Sendai Plain Research Articles

Analysis of Spatio-Temporal Tsunami Source Models for Reproducing Tsunami Inundation Features

A powerful tsunami triggered by the Mw 9.0 Tohoku earthquake struck the northern Pacific coast of Japan in 2011, destroying several coastal communities in Iwate Prefecture, Miyagi Prefecture, and Fukushima Prefecture. Here, we investigate a new spatio-temporal slip model (model B) developed by the Cabinet Office of the Government of Japan. This slip model was compared against the non-uniform slip model estimated using tsunami waveform data (model A). We focused our analysis on two areas that were destroyed by the tsunami. The town of Onagawa and the coastal Sendai Plain area were selected because they were located in front of the epicentre, where the most significant slips were registered. Our simulation results revealed that the spatio-temporal slip distribution better replicated the observed data. Regarding the tsunami waveforms from the coastal tide gauge station and offshore stations, the Cabinet Office’s slip model showed an approximately 30% better accuracy relative to the non-uniform slip model. Furthermore, by comparing the local inundation features at two locations with unique topographic and coastal morphological characteristics, we also found that model B better replicated the measured inundation depths. Finally, considering that tsunami-induced damage is a direct function of various inundation features such as the flow depth and flow velocity, this new slip model can generate more realistic damage scenarios for future tsunami assessments.

Inverse Tsunami Flow Modeling Including Nonequilibrium Sediment Transport, With Application to Deposits From the 2011 Tohoku‐Oki Tsunami

Tsunami deposits provide important clues to understand ancient tsunami events. Several inverse models have been proposed to estimate the magnitude of paleotsunamis from their deposits. However, existing models consider neither nonuniform transport of suspended sediment nor turbulent mixing, which are essential factors governing sedimentation from suspension in tsunami flows. Here we propose a new inverse model of tsunami deposit emplacement, considering both transport of nonuniform suspended load and entrainment of basal sediments. This inversion model requires the spatial distribution of deposit thickness and the pattern of grain size distributions of the tsunami deposit along a 1‐D shoreline‐normal transect as input data. It produces as output run‐up flow velocity, inundation depth, and concentration of suspended sediment. To solve for advection of nonuniform suspended load, a transformed coordinate system is adopted, which increases computational efficiency. Tests of model inversions using artificial data successfully allow reconstruction of the original input values, suggesting the effectiveness of our optimization method. We apply our new inversion model to the 2011 Tohoku‐Oki tsunami deposit on Sendai Plain, Japan. The thickness and grain size distribution of the tsunami deposit was measured along a 4 km long transect normal to the coastline. The result of our inversion fits well with the observations from aerial videos and field surveys. We conclude that this method is suitable for the analysis of ancient tsunami deposits and that it has the advantage of requiring relatively little information about the condition of the emplacing paleotsunami for reconstruction.

Probabilistic Tsunami Loss Estimation Methodology: Stochastic Earthquake Scenario Approach

This study develops a probabilistic tsunami loss estimation methodology for enhancing community resilience against tsunami disasters. The method is based on novel stochastic earthquake source modeling and state-of-the-art tsunami fragility modeling. It facilitates the quantitative evaluation of tsunami loss for coastal community by accounting for uncertainties of earthquake occurrence and rupture characteristics. A case study is set up to illustrate an application of the developed method to the Sendai Plain area by focusing on possible tsunami events in the Tohoku region of Japan. The quantitative tsunami hazard as well as risk assessment results serve as effective means to make decisions regarding tsunami disaster risk reduction.

Estimation of fatality ratios and investigation of influential factors in the 2011 Great East Japan Tsunami

The 2011 Great East Japan earthquake caused a wide-ranging and devastating tsunami that reached a maximum height of 40m and caused 19,000 fatalities, particularly along the Tohoku coast of Japan. The purpose of this study is to present a new estimation of the fatality ratios based on tsunami arrival times and evacuation perspectives and to present lessons learned for future tsunami loss assessments in other areas. In addition, this study investigates influencing factors, such as age, gender, and two coastal topography types (Sanriku ria-coast and Sendai plain). The fatality ratio is calculated by the number of fatalities divided by the population at the town scale along the shoreline, and the tsunami arrival time is calculated using the TUNAMI model with nesting grids of 1350m, 450m, 150m, and 50m. Then, linear and nonlinear regression analyses are performed to develop a relationship between the fatality ratios and tsunami arrival times. For different topography types, different distributions of fatality ratios with tsunami arrival times were observed; the fatality ratios of the Sendai plain were generally higher than those of the Sanriku ria-coast for the same arrival time. Based on the results, a strong inverse correlation between the fatality ratios and the tsunami arrival times was found in the Sendai plain, while the Sanriku ria-coast must be divided into two groups to obtain this correlation. Furthermore, other influencing factors, such as age and gender, contributed to differences in estimating the fatality ratios.

