1. Seismology – Summary

2. Earthquakes -- yearly
Number of events per km2

3. Earthquakes -- Why?
Stresses applied to rocks cause brittle failures

4. Fault types Normal: Spreading ridges Mid-Atlantic Ridge
Thrust: Subduction zones
Cascadia
Strike-slip: Transform boundaries
San Andreas

5. Beachballs
Normal (Mid-Atlantic Ridge)

6. Beachballs
Reverse thrust (Cascadia)

7. Beachballs
Reverse oblique (Aleutians)

8. Beachballs
Strike-slip (San Andreas)

9. Beachball -- summary
Beachball equivalents for various fault types

10. Earthquake moment tensor
Components of the moment tensor – 9 elements

11. Fault Plane Scale of earthquake:
Mo = fault plane area x slip x breaking strength of rock

12. First motion wave field
We see two quadrants of compression and two of dilatations

13. Earthquake wave types Body waves (travel through volume of Earth):
P-waves (sound)
S-waves (shear)
Surface waves (travel across surface of Earth:
Love waves (horizontal)
Raleigh waves (vertical and horizontal)

14. Travel-time curves

15. P-to-S travel times
P-wave velocity through rock is usually about 70% faster than S-wave velocity.
The delay between P-wave and S-wave arrivals at a seismograph determines distance from source event.

16. Locating a seismic event
If we can determine the distance of an event from 3 or more seismographs, we can locate its epicentre.
...from Press and Seiver

17. Benioff Zone
Cascadia subduction zone

18. Earth Structure
Schematic of interior structure of the Earth