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

202G. E. Fiedler B.FI1 IFigure 1. The Caracas earthquake of October 29, 1900, recorded by a Milne horizontalseismograph, installed in the Pamplemousses Observatory, Mauritius Island, at an epicentraldistance of about 13,900 kin. A photomounting, made by G. Fiedler was necessary,due to problems with the copy. The period of the pendulum was 20.4 sec, damping ratio1.04, static magnification = 11, mass = 310 gm, the photomounting amplification is 1.17:l(enlarged). Directionality must be checked.Figure 2. The Caracas earthquake of October 29, 1900, H = 9:11:00 GMT, recordedby the Milne horizontal seismometer, EW component, at the Kew Observatory, England(scale 1:l).2. Interpretation of Seismograms of the Caracas Earthquake of 19002.1 Instrumental ConstantsThe E-W component of the Mihe seismograph of the Kew Observatory, followingReid (1910) and Wood (1921), had the intrinsic constants: To = 18 to 19 sec,static magnification V = 6.1, length of the indicator J = 520 meters, damping ratioE = 1.114 (in this case and after Reid (1910) is the amplitude ratio of a maximumto the next following minimum, i.e., over one half period or T), friction dampingzero (photographic recording), angular displacement 1 mm = 0.55”, mass M = 255gm, distance of the center of oscillation from the axis of rotation L = 15.6 cm.From J = 520 meters and V = 6.1, we obtain by the relations V = J/L andTo = 27rm the equivalent pendulum length L = 85.2 meters and the naturalperiod To = 18.5 sec, as given by Reid (1910) and Wood (1921). Both published alsothat for angular displacement, 1 mm = 0.55”. But it is J/(360” x 3600”/27r) = 2.52nun/’’ or 1 nim trace amplitude corresponds to ground tilt of 0.39”, unless anyunmentioned unknown is still involved.