Instrument Info
What is a Seismometer?
A seismometer is a very sensitive instrument that can detect movements of the Earth’s surface. The surface is in constant motion due to natural and man-made phenomena. For example, the solid Earth tide is the stretching of the Earth due to the gravitational pull of the moon. Phoenix, Arizona slowly rises and falls twice a day about 40 cm due to the moon orbiting overhead. Mining explosions in Morenci, Arizona create seismic waves which shake Phoenix daily. The amplitude of these vibrations are less than a micrometer and the oscillations have a frequency of several Hz.
The wide variety of ground motion, both in terms of period of oscillation and amplitude of vibration make it impossible for a single seismometer to record all types of motion. The AS-1 seismometer is a simple inertial system that can detect local mining explosions as well as moderate or large earthquakes that occur globally.
The seismometer is called inertial because it is based on Newton’s 1st Law: A body at rest will remain at rest. In the AS-1 seismometer a magnet is suspended by a spring from a boom. When the ground vibrates the boom moves, but the mass wants to remain at rest, so the spring is stretched. The relative motion between the boom and mass is proportional to the ground shaking and causes a tiny electric current to be generated that is sent to the seismograph to be converted into a seismogram. Unfortunately, the spring will eventually pull the mass, so the seismometer requires damping. Oil or a magnetic damping system is used to dampen the AS-1 Seismometer.
Resources for the AS-1
AS-1 Seismometer
The AS-1 is a portable effective classroom tool for teaching about earthquakes and the instruments that record them. The AS-1 has been loaned to many teachers through the Seismographs in Schools program. The network is still growing and more instruments are being loaned to teachers who participate in IRIS workshops.
Guralp CMG-PEPPV
The CMG-PEPPV records vertical (up and down) ground motion. It may look rather simple on the outside but it is a complex and sensitive instrument that can record earthquakes worldwide.
