# Presentation: 709010

## Session I.3

- The Earth’s Structure and Seismicity

## The Earth’s Layers

**The Earth’s Layers**

**Notes:**
Mechanical layers and chemical layers
Primary divisions
Crust
Oceans 7-8 km thick, less than 200 Ma old and relatively constant in properties
Continents <80 km thick, average ~40 km, can be old or young, highly heterogenious
Mantle: chemically and physically distinct, richer in Mg and Fe, fluid.
Outer core: Liquid, mostly Fe with some Ni O S
Inner core: Solid, Fe
smaller features and other divisions
Lithosphere (rigid, the plates in Plate Tectonics, thermal boundary layer)/Asthenosphere (fluid
Upper mantle/lower mantle
Mantle transition zone – olivine (Fe,Mg silicate) -> spinel (410km) -> perovskite (670 km)
D’’ layer – low and high velocity zone (post-perovskite phase)

## Slide 3

**Notes:**
The lithosphere is a the cool rigid layer, like the skin that forms on a cooling liquid. This skin is broken into plates which move relative to one another.
Where plates move apart new oceanic crust and lithosphere is generated – “constructive boundaries”.
Where plate move together – “destructive boundaries” - what happens depends on the natures of the lithosphere at the boundary:
Old oceanic lithosphere is denser than the mantle beneath it and therefore unstable and can be subducted, continental crust is very low density compared to the mantle and so the continental lithosphere cannot be subducted.
So, if one or both plates are oceanic in nature than one of the plate subducts down into the mantle
But, If both of the plates are continental than the two plate plough into one another and crumple up to accommodate the motion, generating large mountain ranges.
Plates can also slide past one another no net movement together or apart, in these cases lithosphere is neither created nor destroyed “conservative boundaries” or “transform boundaries”.

The forces are generated by the cooling of the plates: 
The largest contribution is from “slab pull” – the cooling oceanic lithosphere becomes denser than the warm mantle beneath leading to gravitational instability and subduction. The density contrast between the plate and the mantle generates a force that pulls the plates down into the Earth. 
The other major contributing effect is “ridge push” – the combination of the upwelling mantle pushing the lithosphere away from the elevated mid-ocean ridges and the gravitational force of the plate ‘sliding’ down from the elevated ridge crest.
Ridge push is about an order of magnitude small then slab pull. Note that the fastest moving plates tend to have the greatest length of subduction zone.
Most plates appear to be moving at a constant speed; therefore the driving forces above must be opposed by resistive forces of the same magnitude. The largest resistive force is likely to be mantle drag (this assumes that the plates are moving faster than the mantle – otherwise it would be a driving force). There is also a contribution from friction at plate boundaries as indicated by the earthquakes.

## Slide 4

**Notes:**
Cartoon of the plate boundaries in 3D. Shows the relative motion of the plates at each of the boundaries and how the boundaries are different for oceanic and continental lithosphere

## Slide 5

**Notes:**
Because the continental crust cannot be subducted and plate driving forces are mostly generated by oceanic boundaries, the continents are long-lasting and have been carried around on the plates through time moving together and apart.

## Slide 6

**Notes:**
Although the plate move at reasonable constant velocity away from the plate boundaries, at the boundaries frictional forces mean that rather than the plates moving smoothly past or way from one another, they exert forces which cause deformation and earthquakes. 
At the boundaries the plates are often “locked” together and there is no relative movement between the two for a period of time, instead the plate motion is taken up as strain and deformation within the plates, then the accumulated strain is released suddenly as an earthquake. 

The vast majority of geological deformation and earthquakes occur at or near plate boundaries.

The global map of earthquakes shows strong bands of earthquakes which pick out the plate boundaries.

The destructive plate boundaries generally show a broader strip of earthquakes than the constructive boundaries

- 0 – 70 km deep

**Notes:**
Earthquake distribution can be looked at in terms of depth as well as geography. 

Here we are looking at how common earthquakes are within certain depth bands.  
Frequency of earthquakes varies around the globe, more common at destructive boundaries than constructive
constructive boundaries only have shallow earthquakes.

