Tectonic Plate Movement: Earthquakes

How slow plate movement produces sudden shaking

Tectonic plates move only a few centimetres each year, yet their movement can produce an earthquake lasting seconds or minutes. The connection is the gradual storage of energy in rocks, followed by its sudden release.

Most earthquakes occur at or near plate boundaries. These boundaries contain faults: fractures along which rocks can move. Earthquakes can also occur within plates, so they are not restricted entirely to plate edges.

From a locked fault to an earthquake

Movement along a fault is not always smooth. Friction can hold the rocks on either side together, temporarily locking the fault even while the surrounding plates continue to move.

This continued movement builds stress in the rocks. They gradually deform, storing energy rather like a ruler that bends when a force is applied. Eventually, the stress becomes great enough to overcome the resistance to movement. The rocks suddenly slip along the fault and spring back towards their previous shape, although the two sides are now displaced relative to one another. This is called elastic rebound.

Stored energy is released as seismic waves. These travel outwards through the Earth and make the ground shake. The earthquake is therefore caused by sudden movement along a fault, not simply by plates moving slowly.

Locating the start of an earthquake

The focus is the point inside the Earth where the earthquake begins. The epicentre is the point on the surface directly above it.

Imagine drawing a vertical line upwards from the focus until it reaches the ground surface: its surface end marks the epicentre. Seismic waves spread away from the focus through the surrounding rocks; they do not travel only along that vertical line.

Why earthquakes occur at different boundaries

Earthquakes occur at all four main types of plate boundary, but the movement producing stress differs:

• Constructive boundaries: plates move apart. The rocks stretch and fracture, and movement along faults produces earthquakes. These tend to be frequent and small.
• Conservative boundaries: plates slide past one another. Friction can lock sections of the boundary before sudden slipping releases energy. These boundaries can produce large, shallow-focus earthquakes, as along the San Andreas fault in the USA.
• Collision boundaries: continental plates move towards one another. Compression crumples and fractures rocks, and movement along faults causes earthquakes.
• Destructive boundaries: one plate descends beneath another. Earthquakes occur both along the contact between the plates and within the descending plate.

The same basic connection applies throughout: plate movement builds stress, and sudden movement releases energy.

Shallow-focus and deep-focus earthquakes

The distinction concerns how far below the surface the earthquake starts. A shallow-focus earthquake begins relatively close to the surface. Shallow earthquakes can occur at all types of plate boundary; conservative boundaries are particularly associated with shallow-focus earthquakes.

Deep-focus earthquakes begin much further beneath the surface. They are associated with destructive boundaries because subduction carries an oceanic plate down into the mantle. Earthquakes can continue to occur within this descending slab, giving subduction zones the deepest earthquake foci.

For example, the oceanic Nazca Plate descends beneath the South American Plate at the Peru–Chile trench. Earthquakes occur near the plate contact and within the slab as it descends. In a cross-section, earthquake foci within the slab therefore extend downwards beneath the overriding plate, rather than remaining close to the surface.

Depth and earthquake size are different properties. A shallow focus does not mean a weak earthquake: conservative boundaries can produce large earthquakes despite their shallow foci.

Earthquake mechanism

Plate movement → friction locks a fault → stress and stored energy build → sudden slip and elastic rebound → seismic waves → ground shaking.

Focus and epicentre

• Focus: underground point where the earthquake begins.
• Epicentre: surface point directly above the focus.
• Waves spread outwards from the focus.

Boundary links

• Constructive: pulling apart; rocks fracture; generally frequent, small earthquakes.
• Conservative: sliding plates lock and slip; shallow earthquakes can be large.
• Collision: compression and fault movement cause earthquakes.
• Destructive: earthquakes at the plate contact and within the descending slab; includes the deepest foci.

Focus depth

• Shallow focus: close to the surface; possible at all boundary types.
• Deep focus: far below the surface; associated with subduction.
• Depth is not magnitude: shallow does not mean weak.