Earthquakes: How the Ground Shakes, Why Faults Slip, and How Science Helps Detect Them

Earthquakes happen when rocks inside Earth suddenly break or slip along a fault, releasing energy that travels through the ground as seismic waves. The shaking may last only seconds, but the effects can range from barely noticeable motion to serious damage, depending on the earthquake’s size, depth, distance, local soil, building conditions, and nearby hazards.

The U.S. Geological Survey explains that an earthquake happens when two blocks of Earth suddenly slip past one another. The surface where they slip is called a fault or fault plane. The underground starting point is the hypocenter or focus, and the point directly above it at the surface is the epicenter. (USGS)

Preparedness note: This page is educational. For an active earthquake, tsunami warning, or emergency situation, follow official alerts from local emergency management, the U.S. Geological Survey, NOAA Tsunami Warning Centers, local officials, and emergency services.

What Is an Earthquake?

An earthquake is shaking caused by a sudden release of energy inside Earth. Most earthquakes happen when stress builds up along a fault until rock suddenly slips. The released energy moves outward as waves, similar to ripples spreading across a pond after a stone is dropped.

A simple way to picture it:

1. Tectonic forces slowly push or pull rock.
2. Rock bends or locks along a fault.
3. Stress builds over time.
4. The fault suddenly slips.
5. Energy travels outward as seismic waves.
6. The ground shakes.

Earthquakes can happen near plate boundaries, within tectonic plates, near volcanoes, or in some cases because of human activities that change underground pressure, such as fluid injection.

Key Earthquake Terms

Earth’s Structure and Tectonic Plates

Earth is not a solid ball of rock all the way through. It has layers.

Earth’s outer shell is broken into large pieces called tectonic plates. These plates move slowly, usually only a few centimeters per year. That may sound tiny, but over decades, centuries, and thousands of years, the motion builds stress in rocks.

Earthquakes are common near plate boundaries because that is where plates collide, pull apart, or slide past one another. But earthquakes can also occur within plates, especially along old or hidden faults.

Types of Plate Boundaries

The largest earthquakes usually happen in subduction zones, where one tectonic plate dives beneath another. These zones can produce very large fault ruptures and may also generate tsunamis if the seafloor moves suddenly.

Faults: Where Earthquakes Begin

A fault is a break in Earth’s crust where rock can move. Not every fault moves often, and not every fault produces large earthquakes. But faults are important because they are the surfaces where many earthquakes start.

Main Fault Types

A fault may stay locked for a long time. During that time, stress builds. When the stress becomes greater than the friction holding the fault in place, the fault slips.

Elastic Rebound: Why Faults Snap

The idea of elastic rebound helps explain earthquakes. Rocks may seem solid and stiff, but under stress they can bend slightly. If a fault is locked, the rocks on either side may slowly deform as tectonic forces continue pushing.

Eventually, the fault slips suddenly. The bent rock “rebounds” toward a less-stressed shape, releasing energy as seismic waves.

A classroom analogy is bending a stick:

• At first, the stick bends.
• Stress builds.
• If the stress becomes too great, the stick snaps.
• Energy is released suddenly.

Faults are much larger and more complex than a stick, but the basic idea is similar: stored energy is released suddenly.

Seismic Waves: How Earthquake Energy Travels

When a fault slips, it sends out seismic waves. Different types of waves move in different ways.

P waves arrive first. S waves and surface waves usually cause stronger shaking. Earthquake early warning systems use the fact that fast electronic alerts can travel faster than the slower damaging seismic waves. USGS explains that earthquake early warning works because alerts can be transmitted almost instantly, while earthquake shaking waves travel through Earth’s shallow layers at roughly one to a few kilometers per second. (USGS)

Magnitude vs. Intensity

People often confuse magnitude and intensity, but they describe different things.

Magnitude

Magnitude describes how much energy was released at the earthquake source. Modern earthquake science usually uses moment magnitude, especially for larger earthquakes.

