Types of Earthquakes: Tectonic, Volcanic, Induced, and More

The word "earthquake" evokes a single phenomenon — the ground shaking — but the processes that cause the ground to shake are remarkably diverse. The vast majority of earthquakes are caused by the slow, relentless motion of tectonic plates, but earthquakes can also be triggered by rising magma beneath volcanoes, by the injection of wastewater deep underground, by the collapse of underground caverns, and even by the calving of glaciers and the tidal stresses on the Moon.

Understanding the different types of earthquakes is more than an academic exercise. The type of earthquake determines its maximum possible size, its depth, its characteristic shaking patterns, and the kinds of secondary hazards (tsunamis, landslides, volcanic eruptions) it can trigger. A subduction megathrust earthquake and a mining-induced collapse event are both "earthquakes" in the sense that they produce seismic waves, but they differ enormously in their causes, their magnitudes, and their consequences.

This article provides a comprehensive classification of earthquake types by cause and by fault mechanics, with real-world examples and verified data for each. What causes earthquakes

Classification by Cause

Tectonic Earthquakes: The Dominant Source

Tectonic earthquakes account for more than 90% of all seismic energy released on Earth. They are caused by the sudden release of elastic strain energy accumulated in the Earth's crust and upper mantle as tectonic plates move relative to one another. The Earth's outer shell is divided into approximately 15 major lithospheric plates and several dozen smaller plates, all in constant motion driven by convective currents in the underlying mantle.

The plates move at rates of a few centimeters per year — comparable to the rate at which fingernails grow. This motion is imperceptible on human timescales, but over decades and centuries, it accumulates enormous elastic strain energy along the boundaries where plates interact. When the accumulated stress exceeds the frictional strength of the rocks along a fault, the fault ruptures suddenly, and the stored energy is released as seismic waves.

The USGS locates approximately 20,000 earthquakes per year — roughly 55 per day — the vast majority of which are tectonic in origin. Most are too small to be felt, but several hundred each year are large enough to be damaging. Understanding tectonic plates

Tectonic earthquakes are further classified by the type of plate boundary where they occur and the style of faulting involved.

Subduction Zone (Megathrust) Earthquakes

The largest earthquakes on Earth — and the only ones capable of reaching magnitude 9.0 or above — occur at subduction zones, where one tectonic plate is thrust beneath another. The contact surface between the overriding and subducting plates, called the megathrust, is typically a gently dipping fault that extends hundreds of kilometers along a plate boundary and can rupture in single, enormous earthquakes.

The three largest instrumentally recorded earthquakes are all megathrust events:

1. 1960 Valdivia, Chile (M9.5): The largest earthquake ever recorded. The rupture extended approximately 1,000 km along the Chilean subduction zone. The earthquake generated a devastating tsunami that crossed the Pacific Ocean, causing fatalities in Hawaii, Japan, and the Philippines.

2. 2004 Indian Ocean (M9.1): The rupture extended approximately 1,300 km along the Sunda megathrust. The resulting tsunami killed an estimated 227,000 people across 14 countries, making it one of the deadliest natural disasters in recorded history.

3. 2011 Tōhoku, Japan (M9.1): The rupture extended approximately 450 km along the Japan Trench. The tsunami reached heights exceeding 40 meters in some coastal areas and triggered the Fukushima Daiichi nuclear disaster.

Megathrust earthquakes are uniquely dangerous because of their enormous magnitude, the large area of fault rupture (which produces long-duration shaking), and their ability to displace the ocean floor and generate catastrophic tsunamis. They occur exclusively at convergent plate boundaries — the "Ring of Fire" that encircles the Pacific Ocean accounts for approximately 81% of the world's largest earthquakes. Ring of Fire

Cascadia Subduction Zone

Continental Collision Earthquakes

Where two continental plates converge — neither of which is dense enough to subduct — the collision crumples and thickens the crust, building mountain ranges and producing large, destructive earthquakes. The Himalayan arc, formed by the collision of the Indian and Eurasian plates, is the most seismically active continental collision zone on Earth.

