Alfred Wegner 1912 - CONTINENTAL DRIFT
- Couldn't explain how it happened but found evidence
- Saw the jigsaw fit of the continents, especially S. America and West Coast of Africa
- Evidence: fossils = glossopeteris (fern) + mesosauraus (dinosaur which couldn't swim) on both continents.
- The glossopteris fossil was found on lots of different continents
- The same structure of rocks on both coasts with similar relief (table mountain cut in half)
- Concluded all the continents were joined together in a big land mass (supercontinent) he called Pangaea 230 million yrs ago.
- His ideas were dismissed because he couldn't explain HOW continental drift happened
- Other scientists thought the ocean floor would rise to let animals walk across then sink again
Heezen, Ewing and Tharp - ECHO SOUNDING 1948
- Added to Wegner's continental drift theory
- Threw explosives down into the ocean and recording the time it took for the sound waves to reflect back from the ocean floor so they could calculate how deep the ocean was
- They wanted to measure the thickness of the oceanic crust
- They also found mountain ranges
- Found MAR = Mid Atlantic Ridge extended 60,000km around the planet
- Mapped these mountain ranges all around the worlds oceans (MOR)
- Found that the mountain range had a huge valley down the centre which is evidence for the plates splitting apart
- The ridge showed an ocean rift which suggested the Earth is moving apart here
Harry Hess - 1962 - Seafloor Spreading
- Added to continental drift theory
- Seafloor spreading - MOR where the plates moving apart creating a conveyor of rock
- Looked at age of rocks on the ocean floor and saw they were newer at the MOR and older at the coast
- He thought that the valley down the centre was a crack from which molten rock was erupting, continuously creating new rock
- Conveyor belt of rock around the world
- Explained ocean ridges and why rocks were volcanic = explains continental drift
Vine & Matthews - 1960s - PALEOMAGNETISM
- matthews used magnetometers to survey the magnetic properties of the volcanic rocks on the ocean floor
- vine used computers to map the direction the rock was facing
- the iron in the lava aligns itself in the direction of the earths magnetic fields
- every 4000,000 years or so the earths magneetic field flips; this means the bands of rock are aligned in different direction
- the last flip was 780,000 yrs ago (171 reversals in 76million yrs)
- shows that new sea floor has moved away from the mor and new sea floor has formed in its place with a different polarity
- symmetrical pattern of magnetic stripes either side of the mor = sea floor spreading
- magnetometer measured magnetism of rocks which showed positive and negative anomalies. stripes found that showed reversals in polarity
- during underwater volcanic eruptions basalatic lava intruded into crust and cooled
- Supports sea floor spreading = pattern of stripes either side of MAR symmetrical
Theory & Geology
- 8 major tectonic plates
- Movement of plates = distribution of continents and features such as MOR and ocean trenches
- The process of sea floor spreading has been helped by the knowledge we have of the structure of the earth by looking at the way earthquake waves bend inside the earth
- Coal deposits = need warm + wet weather not found in N. America, UK, NW. Europe + Antarctica where the deposits are found
- Desert Sandstones in Cheshire
- Land must haave moved
Structure of the Earth
Crust varies in depth between 100km (below mountain ranges) to 5km (beneath oceans)
Continental - 2.6 g/cm3 (lighter), 30-70km, composition is mainly granite; silicon, aluminium, oxygen
Oceanic - 3.0 g/cm3 (denser), 6-10km thick, composition is mainly basalt; silicon, magnesium, oxygen
Crust to mantle = changes in CHEMICAL composition
- Crust - high % of silicon and aluminium, least dense, 5km ocean, 200km oceanic, made up of plates
- Mantle - high % of iron and magnesium, dense rock, 2900km thick, convection currents
- Mohorovijiv discontinuity (MOHO) - boundary between crust and mantle 80-90km thick
- Mantle - around 2900km thick; made from dense rock & radioactive material; consistency of toffee; up to 4000C - slow circulation creates convection currents which move plates
- Outer core - intensely hot layer of liquid metal (mainly iron with some nickel); movement of liquid generates Earth's magnetic field
- Inner core -- 2400km wide ball of solid iron and nickel over 5500C hot
Lithosphere and Asthenosphere
Lithosphere (crust and top of mantle):
- Crust and the rigid upper section of mantle
- Approx. 80-90km thick
- Divided into tectonic plates
- Boundary between crust and mantle = MOHO
Asthenosphere (layer of mantle below lithosphere):
- Earth's mantle where the rocks are soft and easily deformed
- Several 100s of km thick
- Temp increases with depth until around 80km where it reaches 1400C
- Upper boundary = MOHO
- Viscous fluid
tectonic plate movement
Holmes Convection Theory (1962)
- Radioactive decay in core heats rocks = less dense = rise
- This hot plume of magma pushes through the lithosphere forming MOR causing plates to push apart
- The rising and falling of hot + cold rocks = convection currents
- Currents drag overlying plates with them
- Where the plates collide the denser oceanic crust subducts pulling the rest of the plate with it
- Convection currents through whole of mantle from radioactive core producing heat
- The mantle is under high temp and pressure = deforms = viscous = convection currents
- The plates are called lithospheric plates because the whole lithosphere moves on the convection currents in the mantle
- Boundary layer theory = different layers of convection currents (subducted crust doesn't go deeper than 700km + different compositions of trace elemts and noble gases in lava)
- Holmes convection theory 1962 = convection currents through whole of mantle from radioactive core producing heat
- Ridge push- upwelling of magma at MOR creates new oceanic crust(seafloor spreadin) causing plates to push apart. plates also slide away due to gravity
- Plate drag - convection currents in asthenosphere drag overlying plates with them
- Trench pull - weight of dense, cold oceanic crust subducting into asthenosphere = pulls plate down
plate margins - constructive
2 plates move apart due to convection currents in the mantle so you get ridge push and plate drag.
