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

STUDIES OF EARTHQUAKES ON THE BASIS OFHISTORICAL SEISMOGRAMS IN BELGIUMM. De Becker and T. CamelbeeckCentre de Ge‘ophysique Interne, Royal Observatory of Belgium9, Avenue Circ ulaire, 11 80 Brussels, BelgiumABSTRACTThe history of the seismological stations in Belgium is told, with aspecial emphasis on the description of the oldest instruments. The firstBelgian station was set up in the Royal Observatory in Brussels in 1899and was equipped with a Rebeur-Ehlert seismometer. This station waslater equipped with two Wiechert penduli (horizontal and vertical components)as well as with three Galitzin seismometers.In 1984, the ‘Centre de Gkophysique Interne” started to review theseismicity of Belgium using historical documents and seismograms. Thebrief study of two main earthquakes that occurred during this centuryin Belgium show the usefulness of the analysis of historical seismogramsto better assess earthquake hazards, especially in regions with quite lowseismicity such as Belgium.The first seismological station in Belgium was set up in the grounds of the RoyalObservatory at Uccle, a suburb of Brussels, at the beginning of 1899. The geophysicscellar, originally the private property of E. Solvay, a well-known patron ofscience, was offered to the government in 1904. The first earthquake was recordedon 21 March 1899. In 1903, stations also were set up in a quarry of Quenast andin the mines at F’rameries, in the hope of recording and predicting explosions offire-damp, which were claiming many victims. No seismograms from either stationhave survived.The instruments at all three stations were Rebeur-Ehlert pendulums made bythe firm of Bosch in Strassburg. Somville (1907) gives a full description of theseinstruments. They consist of three identical horizontal pendulums oriented in directions120’ apart, and enclosed in a heavy brass box provided with levelling screws,and covered with a glass plate. Each pendulum carried a mirror, which enabledits movements to be recorded photographically. The total weight of the instrumentwas 68.3 kg.The pendulums were regularly calibrated, and operated at a period of about10 sec. With the axis placed horizontally (as in determining a reduced pendulumlength) the period was 0.64 sec. A static magnification of about 160 was obtained.Because of the slow speed of the recording drum, times cannot be read reliably tobetter than about 8 sec. The Royal Observatory still has the instrument and someof its records.In June 1906, two “Strassburg type” heavy horizontal pendulums, with a mass of100 kg, were installed at Uccle. These “tronometers” oriented N-S and EW, weremuch less sensitive than the Rebeur-Ehlert instruments, and their periods provedto be very unstable, varying between 10 and 25 sec (Somville, loc. cit.). On 18 May1907, the EW component from the Rebeur-Ehlert instrument was provided withdamping, obtained by means of a small glass fibre trailing in oil.Historical Seismogramsand Earthquakes of the World225Copyright ,CC1988 by Academic Press, Inc.All rights of reproduction in any form reserved.