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The Hyuga-nada region is located in the southwestern part of the Nankai Trough in the Pacific Ocean, where M7 class interplate earthquakes have been repeatedly occurring because of the subduction of the Philippine Sea Plate. The largest earthquake recorded in history for the Hyuga-nada region was the Hyuga-nada earthquake of 1662, which occurred off Miyazaki Prefecture in the southeastern area of Kyushu region, Japan, generating a tsunami. The region is also an area where slow earthquakes are active at the shallow part of the plate boundary. It is confirmed by the 2011 Tohoku earthquake that the active area of shallow slow earthquakes also became a tsunami source area. We hypothesize that the unusually large tsunami of 1662 was caused by the coseismic slipping of the active source area of shallow slow earthquakes. We constructed the fault model of the 1662 Hyuga-nada earthquake based on recent geophysical observations. A numerical simulation of the tsunami was carried out using the fault model constructed based on hypotheses. The total seismic moment was 9.8 × 10²⁰ Nm (Mw 7.9) estimated from the fault model. The estimated fault model can explain the distribution of the likely tsunami event deposits which were recognized in this study and also have been reported by other studies and tsunami heights in historical records. Hypotheses based on geophysical findings have been found to be correct., 1662年日向灘地震の新たな断層モデルを構築 --地球物理学と地質学の検証に基づく初の成果--. 京都大学プレスリリース. 2023-01-24.

The 1662 Hyuga-nada Earthquake is one of the largest earthquakes to have occurred in the Hyuga-nada region, Southwest Japan, rupturing the western part of the Nankai Trough subduction zone. Strong ground motion and a large tsunami with an estimated height of at least 4–5 m were reported along the coast of Miyazaki Prefecture, Kyushu Island, with extensive damage reported across this region. Therefore, developing a more complete picture of the 1662 Hyuga-nada Earthquake will improve our understanding of tsunami risk along the Pacific coast of Southwest Japan. Here we use the most recent geophysical data from the region to propose a novel source model for the 1662 Hyuga-nada Earthquake that incorporates our current understanding of the interactions between slow earthquakes and great earthquakes. The source area in our proposed model extends from the focal region of recurrent M7-class interplate earthquakes to the region of slow earthquakes that occur at relatively shallow depths along the plate boundary. Tsunami simulations indicate that the predicted coastal tsunami heights are broadly compatible with previously reported tsunami heights. This new model suggests that the 1662 Hyuga-nada Earthquake was probably a M8-class earthquake, and that previous magnitude determinations may have underestimated the strength of this historic earthquake. However, the model uncertainty remains large, and the calculated tsunami heights for the southern coast of Miyazaki Prefecture are very high. We suggest that future verification, including an analysis of tsunami-related deposits along the coast, will be important to more accurately determine the source area of the 1662 Hyuga-nada Earthquake.

Hyuga-nada region is located at the south-western part of Nankai Trough, in the Pacific Ocean. M7-class interplate earthquakes are repeatedly occurred by the subducting Philippine Sea plate beneath the Eurasian plate. The largest earthquake in this area was the 1662 Hyuga-nada earthquake (M=7.6) which occurred off Miyazaki Prefecture, south-eastern area of Kyushu region, Japan, and generated tsunami (after called the 1662 tsunami). Strong ground motion hit and many structures were broken near the coast of Miyazaki Prefecture. The tsunami heights were estimated at least 4-5 m along the coast of Miyazaki city, and more than 200 people died by the earthquake and tsunami by historical records. This region is also active area of the shallow slow earthquakes. The 1662 tsunami was much larger than tsunamis generated by usual M7-class interplate earthquakes. It is known by the 2011 Tohoku earthquake that focal area of shallow slow earthquakes also become a tsunami source area. So, we hypothesized that the 1662 unusual large tsunami was caused by the coseismically slipping of focal area of shallow slow earthquakes. We firstly constructed the fault model of the 1662 earthquake based on the recent result of geophysical observation. To examine the tsunami source of the 1662 earthquake, we surveyed the 1662 tsunami deposits in the lowland along the coast of south-eastern Kyushu region. As a result, sandy event deposits interbedded with clay (organic clay) were recognized at several surveyed points. Based on facies features, these event deposits were possibly formed by the 1662 tsunami. Numerical simulation of the tsunami was carried out using the constructed fault model. Calculated tsunami inundation area can explain distribution of the likely tsunami event deposits at Komei, Miyazaki Prefecture. Furthermore, this study compares calculated tsunami inundation areas, distribution of other surveyed tsunami deposits and tsunami heights of historical records. Tsunami source of the 1662 earthquake proposed by our study could better explain geophysical, geological and historical records.

