7Important 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.
14Rupture continues to expand as a crack along the fault plane. 0 SecondsRupture expands circularly on fault plane, sending out seismic waves in all directions.FocusFault cracksat surface5 SecondsRupture continues to expandas a crack along the fault plane.Rocks at the surface begin torebound from their deformedstate.Fault crackextends10 SecondsThe rupture front progressesdown the fault plane, reducingthe stress.20 SecondsRupture has progressed alongthe entire length of the fault.The earthquake stops.
15The fault plane of the Landers earthquake (eastern California shear zone; 1992)Displacement on fault plane
16Earthquake nomenclature EpicenterGroundHypocenter (‘focus’)Fault planeOther side of the earthAnticenter
17P waves — a body wave analogous to sound Wave direction
18S waves— a body wave analogous to light Wave direction
19Surface waves Rayleigh wave (analogous to ocean surface) Wave directionLove 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 areca. tens of minutes
21Speeds of seismic waves • Surface and normal modes have complex velocity dependencies; take 11d tolearn about these!• Body waves are simpler (and more important for studying earth’s interior)elastic modulus’ (stiffness)Velocity is proportional todensity (momentum)F/m2 — kg/s2mstressstrainElastic modulus =Unitless; e.g., ∂Volume/VolumeTwo elastic moduli:• Bulk modulus (): isotropic compression; springiness of bonds• Shear modulus (): resistance to change in shape
22Speeds of seismic waves General relation:V = (modulus/)0.5VP = ([+4/3]/)0.5VS = (/)0.5• For finite and , VP must be faster than VS• = 0 in fluids, so VP drops sharply and VS goes to 0 when waveshit a solid/fluid boundary
23Moment = Slip x Area x Elastic modulus Moment magnitudeMoment = Slip x Area x Elastic modulusN.meters(i.e., work)MetersKg/s2.m(i.e., force per area)Meters2Log10 of moment
24The Mercalli Intensity scale (earthquake intensities for people who don’t like numbersand are easily scared)
25Deaths from earthquakes # of earthquakesDeaths from earthquakesCourtesy of James JacksonPopulation growth