slide © ubc-eosc 2001 the shaking earth: human impact
TRANSCRIPT
slide © UBC-EOSC 2001
The Shaking Earth: human impact
slide © UBC-EOSC 2001Outline for “The Shaking Earth”
We investigate five aspects of earthquake science:• Effects and global distribution
– Local context, global distribution, effects, plate tectonics.
• Earthquake sources– Forces, deforming rocks, faults, energy, tectonics.
• Seismic waves– Ground motion, wave types, propagation, using seismic waves.
• Human impact and engineering– What kills?, magnitude vs intensity, buildings, soils.
• Mitigation and Prediction– Minimizing effects, predicting what?
Eosc115 lab exercises …- Using seismic signals to find location, magnitude, and energy.- Investigating effects of ground motion.
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Analyzing seismic signals:
Waves from the earthquake source (Northridge CA, 1994, M=6.7)
Quiet - waves have not yet arrived
P-waves arrive first
S-waves arrive later Slow large surface waves,mixed with remaining P, S waves
Time in seconds
ts - tp = 7 seconds
Vp ~ 6.7km/s, Vs ~ 3.8km/s
slide © UBC-EOSC 2001Converting seismic wave time into distance
In eosc115 lab exercise
order
slide © UBC-EOSC 2001Finding earthquake’s source location
ts - tp = 7 seconds
Vp ~ 6.7km/s, Vs ~ 3.8km/s
P-waves S-waves
Procedure:
A. Convert P-S time to distance
B. Draw circle at distance.
C. Do it at 3 or more places.
Intersection is the epicentre.
slide © UBC-EOSC 2001Outline for “The Shaking Earth”
We investigate five aspects of earthquake science:• Effects and global distribution
– Local context, global distribution, effects, plate tectonics.
• Earthquake sources– Forces, deforming rocks, faults, energy, tectonics.
• Seismic waves– Ground motion, wave types, propagation, using seismic waves.
• Human impact and engineering– What kills?, magnitude vs intensity, buildings, soils.
• Mitigation and Prediction– Minimizing effects, predicting what?
Eosc115 lab exercises …- Using seismic signals to find location, magnitude, and energy.- Investigating effects of ground motion.
slide © UBC-EOSC 2001
Outline of Human Impact:
• Earthquakes don’t kill …
• Causing damage:
– Acceleration, duration
– Factors affecting ground motion
– Building characteristics
– Soil liquefaction
NB: Readings & lectures are complementary - some unique coverage in both.
Indicates slide that differs fromcustom course materials
slide © UBC-EOSC 2001
“Earthquakes don’t kill” …
• If buildings and structures never fell, earthquakes would not be considered disasters. (Abbot’s theme for chapter 2)
Alaska: Exposed sea floor … 33 ft of uplift!
• Alaska 1964; Mw=9.2 (second largest ever recorded) (pg 99)– 131 killed, $ ??
Kobe: Destruction in urban areas
• Kobe 1995; Mw=6.9 (Abbott 4th ed., pg 117)– 6,425 killed, $2,716 Millions
slide © UBC-EOSC 2001Earthquakes in the wilderness: Alaska
M 7.9, Nov3, 2002
- Catastrophe?
- People involved?
- Infrastructure?
??
slide © UBC-EOSC 2001Breaking buildings
• How to break it? Apply a force.– F = ma. Force on a building is proportional to acceleration. – Passing waves cause ground to accelerate then decelerate.
• Vertical acceleration? 9.8m/s2 = 1.0g. – Buildings ARE good at withstanding 1.0 ± 1g.
• Horizontal acceleration? 0.0m/s2 = 0.0g. (In a train or boat aX = 0.1g or 0.2g)
– Buildings are NOT good at withstanding 0.0 ± 1g.
at UBC: Education bldg, and EOS-East reinforcing project.
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What type of seismic energy will cause damage?
Which direction must ground move to damage buildings?
? ?
??
Recall the three wave types …
Which is most likely to cause damaging ground motion?(Consider direction and amount of ground motion.)
slide © UBC-EOSC 2001Intensity: violence of shaking and damage
QuickTime™ and aCinepak decompressorare needed to see this picture.
powerful shaking?
• Five important factors (Abbott 4th ed., pg 80).
1. Earthquake magnitude
2. Ground type
3. Distance from epicentre
4. Duration of shaking
5. Building characteristics
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“Size” of an earthquake’s source
• Distinguish between magnitude and intensity– magnitude indicates how much energy was released.– intensity is how strong the ground motion is at the felt location.
• Consider a light bulb …
Fixed magnitude Local intensity Local intensity
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Earthquake magnitude
• Estimated indirectly since we can never monitor the focus.
• Use seismic wave amplitude, distance and instrument.
• (Eosc115 - details in the earthquake web-based lab exercise.)• Richter Magnitude, ML
– Based upon largest amplitude
(surface waves)
– Strictly “correct” only for
Southern California but still
widely used.
Logarithmic scales are needed.
Energy of an M = 5 quake is 43 times M = 6.Abbott 4th ed., pgs 75-77
and only correct for “Wood-Anderson” seismograph
since only high frequencies are recorded
Magnitude estimates for large quakes are too low!
