Frequency-magnitude distributions are generally expressed by the Gutenberg-Richter (G-R) law. However, frequency-magnitude distributions are sometimes a convex-upward curve rather than a straight line, departing from the G-R law. An η value originally introduced by Utsu (1978) is an indicator that represents the degree of deviation from the G-R law. We investigate η values before and after six M7-9 class mainshocks off the Pacific coast of eastern Japan. The η values tend to become small (i.e., the distribution deviates from the G-R law) before the mainshocks, and then increase (i.e., recovering to the G-R law). Taking this characteristic into account, we suggest a simple and challenging earthquake forecast model based on η values. Probability gain of the optimized forecast model by a retrospective test becomes 2.24-3.03, and the alarm rate and the truth rate become 100% and 0.14-0.47%, respectively. According to the result of the forecast model applied to the latest seismicity, we should pay attention to seismicity off the coast southeast of Kanto district.
A large earthquake (M7.0~7.5) in Hyuga-nada Sea on the east side of Kyushu along Nankai trough in Mi-no-koku (approximately 9-11 a.m.) on June 30th, 1498 on Julian calendar, which was based on the descriptions about serious earthquake damage in Kyushu in the war chronicle Kyushu-gunki, has been accepted by many seismologists as the 1498 Meio Hyuga-nada earthquake. Kyushu-gunki is one of the popular novels which was written more than 100 years after the 1498 event. The damage descriptions have been used to estimate the location of this earthquake without evaluating the reliability. In this study, in order to assess credibility of the description, we carefully examined writing process of Kyushu-gunki and damage descriptions in this war chronicle. Our findings are summarized as follows: (1) Kyushu-gunki was completed in the current Saga prefecture (northwestern part of Kyushu) in 1607, hence the writers’ experience of the contemporary 1596 destructive earthquake in northeastern Kyushu might have influenced the description of the damage of 1498 earthquake; (2) Damage descriptions of Kyushu-gunki were generic without location information and most of them were cited from descriptions of damage in Kyoto due to the large earthquake on Aug. 6th, 1185 in the war chronicle Gempei-josui-ki; (3) Origin time (Mi-no-koku) of this event is described only in Kyushu-gunki and it is close to that of the great Tokai earthquake on Sep. 11th, 1498 (Tatsu-no-koku: approximately 7-9 a.m.); (4) The chapter of the earthquake damage also includes serious famine in Kyushu in 1503 and pains of people by many disasters, making this chapter a stage setting for later stories. To make a story of the war chronicle more interesting, writers seem to have created fictitious story of serious damage due to a large earthquake in Kyushu based on old war chronicles and some records of the great Tokai earthquake. Therefore, we concluded that the 1498 Meio Hyuga-naga earthquake is a fake earthquake and it should be deleted from Japanese historical earthquake catalog. Other documents in Kyoto and Nara recorded an earthquake in Saru-no-koku (approximately 3-5 p.m.) on June 30th, 1498, but the location and magnitude are not clear.
The southern part of the Sendai Plain is located between the Futaba active fault zone and the Nagamachi-Rifu active fault zone. In the existing research, only 6.5km long of the active fault trace was estimated along the eastern foot of Medeshima Hills because the geomorphic surface of the Medeshima Hills is warping toward east. Besides the above, no other active fault trace had been clearly identified in this area. To reveal the subsurface structure, we conducted 5.3-km-long of seismic reflection survey across the southern part of the Sendai Plain, near the mouth of the Abukuma river, which is the northern extension of Futaba active fault zone. The result of seismic profile analyzed with a standard CMP method shows the existence of concealed active fault beneath the Sendai Plain and 30-40m of dislocations in seismic reflectors from Miocene to Quaternary layer. The activity of the concealed fault is estimated as a C-class (Long-term rate of faulting, 0.01-0.1mm/yr). The survey of relative gravity measurement at intervals of 200 m was also conducted, including the seismic reflection survey line. Steep gradient of the Bouguer gravity anomaly were observed in the vicinity of the concealed active fault that is revealed from the seismic reflection survey. Analysis with the 2-D density structure analysis demonstrates that steep gradient of Bouguer gravity anomaly was responsible for large deformation of pre-Miocene basement rocks by repeated faulting of the concealed active fault. In addition, to discuss a tectonic evolution of the Sendai Plain, we made topographic profiles, using 1m-DEM and 2m-DEM data across the plain. Topographies of these profiles show that discontinuity of 1.5-2.2m on both sides of the westernmost beach ridge in the southern part of the Sendai Plain. Active faulting of the concealed fault has possibly contributed to these topographic discontinuities.
The Dogo hot spring, situated in Matsuyama City, Ehime Prefecture, Japan, is one of the oldest and most famous hot springs in Japan. The well water level or discharge at the spring often decreased coseismically and increased postseismically related to the past Nankai earthquakes. We analyzed well water level data recorded at the spring immediately after the 1946 Nankai earthquake and over the period from 1985 to 2015. From this analysis, we have got five postseismic well water level increases related to the earthquakes whose seismic intensities were four or greater at Matsuyama city in JMA scale. The pattern of the five postseismic increases is very similar and shows a tendency of exponential convergence. We found that these postseismic increases can be explained by a basic equation of groundwater motion, which is a kind of diffusion equation. We also tried to detect the change in the diffusion coefficient or hydraulic diffusivity. However we did not detect it.