The ground beneath your feet is not as solid as it seems. Earth's outermost layer is broken into massive slabs of rock called tectonic plates, and these plates are constantly in motion — colliding, pulling apart, and grinding past one another. Earthquakes are the result of sudden energy release at the boundaries and fractures where this motion creates stress that eventually exceeds the strength of the rock.
Understanding what causes earthquakes is foundational to everything from building safer structures to predicting where the next major disaster might strike. This article covers the complete science: plate tectonics, fault mechanics, the different types of earthquakes, where they occur most frequently, and why some seemingly "safe" regions are anything but.
For a deeper look at how earthquake energy is quantified, see our guide to the earthquake magnitude scale. To understand the waves earthquakes produce, visit earthquake waves explained.
Plate Tectonics: The Engine Behind Earthquakes
The Structure of Earth's Interior
Earth's interior is layered. At the center is a solid inner core of iron and nickel, surrounded by a liquid outer core. Above that sits the mantle — a thick layer of semi-solid rock that behaves like an extremely viscous fluid over geological timescales. The outermost layer is the crust: a thin, rigid shell ranging from about 5 km thick beneath the oceans to 70 km thick under mountain ranges.
The relevant distinction for earthquake science is between the lithosphere and the asthenosphere. The lithosphere includes the crust and the uppermost part of the mantle, forming rigid plates typically 100–200 km thick. Beneath it, the asthenosphere is a hotter, weaker zone of the upper mantle where rock can flow slowly. This flow is what allows tectonic plates to move.
What Are Tectonic Plates?
Earth's lithosphere is divided into 15 major tectonic plates and several smaller ones. The largest — the Pacific Plate — covers approximately 103 million km². The plates are not static; they move at rates ranging from about 1 cm/year to over 10 cm/year, driven by forces including mantle convection, ridge push at mid-ocean ridges, and slab pull where dense oceanic lithosphere sinks into the mantle at subduction zones.
According to the USGS, the movements of these plates are responsible for the vast majority of earthquakes, volcanic eruptions, and mountain-building events on Earth. The theory of plate tectonics, formalized in the 1960s, unified decades of observations about continental drift, seafloor spreading, and global seismicity patterns into a single framework USGS Science of Earthquakes.
The Three Types of Plate Boundaries
Earthquakes concentrate along plate boundaries, but not all boundaries produce earthquakes in the same way. The three fundamental types of plate boundaries each generate distinct seismic activity.
Convergent Boundaries (Collision Zones)
At convergent boundaries, plates move toward each other. When oceanic lithosphere meets continental lithosphere, the denser oceanic plate dives beneath the continental plate in a process called subduction. This generates the world's largest and deepest earthquakes.
The 2011 Tōhoku earthquake (M9.1) off the coast of Japan occurred at the convergent boundary where the Pacific Plate subducts beneath the Okhotsk Plate. The 2004 Indian Ocean earthquake (M9.1) was caused by subduction of the Indo-Australian Plate beneath the Burma Plate. The Cascadia Subduction Zone off the Pacific Northwest coast of the United States is a convergent boundary capable of producing M9+ earthquakes — the last occurred in 1700.
When two continental plates converge, neither subducts easily. Instead, the crust crumples and folds, building mountain ranges. The Himalayas are the result of the ongoing collision between the Indian and Eurasian Plates, which produces frequent and destructive earthquakes across Nepal, northern India, and Pakistan. The 2015 Gorkha earthquake (M7.8) in Nepal killed nearly 9,000 people along this boundary.
Divergent Boundaries (Spreading Zones)
At divergent boundaries, plates pull apart. Magma rises from the mantle to fill the gap, creating new oceanic crust. The Mid-Atlantic Ridge is the longest divergent boundary on Earth, stretching over 16,000 km from the Arctic to near Antarctica.
Earthquakes at divergent boundaries are typically shallower and smaller than those at convergent boundaries, usually below M8. Iceland sits directly on the Mid-Atlantic Ridge, where the North American and Eurasian Plates are separating at about 2.5 cm/year. This produces frequent moderate earthquakes and volcanic activity across the island.
