AS OCR Geology - Earthquakes

module 1.2 - Earthquakes

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SIESMIC WAVES

SIESMIC WAVES: a wave that travels through the earth. The particles of a rock vibrate, transmitting energy from one particle to another.

BODY WAVES: travel through the interior of the earth. There are two types; P and S waves.

The properties of the rock govern how quickly the wave travels:

  • DENSITY: the denser a material, the harder it is for a rock to pass through it and the move the wave is slowed down.
  • INCOMPRESSIBILITY:  P waves cause rapid compression then the material springs back and passes the energy along. The faster the material reounds the faster the waves can travel through it.
  • RIGIDITY: how much a material resists a bending force. A liquid has zero rigidity.
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P - WAVES

  • PRIMARY - travel fastest and arrive first
  • PUSH - are longitudinal or compressional waves so the vibrations are back and forth. They can travel through any type of material.
  • PRESSURE - the particles alternativly more together (compression) and apart (rarefaction) in the direction of travel of the wave: longnitudinal wave.

P - WAVES shadow zone between 103 and 142. Between 142 and 142 the P waves are late arriving because they are slowed down by the liquid outer core. However they donot arrive as late as exspected if the whole core was liquid suggesting the inner core must be solid, due to the immense pressure at those depths.

The P WAVE velocity depends on density, incompressibility and ridgity.

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S - WAVES

  • SECONDARY - travel slower then p waves (about 60% of velocity) so arrive second.
  • SHEAR - the movement of particles is always sideways, at right angles to direction of travel in the waves. It is a transverse wave - the groud moves alternativly from one side to another.
  • SEVERAL times larger in amplitude than P waves.

The S WAVE shadow zone is between 103 and 103.

The S WAVE velocity depends on density and ridgity.

S WAVES donot travel through liquids. Hence donot travel through molten outer core.

S WAVES can be generated by P WAVES in the inner core.

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SURFACE (L) WAVES

  • LONG - the wave length of the wave is longer than for P and S waves.
  • LAST - they arrive last of the 3 waves

Most destructve type of wave due to low frequency, long duration and large amplitude. These waves cause the most destrcuction to buildings.

There are two types of L waves:

  • RAYLEIGH: vertical movement
  • LOVE (L waves): with horizontal movement.

They oscillation in a circular motion so the waves lose energy quickly as they travel away from the epicentre.

L waves are confined to the surface layers of the earth.

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LOCATING EARTHQUAKES

FOCUS:  the point within the earth at which the earthquake orginates.

  • shallow-focus 0-70km
  • intermediate 70 - 300km
  • deep-focus 300 - 700km
  • earthquakes donot orginate at depths greater then 720km because deeper, warmer rocks are not brittle enough to fracture so faults donot occur.

EPICENTRE: the point at the earth's surface directly above the focus. This is where the greatest amount of damage is likely to occur and the greatest intensisty.

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Earthquake Intensity - Mercalli Scale

INTENSITY: is a measure of the surface damage caused by an earthquake.

The MERCALLI SCALE measures the intensity of an earthquake and is based on the effects that are felt in that area.

  • It is an arbitray scale, meaning it depends on the opinions of the observer and has no mathmatical base.
  • Some witnesses may exaggerate the effects of the earthquake or you may find no witnesses agree on how bad things were during the earthquake.
  • The intensities ar measured in many different locations and recorded onto  a map.
  • Isoseismal lines are then constructed, which joins areas of equal intensity.
  • As waves travel away from the focus there energy spreads out and decreases.
  • There is a limit of up to 12 on the scale.
  • Takes time to map out damage.
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Earthquake Magnitude - Richter Scale

MAGNITUDE: a measure of the amount of strain energy released by an earthquake.

The RICHTER SCALE measures the magnitude of the earthquake by recording the amplitude of the earthquake waves.

