What causes earthquakes, and what types of earthquakes are there? To answer these questions, it is first helpful to have an understanding of Earth’s composition.
Earth is made up of
a solid inner core
a molten outer core
the thick and mostly solid mantle, which occupies approximately 84 percent of the earth's total volume
the comparatively thin crust, which varies between 5 to 50 km in thickness.
Earth's outer skin is not a continuous surface. Instead, it is composed of massive segments called tectonic plates. Earthquakes occur along the boundaries between tectonic plates or at the site of cracks within the plates, which are called faults.
What causes earthquakes?
The tectonic plates that make up the earth's crust are moving constantly. As the edges of these plates slide against each other in fault zones, friction can slow them down, leading to the buildup of pressure over long periods of time. When the force of movement finally overcomes the friction, sections of the crust suddenly break or become displaced, releasing the pent-up pressure in the form of seismic waves. This is a naturally occurring earthquake, sometimes referred to as a tectonic earthquake.
While tectonic earthquakes can occur at any location around the world, the majority of large earthquakes—about 80 percent—occur at the circum-Pacific seismic belt found along the rim of the Pacific Ocean. Two other regions regularly exhibiting earthquakes include the Alpide belt, extending along the southern margin of Eurasia through the Himalayan Mountains, Sumatra, and Java; and the Mid-Atlantic Ridge running along the floor of the Atlantic Ocean.
When an earthquake occurs, different types of energy waves are generated.
"P waves" or "primary waves" are the first waves to be detected. These are compressional waves that push and pull as they move through rock and fluids.
"S waves" or "secondary waves" are the next waves to be detected. These waves move only through rock. They move up and down or side to side, perpendicular to the direction in which the wave is moving.
Surface waves follow P and S waves. They travel along the surface of the earth and thus cause the most damage. Surface waves can be characterized as Love waves, which are faster and move the ground from side to side, and Rayleigh waves, which roll like waves on the surface of oceans and lakes.
In addition to tectonic earthquakes, seismologists have classified three more earthquake types:
Volcanic: Earthquakes that occur in conjunction with volcanic activity
Collapse: Smaller-scale earthquakes that result from the subterranean collapse of caverns or mines
Explosion: Earthquakes caused by underground explosions of nuclear or chemical devices.
Does fracking cause earthquakes?
"Fracking" is an informal term for hydraulic fracturing, a process used to increase the flow of oil or gas to a production well. It involves the injection of large volumes of water, sand, and chemicals under high pressure into a bedrock formation to create new fractures in the rock or increase the size, extent, and connectivity of existing fractures, leading to more permeability.
On rare occasions, fracking can lead directly to earthquakes. More frequently, earthquakes are induced by the disposal of waste water associated with the oil production into underground wells. Seismic events associated with fracking and waste fluid disposal tend to be low-level and dependent on conditions such as the injection rate and total volume injected, the proximity of faults near the injection site and the pathways permitting pressure to travel from the injection site to the fault, and the presence of stresses on faults that are large enough to produce earthquakes.