Tectonic Processes & Hazards - EQ1 - 1.3A - Earthquakes Flashcards
What is the Benioff zone
- At a convergent plate boundary the subduction process involves the descent into the mantle of a slab of cold hydrated oceanic lithorsphere, about 100 km thick, that carries a relatively thin cap of oceanic sediments
- The Benioff zone is an area of seismicity corresponding with the slab being thust downwards in the subduction zone
- The different ……..&………. of rock at this point produce numerous earthquakes.
- It is the site of …………../…….. earthquakes.
- This theory is an important factor in determining …………. ……….., since it determines the ………..&………….
- The different speeds and movements of rock at this point produce numerous earthquakes.
- It is the site of intermediate/ deep-focused earthquakes.
- This theory is an important factor in determining earthquake magnitude, since it determines the position and depth of the focus
This theory is an important factor in determining….
earthquake magnitude, since it determines the position and depth of the focus
Define epicentre
point on the earth’s surface vertically above the focus
Define fault
- a fracture or zone of fractures between 2 blocks of rock
Define focus
- place inside the earth’s crust where the earthquake originates and a pulse of energy radiates out in all directions
What is a fault scarp
- In some cases, the earthquake motion displaces the surface, so a fault scarp can be seen
Sequence of an Earthquake
How does an earthquake start? (1)
- There is gradual build-up of tectonic strain which stores elastic energy in crustal rocks
Sequence of an Earthquake
(2) When the ……………………. exceeds the …………….., the rock ………..
When the build up of pressure exceeds the strength of the fault, the rock fractures
Sequence of the Earthquake
(3) What happens when the rock fractures
- This produces a sudden release of energy, creating Seismic waves that radiate away from the point of fracture (focus).
Sequence of the Earthquake
(4) This produces a sudden release of energy, creating Seismic waves that radiate away from the point of fracture (focus)……….
- The brittle crust then rebands either side of the fracture, which is the ground shaking, that is, the earthquake felt on the surface
An earthquake’s energy is released as ………… These waves radiate out from the ……….
- An earthquake’s energy is released as seismic waves. These waves radiate out from the focus
Primary & Secondary waves are called ……waves, because…
Primary & Secondary waves are called body waves, because they travel through the Earth’s body
Love waves & Rayleigh waves are …….. waves, because….
Love waves & Rayleigh waves are surface waves, because they travel along the Earth’s surface.
The overall severity of an earthquake is linked to the ………….&………. of these wave types
- amplitude and frequency
Primary Waves (P) - body or surface?
body
Primary Waves (P) - speed?
- Fastest (rate of about 8 km/sec)
- Reach the surface first
Primary Waves (P) - travel through?
- Travel through liquids and solids
Primary Waves (P) - motion?
- Cause backwards and forwards shaking - pushes & pulls ( these are vibrations caused by compression) in the direction of travel
Primary Waves (P) - damage?
least damaging
Secondary Waves (S) - body or surface?
body
Secondary Waves (S) - speed?
- Slower than P waves (about 4 km / sec)
Secondary Waves (S) - travel through?
- Only travel through solids
Secondary Waves (S) - motion?
- up and down movement
Secondary Waves (S) - damage?
Most damaging - higher amplitude
Love Waves (L) - body or surface?
surface
Love Waves (L) - speed?
- Arrive last, as they travel only on the surface, after P & S waves have weakened buildings
-fastest of the surface waves
Love Waves (L) - travel through?
- Only travels through the surface of the crust
Love Waves (L) - motion?
- moves from side to side (horizontal) as it moves forward
Love Waves (L) - damage?
- most damaging of the surface waves
Rayleigh Waves (R) - Body or surface?
surface
Rayleigh Waves (R) - Travel through?
- Only travels through the surface of the crust
Rayleigh Waves (R) - motion?
in a rolling motion, the ground is moved up and down, side to side
Rayleigh Waves (R) - damage?
- Responsible for the shaking felt by people
Earth scientists use two characteristics - …….&……. - to measure earthquakes.
- magnitude and intensity
Earthquake magnitude
Magnitude measures the amount of energy released at the ……
epicentre
Earthquake magnitude
Several ……… are used to measure magnitude, but the…………………. is generally preferred
- scales
- Moment Magnitude Scale (MMS)
Earthquake magnitude
Why is the Moment Magnitude Scale (MMS) preferred
it’s accurate and better at measuring large earthquakes
Earthquake magnitude
What is the Moment Magnitude Scale (MMS) measuring
- the total energy released by an earthquake at the moment it occurs (called the seismic moment)
Earthquake magnitude
How does the MMS measure the seismic moment
- size of the seismic waves
- amount of slippage or rock movement
- area of the fault surface broken by the earthquak
- resistance of the affected rocks
Earthquake magnitude
MMS - What does the scale go from
- The scale goes from 1 (smallest) and is infinite
- but it generally stops at 10, since the largest earthquake ever recorded was a magnitude 9.5 (in Chile in 1960)
Earthquake magnitude
MMS - The scale is….
