What Happens Underground During an Earthquake?
What's an Earthquake?
An earthquake is the sudden shaking of the Earth, which is created by the movement or vibrations of rocks and soil along fractures in the earth's crust. During an earthquake, the earth is suddenly shaken by the readjusting of tectonic plates which compose the lithosphere. This shift produces vibrations on the surface rocks and soil known as seismic waves. When a big earthquake occurs, the aftershocks create several earthquakes of all magnitudes which can last for months. The shifts and shockwaves represent the way in which the Earth's crust is settling into new ground arrangements.
Most earthquakes occur along continental and oceanic plate boundaries where these plates converge, diverge or slip past one another. The Earth’s lithosphere comprises seven distinct huge tectonic plates, along with many other small plates. The great majority of earthquakes occur around the Ring of Fire in the Pacific Ocean at depths of no more than a few tens of kilometers. On continental land, earthquakes may occur at depths of less than 70 km (43 miles), and they're termed shallow earthquakes. Those occurring at depths ranging from 70-300 km (43-186 miles) are known as intermediate. If the focus of an earthquake is above 300 km (186 miles), they’re called deep earthquakes.
Oceanic and Continental Plates Boundaries
The sudden movement of tectonic plates is caused by the motion of a layer lying underneath the crust called the mantle. It is believed that convection currents occurring in the upper mantle are the principal driving force producing the movement of the tectonic plates. Earthquakes also occur along cracks in the Earth’s crust, known as faults, where the release of built up stress produces a sudden slip among locked up plates. When these plates pull apart, collide or slip sideways, they produce ground vibrations. These ground vibrations are known as seismic waves. Seismic waves travel through the interior of the Earth’s crust.
Seismic Waves Radiating From the focus
Faults in the Earth's Crust
Faults are fractures in the Earth's crust where rock mass has moved against another rock mass in the oceanic or continental crust. The blocks on either side of a fault may just slide and pass one another without producing an earthquake; however, if the blocks get interwined, over time, they will reach a certain point where they will have stored enough energy, causing a sudden break between them, and releasing tremendous energy in the form of an earthquake.
Faults may cause breaks through the ground surface or they may go deep into the Earth's interior. The location at which a rock slip first occurs is called the focus, and the position directly above it on the surface of the Earth is known as the epicenter. Normally, earthquakes take place at some depth within the Earth. Based on the depths of the focus, earthquakes are classified as shallow if the focus is at 60 km; intermediate if the focus is between 60-300 km; and deep if the focus is above 300 km.
How Does an Earthquake Occur?
1. Break one block of foam board into two pieces
2. Place both broken pieces on a table
3. Place both of your hands on top of each piece of foam
4. Push half of one block with your left hand. At the same time pull the other half of the foam with your right hand.
5. Maintain pushing and pulling until the two blocks slide freely
This is what occurs during an earthquake:
The crack between the two blocks represent the fault, and the point at the very center of the two blocks is where the friction unlocked the tension accumulated throughout a very long expanse of time.
The blocks represent the Earth's crust, and the force exerted upon the blocks with the hands represents the movement during an earthquake.
Soon after an earthquake strikes, the energy is released in the form of seismic waves, moving in all directions. These waves can either radiate underground from the point of the focus, and are known as body waves, or from above the ground, and are known as surface waves. Body waves radiate through the Earth's interior, while surface waves radiate only within the outer surface layer of the crust. There are two types of body waves, P waves and S waves. P waves move back and forth, and they can go through liquids, solids and gases. P waves move at about 6 km/ second (4 miles/second), and cause lots of damage.
P waves can temporarily change the density and volume of the ground where buildings are built on. When the ground contracts or expands during an earthquake, many constructions tend to fall down because of the ground surface modifications. Many public services in big cities become affected. For example, many subterranean ducts and pipelines may rupture or explode due to sudden ground movements. This is the reason why expolsions and fires are often seen during an earthquake.
S waves usually follow P waves. S waves can only be transmitted by solids not liquids. S waves shake ground material at right angles, or sideways motions, to the direction the wave is traveling. This type of motion produces changes in the shape of the material they encounter, although not its volume. S waves move at the speed of 3.5 km/second (2.2 miles/second). They're responsible for knocking down buildings from their foundations. Although, S waves travel slower than P waves, they cause more damage than P waves due that they shake the ground in a perpendicular direction to the direction the wave is traveling.
Surface waves, including Rayleigh and love waves, can be dramatically destructive due to their complicated circular and twisting motions. A Rayleigh wave produces elliptical motions on the ground with no perpendicular or transverse motion. A love wave has a horizontal motion to the direction in which the wave is traveling. Surface waves move slower than body waves; however, because of their large amplitud and duration, they can be the most destructive types of seismic waves.
Diffracted P waves at the Mantle
Seismic wave velocities change significantly when they reach the mantle. The change in velocity is determined by a denser mantle. P and S waves vary in speed at about 35-40 km (22-25 miles) under the continents, and at approximately 10 km (6.2 miles) below the oceans. Below high mountain ranges, the speed of seismic waves may change at depths of approximately 70 km (43 miles).
This boundary where the seismic waves tend to change is known as the Mohorocicic discontinuity. Seismic waves usually travel an approximate 20% slower speed below this discontinuity than above it, and the Mohorocicic discontinuity is considered to be the deepest limit of the Earth's crust. Seismic wave velocity tends to increase with depth; however, the heat present in the mantle slows down seismic waves. Partially molten areas within the mantle tend to slow down P waves, and since S waves are not able to move through liquids, they also prevent the displacement of S waves.
The following video describes with detail the locations at which earthquakes most commonly occur, as well as the geological processes involved in their creation.
Interesting Information About Earthquakes
Alaska is the most seismically active regions in the world, and experience a magnitude 7 earthquake every year, as well as a magnitude 8 earthquake every fourteen years.
Since millions of earthquakes are produced on the planet, there is no way one can be predicted; however, scientists are studying the problem.
The worst earthquake in terms of loss of human lives was one that occurred in Shensi, China, on January 23, 1556. This earthquake claimed the lives of 830,000 people.
The Largest earthquake of the twentieth century was one registered in Chile on May 22, 1960. This earthquake reached a magnitude of 9.5 on the Richter scale.
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