Sendai Plain Research Articles

Understanding flow characteristics from tsunami deposits at Odaka, Joban Coast, using a deep neural network (DNN) inverse model

Abstract. The 2011 Tohoku-oki tsunami inundated the Joban coastal area in the Odaka region of the city of Minamisoma, up to 2818 m from the shoreline. In this study, the flow characteristics of the tsunami were reconstructed from deposits using the DNN (deep neural network) inverse model, suggesting that the tsunami inundation occurred in the Froude supercritical condition. The DNN inverse model effectively estimated the tsunami flow parameters in the Odaka region, including the maximum inundation distance, flow velocity, maximum flow depth, and sediment concentration. Despite having a few topographical anthropogenic undulations that caused the inundation height to fluctuate greatly, the reconstructed maximum flow depth and flow velocity were reasonable and close to the values reported in the field observations. The reconstructed data around the Odaka region were characterized by an extremely high velocity (12.1 m s−1). This study suggests that the large fluctuation in flow depths on the Joban Coast compared with the stable flow depths in the Sendai Plain can be explained by the inundation in the supercritical flow condition.

Sensitivity of slip distribution on tsunami trace heights and geological evidences: a case study of the 2011 Tohoku-Oki earthquake

We examined whether it is possible to estimate the tsunami source model of the 2011 Tohoku-Oki earthquake from a comparison of numerical simulations of tsunami propagation and sediment transport, the measured trace heights, and the sediment thickness of tsunami deposits. Twelve models with different subfault numbers were prepared based on a reference model inferred from tsunami waveform inversion. The reference model with 55 subfaults considering rupture propagation and the model with instantaneous slip successfully reproduced both the tsunami trace heights and sediment thickness distribution of tsunami deposits in the Idagawa Lowland and Sendai Plain. Other models with the same moment magnitude but fewer subfaults could not reproduce the observed trace heights, and the reproducibility of sediment thickness distribution strongly depended on the slip distribution. Models with increased slip amounts and moment magnitude could reproduce the trace heights; however, the simulated sediment thickness was underestimated for the Idagawa Lowland while overestimated for the Sendai Plain. Our results indicate that the combination of trace heights, sediment thickness of tsunami deposits, and numerical simulations of tsunami propagation and sediment transport can be used to estimate historical earthquakes and tsunamis. Efforts should be made to increase the number of subfaults for the study of historical events, although the obtained solutions may not be unique because of fewer trace heights or tsunami deposit data.

To what extent tsunami source information can be extracted from tsunami deposits? Implications from the 2011 Tohoku-oki tsunami deposits and sediment transport simulations

A quantitative understanding of paleotsunamis is a significant issue in tsunami sedimentology. Onshore tsunami deposits, which are geological records of tsunami inundation, are used to reconstruct paleotsunami events. Numerical models of tsunami hydrodynamics and tsunami-induced sediment transport are utilized in such reconstructions to connect tsunami deposit characteristics, flow conditions, and (paleo-) tsunami sources. Recent progress in tsunami numerical modeling has increased the possibility of developing a methodology to estimate paleotsunami sources from tsunami deposits. Several previous studies have estimated paleotsunami sources using tsunami sediment transport simulations. However, the accuracy of paleotsunami source estimation has not yet been explored. Thus, to bridge this research gap, in this study, we showed the potential and limitations of deposit-based tsunami source estimation based on the 2011 Tohoku-oki tsunami deposit data on the southernmost part of the Sendai Plain, northeastern Japan. The tsunamigenic megathrust along the Japan Trench was divided into ten subfaults having similar lengths and widths. The hypothetical source models with varying slips on each subfault were examined by comparing the depositional volume and sediment source of onshore tsunami deposits. Due to limited information on the depositional area of the tsunami deposits used in the modeling, slips only in some parts of the entire tsunami source region could be estimated. The fault slip was slightly overestimated but could be compared with previous well-constrained source models. Thus, these results indicated that vast high-quality datasets of tsunami deposits can improve the accuracy of paleotsunami source estimation. It is also suggested that the amplitude of the receding wave affects the erosion pattern from the shoreface to the nearshore area. Although sufficient data for paleotsunami source estimation are lacking, an effective combination of tsunami deposit data and sediment transport simulations potentially improves the accuracy of the source estimation. The results will contribute to developing a framework of deposit-based paleotsunami source modeling and assessing its accuracy.

