Folds, Faults & Earthquakes
Rocks under stress can either bend or break
both are referred to as deformation
Any stress can deform rocks
compressional (squeezing) = folded rocks
anticlines and synclines
extensional (stretching) = “thinned” beds
Whether or not the rocks fold or break depends
on the amount of stress, the strength of the rock
and the duration and rate that the stress is
applied to the rocks
Folded
Rocks- syncline
- syncline = U-shaped fold
Folded Rocks
Anticline = A-shaped fold (in this case cut in half by a
river
Large scale folding in the Appalachian Mountains
If the stress is applied too fast or if it greatly
exceeds the rock strength…
the rocks break => releasing the stored energy
The break is called a fault, the energy released is
called an earthquake
Small earthquakes are cool- everyone feels them, no
one gets hurt, minimal damage
Big ones suck. They collapse buildings and other
structures that kill people and create secondary events
that do the same
Significant Earthquakes
Year
1964
1970
1971
1975
1976
1976
1985
1988
1989
1990
1992
1992
1992
1993
1994
1994
Location
USA (Alaska)
Peru (Chimbote)
USA (San Fernando, California)
China (Haicheng)
Guatemala
China (Tangshan)
Mexico (Mexico City)
Armenia
USA (Loma Prieta, California)
Iran
Turkey
USA (Landers, California)
Egypt
India
USA (Northridge, California)
Bolivia
Magnitude
8.6
7.8
6.6
7.3
7.5
8.0
8.1
7.0
7.1
7.3
6.8
7.5
5.9
6.4
6.7
8.2
Deaths
131
25,000
65
?
23,000
242,000
9,500
25,000
63
40,000
570
1
500+
30,000+
61
?
• 13 million+ lives have been lost in the past 4000
years due to earthquakes and related phenomena.
Earthquakes
• Shaking caused by the sudden release
of energy in the Earth, resulting from
displacement of rock along a fault.
• Most are caused by faults associated
with plate movement.
Fault- a break in the lithosphere that moves
Epicenter- a projection of the location on the
Earth’s Surface
Focus- the location of the energy release
(rupture) along the fault
Relationships
between a
fault plane,
epicenter,
focus, and
energy waves
Look for relative motion-helps determine how force was applied
Vertical Faulting
Lateral Faulting
HW is a normal fault
Opposite block moves right
means right lateral
HW is a reverse fault
Head on Headwall
Feet on Footwall
Fault Plane
Fault type tells us something about motion, stress and
forces needed to create them and thus gives hints as
to plate tectonics setting
Normal fault = extensional force
Reverse fault = compressional force
Lateral fault = shearing force
What statement is most accurate concerning earthquakes?
A) The stronger the earthquake, the more people are
killed.
B) All earthquakes occur close to a tectonic plate edge.
C) The poorer the country, the more deaths are likely to
occur due to moderate to severe earthquakes
D) Earthquake epicenters are always associated with faults
at Earth’s surface
Seismic Waves (1:3)
• cause shaking and destruction.
• Two types of seismic waves:
• Body Waves travel through the
Earth.
• Surface Waves travel only along
the surface of the Earth
Lab exercise using P- and S-waves to locate an earthquake
Body Waves
•
Primary Waves (P-waves)
•travel fastest
• move through solid, liquid or gas.
•P-waves are compressional.
•
Secondary Waves (S-waves)
• travels slower than P-waves
• solids only!
• S-waves
- shear waves that move material
perpendicular to their direction of travel.
QuickTime™ and a
Sorenson Video decompressor
are needed to see this picture.
QuickTime™ and a
Sorenson Video decompressor
are needed to see this picture.
Surface Waves
Rayleigh Waves (R-waves)
• the slower of the surface waves.
• Love Waves (L-waves)
•
• move similar to S-waves
• displacement of material occurs in a
horizontal plane perpendicular to direction of
wave movement.
Movie clip
Seismograph
• An instrument that measures and records earth movements produced by an
earthquake.
Time-Distance Graphs
• The time difference between P- and
S-waves arrivals allows calculation
of the distance from the epicenter.
Dt = d {(Vp-Vs)/(VpVs)}
solve for d
where Dt = Time interval between arrival of P and S
waves at a station measured from seismograms
D = distance of station from epicenter
Vp ; Vs = velocity in Km/sec of P waves and S waves
- generally Vp = 6.0 km/sec; Vs = 3.5 km/sec *
making the (Vp-Vs)/(VpVs) term = (2.5/21) = 0.119
* Within 1000 km
Graphical version of the distance calculation.
Location of earthquakes
• Using three seismograph locations
the location can be pinpointed on a
map
• Triangulation
• Time-distance
graphs only tell
us how far we
are from the
epicenter.
• Triangulation
of three stations
allows us to
pinpoint the
epicenter.
Earthquake website
Earthquakes and Plate Margins
A seismic hazard map
Exceptions to
Plate Boundary
earthquakes
'quakes
New Madrid, MO
in 1811
1811 and
and 1812
1812 aa series
series
•• in
of strong
strong intraplate
intraplate
of
earthquakes occurred
occurred near
near
earthquakes
New Madrid,
Madrid, MO.
MO.
New
•• Only twenty people died
(imagine the
the toll
toll if
if this
this
(imagine
occurred in
in the
the region
region
occurred
today), and
and the
the quake
quake was
was
today),
felt in
in Boston,
Boston, 1600
1600 km
km
felt
away.
away.
Focal Depth & Plate Boundary Type
• Focal depth increases in a "Benioff Zone" adjacent to deep ocean
trenches that define convergent boundaries.
Benioff zone
Categories-Depth
• Shallow-focus:
• focal depth less than 70 km.
• Intermediate-focus:
• focal depth between 70 and 300 km.
• Deep-focus:
• focal depth greater than 300 km.
• most earthquakes (~80%) occur at
depth of less than 100 km.
Movie clip
Earthquake Intensity & Magnitude
•
Intensity (qualitative data)
•A measure of earthquake damage.
•MMI - scale (Modified Mercalli Intensity)
-ranges from 1 to 12
12, expressed as roman numerals (I, II, III, IV)
-observation of damage
damage, semi-independent of energy released
•Magnitude (quantitative data)
•a quantitative measure of the amount of energy released
by an earthquake.
-measured on the Richter Scale- max energy in one instant
-each step is multiplied by a factor of 30
•Moment magnitude is energy released over the entire area
over the entire time
Earthquake Prediction
• Short term predictions are still unsuccessful.
• Long term forecast maps are more reliable
• offer a magnitude and probability of
occurrence for a given interval of time
• Damage maps are developed based on
intensity of past earthquakes and current
geologic maps.
Tsunamis and Ground failures
• Seismic sea-waves
• produced by earthquakes on the sea floor.
• Travel at very high speeds
• 100's of km/hr in the open ocean
- Barely noticeable in open ocean
• but may reach heights of 65 m as they near shore.
Mass movements
• Earthquake-triggered
landslides cause much
damage in mountainous
areas.
• 1959 Madison County
(Montana) earthquake
Notice
the Pwaves
and Swaves
traveling
outward
from the
focus
Movie clip
QuickTime™ and a
Photo - JPEG decompressor
are needed to see this picture.