Physical Geology
Earthquakes
Vibrations of the Earth caused by the rupture and sudden movement of rocks that have been strained beyond their elastic limits.
- - similar to what happens when a stick is bent, then breaks.
- - 3 types of waves generated by an earthquake shock, P, S, and Surface
P waves
- like a spring, moving backward and forward along the direction of movement
S waves
- waves that oscillate side to side along the direction of travel, at 90 degree angles
Surface waves
- slower waves, moving in circular orbits like waves in water
P waves are the first the fastest, followed by S then Surface.
The epicenter is where the Earthquake happened on the surface of the Earth, a point directly above the focus, the point where the slippage actually occured.
- - Shallow earthquakes happen at the surface to a depth of 70 km
- - Intermediate earthquakes happen between 70 and 300 km deep
- - Deep earthquakes happen from 300 to 700 km deep and beyond
- - most earthquakes deeper than 70 km never reach the surface.
Intensity:
- evaluation of the severity of an earthquake at a given location.
- depends on:
- 1) distance from quake
- 2) total amount of energy released by the quake
- 3) type of rock structure is on
- - hard rock is best
- - unconsolidated sediment is the worst, cannot absorb waves and shakes too much: liquefaction
- - in S.F. in 1989, most of the damage was too buildings built on an old landfill.
Magnitude:
- measure of the amount of energy released by the quake
- world-wide standard for this measurement is the Richter Scale
- each step in the scale is a 10 times increase in the size of the quake
- quakes 8 and above are the worst -- they do the total destruction
- 6's and 7's can do some damage in populated areas, like in Kobe a few years ago.
Application to Plate tectonics:
- distribution of quakes around the world exactly deliniates the edges of the major tectonic plates
- shallow quakes follow the MOR and transform plate boundaries
- quakes at subduction zone boundaries occur in a zone inclined downward beneath the adjacent plate to define the Benioff Zone
- - quakes get deeper and deeper farther from the subduction zone, indicating that the slab of o.c. is slipping downward into the mantle.
- some shallow quakes occur in the middle of the plates, indicating that the plates do some flexing.
Application to the Earth's interior:
- P waves travel through the solid inner core slower than the mantle, and they are also refracted
- - results in a P wave shadow zone between 103 and 143 degrees from a quake's focus
- S waves cannot travel through liquid, so will not travel through the liquid outer core at all, allowing for a huge S wave shadow zone, extending almost half way around the Earth from a quake's focus.
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