The East African Rift System is a continental divergent boundary where the African Plate is splitting into the Nubian and Somali Plates. This rift produces significant seismic activity across Ethiopia, Kenya, Tanzania, and surrounding countries.
Transform Boundaries (Sliding Zones)
At transform boundaries, plates slide horizontally past each other. The San Andreas Fault in California is the world's most studied transform boundary, where the Pacific Plate moves northwest relative to the North American Plate at approximately 50 mm/year. The San Andreas Fault accommodates roughly 20–28 mm/year of this total motion, with the remainder distributed across parallel faults including the Hayward, Calaveras, and San Jacinto Faults.
Transform boundary earthquakes are generally shallow (less than 20 km deep) and can be very destructive because they occur close to the surface. The 1906 San Francisco earthquake (M7.9) and the 2010 Haiti earthquake (M7.0) both occurred on transform faults.
| Boundary Type | Plate Motion | Typical Max Magnitude | Typical Depth | Real-World Examples |
|---|---|---|---|---|
| Convergent (subduction) | Plates move toward each other; one sinks beneath the other | M9.5 | Shallow to deep (0–700 km) | Japan Trench, Cascadia, Peru-Chile Trench, Sumatra |
| Convergent (continental collision) | Two continental plates collide | M8.5 | Shallow to intermediate (0–200 km) | Himalayas, Zagros Mountains (Iran) |
| Divergent | Plates pull apart | M8.0 | Shallow (0–30 km) | Mid-Atlantic Ridge, East African Rift, Iceland |
| Transform | Plates slide past each other | M8.5 | Shallow (0–20 km) | San Andreas (CA), Alpine Fault (NZ), North Anatolian (Turkey) |
World map showing tectonic plate boundaries and major earthquake zones
Features: All 15 major plates labeled; convergent boundaries in red, divergent in blue, transform in green; dots for M7+ earthquakes in the past 50 years color-coded by depth
Fault Mechanics: Where Rock Breaks
A fault is a fracture in Earth's crust along which blocks of rock have moved relative to each other. Faults are classified by the direction of movement, which reflects the type of stress acting on the rock.
Normal Faults
Normal faults form under extensional stress — when the crust is being pulled apart. The rock above the fault plane (the hanging wall) drops downward relative to the rock below (the footwall). Normal faults are common at divergent boundaries and rift zones.
The Wasatch Fault along the front of the Wasatch Range in Utah is a major normal fault capable of producing earthquakes up to approximately M7.5. The Basin and Range Province across Nevada and western Utah is defined by hundreds of normal faults created by crustal extension.
Reverse (Thrust) Faults
Reverse faults form under compressional stress — when the crust is being pushed together. The hanging wall moves upward relative to the footwall. When the fault plane is angled at less than 45°, it is called a thrust fault. These produce many of the world's largest earthquakes.
The fault that produced the 2011 Tōhoku M9.1 earthquake was a thrust fault in the Japan Trench. The Himalayan Main Frontal Thrust is a massive thrust fault responsible for building the Himalayas and for devastating earthquakes throughout the region.
Strike-Slip Faults
Strike-slip faults form under shear stress, with blocks of rock moving horizontally past each other. If the far side of the fault moves to the right, it is a right-lateral (dextral) fault; if it moves to the left, it is a left-lateral (sinistral) fault.
The San Andreas Fault is a right-lateral strike-slip fault. The North Anatolian Fault in Turkey, which produced the devastating 2023 Kahramanmaraş earthquake sequence (M7.8 and M7.5), is also right-lateral. The Dead Sea Transform between the Arabian and African Plates is left-lateral.
Elastic Rebound Theory
In 1910, geologist Harry Fielding Reid studied the aftermath of the 1906 San Francisco earthquake and proposed the elastic rebound theory, which remains the foundational model for understanding how earthquakes occur.
The theory describes a cycle: tectonic forces slowly deform rocks on either side of a locked fault. The rock bends and stores elastic strain energy, like a compressed spring. When the accumulated stress exceeds the frictional strength of the fault — the point of failure — the rock snaps back to its undeformed shape, releasing the stored energy as seismic waves. The fault slips, and an earthquake occurs.