  • It is a logarithmic scale due to such a large range in values of the magnitude of earthquakes.
  • For example on the Richter Scale each increase of one of the scale means that the amount of energy released by the earthquake increases by a factor of around 30.
  • There is no limit to this scale, however the maximum recorded is 8.9.
  • Immediatly the readings can be taken.
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The Elastic Rebound Theory

  • The two parts of the rock are under stress due to opposing forces acting on the rock. These forces ar usually due to plate tectonics.
  • The body of rock is slowly deformed and put under strain. This energy is potential or strain energy within the rock.
  • The deformation continues until the stress overcomes the strength of the rock and it fractures.
  • The two piece of rock move relative t each other and there is displacement along the fault or fracture.
  • The strain energy that is stored in the rock is released which causes the ground to vibrate as an earthquake.

ROCK STRESS CAUSES DEFORMATION - ROCK UNDER STRIAN - CAUSES DISPLACEMENT ALONG FAULT - EARTHQUAKE CAUSED

Incompetent rocks (mudstones, shales and ho metamorphic rocks) steadily deform under the stress by bending or moving as plastic flow and therefore donot fracture.

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Detecting Waves - The Seismometer

A SEISMOMETER detects and records ground motion and then converts this into signal that can be recorded.

The recoring is called a SEISMOGRAM. A seismogram will have three vibrations after an earthquake: P, S, and L waves.

The time gap between P and S waves increases with distance from the epicentre. This time gap can be used to record the distance from the epicentre. To figure out the location the distances of the epicentre from 3 seismographs can be used.

The magnitude can then be calculated from the distance between the epicentre and the seismometer. The magnitude depends on the amount of energy released at the focus whereas the amplitude depends on a siesmograph depends on the distance away from the epicentre.

A seismometer together with the unit recording the signal is called the SEISMOGRAPH.

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The Effects of Earthquakes (1)

When the crust suddenly fractures elastic energy is released.

GROUND MOVEMENT

  • Stronger, larger amplitude earthquakes cause shear or lateral movement -which is more damaging.
  • The upward acceleration of the ground may be greater than that of gravity, in which case loose objects can be thrown upwards.

DAMAGE TO STRUCTURES - ground movment separates part of structures:

  • Bricks and stonework separate aloong mortar causing walls to collapse.
  • Floors separate from supporting walls, causing them to pancake on top of one another.
  • Bridges built in sections separate from thier supporting piers.
  • Sections of gas, water and drainage pipes separate from each other.
  • Building are caused to sway when thier foundations move sideways.
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The Effects of Earthquakes (2)

LIQUEFACTION

  • In wet sand (and other unconsolidated materials) the vibrations cause the water to separate from the soil particles and rise to the surface.
  • Therefore buildings built on alluvial fans can suddenly find themselves standing on water and large amounts of damage are caused.

LANDSLIPS

  • On steep slopes made unstable by high rainfall the vibrations may trigger a landslide and mud flows which are partly assisted by liquifaction - burail!

AFTER SHOCKS

  • The main movement along a fault releases energy, but the subsequant movements (minuites, hours or even days later) can cause already weakened buildings to collapse.
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Tsunamis

A TSUNAMIS ia a water wave which caused by vertical displacement of a large volume of water by the movement of a large section of the crust on the sea floor. They can also result from the displacement of water by a large landslide e.g. when the flanks of the volcano collapse into the sea.

Speeds of up to 700km/h. Amplitude  of a meter. Wavelength of hundreds of km.

when the tsunami approached the shallow water of the coast, the hieght of the wave increases dramatically, suring across low-lying coastal areas with devasting effect.

The most immediate warning of a tsunami is that the sea drains away from the shore line.

