9-14 Flashcards
Three stages of earthquake cycle
- Interseismic strain accumulation
- Coseismic Slip
- Postseismic deformation
Interseismic strain accumulation
Cool upper crust warped elastically - strain builds up.
- depends on rate of motion across fault zone and the slip from the resulting earthquake
Coseismic slip
The earthquake
- elastic strain 10-4 to 10-5
- cannot be supported elastically
- break of rocks - seconds to minutes depending on size
Postseismic deformation
stress changes due to slip are relaxed - causing deformation
Study earthquake cycles - 3 ways
- Location of event + map ruptures
- Satellite data - measure ground motion
- Seismology - so the waves produced
How to find location of earthquake with seismology?
P wave - faster than s
- so differential time can show how far it is
- TRIANGULATE FROM 3
- though they can use the direction of first wave motion as on Mars and only use one, but not very accurate
How to find earthquake depth
depth phases - waves travelling upward from source - p or s - bounce off surface near source and follow same path as direct wave to seismometer
- so difference from depth phases to direct waves indicate the depth
How to find magnitude of earthquake
mainly use moment magnitude
seismic moment
=Mw = 2/3(M0) - 10.73 where M0=μAu
μ= shear modulus A = fault area u = average fault slip
What is a faults geometry
describe fault plane with strike and dip
- direction of motion = rake = relative strike direction in the plane of the fault
Rake of strike-slip faults
0 or 180 degrees
what rake represents thrust faulting
90
What rake is normal faulting
270 or -90
Auxiliary plane
plane that runs at 90 degrees to fault plane
Νodal planes
fault and auxiliary planes
P and T axes
orientation with most compression and extension
Hotspot reference frame
The theory that cause use the movement of plates over hotspots in order to determine absolute movement of plates
- atm its just relative plate motion
- just dont know if plumes are fixed in place
Triple junction
point at which three plates meet
Stability lines
lines that tell us where the triple junction can sit on a vector diagram
2 reasons why configurations of plate boundaries change over time
- one plate entirely subducted under another - like what will happen to Juan de Fuca and Nazca plates
- change in the distribution og forces acting on the plate
4 plate driving forces
- Slab pull
- Ridge push
- Mountain range buoyancy forces
- Basal drag
What is slab pull
the force resulting from the cold and dense subducting slabs sinking into the mantle - pull the plate along
What is ridge push
mid-oceans ridges are elevated above older and cooler sea floor
- so exert an outward force that pushes the bounding plates away from the ridge
What are mountain range buoyancy forces
the tendency of mountains to spread under their own weight
- the gravitational potential energy differences
What is basal drag
the shear force on the plate base due to relative motion between plate and underlying mantle
example of abandoned plate boundaries
central Indian ocean - wharton ridge abandoned when convergence rate between india and asia rapidly decreased - changing forces on the entire plate
ways of accommodating continental convergence
- Mountain building (continental thickening)
2. Lateral expulsion
why can deep earthquakes (depths of over 600km) even exist - theories x 3
- Presence of high-pressure fludis that have been brought down with the slab, or released by metamorphic dehydration reactions, reduced effective normal stress - allow faulting to occur
- Mineral volumetric changes due to metamorphic reactions generate large enough stresses to cause faulting
- a thermal run away effect - any fault motion pressure so high that friction causes melting of plane - so slip continues
- but that needs slip to start
why do passive margins often have layers of salt
when little extension has occured, like in east african rift, above sea level with some lakes in the rift valleys which can lead to evaporates
Lifecycle of extension - doesnt always run to completion
- Continental stretching - East African Rift
- Surface drops below sea level due to isostacy - The Aegean
- Seafloor spreading - Red seas and Gulf of Aden
What are characteristic differences between slow and fast spreading ridges?
slow = mid atlantic ridge - well-developed central rift valley and many normal faulting earthquakes
fast = east pacific rise - lake a central valley and have few moderate or large mag earthquakes
how thick is oceanic crust
roughly 7 km
how thick is continental crust
30km
why is the crust below iceland thick
becuase it sits over a mantle plume, meaning there is increased mantle temperatures and increased volume of melt - this cools and causes thicker crust
how are natural non-tectonic earthquakes produced
motion of fluids - either melt or hydrous fluids
- can be at high temperatures and reduce effective stress on pre-existing faults
- or produce new extensional fractures - fluids can move fast - strain rates can be high
Human induced earthquakes
- reservoir filling
- Fracking
- Mining
- Disposal of wastewater in subsurface
How can reservoir filling cause earthquakes
- example = Thrust-faulting mag 5 in Thompson Australis 1996
- suggest increased pore-fluid pressure - because of thrust faulting which would not occur without as pressure on surface
What is a hazard
something that can cause harm
What is risk
The likelihood of harm from a hazard
How do we work out hazard by earthquakes
- recent earthquakes - time between
2. tectonic geomorphology - signs of past earthquakes in the landscaps
How to reduce earthquake risk
- policy and making buildings better
2. warning systems
