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

Philippine Historical Earthquakes 293Table 2. Comparison of Intensity ScalesMas0 R-F (original) R-F (adapted) MMI Perceptible 2-3 1 2I1 Light 4 2 3I11 Normal 5 3 4IV Strong 6-7 4 5V Violent 8 5 6VI Destructive 9-10 6 7-8 (lower)7 8 (upper) -9 (lower)8 9 (upper) -109 11-125. Intensity Attenuation DataIsoseismal maps for 35 earthquakes were used for calculating attenuation functions.An equation derived by Su (1980) for the Philippine region in the form:I(R) - I,, = u - bR - clog R,where I(R) = intensity as a function of hypocentral distance, I,, = intensity at theepicenter, and R = hypocentral distance in kilometers, was used in obtaining attenuationcurves. In this equation, the coefficient b is related to the absorption factorwhile the coefficient c is associated with the geometric spreading factor (Howell andSchultz, 1975).Inasmuch as earthquakes in the Philippines are either inland or offshore earthquakes,two methods of determining I,, were employed. For the first category, 1,’swere taken to be the maximum observed intensity while for the second category,Io’s were extrapolated values from the individual intensity decay curves.A regression analysis was performed on the data from the 35 earthquakes. Table 3gives the list of events and their respective a, b, and c values. Results were then correlatedwith the seismic source zones delineated by Su (1985) as shown in Figure 2.Events were grouped according to zones as follows:ZoneNo. of eventsper zone1 8.2 83 14 125 06 07 6The values of a, b, and c for all zones, except zones 5 and 6 where there are nosample events, are listed in Table 4.In this study, three events were screened out due to either very large or verysmall attenuation rates deviating greatly from the average values for their respectivegroups. Further analysis of these earthquake events needs to be done. Figure 3shows the attenuation curves for the different zones.