GL 107 Earth Systems Science
K. Hannula
Discussion questions for 9/26/07
1. Define:
Focus (of an earthquake): The point on a fault at which an earthquake begins. (See Figure 7.2, p. 189 in textbook.)
Fault: A crack in rock along which rock has slipped.
P-waves: Also known as "primary waves," these are seismic waves that travel through the body of rock, compressing and expanding rock as they travel. They are the fastest seismic waves, and the first to arrive after an earthquake occurs. (See Figure 7.8 A and B, p. 195 in textbook.)
S-waves: Also known as "secondary waves," these are seismic waves that travel through the body of rock, but which shear rock (deforming it perpendicular to the direction the waves travel). They are slower than P-waves, and are the second waves to arrive after an earthquake occurs. (See Figure 7.8 C and D, p. 195 in textbook.)
surface waves: Surface waves travel along the surface of the earth (unlike P and S waves, which travel through rock). The shaking that results from surface waves is stronger than that of P and S waves at the earth's surface, but weakens underground. Surface waves are slower than both P and S waves.
epicenter: The point at the earth's surface directly about the focus of an earthquake. The point used to show the location of an earthquake on a map. (See Figure 7.2, p. 189 in textbook.)
2. Explain the "elastic rebound" theory.
Rocks can deform a small amount without breaking, but if they are twisted too much, they snap, creating an earthquake.
Between earthquakes, movements of plates continue far away from the faults. Eventually, the rocks become too twisted to withstand breaking, and they snap in an earthquake. (See Figure 7.5, p. 191 in textbook.)
3. Explain how seismologists determine the location of earthquake epicenters.
See Figures 7.9 and 7.10, p. 196 in textbook.
1) Use three different seismograms from three different seismograph locations.
2) Determine the time difference between the arrival of the P and S waves on each seismogram to determine the distance to the epicenter from each seismograph.
3) Draw a circle around each seismograph station representing the distance to the epicenter. The three circles should meet at one point: the epicenter.