USGS explains that each whole-number increase in magnitude represents a tenfold increase in measured wave amplitude on a seismogram. A magnitude 6.3 earthquake records waves about 10 times larger in amplitude than a magnitude 5.3 earthquake. (USGS)

This is why a magnitude 7 earthquake is not “a little bigger” than a magnitude 6. It releases much more energy.

Intensity

Intensity describes what people feel and what happens at a location. The same earthquake may produce weak shaking far away and strong shaking near the fault. Local soil and geology can also amplify shaking.

Intensity is often described using the Modified Mercalli Intensity Scale, which ranges from low levels that only a few people feel to severe shaking that can cause heavy damage.

Why Two Places Feel the Same Earthquake Differently

The strength of shaking at a location depends on several factors.

A smaller shallow earthquake nearby may feel stronger than a larger deep earthquake farther away.

Common Earthquake Hazards

Earthquakes are multi-hazard events. The shaking itself is only one part of the risk.

USGS notes that earthquake-triggered landslides and liquefaction can have significant impacts, including damage to structures, roads, ports, industrial areas, commercial facilities, and rivers blocked by unstable landslide dams. (USGS)

Surface Rupture

Surface rupture happens when a fault slip reaches the ground surface. The land may shift horizontally, vertically, or both. Surface rupture is usually most important close to the fault.

Surface rupture can affect:

• Roads
• Rail lines
• Pipelines
• Canals
• Fences
• Buildings
• Power lines
• Water systems

Not every earthquake creates visible surface rupture. Some ruptures remain underground.

Liquefaction

Liquefaction happens when loose, water-saturated soil temporarily loses strength during shaking. The soil begins to behave more like a liquid than a solid.

This can cause:

• Buildings to tilt or settle
• Roads to crack or sink
• Underground pipes to float or break
• Sand boils to appear at the surface
• Ports and waterfront areas to deform
• Foundations to lose support

Liquefaction is most common in loose sandy or silty soils with shallow groundwater, especially near rivers, bays, deltas, and reclaimed land.

Landslides and Rockfalls

Earthquake shaking can loosen slopes. Landslides may occur during or after shaking, especially where slopes are steep, rock is fractured, soil is weak, or rain has already saturated the ground.

Earthquake-triggered slope failures include:

Mountain roads, canyons, coastal bluffs, and steep hillsides may be especially vulnerable.

Tsunamis

Some earthquakes can generate tsunamis. A tsunami is not a normal wind wave. It is a series of long waves usually caused by sudden movement of the seafloor.

USGS explains that tsunamis can be generated when the seafloor experiences rapid vertical displacement, such as during large shallow earthquakes or submarine landslides. (USGS)

Tsunamis are most likely when:

• The earthquake occurs under or near the ocean.
• The earthquake is large.
• The rupture is shallow.
• The seafloor moves vertically.
• The earthquake occurs in a subduction zone.

NOAA’s Tsunami Warning Centers use seismic and sea-level networks with tsunami forecast models to refine warning messages. (NOAA)

Aftershocks

Aftershocks are earthquakes that follow a larger earthquake. They happen because the crust is adjusting after the main fault slip.

Aftershocks usually:

• Are most frequent soon after the mainshock
• Become less frequent over time
• Can continue for weeks, months, or longer
• May be strong enough to cause additional damage
• Can occur on the same fault or nearby faults

USGS provides aftershock forecasts that estimate expected aftershock numbers and the probability of larger aftershocks after some earthquakes. These forecasts are for situational awareness, not exact predictions of individual aftershocks. (USGS Earthquake Hazards Program)

Foreshocks

A foreshock is an earthquake that happens before a larger earthquake in the same area. The tricky part is that scientists usually cannot know an earthquake is a foreshock until after the larger earthquake happens.

Most small earthquakes are not followed by large earthquakes. That is one reason exact earthquake prediction is so difficult.

