1. Seismic wave types and velocities
GE 11a, 2014, Lecture 4
Seismic wave types and velocities

2. Seismic activity on earth is widespread, frequent, localized and powerful

3. The earth may be unique in the solar system in this respect
Weak, diffuse moonquakes

4. Basic types of faults Ground Hanging wall Foot wall Fault plane
Dip-slip (cut-away view)
Normal: Hanging wall down
Thrust (‘reverse’): Hanging wall up
Strike-slip
(bird’s eye view)
Right lateral
Left lateral
Fault trace

5. continuous plastic shear
The broader context of faulting
Fault plane;
episodic rupture
Brittle
Ca km deep
Ductile
Broad zone;
continuous plastic shear

6. ‘Knife-sharp’ faults
Guatemala
Somewhere else

7. Important field trip guidelines:
• Depart Friday, Oct. 17tt, return Sunday, Oct. 19th, etc. 6 pm
• Notebook (preferably something tough) and pens
• Show up on the S. side of Arms by 11:30
• Bring a sleeping bag or a heavy blanket (pillows, sheets, etc. are provided)
• Warm-ish clothes
• Walking shoes
• At least one piece of rain gear
• Maybe a swim suit? Depends on timing and hot spring access
• There are consequences for getting crunk.

8. Fault ‘zones’

9. Plastic deformation near and in faults
Antelope valley

10. ‘Breccia’
‘Cataclasite’
More fine grained
Blocks are breccia; i.e.,
clearly multiple stages
‘Mylonite’
Sorting and stretching into layers

11. Distributed fault systems — like a ‘mega-fault zone’

12. Plastic deformation in a shear zone
Little
Medium
Big

13. Earthquakes! The sources of seismic waves

14. Rupture continues to expand as a crack along the fault plane.
0 Seconds
Rupture expands circularly on fault plane, sending out seismic waves in all directions.
Focus
Fault cracks
at surface
5 Seconds
Rupture continues to expand
as a crack along the fault plane.
Rocks at the surface begin to
rebound from their deformed
state.
Fault crack
extends
10 Seconds
The rupture front progresses
down the fault plane, reducing
the stress.
20 Seconds
Rupture has progressed along
the entire length of the fault.
The earthquake stops.

15. The fault plane of the Landers earthquake
(eastern California shear zone; 1992)
Displacement on fault plane

16. Earthquake nomenclature
Epicenter
Ground
Hypocenter (‘focus’)
Fault plane
Other side of the earth
Anticenter

17. P waves — a body wave analogous to sound
Wave direction

18. S waves— a body wave analogous to light
Wave direction

19. Surface waves Rayleigh wave (analogous to ocean surface)
Wave direction
Love wave (analogous to a snake or shaken rope)
Wave direction

20. (‘natural’ or ‘harmonic’ oscillations)
Normal modes
(‘natural’ or ‘harmonic’ oscillations)
Spheroidal (radial motion)
Toroidal (torsional, shearing motion)
On earth, periods are
ca. tens of minutes

21. Speeds of seismic waves
• Surface and normal modes have complex velocity dependencies; take 11d to
learn about these!
• Body waves are simpler (and more important for studying earth’s interior)
elastic modulus’ (stiffness)
Velocity is proportional to
density (momentum)
F/m2 — kg/s2m
stress
strain
Elastic modulus =
Unitless; e.g., ∂Volume/Volume
Two elastic moduli:
• Bulk modulus (): isotropic compression; springiness of bonds
• Shear modulus (): resistance to change in shape

22. Speeds of seismic waves
General relation:
V = (modulus/)0.5
VP = ([+4/3]/)0.5
VS = (/)0.5
• For finite  and , VP must be faster than VS
•  = 0 in fluids, so VP drops sharply and VS goes to 0 when waves
hit a solid/fluid boundary

23. Moment = Slip x Area x Elastic modulus
Moment magnitude
Moment = Slip x Area x Elastic modulus
N.meters
(i.e., work)
Meters
Kg/s2.m
(i.e., force per area)
Meters2
Log10 of moment

24. The Mercalli Intensity scale
(earthquake intensities for people who don’t like numbers
and are easily scared)

25. Deaths from earthquakes
# of earthquakes
Deaths from earthquakes
Courtesy of James Jackson
Population growth

27. Tehran North Tehran Fault > 5,000,000 1,000,000 to 5,000,000

28. Some examples of large potential seismic disasters
Tehran
Lebanon etc.
Himalayan front
Some examples of large potential seismic disasters
Myanmar, Bangladesh
western Sumatra

29. Ratnal, India after the 2001 magnitude 7.7 event

30. Youtube it! (ground motions) (Building motions)
(Building motions)

32. Topagraphy near Sumatra

33. 2011 Japan event

34. Youtube model 2011 Japan event model

35. Youtube it! Summatra Japan http://www.youtube.com/watch?v=HNpi9nCnMPo