A brief history of seismology

Earthquake recording instruments can be broadly divided into two groups, seismoscopes and seismographs. Seismoscopes record the fact that there has been a earthquake, but give little or no information on the event's time, its magnitude or duration. Seismographs however generally give some indication of the degree of shaking versus time.

Most instruments before the 1800's were seismoscopes. The Chinese  and Egyptians are credited with the first such instruments.  The first known instrument was designed by Chinese philosopher Chang Heng in AD 132. Balls or marbles were released from a vessel when shaking was severe enough, and the direction that the balls travelled in gave some indication of the direction of the earthquake. Chang Heng's instrument was believed to resemble a wine jar, 2 metres across, and with 8 dragon heads symmetrically around it. In each dragon head was a ball, and, if dislodged, the ball fell into the open mouth of a stone toad placed beneath it. The instrument was reputedly used for over 400 years.

Significant advances were made by Italian inventors in the 1700's. Nicolas Crillo used simple pendulums to study a series of earthquakes in Naples in 1731. He discovered that amplitude decreased as the square of distance. Significant advances were made during times of increased earthquake activity, and the series of Calabrian events in 1783 was notable for the research which emanated from it.

In 1839, a series of small earthquakes occurred near Perthshire in Scotland, and research during this period resulted in the invention of an "inverted pendulum" seismometer by James Forbes in 1844.

In 1856 in Italy, Luigi Palmieri introduced an instrument designed to measure direction, intensity and duration of earthquakes on Mount Vesuvius. It was called a "sismografo elettro-magnetico". It was later used for 10 years to detect earthquakes in Tokyo.

Modern instrumentation can be traced back to English scientists working in Japan in the second half of the 19th century. They developed their own intruments to replace Palmieri's in 1885, when routine recording of earthquakes in Tokyo began. In particular, the work of Milne, Gray and Ewing needs to be noted. They were all visiting professors at the Imperial College of Engineering at Tokyo.

The first recording of a teleseism was made by Von Rebeur-Paschwitz in 1889. The recording was made optically on a continuously moving photographic medium. The first seismometer with viscous damping was introduced by Weichert in 1898

INTENSITY SCALES

Various intensity scales are in use around the world, many indigenous to earthquake prone areas. One of the earliest scales was devised  by the Italian De Rossi in collaboration with the Swiss seismologist, Forel. The Rossi-Forel scale is still in use in many places around the world. It is a 10 point scale, devised in 183, and was a combination of earlier scales the two had devised independently in 1874 and 1881.

Guiseppe Mercalli studied volcanoes in Italy, and he amended the Rossi-Forrel scale to a 12 point scale in 1902. This was later amended  (in 1931) by the Americans Harry Wood and Frank Neumann, and became known as the Modified Mercalli Scale, which is the most common scale in use in the world today.

In Japan, the JMA (Japanese Meteorolgical Agency) scale is used. The maximum intensity on this scale is 7. It is based on the work of the famous Japanese seismologist, Omori.

In Europe, the MSK scale was introduced in 1964 by Medvedev, Sponheuer and Karnik. This was based to a large extent on earlier work by Medvedev in Russia.