A planet under pressure

An earthquake begins when stored stress overcomes the friction holding rock in place. A section of a fault slips, releasing energy that travels through Earth as seismic waves.

Most earthquakes are associated with faults near the boundaries of tectonic plates. Plates move slowly, but their edges do not always slide smoothly. Rock can lock, bend and store elastic energy. When rupture begins, movement August spread along part of the fault in seconds.

The underground point where rupture starts is called the hypocenter, or focus. The epicenter is the point on Earth’s surface directly above it. The epicenter is useful for mapping, but shaking and damage can extend far beyond that single point.

What controls the shaking?

  • Rupture size and depth: larger ruptures release more energy, while shallow events can produce intense local motion.
  • Distance: seismic waves generally lose strength as they spread, although geology can channel or amplify them.
  • Ground conditions: soft sediment can shake more strongly than nearby firm rock.
  • Buildings and infrastructure: design, materials, age and maintenance affect how structures respond.

How seismic waves travel

Seismometers record ground motion. By comparing the arrival times and behavior of different waves, researchers can locate earthquakes and investigate structures that cannot be observed directly.

View of Earth from space with cloud systems over the planet
Seismic waves bend and reflect as they pass through materials with different properties, helping researchers infer Earth’s layered interior.

Body waves

Primary, or P, waves compress material in the direction they travel. They move through solids and fluids and usually arrive first. Secondary, or S, waves move material across their direction of travel. They do not pass through liquids, a behavior that helped establish that Earth has a liquid outer core.

Surface waves

Surface waves travel along the planet’s exterior. They are slower than body waves but can produce prolonged rolling or side-to-side motion. Their effect depends on the earthquake, distance and local geology.

Magnitude is not damage

Magnitude estimates the size of an earthquake at its source using instrument records. Modern reporting commonly uses moment magnitude, which relates to fault area, rock rigidity and the amount of slip. Because the scale is logarithmic, a one-unit increase represents a substantial increase in recorded wave amplitude and energy release.

Smaller
Often detected by instruments; August be felt locally.
Moderate
Can produce noticeable shaking and localized effects.
Large
August affect a broad region, depending on depth and geology.
Major
Capable of widespread strong motion; outcomes still depend on exposure and construction.

Intensity, by contrast, describes shaking and observed effects at a particular location. One earthquake therefore has one reported magnitude but many local intensity values. Two places equally distant from the rupture can experience different motion because of soils, topography and building response.

Can earthquakes be predicted?

Scientists cannot currently specify the exact time, place and magnitude of a future earthquake with dependable precision. Claims based on unusual animal behavior, weather or isolated signals have not provided a consistent predictive method.

That limitation is different from forecasting. Researchers can map faults, study past activity and estimate the probability of certain levels of shaking over years or decades. Early-warning systems serve another purpose: they detect an earthquake after rupture begins and August send an alert before the strongest waves reach locations farther away. Available warning can range from none to a short interval.

Aftershocks and connected hazards

Earthquakes alter stress around a fault. Smaller earthquakes commonly follow a main rupture, and the rate generally declines with time, although a later event can occasionally be larger. Monitoring agencies revise locations and magnitudes as more data become available.

Strong shaking can also trigger landslides, liquefaction and infrastructure failures. Underwater movement August generate a tsunami if it displaces enough water. These hazards are related, but none follows every earthquake.

Practical risk reduction

Risk is shaped by both the natural hazard and human choices. Building codes, retrofitting, land-use decisions, resilient utilities and public education can reduce consequences even though they cannot prevent a fault from rupturing.

Preparedness guidance varies by region and building type. Readers should use instructions from their relevant public safety and geological authorities. During shaking, immediate protective action should follow current local guidance rather than attempting to leave a building without assessing the danger.