ISNS 4359 Earthquakes and VolcanoesISNS 4359 Earthquakes and Volcanoes(aka shake and bake)(aka shake and bake)

Lecture 5: Faults and Seismic Waves

Fall 2005

What is an Earthquake?What is an Earthquake? An event in which the An event in which the Earth quakesEarth quakes, and vibrations are felt or recorded, and vibrations are felt or recorded Caused by volcanic activity, meteorite impacts, undersea landslides, explosions of Caused by volcanic activity, meteorite impacts, undersea landslides, explosions of

nuclear bombs or most commonly, by movement of the Earth across a faultnuclear bombs or most commonly, by movement of the Earth across a fault FaultFault: fracture in the Earth across which the two sides move relative to each other: fracture in the Earth across which the two sides move relative to each other StressesStresses build up until great enough that rocks fracture and shift, sending off build up until great enough that rocks fracture and shift, sending off

waves of energy felt as earthquakewaves of energy felt as earthquake Rock hitting water radiates ripples like earthquakeRock hitting water radiates ripples like earthquake

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Faults and Geologic Faults and Geologic MappingMapping

1919thth century recognition that fault movements century recognition that fault movements cause earthquakes led to identification of cause earthquakes led to identification of earthquake-hazard beltsearthquake-hazard belts

Understanding faults begins with Understanding faults begins with understanding rock relationships, formalized by understanding rock relationships, formalized by Steno:Steno: Law of original horizontality: sediments are originally Law of original horizontality: sediments are originally

deposited in horizontal layersdeposited in horizontal layers Law of superposition: in undeformed sequence of Law of superposition: in undeformed sequence of

sedimentary rock layers, each layer is younger than sedimentary rock layers, each layer is younger than the layer beneath it and older than the layer above itthe layer beneath it and older than the layer above it

Faults and Geologic Faults and Geologic MappingMapping

Faults and Geologic Faults and Geologic MappingMapping

Faults and Geologic Faults and Geologic MappingMapping

Law of original continuity: sediment layers are Law of original continuity: sediment layers are continuous, ending only against a topographic high, continuous, ending only against a topographic high, by pinching out from lack of sediment, or by by pinching out from lack of sediment, or by gradational change from one sediment to anothergradational change from one sediment to another

• If sedimentary layer ends abruptly, may have been eroded If sedimentary layer ends abruptly, may have been eroded by water action or truncated by fault passing through layerby water action or truncated by fault passing through layer

• Identifying truncated sedimentary layers and recognizing Identifying truncated sedimentary layers and recognizing their continuation elsewhere allows determination of length their continuation elsewhere allows determination of length of faultsof faults

• Length of fault determines size of earthquake possible on Length of fault determines size of earthquake possible on that fault, as longer lengths of fault rupture create bigger that fault, as longer lengths of fault rupture create bigger earthquakesearthquakes

• Understanding fault offset can also have financial rewards, Understanding fault offset can also have financial rewards, if ore-bearing unit exists two different places on either side if ore-bearing unit exists two different places on either side of fault (example of gold-bearing gravels 480 km apart in of fault (example of gold-bearing gravels 480 km apart in New Zealand)New Zealand)

Faults and Geologic Mapping—Faults and Geologic Mapping—New Zealand—480km of lateral offsetNew Zealand—480km of lateral offset

Faults and Geologic Mapping—note relation Faults and Geologic Mapping—note relation of earthquakes laterally and verticallyof earthquakes laterally and vertically

Types of FaultsTypes of Faults JointingJointing – brittle lithospheric rocks fracture and crack – brittle lithospheric rocks fracture and crack Large stress differential on either side of a fracture results in Large stress differential on either side of a fracture results in

movement: fracture becomes a movement: fracture becomes a faultfault Movement ranges from millimeters to hundreds of kilometers, Movement ranges from millimeters to hundreds of kilometers,

resulting in tilting and folding of layersresulting in tilting and folding of layers Use Use strikestrike and and dipdip to describe location in 3D space of a geologic to describe location in 3D space of a geologic

contact or layer which would include a deformed rock layercontact or layer which would include a deformed rock layer DipDip: angle of inclination from horizontal of tilted layer: angle of inclination from horizontal of tilted layer StrikeStrike: compass bearing of horizontal line in tilted layer: compass bearing of horizontal line in tilted layer

