Ch 3 Flashcards
The Earth’s surface shakes due to:
- Volcanoes, meteorites, landslides, nuclear bomb detonation
- Fault release
Fault release
Fault: fracture in Earth’s crust
- Stress across fault releases suddenly causing sudden deformation (strain)
- Strain propagates as seismic waves
Law of original horizontally:
Sediments gather in horizontal layers
Law of superposition:
Old sediment on bottom, young sediment on top
Law of original continuity:
Layers are continuous horizontally
Truncation can indicate a fault
Fault offset: distance of relative motion
Fault length: length of rupture
Fault area: (~length*offset) related to energy
Joints
- Fractures and cracks in brittle lithospheric rocks (no motion across)
- Stress differences on either side of a fracture result in offset: fracture -> fault
- Offset ranges from mm to 100s of km
Strike
compass pointing parallel to fault, horizontal
Dip
Angle of inclination from horizontal
Dip-Slip faults
- Dominated by vertical offset
- Ore veins often form in fault zones, so many mines are dug out along faults
Normal fault
- Offset under extensional stress (<—–>)
- H.W moves down relative to F.W
- Layers missing in a vertical bore
- Continental/oceanic crust divergent boundaries
Reverse fault
- Compressional stresses (—> <—)
- H.W moves up relative to F.W
- Layers are repeated in a vertical bore
- Convergent plate boundaries: collision and subduction
Strike-Slip Fault
- Shear stresses
- Right lateral fault: opposite block moves to observer’s right
- Left lateral fault: opposite block moves to observer’s left
Faults
- Complex zones of breakage with irregular surfaces, miles wide and many miles long
- Stress builds up over years until rupture occurs at weak point and propagates along fault surface
- Energy is released as seismic waves
- Fault rupture is a series of events over weeks to months to years, with largest event referred to as ‘the earthquake’
Transform Faults
- Divergent plates at MORs slide past other plates (b/c spherical Earth) along transform faults
- Transform faults link divergent and convergent plates
Development of Seismology
Detectors: seismometers
Recorders: seismographs (record 3D ground movement: N-S, E-W, and vertical)
Amplitude (V)
velocity or acceleration of detector
Wavelength (m)
distance between successive waves
Period (s)
of seconds between waves
Frequency (Hz)
of waves in 1 second
Body waves (P and S)
fastest, high frequency, travel all the way through the Earth
Surfaces waves (love and Rayleigh)
body waves combined at Earth’s free surface (stuck at surface), most destructive, slowest
P (primary) waves
- Fastest of all, arrive 1st at detector
- Compressional strain
- Supported by all phases of matter (g/l/s)
- ~5.3km/s P-wave velocity in granite
- Varies with compressibility, shear strength, and density of matter
S (secondary) waves
- Follows primary to detector
- Shear strain along path of propagation
- Cannot travel through liquid/gas
- ~3km/s in granite
- Varies with shear strength and density of matter
From the Earth’s surface down:
- Wave speed increases with depth (slows at asthenosphere b/c weak shear)
- Wave speeds increase through mantle to core boundary (S waves disappear at outer core, P waves slow at outer core b/c no shear)
- P-wave speed increases through outer core, increases suddenly at inner core
Love waves
- Faster than Rayleigh waves
- Shear strain normal to propagation direction
Rayleigh waves
- Longer period than body waves
- Slowest
- Travel great distances and are energetic at great distances
- Backward-rotating (retrograde), elliptical motion produces horizontal and vertical shaking
- More energy is released as Rayleigh waves when earth hypocenter is close to surface
Locating the source of an earthquake
- P waves travel ~1.7x faster than S waves
- Farther from hypocenter, greater lag time of S wave behind P wave (S - P)
- S - P time indicates how far away the earthquake was from the station (no direction though)
- Epicenter of earthquake located by triangulation from multiple stations
Seismic moment (M0)
Strain energy released from entire rupture surface, more accurate for large earthquakes
M0 = shear strengthrupture areadisplacement
Moment magnitude scales uses seismic moment