Heat flow data and thermal structure in northeastern Japan

New heat flow data corrected for climate change over Northeastern Japan were obtained using the temperature profile of the borehole of the High Sensitivity Seismograph Network (Hi-net). The obtained spatial distribution of heat flow shows low heat flow on the forearc side, high heat flow along the Ou Backbone Range, and low heat flow in the plains on the back-arc side. However, the distribution is not clearly divided into high and low heat flow along the VF front; for example, the low heat flow extends from near the northern Kitakami Mountains on the forearc side to the Ou Backbone Range crossing the VF, while the high heat flow extends to the central Kitakami Mountains and Sendai plain on the forearc side. In addition, a crustal temperature structure model was developed that considers into account the presence of sedimentary layers, the temperature dependence of thermal conductivity, and differences in heat generation due to lithology. There is a good correlation between this temperature structure and the lower limit of the seismogenic layer, which is between 400 and 450 °C. Compared to previous studies, the crustal thermal structure calculation method assumed is a model whose estimated temperature distribution is sensitive to structural differences; however, a more accurate estimation of the temperature structure is possible if detailed structural information is available. On the other hand, it seems necessary to treat fluid behavior in more detail in areas of high heat flow. However, the estimation of crustal temperature structure, especially in regions with thick sedimentary layers, is considered an improvement over the previous study.Graphical

Analytical solution for time-dependent shoreline position response to the tectonic recovery process in the Sendai Plain, Japan, after the 2011 Great East Japan Earthquake

The 2011 Great East Japan Earthquake on March 11, 2011 triggered not only giant tsunami waves, but also caused significant land subsidence followed by land rise in the Sendai Coastal Plain areas. These impacts resulted in severe damages to the coastal infrastructure and induced coastal morphology changes along Japan's northeast coast. It was observed that the maximum downward deformation in the Sendai Coastal Plain was initially about - 0.3 m. Immediately after the earthquake, the land started to rise and it has already recovered back to its original position at some locations after 10 years. Thus, the high-quality measured beach profile data sets after the 2011 Tohoku Earthquake are valuable for exploring the impact of sea level rise (SLR) on shoreline position evolution, since the bed level change is inversely related to the sea level rise process. The data also provide a unique opportunity to evaluate the Equilibrium Beach Profile (EBP) theory and validate the Bruun concept. In the present study, a new simple analytical solution for time-dependent shoreline position response to the tectonic recovery process is developed based on the convolution method. The model results are able to predict the time-dependent shoreline evolution, including the maximum potential response and the characteristic erosion time scale at which the profile evolves towards a new equilibrium state.

Analysis of the heat transfer in subsurface porous media with considering Robin-type boundaries and arbitrary surface temperature variations

Heat occurs everywhere in the environment and has been used as a natural groundwater tracer in many hydrogeological studies. Several studies have developed various mathematical models to analyze the heat transfer in subsurface porous media. However, most of these models have two limitations: (1) the media have semi-infinite thicknesses, and (2) the surface temperature variations follow specific temporal patterns. As a result, the models are qualified for only some specific situations. To handle these geothermal problems effectively, it is essential to develop a general model. This study therefore develops a heat transfer model for simulating spatiotemporal distributions of the subsurface temperature (i.e., geotemperature) in a finite-thickness porous medium. This model comprises a heat conduction-advection equation that is subject to the condition of arbitrary surface temperature variations and a Robin-type condition for the bottom convective boundary. The solution of this model is obtained via the Laplace transform and Duhamel's principle and validated by a finite-difference solution. In a case study, we consider saturated sandy soils with the thermal properties of heat capacity cρ = 2.96×106 J/m3/ °C and thermal conductivity λ = 2.2 W/m/ °C for analyzing the geotemperature via the present solution. By way of the solution, the results show that the model can reduce to a pure-conduction model at the thermal Peclet number Pe≤10−4 and to a pure-advection model at Pe≥102. The Robin boundary condition can become the Dirichlet boundary condition at the dimensionless heat transfer coefficient Hf ≥ 103 and the Neumann boundary condition at Hf ≤ 10−2. The responses of the geotemperature to the changes in the thermal parameters are inspected in a sensitivity analysis. The present solution agrees better with the field data from the Sendai Plain in Japan than the existing solutions subject to the limitation of semi-infinite-thickness porous media on predicting geotemperature-depth profiles. As stated above, the developed solution can solve relatively comprehensive geothermal problems and greatly benefits the studies on the heat transport in subsurface porous media.

