# Tectonic hazards/Earthquake

An earthquake is a violent movement of the rocks in the Earth's crust which create waves of energy travelling through the Earth. Earthquakes are usually quite brief (and even somewhat repeated itself, known as an aftershock), but may repeat over a period of time. The study of earthquakes is called seismology.

## Occurrence and Cause

Earthquakes usually occur where tectonic plates are in constant motion, in which these areas of constant motion (or violent motion) are usually at the edges of the plates. For example, several and several earthquakes have been recorded in the Western part of South America due to the constant motion between the South American plate and the Nazca plate. Earthquakes usually occur on faults, which are breaks in the Earth's crust formed by the sliding, pushing, or pulling of tectonic plates.
What causes earthquakes is elastic deformation, which is when rocks change in shape when pressure/forces is applied on them, but then return to its normal state after the pressure/force has gone (see https://www.youtube.com/watch?v=zGcnUUaYXv4 for better understanding). This is what causes earthquakes, and not plastic deformation, when permanent deformation occurs on a rock.
The cause of tectonics and M6.2+ strong earthquakes is in the externally forced magnification of Earth's oscillations. The magnification arises due to cumulative secondary gravitational effects making Earth masses overstir under the sum of gravitational forces from 2 other heavenly bodies in Solar system as they "align with Earth" (their angular velocities get in step with the Earth's) for more than 3 days. (see https://youtu.be/nTamffiES84 for better understanding). The discovery enables seismic antiforecasting (predicting of times without catastrophic earthquakes) anywhere on Earth, months ahead.

### Elastic Rebound

This term is used to describe the abrupt return of elastically deformed rock to its original shape. Elastic Rebound is somewhat similar to a rubber band being stretched and then ripped. After the band was ripped into one whole string, the shape of the string came back to its original shape. This occurs when stress and stress is applied to a rock. So much stress, to the point the rock cannot withstand it! During the process of elastic rebound, energy is released. Some of this energy travels in what is known as "seismic waves", and these waves are the originators of some earthquakes.

### Plate Motions and Earthquake Zones

    See more at Plate tectonics and the structure of the Earth's crust#Faults
There is a specific plate motion, which can cause a specific fault (listed fault is the major type of fault at this plate motion):
* Transform Motion occurs when two plates slip/shift past each other. This motion creates a strike-slip fault, which is when two blocks of Earth's crust shift horizontally.
* Convergent Motion occurs when two plates collide with each other. This motion creates a reverse fault, which is when two blocks of crust are pushed together.
* Divergent Motion occurs when two plates pull away from each other. This motion creates a normal fault, which is when two blocks of crust are pulled away from each other (or stretched).
Each of these motions creates a specific type of fault, which can cause earthquakes.
Earthquakes can happen on earthquake zones. Earthquake zones are places where a large number of faults are at/a lot of seismic activity occurs. An example of an earthquake zone is the San Andreas Fault Zone located in California. The San Adreas Fault was formed in the Cenozoic (also Cænozoic, Caenozoic or Cainozoic) era when the North American plate and the Pacific plates slid past each other, which formed a strike-slip fault known as the San Andreas Fault. This fault is the cause of several earthquakes in California, including the 1906 San Francisco earthquake.
Earthquake zones are places where a large number of faults are at. An example of this is the San Andreas Fault Zone in California.

## Waves of an Earthquake

Waves of energy, or waves containing energy, that travel through the Earth are called seismic waves. Seismic waves that travel through the Earth's interior/body are called Body Waves. These body waves are divided into two types of waves: P-waves and S-waves. Seismic waves that travel along the Earth's surface are called surface waves.
P waves, also known as pressure waves, travel through solids, liquids, and gases. These waves are the fastest type of body waves. They travel ahead of seismic waves, and thus being the fastest, are the first to be detected in an earthquake. These waves, when coming through rock, cause particles of that rock to move in a back-forth type of direction. To better comprehend this, imagine tapping jello. Jello, when taped, is momentarily deformed (by the pressure inside the jello), and then [the jello] reacts to the deformation by moving back to its original shape. This is a great example of how P-waves affect rocks.
Rock can be, in addition to being deformed back and forth, deformed side to side. After being deformed from side to side, the rock returns to its original position, and this is how S-waves, or shear waves, are created. These waves are the second fastest seismic waves. These waves can shear rock side to side, which means they stretch rock sideways. Though, S-waves cannot travel through parts of the Earth that are liquid, and are also slower than P-waves and arrive later. Thus, S-waves also have the name of "secondary waves".
Surface Waves move along the Earth's surface and produce motion mostly in the few kilometers of the Earth's crust. There are two types of surface waves: One type of surface wave produces motion up, down, and around. The other type produces back-and-forth motion like the motion produced by P-waves. Surface waves are different from body waves. How? Surface waves travel way more slowly and are a lot more destructive!

## Tsunami

When the earth moves offshore in the ocean, it can cause a tsunami. A tsunami can cause just as much death and destruction as an earthquake. Landslides can happen, too. This is an important part of the Earth's cycle.
Earthquakes are measured using observations from seismometers. The magnitude of an earthquake and the intensity of shaking are measured on a numerical scale. On the scale, 3 or less is scarcely noticeable, and magnitude 7 (or more) causes damage over wide areas.