Tsunami deposits were collected along the coast of southwestern Hokkaido and Okushiri Island, northern Japan. The distribution of these deposits suggested that large earthquakes and tsunamis have repeatedly occurred off southwestern Hokkaido. Along the southern coast of Okushiri Island, five tsunami sand/gravel layers have been deposited during the last 3000 years. The latest was deposited by the 1741 Oshima–Oshima landslide tsunami and the second by the 12th century tsunami. The later tsunami was probably generated by a large earthquake because submarine seismo-turbidites with similar age exist in the region and a large inland landslide had occurred in Okushiri Island in approximately the 12th century. The ages of paleo-tsunami events prior to the 12th century are 1.5–1.6, 2.4–2.6, 2.8–3.1 ka. In this study, a fault model of the 12th century earthquake was estimated by comparing tsunami deposit distributions and calculated tsunami inundation areas at five sites in Okushiri Island and Hiyama region. Fault model F17, a submarine active fault in the Japan Sea near Oshima–Oshima, is a probable source for this tsunami. Numerical simulation of the tsunami was performed based on fault model F17; we modified the fault parameters (length and slip amount) from the original model to explain tsunami deposit distributions. A shorter length of 104 km and a larger slip amount of 18 m were appropriate for the fault model on the basis of parametric studies. The seismic moment of the earthquake was calculated to be 9.95 × 1020 Nm (Mw 7.9) assuming a rigidity of 3.43 × 1010 N/m2. The estimated fault model is located between the focal regions of the 1993 Hokkaido Nansei-oki earthquake and the 1983 Japan Sea earthquake.

The 2011 Tohoku-oki earthquake generated a large tsunami that caused catastrophic damage along the Pacific coast of Japan. The major portion of the damage along the Pacific coast of Tohoku in Japan was mainly caused by the first few cycles of tsunami waves. However, the largest phase of the tsunami arriving surprisingly late in Hakodate in Hokkaido, Japan; that is, approximately 9 h after the origin time of the earthquake. It is important to understand the generation mechanism of this large later phase. The tsunami was numerically computed by solving both linear shallow water equations and non-linear shallow water equations with moving boundary conditions throughout the computational area. The later tsunami phases observed on southern Hokkaido can be much better explained by tsunami waveforms computed by solving the non-linear equations than by those computed by solving the linear equations. This suggests that the later tsunami waves arrived at the Hokkaido coast after propagating along the Pacific coast of the Tohoku region with repeated inundations far inland or reflecting from the coast of Tohoku after the inundation. The spectral analysis of the observed waveform at Hakodate tide gauge shows that the later tsunami that arrived between 7.5 and 9.5 h after the earthquake mainly contains a period of 45–50 min. The normal modes of Hakodate Bay were also computed to obtain the eigen periods, eigenfunctions, and spatial distribution of water heights. The computed tsunami height distributions near Hakodate and the fundamental mode of Hakodate Bay indicate that the large later phases are mainly caused by the resonance of the bay, which has a period of approximately 50 min. The results also indicate that the tsunami wave heights near the Hakodate port area, the most populated area in Hakodate, are the largest in the bay because of the resonance of the fundamental mode of the bay. The results of this study suggest that large future tsunamis might excite the fundamental mode of Hakodate Bay and cause large later phases near the Hakodate port.

