Seismic Hazards

Question 1

Steps to seismic waves

Answer 1

1) Pressure builds at the point where 2 plates meet
2) Sudden release (due to rock failure creates waves)
3) The waves cause the ground to shake
4) The intensity of the shaking is dictated by the depth of the focus and the energy release
5) The result of this is the earthquake hazard

Question 2

P-waves (primary)

Answer 2

Fastest waves, moving through solid and liquids causing the earth to move backwards and forwards.

Question 3

S-waves (secondary)

Answer 3

Slower waves which move through a sideways motion, shaking the Earth at right angles to the direction of travel. Can’t move through liquids but do much more damage then P-Waves.

Question 4

Love waves

Answer 4

Surface waves which are the slowest. Cause a sideways motion and the most damage.

Question 5

Rayleigh waves

Answer 5

Radiate along the sirface in a low-frequency rolling motion.

Question 6

What causes a tsunami?

Answer 6

. Large ocean wave that is caused by sudden motion on the ocean floor
. Could be an earthquake, a powerful volcanic eruption, an underwater landslide or a large meteorite
. They travel across the open ocean at great speeds and build into large deadly waves in the shallow water of a shoreline

Question 7

Where are tsunami’s generated?

Answer 7

. Most tsunamis are caused by earthquakes generated in a subduction zone (an area where an oceanic plate is being forced down into the mantle by plate tectonic forces)
. The friction between the subducting plate and the overriding plate is enormous
. This friction prevents a slow and steady rate of subduction and instead the two plates become “stuck”.

Question 8

Describe the formation of a tsunami

Answer 8

. Most caused by earthquakes at destructive plate boundaries where oceanic plate is subducted into the mantle beneath a continental plate
. The movement causes friction and the plates then stick
. Energy accumulates and when the energy exceeds the friction the plates snap back into position
. The movement thrusts the water above causing a wave to form
. When the wave reaches shallower water the wave slows, the height of the wave increases by several metres and the waves get closer together
. A sudden shift in plate movement causes water displacement at the epicentre and large waves move along the seabed away from the epicentre

Question 9

Tsunami wave train

Answer 9

. Tsunamis consist of multiple waves

Question 10

Tsunami formation simple

Answer 10

. Rapid movement of the ocean floor displaces a column of water
. A series of waves outward at heights believed to be less than three feet on the open
. As a wave approaches land, its energy compresses into a smaller space, forcing it to gain height

Question 11

Landslides

Answer 11

. Groundshaking due to earthquakes destabilises cliffs and steep slopes, causing landslides and rock falls as a significant side-effect

. Heavy rain and unconsolidated or fractured rock are exacerbating factors

. Subsidence, or lowering of the ground surface, often occurs during earthquakes

Landslides are common in highland areas, many of which are fold mountains meaning they sit on unstable plate boundaries (e.g. the Andes and the Himalayas)

. Relief rainfall in these upland areas means the ground is significantly more likely to become saturated and therefore more likely to fail

Question 12

Liquefaction

Answer 12

. Buildings can sink into the ground if soil liquefaction occurs

. Liquefaction is the mixing of sand or soil and groundwater (water underground) during the shaking of a moderate or strong earthquake

. When the water and soil are mixed, the ground becomes very soft and acts similar to quicksand

. If liquefaction occurs under a building, it may start to lean, tip over, or sink several feet

. The ground firms up again after the earthquake has past and the water has settled back down to its usual place deeper in the ground

. Liquefaction is a hazard in areas that have groundwater near the surface and sandy soil.