Mountain Lecture 2- Earthquake! How seismicity builds mountain ranges Flashcards
What gives rise to stick-slip behaviour and why?
The frictional properties of the Earth’s brittle upper crust (< 15km or so).
This is due to the sides of the fault being loaded by relative plate motion.
What are three faults with the earthquake cycle?
- Local rock strength is neither constant in time nor uniform in space along a fault.
- The rate at which stress accumulates in the crust is not constant.
- Each earthquake affects the stress on other faults nearby.
What are the four alternative models?
Periodic/characteristic model
Time-predictable model
Slip-predictable model
Clustered model
What is the periodic model?
Same size earthquake occurs after the same amount of stress has accumulated. Earthquakes at a constant interval.
With a known interval, we can predict the timing and magnitude of future events.
What is the time-predictable model?
Strain required for earthquake is constant, but the size of the earthquake can vary.
After a large earthquake, it takes longer to reach the threshold strain energy again.
The time to next earthquake can be estimate but not its magnitude.
What is the slip-predictable model?
Strain to cause earthquake varies, but the earthquake always releases enough energy to return to same state.
The longer since the last earthquake, the larger the earthquake.
The magnitude of next earthquake can be estimated (based on how long since the last) , but not its timing.
What is the clustered model?
In areas of insufficient data, earthquakes seem to occur in clusters of lots of events, separated by times of relative quiet (10-10000 years long).
Clusters can follow one of the previous models or be apparently random.
Neither the magnitude nor the timing of the next earthquake can be predicted.
What model do most faults follow?
Clustered model
What do alternative models seek to do?
Relate short-term coseismic activity to long term slip between the plates.
What are moving towards instead of models to predict earthquakes?
Designing early warning systems and protocols based on the worst-case scenario.
What is the best way to assess the magnitude of an earthquake?
The amount of energy it releases
What is the amount of energy an earthquake proportional to?
Seismic moment
What is the seismic moment equation?
Seismic moment = constant (around 3 x 10¹⁰ N/m²) x average slip of the earthquake x area of the fault that has ruptured
What is the seismic moment equation in symbols?
Mₒ = ∝UA
What is the equation for the area of the fault that has ruptured (A)?
A = length of rupture x depth of rupture (no more than around 10-20km for continental earthquakes)
What is the unit for seismic moment?
Nm (force x length) - the units of energy.
Can seismic moment be converted into a moment magnitude?
Yes
Moment magnitude
Mᵥᵥ
What does increasing the magnitude by +1 mean?
Around 30x the energy release
What are the three biggest earthquakes?
1960 Chile
2004 Sumatra
1964 Alaska
What is magnitude of 1960 Chile earthquake?
Mᵥᵥ 9.5
What is magnitude of 2004 Sumatra earthquake?
Mᵥᵥ 9.3
What is magnitude of 1964 Alaska earthquake?
Mᵥᵥ 9.2
What is exhumation?
Movement of rock to the Earth’s surface
What happens during oceanic-continental convergence?
The continental crust warps as stress builds up along fault (subsistence at fault, uplift of mountains).
Earthquake releases the stress and the crust ‘pings back’ (uplift at fault, subsistence of mountains).
What happens during oceanic-continental convergence (inter-seismic phase)?
Fault is locked.
Upper plate bends (continental), leads to subsistence close to fault and uplift farther inland.
What happens during oceanic-continental convergence (co-seismic phase)?
Fault slips.
Continental plate relaxes and pattern of uplift and subsistence reverse.
What pattern at the fault for oceanic-continental convergence can be identified?
A repeated, predictable pattern of uplift and subsidence at any one point near the fault.
What are the three phases of the earthquake cycle?
Pre-seismic
Coseismic
Post-seismic
What happens in the pre-seismic phase - oceanic-continental convergence?
Mostly elastic (recoverable) strain accumulation; no fault movement.
Mainly longer term (10-10000 years)
What happens in the co-seismic phase- oceanic-continental convergence?
Rapid strain release in an earthquake (seconds-hours).
What happens in the post-seismic phase- oceanic-continental convergence?
Relaxation and more rapid strain accumulation, but decaying with time (hours-years).
What is an example of uplift at continental-continental convergence?
Main Himalayan Thrust
What happens at continental-continental convergence in terms of uplift?
Thicker crust means that faults can be much deeper than oceanic crust and therefore behave differently (more ductile).
Patters of uplift vary in space and time.
Size of earthquake affect how much mountain building is achieved.
What happens in the pre-seismic phase - continental-continental convergence?
Broad and diffuse uplift due to motion (ductile deformation) along thrust.
What happens in the co-seismic phase - continental-continental convergence?
Small earthquake (only a part of the fault moves), so only part of the surface is affected. This can be seen with 2015 Gorkha earthquake.
What happens in the post-seismic phase - continental-continental convergence?
large earthquake (most of the fault moves), so a much larger area is affected and the uplift is greater.
This can be seen with the 2008 Wenchuan earthquake.
What did the 1983 Borah Peak earthquake (Idaho) cause and show us?
Caused a break/scarp which was 2-3m high.
The pattern of slip shows that the mountains have uplifted in this way over time by earthquake events.
What are mountains built by?
Repeated and relatively small earthquakes and so takes a long time.
Name an earthquake which caused a large fault scarp.
2016 Kaikoura earthquake in New Zealand.
What are mountain belts composed of?
Multiple faults and folds.
How can we understand the dynamics of an entire mountain range?
Once we know how a single fault growa and accumulates displacement.
Is tectonic rock uplift the same as surface uplift?
No
Why is tectonic rock uplift not the same as surface uplift?
Because we must also allow for erosion (denudation)
How do we work out surface uplift?
Surface uplift = rock uplift - denudation
What happens if denudation is faster than rock uplift?
Negative surface uplift (mountain elevation decreases even if these is tectonic activity).
What is isostatic rock uplift?
Continental crust ‘floats’ at the surface of the denser mantle.
Adding mass to the crust causes the crust to sink.
Removing mass causes the crust to rebound or rise.
Denudation of mountains can therefore lead to isostatic rock uplift/rebound (around 80% of total denudation).
Net surface loweing after denudation is smaller than we may think.
What happens if erosion is non-uniform?
Isostatic rock uplift can cause the peaks to rise higher than their original elevation, even without tectonic activity.
Erosion can cause surface uplift and build mountains.
What is the difference between seismic moment and moment magnitude?
Seismic moment is the amount of energy released by an earthquake. The moment magnitude is the measure of the earthquake’s magnitude (‘size/strength’) is based on the seismic moment (a scale that allows us to compare earthquakes).
If erosion is non-uniform, what can cause mountain peaks to rise higher than their original elevation without tectonic activity?
Isostatic rock uplift
If denudation rates exceed rock uplift rates, what happens to the surface elevation of your mountain range?
Surface uplift would be negative, which means your net surface uplift would be less than zero – or a lowering of the surface.
With oceanic-continental convergence, where do we get subsidence and uplift? Fill in the gaps with either ‘uplift’ or ‘subsidence’. The continental crust warps as stress builds up along fault (______ at fault, ______ of mountains). Earthquake releases the stress and the crust ‘pings back’ (______ at fault, ______ of mountains).
The continental crust warps as stress builds up along fault (subsidence at fault, uplift of mountains). Earthquake releases the stress and the crust ‘pings back’ (uplift at fault, subsidence of mountains).
