How Seismographs Work Scientists who weren't in Chile during
this morning's aftershocks nevertheless knew the moment the
rumbling started, thanks to a global network of quake-detecting
instruments called seismographs. Seismographs are securely
mounted to the surface of the Earth, so when the ground starts
shaking, the instrument's case moves. What doesn't move,
however, is a suspended mass inside the seismograph, called the
seismometer. During an earthquake, the seismometer remains still
while the case around it moves with the ground shaking.
Traditionally, the suspended mass was a pendulum, but most
modern seismometers work electromagnetically. A large permanent
magnet is used for the mass and the outside case contains
numerous coils of fine wire. Movements of the magnet relative to
the case generate small electric signals in the wire, which can
be sent to a computer or recorded onto paper to create a
seismogram. Seismographs can detect quakes that are too small
for humans to feel. During an earthquake, grou