Sequence of events which forms a Mid Oceanic Ridge (MOR)
- Convection currents in mantle cause tectonic plates to move apart making a rift zone (where 2 plates move apart where magma rises)
- Magma flows to surface and reaches the top volcanoes which erupt
- Because the plates are moving apart the further away the volcano gets from the magma chamber it will become extinct as it has no magma source. - extinct volcano - flat topped (erosion from sea)
Key features of MOR
- Ridge extends down the Atlantic Ocean continues for 60,000kms
- Plate spreads apart at 10-51mm per year in the Atlantic
- In the pacific the plates are spreading at 90mm per year
constructive plate margin - rift valley
- A long narrow block sunk between two parallel normal faults
- Graben (floor of rift valley)
- Floor drops when magma has moved somewhere else
- Example: Great African Rift Valley, East Africa, which will widen over time and eventually floor with seawater from the red sea. This rift valley is in a Constructive PB in continental area. African and Arabian plates.
- Magma has risen to surface and formed volcanoes - e.g. Kilamanjaro
- Lakes in rift valley e.g. Lake Malawi where ground has dropped below ground water level
constructive plate margin
transform faults occur on MAR
- results of tension stresses due to plate movement not being constant - not all plates move at the same time
- Fault lines that cut across the main ocenic/continental rift which offsets the main ridge
- Shallow focus earthquakes also occur on the MAR and along the transform faults
Other features found on seafloor
- PILLOW LAVA - Lava erupted under ice of water quickly forms a skin as it is chilled. Fresh lava is then squeezed out through cracks in the skin. Each resulting blob of lava forms its own skin and may pull away from the parent flow, behaving like a liquid filled balloon,.
- HYDROTHERMAL VENT - A fissure on the sea floor out of which flows water that has been heated by magma. They form ecosystems for microbes and animals that can withstand the hostile environments. The hottest hydrothermal vents are called black smokers because they spew out black compounds of iron and sulphide. = Basically a fissure on the sea floor out of which flows water thats been superheated by magma
constructive plate boundaries
The plate diverge because;
- convection currents in lower mantle and asthenosphere leading to less dense magma rising, causing ridge push - the upwelling of magma creating new oceanic crust (seafloor spreading) pushing the plates apart and also plate drag - the convection currents of the asthenosphere drag the overlying plate with it.
- A rift zone forms - rising magma exits crust via conduit, then finally exists the active shield volcanoes which form upon the ridge, due to non viscous, basaltic lava.
- Processes continues, new plate material formed, existing volcanoes are moved away from magma source, becoming extinct = forms under water seamount as it moves off the magma source and becomes eroded over time.
Destructive plate boundaries
- 80% of the worlds active volcanoes occur at destructive plate boundaries
- earthquakes are created when friction occurs in the benioff zone
- a subducting playe contains water which reduces the melting point of the litosphere crust
- molten magma rises through overlying crust in large bodies known as diapirs/ they may cool to form plutons.
- as magma rises pressure is released and it becomes more fluid like, the magma is high in silica which makes it andesitic or rhyolitic.
- find reverse and thrust faults and anticilines, synclines and recumbent folds
- example: nevada del ruiz, columbia 1985
destructive plate boundaries-oceanic/oceanic
Oceanic oceanic convergent margin with subduction zone
- the key processes are the same as a continental-oceanic boundary
- Japan trench is an example of this where the Pacific plate and North American plates are convergings off the East coast of Japan
- The pacific plate subducts (more dense, moves faster and only oceanic).
- The Japan Trench marks the point of subduction.
- Usual melting occurs, rising of diapirs of magma, creating explosive volcanoes on the surface the magma is andesitic or rhyolitic. (composite volcanoes)
- The volcanoes that occur will be on the oceanic crust that hasn't subducted means volcanoes will begin underwater (Seamounts) and eventually grow above sea level appearing as lines, or island arcs (.e.g japan).
- Ocean trench - as the oceanic plate is subducted the pressure cases rocks and sediments to deform and buckle (Accretionary ridge)
- Benioff zone - friction = EQ, shallow, intermediate and deep focus (up to 700km). The water reduces melting point of rock = high silica content = viscous magma = rises in blobs called diapirs.
destructive plate boundaries-continental/continent
Continental-continental collision zone
- A destructive plate boundary with no volcanoes
- formation of himalayas: 2 continental plates of similar density meet under Himalayas. The Tethys sea in between these 2 plates disappeared about 20MYA leading to continental collision between India and the rest of continent of Asia. Sediments of sea floor have been thrust upwards to form Himalayas.
- The Indo Australian plate has spent the last 10 million yrs drifting North towards the Eurasia plate; these 2 similar plates met and formed Himalayas.
- Magma = no volcanoes. Magma rises, = batholiths = intrusion of magma = a large pool cooling slowly = large crystals
- Mountain root = formed when the crust is compressed and thickens. As it sinks = heated = melts to form magma.
- Fold mountains (Himalayas) = reverse thrust faults caused by compressional forces = one block forced above the other. Lots of faults = lots of EQ's.
