Tectonics and Geodynamics (L9-14) Flashcards
What makes up the earthquake cycle?
Interseismic strain accumulation
Coseismic slip
Postseismic deformation
Outline the interseismic strain accumulation part of the earthquake cycle
Cool upper crust warped elastically and strain builds up
Can take decades to tens of thousands of years
Outline the coseismic slip part of the earthquake cycle
Elastic strains built up enough to not be supported elastically
Rocks break along a fault
Elastic strains relieved by fault slip
Outline the postseismic deformation part of the earthquake cycle
Stress changes from slip is relaxed
Minor deformation
What are the three ways to study earthquake cycles?
Field observations
Seismology
Satellite data
How can the location of an earthquake be estimated from seismology?
Time delay between P- and S-waves relates to distance travelled in the Earth
Using three or more seismometers gives a location
How can the depth of an earthquake be estimated from seismology?
Use depth phases: waves that travel up from the source and bounce off the surface and travel directly to the seismometer
Time delay between direct wave and depth phase gives the earthquake depth
How can the magnitude of an earthquake be estimated from seismology?
Higher-magnitude earthquakes produce higher-amplitude waves
What are the three measurements of fault geometry?
Strike and dip of the fault plane Direction of motion on the fault plane = rake Rake = 0/180 = strike-slip Rake = 90 = thrust Rake = 270 = normal
How are focal mechanisms represented on a stereonet?
White quadrant = ground moving away = dilatational
Coloured quadrant = ground moving towards = compressional
Fault + auxilliary plane = nodal planes = no P-waves received
What does the focal mechanism of a normal-faulting earthquake look like?
White in the centre
Colour on either side
What does the focal mechanism of a thrust-faulting earthquake look like?
Colour in the centre
White on each side
What does the focal mechanism of a strike-slip-faulting earthquake look like?
Four visible quadrants
2 coloured, 2 white
What tool is used to estimate plate motions?
Velocity vector diagrams
For plate motion, what would a velocity vector of B_V_A represent?
The motion of plate A relative to plate B
How can the geometry of plate motion be expressed on a sphere?
A rotation about an axis that passes through the centre of the Earth (an Euler pole)
How can a mid-ocean ridge be used to establish the location and rate of an Euler pole?
Transform faults are perpendicular to the local direction of the Euler pole
Lines perpendicular to transform faults meet at the Euler pole
How can the location and rate of an Euler pole be measured using satellite data?
If a large enough area of both plates is above sea-level, satellites can measure relative motion
What are stability lines used for in a velocity vector diagram?
Motion of a triple junction relative to the bounding plates
Whether the triple junction is stable
What are the rules for the stability line of an extensional plate boundary?
Triple junction must move at the same rate relative to both plates, as motion is symmetric
Stability line is perpendicular to AB
What are the rules for the stability line of a strike-slip plate boundary?
Triple junction must move along the boundary between the two plates
Stability lines is along AB
What are the rules for the stability line of a compressional plate boundary?
If plate A is consumed: triple junction can only move along a line parallel to strike of subduction passing through B
What classifies as an unstable triple junction?
What must happen?
If there is no coherent motion relative to all three plates
Evolves to stable geometries if the plates keep moving
Why don’t stable triple junctions exist forever?
One of the plates is being consumed (subduction)
Changes to the force balance driving the plates
What are the most important forces acting on plates?
Ridge push
Slab pull
Mountain range buoyancy
Basal drag
What is slab pull?
Force from a cold, dense, subducting slab pulling the plate down into the mantle
What is ridge push?
Force from bounding plates being pushed from the ridge
What is mountain range buoyancy?
Force from mountain range pushing against bounding plates to spread out due to its weight
What is basal drag?
Shear force on base of plates from relative motion between the plate and underlying mantle
What is the consequence of an earthquake on individual normal faults?
Subsidence of the hangingwall
Uplift of the footwall
What controls the widths of basins and mountains produced on normal faults?
Thickness of the brittle upper crust
What are the two ways present-day faulting can be distributed?
What is the implication of this?
Closely following old deformation belts
Spatially distributed
If pre-existing strength contrasts are too low, wide regions stretch
If strength contrasts are large then isolated rift valleys form along pre-existing weaknesses
What is the progression of extension to mid-ocean ridge formation?
Continental stretching
Crustal thickness reduces
Surface sinks below sea level due to isostasy
Extension continues enough for mid-ocean ridge formation
Why is the extension in ocean basins focused at the ridge axis?
Large strength contrast between a ridge axis and older oceanic lithosphere
What is the morphology of a mid-ocean ridge affected by?
How so?
Rate of magma production thus rate of extension
Slow-spreading: well-developed central rift valley and lots of normal-faulting earthquakes
Fast spreading: no central valley, often only earthquakes at the transform faults
What does the thickness of oceanic crust produced at a mid-ocean ridge depend on?
Temperature of underlying mantle that is upwelling and undergoing decompression melting
Hotter mantle = deeper solidus intersection = more melt
What is the implied temperature of underlying mantle from normal oceanic crust?
Normal oceanic crust = 7km
Therefore mantle has a potential temperature of 1300
What else does mantle potential temperature have an effect on?
The nature of the passive margins left behind on the edges of the continents after seafloor spreading begins
How does mantle potential temperature effect the nature of ocean-continent passive margins?
Normal-temp mantle = mantle too cool to melt = no large scale magmatism pre spreading= continental margin of thinned continental crust with rift sediments
Hotter mantle = significant magmatism pre spreading = intrusive + extrusive igneous rocks in sediments
What are natural non-tectonic earthquakes produced by?
Motion of fluids (melt or hydrous fluids) through the rocks
What are the two main effects of fluids moving through rocks in relation to natural non-tectonic earthquakes?
Fluids can be at high pressures = reduces effective normal stress to very low value or if above lithostatic pressure then produce new extensional fractures
Fluids can move very fast = very high strain rates
What is the result of the effects of fluids moving through rocks in relation to natural non-tectonic earthquakes?
Earthquakes associated with magma intrusion and hydrothermal circulation can occur in tectonically unexpected places
What are the ways humans induce earthquakes?
Fluids: reservoirs of water, moving fluids in the subsurface
Mine-related
What are the two effects of fluids moving in the subsurface?
Pore fluid pressure change
Volume/mass change results in stress changes in surrounding material
What are the two main settings in which earthquakes are intentionally produced by fluid injection?
Fracking for hydrocarbon extraction
Opening fractures for geothermal energy production
How does mining induce earthquakes?
Intentional blasts in active mines
Collapse of old mines
