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
What is seismology?
oGeophysics: physics of planet earth
oSeismology: use measures of earthquakes to measure the Earth’s
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
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
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
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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
oTime-distance graph
oEarthquake Magnitude
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