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Published byJoella Brown Modified over 9 years ago
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Continuation of plate tectonics- convection in the asthenosphere is still the driving force of moving lithospheric plates.
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At plate boundaries where pressure builds up and eventually breaks rock Usually not in the middle of plates (1%) We call these Intraplate earthquakes Faults occur where boundaries meet and rocks “ pass their elastic limit ” Earthquakes often occur near these fault lines Three types of faults See Exploring Earth Website
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1. Normal faults Caused by TENSIONAL forces Forces PULL OR STRETCH rocks
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2. Reverse- - caused by COMPRESSIONAL forces - rocks are PUSHED toward each other
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3. Strike-slip fault - Caused by SHEAR forces - Rocks move PAST EACH OTHER with little up or down motion.
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For each type of fault: NAME DEMONSTRATE DESCRIBE how the rocks move at the fault line – away, toward, past FORCES - compressional, tensional or shear Type of BOUNDARY FOOTWALL/HANGING WALL
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Point where plate movement occurs and energy is originated is called the focus (can be miles below the surface). Point on Earth ’ s surface above the focus is called the epicenter
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Let’s see if we can figure out from our data first!!! Now let’s consult a textbook’s website and see what you think. Normal (divergent): shallow Strike-slip (transform): shallow Intraplate: shallow Reverse (convergent) C/C - shallow C/O - all depths (up to 700 km – 435 mi) Focus Depths increase farther from the coast Due to subduction
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All types of seismic waves (s,p and surface) are detected by seismographs and recorded on seismograms.
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An earthquake creates seismic waves that travel away from the epicenter of an earthquake. Remember, epicenter is the place on the Earth ’ s surface directly above where the earthquake occurred. Seismic waves are detected by an instrument called a seismograph, which our book calls a seismometer. Energy shown on a seismogram. Studied by seismologist!
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Three types of seismic waves: Primary waves (P waves) travel fastest ~6 km/sec in the upper crust Move particles back and forth in the same direction Cause little destruction Secondary waves (S-waves) travel slower, cause more damage ~3.5 km/sec in the upper crust Move particles back and forth at a ninety degree angle to wave motion Surface waves travel slowest, cause the most damage Move particles side-to-side and in a swaying motion Where does each wave type originate?
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The difference in arrival time between p-waves and s-waves can be timed to determine how far away from the seismograph station the earthquake occurred. With at least three stations reporting, we can pinpoint the earthquake ’ s location using TRIANGULATION (see next slide or page 312 for example).
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How far away is the epicenter of an earthquake if there is 5 minutes between wave arrival time? If an earthquake occurs 4700 miles away from a seismograph station: How long after the earthquake will the P-wave arrive? How long after the earthquake will the S-wave arrive? What will be the difference in arrival time?
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In three steps, seismologists can find the epicenter of an earthquake. 1. Find the time difference between P&S wave arrival 2. Use the time difference to find the distance from the seismograph station to the epicenter Gives a radius around the city 3. Compare with minimum three locations to find epicenter
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Richter Scale – 1935 Measures earthquake magnitude Determined by height of waves from seismogram Scale is infinite (depends on sensitivity of equipment) Largest recorded - 9.5 1960 Chile Earthquake Base-10 logarithm - √1000 ≈ 31.6 x Quantitative
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Mercalli Scale – late 1800’s - modified Measures earthquake intensity Determined by human observation and structural damage I – XII Qualitative
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Mercalli Scale What happens to Mercalli Intensity as distance from epicenter increases?
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Magnitude Shallow/deep Liquefaction Tsunami Time of day Shadow zone Landslides/mudslides/avalanches - terrain Structures (codes, poverty, expected?) Gas Pipes/Fires Warning – Pacific Tsunami Warning Center Duration (how long did shaking occur?) Government Aid
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By Permission Only Location, date and time Pictures Damage (cost), deaths, injuries Magnitude Difference in P/S waves in Punxsy Search USGS Top Ten for list of earthquakes. Also a list on p. 318. Other research Plates involved
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Occur far from plate boundaries Faults can still exist, even where two tectonic plates do not meet Example: New Madrid Fault
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Explain how Primary and Secondary waves are used to pinpoint the location of an earthquake ’ s epicenter Calculate the difference in earthquake magnitude using the Richter Scale
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