Seismograph

A seismograph is an instrument that makes a record of seismic waves caused by an earthquake, explosion, or other Earth-shaking phenomenon. The record it produces is a seismogram, the trace shown on a display screen or paper printout.

The terms seismograph and seismometer are often used interchangeably, but only a seismograph possesses the capacity to record the phenomena it detects. A seismometer is a very sensitive instrument that can detect movements of the Earth's surface, and a seismograph is the instrument used to record and measure the vibrations of the ground. Traditional seismographs combined a sensing element called a seismometer with an amplifier and a hardcopy display unit.

Almost all seismometers are based on the principle of inertia, in which a suspended mass tends to remain still when the ground moves. In practice a seismograph has a base fixed in the ground and a heavy weight hanging free, so that when the ground shakes the base shakes too while the hanging weight does not, and the difference in position between the two is what is recorded. A seismometer is called inertial because it rests on Newton's first law, that a body at rest will remain at rest.

In the AS-1 seismometer a magnet hangs from a spring on a boom, and when the ground vibrates the boom moves while the mass stays at rest and stretches the spring. Because the spring will eventually pull the mass, the seismometer needs damping, supplied by oil or by a magnetic damping system.

Most modern seismometers work electromagnetically, with a permanent magnet as the mass and a case carrying windings of fine wire in which small electric signals are generated. In modern seismographs the display is replaced or augmented by a digitizer, and the data stream may be sent to a central facility by radio, telephone or the Internet.

Seismographs are built in distinct classes according to the ground motion they are meant to capture. Short-period, long-period and broadband seismographs divide the work of ordinary observation, while low-gain strong-motion instruments are reserved for large nearby earthquakes. The division follows the behaviour of the waves themselves: high-frequency seismic waves attenuate most severely, so an instrument intended for local earthquakes must respond to different frequencies than one recording distant events.

The performance of the classes differs sharply. Short-period seismographs magnify ground motion several hundred thousand times and can detect movements too small for a human being to feel, whereas strong-motion seismographs apply magnification of less than 100x and are sensitive to ground acceleration during large nearby earthquakes. Modern systems amplify and record ground motion as a function of time, typically at periods between 0.1 and 100 seconds.

The seismoscope stands at the beginning of seismic instrumentation as a device of indication rather than record: it announced that the ground had shaken without preserving any account of the oscillations in time. Its most celebrated form is associated with Zhang Heng, who lived from 78 to 139 CE and worked as a Chinese mathematician, astronomer and geographer. His instrument, dating to roughly the second century of the common era, belongs to the earliest stratum of earthquake observation.

As the ancient texts describe it, the device was an urn-like vessel, apparently cylindrical, with a central pendulum suspended within and eight dragons' heads arranged around its upper circumference, each holding a ball. A tremor unbalances the pendulum and activates a set of levers inside, whereupon one of the eight dragons releases a bronze ball into a toad's mouth to announce the direction of the quake. The History of the Later Han Dynasty is the source of this description.

The instrument is credited in the record with detecting an earthquake in February of the year 138 at a distance of about 600 kilometers from Luoyang, then the capital of China. Interest in Zhang Heng's work and in replications of his seismoscope revived with the rise of the science of seismology in the late nineteenth century. The Didong Yi was first reconstructed by a Japanese scholar in 1875, and a widely known model was redesigned by the Chinese museum researcher Wang Zhenduo in 1951 for display. None of the older replicas could detect an earthquake, and scholars disagree about the exact scientific principles described in the ancient texts.

By the close of the eighteenth century, Ascanio Filomarino had already constructed a mechanical seismograph capable of recording the amplitude of seismic waves, the incoming direction of an earthquake and its starting time.

Luigi Palmieri, who lived from 1807 to 1896, was appointed Director of the Vesuvius Observatory in 1855, and in that same year designed a seismograph composed of several U-shaped tubes filled with mercury, each oriented toward a different point of the compass. He went on to realize the first model of an electromagnetic seismograph. The uninterrupted use of his electromagnetic seismograph at the observatory marked the first step toward geophysical surveillance of the Mt.

Italian seismologists regard the first true seismograph as the instrument created in 1875 by the physicist Filippo Cecchi, which recorded the relative motion of pendulums with respect to the ground as a function of time.

John Milne had his first experience of an earthquake on the night of his arrival in Tokyo, and he came to devote the greater part of his time to the subject with the co-operation of the Japanese government.

Milne designed one of the first reliable seismographs and travelled widely in Japan to set up 968 seismological stations for a survey of the country's earthquakes.

The Kew Observatory instrument was one of the first devices in the worldwide network of horizontal-pendulum seismographs established from 1897 by Milne and the British Association.

This horizontal pendulum design detected small vibrations from distant earthquakes, and observations from similar instruments in different locations could be compared in order to determine earthquake epicentres and reveal the interior structure of the Earth.