The time between major earthquakes on a given fault is called the recurrence interval. On the southern San Andreas Fault, the average recurrence interval for major earthquakes is estimated at roughly 150 years, according to paleoseismic studies — and the last major event on that segment was in 1857, over 165 years ago USGS Earthquake Hazards — Faults.
Types of Earthquakes
Tectonic Earthquakes
The vast majority of earthquakes — over 90% — are tectonic, caused by the movement and interaction of tectonic plates. These range from imperceptible microearthquakes to catastrophic M9+ megathrust events. Every example discussed above in the plate boundaries section involves tectonic earthquakes.
Volcanic Earthquakes
Volcanic earthquakes are caused by the movement of magma beneath a volcano. As magma forces its way through rock, it fractures the surrounding material and generates seismic waves. These earthquakes are typically smaller than major tectonic events (usually below M5) but can serve as critical warnings of impending eruptions.
Before the 1980 eruption of Mount St. Helens, seismologists recorded thousands of small earthquakes — including a M4.2 event on March 20, 1980 — that signaled magma was rising. According to the USGS, volcanic seismicity is now one of the primary tools used to forecast eruptions USGS Volcano Hazards Program.
Long-period (LP) volcanic earthquakes, caused by the resonance of fluid-filled cracks, and volcanic tremor — continuous rhythmic seismic signals — are distinct from ordinary tectonic earthquakes and are monitored continuously at active volcanoes worldwide.
Induced (Human-Caused) Earthquakes
Human activities can trigger earthquakes through several mechanisms:
Wastewater injection. The injection of large volumes of fluid underground — particularly wastewater from oil and gas operations — can increase pore pressure on pre-existing faults, reducing the effective friction and allowing them to slip. Oklahoma experienced a dramatic increase in seismicity beginning around 2009, going from an average of about two M3+ earthquakes per year to over 900 in 2015, according to the Oklahoma Geological Survey. The M5.8 Pawnee earthquake in September 2016 was the largest instrumentally recorded earthquake in Oklahoma's history and was linked to wastewater disposal. After injection volumes were reduced through regulatory action, seismicity decreased significantly.
Reservoir-induced seismicity. The weight of water behind large dams can alter stress conditions on underlying faults. The 1967 Koyna earthquake (M6.3) in India, which killed approximately 180 people, occurred near the Koyna Dam and is one of the best-documented cases of reservoir-induced seismicity. The filling of Lake Mead behind Hoover Dam in the 1930s also triggered hundreds of small earthquakes.
Mining. The removal of large volumes of rock and subsequent redistribution of stress can trigger earthquakes. South Africa's deep gold mines experience frequent mining-induced seismicity, with some events reaching M5+.
Hydraulic fracturing (fracking). While fracking itself typically causes only small microearthquakes (M < 1), the process can occasionally induce felt events. More commonly, it is the disposal of fracking wastewater through injection wells — rather than the fracturing process itself — that triggers larger induced earthquakes.
Where Earthquakes Happen Most
The Ring of Fire
The Pacific Ring of Fire is a roughly 40,000 km horseshoe-shaped zone of intense seismic and volcanic activity encircling the Pacific Ocean. It runs from New Zealand, along the eastern edge of Asia, north across the Aleutian Islands, and south along the western coast of the Americas from Alaska to southern Chile.
According to the USGS, approximately 81% of the world's largest earthquakes occur along the Ring of Fire. This zone includes numerous subduction zones where oceanic plates dive beneath continental plates, producing the most powerful earthquakes on Earth USGS Earthquake Hazards Program.
The five largest instrumentally recorded earthquakes all occurred along the Ring of Fire:
- 1960 Valdivia, Chile — M9.5
- 1964 Great Alaska earthquake — M9.2
- 2004 Indian Ocean (Sumatra) — M9.1
- 2011 Tōhoku, Japan — M9.1
- 1952 Kamchatka, Russia — M9.0
For more on this region, see our complete guide to the Ring of Fire.
