What is an earthquake? An earthquake is the shaking of the ground caused by a sudden release of energy within Earth’s crust or upper mantle. The energy travels outward as seismic waves. Most earthquakes occur when rocks on either side of a fault suddenly slip after stress has built up, although volcanic activity, human activities and other processes can also produce earthquakes.
The important idea is that an earthquake begins underground, while people experience its effects at the surface. The point where rupture begins is the focus or hypocentre; the point directly above it on Earth’s surface is the epicentre. The shaking measured at a particular place depends on more than distance: earthquake magnitude, depth, local geology, building design and construction all matter.
Quick notes: A fault is a fracture or zone of fractures where rocks move. Seismic waves carry energy through Earth. Magnitude describes the size of an earthquake at its source; intensity describes the shaking and effects at a particular location. The moment magnitude scale is widely used for large earthquakes. Not every earthquake causes a tsunami, and not every earthquake is felt by people.
How an earthquake happens
Earth’s outer shell is divided into tectonic plates that move slowly over geological time. Where plate boundaries meet, rocks can be compressed, pulled apart or sheared sideways. Fault surfaces may remain locked by friction even while tectonic forces continue to push or pull the surrounding rocks. Stress then builds up in the surrounding rock.
When the stress on a fault overcomes the resistance holding it in place, part of the fault can slip suddenly. The stored elastic energy is released, and seismic waves travel away from the rupture. This process is often described using elastic rebound: rocks deform as stress accumulates, then move towards a less strained configuration when the fault slips.
- Plate motion applies force: tectonic plates continue moving, even when a fault is temporarily locked.
- Stress accumulates: rocks on and around the fault deform as forces act on them.
- Fault rupture begins: a section of the fault slips when the conditions for movement are reached.
- Seismic energy travels: waves move through Earth and along its surface.
- The ground shakes: buildings, roads and people experience motion according to local conditions.
- Aftershocks may follow: surrounding stresses adjust, and additional earthquakes can occur near the original rupture.
The simplified sequence explains many tectonic earthquakes. Real fault systems can be complex, with rupture spreading over a large area, jumping between fault segments or interacting with nearby structures. Scientists use geological mapping, seismology and geodetic measurements to investigate these processes.
What is a fault?
A fault is a fracture or zone of fractures in rock along which displacement has occurred. Some faults are visible at the surface; others are buried underground. A fault may be locked, creeping slowly or slipping during an earthquake. Faults are classified by the relative motion of the rocks on either side.
- Normal fault: the hanging-wall block moves down relative to the footwall, commonly in a region being extended.
- Reverse fault: the hanging-wall block moves up relative to the footwall under compression. A low-angle reverse fault is called a thrust fault.
- Strike-slip fault: blocks move mainly sideways past one another, with right-lateral or left-lateral motion described from a chosen viewpoint.
These names describe the geometry and movement of a fault, not the size of an earthquake. A small event and a very large event can occur on faults of the same broad type.
Focus and epicentre: two different locations
The focus, also called the hypocentre, is the point within Earth where the rupture starts. The epicentre is the point on the surface directly above that focus. News reports often give the epicentre because it is a convenient surface location, but the depth of the focus is also important.
A shallow earthquake can produce strong shaking near the rupture, while a deeper event may be felt over a wider area but often produces different surface effects. This is a general tendency, not a prediction for every event. The earthquake’s magnitude, mechanism, local geology and distance all contribute.
What are seismic waves?
Seismic waves are vibrations that carry energy through Earth or along its surface. Seismologists use their arrival times, amplitudes and waveforms to locate earthquakes and study Earth’s interior.
P waves
Primary or P waves are compressional waves. Particles move back and forth in the direction the wave travels. P waves can travel through solids, liquids and gases, which is why they can pass through Earth’s liquid outer core.
S waves
Secondary or S waves are shear waves. Particle motion is perpendicular to the direction the wave travels. S waves travel through solids but not through liquids in the same way, a property that helps scientists infer the structure of Earth’s interior.
Surface waves
Surface waves travel along or near Earth’s surface. They can produce strong ground motion and often contribute substantially to damage, depending on the earthquake and local conditions. Different surface-wave types move the ground in different ways.
P waves generally arrive before S waves at a given recording station because they travel faster through the same material. Seismologists use the difference in arrival times, together with data from multiple stations, to locate an earthquake. People should not wait to identify wave types before taking protective action during strong shaking.
Magnitude versus intensity
Magnitude is a measure of the size of an earthquake at its source. Modern seismology commonly reports moment magnitude, which is based on the seismic moment and is suitable for comparing events across a wide range of sizes. It is logarithmic: an increase of one magnitude unit corresponds to a much larger measured seismic-wave amplitude and roughly 32 times the energy release in the simplified comparison commonly taught.
Intensity describes how strongly an earthquake is felt and what effects it produces at a particular place. It can differ from one neighbourhood to another during the same earthquake. Distance from the rupture, soil conditions, building design and the direction in which rupture spreads can influence observed intensity.
A single earthquake therefore has one commonly reported magnitude estimate, but many local intensity observations. Do not say that a magnitude 6 earthquake has ‘intensity 6’ as though the two numbers measure the same thing.