Estimation of Tsunami Characteristics from Deposits: Inverse Modeling Using a Deep‐Learning Neural Network

Tsunami deposits provide information for estimating the magnitude and flow conditions of paleotsunamis, and inverse models have potential for reconstructing hydraulic conditions of tsunamis from their deposits. The majority of the previously proposed models are based on oversimplified assumptions and possess some limitations. We present a new inverse model based on the FITTNUSS model, which incorporates nonuniform and unsteady transport of suspended sediment and turbulent mixing. The present model uses a deep neural network (DNN) for the inversion method. In this method, forward model calculations are repeated for random initial flow conditions (e.g., maximum inundation length, flow velocity, maximum flow depth, and sediment concentration) to produce artificial training data sets of depositional characteristics such as thickness and grain‐size distribution. The DNN was then trained to establish a general inverse model based on artificial data sets derived from the forward model. Tests conducted using independent artificial data sets indicated that this trained DNN can reconstruct the original flow conditions from the characteristics of the deposits. Finally, the model was applied to a data set of 2011 Tohoku‐oki tsunami deposits. The predicted results of flow conditions were verified by the observational records at Sendai plain. Jackknife resampling was applied to estimate the precision of the result. The estimated results of the flow velocity and maximum flow depth were approximately 5.4 ± 0.1 m/s and 4.1 ± 0.2 m, respectively, after the uncertainty analysis. The DNN shows promise for reconstruction of tsunami characteristics from its deposits, which would help in estimating the hydraulic conditions of paleotsunamis.

Reduction effect of tsunami sediment transport by a coastal forest: Numerical simulation of the 2011 Tohoku tsunami on the Sendai Plain, Japan

We examined the effect of the coastal forest on tsunami sediment movement on the Sendai Plain, Japan, during the 2011 Tohoku tsunami using a numerical simulation of sediment transport. To account for the variable roughness coefficient in the coastal forest, a composite equivalent roughness coefficient was adopted. The simulated mean concentration of suspended sediment, wave height, flow velocity, and inundation limit were significantly reduced by the coastal forest. For fine to medium sand, the volumes of maximum deposition and erosion were reduced to 57–83% of those in the simulation without the coastal forest; these reductions were particularly notable for fine sand. The maximum extent and thickness of the sand layer were also reduced. The simulated sediment thickness and extent with the coastal forest for finer grain sizes roughly agreed with the observations, while the simulated results for coarser sand underestimated the observations. The reduction of sediment transport due to the coastal forest suggests that land use conditions and the vegetation environment at the time of a tsunami should be considered when studying historical and paleo-tsunamis using the geological evidence.

