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

Historical Seismograms in China 42 1number of stations on the China mainland increased to 20 by 1957. In July of 1957,12 of these stations were upgraded with SK Type (Kirnos Type) medium-periodbroadband seismographs, and SH Type (Halin Type) short-period seismographs.Since 1959, networks consisting of individual stations having seismographs withelectronic amplification have been set up around Beijing, Xinfengjiang and Xichangfor microearthquake observations. By April 1966, a telemetered seismograph networkhad been installed around Beijing and had begun recording. Because of thesuccessive occurrence of many strong earthquakes, the Chinese government andpeople have given tremendous support to seismology studies. The number of seismographstations in China has increased rapidly and now exceeds 400. Smokedpaperand/or pen-and-ink recorders and quaxtz-crystal clocks are commonly used.To increase the ability for detecting microearthquakes in the plains regions, Chinahas installed bore-hole seismographs at some stations.3. Seismic Observation SystemAt present, most seismic stations in the country of China are engaged in comprehensiveobservations; i.e., besides seismic observations, they also make observationson geomagnetics, gravity, crustal deformation, and others. These stations can beroughly divided into three types, as shown in Figure 1:(1) Standard seismic stations equipped with three-component short-period seismographs,three-component broadband seismographs, and three-component longperiodseismographs; there axe 22 stations of this type in operation.(2) Regional seismic stations having three-component short-period seismographs;there are now about 70 such stations and over 300 local seismic stations.(3) Regional telemetered seismograph networks; there are six such networks atpresent.4. Instrument SystemAt present, the long-period seismograph used on the China mainland is Type 763;photographic recording is provided by a galvanometer (Figure 2). The broadbandseismographs used are the SK (Kirnos) or GW (Galitzin-Wilip) Type, and also havephotographic recording provided by galvanometers. The DK- 1 seismograph, whichhas electronic amplification and pen-and-ink recording, is used only for monitoringregional seismicity and quick-reports of seismic events. Besides the DK-1, othertypes of short-period seismographs that also axe used, can be summarized as follows:(1) with photographic recording - Type 62, Type SH (Halin), Type VGK; and(2) with pen-and-ink recording - such instruments consist of three units, the seismometer,the amplifier, and the recorder, as listed below (Figure 2):Seismometer Amplifier RecorderDD-1 DF-1 DD- 1Type 64 Type 63-A Type 63-AType 65 Type 581 Type 768VGK Type 67BJ-1 (bore-hole)