Wood-Anderson seismograph

Wood-Anderson seismograph is a short-period torsion seismometer designed by Harry O. Wood and J. A. Anderson in the 1920s at the Caltech Seismological Laboratory. With a natural period of approximately 0.8 seconds and a specific damping characteristic, it became the reference instrument for Charles Richter's 1935 magnitude scale, effectively defining what a magnitude measurement meant for three decades. The instrument's design was optimized for California's crustal structure and moderate earthquake sizes; its 0.8-second period means it is blind to the long-period energy released by the largest earthquakes, a limitation that directly caused the saturation problem in the Richter magnitude scale.\n\nThe seismograph used a small mirror attached to a taut wire that rotated in response to ground motion, reflecting a light beam onto photographic paper. This elegant mechanical design produced recordings of sufficient fidelity to measure peak amplitudes reliably, but it also constrained the measurable frequency band. The instrument did not merely record earthquakes; it selected them.\n\nThe Wood-Anderson seismograph is a case study in how instruments define domains. It was not a neutral window onto seismic activity but a selective filter that made certain earthquakes visible and others invisible. The scale that depended on it could not outgrow the instrument's assumptions.\n\n\n\n