Earthquake Unit: Lesson 2

• Recording Earth Quakes and Seismic Waves

Vocabulary

• Seismic Wave: An elastic wave generated by an impulse such as an earthquake or an explosion. Seismic waves may travel either along or near the earth’s surface or through the earth’s interior.

• Seismograph: detects and records seismic waves. Records the vertical and horizontal motions of the earth, by tracing wave shaped lines on paper or by translating the motion into electronic signals

Primary Waves or P-waves

– Fastest moving waves– First to be recorded by a seismograph– P waves can travel through solids and liquids– More rigid the material the faster the P waves

travel – Cause rock particles to move together and apart

alone the direction of the waves

Secondary waves or S-waves

• Second waves to be recorded• Travel only through solid material• Cause rock particles to move at right angles to

the directions in which the waves are traveling• Cannot be detected on the side of the earth that

is opposite the earthquakes’ epicenter. Why? • (Because they cannot travel through the liquid

part of the earths outer core)

Long waves or L-waves

• Also called surface waves• When p waves and s waves reach the earth’s surface

their energy is converted into this type of wave• Slowest moving waves• Last to be recorded on a seismograph• Travel slowly over the earth’s surface I a movement

similar to that of ocean waves• Cause the surface of the earth to rise and fall

particularly destructive when traveling through loose earth

Locating an Earthquake

• To find the epicenter of an earthquake scientist analyze the difference between the arrival times of the P waves and the S waves

• Scientist plot the difference in arrival time of the S and P waves – called lag time

• Must plot the data from at least three different seismograph stations

• Compare the results and epicenter of the earthquake may now be located.

Richter Scale

• Measures the magnitude of the earthquake• Magnitude is the measure of the energy released by

an earthquake.• Largest earthquake so far had a magnitude of 9.6• Major earthquakes have a magnitude of 7 or above• Moderate earthquakes have a magnitude of between 6

or 7• Minor earthquakes have a magnitude of between 2.5

and 6• Micro quakes have a magnitude of less that 2.5 and

are usually not felt by people

Mercalli scale

• Expresses the intensity of an earthquake• The amount of damage it causes• Measured in roman numerals from I to XII• Ex. II-low intensity. Felt by only a few persons at rest.

Delicately suspended objects may swing.• Intensity X- some well built wooden structures

destroyed. Most masonry and frame structures destroyed-foundations badly cracked

• Landslides, shifted sand and mud, water splashed over banks

• Intensity XII- total destruction

Haiti Earthquake

Earthquake in China

Japan Earthquake

Indonesia Earthquake

Study or Quiz questions

• What instrument is used to record seismic waves?

• Explain the three types of seismic waves. • How is the epicenter of an earthquake

located? • How do scientist measure the magnitude of

an earthquake? • Why do P waves travel faster through the

lithosphere that through the asthenoshpere?