Hazards from Seismic Activity

Question 1

Earthquake basics

Answer 1

An earthquake is a release of stress in the Earth’s crust. A series of shock waves originate from the earthquake focus (the location where the stress is released) and on the Earth’s surface this point is known as the epicentre. Fore-shocks can be released before the main earthquake event. The main locations for earthquake activity are mid-ocean ridges, ocean trenches and island arcs, collision zones and conservative plate margins

Question 2

The three types of seismic waves

Answer 2

• Primary waves: fast, low frequency
• Secondary waves: half the speed of primary, high frequency
• Surface waves: slow, low frequency

Question 3

Shallow-focus earthquakes

Answer 3

• Surface down to approximately 70km
• Often occur in brittle rocks
• Generally release low levels of energy but high-energy shallow quakes can cause severe impacts

Question 4

Deep-focus earthquakes

Answer 4

• 70-700km
• Increasing depth leads to high pressure and temperature
• Less frequent but very powerful
• Full understanding of deep-focus earthquakes is evolving; water and change in minerals may be contributing factors

Question 5

Richter scale

Answer 5

• Developed in 1935
• Uses the amplitude of seismic waves to measure magnitude
• Scale is logarithmic from 1 to 9 (although there is no upper limit); each integer increase is a ten-fold increase

Question 6

Moment Magnitude scale

Answer 6

• Scale of 1.0 to 9.0
• Measuring energy release as related to geology, the area of the fault and the amount of movement on the fault
• Accurate for large earthquakes as it uses the physical movement caused by the earthquake
• It is not used for small earthquakes

Question 7

Modified Mercalli scale

Answer 7

• Measures earthquake intensity and impact

- It relates to impacts felt and seen by those affected; it is qualitative not quantitative

Question 8

Ground shaking and ground displacement

Answer 8

This is the vertical and horizontal moving of the ground. Severity depends on the earthquake magnitude, distance from the epicentre and geology

Question 9

Liquefaction

Answer 9

When violently shaken, soils with a high water content lose their mechanical strength and become fluid

Question 10

Landslides and avalanches

Answer 10

Slope failure as a result of ground shaking

Question 11

Tsunamis

Answer 11

A tsunami is a giant sea wave generated by shallow-focus underwater earthquakes (also volcanic eruptions and large landslides into the sea). Tsunamis have long wavelength (often over 100km) and low wave height (under 1m) in the open ocean. They travel quickly (speeds over 700km/h) but on reaching the shallow water bordering land they increase in height. A wave trough forms in front of the tsunami where sea level is reduced: this is called a drawdown. Behind this comes the tsunami itself, sometimes as high as 25m or more

Question 12

Flooding

Answer 12

Earthquakes can indirectly cause flooding in a number of ways: triggering tsunamis, destabilising/destroying dams, destroying and/or lowering protective levees

Question 13

The effects of earthquakes on landforms and landscapes

Answer 13

Earthquakes have had a dramatic and widespread effect on global landscapes across different geological timescales. Mountain ranges have been created (e.g. the Himalayas-Karakoram Range in Asia), as well as major fault systems, rift valleys (e.g. East Africa) and escarpments