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Lecture 5

Geography 1100 Lecture Notes - Lecture 5: Ground Vibrations, Wind Wave, Seismic Wave


Department
Geography
Course Code
GEOG 1100
Professor
D.Kim Holland
Lecture
5

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Earth Sciences 1022b February 11 th Lecture EARTHQUAKES
Cause: mainly by elastic- rebound of the crust as stress builds up as rocks bend, the stress
builds up until they rupture and they start begin again or slip along the focus and energy
radiates in all directions. Commonly happens at plate boundaries. Occurs below the spot
platted on a map called the epicentre
- foreshocks and aftershocks accompany the main event with some earthquakes
Seismology: uses tremor -detecting seismographs to record vibrations as surface waves, and
Body waves (compressional and Shear) that travel through rock:
Surface waves : slowest, long l waves travel on the surface like ripples on water
Compressional : push- pull - faster, primary p waves travel in the direction of initiation, like a
slinky
Shear : slower, secondary s waves travel perpendicular to the direction of initiation, moving
rope up and down
Locating the source of an earthqu ake: differences in arrival times of p and s waves yield
wave travel distances from the epicentre - plot these distances from 3 stations as arcs on a
map and their intersection gives the location of the epicentre
- seismic stations around the world indicate that earthquakes occur mainly at plate boundaries
and are shallow, intermediate, deep (>300m); shallow most destructive
Measuring the size of earthquakes: the Richter scale measures earthquake magnitude
based on the amplitude of the largest wave on the seismogram which depends on the
energy released ; largest earthquakes recorded in history ~8.9
- each number 10 times the largest wave amplitude, or ~32 times the energy of last number
Moment magnitude also considers the size and displacement of the fault, as well as rock
strength - better measure of total energy released during largest earthquakes
Destruction: largely depends on the underlying geology of the area affected
Ground vibrations - buildings cannot stand much horizontal movement and soft sediment
amplifies vibrations whereas solid bedrock is more resistant to shaking
Liquefaction - normally stable soft sediment can turn into unstable mud from shaking;
buried objects can float to ground surface while buildings can sink and collapse
Tsunami (seismic sea wave) - following rupture of the sea floor or earthquake- induced
submarine landslides, travels rapidly across the ocean, damages coasts
Landslides and ground subsidence occurs in soft rocks and sediment in hilly areas
Fire mainly due to broken gas and power lines; made worse when water lines break
Prediction: short- range predictions (hours to days) are not yet feasible
- long- range predictions are made where seismic gaps occur along plate boundaries –that is,
where no big earthquakes have happened for at least 100 years; overdue
Earths Interior: when seismic waves hit a material boundary, part of the seismic wave is
reflected , part refracted
Major Boundaries
- Crust mantle: both waves speed up
- Mantle- Core: p waves slow down, s waves disappear in liquid core
- Inner Core: p waves speed up in solid inner core
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