Download Earthquakes

Earthquakes
Earthquakes
• When plates move very slowly and
over a long period of time
• Eventually a sudden shaking of the
crust occurs at a particular place:
Earthquake
Earthquakes
• The majority of
earthquakes occur at
the boundaries of
tectonic plates.
• Individual earthquakes
also occur where there
is a fault.
• A fault is a zone of
weakness in the crust
along which some
movement of rock
occurs.
Three Types of Faults
1) Reverse – compression causes horizontal and
vertical movement
2) Normal: Tension causes horizontal and vertical
movement
3) Strike-Slip – shear causes horizontal and vertical
movement
Earthquake Terms
• Focus: the area in the crust (below surface)
where energy is released during an
earthquake (where the earthquake begins)
– Shallow: 0-70 km
– Intermediate: 70-300 km
– Deep: over 300 km
• Epicenter: The point on the surface directly
above the focus of an earthquake
Process
• As tectonic plates move, friction
causes the rocks at plate
boundaries to stretch or compress.
• Like a stretched rubber band or a
compressed spring, these rocks
energy called the
Elastic Rebound Theory.
store
• When the pressure builds up within
the crust, the stored energy is suddenly released as
the rocks slip along the fault, and an earthquake
occurs.
Process
• More than ½ of all earthquakes occur
along the edges of plates
– Subduction zones
– Mid-ocean ridge
– Collision zones (continent-continent
convergence)
Seismology
• The study of earthquakes
• Scientists cannot predict the exact date
and time of an earthquake
• But… they can identify areas likely to have
an earthquake in the next 10 years.
Seismology
• A seismograph
measures earthquakes,
and seismologists use
seismic waves to study
• The energy released by
an earthquake travels
away from the focus in
waves
• Magnitude: relative size
of an earthquake
depends on amount
energy released
How Seismographs Work
the pendulum remains
fixed as the ground
moves beneath it
http://www.uwgb.edu/dutchs/EarthSC-102VisualsIndex.HTM
Seismology
• Seismographs show the kinds of waves,
their amplitude, and the timing of the
waves
• Foreshocks are small tremors that
precede an earthquake
• Aftershocks are small tremors that follow
after an earthquake
Scales
• Magnitude is expressed on the Richter Scale
– Study the waves to determine the magnitude and
location of the earthquake
– Uses number 1 & up
– Each # indicates ten times stronger than the # below
• Ex: A rating of 8 has a magnitude ten times as great as an
earthquake with a magnitude of 7
– 7 or higher indicates a major earthquake ( 10-felt all
over the earth)
Scales
• Modified Mercalli Scale: use to rate
intensity
– Roman numerals I to XII describes damage
done by the quake
– XII : total destruction
– An earthquake with one magnitude, damage
can vary depending on the location
How are Earthquakes Measured?
Mercalli Intensity Scale
Click Link for Interactive Demo
http://elearning.niu.edu/simulations/images/S_portfolio/Mercalli/Mercalli_Scale.swf
Seismic Waves
• The conversion of potential to kinetic
energy results in seismic waves
• Travel through the ground 20 times faster
than the speed of sound
• Seismic waves reach the ground at the
epicenter
Seismic Waves
• Body waves – originate from the focus of
the earthquake
– P waves (primary waves) are compression
waves that push and pull rock as they travel
• Cause particles to move back & forth
• Move through solids, liquids and gases
• 1st waves to arrive
Seismic Waves
– S waves – (shear/ secondary waves) Body
waves that have reached the surface
• Cause particles to move side to side
• Do not travel through liquids
Seismic Waves
• Surface Waves: Third & Fourth waves
– Love waves & Rayleigh waves
• Travel along the surface & more slowly
• Cause the most damage
• Love: no vertical movement of the surface
• Rayleigh: both horizontal and vertical
movement
How to determine the distance to
the epicenter
• Need data from three seismographic
stations including the spread of S and P
waves and the time between the arrival of
the s and p waves.
• Forecasting
– Based on calculating the probability
– 2 factors
• History of area
• Rate at which strain builds up
Locating Earthquakes
http://www.uwgb.edu/dutchs/EarthSC-102VisualsIndex.HTM
Locating Earthquakes
http://www.uwgb.edu/dutchs/EarthSC-102VisualsIndex.HTM
Locating Earthquakes
http://www.uwgb.edu/dutchs/EarthSC-102VisualsIndex.HTM
Earthquake Hazards
• Factors that determine the severity of damage
1) Design of buildings
- Wooden structure = less damage
2) Ground shaking
- tall building – vibrations too rapid
-short building – vibrations too slow
- 5-15 stories collapsed
Earthquake Hazards
• 3) Soil Fracture
– Sloping area = landslides
– Liquefication = sand becomes liquidy due to
vibrations
– Soft materials amplify the motion
– Hard/resistant materials lessen the motion
Causes
• sinking, falling over, buildings
• Underground pipes & tanks rise to the surface
Earthquake Hazards
• 4) Tsunami: A huge wave generated by an
underwater earthquake or landslide
– Caused by vertical motion on the ocean floor
• Open ocean
– Height less than 1 m
– Speed 500-800 km/h
Earthquake Hazards
• Shallow water
– Height increases due to breakers/shallow
water
– Height may exceed 30 m
Tsunami Warning System
http://isu.indstate.edu/jspeer/Earth&Sky/EarthCh11.ppt