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seismic waves

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-propagation of movement/motion through rock; waves of energy that travel through earth's layers -3km/sec average

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amplitude

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-height from crest (or trough) to middle average

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frequency

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-number of waves that pass in a unit of time ie: cycles per second

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period

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-time between wave crests passing a certain point

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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

answer

-

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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

answer

-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

answer

-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

answer

-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

answer

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

answer

-S waves biggest

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distance quakes

answer

-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

answer

-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)

answer

-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)

answer

-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)

answer

-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

answer

-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

answer

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

answer

1. Plate-scale controls 2. Regional subsurface geology

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Site Effects

answer

1. local geology -bedrock vs. weak soil/sediment basins -sediment vs. bedrock shaking -sediment amplification (Vs30)

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sedimentary basin effects

answer

-LA basin 10km depth 11x amplitude on sedimentary basin bedrock

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fault rupture velocity

answer

3km/second

Geol 240: Earthquakes Midterm 2 – Flashcards

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