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

300 R. Muir Wood, G. Woo, and H. Bungumand historically 'opaquen listing of locations and intensities. From 1938 even theindividual intensity assignments were dropped as the events were indicated solelywith a map marked with a felt area boundary or isoseismals. The number of smallevents reported in this period also showed a marked decline. However, the datagathering, the posting of directed questionnaires, continued, and when in 1983Sellevoll, Almaas and Kijko finally caught up with the backlog of data from 1953-1975 they provided maps with individual intensity points noted, also marked withthe instrumentally located epicenters.3. Analysis of Macroseismic DataIn order to derive important seismological parameters from the record of historicalearthquakes, it is important that the data have been processed and mappedaccording to a uniform methodology. Intensity is a compound measure of the amplitude,spectral content and period of earthquake waves & a surface location.Intensity at a given site varies according to the properties of the geology, groundand resonant frequency of the building in which an observer is located. Thus assessmentsof intensity for different observers in the same village or town may varyby up to one intensity grade. The intensity scale utilized here is the standard EuropeanMSK scale, with some specific adaptations made for the circumstances ofNorwegian wooden houses.Intensity assessment cannot be divorced from the means by which the informationwas gathered. For example, solicited questionnaires placed in newspapers will tendto gain extreme value, rather than average observations, since correspondants willtend to be those with the most interesting effects to report. The subsequent isoseismalswill therefore also be of extreme value, including all the outermost locationswhere a given intensity grade was perceived. In contrast, the intensities within thispresent study have been plotted as modal community intensities using grid averages,taking into consideration the range of perceptions, including also informationas to where the earthquake was not felt. The individual intensity points have thenbeen contoured, and wherever possible the individual intensity assignments havebeen made from the primary data. However, where only intensity maps have beenpublished without individual intensity assignments (as between 1937 and 1953),these have been recontoured according to a comparison of both intensity scales andthe original method of contouring. An example of an important relocalization isgiven in Figure 1.The collection of isoseismal maps for felt earthquakes in the North Sea regionforms the basis for a quantitative analysis of macroseismic data. From historicalobservations of earthquakes, the essential seismological parameters of event locationand magnitude can be estimated even though no instrumental records may exist.The fact that this is possible is due mainly to the availability of 20th centurymacroseismic and instrumental data which allow earlier events to be quanti6ed viaa correlation procedure.The need for a correlation procedure to determine the epicenters of felt earthquakesis a consequence of the number of important offshore events in the region.For such events, the land intensity distribution is often inadequate to locate theepicenter geometrically with enough precision. As Figure 2 illustrates, there isa possible solution to this problem if another event can be found with a similarintensity distribution, and which has been instrumentally located. This event, if it