Application of Advanced Agricultural Methods to Restore Agricultural Areas Devastated by a Tsunami on the Sendai Plain

Application of Advanced Agricultural Methods to Restore Agricultural Areas Devastated by a Tsunami on the Sendai Plain

A geochemical approach for identifying marine incursions: Implications for tsunami geology on the Pacific coast of northeast Japan

Potential tsunami inundation areas can be predicted by the distribution of paleo tsunami deposits on land, which are mainly composed of marine-derived sands and muds. These exotic sandy and muddy layers have been identified by multiple approaches. However, there still remain uncertainties regarding the sources and weathering trends over the long term, and other useful proxies need to be investigated to identify of tsunami deposits. We examined the geochemical signatures of modern tsunami deposits from the Pacific coast of northeast Japan (2011 Tohoku tsunami) and those paleo tsunami deposits taken from the Tohoku District (Jogan, ~1080 calibrated ages before present [cal BP] and Yayoi, ca. 2000 cal BP). A geochemical ternary diagram (Seawater-Rock-(As + Metals)) showed the weathering trends of tsunami deposits over approximately 2000 years in the Sendai plain in the Tohoku. In the tsunami layers from the Tohoku, the Na/Ti atomic ratios markedly increased to 23.4 with an average value of 19.3 ± 3.0 (Jogan and Yayoi tsunami deposits), which was clearly higher than those of other layers (soil deposits, 10.3 ± 3.5 on average). These results show that Na/Ti ratios and other geochemical proxies are useful indicators of marine incursions in our case, and may contribute to the detection of muddy tsunami deposits close to the limit of inundation.

Proposal for a Method Predicting Suitable Areas for Installation of Ground-Source Heat Pump Systems Based on Response Surface Methodology

The installation potential of ground-source heat pump (GSHP) systems has been studied based on the spatial interpolation of numerical simulation results using ground heat exchanger (GHE) models. This study is the first to create an estimation formula for the heat exchange rate (HER) to obtain a solution equivalent to the numerical analysis results considering the average method when supplying three-dimensional (3D) hydrogeological information that affects the HER to a two-dimensional (2D) map. It was found that the main factors affecting the HER were groundwater flow velocity, subsurface temperature, and thermal conductivity. The response surface methodology was utilized to approximate the HER using the above-mentioned three parameters. The estimated HER showed very strong agreement with that calculated by the GHE models. The application of the estimation formula to the simulation of the 3D groundwater flow and heat transport model of the Sendai Plain (Japan) better reflects the hydrogeological information of the regional model than conventional maps. The proposed method improves the spatial resolution of maps and allows for the easy creation of the HER estimation formula.

Spatial distribution and sources of tsunami deposits in a narrow valley setting - insight from 2011 Tohoku-oki tsunami deposits in northeastern Japan

Sedimentary processes and spatial distributions of tsunami deposits in valleys have poorly been understood despite many paleo-tsunami deposits have recently been discovered from sedimentary sequences in valleys. We conducted an exhaustive investigation of 2011 Tohoku-oki tsunami deposits in a narrow valley at the south end of Sendai Plain, northeastern Japan, to collect sedimentological data of the tsunami deposits in the valley and to correlate them with tsunami inundation and associated sedimentary processes. The tsunami deposits were investigated at 174 sites to cover the entire inundation area of the narrow valley. We analyzed thickness, sedimentary structures, and grain size of the tsunami deposits to correlate their spatial variation with tsunami flow hydrodynamics and topographic features. The tsunami deposits, composed of sand and mud layers, were found to be 0–40 cm thick. Sand thickness generally decreased inland with local fluctuation. The sand layer was mostly composed of single unit on the upper main valley and sub-valleys, although it was mainly composed of 2–6 sub-units on the lower main valley and in the pond. Mud thickness is strongly controlled by local sources, namely, the rice paddy and the pond. The depositional area of the mud layer coincides with the distribution of the rice paddies and the pond location. The mud layer also considerably thicker in and around the pond. These results indicate that sediment thickness and sedimentary structures are highly varied on a local scale, but they are generally controlled by local topography. Sediment budget in the valley was estimated, and it may provide implications to general sedimentary process. Total depositional volume of the tsunami sand deposited on land is half of the volume of sediments disappeared from the sandy beach and sand dune, suggesting the rest must have been discharged into the sea by the backwash. Spatial distribution of sediment thickness and sedimentary structures as well as sedimentary processes inferred from 2011 Tohoku-oki tsunami deposits will benefit searching and identification of paleo-tsunami deposits in valleys of other coastal environments.

