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

0.3244 Pao Hua Lee(3) Ryukyu-Taiwan Seismic Zone: This zone extends from Ryukyu Island southwestwardto Hwalien and Nand. Focal depths in this zone vary from shallow up toas much as 300 km.Further, we have studied the distribution and energy release [calculated by usingthe Gutenberg-Richter formula: log E = 11.8 + 1.5M; E = Energy release (erg);M = Earthquake magnitude] of major earthquakes, from 1935 to 1984 having magnitudesM > 5.0. To accomplish this, we have divided the area into 72 equal grids,extending from 21" N to 25.5" N, and 119" E to 123" E (see Figure 3). According toFigure 3, earthquake occurrence west of Taiwan is 106; the percentage in most gridsis less than 1.0%, except in the region between 23.0" N to 24.5' N and 120.5" E to121.0" E, where the percentage ranges between 1.0% and 2.0%.126> 1:--T---3!6II252z2:I3.9 I 7.9(1.5) I (1.8)I_-----I-----=Ic8.9 3.9(10.0) i (7.1)II24I8.2 4.4(4.6) / (1.1)--___-IT------I0.6 1 0.7(5.34 (0.2)I0 . 7 : 0.8(7.3) 1 (0.4)I------+------\0.4 1 0.2252320.1 0.1(0.1) j (X)0.7 ~(0.9) 1 (0.1)22I" 3LI119 120 121 122 1213Figure 3. Number and energy release (in percent) of major earthquakes (A4 2 5.0) inthe Taiwan area, from 1935 to 1984. In each grid the upper numerals represent percentof earthquake occurrence and the lower numerals in parentheses represent the percent ofenergy release of the earthquakes. X represents energy release of less than 0.1%.