The Alpide Belt
The second most seismically active zone on Earth is the Alpide Belt, stretching from the Mediterranean through Turkey, Iran, and the Himalayas to Southeast Asia. This belt accounts for approximately 17% of the world's largest earthquakes and includes the collision zones of the African, Arabian, and Indian Plates with the Eurasian Plate.
The 2023 Turkey-Syria earthquake sequence, the 2005 Kashmir earthquake (M7.6, ~87,000 deaths), and the 2015 Nepal earthquake (M7.8) all occurred along the Alpide Belt.
Pie chart or donut chart — Distribution of world's largest earthquakes by tectonic zone
Data: Ring of Fire ~81%, Alpide Belt ~17%, Other (mid-ocean ridges, intraplate) ~2%. Source: USGS.
Mid-Ocean Ridges
Mid-ocean ridges produce frequent but generally moderate earthquakes. These divergent boundaries are mostly underwater and rarely cause damage, but they account for a large number of recorded seismic events globally.
Most Seismically Active Countries
Earthquake frequency varies dramatically by country. The following table ranks countries by the average number of significant earthquakes (M5+) per year, based on data from the USGS and NOAA Significant Earthquake Database for the period 1900–2024.
| Rank | Country | Avg. M5+ Earthquakes/Year | Largest Recorded Earthquake | Tectonic Setting |
|---|---|---|---|---|
| 1 | Indonesia | ~150 | M9.1 (2004, Sumatra) | Subduction (multiple trenches) |
| 2 | Japan | ~100 | M9.1 (2011, Tōhoku) | Subduction (Pacific, Philippine Sea Plates) |
| 3 | China | ~50 | M8.0 (2008, Sichuan) | Continental collision (India-Eurasia) |
| 4 | Iran | ~40 | M7.8 (1990, Manjil-Rudbar) | Continental collision (Arabia-Eurasia) |
| 5 | Turkey | ~30 | M7.8 (2023, Kahramanmaraş) | Transform/collision (Anatolian Plate) |
| 6 | Peru | ~25 | M8.0 (2007, Pisco) | Subduction (Nazca Plate) |
| 7 | Chile | ~25 | M9.5 (1960, Valdivia) | Subduction (Nazca Plate) |
| 8 | Philippines | ~25 | M8.0 (1918, Celebes Sea) | Subduction (multiple plates) |
| 9 | United States | ~20 | M9.2 (1964, Alaska) | Subduction (Cascadia, Alaska); transform (San Andreas) |
| 10 | Mexico | ~15 | M8.1 (1985, Michoacán) | Subduction (Cocos Plate) |
US map showing major fault lines and seismic hazard zones
Features: Major fault lines colored by type (normal, reverse, strike-slip); background shading showing peak ground acceleration with 2% probability of exceedance in 50 years; label major faults: San Andreas, Cascadia, Wasatch, New Madrid Seismic Zone, Ramapo
Earthquake Depth Classifications
Earthquakes are classified by the depth of their focus (hypocenter) — the point underground where rupture initiates. Depth profoundly affects both the intensity of shaking at the surface and the area over which an earthquake is felt.
| Classification | Depth Range | Percentage of All Earthquakes | Characteristics | Examples |
|---|---|---|---|---|
| Shallow | 0–70 km | ~72% | Most destructive; strong shaking concentrated near epicenter | 1906 San Francisco (10 km), 2010 Haiti (13 km), 2023 Turkey (10 km) |
| Intermediate | 70–300 km | ~21% | Felt over wider area; less intense surface shaking | 2001 El Salvador (60–100 km), Hindu Kush region events |
| Deep | 300–700 km | ~7% | Rarely cause damage; felt over very large areas with low intensity | 2013 Okhotsk Sea M8.3 (609 km), 2015 Bonin Islands M7.9 (682 km) |
Shallow earthquakes are by far the most destructive because seismic energy has less distance to travel and less material to attenuate it before reaching the surface. The devastating 2010 Haiti earthquake (M7.0) was so destructive in part because its hypocenter was only 13 km deep and directly beneath a densely populated area.
