█ LAURIE DUNCAN
A seismograph is an instrument that measures and records elastic ground vibrations called seismic waves that are generated by earthquakes and man-made explosions. By recording the arrival of seismic waves at remote seismograph stations, seismologists deduce information about the initial earthquake fault rupture or explosion, and about the physical properties of earth materials between the seismic source and the seismograph. Much of our present knowledge of Earth's large-scale interior structure came from analysis of seismograph records. Academic, petroleum, and mining geologists use other seismic techniques to study the structure of Earth's outer sedimentary layers, to prospect for petroleum, and to assess mineral ore bodies. Academic seismograph networks designed to detect earthquakes or planned survey explosions also perform double-duty as monitoring systems that detect military explosions that may indicate violations of international weapons bans.
A modern seismograph includes five basic parts: a clock, a sensor called a seismometer that measures intensity of shaking at the instrument's location, a recorder that traces a chart, or seismogram, of the seismic arrivals, an electronic amplifier, and a data recorder that stores the information for later analysis. The clock records precise arrival times of specific seismic waves. The seismometer mechanically measures ground movement by comparing the motion of a support structure that moves with the land surface to a stationary or inertial mass. To measure vertical motion, the inertial mass hangs from the support by a spring; to measure horizontal motion it is suspended on a hinge. The recording device registers seismic vibrations with a pen attached to the inertial mass, and a roll of paper that moves along with the Earth's vibrations. As the paper cylinder oscillates and unwinds at a constant rate, the stationary pen traces a seismogram that shows the amplitude and frequency of shock waves that arrive over time. Today's seismographs often contain electronic sensors and recorders that perform these tasks, but the principles of their operation remain the same.
Scientists have used tools to detect ground motion since the ancient Han Dynasty when Chang Heng, a Chinese astronomer and mathematician, invented the first seismometer in about 132 A.D. Heng's "earthquake weathercock" seismoscope consisted of a pendulum that swung inside a jar surrounded by eight balanced dragon heads, each holding a bronze ball in its moveable jaws. During an earthquake, the pendulum would swing away from the approaching seismic waves, hit one of the dragons, and knock the ball out of its jaws, indicating the direction of the shock waves.
Seismographs have undergone considerable refinement since Heng's time. European scientists of the 1700s and early 1800s developed a series of mercury-filled seismoscopes and pendulum seismometers that attempted to measure the amplitude and frequency of seismic waves, as well as their propagation directions. British seismologist, John Milne, and his colleagues developed the first modern seismographs to observe Japanese earthquakes in the late 1800s. Their seismographs, however, recorded only a limited range of wave sizes and seismic events, the instruments were fairly inaccurate, and they required difficult mechanical calibration. German seismologist, Emil Weichert, invented an inverted, mechanically damped pendulum seismometer that considerably improved the sensitivity and accuracy of Milne's seismometer in 1899. In 1906 Boris Golitsyn, a Soviet physicist and seismologist, devised an electromagnetic seismograph that operated without mechanical levers, an enhancement of Weichert's instrument. The first modern seismographs in the United States were installed at the University of California at Berkeley and the Lick Observatory at Mount Hamilton, California in 1877. They recorded the 1906 earthquake that devastated San Francisco.
Development of precise seismographs led immediately to discoveries of Earth's interior structure and delineation of its major physical layers: solid inner core, liquid outer core, solid lower mantle, plastic upper mantle, and rigid lithosphere. British seismologist, Richard Oldham (1858 – 1936) observed that seismic events produce three of different types of waves that travel away from an earthquake focus at different speeds, and named them surface waves, P (Primary or Pressure) waves, and S (Secondary or Shear) waves. Oldham and Weichert confirmed the existence of Earth's core in 1906 by comparing the paths of P waves and S waves through the planet's interior. Yugoslavian seismologist and meteorologist, Andrija Mohorovicic (1857–1936) used seismograph records to define the Mohorovicic seismic discontinuity, or Moho, at the boundary of the iron-rich mantle and the silica-rich crust in 1909. The Danish seismologist, Inge Lehmann, discovered of the boundary between Earth's liquid outer and solid inner core in 1914.
Today, seismologists continue to use seismograph records to make discoveries about Earth's interior structure, to prospect for petroleum and minerals, and to monitor large military explosions. The Incorporated Research Institutions for Seismology (IRIS) consortium, for example, operates the Global Seismograph Network (GSN) of about 120 permanent seismographs that continuously record seismic events around the planet and transmit their data to a publicly available data base. The GSN, like its precursor, the World-Wide Seismograph Network (WWSN), detects all but the smallest earthquakes worldwide, as well as seismic waves emitted by nuclear explosions and detonations of large conventional weapons. The academic members of IRIS provide data and analyses in support of the international Comprehensive Test Ban Treaty (CTBT) that seeks to monitor international weapons tests, and identify treaty violations.
█ FURTHER READING:
Fowler, C. M. R. The Solid Earth. Cambridge: University Press, 1990.
Press, Frank and Raymond Siever. Understanding Earth New York: W.H. Freeman and Company, 2000.
Incorporated Research Institutions for Seismology. "Welcome to the IRIS Homepage." December 3, 2001. < http://www.iris.edu > (December 28, 2002).
United States Geological Survey Earthquake Hazards Program. "Seismology." National Earthquake Information Center and World Data Center for Seismology, Denver. April 5, 2001. < http://neic.usgs.gov/neis/seismology/ > (December 28, 2002).