Continental collision earthquakes are typically shallow (less than 30 km deep) and occur on complex networks of thrust and strike-slip faults. Because they occur beneath some of the most densely populated regions on Earth, they are among the deadliest. The 2005 Kashmir earthquake (M7.6, approximately 87,000 deaths), the 2008 Wenchuan, China earthquake (M7.9, approximately 87,500 deaths), and the 2015 Gorkha, Nepal earthquake (M7.8, approximately 9,000 deaths) are all continental collision events.

Other active continental collision zones include the Zagros Mountains (Iran-Iraq border), the Alps (Europe), the Caucasus Mountains, and the mountains of Central Asia.

Strike-Slip Earthquakes

Strike-slip faults are vertical or near-vertical faults where the two sides move horizontally past each other. The San Andreas Fault in California is the world's most studied strike-slip fault. It accommodates roughly 20–28 mm/year of the approximately 50 mm/year of total Pacific–North American plate motion, depending on the segment.

Strike-slip earthquakes are typically shallow (less than 20 km deep) and can reach magnitudes of approximately 8.0. The 1906 San Francisco earthquake (M7.9), which ruptured approximately 477 km of the San Andreas Fault and killed approximately 3,000 people, is the most famous strike-slip earthquake in U.S. history. The 1999 İzmit, Turkey, earthquake (M7.6) on the North Anatolian Fault and the 2023 Türkiye–Syria earthquake sequence (M7.8 and M7.7) on the East Anatolian Fault are other major strike-slip events.

Strike-slip faults also form transform boundaries between tectonic plates. The San Andreas, the North Anatolian Fault, the Alpine Fault in New Zealand, and the Dead Sea Transform are all plate-boundary transform faults.

Normal Fault Earthquakes

Normal faults occur in extensional tectonic settings — regions where the crust is being pulled apart. One side of the fault drops down relative to the other, accommodating the extension. Normal faulting is characteristic of continental rift zones (such as the East African Rift), back-arc basins behind subduction zones, and regions of broad crustal extension (such as the Basin and Range Province of the western United States).

Normal fault earthquakes are generally smaller than subduction megathrust events, typically reaching magnitudes of 7.0–7.5. However, they can be very destructive because they tend to be shallow. The 2009 L'Aquila, Italy, earthquake (M6.3, 309 deaths) and the 2016 Amatrice, Italy, earthquake (M6.2, 299 deaths) were both normal faulting events in the central Apennines, an extensional tectonic setting.

The largest recorded normal fault earthquake is debated, but the 1556 Shaanxi, China, earthquake — estimated at approximately M8.0–8.3 and responsible for an estimated 830,000 deaths (making it the deadliest earthquake in recorded history) — may have involved normal faulting in the extensional Weihe Graben.

Volcanic Earthquakes

Volcanic earthquakes are caused by the movement, accumulation, and pressure of magma and volcanic gases within and beneath volcanic structures. They are an important tool for monitoring volcanic activity, as changes in earthquake patterns often precede eruptions.

Volcanic earthquakes are generally smaller than tectonic earthquakes — typically below magnitude 5.0 — though exceptions exist. They occur in swarms (clusters of many small earthquakes over hours to weeks) rather than in the mainshock-aftershock pattern characteristic of tectonic earthquakes.

There are several subtypes:

Volcano-tectonic (VT) earthquakes: These result from the fracturing of rock caused by stress changes from magma intrusion. They are essentially small tectonic earthquakes triggered by volcanic processes and produce sharp, impulsive seismic signals similar to ordinary earthquakes.

Long-period (LP) earthquakes: Also called low-frequency events, these are caused by the resonance of fluid-filled cracks and conduits within the volcanic edifice. Their waveforms are characterized by a dominant frequency lower than that of VT events.

Volcanic tremor: A continuous, rhythmic seismic signal produced by the sustained movement of magma or volcanic gases through conduits. Volcanic tremor is distinct from discrete earthquakes and often indicates that an eruption is imminent or underway.

