The Steel Globe of the International Seismological Summary

A brief historical note by Anthony A. Hughes (ISS Staff Member 1963 - 1968, later Director of the ISC)

Received: 11 March 2013

At the end of the First World War in 1918 and 1919, Professor H H Turner, the Savilian Professor of Astronomy at Oxford University decided to continue the work of his old friend Professor John Milne in assembling and publishing information and instrumental data of earthquakes in chronological order of occurrence. In 1923 the International Association of Seismology gave this project its support and agreed to the publication being called the International Seismological Summary (ISS). Assembly of the information was undertaken at the astronomical observatory in South Parks Road, Oxford under Turner’s direction, generally arriving by post in the form of station bulletins, seismograms and newspaper reports. By the mid 1920s, the ISS had a small staff including Mr J S Hughes, who remained with the ISS until 1965, and Miss Ethel Bellamy who retired after the Second World War. A list of stations with locations reporting data had been established and an early set of travel times of primary earthquake waves around the world had been derived by Turner. This set is known as the Turner – Zöpritz Tables and is published in the ISS for 1918, the first year of publication. Using these tables, the station locations and their instrumental readings, a method was needed to quickly derive earthquake epicentres (location of origins) and times of occurrence. About this time either Turner or a staff member witnessed an experimental globe being used for such a task. With station locations and onset times of primary earthquake waves arriving at these stations, arcs could be drawn on the globe using a theoretical origin time, giving the locus of a possible epicentre. The origin time is adjusted until the arcs from at least three stations intersect at a single point. This epicentre was confirmed by as many readings as possible. This came to be known as the “Swinging Arcs” procedure. Not only preliminary epicentres could be derived quickly but readings could be checked for consistency. However it was noticed that this work on the experimental globe was very wearing on its surface with pencil arcs being drawn and erased, and pointed instruments like dividers held on the surface.

In the late 1920s, Professor Turner and his staff decided to acquire a globe for the ISS particularly for deriving earthquake epicentres quickly. The steel globe was chosen so as to withstand the harsh treatment of its surface. The engineering company of Casella, London with experience of building precision instruments was approached and the steel globe was built and delivered to the ISS at Oxford Observatory around 1930. It was delivered with a circular cradle on short legs standing on a robust table with a working surface beside the globe. A detachable arc was mounted on the cradle so that a precision ruler could be suspended vertically above the globe. The globe itself comprised of two hemispherical shells about 6 cm thick welded together at the equatorial latitude. Lines of latitude and longitude were engraved on the globe. Subsequently the ISS had quite a number of points drilled to indicate location of stations reporting data and the alphabetic codes of some stations names were also engraved.

The globe was first exhibited publicly at the meeting of the British Association for Advancement of Science in Edinburgh in 1930. After about two years use one of the shells collapsed and the globe was returned to Casella who rebuilt it. It was in frequent use from 1930 to 1955 at the ISS which was relocated to the Kew Observatory, Surrey, UK in 1948. By the mid 1950s use diminished due to the advent of reliable preliminary determination of epicentre services which became available from such organisations as the US Geological Survey in Washington and the Bureau Central International in Strasbourg.

Around 1950 the ISS received a delegation of Indian seismologists who wished to examine the ISS globe and its functions. It is said that two copies were built by Casella and shipped to India. In 1968 the ISS was merged with the International Seismological Centre (ISC) in Edinburgh. The ISS published information on earthquakes occurring in 1918 to 1963, after which the Bulletin of the ISC succeeded it.

Picture taken in mid-1980s