Earthquake Science Explained: Causes, Seismic Waves, Magnitude, Aftershocks and Fault Rupture

What Causes Earthquakes?

Most earthquakes are caused by the movement of tectonic plates. Earth’s outer shell is divided
into large and small plates that move slowly across the softer mantle below. These plates
collide, separate and slide past one another, creating stress within the crust and upper mantle.

Faults are fractures or zones of weakness where blocks of rock can move relative to one another.
Although tectonic plates may move continuously at rates of only a few millimeters or centimeters
per year, individual faults often remain locked by friction. Stress gradually builds until the
fault can no longer resist it.

When the fault slips, stored elastic energy is released. The rupture may travel across only a
few meters of rock or extend for hundreds of kilometers. The sudden movement generates seismic
waves that radiate away from the source and shake the ground.

The hypocenter and epicenter

The underground point where earthquake rupture begins is called the hypocenter,
or focus. The point on Earth’s surface directly above the hypocenter is the
epicenter.

The epicenter is useful for mapping an earthquake, but it does not always mark the location of
the strongest shaking or greatest damage. Large earthquakes rupture long fault segments, and
severe shaking may occur far from the epicenter along the direction of rupture.

Tectonic earthquake settings

Earthquakes occur in several major tectonic environments:

• Subduction zones: One tectonic plate descends beneath another, producing
  shallow, intermediate and deep earthquakes. The largest earthquakes on Earth are generally
  megathrust events along subduction boundaries.
• Transform boundaries: Plates slide horizontally past each other along
  strike-slip faults such as the San Andreas Fault.
• Collision zones: Continents converge and thicken the crust, creating
  widespread thrust faults and destructive shallow earthquakes.
• Rift zones: The crust stretches and breaks along normal faults, as in the
  East African Rift and Iceland.
• Intraplate regions: Earthquakes occur within tectonic plates, often by
  reactivating ancient faults and zones of crustal weakness.

Volcanic earthquakes

Some earthquakes are linked to volcanic activity. Rising magma can fracture surrounding rock,
pressurize cracks and produce clusters of small earthquakes. Volcanic tremor may be generated
by the movement of magma, gas or hydrothermal fluids.

Not every earthquake swarm near a volcano indicates that an eruption is imminent. Scientists
compare seismicity with ground deformation, gas emissions, thermal changes and other monitoring
data before interpreting volcanic unrest.

Human-induced earthquakes

Human activity can also change stress and fluid pressure underground. Wastewater injection,
reservoir filling, geothermal operations, mining and hydraulic fracturing can trigger
earthquakes under certain geological conditions.

Elastic Rebound Theory

Elastic rebound theory explains how stress accumulates and is suddenly released along a fault.
It became one of the foundations of modern earthquake science after scientists studied ground
deformation associated with the 1906 San Francisco earthquake.

Rocks on opposite sides of a locked fault continue to be pushed or pulled by tectonic motion.
Because the fault does not immediately slip, the surrounding rock slowly bends and deforms.
The rock behaves elastically, storing energy in much the same way as a stretched spring.

Eventually, stress exceeds the fault’s frictional resistance. The fault ruptures, the rock
blocks move and the deformed crust rebounds toward a less strained shape. The released energy
produces seismic waves.

The elastic rebound cycle

1. Interseismic period: The fault is locked while tectonic stress accumulates.
2. Nucleation: A small part of the fault begins to slip.
3. Dynamic rupture: Slip accelerates and spreads along the fault.
4. Seismic-wave release: Energy radiates through the Earth.
5. Postseismic adjustment: Aftershocks, afterslip and crustal deformation
   continue after the main rupture.
6. Reloading: Tectonic motion begins rebuilding stress on the fault system.

Why earthquakes do not follow a perfect cycle

Elastic rebound is a useful model, but real fault systems are irregular. Fault surfaces contain
bends, branches, rough patches and zones with different frictional properties. Fluids can alter
effective pressure, and stress may be transferred between neighboring faults.

