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

The Historical Seismograms of Colombia 471The quantity of records for most of these years was greater at the beginningof the period, but decreasing appreciably and gradually for the last years. Thetotal number of seismograms is about 554 for the last 12 years, and with the 97seismograms recorded in the first 5 years, we have a total of about 651 for the firstperiod of the Bogoti seismic station from 1923 to 1940.3. Second Period (1942-1961)The second period of the seismic station at Bogoti began with the establishmentof the Instituto Geofisico de Los Andes Colombianos on September, 1941, in theSan BartolomC College "La Merced", situated to the north of the former and to thenortheast border of the city of Bogoti.The geographic situation of the new site is given by the following coordinates:4" 37' 23" N and 74' 03', 54" W at an elevation of 2,653 m above the mean sea level.For the new station, a place was selected as isolated as possible from the city traffic,and the site was prepared for the best conditions according to the specificationsof modern seismology. By taking advantage of a small cavity in a rocky escarpmentwithin the College campus, a cave was excavated in the rock, and the place wasprepared so that the seismic instruments were installed on cemented bases over therocky ground. Of the three previous mechanical seismographs, only the Wiecherttypewas kept and installed in the new place. The other two instruments weredismantled, and most of their parts were dispersed or destroyed; only one recordingsystem, the Cartuja mass of 1000 kg, and the clocks were preserved. In addition,an electromagnetic seismograph was acquired: a short-period, photographic, Benioffmodel, constructed under direction of the same author at Pasadena, California.The seismic station was completed with an electric clock of the Standard ElectricTime Co., one radio transmitter by Johnson Viking, and a Hallicrdter receiver to receivethe international time. A three-component electromagnetic and photographicseismograph of the Galitsin-type was ordered from the Masing-Wilip workshop inEstonia, but it was never received because of the Second World War. Later on, aseismograph of two horizontal components was ordered from Sprengnether at St.Louis, Mo. (U.S.A.).The new station was ready to start operation at the end of 1941, but its regularfunction did not begin until the month of March, 1942. The period of the 100 kgBenioff vertical seismograph was 1 sec and that of its galvanometer was 0.3 sec;its damping was critical and its static magnification was 1,200, but its polarity isunknown. Its drum is 32 cm in length and its circumference 91 cm; its rotationis 1 mm/sec and the spacing between lines is 2.5 mm. The size of the recordingphotographic paper is the same as that used in the Vela project.The Wiechert seismograph started operation again at the beginning of 1943. Thehorizontal seismograph (ordered from Sprengnether to replace the Masing- Wilip)reached Bogoti in 1946; one component had to be repaired, and therefore the wholeinstrument installation was made in July of 1947.This equipment is composed of two horizontal components, Sprengnether modelH, galvanometers by Leeds & Northrup of Philadelphia (U.S.A.), and a triple drumfor photographic paper of 30 cm by 90 cm. The drum rotation is 30 cm per minute.The natural period of this seismograph was 16 sec, and the components orientationwas N-S and EW; other constants, such as damping, polarity, and magnification,were unknown.