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

212 D. Muiioz and A. Udias3 wFigure 2. Area of greater damage (VIII-IX).36'25 N#5 wcoordinates to historical earthquakes. These are based on the macroseismic dataand are located at the center of the maximum intensity area. In the case of theMBlaga earthquake, the isoseismal lines are not closed and it is possible that theepicenter was situated offshore. However, besides the absence of direct evidence oftsunami effects, there are reports in M6laga that the shock came from the west,and the area of maximum damage is located at the Guadalhorce basin west ofMBlaga where there exist deep geological faults oriented approximately N 110' E.The orientation of these faults agrees with the elongation of the maximum intensityarea. These arguments do not completely exclude the location of the epicenter atsea, but better support a location inland to the west of Mblaga. From the mapshown in Figure 2, the estimated coordinates are 36.7" N, 4.7" W.Origin time cannot be given with better precision than the hour. Accounts give7 a.m. and 7:15 a.m. local time and since there is no knowledge of the state ofclocks, this may be taken as the best approximation.From the intensity map the attenuation coefficient and depth has been calculatedfrom the relationI, - I = a log (&GTG/h)Two directions of maximum and minimum attenuation, corresponding to west andnorth respectively have been taken. F'rom them, an attenuation coefficient of a = 6