Download Getting to Know: Where Earthquakes Occur

Getting to Know: Where Earthquakes Occur
Have you seen pictures of the damage caused by the earthquake that struck Haiti in 2010? The damage devastated entire city blocks. Often, natural disasters happen in specific
areas. Major flooding occurs near large rivers. Hurricanes occur near the oceans. Avalanches
happen in snow-covered mountain ranges. Where do earthquakes occur? As you explore this lesson, you will discover that there are places where earthquakes are most likely to occur, and you will learn why earthquakes occur
in those locations.
Where are earthquakes most
likely to occur?
The 2010 earthquake that struck Haiti
caused serious damage.The earthquake
was caused by the movement of
tectonic plates.
To learn where earthquakes occur, you first have to understand tectonic plates. The outermost
layer of Earth’s crust is made of many large plates. These plates make up both the continents
and the ocean floors. Tectonic plates move slowly over time. In some parts of the world,
tectonic plates are sliding against each other in opposite directions. In other places, one
tectonic plate is sliding underneath another tectonic plate. Tectonic plates can also collide.
The movement of tectonic plates frequently shakes parts of Earth’s crust. We call these events
earthquakes.
Tectonic forces can cause Earth’s crust to crack or fracture. These fractures, called faults, are
usually located near plate boundaries. Earthquakes usually occur when two pieces of the
Earth’s crust suddenly move along a fault. That is why earthquakes typically occur along the
edges of plate boundaries. In fact, if you examined a map of the major earthquakes that
happen around the world, you would see that most earthquake sites trace the boundaries of
the tectonic plates.
Misconception 1: Is it true that earthquakes can only happen on continents?
That is not true. Earthquakes can happen anywhere on a plate boundary or at a fault.
Remember that Earth’s plates also cover the ocean floor. Earthquakes that occur in the
oceans can cause giant waves, called tsunamis.
How do tectonic plates move in relation to each other?
Geologists classify the different types of tectonic plate boundaries by the way that the plates
move in relation to each other. There are three main types of plate boundaries: convergent,
divergent, and transform. At divergent boundaries, tectonic plates move away from each
other. Tectonic plates move toward each other at convergent boundaries. At a transform plate
boundary, tectonic plates move past each other. Earthquakes are possible at any of these
plate boundaries.
Concept: Where Earthquakes Occur
Getting to Know
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Do earthquakes occur more frequently
at different types of plate boundaries?
Earthquakes are most likely to occur at transform and convergent plate boundaries. This is because earthquakes happen when pressure is
released along plate boundaries or faults. Imagine if you and
a friend touch the tips of your fingers together. At first, you
might be able to push against each other’s fingertips with
quite a bit of force. If one finger slips, however, both fingers
will suddenly move, and your finger might shoot past your
friend’s finger. A similar process occurs at transform and
convergent plate boundaries.
Pressure can build up at plate boundaries. If there is too
much pressure, the plates can quickly move and release the
energy that is stored at the plate boundary. Transform and
convergent plate boundaries are more likely to build up this
kind of pressure. Some plate boundaries experience frequent
earthquakes, whereas other plate boundaries might go
hundreds of years without an earthquake.
Pressing two fingertips
together is a simplified model
of the pressure that builds at
convergent plate boundaries.
Divergent plates, as you’ll remember, are moving away from each other. That’s like having
two fingertips that are slowly moving apart: when one finger slowly moves, it does not cause
the other fingertip to move. Of course, this doesn’t mean that earthquakes never happen
at divergent plate boundaries. As the plates move apart, magma from Earth’s core rises up
and new crust is created. This event also releases a lot of energy and can sometimes produce
earthquakes.
Misconception 2: Earthquakes only happen at very shallow depths in Earth’s
crust. Is this correct?
That is incorrect. Earthquakes can occur at various depths in Earth’s crust. Many
earthquakes are shallow, especially those that occur at divergent plate boundaries.
However, some earthquakes can occur at extreme depths in Earth’s crust. In fact, some
earthquakes occur so far below Earth’s surface that they cannot be detected at the
surface. Scientists classify earthquakes by their depth: shallow (0–70 km), intermediate
(70–300 km), and deep (300–700 km).
You’ll learn much more about where earthquakes occur, including information about the
different types of plate boundaries and types of faults, as you explore this lesson in greater depth.
Concept: Where Earthquakes Occur
Getting to Know
www.discoveryeducation.com
2
© Discovery Education. All rights reserved.
Discovery Education is a subsidiary
of Discovery Communications, LLC.