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

The Mdlaga Earthquake of 9 October 168021s0-2-4-Ic- 6ra-a=6h= 50 km10 --I I 1 1 1 1 1 1 I 1 1 I I I I I I I I 1 I I l lFigure S. Plot of I, - I versus A (km). Observational values are plotted as dots, andthe curve is based on Equation (1).and an average depth of 50 km are obtained (Figure 3). It must be noticed that thismethod gives pairs of values for attenuation and depth, that are not independent.The relatively high attenuation value agrees with values obtained for other earthquakesin this zone (Muiioz et al., 1974). The average value for the depth, locatesthe hypocenter under the crust which in this region has a value of approximately25 km (Ansorge and Banda, 1980). This depth of the focus is not unusual in thisregion since in the seismic catalogue (Mezcua and Martinez Solares, 1983) there areseveral earthquakes with depths between 30 km and 100 km. The values obtainedfor attenuation coefficient and depth are not very precise because the intensity zonesare not well defined, due to the low density of points, except for the highest grades.From the maximum intensity and depth, a value of magnitude may be estimatedusing the formula of Sponheuer (1960)M = 0.6611, + 1.7 log h - 1.4. (2)The value obtained is between 6.8 and 7.4 using h = 50 km and taking h = 30 km,6.4 and 7.1 (lower and upper limits correspond to I, = VIII or IX). Another expressionthat relates magnitude and intensity is2M = 1 + -Io.3(3)This relation gives a magnitude between 6.3 and 7.Seismic moment can be estimated from this value of magnitude according toBrune (1968) giving a value between 10l8 Nm and lo1’ Nm. This parameter canalso be estimated directly from the area of intensity VI, according to the results