The ground can shake.
Most shakes happen at the edges of big plates. But some shakes happen in the middle. These can be very strong. They can hurt buildings.
Have you ever felt the ground move?
The Earth is made of big plates.
Most shakes happen at the edges of these plates. But some shakes happen in the middle. These are called intraplate earthquakes.
They can happen in old cracks in the ground. They might also happen when fluids move deep down. These shakes can be very strong.
Many cities are not ready for them. This can cause a lot of damage to buildings.
One big shake happened in India in 2001. It hurt many people and homes. These shakes are rare but very important to study.
The Earth's outer shell is made of many large plates. These plates move very slowly. Most earthquakes happen at the edges where plates meet. But sometimes, shakes happen in the middle of a plate. We call these intraplate earthquakes.
These shakes are rare. They often happen at old, weak spots in the crust. These spots might be old cracks or failed rifts. Sometimes, fluids moving deep down can cause them. They might also happen because of erosion or melting ice.
One kind is an intraslab earthquake. This happens inside a plate that is sinking into the Earth. These can be very deep. They can reach depths of over 500 km.
Intraplate earthquakes can be very dangerous. Many cities are far from plate edges. They do not have special building tools to stay safe. In 2001, a big quake hit Gujarat, India. It caused much damage because the area was not prepared. Other big shakes happened in New Madrid and Charleston. Scientists use tools called seismometers to study these shakes. This helps them find patterns in the ground.
Most earthquakes happen at the edges of tectonic plates. The Earth's crust is made of many different plates. There are seven primary plates and eight secondary plates. Dozens of smaller microplates also exist. These plates move slowly on top of the mantle. They often collide or slide past each other. This movement creates frequent earthquakes at the plate boundaries.
An intraplate earthquake is different because it happens in the middle of a plate. These events are much rarer than boundary earthquakes. They often occur at old, weak spots in the crust. These spots might be ancient failed rifts or partial fractures. Sometimes, fluids move up the crust along these old zones. Other times, erosion or melting ice might cause the shake.
One special type is called an intraslab earthquake. This happens inside a plate that is sinking into the mantle. These sinking pieces are known as slabs. Older slabs are colder and produce these quakes more often. Younger slabs are warmer and rarely produce them. These can happen at depths over 500 km. Shallow intraslab quakes at 20 to 60 km can damage cities.
These earthquakes can be very dangerous to people. Many cities are far from plate edges. These places often lack seismic retrofitting to protect buildings. In 2001, a large quake hit Gujarat, India. It was far from any plate boundary. The Kutch district suffered huge damage. Over 12,000 people died in Kutch, and the total death toll was over 20,000.
History shows many large intraplate events. The New Madrid earthquakes happened in 1811 and 1812. They may have reached a magnitude of 8.6. In 1886, a magnitude 6.5 to 7.3 quake hit Charleston. The 1755 Boston earthquake was also large. In 1888, an earthquake hit Río de la Plata. It caused a tsunami in Uruguay. Scientists use many seismometers to study these patterns today.
An intraplate earthquake is a seismic event that occurs within the interior of a tectonic plate. Most people are more familiar with interplate earthquakes. Those occur at the boundaries where plates meet. The Earth's crust is composed of seven primary plates and eight secondary plates. Dozens of smaller tertiary microplates also exist. These large plates move slowly atop convection currents in the mantle. Because they move in different directions, they often collide or slide past one another. This constant movement makes earthquakes at plate boundaries very frequent.
Intraplate earthquakes are relatively rare compared to boundary events. They do not happen at the edges of the plates. Instead, they occur in the middle of a plate's surface. These quakes often happen at ancient failed rifts. They may also occur along partial fractures in an existing plate. These old features leave weaknesses in the crust. These weaknesses become vulnerable to regional tectonic strain. Sometimes, the exact cause remains uncertain. Scientists believe faults may be deeply buried and hard to find. Some studies suggest fluids moving up the crust along ancient fault zones cause them. Other factors like erosion or deglaciation might also play a role.
A specific type of intraplate event is the intraslab earthquake. This occurs within a subducting plate, which is a plate sinking into the mantle. These sinking sections are called slabs. Intraslab earthquakes are more frequent in older slabs. This is because older slabs are colder. Younger slabs are warmer and rarely produce these earthquakes. These events can occur at extreme depths exceeding 500 km. They are the source of intermediate and deep-focus earthquakes. However, shallow intraslab earthquakes can also occur. These happen at depths of 20 to 60 km. Such shallow events can be very destructive to cities.
These earthquakes pose a unique danger to many populations. Many cities are located far from active plate boundaries. Because of this, these buildings rarely have seismic retrofitting. Seismic retrofitting is the process of strengthening structures to withstand shaking. When a large intraplate earthquake strikes, the damage can be heavy. For example, the 2001 Gujarat earthquake in India was devastating. It occurred far from any plate boundaries. The Kutch district suffered tremendous damage. The death toll in Kutch was over 12,000 people. The total death toll for the earthquake was higher than 20,000.
History provides many examples of significant intraplate activity. The New Madrid earthquakes occurred in 1811 and 1812. These may have reached an estimated magnitude as high as 8.6. In 1886, the Charleston earthquake hit South Carolina. It had an estimated magnitude between 6.5 and 7.3. This event was surprising because the area had almost no history of minor quakes. The 1755 Boston earthquake was also large, with a magnitude between 6.0 and 6.3. In 1737 and 1884, earthquakes were felt in New York City. Those events had an estimated magnitude of 5.5.
Other notable events have occurred in different parts of the world. In 1989, an earthquake shook Newcastle, New South Wales. A 2017 earthquake occurred in eastern Botswana. This magnitude 6.5 event was 24 to 29 km deep. It happened over 300 km from the nearest active plate boundary. In 1888, an earthquake hit the Río de la Plata region. It involved reactivated faults in the Quilmes Trough. This quake had a magnitude greater than 5.0. It was felt in Buenos Aires and La Plata. The event even generated a tsunami that hit Uruguay.
One major difference involves how seismic energy is measured. Intraslab earthquakes radiate more energy than megathrust earthquakes of a similar magnitude. Megathrust earthquakes are common interplate events. Because of this, seismic energy is a better measure for potential macroseismic effects. This is different from the common seismic moment used to calculate magnitude (Mw). Scientists use detailed micro-seismic monitoring to understand these patterns. They use dense arrays of seismometers to track very small earthquakes. This helps them locate causative faults with great accuracy. These small quakes often line up in patterns consistent with faulting.
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