## Slide 8

**Notes:**
Spreading centers in the Indian Ocean have only shallow EQs whereas the subduction zones have shallow and deep earthquakes.

The broader geographical spread of earthquakes at subduction zones is related to the depth distribution. The deeper earthquakes are not located directly under the shallow earthquakes. This distribution follows the subducting slab and is called the “Wadati-Benioff zone”.

## Slide 9

**Notes:**
Look more closely at spreading centers

Passive upwelling of mantle to fill the space created as lithosphere pulled apart.

Mantle melts adiabatically (i.e. without input of heat), this melted material cools to form the new ocean crust. The structure of the crust varies depending on the spreading rate of the ridge, but because the ocean crust is young and the processes forming at the ridge are relative constant, the ocean crust is remarkably uniform in properties across the ocean.

The lithosphere is almost zero thickness at the ridge, but thickens away from the spreading centre as the lithosphere cools. It thickens to about 100 km thickness then remains constant. 

**Notes:**
Earthquakes occur along the axis of the ridge. 

Because the lithosphere is so thin, the earthquakes are limited to shallow depths

## Slide 11

**Notes:**
Spreading centers are not restricted to oceans. They can be seen in the continental lithosphere through East Africa.

Here spreading seems to be dominated by faulting, often along existing geological weaknesses or boundaries, such as the partitioning of the spreading to rifts either side of Lake Victoria. – Lake Victoria sits on a Craton, so rather then spliting this strong old crust, the strain is focused into rifts at the boundaries of the Craton, where the crust is weaker.

The Red Sea and Gulf of Aden are young oceans, here the processes are pretty much the same as in the mature ocean basins. 

So Ethiopia sits at the transition from faulting dominated spreading in Kenya to full sea floor spreading (dominated by magma generation, movement and cooling) to the north. Here we see a transition between the two processes.

## Slide 12

**Notes:**
Cartoon of this evolution of spreading centers

## Slide 13

**Notes:**
At Ocean-Ocean destructive boundaries we normally see a trench marking the plate boundary and a volcanic island arc on the over-riding plate.

The volcanism is the result of water driven off the subducting slab as the slab heats up. This water reducing the solidus temperature in the mantle above the slab resulting in partial melting.

## Slide 14

**Notes:**
At Ocean-Continent destructive boundaries the processes are very similar, but here the melt rises into the over-riding content volcanoes and Andean like mountain belts.

## Slide 15

**Notes:**
The earthquakes associated with subduction zones are divided into three groups depending on their depth.

Shallow earthquakes 
< 70 km deep
Associated with interaction between the two plates:
thrust fault earthquakes at the plate boundaries – the biggest recorded earthquakes are of this type (e.g. Chile 1960, Alaska 1964, Sumatra 2004)
normal fault earthquakes may be seen at the hinge associated with the bending of the plate. These can also be very large earthquakes (e.g. Rat Island 1965)
occur more frequently than intermediate and deep earthquakes

Intermediate earthquakes  
70 – 300 km deep
Occur in the interior of the subducting slab, near the slab top
mainly indicate down-dip tension

Deep earthquakes. 
> 300 km deep (separated from intermediate depth eqs because there is a minima in the frequency of events at ~300 km depth)
Occur within the subducting slab
Can be either a single plane or earthquakes or two distinctly separate planes.
mainly down-dip compression
depth and dip varies considerably
maximum depth (can be almost 700 km) of earthquakes appears to be related to the vertical decent rate and age of lithosphere. Deeper earthquakes occur where decent rate is higher and lithosphere is older. Therefore the maximum depth of earthquakes is probably limited by temperature probably related to temperature (the slab heats as it descends and this causes the rigidity to drop. Eqs stop when the slab can no longer support stresses high enough to cause significant eqs)

