20 * Re-Evaluation of Isoseismal Maps and Magnitudes from Two Big Earthquakes along the Subduction Zone of Kyushu and Ryukyu Islands Early in the 20th Century Masayuki TAKEMURA, Katsuhisa KANDA Kobori Research Complex, Tokyo, 107-8502 Japan Tetsuo ABIRU, Hiroaki HARA Chugoku Electric Power Company, Hiroshima, 730-8701 Japan Two big earthquakes occurred along the subduction zone of Kyushu and Ryukyu Islands early in the 20th century. One is the event under West Miyazaki Prefecture of Kyushu in 1909 and the other is the event near the Kikai-jima Island of Ryukyu in 1911. Damage data and seismic intensity data were re-examined to evaluate isoseismal maps for the earthquakes. They show strong anisotropy along the island arcs. The anisotropy of the isoseismal map of the 1909 event is similar to that of the 2006 event in West Oita Prefecture of M=6.2 with focal depth of 145km, while the absolute values of seismic intensity from the 1909 event are one rank higher than those from the 2006 event. Optimal magnitudes of the 1909 and the 1911 events were obtained to be 7.3 and 7.9, respectively, by the method of an attenuation curve fitting for the seismic intensity data. Keywords: Seismic intensity, Intermediate-depth earthquake, JMA magnitude, Kyushu-Ryukyu Islands * 107-8502 6-5-30 730-8701 4-33 : takemurm kajima.com - 7 -
1 1909 Table 1 Summary of magnitude and location of hypocenter for the 1909 event 2 1911 Table 2 Summary of magnitude and location of hypocenter for the 1911 event - 8 -
3 Table 3 Historical change of the definition of seismic intensity in the Japan Meteorological Agency (JMA) scale - 9 -
4 Table 4 List of the news papers as sources of damage data - 10 -
5 Table 5 Correlation between damage and seismic intensity assignment - 11 -
1 1909 Fig.1 Seismic intensity data revised mainly through the damage-base estimation for the 1909 event. - 12 -
2 1909 3 Fig.2 Seismic intensity data at the stations of network of the Central Meteorological Observatory(CMO) for the 1909 event 3 1909 (1985) 3 Fig.3 Seismic intensity data summarized by Hamamatsu and Usami (1985) for the 1909 event 4 2006 6 12 (M6.2 145km) Fig.4 Seismic intensity data for the 2006 event (M6.2) in West Oita Prefecture reported by JMA - 13 -
5 1911 Fig.5 Seismic intensity data revised mainly through the damage-base estimation for the 1911 event. - 14 -
6 1911 3 Fig.6 Seismic intensity data at the stations of network of the Central Meteorological Observatory (CMO) for the 1911 event 7 2000 6 6 (M6.2 28km) Fig.7 Seismic intensity data for the 2000 event (M=6.2) near Amami-oshima Island reported by JMA 8 2005 11 22 (M6.0 146km) Fig.8 Seismic intensity data for the 2005 event (M=6.0) near Tanegashima Island reported by JMA - 15 -
6 Fig.6 List of the recent three earthquakes used for the determination of magnitudes of the 1909 and 1911 events. (a) 2006 6 (b) 2000 6 (c) 2005 11 (1) Fig.9 Attenuation formula of eq.(1) and measured seismic intensity data of the recent three earthquakes. (a) 2006 6 (b) 2000 6 (c) 2005 11 10 Fig.10 Relative intensity distribution for site correction from the data of the recent three earthquakes. - 16 -
11 1909 M=7.28 Fig.11 Attenuation formula of eq.(1) for the optimized M(=7.28) and measured seismic intensity data of the 1909 event after the site correction by the relative intensity in Fig.10(a) 12 1911 (M=7.85) Fig.12 Attenuation formula of eq.(1) for the optimized M(=7.85) and measured seismic intensity data of the 1911 event after the site correction by the relative intensity in Fig.10(b) 13 1911 (M=7.94) Fig.13 Attenuation formula of eq.(1) for the optimized M(=7.94) and measured seismic intensity data of the 1911 event after the site correction by the relative intensity in Fig.10(c) - 17 -
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1 1909 1/6-20 -
1 1909 2/6-21 -
- 22-1 1909 3/6
1 1909 4/6-23 -
1 1909 5/6-24 -
1 1909 6/6-25 -
- 26-2 1911 (1/6)
2 1911 (2/6) - 27 -
- 28-2 1911 (3/6)
- 29-2 1911 (4/6)
2 1911 (5/6) - 30 -
2 1911 (6/6) - 31 -