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

302 R. Muir Wood, G. Woo, and H. BungumFigure 2. Macroseismic map for the 1895/2/5 earthquake (right) and the 1958/1/23earthquake (left). The epicenter to the left is instrumentally determined, and the figureillustrates the use of model (or template) events for locating historical earthquakes.This correlation does not include an explicit depth dependence. The focal depths ofNorth Sea events of engineering significance (> 4.0 Ms) are probably fairly homogeneous,and there is thus little need to introduce a depth dependence. Hypocentraldepths in this region are moreover known only with poor precision, and any attemptto include a depth factor would therefore have to rely on a choice of average depthswhich would lead to very similar results.The magnitude-felt area correlation given above has several import ant features:(a) magnitudes can be assigned when either A111 or A I is ~ known, and when bothare known a consistency check is possible; (b) the logarithmic dependence on feltarea ensures the stability of the correlation against macroseismic error.The first feature enables magnitudes to be assigned for most documented feltearthquakes, even though during the past centuries the observation threshold hasvaried substantially in the region. The second feature allows Ms values to beassigned to offshore events for which an estimate of felt area has to be based onan incomplete arc of data. Because of the logarithmic dependence of Ms on feltarea, and because of the low regional attenuation which results in comparativelysmall coefficients for this logarithmic dependence, the error in Ms due to incompletedata is only about fO.l or f0.2. In the catalogue shown in Figure 4 and Table 1,the Ms values for felt events are based on the above correlation, except where aninstrumental value has been measured.Because of the varying observational geographical and temporal detection thresholds,the map of felt events (Figure 4) is not a complete picture of the historicalseismicity of the region. The same can be said of the ISC instrumental catalogue