physical geography lecture 14 - folding, faulting, and earthquakes 112816

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Folding, Faulting,Folding, Faulting,and Earthquakesand Earthquakes

Chapter 9Chapter 9

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DiastrophismDiastrophism

• Folding and faulting causing deformation of Earth’s crust on a large scale is called diastrophism

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Compression, Tension, and Compression, Tension, and Shearing stresses Shearing stresses

• Compression is force exerted inward

• Tension is a pulling apart

• Shearing occurs when force is exerted in opposite directions, but parallel to one another

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Crustal Fold StructuresCrustal Fold Structures• monocline—a one-sided slope• syncline—a downfold that creates a U-shape• anticline—an upfold that creates an n-shape• overturned fold—similar to an anticline, but tipped to

one side• overthrust fold—an overturned fold pushed completely

over on its side, so that the entire fold lays on top of the section in front of it

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FaultsFaultsEventually, even the plastic crust will break…

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Fault Block MountainsFault Block MountainsThe Sierra NevadasThe Sierra Nevadas

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Strike-slip/Transform/Transcurrent FaultStrike-slip/Transform/Transcurrent Fault(shear stress)(shear stress)

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The San Andreas runs right The San Andreas runs right through the town of Hollister, CA…through the town of Hollister, CA…

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Common Structures Associated Common Structures Associated with Transform Faultswith Transform Faults

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EarthquakesEarthquakes

or Hypocenter

Earthquake Measurement Seismic waves—Energy waves propagated during an

earthquake Seismograph—Recording device for measuring the amount of

shaking which occurs during an earthquake Seismogram—The printed record made by the seismograph

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Quantitative vs. QualitativeQuantitative vs. QualitativeSeismic MeasurementsSeismic Measurements

• Quantitative—Objective, fact-based measurement; mathematical

• Qualitative—Subjective; each person’s interpretation of the same event may be different

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Measuring Seismic WavesMeasuring Seismic Waves• Richter Scale—A numerical expression of the

amount of energy released during an earthquake event (Quantitative)– Based on the physical force exerted by the surface

movement of earthquake waves– Logarithmic scale…the difference between one order

of magnitude and the next represents 10 times the amount of force

– Only useful for expressing surface motion• Moment Magnitude Scale—More accurate at

higher magnitudes (Quantitative)– Equations used to compare larger quakes (>4.0)--

best for 7.0+

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Measuring Seismic WavesMeasuring Seismic Waves

• Mercalli Scale– Measures an earthquake’s intensity

(Qualitative)– Based on what people feel– Uses questionnaires and personal accounts

• Good for:– Earthquakes of the past for which no other records exist– Areas where existing development makes geologic

studies more difficult– Can help urban agencies plan for future earthquakes in

areas needing upgrades or retrofitting

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Loma Prieta Quake, 1989Loma Prieta Quake, 1989

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Loma Prieta QuakeLoma Prieta Quake• On October 17, 1989, at 5:04:15 p.m. (PDT)• Magnitude 6.9 (moment magnitude)• Severely shook the San Francisco and Monterey Bay

regions• Epicenter located near Loma Prieta peak in the Santa

Cruz Mountains, approximately 14 km (9 mi) NE of Santa Cruz and 96 km (60 mi) S-SE of San Francisco

• The Pacific and North American Plates abruptly slipped as much as 2 meters (7 ft) along the San Andreas fault

• The rupture began at a depth of 18 km (11 mi) and extended 35 km (22 mi) along the fault, but it did not break the surface of the Earth

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Loma Prieta Quake, 1989Loma Prieta Quake, 1989

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Seismic WavesSeismic Waves

• Body waves– Travel deep beneath the surface

• P-waves• S-waves

• Surface waves– Travel at or near the surface

• L-waves• R-waves

Seismic Waves P-waves: Pressure or

Primary waves Travel fastest

First to arrive Travel through all mediums

(solid, liquid, gas) If big enough, they can be felt

on the other side of the planet

Seismic Waves S-waves: Secondary or

Shear waves Slower than P-waves

Second to arrive Travel only through solids

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Seismic WavesSeismic Waves

• Love waves– Push rocks from side to side as the motion of the

wave follows a horizontal ellipse which travels forward

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Seismic WavesSeismic Waves

• Rayleigh waves produce an up and down motion created by a forward roll, much like that of an oscillating water wave on the open ocean– Slower than Love waves

• 10 times the speed of sound

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Both L-waves and R-waves:Both L-waves and R-waves:• Can travel only through solids• May not be felt at every earthquake event• Motion decreases exponentially the further the

focus is from the surface• Motions do not dissipate quickly• May continue for an extended period at the tail

end of a quake• Can travel much longer distances than body

waves• Tend cause the most damage to structures and

to landforms prone to mass movement

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L - waves R - waves

S - waves

P - waves

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Earthquakes and theEarthquakes and theRelationship to Plate TectonicsRelationship to Plate Tectonics

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Pinpointing an EarthquakePinpointing an Earthquake

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Pinpointing an EarthquakePinpointing an Earthquake

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Earthquake Hazard MapEarthquake Hazard Map

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Earthquake HazardsEarthquake Hazards

• Falling debris and rock material• Crumbling buildings• Ground cracks• Broken bridges• Landslides• Liquefaction• Tsunamis

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LiquefactionLiquefaction

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LiquefactionLiquefaction• Liquefaction

– (from Latin liquefacere meaning “to liquefy”)– Settling of solid material and rising of water normally

stored between the pore spaces, turning surface material into quicksand

– Danger where the water table is near the surface and surface material consists of loose, unconsolidated, water-saturated sediments

• Land may subside• Structures (and people!) sink

– Only occurs during shaking• After shaking, settled material becomes solid again and any

water on the surface either percolates back down or runs off into streams.

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Liquefaction under Kawagishi-cho Liquefaction under Kawagishi-cho apartment buildings, Niigata quake, 1964apartment buildings, Niigata quake, 1964

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San Francisco: Original ShorelineSan Francisco: Original ShorelineLiquefaction Potential?Liquefaction Potential?

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Tidal waves…Tidal waves…

Truro, Nova Scotia, Canada

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You meant a You meant a TsunamiTsunami, , right?right?

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• Produces:– Big earthquakes and volcanic islands, called

“island arcs”– A deep ocean trench–High potential for tsunamis

Oceanic-Oceanic SubductionOceanic-Oceanic Subduction

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The Pacific Ring of Fire: The Pacific Ring of Fire: Tsunami DangerTsunami Danger

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Subduction zones are prone to tsunami Subduction zones are prone to tsunami waves after large earthquakeswaves after large earthquakes

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You meant a You meant a TsunamiTsunami, , right?right?

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TsunamisTsunamis• Waves caused by undersea volcanic or tectonic

events (earthquakes)• Unnoticed by observers on the open ocean• Can reach up to 100 feet in height when they

enter a coastal zone• Can be anticipated wherever deep-focus

undersea earthquakes occur• Can be detected with special sensors, allowing

time for evacuation