3.a. There is a variety of earthquake activity and resultants landforms and landscapes Flashcards
What is an earthquake?
A release of stress that has built up within the Earth’s crust caused by tension, compression, and shearing of rocks.
A series of seismic shock waves originate from what? What is this?
The earthquake focus.
An earthquake focus is the term used to describe the location where the stress is suddenly released.
What is the earthquake epicentre?
The point immediately above the earthquake focus, at the Earth’s surface.
What are some earthquake events preceded by? What do these indicate?
Preceded by several fore-shocks, often indicating a large event is likely.
This can give some populations several hours warning.
What often happens after an earthquake event?
A series of after-shocks follow the main quake, gradually reducing in intensity.
Which produce greater impacts, volcanoes or earthquakes?
The impacts of earthquakes significantly out-weigh the impacts of volcanoes.
Earthquake activity tends to be concentrated in one of four locations. What are they?
Ocean trenches and island arcs (convergent).
Transform plate margins.
Collision zones (convergent).
Mid-ocean ridges (divergent).
Which is the one of the four earthquake locations produces the strongest events? Why?
Ocean trenches and island arcs (convergent).
This is because there are compressive forces associated with the subduction of one plate below another.
Which is the one of the four earthquake locations produces the second strongest events? Why?
Transform plate margins.
This is because there are shearing forces associated with the intermittent movement of one plate past another.
Which is the one of the four earthquake locations produces the second weakest events? Why?
Collision zones (convergent).
This is because it features compressive forces associated with the grinding together of plates carrying continental crust.
Which is the one of the four earthquake locations produces the weakest events? Why?
Mid-ocean ridges (divergent).
This is because there are tensional forces associated with spreading processes and subsequent faulting and rifting.
What are seismic waves?
Waves that can travel both along the surface and through the layers of the Earth.
There are three types of seismic wave.
Outline primary (P) waves.
Fast-travelling.
Low-frequency compressional waves.
They vibrate in the direction in which they travel.
Travel through solids and liquids.
Outline secondary (S) waves.
Half the speed of P waves.
High-frequency waves.
They vibrate at right angles to the direction in which they travel.
Can only travel through solid material.
More powerful than S waves.
Outline surface (L) waves.
Slowest of the three types.
Low-frequency waves.
Some L waves have a rolling movement that moves the surface vertically, while other move the ground at right angles to the direction of movement.
How are earthquakes often categorised?
Categorised according to their depth of focus.
Outline shallow focus earthquakes.
Surface down to about 70 km.
Shallow quakes occur in cold, brittle rocks resulting from the fracturing of rock due to stress within the crust.
They are very common, with many releasing only low levels of energy, although other high-energy shallow quakes are capable of causing severe impacts.
What is triangulation?
The process by which multiple seismometers are used to locate an earthquake event.
Deep focus earthquakes occur in what range?
70 km - 700 km.
As you get closer to the core, what happens? What may happen to minerals? How does this relate to deep focus earthquakes?
Increasing depth, pressure - with temperatures increasing to very high levels.
Minerals change type and volume, which may contribute to a release of energy.
It is likely that dehydration of water in subducting plates plays a significant role in these events but scientists continue to evolve their ideas about these less frequent but often powerful quakes.
What does the Richter scale assess?
(The Richter Scale)
(Different measures of assessing earthquake magnitude)
Earthquake energy.
When was the Richter scale developed? How does it measure magnitude?
(The Richter Scale)
(Different measures of assessing earthquake magnitude)
1935, uses amplitude of seismic waves to determine earthquake magnitude.
What type of scale is the Richter scale? What is this?
(The Richter Scale)
(Different measures of assessing earthquake magnitude)
A logarithmic scale, so each whole-number increase in magnitude represents a ten-fold increase in the amplitude of the seismic wave.
Each whole-number increase also represents a 30-fold increase in the release of energy.
What is the upper limit of the Richter scale? What are the most powerful events seen?
(The Richter Scale)
(Different measures of assessing earthquake magnitude)
Has no upper limit, although the most powerful earthquakes record a magnitude of around 9 (e.g. Tohoku, Japan in 2011).
