Patterns of Earthquakes

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Where are the earthquakes?

Earthquakes are mainly confined to active seimic 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 dislocation and therefore no earthquakes. Aseismic areas are generally the continental shields of stable rocks and the ocean basins. Britain is an aseismic area.

Depth of Focus

The wrold map of earthquakes shows that there are separate zones of shallow-focus and dep-focus earthquakes, the depth of focus is related to the plate boundaries,

Mid-Ocean Areas

MORs: Shallow Focus Earthquakes

The high heat flow and eruption of basaltic pillow lavas at the MOR indicates rising magma. When magma moves, it vibrates to produce harmonic waves , which are detected as small, shallow-focus earthquakes. It is the same phenomenon that makes old pipes rattle as water flows through them.

Axial Rift System: shallow focus Earthquakes

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.

Transform fault: Shallow Focus Earthquakes

The oceanic crust spreads away from the MOR in sections between transform faults. Horizontl movement alonf the transform faults cause earthquakes.

Subduction Zones

Deep Ocean Trench and Fold Mountains

Earthquakes are shallow uinder 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 the earthquakes gets deeper along a zone that slopes away from the trench at about 45 degrees. This marks the top of the boundary betwen the two plates where one moves beneath the other. Called the Benioff Zone.

The earthquakes are due to friction between two plates as they slide past each other or get stuck and suddenly move.

This active zone is present underneath the full length of the mountain range. It is also present under island arc systems.

When an area of sea floor spreading reaches the edge of a continent, the denser oceanic plate rocks are forced down under the light contintental 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 onr of stress and displacement, causing deeper earthquakes. This is called the subdduction zone. Rising magma under sland arcs causes shallow focus earthquakes.

Collision Zones

Where two continents collide, such as India and Asia, there will be shallow and medium earthquakes along deep fault. Compression is taking place as the Indian plate is still being pushed north from the INdan Ocean MOR.

Where plates slide past each other at conservative plate margines, shallow earthquakes are common.

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