(1.2) Global Seismology and Potential Field Techniques Flashcards
How are earthquake first motion pulls represented on a diagram?
White
How are earthquake first motion pushes represented on a diagram?
Black
What is a lower focal hemisphere plot?
“Beach-ball” diagram - plotting first motion pull and pushes of an earthquake
What is the correct name for a diagram showing first motion pulls and pushes of an earthquake?
Lower Focal Hemisphere Plot
What does the red line show on the following diagram?
Dextral fault trending NE
What does the red line show on the following diagram?
Normal, steep fault downthrowing to SW
What does the red line show on the following diagram?
Thrust. steep fault with top to NW
What does the red line show on the following diagram?
Normal plus Strike slip on shallow fault tranding NNE
What does the blue line show on the following diagram?
Sinistral fault trending SE
What does the blue line show on the following diagram?
Normal, shallow fault downthrowing to NE
What does the blue line show on the following diagram?
Thrust, shallow fault with top to SE
What does the blue line show on the following diagram?
Normal plus Strike slip on shallow fault tranding NNW
What is an Anisotropic Medium?
A medium which has a preferred direction of seismic wave travel
What is an Isotropic Medium?
A medium which acts the same in all directions
Give two prominent examples of an Anisotropic Mediums in modern Earth?
Olivine crystals which have undergone strain, Fluid filled voids with a preferential direction
What can anisotropic Olivine give an indication of?
Strain in upper mantle
How can the theory of anisotropy be used for reservoir identification in basins?
The presence of preferentially orientated fractures containing hydrocarbons
Give a modern day example where the presence of magma is aiding rifting by reducing friction
East African Rift Valley
How does the theory of anisotropy relate to the East African Rift Valley
There is a preferred orientation of fractures running parallel to the rift which contain magma in abundance
Why are nearly all of the biggest earthquakes occurring around the edges of the Pacific Ocean?
Rocks are stronger in compression than tension, so store more energy before brittle failure occurs. Pacific margins are areas of compression and so release higher amounts of energy than a spreading centre
What was the biggest ever earthquake?
Chile, 22/05/1960, Mw 9.5
What is the CTBT and why are seismometers important to it?
Comprehensive Test Ban Treaty for Nuclear Weapons Development. These types of explosions actually have a unique signature in the seismological record and thus can be recognised over a normal seismic release
What did the study by Earle and Shearer (1994) set out to achieve?
Creation of an automatic phase picker for P and S wave arrival times
What was the main advantage of the algorithm created by Earle and Shearer (1994)?
Lots of data can be processed quickly and efficiently to determine source of earthquake
Who were the researchers whom created the arrival time algorithm?
Earle and Shearer (1994)
What was the main disadvantage of the arrival time algorithm created by Earle and Shearer (1994)?
Humans are better able to destinguish the arrival time manually
What is the name of the researchers whom studied the Northern Ethiopian Rift?
Kendall et al (2005)
What did the model created by Kendall et al. (2005) determine?
Very likely that melt induced anistropy aided in rift
Who were the researchers whom observed six plumes stretching into the lowermost mantle?
Montelli et al. (2010)
Which were the six plumes which Montelli et al. (2010) determined streched into the lowermost mantle?
Ascension, Azores, Canary, Easter, Samoa, Tahiti
What did Montelli et al. (2010) suggest for those plumes which were restricted to the upper mantle?
The possibility of the existance of two depth regiemes in plume development
How did Montelli et al. (2010) suggest that mantle plumes effected the entire heat regieme of the earth?
Plumes contribute towards a substantial fraction of internal heat escape
How large did Montelli et al. (2010) suggest mantle plumes are based on their tomographic findings?
Several hundreds of km’s
Who were the researchers whom used tomography to determine the process of the Indian plate collision with Asia?
Replumaz et al. (2004)
What did Replumaz et al. (2004) suggest happened to the Indian Plate’s mantle?
Over-ridden by the Indian plate, not significantly thrusted beneath Tibet
How much deformation was absorbed by the Asian plate during the Indian collision according to Replumaz et al. (2004)?
~1500km
What does the Indian plates slab look like at 1100 km and 700 km?
