Flashcards in Seismicity- Content Deck (51)
What are the three different earthquake depths?
1. Shallow focus
2. Intermediate focus
3. Deep focus
How deep is a shallow focus earthquake likely to be
How deep is an intermediate focus earthquake likely to be
How deep is a deep focus earthquake likely to be
Which type of focus has the greatest damage?
Shallow focus (0-70km)
What are seismometers?
Seismometers are sensitive instruments that measure seismic waves. They give us information about earthquakes and the internal structure of the Earth.
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
What are the two main types of seismic wave?
1. Primary waves
2. Secondary waves
What are the characteristics of primary waves
- Shake ground up then down in longitudinal movement
- can also causes back and forth movement
- relatively weak
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
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
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
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
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
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
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
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
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
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
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
What are examples of indirect hazards caused by an earthquake
What are the primary effects of earthquakes
2. Soil liquefaction
3. Landslides and avalanches
4. Effects on people and their build environment
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
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
What are tsunamis?
Tsunamis are giant sea waves generated by shallow focus underwater earthquakes
What is an example of a tsunami
Indonesia December 2004
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'
What are the primary effects on people and their built environment
- buildings, roads and bridges collapse
- disruption to gas, electricity and water supplies
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