Seismograph (World of Earth Science)

A seismograph is an instrument used to measure and record ground vibration caused by explosions and earthquake shock waves. In the late 1800s, John Milne (1850913), an English mining engineer, developed the first precise seismometer, the sensor in a seismograph that detects and measures motion. Since then, seismograms, the data recorded by a seismograph, have helped seismologists predict much more than Earth movement. These devices have also led to discoveries about the nature of the earth's core.

The process of using a tool to detect ground motion dates back to the ancient Han Dynasty when Chang Heng, a first-century Chinese astronomer and mathematician, invented the first seismometer. He used a pendulum connected to an eight-spoked wheel in which each spoke was connected to a mounted dragon's head with moveable jaws. When the pendulum moved during an earthquake, a bronze ball in each of the heads would pop out if hit by the spoke of the pendulum wheel. While this did not lend clues about the force of an earthquake, it gave the ancient scientist an idea of the direction of the shock waves and their source. Since that time, Heng's concept has been refined considerably. Later seismographs employed a heavy pendulum with a stylus, or needle, suspended above a revolving drum. The drum contained a device on which the etchings from the needle could be recorded. During an earthquake, the pendulum and needle remained motionless while the drum on the base moved, recording the earth's movement. As much as these later pendulum seismographs improved upon the ancient Chinese method, they still fell short of providing answers to the many questions that arose with more precise readings. For example, once a strong motion set off a seismograph's pendulum, the pendulum would swing indefinitely, failing to record aftershocks that followed the initial disturbance. Additionally, the seismographs of the late 1800s recorded only a limited range of wave sizes and numbers. The inverted pendulum, invented by German seismologist Emil Weichert in 1899, helped overcome many of these limitations. Weichert employed a system of mechanical levers that linked the pendulum movement more closely to the earth's vibrations.

In 1906, Boris Golitsyn (1862916), a Soviet physicist and seismologist, devised the first electromagnetic seismograph; for the first time, a seismograph could be operated without mechanical levers. Although many of the modern seismographs are complicated technical devices, these instruments contain five basic parts. The clock records the exact time that the event takes place and marks the arrival time of each specific wave. The support structure, which is always securely attached to the ground, withstands the earth's vibrations during the earthquake or explosion. The inertial mass is a surface area that does not move although the earth and the support structure oscillate around it. The pivot holds the inertial mass in place, enabling it to record the earth's vibration. The vibrations are registered through the recording device: essentially a pen attached to the inertial mass and a roll of paper. The paper moves along with the earth's vibrations while the pen remains stationary. This shows the pattern of shock waves by recording thin, wavy lines, revealing the strength of the various waves as well as the frequency with which they occurred.

After the first modern seismograph was installed in the United States at the University of California at Berkeley, it recorded the 1906 earthquake that devastated San Francisco. Not long before that, Weichert and fellow scientist Richard Oldham (1858936) were finally able to determine the existence of the earth's core through precise recordings of seismic waves. In 1909, the use of a seismograph helped Yugoslavian seismologist and meteorologist Andrija Mohorovicic (1857936) discover the location at which the earth's crust meets the upper mantle. That discovery was followed in 1914 by Inge Lehmann's discovery of the boundary between the earth's outer and inner core. These important findings finally secured knowledge about the existence of boundaries for all of the earth's major layers: the inner core, the outer core, the mantle, and the crust. Seismographs also help miners determine the amount of dynamite needed for quarry blasts. Seismographs detect the force of atomic blasts and nuclear explosions, and are also used to detect the speed of seismic waves traveling in the earth. This data provides valuable information about the substances of which the earth is comprised, such as the natural resources oil and coal.

See also Earth, interior structure; Faults and fractures; Mohorovicic discontinuity (Moho); Richter scale