earthquakes

Question 1

what is the benioff zone?

Answer 1

this is the area where rock is thrust downwards at convergent plate boundaries in a SUBDUCTION ZONE, the different speeds and movements of rock result in numerous earthquakes. it is the sight of intermediate/ deep focus earthquakes. (therefore an important factor in determining magnitude as it dertermines postion and depth of hypocentre)

Question 2

what is the hypocentre?

Answer 2

the hypocentre the focus point within the ground where the strain energy of the earthquake in the rock is first released. it can occur any depth between the earths surface and about 700km below.

Question 3

why do so many earthquakes occur at the benioff zone?

Answer 3

the different speeds and movements of rock at the subduction zone produces lots of deep focus earthquakes which is also an important factor in determining the earthquakes magnitude, based on the position of the hypocentre

Question 4

what causes intraplate earthquakes?

Answer 4

earthquakes that do not occur at plate boundaries, rather they occur within a plate. movement of tectonic plates introduce faults and weaknesses within the plate. they are relatively rare however they can cause more damage as they are not expected so buildings may not be built to seismic code etc.

Question 5

how are earthquakes measured?

Answer 5

using one of three methods:
1. Richter Scale (0-9)
measures height (complitude) of waves produced. absolute scale: wherever an earthquake is recorded, it’ll measure the same.

2. Mercalli Scale (I-XII)
   measures experienced impact of an earthquake. descriptive scale of damages. relative scale: people experience different amounts of shaking in different places
3. Moment Magnitude Scale (0-9)
   modern measure used by seismologists to describe earthquakes in terms of energy released. magnitudes based on the seismic moment which is calculated from: amount of slip on the fault, area affect, earth-rigity factor. USGs uses MMS to estimate magnets for all earthquakes.

Question 6

what is a primary hazard?

Answer 6

primary is a direct result of the earthquake. hazard is the potential to cause harm.

1. ground shaking- most earthquake damage results from the shaking of seismic waves passing beneath buildings, roads etc.
2. crustal fracturing- when energy released during an earthquake causes earths crust to crack, leaving gaps.

Question 7

what is a secondary hazard?

Answer 7

secondary is an after result of the earthquake. hazard is the potential to cause harm.

1. soil liquefaction- where fully/ partially saturated soil looses strength and stiffness (as spaces in between grains are filled with water) in response to applied stress, the shaking causes it to behave like a liquid. can cause buildings to settle, titlt and sometimes collapse.
2. landslides- ground shaking places stress on slopes causing them to become unconsolidated and result in them falling. account for a large proportion of damage and injuries caused by an earthquake.
3. tsunamis- destructive and high risk.

Question 8

what are seismic waves?

Answer 8

earthquakes energy is released as seismic waves that radiate out from the focus. there are three main types of wave.

Question 9

what are the three main types of wave?

Answer 9

1. primary waves
   
   • fastest and first to reach the surface
   • travel through both solid rock and fluids (‘push’ through crust mantle and core)
   • only damaging in powerful earthquakes
2. secondary waves
   -slower (60% slower than P)
   -travel through solids only
   (‘shake’ at right angles to direction of travel-through crust and mantle only)
   -do more damage than P
3. love waves
   
   • slowest and last to arrive
   • surface waves: larger and focus all their energy on the earth surface
   • cause most damage, shake side to side