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.

This study develops a three-dimensional viscoelastic model using the Finite Element Method to understand the postseismic deformation that followed the 2004 off the Kii peninsula earthquake. The questions how long the viscoelastic relaxation continues to the surface deformation and how much the viscoelastic relaxation affects the surface deformation are of particular importance. We first detected the long lived postseismic deformation up to end of 2016. This long lived postseismic deformation is explained by the viscoelastic relaxation caused by the main rupture. Our viscoelastic model consists of three viscoelastic media—the mantle wedge, oceanic asthenosphere and Lithosphere and Asthenosphere Boundary (LAB). The optimal viscosities for three viscoelastic media are the mantle wedge with 2×10¹⁸ Pa·s, the oceanic asthenosphere with 1×10¹⁹ Pa·s, and the LAB with 5×10¹⁸ Pa·s. The viscoelastic relaxation produces southward motion across the entire Chubu district, and continues for at least a few decades. The maximum horizontal displacement predicted by the viscoelastic relaxation reached 6.2 cm cumulative over 10 years at the tip of the Shima peninsula. The viscoelastic model explains most of the southward motion observed after the main rupture up to end of 2016. It is obvious that observed crustal deformation in the Chubu district contains the effect of viscoelastic relaxation caused by the 2004 off the Kii peninsula earthquake. Therefore we should consider these effects when we interpret the crustal deformation in this area. Otherwise we overestimate or underestimate the mechanism, such as the interplate coupling or the interplate fault slip.

An M_JMA 6.7 (Mw 6.2) earthquake occurred in Northern Nagano, Japan, on November 22, 2014. While the damage in the city center of Nagano was relatively minor, 65 stone lanterns, among 182, standing in the precinct of the Zenkoji Temple, approximately 25 km from the epicenter, were toppled by the ground motion of this earthquake. Damage of the surrounding residential area was minor. Directions of the collapse were dominantly in the north-south. Strong motion seismograms recorded at nearby JMA Nagano Local Meteorological Observatory were rich in high frequency, especially in the NS component, which explains collapse of stone objects whose natural periods are few tenths of a second. Similar damage was documented in a historic earthquake in 1714, and recurrence of such damage implies that high frequency ground motions from large earthquakes in this epicentral area have been repeated threats to the Zenkoji Temple and Nagano City.

A source process inversion for an Mj 6.6 earthquake which occurred on October 21, 2016 in central Tottori Prefecture, was performed using near-field strong motion waveforms. The results showed that a left-lateral strike-slip component was dominant on the fault plane, the strike of which was in a NNW-SSE direction and the dip almost vertical. The moment magnitude was estimated as 6.2. A large slip area (asperity) was determined around the hypocenter, including its northern shallow part, in which the maximum slip was nearly 1.2 m. Another small asperity was estimated also near the northern edge of the fault plane. These two asperities corresponded to the two distinct wave packets observed at the stations near the source region. Furthermore, the results suggested that a discrepancy in frequency characteristics between these asperities existed, by comparing them with waveforms obtained just above the source region. We also estimated the stress drop on the fault plane by the final slip distribution, and the maximum value was about 16 MPa. This result meant that the strong ground motion excitation of this earthquake was of an average level compared to past crustal earthquakes in Japan.

It has long been believed that a M6.1 earthquake occurred on November 14, 1831 in Saga, southwest Japan. The event relies on a single entry from note of “Tenpo Zakki,” which is a collection of miscellaneous notes between 1831 and 1844. Here we propose that the location for the earthquake is likely misinterpreted. Reexamination of the note shows that the earthquake occurred on November 13, 1831, as was recognized in 1919. The original location was thought to be in Saga because according to the note the earthquake was reported from “Hizen-no-kami” (lord of Saga). Analyses of the time it took for the news to reach Edo (Tokyo) show that the location of the earthquake is possibly not Saga, but Aizu, in northeast Japan. The note of “Tenpo Zakki” shows that the information of the event reached Edo in 5 days. However, it was impossible to deliver a letter from Saga to Edo in 5 days at that time. No description on the earthquake was found in diaries written around Saga. “Tenpo Zakki” might have mistaken “Higo-no-kami” (lord of Aizu) for “Hizen-no-kami.” This result contributes to improvement of the list of historical earthquakes for Japan.

