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San Francisco, April 18, 1906 San Francisco City Hall after the 1906 EQ

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Objectives for this Lab: Understand how to interpret seismograms and record data Use seismic data and determine magnitude an earthquake Use seismic data to determine the location of earthquake epicenters Objectives for this Lab: Understand how to interpret seismograms and record data Use seismic data and determine magnitude an earthquake Use seismic data to determine the location of earthquake epicenters

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Consider a rock that is dropped in calm water. What happens? Earthquake – vibrations within the earth that produce rapid release of energy produce rapid release of energy in the form of seismic waves in the form of seismic waves

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Good Vibrations – released seismic wavesGood Vibrations – released seismic waves Three major seismic waves are released: The P-waveThe P-wave The S-waveThe S-wave The L-wave (surface wave)The L-wave (surface wave) Three major seismic waves are released: The P-waveThe P-wave The S-waveThe S-wave The L-wave (surface wave)The L-wave (surface wave) What measures the incoming seismic waves? The SeismographThe Seismograph What measures the incoming seismic waves? The SeismographThe Seismograph The inertia of the suspended mass keeps the seismograph motionless while the seismic waves vibrate the recording drum anchored to the bedrock – produces a seismogram The inertia of the suspended mass keeps the seismograph motionless while the seismic waves vibrate the recording drum anchored to the bedrock – produces a seismogram The Seismograph

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P-wave (primary wave) the fastest compressional wave moves through solids and liquids P-wave (primary wave) the fastest compressional wave moves through solids and liquids S-wave (secondary wave) 2/3 speed of P-wave shearing wave moves through solids only S-wave (secondary wave) 2/3 speed of P-wave shearing wave moves through solids only Love wave (surface wave) slowest waves side-to-side (surface) We feel these waves Love wave (surface wave) slowest waves side-to-side (surface) We feel these waves Rayleigh wave (surface wave) slowest wave rolling wave like floating on a boat type motion Rayleigh wave (surface wave) slowest wave rolling wave like floating on a boat type motion

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Whats the difference between an 8 magnitude and a 6.5 magnitude? Did you feel the earthquake? – Measuring an EQ Two ways to measure EQs: Mercalli Intensity Scale Richter Scale

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Modified Mercalli Intensity Scale measures the intensity of the EQ – how much measures the intensity of the EQ – how much damage occurs damage occurs closer to the epicenter-- more damagecloser to the epicenter-- more damage farther from the epicenter -- less damagefarther from the epicenter -- less damage people report the damage people report the damage destruction is assigned a Roman Numeral (I – XII) destruction is assigned a Roman Numeral (I – XII) I – least intensityI – least intensity XII – greatest intensityXII – greatest intensity Less intense More intense 23

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Richter Scale (Charles Richter – 1935) measures the intensity of an EQ using a seismogram measures the intensity of an EQ using a seismogram maximum intensity = largest amplitude recorded maximum intensity = largest amplitude recorded EQ intensity is based on the logarithmic scale. EQ intensity is based on the logarithmic scale. each step (M1…M2…M3..) increases the amplitude X10 each step (M1…M2…M3..) increases the amplitude X10 M5 has an amplitude 10 times larger than an M4 M5 has an amplitude 10 times larger than an M4 M5 has an amplitude 100 times larger than an M3 M5 has an amplitude 100 times larger than an M3 What about the energy released? What about the energy released? Each increase in Richter amplitude (by 10) increases energy 32 times so,Each increase in Richter amplitude (by 10) increases energy 32 times so, An M6 releases 32 more times energy than an M5.An M6 releases 32 more times energy than an M5. An M6 releases _____ more times energy than an M4.An M6 releases _____ more times energy than an M4. An M6 releases _____ more times energy than an M3.An M6 releases _____ more times energy than an M3. An M6 releases ________ more times energy than an M2.An M6 releases ________ more times energy than an M2.32,76832,768 1,0241,024 1,048,5761,048,576 What observations can you make about the increasing energy release from one magnitude to the next? largest amplitude

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Interpretation of a seismogram S-P time interval P-wave S-wave S-P interval (S-wave subtract P-wave) = in seconds Amplitude P-wave S-wave P-wave S-wave The height of the wave (mm) The height of the wave (mm)

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Calculate the S-P interval: P-wave arrivalS-wave arrival 21hr 32m 40sec21hr 33m 10sec 15hr23m19sec15hr24m5sec 3 hr16m32sec3 hr17m13sec 19hr43m12sec19hr43m52sec 20hr58m42sec20hr 59m16sec 21hr 32m 40sec21hr 33m 10sec 15hr23m19sec15hr24m5sec 3 hr16m32sec3 hr17m13sec 19hr43m12sec19hr43m52sec 20hr58m42sec20hr 59m16sec 30 sec 46 sec 41 sec 40 sec 34 sec S-P interval

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Calculating the Richter Magnitude P S magnitude 220 km distance

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SAF 8M Seismograph Stations A B C D S-P What is the magnitude at each station? How would the Mercalli scale relate to each station? What is the magnitude at each station? How would the Mercalli scale relate to each station?

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Interpreting magnitude at various seismic stations A B C D Although, there are different size triangles, the ratio of (2) remains the same. Although, there are different size triangles, the ratio of (2) remains the same. Although, there are different S-P interval seismograms, the magnitude remains the same Although, there are different S-P interval seismograms, the magnitude remains the same Analogy How does magnitude relate to the Mercalli scale?? More damage Less damage closest

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Triangulation – locating the epicenter of an EQ Epicenter SS-A SS-B SS-C SS = seismic station D = distance to epicenter SS = seismic station D = distance to epicenter D D D D D D

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Finding the epicenter using triangulation intersection of three circles = epicenter location Finding the epicenter using triangulation intersection of three circles = epicenter location the radius of circle = distance to epicenter from a single earthquake the radius of circle = distance to epicenter from a single earthquake S-P interval (min, sec) S-P interval (min, sec) Distance (km, mi) use the S-P interval vs. Distance chart to find the distance to the epicenter from a single EQ use the S-P interval vs. Distance chart to find the distance to the epicenter from a single EQ

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