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. 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. Again, this energy travels in the form
Seismograms record the energy emitted by seismic or
earthquake waves. Seismograms are created by machines called seismographs.
Seismograms are used by seismologists (scientists who study earthquakes) to
mathematically describe the magnitude of an earthquake. Seismologists have
established a worldwide network of seismographic stations. They are
especially concentrated in areas of active earthquakes.
When an earthquake occurs, data from the seismographs
feeds into a major research center, where the seismograms are analyzed. This
analysis determines where the earthquake occurred, and determines its
magnitude. Major earthquake data centers in the United States include Menlo
Park and Los Angeles, both in California, and Boulder, Colorado.
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, hence their names.
Seismograms will record P-wave arrivals before S-wave arrivals.
- The worksheet shows a copy of six real seismograms recorded at
different seismographs near Mammoth Lake, California. Notice that
different locations did not record the seismic waves at the same time,
because they were different distances from the epicenter. The first
arrivals of P- and S-waves are labeled with a small "P" or
"S" respectively on the diagram. The lowest two lines on the
seismogram indicate time. This is used by seismologists to determine the
distance of the seismograph from the earthquake’s epicenter.
- Explain P-waves and S-waves to the class. Emphasize that P-waves are
faster than S-waves, and will be recorded first on a seismogram.
Answers:1) 6; 2) look on seismogram above; 3) time.
- Instruct the students to find other seismograms on the Internet.
Seismologists make extensive use of the Internet to post data derived
from earthquakes. The U. S. Geological Survey maintains an excellent
website that has real-time earthquake data for the whole United States,
It also has good links to other earthquake information.