Understanding Earthquake Epicentre Location

The epicentre of an earthquake is the point on the Earth's surface directly above the hypocentre or focus, which is where the earthquake originates underground. Locating the epicentre is a crucial step in understanding the earthquake and its potential impact.

Seismic stations are equipped with instruments called seismographs that detect and record ground motion caused by seismic waves generated during an earthquake. When an earthquake occurs, different types of seismic waves are produced, including P-waves (primary waves) and S-waves (secondary waves). These waves travel at different speeds; P-waves are faster than S-waves.

How Seismic Stations Determine Distance

Because P-waves travel faster than S-waves, they arrive at a seismic station earlier. The time difference between the arrival of the first P-wave and the first S-wave (known as the S-P time) is directly related to the distance of the seismic station from the earthquake's epicentre. The larger the S-P time difference, the farther away the earthquake is.

Using this S-P time difference and known wave speeds, seismologists can calculate the distance from a specific seismic station to the epicentre. This distance calculation provides a radius.

Why Multiple Seismic Stations Are Needed for Epicentre Mapping

Locating the exact position of the earthquake epicentre requires data from multiple seismic stations using a technique called triangulation.

• One Seismic Station: Data from a single seismic station gives the distance to the epicentre. This means the epicentre could be located anywhere on a circle drawn on a map with the seismic station as the center and the calculated distance as the radius. A single station does not provide a unique location.
• Two Seismic Stations: Data from two seismic stations provides two different distances. Drawing circles around each station with their respective calculated distances as radii will result in two intersection points. The epicentre is located at one of these two points. Two stations narrow down the possibilities but still do not provide a unique location.
• Three Seismic Stations: Data from three seismic stations provides three different distances. Drawing circles around each of the three stations with their respective calculated distances as radii will result in the three circles intersecting at a single point. This unique intersection point is the location of the earthquake epicentre.

Therefore, a minimum of three seismic stations is required to accurately pinpoint and map the epicentre of an earthquake.

Summary of Epicentre Location by Number of Stations

Revision Table: Key Earthquake Terms

Additional Information: Seismic Wave Properties

Understanding the behaviour of seismic waves is fundamental to locating earthquake epicentres and studying Earth's interior. Here's a brief look at P-waves and S-waves:

• P-waves (Primary waves):
  • Are the fastest type of seismic wave.
  • Are compressional waves, meaning they cause particles to move back and forth in the same direction the wave is travelling.
  • Can travel through solids, liquids, and gases.
  • Are the first to arrive at a seismic station.
• S-waves (Secondary waves):
  • Are slower than P-waves.
  • Are shear waves, meaning they cause particles to move back and forth perpendicular to the direction the wave is travelling.
  • Can only travel through solids. They cannot pass through liquids or gases.
  • Are the second to arrive at a seismic station (if the medium is solid).

The difference in speed between P-waves and S-waves is what allows seismologists to calculate the distance from the seismic station to the earthquake source.