As a result, earthquakes do not occur at perfectly regular intervals. One rupture may release
only part of the accumulated strain, while another may jump across several fault segments.
Recurrence estimates are therefore probabilities rather than countdown clocks.

How Seismic Waves Travel Through Earth

Earthquakes generate several types of seismic waves. Their speeds, directions and interactions
with different materials allow scientists to locate earthquakes and investigate Earth’s
interior.

Primary waves

Primary waves, or P waves, are compressional waves. They alternately squeeze
and expand material in the direction the wave is traveling. P waves move through solids,
liquids and gases and are usually the first waves recorded by a seismograph.

Secondary waves

Secondary waves, or S waves, move material perpendicular to the direction of
travel. They are slower than P waves and cannot pass through liquids. The absence of S waves
through Earth’s outer core helped scientists determine that the outer core is liquid.

Surface waves

Surface waves travel along Earth’s exterior and often produce the strongest prolonged shaking
during large shallow earthquakes. Love waves move the ground horizontally from side to side,
while Rayleigh waves create a rolling, elliptical motion.

Why some places shake more strongly

Seismic waves are affected by local geology. Soft sediment can amplify shaking, while deep
sedimentary basins may trap seismic energy and prolong ground motion. Bedrock generally shakes
differently from loose, water-saturated soil.

Distance alone therefore does not determine earthquake damage. Fault geometry, rupture
direction, earthquake depth, soil conditions and building design all influence the severity
of shaking.

Earthquake Magnitude vs. Intensity

Magnitude and intensity describe different aspects of an earthquake. They are often confused,
especially in news reports, but they are not interchangeable.

What is earthquake magnitude?

Magnitude measures the size of an earthquake at its source. A single earthquake receives one
principal magnitude value, although early estimates may be revised as more seismic data become
available.

Modern seismologists commonly use moment magnitude, written as Mw, for moderate
and large earthquakes. Moment magnitude is calculated from the rigidity of the rocks, the area
of the fault that slipped and the average amount of slip.

The magnitude scale is logarithmic. An increase of one whole magnitude unit represents roughly
ten times greater recorded wave amplitude and about 32 times more energy release.

What happened to the Richter scale?

The original Richter magnitude scale was developed for local earthquakes recorded in Southern
California. It remains familiar in popular language, but it is not the preferred measure for
most large global earthquakes.

Moment magnitude performs better across a broad range of earthquake sizes and does not
underestimate the largest events as severely as some older magnitude scales.

What is earthquake intensity?

Intensity describes the effects of shaking at a particular location. These effects may include
whether people felt the earthquake, how objects moved, whether chimneys cracked and whether
buildings suffered serious structural damage.

One earthquake can produce many intensity values. Intensity is generally highest near the
rupture, but local soil conditions and construction quality can create pockets of severe damage
farther away.

Magnitude and intensity compared

Why depth matters

A shallow earthquake often produces stronger surface shaking than a deeper earthquake of the
same magnitude because seismic waves travel a shorter distance before reaching populated areas.

Deep earthquakes may be felt across extremely wide regions, but their energy is usually more
dispersed by the time it reaches the surface.

Foreshocks Explained

A foreshock is an earthquake that occurs before a larger earthquake in the same general region.
The larger event is called the mainshock. Importantly, an earthquake can only be identified as
a foreshock after the larger earthquake has occurred.

Before the mainshock, the smaller event is simply an earthquake. There is usually no reliable
way to determine whether it will remain isolated, become part of a swarm or be followed by a
much larger rupture.

Do all major earthquakes have foreshocks?

No. Many damaging earthquakes occur without clearly recognized foreshocks. Others are preceded
by one or more smaller earthquakes minutes, days, months or occasionally longer before the
main rupture.

Why foreshocks matter

Foreshock sequences can provide information about how a fault approaches failure. Researchers
study whether small ruptures progressively weaken a fault, redistribute stress or reveal a
larger nucleation process.

