What Happens During an Earthquake?

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 basics

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.
Credit: © Agata Nowicka/Marlena Agency for Caltech Science Exchange

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.

Credit: © Agata Nowicka/Marlena Agency for Caltech Science Exchange

What other types of earthquakes exist?

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.

An earthquake is caused by a sudden slip on a fault. The tectonic plates are always slowly moving, but they get stuck at their edges due to friction. When the stress on the edge overcomes the friction, there is an earthquake that releases energy in waves that travel through the earth's crust and cause the shaking that we feel.
In California there are two plates - the Pacific Plate and the North American Plate. The Pacific Plate consists of most of the Pacific Ocean floor and the California Coast line. The North American Plate comprises most the North American Continent and parts of the Atlantic Ocean floor. The primary boundary between these two plates is the San Andreas Fault. The San Andreas Fault is more than 650 miles long and extends to depths of at least 10 miles. Many other smaller faults like the Hayward (Northern California) and the San Jacinto (Southern California) branch from and join the San Andreas Fault Zone.
The Pacific Plate grinds northwestward past the North American Plate at a rate of about two inches per year. Parts of the San Andreas Fault system adapt to this movement by constant "creep" resulting in many tiny shocks and a few moderate earth tremors. In other areas where creep is NOT constant, strain can build up for hundreds of years, producing great earthquakes when it finally releases.

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.