ISNS 4359 Earthquakes and Volcanoes
(aka shake and bake)
Lecture 5: Faults and Seismic Waves
Fall 2005
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What is an Earthquake?
b
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 great enough that rocks fracture and shift, sending off
waves of energy felt as earthquake
 Rock hitting water radiates ripples like earthquake
Faults and Geologic
Mapping
19th century recognition that fault movements
cause earthquakes led to identification of
earthquake-hazard belts
 Understanding faults begins with
understanding rock relationships, formalized by
Steno:
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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
Faults and Geologic
Mapping
Faults and Geologic
Mapping
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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 continuation elsewhere allows determination of length
of faults
• Length of fault determines size of earthquake possible on
that fault, as longer lengths of fault rupture 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 480 km apart in
New Zealand)
Faults and Geologic Mapping—
New Zealand—480km of lateral offset
Faults and Geologic Mapping—note relation
of earthquakes laterally and vertically
Types of Faults
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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 a
geologic contact or layer which would include a deformed
rock layer
Dip: angle of inclination from horizontal of tilted layer
Strike: compass bearing of horizontal line in tilted layer
Types of Faults-orientation
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
 Fault rupture is series of events, with largest one referred to as ‘the
earthquake’
 Smaller events preceding it are foreshocks
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Impossible to identify as foreshock until after ‘the earthquake’ has
occurred
 Smaller events after it are aftershocks
Dip-Slip Faults
Terminology:
 Caused by pushing or pulling force
 Where dominant force is extensional, normal fault occurs
when the hangingwall moves down relative to the
footwall, and zone of omission results
 Where dominant force is compressional, reverse fault
occurs when the hangingwall moves up relative to the
footwall, and zone of repetition results
Dip-Slip Faults
Dip-Slip Faults
Sand and limestone
bedding planes
in wavecut platform
30m high cliff
Gravels
View looking NE
Lower Jurassic interbedded pelagic limestones and marls
cut by normal faults. To build a 3-D structural model of the
outcrops.
Aerial and ground photo of
foreshore at Kilve. To be
intergated with ground scanning
and helicopter photos
McLinjoy mapping onto 3D model
Strike-Slip Faults
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Strike-slip faults are dominated by horizontal
movement
 When straddling a fault, if the right-hand side
has moved towards you, it is a right-lateral
fault
 When straddling a fault, if the left-hand side
has moved towards you, it is a left-lateral fault
 Convention works in either direction
Strike-Slip Faults
Southern California—looking north
Steps in Strike-Slip 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
Right step in right-lateral fault or left step in left-lateral fault:
 Extension, down-dropping, basins and valleys
Transform Faults
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As oceanic plates spread apart at mid-ocean ridges, they must slide past
other plates
Sliding takes place on transform faults
Transform faults link spreading centers or connect spreading center to
subduction zone
Between two spreading centers, motion on transform faults is same as on
strike-slip faults
Outside two spreading centers, plates are moving at same rate so there is
no offset – fracture zone
Transform Faults
Development of Seismology
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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
Waves
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Amplitude: displacement
 Wavelength: distance between successive
waves
 Period: time between waves
 Frequency: number of waves in one second
(1/period)
Seismic Waves
Seismic waves come in two families: those that
can pass through the entire Earth (body
waves) and those that move near the surface
only (surface waves)
 Body waves: faster than surface waves, have
short periods (high frequency – 0.5 to 20 Hz),
most energetic near the hypocenter
 Two types of body waves:
 P waves and S waves
Body 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
Body Waves
Body 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
Seismic
Waves
Seismic Waves and the Earth’s
Interior
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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
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)
Seismic Waves and the Earth’s
Interior
Surface Waves
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Surface waves
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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
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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 the
hypocenter is close to the surface