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

1. IntroductionHISTORICAL SEISMOGRAMS RECORDEDIN VENEZUELAG. E. Fiedler B.Chief, Seismological Institute CAR (ret.)Apartado postal 5407 Carmelitas, Caracas lOlO-A, VenezuelaThe Cagigal Observatory in Caracas, Venezuela, has been recording earthquakessince 1933. This observatory was created by governmental decree on September 8,1888, basically for astronomy and meteorology research. Figure 1 gives an idea ofthis historical building. After the large and destructive Caracas earthquake of October29, 1900, seismology was included by name in the mission of the observatory.The director, Dr. Luis Ugueto, asked for financial help to buy two Agamennoneseismographs for about Bolivares 260 each at that time, but he could not raise thefunds. Venezuela, therefore, had to wait until 1932-33, when a vertical and a horizontalmechanical seismograph designed by Emil Wiechert were first installed atthe observatory.The vertical Wiechert seismograph had a mass of 80 kg, an equivalent pendulumlength of 9.91 m, and a static magnification of 62. Unfortunately, because ofproblems in the spring-suspension system from the beginning, this instrument neverproduced acceptable records.Like the vertical, the Wiechert horizontal astatic mechanical seismograph camefrom the Spindler and Hoyer factory of Gottingen, Germany (Figure 2). Accordingto Centeno-Grau (1940, p. 162), ‘This inverted pendulum had a mass of 80 kg,the amplification of the indicator was 101, and the equivalent pendulum length was9.85 m for one of the two components and 9.01 m for the other.” From these datawe derive that the free period of the pendulum was about 6 to 7 sec (the factorygives 4 to 12 sec). Other important data like damping and dynamic response ofthe system have to be estimated from the records. The speed of rotation of therecording drum (paper speed) was 5.44 sec/mm record or 0.184 mm/sec.The Wiechert seismographs are no longer in operation. They were removed in1953-54 when the observatory building was razed. Seismographic recordings inVenesuela continued in 1955-56 in the newly constructed Seismological Institute ofthe “Observatorio Astron6mico Sismoldgico Geomagnktico Juan Manuel Cagigal.”The new instruments were 3 Galitzin-Wilip, 5 Sprengnether, 1 Benioff vertical,and a 20-ton horizontal Wiechert pendulum recording the ground motion in thetwo components EW and NS, on smoked paper. The intrinsic constants of thisinstrument, also made by Spindler and Hoyer, installed and calibrated by the authorof this paper, are the following: mass of the pendulum 19,600 kg, free period 1.62 sec,equivalent pendulum length 0.64 meters, indicator-length 1577 meters (lever-armsystem and torsions), static magnification about 1800, damping ratio for half period(overshoot ratio) 4:1, resonant gain 2400. This instrument is still recording. It wasnot until April 1962, when the World-Wide Standardized Seismograph Network(WWSSN) crystal clock timing system replaced the mechanical Riefler clock. Thisinvar-steel pendulum clock was held within the accuracy of f 2.5 sec per day. Theproblem is, before 1955, no time marks like minute or hour marks were connected toHistorical Seismogramsand Earthquakes of the World462Copyright 0 I9XX by Academic Press, Inc.All rights of reproduction in any form reserved.