|
Seismographs
Q: What
is a seismometer? What are seismographs? How do they work?
|
|
A: A
seismometer is the internal part of the seismograph, which
may be a pendulum or a mass mounted on a spring; however,
it is often used synonymously with "seismograph".
Seismographs are instruments used to record the motion of
the ground during an EQ--installed in the ground throughout
the world and operate as seismographic network. The first
one was developed in 1890. The earliest "seismoscope"
was invented by the Chinese philosopher Chang Heng in A.D.
132. This did not record earthquakes, however. It only indicated
that there was one occurring.
A seismograph is securely mounted onto
the surface of the earth so that when the earth shakes,
the entire unit shakes with it, EXCEPT for the mass on the
spring which has inertia, and remains in the same place.
As the seismograph shakes under (in the example below) the
mass, the recording device on the mass records the realtive
motion between itself and the rest of the instrument, thus
recording the ground motion. In reality, these mechanisms
are no longer manual, but instead work by measuring electronic
changes produced by the motion of the ground with respect
to the mass.
For further information see:
Seismographs:
Keeping Track of Earthquakes - NEIC
|
Q: What
are seismograms? How do you read them?
|
|
A: Seismograms are the records
(paper copy) produced by seismographs used to calculate
the location and magnitude of an EQ. They show how the ground
moves with the passage of time.
On a seismogram, the HORIZONTAL axis
= time (measured in seconds) and the VERTICAL axis= ground
displacement (usually measured in millimeters). When there
is NO EQ reading there is just a straight line except for
small wiggles caused by local disturbance or "noise" and
the time markers.
|
Q: How is
the movement of the seismometer converted into a seismogram?
|
|
A: There are several ways:
(1) a pen drawing an ink line on
paper revolving on a drum
(2) a light beam making a trace on a moving photographic
film
(3) electromagnetic system generating a current that is
recorded electronically on tape
|
Q: How can
I make my own seismometer?
|
|
A: It is relatively easy to
acquire the necessary materials and build your own seismometer.
The links here are to various sources with information on
how to build a seismometer. They range from very simple
and inexpensive to sophisticated and pricey.
|
Back to Earthquake FAQ Main
page
|