After the British Association for the Advancement of Science suggested it, the Government of Canada established seismograph stations in eastern and western Canada, purchasing two Milne seismographs; the instrument installed at Toronto in September of 1897 became the first to record continuously in Canada. The second Milne instrument was serial number ten of the original group of its design and began operations in Victoria, British Columbia on January 1 of 1899, recording on paper moving at 250 mm per hour. In 1905 responsibility for seismological observations in Canada passed to the newly created Dominion Observatory, and in January of 1906 a pair of Bosch-Omori photographic seismographs began operation in Ottawa and captured the great San Francisco earthquake of that year. Until 1927 stations in Canada could only detect large distant earthquakes, with weaker events of magnitude under 5 1/2 beyond their reach.

Switzerland is the first country to establish an official and permanent organisation for monitoring earthquakes, followed by Japan and Italy. The Erdbebenkommission, the Swiss earthquake commission, was established in 1878 at the annual meeting of the Swiss Society of Nature Sciences in Bern. On 1 April 1914 the Swiss seismic service, from then on known as the Swiss Seismological Service, became an official institution instead of a task manned by volunteers. The first Swiss earthquake surveillance station opened at Degenried in 1911 with a Mainka horizontal seismometer and a Wiechert vertical seismometer, and in 1922 a universal seismometer developed by Alfred De Quervain and August Piccard and weighing 21 tonnes began operation at the Degenried station near Zurich.

More than 20 metric tons of the old universal seismograph were turned into 13 kilograms in the digital broadband network, whose instruments record signals ten thousand times smaller. The modern Swiss national seismic network encompasses more than 200 stations whose data are transmitted in real time instead of being collected station by station.

The Seismo Lab was founded in 1921 by the prominent seismologist Harry Wood, and it reached its centennial milestone a century later. Charles Richter joined the staff in 1927. Beno Gutenberg joined Caltech in 1930 and in 1946 became the director of the lab. In 1937 the Seismo Lab, its programs and most of its personnel were transferred from the Carnegie Institution to Caltech. The Seismo Lab and the United States Geological Survey began a cooperative program that developed into the Southern California Seismic Network.

Wood worked with the Mount Wilson Observatory astronomer John Anderson to develop the Wood-Anderson torsion seismometer, originally completed in 1922 and destined to become the standard for the field with examples in use to this day.

Hugo Benioff constructed advanced instruments for detecting seismic waves, including the Benioff seismograph in 1932 and a strain instrument for detecting the stretching of the Earth. The Valdivia quake allowed Benioff, Press and Stewart Smith to observe and measure the so-called free oscillation, the ringing of the Earth, for the first time.

The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes. Richter's original definition held only for California earthquakes occurring within 600 km of a particular type of seismograph, the Wood-Anderson torsion instrument. The moment magnitude scale, which replaced the Richter scale as the world standard, was developed in 1979 by Thomas Hanks of the USGS and Hiroo Kanamori of Caltech and is based on seismic moment. The seismic moment is the product of the rigidity of the rock, the area of the fault that slipped and the distance the fault moved, so stronger rock or a larger rupture produces a larger magnitude.

In 1977 the federal government launched the National Earthquake Hazards Reduction Program, making the USGS the lead agency for federal seismic monitoring.

From 1961 to 1996 the Albuquerque Seismological Laboratory installed and operated the World-Wide Standardized Seismograph Network. Each station of the network carried three Benioff short-period sensors and three Sprengnether Press-Ewing long-period sensors along with recording, timing and calibration equipment.

Approximately 3.7 million single-day record film chips covering the period from 1962 to 1978 were created from the station records. Two almost complete copies of the film chips still exist, at the Albuquerque Seismological Laboratory and at the Lamont-Doherty Earth Observatory.

The seismometer left on the Moon by Apollo 11 returned data for just three weeks, yet in that span it recorded moonquakes and traced how seismic waves travel through the lunar body, giving the first detailed view of the Moon's internal structure. Through the 340-hour lunar night, when temperatures can plummet to minus 170 degrees Celsius, a radioisotope heater kept the instrument above minus 54 degrees — the first major use of nuclear energy in a NASA manned mission. More advanced seismometers deployed at four further Apollo landing sites transmitted until September of 1977, and each measured all three components of ground displacement.

On Mars, the seismometer SEIS was a round, dome-shaped instrument that sat on the Martian surface and took the planet's pulse. A suite of wind, pressure, temperature and magnetic field sensors helped fine-tune the seismometer's measurements of surface vibrations generated by dust storms and dust devils.

A seismogram is the record of the ground motions produced by seismic waves, set down by the seismograph or seismometer. Among the waves it captures are S-waves, transverse waves whose motion is perpendicular to the direction of travel and which are slower than P-waves. Because S-waves do not travel through fluids, they do not exist in the Earth's liquid outer core.

