Earthquakes Lecture Notes

GeoClassroom Physical Geology Historical Geology Structure Lab

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.