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

468 J. Rafail Goberna, S. J.occurred at the Perli-Colombian border on the night of the 16th to 17th of Januaryof 1922, at H = 03h 50m 33.0s GMT with the epicenter about 800 km to the southeastof Bogoti at 2.5' N and 71.0D W, according to Gutenberg's determination; itsdepth was about 650 km and its magnitude about 7.6. It was selected and studiedby Turner (1922) to prove the occurrence of very deep earthquakes and it was theobject of a very detailed analysis made by Vicente Inglada (1934).The trace amplitude was large with a peak of 20 cm. This led Fr. Carlos Ortizto calculate the magnification of the seismograph, making use of Wiechert formulasand of the mass, period, friction, damping, etc. of the instrument. The result wasa magnification of 4.7 K. In the following year, Rev. El. Sarasola, Director of theSan Bartolomd Meteorological Observatory, installed this instrument in a betterprotected and permanent underground place at the southeast corner of the samebuilding (Carrera 6 and Calle 9), although this site was not yet entirely satisfactory.The seismometer mass weighted 200 kg, but other constants and specifications wereunknown, except for the provisionally determined magnification of about 4,700.The components were orientation NNE-SSW and ESEWNW.The clock used for time marks as well as the method for its correction werealso unknown. There are two old pendulum clocks with their weight chains andminutes contacts, which might be used for recording time marks. Although Rev.Sarasola usually kept the time in the Observatory adjusted through astronomicalobservations by an electric clock of Rietler type, the seismic clock did not keep timewith same precision, as can be seen from notes found sometimes on the seismogramsindicating corrections of two or more minutes.The drum was of metal and its size was 28 cm in length by 18 cm of radiuswith recording paper of 60 cm in length. Its motion was through a clockwork withrotation varied from 1.5 cm to over 2.0 cm. Its displacement was obtained througha wormscrew, keeping a distance between lines of about 2 mm; this is the only piecethat today remains at the station and maybe also its clock. This equipment waskept functioning from July, 1923, until the end of 1925. From 1926 until April of1928, the station operation was suspended for unknown reasons. However it wasoperated again in April of 1928 until the end of the same year.In 1928 a new seismograph was acquired in Europe, constructed by Spindlerand Hoyer of Gottingen (Germany) of the Wiechert type; this instrument was keptfunctioning until to the present time with some occasional interruptions. Its originalconstants were determined at the factory before being shipped to BogotA. It is ahorizontal astatic and mechanical pendulum with a mass of 200 kg. One componenthas a period of 5.78 sec and the other one of 5.63 sec; its magnification is about92 and 96, and its damping is about 4.5 and 4.6 respectively. The components'orientations were N-S and EW, but their polarization was unknown. However, itwas determined later since its position was never changed.Again in this case, it is not known with certainty the type of clock used for timemarking. Most probably, it was a clock made by the same Spindler and Hoyerfactory for the Wiechert seismograph, but it can not be found at the station now.The Wiechert drum was also somewhat different from the previous one. It was abit shorter in length, but a bit longer in radius, so that its records are 21.5 cm inwidth and 62 cm in length. Its rotation is also slower and is obtained through awormscrew moved by a weight. The rotation is also of opposite direction: it is fromright to left in the Granada type, opposite to the clock hands movement, and it is