test 2
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mean: 75, median: 79
multiple choice: 42 questions, 2 points each
short answer: 4 questions, 4 points each
100 total: circled number inside front cover of
blue book
• answer key to multiple choice is posted on
blackboard under ‘test materials’
Earthquakes and Related Phenomena
chapter 8
EQ terms
• fault: break (fracture)
in a rock
• rupture: breaking of
rock along a fault
• hypocenter (focus):
rupture point of rock
below surface
• epicenter: point on
earth surface directly
above hypocenter
• scarp: topographic
(elevation) display of
a ruptured fault
how to quantify the size of an event?
• describes the energy released by an EQ
• different types based on different parameters:
– Modified Mercalli Scale: based on how much people felt
the ground shake
• very subjective, but all we have for past events
• scale from I to XII
– Richter magnitude: based on size (amplitude) of largest
seismic wave
• easy to measure
• scale from 0 to ~9
– moment magnitude: based on amount of energy released
by an event
• more accurate because takes into account specific factors about
the fault (slip area, rigidity of rock, etc.)
• scale from 0 to ~10
how do we record events?
• observations, journals (Mercalli)
• seismograms (Richter)
– record movement (displacement)
of earth in three directions
• vertical (up and down)
• horizontal (left and right)
• transverse (front and back)
• known information about
specific fault (moment)
how to interpret the scales
• Richter magnitude (M) is predominantly used
– for each 1 step up in M, there are ~1/10 of the
number of events from before
– for each 1 step up in M, there is an ~10x increase
in ground shaking
– for each 1 step up in M, there is an ~30x increase
in energy release
where do EQ’s happen?
where do EQ’s happen
• plate boundary EQ’s:
– transform boundaries
(California)
– convergent boundaries
(Japan)
• intraplate EQ’s
– seismically active regions
not directly linked to
plate boundaries (rifts)
– New Madrid (Missouri,
Tennessee, Arkansas):
Reelfoot rift
where do EQ’s happen?
fault types
• strike slip
– right lateral
– left lateral
• reverse
– hanging wall up
• normal
– footwall up
fault activity
• use geologic record to determine when was the
latest rupture on a fault
• active faults: movement in the last 10000 years
• partially active: movement in the last 1.65
million years
• inactive: no evidence of movement within the
last 1.65 million years
• public policy relationship: US Nuclear
Regulatory Commission: “active” faults are
movement within the last 50000 years
activity measurements
• recurrence interval (how often a fault is
active) found by:
– paleoseismic data: average time between events,
according to geologic data
– slip rate: average movement (displacement)
divided by movement per year
– seismicity: averaging time between historical
earthquakes
seismic waves
• actual displacement and shaking of earth is
due to seismic waves (energy release when
fault ruptures)
• seismic waves travel several kilometers per
second through the earth
• types:
– P waves (primary, compressional)
– S waves (secondary, shear)
– surface waves
P waves
• faster of seismic waves
types
• can travel through
solids, liquids, and
gasses
• moves in same
direction as wave
motion
• moves like pushing a
slinky together and
pulling it apart
S waves
• slower than P waves
• can only travel
through solids
• moves perpendicular
to wave motion
• moves like a cracking
a whip or a snake
along the ground
surface waves
• slowest seismic
wave type
• complex vertical
and horizontal
motion patterns
• cause most
damage to
buildings, etc.
how we record EQ’s (again)
• each wave type has
a characteristic size
• by looking at a
seismogram that
recorded an EQ, we
can see when the
different wave
types arrived
how do we know where EQ’s happen?
• can analyze multiple
seismograms to tell
where an EQ’s
hypocenter was
– we know the time the
difference between the
amount of times the
different wave types took
to get to the seismogram
– we know how long it
takes those wave types to
travel a certain distance
ground material effects
• different earth material respond differently to
seismic waves
• amplitude: vertical movement of shaking
• hard rocks resists shaking, unconsolidated
sediments are vulnerable to shaking
ground acceleration
• ground shaking is recorded as acceleration
• how fast the shaking of the ground changes
speed (horizontally and vertically)
• recorded in comparison to gravity acceleration
(9.8 meters/second2)
– if an earthquake has 1g ground acceleration, that
means the shaking of the ground was increasing at
9.8 meters/second2
– an earthquake of M7.0 has ~25%g ground
acceleration
EQ cycle
• aftershocks (following a
large event)
• seismic inactivity
• strain accumulation
• foreshocks
• main event
effects of EQ’s
• shaking and ground rupture
– immediate, damage to buildings, loss of life
• fires
– from gas leaks, electrical lines, etc.
• disease
– from dust with bacteria or contaminated drinking
water
effects of earthquakes
• tsunamis
estimating risk
• US Geologic Survey (USGS) produces ‘seismic
hazards maps’ for the US and other regions
• maps are based on the percentage likelihood
that a certain size event will happen in a given
timeframe
seismic hazard maps
• shows a 2% chance that the color percent gravity
shaking will be exceeded in the next 50 years
• public policy relationship: ability to properly place
emergency response facilities and specify building
codes
forecasting methods
• we use ‘forecast’ instead of ‘predict’ because:
– forecast includes the percentage chance that an
event will happen
– predict indicates that the event will happen
• public policy relationship: how much of a risk
does a possible event need to be before we
take action?
forecasting methods: foreshocks
• foreshocks
– M9.0 EQ in Japan occurred on a Friday, on that
Wednesday they had experienced a M7.2, and on
that Thursday they experienced three M6.0+
– should we have forecasted a major event?
forecasting methods: radon gas
• radon gas is naturally present in rocks
• it is believe that before an EQ, rocks expand
and take in water
• radon is removed from the rock as the water
leaves
• the radon gas rises to the top of the
atmosphere, and can be seen as a
temperature increase
forecasting methods: radon gas
forecasting methods: seismic gaps
• based on the idea that the entire length of a
fault should experience similar amount of
seismic activity
forecasting methods: past EQ’s
• develop by Dr.
Alan Kafka of BC
• EQ’s are more
likely to occur
where EQ’s have
occurred in the
past
response to EQ hazards
• US National EQ Hazard Reduction Program
– develop an understanding of the EQ source
– determine EQ potential
– predict effects of EQ’s
– apply research results
– what can we do with new information we gather?
adjustments to EQ activity
• structural protection
– stricter building codes
– flexible gas and electric lines
• EQ warning systems
– ability of dangerous materials
to shut themselves off
• land use planning
– putting important structures
(schools, hospitals,
government offices) away
from active areas
– putting emergency response
facilities close enough to be
available to those who will
most likely need them
Japan
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death toll will be over 10000
tsunami, flooding, fires
nuclear concern
building codes to allow tall buildings to sway
but not break
• how much should the US help another
country?