Points to Consider
- Think about stresses in the ocean basins. Where in the ocean basis do you think you
would find the features that indicate tensional stresses? Where would you find the
features that indicate compressional stresses? - Earthquakes are primarily the result of plate tectonic motions. List the three types
of plate boundaries and what you think the stresses are that would cause earthquakes
there. - Which type of plate boundary do you think has the most dangerous earthquakes? How
do earthquakes cause the greatest damage?
7.2 Nature of Earthquakes
Lesson Objectives
- Be able to identify an earthquake focus and its epicenter.
- Identify earthquake zones and what makes some regions prone to earthquakes.
- Compare the characteristics of the different types of seismic waves.
- Describe how tsunamis are caused by earthquakes, particularly using the 2004 Boxing
Day Tsunami as an example.
Introduction
Anearthquakeis sudden ground movement caused by the sudden release of energy stored
in rocks. The earthquake happens when so much stress builds up in the rocks that the
rocks rupture. An earthquake’s energy is transmitted by seismic waves. Each year there are
more than 150,000 earthquakes strong enough to be felt by people and 900,000 recorded by
seismometers.
Causes of Earthquakes
Almost all earthquakes occur at plate boundaries. All three boundary types—divergent,
convergent and transform—are prone to earthquake activity. Plate tectonics causes the
lithospheric plates to move. As you might imagine, having giant slabs of lithosphere moving
about on a spherical shape is not smooth. When stresses build, they first cause the rocks
to bend elastically. If the stresses persist, energy continues to build in the rocks. When the
stresses are greater than the internal strength of the rocks, the rocks snap. Although they
return to their original shape, the stresses cause the rocks to move to a new position. This
movement releases the energy that was stored in the rocks, which creates an earthquake.
During an earthquake the rocks usually move several centimeters or maybe as much as a