EESA06H3 Lecture 2: EESA06 Lecture 2

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8 Aug 2016
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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
- 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
oMorphological – 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
- Body waves – goes thru the interior
oPrimary and Secondary waves
- Surface waves – go around the surface
- Primary waves – the quickest 7km/sec, compression + extension spring
ocan go thru fluids, go all the way thru the interior of planet, thru the core
oRefraction effects create “shadow zones” – no direct P waves
- Secondary waves - are smaller 5km/sec, wave on a rope
oCannot 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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EESA06 Lecture 2 Page 2
Picture of the Core by Julain Lowman
- Large plumes protruding from the core
- Green slabs going down into core
- Plates are moving around due to convection in this interior
Rift valley on ridge crest (in textbook!) – seems like a conveyer belt
- Hot mantle rising and cold mantle sinking
- Oceanic trench - Oceanic crust meeting oceanic crust (Older oceanic crust more dense goes
under less dense, younger oceanic crust, continental crust goes on top, least dense)
- Island arc – considered continental crust
- Andesitic volcano
- Mid-oceanic ridge Rift valley
Finger nails grow at average of 3.7cm/year, Atlantic ocean opening growing at 3.7cm/year
Crust types: two (continental and oceanic) and so plate interact in four ways:
1) divergent plate boundaries (mid ocean ridges, continental rifts)
2) oceanic crust converging with ocean or continental (“subduction”)
3) continental crust converging with continental (“obduction”)
4) Both types of sliding past each other (“transform” boundaries)
Continental Drift: (Alfred Wegener)
- 1912 published theory of continental drift
- Proposed a supercontinent Pangea – fitting the continents together
Pangea consists of:
- Laurasia (N.America & Asia)
- Gondwana (Southern continents)
- Wegner suggested continents moved by plowing thru sea floor but this mechanism was rejected
by most scientists…
- Put a map together, by identifying rocks of the same age, evidence that the plates were once
Striation - Scratches in rocks made by glaciers (thick mass of ice that is flowing, usually has rock debris)
- Tell us about the direction of ice flow in the last ice age about 10thousand years ago
Ocean floor gets mapped
- Sound source - mimics energy that is produced by earthquakes – generate seismic signals
- Hydrophone – picks up the bounced off energy from the ocean floor, reconstruct ocean floor
- Swath Bathymetry – higher frequency waves, bounce off the surface of ocean floor
- Can map subduction zones, mid-ocean ridges
The Challenger Deep aka Marianna trench – deepest point of Pacific ocean, 11km deep, just above a
subduction zone
Topography of the ocean floor
- Fractures – transformed faults, horizontal lines – offsets mid-ocean ridges
Harold Hess (1962) – introduced the idea of the sea floor spreading
- Sea floor created at Mid-Ocean Ridges (MOR)’s recycled into mantle at trenches
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EESA06 Lecture 2 Page 3
- Periodically the Earth’s magnetic field flips
- North magnetic pole changes to south
- Geographical pole and magnetic pole do not coincide
Direction of Earth’s magnetic field
- Cooling lava <- Magnetic alignment preserved in magnetic records orientation of Earth’s fields
- Geomagnetic Time scale - Layers of rocks – lava flows showing reverse magnetism
- Magnetic reversals in lavas average every 500k years
- Takes 10k years or less for a reversal to occur
- Magnetic epochs: Brunhas -> Matuyama -> Gausis -> Gilbert
Magnetic Stripes
- Magnetometer – tow behind ship, measure magnetism on ocean floor (cannot see)
- Magnetometer survey of ocean floors identified patterns of anomalies on each side of MOR’s
- Symmetric pattern on either side of the ridge
- Patterns Matched pattern of magnetic reversals
- Proved the sea floor spread
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