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Explain the patterns of seismic activity
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Explain the patterns of seismic activity
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Explain the patterns of seismic activity (8)1. Known pattern of tectonic boundaries recorded along destructive, constructive and transform boundaries
2. Here, friction builds as the lithospheric plates move in alternate direction
3. The frequency of events is particularly high along destructive margins including around the Pacific’s ‘Ring of Fire’
4. Although there is evidence that activity around these margins is frequent in occurrence, the epicentre of seismic
events may be located slightly further distance from the precise margin itself; creating the appearance of a thicker
band of activity like along the margin of the Australian and Pacific plates and along the Andes where the Nazca plate is
subducted below the South American plate
5. In each of these, a dense oceanic plate is subducted deep beneath a thicker, less dense continental plate resulting in
friction created within the Benioff zone which can generate earth tremors with a focus depth of up to 700km
6. Along collision margins too, earthquake events are numerous, like in the Himalayan region. Here the Eurasian
continental plate is uplifted above the Indian continental plate and as it is put under pressure by this collision, faults in
the rock are formed which becomes points of pressure release during earthquakes
7. Seismic activity is present along transform plate margins like the San Andreas fault in California where friction builds
as the Pacific and North American plate move north westwards but at slightly different speeds.
8. Very narrow bands of seismic activity are also recorded along constructive plate margins like along the Mid-Atlantic
ridge, caused by the upwelling of magma linked to volcanic activity and nearby transform faults running perpendicular
to the main margin. As there is no deep subduction in these areas, epicentres form a much narrower pattern and
more precisely follow the plate margin itself. The seismic activity along the East African Rift is caused by the Somalian
Plate splitting from the African plate, which causes tremors
9. Figure 1 also has evidence of intraplate seismic activity, some of which can be linked to known hotspots like in Hawaii
in the centre of the Pacific plate. These earth tremors are likely relatively low in magnitude but are caused by a
significant upwelling of magma from within the mantle, where the resultant volcanic eruptions can cause earth
tremors as rocks are deformed.
10. Other areas of intra-plate activity not associated with hotspots could be in areas like the eastern US. As the larger
plates are pulled and deformed by slab pull motion, this can result in small faults developing away from the plate
margin like Ramapo Fault. These fault lines can then become points of stress release, causing earth tremors
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