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ANTH 2150 Chapter Notes -Surface Wave, Mid-Ocean Ridge, Richter Magnitude Scale

Course Code
ANTH 2150
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of 8
-depth of earthquake important for amount of damage and effects at the surface
-pacific rim (aka “ring of fire”)
what is an earthquake?
a shaking of the earth; when the plates shift and move, they lock; the energy is
released (seismic waves travel throughout the earth’s surface)- shape the earth, hence
the earthquake; caused by shifting tectonic plates
vibrations from earthquakes aren’t always felt, but are still recorded; stronger energy
released for it to actually be perceptible
volcanic activity can also be responsible for earthquakes; meteor impacts; landslides
(mid-oceanic ridge, for example), mountainous underwater landslides; explosions of
nuclear bombs
shifting of plates along a fault- cause of earthquakes; stress between locked area
builds over time, some point the rocks can no longer take strain, suddenly released.
movement along a fault
-fault is an area of weakness or previous fracture
-energy is released along a fault line..sow movement among the fault is called
fault creep
-sudden movement= earthquake
-creep is when constant slow movement; plates aren't being locked; sudden movement
is due to shifting of locked plates; displacement can be quite large ie. 20m in a matter
of secs.
-faults are three-dimensional
hypocentre- where the release of energy takes place
epicenter- where on the earth’s surface directly above the hypocentre is
fault- fracture in the earth across which the two sides move relative to each other; any
area of geologic weakness; rupture occurs at weak point and propagates along fault
earthquakes- vibrations and movements; things associated with them cause the
Nicholas Steno:
1) law of horizonality: as sediment are washed into a basin/ocean, you end up with a
layer of rock that builds, and continues to layer on top of each other; when they’re not
disturbed, end up with flat layers; sedimentary rock sequence, can tell they haven't
gone through any change;
2) law of superposition : as long as these have been undeformed, then you’ll end up with
the bottom rocks being the oldest, the ones at the top being the youngest; able to tell
age of the rocks as long as they are horizontally layed down
3) law of original lateral continuity: you’l end up with original layers layed down in the
continuous way, but will still be able to tell the way they were layed down; would be
continuous if there was no erosion; when they become deformed, due to folding, and
faulting, you end up with a different layout of the layers of rock; the warmer the rocks
are, the more they can deform; the colder they are, the more likely they are to be
brittle and/or crack
1) Anticlines:
-layers are folded upwards
-fall away from the axis in a symmetrical pattern
2) Syncline:
-layers are folded downwards
-axis is centre point of the fold
-for rocks to form this way, lots of pressure and high heat is needed
-pressure needs to be applied slowly so the rock has time to form without cracking or
faults and geologic mapping
when the sedimentary layers end abruptly, often due to erosion by water, or displaced
by a fault
truncated sedimentary layers and their offset= determination of fault length
fault length determines size of earthquake possible
understanding fault offset can also indicate ore-baring veins
can tell when movement has happened in the past, or past active faults
larger the rupture, the larger the earthquake when the rupture is activated
original mining shafts often based on fault line
-below the fault if called the footwall, above is called the hanging-wall
dipsip faults- vertical displacement
strike-slip faults- horizontal movement
normal fault
the movement that occurs is the hanging wall moving down in relation to the footwall
downward movement
extension in this fault- pulling apart; results in hanging wall dipping down in
comparison to the footwall
get higher ground, erosion, mountain ranges, then is flattened out over a very long
period of time
sedimentary layers change; the higher land erodes more readily than the lower land
reverse fault/thrust fault
-vertical movement because on compression; footwall is pushed up by the compression
- clickface is formed due to the displacement
-whatever ground if higher is eroded more quickly
-thrust fault- footwall overrides the hanging wall
-results in displacement; pushing over top as well; these are called mega-thrust
-mega thrust means a lot of material is displaced over the hanging wall; a vertical and
horizontal movement
strike-slip faults
-mostly horizontal movements
-sheer stress where both slides of the fault are moving past each other
-results in right-lateral fault or left-lateral fault
-depends on which part of the fault is moving
-When straddling a fault, if right-hand side moved towards
you = right-lateral fault
- When straddling a fault, if the left-hand side has moved
towards you = left-lateral fault
-if nothing moves towards you, it has to be the opposite
Elastic Rebound Theory
• elastic stress builds up in rocks as they slowly bend
strain released when rocks suddenly snap back
pressure builds up; some deformation of the material
Seismology: study of earthquakes • Instruments to record earthquake
waves: seismographs
Capture movement of Earth in three components: north- south, east-west and
Seismic waves
A seismic wave is a wave that travels through the Earth
– most often as the result of an earthquake (sometimes from an explosion)