e arthquake w aves. s eismic w aves seismic waves - waves generated by an earthquake focus – point...
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EARTHQUAKE WAVES
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SEISMIC WAVES
Seismic waves - Waves generated by an earthquake
Focus – point in earth where EQ takes place
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TYPES OF EQ WAVES
P wave – primary - push/pull -
compression -back and
forth motion - fastest
wave - travel
through solid, liquid, gas
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S waves – secondary - shear
waves - waves
travel at rt angles to wave
- second fastest
- only travel through solids
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Surface waves – cause most of the damage of EQ.
-rolling waves like the ocean waves
-slowest -
produced when seismic waves reach the surface
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Epicenter – location where EQ occurs on the earth’s surface
- directly above the focus pt.
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MEASURING SEISMIC WAVES
Seismograph – instrument to measure seismic waves and arrival times of each
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http://www.wwnorton.com/college/geo/egeo/flash/8_3.swf
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SEISMOGRAM
•
seismogram
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SEISMOGRAPH STATIONS
Each type wave reaches the station at a different time
3 or more stations needed to pin point the epicenter of an EQ
Intersection of the circles is epicenter
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EQ P and S travel time graph Used to find
distances from epicenters
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ANCIENT CHINESE SEISMOGRAPH
Zhng Heng's seismograph was an urn-like instrument with a central pendulum. An earth tremor would cause the pendulum to loose balance and activate a set of levers inside. Then, one of the eight dragons placed in eight directions outside the urn would release a bronze ball held in its mouth.
The ball would fall into the mouth of a toad and give off a sound in the meantime, letting people know when and in which direction an earthquake had occurred.
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HOW WAVES TRAVEL THROUGH EARTH
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SHADOW ZONE
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Four seismograph stations receive data from the same earthquake. The table below shows the differences in travel times for the P- and S-waves recorded at each station. Which station is the closest to the epicenter of the earthquake?
StationTravel-Time Difference
A 4 min 32 sec
B 3 min 52 sec
C 3 min 10 sec
D 4 min 17 sec
1.A 2.B 3.C 4.D
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Seismograph station D is 7,700 kilometers from the epicenter. If the P-wave arrived at this station at 2:15 p.m., at approximately what time did the earthquake occur?
1.1:56 p.m. 2.2:00 p.m. 3.2:04 p.m. 4.2:08 p.m.
Seismograph station B recorded the arrival of P-waves at 2:10 p.m. and the arrival of S-waves at 2:15 p.m. Approximately how far is station B from the earthquake epicenter?
1.1400 km 2.2400 km 3.3400 km 4.4400 km
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DIFFERENCE IN P & S ARRIVAL TIMES
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LOCATING EPICENTER