GHC Ch 3: Earthquake Geology and Seismology Flashcards
What are earthquakes most commonly caused by?
Caused most commonly by movement of the Earth across a fault.
Fault
Complex zones of breakage in the Earth across which the two sides move relative to each other. Has irregular surfaces, and is many miles wide and long.
How does an earthquake occur?
Stresses build up until they are enough to cause rocks to fracture and shift, sending off waves of seismic energy, felt as earthquake.
Understanding faults begins with understanding what rock relationships, formalized by Steno?
Law of original horizontality and Law of superposition
Law of original horizontality
sediments are originally deposited in horizontal layers
Law of superposition
in un-deformed sequence of sedimentary rock layers, each layer is younger than the layer beneath it and older than the layer above it
Law of original continuity
sediment layers are continuous
If a sedimentary layer ends abruptly, it may have been…?
eroded by water action or truncated by fault passing through layer
Identifying truncated sedimentary layers and recognizing their offset continuation allows determination of…?
fault length
Length of fault determines size of…?
earthquake possible on fault
Longer fault ruptures create bigger or smaller earthquakes?
bigger earthquakes
Where does ‘jointing’ occur?
where brittle lithospheric rocks fracture and crack
Large stress differential on either side of a fracture results in movement, and then…?
fracture becomes a fault
Movement, ranging from millimeters to hundreds of kilometers, results in what?
tilting and folding of layers
Use strike and dip to describe what?
describe location in 3D space of deformed rock layer
Dip
angle of inclination from horizontal of tilted layer
Strike
compass bearing of horizontal line in tilted layer
Dip-slip faults are dominated by what movement?
vertical
Miners refer to the block beneath them as the…?
footwall (block beneath the fault)
Miners refer to the block above them as…?
the hangingwall (block above the fault)
what are dip-slip faults caused by?
pushing or pulling force
Strike-slip faults are dominated by what movement?
horizontal
right-lateral strike-slip fault
When straddling a fault, if right-hand side moved towards you
left-lateral strike-slip fault
When straddling a fault, if the left-hand side has moved towards you
How does an earthquake occur at a fault?
Stress produces strain , which builds up until rupture occurs at weak point and propagates along fault surface
hypocenter or focus
Point where rupture first occurs
epicenter
Point directly above hypocenter on surface
Fault rupture
series of events over weeks to months to years, with largest event referred to as ‘the earthquake’
foreshock
Smaller events preceding ‘the earthquake’
Can you identify a foreshock before the earthquake has occurred?
no
aftershocks
Smaller events after ‘the earthquake’
Left step in right-lateral fault or right step in left-lateral fault
Compression, uplift, hills and mountains
Right step in right-lateral fault or left step in left-lateral fault
Extension, down-dropping, basins and valleys
transform faults
faults link spreading centers, or connect spreading center to subduction zone. transform fault motion is same as strike-slip fault
Seismology
study of earthquakes
seismometers
Instruments to detect earthquake waves
seismographs
Instruments to record earthquake wave
How does a seismograph work?
One part stays as stationary as possible while Earth vibrates: heavy mass fixed by inertia in frame that moves with the Earth, and differences between position of the frame and the mass are recorded digitally
Amplitude of a wave indicates
displacement
Wavelength indicates
distance between successive waves
Period
time between waves (= 1/frequency)
Frequency
number of waves in one second (hertz)
Seismic waves come in what two categories?
those that can pass through entire Earth (body waves) and those that move near surface only (surface waves)
Are body waves faster or slower than surface waves?
faster
Where are body waves most energetic?
near earthquake hypocenter
Do body waves have short or long periods?
short
What are the two types of body waves?
P and S
Which is the fastest of all waves?
P waves
Are the P or S waves the first to reach a recording station?
P waves
How does a P wave move?
as push-pull – alternating pulses of compression and extension, like wave through Slinky toy
Where can a P wave travel?
Through solid, liquid, and gas.
What kind of motion does an S wave exhibit?
transverse motion – shearing or shaking particles at right angles to the wave’s path (like shaking one end of a rope)
Where can an S wave travel?
Only through solids
What is interesting about the recording of large earthquakes?
Waves from large earthquakes can pass through the entire Earth and be recorded all around the world
Where do surface waves travel?
near the Earth’s surface
What creates surface waves?
body waves disturbing the surface
Do surface waves have longer or shorter periods than body waves? What does this mean?
Longer period than body waves. Means surface waves carry energy farther.