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Factors effecting the scale of damage

  • BEFORE THE EARTHQUAKE
    • Frequency and magnitude of earthquakes in the area.
    • Quality of forward planning and training.
  • DURING THE EARTHQUAKE
    • Population density and number of people affected.
    • Magnitude of the earthquake and proximity to the epicentre
    • Amount of damage caused.
  • THE RESCUE
    • Quality of emergency services - fire, search & rescue teams, police
    • Availability of medical supplies and services.
    • Availability of emergency food, water, shelter and sanitation.
    • Viability of communications.
  • THE AFTERMATH
    • Healthcare, hygiene and sanitation
    • restoration of electricty and water.
    • Rebuilding costs, materials and labour
    • Employment opportunities.
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Seismic Gap Theory

  • Historical records along a fault can show period of elapsed time between active areas - where the fault is locked and stress can steadily build up. This can be used to predict the date of the next earthquake.
  • Section of faults that move regually dissipate their elastic energy in many small, less destructive earthquakes.
  • The problem with this theory is that it suggest that frictional forces and other properties are constant, which they are not.
  • But if the theory suggests an earthquake is due in a certian area then that area can be monitered.

Example: North Anatolian Fault, Turkey

  •  
    • The SGT suggested an earthquake was ready to hit Izmit. The Geologists were right and in 1999 a magnitude 6.7 earthquake hit the city.
    • According the SGT Istanbul would be the next, but instead the earthquake occured right at the begining of the fault.
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Signs for an earthquake (1)

CHANGES IN GROUND LEVEL - the area around the focus may tilt due to deformation or swell due to micro cracks. Tiltmeters measure the slope of the ground level using GPS and laser technology. Strain gauges in borholes measure deformation and therefore any increase in stress.

DETAILED MEASUREMENTS OF GAS - Radon perculates up through cracks and accumulates in water wells (as it is a heavy gas) and is easily detected. If higher levels of radon are being detected then new pathways are opening up for the gas to escape through.

CHANGES IN WATER LEVELS IN WELLS -  groudwater perculated through cracks which lowers the water levels in wells.

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Signs for an earthquake (2)

PHYSICAL PROPERTIES - the number of foreshock before the main event increase. P wave velocity increase decreases, then increase before the main event. Water raises the electrical conductivity of the ground, lowering its resistivity. Immediatly before an earthquake coloured lights in the sky may indicate a change in quartz and other mineral levels.

ANIMAL BEHAVIOUR - Research in China confirms that animals show distressing behaviour just before an earthquake is ready to hit. Animals may be able to feel small vibrations or a change in the earth's magnetic field.  Ground living bird perch on trees, snakes leave thier burrows.

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Reducing the Impact of earthquakes (1)

PLANNING - The risk is assessed by forecasting the likly number of future earthquakes and their magnitude. Local authorities may ban building from the fault line itself and from areas that might suffer liquifaction (e.g. alluvial deposits) or landslides. Also plan emergency procedures for if earthquake does strike.

BUILDING DESIGN - Design to protect people from total collapse, falling structures and broken glass. Flexiable wooden structures absorb a certian amount of strain. For larger buildings, steel-reinforced concret is safer than bricks and masonry. Foundations can be reinforced by pumping liquid cement into a series of micropiles drilled into the ground.

GROUND OR BASE ISOLATION SYSTEMS - A building rests on large rollers, rubber pads, springs or sliders coated with non stick teflon. The mass of the building keeps in stationary or allows small amounts of movement without damage. Older buildings can have the system inserted for future protection e.g. Utah State Capitol.

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Reducing the Impact of earthquakes (2)

RESISTING SHEARING FORCES - Each part of the structure is attached to other parts to prevent collapse. Diagonal braceing by cables or ridgid girders strengthens the frameworks. Large open spaces are not included and walls are fixed to the floor to add rigidity and prevent pancakeing. Shear walls, which extend to the full height of the bulding  without any openings add to the ridgity.

ABSORBING SWAY - tall buildings are designed to sway and absorb energy throgh flexiable supports and materials made or rubber. Hydrualic systems sometimes computer controlled, dampen the movement, like shock absorbers in cars. Flexiable connections between different parts help to counter movement.

SERVICES - broken gas mains and power lines cause fires and fractured water main prevent the fires from being put out. These problems can be avioded by using flexiable piping to allow for movement.

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