CHAPTER 12EARTHQUAKES
MOVEMENTS OF THE EARTH THAT ARECAUSED BY A SUDDEN RELEASE OFENERGY WHEN ROCKS MOVE ALONG AFAULT.
San Fransisco, CA (1906)
Anatomy of an Earthquake
•Focus - the location within Earth where the firstmotion occurs along a fault.
–90% of earthquakes have a shallow focus. (< 70 km deep)
–Any given shallow focus earthquake will cause lessdamage than most deep focus earthquakes
–Shallow focus quakes cause the most destruction overall
•Epicenter- the point on the earth’s surface directlyabove the focus.
Seismic Waves
•As rocks move, the rocks release energy in theform of vibrations called seismic waves.
•These waves travel outward in all directionsfrom the focus through the surrounding rock.
Two Main Categories of Waves
BODY WAVES
•Body waves travel through thebody of a medium.
•P Waves or primary waves orcompression waves
–Fastest; fastest waves (1st detected)
–Cause rock to move back and forthparallel to the direction the wavesare traveling
–Can move through solids, liquids, andgases.
–Travel faster through more rigidmaterials.
•S Waves or secondary waves orshear waves
–Second fastest waves
–Arrive after P waves
–Cause rock to move side to sideperpendicular to the direction inwhich the waves are traveling.
–Only move through solids.
SURFACE WAVES
•Surface waves travel alongthe surface of a body ratherthan through the middle.
•Form from motion along ashallow fault or from theconversion of energy whenP waves and S waves reachEarth’s surface.
•Slowest moving
•Cause the greatest damage
Seismic Waves and Earth’s Interior
•Seismic waves can be used to understandearth’s interior.
•The composition of the material throughwhich P and S waves travel affect the speedand direction of the waves.
•By studying the speed and direction of seismicwaves, scientists can learn more about themakeup and structure of Earth’s interior.
•Andrija Mohorovicic, 1909…discovered that thespeed of seismic waves increases abruptly at 30km beneath the surface of continents.
•Because the mantle is denser than the crust.
•Therefore, this marks the boundary between themantle and crust.
•The depth of this boundary varies from 10 kmunder the oceans to 30 km under the continents.
•Earth is composed of 3 composition layers:
–crust, mantle, core
•Earth is composed of 5 mechanical layers:
–Lithosphere, asthenosphere, mesosphere, outercore, inner core.
Shadow Zones
•Shadow zones are locations on earth’s surface whereno body waves (P and S) can be detected.
•Shadow zones exist because the materials that makeup the earth’s interior are not uniform in rigidity(some are liquid; some are solid; furthermore, someare rigid and some are “plastic”).
•A large S wave shadow zone would be foundcovering the side of earth opposite the earthquake.
•The S waves cannot pass through the liquid outercore.
Earthquakes and Plate Tectonics
•Earthquakes are a result of stresses in Earth’slithosphere.
•Most earthquakes occur at
–Convergent oceanic environments
•Convergent plate boundaries - colliding
–Divergent oceanic environments
•Divergent plate boundaries – spreading
–Continental environments
•Convergent, divergent, and transform boundaries
Most Earthquakes occur at PlateBoundaries
Earthquakes away from PlateBoundaries
•Not all earthquakes occur at active knownplate boundaries; sometimes ancient faultzones are buried and result in earthquakes.
•1811/1812…New Madrid, Missouri
–Felt as far away as S. Carolina
Studying Earthquakes
•Seismology
•Earthquakes are the best tool for investigatingEarth’s internal structure and dynamics.
•Seismometer/Seismograph – equipment thatcan detect and record the vibrations of anearthquake.
•Seismogram – the tracing of vibration intensityrecorded by a seismograph.
–P waves are recorded first, then S waves.
Locating an Earthquake
•Determining the epicenter of an earthquake
•Triangulation
–3 seismic stations are used
–P Waves travel faster than S Waves
–The arrival of P Waves and S Waves are recorded
–The longer the lag time between the P and SWaves, the farther away the earthquake.
Earthquake Measurement
•Measure the amount of energy released
•Measure the amount of damage caused
•Magnitude = the measure of the strength of theearthquake.
•Richter Scale measures magnitude, 20th century
•Moment Magnitude Scale, 21st century
–More accurate for large earthquakes
–Measures the strength based on the size of the area of thefault that moves, the average distance that the fault blocksmove, and the rigidity of the rocks in the fault zone.
–The larger the number, the stronger the earthquake.
–Largest recorded…9.5
–Magnitudes of less than 2.5 are usually not felt by people.
Intensity: Modified Mercalli Scale
•Before the development of magnitude scale, thesize of an earthquake was determined based onthe earthquake’s effects = intensity.
•Expresses intensity in roman numerals. From I toXII and provides a description of the effectscaused by the earthquakes.
•XII – highest intensity and total destruction.
•Depends on the earthquakes magnitude, thedistance between the epicenter and the affectedarea, the local geology, and the earthquake’sduration.
Modified Mercalli Intensity Scale
Expected Damages
•Most damage is caused by the collapse ofbuildings or falling objects and flying debris.
•Other damage is caused by landslides, fires,explosions (broken electric and gas lines), andfloodwater (collapsing dams).
•CAN YOU THINK OF ANY OTHER DAMAGE?
Tsunamis
•Caused by an earthquake that occurs on theocean floor.
•The shifting ocean floor creates a Tsunami(giant ocean wave) as energy is transferred.
Earthquake Warnings and Forecasts
•Scientists are trying to predict whereearthquakes might occur.
•They think that areas along faults that haveNOT shown movement (seismic gaps) mightbe likely areas for future quakes since thestress may be building in these areas.
Prediction of Earthquakes
•Foreshocks – little earthquakes that mayprecede the main event.
•May be a few seconds up to a few weeks priorto main event.
•Sensors may also detect slight tilting of theground and may be able to detect the stressand strain building along fault zones.
•Predicting earthquakes is mostly unreliable.