Tectonic processes and hazards Flashcards
Seismic hazards
Generated when rocks within 700km of the Earth’s 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 Earth’s crust from which molten magma, volcanic rock, and gases are ejected or extruded
Plate tectonics
A theory developed more than 60 years ago to explain the large-scale movements of the lithosphere (the outermost layer of the Earth). It was based around the evidence from sea floor spreading and ocean topography, marine magnetic anomalies, paleomagnetism and geomagnetic field reversals. A knowledge of the Earth’s 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 tectonic plates.
Paleomagnetism
Results from the zone of magma ‘locking in’ or ‘striking’ the Earth’s magnetic polarity when it cools. Scientists can use this tool to determine historic periods of large-scale tectonic activity through the reconstruction of relative plate motions. They create a geo-timeline.
Subduction zones
Are 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 given threshold, a sudden failure occurs along the fault plane that can result in a ‘mega-thrust’ 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 sheer stress across the fault, that is, it 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 mega-thrust locked fault (subducting Indian Plate) 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 roc is first released. The distance between this and the epicentre on the surface is called the focus 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 (ones in thwich the pore space between individual particles is completely filled with water). 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 gorund shaking. It is the ground shaking that causesn building damage and collapse, and the loss of life from the hazard.
Magnitude
The magnitude of an earthquake is related to the amount of movement, or displacement, in the fault, which is in turn a nmeasure of energy release. The 2004 earthquake in Indonesia was very large (M=9.1) because a large vertical displacement (15m) occurred along a very long fault distance, approximately 1500km. (Earthquake magnitude is measured at the epicentre, the point on the Earth’s surface directly above the hypocentre).
Epicentre
The location on the Earth’s surface that is directly above the earthquake focus, i.e. the point where an earthquake originates.
Tsunami
The word comes from two Japanese words: tsu (port or harbour) and nami (wave or sea). Tsunamis are initiated by undersea earthquakes, landslides, slumps and, sometimes, volcanic eruptions. They are characterised by: long wavelengths (150-1000km), low amplitude (0.5-5m), fast velocitie (up to 600kph in deep water).
