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

470 J. Rafae'l Goberna, S. J.The seismograms of the first period have not been analyzed, and therefore, therecorded events are not yet identified. This is a job that should be made before orat the same time of being photocopied, especially in the case of having to select theseismograms which contain valuable seismic information. It should be also knownthat not all records of the period have been preserved. It seems that only thoserecords, which the occurrence of some seismic event have been noted or identified,have been preserved. The polarity of the components is not known, but it may bepossible to determine by the same construction of the instruments.The seismograms corresponding to the years 1923 to 1925 were recorded on ordinary,but thin paper, and therefore they are brittle and easily perishable. Theirsize is 28 cm in width and 59 cm in length. The smoke coating was very thin and sowas the recording. Consequently, it is often very difficult to identify the beginningof a seismic event and some of its phases. This happens very often in one of thecomponents, and sometimes the recording of one component is also missing. Thissurely will make copying of these seismograms very difficult.The quantity of seismograms for these two and half years is quite small, and theamplitudes of the events recorded are also small or moderate. This makes dubiousthe magnification of 4,700 supposedly assigned to this instrument by I+. C. Ortiz.Only 35 seismograms are preserved for 6 months of 1923, 40 seismograms for 1924,and 22 seismograms for 1925 (a total of 97 seismograms).No seismograms are preserved for 1926 and 1927, probably because the seismographwas damaged. It started functioning again in April of 1928, but only withone component. On June 16, 1928, the Wiechert seismograph made its first record;and for the rest of the year, both instruments were operating simultaneously. Thinordinary paper was used in both instruments, but with better smoke coating, sothat the time marks and the seismic traces are clearer. Therefore, copying theseseismograms can be made without any difficulty. The size of paper for the Wiechertrecording is narrower and somewhat longer than that for the Madrid instrument:21.5 cm in width and 62 cm in length. The drums rotation is opposite; the Wiechertdrum rotates from right to left, and the other from left to right. Hence, the seismogramreads from left to right for the Wiechert instrument, and from right to leftfor the Madrid instrument.No records are preserved for 1929, and the reasons are unknown. The Wiechertseismograph had not been functioning, except for occasional days in January, untilthe month of October in 1930. However, on June of this year, a new mechanicalhorizontal seismograph of the Cartuja-type was installed; its mass was of 1000kg. Since the month of October, 1930, both the new Cartuja and the Wiechertseismographs were operating simultaneously at many times. The paper used from1930 onwards was somewhat thicker and more solid, and therefore better preserved.The smoke coating was often very dense so that reading of these seismograms issometimes very difficult, and this may cause problems in copying. The paper sizeis the same for both instruments; the direction of drums' rotation is also the same.However, the rotation speed of the Cartuja instrument is a bit greater so that withthe spacing between lines and time marks are larger. Also the line spacing in oneof its components is greater than the other, as if each component were running indifferent drum. Consequently, part of one record is sometimes superposed on theother, making it difficult to read and identify phases in the seismic events.