Seismicity Flashcards
What is an earthquake
When there’s a small build up of stress in the earths rocks. When stress is suddenly released parts of Earth experience intense shaking lasts few sec- few min
What is the focus of an earthquake
Point where pressure release occurs
What depths are a shallow, intermediate and deep focus?
Shallow -0-70km
Intermediate -70-300km
Deep - 300-700km deep
What type of focus causes the greatest damage?
Shallow
What is the place on the earth called above the focus
Epicentre and receives highest amount of energy
How are seismic waves measured
With a seismometer recorded on seismograph
What types of seismic waves are there?
Primary waves
Secondary waves
Surface waves (love and Raleigh)
How are primary waves characterised
Fastest, move earth back and forth in compressional motion
Move through solids and liquids
How are secondary waves characterised
Half the speed of p waves.move in sideways/perpendicular shaking earth at right angles to direction of travel.
Can’t move through liquids.do more damage than p waves
How are surface waves characterised
Slowest travel near to ground surface. Most destructive.
Include love waves - ground move up and down
Raleigh waves - complex heaving/rolling motion
What is the magnitude of an earthquake
The amount of energy released by the event usually measured on Richter scale
Describe the Richter scale
Logarithmic scale
Each unit 10 fold increase in strength and 30fold increase in energy.It’s an objective measurement based on scientific readings from seismographs
Describe the modified mercalli scale
Measures the intensity and impact of event
12 point scale level 1-12
Uses observations to measure effects
Examples of primary effects of earthquakes
Ground shaking
Death and destruction
People being buried by collapsing buildings
Fires
Secondary effects of earthquakes
Soil liquefaction
Landslides and avalanches
Tsunamis
Effects on people and built environment
What is soil liquefaction
When soils with a high water content lose their mechanical strength and behave like a fluid after been violently shaken. Surface appears dry but water comes to top by cracks. Creates soil volcanoes . Buildings can “sink”
Long term effects of earthquakes
Higher unemployment Repair and reconstruction (months/years) Longer term illness Trauma and grief (months(/year recover ) Loss of farmland and food production Emergency building can become permanent
What causes earthquakes
They originate along faults where part of the crust is being forced to move in opposite directions. They can get stuck and forces continue building up stress eventually they are released as large amounts of energy- seismic waves
Distribution of earthquakes
Broad uneven belts
Found around plate boundaries
Most powerful- destructive plate
At conservative marked by a series of faults
Some on old fault lines away from boundaries
What is a tsunami
Giant sea waves generated by shallow focus underwater earthquakes (mostly) or volcanic eruptions, underwater debris slides and large landslide into the sea
Causes of tsunamis
Generated by seismic activity NOT by wind
Caused by ocean floor earthquakes or submarine volcanic eruption
Characteristics of tsunamis
- Very long wavelength
- Low wave height - deep water
- Travel quickly- greater than 700km/hr
- When reach shallow water rise rapidly in height and slow
- first sign is wave trough in front of tsunami and reduction in sea level - drawdown
Effects of tsunamis depend on
Height of waves and distance travelled Length of event that caused tsunami Extent to which warnings were given Coastal physical geography Coastal land use and population density
Hazard definition
Something that’s a potential threat to human life or property