- Himalayas due to converging of Indo Australian Plate and Eurasian Plate
destructive plate boundaries-continental/continent
ANTICILINES :( (+SYNCLINES :) )
- Compression = fold into bends
- Initially symmetrical
- As more pressure applied = asymmetrical
- Greater compression = overturned folding
- Even more compression = recumbent fold faulting (reverse thrut fault)
Destructive PB- oceanic/continental
- 80% of worlds volcanoes on destructive PB, but only found at 2 of 3 boundaries: oceanic/continental & oceanic/oceanic
- oceanic crust is denser than continental
- earthquakes are created at all depths up to 700kn depth along Benioff zone as oceanic plate is subducted; subduction is sustained by trench pull
- subducting plate contain water, reducing melting point of lithosphere and crust
- molten magma rises up to the surface through overlying crust as diapirs, may cool to form plutons
- as the magma rises pressure is released, it may become more fluid like
- the magma has a high silica content making it either rhyolitic or andesitic; both are viscous
- when the magma breaks the surface it creates explosive eruptions, e.g. mt st helens 1980 or NDR 1985
- ocean trenches form e.g. peru-chile trench (5900km long, 8km deep + 64km wide) representing the point of subduction
- at the ocean trench pressure between the plates causes rocks/sediment to deform and buckle to form folds leading to fold mountain range e.g. himalayas
Destructive PB- oceanic/continental
- folds, long ridges//to each other and the plate boundary
- reverse thrust faults caused by compressional forces = one block forced above the other results in earthquakes
Nazca & South American plate (example)
Sinking convection currents, currents converge & plate drag & trench pull occur
- magma rises=volcanoes formed on mountain chain e.g. NDR 1985
- composite volcanoes=rhyolitic/andesitic = explosive
mountain root formed when crust is compressed and thickens; if it's deep enough it melts
- found at CONSTRUCTIVE PB
- result of tension and rifiting with rocks displaced in the direction of the fault plane/line
- top block moves down
Strike slip faults
- found at CONSERVATIVE PB
- Movement is a horizontal direction & shallow focus EQ
- found at DESTRUCTIVE PB
- is the product of compression where one side of the fault plane is thrust over the other. top block moves up
- found at CONSTRUCTIVE & CONSERVATIVE PB
- at right angles to plate boundary
- caused by different rates of spreading along the ridge due to the curve in plates (circular planet)
Destructive plate boundaries can form mountain ranges, this process is known as orogenesis, resulting in the largest mountain ranges on the planet (e.g. himalayas, andes in chile, rockies in USA)
What are folds?
- they're closely associated with DESTRUCTIVE PB
- Within the mountains the flat layers (of sediment) is bent or buckled, the rock layers are warped from compression
Types of folds in order of formation:
- Initial compression of plates forms Anticlines & synclines
- Greater compression leads to asymmetric anticlines and synclines
- Even greater pressure leads to overturned folding of rock forming a recumbent fold
- Continued compression leads to the recumbent fold faulting, forming a nappe
Conservative plate boundaries
- No crust is created or destroyed
- No subduction so no volcanic activity
- Movement of plate is parallel to plate margin
- Movement of plates creates stresses between the plate edges
Sections of plates slide past each other in opposite or same direction, but at different speeds creating relative motion in the opposite direction (E.g. on San Andreas fault line, the N. American plate is moving 2-3cm faster than the Pacific plate)
Release of friction in the benioff zone causes shallow focus earthquakes
Only occur on continental crust
The San Andreas fault system is an example of a strike slip fault as parts are creeping and other sections are stuck due to a build up of pressure and friction. This stick slip motion results in the release of pressure as shallow focus earthquakes.
Strike slip fault=build up of friction released=seismic waves
Hotspot theory and intraplate volcanicity
Usually the theory of plate tectonics explains that mosst volcanoes are associated with plate boundaries.
Some volcanoes however occur away from plate boundaries and so need the hotspot theory to explain them.
The Hawaiin Islands is an example of this, as Hawaii is nowehere near a plate boundary (lie in the centre of the Pacific plate) and yet all the islands are volcanic.
How do hotspots form?
The hotspot theory was first proposed by Wilson in 1963 where mantle plumes are thought to cause hotspots and associated volcanic activity.
The Hawaiin island chain is the product of a single 'hot spot' fixed in the mantle. A mantle plume is a long column of hot rock ascending from deep within the mantle:
The hot mantle rock rises very slowly to the surface where lower pressure results in decompressional melt and magma production to form a hot spot underneath the lithospheric plate
The magma bursts through to the surface and initially forms a submarine volcano (seamount) which eventually surfaces above sea level to form a volcanic island
As the plate moves over the hot spot, this process is repeated and a chain of volcanic islands form
Eventually, crust subsidence and marine erosion results in the oldest volcanoes becoming flat topped seamounts (called guyots)
How do hotspots form
Excessive heat at the core-mantle boundary (possible from a conc. of radioactive elements)
Molten rock rises as a mantle plume. It is less dense than the surrounding rock and the surrounding asthenosphere
Hot spot remains in a fixed position
Magma is extruded through the lithosphere and forms seamounts. These eventually get larger as more lava erupts to form a volcanic island
Plates move away/off the hot spot and so the volcanoes become extinct. They are undersea volcanoes due to subsidence of the crust, and flat-topped due to marine erosion
There's ongoing debates on the hotspot theory and what's happening beneat the Earth's surface.
- Seismic tomography seems to show only shallow mantle plumes and not deep mantle plumes.
- Many volcanic island chains are not time progressive like the Hawaiin chain. E.g. the Cameroon volcano chain in West Africa is not time progressive.
- The hot spot may not be fixed. E.g. the change in orientation of the Emperor Seamount chains (guyots) compared to the rest of the Hawaiin island chain may have been caused by a change in the position of the mantle plume and so hot spot rather than a change in the direction of the Pacific plate.
In 2003, she suggested that some hot spots do not always form from a deep mantle plume but instead have formed as a shallow hot spot.
This is because plate tectonics result in extensional stresses (Stretching and thinning) of the lithospheric plates and so lower pressure conditions in the upper mantle and decompressional melting of mantle rock create a hot spot.
She argues that all tectonic plates will have 'scars' from former collisions or divergence, and that larger plates such as the Pacific plate will be stretched towards the centre as the edges of the plate are pulled down into subduction zones.