Types of Faults-orientationTypes of Faults-orientation

FaultsFaultsFaultsFaults are complex zones of breakage with irregular surfaces, many miles are complex zones of breakage with irregular surfaces, many miles

wide and longwide and long Stress builds up until Stress builds up until rupturerupture occurs at weak point and propagates along occurs at weak point and propagates along

fault surfacefault surface Point where rupture first occurs is Point where rupture first occurs is hypocenterhypocenter or focus or focus Point directly above hypocenter on surface is Point directly above hypocenter on surface is epicenterepicenter Fault rupture isFault rupture is series series of events, with largest one referred to as ‘the of events, with largest one referred to as ‘the

earthquake’earthquake’ Smaller events preceding it areSmaller events preceding it are foreshocks foreshocks

Impossible to identify as foreshock until afterImpossible to identify as foreshock until after ‘the earthquake’‘the earthquake’ has has occurredoccurred

Smaller events after it areSmaller events after it are aftershocks aftershocks

Dip-Slip FaultsDip-Slip FaultsTerminologyTerminology:: Caused by pushing or pulling forceCaused by pushing or pulling force Where dominant force is extensional, Where dominant force is extensional, normal faultnormal fault occurs when occurs when

the hangingwall moves down relative to the footwall, and zone of the hangingwall moves down relative to the footwall, and zone of omission resultsomission results

Where dominant force is compressional, Where dominant force is compressional, reverse faultreverse fault occurs occurs when the hangingwall moves up relative to the footwall, and zone when the hangingwall moves up relative to the footwall, and zone of repetition resultsof repetition results

Dip-Slip FaultsDip-Slip Faults

Dip-Slip FaultsDip-Slip Faults

Sand and limestone bedding planes in wavecut platform

View looking NE

Gravels

30m high cliff

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 modelMcLinjoy mapping onto 3D model

Strike-Slip FaultsStrike-Slip Faults Strike-slip faultsStrike-slip faults are dominated by horizontal are dominated by horizontal

movementmovement When straddling a fault, if the right-hand side When straddling a fault, if the right-hand side

has moved towards you, it is a has moved towards you, it is a right-lateral faultright-lateral fault When straddling a fault, if the left-hand side has When straddling a fault, if the left-hand side has

moved towards you, it is a moved towards you, it is a left-lateral faultleft-lateral fault Convention works in either directionConvention works in either direction

Strike-Slip FaultsStrike-Slip Faults

Southern California—looking north

Steps in Strike-Slip FaultsSteps in Strike-Slip FaultsEarth does not rupture along clean, straight line but with several Earth does not rupture along clean, straight line but with several

breaks that stop and start and bendbreaks that stop and start and bend

Left step in right-lateral fault or right step in left-lateral fault: Left step in right-lateral fault or right step in left-lateral fault: Compression, uplift, hills and mountainsCompression, uplift, hills and mountains

Right step in right-lateral fault or left step in left-lateral fault:Right step in right-lateral fault or left step in left-lateral fault: Extension, down-dropping, basins and valleysExtension, down-dropping, basins and valleys

Transform FaultsTransform Faults As oceanic plates spread apart at mid-ocean ridges, they must slide past As oceanic plates spread apart at mid-ocean ridges, they must slide past

other platesother plates Sliding takes place on Sliding takes place on transform faultstransform faults Transform faults link spreading centers or connect spreading center to Transform faults link spreading centers or connect spreading center to

subduction zonesubduction zone Between two spreading centers, motion on transform faults is same as on Between two spreading centers, motion on transform faults is same as on

strike-slip faultsstrike-slip faults Outside two spreading centers, plates are moving at same rate so there Outside two spreading centers, plates are moving at same rate so there

is no offset – is no offset – fracture zonefracture zone

Transform FaultsTransform Faults

Development of SeismologyDevelopment of Seismology Seismology: study of earthquakesSeismology: study of earthquakes Earliest earthquake device: China, 132 B.C.Earliest earthquake device: China, 132 B.C. Instruments to detect earthquake waves: Instruments to detect earthquake waves: seismometersseismometers Instruments to record earthquake waves: Instruments to record earthquake waves: seismographsseismographs Capture movement of Earth in three components: Capture movement of Earth in three components: north-south, east-west and verticalnorth-south, east-west and vertical One part stays as stationary as possible while Earth vibrates: heavy mass fixed by 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 inertia in frame that moves with the Earth, and differences between position of the frame and the mass are recorded digitallyand the mass are recorded digitally

