Seismic Hazards Flashcards
Seismic Hazards
Earthquakes are caused by the tension that builds up at all three types of plate margin.
When plates jerk past each other it send out shockwaves. These vibrations are the earthquake
The shockwaves spread out from the focus. Near the focus the waves are stronger and cause more damage.
The epicentre is the point on the earth’s surface where the earthquake is felt first. Its straight above the focus.
Measuring Earthquakes
Earthquakes can be measured using three different scales.
- The Richter scale – measures the magnitude of the earthquakes. It’s logarithmic - this means that an earthquake with a magnitude of 5 is ten times greater than one with a magnitude of 4.
- The moment magnitude scale (MMS) - is based on the total amount of energy released by the earthquake. It is more accurate than the Richter scale, it’s more widely used.
- The Mercalli scale - measures the impact of the earthquakes using observations of the event. The scale is between 1 and 12, with one being barley detected to 12 being total destruction.
Seismic Waves
An earthquake generates three main types of wave
Primary - Displace particles in the direction of movement. They are the fastest wave, they pass through solid and liquids and gases, so will pass through the mantle and core.
Secondary - Displace particles at right angles to direction of movement. They are slower 3/5 times the speed of primary waves. They do not pass through liquids.
Love - These only travel along the surface. They are the slowest wave. The motion is like a swaying boat. They are particularly damaging to building foundations.
Hazards caused by earthquakes
Tsunamis
- Large waves caused by the displacement of large volumes of water.
- They can be triggered by underwater earthquakes which cause the seabed to move, which displaces water. Waves radiate out from the epicentre. The greater the movement of the sea floor, the greater the volume, the bigger the wave
Landslides and Avalanches
- Shaking of the ground can dislodge rock, soil or snow causing landslides that move downslope quickly
- Shaking can also loosen ground material, making it easier for water to infiltrate. The weight of the water may trigger a landslide even after the ground has stopped shaking.
Soil Liquefaction
- When the soil is saturated with water, the vibrations of an earthquake can cause it to act like a liquid.
- This makes the soil weaker and easier to deform, so its likely to subside, especially where it has a heavy weight on top of it (buildings.)
Earthquake Prediction
The prediction of earthquakes are very difficult. However, some attempts to predict earthquakes include:
- Seismic Gaps - A study of pattern of the events, to try and predict when the earthquake will happen.
- Animal Behaviour - Snakes can detect micro tremors and scurry away. Only one successful example of this prediction type - China Haichng Earthquake 1975.
- Now Casting - Warns people that an earthquake has already happened. May give people in San Francisco 30 seconds to prepare.
Fault Maps - Close studies of fault line can sometimes indicate the point along the fault where the next earthquake could be due.
Groundwater levels - Rocks are compressed or stretched due to tectonic forces at fault lines. This can increase of decrease the volume of cracks within the rocks and therefore water contained within the rock. If levels increase it suggests compressional forces.
Earthquake Protection
Earthquake proof buildings - Narrower towards the top, liquefaction drains, Cross bracing to prevent pancaking.
Education - Instructions and education given to help prepare for such hazardous events.
Land-Use planning - Certain types of buildings should be put in areas of low risk (Schools, Hospitals)
Insurance - Take out insurance to cover peoples loses.
Emergency Services - Need carful organistion and planning to take hazards