Can Scientists Predict Earthquakes?

No reliable method can currently predict the exact time, location, and magnitude of a future major earthquake.

USGS states that neither USGS nor any other scientists have ever predicted a major earthquake and that a true prediction would need three elements: date and time, location, and magnitude. USGS scientists can calculate probabilities of significant earthquakes in specific areas over certain time periods, but that is not the same as exact prediction. (USGS)

This distinction is important. Earthquake forecasting is real. Earthquake early warning is real in some areas. Exact earthquake prediction is not currently possible.

Earthquake Early Warning

Earthquake early warning does not predict earthquakes before they start. It detects an earthquake that has already begun.

USGS describes ShakeAlert as a system that detects an earthquake that has already started, estimates location, magnitude, and shaking intensity, and issues a message if the earthquake is large enough to meet alert thresholds. (USGS)

How Early Warning Works

1. An earthquake begins underground.
2. Nearby sensors detect fast-arriving P waves.
3. Computers estimate the earthquake’s location and size.
4. The system estimates where strong shaking may occur.
5. Alerts are sent to people and automated systems before stronger shaking arrives, when possible.

The warning time may be only a few seconds, and some people very close to the epicenter may receive no warning before shaking begins. But even a few seconds can be useful for alerts, slowing trains, pausing machinery, opening firehouse doors, or prompting people to take protective action where systems are set up.

The ShakeAlert system monitors for significant earthquakes and issues alerts for expected strong shaking. In the United States, USGS describes ShakeAlert coverage as being developed for California, Oregon, and Washington. (USGS Earthquake Hazards Program)

How Earthquakes Are Detected

Earthquakes are detected by instruments called seismometers. A seismometer measures ground motion. The recorded ground motion is called a seismogram.

Modern earthquake detection uses networks of instruments. A single station can record shaking, but a network helps scientists locate the earthquake and estimate its size.

Basic Detection Process

1. Seismic waves reach several stations.
2. Computers identify P-wave and S-wave arrivals.
3. Arrival-time differences help locate the earthquake.
4. Wave amplitudes and other measurements help estimate magnitude.
5. Results are reviewed and updated as more data arrives.

USGS says the National Earthquake Information Center determines the location and size of significant earthquakes worldwide, disseminates information quickly, maintains an online database, and performs research. (USGS)

Seismic Networks

A seismic network is a group of seismometers connected to data systems. Networks may be local, regional, national, or global.

USGS describes the Global Seismographic Network as a permanent digital network with approximately 150 modern seismic stations distributed globally, providing near-uniform worldwide monitoring. (USGS)

How Scientists Locate an Earthquake

Scientists locate an earthquake by comparing when seismic waves arrive at different stations.

P waves travel faster than S waves. The farther a station is from the earthquake, the bigger the time gap between P-wave and S-wave arrival.

A simplified process:

1. Station A detects P and S waves.
2. Station B detects P and S waves.
3. Station C detects P and S waves.
4. The time gaps estimate distance from each station.
5. Circles from multiple stations overlap near the epicenter.
6. Computer models refine the hypocenter depth and location.

This is similar to triangulation, though real earthquake location uses more advanced math and Earth models.

Earthquake Maps and Public Reports

Modern earthquake information is not just instrument-based. Public reports also help scientists understand how shaking was felt.

USGS’s Did You Feel It? system collects information from people who felt an earthquake and creates maps showing what people experienced and the extent of damage. (USGS Earthquake Hazards Program)

Public reports are especially useful because local soil, buildings, and distance can make shaking feel different from one neighborhood to another.

ShakeMap and PAGER

After significant earthquakes, scientists and emergency managers need rapid information about where shaking was strongest.

ShakeMap estimates shaking intensity across a region. PAGER estimates potential impacts based on shaking, population exposure, and vulnerability information.