WavesWaves AmplitudeAmplitude: displacement: displacement WavelengthWavelength: distance between successive : distance between successive

waveswaves PeriodPeriod: time between waves: time between waves FrequencyFrequency: number of waves in one second : number of waves in one second

(1/period)(1/period)

Seismic WavesSeismic WavesSeismic wavesSeismic waves come in two families: those that come in two families: those that

can pass through the entire Earth (can pass through the entire Earth (body body waveswaves) and those that move near the surface ) and those that move near the surface only (only (surface wavessurface waves))

Body wavesBody waves: faster than surface waves, have : faster than surface waves, have short periods (high frequency – 0.5 to 20 Hz), short periods (high frequency – 0.5 to 20 Hz), most energetic near the hypocentermost energetic near the hypocenter

Two types of body waves:Two types of body waves: P waves and S wavesP waves and S waves

Body WavesBody Waves

P (primary) wavesP (primary) waves FastestFastest of all waves of all waves Always Always firstfirst to reach a recording station (hence primary) to reach a recording station (hence primary) Move as Move as push-pullpush-pull – alternating pulses of compression – alternating pulses of compression

and extension, like wave through Slinky toyand extension, like wave through Slinky toy Travel through solid, liquid or gasTravel through solid, liquid or gas

Velocity depends on density and compressibility of Velocity depends on density and compressibility of substance they are traveling throughsubstance they are traveling through

Velocity of about 4.8 km/sec for P wave through graniteVelocity of about 4.8 km/sec for P wave through granite Can travel through air and so may be audible near the Can travel through air and so may be audible near the

epicenterepicenter

Body WavesBody Waves

Body WavesBody Waves

S (secondary) wavesS (secondary) waves SecondSecond to reach a recording station (after primary) to reach a recording station (after primary) Exhibit Exhibit transversetransverse motion – shearing or shaking particles at motion – shearing or shaking particles at

right angles to the wave’s path (like shaking one end of a right angles to the wave’s path (like shaking one end of a rope)rope)

Travel only through solidsTravel only through solids S wave is reflected back or converted if reaches liquid S wave is reflected back or converted if reaches liquid Velocity depends on density and resistance to shearing of Velocity depends on density and resistance to shearing of

substance substance Velocity of about 3.0 km/sec for S wave through graniteVelocity of about 3.0 km/sec for S wave through granite Up-and-down and side-to-side shaking does severe Up-and-down and side-to-side shaking does severe

damage to buildingsdamage to buildings

Seismic Seismic WavesWaves

Seismic Waves and the Earth’s Seismic Waves and the Earth’s InteriorInterior

Waves from large earthquakes can pass through the entire Earth Waves from large earthquakes can pass through the entire Earth and be recorded all around the worldand be recorded all around the world

Waves do not follow straight paths through the Earth but Waves do not follow straight paths through the Earth but change change velocity and directionvelocity and direction as they encounter different layers as they encounter different layers

From the Earth’s surface down:From the Earth’s surface down: Waves initially speed up then slow at the asthenosphereWaves initially speed up then slow at the asthenosphere Wave speeds increase through mantle until reaching outer Wave speeds increase through mantle until reaching outer

core (core (liquidliquid), where S waves disappear and P waves ), where S waves disappear and P waves suddenly slowsuddenly slow

P wave speeds increase gradually through outer core until P wave speeds increase gradually through outer core until increasing dramatically at inner core (increasing dramatically at inner core (solidsolid))

Seismic Waves and the Earth’s Seismic Waves and the Earth’s InteriorInterior

Surface WavesSurface Waves Surface wavesSurface waves

Travel near the Earth’sTravel near the Earth’s surface surface, created by, created by body body waveswaves disturbing disturbing the surfacethe surface

Longer period than body waves (carry energy farther)Longer period than body waves (carry energy farther) Love wavesLove waves

• Similar motion to S waves, butSimilar motion to S waves, but side-to-side side-to-side in horizontal in horizontal planeplane

• Travel faster than Rayleigh wavesTravel faster than Rayleigh waves• Do not move through air or waterDo not move through air or water

Rayleigh wavesRayleigh waves• Backward-rotating, elliptical motionBackward-rotating, elliptical motion produces horizontal produces horizontal

and vertical shaking, which feels like rolling, boat at seaand vertical shaking, which feels like rolling, boat at sea• More energy is released as Rayleigh waves when the More energy is released as Rayleigh waves when the

hypocenter is close to the surfacehypocenter is close to the surface• Travel great distancesTravel great distances