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Optional - check the time is < 15mins
Earthquake magnitudeGraphical calculation of ML
D(km) , sec ML A(mm)Time between P and S arrivals
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Optional - check the time is < 15mins
Earthquake magnitudeGraphical calculation of ML
D(km) , sec ML A(mm)Largest amplitude
slide © UBC-EOSC 2001
Optional - check the time is < 15mins
Earthquake magnitudeGraphical calculation of ML
D(km) , sec ML A(mm)Combine
OR: use an equation:
ML = 2.76 logD - 2.48 + logA
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Magnitude vs Energy
• Several forms of magnitude … – ML Richter magnitude
– Mp (based upon P-waves)
– Ms (based upon S-waves)
– Mw (based upon energy at focus)
• Recall earthquakes lecture 2.
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Does energy affect extent of catastrophy?
• Yes! Alaska, 1964– 1964; Mw=9.2 was the 2nd largest ever recorded!
• But smaller earthquakes also may be devastating!– Iran, 2003; Mw=6.6
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Intensity: five factors
1. Earthquake magnitude
2. Distance from epicentre
3. Ground type
4. Duration
5. Building type
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Intensity factor: Distance from epicentre
• Here is the felt zone map for Nisqually
• Abbott discusses– Loma Prieta – Mexico city
• Why so
complicated?
Variations in
ground.
Strong to very strong shaking.Acceleration = 10 - 30%g
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Intensity factor: Distance from epicentre
Felt zones for 4 earthquakes in SW BC.
slide © UBC-EOSC 2001Intensity factor: Ground type
• Harder rocks– no amplification– a mixture of
frequencies(Abbot 4th ed., Fig 4.29)
• Softer rocks
– shaking is amplified
– low-frequencies may
reverberate in basins, plus soft
rocks absorb high frequencies
slide © UBC-EOSC 2001Ground types in Vancouver
• Harder rocks (bedrock - North Vancouver)– no amplification
– all (high and low) frequencies present
• Softer rocks (sediments - Richmond)– much amplification
– loss of high frequency wave energy
– reverberating low frequency waves
Tidal mud, Richmond
Photos by Clague, GeoMap Vancouver, http://sts.gsc.nrcan.gc.ca/page1/urban/geomap/units.htm
Queen Elizabeth Park (basalt quarry)
Which area might suffer more in an earthquake?
slide © UBC-EOSC 2001Strong Motion Seismographs
• These instruments record motions that will cause damage.• Many are in place around BC’s Lower Mainland.
slide © UBC-EOSC 2001Intensity factor: Building characteristics
• Buildings will resonate at frequencies that depend upon construction type, height and mass.
• demo with “card” buildings as per lab exercise.
Resonance depends on height and mass.
Motion can be “damped”.
QuickTime™ and aCinepak decompressorare needed to see this picture.
slide © UBC-EOSC 2001Effects of building type at Kobe, 1995
Buildings in central Kobe (Chuo Ward).
Foreground: The complete collapse of a two- or three-story traditional Japanese wood-frame building with a heavy tile roof.
Background: A six- or seven-story office building of 1960s' or 1970s' vintage. This reinforced concrete building is a typical example of a mid-height story collapse.
Left: The high rise is post-1981 office building that has no apparent damage. Ground settlement in the vicinity of these buildings was between 30 and 60 centimeters.
The January 17, 1995 Kobe Earthquake. An EQE Summary Report, April 1995 at http://www.eqe.com/publications/kobe/building.htm
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Interactions between two buildings
http://www.geo.arizona.edu/K-12/azpepp/education/history/mexico/damage.html
Mexico City, 1985. See also Abbott 4th ed., pg 102
Adjacent buildings may behave differently and damage each other.
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Summary so far …
• Next: Other factors affecting costs.
• Do Earthquakes kill?
• What type of motion breaks buildings?
• List 3 factors affecting ground motion.
• Is FELT motion related simply to distance?
• Which rock type supports higher frequencies? Which is less elastic (attenuates signals faster)?
• Building resonance - what are two factors?
No … no buildings => no disaster
Side to side accelerations
Hard rocks.Soft rocks.
height and mass
1. Earthquake magnitude2. Distance from epicentre3. Local ground characteristics
No
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Other contributors to cost in $$ and lives
Fire ! (Kobe, Japan, 1995) (See Abbott, colour plates)
Loss of services: Water; Transport; Communications.
Loma Prieta, CA, 1989
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More contributors to cost in $$ and lives
(http://www.ce.washington.edu/~liquefaction/html/main.html)• Soil liquefaction …
Niigata, Japan, 1964
Lower Van Norman Dam
(Abbott 4th ed. Pgs 114)
QuickTime™ and aYUV420 codec decompressorare needed to see this picture.
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Soil liquefaction
• Soil grains may be loosely packed.• When saturated, ground motion may increase pore
pressure.• Soil loses strength and flows like a liquid. (More in
eosc115.)
Where are … Recent sediments - Older, harder, glacial deposits - Solid rock ??
in the Fraser Valley
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Outline of “Human Impact”:
• Earthquakes don’t kill …
• Causing damage: intensity
– violent shaking (acceleration)
– long-duration shaking
– buildings prone to collapse
• Soil liquifaction
Next class … • Mitigation - minimizing the impact• Prediction
magnitude, distance, and soil type
slide © UBC-EOSC 2001
Outline for “The Shaking Earth”
We investigate five aspects of earthquake science:• Effects and global distribution
– Local context, global distribution, effects, plate tectonics.
• Earthquake sources– Forces, deforming rocks, faults, energy, tectonics.
• Seismic waves– Ground motion, wave types, propagation, using seismic waves.
• Human impact and engineering– What kills?, magnitude vs intensity, buildings, soils.
• Mitigation and Prediction– Minimizing effects, predicting what?
EOSC 115 lab exercises …- Using seismic signals to find location, magnitude, energy.- Investigating effects of ground motion.