However, because most small earthquakes are not followed by a major event, foreshocks cannot
currently be used as reliable universal earthquake predictions.

Foreshocks and short-term probability

A moderate earthquake may temporarily increase the statistical probability of a larger nearby
earthquake. Seismic agencies may issue advisories when the short-term chance rises above normal
background levels.

An elevated probability still does not mean that a major earthquake is certain. Most sequences
decay without producing a larger mainshock.

Aftershocks Explained

Aftershocks are earthquakes that occur after a larger mainshock within the surrounding fault
system. They result from stress changes caused by the original rupture and from continued
adjustment of the crust.

Some aftershocks occur on the same fault that produced the mainshock. Others occur on nearby
faults or fault branches that were brought closer to failure.

How long do aftershocks last?

Aftershocks are most frequent immediately after the mainshock and generally decrease with time.
A major earthquake may produce noticeable aftershocks for months, years or even decades.

The duration of a sequence depends partly on the size of the mainshock, regional tectonic
conditions and the characteristics of the fault network.

Can an aftershock be larger than the mainshock?

Yes. If a later earthquake is larger, scientists usually reclassify the earlier event as a
foreshock and the larger event as the mainshock.

Why aftershocks are dangerous

Aftershocks can collapse structures already weakened by the mainshock, trigger additional
landslides and complicate rescue operations. Even an aftershock much smaller than the mainshock
may be damaging if it occurs directly beneath a vulnerable community.

Typical aftershock behavior

Larger mainshocks generally produce more numerous, more energetic and longer-lasting aftershock
sequences. The largest aftershock is often about one magnitude unit smaller than the mainshock,
although this is a statistical tendency rather than a fixed rule.

Earthquake Swarms Explained

An earthquake swarm is a sequence of earthquakes occurring in a limited area over a relatively
short period without one clearly dominant mainshock. Swarms may contain dozens, hundreds or
thousands of events.

Unlike a typical mainshock-aftershock sequence, swarm activity may rise and fall repeatedly,
migrate through the crust or continue for weeks and months.

What causes earthquake swarms?

Swarms can be produced by several processes:

• Movement of magma beneath volcanic regions
• Migration of groundwater or hydrothermal fluids
• Slow-slip events along faults
• Stress transfer through complex fault networks
• Changes in underground pressure caused by industrial activity

Do earthquake swarms mean a major earthquake is coming?

Usually not. Most earthquake swarms end without a major earthquake. However, some swarms are
associated with volcanic unrest or changes in a fault system, so scientists monitor them
carefully.

Interpretation depends on location, depth, migration pattern, fault orientation, ground
deformation and other geological evidence.

Volcanic swarms

Beneath volcanoes, migrating magma can crack rock and produce swarms that move upward or
laterally. Long-period earthquakes and volcanic tremor may indicate fluid movement rather than
ordinary brittle fault rupture.

A swarm alone does not confirm that an eruption will occur. It must be interpreted alongside
gas measurements, thermal observations and deformation data.

Deep-Focus Earthquakes

Earthquakes are commonly classified by depth:

• Shallow-focus: Less than about 70 kilometers deep
• Intermediate-focus: Roughly 70 to 300 kilometers deep
• Deep-focus: More than about 300 kilometers deep

Nearly all deep-focus earthquakes occur inside tectonic plates descending into the mantle at
subduction zones. These inclined zones of earthquake activity are known as Wadati–Benioff
zones.

How can earthquakes happen so deep?

At great depth, pressure and temperature should cause rock to deform plastically rather than
fracture in the brittle manner typical of shallow earthquakes. Deep earthquakes therefore
require different physical explanations.

Proposed mechanisms include mineral phase transformations, dehydration reactions, thermal
instability and rapid shear failure inside the cold interior of a subducting slab.

How deep can earthquakes occur?

The deepest known earthquakes occur at depths approaching 700 kilometers. Below this level,
seismicity becomes extremely rare because subducted rock is increasingly heated and transformed
into mantle material.

Are deep earthquakes dangerous?

