5.4: Seismic Hazards

Question 1

What is an earthquake?

Answer 1

As the crust of the Earth is constantly moving, there tends to be a slow build up of stress within the rocks. When this pressure is released, parts of the surface experience, for a short period, an intense shaking experience.

Question 2

What is the focus of an earthquake?

Answer 2

The point at which the pressure release occurs within the crust.

Question 3

What is the epicentre?

Answer 3

The point immediately above the focus on the Earth’s surface.

Question 4

How deep is a shallow focus earthquake?

Answer 4

0-70km

Question 5

How deep is an intermediate focus earthquake?

Answer 5

70-300km deep

Question 6

How deep is a deep focus earthquake?

Answer 6

300-700km deep

Question 7

What kind of earthquake causes the greatest damage (shallow, intermediate or deep focus earthquake)?

Answer 7

Shallow focus (0-70km deep).

Accounts for 75% of all the earthquake energy released.

Question 8

Where do the vast majority of earthquakes occur?

Answer 8

Along destructive plate margins.

Question 9

Name the most famous fault line (constructive plate margin) that can cause earthquakes.

Answer 9

The San Andreas Fault in California.

Question 10

What are the different ways in which the magnitude of earthquakes can be measured?

Answer 10

• Richter scale
• Moment magnitude scale (MMS)
• Mercalli scale

Question 11

Outline the Richter scale.

Answer 11

Measures the magnitude of earthquakes. Uses logarithmic scale on a scale of 1-10.

Question 12

Outline the the MMS (moment magnitude scale)

Answer 12

Has replaced the Richter scale to identify the energy release from earthquakes - thus measuring magnitude.

Question 13

Outline the Mercalli scale.

Answer 13

Mercalli scale measures the intensity of an event and its impact. On a 12 point scale (from ‘I. not felt’ to ‘XIII. extreme’).

Question 14

Give examples of events that occur just before an earthquake strikes.

Answer 14

• bulging of the ground
• microquakes before the main tremor
• raised groundwater levels
• curious animal behaviour
• increased argon gas content in the soil

Question 15

How can GIS be useful to earthquakes?

Answer 15

GIS are used to prepare hazard maps that show the areas at greatest risk, and thus can planning of urban growth and development.

Question 16

Outline some primary effects of earthquakes.

Answer 16

• ground shaking: causing buildings to collapse, windows to shatter, sewers to fracture, power lines to collapse.
• ground rupture: the visible breaking and displacement of the Earth’s surface.
• immediate deaths and injuries from crushing, falling glass and transport accidents.

Question 17

Outline some secondary effects of earthquakes.

Answer 17

• soil liquefaction: when violently shaken, soils with a high water content lose their mechanical strength and start to behave like a fluid.
• tsunamis: giant sea waves generated by shallow focus underwater earthquakes involving movements of the sea bed.
• fires: due to broken gas pipes and collapsed electricity transmission systems.
• landslides: slope failure due to ground shaking.
• effects on people: collapsing buildings, destruction of transport links, flooding, disease spreading, food heritages

Question 18

What is a tsunami?

Answer 18

Giant sea waves generated by shallow focus underwater earthquakes, violent volcanic eruptions, underwater debris slides and landslides into the sea.

Question 19

Do tsunamis have a short or long wavelength in open ocean?

Answer 19

Long wavelength

Question 20

Do tsunamis have a low or high wave height in open ocean?

Answer 20

Low wave height (under 1metre)

Question 21

How quickly to tsunamis travel in the open ocean?

Answer 21

700km per hour

Question 22

What happens to the wave height of tsunamis as they reach shallow water bordering land?

Answer 22

Rapidly increase in height

Question 23

What is the first warning sign of a tsunami?

Answer 23

The wave trough in front of the tsunami, which results in a reduction in sea level (known as a drawdown)

Question 24

Once on land, how high can a tsunami reach?

Answer 24

In excess of 25m.

Question 25

When a tsunami reaches land, its effects will depend upon…

Answer 25

• the height of the waves and distance they’ve travelled
• the extent to which warnings could be given
• coastal land use and population density

Question 26

How can earthquakes be predicted?

Answer 26

Regions at risk can be identified through plate tectonics.

Attempts to predict the event a few hours before are based upon monitoring groundwater levels, release of radon gas and unusual behaviour.

Question 27

What is a seismic gap? And how can this help to predict an earthquake?

Answer 27

An area (on fault line) which has not had any real seismic activity for the past 20 years

Question 28

How can it be possible to prevent an earthquake?

Answer 28

Schemes look at keeping plates sliding past each other (rather than sticking and releasing, which is the main cause of earthquakes).

Could be done by lubricating this movement using water and oil, or nuclear explosions at depth.

Question 29

How can protection help to manage earthquakes?

Answer 29

For example, in the USA, the Federal Emergency Management Agency’s programme has ten following objectives:

• to promote understanding or earthquakes and their effects
• to work better to identify earthquake risk
• to improve earthquake resistant design and construction technique
• to encourage the use of earthquake safe policies and planning practices.

Question 30

In what was can protection manage the seismic hazard of earthquakes?

Answer 30

They prepare for the event by modifying the human and built environments in order to decrease vulnerability. Also includes attempts to modify the loss by insurance and aid.

Question 31

Outline the 8 ways to protect the environment for an earthquake (seismic hazard).

Answer 31

• hazard resistant structures
• education
• fire prevention
• emergency services
• land use planning
• insurance
• aid
• tsunami protection

Question 32

Outline how hazard resistant structures are used as protection for managing seismic hazards.

Answer 32

Buildings designed to be aseismic, so earthquake resistant.

• putting large rubber shock absorbers in foundations which allow some movement of the building.
• putting a large concrete weight on too often building which will move, with the use of computer programme, in the opposite direction to the force of the earthquake to counteract stress.
• adding cross bracing to the structure to hold it together better when it shakes.

Older buildings can be retrofitted with such devices to make them more earthquake proof.

Question 33

Outline how education is used as protection

Answer 33

Instructions are issued by authorities in how to prepare for events by securing homes, appliances and heavy furniture, and assembling earthquake kits.

Children in schools, as well as offices and factories have earthquake drills.

After the Loma Prieta event, the American Red Cross issued a list of supplies that people should keep at hand including water, foodstuffs (high energy, canned), clothing first aid kit, radio, torch batteries, matches, fire extinguisher etc.

Question 34

Outline how fire prevention can be used as protection for managing seismic activities.

Answer 34

‘Smart meters’ have been developed that can cut off the gas if an earthquake of sufficient magnitude occurs.

In Tokyo, the gas company has a network that transmits seismic info to a computer which then informs employees where to switch off major pipelines, so reducing the number of fires.