Seismic activity is spatially concentrated, defining earthquake clusters, and varies temporally with the occurrence of large earthquakes and earthquake swarms. However, earthquake clusters that occur in isolation from surrounding earthquakes are rarely observed. To investigate the seismic activity in a given area, it is preferable that an earthquake cluster is objectively extracted. In this study, we extracted earthquake clusters in and around the Japanese islands based on hypocenter information listed in the unified catalog of the Japan Meteorological Agency. As a result, we identified that earthquake clusters were efficiently extracted in nearly all areas in which hypocenters are well determined. The seismic characteristics of the earthquake clusters clarify the spatial-temporal heterogeneity of earthquake occurrence and illustrate the temporal changes in seismic activity in each area. Many repeating earthquakes are located in the Pacific Plate subducting from the Kuril-Japan Trench and Philippine Sea plate subducting from the Ryukyu Trench. Earthquake clusters with a high incidence of repeating earthquakes are concentrated near the deepest limit of the inter-plate coupling areas at the Pacific plate. In contrast, most earthquake clusters in regions of subduction-related inter-plate earthquakes are characterized as sequences of mainshock - aftershock type, owing to the occurrence of aftershocks following large earthquakes. Using this method of focusing on earthquake cluster, additional earthquake data are easily analyzed. Moreover, this approach is useful in investigating temporal changes in seismic activity and detecting previously unknown earthquake clusters. The intensive monitoring of earthquake clusters will provide a better understanding of the spatial-temporal changes in seismic activity throughout the Japanese islands.

Only a few large or medium-sized repeating earthquakes are known in many sequences due to long recurrence intervals in comparison with observation period. Recurrence intervals are used to estimate long-term earthquake probability; however, the effect of the number of recurrence intervals on prediction performance is unclear. We studied the predictability dependence on the number of recurrences using small interplate repeating earthquakes along the Japan Trench. These earthquakes were extracted from Tohoku University’s catalog, and this data was used in the probability forecast experiments from 2006 to 2010. The number of forecasts is 524. Two to five events just prior to the forecasts are picked from each sequence to calculate the probabilities. We calculated the probabilities using the Bayesian statistics log-normal distribution model (LN-Bayes), the log-normal distribution model based on the small sample theory (LN-SST), and the exponential distribution model (Exp-pin). We then evaluated the forecast results using mean log-likelihood and Brier score. The performance of the LN-SST and LN-Bayes models was better than that of the Exp-pin model for almost all cases. In addition, we conducted some statistical tests to measure the consistency of forecast with the observed catalog and confirmed the tendency of underestimation of probabilities and the accuracy dependence of probabilities on the number of recurrences for all models. The LN-Bayes and LN-SST models were examined by random number experiments using a large number of simulated sequences of earthquakes. In statistical tests, we changed the number of repetitions and the elapsed time from the most recent earthquake. As a result, performance improvement, along with the increase of repetition number, was evident when the repetition number was small for both models. The LN-Bayes model is generally better than the LN-SST model if the number of repetitions is very small. However, one of the features of the LN-Bayes model is that probability is saturated when the number of repetitive events is small and longer time has passed since the last event.

On July 28, 1889 (Meiji 22), an earthquake (M＝6.3) occurred in the western part of Kumamoto city. The damage statistics for each municipality are reported in the Official Gazette. The data on ground fissures and the seismic damage to houses, bridges, and stonewalls was used to estimate the seismic intensity based on the relation between seismic intensity and seismic damage. We obtained the seismic intensity distribution of this event. The obtained seismic-intensity map was compared with the site amplification factor data provided by the National Research Institute for Earth Science and Disaster Resilience (NIED). It seems that the regions with large site amplification factors tend to experience high seismic intensities. The seismic intensity of some areas estimated in this study were 2-3 levels greater than that estimated from the data on collapsed houses. The houses of the Meiji period were generally located on relatively strong ground with a small amplification factor. In contrast, bridges often were built on weak ground. Thus, the seismic intensity determined only from the damage to houses may sometimes be smaller than that estimated from damage to other structures. Nowadays, residential areas exist not only on strong ground but also on relatively weak ground. In the event of a large earthquake, the seismic damage to houses may be more severe than that in the Meiji period.