Deep earthquakes can be felt across broad regions because their seismic waves spread through
large volumes of rock. However, they generally cause less concentrated surface damage than
shallow earthquakes of similar magnitude.

Exceptions are possible where vulnerable buildings, basin amplification or unusual rupture
characteristics increase shaking.

Slow Earthquakes and Slow-Slip Events

Not all fault movement happens through violent shaking. Some faults release strain gradually
in events that are too slow to generate strong ordinary seismic waves.

These phenomena are broadly known as slow earthquakes. They include slow-slip
events, tectonic tremor, low-frequency earthquakes and very-low-frequency earthquakes.

What is a slow-slip event?

A slow-slip event occurs when part of a fault moves over hours, days, months or even years
instead of rupturing in seconds. The amount of slip may equal that of a moderate or large
earthquake, but the energy is released so gradually that people usually feel nothing.

Where do slow-slip events occur?

Slow slip is especially common near subduction zones, often at the transition between a locked
shallow megathrust and a deeper zone that slides more freely. Slow-slip events are also observed
on some continental faults.

How are slow earthquakes detected?

GPS stations can record subtle movements of the ground during slow-slip events. Borehole
instruments, strainmeters and sensitive seismic networks may detect associated tremor and
low-frequency earthquakes.

Can slow slip trigger major earthquakes?

Slow-slip events can redistribute stress and sometimes coincide with increased seismicity.
In certain cases, slow slip appears to have contributed to the preparation or triggering of
larger earthquakes.

However, many slow-slip events occur without a subsequent destructive earthquake. They are an
important part of the fault-loading process, not reliable standalone predictions.

Silent earthquakes

Slow-slip events are sometimes called silent earthquakes because they involve measurable fault
movement without strong shaking. The term is useful descriptively, but these events are not
truly silent to modern geodetic instruments.

Supershear Earthquakes

In most earthquakes, the rupture front travels along a fault more slowly than shear waves move
through the surrounding rock. In a supershear earthquake, part of the rupture travels faster
than the local shear-wave speed.

This behavior is sometimes compared with a jet exceeding the speed of sound. Instead of a sonic
boom, a supershear rupture can produce a concentrated seismic shock front.

Where do supershear ruptures occur?

Supershear propagation has most often been identified in large strike-slip earthquakes along
relatively straight fault segments. Long, smooth fault geometry may allow rupture to accelerate
to unusually high speeds.

Why supershear earthquakes matter

Supershear ruptures can direct intense seismic energy along the fault, expanding the area exposed
to strong shaking. Communities located far along the rupture path may experience unexpectedly
severe ground motion.

The effect is especially important for long infrastructure corridors, pipelines, bridges and
cities built near major strike-slip faults.

Rupture directivity

Even non-supershear earthquakes can concentrate energy in the direction of rupture propagation.
This directivity effect helps explain why some locations experience much stronger shaking than
others at similar distances from the fault.

Earthquake Triggering

Earthquakes change the stress field in the surrounding crust. Some faults are moved closer to
failure, while others are temporarily pushed farther from failure. This process can trigger
additional earthquakes.

Static stress triggering

Static stress changes remain after a fault has slipped. They are strongest near the rupture and
can influence neighboring faults or fault segments.

Aftershocks are a common expression of static stress redistribution, although fluid movement
and other processes may also contribute.

Dynamic triggering

Dynamic triggering occurs when passing seismic waves from a distant earthquake disturb faults,
geothermal systems or volcanic regions. Large earthquakes can alter seismicity thousands of
kilometers away.

Distant triggering usually produces small earthquakes, tremor or changes in hydrothermal
activity. It does not mean that every large earthquake will trigger another destructive event
across the planet.

Can one major earthquake cause another?

Yes, but the relationship is probabilistic. A large earthquake can increase stress on another
fault and contribute to a later rupture. Demonstrating direct causation becomes more difficult
as distance and time increase.

Some major earthquakes occur in regional sequences, where one rupture transfers stress to
adjacent faults. Others remain isolated.