QUANTIFYING NUMERICAL MODEL ACCURACY AND VARIABILITY

On March 11, 2011 the Tohoku tsunami event caused the death of thousands of people and generated billions of dollars in damages. Following this and previous tsunami events, there has been an effort to improve tsunami risk mitigation for coastal communities. Currently, numerical models are being used in a state-of-the-art methodology to estimate tsunami risk from near and far field sources. Model predictions are essential for the development of Tsunami Hazard Assessments (THA). By better understanding model bias and uncertainties and if possible minimizing them, a more accurate and reliable THA will result. In this study we compare runup height, inundation lines and flow velocity field measurements between GeoClaw and the Method Of Splitting Tsunami (MOST) predictions in the Sendai plain. Runup elevation and average inundation distance was in general over predicted by the models. However, both models agree relatively well with each other when predicting maximum sea surface elevation and maximum flow velocities. Model predictions show that the flow velocity increases as the tsunami wave front reaches the shoreline and makes its way inland for a couple of kilometers, contrary to what it is generally assumed. The tsunami models used in this study show much more variability when predicting flow velocity than predicting runup elevations and inundation lines. The results provided in this study will help understand the uncertainties in model predictions and locate possible sources of errors within a model.

Effects of topography on particle composition of 2011 tsunami deposits on the ria-type Sanriku coast, Japan

A geological survey was conducted in a small valley along the Sanriku coast, and the grain compositions of both autochthonous gravels and the 2011 tsunami deposit from the Tohoku-oki earthquake were investigated to reveal the depositional form of the tsunami deposits on the ria-type coast. The features of autochthonous gravels in the beach, riverbed, and slope areas are characterized by a combination of their rock type and roundness. The roundness values of the beach and slope gravels are large (0.8–0.9) and small (0.1–0.2), respectively, regardless of rock type, and the riverbed gravels are typically granite rocks with an intermediate roundness (0.4–0.5). The 2011 tsunami deposits along the survey transect include the above autochthonous gravels and show several characteristic features, such as a sharp erosional contact with the underlying peaty soil, normal and/or inverse grading structures, mud sheets, and richness in gravels in the lower unit, while the thickness shows no systematic changes with the distance from the coast. The presence of slope gravels suggests high tsunami inundation and flux, which eroded bare rock and talus and transported them to the valley plain by backwash current. The mixture of beach and riverbed deposits implies the formation of both an upstream current from the beach, which transported beach pebbles inland, and a backwash current along the channel, which accumulated sub-angular riverbed gravels around the river mouth. The characteristics of the 2011 tsunami deposit on the Sanriku coast, where narrow valleys are surrounded by high cliffs with steep slopes, are significantly different from those of sandy tsunami deposits in Sendai Plain, where a flat plain develops within several kilometers of the coastline. The differences among sedimentary facies are affected by the topography and autochthonous deposits.

Comparison of reduction of tsunami fluid force and additional force due to impact and accumulation after collision of tsunami-produced driftwood from a coastal forest with houses during the Great East Japan tsunami

Coastal forests on the Sendai Plain reduced the force of the 2011 Great East Japan tsunami, but the tsunami also produced driftwood that increased the impact force or additional drag force due to accumulation in front of houses (damming). The advantage of reducing the tsunami force and disadvantage of increasing the force on houses after collisions of driftwood were repeatedly pointed out after the tsunami, but have never been compared quantitatively. Therefore, a driftwood model was developed to evaluate tree washout, motion of the drifting trees, and collisions with houses. Two-dimensional non-linear long wave equations were coupled with the equation for motion of driftwood, and the impact force was solved assuming the collision to be inelastic. Experiments were conducted to clarify the driftwood movement and determine the impact duration in the simulation. The ratio of the impact force of driftwood to the maximum drag force on a house was comparatively small, around 0.01–0.10. On the other hand, when driftwood piled up in front of a house, the increasing water depth caused additional fluid force. The rate of accumulation of driftwood was not large in experiments, but the magnitude of the drag force increase after accumulation was far larger than that of the impact force of collision. However, in comparison with the reduction of fluid force by the coastal forest, the fluid force due to the damming in front of a house was found to be smaller and not a main factor in the washout of houses.

