EESA06 Lecture 2.docx

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21 Apr 2012
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EESA06 Lecture 2 Page 1
EESA06 Lecture 2
The deepest mine in the world is 4km underground, any deeper it will be too hot, hole will begin to close
due to temperature and movement
Geophysics application of physics to the world
Earthquakes generate energy
- Fault plane surface (between moving bocks) which blocks have moved, we feel as an
earthquake
- Focus is where the energy is released, the trigger of the earthquake, always underground, can
predict how much damage it can do how deep down the focus is
- Epicenter directly above the focus on the earth’s surface
- Seismic risk assessment gauge any movement along any fault, looks at fault scarp & fault trace
- Fault scarp cliff, bluff, the steep slope that results after earthquake (ex: 1891 Japan)
- Fault trace fairly straight lines, many river valleys along traces of faults
o Morphological what shapes on earth’s surface created by faults
- Not all faults create earthquakes, some “creep” don’t result in earthquake, the “lock” ones
create earthquakes
Waves:
- Body waves goes thru the interior
o Primary and Secondary waves
- Surface waves go around the surface
- Primary waves the quickest 7km/sec, compression + extension spring
o can go thru fluids, go all the way thru the interior of planet, thru the core
o Refraction effects create “shadow zones” – no direct P waves
- Secondary waves - are smaller 5km/sec, wave on a rope
o Cannot go thru fluids, stops at core, very large shadow zone
Earth Core:
- Outer most part of core is liquid preventing s-waves, refracting p-waves
- Inner core is solid
Seismograph station closer to focus = time lag of arrival of P&S waves not much, farther from focus =
time lag from P&S wave is bigger
- Allow to determine where the epicenter was
- Networks was setup to determine nuclear bomb testing
Major Epicenters Circum-Pacific belt (most damaging, deepest), Indonesian belt, Mediterranean-
Himalayan belt (where India is colliding with China)
- Deep focus earthquakes - Along the plate margins
- Deepest earthquake is 7km down
- Shallow-focus earthquakes, where plates are diverging, mid ocean drifts
- Low magnitude earthquakes, magma coming up
- Earthquakes can be in the middle of continents
- Intracratonic earthquakes are very difficult to explain and predict (ex: Toronto)
Ancient China used missing farm animals to predict earthquakes
Using Heat Flow measurements to map mantle structure and plumes (very hot rock)
- Black lines are continents cooler areas
- White lines are the plate boundaries higher heat flow below the oceans
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