Explosion earthquakes: Produced by volcanic explosions themselves, these events are associated with the surface expression of eruptions.

The 1980 eruption of Mount St. Helens in Washington state was preceded by approximately two months of increasing earthquake activity. The swarm began on March 16, 1980, with the eruption occurring on May 18. A M5.1 earthquake triggered the catastrophic flank collapse and lateral blast that killed 57 people. The precursory seismicity was instrumental in prompting the evacuation of the surrounding area — though the scale of the eruption exceeded expectations.

The 1991 eruption of Mount Pinatubo in the Philippines was one of the most successful eruption predictions in history, based heavily on seismic monitoring. The evacuation of over 60,000 people from surrounding areas is credited with saving tens of thousands of lives.

Induced Earthquakes: Human-Caused Seismicity

Induced earthquakes are caused by human activities that alter the stress state or fluid pressure in the Earth's crust. While the concept is not new — mining-related earthquakes have been documented for over a century — the scale of induced seismicity has increased dramatically in the 21st century, primarily due to wastewater injection from oil and gas operations.

Wastewater Injection

The most consequential source of modern induced seismicity is the deep injection of wastewater produced as a byproduct of oil and gas extraction — particularly from hydraulic fracturing ("fracking") operations. The injected fluid increases pore pressure in the rock, which can reduce the frictional strength of pre-existing faults and trigger slip.

The state of Oklahoma provides the most dramatic example. Before 2009, Oklahoma averaged approximately two earthquakes of magnitude 3.0 or greater per year. By 2015, that number had risen to more than 900 — making Oklahoma more seismically active than California in terms of M3+ earthquakes. The increase was directly linked to the injection of massive volumes of wastewater from oil and gas operations into the Arbuckle Formation, a deep sedimentary unit.

The 2016 Pawnee, Oklahoma, earthquake (M5.8) was the largest instrumentally recorded earthquake in the state and was classified as induced. Following regulatory reductions in injection volumes, the rate of induced seismicity in Oklahoma has declined significantly, but remains elevated above historical background levels. Oklahoma Geological Survey

Reservoir-Induced Seismicity (RIS)

The impoundment of large reservoirs can trigger earthquakes by increasing the load on the crust and raising pore pressure in the underlying rock. The most notable case is the 1967 Koyna earthquake (M6.3) in Maharashtra, India, which killed 177 people. The earthquake occurred near the Koyna Dam, which had been impounded to a depth of approximately 100 meters in the years preceding the event. While the correlation between reservoir filling and seismicity is well-established at Koyna, reservoir-induced seismicity is generally associated with smaller magnitudes (M5 or less) and specific geological conditions.

Other documented cases of RIS include earthquakes near China's Zipingpu Dam (which some researchers have linked to the 2008 Wenchuan earthquake, though this remains disputed), Lake Mead (Nevada/Arizona), and the Aswan Dam in Egypt.

Mining-Induced Seismicity

Deep mining operations, particularly in South Africa's gold mines (which reach depths of 4 km), regularly produce induced earthquakes. The removal of rock mass changes the stress field, and the mines intersect pre-existing faults that can slip in response. South Africa's deep gold mines experience thousands of small seismic events annually, with occasional events exceeding M5.0.

Geothermal Energy

Geothermal energy extraction — particularly Enhanced Geothermal Systems (EGS), which involve injecting fluid to fracture hot rock at depth — has induced earthquakes in several locations.

The 2006 Basel, Switzerland, EGS project was halted after fluid injection triggered a M3.4 earthquake that caused minor damage in the city. The project was permanently abandoned.

The 2017 Pohang, South Korea, earthquake (M5.4) caused 135 injuries and significant building damage. A government investigation concluded in 2019 that the earthquake was induced by fluid injection at a nearby EGS geothermal pilot project. The finding was significant because M5.4 is unusually large for an induced event and the earthquake caused substantial damage in a densely populated area.