Developing estimating equations of fatality ratio based on surveyed data of the 2011 Great East Japan Tsunami

The 2011 Great East Japan tsunami caused a wide range of devastating tsunami with maximum tsunami height of 40 m and 19,000 casualties especially along the Tohoku coast of Japan. The purpose of this study is to develop estimating equations of fatality ratio from tsunami arrival time for future tsunami loss assessment and investigate the effect of two coastal topography types namely, Sanriku-ria coast and Sendai plain. In this study, fatality ratio was defined as number of fatality divided by total number of people in a small scale of towns along the shoreline and tsunami arrival time was calculated from TUNAMI modelling with nesting-grids of 1350 m, 450 m, 150 m, and 50 m. Then, linear and nonlinear regression analysis were performed to develop a relationship model between fatality ratio and tsunami arrival time. Based on the results, a strong correlation that fatality ratio decreases with longer arrival time was found in both Sanriku-ria coast and Sendai plain. For different coastal types, different distributions of fatality ratio with tsunami arrival time are observed, in which fatality ratio of Sendai plain is higher than that of Sanriku ria-coast at the same arrival time generally.

The Search for Tsunami Evidence in the Geological and Archaeological Records, With a Focus on Japan

Tsunami damage to archaeological sites in Japan has been recognized since the 1980s, but the Great Tōhoku-oki Earthquake and tsunami of 11 March 2011 stimulated geologists and archaeologists to find evidence of previous tsunami in Japan, investigate the responses of earlier inhabitants to tsunami, and assess the probability of future occurrences. Excavated sites on the Sendai Plain, partially inundated in this recent tsunami, have been crucial in this endeavor, with recovered data at times contradicting historical sources. Great progress has been made in the science of identifying tsunami deposits and understanding their nature and distribution, aiding in their recognition at archaeological sites. This article provides an introduction to the nature of tsunami waves and their causes, resources available for studying past tsunami worldwide, and difficulties in identifying tsunami sediments. Seventeen case studies of sites where tsunami deposits have been investigated throughout the Japanese and Ryukyu archipelagos are presented. Tsunami can be included within my conception of ‘tectonic archaeology,’ archaeology that must methodologically deal with the influence of plate tectonics on the islands. Earthquakes, volcanoes, and most tsunami relate to the subduction zone setting of Japan; thus, to fully understand the site remains of previous tectonically derived disasters demands knowledge of plate tectonics, seismology, volcanology, sedimentology, and wave physics among others. Integrating these spheres of knowledge into archaeological research opens new avenues of interpretation, including understanding why many Middle Yayoi settlements on the Sendai Plain were abandoned, not to be reoccupied for 400 years.

The relationship between the density of <I>Dianthus superbus</I> L. var. <I>longicalycinus</I> (Maxim.) Williams and different soil disturbance intensity on the tsunami-stricken coastal forest in Sendai Plain

The relationship between the density of <I>Dianthus superbus</I> L. var. <I>longicalycinus</I> (Maxim.) Williams and different soil disturbance intensity on the tsunami-stricken coastal forest in Sendai Plain

反射法地震探査および重力探査から明らかになった仙台平野南部の伏在活断層

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.

An Analysis of Fatality Ratios and the Factors That Affected Human Fatalities in the 2011 Great East Japan Tsunami

This study presents a new analysis of spatial variation in fatality ratios in the 2011 Great East Japan tsunami, in order to overcome the limitations of previous studies that tended to underestimate the fatality ratios. In addition, this analysis was performed in a manner that allows the results to be compared to those from analyses of fatality ratios in other historical tsunamis. To do this, it uses population and fatality data at the village scale in areas of less than 3 km2 where the inundation ratio was greater than 70%, rather than the lower-resolution data used in previous studies. The median value of the tsunami inundation depth at each location was extracted at the original 5-m grid resolution. All of the data were obtained from the Reconstruction Support Survey Archive. Based on the results, a strong correlation between the fatality ratio and inundation depth was found only in some areas of the Sendai Plain, whereas no strong correlation was observed along the Sanriku ria coastline. Fatality ratios in Sanriku were likely related not only to the force of the tsunami but also to other factors, such as the ria topography and the population’s experience of past historical tsunamis. Data from other tsunamis in regions where tsunamis frequently occur also indicate that historical tsunami experience is a key factor in reducing fatality ratios. In contrast, the Sendai Plain shows smaller variation in local tsunami amplification effect compared to that of the Sanriku ria coastline, as well as fewer coastal defence structures. Therefore, the fatality ratio in that region was predominantly affected by the force of the tsunami and the residents’ individual characteristics. On the Sendai Plain, Ishinomaki City exhibited a strong correlation between the fatality ratio and inundation depth, as well as between fatality ratio and building damage because its low evacuation ratio meant that many fatalities occurred in victims’ homes. Therefore, the fatality ratio in Ishinomaki City was higher than those in other areas at the same inundation depth. Simple empirical formulas were developed for estimation of human fatalities based on inundation depths and building damage ratios.