The 1969 and 1975 great Kurile earthquakes occurred along the Kurile trench. Tsunamis generated by these earthquakes were observed at tide gauge stations around the coasts of the Okhotsk Sea and Pacific Ocean. To understand rupture process of the 1969 and 1975 earthquakes, slip distributions of the 1969 and 1975 events were estimated using tsunami waveform inversion technique. Seismic moments estimated from slip distributions of the 1969 and 1975 earthquakes were 1.1 × 1021 Nm (Mw 8.0) and 0.6 × 1021 Nm (Mw 7.8), respectively. The 1973 Nemuro-Oki earthquake occurred at the plate interface adjacent to that ruptured by the 1969 Kurile earthquake. The 1975 Shikotan earthquake occurred in a shallow region of the plate interface where was not ruptured by the 1969 Kurile earthquake. Further, like a sequence of the 1969 and 1975 earthquakes, it is possible that a great earthquake may occur in a shallow part of the plate interface a few years after a great earthquake that occurs in a deeper part of the same region along the trench.

Paleotsunami researches revealed that a great earthquake occurred off eastern Hokkaido, Japan and generated a large tsunami in the 17th century. Tsunami deposits from this event have been found at far inland from the Pacific coast in eastern Hokkaido. Previous study estimated the fault model of the 17th century great earthquake by comparing locations of lowland tsunami deposits and computed tsunami inundation areas. Tsunami deposits were also traced at high cliff near the coast as high as 18 m above the sea level. Recent paleotsunami study also traced tsunami deposits at other high cliffs along the Pacific coast. The fault model estimated from previous study cannot explain the tsunami deposit data at high cliffs near the coast. In this study, we estimated the fault model of the 17th century great earthquake to explain both lowland widespread tsunami deposit areas and tsunami deposit data at high cliffs near the coast. We found that distributions of lowland tsunami deposits were mainly explained by wide rupture area at the plate interface in Tokachi-Oki segment and Nemuro-Oki segment. Tsunami deposits at high cliff near the coast were mainly explained by very large slip of 25 m at the shallow part of the plate interface near the trench in those segments. The total seismic moment of the 17th century great earthquake was calculated to be 1.7 × 10 22 N m ( M w 8.8 ). The 2011 great Tohoku earthquake ruptured large area off Tohoku and very large slip amount was found at the shallow part of the plate interface near the trench. The 17th century great earthquake had the same characteristics as the 2011 great Tohoku earthquake.

Paleotsunami studies have shown that several large tsunamis hit the Pacific coast. Many tsunami deposit data were available for the 17thcentury tsunami. The most recent tsunami deposit study in 2013 indicated that the large slip of about 25 m along the plate interface near the Kurile trench would be necessary and the seismic moment of this 17thcentury earthquake was 1.7 × 1022Nm. If a great earthquake like the 17thcentury earthquake occurs off the Pacific coast of Hokkaido, the devastating disaster along the coast is expected. To minimize the tsunami disaster, a development of the real-time forecast of a tsunami inundation area is necessary. Estimating a tsunami inundation area requires tsunami numerical simulation with a very fine grid system of less than 1 arcsecond. There is not enough time to compute the tsunami inundation area after a large earthquake occurs. In this study, we develop a real-time tsunami inundation forecast method using a database including many tsunami inundation areas previously computed using various fault models. After great earthquakes, tsunamis are computed using linear long-wave equations for fault models estimated in real time. Simulating such tsunamis takes only 1-3 minutes on a typical PC, so it is potentially useful for forecasting tsunamis. Tsunami inundation areas computed numerically using various fault models and tsunami waveforms at several locations near the inundation area are stored in a database. Those computed tsunami waveforms are used to choose the most appropriate tsunami inundation area by comparing them to the tsunami waveforms computed in real time. This method is tested at Kushiro, a city in Hokkaido. We found that the method worked well enough to forecast the Kushiro’s tsunami inundation area.

The 1963 great Kurile earthquake was an underthrust earthquake occurred in the Kurile–Kamchatka subduction zone. The slip distribution of the 1963 earthquake was estimated using 21 tsunami waveforms recorded at tide gauges along the Pacific and Okhotsk Sea coasts. The extended rupture area was divided into 24 subfaults, and the slip on each subfault was determined by the tsunami waveform inversion. The result shows that the largest slip amount of 2.8 m was found at the shallow part and intermediate depth of the rupture area. Large slip amounts were found at the shallow part of the rupture area. The total seismic moment was estimated to be 3.9 × 1021 Nm (Mw 8.3). The 2006 Kurile earthquake occurred right next to the location of the 1963 earthquake, and no seismic gap exists between the source areas of the 1963 and 2006 earthquakes.