When these more vulnerable parts of the crust pass over previously subducted material that melts easily in conditions of lowered pressure a hot spot is created and volcanic activity will occur. E.g. the Hawaiin Islands.
hotspot formation-tuzo wilson mantle plume theory
Excessive radioactive elements (uranium) in outer core heat up mantle above
Mantle plume of hot rock moves upwards to surface
Creates a hotspot below lithosphere in asthenosphere (lower pressure=decompressional melt=magma)
Lava extruded as a seamount (e.g. Loihi)
As more pillow lava (lots of basaltic magma=frequent eruptions)is added the seamount grows and forms a volcanic island (e.g. Hawaii)
As the plate moves over the hot spot volcanoes become extinct (e.g. Kauai) = chain of seamounts (E.g. Emperor seamount chain showing direction of plate movement)
These extinct volcanoes become flat topped due to sea erosion = guyots
Guyots=pressure on crust=subsistence=moved under sea level
Intraplate volcanoes. Gillian Foulger
- Pacific plate being stretcghed because of subduction causing trench pull=centre is weak/thin
- The subducting plate will melt to form magma which moves + is extruded at the thinner area
- Time progression of Hawaiin islands might be explained by a propagating fracture moving along the plate
- Seismic tomography shows only shallow mantle plumes
- Hotspots not stationary
- Not all island chains have time progression like Hawaii (e.g. islands off Cameroon Africa)
- Emperor seamount chain in diffrent direction than Hawaii.
What is an earthquake?
- Result of sudden failure of rocks in lithosphere due to build up of pressure, this pressure is caused by plate movement or isostatic change
- The point of failure is where shock waves originate from an area called the focus
- The rock movement can result in cracks or faults
- Directly above the focus on the surface is the epicentre - most damage occurs here
- The shaking of the earth from the focus causes seismic waves to move through the rocks
Types of earthquakes:
- Shallow focus: 0-70km depth (75% of all EQ)
- Intermediate focus: 70-300km depth
- Deep focus: 300-700km depth (700km represents 1400degrees isotherm where there is a change in properties)
Seismic waves-vibrations sent through earths surfa
Primary waves (P-Wave)
- Body wave
- Reduced speed by viscosity
- Vibrating in direction in which they travel
- Travels through solid, liquid and gas
- Moves fastest in denser rock (lithosphere) than less denser (asthenosphere)
- Travel in interior of Earth
Secondary waves- S waves
- Travel at half the speed of P-waves
- Slower than P-waves
- Cannot travel through liqud
- Shear rock by vibrating at right angles to direction of travel
- Travels through sold only (doesn't go in outer core)
- Travels in Earth's interior
- Body wave
Surface waves - L waves
- Travel slowest and near to the ground surface
- Travel in lithosphere in Earth's crust
- Some surface waves shake the ground at right angles to the direction of wave movement and some having a rolling motion that produces vertical ground movement
Used to measure seismic waves
- Base of instrument is fixed to solid ground (but away from traffic or human interference)
- As the drum rotates, the pen records a seismic trace onto papaer producing a seismogram
- Inert weight keeps the pen stationary while the drum moves with the ground shaking
middle to the top of wave = wave amplitude
RS measures magnitude of quake (energy released from wave amplitude)
P and S shadow zones
P-wave shadow zone (solid and liquid)
Travel through solids and liquids. P-waves woll travel through the core. However, there's still a shadow zone because the P-waves are refracted by the liquid outer core.
S-wave shadow zone (solid)
Travel through solid only. Don't travel through outer core as it's liquid. This creates a shadow zone where no S-waves are recorded by seisometers. Waves are deflected by less rigid material.
Distribution of earthquakes
The majority of earthquakes are located at plate boundaries around the world where there is extreme pressure build up and faulting. There's also intraplate EQ formed fracking, reservoirs isostatic change etc.
Scale, magnitude, intensity+frequency of EQ
- Scale: usually talking about the size of the area affected
- Scale of measurement: richter scale, mecalli scale
- Magnitude: level ofenergy released by the earthquake and most commonly measured using RS
Richter scale (devised in 1935):
- Logarithmic scale - the whole no. jump in the scale indicates a 10 fold increase in wave amplitude.
- An event measured at 7 on the scale has an amplitude of sesimic waves 10x greater than one measured at 6 on the scale.
- The energy released is proportional to the magnitude, so that for each unit increase in the scale, the energy released increases by 30 times.
- Largest event was 8.9
- Ground tremors record a magnitude of 2 and damage to buildings/structures occurs at scale 6+.
- Gets data from energy released from ground tremors
- Measures seismic wave amplitude
- Each unit is a x10 increase in the seismic wave amplitude
Moment magnitude scale (MMS)
Replaced the richter scale. MMS measures the size of EQ in terms of the energy released. The magnitude is based on the seismic movement of the EQ which is equal to the rigidity of the Earth multiplied by the average amount of slip on the fault and the size of the area that was slipped.
Different from RS because: used to measure EQ mag. taking into account the size of fault rupture, the rocks stiffness and the amount of the movement of the fault using values that can be estimated from the size of several types of seismic waves. However, RS is a numerical scale used to measure the mag. of an EQ using values based on the size of the EQs largest seismic wave.
MMS now used because:
- more accurate for medium-large earthquakes
- richter scale under estimates the mag. of large EQ events above 6
RS still useful for small shallow EQ with MMS less than 3.5
- Measures the intensity of an EQ
- Guiseppe Mecalli's 12 point system is a subjective form of measurement
- Main benefit of the Mercalli scale over the RS is that the intensity figures can be plotted onto maps (isoseismal map), creating isolines, to show the difference in damange between areas.
Frequency: how often an EQ occurs in a given area
Generally, the larger the EQ, the less often that it will occur. The reason that large EQ's are less frequent than small EQ's is that they require a large build up of pressure, which takes more time.
Frequency will also depend on location. If you live in the centre of a tectonic plate (intraplate) then EQ's will be less frequent than if you live close to a plate boundary where plate movement causes rapid pressure build up and subsequent release.
Primary and secondary effects of an EQ
The Primary Effect of an earthquake is that of Ground Shake associated with the high amplitude L waves travelling through the surface layers of the earth. These are most destructive closest to the epicentre, as we have seen with Mercalli’s isoseismal lines.Very few people are directly killed or injured from ground shake itself.
