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
