Categories of Seismometers

There are primarily three types of seismographs used in earthquake research, each characterized by a specific period-the duration required for the instrument's pendulum to complete a single oscillation (or one full back-and-forth swing)-that corresponds to the amplitude (velocity and intensity) of the seismic vibrations it is designed to measure.

Short-Period
Generally employed to study primary (P) and secondary (S) waves, these instruments measure the fastest-moving seismic waves. This is because these waves travel at such high velocities that a short-period seismograph can complete a full oscillation in less than one second; furthermore, it amplifies the recorded seismograms, enabling researchers to discern the precise trajectory of instantaneous crustal movements.

Long-Period
The pendulums in these instruments typically require approximately 20 seconds to complete a single oscillation; they are utilized to measure the slower crustal movements that follow the initial primary and secondary seismic waves. This type of instrument is commonly deployed within seismic detection networks.

Ultra-Long-Period or Broadband
Seismographs featuring the longest pendulum oscillation periods are classified as ultra-long-period or broadband instruments. Broadband seismographs are becoming increasingly prevalent in practice, as they typically provide more comprehensive information regarding crustal movements occurring across the globe.