The term earthquake describes any event that causes the shaking of the Earth’s surface, resulting from a sudden release of stored energy within the planet’s lithosphere. This release generates seismic waves that travel through the rock, causing the ground to vibrate. Seismic events are complex phenomena triggered by various geological and non-geological processes. Scientists classify them into distinct categories based on their underlying cause and their depth of origin to better understand their behavior.
The Primary Mechanism: Tectonic Earthquakes
The vast majority of significant seismic activity worldwide is attributed to tectonic earthquakes, which are generated by the movement and interaction of the Earth’s large tectonic plates. These enormous sections of the lithosphere are constantly grinding against, pulling away from, or colliding with one another, causing immense stress to build up in the surrounding rock. When the accumulated strain exceeds the strength of the rock mass, the rock fractures and slips along a fault plane, releasing the energy in seismic waves.
This plate interaction defines three primary types of faults and their associated earthquakes. Strike-slip faults, such as the San Andreas Fault in California, occur where two blocks of crust slide horizontally past each other due to shearing forces. The movement is predominantly lateral, classified as right-lateral or left-lateral depending on the direction of motion.
Normal faults form in areas where the crust is being pulled apart by extensional forces, such as in rift valleys. The rock mass above the fault plane moves downward relative to the block below it, creating space and lengthening the crust. These events are generally less destructive than those caused by other fault types.
Reverse faults, or thrust faults when the angle is shallow, are found in zones of compression where plates are pushing together. The rock mass above the fault plane is forced upward over the block below, leading to crustal shortening and the formation of mountain ranges. This type of faulting is responsible for the most powerful seismic events, including megathrust earthquakes in subduction zones.
Earthquakes Not Caused by Plate Movement
While tectonic events dominate global seismicity, several categories of earthquakes are generated by different mechanisms. Volcanic earthquakes are caused by the movement of magma and gases beneath the surface of a volcano. As magma rises, it fractures the surrounding rock, generating swarms of small, high-frequency seismic events known as volcano-tectonic earthquakes.
The pressure from volcanic fluids can also cause signals like long-period earthquakes and volcanic tremor, which indicate the resonance of fluid-filled cracks. These events help scientists monitor the stability of a volcano and predict potential eruptions.
Collapse earthquakes represent another distinct type, occurring when underground cavities, such as those found in karst topography or mines, suddenly cave in. These events are usually shallow and of a small magnitude, often triggered when the structural support is weakened or removed by human activity like mining.
Induced seismicity refers to earthquakes caused by human industrial activity. The most widely recognized cause is the injection of wastewater deep underground, which increases the pore pressure within rock formations. This increased pressure can effectively “lubricate” pre-existing faults, overcoming the friction that held them locked and allowing them to slip. Other activities that can trigger induced earthquakes include the impoundment of large reservoirs behind dams and the high-pressure injection of fluids for enhanced geothermal energy systems.
Classification Based on Depth of Origin
Seismologists also classify earthquakes based on the depth of the hypocenter, the point of initial rupture below the Earth’s surface. Earthquakes are divided into three main depth categories, which affect the resulting ground shaking intensity.
Shallow-focus earthquakes originate at depths from the surface down to about 70 kilometers and are the most common type of seismic event. Because their energy is released closest to the surface, they are responsible for the most intense and destructive shaking experienced by human populations and infrastructure.
Intermediate-focus earthquakes occur between 70 and 300 kilometers deep, typically within the subducting slab of a plate boundary. The seismic waves from these events must travel a greater distance through the crust, causing the energy to dissipate more before reaching the surface.
Deep-focus earthquakes originate at depths between 300 and 700 kilometers. These events are the least common, and the extreme depth means the seismic waves lose substantial energy as they propagate. The resulting ground shaking at the Earth’s surface is much less intense compared to that of a shallow-focus event of similar magnitude.