Anthropogenic pollutants and biomarkers for the identification of 2011 Tohoku-oki tsunami deposits (Japan)

Organic geochemistry is commonly used in environmental studies. In tsunami research, however, its applications are in their infancy and it is still rarely used. We present results for two types of organic geochemical markers, biomarkers and anthropogenic markers, present in deposits left by 2011 Tohoku-oki tsunami on the Sendai Plain, Japan. As the tsunami inundated the coastal lowland up to 4.85 km inland, sediments from various sources were eroded, transported and deposited. This led to the distribution of biomarkers from different sources across the Sendai Plain creating a unique geochemical signature in the tsunami deposits. The tsunami also caused destruction along the Sendai coastline, leading to the release of large quantities of environmental pollutants (e.g., fossil fuels, tarmac, pesticides, plastics, etc.) that were distributed across the inundated area. These anthropogenic markers, represented by three main compound groups (polycyclic aromatic hydrocarbons, pesticides, and halogenated compounds), were preserved in tsunami deposits (at least until 2013, prior to land clearing). Their concentrations differed significantly from the pre- and post-tsunami background contamination levels. Organic proxy concentrations can differ for sand and mud deposits due to various factors (e.g., preservation, dilution, microbial alteration). However, it can be concluded that anthropogenic markers and biomarkers have the potential to be a valuable proxy for future studies of recent tsunami deposits because of their high source specificity and relatively good preservation potential providing information about sediment sources and transport pathways (e.g., marine source, evidence of backwash).

Diatom assemblages in the 2011 Tohoku-oki tsunami deposits and underlying soil in the Sendai Plain

Diatom assemblages in the 2011 Tohoku-oki tsunami deposits and underlying soil in the Sendai Plain

The survival situation of the coastal forest eight years after a tsunami struck the Sendai plain

The survival situation of the coastal forest eight years after a tsunami struck the Sendai plain

Factors Affecting the Installation Potential of Ground Source Heat Pump Systems: A Comparative Study for the Sendai Plain and Aizu Basin, Japan

Evaluating the installation potential of ground source heat pump (GSHP) systems based on the hydrogeological condition of an area is important for the installation and sustainable use of the system. This work is the first to have compared the distributions of heat exchange rate in the Sendai Plain and Aizu Basin (Japan) in terms of topographical and hydrogeological conditions. A regional groundwater flow and heat transport model was constructed for the Sendai Plain. Suitability assessment was conducted for an identical closed-loop system by preparing the distribution maps of heat exchange rate for space heating for the plain and basin. For both locations, the upstream area showed a higher heat exchange rate than the downstream area. Multiple regression analysis was conducted using heat exchange rate as a response variable. Average groundwater flow velocity and average subsurface temperature were considered as explanatory variables. The heat exchange rate for the plain, whose Péclet number ranged from 3.5 × 10−3–7.3 × 10−2, was affected by groundwater flow velocity and subsurface temperature. The exchange rate for the basin, whose Péclet number ranged from 8.5 × 10−2–5.8 × 10−1, was affected by groundwater flow velocity. Inland basins are likely to be more suitable for GSHP system installation utilizing groundwater flow than coastal plains in terms of inclination of slope. This study showed that multiple regression analysis can reveal factors affecting the heat exchange rate as well as the degree to which they affect it.

Near-field tsunami inundation forecast method assimilating ocean bottom pressure data: A synthetic test for the 2011 Tohoku-oki tsunami

An approach for forecasting near-field tsunami inundation was developed by combining two methods. The first method computes tsunami by assimilating pressure data observed at numerous ocean bottom sensors without tsunami source information, and the second method forecasts near-field tsunami inundation by selecting a site-specific scenario from a precomputed tsunami inundation database. In order to evaluate the validity of this combined method, we performed a synthetic forecast test for the 2011 Tohoku-oki tsunami along the Pacific coast in Japan. A tsunami computation test performed using the assimilation of synthetic pressure data reveals that the method reproduced well the tsunami field for the 2011 Tohoku-oki tsunami. A synthetic near-field tsunami inundation forecast at four sites, Kamaishi, Rikuzentakata, Minamisanriku, and the Sendai Plain for the 2011 Tohoku-oki tsunami also worked. The results indicate that an accurate tsunami inundation forecast method by this combined approach using pressure data from numerous ocean bottom sensors is now available.

