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

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Presentation on theme: "GE 11a, 2014, Lecture 4 Seismic wave types and velocities."— Presentation transcript:

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

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

6 ‘Knife-sharp’ faults Guatemala Somewhere else

7 Important field trip guidelines: Depart Friday, Oct. 17 tt, return Sunday, Oct. 19 th, etc. 6 pm Depart Friday, Oct. 17 tt, return Sunday, Oct. 19 th, etc. 6 pm Notebook (preferably something tough) and pens Notebook (preferably something tough) and pens Show up on the S. side of Arms by 11:30 Show up on the S. side of Arms by 11:30 Bring a sleeping bag or a heavy blanket (pillows, sheets, etc. are provided) Bring a sleeping bag or a heavy blanket (pillows, sheets, etc. are provided) Warm-ish clothes Warm-ish clothes Walking shoes Walking shoes At least one piece of rain gear At least one piece of rain gear Maybe a swim suit? Depends on timing and hot spring access Maybe a swim suit? Depends on timing and hot spring access There are consequences for getting crunk. 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 Focus 0 Seconds Rupture expands circularly on fault plane, sending out seismic waves in all directions. 5 Seconds Rupture continues to expand as a crack along the fault plane. Rocks at the surface begin to rebound from their deformed state. 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. Fault cracks at surface Fault crack extends

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

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

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

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

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

20 Normal modes (‘natural’ or ‘harmonic’ oscillations) Toroidal (torsional, shearing motion) Spheroidal (radial 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) Velocity is proportional to elastic modulus’ (stiffness) density (momentum) Elastic modulus = stress strain Unitless; e.g., ∂Volume / Volume F/m 2 — kg/s 2 m Two elastic moduli: Bulk modulus (  ): isotropic compression; springiness of bonds Shear modulus (  ): resistance to change in shape

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

23 Moment magnitude Moment = Slip x Area x Elastic modulus N. meters (i.e., work) Meters Meters 2 Kg/s 2. m (i.e., force per area) Log 10 of moment

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

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

26

27 Tehran North Tehran Fault > 5,000,000500,000 to 1,000,000 1,000,000 to 5,000,000

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

29 Ratnal, India after the 2001 magnitude 7.7 event

30 Youtube it! (ground motions) (Building motions) PnE nI 62Ti5_6s UG5c

31

32 Topagraphy near Sumatra

33 2011 Japan event

34 Youtube model Lc 2011 Japan event model

35 Youtube it! Japan MPo Summatra


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