## Causes of intermediate and deep earthquakes

- Body forces, with increased resistance caused by lower mantle density increase

- variation in negative buoyancy caused by altered depth of phase transitions

**Notes:**
There are 2 possible causes for intermediate and deep earthquakes
Tension in the upper portion of the slab caused by the weight of the descending slab causes intermediate eqs, while deep earthquakes result from compression generated by increased density (and therefore resistance to subduction) in the lower mantle.
Reduced temperature in the slab (w.r.t. the surrounding mantle) causes up-warping of the olive-spinel (410 km) transition and down-warping of the spinel-perovskite transition resulting in increased and reduced buoyancy-related forces at the two transitions

## Cause of shallow thrust earthquakes

- Stein & Wysession, 2003

## Slide 18

**Notes:**
At continent-continent collision zones subduction is impossible, instead there is a very broad zone or deformation and major mountain belts may be formed.
There is no Wadati-Benioff zone and no intermediate or deep earthquakes.
In major collision zones such as the Himalayas the earthquakes are not only broadly distributed, but indicate the complex nature of these boundaries, with not only thrust earthquakes, but also strike-slip and normal faulting occurring in areas.

## Slide 19

**Notes:**
Simplified cartoon to illustrate the formation of the Himalayas by the collision of India into Eurasia.

## Transform boundaries

**Notes:**
Transform boundaries occur where one plate slides past another without significant convergence or divergence 

Transform boundaries occur on a range of scales, from relatively short sections along offsets on mid-ocean ridges to very large fault zones such as the San Andreas 

**Notes:**
Transform faults on spreading centers

## The San Andreas Fault System is a 

***The San Andreas Fault System is a ***
***Major Transform Fault***

## South East Asia

**COMPLEX PLATE BOUNDARY ZONE**
**Northward motion of India deforms the region**
**Eastward motion in China & SE Asia**
**Many small plates (microplates) and blocks**
**India subducts beneath Burma microplate**

- Molnar & Tapponnier, 1977

- SUNDA PLATE

- INDIAN PLATE

- EURASIAN
- PLATE

- BURMA
- MICROPLATE

## Partitioning of strain between thrust motion at the trench and strike-slip motion on the Sumatra fault

- Stein & Wysession 2003

## Regions of compression, tension and shearing stresses

-  – accommodated by   faulting

## The relationship between faults and earthquakes – 

***The relationship between faults and earthquakes – ***
***Elastic Rebound Theory***

## Crustal rocks at rest

***Crustal rocks at rest***
***Deformation; dilatancy and development of cracks***
***Instant rupture (earthquake)***
***Sudden drop in stress after earthquake***

## Classification of faults

**Notes:**
Types of fault

## Reverse faulting

## Slide 30

**Notes:**
examples of real faults

## Faults occur at many scales

## Brittle

- Ductile

## Slide 33

## Normal faulting

## Slide 35

**Notes:**
Examples of normal faults

## Ductile

**Notes:**
normal faults give way to shear zones at depth

## Strike-Slip

***Strike-Slip***
***Fault***

## Identifying faults - Fault Scarps

***Identifying faults - Fault Scarps***

## Normal and reverse faults both generate scarps

## Slide 40

**Notes:**
larger scale fault geomorphology

## Block Diagram showing features

***Block Diagram showing features***
***along a Strike-Slip Fault***

## San Andreas Fault

**San Andreas Fault**

- Wallace Creek

- Wallace Creek

## Faults at

***Faults at***
***Sea!!***

## Review

- The Earth lithosphere is broken into numerous plates, which move independently of each other.
- This results in regions of divergence (destructive boundaries), convergence (constructive boundaries) and regions where the plates slide past one another (transform or conservative boundaries)
- The vast majority of earthquakes occur at plate boundaries. The nature of these earthquakes is related to the processes at the boundaries.
- Earthquakes occur when strain built up over many years is released rapidly as brittle failure –  Elastic Rebound Theory.
- 3 end members of fault: strike-slip, normal, reverse/thrust.