Earthquakes Flashcards
What is the definition of a earthquake
the vibration generated by the
sudden release of energy associated
with the rapid movement of rock
along a fault
Define a fault
*A fracture (or series of fractures) in the Earth’s crust
*Can occur at locations in the crust where rocks are
under stress and break
*Can also describe the boundary between two plates
What are the features of faults
Epicenter, Fault surface, Focus, Seismic waves, Footwall, hanging wall
What is an epicenter
a location on
the Earth’s surface
directly above the focus.
What is a fault surface
the
fracture surface between
one block and another
along which movement
occurs
What is the focus
‘origin’ of the
earthquake. The point
within the Earth’s crust
where movement first
occurred
What are seismic waves
waves
of energy that travel
like shock waves from
the focus to the
surrounding area
What are the two most important kinds of seismic waves
P-waves and S-waves
Describe the motion of a P-wave
This is a type of compression wave. Imagine a slinky and your hand is motion it left and right, so the slinky becomes compressed. The wave travels in the same path of motion as your hand does.
Describe the motion of a S-wave
This is a shear wave. The hand motion travels up and down and the wave travels left with it looking like a sine wave.
What are the three main kinds of faults + one that is another variation of the reverse fault.
Strike-slip fault, Normal Fault, Reverse Fault
Thrust fault are low-angle reverse faults
Is the strike slip tension or compression?
Shearing tension - pulling in opposite directions along a plane
Is the Normal fault tension or compression and where would we expect to see it
Tension
occurs mainly at divergent plate boundaries
Is a thrust fault tension or compression and where would we expect to see it
Compression
Convergent plate boundary
Is a reverse fault tension or compression and where would we expect to see it
Compression
Convergent plate boundary
How do we measure earthquakes
Seismometers and seismographs
________ ________ are detected by an instrument called a seismometer and recorded as a paper or electronic trace known as a ____________
Seismic waves
Seismogram
Earthqaukes often occur in clusters.
What is the name to describe each of these definitions
1. Biggest earthquake in the cluster
2. occurs before the main shock
3. occurs after the main shock
- Main shock
- Foreshock
- Aftershock
Prior to the earthquake the _____________ foci are close to the focus.
After the earthquake the _____________ foci travel away from the focus
Foreshock
aftershock
What are the scales we use to measure earthquakes
Moment magnitude scale (Mw)
Logarithmic scale
What scale did the moment magnitude scale replace
Richter scale
Identify each of the scales as quantitative and qualitative
Richter scale
Moment magnitude
Modified Mercalli intensity scale
Peak ground acceleration
Richter Scale: the oldest and best known, quantitative
Moment Magnitude (Mw
): used by seismologists, quantitative
Modified Mercalli Intensity Scale: qualitative
Peak ground acceleration: not a scale technically, used by
engineers, quantitative, site specific
What is the Richter magnitude calculations based on
difference between P and S wave arrival times
maximum seismic wave amplitude on a seismograph
Limitations of Richter Magnitude
- Not accurate for describing large earthquakes (8 or greater)
- Not accurate for deep earthquakes or earthquakes a long way
away from the seismograph station
The moment magnitude is based on _______ ________
energy released
The moment magnitude is similar values to ______ _______ for small to large earthquakes. However, the Moment magnitude is a more precise scale for measuring magnitude larger than __
Richter Magnitudes
8
What are the measurements used to determine the moment magnitude
based on seismograph measurements and measurements of fault displacement
How is peak ground acceleration measured and what do the measurements depend on
Using an accelerogram the maximum ground acceleration during an earthquake, in horizontal and vertical directions
measures ground shaking at a specific location - not magnitude of the earthquake
These measurements depend on the local geology (sand shakes more than solid rock)
What does PGA represent
Peak Ground Acceleration
Seismic risk maps present what?
Seismic risk maps present expected PGA values for a
region if an earthquake were to occur
What is the modified Mercalli intensity scale based on
This scale is based on
observations and
perceptions of people who
felt the earthquake,
therefore it is subjective
(based on personal
experience) and
qualitative.
How many degrees of intensity does the modified Mercalli intensity scale have? What do they depend on?