Deep earthquakes occur exclusively in subduction zones, where oceanic lithosphere descends deep into the mantle. The deepest recorded earthquake occurred at approximately 700 km depth near the Flores Sea in Indonesia in 2018. Below about 700 km, the increasing temperature and pressure cause rock to deform plastically rather than fracturing, so earthquakes do not occur at greater depths.
How Often Do Earthquakes Happen?
Earth is seismically active every single day. The USGS National Earthquake Information Center (NEIC) locates approximately 20,000 earthquakes worldwide each year — roughly 55 per day. Scientists estimate the true total, including events too small for the global network to detect, may exceed 500,000 annually. Of recorded earthquakes, roughly 100 per year are large enough to cause damage USGS Earthquake Facts.
The Gutenberg-Richter relationship, established by seismologists Beno Gutenberg and Charles Richter in the 1940s, describes the frequency-magnitude distribution of earthquakes: for each unit increase in magnitude, the number of earthquakes decreases by approximately a factor of 10. This means M5 earthquakes are roughly 10 times more common than M6 earthquakes.
| Magnitude | Description | Estimated Annual Average Worldwide | Equivalent Energy |
|---|---|---|---|
| 2.0–2.9 | Generally not felt; recorded by instruments | ~1,300,000 | Small lightning bolt |
| 3.0–3.9 | Often felt near epicenter; rarely causes damage | ~130,000 | — |
| 4.0–4.9 | Felt widely; minor damage possible near epicenter | ~13,000 | 6 tons of TNT |
| 5.0–5.9 | Damaging to poorly constructed buildings | ~1,300 | ~32 kilotons of TNT (approximately 2 Hiroshima bombs) |
| 6.0–6.9 | Destructive in populated areas up to ~160 km from epicenter | ~130 | ~1 megaton of TNT |
| 7.0–7.9 | Major earthquake; serious damage over large areas | ~15 | ~32 megatons of TNT |
| 8.0+ | Great earthquake; severe destruction near epicenter, damage at great distances | ~1 | ~1 gigaton of TNT |
Data: USGS Earthquake Facts and Statistics. Estimated annual averages are based on observations from 1900 to present and extrapolation for smaller magnitudes that are not fully detected by the global network. The NEIC locates about 20,000 of these per year. Energy equivalents are approximate and calculated from the standard log-linear energy-magnitude relationship (log₁₀E = 1.5M + 4.8, energy in joules).
[CHART: Bar chart — Estimated annual earthquake frequency by magnitude (M2 through M8+)] Data: USGS estimated annual averages. Y-axis: count (log scale). X-axis: magnitude range. Label each bar with the count.
To see current earthquake activity worldwide, visit our live earthquake tracker.
Why "Safe" Areas Still Get Earthquakes
Intraplate Earthquakes
Most earthquakes occur near plate boundaries, but some of the most surprising and historically destructive earthquakes have struck far from any boundary. These are called intraplate earthquakes, and they pose a particular challenge for seismic hazard assessment because they occur in regions where earthquakes are rare and buildings are typically not designed to withstand strong shaking.
The New Madrid Seismic Zone
The central United States experienced a sequence of four massive earthquakes during the winter of 1811–1812, centered near New Madrid, Missouri. Three of these events are estimated to have been M7.0 or larger — likely between M7.2 and M8.0, based on felt reports and geological evidence. These earthquakes rang church bells in Boston (over 1,600 km away), caused the Mississippi River to temporarily flow backward in some sections, and created Reelfoot Lake in northwestern Tennessee.
The New Madrid Seismic Zone (NMSZ) remains active. The USGS estimates a 25–40% probability of a M6.0 or larger earthquake in the zone within the next 50 years. The underlying cause is not fully understood, but it may relate to ancient rifting structures (the Reelfoot Rift) that created zones of weakness in the continental crust USGS — New Madrid Seismic Zone.
The 1886 Charleston, South Carolina Earthquake
On August 31, 1886, an estimated M7.0 earthquake struck near Charleston, South Carolina, killing at least 60 people and causing extensive damage. It remains the largest earthquake in the southeastern United States in recorded history. The earthquake was felt across roughly two-thirds of the eastern United States.
