Download EARTHQUAKES

EARTHQUAKES
• An earthquake is the sudden movement
of the earth's crust caused by the release
of stress.
• This stress is built up along where two
plates are moving past, away, or into one
another.
• Earthquakes can also occur within a plate
along cracks called faults, but most
earthquakes occur along plate boundaries.
When Seismicity Occurs
Earthquakes Occur
• along faults
• Sudden change in the arrangement of the
minerals of rocks.
• Magma pushing and causing earth’s crust to
break.
• Volcano explosions.
• Giant landslides.
• Meteorite impacts.
• Underground nuclear bomb test.
Elastic Rebound
• Any place where movement occurs along
a crack in the earth’s crust, this is called a
fault.
• Faults can build up energy, until the
energy stored is greater than the strength
of the rock itself, and the rock “snaps” to a
rest position. This is known as elastic
rebound, and this is how earthquakes
occur.
Energy!!!
• Magnitude 4.0 = Appx. 1200 tons of TNT
• Magnitude 7.0 = 32million tons of TNT
• Magnitude 9.0 = 99 million tons of TNT
• Note – Hiroshima Bomb = appx. 19,000
tons of TNT
Why Is Energy Stored?
• Diff. Heating
• Convection
• Plate Motion
How is energy released?
• Energy is transformed from Potential
Energy to
– Thermal
– Seismic Wave
– Kinetic Energy along the fault
Frequency
• Per month..........................................Approxim
ately
80,000
• Per day.........................................Approximately
2,600
• Per minute..................................Approximately 2
Deformation of the Crust
• Ductile deformation – bending without
breaking.
• Brittle deformation – shattering of the
earth’s crust.
• Fault creep – motion along a fault line
where no seismic energy is generated.
Seismic Waves
• Body Waves
– P- waves
– S-waves
Surface Waves
Rayleigh – up and down.
Love – side to side
Seismic Waves
1. Earthquakes generate waves that travel through the earth
Most Earthquakes Occur Along
Plate Boundaries
Earthquakes Occur Along Faults
Faults Con’t
• Active
• Inactive
• Trace – fault that intersects the earth’s
surface
• Fault scarp – normal, or reverse faults can
create a small step.
• Blind faults – faults that exist below the
surface.
Epicenter/Focus
Foreshock/Aftershock
• Foreshock- development of minor cracks
before crust fails may lead to smaller
tremors.
• Aftershock – main quake may activate
nearby faults. These quakes are generally
smaller.
Shadow Zone
•
The shadow zone is the
area of the earth from
angular distances of 104
to 140 degrees that, for a
given earthquake, does
not receive any direct P
waves. The shadow zone
results from S waves
being stopped entirely by
the liquid core and P
waves being bent
(refracted) by the liquid
core.
Shadow Zone Con’t
• Through measuring how P and S waves
travel through the earth and out the other
side, a seismic wave shadow zone was
discovered in about 1910. From the lack of
S waves and a great slowing of the P
wave velocity (by about 40%) it was
deduced that the outer core is made of
liquid. The shadow zone also defined the
diameter of the core.
Recording Earthquakes
• Seismograph - An instrument used to
record earthquakes.
• Seismogram - The record of an
earthquake from a seismograph.
Measuring Earthquakes
• Mercalli Intensity Scale
• Richter Scale
• Seismic Moment Scale – amount of
slippage x length of rupture x depth of
rupture x rock strength
Moment Magnitude
Locating An Earthquake
• 3 seismic stations are needed.
• Each station must determine the s-p
interval and then based on average s and
p wave travel times, determine the
distance from each station.
• Triangulation is used to pinpoint the
epicenter.
Determining Magnitude
•Richter magnitude uses the amplitude of the largest wave on the
seismogram.
•This information along with logarithmic calculation, and the a
distance factor can be used to provide a magnitude of the
earthquake.
•A nomogram can be used, with the s-p lag time to more easily
identify the richter magnitude.
Tsunami
• NOT A TIDAL WAVE!!!
• Different than a Mega-Tsunami which is
created by mass movements into a body
of water.
Deep Focus Earthquakes
• Wadati-Benioff Zone – at subduction
zones, e.q.’s occur in a pattern that would
represent the subducting plate.
Earthquake Destruction
• Earthquake destruction depends upon the
following factors:
– Intensity
– Duration of vibration
– Type of material
– Structure design
– Location
– Depth
Earthquake Predictions
• Short Term
• Long Term
• http://redirect.conservation.ca.gov/cgs/rgh
m/pshamap/pshamain.html