The record also serves as the basis for locating an earthquake. The time difference between the arrivals of the P- and S-waves at a seismometer determines the distance of the earthquake from that station. Once the distance is known from three stations, circles drawn around each station intersect at the location of the earthquake.

Intensity and magnitude describe different things. Intensity is a qualitative measure of the strength of shaking, determined from the observed effects on people, objects and buildings. Earthquake magnitude is a measure of the energy released, estimated from the amplitude of ground motion, and it does not depend on distance from the epicentre. Magnitude scales are logarithmic: each whole number increase represents a tenfold increase in the measured amplitude, while in energy terms it corresponds to about 32 times more energy release.

The Modified Mercalli Intensity Scale currently used in the United States was developed in 1931 by the American seismologists Harry Wood and Frank Neumann and has 12 increasing levels of intensity designated by Roman numerals.

Inge Lehmann trained in mathematics at Copenhagen and Cambridge and analysed how energy from earthquakes travels through the Earth. While studying a large New Zealand earthquake of 1929 she noticed seismic waves appearing in unexpected locations, as if deflected off a boundary in the Earth's core. In 1936 she proposed that the Earth's core has two parts, a solid metal inner core surrounded by a liquid outer core, overturning the accepted theory of an entirely liquid core.

The Raspberry Shake classroom seismograph is based on the Raspberry Pi computer and integrates sensitive monitoring sensors with easy-to-use software.

• 132 CE
  Zhang Heng seismoscope
  Zhang Heng's seismoscope is dated to this year.
• 138
  Recorded detection near Luoyang
  A detection near Luoyang was recorded.
• 1855
  Palmieri at Vesuvius Observatory
  Luigi Palmieri was appointed Director of the Vesuvius Observatory and realized the first model of electromagnetic seismograph.^{9Luigi Palmieri: first scientific bases for geophysical surveillance in Mt. Vesuvius area · Annals of Geophysics (Istituto Nazionale di Geofisica e Vulcanologia) ↗}
• 1875
  Cecchi's seismograph, Milne's Tokyo professorship, Didong Yi reconstruction
  Filippo Cecchi produced the first true seismograph; Milne held a Tokyo professorship; the first reconstruction of the Didong Yi was made.
• 1878
  Swiss Erdbebenkommission
  The Swiss Erdbebenkommission was established.
• 1894
  Milne's return to England
  Milne returned to England.
• 1897
  Milne–British Association worldwide network; Toronto Milne instrument
  The worldwide network of seismographs was established by Milne and the British Association; the Kew Observatory instrument was one of the first devices in it.^{12Horizontal pendulum seismograph designed by John Milne and operated at Kew Observatory (1939-385) · Science Museum Group Collection ↗}
• 1899
  Victoria Milne instrument
  The Victoria Milne instrument was installed.
• 1905
  Dominion Observatory
  The Dominion Observatory was founded.
• 1906
  Ottawa Bosch-Omori instruments; San Francisco earthquake
  In January a pair of Bosch-Omori photographic seismographs began operation in Ottawa and captured the great San Francisco earthquake of that year.^{20Early History of the Canadian Seismograph Network · Natural Resources Canada, Earthquakes Canada ↗}
• 1911
  Degenried station
  The Degenried station was established.
• 1914
  Swiss Seismological Service
  The Swiss Seismological Service was founded.
• 1921
  Founding of the Seismo Lab
  The Seismo Lab was founded.
• 1922
  Wood-Anderson torsion seismometer; De Quervain–Piccard universal seismometer
  The Wood-Anderson torsion seismometer and the De Quervain–Piccard universal seismometer were introduced.
• 1927
  Richter joins the Seismo Lab; limit of Canadian detection
  Richter joined the Seismo Lab; the limit of Canadian detection was reached.
• 1930
  Gutenberg joins Caltech
  Gutenberg joined Caltech.
• 1931
  Modified Mercalli scale
  The Modified Mercalli scale was introduced.
• 1932
  Benioff seismograph
  The Benioff seismograph was developed.
• 1935
  Richter scale
  The Richter scale was introduced.
• 1936
  Lehmann's two-part core
  Lehmann proposed the two-part core.
• 1937
  Transfer of the Seismo Lab to Caltech
  The Seismo Lab was transferred to Caltech.
• 1946
  Gutenberg becomes director
  Gutenberg became director.
• 1951
  Wang Zhenduo model
  Wang Zhenduo produced a model.
• 1961–1996
  WWSSN
  The WWSSN operated during this period.
• 1977
  National Earthquake Hazards Reduction Program
  The National Earthquake Hazards Reduction Program was established.
• 1979
  Moment magnitude scale
  The moment magnitude scale was introduced.