Like New Madrid, the Charleston seismic zone exists far from any plate boundary. Research published by the South Carolina Geological Survey suggests that ancient buried faults, possibly related to the breakup of the supercontinent Pangaea, are reactivated by modern compressive stresses.
The 2011 Virginia Earthquake
On August 23, 2011, a M5.8 earthquake struck near Mineral, Virginia, and was felt by an estimated one-third of the U.S. population — more people than any other earthquake in North American history. The earthquake was felt from Georgia to Maine and as far west as Chicago.
Eastern North American earthquakes are felt over much larger areas than comparable West Coast events because the crust in the east is older, colder, and more rigid, transmitting seismic waves more efficiently with less attenuation.
Why Intraplate Earthquakes Happen
The causes of intraplate earthquakes are still an active area of research. Several mechanisms have been proposed:
- Ancient zones of weakness. Failed rifts, old sutures from past plate collisions, and other buried structures create weak zones in otherwise stable continental crust. These structures can be reactivated by far-field stresses transmitted from plate boundaries.
- Glacial rebound. In regions that were covered by ice sheets during the last glacial period (ending ~12,000 years ago), the crust is still slowly rebounding from the removal of the ice load. This changing stress field can trigger earthquakes, as observed in Scandinavia and eastern Canada.
- Mantle dynamics. Deep-seated mantle processes, including hotspot activity and variations in the thickness and temperature of the lithosphere, can create localized stress concentrations far from plate boundaries.
Earthquake Prediction vs. Forecasting
No scientist or agency can predict earthquakes — that is, specify the exact time, location, and magnitude of a future event. Despite decades of research, reliable short-term prediction remains elusive. The USGS explicitly states that neither they nor any other scientists have ever predicted a major earthquake and do not expect to be able to do so in the foreseeable future USGS — Can You Predict Earthquakes?.
What scientists can do is forecast earthquakes — estimate the probability of an earthquake of a given magnitude occurring in a specified region over a given time period. These probabilistic seismic hazard assessments inform building codes, insurance rates, and emergency planning.
The Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3), published in 2015, estimated a 72% probability of at least one M6.7 or larger earthquake in the San Francisco Bay Area between 2014 and 2043, and a 93% probability of at least one M6.7+ earthquake somewhere in Southern California in the same period.
Earthquake early warning systems — such as ShakeAlert in the western United States — do not predict earthquakes but detect them as they begin. These systems can provide seconds to tens of seconds of warning before strong shaking arrives at a location, enough time to take protective actions like Drop, Cover, and Hold On.
Current Frontiers in Earthquake Science
Slow-Slip Events and Episodic Tremor
Scientists have discovered that some faults slip slowly and silently over days to weeks, releasing energy without producing a traditional earthquake. These slow-slip events, first identified along the Cascadia Subduction Zone in the early 2000s, are often accompanied by a type of seismic signal called tectonic tremor. Research from the University of Washington and Natural Resources Canada suggests that slow-slip events along Cascadia occur roughly every 14 months and may incrementally load stress onto the locked portion of the subduction zone — potentially bringing it closer to a major rupture.
Machine Learning in Seismology
Artificial intelligence and machine learning are transforming earthquake science. Algorithms trained on vast datasets of seismic recordings can now detect small earthquakes that human analysts and traditional methods miss, potentially increasing the number of cataloged events by an order of magnitude. Research published in journals including Nature and Science has shown that machine learning models can identify patterns in seismic noise that may precede earthquakes, though this work is still in early stages and does not constitute prediction.
Fiber-Optic Seismology (DAS)
Distributed Acoustic Sensing (DAS) repurposes existing fiber-optic telecommunications cables as dense seismic sensor arrays. A single fiber-optic cable can function as thousands of individual seismic sensors, providing unprecedented spatial resolution. This technology has been deployed on the ocean floor, along fault zones, and beneath cities, offering new data on earthquake processes in areas that are difficult to instrument with traditional seismographs.