- collapsing buildings and infrastructure destroyed
- fires from broken gas pipes and collapsed pylons
- liquefaction-particles settle and water comes up
Secondary effects of EQ
Building damage: "EQ's dont kill people, buildings do." As Karl Marx stated, damage to infrastructure is often the most common cause of loss of life in EQ disasters.
- Buildings may just topple over, or their internal walls can give way causing the floors to "pancake". In some causes the ground shake frequency has matched the 'sway' frequency of the building, leading to resonance which increases the swag movement and leads to collapse.
- Supporting piers/legs of bridges can give way, causing them to crumble and sections of bridges come apart. (E.g. Road ripped apart by Sichuan EQ in China).
- E.g. January 2010 Haiti EQ. The death toll of 200,000 would probably have been much lower if they'd had aseismic-designed or retrofitted buildings.
Damage to gas, electricity and water systems:
- Fires can result from severing gas/electricity lines, creating additional hazards, especially if near wood infrastructures. E.g. 1995 Kobe EQ, Japan. Suffered devastating fires, causing 70% of deaths.
If the ground is highly saturated (high h2o content), the shaking from a EQ can bring the water up to the surface, and the particles sink. This makes the soil more mobile, resulting in building damage from flooding or sinking foundations.
Landslides/snow avalanches: As the ground shakes, unstable slopes can collapse. E.g. San Salvador EQ 2011, 585 bodies found from the landslide, and still 600 that were never acounted for.
Aftershocks: As the moving plates settle back in to a stable position following the initial quake movement, shake waves are created. The pop is very vulnerable at this stage as buildings may collapse further.
EQ management; Prevention, Prediction, Protection
There has been no successful intervention of an EQ to date. Only way of preventing an EQ would be to trigger smalller ones before pressure builds too high. This may be possible through lubrication.
Predicting an EQ requires accurate measurements of change within an EQ zone. This first needs long term measurements of what normal conditions are like, involving expensive research, money for which is not available in some countries. (Can predict location but not the time also)
Seismic gap theory
E.g. Loma Prieta EQ 1989 (near San Francisco). Conservative PB. PB location = N American Plate and Pacific Plate. San Andreas Fault - strike slip faulting.
Prior to the EQ in 1989, that segment of the San Andreas fault system recorded must less seismic activity than other parts of the fault - there seem to be "gaps" in the activity, where few or no EQ's occured around Loma Prieta. Due to the lack of EQ's the pressure builds up causing a large EQ when the pressure releases. The large EQ moves down the fault line, this is called fault unzipping.
The downside to this method is that the fault line must be recognised already, as some are hidden, and the date of the large EQ cannot be accurately predicted.
E.g. the parkfield EQ occured after Loma Prieta. EQ shows pressure transferred eastwards along san andreas fault line.
The main shock and aftershock of the 1989 EQ event occured within the previous seismic gap.
Rupturing = the movement of fault line
Seismic gap theory
Fault unzipping: fault is like a stuck zip. As you try open it tension builds up in 1 part which is eventually overcome and the zip jerks downwards (the plate moves). But rapidly gets stuck again. Thus pressure is exerted further along the zip (or fault) until this too gives allowing sudden movement.
The Parkfield experiment used Seismic Gap Theory to set up for an EQ in 1993, however the EQ happened in 2004 suddenly and was not predicted.
At the Haicheng EQ, China in 1975, animals are reported of being recorded behaving strangely by some inhabitants, who claimed that this aided the evacuation efforts before the EQ struck. However, this has never been replicated and the reports themselves are by amaeturs and cannot be fully trusted.(E.g. pigs squeal, dogs bark, snakes hit themselves against the wall until they die)
Changes in ground level and shape
- The area around the focus may tilt due to deformation and swell slightly due to the microcracks
- Tiltmeters using lower technology and GPS measure the slope of the ground level very accurately. Strain gauges in boreholes measure deformation and therefore any increase in stress.
Release of gases (especially Radon)
- Radon, a radioactive decay product of Uranium in granite, percolates up through microcracks. As a heavy gas, radon accumulates in water wells in which its easily detected by its radioactivity. If the amount of radon increases new pathways are opening up for the gas, and an EQ may be imminent.
Changes in water level
- Groundwater percolates into the microcracks, lowering the level of water in wells. The levels return to normal and the water is replenished before the EQ occurs.
Evacuation/shut down of utilities: Both of these require reliable prediction of the EQ event. If a false prediction is made, then the population will lose confidence in the system and may not respond appropriately the next time. Only successful once as it requires prediction of the EQ, Haicheng, China 1975:
a) Evacuation. Winter, 1975: Chinese officials ordered the evacuation of Haicheng (pop about 1 million) Reason: reports from scientists and other observers: over a period of months, changes in land elevation and ground water levles also widespread accounts of peculiar animal behaviour (these 2 were precursours); also regional increase in seismicity (which later was recognised as foreshocks) had triggered a low level alert. Increase in foreshock activity triggered the evacuation warning.
Feb 4, 1975: mag 7.3 EQ struck few days after evacuation. 2041 people died, 27,538 were injured. It was estimated that the number of fatalities and injured would have exceeded 150,000 if no EQ prediction and evacuation had been made.
Shinkansen Urgent Earthquake Detection and Alarm System (UrEDAS), Japan
- Determines the magnitured and epicentre of an EQ from P waves and automatically gives an alarm after judging the observation degree of possible damage that is estimated by the magnitude and location of epicentre.
- The Shinkansen Bullet Train is stopped as soon as warning is given
- In March 2011 EQ reached a 9 on RS destroyed train station buildings and tracks but trains remained undisturbed.
- Warning time- Seisometers to detect P waves and give a warning to people in Japan that EQ is to be detected and to stop bullet trains. The bigger wave amplitude=bigger damage. Warning time increases with distance from the epicentre.
"SMART" Meters - used to cut off gas and electricity supplies to homes when an EQ happens.
