Geology Reference
In-Depth Information
bends, it deforms and eventually reaches the point at which it
breaks. When this happens, the two pieces of the original stick
snap back into their original straight position. Likewise, rocks
subjected to intense forces bend until they break and then re-
turn to their original position, releasing energy in the process.
When an earthquake occurs, energy in the form of
seismic
waves
radiates out from the point of release (
Figure 8.3).
These waves are somewhat analogous to the ripples that move
out concentrically from the point where a stone is thrown into
a pond. Unlike waves on a pond, however, seismic waves move
outward in all directions from their source.
Earthquakes take place because rocks are capable of stor-
ing energy; however, their strength is limited, so if enough
force is present, they rupture and thus release their stored en-
ergy. In other words, most earthquakes result when movement
occurs along fractures (faults), most of which are related, at
least indirectly, to plate movements. Once a fracture begins,
it moves along the fault at several kilometers per second for
as long as conditions for failure exist. The longer the fracture
along which movement occurs, the more time it takes for
the stored energy to be released, and therefore the longer the
◗
Seismology,
the study of earthquakes, emerged as a true sci-
ence during the 1880s with the development of
seismographs,
instruments that detect, record, and measure the vibrations
produced by an earthquake (
Figure 8.2). The record made
by a seismograph is called a
seismogram
. Modern seismo-
graphs have electronic sensors and record moments precisely
using computers rather than simply relying on the drum strip
charts commonly used on older seismographs.
◗
◗
Figure 8.2
Seismographs
Seismographs record ground motion during an earthquake.
The record produced is a seismogram. This seismograph records
earthquakes on a strip of paper attached to a rotating drum.
a
North
Spring
Support
Suspended
mass
Cable
Support
Suspended mass
Hinge
Rotating drum
Base anchored
into bedrock and
moves with it
Marker
Rotating drum
Base anchored
into bedrock and
moves with it
Marker
Bedrock
Bedrock
A vertical-motion seismograph. This seismograph
operates on the same principle as the horizontal-motion
instrument and records vertical ground movement.
c
A horizontal-motion seismograph. Because of its inertia,
the heavy mass that contains the marker remains stationary
while the rest of the structure moves along with the ground
during an earthquake. As long as the length of the arm is
not parallel to the direction of ground movement, the marker
will record the earthquake waves on the rotating drum. This
seismograph would record waves from west or east, but to
record waves from the north or south, another seismograph
at right angles to this one is needed.
b
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