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

SEISMOLOGICAL MEASUREMENT IN HONG KONG1. IntroductionH. K. LamRoyal ObservatoryNathan Road, Kowloon, Hong KongSeismological measurements in Hong Kong began in 1921 by the Royal Observatory,Hong Kong. Apart from the period 1941-1950, continuous recordings havebeen made.2. SeismographsTable 1 shows the seismological instrumentation used since 1921. Before WorldWar TI, the instrumentation consisted of two horizontal-component Milne-Shaw seismometerseach having a natural period of 12 seconds. The recording was made photographicallyusing a galvanometer of similar period, the exact values of which areno longer on record. The seismometers were located on a pillar in a double-walledcellar about 6 meters below the Royal Observatory grounds. These instruments wereremoved during the war, and so no records are available for the period 1941-1950.Seismological recordings resumed in 1951 with the acquisition of two horizontalcomponentand one vertical-component Sprengnether seismometers. The naturalperiod of the horizontal seismometers was 14.5 seconds, and that of the verticalseismometer was 1.5 seconds. Recordings were made photographically using mirrorgalvanometers having periods of 18.0 seconds and 1.5 seconds, respectively; for thehorizontal and vertical components. The operation of this set of seismographs wasterminated in 1976 due to frequent seismometer and galvanometer faults.As part of the programs for the International Geophysical Year, a set of threecomponentlong-period seismographs with 15-second Press-Ewing seismometers and105-second galvanometers was set up in 1958 in a cooperative experiment withthe Lamont-Doherty Geological Observatory of Columbia University. It operateduntil 1976 when the supply of spares and consumables from Lamont ceased. TheRoyal Observatory later acquired electronic recording equipment for the Lamontseismometers. Pen-and-ink recordings resumed in 1979.In 1963, a set of three-component long-period seismographs with 15-second Sprengnetherseismometers and 100-second galvanometers and another set of three-componentshort-period seismographs with 1-second Benioff seismometers and 0.75-second galvanometers were installed as part of the U.S. Worldwide StandardizedSeismograph Network (WWSSN). These sets are currently operational.In recent years, there has been increased interest in local seismology in connectionwith the design of many extensive engineering projects that might be subject to theearthquake hazards. This interest was heightened by the occurrences of disastrousearthquakes in China, such as the Tangshan earthquake in 1976, and by earthquakepredictions made at various times. Despite such interests, the Royal Observatorysite was increasingly affected by ground noise from urban development.In order to monitor in more detail the location and magnitude of earthquakesin the vicinity of Hong Kong, a three-station network of short-period seismographswas installed in 1979. The stations, each equipped with a Teledyne Geotech S-13Historical Seismogramsand Earthquakes of the World476Copyright 0 I988 by Academic Press, Inc.All rights of reproduction in any form reserved.