Spatial variations of hazards such as strong ground motion and tsunami inundation are a key element for obtaining a geographical understanding of natural disasters. However, detailed distribution of tsunami run-up heights for the devastating tsunami associated with the 2011 off the Pacific coast of Tohoku earthquake is not available. A GIS analysis of tsunami inundation areas is conducted from data collected by the Tsunami Damage Mapping Team and from post-tsunami 2-m mesh and 5-m mesh digital elevation models (DEM) after the Geospatial Information Authority of Japan, in order to produce the Tsunami Run-up Height Map, which includes polygon data of inundation areas with elevation data at each point. Horizontal shifts of orthophotos taken just after the tsunami are corrected using a Helmert transformation. The map covers Iwate Prefecture, Miyagi Prefecture, and the northern part of Fukushima Prefecture continuously at high resolutions, and reveals spatial variations of tsunami run-up heights in detail. These variations are caused by: 1) landforms at each site, such as coastal plains, valleys, bays, and beach ridges, as well as their directions and magnitudes, and 2) source locations, interference, and wavelengths of the tsunami, as implied by a previous study. The map supports examination carried out on source fault models and simulation results of tsunamis from a geographical viewpoint. At the same time, the methodology to produce the map would be useful for systematically revealing run-up height distribution, in addition to inundation areas immediately after future tsunamis.
The distribution of tsunami run-up heights generally has spatial variations, because run-up heights are controlled by coastal topography including local-scale landforms such as natural levees, in addition to land use. Focusing on relationships among coastal topography, land conditions, and tsunami run-up heights of historical tsunamis—Meiji Sanriku (1896 A.D.), Syowa Sanriku (1933 A.D.), and Chilean Sanriku (1960 A.D.) tsunamis—along the Sanriku coast, it is found that the wavelength of a tsunami determines inundation areas as well as run-up heights. Small bays facing the Pacific Ocean are sensitive to short wavelength tsunamis, and large bays are sensitive to long wavelength tsunamis. The tsunami observed off Kamaishi during the 2011 off the Pacific coast of Tohoku Earthquake was composed of both short and long wavelength components. We examined run-up heights of the Tohoku tsunami, and found that: (1) coastal areas north of Kamaishi and south of Yamamoto were mainly attacked by short wavelength tsunamis; and (2) no evidence of short wavelength tsunamis was observed from Ofunato to the Oshika Peninsula. This observation coincides with the geomorphologically proposed source fault model, and indicates that the extraordinary large slip along the shallow part of the plate boundary off Sendai, proposed by seismological and geodesic analyses, is not needed to explain the run-up heights of the Tohoku tsunami. To better understand spatial variations of tsunami run-up heights, submarine crustal movements, and source faults, a detailed analysis is required of coastal topography, land conditions, and submarine tectonic landforms from the perspective of geomorphology.
Generally, the impacts of a tsunami can be understood in terms of the corresponding relationships between hazard scale and extent of building damage and loss of human life. Focusing on community-scale statistics of the municipalities of Kamaishi, Kesen'numa, Minami-sanriku, and Yamamoto, which were severely affected by the 2011 Tohoku Earthquake, significant variations are observed in the mortality rates at affected villages that experienced the same levels of building damage. Moreover, differences in the geographical locations of the villages also impact their mortality rates. A village with a high mortality rate is situated either on a coastal plain or on an inland valley plain some distance from the sea, whereas a village with a low mortality rate is paradoxically in close proximity to the sea, often facing a small bay. Close interrelationships are identified among geographical conditions and possible evacuation activities, which relate to the geographical imaginations of inhabitants based on their inherent local knowledge and interactions with physical and built environments. Interviews with survivors indicate decisions to escape could not be made quickly after the earthquake occurred. Rather, people were confused regarding the occurrence and magnitude of the tsunami and the provision of evacuation sites. The tsunami waves were often different from those that were formally forecast and broadcast, which compelled people to act flexibly. Consequently, it is argued that it is necessary to emphasize the effects of local geographies on interactions between tsunami waves and evacuation activities from a grassroots perspective when preparing for future tsunamis.
The Geospatial Information Authority of Japan (GSI) supports victims and regions affected by the 2011 off the Pacific coast of Tohoku Earthquake, by promptly surveying and providing geospatial information such as maps and aerial photos. In particular, the “Tsunami Flood Area Overview Map” is useful for various geographical analyses of tsunami damage. This paper summarizes the results of analyses of the relationships between tsunami damage interpreted by aerial photos and geographic conditions such as land condition, elevation and land use. Based on our GIS-based overlay analyses of relationships between tsunami damage levels and other geographic data such as inundation depth, landform classification, elevation and land use, the following are clarified: 1) damage levels are closely related to inundation depths; 2) completely destroyed areas are located within 1 km from the coastline; 3) differences in landform and land use of coastal areas influence the damage level of the hinterland area.
The 2011 Tohoku-oki tsunami caused serious damage on the Pacific coast of the Tohoku district. This study focuses on fallen utility poles to clarify tsunami behavior in the northern and central Sendai coastal plain, because utility poles are distributed throughout the plain in high densities. The locations of utility poles are identified using high-spatial resolution satellite images acquired before the tsunami. The fallen poles and the direction in which they fell are interpreted by analyzing aerial photographs taken immediately after the tsunami. Fallen and/or missing utility poles are common within approximately 2 km of the coastline. The area corresponds to an inundation depth of 2 to 3 m. In addition, almost all of the poles fell to landward and are almost perpendicular to the shoreline. This indicates that they fell during the run-up flow. It seems that fallen poles in the northern part of the study area (Nanakita River to Sendai Airport) occur further inland than in the southern part (Sendai Airport to Abukuma River). In particular, fallen poles occur more than 2.5 km from the coastline at Yuriage to Kozukahara, Natori City, where the death rate was also relatively high. In contrast, fallen poles were less common within 1 to 1.5 km from the coastline at the southern part of Iwanuma City, where the death rate was relatively low. Although the height of the tsunami (i.e., scale of hazard) varied along the coast, the locations and scales of settlements near the shoreline specified the amount of rubble produced by the tsunami, and possibly influenced differences in the number of fallen poles.
