ge 11a, 2014, lecture 4 seismic wave types and velocities
TRANSCRIPT
GE 11a, 2014, Lecture 4Seismic 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
Normal: Hanging wall down
Thrust (‘reverse’): Hanging wall up
Strike-slip
Left lateral Right lateral
Ground
Fault plane
Foot wallHanging wall
Fault trace
(bird’s eye view)
Dip-slip (cut-away view)
Brittle
Ductile
Fault plane;episodic rupture
Broad zone;continuous plastic shear
Ca. 10-30 km deep
The broader context of faulting
‘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
Focus0 SecondsRupture expands circularly on fault plane, sending out seismic waves in all directions.
5 SecondsRupture continues to expand as a crack along the fault plane. Rocks at the surface begin to rebound from their deformed state.
10 SecondsThe rupture front progresses down the fault plane, reducing the stress.
20 SecondsRupture has progressed alongthe entire length of the fault.The earthquake stops.
Fault cracksat surface
Fault crackextends
The fault plane of the Landers earthquake(eastern California shear zone; 1992)
Displacement on fault plane
Earthquake nomenclature
Ground
Hypocenter (‘focus’)Fault plane
Epicenter
AnticenterOther side of the earth
Wave direction
P waves — a body wave analogous to sound
Wave direction
S waves— a body wave analogous to light
Wave direction
Love wave (analogous to a snake or shaken rope)
Rayleigh wave (analogous to ocean surface)
Wave direction
Surface waves
Normal modes(‘natural’ or ‘harmonic’ oscillations)
Toroidal (torsional, shearing motion)
Spheroidal (radial 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)
Velocity is proportional to elastic modulus’ (stiffness)
density (momentum)
Elastic modulus = stressstrain
Unitless; e.g., ∂Volume/Volume
F/m2 — kg/s2m
Two elastic moduli:
• Bulk modulus (): isotropic compression; springiness of bonds
• Shear modulus (): resistance to change in shape
Speeds of seismic waves
V = (modulus/)0.5
VP = ([+4/3]/)0.5
General relation:
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 modulusN.meters(i.e., work)
MetersMeters2
Kg/s2.m
(i.e., force per area)Log10 of moment
The Mercalli Intensity scale
(earthquake intensities for people who don’t like numbers
and are easily scared)
Population growth
Deaths from earthquakes
Courtesy of James Jackson
# of
ear
thqu
akes
Tehran
NorthTehranFault
> 5,000,000 500,000 to 1,000,0001,000,000 to 5,000,000
Lebanon etc.
Tehran Himalayan front
western Sumatra
Some examples of large potential seismic disasters
Myanmar, Bangladesh
Ratnal, India after the 2001 magnitude 7.7 event
Youtube it!(ground motions)
http://www.youtube.com/watch?v=HB2jgJJG2is&feature=related(Building motions)
http://www.youtube.com/watch?v=OyPleemSPnE
http://www.youtube.com/watch?v=ikZDBhP_inI
http://www.youtube.com/watch?v=JhJzdtzl6KY
http://www.youtube.com/watch?v=4Y-62Ti5_6s
http://www.youtube.com/watch?v=UPwSN9gUG5c
Topagraphy near Sumatra
2011 Japan event
Youtube model
http://www.youtube.com/watch?v=PBZGH3yieLc
2011 Japan event model
Youtube it!
http://www.youtube.com/watch?v=ceym2c18OQMJapan
http://www.youtube.com/watch?v=HNpi9nCnMPo
Summatra