Mountain Lecture 2- Earthquake! How seismicity builds mountain ranges

Question 1

What gives rise to stick-slip behaviour and why?

Answer 1

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.

Question 2

What are three faults with the earthquake cycle?

Answer 2

1. Local rock strength is neither constant in time nor uniform in space along a fault.
2. The rate at which stress accumulates in the crust is not constant.
3. Each earthquake affects the stress on other faults nearby.

Question 3

What are the four alternative models?

Answer 3

Periodic/characteristic model
Time-predictable model
Slip-predictable model
Clustered model

Question 4

What is the periodic model?

Answer 4

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.

Question 5

What is the time-predictable model?

Answer 5

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.

Question 6

What is the slip-predictable model?

Answer 6

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.

Question 7

What is the clustered model?

Answer 7

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.

Question 8

What model do most faults follow?

Answer 8

Clustered model

Question 9

What do alternative models seek to do?

Answer 9

Relate short-term coseismic activity to long term slip between the plates.

Question 10

What are moving towards instead of models to predict earthquakes?

Answer 10

Designing early warning systems and protocols based on the worst-case scenario.

Question 11

What is the best way to assess the magnitude of an earthquake?

Answer 11

The amount of energy it releases

Question 12

What is the amount of energy an earthquake proportional to?

Answer 12

Seismic moment

Question 13

What is the seismic moment equation?

Answer 13

Seismic moment = constant (around 3 x 10¹⁰ N/m²) x average slip of the earthquake x area of the fault that has ruptured

Question 14

What is the seismic moment equation in symbols?

Answer 14

Mₒ = ∝UA

Question 15

What is the equation for the area of the fault that has ruptured (A)?

Answer 15

A = length of rupture x depth of rupture (no more than around 10-20km for continental earthquakes)

Question 16

What is the unit for seismic moment?

Answer 16

Nm (force x length) - the units of energy.

Question 17

Can seismic moment be converted into a moment magnitude?

Answer 17

Yes

Question 18

Moment magnitude

Answer 18

Mᵥᵥ

Question 19

What does increasing the magnitude by +1 mean?

Answer 19

Around 30x the energy release

Question 20

What are the three biggest earthquakes?

Answer 20

1960 Chile
2004 Sumatra
1964 Alaska

Question 21

What is magnitude of 1960 Chile earthquake?

Answer 21

Mᵥᵥ 9.5

Question 22

What is magnitude of 2004 Sumatra earthquake?

Answer 22

Mᵥᵥ 9.3

Question 23

What is magnitude of 1964 Alaska earthquake?

Answer 23

Mᵥᵥ 9.2

Question 24

What is exhumation?

Answer 24

Movement of rock to the Earth’s surface

Question 25

What happens during oceanic-continental convergence?

Answer 25

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).

Question 26

What happens during oceanic-continental convergence (inter-seismic phase)?

Answer 26

Fault is locked. Upper plate bends (continental), leads to subsistence close to fault and uplift farther inland.

Question 27

What happens during oceanic-continental convergence (co-seismic phase)?

Answer 27

Fault slips. Continental plate relaxes and pattern of uplift and subsistence reverse.

Question 28

What pattern at the fault for oceanic-continental convergence can be identified?

Answer 28

A repeated, predictable pattern of uplift and subsidence at any one point near the fault.

Question 29

What are the three phases of the earthquake cycle?

Answer 29

Pre-seismic Coseismic Post-seismic

Question 30

What happens in the pre-seismic phase - oceanic-continental convergence?

Answer 30

Mostly elastic (recoverable) strain accumulation; no fault movement. Mainly longer term (10-10000 years)

Question 31

What happens in the co-seismic phase- oceanic-continental convergence?

Answer 31

Rapid strain release in an earthquake (seconds-hours).

Question 32

What happens in the post-seismic phase- oceanic-continental convergence?

Answer 32

Relaxation and more rapid strain accumulation, but decaying with time (hours-years).

Question 33

What is an example of uplift at continental-continental convergence?

Answer 33

Main Himalayan Thrust

Question 34

What happens at continental-continental convergence in terms of uplift?

Answer 34

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.

Question 35

What happens in the pre-seismic phase - continental-continental convergence?

Answer 35

Broad and diffuse uplift due to motion (ductile deformation) along thrust.

Question 36

What happens in the co-seismic phase - continental-continental convergence?

Answer 36

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.

Question 37

What happens in the post-seismic phase - continental-continental convergence?

Answer 37

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.

Question 38

What did the 1983 Borah Peak earthquake (Idaho) cause and show us?

Answer 38

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.

Question 39

What are mountains built by?

Answer 39

Repeated and relatively small earthquakes and so takes a long time.

Question 40

Name an earthquake which caused a large fault scarp.

Answer 40

2016 Kaikoura earthquake in New Zealand.

Question 41

What are mountain belts composed of?

Answer 41

Multiple faults and folds.

Question 42

How can we understand the dynamics of an entire mountain range?

Answer 42

Once we know how a single fault growa and accumulates displacement.

Question 43

Is tectonic rock uplift the same as surface uplift?

Answer 43

No

Question 44

Why is tectonic rock uplift not the same as surface uplift?

Answer 44

Because we must also allow for erosion (denudation)

Question 45

How do we work out surface uplift?

Answer 45

Surface uplift = rock uplift - denudation

Question 46

What happens if denudation is faster than rock uplift?

Answer 46

Negative surface uplift (mountain elevation decreases even if these is tectonic activity).

Question 47

What is isostatic rock uplift?

Answer 47

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.

Question 48

What happens if erosion is non-uniform?

Answer 48

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.

Question 49

What is the difference between seismic moment and moment magnitude?

Answer 49

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).

Question 50

If erosion is non-uniform, what can cause mountain peaks to rise higher than their original elevation without tectonic activity?

Answer 50

Isostatic rock uplift

Question 52

If denudation rates exceed rock uplift rates, what happens to the surface elevation of your mountain range?

Answer 52

Surface uplift would be negative, which means your net surface uplift would be less than zero – or a lowering of the surface.

Question 53

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).

Answer 53

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).