A seismogram is a graph output by a seismograph. It is a record of the ground motion at a measuring station. The energy measured in a seismogram may result from an earthquake or from some other source, such as an explosion.A recording of earth motion as a function of time is called a seismogram.
A set of seismograms for an earthquake from the USGS (click to see large version)Historically, seismograms were recorded on paper attached to rotating drums. Some used pens on ordinary paper, while others used light beams to expose photosensitive paper. Today, practically all seismograms are recorded digitally to make analysis by computer easier. Some drum seismometers are still found, though, especially when used for public display. Seismograms are essential for measuring earthquakes using the Richter scale.
In the history of the innovations surrounding earthquake study, we have to look at two things: the devices that recorded earthquake activity and the measurement systems written to help interpret that data. For example: the Richter Scale is not a physical device, it is a mathematical formula. Around 132 AD, Chinese scientist Chang Heng invented the first seismoscope, an instrument that could register the occurrence of an earthquake. Heng's invention was called the dragon jar (see picture right). The dragon jar was a cylindrical jar with eight dragonheads arranged around its brim; each dragon had a ball in its mouth. Around the foot of the jar were eight frogs, each directly under a dragonhead. When an earthquake happened a ball dropped from a dragon's mouth and was caught by the frog's mouth.
Definition of Intensity and Magnitude Scales
Magnitude measures the energy released at the source of the earthquake. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded on a seismogram at a certain period. Intensity measures the strength of shaking produced by the earthquake at a certain location. Intensity is determined from effects on people, human structures, and the natural environment. Intensity does not have a mathematical basis; determining intensity is based on observed effects. The first reported use of any measurement of earthquake intensity has been attributed to the Italian Schiantarelli, who recorded the intensity of the 1783 earthquake that occurred in Calabrian, Italy.
Types of Seismograms
The credit for the first modern intensity scales goes jointly to Michele de Rossi of Italy (1874) and Francois Forel of Switzerland (1881), who both independently published similar intensity scales. Rossi and Forel later collaborated and produced the Rossi-Forel Scale in 1883. The Rossi-Forel Scale used ten degrees of intensity and became the first scale to be widely used internationally. In 1902, Italian volcanologist Giuseppe Mercalli created a twelve-degree scale of intensity.
Modified Mercalli Intensity Scale
Although numerous intensity scales have been developed over the last several hundred years to evaluate the effects of earthquakes, the one currently used in the United States is the Modified Mercalli (MM) Intensity Scale. It was developed in 1931 by the American seismologists Harry Wood and Frank Neumann. This scale, composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, is designated by Roman numerals. It does not have a mathematical basis; instead, it is an arbitrary ranking based on observed effects.
Richter Magnitude Scale
The Richter Magnitude Scale was developed in 1935 by Charles F. Richter of the California Institute of Technology. On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value. At first, the Richter Scale could be applied only to the records from instruments of identical manufacture. Now, instruments are carefully calibrated with respect to each other. Thus, magnitude can be computed from the record of any calibrated seismograph.
Definition of a Seismograph
Seismic waves are the vibrations from earthquakes that travel through the Earth; they are recorded on instruments called seismographs. Seismographs record a zigzag trace that shows the varying amplitude of ground oscillations beneath the instrument. Sensitive seismographs, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world. The time, location and magnitude of an earthquake can be determined from the data recorded by seismograph stations. The sensor part of a seismograph is referred to as the seismometer, the graph capability was added as a later invention.
Water & Mercury Seismometers
A few centuries later, devices using water movement and later mercury were developed in Italy. In 1855, Luigi Palmieri of Italy designed a mercury seismometer. Palmieri's seismometer had U-shaped tubes filled with mercury and arranged along the compass points. When an earthquake happened, the mercury would move and make electrical contact that stopped a clock and started a recording drum on which the motion of a float on the surface of mercury was recorded. This was the first device that recorded the time of the earthquake and the intensity and duration of any movement.
John Milne was the English seismologist and geologist who invented the first modern seismograph and promoted the building of seismological stations. In 1880, Sir James Alfred Ewing, Thomas Gray and John Milne, all British scientists working in Japan, began to study earthquakes. They founded the Seismological Society of Japan and the society funded the invention of seismographs. Milne invented the horizontal pendulum seismograph in 1880. The horizontal pendulum seismograph was improved after World War II with the Press-Ewing seismograph, developed in the United States for recording long-period waves. It is widely used throughout the world today. The Press-Ewing seismograph uses a Milne pendulum, but the pivot supporting the pendulum is replaced by an elastic wire to avoid friction.