Tectonics Flashcards
Seismic hazards
Generated when rocks within 700km of the Earths surface come under such stress that they break and become displaced
Volcanic hazards
Associated with eruption events
Intra-plate earthquakes
These occur in the middle or interior of tectonic plates and are much rarer than boundary earthquakes
Volcano
A landform that develops around a weakness in the Earths crust from which molten magma, volcanic rock and gases are ejected or extruded
Plate tectonics
Theory developed to explain the large scale movements of the lithosphere. It was based around the evidence from sea floor spreading and ocean topography marine magnetic anomalies paleomagnetism and geomagnetic field reversals. A knowledge of earths interior and outer structure is essential for understanding plate tectonics
Lithosphere
The surface layer of the Earth is a rigid outer shell composed of the crust and upper mantle. It is on average 100km deep. The lithosphere is always moving but very slowly fuelled by rising heat from the mantle which creates convection currents. The distinction between lithosphere and asthenosphere is one of physical strength rather than a difference in physical composition. The lithosphere is broken into huge sections which are the tectonic plates
Subduction zones
Broad areas where two plates are moving together often with the thinner more dense oceanic plate descending beneath a continental plate. The contact between the plates is sometimes called a thrust or megathrust fault. Where the plates are locked together frictional stress builds. When that stress exceeds a threshold a failure occurs along the fault plane that can result in a megathrust earthquake releasing strain energy and radiating seismic waves. It is common for the leading edge to lock under high friction. The locked fault can hold for hundreds of years building up enormous stress before releasing. The process of strain stress and failure is referred to as the “elastic-rebound theory”
Locked fault
A fault that is not slipping because the frictional resistance on the fault is greater than the shear stress across the fault is stuck. Such faults may store strain for extended periods that is eventually released in a large magnitude earthquake when the frictional resistance is eventually overcome. The 2004 Indian Ocean tsunami was the result of a megathrust locked fault with strain building up at around 20mm per year. It generated huge seismic waves and the devastating tsunami
Hypocentre
Is the focus point within the ground where the strain energy of the earthquake stored in the rock is first released. The distance between this and the epicentre on the surface is called focal length
Soil liquefaction
The process by which water-saturated material can temporarily lose normal strength and behave like a liquid under the pressure of strong shaking. Liquefaction occurs in saturated soils. An earthquake can cause the water pressure to increase to the point where the soil particles can move easily especially in poorly compacted sand and silt
Intensity
A measure of the ground shaking. It is the ground shaking that causes building damage and collapse and the loss of life from the hazard
Magnitude
The magnitude of an earthquake in related to the amount of movement or displacement in the fault which is in turn a measure of energy release. It’s measure at the epicentre
Epicentre
The location on the earth surface that is directly above the earthquake focus ie the point where an earthquake originates
Tsunami
Characterised by: Long wavelengths (150-1000km) Low amplitude (0.5-5m) Fast velocities (up to 600 kph in deep water)
Hazard
A perceived natural/geophysical event that has the potential to threaten both life and property