What are love waves?
Similar motion to S waves, but side-to-side in horizontal plane
Are love or Rayleigh waves faster?
Love
Where do love waves not move?
through air or water
How do rayleigh waves move?
Backward-rotating, elliptical motion produces horizontal and vertical shaking, which feels like rolling, boat at sea.
When is more energy released as Rayleigh waves?
when earthquake hypocenter is close to the surface
How far can a rayleigh wave travel?
Great distances
You need the distance of an earthquake from how many stations to pinpoint location of earthquake?
three
How does the Richter scale work?
Bigger earthquake means greater shaking, which produces greater amplitude of seismogram lines
What does an increase by one mean on the Richter scale in regards to magnitude?
10 fold increase in recorded amplitude
What does an increase by one mean on the Richter scale in regards to energy?
the energy release increases by about 45 times, but energy is also spread out over much larger area and over longer time
Bigger earthquake means more people will experience shaking and for longer or shorter time?Does this increase or decrease damage to buildings?
longer. increase.
What is one problem with the Richter scale?
Does not work well for distant or large earthquakes
Are all magnitude scales equivalent? Why?
no. Larger earthquakes radiate more energy at longer periods which are not measured by Richter scale or body wave scale, so large or distant earthquake magnitudes are underestimated
What does a Seismic moment (Mo) measure?
Measures amount of strain energy released by movement along whole rupture surface
Is the Moment Magnitude Scale more accurate for large or small earthquakes?
large
How is a Seismic moment (Mo) calculated?
Calculated using rocks’ shear strength times rupture area of fault times displacement (slip) on the fault
What is the equation for a seismic moment?
Mw = 2/3 logsub10 (Mo) – 6
Large earthquakes are not just single events but…?
part of series of earthquakes over years
mainshock
Largest event in series of earthquakes.
foreshocks
Smaller events preceding mainshock
aftershocks
Smaller events following mainshock
Fault-rupture length greatly influences what?
magnitude
Fault-rupture length and duration influence
seismic wave frequency
Short fault rupture and duration mean
high frequency seismic waves
Long fault rupture and duration mean
low frequency seismic waves
Seismic wave frequency influences
damage
What do high frequency seismic waves do?
cause much damage at epicenter but die out quickly with distance from epicenter
Where do low frequency seismic waves do the most damage?
farther away
Where do low frequency seismic waves travel?
great distance from epicenter
What direction of forces are buildings designed to handle? How does this affect building damage in an earthquake?
Vertical. Horizontal shaking during earthquakes can do massive damage to buildings.
How is movement measured in an earthquake?
Measure in terms of acceleration due to gravity
Weak buildings suffer damage from horizontal accelerations of more than
0.1g
Periods of swaying are about how many seconds per story?
0.1
What affects building periods?
building materials
What induces longer periods of shaking?
Flexible materials (wood, steel)
What induces shorter periods of shaking?
Stiff materials (brick, concrete)
Velocity of seismic wave depends on what?
on material through which it is moving
When waves pass from harder to softer rocks, they slow down and must therefore increase their what in order to carry same amount of energy?
amplitude
resonance
If the period of the wave matches the period of the building, shaking is amplified and resonance results
Mercalli scale
developed to quantify what people feel during an earthquake
What should one do before an earthquake?
Inside and outside your home, visualize what might fall during strong shaking, and anchor those objects by nailing, bracing, tying, etc.
Inside and outside your home, locate safe spots with protection – under heavy table, strong desk, bed, etc.
What should one do during an earthquake?
Duck, cover and hold
Stay calm
If inside, stay inside
If outside, stay outside
What are the variables of the Mercalli scale?
Earthquake magnitude
Distance from hypocenter
Type of rock or sediment making up ground surface
How can one eliminate resonance?
Change height of building
Move weight to lower floors
Change shape of building
Change building materials
Change attachment of building to foundation
Hard foundation (high-frequency vibrations) build tall, flexible building
Soft foundation (low-frequency vibrations) build short, stiff building
Shear Walls
Designed to receive horizontal forces from floors, roofs and trusses and transmit to ground
Braced Frames
Bracing with ductile materials offers resistance
Base Isolation
Devices on ground or within structure to absorb part of earthquake energy
What kind of houses houses perform well in earthquakes?
Modern 1-, 2-story wood-frame houses
What are 5 ways a building can be made to withstand earthquakes?
(1) brace it (2) Infill it (3) frame it (4) buttress it (5) isolate it
