How Earthquakes Are Located and Measured

An earthquake begins when stored stress in rock is released suddenly along a fault. The starting point underground is called the focus or hypocentre. The point on Earth's surface directly above it is the epicentre.

The released energy travels outward as seismic waves. P waves travel fastest and can move through solids and liquids. S waves travel more slowly and move only through solids. Surface waves travel along Earth's exterior and can produce strong shaking over populated areas.

A seismometer detects ground movement and records it as a seismogram. The pattern shows when different waves arrive and how strongly the ground moved at that station.

Because P waves move faster than S waves, they usually reach a station first. The farther the station is from the earthquake, the larger the time gap between the P wave and S wave arrivals. Scientists use known wave speeds and travel-time curves to estimate the distance from the station to the source.

One station gives a distance but not a unique direction. On a map, every point at that distance lies somewhere on a circle around the station. A second station narrows the possibilities, while data from three or more stations can locate the epicentre much more precisely.

Modern earthquake location uses many instruments and computer models, so the real method is more sophisticated than drawing three perfect circles. The simple triangulation model is still useful because it shows why information from several places is needed.

Magnitude describes the size of the earthquake at its source. Modern seismologists commonly use moment magnitude, which is based on factors including the area of fault movement, the amount of slip and the stiffness of the rocks. The scale is logarithmic, so a small numerical increase represents a much larger physical event.

Intensity describes the effects at a particular location, such as shaking and damage. The same earthquake can have different intensities in different places because distance, ground conditions and buildings matter.

Keep the terms separate:

• •Focus means the underground starting point.
• •Epicentre means the surface point above the focus.
• •P waves usually arrive before S waves.
• •Magnitude describes earthquake size.
• •Intensity describes effects at a particular place.

Scientists locate earthquakes by comparing seismic wave arrival times at several stations and measure their size using magnitude scales. Seismograms turn brief ground motion into evidence that can be analysed long after the shaking stops. Separate location, magnitude and local intensity, and earthquake reports become much easier to read.