After an earthquake begins, the break spreads out in all directions along the fault surface. This spreading continues as long as there is enough stored energy in the rocks. Think of it like a crack in glass that keeps growing.
Even though it starts spreading everywhere, the break often moves mostly in one main direction. If the earthquake is big enough and close to the surface, the break might reach the ground. This creates a visible crack on the surface called a surface rupture. The break also goes deep into the Earth. It stops when the rocks become too soft and bend instead of breaking.
Sometimes, an earthquake break can jump from one fault to another. For example, the 2002 Denali earthquake in Alaska started on one fault. Then it jumped to the Denali Fault, and later to another fault. The 2016 Kaikōura earthquake in New Zealand was even more complex. Its break was seen on at least 21 different faults!
How Earthquakes Stop
An earthquake rupture stops when it runs out of energy. This can happen if the rocks ahead of the break have already moved. Or, it might be because the rocks are slowly creeping instead of building up stress.
Some places along faults act like strong walls or barriers. These barriers can stop an earthquake from spreading further. This helps limit how big an earthquake can get.
The length of the break is related to how strong the earthquake is. A smaller earthquake (like magnitude 5-6) might have a break a few kilometers long. A very strong earthquake (like magnitude 7-9) can have a break hundreds of kilometers long.
How Fast Earthquakes Spread
Most earthquake ruptures spread very fast. They move at about half to three-quarters the speed of shear waves. Shear waves are a type of seismic wave that shakes the ground sideways.
Some earthquakes can spread even faster. These are called supershear earthquakes. They move faster than shear waves, sometimes up to 3.5 kilometers per second! This usually happens with strike-slip earthquakes. These are earthquakes where the ground moves mostly sideways.
On the other hand, some earthquakes spread much slower than normal. This often happens in places where the fault has weaker material. For example, some very large earthquakes called megathrust earthquakes can be slow. Their breaks might only move about 1 kilometer per second.
These slow earthquakes are sometimes called tsunami earthquakes. They are dangerous because they don't shake the ground much. This means people near the coast might not feel them strongly. But they can still cause huge tsunamis. The 1896 Sanriku earthquake in Japan was like this. It was barely felt, but its tsunami killed over 22,000 people.
There are also extremely slow ruptures. These can take hours or even weeks to happen. They are called slow earthquakes. They occur deeper in the Earth than regular earthquakes.
See also
- Asperity (faults)
- Earthquake duration magnitude
- Earthquake magnitude
- Epicentral distance
