Earthquakes

1. Earthquakes

2. Earthquakes • An earthquake is the shaking and trembling that results from the movement of rock beneath Earth’s surface. • Not all earthquakes occur at plate boundaries. Sometimes they happen in the middle of a tectonic plate. • Earthquakes can happen both near the Earth’s surface or far below it.

3. Earthquakes always begin in rock below the surface. • Most earthquakes begin in the lithosphere within 100 kilometers of the surface. • The focus is the point beneath Earth’s surface where rock that is under stress breaks, triggering an earthquake. • The point on the surface directly above the focus is called the epicenter.

4. Seismic Waves • Seismic waves that travel through the Earth are called body waves. • Seismic waves are vibrations that travel through Earth carrying the energy released during an earthquake. • Seismic waves that travel along Earth’s surface are called surface waves. • Each type of seismic waves travels through Earth’s layers in a different way and at a different speed.

5. Body Waves • There are two types of body waves: • P waves • S waves

6. P-waves Click on picture to view animation after reading the notes • The first waves detected in an earthquake are p waves, or pressure waves. • P waves compress and expand the ground like an accordion. • P waves can travel through solids, liquids, or gases.

7. S waves Click on picture to view animation after reading the notes • After P waves come secondary waves, or S waves. • S waves are earthquake waves that vibrate from side to side and thrust the ground up and down, or back and forth. • When S waves reach the surface, they shake structures violently. • S waves cannot move through liquids.

8. Surface Waves • When P and S waves reach the surface, some of them are transformed into surface waves. • Surface waves move more slowly than P and S waves, but they produce the most severe ground movements. • They can actually make the ground roll like ocean waves. • Other surface waves shake the ground from side to side.

9. All 3 waves Click on picture to view animation

10. Detecting Seismic Waves Seismograph • Geologists use a seismograph to record and measure the vibrations of seismic waves. • When the waves reach a seismograph, the instruments creates a seismogram. • A seismogram is a tracing of earthquake motion. • Until recently, scientists used mechanical seismographs, like the one in the picture. Seismogram • Today they use electronic seismographs that convert ground movements into a signal that can be recorded and printed.

11. Finding the Epicenter • One method they use is called the S-P Time Method. • They collect readings for the same earthquake from seismographs stations at different locations. • Scientists use seismograms to find the earthquakes epicenter. • They then use this data to determine the distance each station is from the earthquake. • They can then triangulate the results to find the epicenter. • It takes a minimum of 3 seismograph readings to find the epicenter of and earthquake.

12. Richter Scale • Magnitude is a measurement of earthquake strength based on seismic waves and movement along faults. • There are many ways that scientists can measure an earthquake. • Charles Richter created the Richter magnitude scale in the 1930s to compare earthquakes by measuring ground motion and adjusting for distance to find their strength. • When magnitude increases by one unit the measured ground motion becomes 10 times larger on the Richter scale. • The Richter scale provides accurate measurements for small, nearby earthquakes, but the scale does not work well for large, or distant earthquakes.

14. Mercalli Scale • Seismologists can also measure the intensity of an earthquake. • The Modified Mercalli Intensity Scale is used to measure the degree to which an earthquake is felt by people and the amount of damage done by it. • The Mercalli scale uses Roman numbers from I to XII to describe increasing earth quake intensity levels.