Caltech Seismological Laboratory

A laboratory in Pasadena that gave earthquakes a number. The Richter magnitude scale of 1935 depended on one instrument type in many places, a lesson as relevant to networks today as it was to Southern California then.

A lab and a seismograph

The Carnegie Institution of Washington set up the Seismological Laboratory in Pasadena in 1921 under Harry O. Wood. Wood and John Anderson developed the Wood–Anderson torsion seismometer, a sensitive, standardised instrument used for local earthquakes. By 1932 six seismographs were in place across Southern California and a routine station bulletin had begun. In 1937 the laboratory became part of the California Institute of Technology.

Beno Gutenberg joined from Germany and with Charles Richter and others built a catalogue of the region's earthquakes.

The magnitude scale

Before 1935, earthquakes were rated by intensity, a measure of damage at a place, which depends on the observer and on local ground. In 1935 Charles Richter published An Instrumental Earthquake Magnitude Scale in the Bulletin of the Seismological Society of America. The scale used the largest amplitude recorded on a Wood–Anderson seismograph at a standard distance of 100 kilometres, on a logarithmic scale anchored at a defined reference: a trace amplitude of one micrometre corresponds to magnitude zero.

The scale worked because the same instrument was used at every station, so that the amplitude recorded at one could be corrected to another. Hiroo Kanamori later developed the moment magnitude scale in 1977, based on the physical size of the earthquake, which replaced Richter's for large events.

From the lab to the network

The Southern California Seismic Network programme began in 1974 with the US Geological Survey, after the San Fernando earthquake of 1971. The laboratory's staff include Hugo Benioff, who developed the Benioff seismograph and showed how earthquakes trace subducting plates. The network now supplies the ShakeMap and earthquake early warning systems, which tell people in Los Angeles of an earthquake seconds before shaking arrives.

A scale with limits

The Richter scale was designed for one instrument, in one region, for earthquakes at a particular range. It saturates for large earthquakes, because the Wood–Anderson seismograph cannot record their long periods fully, and it was never meant to be a global measure. Seismologists used it as a stopgap for decades, and the public still knows it by Richter's name even though the agencies now report moment magnitude. The episode is an example of a scale that outlives its design limits: the name and the logarithmic idea persisted, while the underlying measurement changed.

Instruments and technologies

  • Wood–Anderson torsion seismometer A small copper cylinder suspended on a fine wire, with a mirror whose deflections are recorded by a light beam on photographic paper.
  • Benioff seismograph A moving-coil electromagnetic seismometer that gave greater sensitivity to a wider range of periods.
  • Modern broadband network Digital broadband sensors in vaults with telemetry to the lab give real-time data for early warning.

The people

  • Harry O. Wood Founder of the laboratory.
  • Charles F. Richter Seismologist; the magnitude scale of 1935.
  • Beno Gutenberg Seismologist; collaborated with Richter on magnitude and global seismicity.
  • Hugo Benioff Seismologist and instrument designer.
  • Hiroo Kanamori Developed the moment magnitude scale in 1977.

Timeline

  • 1921 Harry Wood establishes the laboratory under the Carnegie Institution of Washington.
  • 1932 Six seismographs in place; routine station bulletin begins.
  • 1935 Richter publishes An Instrumental Earthquake Magnitude Scale in the BSSA.
  • 1937 Responsibility for the laboratory transfers to Caltech.
  • 1974 Southern California Seismic Network programme begins with the USGS.
  • 1977 Hiroo Kanamori develops the moment magnitude scale.

Key lessons for observation today

  • One instrument type makes a scale possible The Richter scale worked because every station used the same seismometer. A standard instrument type across a dense network is how a universal scale is made.
  • Scales must be redefined when they fail The moment magnitude scale replaced Richter's for large earthquakes. Standards need versions and a path for change.
  • From a catalogue to an alert The same network now feeds early warning. Data held for decades can be reused for real-time services if it is structured and open.

Sources

  • Caltech Seismological Laboratory, A brief history of the Seismo Lab
  • Richter (1935), An instrumental earthquake magnitude scale, BSSA 25(1), Caltech repository
  • Wikipedia, California Institute of Technology (coordinates, secondary)