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

1. Seismological ObservationsThe Standardization of Seismological Data 43 3The ILOM was established to observe latitude variations for investigating themechanism of polar motion and the rotation of the Earth. The amplitude andphase of the polar motion appear to change with time. Though some remarkablepart of the excitation of the Chandler Wobble is considered to be due to atmosphericand synchronized phenomena, the remaining part may be caused by seismic activityand accompanying crustal movements. At the end of the 19th century, an importantproblem was to determine the internal structure of the Earth by observing earthtides, deflection of the vertical, and velocities of seismic waves.The ILOM began seismological observations in 1902 using Omori horizontal pendulums,which recorded on smoked paper. Nasu seismographs, which consisted ofthree components, were also installed in the same room in 1933. Though all componentsof the Nasu instruments recorded on the same paper, readings from only thevertical component were reported along with readings from the horizontal componentsof the Omori seismographs. All seismographs had no damping devices until1956. And after that time, magnetic dampers were installed on each seismographexcept the N-S component of the Omori seismograph. As the instrumental constantswere sometimes slightly changed, the values for 1970 are shown in Table 1as an example. Time marks were provided by electric contacts each minute usinga pendulum clock. Time corrections were made by referring to signals given bywireless (-1902), telephone (-1906), and radio (-1924) time service. The adoptedtime-mark and the recording systems are shown in Figure 1.In 1966, these instruments were moved into a new building located about 100 mfrom the previous station. Then, in 1970, they were replaced by Sax-type longperiodelectromagnetic seismographs and new recording systems. The location ofthe new station and the characteristics of the new seismographs are shown in Tables2 and 3, respectively.2. Standardization of Seismological DataA 3-year project to standardize seismological data recorded by the Omori andNasu seismographs was begun in 1981. As shown in Figure 2, the main tasks wererepairing and filming the seismograms and standardizing the observed data. Thestandardized data were published in March 1984.Table 1. Constants of Old Seismographs for the Year 1970Omori Seismograph Nasu SeismographE-W N-S U-DProper period (sec) 10.8 33.0 4.8Statical magnification 100 20 25Coefficient of friction 3.8 2.5 1.7Damping ratio 2 (no damper) 2Mass of weight (kg) 40.5 17.6 4.4Paper speed (mm/min) 22 25 50