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

440 Yasuhiro Umeda and Katsuyoshi It0I Wiechert1 Strong-motion III19?0 Go 'go '60 ?O '80II ..LOW- gain[:.,..::,~;:,.-..::;.-* ........ ..:*: i :*.:.*.: . .. ,::-.-.::':..-.._. ._I ong - period=I I 1 I 1 I IFirst year m?? Second year mi Third yearFigure 1. Microfilming schedule.The first frame of the microfilm is a resolution chart for the focus and the exposurecorrections. The second to the eighth frames contain information on how touse the microfilm. The second frame shows the roll number and the dates of theseismograms being filmed. The third frame shows the station name, address, latitude,longitude, etc. The fourth frame (Figure 2) shows the name of the instrumentand its constants along with the response curves. The fifth frame (Figure 3) showsphotographs of the Wiechert seismographs; the direction of ground displacement onthe original seismogram is also drawn on the right side of this frame. The originalseismograms are smoked paper records.The sixth frame (Figure 4) indicates the filming format. The seismogram of theWiechert instrument is a ring-shaped paper with a total length of 180 cm. Eachseismogram is folded in two, as shown in the upper left of Figure 4. Two sides ofthe seismogram, i.e. A- and B-plane, are filmed in two frames of the microfilm. Oneside of the three-component seismogram is filmed on one frame, and the other sideis filmed on the successive frame, as shown in the bottom of Figure 4. Generally,the side on which some information is described on the seismogram is chosen asthe A-plane. The three-component seismogram is arranged from top to bottomin the order of vertical, east-west, and north-south components. If the east-westcomponent is missing, the middle place in the frame is blank, as shown in Figure4. A small label (10 x 3.5 cm) is affixed in the margin of each seismogram. Thestation, instrument, A- or B-plane, year, component, data on/off time, and a scaleindicating the original record length per minute are given on this label. Since thedrive speed of the recording sheet is 3 cm per minute, the length of one recordline (180 cm) is one hour. Time is Japanese Standard Time (JST), and UniversalCoordinate Time (UTC) is JST minus 9 hours.The seventh frame, sometimes 7th to 9th frames, is an inventory sheet (Figure 5 )indicating whether the seismograms are present or missing. A blank column indicatesthat the records are saved, whereas an oblique line in a column indicatesa missing record. When large amplitude seismic waves were recorded and/or theinstrument was adjusted, a new recording sheet was set on twice or three times ina day. In this case, the number of seismograms per day is described in the column."C" in the column indicates that the calibration signals, i.e. free period, dampingratio and the static magnification of the instrument are recorded on the seismogram.