- Copy, download, or print the crossword puzzle. (1
- Using the suggested website links, search the sites to answer
the questions on the crossword puzzle. Begin with Earth
Shakes and Modern Seismographs before accessing Volcano World
and other great sites.
is an earthquake?
You may know that during an earthquake the ground rumbles,
household items like dishes rattle, and walls sometimes
shake. The earth is constantly creating earthquakes,
but what causes them?
The Earth consists of a thin outer layer called the crust
which is mostly cold and brittle rock. Large and small
cracks called faults are located all over the crust.
Because these faults are burried deep in the ground and
compressed together very tightly, they cannot be seen.
Faults are compressed by powerful forces and stay together
for many years. Eventually, the forces pushing the
crustal pieces together break apart and move. This
sudden shift in the crust shakes all of the rock around
the fault causing vibrations, called seismic waves, which
travel outward in all directions. The vibration of
the earth caused by suddent shifts in the crust is called
type of stress is associated with which faults?
When rocks are compressed with enough force, they can fracture
(which means break) or fold (which means bend).
Geologists use the term anticline to describe the formation
created when rocks fold upward and the term syncline to
describe the formation created when rocks fold downward.
(Remember: vowels go together - anticline upward; constanants
go together - syncline downward)
The action of rocks breaking and moving is called faulting.
Altough there are many different kinds of faults, this site
limits the discussion of faults to three types.
faults occur when one block of rock has slipped down in
relation to the other. Normal faults are produced
by tension, as the rocks are pulled apart. The Sandia
Mountains in New Mexico is an example of a normal fault.
(Remember: Normal Faults - down, tension).
faults (also called thrust faults) occur when one block
apears to have moved up over the other. Reverse
faults are caused by compression, as the rocks are pushed
together. Mount Gould in Glacier National Park is
an example of a reverse fault. (Remember: Reverse
Faults - up, compression).
faults (also called lateral faults) occur when the rock
blocks move horizontal to each other. Strike-slip
faults are caused by shearing, as rocks slide past each
other. The San Andreas fault in California is an
example of a strike-slip fault. (Remember: Strike-Slip,
Slide past each other, Shearing force)
Today earthquake waves are measured by geologists with a
seismograph. The workings of a typical seismograph
is very simple.
A horizontal rod is attached to a support pole. Hanging
from the horizontal rod is a wire or spring attached to
a heavy weight. On the other end of the weight is
an ink pen that is free to swing from side to side when
the ground shakes. Directly underneath the pen is
a peice of paper rolled around a cylinder or drum.
If the ground does not move, the rod does not swing.
Therefore, the pen stays in place and a smooth line is drawn.
If the ground shakes, the rod swings and the pen draws a
zigzag line called a seismogram on the paper. As the
paper on the cylinder or drum rotates, the vibrations are
recorded. Earthquakes of high intensity are cause
by more shaking and are recorded by sharper zigzags.