Earthquake

An earthquake is a sudden release of energy inside the Earth that produces shaking at the surface. The energy travels outward as seismic waves, which can be felt as a brief tremor or, in severe cases, cause buildings to collapse and the ground to crack. Earthquakes occur most often along the boundaries of the tectonic plates that make up the planet's outer shell, but they can also strike far from plate edges.

Because they arrive with little or no warning, earthquakes are among the most feared natural hazards. Understanding why they happen, how they are measured, and how communities can prepare helps reduce the toll they take on lives and property.

What causes earthquakes

The outer layer of the Earth is broken into enormous slabs called tectonic plates, which move slowly over the hotter, softer rock beneath them. As the plates grind against each other, stress builds up in the rocks along their edges. When the stress exceeds the strength of the rock, it breaks or slips suddenly along a fault, releasing stored energy. The point underground where this rupture begins is the focus, and the point directly above it on the surface is the epicentre. Earthquakes can also be triggered by volcanic activity, human activities such as mining and dam filling, and the injection of fluids underground.

Seismic waves

The energy from a rupture radiates in several kinds of waves. Some shake the ground back and forth and travel fastest, arriving first at distant places. Others travel more slowly but can cause stronger swaying, especially in soft ground. Surface waves, which move along the top of the Earth, often do the most damage to buildings because they have larger amplitude. Scientists study the arrival times and strengths of these waves to locate earthquakes and to understand the structure of the planet's interior.

Measuring earthquakes

Earthquakes are described in two main ways: magnitude and intensity. Magnitude measures the size of the earthquake itself at its source and is expressed on a logarithmic scale, meaning each whole number represents a large increase in energy released. Intensity describes how strongly the shaking was felt and how much damage occurred at a particular place; it varies with distance and local conditions. Older accounts sometimes refer to the Richter scale, while modern measurements often use the moment magnitude scale, which works well for very large quakes.

Hazards

The shaking itself is only part of the danger. Ground motion can collapse buildings and bridges, break gas and water lines, and start fires. In some areas, water-saturated soil can behave like a liquid during shaking, a phenomenon called liquefaction, which undermines foundations. Earthquakes beneath the ocean floor can displace water and generate tsunamis. On steep slopes, shaking may trigger landslides and rockfalls. Aftershocks, smaller quakes that follow the main event, can bring down already weakened structures and hamper rescue efforts.

Where earthquakes happen

Most earthquakes occur along plate boundaries, and a few regions account for a large share of the world's largest events. Subduction zones, where one plate dives beneath another, produce the most powerful earthquakes and, often, tsunamis. Fault lines far from plate edges, such as those in some continental interiors, can still generate damaging quakes, though less frequently. Some regions have long gaps between major events, which can make the risk seem lower than it really is.

Prediction and preparation

Scientists cannot yet predict exactly when and where an earthquake will strike. Instead, they estimate the probability of earthquakes over years or decades and focus on preparation. Building codes that require structures to withstand shaking are among the most effective measures. Other steps include securing heavy furniture, practising drills, preparing emergency supplies, and designing warning systems. In some regions, early-warning systems detect the first fast waves of a quake and send alerts seconds before the slower, damaging waves arrive, giving people a brief but valuable head start.

Common misconceptions

People sometimes think earthquakes happen only in well-known "earthquake countries", but they occur in many places, including regions with no recent history. Another misunderstanding is that animals reliably predict earthquakes; while some studies have looked for patterns, no reliable method has been demonstrated. It is also wrong to assume the first quake is always the largest; in some cases a smaller foreshock precedes the main event, but this is not a dependable sign.

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