Outline a limitation of the Richter scale.
(The Richter Scale)
(Different measures of assessing earthquake magnitude)
Cannot express damage as it does not consider population.
What does the MMS measure?
(The Modified Mercalli Scale)
(Different measures of assessing earthquake magnitude)
Measures earthquake intensity and its impact.
Outline the MMS.
(The Modified Mercalli Scale)
(Different measures of assessing earthquake magnitude)
It relates ground movement to impacts that can be felt and seen by anyone in the affected location.
It is a qualitative assessment based on observation and description.
What does the MW measure?
(The Moment Magnitude Scale, (MW))
(Different measures of assessing earthquake magnitude)
Measures the energy released from earthquakes.
Fault movement is always measured to assess the earthquake.
Outline a limitation and a strength of the MW.
(The Moment Magnitude Scale, (MW))
(Different measures of assessing earthquake magnitude)
Only effective for larger earthquakes.
However, unlike the Richter scale, it considers geological properties, such as surface faults, e.g. San Andreas Fault.
The MW measures the energy released from earthquakes, with fault movement always being needed to assess the earthquake. All of these factors relate to a focus on what?
(The Moment Magnitude Scale, (MW))
(Different measures of assessing earthquake magnitude)
A focus on the physical movement of the event itself.
What type of scale is the MW?
(The Moment Magnitude Scale, (MW))
(Different measures of assessing earthquake magnitude)
A logarithmic scale.
What is the MW baseline? Are these common?
(The Moment Magnitude Scale, (MW))
(Different measures of assessing earthquake magnitude)
A MW 5.0 is the baseline for recorded earthquakes of importance (equal to an atomic bomb).
There are usually thousands of MW 5.0 events each year.
Anything 2.0 MW or lower on the MW scale is referred to as a what? What happens to these?
(The Moment Magnitude Scale, (MW))
(Different measures of assessing earthquake magnitude)
A micro-earthquake; these are not significant enough to produce an accurate reading so are largely ignored.
What is the maximum value of the MW scale?
(The Moment Magnitude Scale, (MW))
(Different measures of assessing earthquake magnitude)
There is no maximum value on the moment magnitude scale.
If there was no earth movement, what would landscapes be like? Why?
(The effects of earthquakes on landforms and landscapes)
The landscape would be more or less flat given the combination of gravity and denudation (erosion, weathering and transport).
On geological timescales and across areas covering many thousands of square kilometres, earthquakes are associated with what?
(The effects of earthquakes on landforms and landscapes)
The formation of entire mountain chains, e.g. the Himalaya-Karakoram Range in Asia.
How was the Himalaya-Karakoram Range in Asia formed?
(The effects of earthquakes on landforms and landscapes)
The northward drift of India into Eurasia and the subsequent continental collision led to a complex pattern of folding and faulting of rocks.
Where is the Tibetan Plateau? What is it?
(The effects of earthquakes on landforms and landscapes)
North of the Himalaya-Karakoram Range.
Averages 4500 m above sea level - covering an area of 2.5 million km2 (ten times the size of the UK).
Major fault systems are evident in the rocks and these indicate considerable movement.
The entire region is tectonically active.
What supports the fact that the Tibetan Plateau region is tectonically active?
(The effects of earthquakes on landforms and landscapes)
2008 (8.0 MW) and 2013 (7.0 MW) earthquakes in Sichuan province, and the 2015 (7.0 MW) event in Nepal.
What evidence supports the effects associated with earthquakes on the morphology of the Earth’s surface?
(Rift valleys and escarpments)
(The effects of earthquakes on landforms and landscapes)
The rift valleys along mid-ocean spreading ridges, in East Africa and Iceland.
What marks the location of faults caused by tension and compression within the crust? What happens to these over time?
(Rift valleys and escarpments)
(The effects of earthquakes on landforms and landscapes)
The inward-facing fault scarps or escarpments of rift valleys.
Rift valleys on the continents are altered by weathering and erosion.
Over time fault scarps are worn away, blending into the landscape, and may even disappear under accumulated sediments.
