WoodAnderson

Wood–Anderson is a historic short-period, single-component seismograph developed in the 1920s that became the standard instrument for recording local earthquakes and calibrating the local magnitude scale. The device is named after its designers and was built to provide a consistent, high-frequency response suitable for measuring near-field seismic waves. It employs a mass-spring system with a relatively short natural period—approximately 0.8 seconds—and records ground motion with a stylus tracing on a rotating drum. The resulting traces serve as a basis for estimating the amplitude of seismic waves; this amplitude, when corrected for distance, forms the data used to compute local magnitudes (ML) in the early magnitude scales. The Wood–Anderson seismograph saw widespread adoption by seismology networks in the mid-20th century and contributed to the standardization of seismic measurements. Later instruments with broader frequency responses and digital recording replaced it, but the Wood–Anderson design remains a milestone in the history of seismology and magnitude computation.