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GOPH375 > Ch 3 > Flashcards

Ch 3 Flashcards

1
Q

The Earth’s surface shakes due to:

A
  • Volcanoes, meteorites, landslides, nuclear bomb detonation
  • Fault release
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2
Q

Fault release

A

Fault: fracture in Earth’s crust
- Stress across fault releases suddenly causing sudden deformation (strain)
- Strain propagates as seismic waves

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3
Q

Law of original horizontally:

A

Sediments gather in horizontal layers

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4
Q

Law of superposition:

A

Old sediment on bottom, young sediment on top

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5
Q

Law of original continuity:

A

Layers are continuous horizontally

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6
Q

Truncation can indicate a fault

A

Fault offset: distance of relative motion
Fault length: length of rupture
Fault area: (~length*offset) related to energy

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7
Q

Joints

A
  • 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
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8
Q

Strike

A

compass pointing parallel to fault, horizontal

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9
Q

Dip

A

Angle of inclination from horizontal

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10
Q

Dip-Slip faults

A
  • Dominated by vertical offset
  • Ore veins often form in fault zones, so many mines are dug out along faults
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11
Q

Normal fault

A
  • Offset under extensional stress (<—–>)
  • H.W moves down relative to F.W
  • Layers missing in a vertical bore
  • Continental/oceanic crust divergent boundaries
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12
Q

Reverse fault

A
  • Compressional stresses (—> <—)
  • H.W moves up relative to F.W
  • Layers are repeated in a vertical bore
  • Convergent plate boundaries: collision and subduction
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13
Q

Strike-Slip Fault

A
  • Shear stresses
  • Right lateral fault: opposite block moves to observer’s right
  • Left lateral fault: opposite block moves to observer’s left
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14
Q

Faults

A
  • 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’
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15
Q

Transform Faults

A
  • Divergent plates at MORs slide past other plates (b/c spherical Earth) along transform faults
  • Transform faults link divergent and convergent plates
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16
Q

Development of Seismology

A

Detectors: seismometers
Recorders: seismographs (record 3D ground movement: N-S, E-W, and vertical)

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17
Q

Amplitude (V)

A

velocity or acceleration of detector

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18
Q

Wavelength (m)

A

distance between successive waves

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19
Q

Period (s)

A

of seconds between waves

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20
Q

Frequency (Hz)

A

of waves in 1 second

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21
Q

Body waves (P and S)

A

fastest, high frequency, travel all the way through the Earth

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22
Q

Surfaces waves (love and Rayleigh)

A

body waves combined at Earth’s free surface (stuck at surface), most destructive, slowest

23
Q

P (primary) waves

A
  • 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
24
Q

S (secondary) waves

A
  • 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
25
Q

From the Earth’s surface down:

A
  • 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
26
Q

Love waves

A
  • Faster than Rayleigh waves
  • Shear strain normal to propagation direction
27
Q

Rayleigh waves

A
  • 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
28
Q

Locating the source of an earthquake

A
  • 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
29
Q

Seismic moment (M0)

A

Strain energy released from entire rupture surface, more accurate for large earthquakes
M0 = shear strengthrupture areadisplacement

30
Q

Moment magnitude scales uses seismic moment

A

Mw= 2/3log10(Mo) - 10.7

31
Q

Seismic wave frequency influences damage:

A

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

32
Q

Horizontal shaking: massive damage to buildings

A

Acceleration
- Measure in terms of acceleration due to gravity (g)
- Damage from horizontal accelerations >0.1g

33
Q

Buildings have natural frequencies and periods:

A
  • Periods of swaying are ~0.1s/story
  • 30-story buildings sways at ~3s/cycle
34
Q

Building materials affect building periods

A
  • Flexible materials (wood, steel) = longer period of shaking
  • Stiff materials (brick, concrete) = shorter period of swaying
35
Q

The velocity of seismic waves depends on the material through which it is moving

A

Faster through hard rocks, slower through soft rocks

36
Q

When waves pass from harder to softer rocks they must slow down and

A
  • Must increase amplitude in order to carry the same amount of energy = greater shaking
  • Shaking tends to be stronger at sites with softer sediments
37
Q

If the period of the waves matches the period of the building, shaking is amplified and resonance results

A
  • Common cause of catastrophic failure of buildings
38
Q

What to do before an earthquake

A
  • Inside/outside your home, visualize what might fall and anchor those objects
  • Locate safe spots with protection - under heavy table, strong desk, bed, etc.
39
Q

What to do during an earthquake

A
  • Duck, cover, hold on
  • Stay calm
  • If inside, stay inside; if outside, stay outside
40
Q

Mercalli Intensity Scale

A
  • 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
41
Q

Mercalli Scale Variables

A
  • Earthquake mag
  • Distance from focus/hypocenter
  • Type of rock or sediment making up ground surface
  • Building style
  • Duration of shaking
42
Q

Mercalli scale: earthquake mag

A
  • Bigger earthquake = more likely death and damage
43
Q

Mercalli scale: distance from focus/hypocenter

A
  • Usually, closer earthquake = more damage
44
Q

Mercalli scale: type of rock or sediment of ground surface

A
  • Hard rock foundations vibrate from nearby earthquake body waves
  • Soft sediments amplified by distant earthquake surface waves
  • Steep slopes can generate landslides when shaken
45
Q

Mercalli scale: building style

A
  • Body waves near epicenter are amplified by rigid short buildings
  • Low-freq surface waves are amplified by tall buildings, esp on soft foundations
46
Q

Mercalli scale: duration of shaking

A
  • Longer shaking time = more buildings can be damaged
47
Q

San Fernando Valley, Cali, Earthquake of 1971

A
  • 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
48
Q

Building in earthquake country

A
  • Eliminate resonance
  • Shear walls
  • Braced frames
  • Retrofit buildings and bridges
  • Houses
  • Base isolation
49
Q

Eliminate resonance

A
  • 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
50
Q

Shear walls

A
  • 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
51
Q

Braced frames

A
  • Brace with ductile materials = resistance
52
Q

Retrofit buildings and bridges

A
  • Increase resistance to shaking
  • Bridges built with alternating layers of steel and concrete
53
Q

Houses

A
  • 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)
54
Q

Base isolation

A
  • 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

GOPH375 (10 decks)

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