What is an Earthquake?

An earthquake is a type of natural disaster. When you feel and earthquake, you feel the Earth shaking under you. Earthquakes can be violent or disastrous, or minor and not doing a lot of damage. Scientists have different ways to measure the strength or size of an earthquake.

What Causes Earthquakes?

The Earth’s crust are broken into different pieces. These pieces are called tectonic plates. The tectonic plates float on the lithosphere’s slippery surface and are always moving towards or away from each other extremely slowly. Junctions between two tectonic plates are called faults. Faults are where stress, or friction between the rock, builds up. Stress is caused because of the constant movement between two plates. The plates get locked and do not move. Overtime, stress builds up where the plates are locked together. Eventually, the stress is too much for the rock. The plates unlock and strike past each other. This creates an earthquake.

Types of Stress

There are different types of stress that builds up between rock that is based on the movement of the rock.

Tension is a type of stress where two masses of rock move away from each other. Tension in the earth’s crust could have caused the supercontinent Pangea to break apart.

Compression is a type of stress in which two masses of rock move towards each other and push against each other. This causes the rock to fold or break.

Shearing is a type of stress in which two masses of rock move in opposite directions. They rub or slide past each other. This causes part of the rock to break, making it smaller.

Types of Faults

There are three types of faults: normal faults, reverse faults, and strike slip faults.

In normal faults, the tectonic plates meet at an angle. This causes part of one plate to be on top of the other plate. The piece of the plate that is above the other plate is called the hanging wall. The hanging wall lies over the footwall. The type of stress in a normal fault is tension. The two plates are moving apart.

A reverse fault also has two plates that meet at an angle. A reverse fault also as a footwall and hanging wall.However, in a reverse fault the type of stress is compression. The two plates are moving toward each other, and the hanging wall gets forced farther above the foot wall.

A strike slip fault does not have two plates that meet at an angle. In a strike slip fault, the two plates move in opposite directions, sliding past each other. The stress that builds up in strike slip faults is shearing.

Parts of an Earthquake

When an earthquake happens, the place where the rock breaks because of stress is called the focus. Right above the focus at the surface of the earth is the epicenter. The epicenter is the middle of the earthquake. Energy is released from the focus. This energy is released as seismic waves.

Seismic Waves

Waves of energy are released during an earthquake. These waves are called seismic waves. Seismic waves are released from the focus as two types of waves: p waves and s waves.

P waves can travel through solids and liquids. They travel fastest,  so they reach the surface first. The first waves you feel in an earthquake are p waves. P waves compress and expand the surface, which causes the vibrations you feel.

S waves cannot travel through liquids. Since they are not as fast as p waves, you do not feel them at the same time. However, they follow close behind. S waves are much stronger than p waves. S waves shake the ground from side to side rather than compressing and expanding it.

When p waves and s waves reach the surface, many of those waves become surface waves. Surface waves travel the slowest, but are the strongest and most violent waves. Surface waves shake the ground up and down, or side to side. They also cause rolling motions in the ground, like that of an ocean wave.

Which Methods do Scientists Use to Measure Earthquakes?

Richter Scale:

The Richter Scale, invented by Charles Francis Richter in 1935, measures the magnitude of an earthquake. The magnitude is assigned to an earthquake based on its strength. The Richter Scale uses seismographs to record the earthquake’s seismic waves and their size, and assign it a magnitude based on that. A seismograph is a machine that measures the size, strength, and location of seismic waves.

Mercalli Scale:

The Mercalli scale, invented by Giuseppe Mercalli in 1902, is used to rate an earthquake based on the amount of damage the earthquake causes. There are different levels, shown in roman numerals, of damage. Earthquakes rated 15 cause the highest damage, while earthquakes rated 1 cause the lowest damage.

Moment Magnitude Scale:

The moment magnitude scale, invented by Tom Hanks and Hiroo Kanamori and introduced to the world in 1979, measures the total energy released by an earthquake. The moment magnitude scale is used the most today because it can measure earthquakes of all distances. It does not matter if the earthquake is near or far from the labs. It also measures earthquakes of all sizes more accurately than the other two scales.

Earthquake Damage

Earthquakes can cause a lot of damage. They could destroy buildings or other structures. Earthquakes can also form landslides, mudslides, or avalanches on mountains. When the epicenter of an earthquake is located near or on water, the vibrations could cause a tsunami. Occasionally, earthquakes might trigger volcanic activity.