GE 11a, 2014, Lecture 4
Seismic wave types and velocities
Seismic activity on earth is widespread, frequent, localized and powerful
The earth may be unique in the solar system in this respect
Weak, diffuse moonquakes
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
The broader context of faulting
Brittle
Fault plane;
episodic rupture
Ca. 10-30 km deep
Ductile
Broad zone;
continuous plastic shear
‘Knife-sharp’ faults
Guatemala
Somewhere else
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.
Fault ‘zones’
Plastic deformation near and in faults
Antelope valley
‘Breccia’
‘Cataclasite’
- More fine grained
- Blocks are breccia; i.e.,
clearly multiple stages
‘Mylonite’
Sorting and stretching into layers
Distributed fault systems — like a ‘mega-fault zone’
Plastic deformation in a shear zone
Little
Medium
Big
Earthquakes! The sources of seismic waves
Focus
0 Seconds
Rupture expands circularly on
fault plane, sending out seismic
waves in all directions.
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.
The fault plane of the Landers earthquake
(eastern California shear zone; 1992)
Displacement on fault plane
Earthquake nomenclature
Epicenter
Ground
Fault plane
Other side of the earth
Hypocenter (‘focus’)
Anticenter
P waves — a body wave analogous to sound
S waves— a body wave analogous to light
Surface waves
Rayleigh wave (analogous to ocean surface)
Wave direction
Love wave (analogous to a snake or shaken rope)
Wave direction
Normal modes
(‘natural’ or ‘harmonic’ oscillations)
Spheroidal (radial motion)
Toroidal (torsional, shearing motion)
On earth, periods are
ca. tens of minutes
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
Elastic modulus =
stress
strain
density (momentum)
F/m2 — kg/s2m
Unitless; e.g., ∂Volume/Volume
Two elastic moduli:
• Bulk modulus (): isotropic compression; springiness of bonds
• Shear modulus (): resistance to change in shape
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
Moment magnitude
Moment = Slip x Area x Elastic modulus
N.meters
(i.e., work)
Log10 of moment
Meters
Meters2
Kg/s2.m
(i.e., force per area)
The Mercalli Intensity scale
(earthquake intensities for people who don’t like numbers
and are easily scared)
# of earthquakes
Deaths from earthquakes
Courtesy of James Jackson
Population growth
North
Tehran
Fault
Tehran
> 5,000,000
1,000,000 to 5,000,000
500,000 to 1,000,000
Lebanon etc.
Tehran
Himalayan front
Some examples of large potential seismic disasters
Myanmar, Bangladesh
western Sumatra
Ratnal, India after the 2001 magnitude 7.7 event
Youtube it!
(ground motions)
http://www.youtube.com/watch?v=UPwSN9g
UG5c
http://www.youtube.com/watch?v=4Y62Ti5_6s
http://www.youtube.com/watch?v=ikZDBhP_i
nI
(Building motions)
http://www.youtube.com/watch?v=HB2jgJJG2is&feature=related
http://www.youtube.com/watch?v=JhJzdtzl6KY
http://www.youtube.com/watch?v=OyPleemS
PnE
Topagraphy near Sumatra
2011 Japan event
Youtube model
2011 Japan event model
http://www.youtube.com/watch?v=PBZGH3yie
Lc
Youtube it!
Summatra
http://www.youtube.com/watch?v=HNpi9nCn
MPo
Japan
http://www.youtube.com/watch?v=ceym2c18OQM