Spatial Statistics of Tsunami Overland Flow Properties

AbstractNumerical models are a key component of methodologies used to estimate tsunami risk, and model predictions are essential for the development of tsunami hazard assessments (THAs). By better understanding model bias and variability and, if possible, minimizing them, more reliable THAs will result. In this study, the authors compare run-up height, inundation lines, and flow-velocity field measurements between an open-source tsunami model and the method of splitting tsunami (MOST) model predictions in the Sendai Plain in Japan. Run-up elevation and average inundation distance are, in general, overpredicted by the models. However, both models agree relatively well with each other when predicting maximum sea surface elevations and maximum flow velocities. Furthermore, to explore the variability in numerical models, MOST is used to compare predictions from six different grid resolutions (90, 60, 30, 20, 15, and 10 m). Results of this work show that predictions of statistically stable products (run-up, in...

CFD prediction of effects of windbreak forest on occurrence frequency of blown sand in coastal agricultural area in Sendai plain

CFD prediction of effects of windbreak forest on occurrence frequency of blown sand in coastal agricultural area in Sendai plain

Anthropogenic disruption to the seismic driving of beach ridge formation: The Sendai coast, Japan

The expected geomorphic after-effects of the Mw 9.0 Tōhoku-oki earthquake of 11 March 2011 (eastern Japan) are summarized by a schematic model of seismic driving, which details seismogenic disturbances to sediment systems that affect the rate or timing of sediment delivery to coastlines over timescales of 102–104years. The immediate physical environmental responses to this high-magnitude earthquake included a large tsunami and extensive region-wide slope failures. Normally, slope failures within mountain catchments would have significant impacts on Japan's river and coastal geomorphology in the coming decades with, for example, a new beach ridge expected to form within 20–100years on the Sendai Plain. However, human activity has significantly modified the rate and timing of geomorphic processes of the region, which will have impacts on likely geomorphic responses to seismic driving. For example, the rivers draining into Sendai Bay have been dammed, providing sediment traps that will efficiently capture bedload and much suspended sediment in transit through the river system. Instead of the expected ~1km of coastal progradation and formation of a ~3m high beach ridge prior to the next large tsunami, it is likely that progradation of the Sendai Plain will continue to slow or even cease as a result of damming of river systems and capture of river sediments behind dams. The resulting reduction of fluvial sediment delivery to the coast due to modification of rivers inadvertently makes seawalls and other engineered coastal structures even more necessary than they would be otherwise.

Erosion of a paleo-tsunami record by the 2011 Tohoku-oki tsunami along the southern Sendai Plain

Erosion of sediment beneath a coastal lake by the 2011 Tohoku-oki tsunami largely removed up to 1100years of the geologic record of tsunami and tephra deposition. That lost geologic record included the 1611 Keicho tsunami deposit and the 915 To-a tephra, amounting to ca. 1.4m of lake-bottom sediment, as judged from comparison of stratigraphy at four locations examined before and after the 2011 tsunami. The tsunami replaced the eroded sediment with a 7–15-cm-thick sand sheet and an overlying 20–60-cm-thick layer of black mud. Both 2011 deposit components were likely derived from sediment eroded along the tsunami flow path, including granite rock fragments downstream of the lake basin and mainly reworked lake-floor sediment. Erosion and redeposition of lake floor sediment by tsunami inundation can introduce uncertainty to sandy tsunami deposit age estimation. The 2011 tsunami sand might be dated a few hundred years older than the actual age if one overlooks the reworked muddy tsunami deposit. In its facies, the mud closely resembles mud deposited from the usual environment but is mixed with sediments of various ages. Therefore, it is important to identify the reworked muddy tsunami deposit based on geochemistry and micropaleontology, to avoid misinterpretation of the depositional age of tsunami deposits. Erosion of deposits beneath the lake floor also erases evidence, leaving an incomplete paleo-tsunami history. Reconstructing geological records of tsunamis in coastal lakes requires careful evaluation of the possible effects of tsunami erosion. Identifying sediment sources, mapping stratigraphic unconformities, and constructing detailed age-depth models to test for gaps in sedimentation can produce important insights.