USGS explains that PAGER products are generally available online within 20–30 minutes for earthquakes larger than magnitude 5.5, and in some domestic regions, especially California, ShakeMap and PAGER results can be produced within about 10 minutes. (USGS Earthquake Hazards Program)

USGS notes that the Earthquake Notification Service is not an early warning system; its messages generally arrive after shaking would be felt. (USGS)

Earthquake Forecasting and Hazard Modeling

Since scientists cannot predict exact earthquakes, they focus on hazard forecasting and risk reduction. Hazard models estimate where damaging shaking is more likely over long periods.

The USGS National Seismic Hazard Model relies on updated datasets, models, maps, source code, and documentation for seismic hazard assessments. (USGS)

Earthquake hazard models may include:

• Known faults
• Past earthquake records
• Plate motion
• Geologic slip rates
• Ground-motion models
• Soil and rock conditions
• Subduction zone behavior
• Historical shaking
• Probability estimates
• Uncertainty ranges

Hazard models are not guarantees. They help communities understand relative risk and prepare for likely shaking patterns over time.

Technology Used to Study Fault Movement

Earthquake science is not limited to seismometers. Scientists also measure how the ground slowly moves between earthquakes.

USGS uses GPS, creepmeters, and strainmeters to monitor deformation in regions such as the San Francisco Bay Area. (USGS)

InSAR, or Interferometric Synthetic Aperture Radar, uses radar images from satellites to map ground deformation. USGS explains that radar can work through most weather clouds and in darkness, making it useful for tracking deformation from space. (USGS)

How Earthquake Technology Has Changed Over Time

Earthquake science has changed from human observation and simple instruments to digital global networks, satellites, real-time alerts, and advanced models.

USGS has scanned World-Wide Standardized Seismograph Network records from 1962–1978, showing how older paper and microfilm seismic records are being preserved and digitized for modern research. (USGS)

From Analog Seismographs to Digital Networks

Early seismographs recorded motion on paper or photographic film. Scientists had to read records manually, compare arrival times, and calculate locations.

Modern systems are different:

• Sensors record digital data.
• Data streams in real time.
• Computers detect events automatically.
• Magnitudes are estimated quickly.
• Maps update as more data arrives.
• Public alerts and data products can be distributed online.

The USGS Earthquake Hazards Program monitors and reports earthquakes, assesses impacts and hazards, and conducts research on earthquake causes and effects. (USGS)

Smartphones, Crowdsourcing, and Citizen Science

Smartphones can contribute to earthquake awareness in two ways:

1. People can report shaking.
2. Phone sensors may detect motion in some systems.

USGS’s Did You Feel It? is a major example of crowdsourced shaking reports. Smartphone apps and citizen-science projects can also help collect felt reports and sometimes sensor data, though official earthquake monitoring still depends heavily on scientific seismic networks.

Crowdsourced reports are not a replacement for seismometers, but they can provide valuable detail about what people experienced.

Artificial Intelligence and Machine Learning

AI and machine learning are increasingly used in earthquake research and monitoring. They can help process large amounts of seismic data, identify small earthquakes, detect seismic phases, estimate earthquake parameters, and reduce noise.

Possible uses include:

• Detecting small earthquakes in noisy data
• Picking P-wave and S-wave arrival times
• Estimating magnitude more quickly
• Recognizing aftershock patterns
• Improving early warning algorithms
• Analyzing satellite deformation data
• Sorting public reports
• Finding hidden fault activity in large datasets

AI does not make exact earthquake prediction possible. It is best understood as a tool for faster detection, better pattern recognition, and improved data processing.

Tsunami Warning Technology

Earthquakes under the ocean can create tsunamis, so tsunami warning systems combine seismic and ocean data.