B) Improving building design/location (incl. landu
Building design: 70/100 largest cities are in EQ prone areas and building collapse is biggest killer.
Base isolation: Rubber pads or ball bearings underneath a building absorb the ground shaking energy keeping them mainly stationary. Buildings need space to move on all sides and utilities such as pipes need to be flexible so they don't break. E.g. The LA County Fire Command and Control Facility in California was built in 1990 with base isolation. E.g. Olive View Medical centre was destroyed in 1971's EQ. Building code introduced in 1973, Building rebuilt with base isolation and survived the EQ.
Building counterweight (e.g. Taipei 101, Taiwan): A large weight on top floor of a tall building moves opposite to the force applied by the EQ counterbalancing the building. Movement of the weight is triggered by computer controlled dampeners which can react very quickly.
- 660 tonne steel pendulum on top of Taipei
- The building is also made up of multiple steel sections which can bend preventing the building shell from giving way and pancaking.
- Every 8th floor, steel reiforcement help to make the building more flexible(cross bracing)
- Base isolation
No EQ proof measures:
- Adobe buildings (mud-brick)
- Unreinforced maonry, non-seismic design
- Reinforced concrete frames, non-seismic design
- Steel frames, non-seismic design
- Reinforced masonry, medium quality, non-seismic design
- Reinforced concrete frames, aseismic design
- Shear wall structures, aseismic design
- Wooden structures, aseismic design
- Steel frames, aseismic design
- Reinforced masonry, high quality, aseismic design
Different materials have different aseismic properties, with wood being the best at 10 on the Mercalli scale; however skyscrapes can't be built out of wood. Aseismic designed buildings are expensive to build, cross bacing is a method that can be retrofitted (refitted) to existing buildings, allowing them to sway and twist, however cannot be afforded in LEDC's.
The Guatemala quake of 1976 was described as classquake as the wealthy people in aseismic buildings survived whereas 20,0000 poorer people in seismic buildings died.
Land Use Mapping: this is where previous EQ intensity areas are marked on a map so important new buildings can be built on low intensity areas. Very useful as it can show the type of ground material and the amount of ground shaking influenced by that material. This can then be used to plan where emergency services and evacuation centres are placed so there will be little damage to those buildings and infrastructure.
Bedrock - solid rock so good to build on as less ground shaking
Soft alluvium-prone to liquefaction. Alluvium=gravel.
c) preparation/education (raising public awareness
This can reduce death toll of a pop as education and people being prepared means that they can evacuate quicker and be less panicked. There are lots of EQ programmes and scheme in EQ prone areas and in some cases are national events.
In Japan: Disaster Prevention Day 1st September, this is where there is a weeklong series of events that is funded by the govt ensuring people are prepared and are confident about how to react when an EQ strikes or tsunami or typhoon.
In the USA: they have a "shake out" day which is a govt. funded organisation that put on annual EQ drills and also provide lots of info, and have a campaign in what to do during an EQ "drop,cover,hold". these events have millions of people join in. America also have the Federal Emergency Management Agency (FEMA) which is a govt run agency that provides aid as soon as the EQ hits so do not have to wait for foreign aid.
Tsunami is a series of waves caused by a sudden disaplacement of water in the ocean. 75% caused by EQ - submarine EQ's on the sea bed).
- ONE: Tsunamis are mainly associated with subduction zones. At a destructive PB, subduction results in compression within the lithosphere. Thus resulting in reverse faulting.
- TWO: This release of pressure along the fault results in an EQ. This causes sections of the sea floor to mvoe up and down by several metres. The displacement of water creates a wave crest and trough. A wave crest=highest point of wave & a wave trough = the lowest point of a wave.
- THREE: In deep water the tsunami wave is barely visible, but it is travelling at about 600mph. The wave height is just 10s of cm's. The wavelength however is up to 100miles. Wavelength of the wave=the horizontal distance between 2 consecutive wave crests.
- FOUR: When the wave enters shallow water, it slows to about 30mph. And then its wavelength is shortened, meaning the waves are closer together, and its height can be up to 30metres. The wave builds in height as the bottom of the wav is slowed down by fricition with the seabed. This acts a breaking effect on the base of the wave. This compressed energy causes the wave height to increase considerably. The water on the beach is "drawn back" by the approaching tsunami, leaving the seabed exposed. This is the 1st sign of a tsunami. The trough of the wave often hits before the crest.
Why waves increase in size as the water reaches the continental shelf (and coastline):
- 90% originate in the Pacific Ocean
- Before tsunami hits the water is drawn back - i.e. sea receeds rapidly from the coast
- As tsunami approaches shallow water, fricition acts as a brak on the leading waves. Waves at the back still advancing quickly. The waves get much taller and closer together/
- Volcanic activity produces a landslide which can then slide into the sea displacing the water, causing a tsunami.
- In the Pacific Ocean, tsunamis can travel up to 500mph.
- In deep water, the large ripples which are the tsunami have a small wave height (less than 5m) and long wavelength (100s of miles)
Tsunamis are caused at destructive PB where the non-subducting plate can get stuck and slow distorts. Eventually, pressure is released causing the seabed to move. This displaces h2o. Tsunamis can also be formed when volcanoes explode, any of the debris from the volcanic eruption falls into the water causing a massive tsunami to form as the water is displaced. Meteor impacts, landslides and other unknown phenomena can cause tsunamis.
How volcanic eruption can cause tsunami: Tsunamis can also be formed when volcanoes explode, any of the debris from the volcanic eruption falls into the water causing a massive tsunami to form as the water is displaced.
How EQ's cause tsunami: Tsunamis are caused at destructive PB where the non-subducting plate can get stuck and slow distorts. Eventually, pressure is released causing the seabed to move. This displaces h2o.
Other potential causes: Meteor impacts (2%), landslides (8%), volcanic events (5%), EQ (75%) and other unknown phenomena (10%) can cause tsunamis.
