How to Locate an Earthquake Epicenter

These two words are often used interchangeably in news reports, but they mean fundamentally different things in seismology:

• Magnitude is a single number that describes the total energy released at the earthquake's source. It is calculated from the amplitude of seismic waves recorded on a seismograph and is the same no matter where you measure it. One earthquake, one magnitude.
• Intensity describes how strong the shaking felt at a particular location. It depends on the magnitude, the distance from the epicenter, the depth of the earthquake, the local geology (soft soil shakes more than bedrock), and the type of buildings. The same earthquake can produce very different intensities in neighbouring towns.

Moment magnitude (Mw) is calculated from the seismic moment, which is proportional to the fault rupture area, the average slip on the fault, and the rigidity of the rock. Because it is tied to the physics of the rupture, Mw can accurately describe earthquakes of any size, from tiny microseisms to the largest megathrust events.

Intensity is most commonly expressed using the Modified Mercalli Intensity (MMI) scale, which runs from I (not felt) to XII (total destruction). It is determined from human reports, building damage, and geological effects. Unlike magnitude, intensity is a map, not a single number — it contours the earthquake's felt effects across a region.

The epicenter is the point on Earth's surface directly above where the earthquake started (the focus or hypocenter). To find it, seismologists use the time difference between P-wave and S-wave arrivals at each station.

Because P-waves are faster than S-waves, the gap between their arrival times grows with distance. At a nearby station, the gap is small; at a distant station, it is large. By measuring this gap, seismologists calculate how far the earthquake is from each station. With distance circles drawn around three or more stations, the intersection of the circles pinpoints the epicenter.

One station gives a distance but not a direction — the earthquake could be anywhere on a circle centred on that station. A second station gives a second circle, narrowing the location to two possible points (where the circles intersect). A third station resolves the ambiguity: its circle will pass through only one of the two intersection points — the true epicenter.

In practice, seismologists use dozens of stations and computer algorithms to find the best-fit location, because real Earth structure is not perfectly uniform and wave speeds vary with depth and rock type. The result is an epicentral location with an uncertainty ellipse, typically a few kilometres across for well-recorded earthquakes.