Environmental Geology Lecture -
Seismology, the science of elastic waves and earthquakes.
What are seismic waves and what information
can obtained from their study?
IRIS Consortium
information.
Definition of elastic deformation.
Character of seismic waves:
- surface waves (Rayleigh and Love waves)
vs. body waves (primary and secondary waves).
- primary or P waves: fastest, 2 to 7.8
km/sec in a variety of rocks, travels through liquids and solids.
- secondary or S waves: slower, 1.1-4.6
km/sec in a variety of rocks, travel only through solids
- surface waves: cause major ground accelerations,
damaging and of most direct concern.
Seismometers: pendulums, dead weights and
springs, seismic networks and computers.
Sources of seismic events:
- earthquakes faults and seismogenic zones.
- magma movement and volcanic explosions.
- impacts.
- assortment of human activity.
Elastic rebound model for earthquakes: creep
vs. stick-slip behavior, the seismic 'cycle' for stick-slip motion
as storage and release of elastic energy, the focus (hypocenter)
as where the tear started and the epicenter.
Where was the earthquake located? Triangulation
from P vs. S wave delay times.
Virtual seismology link.
How to measure the size of an earthquake?
- Mercalli scale:
a human scale, intensity of damage.
- Richter scale:
energy release from calibrated P wave arrival.
- Moment scale:
function of area that slipped and force involved. A more precise
geophysical determination.
How did the earth move? First motion
studies, and the pattern of first ground motions as a function
of fault geometry and slip direction.
Using elastic waves to image the earth:
reflection and refraction events.
What determines the size of an earthquake?
- rate of slip.
- frictional resistance.
- size of area on fault that slips.
- previous history and chaos.
- log-log frequency-size relationship.
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