Earthquakes are caused by the sudden movement and
fracturing of rock masses along preexisting faults. A fault is a broken
surface within the Earth’s crust. The point on the fault at which the
displacement begins is called the focus of the earthquake. The point on the
surface of the Earth directly above the focus is the epicenter. Your
students need to understand that an earthquake happens in rocks that have
been stressed. This stress is stored until the strength of the rock is
exceeded. The actual break (the earthquake) then releases the energy. As
described in the Pre Lab, this energy travels in the form of waves.
The seismic waves generated by an earthquake can be
recorded and measured on a seismograph. The record produced by a seismograph
is called a seismogram. The interpretation of the waves provides
seismologists with a way of "seeing" into the inside of the Earth.
The waves produced by earthquakes travel through the Earth and bounce off
different features of the Earth's interior. The patterns they form after
bouncing off these features can be used to create images of the interior.
Earthquakes generate many different types of seismic
waves. Two major types are P (push/pull; compressional, or primary) and S
(shear or secondary). P-waves are faster than S-waves and are recorded on
the seismogram first. Secondary waves are recorded second, and is shown as a
sharp increase on the seismogram. Other waves like Raleigh waves, Love
waves, and over 200 different types of waves are recorded afterwards.
- Ask the students to predict what causes damage during an earthquake
before showing the beginning of the 1964 Alaskan Earthquake video. Write
their comments on the board or on the screen if you are using a
projector. Use a chart similar to the one below.