Seismic instruments are used to measure low-frequency ground motion caused by earthquakes. They detect the seismic waves created by subsurface ruptures and convert ground motions into electronic signals which are suitable for transmission. Seismometers are spring-mounted seismic instruments which resemble a pendulum. They detect seismic waves and magnify ground movement by mechanical, electronic, electromagnetic, or mechanical-optical methods. Seismometer outputs are transmitted to seismographs, seismic instruments that produce seismographs and seismograms. A seismograph records the movement of the earth. Typically, the output of a seismograph is proportional to ground velocity; however, special seismographs called accelerographs provide outputs that are proportional to ground acceleration. A seismogram is a paper document that displays summary information about the magnitude and location of an earthquake. There are four major types of seismograms: tectonic-like earthquakes; shallow, volcanic earthquakes; surface events; and harmonic tremors.