2013 年 65 巻 3 号 p. 231-242
The hypocenter of the 1911 great earthquake occurred around Kikai-jima, Japan is thought to be 28.0°N, 130.0°E (about 30km southward from Kikai-jima) and 100km depth in general. This implies that the 1911 event occurred within the subducting slab at the Ryukyu trench. However, it is not clearly known what kind of seismic data were used to determine the epicenter and how the depth of hypocenter was estimated quantitatively. In the present study, we determine hypocenter of this event by using selected S-P time data not only reported ones but also re-measured ones from smoked-paper seismograms. The hypocenter is revealed to be 28.7°N, 130.6°E and 60km in depth, about 60km northeastward from Kikai-jima. The focal depth is, however, considered to be about 10km instead of 60km by referring to the present hypocenter distribution because of the low reliability of focal depth in the relocation. Recent epicenter distribution shows the low seismic activity area on the plate boundary northeast off Kikai-jima which extends about 70km length parallel to the Ryukyu trench axis. The 1911 great earthquake seems to be occurred at the shallower rim of this low seismic activity area. If the low seismic activity is the result of strong coupling on plate boundary, this area might be the asperity of the 1911 great earthquake. Tsunami heights accompanied with this event are reported recently to be more than 5m at Kikai-jima and Amami-oshima, which supports the present study that the 1911 great earthquake is the interplate event rather than the intraslab one.