Chapter 3
Earthquake Geology and
Seismology
Lecture PowerPoint
1
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The Lisbon Earthquake of 1755
• Morning of November 1, 1755: Lisbon experienced
two major earthquakes in close succession, the
first of which caused widespread fires and the
second of which caused sea waves which swept
many away
• A few hours later, Lisbon was again shaken by an
earthquake in Fez, Morocco (550 km away)
• 70,000 people killed and 90% of structures
destroyed or damaged
• Changed people’s attitudes about the world
What is an Earthquake?
• An event in which the Earth quakes, and vibrations are
felt or recorded
• Caused by volcanic activity, meteorite impacts, undersea
landslides, explosions of nuclear bombs or most
commonly, by movement of the Earth across a fault
• Fault: fracture in the Earth across which the two sides
move relative to each other
• Stresses build up until
enough to cause rocks to
fracture and shift, sending
off waves of seismic
energy, felt as earthquake
Figure 3.2
Faults and Geologic Mapping
• 19th century recognition that fault movements cause
earthquakes led to identification of earthquakehazard belts
• Understanding faults begins with understanding
rock relationships, formalized by Steno:
– Law of original horizontality: sediments are
originally deposited in horizontal layers
– Law of superposition: in undeformed sequence
of sedimentary rock layers, each layer is younger
than the layer beneath it and older than the layer
above it
Faults and Geologic Mapping
– Law of original continuity: sediment layers are
continuous, ending only against a topographic high, by
pinching out from lack of sediment, or by gradational
change from one sediment to another
• If sedimentary layer ends abruptly, 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 earthquakes)
• Understanding fault offset can also have financial rewards, if
ore-bearing unit exists two different places on either side of fault
(example of gold-bearing gravels 840 km apart in New Zealand)
Faults and Geologic Mapping
Figure 3.5
Figure 3.4
Types of Faults
• Jointing – brittle lithospheric rocks fracture and crack
• Large stress differential on either side of a fracture results in
movement: fracture becomes a fault
• Movement ranges from millimeters to hundreds of kilometers,
resulting in tilting and folding of layers
• Use strike and dip to describe location in 3D space of deformed
rock layer
Types of Faults
• Use strike and dip to
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
Figure 3.7
Types of Faults
Dip-slip faults:
• Dip-slip faults are dominated by vertical movement
• Ore veins often form in fault zones, so many mines are
actually dug out along faults
• Miners refer to the
block beneath them as
the footwall (block
beneath the fault) and
the block above them
as the hangingwall
(block above the
fault)
Figure 3.8
Types of Faults
Dip-Slip Faults:
• Caused by pushing or pulling force
• Where dominant force is extensional (pulling), normal fault
occurs when hangingwall moves down relative to footwall, and
zone of omission results
Figure 3.9
Types of Faults
Dip-Slip Faults:
• Caused by pushing or pulling force
• Where dominant force is compressional, reverse fault occurs
when hangingwall moves up relative to footwall, and zone of
repetition results
Figure 3.10
Types of Faults
Strike-slip faults:
• Dominated by horizontal
movement
• When straddling a fault, if
right-hand side moved
towards you, it is a rightlateral fault
• When straddling a fault, if
the left-hand side has moved
towards you, it is a leftlateral fault
• Convention works in either
direction
Figure 3.11
Types of Faults
Faults are complex zones of breakage with irregular
surfaces, many miles wide and long
• Stress builds up until rupture occurs at weak point and
propagates along fault surface
• Point where rupture first
occurs is hypocenter or
focus
• Point directly above
hypocenter on surface
is epicenter
Figure 3.12
Types of Faults
• Fault rupture is series of events over weeks to months to
years, with largest event referred to as ‘the earthquake’
• Smaller events preceding ‘the earthquake’ are foreshocks
– Impossible to identify as foreshock until after ‘the
earthquake’ has occurred
• Smaller events after ‘the earthquake’ are aftershocks
Types of Faults
Steps in StrikeSlip Faults:
• Earth does not
rupture along
clean, straight
line but with
several breaks
that stop and start
and bend
• Left step in right-lateral fault or right
step in left-lateral fault:
– Compression, uplift, hills and
mountains
Figure 3.13
Types of Faults
Steps in StrikeSlip Faults:
• Right step in rightlateral fault or left
step in left-lateral
fault:
– Extension,
downdropping,
basins and
valleys
Figure 3.14
Types of Faults
Transform Faults:
• As oceanic plates spread apart at mid-ocean ridges, they must slide
past other plates, along transform faults
• Transform faults link spreading centers, or connect spreading center
to subduction zone
• Between two spreading centers, transform fault motion is same as
strike-slip fault
• Outside two
spreading centers,
plates are moving
at same rate so
there is no offset –
fracture zone
Figure 3.15
Development of Seismology
•
•
•
•
•
Seismology: study of earthquakes
Earliest earthquake device: China, 132 B.C.
