What are the main hazards generated by seismic activity?

Question 1

Where do earthquakes tend to be concentrated?

Answer 1

Mid-ocean ridges
Ocean trenches
Collision zones
Conservative plate margins

Question 2

What are the different types of seismic waves?

Answer 2

Primary P waves
Secondary S waves
Surface L waves

Question 3

What are P waves?

Answer 3

Fast-travelling, low-frequency compressional waves.
They vibrate in the direction they travel

Question 4

What are S waves?

Answer 4

Half the speed of P waves, high-frequency. They vibrate at right angles to the direction they travel

Question 5

What are L waves?

Answer 5

Slowest, low-frequency. Some have a rolling movement. L waves travel through the outer crust only (and so cannot travel through the outer core). While P waves travel through the Earth interior

Question 6

What does each whole-number increase on the Richter scale represent?

Answer 6

A ten-fold increase in amplitude of seismic wave
A 30-fold increase in release of energy

Question 7

What are the limitations of the Richter scale?

Answer 7

Not used to express damage. An earthquakes damage is determined partly by magnitude but also by other factors.

Question 8

What are the different measurements for Earthquakes?

Answer 8

Richter scale
Modified Mercalli Scale
Moment Magnitude Scale

Question 9

What is the Modified Mercalli scale and how does it compare to the Richter Scale?

Answer 9

Measures earthquake intensity and its impact. It relates ground movement to impacts that can be felt and seen by anyone in the affected location. In this respect - and unlike the Richter scale- it is a qualitative assessment based upon observation and description

Question 10

Name some of the measurements of the Modified Mercalli scale

Answer 10

I- Scarcely felt
VII- People run outdoors, well-designed buildings safe but poorly designed buildings damaged
XII- Total structural damage and ground disturbance

Question 11

How does the Richter Scale and Modified Mercalli scale relate to damage?

Answer 11

At around 2-3 on the Richter Scale and IV-V on the Modified Mercalli scale there begins sensory effects e.g. felt by most
At around 6 on the Richter Scale and IX-X on the Modified Mercalli Scale, structural damage occurs in all buildings e.g. well-designed structures begin to fall

Question 12

What is the Moment Magnitude Scale?

Answer 12

Measures energy released by the earthquake more accurately than the Richter Scale. The amount of energy released is related to the geological properties such as rock rigidity.
Measures the most accurately large earthquakes however it is not used for small earthquakes

Question 13

What are the factors that determine the severity of ground shaking?

Answer 13

Earthquake magnitude
Distance from the epicentre
Local geology

Question 14

What is the impact of ground shaking on buildings?

Answer 14

In general, buildings can withstand vertical movements better than horizontal ones; it is the swaying of structures that is so dangerous to their stability. Ground movements that cause displacement of rocks along fault lines can rip apart pipelines and sewers, sever rigid structures such as railway tracks and roads and cause buildings to collapse

Question 15

What is the impact of ground shaking on the landscape?

Answer 15

The displacement of the surface can disrupt natural drainage, divert streams and rivers and affect the movement of groundwater in aquifers. This can have serious implications for public water supplies and irrigation for agriculture

Question 16

When does liquefaction occur?

Answer 16

When an earthquake strikes an area with surface materials of fine-grained sands, alluvium and landfill with a high water content, the vibrations can cause these materials to behave like liquids

Question 17

What places are vulnerable to landslides or avalanches?

Answer 17

Steep slopes in mountainous regions e.g. Himalayan region, where their vulnerability is increased by deforestation and heavy monsoon rains, so that even small tremors can trigger landslides

Question 18

How severe can landslides be?

Answer 18

Can block transport routes or rivers which can create temporary lakes, which can threaten areas downstream with floods e.g. in Kashmir in 2005