1. The Focus and Epicenter of an Earthquake
The point within Earth where faulting begins is the focus, or hypocenter
The point directly above the focus on the surface is the epicenter

2. Benioff Zone
Benioff Zone is an area of increasingly deeper seismic activity, inclined from the trench downward in the direction of the island arc.

3. Where Do Earthquakes Occur and How Often?
- 80% of all earthquakes occur along the Pacific Plate known as the circum-Pacific belt
most of these result from convergent margin activity
~15% occur in the Mediterranean-Asiatic belt
remaining 5% occur in the interiors of plates and on spreading ridge centers
more than 150,000 quakes
strong enough to be felt are
recorded each year

4. What are Seismic Waves?
Response of material to the arrival of energy released by rupture
Two types:
Body waves
P and S
Surface waves
R and L

5. Body Waves: P and S waves
P or primary waves
fastest waves
travel through solids, liquids, or gases
compressional wave-material movement is in the same direction as wave movement
S or secondary waves
slower than P waves
travel through solids only
shear waves - move material perpendicular to wave movement

6. Surface Waves: R and L waves
Travel just below or along the ground’s surface
Slower than body waves; Especially damaging to buildings
Two Types:
Rayleigh waves- rolling
Love waves- side-to-side

7. Seismograph records earthquakes events.
Horizontal Seismograph
Vertical Seismograph

8. A seismogram records wave amplitude and time.
Amplitude- maximum height in a peak of a wave
Which waves have the largest amplitude?

9. How is an Earthquake’s Epicenter Located?
Seismic wave behavior
P waves arrive first, then S waves, then L and R
Average speeds for all these waves is known
After an earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter.

10. How is an Earthquake’s Epicenter Located?
Time-distance graph showing the average travel times for P- and S-waves. The farther away a seismograph is from the focus of an earthquake, the longer the interval between the arrivals of the P- and S- waves

11. Triangulation: Locating an Epicenter
Three seismograph stations are needed to locate the epicenter of an earthquake
A circle where the radius equals the distance to the epicenter is drawn
The intersection of the circles locates the epicenter

12. Clues to Earth’s Interior
Seismic waves change speed and direction when they encounter different materials in Earth’s interior.
P-waves and S-waves traveling through the mantle follow fairly direct paths.
P-waves that strike the core are refracted, or bent, causing P-wave shadow zones where no direct P-waves appear on seismograms.
S-waves do not enter Earth’s outer core because they cannot travel through liquids and do not reappear beyond the P-Wave shadow zone.

13. Clues to Earth’s Interior

14. Clues to Earth’s Interior
This disappearance of S-waves has allowed seismologists to reason that Earth’s outer core must be liquid.
Detailed studies of how other seismic waves reflect deep within Earth show that Earth’s inner core is solid.