Types of earthquakes
Interplate earthquakes occur along the three types of plate boundaries: (1) mid-ocean spreading ridges, (2) subduction zones, and (3) transform faults.
Mid-ocean spreading ridges
Mid-ocean spreading ridges are places in the deep ocean basins where the plates
move apart. As the plates separate, hot lava from Earth's mantle rises between
them. The lava gradually cools, contracts, and cracks, creating faults. Most of
these faults are normal faults. Along the faults, blocks of rock break and slide
down away from the ridge, producing earthquakes.
Near the spreading ridges, the plates are thin and weak. The rock has not cooled
completely, so it is still somewhat flexible, thus large strains cannot build,
and most earthquakes near spreading ridges are shallow.
Subduction zones
Subduction zones are places where two plates collide, and the edge of one plate
pushes beneath the edge of the other in a process called subduction. Because of
the compression in these zones, many of the faults there are reverse faults.
About 80 per cent of major earthquakes occur in subduction zones encircling the
Pacific Ocean. In these areas, the plates under the Pacific Ocean are plunging
beneath the plates carrying the continents. The grinding of the colder, brittle
ocean plates beneath the continental plates creates huge strains that are
released in the world's largest earthquakes.
Faults are divided into three main groups. Reverse fault - when two plates collide and one side of the fracture moves on top of another; normal fault -when two plates are moving apart; and strike-slip or lateral - when two plates slide past each other.
In a normal fault, the block of rock on the upper side of the sloping fracture slides down. In a reverse fault, the rock on both sides of the fault is greatly compressed. The compression forces the upper block to slide upward and the lower block to thrust downward. In a strike-slip fault, the fracture extends straight down into the rock, and the blocks of rock along the fault slide past each other horizontally.
As the tectonic plates move about on the asthenosphere, they interact with one another at their boundaries. There are three types of boundaries: (1) divergent, where plates move apart from each other, (2) convergent, where plates move toward each other, and (3) transform, where plates slide alongside each other.
Divergent plate boundaries
Divergent plate boundaries are mostly on ocean floors. There, the separation of
plates, or rifting, creates lithosphere.The rifting of the ocean floor enlarges
the floor. Magma (liquefied rock) rises from the asthenosphere, filling the gap
between the separating plates. The magma hardens, creating equal amounts of new
crust on the edges of the two plates. The process of separation of plates and
formation of new crust is called sea-floor spreading.
The build-up of
ocean crust on plate boundaries generates long underwater mountain ranges called
ocean ridges. Some of these mountain ranges occur along the center of ocean
basins and are called mid-ocean ridges.
Earthquakes occur at ocean ridges when one plate edge drops down and grinds
against the edge of a neighboring plate. These earthquakes occur a short
distance beneath the surface of the plates, indicating that newly formed plate
edges are very thin.
The rifting of continents creates new seas as ocean waters fill a gap in
continental crust.
In the early stages of rifting, the gap is not yet deep enough to become filled
with ocean water from the Indian Ocean. However, scientists believe that in 50
million years an extension of that ocean may cut into southeastern Africa.
Convergent plate boundaries
Convergent plate boundaries are places where lithosphere created at divergent
boundaries is destroyed by recycling into the mantle. At a convergent boundary,
the edge of a plate sinks, thrusting under the margin of its neighboring plate.
This process is called subduction. The sinking plate can create deep ocean
trenches where it plunges into the asthenosphere. Because Earth is not changing
in size, scientists believe that subduction zones consume the same amount of
ocean crust as ocean ridges create.
The subducting plates generate powerful earthquakes and usually create a line of
volcanoes along the overriding plate boundary. A volcano forms when magma, hot
gases, and fragments of rock burst through the surface. Subduction zones
generate magma at a depth of about 75 miles (120 kilometers) by melting three
kinds of material: oceanic crust at the top of the descending plate, ocean
sediment dragged to great depths, and asthenosphere caught in the corner between
the converging plates.
At some convergent plate margins, the overriding plate scrapes a thick mass of
sediment off the descending plate. This process of subduction accretion
(pronounced uh KREE shuhn), adds material to the edge of the overriding plate.
At other convergent plate boundaries, the edge of the descending plate, all its
cover of sediment, and even pieces from the edge of the overriding plate
disappear beneath the overriding plate. This process, subduction erosion, causes
continents to shrink.
At boundaries where plates carrying continents collide, layers of rock in the
overriding plate crumple and fold like a tablecloth that is pushed across a
table.
Transform plate
boundaries
Transform plate boundaries, where plates slide horizontally against each other,
neither create nor destroy lithosphere. However, at these boundaries, or
transform faults, powerful earthquakes can occur.
Although the tremendous force of tectonic plates grinding against each other is
responsible for many earthquakes, we humans can cause them as well.
In some areas, severe earthquake damage is the result of liquefaction of soil. In the right conditions, the violent shaking from an earthquake will make loosely packed sediments and soil behave like a liquid. When a building or house is built on this type of sediment, liquefaction will cause the structure to collapse more easily. Highly developed areas built on loose ground material can suffer severe damage from even a relatively mild earthquake. Liquefaction can also cause severe mudslides.