Lecture 7 Flashcards
Why did the WWSSE come about?
The world wide standardised seismograph network=global network of seismometers
- 1963 when united test ban treaty
What are the properties of trenches?
They are long, arcuate and usually on the edge of ocean basis
- often convex towards the ocean
- upto 11km deep
- associated with island arcs (like Japan) or continental mountains (andes)
- sites of the worlds largest earthquakes - on thrust faults
- big gravity anomalies and aren’t in isostatic balance - shape maintained by flexure
What are the volcanoes associated with trenches made of?
Andesite magma
Normal vs thrust fault
Normal fault involves extension compared to trust faults which involve shortening
GRAPH
What is the process of subduction?
The process at trenched where the oceanic lithosphere is returned to the asthenosphere
- generally more than 70mm per year
What are Benioff zones?
The inclined zones where the subducted oceanic slabs are still cold enough to have earthquakes
- deepest earthquakes at roughly 670km by Japan
What do seismic images of subducted slabs show?
Low attenuation (damping)
high seismic velocity
- consistent with being colder than surrounding mantle
Where do andesitic volcanoes usually lie?
Where the dipping earthquake zone reaches a depth of roughly 100km
Roughly what depth do you need to be above to have earthquakes?
(deepest earthquakes)
670km
When do earthquakes occur?
when faults slip
- in oceans usually confined to narrow linear zones
- plate boundaries
- Shallow at ridges and only deep at trenches
- on normal faults at mid-ocean ridges, thrust faults at trenches and strike-slip faults at transforms
What is attenuation (Q)
the gradual loss of flux due to dissipation of energy
- so High Q means cold and not much damping effect
- low means lots of damping and so probably hot
What are the nature of the plate-boundaries
- ridges
- trenches
- transform faults
- constructive
- destructive
- conservative
Euler poles
the points where the theoretical axis emerges from earth when thinking about the horizontal motion of the surface as the angular rotation about said axis
- directions of this motion then follow small circles about that pole
Where are the Euler poles of transform faults?
motion is horizontal
- must lie perpendicular to their strike
- if faults bond rigid plates then the same euler pole must work for all of them
- so the perpendicular directions to faults on same plate boundary should all converge at a point—>a euler pole
What do euler poles found via perpendicular to strike lines, show?
describe relative motion between plates
Why do the interior of plates not deform?
They are rigid so deformation restricted to the plate boundaries
How to test if the plates are rigid?
- see whether transformation faults define a unique euler pole
- using mercator projection (where all lines of latitude are parallel, and the euler pole as the projection pole the horizontal directions of slip should be parallel to each other—>they are
What is a mercator projection?
a map drawn where the lines of latitude are parallel - usually with North pole as the projection pole
What is Geodesy
surveying on a planetary scale
- concerned with accurate measurement of the shape of the earth, positions of points on earth and distance between them
techniques used in space geodasy
- show that relative motions at plate boundaries are very close to estimates
- also that interiors of major plates really rigid <1mm per year
- VLBI=Very Long Baseline Interferometry
- radio telescopes to looks at distance radio sources->big and cumbersome and not really portable - SLR = Satellite laser ranging
- distance to an artificial satellite using ground-based lasers
- cumber some and not portable - GPS = global positioning system
- records signals for several orbital satellites and measures distance between them
- portable and easy to use - capable of mm precision over several hundred km
What is a back-arc basin?
occur on landward, concave side of many subduction zones
where sea-floor spreading above sinking slab
- helped by hot region above sinking slab near volcanic arch
