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The distinctive bathymetric feature exists in the Suruga Bay, Japan. It has been called as Senoumi (Stone flower sea) from old times. Senoumi is a 30 km wide and 20 km long concave feature. Its origin has not been explained yet; however, the feature might be a combined consequence of intensive tectonic activity in the plate border, landslides, and a submarine flow coming from the Oi River. If the Senoumi was caused by a landslide, the latter would be larger than any on-land landslide in Japan. The downshelf “exit” from this feature is much narrower than its central part. This is not usual shape of landslides, but it is similar to the liquefied landslides such as those in quick clays which mobilize great strength reduction after failure. To study Senoumi as a landslide, the shear behaviors of the following three soil samples were investigated by the cyclic and seismic undrained stress control ring shear tests. One sample is volcanic ash taken from the base of landslide deposits (mass transport deposits), from 130 to 190 m deep layer below the submarine floor which was drilled and cored by the Integrated Ocean Drilling Program Expedition 333. Another two samples are the Neogene silty–sand and silt taken from the Omaezaki hill adjacent to the Senoumi, because the shear zone might have been formed in Neogene layers extending from on-land to the continental shelf. The largest strength reduction from peak to steady-state shear resistance in the undrained cyclic loading test was found in volcanic ash. The strength reduction in Neogene silty–sand was smaller than volcanic ash, while the Neogene silt mobilized the least post-failure strength reduction. An integrated model simulating the initiation and motion of earthquake-induced rapid landslides (landslide simulation (LS)-RAPID, Sassa et al. Landslides 7–3:219–236, 2010 ) was applied to this study. The steady-state shear resistance and other geotechnical parameters measured by the undrained ring shear tests and the greatest strong motion record in the 2011 off-the-Pacific Coast of Tohoku earthquake ( M w 9.0), also known as “2011 Tohoku Earthquake” at the observation point MYG004 (2,933 gal) were input to this model. As the result, it was found that landslides would be triggered by 0.30–1.0 times of MYG004 in volcanic ash, 0.4–1.0 times of MYG004 in Neogene silty–sand and Neogene silt, though the depth and area of triggered landslides were different in soils and intensity of shaking. Feature, created by LS-RAPID using the parameters of volcanic ash, was most similar to the Senoumi in depth and extent. The result obtained from this study includes a hypothesis to be proved, but presents the strong need to investigate the risk of the large-scale submarine landslides which could enhance tsunami wave and possibly enlarge the submarine landslide retrogressively into the adjacent coastal plain by the upcoming mega earthquake in the Nankai Trough.

On 28 September 2018, a strong earthquake with a moment magnitude of 7.5 occurred on the island of Sulawesi, Indonesia. This earthquake caused extensive liquefaction and liquefaction-induced flow slides inland. Despite a strike-slip fault, which typically displaces land horizontally, being unlikely to produce significant tsunamis, the earthquake in fact caused devastating tsunamis. Our field investigations showed that there was an occurrence of extensive liquefaction in coastal areas. Significant coastal liquefaction can result in a gravity flow of liquefied soil mass that can cause a tsunami. A comparison with a past disaster of the strike-slip fault Haiti earthquake tsunami indicated that essentially the same occurred at the Palu coast of Central Sulawesi. Namely, liquefaction-induced total collapse of coastal land caused liquefied sediment flows, resulting in a tsunami. An important difference between this time and Haiti was that such total collapses and flows of coastal land due to liquefaction occurred at several (at least nine) places, resulting in multiple tsunamis. Analysis of the tidal data implied that less than 20% of the tsunami height was related to tectonic processes, and the majority was caused by the coastal and submarine landslides as characterized by liquefied gravity flows.

Submarine landslides are a global geohazard that can displace huge volumes of loose submarine sediment, thereby triggering enormous tsunami waves and causing a serious threat to coastal cities. To investigate the generation of submarine landslide tsunamis, a three-dimensional numerical model based on the smoothed particle hydrodynamics (SPH) method is presented in this work. The model is first validated through the simulation of two underwater landslide model tests, and is then applied to simulate the movement of the Baiyun landslide in the South China Sea (SCS). The kinetics features of the submarine landslide, including the sliding velocity and runout distance, are obtained from the SPH simulation. The tsunami waves generated by the Baiyun landslide are predicted. In addition, sensitivity analyses are conducted to investigate the impact of landslide volume and water depth on the amplitude of the tsunami waves. The results indicate that the amplitude of tsunami waves triggered by submarine landslides increases with the landslide volume and decreases with the water depth of the landslide.

