3. 33
Compression, Tension, andCompression, Tension, and
Shearing stressesShearing stresses
• Compression is force exerted inward
• Tension is a pulling apart
• Shearing occurs when force is exerted in
opposite directions, but parallel to one
another
4. 44
Crustal Fold StructuresCrustal Fold Structures
• monocline—a one-sided slope
• syncline—a downfold that creates a U-shape
• anticline—an upfold that creates an n-shape
• overturned fold—similar to an anticline, but tipped to one
side
• overthrust fold—an overturned fold pushed completely
over on its side, so that the entire fold lays on top of the
section in front of it
11. Earthquake Measurement
Seismic waves—Energy waves propagated during an
earthquake
Seismograph—Recording device for measuring the amount of
shaking which occurs during an earthquake
Seismogram—The printed record made by the seismograph
12. 1212
Quantitative vs. QualitativeQuantitative vs. Qualitative
Seismic MeasurementsSeismic Measurements
• Quantitative—Objective, fact-based
measurement; mathematical
• Qualitative—Subjective; each person’s
interpretation of the same event may be
different
13. 1313
Measuring Seismic WavesMeasuring Seismic Waves
• Richter Scale—A numerical expression of the
amount of energy released during an
earthquake event (Quantitative)
– Based on the physical force exerted by the surface
movement of earthquake waves
– Logarithmic scale…the difference between one order
of magnitude and the next represents 10 times the
amount of force
– Only useful for expressing surface motion
• Moment Magnitude Scale—More accurate at
higher magnitudes (Quantitative)
– Equations used to compare larger quakes (>4.0)--
best for 7.0+
14. 1414
Measuring Seismic WavesMeasuring Seismic Waves
• Mercalli Scale
– Measures an earthquake’s intensity
(Qualitative)
– Based on what people feel
– Uses questionnaires and personal accounts
• Good for:
– Earthquakes of the past for which no other records exist
– Areas where existing development makes geologic
studies more difficult
– Can help urban agencies plan for future earthquakes in
areas needing upgrades or retrofitting
16. 1616
Loma Prieta QuakeLoma Prieta Quake
• On October 17, 1989, at 5:04:15 p.m. (PDT)
• Magnitude 6.9 (moment magnitude)
• Severely shook the San Francisco and Monterey Bay
regions
• Epicenter located near Loma Prieta peak in the Santa
Cruz Mountains, approximately 14 km (9 mi) NE of
Santa Cruz and 96 km (60 mi) S-SE of San Francisco
• The Pacific and North American Plates abruptly slipped
as much as 2 meters (7 ft) along the San Andreas fault
• The rupture began at a depth of 18 km (11 mi) and
extended 35 km (22 mi) along the fault, but it did not
break the surface of the Earth
18. 1818
Seismic WavesSeismic Waves
• Body waves
– Travel deep beneath the surface
• P-waves
• S-waves
• Surface waves
– Travel at or near the surface
• L-waves
• R-waves
19. Seismic Waves
P-waves: Pressure or
Primary waves
Travel fastest
First to arrive
Travel through all mediums
(solid, liquid, gas)
If big enough, they can be felt
on the other side of the planet
20. Seismic Waves
S-waves: Secondary or
Shear waves
Slower than P-waves
Second to arrive
Travel only through solids
21. 2121
Seismic WavesSeismic Waves
• Love waves
– Push rocks from side to side as the motion of the
wave follows a horizontal ellipse which travels
forward
22. 2222
Seismic WavesSeismic Waves
• Rayleigh waves produce an up and down
motion created by a forward roll, much like that
of an oscillating water wave on the open ocean
– Slower than Love waves
• 10 times the speed of sound
23. 2323
Both L-waves and R-waves:Both L-waves and R-waves:
• Can travel only through solids
• May not be felt at every earthquake event
• Motion decreases exponentially the further the
focus is from the surface
• Motions do not dissipate quickly
• May continue for an extended period at the tail
end of a quake
• Can travel much longer distances than body
waves
• Tend cause the most damage to structures and
to landforms prone to mass movement
29. 2929
LiquefactionLiquefaction
• Liquefaction
– (from Latin liquefacere meaning “to liquefy”)
– Settling of solid material and rising of water normally
stored between the pore spaces, turning surface
material into quicksand
– Danger where the water table is near the surface and
surface material consists of loose, unconsolidated,
water-saturated sediments
• Land may subside
• Structures (and people!) sink
– Only occurs during shaking
• After shaking, settled material becomes solid again and any
water on the surface either percolates back down or runs off
into streams.
38. 3838
Produces:
– Big earthquakes and volcanic islands, called
“island arcs”
– A deep ocean trench
–High potential for tsunamis
Oceanic-Oceanic SubductionOceanic-Oceanic Subduction
39. 3939
TsunamisTsunamis
• Waves caused by undersea volcanic or tectonic
events (earthquakes)
• Unnoticed by observers on the open ocean
• Can reach up to 100 feet in height when they
enter a coastal zone
• Can be anticipated wherever deep-focus
undersea earthquakes occur
• Can be detected with special sensors, allowing
time for evacuation