Instruments to detect earthquake waves: seismometers
Instruments to record earthquake waves: seismographs
Capture movement of Earth in three components: north-south,
east-west and vertical
• 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
Figure 3.16
Development of Seismology
Waves:
•
•
•
•
Amplitude: displacement
Wavelength: distance between successive waves
Period: time between waves (= 1/frequency)
Frequency: number of waves in one second
Figure 3.17
Seismic Waves
Seismic waves come in two categories: those that can
pass through entire Earth (body waves) and those
that move near surface only (surface waves)
• Body waves: faster than surface waves, have short
periods (high frequency – 0.5 to 20 Hz), most
energetic near earthquake hypocenter
• Two types of body waves:
– P waves and S waves
Seismic Waves
P (primary) waves
• Fastest of all waves
• Always first to reach a recording station (hence primary)
• Move as push-pull – alternating pulses of compression and
extension, like wave through Slinky toy
• Travel through solid, liquid or gas
– Velocity depends on density and compressibility of substance they are
traveling through
– Velocity of about 4.8 km/sec for P wave through granite
– Can travel through air and so may be audible near the epicenter
Figure 3.18a
Seismic Waves
S (secondary) waves
• Second to reach a recording station (after primary)
• Exhibit transverse motion – shearing or shaking particles at right
angles to the wave’s path (like shaking one end of a rope)
• Travel only through solids
–
–
–
–
S wave is reflected back or converted if reaches liquid
Velocity depends on density and resistance to shearing of substance
Velocity of about 3.0 km/sec for S wave through granite
Up-and-down and side-to-side shaking does severe damage to buildings
Figure 3.18b
Seismic Waves
Seismic Waves and the Earth’s Interior
• Waves from large earthquakes can pass through the entire Earth and
be recorded all around the world
• Waves do not follow straight paths through the Earth but change
velocity and direction as they encounter different layers
Figure 3.19
Seismic Waves
Seismic Waves and the Earth’s Interior
• From the Earth’s surface down:
– Waves initially speed up then slow at the asthenosphere
– Wave speeds increase through mantle until reaching outer core
(liquid), where S waves disappear and P waves suddenly slow
– P wave speeds
increase
gradually
through outer
core until
increasing
dramatically at
inner core
(solid) Figure 3.19
Seismic Waves
Surface waves:
• Travel near the Earth’s surface, created by body waves
disturbing the surface
• Longer period than body waves (carry energy farther)
• Love waves
– Similar motion to S waves, but side-to-side in horizontal plane
– Travel faster than Rayleigh waves
– Do not move through air or water
Seismic Waves
Surface waves:
• Rayleigh waves
– Backward-rotating, elliptical motion produces horizontal and
vertical shaking, which feels like rolling, boat at sea
– More energy is released as Rayleigh waves when earthquake
hypocenter is close to the surface
– Travel great distances
Figure 3.18c
Seismic Waves
Sound Waves and Seismic Waves:
• Seismologists record and analyze waves to determine where an
earthquake occurred and how large it was
• Waves are fundamental to music and seismology
• Similarities:
– More high frequency waves if short path is traveled
• Trombone is retracted, short fault-rupture length (small
earthquake)
– More low frequency waves if long path is traveled
• Trombone is extended, long fault-rupture length (large
earthquake)
Locating the Source of an Earthquake
• P waves travel about 1.7 times faster than S waves
• Farther from hypocenter, greater lag time of S wave
behind P wave (S-P)
Figure 3.22
Locating the Source of an Earthquake
• (S-P) time indicates how far away earthquake was from
station – but in what direction?
