Patterns of earthquakes

Earthquakes are mainly confined to active seismic zones around the world, where stresses are acting in the crust and upper mantle

Between these zones are aseismic areas with no stresses to cause dislocatin and therefore no earthquakes

Aseismic areas are generally the continental shields of old stable rocks and the ocean basins

Britain is, at the present time, an aseismic area (other than the minor tremors we occassionally feel)

The world map of earthquakes shows that there are separate zones of shallow-focus and deep-focus earthquakes. 

The depth of focus is related to the plate boundaries 

The high heat flow and reuption of basaltic pillow lavas at the MOR indicates rising mangma. When magma moves, i vibrates to produce harmonic waves (similar to the wave patterns in some music instruments), which are detected as small, shallow-focus earthquakes. It is the same phenomenon that makes old pipes rattle and bang as water flows through them. 

Extension of the crust at the rift valley along the centre of the MOR causes formation of normal faults. Movement along the faults, usually subsidence, causes earthquakes 

The oceanic crust spreads away from the MOR in sections between transform faults. Horizontal movement along the transform faults causes earthquakes 

When an oceanic plate is subducted beneath a continental plate, the depth of the origin of earthquakes ranges from shallow to deep 

• earthquakes are shallow under the deep-ocean trenches and along the oceanic sides of fold mountains, such as the Pacific coast of the Andes
• the trench marks the line at the surface where two plates meet
• further inland, towards the fold mountains, the foci of earthquakes get progressively deeper along a zone that slopes away from the trench at about 45 degrees
• this marks the top of the boundary between the two plates where one moves beneath the other
• this zone is called the Benioff zone
• the earthquakes are due to friction between the two plates as they slide past each other or get stuck and then suddenly move 
• this action zone is present underneath the full length of the moutain range
• it is hardly surprising that Peru and Chile experience many devastating earthquakes
• the Benioff zone is also present under island arc systems
• when are area of sea floor spreading reaches the edge of a continent, the denser oceanic rocks are forced down under the light continental rocks in the process of subduction 
• the inclined boundary between the plates, starting at the trench and ending where the descending plate melts, is an active zone of stress and displacement, causing progressively deeper earthquakes
• this is called the subduction zone
• rising magma under island arcs causes shallow-focus earthquakes

• Where two continents collide, sch as India and Asia, there will be shallow and medium earthquakes along deep faults
• compression is taking place as the Indian palte is still being pushed north from the Indian Ocean MOR
• where plates slide past each other at conservative plate margins, shallow earthquakes are common