Physical factors affecting impacts of tsunami: mag of EQ, distance from epicentre, frequency of tsunamis, time of day and year
Human factors: building design, such as homes on stilts, tsunami shelters, urbanisation, primary wave detection system, education programmes and tsunami drills, primary wave detection system
Japanese Tsunami 11th March 2011
- Destructive PB (Oceanic/Oceanic) offshore
- Chain of volcanic islands = island arcs
- Caused by EQ - Mt. Sakrujima
- The epicentre of EQ 130km (80 miles) from coastline
- N. American plate (which doesn't subduct) is slowly distorted and when pressures released the seabed moved.
- EQ was 9 on RS.
- Took 8mins for 1st wave to reach Sendai (coast)
- P wave warning system-1st warning
- Little resistance due to flat landscape
- Thick deposits of mud
- Fukushima nuclear plant exploded due to the cooling system failing. It led to radioactive material being released into the atmosphere.
Effects of tsunami
- Land subsided by 1m due to EQ so contributed to devastating effects of tsunami
- Destroyed 95% of vegetation
- Waves surged 8 miles inland
- Erosion of beaches and thick mud deposits covered land
- Destroyed trees
- Salinisation (salt water) of soil will result in decreased soil fertility for several years
- Reclaimed land suffered liquefaction - 100,000 homes destroyed on soft alluvial plains.
- Seawall covered 40% of Japanese coastline to protect from tsunamis was overtopped
- Destroyed buildings - over 95% of buildings in 1 town
- Nuclear radiation threat of the Fukishima plant. Tsunamis damaged cooling system of nuclear plant causing 4 reactors to shut down. More than 100,000 people evacuated from 12 mile radius
- 20,000 killed. 3000 never accounted for
- 100,000 homes destroyed
- Cars swept away
Protection measures during Japanese Tsunami:
- Preparedness drills
- 90 sec EQ warning by Japanese early warning - sent warning on mobiles, TV, radio
- Buildings absorb violent sideways shaking (Cross bracing and base isolation)
- 10m high seawalls(40% of coastline)
- $25 billion for reconstruction
Boxing Day Tsunami, Indian Ocean 2004
- Occured at destructive PB. Indo Australian plate subducted underneath the Eurasian plate
- EQ occured at 7:59am. Epicentre was 160km from the coastline and there was a gentle sloping continental shelf
- Mag 9 on RS resulted in 230,000 deaths
- 20m high waves, surged inland for 2 miles
- No sea defences and with the exception of Thailand, none of the countries affected had any warning system in place. This was due to infrequency of tsunamis in this part of the world as well as lack of money available to spend.
- It was clear from the response of many people, that people had no awareness of the potential hazards represented by a tsunami. E.g. Sri Lanka people returned beach after 1st wave to collect fish not knowing it's a tsunami and were swept away and died by the second more powerful wave
- Vast majority of local authorities did not have emergency supplies or plans for the disasted.
- Tsunami occured on boxing day, so many tourists on beaches also at 8am and people were asleep so didn't know
Boxing day tsunami
Vertical evacuation, where people head to higher ground; however the effectiveness depends on a warning system, the amount of education, accessibility to higher ground and the distance from the epicentre, which will determine the amount of warning time.
The Pacific Tsunami Warning Centre (PTWC) is located in Hawaii, in the middle of the Pacific Ocean. There is a network of buyos across the Pacific Ocean, which float on the sea surface, but are connected to an EQ detection equipment on the seabed. When an EQ occurs, information from the buoy is transmitted via satellite to the Pacific Tsunami Warning Centre, which enables a tsunami warning to be sent to coastal locations.
After the Boxing Day tsunami, a warning system was sent up among the countries that border the Indian Ocean. This would have been little use in Northern Sumatra, as it was too close to the epicente of the EQ, but other countries would have benefitted from some warning.
- Had no sea defences
- No warning system and lack of money
- Japan were better prepared - indian ocean smaller than pacific ocean = more countries affected
- Offshore breakwaters- these are designed to break the force of the tsunami before it arrives at the coast. However, they can be overtopped.
- Evacuation Routes and Public Awareness - the coastal areas are covered with tsunami warning signs to inform people of indicating the quickest routes to higher, safer ground in the event of an emergency.
- Building design - buildings can be built to withstand the force of the tsunami. E.g. raised, open foundations allow water under buildings and strong materials such as concrete can reduce the damage of a tsunami
case studies: nature of EQ hazard
An EQ occured in Sichuan in China at 2:30pm on 12th May 2008. Represents a seismic event from an LEDC. Occured in Sichuan, a poor part of central China
An EQ occured in Northridge in LA at 4:30am on Martin Luther King Day (a holiday) in 1994. It represents a seismic event from an MEDC.
- Conservative PB. San Andreas fault. Pacific (12cm p.a.) & N. American (6cm p.a.)
- Lower mag at 6.7 on RS. Slightly lower mag aftershock at 5.6 on RS.
- Fewer aftershocks
- Epicentre of EQ is 21km North of city of Santa Monica and 30km NW of downtown LA
- Blind thrust fault-fracture in earths crust which doesn't reach the surface
- Occured at 4:30am
- Unknown fault
- Reverse thrust fault
Nature of EQ hazard-Sichuan
- 7.9 on RS
- Near destructive boundary: C/C. Indoaustrlian plate collided with the Eurasian plate resulting in fold mountains which are heavily folded and faulted, resulting in large mag EQ with a range of focus depths
- Indian plate and Eurasian late
- Slightly higher mag aftershocks at 6.0 on RS
- Occured at 2:30pm
- Fault line in which caused the EQ follows the mountain front separating the Tibetan plateau from the flat Sichuan Basin. The fault dipped at 30 degrees and ripped through the landscape for approx. 300km with a displacement of 5-20m in less than 2 mins.
