Geologic & Natural Hazards
Natural
Hazards
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Earthquakes
Tsunamis
Floods
Monsoons
Volcanoes
Asteroids
Earthquakes : Seismic waves Video of waves
Waves move in specific directions
Body Waves
P : Primary waves
S : Secondary waves
The epicenter, not the “earthquake” is directly
above the focal mechanism (release of energy)
Surface Waves
Love
Rayleigh
Earthquakes :Fault motion
• Fault motion describes type
of earthquake
• Dip – Slip faults
– Normal faults
– Reverse faults
• Strike-slip faults
Earthquakes : Seismic waves
X
• The pattern of "ups" and
"downs" recorded around
the earthquake epicenter
will indicate whether the
fault was a, reverse, normal,
or strike slip.
Elastic Rebound Theory
Distant forces cause a gradual build up of stress in the earth
over tens or hundreds or thousands of years, slowly
distorting the earth underneath our feet. Eventually, a preexisting weakness in the earth--called a fault or a fault zone-can not resist the strain any longer and fails
catastrophically.
What is an earthquake?
What is the role of a model in science?
Calvin and Hobbes by Bill Watterson
EQ Machine - Lite
Bulk of the Plate
Top View
B
C
Elastic Properties of
Earth Materials
Edge of the Plate
Plate has Constant
Velocity Here of
1cm/year
Visualizing magnitude with the model
B
Seismic Moment
Mo = fault length x fault width x displacement x rigidity
Moment Magnitude = Mw = log Mo/1.5 – 10.7
Where is the Fault line?
Map of Homes on Fault Line
Seismic Risk Assessment
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Earthquake Prediction (?): Numerous factors have been proposed as possible precursory signals that an earthquake is imminent including changes in
low magnitude seismic activity in the weeks and months preceding a major quake, changes to groundwater levels, radon and other gases in
groundwater wells, changes in the electrical resistivity of the crust (related to changes in groundwater distribution in rock as it begins forming
microcracks immediately prior to an earthquake, changes in seismic wave velocity in the crust surrounding a fault that is beginning to fail prior to an
earthquake, even strange behavior by animals just prior to a major earthquake. To date, no predictive factor or group of factors has been found to
allow the prediction of an imminent earthquake.
Earthquake Probability: Seismologists are, however, able to estimate the probability of an earthquake of a given size occurring in a given period of
years on a particular segment of a fault. Probability estimates utilize such information as the past history of earthquakes on the fault (size and
average time between), magnitude and age of the most recent large earthquake (and the amount of stress released), rate of stress buildup in the
plate boundary region based on the velocity of relative plate motion and regular monitoring of elastic strain buildup through survey techniques, and
estimated strength of the fault (rock strength and friction).
While the frequency of earthquakes is much greater in areas around tectonic plate boundaries, where the stresses build up quickly, there are locales
in the middle of plates, far from active faulting where stresses nevertheless build up on ancient faults potentially leading to major earthquakes (for
example, the New Madrid fault zone on the Mississippi River).
Seismic Gaps and Sequences: Segments of faults that have not had significant earthquakes (stress release) in some time are more likely to have an
earthquake sooner than segments that have had more recent earthquakes.
In Turkey, since 1939 there has been a westward progression of damaging earthquakes on segments of the North Anatolian fault. The most recent
one was a 1999 earthquake that caused major damage and many deaths in the city of Izmet. Based on the sequence, the great city of Istanbul
appears to be next.
Damage Factors:
- Loose, unconsolidated sediments, and especially saturated sediments experience stronger ground motions in an earthquake as compared to solid
bedrock. Damage to structures is least where built on solid bedrock.
- Buildings have their own natural vibration frequency, like a tuning fork, depending on their height and rigidity. Given the natural vibration
frequency of the rocks or sediments upon which the building are constructed, buildings of a particular height range are most susceptible to damage
in strong earthquakes.
- Unconsolidated sediments may undergo liquefaction in a strong earthquake allowing buildings to sink, usually one side more than the other so that
the building topples.
- A "seismic bounce" causes stronger than expected ground motion at a distance from an earthquake epicenter where reflected seismic waves
combine with direct-arriving seismic waves.
- Strong earthquakes typically break underground gas lines leading to fires. Water lines also break making it difficult to fight the fires.
- Strong earthquakes that offset the seafloor produce tsunamis, which are not normally discernible from a ship at sea with their very long
wavelengths, but as theses very rapidly moving waves come into shallow water approaching the shoreline they slow greatly, getting steeper and
much higher innundating low-lying coasts.
- Earthquakes may also set landslides and mudslides in motion, burying whole neighborhoods.
• Earth’s Structure: Animation & Quiz
• Plate convergence : USGS
• Fault quiz
“A” is the
Fault plane
Foot wall
That is CORRECT
Fault trace
Hanging wall
That is Incorrect