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

Historical Seismograms in Sweden 6520c1ocZ-a(35c2cI I I2 5 10 20PERIOD (SECIFigure 1. Amplitude response curves for the Uppsala Wiechert horizontal instrument.ITable 1. Historical Seismograph Stations in SwedenStation Lat. Long. Seismometer Recording Time ofON OE ODerat ionUp ps a1 a 59.86 17.63 Wiechert E,N smoked paper 1904-Vassijaure 68.42 18.18 Wiechert Z smoked paper 1906-1915Abisko 68.34 18.82 Wiechert Z smoked paper 1915-1951Galitzin E,N,Z photogr. paper 1919-1943Lund 55.70 13.19 Wiechert E,N smoked paper 1917-1953operating without interruptions. The available historical, as well as more recentrecords, are invaluable in revising the existing and in compiling new earthquakecatalogues, in making re-evaluations and/or determinations of magnitudes and seismicmoments and in illuminating source processes, for example. When determiningmagnitudes for larger data sets (in time and space) essentially two requirements arecrucial. First, the same magnitude has to be applied throughout the event series.Second, the way in which magnitudes were determined should be explicitly specifiedto make calculations of other source parameters possible. For example, when compilingthe earthquake catalogue for Turkey for the period 1913-1970 (Alsan et al.,1975), surface-wave magnitudes, Ms , deduced from Uppsala Wiechert records wereintroduced. There were only a few Turkish earthquakes with clearly recorded bodyphases in Uppsala which a priori disqualifies the use of body-wave magnitudes, mb.Besides, Ms-values often are more stable when compared with corresponding mb estimates,Many earthquakes in Turkey occur at shallow depths, and so focal-depthinfluence does not create any additional problem. Uppsala Wiechert records areavailable for the entire time interval considered, which in turn, guarantees magnitudesof high homogeneity, an essential requirement for any statistical analysis.