EARTHSC 1121 Lecture Notes - Lecture 8: Fault Scarp, Seismic Wave, Thrust Fault

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Earthquakes
Key
1. Definition
2. My explanatory notes
What is an Earthquake?
Earth shaking caused by a rapid release of energy
o Due to tectonic stresses that cause rocks to break
o Energy moves outward as an expanding sphere of waves
o This waveform energy can be measured around the globe
Earthquakes destroy buildings and kill people
o 3.5 million deaths in the last 2,000 years
Earthquakes are common
Seismicity
Seismicity (earthquake activity) occurs due to…
Motion along a newly formed crustal fracture
A sudden change in mineral structure
Motion on an existing fault
Inflation of a magma chamber
Volcanic eruption
Giant landslides
Meteorite impacts
Nuclear detonations
Faults and Earthquakes
Most earthquakes occur along faults
o Faults are crustal fractures that move rock masses
o The amount of movement is termed displacement
o Displacement is also called offset, or slip
Markers may reveal the amount of offset
Earthquake concepts
Hypocenter (focus): the spot within the Earth where earthquake waves originates
o Usually occurs on a fault surface
o Earthquake waves expand outward from the hypocenter
Epicenter: land surface above the hypocenter
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**Epicenter and hypocenter can be confusing because the fault is not necessarily
vertical so that is why they are in different spots
Faults and Fault Oceans
Faults are planar breaks in blocks of crust
o Normal fault (divergent)
Hanging wall goes down relative to footwall
Due to crustal stretching
o Thrust fault (convergent)
Hanging wall goes up relative to footwall
Due to crustal shortening
Slope (dip) of fault is not steep
o Strike-slip fault (transform)
No vertical motion
One block slides sideways (laterally) past the other
Fault surface is nearly vertical
Faults are common place in the crust
o Active faults: ongoing stresses produce motion
o Inactive faults: motion occurred in the geologic past
Displacement can be visible
o Fault trace: a surface tear due to the intersection of the fault plane with the
ground surface
o Fault scarp: a small cliff
Blind faults are invisible
Fault Initiation
Tectonic forces add stress to unbroken rocks
The rock deforms slightly (elastic strain)
Continued stress will cause growth of cracks
Eventually, cracks grow to the point of failure
When the rock breaks, elastic strain transforms into brittle deformation, releasing
earthquake energy
Fault Motion
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faults move in jumps
once motion starts, it quickly stops due to friction
eventually, strain will build up again, causing failure
this behavior is termed stick slip behavior
o stick: friction prevents motion
o Slip: friction briefly overwhelmed by motion
when rocks break, stored elastic strain is released
this energy radiates outward from the hypocenter
the energy, as waves, generates vibrations
vibrations cause motion, as when a bell is rung
foreshocks and aftershocks are common
Amount of Displacement
Displacement scale varies from small to large
o Large events tear huge fault segments 100s of kms long
o Displacement maxima near hypocenter/epicenter
o Displacement diminished with distance
Faulting may result in uplift or subsidence
Changes are measureable from satellites
Seismic Waves
Body waves pass through Earth’s interior
o Compressional or primary (P) waves
Push-pull (compress and expand) motion
Travel through solids, liquids, and gases
Fastest
o Shear or Secondary (S) Waves
“shaking” motion
travel only through solids; not liquids
slower
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Document Summary

Seismicity: seismicity (earthquake activity) occurs due to , motion along a newly formed crustal fracture, a sudden change in mineral structure, motion on an existing fault, inflation of a magma chamber, volcanic eruption, giant landslides, meteorite impacts, nuclear detonations. Faults and earthquakes: most earthquakes occur along faults, faults are crustal fractures that move rock masses, the amount of movement is termed displacement, displacement is also called offset, or slip, markers may reveal the amount of offset. Earthquake concepts: hypocenter (focus): the spot within the earth where earthquake waves originates, usually occurs on a fault surface, earthquake waves expand outward from the hypocenter, epicenter: land surface above the hypocenter. **epicenter and hypocenter can be confusing because the fault is not necessarily vertical so that is why they are in different spots. Seismology: seismology is the study of earthquake waves, seismographs instruments that record seismicity, worldwide, they detect earthquakes anywhere on earth, seismology reveals size and location of earthquakes.

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