Seismic Hazards Flashcards
Where do most earthquakes occur
Along plate margins or deep under continents. Their location can also be linked to the distribution of certain geological characteristics, such as conservative a plate margins and associated transform faults (low frequency but high predictability), ocean trenches and beneath fold mountains (greater frequency but high predictability)
What is an earthquake
As the earths crust is mobile, there can be a slow build up of stress within the rocks where movement is taking place. When this stress is suddenly released, parts of the surface experience an intense shaking motion that lasts for just a few seconds. This is an earthquake
Define seismicity
The geographic and historical distribution of earthquakes. Again, their distribution is closely associated with plate margins.
Define focus
The point below the earths surface at which an earthquake occurs
Define epicentre
The point on the earths surface directly above the focus of an earthquake
How is the depth of focus of an earthquake significant
Shallow earthquakes (0-70km) (75% of all energy released) cause the most damage
Intermediate (70-300km) and deep (300-700km) earthquakes have much less effect on the surface.
Where do earthquakes originate
Originate along faults. Parts of the crust are being forced to move in opposite directions, or in the same direction by at different speeds. These huge masses of rock get stuck, but the forces on them continue, building up stresses in the rocks. Eventually the strain overcomes the elastic strength of the rocks and they fracture, releasing large amounts of energy. At the moment of fracture the rocks regain their original shape but a new position.
The quaking and shaking takes place during the sudden movement of the rock back to its original shape, after the stress is released. This energy is transferred to the surrounding rocks, travelling through them as seismic waves. A lot of the energy is transferred vertically to the surface and then moves outwards from the epicentre.
When is a seismic event deemed hazardous
Only when it has an impact on people.
The impacts of Hazardous effects can range from the area close to the event to large parts of the planet.
List the hazardous effects of a seismic event
Earthquakes
Aftershocks
Tsunamis
Liquefaction
Landslides
Tell me about the hazardous of effect of a seismic event: earthquakes
A series of vibrations and shock waves initiated by movements along the margins of tectonic plates.
Tell me about the hazardous of effect of a seismic event: aftershocks
Smaller earthquakes that occur after a previous large earthquake in the same area
Tell me about the hazardous of effect of a seismic event: tsunamis
Large waves that flood areas along a coastline, often caused by submarine earthquakes. When they are out at sea, they have a very long wavelength, often in excess of 100km. They are very short in amplitude, around 1m in height. They travel very quickly, often at speeds of 700km an hour, for example taking less than a day to cross the Pacific. When they reach land they rapidly increase in height, up to over 25m in some cases. They are often preceded by a localised drop in sea level (drawback) as the approaching tsunami draw water back and up.
Tell me about the hazardous of effect of a seismic event: liquefaction
Where soils with high water content lose their mechanical strength when shaken violently during an earthquake. They behave like a fluid.
Tell me about the hazardous of effect of a seismic event: landslides
Mass movement of rock down a mountainside, triggered by the shaking of the ground during an earthquake. I
What are the 3 ways to measure an earthquakes magnitude
An earthquakes magnitude is measured by a number of scales including (in order of date of development) the Mercalli scale, the Richter scale and the moment magnitude scale (MMS)
