ge 11a, 2014, lecture 4 seismic wave types and velocities

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