Why Does Earth Earthquake Happen? - Seeker's Thoughts

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Why Does Earth Earthquake Happen?

Earthquakes are caused by the movement of large sections of rock called tectonic plates. These plates are constantly moving and are driven by convection currents inside the Earth's mantle.


When the edges of tectonic plates meet, they can become stuck and the friction causes them to build up pressure. When the pressure gets too great, they break loose and cause shock waves to shake Earth.


Tectonic Plates


Tectonic plates are the pieces of Earth’s outer layer that shift and move, reshaping the surface of the planet. This motion can create earthquakes, volcanoes and mountains as the crust changes.


During 4.6 billion years of the planet’s history, tectonic plates have gradually clustered together and separated again and again. This repeated shifting of the lithosphere is called plate tectonics and is responsible for much of the earthquake and volcanic activity on our planet.


The tectonic plates are made of two types of crust – the oceanic crust and the continental crust. The oceanic crust is composed of thin layers, while the continental crust is thicker.


At convergent plate boundaries, the lower crust is subducted beneath the upper crust. This subduction process creates new crust along divergent margins. The area of the subducted lower crust is balanced by the formation of new oceanic crust at mid-ocean ridges.


These mid-ocean ridges are where the older, colder crust of one plate is pushed up by another plate that is warmer and thicker. This process is also known as slab pull.

When a warm plate pushes against a cooler plate, it causes a build-up of tension that eventually produces an earthquake. It is this tension that releases the energy in seismic waves and causes the shaking on the Earth’s surface.


During the past 750 million years, plate tectonics has produced a variety of earthquakes and volcanoes all over the world. During this time, scientists have learned to map out the epicentres of earthquakes by using seismographs, which can tell where the earth’s crust is slipping.


Scientists have also found that when the plates move apart, magma fills the gap with rock. This process is called seafloor spreading and palaeomagnetism can help us understand it.


Palaeomagnetism is the study of how rocks and sediment are affected by the Earth’s magnetic field. When the earth’s magnetic poles change, the magnetic grains in the rocks align in a different direction from each other. This explains how the north and south magnetic poles flip and switch over time.


During the 20th century, scientists discovered that earthquakes occur along oceanic trenches and spreading ridges. These zones are usually inclined 40-60 deg from the horizontal and extend several hundred kilometers into the Earth’s crust.




Most earthquakes are caused by faults that form along the boundaries of tectonic plates. These quakes are triggered when two plates converge, diverge or slide across each other, which can cause them to collide, causing damage to the Earth’s surface and potentially triggering volcanoes.


Plate boundaries are where the 15 major tectonic plates of Earth's crust meet and move together. The most powerful quakes occur near these boundaries, but there are also some large intraplate earthquakes that happen when the plates move within each other.


In a normal fault, the two blocks of rock that are pulled apart by the movement of the plates slide past each other on a very steep plane called a fault. Seen from above, these plate boundaries look like broad zones of deformation with many faults braided together.


"The most common normal faults are on subduction zones," said van der Elst. These zones involve the movement of one tectonic plate beneath another and often generate some of the largest earthquakes in the world, such as the 2011 Tohoku Japan quake.


As the plates slip, they exert stress on each other and overcome the friction that keeps them from sliding past each other. As the pressure gets too great, rocks on either side of the fault begin to jerk past each other in an earthquake. The energy released from the jerking is stored as elastic strain energy, which then releases as heat and seismic waves that cause the Earth's surface to shake.


For most faults, the angle of the fault with respect to the surface (known as the dip) and the direction of the slip along the fault are used to classify them. Faults that slip along the dip are called dip-slip faults and strike-slip faults are classified as right-lateral or left-lateral.


Strike-slip faults are those that slip horizontally across the fault surface, such as the San Andreas Fault in California. They are usually described as right-lateral faults if the footwall moves to the right, and left-lateral faults if it moves to the left.


Reverse dip-slip faults, on the other hand, are those that move downward with respect to the dip plane, such as the Northridge fault in Southern California. These are reverse dip-slip faults that dip less than 45 degrees, resulting from compressional forces in the Earth's crust that shorten or compress it. These faults are found in areas of compression, such as the Himalayas and subduction zones along the Pacific coast of South America.


Elastic Rebound Theory


Tectonic plates move slowly relative to each other, about a few centimetres per year, but this does not stop them from causing huge amounts of deformation at their plate boundaries. These deformations can cause a range of problems, including earthquakes.


Earthquakes occur when there is a sudden release of energy in the crust of the earth (called seismic waves). The energy released during an earthquake can cause many different effects, including shaking your house as if someone were driving a large truck past you, falling objects or even entire buildings and bridges collapsing.


The most common theory to explain why these earthquakes happen is the elastic rebound theory. It is based on the idea that rocks on opposite sides of a fault accumulate energy as they deform until their internal strength is exceeded. This accumulated energy is then released when they suddenly shift along the fault.


This theory was first introduced by Henry Fielding Reid, who observed that the ground he studied around the San Andreas Fault in California had shifted a number of meters during a period of time before the 1906 earthquake. He compared this to previous measurements and found that the quake must have involved an elastic rebound of previously stored elastic strain energy in the rocks on either side of the fault.


Another type of elastic rebound occurs in subduction zones, where an oceanic plate is subducting beneath a continental plate. This causes significant straining in the crust.


These areas are characterized by active volcanoes and periodic release of the strain, which leads to earthquakes. This can occur repeatedly over a 100-500 year cycle.


Throughout the world, we see a wide variety of different faults. These can vary in size and shape. Most of the time, these faults are curved or narrow and have asperities or irregularities along them that increase frictional resistance. This increases the amount of energy that must be stored in order to push the edges of the rock blocks apart.


Seismic Waves


When two moving tectonic plates come into contact, they release energy that causes a sudden shock, or earthquake. These shocks happen because the blocks of the Earth's crust (called tectonic plates) slip past each other along what is called a fault.


There are many kinds of faults and each type of fault has its own way of interacting with the other. The most common type of fault is a strike-slip fault, which means the plates are sliding against each other rather than colliding with each other.


Another type of fault is a shear fault, which involves the movement of the rock particles in the ground perpendicular to their direction of travel. These waves are called secondary or shear waves, and they are 60 percent slower than P waves, which move particles up and down, perpendicular to the direction that they're traveling.


Primary or P waves are the first waves to hit the seismographs and the ones that cause the sharp jolts we feel during an earthquake. They have speeds of 1.5 to 13 km/s, and they can only move through solids.


Seismic waves occur when energy is released at a focal point, which is often found deep in the earth's core or at a shallow depth from the surface. These shocks then spread out in all directions and can cause a variety of different ground motions.


There are several types of seismic waves that can be produced during an earthquake, including primary and secondary waves, which are also called body waves, and surface waves, which are sometimes referred to as "waves" or "ripples". The speed and type of motion that each type of wave has is important for the kind of damage it does.


The most destructive type of seismic waves is the Love wave, which can do a lot of damage to buildings. These transverse waves vibrate the ground in a horizontal direction, and they're dispersive, which means that different periods of the wave travel at different velocities.


Another type of wave is the Rayleigh wave, which moves the ground both vertically and horizontally and side-to-side. This type of wave, named for the 3rd Baron Rayleigh, John William Strutt, is about 10 percent slower than secondary waves and can cause a lot of damage to structures.


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