Collapse Earthquakes

Collapse earthquakes are caused by the sudden collapse of underground cavities — natural caves, mine workings, or other voids — when the overlying rock can no longer support its own weight. These are typically very small events (usually below M3.0) and are felt only in the immediate vicinity, if at all.

Collapse earthquakes are most common in areas with extensive underground mining or in karst terrain — regions underlain by soluble rock (limestone, dolomite, gypsum) where dissolution has created cave systems. The resulting seismic signals are often implosive rather than the double-couple radiation pattern of tectonic earthquakes.

Ice Quakes (Cryoseisms)

Ice quakes are seismic events generated by the fracturing and movement of ice — in glaciers, ice sheets, or even in frozen ground. The largest glacial earthquakes are produced by the calving of large icebergs from the Greenland and Antarctic ice sheets, which can generate seismic waves equivalent to magnitude 5.0 events detectable by seismometers worldwide.

Research published in the journal Science documented that large glacial earthquakes in Greenland increased sevenfold between 1993 and 2005, correlating with the acceleration of glacier flow associated with climate change. Monitoring glacial earthquakes has become a tool for studying ice sheet dynamics.

Surface cryoseisms — also called frost quakes — occur when water in the ground freezes rapidly and expands, cracking the frozen ground. These events are common in northern climates during sudden cold snaps and can produce sharp booms and shaking felt by residents, though they are not detectable by distant seismometers.

Moonquakes

Seismometers placed on the Moon by Apollo astronauts (Apollo 12, 14, 15, and 16, from 1969 to 1972) detected thousands of moonquakes during their operation until 1977. Lunar seismicity differs fundamentally from Earth's and has been classified into four categories:

Deep moonquakes: Occurring at depths of approximately 700–1,200 km, these small events are triggered by tidal stresses from Earth's gravitational pull. They recur at regular intervals corresponding to the lunar tidal cycle.

Shallow moonquakes: Less common but more energetic, reaching estimated magnitudes of approximately 5.0. Their cause is poorly understood but may involve tectonic stresses from the cooling and contraction of the lunar interior.

Thermal moonquakes: Caused by the expansion and contraction of the lunar surface due to extreme temperature changes between lunar day and night (a range of approximately 280°C).

Meteorite impact quakes: Produced by the impact of meteorites on the lunar surface.

Unlike Earth's earthquakes, which typically last seconds to minutes, moonquakes can continue for extended periods — deep moonquakes sometimes lasted more than 10 minutes, and one shallow moonquake continued for over an hour. The long duration is attributed to the absence of water in lunar rocks, which means less energy attenuation.

Classification by Fault Type

Independent of their cause, earthquakes involving fault slip can be classified by the geometry and direction of motion on the fault. This classification system — based on fault type — describes the mechanics of the rupture.

Normal Faults

On a normal fault, the hanging wall (the block of rock above the fault plane) moves downward relative to the footwall. Normal faults accommodate crustal extension — the crust is being pulled apart. The fault plane typically dips at 45–60 degrees.

Normal faulting produces basins (grabens) flanked by uplifted blocks (horsts). The Basin and Range Province of the western United States — a broad region of alternating mountain ranges and valleys stretching from Utah to California — is the product of millions of years of normal faulting. The East African Rift Valley, where the African plate is slowly splitting apart, is another major zone of normal faulting.

Reverse (Thrust) Faults

On a reverse fault, the hanging wall moves upward relative to the footwall, driven by compressive forces. Reverse faults with shallow dip angles (less than 45 degrees) are called thrust faults. Subduction zone megathrust faults are the largest and most energetic thrust faults on Earth.

Thrust faulting builds mountain ranges and thickens the crust. The Himalayas, the Andes, and the Alps are all products of reverse/thrust faulting at convergent boundaries.

Strike-Slip Faults

On a strike-slip fault, the two sides move horizontally past each other. If the far side moves to the left, the fault is called left-lateral (sinistral). If the far side moves to the right, it is right-lateral (dextral). The San Andreas Fault is a right-lateral strike-slip fault.

