chapter 10 objectives 1. discuss the elastic rebound theory. 2. explain why earthquakes generally...
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•Chapter 10 Objectives1. Discuss the elastic rebound theory.2. Explain why earthquakes generally occur at plate boundaries.3. Compare the 3 types of seismic waves.4. Discuss the method scientists use to pinpoint an Earthquake (EQ).5. Discuss the method most commonly used to measure the magnitude of EQs.
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Earthquakes & Plate Tectonics
•Earthquakes (EQs): vibrations of the Earth’s crust– Occur when rocks
under stress suddenly shift
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•Elastic Rebound Theory: the theory geologists use to explain many EQs– Rocks on each side of fault are moving– If fault is locked, stress increases– At certain point rocks fracture & rebound
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•Focus: the area along a fault where slippage 1st occurs.–Depths vary
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•Epicenter: the point on the Earth’s surface directly above the focus.
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•Aftershocks: series of smaller tremors.–Caused by released energy (seismic waves)•Increases stress in other rocks along the fault
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•Focus depths vary–Shallow focus - w/in 70 km of surface•Tend to cause the most damage
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•Focus depths vary– Intermediate focus - 70 to 300 km
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•Focus depths vary–Deep focus (occur in subduction zone)•300 to 650 km
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•Focus depths vary–About 90% of Continental EQs are shallow focus
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3 Major EQ Zones•Links btwn EQs & Plate Tectonics
– Plate movements = stress buildup
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•1. Pacific Ring of Fire: ring of tectonic/volcanic activity surrounding the pacific.
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•2. Mid-ocean ridges: spreading motion = stress
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•3. Eurasian-Melanesian mountain belt: Eurasian plate colliding w/African & Indian plates = stress
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•Fault Zones: are groups of interconnected faults.– Form at plate
boundaries b/c of intense stress•e.g. San Andreas
fault
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•Not all EQs occur at plate boundaries!– e.g. New Madrid, Missouri, 1812
•Ancient fault? (600 million yrs ago)
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3 major Types of Seismic Waves
•P waves: primary wave.– Move fastest– 1st to be recorded– Travels through solids & liquids
•More rigid matl. = faster
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3 major Types of Seismic Waves
•S waves: secondary waves.– 2nd waves to be recorded– Only travel through solids
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3 major Types of Seismic Waves
•L waves: surface waves, also called long waves.– Converted from P & S waves when they
reach the surface– Slowest-moving waves
•Last to be recorded
– Cause surface to rise & fall
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Recording EQs•Seismograph: instrument used to
detect and record seismic waves.– 3 separate devices
•1 records vert. motion•2 record horz. motion
– One N to S– One E to W
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•Seismogram: seismograph’s record of waves; has wavy lines recorded on paper.
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Locating an EQ•Scientists compare time diff. btwn
P & S waves.– Plot on graph to get distance to epicenter
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Locating an EQ•Need 3 plots to locate epicenter
– Point where all 3 circles meet is location of epicenter
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Blue primary waves followed by red secondary waves move outward in concentric circles from the epicenter of an earthquake off British Columbia and Washington State.
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EQ Measurement•Richter Scale: used to express
magnitude of an EQ.– Measure of energy released
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EQ Measurement•Richter Scale: used to express
magnitude of an EQ.– Measure of energy released– Major EQ = 7 or above
•Causes widespread damage
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EQ Measurement•Richter Scale: used to express
magnitude of an EQ.– Measure of energy released– Moderate EQ = 6 to 7
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EQ Measurement•Richter Scale: used to express
magnitude of an EQ.– Measure of energy released– Minor EQ = 2.5 to 6
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EQ Measurement•Richter Scale: used to express
magnitude of an EQ.– Measure of energy released– Microquakes = < 2.5
•Not felt by people
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EQ Measurement•Mercalli Scale: expresses the
intensity of an EQ.
•Intensity: the amount of damage the EQ causes.– Roman numerals from I to XII
Seismic Monitor
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EARTHQUAKE DAMAGEMovement of the ground itself seldom causes many deaths or injuries. Most injuries result from the collapse of the buildings, flying glass and falling objects.
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EQs and under water landslides can cause huge ocean waves called tsunamis. Tsunamis can cause massive death and damages.
EARTHQUAKE DAMAGE