Slide © UBC-EOSC 2001 The Shaking Earth: human impact

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.


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”







Eosc115 lab exercises …- Using seismic signals to find location, magnitude, and energy.- Investigating effects of ground motion.


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


“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.


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


“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


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!


Optional - check the time is < 15mins

Earthquake magnitudeGraphical calculation of ML

D(km) , sec ML A(mm)Time between P and S arrivals




D(km) , sec ML A(mm)Largest amplitude




D(km) , sec ML A(mm)Combine

OR: use an equation:

ML = 2.76 logD - 2.48 + logA


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.


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


Intensity: five factors

1. Earthquake magnitude

2. Distance from epicentre

3. Ground type

4. Duration

5. Building type


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


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


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.


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


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


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.


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


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


Outline for “The Shaking Earth”







EOSC 115 lab exercises …- Using seismic signals to find location, magnitude, energy.- Investigating effects of ground motion.

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Slide © UBC-EOSC 2001 The Shaking Earth: human impact