Geol 240: Earthquakes Midterm 2 – Flashcards
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wavelength
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-distance from crest to crest (or any relative part of wave)
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velocity
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-tsunami velocity = 800km/hr
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ocean waves
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-driving force:
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tsunami waves
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-high velocity (approx. 800km/hr in open ocean) -low frequency (.6 cycles/minute) -huge wavelength (200km) -generated at shallow (30km depth) part of megathrust fault near oceanic trench (typically 6-10km deep) where there is seafloor uplift (up to 10km)
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tsunami causes
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-oceanic megathrust quakes -deep ocean landslides -large-scale water displacement -seafloor surface rupture is effective at raising seafloor
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oceanic megathrust faults
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-extremely long (1000km) -trench depth 6-10km -10m seafloor uplift in earthquake -slip amount:
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epicenter
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-point at surface above hypocenter
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hypocenter
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-point in 3D space where fault starts slipping, creating the earthquake -also called focus
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2011 Tohoku Mw. 9.0
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-15,000-25,000 deaths
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2004 Sumatra Mw. 9.2
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-250,000 deaths
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elasticity
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1. distortion (strain) will disappear after applied stress is removed and material will revert to original size and shape (ie: below failure stress, no permanent strain 2. distortion (strain) is proportional to the applied stress (Hooke's Second Law)
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3 Elastic Moduli
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-distortion/strain will disappear after applied stress is removed -material will resort to original size and shape if below failure stress 1. Young's Modulus 2. Bulk Modulus 3. Shear Modulus
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Hooke's Law
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-distortion (strain) is proportional to the applied stress -linear stress to strain relationship
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Young's Modulus (Y)
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-defines relationship of a specific material to an applied stress -Linear Elastic Strain -Hooke's Law -the larger Y is, the less the material will be distorted
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Bulk Modulus (k)
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-measure of resistance of material to changes in volume, due to pressure or isotropic stress -also called rigidity or incompressibility -think about a tennis ball vs. shotput and response to isotropic stress
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Shear Modulus (M)
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-resistance of a material to change in shape -gases and liquids have a M=0, which is why S waves cannot travel through them
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ray path
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-direction of energy travel or wave propagation
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body waves
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-propagate throughout solid earth -include wave fronts and ray paths -includes P and S waves -body waves lose energy faster with distance than surface waves by 1/r2 -body waves are high frequency and lose energy with distance
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P waves
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-"primary" or "pressure" waves -always the first to arrive, fastest moving -parallel to ray path -compressional motion, adding elastic strain to system -don't carry much energy Vp= root (k+4/3M)/density
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S waves
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-"shear" or "secondary" waves -carry most energy in a local earthquake -particle motion perpendicular to ray path -cannot propagate thru gases/liquids because M=0 Sv= vertically polarized S wave Sh= horizontally polarized S wave Vs= root M/density
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surface waves
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-propagate along surface of earth (mostly in crust) -biggest wave phases for distance earthquakes
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Rayleigh waves
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-retrograde ellipse wave pattern
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Love waves
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-horizontally polarized shear waves (Sh) that are trapped in a low velocity layer by total internal reflection (in the crust) -near surface of Earth -what lead to total internal reflection concept
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geothermal gradient
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-1km/30C @ 5km = 120-150C @ 15km = 450C @350-450C, rocks begin to metamorphose and exhibit elastic behavior
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crustal earthquake depth
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-deepest quakes at Wadati-Benioff Zone, 700km
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polymorphic phase transformations
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-400km, corresponding to big wave velocity jump -olive to spinel @650km, spinel to perovskite; another velocity jump
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low velocity zone
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-LVZ occurs close to the boundary between the lithosphere and the asthenosphere in the upper mantle -characterized by unusually low seismic shear wave velocity compared to the surrounding depth intervals -80-300km depth
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mantle zones
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Crust = 5-80km (rock) Mantle = 2885km (rock) Outer Core = 2270km (metal) Inner Core = 1216km (metal)
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core formation
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-iron core accumulation during accretion/differentiation -core accumulation due to gravity (metal pond theory) -heat LOSS from core drives plate tectonics as a cooling system
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early Earth heating
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1. radioactive decay of metals 2. compression due to gravity (compression = heat) 3. bombardment