Why some places shake more than others
- Distance and depth: shaking often weakens with distance, but the pattern depends on the source and geology.
- Local soil and sediment: soft sediments can amplify certain ground motions compared with hard rock.
- Basin effects: deep sedimentary basins can trap and prolong seismic waves.
- Building design: structure, height, materials, maintenance and compliance with building codes influence performance.
- Rupture direction: the direction and speed of fault rupture can affect where stronger motion occurs.
- Ground failure: liquefaction, landslides and surface rupture can cause additional damage.
What is liquefaction?
Liquefaction can occur when water-saturated, loose sediment loses much of its strength during strong shaking. The ground may behave temporarily more like a fluid, allowing buildings to settle or tilt and buried infrastructure to move. Liquefaction is not simply ‘soil turning into water’; it is a change in the strength and behaviour of saturated granular material as pore-water pressure rises.
The risk depends on soil type, groundwater conditions, ground shaking and other local factors. Engineers investigate these conditions when planning buildings, bridges, ports and lifeline infrastructure.
Can earthquakes be predicted?
Scientists cannot currently predict the exact time, place and magnitude of a future earthquake with reliable precision. Earthquake forecasts and hazard assessments can estimate probabilities over specified areas and periods, but they do not provide a dependable countdown to an individual event.
Monitoring fault movement, past earthquakes, ground deformation and seismic activity improves understanding of hazard. Early-warning systems are different from prediction: they detect an earthquake after it begins and may send an alert before stronger shaking reaches some locations. The warning time can be only seconds, and places near the source may receive little or no advance warning.
Do earthquakes cause tsunamis?
Some undersea earthquakes generate tsunamis when they move the seafloor vertically and displace a large volume of water. But most earthquakes do not create tsunamis. An event’s location, depth, magnitude, fault movement and the geometry of the seafloor all matter. Undersea landslides and volcanic processes can also generate tsunamis.
For a plain-language explanation of the wave itself, see What Is a Tsunami in G3 Geography?. For a practical guide to earthquake response in Japan, read What to Do When There Is an Earthquake in Japan.
What should people do during an earthquake?
The safest action depends on where you are and local emergency guidance. A widely used protective instruction in many settings is to Drop, Cover and Hold On: get low, protect your head and neck under sturdy furniture if possible, and hold on until the shaking stops. Stay away from windows and objects that may fall. Do not run outside while strong shaking is occurring.
- Indoors: drop, cover and hold on; keep clear of glass and heavy furniture that could topple.
- Outdoors: move away from buildings, streetlights, trees and overhead wires where it is safe to do so; stay in an open area.
- In a vehicle: pull over safely away from bridges, overpasses, buildings and power lines; remain inside until shaking stops and follow local instructions.
- Near the coast: after strong or long shaking, move inland or to high ground if local tsunami guidance says to evacuate; do not wait for an official alert if natural warning signs are present.
After the shaking stops, expect aftershocks. Check for injuries and immediate hazards, avoid damaged buildings and follow official emergency information. Do not use lifts if the building may be damaged. Use text messages where possible to reduce pressure on voice networks, and avoid spreading unverified reports.
Why aftershocks happen
Aftershocks are earthquakes that follow a larger event in the same general region. The main rupture changes stress on surrounding faults and sections of the crust. Some areas become more likely to slip, so further earthquakes can occur. Aftershock frequency usually decreases over time, but damaging aftershocks can still happen.
An aftershock is not necessarily harmless because it is smaller than the mainshock. Buildings weakened by the first earthquake may be more vulnerable to further shaking. Follow local instructions and do not re-enter a damaged structure simply because the initial shaking has ended.
Earthquakes in Singapore’s regional context
Singapore is not located on a major active plate boundary, but strong earthquakes in the wider region can sometimes be felt here, depending on their magnitude, depth, distance and the local conditions. Regional awareness is useful for travel, infrastructure planning and interpreting official advisories. A felt tremor does not automatically mean that an earthquake occurred beneath Singapore.
For science learning, read Understand Earthquakes, Seismic Waves and Risk and How Seismometers Detect Earthquakes. These companion articles explore measurements and hazard more deeply.
Frequently asked questions about earthquakes
What causes most earthquakes?
Most earthquakes are caused by sudden slip on faults as stress builds up in Earth’s crust and upper mantle, often in association with tectonic plate movement.
What is the difference between focus and epicentre?
The focus is where an earthquake rupture begins inside Earth. The epicentre is the surface point directly above it.
What is the difference between magnitude and intensity?
Magnitude measures the earthquake source size. Intensity describes shaking and effects at a particular location.
Can scientists predict earthquakes?
They can assess seismic hazard and estimate probabilities, but they cannot reliably predict the exact time, place and magnitude of an individual future earthquake.
Are aftershocks dangerous?
Yes. They can cause further damage, especially to structures already weakened by the main earthquake.
Does every undersea earthquake cause a tsunami?
No. The seafloor movement, earthquake depth, location and other factors determine whether enough water is displaced to generate a tsunami.
Sources and further reading
The US Geological Survey Earthquake Hazards Program explains earthquake processes, monitoring and hazard. The Earthquake Country Alliance provides protective-action guidance. Follow local emergency-management instructions wherever you are; this article is general education, not a substitute for local official alerts.