Transcript Earthquakes

Earthquakes
Reading
• Smith Ch 6
Tectonic Hazards
• Tectonic: forces of ‘earth-building’
– Volcanoes
– Earthquakes
• Very much connected
– plate tectonics
– most earthquakes on plate boundaries, fault
lines driven by plate movements.
Earthquakes
• Sudden crustal movements
• Usually shallow 0-700 km deep
Earthquakes
• Slow build-up of strain
– driven by mantle convection?
• Sudden release as crust moves: earthquake
• Seismic waves radiate outwards
Earthquakes
• Most damaging earthquakes are shallow
– >40 km depth
San Fernando EQ 1971
• Richter M 6.4
• Depth: 13 km
Tangshan EQ 1976
• At shallow depth
• Killed 250,000 to 750,000
Earthquakes
• Hypocentre:
– underground centre of earthquake
• Epicentre
– the point on the surface above the hypocentre
Earthquake Magnitude
• Richter Scale
• Moment Magnitude
• Mercalli Scale
Richter Scale
• a.k.a. Local magnitude
• Energy of seismic waves 100 km from
source
• =logarithm of ground motion in
micrometres 100km from epicentre
• Scale is logarithmic
– M7 is 10x motion of M6
Richter Scale
• >M 5.5 for a major disaster
• Most Eqs are small
• Most EQ energy released in the Big Ones
Moment Magnitude
• Surface area of fault displaced
• <M6.5 use Richter
• >M6.5 use Moment magnitude
Kobe EQ 1995
• M6.9
• Strong shaking on soft soils, landfill
• Dense urban area nearby
– Older homes with heavy clay-tile roofs not
earthquake proof
Problems with these scales
• Richter & Moment Magnitude don’t
measure shaking and overall impact, just
seismic energy
• Can only be used if ground movement,
epicentre, fault area is known
• Useless for “historic” quakes
Modified Mercalli Scale
•
•
•
•
MM scale: intensity of damage
Roman numerals
MM: I = not felt at all
MM: XII= widespread destruction
Seismic Waves
• Vertical stress, short wave period
– ‘P’ or primary waves
– ‘S’ or secondary waves
– Moment Magnitude measures this
• Horizontal stress, long wave period
– Love Waves
– Rayleigh waves
– Richter scale /local magnitude measures this
P Waves
• compression-dilation
• 8km/sec in solid & liquid
S Waves
• Sine waves
• 4km/sec in solid only
• can be destructive
Love & Rayleigh Waves
• Horizontal shaking
• at 90 degrees to wavefront
• Most damaging to buildings
Other issues
• Ground acceleration
• Wave frequency
Mexico City EQ 1985
• High rise buildings collapse especially if in
22-25 storey range
• Taller and shorter buildings less damaged
• EQ has just the right resonance to tear
certain buildings apart
Small buildings
• Resonant to short wave periods
• P&S waves knock them down
Tall buildings
• Resonant to long wave periods
• Love & Rayleigh waves knock them down
Effects of Local Topography
• Soft bouncy sediments amplify wave effects
– ash
– alluvium
– landfill
• Mexico City EQ 1985, San Salvador EQ
1986: cities on bouncy sediments
Secondary Effects
• Soil failure
• Slope failure
• Tsunamis
Soil Liquifaction
• Wet soil can liquify
• Loses load-bearing properties
• Richmond BC:
– If Richter M6.5 EQ, high-rises may fall like
dominoes in Fraser delta sediments
Lateral Spread
• On shallow slope soil moves sideways
Ground Oscillation
• Soil blocks rotate and surface crumples
Loss of Bearing Strength
• Buildings sink into soil
Flow failure
• On steeper slopes, slope collapses
• Can cause damage over extensive area, long
distance
Landslides, Rock & Snow
Avalanches
• EQ triggers slope failure
Huascaran Mountains, Peru 1970
•
•
•
•
•
30m high wave of rock & mud
Travelled 70-100 m/sec
buried Yungay & Ranrahirca
Debris 10 m deep,
Killed 18,000 in 4 minutes
Tsunami
• a.k.a. “tidal waves”
• Large shallow submarine EQ triggers
shockwave in sea
• Difficult to spot out at sea
• Moves fast
Tsunami
• Wave height & speed increase as wave
moves into shallow coastal water
• Begins with water drawdown
• Then violent oscillation for hours
Human Vulnerability
• Many EQ zones well-known but densely
inhabited
• Populations often in coastal zone, valley
lands
– vulnerable to Tsunamis, debris flows
– cities built on wobbly sediments
Human Vulnerability
• Increased by poor construction techniques
– not designed for horizontal shear
– failure to avoid vulnerable sites, soil conditions
– failure to avoid vulnerable zones
Maharashtra 1993
Timing
• Deathtoll and damage rises if timing is
unfortunate
– 1923 Tokyo Bay EQ struck at lunchtime, starts
fires, 130,000 die
– 1976 Tangshan EQ struck at night, people
trapped indoors
Trigger Earthquakes
• by crustal loading
– build a large dam
– storm surge in cyclone conditions
• 1923 Tokyo Bay EQ
Prediction
• Easier to predict where an EQ will happen
than when
– spotting “locked” areas surrounded by zones of
movement
• Forecasting not advanced
– West: relies on science
• Failed to spot Kobe 1995, Northridge 1994
– China: relies on culture (more accurate)
• Failed to spot Tangshan 1976