GEO 303 Lecture Notes - Lecture 14: Longitudinal Wave, Transform Fault, Seismic Wave

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10 May 2016
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Geo 303 Lecture 14: Earthquakes and Seismology, Chapter 16:
Next lecture: Chapter 16
In lab: Topographic maps Chapter 17
Lecture:
What is seismology?
oGeophysics: physics of planet earth
oSeismology: use measures of earthquakes to measure the Earth’s
interior
oStudy of seismic waves, ground vibrations produced by rock
movement along faults (or man-made explosions)
o1906: 7.8 magnitude quake in San Francisco, really started the field
o1989: Loma Preita Earthquake (6.9), Testing how well we did
Very limited damage
Earthquakes and faults
oEarthquake: Seismic event generated by a sudden shift or rocks across
a fault
oRocks under stress fail when stress exceeds strength. Strain results
from stress.
oWhen stress grows across a fault or shear plane, rocks bend until they
can no longer resist and then break causing a fault rupture
oElastic rebound theory (H.F. Reid)
Stress builds across a fault or plane of weakness, rocks bend
until they can no longer resist. Blocks slip suddenly, rocks snap
back into place, and stress is temporarily relieved
oDifferent types of faults:
Normal fault: things are pulled apart, block sliding down the
middle
Reverse Fault: things are being compressed, one is slipped up
relative to another
Strike-slip fault: Forces are horizontal, two plates are sliding
along each other
Transform fault: contact between two plates, that slide
horizontally past one another, commonly connected
two mid-ocean ridges
Oblique Fault: one goes left/right, and another goes
vertical
oFocus and Epicenter:
Focus: point of first fault rupture, at depth in crust
Epicenter: point on land surface directly above focus
Seismic waves
oEmanate from earthquake focus in all directions. Wave forms travel,
but substance doesn’t.
Ray paths vs. wave fronts
find more resources at oneclass.com
find more resources at oneclass.com
Vibrations radiate outward. Ray path is the path
perpendicular to the wave front.
Body waves: travel through the earth
P Waves: Primary, push-pull (compressional), travel
fastest, arrive at distance point first
S Waves: Secondary, shear motion, do not travel
through liquid, arrive second
Surface (L) waves: travel around Earth
Arrive after body waves, long wavelength
Include Love and Rayleigh waves.
P-Wave (compressional wave) animation
Measuring earthquakes
oSeismographs and seismograms
Seismic Waves
Waves travel out from focus through Earth’s layered
interior.
oTime-distance graph
oEarthquake Magnitude
find more resources at oneclass.com
find more resources at oneclass.com