Linear at 1100 km, becomes more deformed with shallowing - so 700 km highly deformed
What do the components of the following equation mean?
F = Force, G is the Universal Gravitational Constant (~6.7 x10^-11), r is the seperation between m1 and m2, the masses which are interacting gravitationally
What happens to gravity with increasing seperation? What is another name for this law?
Rapidly decreases with increasing seperation - inverse square law
Substitute Newton’s second Law into the following equation and simplify.
F= G m1 m2 / r^2
F = ma where m is the smaller second mass, m2, giving:
How much does “g” vary in gravitational surveys?
up to 100 micrometer s-2
What are the two controlling factors influencing changes in “g” in gravitational surveys?
Densities of rocks, pore fluids
What does 1 Gal =?
0.01 m s-2
Who were the researchers whom observed amagmatic ridge segments?
Dick et al. (2003)
What is the cause of amagmatic ridge segments according to Dick et al. (2003)?
Ultraslow spreading ridges
What defines a ultraslow spreading ridge according to Dick et al. (2003)?
12 mm yr-1 to 20 mm yr-1
What are the two examples of ultraslow spreading ridges given by Dick et al. (2003)?
Southwest Indian and Arctic ridges
What are the two features of ultraslow spreading ridges given by Dick et al. (2003)?
Intermittent volcanism and lack of transform faults
What is special about the formation of amagmatic segments and their potential relationship to transform faults according to Dick et al. (2003)?
Assume any orientation and hence have no requirement for transform faults
What is the Free Air Anomaly correlated to? (3)
Topography, components of mass burial, non-spheroidal shape of Earth
What is the Bouguer Anomaly?
What remains after gravitational effects of topography taken into account
Why does the Bouguer Anomaly have an inverse correlation with topography?
Isostatic compensation at deeper levels
How can we correct the Bouguer Anomaly so that the signals due to mass variations in the uppermost mantle are isolated?
Using models of crustal thickness
What would be a good measurement scale for a gravimeter?
1 micro m Gal
What do gravimeters measure?
Change in gravity of a mass at various locations
Why does altitude need to be corrected for when measuring gravity?
Takes the measurement further away from the center of mass (Earth)
What is a common datum used in the Free Air Anomaly?
Sea level due to good approximation of geoid
How do we correct the geoid using the free air correction?
Free air correction is 0.3086 mGal/m * the height in m above sea levels giving the answer in mGal
Why does the Free Air Anomaly retain a strong positive correlation with topography?
Density contrast between rocks and air is large, so need to correct using Bouguer Anomaly
What are the two steps to calculating the Bouguer Anomaly?
Slab correction then Terrain correction
What is the slab correction in calculating the Bouguer Anomaly?
A hypothetical slab of rock is added ontop of the Free Air anomaly which equates to the height of the site of measurement (2_G_h ms-2)
What is the terrain correction in calculating the Bouguer Anomaly?
Removes the effect of terrain variablilty - i.e. compensates for the slab correction stage
What is the standard product of most gravity surveys?
Bouguer Anomaly
Why is the Bouguer Anomaly useful?
With elevation and terrain effects removed, it highlights rock density variations within the crust and mantle
Why might the Bouguer Anomaly be less useful over larger scales, such as those including both Oceanic and Land surveys?
Oceanic particularly dense, thus anticorrection occurs
What might an image of the Bouguer Anomaly show over a wide expanse of oceanic mantle?
Density differences due to expanded/less dense/hotter mantle
How can we attain a image of the Bouguer Anomaly of the oceanic mantle? Why is this harder to achieve with Continental imagary?
Oceanic lithosphere generally uniform density - can remove it’s effect. Doesn’t work with continental due to mountain roots
What are the four corrections needed when using the sea as an estimation of the geoid?
Salinity, Currents, Waves, Ice
How do we attain gravity information for the geoid?
Radar altimeters fitted to satelites
What resolution is given for the geoid using satellite imagary?
15km resoultion
Why are satellite missions with altimeters not a full picture of the geoid?
They rely on the geoid to compute the height of the orbit
What are the two satellite missions set to overcome the geoid altimeter problem?
GRACE and GOCE
How does GRACE work?