Shorter intervals between great earthquakes near Sendai: Scour ponds and a sand layer attributable to A.D. 1454 overwash

AbstractA sparsely documented tsunami in 1454 may subdivide the recurrence interval between the 869 and 2011 tsunamis near Sendai, as judged from geomorphic, stratigraphic, and archival evidence. Pond‐filled breaches cut across beach ridges on century‐old topographic maps. The basal pond deposit in one of these breaches postdates 1454. Stratigraphy on Sendai Plain includes a sand sheet that contains marine and brackish diatoms. Radiocarbon ages suggest that the sheet dates to 1406–1615 (2σ), and written records for this interval in Tohoku mention a tsunami in 1454. The inferred inundation extended 1.0–2.5 km inland from an approximate medieval shoreline. Simulated tsunamis that best account for the sand sheet require a thrust earthquake of moment magnitude 8.4 or larger. If the sand sheet represents the 1454 tsunami, the two most recent intervals between great thrust earthquakes in Sendai region spanned 585 and 557 years.

OSL dating of the AD 869 Jogan tsunami deposit, northeastern Japan

Sediments deposited by the AD 869 Jogan tsunami offer an opportunity to test the reliability of optically stimulated luminescence (OSL) dating of relatively old historical tsunami deposits. We collected a geoslicer sample from sand deposited on the Sendai Plain, northeastern Japan, by the Jogan tsunami and applied quartz OSL dating to it. We then compared the OSL ages with the known age of the tsunami event. In ascending order, the sedimentary sequence in the geoslicer sample consists of the beach–dune sand, lower peat, Jogan tsunami deposit, upper peat, pre-2011 paddy soil, and the 2011 tsunami deposit. To obtain equivalent dose (De,bulk), a standard single-aliquot renegerative-dose (SAR) protocol was applied to large aliquots of the 180–250 μm fraction of two samples from the beach–dune sand, and four samples from differing levels of the Jogan tsunami deposit. The OSL decay curves were dominated by the medium component; thus, for two samples from the Jogan deposit the fast-component OSL signal was isolated and used to determine the equivalent dose (De,fast). Using De,bulk, OSL ages of the tsunami deposit were underestimated by ∼40%, and even the beach–dune sand was dated younger than AD 869. In contrast, De,fast provided a robust age estimate with only slight underestimation. A pulse annealing test showed that the bulk and medium-component OSL signals were thermally unstable. The medium component in the natural OSL was clearly truncated in comparison to the regenerated OSL; the medium component is thus considered to be the main cause of the underestimated ages. Similar effects of a dominant medium-component OSL have been reported in tectonically active regions, which are also prone to tsunamis. The effect of this dominance should be carefully considered in quartz OSL dating of tsunami deposits.

仙台平野中部亘理町逢隈地区のイグネの特徴と津波の影響

仙台平野中部亘理町逢隈地区のイグネの特徴と津波の影響

A methodology for near‐field tsunami inundation forecasting: Application to the 2011 Tohoku tsunami

AbstractExisting tsunami early warning systems in the world can give either one or a combination of estimated tsunami arrival times, heights, or qualitative tsunami forecasts before the tsunami hits near‐field coastlines. A future tsunami early warning system should be able to provide a reliable near‐field tsunami inundation forecast on high‐resolution topography within a short time period. Here we describe a new methodology for near‐field tsunami inundation forecasting. In this method, a precomputed tsunami inundation and precomputed tsunami waveform database is required. After information about a tsunami source is estimated, tsunami waveforms at nearshore points can be simulated in real time. A scenario that gives the most similar tsunami waveforms is selected as the site‐specific best scenario and the tsunami inundation from that scenario is selected as the tsunami inundation forecast. To test the algorithm, tsunami inundation along the Sanriku Coast is forecasted by using source models for the 2011 Tohoku earthquake estimated from GPS, W phase, or offshore tsunami waveform data. The forecasting algorithm is capable of providing a tsunami inundation forecast that is similar to that obtained by numerical forward modeling but with remarkably smaller CPU time. The time required to forecast tsunami inundation in coastal sites from the Sendai Plain to Miyako City is approximately 3 min after information about the tsunami source is obtained. We found that the tsunami inundation forecasts from the 5 min GPS, 5 min W phase, 10 min W phase fault models, and 35 min tsunami source model are all reliable for tsunami early warning purposes and quantitatively match the observations well, although the latter model gives tsunami forecasts with highest overall accuracy. The required times to obtain tsunami forecast from the above four models are 8 min, 9 min, 14 min, and 39 min after the earthquake, respectively, or in other words 3 min after receiving the source model. This method can be useful in developing future tsunami forecasting systems with a capability of providing tsunami inundation forecasts for locations near the tsunami source area.