- logarithmic (each number is ten times the magnitude of the number before),
- so a magnitude 5 earthquake is ten times more powerful than a magnitude 4
Earthquake intensity
What is an earthquake’s intensity
An earthquake’s effect on people, structures and the natural environment
Describe a scale used to measure earthquake intensity
- The Modified Mercalli Intensity Scale,
- which takes observations from people who experienced the earthquake
- and rates them on a scale from I (hardly noticed) to XII (catastrophic)
When you look at a seismic trace. You can see the ……..shock, ……..shocks (they don’t always occur) and………shocks
When you look at a seismic trace. You can see the main shock, foreshocks (they don’t always occur) and aftershocks
Seismic Trace
The main shock is the one with the …… ……
largest magnitude
When does an after shock become foreshock
- If a subsequent event is larger than the one deemed a main shock,
- it can be redefined as a foreshock,
- for example, Tohoku, Japan (2011)
Physical Factors affecting how destrcutuve an earthquake can be
How can destructive plate boundaries affect earthquake magitude?
- Earthquakes on a destructive plate boundary along the Benioff Zone tested to be higher in magnitude, e.g. 9.5M earthquake in Chile (1960)
Physical Factors affecting how destrcutuve an earthquake can be
How can transform plate boundaries affect earthquake magitude?
- They can also be high on transform plate boundaries, e.g. 7.6M earthquake on the San Andres Fault
Physical Factors affecting how destrcutuve an earthquake can be
How can the nature of the ground affect the earthquake?
- The nature of the ground at the surface of an earthquake can have a profound influence on the level of damage
- Loose, sandy, soggy soil, like in Mexico City, can liquefy if the shaking is strong and long enough, for example
Physical Factors affecting how destrcutuve an earthquake can be
Why are shallow focus earthquakes more intense
- Earthquakes with shallow focus cause the most damage/ are the most destructive
- This is because seismic waves have less distance to travel from the focus to the epicentre meaning when they arrive - they have more energy
What is a primary hazard
a hazard directly linked to the seismic wave energy from the earthquake
What is a secondary hazard
a consequence of the pirmary hazards
Crustal Fracturing - primary or secondary?
primary
Ground Shaking - primary or secondary?
primary
Liquefaction - primary or secondary?
secondary
Landslides - primary or secondary?
secondary
Tsunamis - primary or secondary?
secondary
Describe the hazard of crustal fracturing
- Shockwaves travel fast through solid rock and can increase stress in it, especially if rock types are different (as waves travel at different speeds through them)
- Stress causes strain until the rock fractures
Describe the hazard of ground shaking
- Shockwave ground movements include (in order of occurrence) pushing and pulling (P wave), up and down (S wave), and side to side (L wave)
- This all happens in less than 60 seconds
Describe the hazard of liquefaction
- Occurs when shockwaves travel through loose unconsolidated material, shaking it so that it acts like a fluid
- For example, Christchurch, 2011
Describe the hazard of landslides
- Occur when there are steep slopes
- Shockwaves loosen rock and cause it to move downslope under the influence of gravity
- For example, Kashmir, 2005
What are the effects of crustal fracturing
- Causes faults, which can rupture the surface, causing subsidence or uplift
- Anything on these faults can get moved and damaged
- Buildings can collapse
What are the effects of ground shaking
- Buildings that are not aseismic (earthquake-proof) may only survive some types of shaking, and will be damaged
- If buildings have the same resonance as the seismic energy, the impact is intensified
What are the effects of liquefaction
- Causes buildings to tilt over or collapse and the ground to crack
- Underground infrastructure such as cables and pipes may be ruptured
What are the effects of landslides
- Landslides may hit settlements directly or destroy or block transport and communication infrastructure
Crustal Fracturing can cause the movement of…
rock or debris down a slope, often caused by earth tremors
Groundshaking causes …
- Unconsolidated materials to lose their load bearing capacity and become more liquid than solid
- Buildings and other structures, may sink into the ground
Some underwater earthquakes generate….
tsunami that cause major problems for coastal areas
Earthquakes frequently generate large landslides as secondary hazards. This is especially true in area of….
- geologically young (and therefore unstable) mountains such as the Himalaya
Landslides accounted for up to …% of the deaths in the 2008 Sichuan and 2005 Kashmir earthquakes.
30%
Where is liquefaction a particular hazard
- in areas where the ground consists of loose sediment such as sand, silt or gravel that is also waterlogged - often found in areas close to the sea or lakes
Explain the relation between water/sediments and groundshaking
- Shaking compacts the loose sediment together, forcing water between the sediment out and upwards
- This undermines foundations, and causes buildings to sink, tilt and often collapse
- It causes water-saturated material to temporarily lose normal strength and behave like a liquid under the pressure of strong shaking
- Water pressure increases to the point where the soil particles can move easily
