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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.

Sector collapse during the 1741 eruption of Oshima‐Oshima volcano (southwestern Hokkaido, Japan) generated a large tsunami in the Japan Sea. The tsunami caused severe damage along the Oshima (Hokkaido) and Tsugaru (Honshu) peninsulas. Tsunami deposits due to the 1741 event were identified along the Okushiri and Hiyama coast in Hokkaido. In this study, we numerically simulated the landslide and tsunami generated by the 1741 Oshima‐Oshima eruption using an improved two‐layer model to explain the depositional area of the landslide, the tsunami heights written in historical records, and the distributions of tsunami deposits. Areas of erosion and deposition by the 1741 landslide were estimated from the bathymetric data on the northern slope of Oshima‐Oshima volcano. In addition, previous topography before the sector collapse was restored. From the bathymetry difference before and after the landslide, the volume of collapsed material was estimated at 2.2 km3. Based on those data, the landslide and tsunami were numerically simulated by solving equations of an improved two‐layer model that incorporates Manning's formula in the bottom friction terms of the lower layer. An apparent friction angle of 2.5 and a Manning's roughness coefficient of 0.15 were selected to explain the area of deposition estimated from the bathymetry analysis and distributions of tsunami deposits. The thickness distribution of the computed landslide mass fits relatively well with the depositional area. Computed tsunami heights match those from historical records along the Hiyama coast. Computed tsunami inundation areas cover most of the distributions of tsunami deposits identified along the coasts.

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 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 9 10 21 Nm (M w 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.

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 × 10 20 Nm (M w 7.9) assuming a rigidity of 3.43 × 10 10 N/m 2. The estimated fault model is located between the focal regions of the 1993 Hokkaido Nansei-oki earthquake and the 1983 Japan Sea earthquake.

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