san francisco, april 18, 1906 san francisco city hall after the 1906 eq
TRANSCRIPT
San Francisco, April 18, 1906San Francisco, April 18, 1906
San Francisco City Hall after the 1906 EQSan Francisco City Hall after the 1906 EQ
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
Consider a rock that is dropped in calm Consider a rock that is dropped in calm water. What happens?water. What happens?Consider a rock that is dropped in calm Consider a rock that is dropped in calm water. What happens?water. What happens?
EarthquakeEarthquake – – vibrationsvibrations within the earth that within the earth that produce rapid release of energyproduce rapid release of energy in the form of in the form of seismic wavesseismic waves
““Good Vibrations” – released seismic wavesGood Vibrations” – released seismic waves““Good Vibrations” – released seismic wavesGood Vibrations” – released seismic wavesThree major seismic waves are released: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: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?What measures the incoming seismic waves?•The SeismographThe Seismograph
What measures the incoming seismic waves?What measures the incoming seismic waves?•The SeismographThe Seismograph
The inertia of the suspended mass keeps the seismographmotionless while the seismicwaves vibrate the recording drum anchored to thebedrock – produces a seismogram
The inertia of the suspended mass keeps the seismographmotionless while the seismicwaves vibrate the recording drum anchored to thebedrock – produces a seismogram
The SeismographThe SeismographThe SeismographThe Seismograph
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
What’s the difference between an 8 magnitude and What’s the difference between an 8 magnitude and a 6.5 magnitude?a 6.5 magnitude?
Did you feel the earthquake? – Measuring an EQDid you feel the earthquake? – Measuring an EQ
Two ways to measure EQ’s:Two ways to measure EQ’s:
MercalliMercalli Intensity ScaleIntensity Scale
Richter Scale Richter Scale
Modified Modified Mercalli Intensity ScaleMercalli 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 damage farther from the epicenter -- less damage
• people report the damagepeople 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 intenseLess intenseLess intenseLess intense
More intenseMore intenseMore intenseMore intense2323
Richter ScaleRichter Scale (Charles Richter – 1935) (Charles Richter – 1935)• measures the intensity of an EQ using a measures the intensity of an EQ using a seismogramseismogram
• maximum intensity = largest amplitude maximum intensity = largest amplitude recorded recorded
• EQ intensity is based on the logarithmic scale. EQ intensity is based on the logarithmic scale.
• each step each step (M1…M2…M3..)(M1…M2…M3..) increases the increases the amplitude X10 amplitude X10 • M5 has an amplitude 10 times larger than an M5 has an amplitude 10 times larger than an M4 M4 • M5 has an amplitude 100 times larger than M5 has an amplitude 100 times larger than an M3 an M3
• What about the energy released?What about the energy released?•Each increase in Richter amplitude (by 10) increases Each increase in Richter amplitude (by 10) increases energy energy 32 times 32 times so,so,
•An M6 releases An M6 releases 3232 more times energy than an M5. more times energy than an M5.
•An M6 releases _____ more times energy than an An M6 releases _____ more times energy than an M4.M4.
•An M6 releases _____ more times energy than an An M6 releases _____ more times energy than an M3.M3.
•An M6 releases ________ more times energy than an An M6 releases ________ more times energy than an M2.M2.
32,76832,76832,76832,768
1,0241,0241,0241,024
1,048,5761,048,5761,048,5761,048,576
What observations can you make about the increasing energyWhat observations can you make about the increasing energy
release from one magnitude to the next?release from one magnitude to the next?
largest amplitudelargest amplitudelargest amplitudelargest amplitude
Interpretation of a seismogram S-P time intervalInterpretation of a seismogram S-P time interval
P-waveP-wave S-waveS-wave
S-P interval (S-wave subtract P-wave) = in secondsS-P interval (S-wave subtract P-wave) = in seconds
Amplitude
P-waveP-wave S-waveS-wave
P-waveP-wave S-waveS-wave
The heightof the wave
(mm)
The heightof the wave
(mm)
Calculate the S-P interval:Calculate the S-P interval:
P-wave arrival S-wave arrivalP-wave arrival S-wave arrival
21hr 32m 40sec 21hr 33m 10sec
15hr 23m 19sec 15hr 24m 5sec
3 hr 16m 32sec 3 hr 17m 13sec
19hr 43m 12sec 19hr 43m 52sec
20hr 58m 42sec 20hr 59m 16sec
21hr 32m 40sec 21hr 33m 10sec
15hr 23m 19sec 15hr 24m 5sec
3 hr 16m 32sec 3 hr 17m 13sec
19hr 43m 12sec 19hr 43m 52sec
20hr 58m 42sec 20hr 59m 16sec
30 sec30 sec
46 sec46 sec
41 sec41 sec
40 sec40 sec
34 sec34 sec
S-P intervalS-P interval
Calculating the RichterMagnitude P S
5 20
5 magnitude220 km distance
SAF
8M
Seismograph Stations
A
B
C
D
S-P
S-P
S-P
S-P
What is the magnitude at each station?What is the magnitude at each station?
How would the Mercalli scale relate to How would the Mercalli scale relate to each station?each station?
What is the magnitude at each station?What is the magnitude at each station?
How would the Mercalli scale relate to How would the Mercalli scale relate to each station?each station?
Interpreting magnitude at various seismic stations Interpreting magnitude at various seismic stations
A
B
C
D 3
4
5
6
8
10
12
16
20
24
32
40
2
2
2
2
Although, there are differentsize triangles, the ratio of(2) remains the same.
Although, there are differentsize triangles, the ratio of(2) remains the same.
Although, there are differentS-P interval seismograms, the magnitude remains the same
Although, there are differentS-P interval seismograms, the magnitude remains the same
AnalogyAnalogy
How does magnitude relateHow does magnitude relateto the Mercalli scale??to the Mercalli scale??
More damageMore damage
Less damageLess damage
closest
Triangulation – locating the epicenter of an EQTriangulation – locating the epicenter of an EQ
Epicenter
SS-ASS-A
SS-BSS-B
SS-CSS-C
SS = seismic stationD = distance to epicenterSS = seismic stationD = distance to epicenter
DDDD
DD
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)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