NOAA describes the U.S. tsunami warning system as using tsunami forecast models combined with data from seismic and sea-level networks to refine messages. (NOAA)

NOAA’s Tsunami Program includes observation systems to rapidly detect tsunami-generating earthquakes and tsunamis, forecast models, messaging, decision-support services, and preparedness activities. (Tsunami.gov)

Why Earthquake Modeling Is Difficult

Earthquake systems are complex. Faults are buried, rocks are uneven, stress is hard to measure directly, and small changes underground can matter.

This is why earthquake science focuses heavily on probabilities, scenarios, monitoring, and building knowledge over time rather than exact prediction.

Earthquakes and Human Activity

Most earthquakes are natural, but some can be induced or triggered by human activity that changes underground stress or pressure.

Possible human-related causes include:

• Wastewater injection
• Geothermal energy operations
• Reservoir filling
• Mining
• Underground fluid extraction or injection
• Some industrial activities

Human-induced earthquakes are still earthquakes: they involve fault slip and seismic waves. The difference is that human activity may change stress or pressure enough to make a fault slip earlier than it otherwise would have.

Earthquakes Away From Plate Boundaries

Not all earthquakes happen in California, Alaska, Japan, Chile, or other famous plate-boundary regions. Earthquakes can happen within tectonic plates too.

Intraplate earthquakes may occur along old faults that are reactivated by modern stress. They can be surprising because they happen in places where earthquakes are less frequent. Also, seismic waves can sometimes travel efficiently through older, colder continental crust, so earthquakes in some central and eastern regions may be felt over wide areas.

How Local Ground Conditions Affect Shaking

Local ground conditions can strongly affect shaking.

This is why earthquake hazard maps consider more than just distance to a fault.

Earthquake Science and Buildings

Earthquake shaking affects buildings differently depending on design, height, materials, soil, and shaking frequency.

This page does not provide engineering advice. For building-specific questions, use qualified professionals and local building authorities.

Comparing Earthquake Size, Shaking, and Damage

A large earthquake far away may cause less local damage than a smaller earthquake directly beneath a city.

Earthquake Forecast Products

Common Earthquake Misunderstandings

Earthquake Vocabulary

Technology Summary

Earthquake technology has changed enormously over time. Earlier earthquake knowledge came mainly from human observations, damage descriptions, and simple instruments. Later, seismographs allowed scientists to record shaking. Global seismic networks made it possible to locate earthquakes around the world. Digital sensors and computers made earthquake information faster and more accurate.

Modern earthquake science now uses:

• Dense seismic networks
• Strong-motion sensors
• Global digital seismographic stations
• GPS/GNSS ground-motion monitoring
• Creepmeters and strainmeters
• Satellite InSAR deformation mapping
• Earthquake early warning systems
• ShakeMap and PAGER impact products
• Tsunami warning networks
• Public felt reports
• Cloud computing
• Machine learning and automated detection
• Long-term seismic hazard models

These tools do not allow exact earthquake prediction, but they do help scientists detect earthquakes faster, understand faults better, estimate hazards more clearly, model shaking more realistically, and communicate information more quickly.

Science Summary

Earthquakes happen when stress builds in Earth’s crust and a fault suddenly slips. The energy travels outward as seismic waves. P waves arrive first, followed by S waves and surface waves that often produce stronger shaking. Magnitude describes the size of the earthquake source, while intensity describes shaking and effects at a specific location.

Earthquake hazards include ground shaking, surface rupture, aftershocks, liquefaction, landslides, tsunamis, fires, and infrastructure disruption. The amount of damage depends not only on the earthquake, but also on distance, depth, soil, building conditions, population exposure, and local geography.

Earthquake forecasting is different from prediction. Scientists cannot reliably predict the exact time, location, and magnitude of a future major earthquake. They can estimate long-term probabilities, produce hazard maps, issue aftershock forecasts, detect earthquakes quickly, and provide early warning in some places after an earthquake has already begun.

The most important science lesson is that earthquakes are sudden, complex geologic events. Modern technology gives us better detection, better maps, better models, faster alerts, and stronger understanding—but not perfect certainty.