Seismic Activity

Question 1

what is an earthquake?

Answer 1

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.

Question 2

distribution of earthquakes

Answer 2

• 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.

Question 3

how do earthquakes form?

Answer 3

1. ) Movements within the Earth’s crust cause stress to build up at points of weakness, causing the rocks to deform.
2. ) When the stress finally exceeds the strength of the rock, the rock fractures along a fault often at a zone of existing weaknesses.
3. ) 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).
4. )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.

Question 4

focus

Answer 4

the plate of origin of an earthquake inside the earth.

Question 5

Epicenter

Answer 5

• point on the Earth’s surface vertically above the focus

- max charge is caused at the epicenter

Question 6

isoseismic line

Answer 6

a line connecting all points on the surface of the earth where the intensity is the same.

Question 7

what are the causes of earthquakes?

Answer 7

• plate tectonics
• slipping of the fault line
• human induced
• volcanic activity

Question 8

causes of earthquakes - plate tectonics - divergent

Answer 8

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’.

Question 9

divergent plate boundary magnitude

Answer 9

• small + frequent

- little risk due to this and their geographic location

Question 10

divergent plate boundary - depth of focus

Answer 10

crust = thin

focus depth = shallow

Question 11

divergent plate boundary example

Answer 11

• Mid Atlantic Ridge
• Eurasian plate moves eastward
• North American plate moves west

Question 12

formation of earthquake - Transform

Answer 12

zone of friction is created as plates move past each other, this happens when the plates move in parallel direction or same direction.

Question 13

transform - magnitude

Answer 13

high level of risk

Question 14

depth - transform

Answer 14

shallow - intermediate

Question 15

transform - example

Answer 15

San Andreas fault

- pacific plate moves north at a different speed to the North american plate

Question 16

convergent - earthquake formaton

Answer 16

as one plate subducts friction is created. when the friction is overcome, the energy is released. these earthquakes have high potential to create tsunamis

Question 17

convergent - magnitude

Answer 17

• 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

Question 18

convergent - depth of focus

Answer 18

• 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

Question 19

convergent - example

Answer 19

Japan Tsunami 2011

• caused by earthquake
• Pacific plate subducted beneath the Eurasian plate

Question 20

what are most earthquakes usually related to?

Answer 20

• compressional or tensional stresses built up at margins of the huge moving lithospheric plates.

Question 21

what does the moving of the lithospheric plates cause~?

Answer 21

shallow earthquakes - sudden release of stress along a fault line, or fracture in the Earth’s crust

Question 22

slipping of land along a fault line

Answer 22

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.

Question 23

how does volcanic activity cause earthquakes?

Answer 23

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.

Question 24

foreshock

Answer 24

a mild earthquake preceding the violent shaking movement of an earthquake is known as a foreshock

Question 25

aftershock

Answer 25

usually, a major or even moderate earthquake of shallow focus is followed by many lesser size earthquakes known as aftershocks

Question 26

swarms

Answer 26

a numerous amount of earthquakes at shallow focus which occur very frequently for a few months without any major earthquake forming

Question 27

fault zones

Answer 27

- 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.

Question 28

plate tectonics

Answer 28

- slipping of land at convergent, divergent and transform plate boundaries cause earthquakes. - convergent = most powerful earthquakes occur at this plate boundary

Question 29

megathrust earthquakes

Answer 29

- 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

Question 30

human induced earthquakes

Answer 30

- minor earthquakes and tremors caused by human activities e. .g. - mining - large scale petroleum extraction - fracking - artificial lakes (reservoirs) - nuclear tests

Question 31

seismic shockwaves

Answer 31

- Primary waves (p) - Secondary waves (S) - Love waves (L)

Question 32

primary waves (P)

Answer 32

- fastest - reach the surface first - small waves - travel through the mesosphere and the core opposite side of the Earth

Question 33

Secondary waves (S)

Answer 33

- 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

Question 34

Earthquake shadow zone

Answer 34

1. ) this is the area of the Earth from angular distances of 104-140 degrees that does not receive any direct P waves. 2. ) the shadow zone results from swarms being stopped entirely by the liquid core and P waves being refracted by the liquid core

Question 35

Crustal Fracturing

Answer 35

- 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

Question 36

ground shaking

Answer 36

energy release - ground shakes - seismic waves are created by sudden movement the greater the energy release the greater the amplitude.

Question 37

liquefaction

Answer 37

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.

Question 38

landslides

Answer 38

- 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

Question 39

Tsunamis

Answer 39

- occur in specific regions: pacific - Only occur in coastal zones - most are generated at subduction/convergent plate boundary around the Pacific ocean

Question 40

Tsunami Stats

Answer 40

- 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

Question 41

How do Tsunamis occur?

Answer 41

- 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.