Two satellites, on the same orbit, small distance appart - first satellite crosses a mass and speeds away from second - overall shape computed
Why is a global view of the geoid useful? Give one example
Water content in the amazon - changes in water can depress geoid by 12mm in a wet season
How does GOCE work?
Flies closer to the surface so smaller gravitational changes are easier to sense
Aside from water changes in the Amazon, give five other uses of the GRACE mission
Changes in mass in the polar ice caps, Exchanges of water vapour between oceans and atmosphere, Earthquake movements, Lava in volcanoes, Minerals prospecting
Why are gravity anomaly surveys not useful on their own?
A gravity signature could be created from a combination of different densities and depths - often these are preliminary to seismic surveys
Who were the authors who reconstrcuted paleo-plate movement in the South Pacific from gravity anomaly data?
Eagles et al (2004)
What did Eagles et al (2004) manage to observe?
Gravity anomalies in the south pacific which lead to the reconstruction of plate movements
What are the units of magnetic field strength?
Tesla
How does magnetic field strength vary with distance seperating the bodies?
Inverse square
Because anomalies in rocks are so small, literature tends to use ____ as a unit
nanoTesla
Magnetic field strength is a ___ quanitiy, as it has both direction and magnitude
Vector
Why do we assume that the core has an electrical origin to drive it’s magnetism?
Permanent magnitisation cannot exist as the core is too hot
What is the name for the overall drive of Earth’s magnetic field?
Geodynamo
What must have initiated Earth’s geodynamo?
Strong external magnetic field - possibly young sun?
What are the names of the satellites which have recorded changes in the magnetic core field between 1980 and 2000?
Oersted and Magsat
What kind of timescale does magnetism operate on, and what is the cause of the shortest timescale?
Very short geological timescales, with solar wind causing hourly and daily fluctuations
Low amplitude magnetic anomalies tend to have a ___ source, where as ___ anomalies are more typical of ____ and ___ rocks found in ___.
Deeper, strong anomales, igneous and metaorphic basin rocks
What does this map of magenic anomales generally indicate about Scotland and England/Wales?
Scotland has higher amplitude anomalies, thus likely to be dominated by shallow igneous/metamorphic. England dominated by deeper basement rocks
Basalt has a high ___ ___ giving the ability to trase magnetic stripes on the sea floor
Remnant magnetism
What is used to measure magnetic susceptibility over large areas, and on what platform?
Helicopters dragging Magnotometers
What is the problem with helicopters having magnetometers on board, and how is it overcome?
Helicopters are slightly magnetically susceptable, thus magnotometer is usually dragged on a tow line (or left in place)
Who are the authors who published on using gravity anomalies to determine the depth and location of the Chicxulub Crater?
Hildebrand et al. (1998)
What did Hildebrand et al. (1998) find in reguards to gravity anomalies in the crater fill?
Strong negative anomaly in the fill
Name some of the uses that Okuma et al. (2001) has described for aeromagnetic surveying in Japan (5 total)
Faulting/fault zones, debris avalance deposits, volcanic activity, subsurface ancient volcanoes, subsurface faulting
Why is care needed when observing gravity and magnetic anomalies on maps?
May be a result of data smearing with distance from anomaly OR may be result of less intense/wider mapping
What happens to a magnetically susceptible body when it interacts with Earth’s inclined field?
The resulting field from the body will also be inclined
The field strength variation of a magnetically susceptable body will ___ and ___ components which are centered where?
Positive and negative - not centered over buried body
When will dipole effects not occur?
When the inducing field is vertical, or if the induced body strikes parallel to azimuth of inducing field lines
What is “reduction to the pole”?
A mathematical proess where magnetic anomalies appear as if they have been recorded in a region with a vertical inducing field - dipole component is removed
Why is “reduction to the pole” useful?
Anomalies become centered over their source bodies allowing for easier interpretation
When may the dipole component remain when using “reduction to the pole”?
When there is already remanent magnetisation
Give an example where reduction to the pole can be used
Over a volcanoe - can see higher magnetisation and thus fresher lava ridge
What is upward continuation?
A mathematical technique in which potential field anomalies decrease in amplitude and increase in width with seperation between detecotr and source body increases