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mechanical layering
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-lithosphere vs. asthenosphere -asthenosphere is mechanically weak but NOT MOLTEN
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compositional layering
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-crust vs. mantle vs. core
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continental crust
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-thin, compositionally different -SiO2, feldspars, tectosilicates -buoyant, low density -isostacy -high variable thickness, avg. 30km but ranges 5-85km
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oceanic crust
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-mafic -low Si content -nesosilicates
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mantle
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-largest part of Earth by volume -composed of ultramafic rocks -olivine is dominant in upper mantle -ridge push vs. slab pull vs. basal drag theories
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teleseisms
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-rays from earthquakes a great distance away from seismic station -first arrival ray path for a teleseism is usually coming from straight up underneath a sesimometer
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first arrival/fastest ray path
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-
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mainshock
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-the largest earthquake in a sequence of earthquakes -occurring in same general area
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foreshock
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-smaller quake that occurs before the mainshock -within a brief period of time before the mainshock (not a well defined time period/definition of brief) -approx. same location as mainshock (again not well defined; usually within 1-2 rupture lengths of mainshock) -%5 is CA along San Andreas are foreshocks (5% chance that any small quake will be followed by a larger quake NOT necessarily a big quake)
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aftershock
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-can be thousands after big quake -set of smaller that come after mainshock -in approx same area (1-2 mainshock rupture lengths) -for every mainshock, there will always be about one aftershock that is 1 Mw smaller; 10 that are 2 Mw smaller, 100 that are 3 Mw smaller, etc.
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Omori's Law
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-rate of decay of aftershocks after mainshocks -#aftershocks/day = alpha/time+beta or 1/t -square curve
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seismometers
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1. short-period: measure high frequency short period body waves 2. long period: measure low frequency long period surface waves -vertical, horizontal EW and horizontal NS components
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earthquake location methods
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1. locate surface rupture 2. location of maximum shaking/damage w/ Shake Maps 3. locating with S-P wave travel times
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S-P travel time curves
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-local: # will be small -teleseism: # will be big -for teleseisms, seismic energy usually coming from directly underneath the station, so P waves will be prominently displayed
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local quakes
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-S waves biggest
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distance quakes
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-teleseisms -surface waves will be biggest -body waves lose energy faster with distance than surface waves by 1/r2 -body waves are high frequency and lose energy with distance -LOW FREQUENCY WAVES LOSE ENERGY SLOWER THAN HIGH FREQUENCY WAVES
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Modified Mercali Intensity Scale
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-a "shake map" 1: no movement **** 3: can't feel indoors 5: almost everyone feels it 6: everyone feels it **** 7: difficulty driving 9: really bad damage *** 12: apocalyptic ****
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Richter Magnitude Scale (Ml)
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-theoretically an open scale -1930s CalTech -measures amplitude of seismic waves on a specific type of seismometer -Wood-Anderson Torsion Seismometer -calibrated to 1s period of shear waves at 100km distance -scale ranges from -4 to 10
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Surface Wave Magnitude Scale (Ms)
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-for Ml greater than 7 -based on measurement of 20s or 40s surface waves -otherwise similar to Richter Scale -10x increase in magnitude for every mag. unit -much better at measuring larger or distant quakes
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Moment Magnitude Scale (Mw)
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-energy released in quake -NOT based on measuring amplitude in seismograms -based on measuring entire frequency range of seismic energy released -based on measuring a torque called "Seismic Moment" (Mo) -Mw increases 32x for every unit increase
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seismic moment
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-a torque measured in Mw scale Mo = M * A * D or = to product of shear modulus, area of fault slip, and fault displacement
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Source Effects
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1. Location 2. Duration 3. Magnitude 4. Frequency content of radiated seismic energy -bigger the quake, more low E frequency release -smaller the quake, more high E release 5. Rupture propagation direction and source directivity
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directivity pulse
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-doppler effect on P waves -rupture propagation velocity typically 80-90% of shear wave velocity -need rupture propagation to be parallel to slip direction -almost always happens in large strike slip fault ruptures
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Path Effects
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1. Plate-scale controls 2. Regional subsurface geology
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Site Effects
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1. local geology -bedrock vs. weak soil/sediment basins -sediment vs. bedrock shaking -sediment amplification (Vs30)
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sedimentary basin effects
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-LA basin 10km depth 11x amplitude on sedimentary basin bedrock
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fault rupture velocity
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3km/second
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