Seismic Activity Flashcards
- Earthquakes - Tsunamis
what is an earthquake?
a sudden shaking or trembling of the earth’s surface caused by the seismic waves or earthquake waves that are generated due to a sudden movement (sudden release of energy) in the Earth’s crust (shallow focus) or upper mantle (some shallow focus and all the intermediate and deep-focus earthquakes.
distribution of earthquakes
- The Pacific Ring of Fire accounts for about 68 per cent of all earthquakes.
- A second belt, known as the Alpine Belt (Himalayas and Alps). The energy released in earthquakes from this belt is about 15 per cent of the world total.
- Circum-Pacific Belt, which affects many populated coastal regions around the Pacific Ocean—for example, those of New Zealand, New Guinea, Japan, the Aleutian Islands, Alaska, and the western coasts of North and South America.
how do earthquakes form?
- ) Movements within the Earth’s crust cause stress to build up at points of weakness, causing the rocks to deform.
- ) When the stress finally exceeds the strength of the rock, the rock fractures along a fault often at a zone of existing weaknesses.
- ) The stored energy is suddenly released of intense vibration, or seismic waves spread out from the initial point of rapture. like a ripple on a pond (or a more violent one).
- )These waves are what makes the ground shake and contract large distances in all directions, near focus line, the waves can be very large, making them extremely destructive.
focus
the plate of origin of an earthquake inside the earth.
Epicenter
- point on the Earth’s surface vertically above the focus
- max charge is caused at the epicenter
isoseismic line
a line connecting all points on the surface of the earth where the intensity is the same.
what are the causes of earthquakes?
- plate tectonics
- slipping of the fault line
- human induced
- volcanic activity
causes of earthquakes - plate tectonics - divergent
As the plates move away from one another movement occurs which can create earthquakes. Further earthquakes can be generated as magma moves up through the ‘spreading ridge’.
divergent plate boundary magnitude
- small + frequent
- little risk due to this and their geographic location
divergent plate boundary - depth of focus
crust = thin
focus depth = shallow
divergent plate boundary example
- Mid Atlantic Ridge
- Eurasian plate moves eastward
- North American plate moves west
formation of earthquake - Transform
zone of friction is created as plates move past each other, this happens when the plates move in parallel direction or same direction.
transform - magnitude
high level of risk
depth - transform
shallow - intermediate
transform - example
San Andreas fault
- pacific plate moves north at a different speed to the North american plate
convergent - earthquake formaton
as one plate subducts friction is created. when the friction is overcome, the energy is released. these earthquakes have high potential to create tsunamis
convergent - magnitude
- earthquake focus is considerable in depth
- shallow earthquakes = movement of magma and subsidence occur at shallow depths
- large earthquakes associated with the benioff zone occur at considerable depths depending on the trajectory of the subducted plate boundary
convergent - depth of focus
- this area experiences a range of earthquakes
- most focused in an area names the Benioff zone
- some of the most largest and damaging earthquakes occur here.
- lower magnitude earthquakes are also frequent
convergent - example
Japan Tsunami 2011
- caused by earthquake
- Pacific plate subducted beneath the Eurasian plate
what are most earthquakes usually related to?
- compressional or tensional stresses built up at margins of the huge moving lithospheric plates.
what does the moving of the lithospheric plates cause~?
shallow earthquakes - sudden release of stress along a fault line, or fracture in the Earth’s crust
slipping of land along a fault line
sudden slipping of rock formations along faults are fractures in the Earth’s which happens due to the constant change in volume and density of rocks due to the intensity in temperature and pressure in the Earth’s interior.
how does volcanic activity cause earthquakes?
Caused by the consequent release of elastic strain energy both by tectonic faults and the movement of magma in volcanoes e.g. Mt St Helens eruption 1980.
foreshock
a mild earthquake preceding the violent shaking movement of an earthquake is known as a foreshock
aftershock
usually, a major or even moderate earthquake of shallow focus is followed by many lesser size earthquakes known as aftershocks
swarms
a numerous amount of earthquakes at shallow focus which occur very frequently for a few months without any major earthquake forming
fault zones
- immediate cause of shallow earthquakes in the sudden release of stress along a fault line rupture in the Earth’s crust.
- sudden slipping of rock formations along fault rapture in the Earth’s crust due to a constant change in volume and density of temperature and pressure in the earth’s lithosphere.
- the longether the length and the wider the length of the earthquake - the catastrophic the earthquake will be.
- longest earthquakes occur at convergent plate boundary.
plate tectonics
- slipping of land at convergent, divergent and transform plate boundaries cause earthquakes.
- convergent = most powerful earthquakes occur at this plate boundary
megathrust earthquakes
- subduction zones
- e.g. Indian Ocean Earthquake 2004 (8.0)
- strike slip faults: continental landforms e.g. San Andreas Fault: pacific plate and North American plate move horizontally relative to the each other causing earthquakes along the fault line
- divergent plate boundary = usually around 7.0
human induced earthquakes
- minor earthquakes and tremors caused by human activities
e. .g. - mining
- large scale petroleum extraction
- fracking
- artificial lakes (reservoirs)
- nuclear tests
seismic shockwaves
- Primary waves (p)
- Secondary waves (S)
- Love waves (L)
primary waves (P)
- fastest
- reach the surface first
- small waves
- travel through the mesosphere and the core opposite side of the Earth
Secondary waves (S)
- half as fast as P waves
- high frequency
- moves like a snake
- shake like a skipping rope
- travel through the mantle but not through the Earth’s core
- cannot be measured as it does not reach opposite end of the earth
Earthquake shadow zone
- ) this is the area of the Earth from angular distances of 104-140 degrees that does not receive any direct P waves.
- ) the shadow zone results from swarms being stopped entirely by the liquid core and P waves being refracted by the liquid core
Crustal Fracturing
- earthquakes caused by sudden release of stress from rocks.
- often occur at faults/fractures
- sudden movement: further crust ebing fractures
- scarps - indication earthquake is going to occur
ground shaking
energy release - ground shakes - seismic waves are created by sudden movement
the greater the energy release the greater the amplitude.
liquefaction
this is where seismic waves travel through loose rock that is saturated.
strength is reduced significantly and its acts as a liquid
causes: buildings to tilt, collapse, slide and/or spread out.
landslides
- Mountainous zones
- shaking makes slope failure more likely usually induce landslides
- major threat to inhabitants living in mountainous areas e.g. Sichuan Earthquake (2008) caused 1/3 of deaths
Tsunamis
- occur in specific regions: pacific
- Only occur in coastal zones
- most are generated at subduction/convergent plate boundary around the Pacific ocean
Tsunami Stats
- 90% of damaging earthquakes, tsunamis occur in the pacific.
- 1/3 of all tsunamis are generated in deep sea trenches bordering Japan
- 25% in Japan/Taiwan Island Arc
How do Tsunamis occur?
- Earthquakes
- landslides in the sea floor
- land slumping in the ocean
- major volcanic eruptions
- large slabs of rock are forced to move past each other and suddenly causing overlying water to move.
