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Geophysical systems

How geophysical processes give rise to geophysical events of differing type and magnitude

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Mechanisms of plate movement including internal heating, convection currents, plumes, subduction and rifting at plate margins

Reading: Prof. Stephen A. Nelson’s paper (Tulane University) on Plate tectonics

 

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Characteristics of volcanoes (shield, composite and cinder) formed by varying types of volcanic eruption; and associated secondary hazards (pyroclastic flows, lahars, landslides)

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Characteristics of earthquakes (depth of focus, epicentre and wave types) caused by varying types of plate margin movement and human triggers (dam building, resource extraction)

Definition:

Mass movement, Landslides and Avalanches

Mass movement denotes the collective term for all gravitational or downslope movement of weathered rock debris. Gravity plays the sole important role in transporting materials mostly down the slop and other transporting medium such as water, wind, waves; glaciers are not involved in the process but they may aggravate or trigger the condition of mass wasting by adding additional weight or by generating seepage pressure or lubrication.  Weakening internal cohesionis the most common cause of mass movements. Landslides denote the perceptible downslope movement of rock, debris or earth. Landslides are caused by a large volume and combination od rock, debris, soil and water, moving downslope under the influence of gravity and water. Avalanches refer to the fall or very rapid down slope movement of snow as a result of structural weakness (Such as reduction of stability by the existence of solid bad rock that acts as a slide planebelow the weak material) or gravity. Avalanches tend to recur in the same places. Methods of stabilizing hill slopes may help to reduce the impact and vulnerability of the avalanche related hazards. Ski resorts are particularly prone to avalanches.

 

Movement categoryTypes of materialsSpeed of onset
Bed rockCoarseFineHigh (slope angle or gradient plays the major role)



Low (water plays an important role)
FallRock fallDebris fallEarth fall
ToppleRock toppleDebris toppleEarth topple
SlideRock slideDebris slideEarth slide
SpreadRock spreadDebris spreadEarth spread
FlowRock flowDebris flowEarth flow

Catastrophic landslides

Avalanches

Soil Liquefaction

Happens when solid materials turns into a liquefied state due to an increase in pressure, most likely as a result of ground shaking during an earthquake. Unconsolidated sediments and materials on the surface may get saturated with water and the entire ground may starts moving, damaging building foundations. Soil liquefaction may cause massive collapse of the buildings and other man made structures such as bridges, dam, pipelines etc. Loosely packed, waterlogged sediments at or near the ground surface are ideal for liquefaction. Liquefaction occurring beneath buildings and other structures can cause major damage during earthquakes. For example, during the California earthquake, 1989, liquefaction of the soils and debris caused major subsidence, fracturing, and horizontal sliding of the ground surface in the Marina district in San Francisco.

 

Transverse faults

Transverse faults occur when a block of rock fractures, and the two blocks of rock slide past each other in opposite directions. Transverse faults strike obliquely or perpendicular to the general structural trend of the region. Unusual transverse faults along the Cascadia margin have been discovered in 1990s using sonar and seismic reflection.

Demonstration of soil liquefaction due to earthquake

Ground Liquefaction

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