Strike-slip faults are typically near-vertical and produce horizontal displacement with little or no vertical motion. They are common at transform plate boundaries and within plates where stress concentrations cause horizontal shearing.

Oblique-Slip Faults

Many real faults exhibit a combination of motion types — for example, both lateral and vertical displacement. These are called oblique-slip faults. The 2010 Haiti earthquake (M7.0, which caused an estimated 100,000 to 316,000 deaths) occurred on a fault system with significant components of both strike-slip and thrust motion.

How Earthquake Depth Affects Damage

Earthquake depth is one of the most important factors determining the level of damage at the surface. Earthquakes are classified into three depth categories:

Shallow-focus earthquakes (0–70 km depth): These account for approximately 75% of all earthquake energy release. Shallow earthquakes, particularly those with hypocenters less than 20 km deep, cause the most intense shaking and the most damage at the surface because the seismic energy has less distance to travel and less opportunity to attenuate before reaching buildings and people. Most of the deadliest earthquakes in history — including the 2010 Haiti earthquake (depth ~13 km), the 2023 Türkiye–Syria earthquakes (depth ~18 km), and the 2015 Nepal earthquake (depth ~15 km) — have been shallow.

Intermediate-focus earthquakes (70–300 km depth): These occur within the subducting slab at convergent boundaries. They are felt over a wider area than shallow earthquakes of the same magnitude but generally cause less intense shaking at the surface due to the greater distance.

Deep-focus earthquakes (300–700 km depth): These occur exclusively within subducting slabs and are among the most scientifically intriguing seismic phenomena. The deepest recorded earthquake occurred at approximately 700 km depth beneath the Sea of Okhotsk in 2013 (M8.3). At these depths, rocks are under enormous confining pressure and temperature, and the mechanism of brittle fracture that produces shallow earthquakes should not operate. The leading explanations involve phase transitions in the subducting slab — the transformation of olivine to its high-pressure polymorph, wadsleyite — which may cause sudden volume changes and localized shear failure.

Deep earthquakes produce broad areas of mild to moderate shaking at the surface but rarely cause significant damage.

[CHART: Histogram — Global Earthquake Depth Distribution] Data: Distribution of global earthquakes by depth (2000–2023, M4.0+, USGS catalog). X-axis: Depth in km (bins: 0–10, 10–20, 20–33, 33–70, 70–150, 150–300, 300–500, 500–700). Y-axis: Number of earthquakes. Approximately 60% of all M4+ earthquakes occur at 0–33 km depth. A secondary peak occurs at 100–200 km (intermediate depth in subduction zones). Deep earthquakes (>300 km) represent less than 4% of all seismicity. The histogram should show the strong concentration of seismicity at shallow depths with a long tail extending to 700 km. Data source: USGS Earthquake Catalog (earthquake.usgs.gov), 2000–2023 Features: Bar histogram with depth bins, annotated with percentage of total for each depth range

The Special Problem of Induced Seismicity

The rapid increase in induced seismicity, particularly in the central United States, has created a novel seismic hazard in regions that historically had little earthquake risk. The USGS now produces separate hazard maps for natural and induced seismicity, and the combined hazard in parts of Oklahoma now rivals that of naturally seismic areas in California.

The legal, regulatory, and public policy implications are significant. Unlike natural earthquakes, induced earthquakes are potentially controllable — reducing injection volumes has been demonstrated to reduce earthquake rates in Oklahoma and other areas. However, the relationship between injection and seismicity is complex: earthquakes can continue for months or years after injection ceases, and the maximum magnitude of an induced earthquake on a given fault is controlled by the fault's geometry and stress state, not by the volume of injected fluid. USGS Earthquake Types USGS Volcanoes and Earthquakes

The 2017 Pohang earthquake in South Korea underscored the stakes: a M5.4 earthquake induced by an EGS geothermal project caused 135 injuries and approximately $52 million in damage (300 million USD in some estimates), leading to the permanent closure of the geothermal site and a government reckoning with the risks of deep fluid injection in seismically active regions.