Elastic Rebound Theory
Deformation, release, rebound
Stress vs strain
Elastic, elastic limit, brittle failure, (plastic = folds)
View short animation from CD or video disc
Seismology study of earthquakes
Seismograph
Seismogram
Seismic waves
Focus vs. epicenter
Shallow, intermediate, deep foci
Relationship to plate boundaries, Tonga trench, Benioff Zones
Frequency and Distribution
95% plate boundaries, 5% plate interiors or spreading ridges
why the interiors of plates?
Seismic waves:
Body waves:
P-waves, primary (first arrival), compressional, like sound waves
S-waves, secondary, shear, motion of a shaken rope
Speeds determined by density and rigidity/elasticity of medium
P: 5-7 km/s in crust; 8 km/s in upper mantle
(sound = 0.34 km/s)
S: 3-4 km/s in crust; none at all in a liquid
Solids vs. liquids
Surface waves:
R-waves, Rayleigh waves, like water, eliptical path for particles
L-waves, Love waves, shear wave but only in horizontal plane
Particularly hard on buildings
Locating an earthquake:
See exercise on CD
See exercise on web-site at:
http://vflylab.calstatela.edu/edesktop/VirtApps/VirtualEarthQuake/VQuakeExecute.html
[1] from seismogram measure time gap between P and S waves = distance
[2] do this for 3 different stations
[3] intersections of 3 circles should give a single point epicenter
Intensity and Magnitude:
Intensity = qualitative Modified Mercalli Intensity Scale
I XII
Magnitude = amount of energy Richter Scale
1 10, largest recorded is an 8.6 (limited by strength of rock)
log scale (remember river exercise?): 6 = 10 x 5 = 100 x 4
1 unit is ~ 30X energy
Destructive Effects:
Ground shaking: depends on magnitude and type of rock/soil
Fire
Tsunami
Ground failure
Earthquake prediction, precursors and control
Microquakes, fore shocks
Gas release
Water levels in wells
Dilatancy