# Chapter 8 & 9

 What causes an earthquake?
 Earthquakes are caused by the vibrations of Earth produced by the sudden release of energy.
 Describe the location of an epicenter and focus.
 Epicenter – location on the surface of Earth directly above the focusFocus – the point within Earth where the earthquake starts
 What is the elastic rebound hypothesis? How does it relate to earthquakes?
 Elastic rebound hypothesis is the release of built-up energy. Most earthquakes are produced by the release of elastic energy stored in rocks that has been subjected to great forces.
 Describe aftershocks and foreshocks.
 Aftershocks – small quakes that come after a major earthquakeForeshocks – small quakes that occur before a major earthquake
 How are earthquakes measured?
 Historically, scientists have used two different types of measurements to describe the size of an earthquake – intensity and magnitude
 Describe the following earthquake waves: surface, S waves, and P waves
 Surface waves – seismic waves that travel along Earth’s outer layers. They move up, down, and side to side. Most destructive earthquake wave. Last to arrive at the seismograph. S waves – shake particles at right angles to their direction of travel. Also known as traverse waves. Can only travel through solids. Slower than P waves but faster than surface waves P waves – They push (compress) and pill (expand) rocks in the direction the wave travels. Also known as compression waves. Can travel through solids, liquids & gas. Fastest wave (arrive 1st to the seismograph) the order should be P,S,Surface
 How do scientist locate earthquakes? (distance and direction)
 The difference in velocities of P waves provides a way to locate the epicenter. Scientists use a travel-time graph, in which one needs at least 3 seismograph station data.
 Where do most earthquakes occur?
 Most of these earthquakes occur around the outer edge of the Pacific ocean. This zone is known as the circum-Pacific belt. Active earthquake in this zone include Japan, the Phillipines, Chile, and Alaska’s Aeulation Islands
 Describe each of the different scales used to measure earthquakes. Which one measures intensity and which one measures magnitude? What is the difference between magnitude and intensity?
 Richter scale –