Mw= 2/3log10(Mo) - 10.7
Seismic wave frequency influences damage:
High freq waves = lots of damage at epicentre, but die out quickly with distance
Low freq waves = travel great distance from epicenter but do most damage father away
Horizontal shaking: massive damage to buildings
Acceleration
- Measure in terms of acceleration due to gravity (g)
- Damage from horizontal accelerations >0.1g
Buildings have natural frequencies and periods:
- Periods of swaying are ~0.1s/story
- 30-story buildings sways at ~3s/cycle
Building materials affect building periods
- Flexible materials (wood, steel) = longer period of shaking
- Stiff materials (brick, concrete) = shorter period of swaying
The velocity of seismic waves depends on the material through which it is moving
Faster through hard rocks, slower through soft rocks
When waves pass from harder to softer rocks they must slow down and
- Must increase amplitude in order to carry the same amount of energy = greater shaking
- Shaking tends to be stronger at sites with softer sediments
If the period of the waves matches the period of the building, shaking is amplified and resonance results
- Common cause of catastrophic failure of buildings
What to do before an earthquake
- Inside/outside your home, visualize what might fall and anchor those objects
- Locate safe spots with protection - under heavy table, strong desk, bed, etc.
What to do during an earthquake
- Duck, cover, hold on
- Stay calm
- If inside, stay inside; if outside, stay outside
Mercalli Intensity Scale
- Developed to quantify what people feel during an earthquake, used for earthquakes before instrumentation and in areas without instrumentation
- Assesses effects on people and buildings
Mercalli Scale Variables
- Earthquake mag
- Distance from focus/hypocenter
- Type of rock or sediment making up ground surface
- Building style
- Duration of shaking
Mercalli scale: earthquake mag
- Bigger earthquake = more likely death and damage
Mercalli scale: distance from focus/hypocenter
- Usually, closer earthquake = more damage
Mercalli scale: type of rock or sediment of ground surface
- Hard rock foundations vibrate from nearby earthquake body waves
- Soft sediments amplified by distant earthquake surface waves
- Steep slopes can generate landslides when shaken
Mercalli scale: building style
- Body waves near epicenter are amplified by rigid short buildings
- Low-freq surface waves are amplified by tall buildings, esp on soft foundations
Mercalli scale: duration of shaking
- Longer shaking time = more buildings can be damaged
San Fernando Valley, Cali, Earthquake of 1971
- NW LA
- 6:01am, 67 deaths
- Mag: 6.6, with 35 mag 4< aftershocks in 7mins after mainshock
- Building style = soft 1st story buildings were major problem, hollow-core bricks at Veteran’s Admin Hospital caused collapse and 47 deaths, collapse of freeway bridges
- Duration of shaking = strong for 12secs, dam almost failed -> only needed 5 more sec to fail
Building in earthquake country
- Eliminate resonance
- Shear walls
- Braced frames
- Retrofit buildings and bridges
- Houses
- Base isolation
Eliminate resonance
- Change height of building
~ hard bedrock: build tall, flexible building
~ soft bedrock: building short, stiff building - Move mass to lower floors
- Change shape of building
- Change building materials
- Change attachment of building to foundation
Shear walls
- Carry horizontal forces to the ground
- Lack of shear walls = structure like parking garages will fail in earthquakes
- Best if firmly attached to floors and roofs
Braced frames
- Brace with ductile materials = resistance
Retrofit buildings and bridges
- Increase resistance to shaking
- Bridges built with alternating layers of steel and concrete
Houses
- Modern 1/2 story wood-frame houses do well in earthquakes
- Additional support = shear walls, bracing, tying walls and foundations and roof together
- Most damage occurs as interior items are thrown around (so bolt down water heaters, ceiling fans, cabinets, bookshelves, electronics)
Base isolation
- Devices on ground or in structure to absorb part of earthquakes energy
- Use wheels, ball bearings, shock absorbers, ‘rubber doughnuts’, etc to isolate building from worst shaking
