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ISNS 4359 Earthquakes and Volcanoes (aka shake and bake). Lecture 5: Faults and Seismic Waves. Fall 2005. What is an Earthquake?. b. An event in which the Earth quakes , and vibrations are felt or recorded

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ISNS 4359 Earthquakes and Volcanoes

(aka shake and bake)

Lecture 5: Faults and Seismic Waves

Fall 2005


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 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:
    • 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 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)

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 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


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
    • Impossible to identify as foreshock until after‘the earthquake’has occurred
  • Smaller events after it are aftershocks

Dip-Slip Faults


  • 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

Sand and limestone bedding planes

in wavecut platform

30m high cliff


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


Strike-Slip Faults

  • 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

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

  • 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

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


  • 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

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 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
  • 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)

Surface Waves

  • Surface waves
    • Travel near the Earth’s surface, created bybody 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
    • 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
      • Travel great distances