We applied a new method for estimation of displacement distribution around the earthquake fault occurred by 2011 Fukushima Hamadori Earthquake, using airborne LiDAR data before and after the earthquake. The results obtained by our method almost conform to distribution of net fault displacement shown by field investigations. Furthermore, the results suggest block tilting and land subsidence caused by the earthquake. Our new method can provide information difficult to get ordinary methods of field investigation, and consequently useful for investigation of displacement distribution just after earthquakes.

View full abstract

 The report on field observations of landslide movements in residential area induced by the 2011 off the Pacific coast of Tohoku Earthquake (M9.0) was conducted. The Midorigaoka 4-chome (Midorigaoka #4) subdivision in the southern part of Sendai City sprawled across the flatland stretching between the terrace and valley floor. The large landslides, the Midorigaoka #4 landslide, occurred in the widening fills of the lower part of this subdivision. The fills consist of soft and loose mixed bedrock material, sand, clay, and sandy silt with gravel. The humid top soil of original ground surface was found at the boundary of the fill and bedrock. The ground water level was very shallow-0.5 m to 1.1 m below the ground surface-indicating that the fills were nearly saturated by ground water. Measurements on ground inclination using borehole inclinometers, and pore water pressure changes were made from June 2011 with high precision time interval of 100 Hz. Surface seismic velocity was observed at the head of the landslide in 200 Hz. The longtime slow landslide movements including bedrocks were observed from the beginning of the observations until December 2011 to January 2012 as the after effect of the main shock. The inclination rate at the top soils was greater than two times of the rate at the bedrock. The contrast in strength between the fill and bedrock is clear, and soft top soils exist at the boundary. Thus the major landslide is thought to move along the bottom of the fill, and the top soils should be considered as the slip layer of the landslide. The largest seismic response of inclination was found at the weak layers, at the top soils of the base of fills and at the fragile lignite layer of bedrocks. In contrast, the reduction of response at the upper part of fills was remarkable in case that the weak layers developed at the lower. These results indicate the self-dumping at the weak layers in ground structure. The effects of self-dumping varied depend on the thickness of weak layers and distance from epicenters. Excess pore water pressure in the landslide increased in direct proportion to horizontal peak ground velocity (PGV) during earthquakes. This relation suggests that the landslide was initiated by the fully loss of shear strength of slip layer caused by increase of excess pore water pressure during the strong seismic motion over 80 cm/s of PGV of the main shock. The failure process at the slip layer of landslide developed from small local failures to landslide movement was found by chance. The performance of landslide during strong motion has not been discussed based on observed facts. Precise field observation on landslide movement during earthquake should be significant to mitigate the landslide risk in urban region.

View full abstract

 Slope disaster in urban residential region happened a lot by “The 2011 off the Pacific coast of Tohoku Earthquake”. These slope disasters brought serious damages such as complete to half collapse houses and broken lifelines, mainly caused by movement of reclaimed area by valley fill or widening of embankment. As a purpose of preventing earthquake disaster at reclaimed area that exist a lot in urban regions in Japan, the features of each slope disaster are categorized in five types and that of each damages are put together based on the investigation results gotten from 42 places. These five types are 1) movement of artificial valley fills, 2) movement of widening of embankment, 3) movement of artificial valley fills and widening of embankment, 4) earth flow of embankment and 5) surface slide. The surface slide that was classified as Type 5 was a damage that appeared to be clear in this earthquake ; the slope become unstable by liquefaction of the foot of slope. Each type of damages happens in the embankment area. Effectiveness of landslide prevention works for reinforcement against earthquake that was constructed after “1978 Miyagiken-oki earthquake” was examined. The examination showed that landslide prevention works (pile work and groundwater drainage work), was effective for large deformation of embankment slope. However, since only landslide prevention works could not control deformation around the foundation of house, it appeared that development of prevention works to improve movement resistance around it is necessary here after. In addition, it appeared to be possible to control mass movement by making a part of uniformly weak movement mass stronger, leading its flank resistance to be increased.

View full abstract

 In this paper, the paleo-environmental changes were presumed by the diatom assemblages taken from the tsunami borehole deposits of 1960 Chili Earthquake Tsunami at the three sites in the Kesennuma Bay, along the Pacific Coast, north east Japan. The tsunami deposits contained a lot of fresh water diatom valves, and their volumes to the all diatom valves exceed 60 %. Because the average frequency of fresh water diatoms in the present bay surface deposits are about 20 %, the tsunami deposits composed by mainly inland deposits those were washed into the bay by backwashed current of tsunami waves. In addition to it, marine species such as Thalassiosira spp., those live at offshore areas along the Pacific Coast increased cyclically in the cores of tsunami deposits. Those peaks of marine species presumably showed the repetition of tsunami waves during the tsunami hazard. The 1960 Chili Earthquake Tsunami continued about two days, and tsunami waves attacked with 1-2 hours intervals to the Kesennuma area.
 The three cores of tsunami deposits could not reach the basement of the tsunami deposits presume by diatom assemblages. We planned the further drilling in this area, however the bay topography of the bay was drastically changed by the East Japan Earthquake Tsunami, caused on March 11, 2011. Our drilling data archived the sedimentary conditions of the bay just before the huge tsunami in 2011.

View full abstract

[in Japanese]

View full abstract

;

View full abstract