12
The degree of intensity will
vary depending on where
the earthquake occurs
For these descriptions, which is a 1 on the modified Mercalli intensity scale and which is a 12
Detected only by seismic instruments; causes no damage
Earthquake waves cause visible undulations of the ground surface; objects are thrown up off the ground; there is complete destruction of buildings and bridges of all types
The first definition would be a 1, the second description is a 12
What are two websites you can report your expirince of a earthquake to then build a MMI map
Canada - NRCan website
World - USGS website
What kind of sediments/rock shake more in an earthquake? Order these from Hard(least seismic wave amplification) to soft(most seismic wave amplification)
Stiff soil(mud), Hard rock(igneous rock), Very dense soil and soft rock(sandstone), soils requiring site-specific evaluation(peat, some clays), Soft soil(artificial fill), Rock(volcanic rock)
Hard Rock
Rock
very dense soil and soft rock
stiff soil
soft soil
soils requiring site-specific evluations
The softer the geological media (soil, rock, or sediment) under a building or
structure, the _______ likely there will be a failure in an earthquake
more
Worldwide, _______ earthquakes happen more than once per month. _________ earthquakes, such as magnitude 2 earthquakes, occur several hundred times a day. To create a mountain system might take several million _________ sized earthquakes over tens of millions of years.
Strong
Smaller
Medium
What are natural causes for earthquakes
- creation of a new fault by rock fractures along a fault plane
- movement along an old fault
- movement of magma below the surface or a volcanic explosion
- huge landslide
What are anthropogenic causes of earthquakes
*underground nuclear
explosive tests
*fracking - injections of
water and sand
*wastewater injection
water pressure increases
in inactive faults and
pushes them apart
btw, anthropogenic means human-made
Where are earthquakes concentrated in Canada
Along the west coast - plate tectonic boundaries
several in Ontario and Quebec along old fault zones, a weakness in the crust
Why would earthquakes occur in northern Canada
Northern Canada
may relate to retreat of
glaciers 1000s of years
ago - land slowly rising
(was depressed by the
glaciers)
What are the magnitude of earthquakes in Estevan and Radville
M4-M5, Iv - V Mercalli
Due to rocks slowly
dissolving at depth,
shifting along faults.
Describe elastic rebound theory
- Energy is stored in the form of elastically deformed rock.
- When strain exceeds rock strength, the rock fractures and energy is released
Stages of elastic rebound theory occurring
- Curst at rest (no strain)
- Crust deformed, strain accumulates (strain build up)
- Ruptured crust causing earthquake (rupture with elastic rebound)
- Fractured crust, Fault zone (strain released)
What is the elastic limit?
The breaking point
What occurs when you exceed the breaking point during an earthquake
The material will fracture
Earthquakes happen when the _______ _______ of the rock along a fault with accumulated stresses is __________
Elastic limit
exceeded
The sudden release of energy, as
the rock ‘snaps’ or fails, generates
a series of ‘shock waves’ called
______ _______, that move through
the rock
Seismic waves
Which of these statements are true:
Stress - Force/area - elastic bending
Strain - Force/area - elastic bending
rupture - Force/area - elastic bending
Stress - deformation - rupture formation
Strain - deformation - rupture formation
Rupture - deformation - rupture formation
Strain - slip and vibration
Stress - slip and vibration
Rupture - slip and vibration
Stress - Force/area - elastic bending
Strain - deformation - rupture formation
Rupture - slip and vibration
Which type of waves travel through the earths interior and consist of P-waves and S-waves
Body waves
Which type of wave travels along the surface of the lithosphere and consist of love waves and Rayleigh waves
Surface waves
Seismic waves are detected by an
instrument called a ____________. They
contain several instruments that detect
motion in different directions.
seismometer
The results are recorded as a paper
or electronic trace known as a
______________. _____ waves arrive first,
then ______ waves, then ______ waves.
seismogram
P
S
Surface
How fast the
waves travel
depends on the
rock they are
travelling
through – ________
materials, faster
travel!