This study examines the characteristics of landforms affected by the 2011 tsunami along the rocky coasts of the Sanriku area in northeastern Japan. We use terrestrial laser scanning to take detailed topographic measurements of slopes near the coast. As unique topographic characteristics, small cliffs in bedrocks frequently found near the height of the maximum tsunami surface along the coastal slopes are ascribed to cumulative erosion by repeated tsunamis in the mid to late Holocene. Shallow landslides whose scarps are located near the tsunami surface also suggest that they were triggered by water saturation and/or stripping by the tsunami flow. Such characteristic landforms require further careful analysis, not only on the Sanriku coast but also in other areas with rocky coasts, to estimate the effects of repeated tsunamis on bedrock morphology.
Landform classification data are useful for assessing land liquefaction. Koarai et al. (2013) suggested a comprehensive risk assessment table for land liquefaction by combining 7.5-arc-second Japan engineering geomorphologic classification data (Wakamatsu and Matsuoka, 2009) with seismic intensity. The Geospatial Information Authority of Japan (2007) suggested a risk assessment standard for land liquefaction using land condition data produced by the Geospatial Information Authority of Japan. Our new hazard assessment standard for land liquefaction is based on land condition data and a risk assessment table produced by Koarai et al. (2013). Furthermore, a landform classification and hazard assessment standard of land liquefaction is suggested to create a simple land liquefaction hazard map. This information allows land liquefaction hazard to be assessed from land condition data or 7.5-arc-second Japan engineering geomorphologic classification data and to interconvert both land liquefaction hazard assessments.
The 2011 off the Pacific coast of Tohoku Earthquake liquefied large areas of the Kanto region. At the reclaimed land of Urayasu and Mihama, Chiba prefecture, liquefaction tended to concentrate in sandy land-fill layers covering thick alluvium. Alluvial plains along the lower reaches are identified as depositional surfaces of the coastal prism (CP). The CP is sandwiched between the present and the last glacial river-profiles. In Japan, the last glacial rivers developed basal gravel layers (BG) along the bottom of the CP. A thick CP with BG lengthens the secondary seismic wave period and its duration because of a slow s-wave velocity and multi-reflection, resulting in increased internal water pressure and liquefaction of the upper sandy layer of the CP. Historic liquefaction sites show close relations with the distribution of the CP. Subduction-zone large earthquakes caused repeated liquefaction in an alluvial plain where the CP was more than 30 m thick. The inland limit of the liquefaction area roughly coincides with the upstream edge of the CP. The three largest rivers in the Kanto regionof Kinu River, Ara River, and Naka River (Furutone River) have the longest CP in Japan. As a result, the Great East Japan Earthquake liquefied large areas of the Kanto region. The Earthquake also liquefied inland basins such as the Koriyama basin with late Quaternary lacustrine sediments. This demonstrates that a mega-thrust earthquake has the potential to liquefy inland sediment-fill basins beyond the inland limit of the CP.
The 2011 off the Pacific coast of Tohoku Earthquake brought severe damage caused by wide-ranging soil liquefaction across the Tohoku and Kanto districts. In Abiko City, Chiba Prefecture, heavy liquefaction damage occurred in a small area, which was not accurately predicted by the existing liquefaction hazard map. The authors consider the reason to be land history, such as landform development and artificial landfill, which was not taken into account in the assessment of liquefaction risk. Accordingly, Abiko City has revised the hazard map to take account of micro-landform classifications using land condition maps, old edition maps, and aerial photographs with the assistance of the Geospatial Information Authority's support team.
Building damage resulting from unequal settling caused by earthquakes occurs often on embankments and along cut-and-fill boundaries in hilly areas. To predict and prevent damage, determining the location of cut-and-fill boundaries is crucial. Boundaries can be determined by comparing digital elevation models (DEMs) of present-day and old terrain. The accuracy of the boundary is dictated by the accuracy of the old terrain data, because present-day 5-m DEMs created from airborne lidar are highly accurate. The Geospatial Information Authority of Japan (GSI) has published procedures formeasuring old terrain, but accuracy is not addressed directly. We create 5-m DEMs from a 1:3000-scale old topographic map published in 1956 and from aerial photographs taken by the GSI in 1965, by a private corporation in 1956, and by the US military in the late 1940s, and validate their accuracy by comparing unchanged areas. There are fourmain outcomes. (1) The highest accuracy (SD = 0.6 m) is achieved using the aerial photograph taken in 1965. The lowest accuracy (SD = 1.14 m) is obtained using the 1:3000-scale topographic map. (2) The accuracy of photogrammetry depends not only on the scale of a photograph but also on the base-to-height ratio. (3) Allmeasures of accuracy are within the range specified in the GSI manual, but better accuracy might be achieved using 1:20000-scale photographs. (4) The accuracy provided when using the 1:3000-scale map is not satisfactory for predicting and preventing damage. Photogrammetry should be carried out where possible to determine the locations of cut-and fill boundaries.