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Flashcards in Seismicity- Content Deck (51)
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1

What are the three different earthquake depths?

1. Shallow focus
2. Intermediate focus
3. Deep focus

2

How deep is a shallow focus earthquake likely to be

0-70km

3

How deep is an intermediate focus earthquake likely to be

70-300km

4

How deep is a deep focus earthquake likely to be

300-700km

5

Which type of focus has the greatest damage?

Shallow focus (0-70km)

6

What are seismometers?

Seismometers are sensitive instruments that measure seismic waves. They give us information about earthquakes and the internal structure of the Earth.

7

How does a seismometer work?

1. The heavy mass doesn't move
2. The whole seismometer moves as the earth it is attached to shakes
3. The recording device measures how far the rest of the seismometer has moved

8

What are the two main types of seismic wave?

1. Primary waves
2. Secondary waves

9

What are the characteristics of primary waves

- Shake ground up then down in longitudinal movement
- can also causes back and forth movement
- relatively weak

10

What are the characteristics of secondary waves

- travel slower through crust
- crust moves side to side at right angles to outward motion of the main wave
- also called transverse waves
- cause most damage

11

What are characteristics of the Richter scale

- logarithmic scale (each unit is a 10 fold increase)
- destructive earthquakes tend to be above 6
- measures the amount of energy released

12

What are the characteristics of the Mercalli scale

- 12 point scale runs from Level 1 (detected by few people) to Level 12 (total destruction)
- measures the intensity of an earthquake and its impact
- seismic detectors only date back to 1848; the Mercalli scale works on observations on the ground of the actual impacts

13

Why do people often fear aftershocks as much as an initial earthquake?

People often fear aftershocks as much as an initial earthquake as they may last for months afterwards and can be quite destructive

14

What is an example of destructive aftershocks

The 9.3 magnitude Indonesian earthquake of 2004 was followed by a series of aftershocks, one of which was a 6.1 in magnitude

15

What are the 7 main factors that influence the impact of earthquakes?

1. Magnitude and depth of the earthquake
2. Geological conditions
3. Distance from the epicentre
4. Population density, preparation and education
5. Design and strength of buildings
6. Time of day/week
7. Impact of indirect hazards

16

How can magnitude and depth affect the impact

- the deeper the focus, the less damage is caused as waves lose energy as they travel through the crust and by the time they reach the surface, the damage can be minimised

17

How do geological conditions affect the impact

- if the area is solid rock there is usually less damage than sands and clays
- on clays liquefaction can occur where water penetrates between the clay particles creating a quicksand-like substance into which buildings sink

18

How can population density, preparation and education affect the impact

- the more densely populated an area, the more potential for loss of life and damage to property
- the more educated, the better prepared a population can be for an earthquake

19

How can design and strength of buildings affect the impact

- buildings can be built to withstand shaking of the earth and to limit loss of life and damage

20

How can time of day/week affect the impact

- weekends mean families are usually together which reduces people trying to move around
- midday during the week, night times and rush hours can result in more damage

21

What are examples of indirect hazards caused by an earthquake

1. Fires
2. Landslides
3. Tsunamis

22

What are the primary effects of earthquakes

1. Tsunamis
2. Soil liquefaction
3. Landslides and avalanches
4. Effects on people and their build environment

23

What is soil liquefaction?

- when violently shaken, soils with a high water content lose their mechanical strength and behave like a fluid
- can cause movement of groundwater
- it can come to the surface through cracks e.g. soil volcanoes
- buildings can sink as a result

24

What is an example of when soil liquefaction occurs

San Francisco 1989;
- the reclaimed Bay Area cost 100,000 million US dollars due to liquefaction

25

What are tsunamis?

Tsunamis are giant sea waves generated by shallow focus underwater earthquakes

26

What is an example of a tsunami

Indonesia December 2004

27

What are landslides and avalanches and what can they lead to

- slope failure as a result of ground shaking
- further deaths and injuries
- flooding from blocked rivers creating 'quake lakes'

28

What are the primary effects on people and their built environment

- buildings, roads and bridges collapse
- disruption to gas, electricity and water supplies

29

What are the secondary effects on people and their built environment

- scarce or contaminated water supplies
- fires from ruptured gas mains
- loss of grade from damage to industries

30

What are the long term effects of earthquakes?

- permanent disruption to natural drainage patterns
- problems with restoring trust in civil authorities/neighbours
- repair and reconstruction of buildings/infrastructure can take years
- higher unemployment; not all businesses recover
- longer term illness and reduced life expectancy
- trauma and grief may take months and years for recovery
- loss of farmland and food production