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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