Historical Seismograms - Evidence from the AD 2000 Izu Islands ...

452 David Denhamnortheast coast of Tasmania between 1883 and 1885. Biggs (1885) constructed hisinstruments 'to arrive at an estimate of the actual magnitude of the surface-motionof the earth; and to gain some idea of the position of the source, or focus, of thedisturbance". Biggs built a three-component instrument that recorded on smokedglass; unfortunately, none of these records are known to be extant.In 1888, H. C. Russell, government astronomer at Sydney, obtained a Ewingseismograph, and in the same year, a Gray-Mane was installed at the MelbourneObservatory (Doyle and Underwood, 1965). Records from these instruments alsoare not known to be extant.The seismological committees of the Australian and British Associations for theAdvancement of Science sponsored a program to establish the first world network ofseismographs, and in 1901 Milne instruments were obtained for Melbourne, Perthand Sydney. These were installed in 1901, 1902, and 1906, respectively, and in1909 an additional Milne was installed in Adelaide (Doyle and Underwood, 1965).Because the magnification of the Milne instruments was low (kc 6) and they wereunderdamped, few events would have been recorded by them.In the 1920's the situation improved considerably when Milne-Shaw seismographswere installed at Perth (1923), Adelaide (1924), and Melbourne (1928). Theseinstruments had a magnification in the range 150-250 and had magnetic damping.The valuable records from these instruments form the core of historical seismogramsfrom Australia. Meanwhile at the Riverview station near Sydney, which beganoperation in 1909, the instrumentation was gradually improved to make it the bestknown, and the best equipped, station in Australia for many years.Wiechert N-S and E-W seismographs operated from March 1909 until June1955; a Wiechert vertical seismograph operated from May 1909 until January 1944;Mainka N-S and EW seismographs operated from August 1910 until December1962; and Galitzin vertical, N-S and EW seismographs operated from January1941 until December 1962. All these instruments were calibrated regularly, andDrake (1985) has made a thorough study of their response characteristics. In his1985 paper Drake describes (1) where to find the instrumental constants of the seismographs,(2) how to find the magnification and phase lead from these constants,and (3) how the constants were determined.3. Historical SeismogramsSeismograms from the Melbourne, Perth and Riverview observatories form thebest historical sets. The Melbourne and Perth records have been properly archivedand readily available for several years. However, the Riverview records have onlyrecently been cataloged and archived. None of these seismograms have been microfilmedand many are fragile.Adelaide seismograms, which had been presumed lost for several years, wererecently found but they are not properly cataloged (S. Greenhalgh, Flinders University,South Australia, pers. com. 1985).Table 1 lists the periods of operation of the early seismographs and the availabilityof the records. The data from this table are based mainly on Doyle and Underwood(1965). Figure 1 shows locations of the early seismograph stations. The historicalseismograms from Australia are reasonably well cataloged and available. However,