Harder
_________ is used to determine the layers inside the earth such as the crust, Mantle, and core
Seismology
The more consolidated / cemented the rock, or the
denser the minerals in the rock, the ______ the speed of
a wave
Faster
What is rarefraction
“bending” of the seismic waves as they
move from one material to another of different density
Why is wave refraction used
is used to identify a discontinuity (change) e.g. a
discontinuity in the rock density and/or composition
What is wave reflection
waves “bounce” off surfaces
If a wave travels from a less dense rock to a more dense rock, will the refracted wave be bent down or up?
Refracted wave bent up
If a wave travels from a more dense rock to a less dense rock, will the refracted wave be bent down or up?
Refracted wave bent down
Can S-waves and P-waves travel through solids and/or liquids?
S-waves travel through solids but not liquids
P-waves travel through solids and liquids
Are s-waves or P-waves detected first at a seismic station?
P-waves
Are P-waves or S-waves analogous to sound waves?
P-waves
Are P-waves or S-waves faster?
P-waves
How many degrees is the P-wave shadow zone?
150 - 103 = 47 degrees
How many degrees is the S-waves shadow zone?
It begins at 103 degrees on both sides, waves cannot travel through the core of the earth.
Do P-waves or S-waves cause the most damage?
S-waves
_______ ________ confirm the existence of
discontinuities in Earth’s interior
Seismic data
How can we tell there are discontinuities in the earths interior using seismic data
S waves are attenuated (stopped) by liquid
outer core layer
* P waves refract when they go from one
layer to another and there is a change in
composition
* By studying seismic data at seismograph
stations around the world, we can see the
evidence for the Earth’s layers!
Describe the movement of love waves
cause horizontal shifting at the surface of the earth
Describe the movement of Rayleigh waves
rotating waves along surfaces, create
vertical displacement (movement), like ripples on water
Divergent Boundaries:
_______ earthquakes within ocean basins, foci are located:
Along _______ _______
Along ________ ______
Shallow
mid-ocean ridges
transform faults
Transform-Fault boundaries:
Transform faults along plate boundaries have _______ earthquakes
_______ _______ mechanism due to shearing forces from tectonic plate movement.
Can produce ______, _________ earthquakes
many
strike-slip fault
large, destructive
Convergent Boundaries:
Largest earthquakes occur at __________ zones.
-called __________ earthquakes
-Overriding plate thrust _______ relative to the subducting plate
The three _______ earthquakes ever measured by seismometers were of this type
Subduction
megathrust
upward
largest
*Zone of seismicity along the plane of the subducting plate is
called
the Wadati-Benioff Zone
The size or magnitude of an earthquake is related to
depth
___% of earthquakes occur at a depth less than 100 km
90
____ severe earthquakes are shallow earthquakes (focus is in the upper 70km)
Most
Are the focus depths off the west coast of BC shallow or deep?
Shallow
The subducting juan de fuca plate is at a very shallow angle. This creates a weak or strong earthquake?
Strong
Intense __________ of the plates where they
collide, causes folding and faulting, thrust faults
compression
Earthquakes at ________-_________ __________
plate boundaries are generally shallow, spread over
a wide area in the region compressed by the
collision
continent-continent convergent
What is an example of a continent-continent convergence
Himalayas
What are the ways earthquakes cause damage
Faulting and shaking – primary hazards
*Landslides
*Liquefaction
*Tsunamis
*Fires
What does liquefaction mean
*Shaking causes water-saturated
sediment to temporarily behave like a
fluid
*Friction decreases as water moves
between particles
*Particles consolidate and pore waters are
expelled
Fill in the blanks for these characteristics of tsunamis:
* Undersea megathrust earthquakes often generate __________
waves
* Thrust faulting leads to ______ seawater displacement, forms
water wave
* Waves travel ________ from epicenter, when reach shore slow
down and amplify vertically
destructive
vertical
outward
A tsunami is only a few _________ high in the deep ocean but can _________ to _______ _______ high close to shore
centimeters
increase
many meters
The more consolidated/cemented the rock, the ______ the speed of a wave
faster
As speed decreases, wave amplitude _________ and
so does the potential for damage
increases
Where is seismic risk higher
*Delta sediments
*Floodplain
sediments
*Mountain slopes
*Saturated
materials
*Thicker sediments
anthropogenic causes of earthquakes
mining, injecting water, oilfield activity
