LOCATING AND MEASURING EARTHQUAKES
 An instrument called a seismograph detects and records waves
produced by an earthquake
 The earthquake may have originated hundreds, even thousands of
kilometres away
 A record sheet called a seismogram gives a print-out of the waves
of the earthquake
Interpreting a Seismogram
 P waves travel faster than S waves
 Therefore, the P waves always arrive at the seismograph station
before the S waves
 The first large zigzag on the seismogram marks the arrival of the P
wave
 The S waves arrive next producing a different pattern
 As the P waves and S waves travel through the ground, the slower S
waves lag further behind
 So, the farther a seismograph station is from the epicentre, the
greater difference in the arrival times of the P and S waves
Locating the Epicentre
 Even though a seismogram tells scientists the distance between the
seismograph station and the earthquake’s epicentre, it does not give
them enough data to located the epicentre
 Because the distance measured from the epicentre provides a circle
with a large radius, the scientists need to know distances for at
least 3 different stations in order to plot the earthquake’s
epicentre location
Measuring an Earthquake’s Magnitude
 Seismograms can be used to determine the strength of an
earthquake
 Magnitude is a measure of the amount of energy released in an
earthquake
 The most widely used scale is the Richter scale
 Each increase of the whole number in Richter magnitude equals a
31-times increase in energy
 The Richter scale has been shown to have limitations
a) it does not indicate accurately the amounts of energy
released in very large earthquakes
 Richter magnitude measures the intensity of ground movements
 Another measure used is moment magnitude, which measures the
energy released at an earthquake’s source
 Moment magnitude more accurately indicates the total energy
involved in an earthquake
 For example:
Earthquake
San Francisco 1906
Alaska 1964
Richter Magnitude
8.3
8.5
Moment Magnitude
7.9
9.2