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How We Measure Them

Introduction

What is an Earthquake?

Why and Where?

Seismic Waves

How We Measure Them

Locating Earthquakes

Measuring the Size of an Earthquake

Intensity

The Structure of the Earth

The Biggest and the Deadliest

Earthquakes in the UK

Links to Seismology Information

Printable PDF of Earthquakes Booklet (2.7 MB)

A seismogram is a record of the seismic waves from an earthquake. A seismograph or seismometer is the measuring instrument that creates the seismogram. Almost all seismometers are based on the principle of inertia: a suspended mass tends to remain still when the ground moves. The relative motion between the suspended mass and the ground will then be a measure of the ground’s motion.

On a seismogram from an earthquake, the P-wave is the first signal to arrive, followed by the slower S-wave, then the surface waves. The arrival times of the P- and S-waves at different seismographs are used to determine the location of the earthquake. Given that we know the relative speed of P- and S-waves, the time difference between the arrivals of the P- and S-waves determines the distance the earthquake is from the seismograph.

Seismogram

 

Issued 10 March 2008