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.

The first widely used magnitude scale was developed in the 1930s by Charles Richter for earthquakes in southern California. It was based on the size of waves recorded on a particular type of seismometer at a known distance. The idea was brilliant, but the scale had limits. It was designed for a specific region and instrument, and for very large earthquakes the readings stopped increasing properly, so huge events could be underestimated. Moment magnitude was later developed to solve that problem.

News reports still sometimes mention the Richter scale, but most modern figures are moment magnitudes, which are designed to give similar numbers for moderate earthquakes. The logarithmic steps are larger than they look. Each increase of one whole number means roughly thirty times more energy released, so a difference of two means around a thousand times more energy. That is why a magnitude seven earthquake is not slightly worse than a magnitude five but an event of a completely different scale.

Nobody has drilled more than a tiny distance into Earth's crust, yet scientists know the planet has a liquid outer core and a solid inner core. The evidence comes from seismic waves. Because S waves cannot travel through liquid, stations on the far side of the planet from a large earthquake receive no direct S waves at all. In the early twentieth century, seismologists noticed this large S wave shadow and concluded that part of the core must be liquid, which also fits the way Earth generates its magnetic field.

P waves can cross liquid, but they bend as they pass from one layer into another, just as light bends entering water. This creates a band at certain distances from an earthquake where direct P waves are weak or missing. In 1936 the Danish seismologist Inge Lehmann noticed faint P waves arriving inside that shadow zone. She argued that they were being reflected from a separate inner core with different properties. Later measurements confirmed that this inner core is solid, despite being even hotter than the liquid around it, because the pressure there is so enormous.

For nearby earthquakes, you can turn the gap between P and S waves into a rough distance yourself. In the upper crust, P waves travel at around 6 kilometres per second and S waves at around 3.5. Working through the arithmetic, the gap grows by about one second for every 8 kilometres of distance. So if a seismogram shows the S wave arriving 20 seconds after the P wave, the earthquake was roughly 160 kilometres away. The rule of thumb breaks down for distant earthquakes, whose waves dive deep into Earth where speeds are higher, which is why seismologists rely on full travel-time curves instead.

The same head start between P and S waves makes earthquake early warning possible. Sensors near a fault detect the faster but usually gentler P waves, computers estimate the location and size within seconds, and alerts race ahead electronically by radio, phone and television before the stronger shaking arrives. Japan launched a nationwide public system in 2007, and its high-speed trains can begin braking automatically. The warning may be only a few seconds to a few tens of seconds, enough to take cover, pause delicate operations or open fire station doors. Places close to the epicentre may get no warning at all, because the shaking arrives before the alert.

Seismometers record underground nuclear tests as well as earthquakes, and the two leave different fingerprints. An explosion pushes outwards in every direction at once, so stations all around it record a first P wave motion that is a push. An earthquake is caused by rock slipping along a fault, which pushes the ground in some directions and pulls it in others, so the first motions form a mixed pattern. Depth helps too. A test has to be set off in a drilled shaft or tunnel only a short way down, while many earthquakes begin far deeper than anyone could place a device.

Another clue is the balance of wave types. A shallow explosion of a given size tends to produce weaker surface waves compared with its P waves than an earthquake of similar strength does, so comparing magnitudes measured from the two kinds of wave separates the groups. After the Comprehensive Nuclear-Test-Ban Treaty was opened for signature in 1996, a worldwide network of monitoring stations was built, including many seismometers. When North Korea carried out its underground tests from 2006 onwards, stations in many countries detected and located them within hours.

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.