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Seismic wave types and velocities

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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

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