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Lecture 4

EARTHSC 2GG3 Lecture Notes - Lecture 4: Epicenter, Logarithmic Scale, 32X

Earth Sciences
Course Code
Eduard Reinhardt

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Thursday, October 11, 2018 2:35 AM
What is an Earthquake?
Motion or trembling of ground caused by sudden displacement of rock
Most common reason: movement of plates
Underground explosions mimic earthquakes -> enough energy
Toll of Earthquakes
Consequences dependent on:
Magnitude - how much energy
Depth - shallower the earthquake -> more damage
Direction of fault rupture
Distance from populated areas
Nature of local earth materials
Engineering and construction practices
Population distribution
Differences of factors:
Why M7 in Haiti (2010) killed 240 000 but M6.9 in California (1989) killed <70
Intensity - damage dependent upon density vs. Magnitude - how much energy released
Result from rupture of rocks along fault
Energy released in form of Seismic Waves
Mapped according to the epicentre
The focus is directly below the epicentre
Where it happened underground
Measured by seismographs
Paper w waves bath and forth
Compared based on
Magnitude - energy released
Intensity - damage + effect on ppl, harder to measure
Measured by moment magnitude
Logarithmic scale - powers of 10
Ground motion of M3 is 10x that of M2
Amount of energy released for M3 is 32x that of M2
Smaller earthquakes are more frequent than larger ones
Almost all occur on west coast
Introplate Earthquake - Charleston, south Carolina
Largest earthquake on record - CHILE

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Most great earthquakes happen on ocean floor
Measured by Modified Mercalli Intensity Scale
Qualitative scale - based on damage to structures and people's perceptions
MMI maps show where damage and perceived shaking is most severe
Data now collected using the internet
Used to be done by surveys
Shake maps use seismographs data to show areas of intense shaking
Earthquakes commonly occur at or near plate boundaries
Plate boundaries may contain FAULTS
Semi-plastic -> can create faults
Friction along plate boundaries -> strain or deformation
STRESS on FAULTS -> sudden movements -> earthquake
When stress on rocks exceeds their strength -> rupture produces seismic waves
Fault Types
A. Dip Slip - Vertical movements --->
Walls on incline defined as:
Hanging Wall
Footwall Block
3 types based on direction of movement
a. Normal Fault
Hanging wall moves downward relative to footwall
If you pull blocks apart
Ex. Mid-oceanic ridges
b. Reverse Fault
Dip Slip
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