The South China Sea region is potentially threatened by tsunami hazards originated from multiple sources: the Manila subduction zone in the east, the Littoral Fault Zone (LFZ) in the north, numerous submarine landslides on the continental slopes and the volcanic islands in the Luzon Strait. Infrequent but potentially devastating tsunami hazard poses a great threat to the populous coastal region, fishery, oil and gas exploitation in the deep sea, etc. Here we review the recent progress in tsunami hazard assessment in the South China Sea region, focusing on two primary sources: submarine earthquakes and landslides. We sort and review the literature by the two commonly used approaches: deterministic and probabilistic tsunami hazard assessment for both source types. By simulating tsunamis generated by typical earthquakes originated from the Manila Trench, the LFZ and landslides in the continental slopes, we investigate their tsunamigenic mechanism and key tsunami characteristics in the South China Sea region. We point out the research gaps and highlight the key issues to be addressed in the future.

Submarine landslides occur globally and have the potential to damage seafloor infrastructure and trigger tsunamis. Recently, diatomaceous weak layers have been hypothesized to play a role in triggering submarine landslides on passive continental margins by generating overpressure. Here, we mixed two types of clay, an illite‐rich glaciomarine Boston Blue Clay (BBC) and a smectite‐rich marine clay from Eugene Island (EI) Block 330 offshore Louisiana, each with marine or lacustrine diatoms at mass ratios of 100:00, 90:10, and 80:20 to investigate the consolidated‐undrained shear strength of diatomaceous mudstones. These samples underwent uniaxial consolidation to 950 kPa before undrained shearing. Failure modes vary from bulging failure (BBC mixtures) to distinct failure planes (EI mixtures). Under peak conditions, BBC shows higher shear strength (202 vs. 176 kPa), effective friction angle (32° vs. 24°), and excess pore pressure (500 vs. 360 kPa) than EI, respectively. Adding 20 wt.% marine diatoms increased the peak undrained shear strength (BBC: +47 kPa, EI: +44 kPa) and effective friction angle (BBC: +9°, EI: +10°) similarly in both lithologies. However, excess pore pressure increased more rapidly in EI mixtures (+250 kPa) than in BBC mixtures (+5 kPa). Therefore, EI mixtures are more susceptible to shear failure with increased diatom content than BBC mixtures because of lower shear strengths and friction angles and faster pore‐pressure buildup. When combined with prior hydromechanical data, the findings indicate that diatom contents exceeding 20 wt.% and/or higher sedimentation rates are needed to produce overpressure sufficient to destabilize a passive continental slope.

A moment magnitude (Mw) 7.5 earthquake occurred on January 1, 2024, at the northern tip of the Noto Peninsula, Central Japan, triggering a large tsunami. Seismological and geodetic observations revealed the rupture of mapped submarine active faults. While proximal segment ruptures have been well resolved by previous research, far offshore segments have posed challenges for onshore-based inversions. This emphasizes the necessity for a comprehensive study on the fault through tsunami modeling. Here, we aimed to examine tsunami propagation and inundation using four different fault models to identify the general characteristics of the tsunami source and evaluate the complexities of earthquake- and submarine landslide-induced tsunamis. We identified the simultaneous rupture of two active fault systems as the most suitable model for explaining observed tsunami height and inundation; however, some inconsistencies with observations remained. The propagation process did not follow a concentric pattern but aligned with bathymetric heterogeneity. The findings also suggested potential amplification effects responsible for the devastation of the coast of Iida Bay and indicated a possible submarine landslide in southern Toyama Bay. The findings of the present work could benefit the exploration of a more realistic tsunami source model, considering the differences between observations and simulations. Such efforts, in collaboration with paleotsunami research, can contribute to the improved assessment of hazards from submarine active faults.