Figure 3.21
Locating the Source of an Earthquake
• Need distance of earthquake from three stations to
pinpoint location of earthquake:
– Visualize circles drawn around
each station for appropriate
distance from station, and
intersection of circles at
earthquake’s location
– Method is most reliable when
earthquake was near surface
– Usually computer calculation
Figure 3.23
Magnitude of Earthquakes
Richter scale:
• Devised in 1935 to describe
magnitude of shallow, moderatelysized earthquakes located near
Caltech seismometers in southern
California
• Bigger earthquake  greater
shaking  greater amplitude of
seismogram lines
• Defined magnitude as ‘logarithm of
maximum seismic wave amplitude
recorded on standard seismogram at
100 km from earthquake’, with
corrections made for distance
Figure 3.24
Magnitude of Earthquakes
Richter scale:
– For every 10 fold increase in recorded amplitude, Richter
magnitude increases one number
– With every one increase in Richter magnitude, 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 time (increases damage to
buildings)
– Many more small earthquakes each year than large ones,
but more than 90% of energy release is from few large
earthquakes
– Richter scale magnitude is easy and quick to calculate, so
popular with media
Magnitude of Earthquakes
Other Measures of Earthquake Size:
• Richter scale is useful for magnitude of shallow, smallmoderate nearby earthquakes
• Does not work well for distant or large earthquakes
– Short-period waves do not increase in amplitude for bigger
earthquakes
– Richter scale:
• 1906 San Francisco earthquake was magnitude 8.3
• 1964 Alaska earthquake was magnitude 8.3
– Other magnitude scale:
• 1906 San Francisco earthquake was magnitude 7.8
• 1964 Alaska earthquake was magnitude 9.2
Magnitude of Earthquakes
Other Measures of Earthquake Size:
• Body wave scale (mb):
– Uses amplitudes of P waves with 1 to 10-second periods
• Surface wave scale (ms):
– Uses Rayleigh waves with 18 to 22-second periods
• All magnitude scales are not equivalent
– 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
Magnitude of Earthquakes
Moment Magnitude Scale:
• Seismic moment (Mo)
– Measures amount of strain energy
released by movement along whole
rupture surface; more accurate for
big earthquakes
– Calculated using rocks’ shear
strength times rupture area of fault
times displacement (slip) on the
fault
• Moment magnitude scale uses
seismic moment:
– Mw = 2/3 log10 (Mo) – 6
Figure 3.25
Magnitude of Earthquakes
Foreshocks, Mainshock and Aftershocks
• Large earthquakes are not just single events but part
of series of earthquakes over years
– Largest event in series is mainshock
– Smaller events preceding mainshock are foreshocks
– Smaller events following mainshock are aftershocks
• Large event may be considered mainshock, then
followed by even larger earthquake, so then reclassified as foreshock
Magnitude of Earthquakes
Magnitude, Fault-Rupture Length, and SeismicWave Frequencies:
• Fault-rupture length greatly influences magnitude:
–
–
–
–
100 m long fault rupture  magnitude 4 earthquake
1 km long fault rupture  magnitude 5 earthquake
10 km long fault rupture  magnitude 6 earthquake
100 km long fault rupture  magnitude 7 earthquake
Magnitude of Earthquakes
Magnitude, Fault-Rupture Length, and SeismicWave Frequencies:
• Fault-rupture length and duration influence seismic
wave frequency:
– Short rupture, duration  high frequency seismic waves
– Long rupture, duration  low frequency seismic waves
• Seismic wave frequency influences damage:
– High frequency waves cause much damage at epicenter
but die out quickly with distance from epicenter
– Low frequency waves travel great distance from
epicenter so do most damage farther away
Ground Motion During Earthquakes
• Buildings are designed to handle vertical forces
(weight of building and contents) so that vertical
shaking in earthquakes is usually safe
• Horizontal shaking during earthquakes can do
massive damage to buildings
• Acceleration
– Measure in terms of acceleration due to gravity (g)
– Weak buildings suffer damage from horizontal
accelerations of more than 0.1 g
– At isolated locations, horizontal acceleration can be as
much as 1.8 g (Tarzana Hills in 1994 Northridge,
California earthquake)
Ground Motion During Earthquakes
Periods of Buildings and Responses of
Foundations:
• Buildings have natural frequencies and periods
• Periods of swaying are about 0.1 second per story
– 1-story house shakes at about 0.1 second per cycle
– 30-story building sways at about 3 seconds per cycle
• Building materials affect building periods
– Flexible materials (wood, steel)  longer period of shaking
– Stiff materials (brick, concrete)  shorter period of shaking
• Velocity of seismic wave depends on material through
which it is moving
– Faster through hard rocks, slower through soft rocks
Ground Motion During Earthquakes
Periods of Buildings and Responses of
Foundations:
• When waves pass from harder to softer rocks, slow down
• Must therefore increase their amplitude in order to carry
same amount of energy  greater shaking
• Shaking tends to be stronger at sites with softer ground
foundations (basins, valleys, reclaimed wetlands, etc.)
