Lesson 2: Measuring Earthquakes Earth Science A Unit 7: Earthquakes and Earth's Interior

Describe the three different ways to measure the size of an earthquake.

Earthquakes can be measured by their intensity (or level of damage done) or by the magnitude (amplitude of seismic waves).

How does a seismograph measure an earthquake?

In concept, a seismograph has a weight which is suspended from a support that is attached to bedrock. When the bedrock shakes, the weight remains stationary which allows it to act as a reference point. The movement of Earth can then be compared to the weight and recorded on a stationary drum.

Which seismic waves travel most rapidly?

P waves

Describe the differences in speed and mode of travel between primary waves and secondary waves.

P waves push and pull rocks in the direction of travel. Their velocity is greater than the velocity of S waves. S waves shake the particles of material at right angles to their direction of travel.

Briefly describe how the epicenter of an earthquake is located.

The epicenter of an earthquake is located using data taken from at least three different seismograph stations. The time that the first P wave arrives at the station is then subtracted from the time that the first S wave arrives. This value can then be turned into a distance using a travel-time diagram. This distance means that the epicenter is that far from the station. A circle is drawn around each seismograph station and the circles meet where the earthquake epicenter is likely to be found.

List the two categories of seismic waves.

The two categories of seismic waves are body waves (P and S) and surface waves.

Describe how scientists detect and record seismic waves.

To detect and record seismic waves, scientists use an instrument called a seismograph. The graphic record produced by a seismograph, called a seismogram, shows when the different types of seismic waves arrived at the seismograph.

What is a seismograph?

a graphic record of an earthquake's seismic waves, made by a seismogram

What is moment magnitude?

a more precise measure of earthquake magnitude than the Richter scale, which is derived from the amount of displacement that occurs along a fault zone and estimates the energy released by an earthquake

A seismogram shows that P waves travel

faster than S waves

Overall, which seismic waves are the most destructive?

surface waves