EarthquakesEarthquakes
EarthquakesEarthquakes
•When plates move very slowly andover a long period of time
shaking•Eventually a sudden shaking of thecrust occurs at a particular place:Earthquake
Earthquakes Earthquakes
•The majority ofearthquakes occur atthe boundaries oftectonic plates.
•Individual earthquakesalso occur where thereis a fault.
•A fault is a zone ofweakness in the crustalong which somemovement of rockoccurs.
Three Types of FaultsThree Types of Faults
1)Reverse – compression causes horizontal andvertical movement
2)Normal: Tension causes horizontal and verticalmovement
3)Strike-Slip – shear causes horizontal and verticalmovement
Earthquake TermsEarthquake Terms
•Focus: the area in the crust (below surface)where energy is released during anearthquake (where the earthquake begins)
–Shallow: 0-70 km
–Intermediate: 70-300 km
–Deep: over 300 km
•Epicenter: The point on the surface directlyabove the focus of an earthquake
ProcessProcess
•As tectonic plates move, frictioncauses the rocks at plateboundaries to stretch or compress.
•Like a stretched rubber band or acompressed spring, these rocks storeenergy called theElastic Rebound Theory.
•When the pressure builds up withinthe crust, the stored energy is suddenly released asthe rocks slip along the fault, and an earthquakeoccurs.
ProcessProcess
•More than ½ of all earthquakes occuralong the edges of plates
–Subduction zones
–Mid-ocean ridge
–Collision zones (continent-continentconvergence)
SeismologySeismology
•The study of earthquakes
•Scientists cannot predict the exact dateand time of an earthquake
•But… they can identify areas likely to havean earthquake in the next 10 years.
SeismologySeismology
•A seismographmeasures earthquakes,and seismologists useseismic waves to study
•The energy released byan earthquake travelsaway from the focus inwaves
•Magnitude: relative sizeof an earthquakedepends on amountenergy released
How Seismographs WorkHow Seismographs Work
http://www.uwgb.edu/dutchs/EarthSC-102VisualsIndex.HTM
the pendulum remainsfixed as the groundmoves beneath it
SeismologySeismology
•Seismographs show the kinds of waves,their amplitude, and the timing of thewaves
•Foreshocks are small tremors thatprecede an earthquake
•Aftershocks are small tremors that followafter an earthquake
ScalesScales
•Magnitude is expressed on the Richter Scale
–Study the waves to determine the magnitude andlocation 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 anearthquake with a magnitude of 7
–7 or higher indicates a major earthquake ( 10-felt allover the earth)
How are Earthquakes Measured?Richter ScaleHow are Earthquakes Measured?Richter Scale
ScalesScales
•Modified Mercalli Scale: use to rateintensity
–Roman numerals I to XII describes damagedone by the quake
–XII : total destruction
–An earthquake with one magnitude, damagecan vary depending on the location
How are Earthquakes Measured?Mercalli Intensity ScaleHow are Earthquakes Measured?Mercalli Intensity Scale
Click Link for Interactive Demohttp://elearning.niu.edu/simulations/images/S_portfolio/Mercalli/Mercalli_Scale.swf
Seismic WavesSeismic Waves
•The conversion of potential to kineticenergy results in seismic waves
•Travel through the ground 20 times fasterthan the speed of sound
•Seismic waves reach the ground at theepicenter
Seismic WavesSeismic Waves
•Body waves – originate from the focus ofthe earthquake
–P waves (primary waves) are compressionwaves 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 WavesSeismic Waves
–S waves – (shear/ secondary waves) Bodywaves that have reached the surface
•Cause particles to move side to side
•Do not travel through liquids
Comparing Seismic WavesComparing Seismic Waves
Seismic WavesSeismic 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 verticalmovement
How to determine the distance tothe epicenterHow to determine the distance tothe epicenter
•Need data from three seismographicstations including the spread of S and Pwaves and the time between the arrival ofthe s and p waves.
•Forecasting
–Based on calculating the probability
–2 factors
•History of area
•Rate at which strain builds up
Locating EarthquakesLocating Earthquakes
http://www.uwgb.edu/dutchs/EarthSC-102VisualsIndex.HTM
Locating EarthquakesLocating Earthquakes
http://www.uwgb.edu/dutchs/EarthSC-102VisualsIndex.HTM
Locating EarthquakesLocating Earthquakes
http://www.uwgb.edu/dutchs/EarthSC-102VisualsIndex.HTM
Earthquake HazardsEarthquake 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 HazardsEarthquake Hazards
•3) Soil Fracture
–Sloping area = landslides
–Liquefication = sand becomes liquidy due tovibrations
–Soft materials amplify the motion
–Hard/resistant materials lessen the motion
Causes
•sinking, falling over, buildings
•Underground pipes & tanks rise to the surface
Earthquake HazardsEarthquake Hazards
•4) Tsunami: A huge wave generated by anunderwater earthquake or landslide
–Caused by vertical motion on the ocean floor
•Open ocean
–Height less than 1 m
–Speed 500-800 km/h
Earthquake HazardsEarthquake Hazards
•Shallow water
–Height increases due to breakers/shallowwater
–Height may exceed 30 m