Submarine landslides shape continental margins, transfer massive amounts of sediment downslope, and can generate deadly and destructive tsunamis. Submarine landslides are common globally, yet constraining hazard potential of future events is limited by a short historical record and a wide range of possible slide dynamics. We test a novel approach to investigate slide dynamics using properties of the deformation zone induced by a large submarine landslide along the Cascadia margin, offshore Oregon. We use a simple model of a line load on a poroelastic half space to show the deformation zone size required rapid transport and deceleration. We argue that the slide moved at high speeds, aided by low dynamic frictional resistance, suggesting this event could have generated a tsunami. This method is applicable where slide‐induced impact zones are observed. Plain Language Summary The Cascadia margin is susceptible to underwater landslides and tsunami hazards due to the active subduction zone that can produce large magnitude earthquakes. However, determining whether future submarine landslides will be tsunamigenic is challenging. While past landslide deposits can easily be identified and in many cases age‐dated, the dynamic process including the initial acceleration and velocity that created the deposit is almost never known. We present a novel approach to back‐analyze slide velocity of past landslides by using the characteristics of the deformation zone that occurred when the landslide deposit came to rest on the seafloor. Our models suggest that the slide hydroplaned while moving at high‐speeds (up to 60 m/s) with a run‐out distance of 10‐km from the source, which match the observations. While our method does not model tsunamis explicitly, the high speeds are consistent with known tsunamigenic slides. This approach provides critically important constraints on the slide velocity and therefore the hazards of submarine landslides that can occur at Cascadia. Key Points A broad zone of impact‐induced deformation is observed immediately adjacent to the massive 44‐N Slide offshore Oregon The deformation of seafloor sediments observed at the slide terminus requires slide deceleration, implying catastrophic emplacement Modeled impact forces require a slide velocity of up to 60 m/s, which may have been sufficient to generate a tsunami

Previous studies have suggested submarine landslides as sources of the tsunami that damaged coastal areas of Palu Bay after the 2018 Sulawesi earthquake. Indeed, tsunami run-up heights as high as 10 m determined by field surveys cannot be explained by the earthquake source alone although the earthquake is definitely the primary cause of the tsunami. The quantitatively re-examined results using the earthquake fault models reported so far showed that none of them could fully explain the observed tsunami data: tsunami waveforms inferred from video footage and the field survey run-up tsunami height distribution. Here, we present probable tsunami source models including submarine landslides that are consistent with the observed tsunami data. We simulated tsunamis generated by submarine landslides using a simplified depth-averaged two-dimensional model. The estimated submarine landslide model consisted of two sources in the northern and southern parts of the bay, and it explained the observed tsunami data well. Their volumes were 0.02 and 0.07 km3. The radius of the major axis and the maximum thickness of the initial paraboloid masses and the maximum horizontal velocity of the masses were 0.8 km, 40 m and 21 m/s in the northern bay, and 2.0 km, 15 m and 19 m/s in the southern bay, respectively. The landslide source in the northern bay needed to start to move about 70 s after the earthquake to match the calculated and observed arrival times.

The rheological behaviors of submarine landslide materials significantly affect the landslide’s motion and the associated tsunami hazard. In this study, the effects of rheological behaviors on the landslide dynamics and tsunami hazard are numerically investigated. The Herschel-Bulkley rheological theory and nonlinear shallow water equation model are adopted. Taking one of the large landslides in the South China Sea (SCS), the Baiyun Slide, as an example, we investigate the impacts of remolding rates and initial/residual yield strengths on the speed and runout of landslide and the resulting tsunami wave. Further comparison of the Newtonian viscous model and the rheological model reveals the importance of the materials’ properties on landslide dynamics and associated tsunami hazard. The resonance characteristics of landslide-generated tsunamis are studied through fast Fourier transform (FFT) analysis. A parameter set of (τy,0, τy,∞, Γ) = (12 kPa, 3 kPa, 0.05) is found to be optimal for the Baiyun Slide. The slide speed and runout distance are mainly affected by the residual yield strength and remolding rate. The computed arrival time is not sensitive to the rheological parameters, but a smaller residual yield strength and a larger remolding rate result in a larger tsunami amplitude. The standing wave and dominant tsunami wave periods with high energy range from 15 and 26 min. This study reveals that the rheological parameters of landslide have considerable effects on submarine landslide motion and landslide tsunami amplitude, which is helpful for the landslide-generated tsunami assessment.

The Maps of Geohazard features of Southern Sardinia produced in the framework of the Magic project (MArine Geohazard along Italian Coasts) are here presented. The MaGIC project (Marine Geohazard along the Italian Coasts) had the aim of mapping the geohazard in the Italian seas. The features were derived from the digital elevation model interpretation of the seafloor morphology and shallow sub-surface. From the marine geo-hazards point of view, the main critical elements are represented by gravitational mass processes in the canyon heads, some of which, as in the Toro Canyon, are exposed to seismic triggering. In other cases, we observed that gravitational dynamics connected to fluid leakage processes (pockforms). Large landslides and debris avalanches have been detected in Cagliari Gulf, whereas in eastern upper slope, crescent bedforms, occurring in the eastern sector of the upper slope testify to the upward migration of hyperpycnal erosional structures linked to flows from nearby river inputs.