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

394 R. Inoue and T. Matsumoto3. Digitization and Correction ProceduresTo obtain the corrected data with equal time intervals, the following steps wereperformed on the original data: (1) The original seismogram was enlarged andprinted on a polyester base. (2) The co-ordinates of peaks, troughs and points ofinflection of the enlarged record were read by a digitizer. (3) Baseline correctionswere performed on the data by the least squares method. (4) The effects of finitearm length and inclination of the arm were eliminated from the data. (5) The datawere interpolated by a cosine curve and equally spaced data were produced. Thetime interval At was chosen as 0.25 sec in this analysis. (6) The corrections forinstrumental characteristics on the data were performed. (7) A bandpass filter wasapplied to the data to remove the high and low frequency noise. As shown inFigure 3, the upper cut-off frequency is 0.5 Hz (2 sec), and the lower is 0.05 Ha(20 sec).These correction procedures of the digitized data are similar to those outlinedin Skoko and Sat0 (1973). The records of the J.M.A. strong motion seismometerswere corrected by the above procedures.However, in the case of the records by the old type seismographs, there aretwo additional problems to be solved. First, the mechanical arm length of theseismographs cannot be found in any of the documents now available. Second,there are a few records in which the apparent zero-line varies while the earthquakeis being recorded (Figure 5). We solved these problems by the following procedures.First, the procedures for estimating the arm length R are as follows (Figure 4):(1) A portion where the amplitude of the time history is very large was chosen.(2) An iso-time circle (Figure 4) which passes through the peak or trough of thelarge amplitude was constructed, and three points, P1, P2 and P3, on the iso-timecircle were read by a digitizer. (3) The perpendicular bisectors of the line P1 . P2and the line P2. P3 were drawn, and the co-ordinates of the point C which is theintersection of the two bisectors was calculated. That point is the position of thepivot of arm. (4) The length of the line segment P1. C (= P2 . C = p3c) wasmeasured. That is the arm length of the seismograph.The values of the arm length for the old type seismographs in Table 2 wereestimated by the above procedures.Second, the procedures for the arm-correction of the above mentioned record areas follows (Figure 5): (1) For the given time history where the apparent zero-linevaries in many stages (Figure 5a), the baseline correction in each section corre-F1 = 0.035 Hz (T = 28.6 sec); F2 = 0.05 Hz (T = 20.0 sec);F3 = 0.5 Hz (T = 2.0 sec); F4 =0.7 Hz (T = 1.4 sec).Figure 3. Characteristics of the bandpass filter.