Earthquakes
EQ Scales
Plate Margins
Earthquakes
Seismic Waves
EQ Hazards
Kobe EQ
Predicting EQ
EQ Enigneering
Pressures that cause EQ are generated by movements of tectonic plates. Common along boundaries where plates are created and where they collide. Strong relationship between distribution of EQ and plate boundaries
Constructive
Oceanic crust= thin
EQ= Shallow focus
Occur in response to- (1) Slumping of crust along normal faults (2) Splitting of rock due to magma injection (3) strike slip movement along transform fault
Destructive/ Convergent
Not powerful (4 or 5 on Richter scale)
Magnitude 8 recorded along transform faults
Occur along subduction zone
Occur along upper margin of subducting plate
Triggered by stick/ slip motion of crust
Inclined pattern of EQ foci (0-300km depth) Arrangement= Benioff Zone
Co-Co Convergent
Occur at great depths, plate remains cold & brittle within deep mantle
Along thrust faults in Co crust (<7)
Conservative
Caused by strike/slip motion
Classic example= San Andreas Fault
Energy released generates seismic waves= radiate from focus
Transmitted as Body Waves (deep) and Surface Waves (surface)
Body Waves
P waves
S waves
Longitudinal/ compression, compress and expand rock, fastest, travel through all states, slow down in more compressible
Transverse/ shear, slower, cannot move through liquid
Surface Waves
Rayleigh
Love
Shakes ground side to side
Rolling sea wave like, Moves ground up and down
Attempted to measure EQs in 2 ways- (1) Magnitude (energy released), (2) Intensity (damage caused)
Magnitude
Measured in Joules
Magnitude 9= 7.9 x10^17 Joules
Richter Scale
1935 Charles Richter devised the scale
Each magnitude defined by equivalent energy released by TNT explosions (Mag 1= 30 pounds, Mag 2= 1 ton, Mag 3= 29 tons etc)
His scale couldn't be applied consistently worldwide
Designed using only 1 type of seismograph (measuring EQs in California at distance 600km
In Richter's scale Q= 2.9 (value varies on type of seismograph and local effects)
1979= new scale proposed= Moment Magnitude Scale
Moment Magnitude scale
More reliable
Derived from factors easily measured (Fault rupture, Displacement)
During slip of fault rock either side rotate with respect to each other. This rotation represents lever that can be measured by its Moment
Moment= F X Perp Distance
Intensity
Measure of damage caused or observations of effects
Most widely used scale proposed by Guiseppi Mercalli
Modified Mercalli Scale
I= not felt (Mo 1-2), II= felt by few people, III= felt noticeable indoors, VI= felt by all, VII= damage negligible in well designed buildings, IX= damage considerable, X= some well built structures destroyed (Mo7-8), XII= Damage total (Mo>8)
Primary
Secondary
Ground Shaking- caused by Love and Rayleigh Waves
Liquefaction= Reduces pressure, causes groundwater pressure > Load pressure, loss of strength, groundwater rises to surface and spills out through cracks and pores
These waves rapidly lose energy as move through surface rocks, damage caused= restricted to small area close to epicentre
Factors that increase destructive power of ground shaking=
- EQ magnitude
- Proximity to Epicentre
- Focus depth
- Rock/ soil type
- Economic development
Landslides and Rockfalls
Tsunamis= energy wave caused by displacement of water, as wave moves into shallow coastal water it slows down and forms into huge wave
Background
One of most active seismic areas
Compressive intersection of 3 plates (Eurasian, Philippine, Pacific)
Summary
5:46am Jan 17 1995
7.2 magnitude (tremors= 20 seconds)
Epicentre= Awaji-shima= Island 20km from Kobe
Strike-slip movements along Median Tectonic Line
Focus= 20km depth= strong horizontal ground movements
Most damaging to strike since Great Kanto EQ (destroyed large areas of Tokyo, Yokohama)
X to XI on Mercalli scale
Devastation caused for 2 reasons (1) Kobe region not thought to be prone to devastating tremors= located well away from intersection of plates, (2) Japan considered highly prepared due to strict building codes, public education programmes
Impacts
5500 deaths
35,000 injuries
300 major fires
104,000 buildings destroyed
300,000 homeless
Hanshin expressway collpased
Liquefaction
20,000 lost jobs
7% covered by insurace
2,900 died from suicide or neglect
62 high rise commercial buildings demolished (19 rebuilt)
$US 200 billion losses
Foreshocks
Seismic Gap Theory
Ground Deformation
Groundwater (gradual lowering, accelerated lowering, rapid rise)
Radon Gas
Earthquake Lights
Electromagnetic Fields
Animal Behaviour
Pendulum (Counteracts swinging of buildings)
Outer Bracing (Strong, flexible steel supports outer walls)
Retro-Fitting (Concrete replaced by Steel supports)
Elevation Control (Pyramid shaped structure reduces upward vibration movement)
Vibration Dampers (Shock absorbers to reduce transfer of vibrations, giant springs)
LightFramed Structures (limit injuries if collapsed)
Flexible Materials (Bend and twist in response to vibrations)