• If the period of the wave matches the period of the
building, shaking is amplified and resonance results
– Common cause of catastrophic failure of buildings
Earthquake Intensity –
What We Feel During an Earthquake
• Mercalli intensity scale was developed to quantify what people
feel during an earthquake
• Used for earthquakes before instrumentation or current earthquakes
in areas without instrumentation
• Assesses effects on people and buildings
• Did You Feel It?: Maps of Mercalli intensities can be generated
quickly after an earthquake using people’s input to the webpage
http://pasadena.wr.usgs.gov/shake
Insert Table 3.6 here
In Greater Depth: What To Do Before
and During an Earthquake
• 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.
• During an earthquake:
–
–
–
–
Duck, cover and hold
Stay calm
If inside, stay inside
If outside, stay outside
Earthquake Intensity –
What We Feel During an Earthquake
Mercalli scale variables:
• Earthquake magnitude
– Bigger earthquake, more likely death and damage
• Distance from hypocenter
– Usually (but not always), closer earthquake  more damage
• Type of rock or sediment making up 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
Earthquake Intensity –
What We Feel During an Earthquake
Mercalli scale variables:
• Building style
– Body waves near epicenter are amplified by rigid short buildings
– Low-frequency surface waves are amplified by tall buildings,
especially if on soft foundations
• Duration of shaking
– Longer shaking lasts,
more buildings can
Insert Table 3.7 here
be damaged
A Case History of Mercalli Variables
San Fernando Valley, California, Earthquake of 1971
• Earthquake magnitude
– Magnitude 6.6, with 35 magnitude 4.0 or higher aftershocks in 7
minutes after main shock
• Distance from epicenter
– Bull’s-eye damage pattern
• Building style
– ‘Soft’ first-story buildings
were major problem
– Hollow-core bricks at V.A.
Hospital caused collapse
– Collapse of freeway bridges
Figure 3.27
A Case History of Mercalli Variables
San Fernando Valley, California,
Earthquake of 1971
• Duration of Shaking
– Strong ground shaking lasted 12
seconds
– Lower Van Norman Reservoir failed by
landslides until stood only 4 ft above
water – had shaking continued 5
seconds longer, dam would have failed,
homes of 80,000 would have flooded
• Learning from the Past, Planning for
the Future
– 1994 Northridge earthquake caused
same kinds of damage
Figure 3.30
Building in Earthquake Country
• 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
Building in Earthquake Country
• Shear Walls
– Designed to receive horizontal forces from floors, roofs and
trusses and transmit to ground
– Lack of shear walls typically cause structures like parking
garages to fail in earthquakes
Figure 3.31
Figure 3.32
Building in Earthquake Country
• Braced Frames
– Bracing with ductile materials offers resistance
• Retrofit Buildings
– Increase resistance to seismic shaking
Figure 3.33
Figure 3.34
Building in Earthquake Country
• Base Isolation
– Devices on ground or within structure to absorb part of
earthquake energy
– Use wheels, ball bearings, shock absorbers, ‘rubber
doughnuts’, etc. to isolate building from worst shaking
Figure 3.35
Building in Earthquake Country
• Retrofit Bridges
– Bridges combine steel with concrete, materials with different
properties in earthquakes
– Rebuild with alternating layers of steel and concrete
Figure 3.36
Building in Earthquake Country
• Houses
– Modern 1-, 2-story wood-frame houses perform well in
earthquakes
– Additional support can be
given by building shear
walls, bracing, tying walls
and foundations and roof
together
– Much damage as interior
items are thrown about
• Bolt down water heaters,
ceiling fans, cabinets,
bookshelves, electronics
Figure 3.37
End of Chapter 3