Energy Grade Line Analysis of Tsunami run-up on the Sendai Plain after the 2011 Tohoku earthquake

Energy Grade Line Analysis (EGLA) of tsunami run-up has been proposed as a simple tool to predict inundation heights and tsunami loads. A large database of more than 500 inundation measurements collected on the Sendai Plain on the east coast of Japan after the Tohoku earthquake of 2011 provided the writers the opportunity to evaluate the accuracy of the general EGLA method and to make adjustments to fit the model to the Sendai Plain. Refinements to the EGLA model included a correction for the influence of large linear topographic obstructions on tsunami flows in order to achieve better agreement with measured inundation heights. The study illustrates that inundation height predictions are sensitive to the choice of Froude number parameter and land use roughness coefficients. An important outcome of this study is a site-specific tool for probabilistic prediction of inundation heights over the Sendai Plain that is of value for tsunami counter measure planning including design of tsunami resistant structures.

Geological evidence of tsunamis in the past 3800 years at a coastal lowland in the Central Fukushima Prefecture, Japan

We found seven event deposits during the past 3800 years in a total of 16 geological core samples from a coastal lowland in Fukushima Prefecture, about 12 km north of the Fukushima Daiichi Nuclear Power Station. The event deposits consisted of well-sorted and rounded fine to coarse sand with single normal grading structures, parallel laminae, mud drapes, rip-up clasts, and erosional basal contacts. They were distributed about 2 km inland from the current shoreline and commonly became thinner landward. The third and fifth sand layers were characterized by many climbing ripples and parallel and trough-cross bedding that were probably caused by storm surges or some other events during the period of absent beach ridges. The other deposits were characterized by sedimentological features common to tsunami deposits. The top layer appeared to have been deposited by the 2011 Tohoku tsunami, and the second layer might have been resulted from the AD 869 Jogan Tohoku tsunami. The other three tsunami deposits corresponded to tsunami events between the second and fourth centuries AD, the sixth and fourth centuries BC, and the twelfth and ninth centuries BC. The average recurrence interval of the paleo-tsunamis was estimated to be 560–950 years. These dates are mostly consistent with previous studies of the Sendai plain, suggesting that paleo-tsunamis that reached the Sendai plain also reached the coast of Fukushima Prefecture. However, no trace was found from an earthquake around the 15th century, which had been considered as a predecessor of the 2011 Tohoku earthquake (Mw 9.0) in the tsunami deposit surveys in Sendai plain.

Are inundation limit and maximum extent of sand useful for differentiating tsunamis and storms? An example from sediment transport simulations on the Sendai Plain, Japan

We examined the quantitative difference in the distribution of tsunami and storm deposits based on numerical simulations of inundation and sediment transport due to tsunami and storm events on the Sendai Plain, Japan. The calculated distance from the shoreline inundated by the 2011 Tohoku-oki tsunami was smaller than that inundated by storm surges from hypothetical typhoon events. Previous studies have assumed that deposits observed farther inland than the possible inundation limit of storm waves and storm surge were tsunami deposits. However, confirming only the extent of inundation is insufficient to distinguish tsunami and storm deposits, because the inundation limit of storm surges may be farther inland than that of tsunamis in the case of gently sloping coastal topography such as on the Sendai Plain. In other locations, where coastal topography is steep, the maximum inland inundation extent of storm surges may be only several hundred meters, so marine-sourced deposits that are distributed several km inland can be identified as tsunami deposits by default. Over both gentle and steep slopes, another difference between tsunami and storm deposits is the total volume deposited, as flow speed over land during a tsunami is faster than during a storm surge. Therefore, the total deposit volume could also be a useful proxy to differentiate tsunami and storm deposits.