Can earthquakes trigger volcanoes?

Powerful earthquakes can disturb magma chambers, fractures and hydrothermal systems. Changes in
pressure or permeability may alter volcanic activity.

A direct eruption is most plausible when a volcano is already pressurized and close to failure.
Most large earthquakes do not trigger volcanic eruptions.

Can weather trigger earthquakes?

Seasonal water loading, rainfall, snowmelt, groundwater changes and atmospheric pressure can
slightly modify stress on some faults. These effects may influence the timing of small or
already-imminent earthquakes in sensitive settings.

Weather does not create the large tectonic stress responsible for major earthquakes. It may
act as a minor final influence where a fault is already close to failure.

Common Earthquake Myths

Myth: Earthquakes only happen at plate boundaries

Most earthquakes occur near plate boundaries, but damaging intraplate earthquakes can occur
far inside tectonic plates. Ancient faults may be reactivated by present-day stress.

Myth: Small earthquakes prevent large earthquakes

Small earthquakes release only a tiny fraction of the energy stored for a major rupture.
Thousands of small events would be required to equal the energy of one very large earthquake.

Minor seismicity can sometimes relieve local stress, but it can also redistribute stress onto
neighboring parts of a fault.

Myth: Earthquake weather exists

Earthquakes occur in hot, cold, wet and dry weather. There is no reliable type of weather that
consistently precedes tectonic earthquakes.

Myth: Earthquakes always happen at night

Earthquakes occur at all hours. Nighttime earthquakes may seem more memorable because people
are sleeping, indoor conditions are quiet and the event is more disruptive.

Myth: Animals can reliably predict earthquakes

Animals may respond to weak shaking, ground vibrations, sounds, gas changes or other
environmental signals. Reports of unusual behavior are common after major earthquakes.

No animal behavior has been shown to predict earthquakes consistently enough to serve as a
dependable warning system.

Myth: The ground opens and swallows people

Fault rupture can produce fissures, scarps and areas of ground collapse, but tectonic faults do
not normally open into vast bottomless cracks. Many dramatic earthquake fissures are caused by
landslides, liquefaction or surface stretching.

Myth: Doorways are always the safest place

Doorways were sometimes stronger than surrounding walls in older unreinforced buildings. In
modern structures, doorways usually offer little special protection and may expose people to
swinging doors or falling debris.

In most indoor situations, the recommended action is to
Drop, Cover and Hold On.

Myth: Scientists know when the next major earthquake will strike

Scientists can estimate long-term earthquake probabilities and identify dangerous faults, but
they cannot reliably predict the exact date, location and magnitude of a future earthquake.

How Scientists Study Earthquakes

Earthquake science combines seismology, geology, geodesy, physics, engineering and computer
modeling. No single instrument reveals the entire process.

Seismometers

Seismometers detect ground motion and record the arrival of different seismic waves. Networks
of instruments allow scientists to calculate an earthquake’s location, depth, magnitude and
rupture characteristics.

GPS and geodetic monitoring

High-precision GPS stations measure the slow movement of Earth’s surface. They reveal where
faults are locked, how strain accumulates and how the crust moves during and after earthquakes.

Satellite radar

Interferometric synthetic aperture radar, or InSAR, compares radar images acquired before and
after an earthquake. It can map ground displacement across large areas with centimeter-scale
precision.

Paleoseismology

Paleoseismologists dig trenches across faults and study displaced sediments, buried soils and
evidence of ancient liquefaction. These records help reconstruct prehistoric earthquakes that
occurred before modern instruments or written history.

Laboratory experiments

Researchers compress and shear rock samples under controlled pressure, temperature and fluid
conditions. These experiments help explain friction, fault weakening and rupture nucleation.

Computer simulations

Numerical models simulate how faults accumulate stress, begin to rupture and generate seismic
waves. Models are also used to estimate ground shaking for earthquake hazard maps.

Continue with the dedicated guide to

earthquake monitoring and forecasting
.