- Focus was 19km below the surface with the epicentre being 80km NW of Chengdu, capital of Sichuan province
Impact of EQ events - northridge
- EQ struck a wealthy area. Bigger economic cost e.g. $30 billion damage-costiest disasted at the time
- 57 dead (72 indirectly)
- 1500 injured
- 20,000 homeless
- 4:30am - people in bed when happened in safe buildings
- Federal holiday (Martin Lurther King Day)
- Building damage (Aseismic building survives)
- Some liquefaction
- Damage and collapse to infrastructure and buildings
- Landslides in surrounded in mountainous areas
- Densley populated area- epicentre 80km from Chengdu
- Over 600 aftershocks recorded, strongest being 5.6 on RS
- 20,000 homeless
- Gas electricity and water supplies damaged and several days after the event, 9000 premises had no electricity, 20,000 had no gas and 48,000 had limited water supply
impact of EQ event - sichuan
- Mountainous areas surrounded chengu are at a low level of development with a poor pop supported mainly by agriculture e.g. losses incl. millions of pigs and livestock
- Bigger social impact (10x higher no. of deaths)
- 6800 deaths (1/3 of children)
- China's policy (1 child) was waivered in Sichuan following this event
- 380,000 injured
- 48 million homeless
- 2:30pm on weekday - people at work, children at school
- Buildings collapsed (incl. schools), EQ lakes (as a result of landslides)
- Office blocks in Beijing and Shangai swayed, despite being over 1500km away
- Flooding occured in the days following the EQ due to the damming of rivers from landslides. 34 'earthquake lakes' had formed and 28 were deemed as being potential risks to human life.
Difference in death toll of the 2 tsunamis:
Boxing Day tsunami had higher death toll because:
- Don't happen frequently in Indian Ocean
- Lack of warning system so people didn't know it was coming
- No defence to protect from waves
- Less economically developed (lack of sea defences)
- No education on tsunamis
- Wooden buildings especially in Indonesia (LEDC) so affected more
The management &responses to event- northridge
- Lessons learnt from last EQ tp hit LA in 1971 has led to better protective measures and can afford them - MEDC
- FEMA co-ordinates emergency response
- Shaking intensity map used to make emergency response more effectove
- Old buildings predating building code have been retrofitted; public buildings have base isolation (e.g. hospitals)
- Strict building code enforced since 1973
- Some hospitals survived - e.g. Olive View Medical Centre due to base isolation
- Annual Shake Out Day - EQ drill practice
- People shown public shelters before storm arrived
- Police, fire departments, medical emergency teams and citizens rescued people and fought fires while emergency managers decided what resources would be needed to shelter people and help them recover
- Teams of relief workers were sent where they were most needed in the days following the EQ. The map helped emergency managers reduce the time for delivery of relief money to individuals from weeks to hours.
- National rescues instigated
- Evacuation of buildings after the EQ
the management&responses to event-sichuan
- Less money to spend as NIC led to inadequate protective measures
- Takes 4 days for 137,000 troops to get involved in resurce. Also international assistance from South Korea, Russia and USA.
- No shaking intensity map
- Old buildings which predate building code were not retrofitted
- Widespread corruption in buildings of newer govt properties e.g. schools pancakes = poor building construction; since the EQ
- Rebuilding at a rapid rate but will still be poorly constructed
- Many hospitals destroyed, leaving fewer care for victims
- No public education day held at first but there's now a disaster prevention day since this event
- Heavy rain=flooding and EQ lakes. Made access to rural areas difficult and tampered rescuse for a few days causing a higher death toll.
- Airport temporarily closed and all flights were diverted as the EQ struck
Why some places suffer more from EQs than others
Earthquake magnitude and depth:
- If an EQ has a very deep focus then they are less likely to affect life and the impact reduces. These are the deep focus EQ's in the Benioff Zone associated with destructive plate margins.
- However, if the EQ's focus and epicentre are close to civilisation (intermediate & shallow focus), the the destruction tends to be much worse as the ground shaking is greater. Tsunamis and landslides are 2 issues that could occur (Japan 2011)
- The magnitude of a EQ affects the suffering caused. Low magnitude EQ's (usually less than 5/6 on RS) are less damaging but higher mag (above 6) tend to cause major threats to human life
Nature of bedrock:
- Some materials become jelly like when shaken - LIQUEFACTION, and is associated with clay and silts.
- Towns and villages built on this material are at risk of their building sinking into the saturated ground (Mexico City, 1985)
Why some places suffer more from EQs than others
Level of development:
- MEDC's can invest in protection measures like aseismic buildings and education of what to do in an EQ. This can help to reduce the impact of an EQ and prevent loss of life.
- MEDC's like Japan can also invest in developing warning systems to allow for better protection measures.
- LEDC's don't have the money to pay for expensive technology and educate their people so they tend to be at greater risk of an EQ destroying their lives (Boxing Day EQ 2004)
- Places with aseismic buildings are more likely to have reduced effects if an EQ occurs as the most common cause of death is due to collapse. These can withstand the shaking in most cases so life is more protected (Taipei 101)-more common in MEDC's
- Adobe buildings/structures (mud) and non-aseismic/seismic buildings pose a greater threat on life as they could pancake or topple over if subjected to enough shaking-these are associated with LEDC's.
Why some places suffer more from EQs than others
- A natural event only becomes a hazard when it impacts on human activity. 70/100 largest cities in the world lie in EQ zones. Many are around the Pacific Rim.
- Their densely packed buildings and raised road systems pose a huge risk to life if a major EQ was to occur.
- Smaller, less populated areas are still at risk but the death toll is usually going to be much less.
- Places which lie on plate boundaries suffer from EQ more than those in the centre of the plates.
- Countries on destructive plate boundaries suffer from a greater amount of EQ's than those at constructive e.g. Japan and S. America.
- Locations like Hawaii suffer from EQ's as they lie above hotspots meaning they are at higher risks than places that aren't above a hotspot.