Early Instrumental Seismology on a Global Scale

World networks of seismographs, such as the Worldwide Standardized Seismographic Network (WWSSN), the Global Digital Seismographic Network (GDSN), and the Incorporated Research Institutions for Seismology (IRIS), are not new. Early outstations were set up by European countries, such as the German station at Apia in Samoa in 1902, but the first serious attempt at global coverage was the network of the British Association for the Advancement of Science (BAAS) established by Prof. John Milne on his return to Britain from Japan in the late 1890s. The Milne network at its peak had about ten stations in the British Isles and nearly thirty elsewhere throughout the world. The network comprised instruments of standard manufacture from which Milne collected readings at his home in the Isle of Wight in the first systematic analysis of global seismicity. From this enterprise, whose results were published as the Shide Circulars of the BAAS, there developed the International Seismological Summary (ISS) and, later, the International Seismological Centre (ISC). Under Milne, for the first time the true pattern of instrumentally determined global seismicity began to emerge, as shown in his earthquake map for 1899, which also shows the distribution of Milne seismographs (fig. 2; Milne, 1900).

We must remember that the knowledge of earth structure at that time was very elementary, and the instruments low in magnification (about 12), long in period (about 12 s natural period), and undamped, with surface waves as the main recordings. The general accuracy of location was surprisingly good, but naturally mistakes are apparent in the light of modern knowledge. One great strength of the network was its uniformity, which has helped in a recent reevaluation of Milne's locations of early events in West Africa (Ambraseys and Adams, 1986). Irrespective of details, it was often possible to establish by a similarity of reporting that certain stations were nearly equidistant from a given earthquake. We found that consistent locations could be found by using the reported time of the maximum oscillation, M , assuming it to be an Airey phase of velocity 2.8 to 3.0 km/s. In this way we could confirm the location of many early events, but some we found to be grossly misplaced. An example is the event of May 21, 1910, originally placed in Niger in Central Africa but found by us to be in Turkey, where there was confirmatory felt information (fig. 3).

Damped instruments of greater sensitivity were developed after about 1910, and fuller details of P and S phases were recorded but at the expense of uniformity of recording. By 1930 the number of recording stations had grown, and a map (fig. 4) compiled by Miss Bellamy (1936) of ISS shows well the main features of global seismicity, but with the details blurred by scatter. The stations at that time are shown in figure 5.

The number of stations grew steadily, and by 1951 about 600 were listed with their direction cosines in an ISS publication. The distribution of sta-

Figure 2
Global earthquakes and seismograph stations in 1899 (Milne, 1900). Epicenters
are approximately indicated by numbers that refer to events in Milne's catalog.

Figure 3
Relocation to Turkey of an event originally placed in Niger (Ambraseys
and Adams, 1986). Arrival times are given for stations with reinterpretation
of phases and derived distances in degrees.

Figure 4
Global earthquakes 1913–1930, showing scatter of features now better defined (Bellamy, 1936).

Figure 5
Seismograph stations in 1936 (Bellamy, 1936).

tions, however, remained very uneven, with strong concentrations in North America and particularly in Europe. At the time the New Zealand station Roxburgh was installed in 1957, at latitude about 45°S, it was the most southerly regularly operated station in the world, still leaving the southernmost thirty percent of the world's surface with no station. The advent of the International Geophysical Year, and the installation of about 120 WWSSN stations in the late 1950s and early 1960s, changed the picture enormously; since then the coverage of recording stations has significantly improved.