1. What is the Elastic Rebound Theory?
Explains how energy is stored in rocks
Rocks bend until the strength of the rock is exceeded
Rupture occurs and the rocks quickly rebound to an undeformed shape
Energy is released in waves that radiate outward from the fault

2. 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

3. Where Do Earthquakes Occur and How Often?
~80% of all earthquakes occur in 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. The Economics and Societal Impacts of EQs
Damage in Oakland, CA, 1989
Building collapse
Fire
Tsunami
Ground failure

5. How is an Earthquake’s Epicenter Located?
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

6. Why do earthquakes occur?
focus
epicenter
Fractures, faults
Energy released
and propagates in all directions as seismic waves causing
earthquakes
Why do rock break?
Review three types of stress: tension, compression, shear
Three types of deformation resulting: elastic, plastic and brittle
We’re concerned with the brittle deformation – causes faults, fractures through which energy, created by the friction, is released
Define Focus, epicenter and seismic waves

7. Crater Lake, Oregon

8. The Stump of Mount Mazama

9. What comes out of a volcano?
Ash

10. What comes out of a volcano?
Gas
What comes out of a volcano?
Most common:
H2O
CO2
SO2
HCl

11. What comes out of a volcano?
Lava

12. Mauna Loa (Hawai’i): A typical shield volcano

13. Mt. St. Helens: A typical composite volcano

14. Mt. St. Helens after its 1980 eruption

15. How Calderas Form

16. Plate-tectonic setting of volcanism
Explosive (andesitic) volcanoes form at subduction zones.

17. Plate-tectonic setting of volcanism
At spreading centers, low pressure triggers
mantle melting—fluid basaltic magma rises.

18. Plate-tectonic setting of volcanism
Within plates, rising plumes of hotter mantle feed
hot spots; varied volcanoes result (basaltic on Hawaii).

20. Mt. St. Helens Pyroclastic Eruption

21. Mount Saint Helens- after

35. Mt. Saint Helens before

36. Phreatic (vapor) eruption

37. Bulge

38. After the eruption

39. Pyroclastic eruption

40. Volcanic landscape: A Caldera (Crater Lake)