Presentation Information
[U09-P01]Subsurface geology and disaster risk in urban areas; what does the subsurface geology directly beneath your house consist of?
*Toru Takeshita1 (1.Adviser, Land Infrastructure Division, Pacific Consultants Co., Ltd.)
Keywords:
Subsurface geology and disaster risk in urban areas,disaster risk literacy,generation mechanisms of disaster,earthquake-related disaster,collapse of roads,debris flow
The Japanese Islands mostly consist of mountains and hills (up to c. 70%), where the populations are concentrated in river terrace and alluvial plain ranging between 0 and 100 m in altitude, which occupies only one fourth of the Islands. As such, in daily life it is important to think about what kinds of natural and ground disasters could occur in urban areas, and how we can prepare for them. However, considering so many people died in the tsunami caused by the Tohoku Pacific Ocean Earthquake, the disaster risk literacy has not become widespread among the citizen. Accordingly, I aim to build the information resources about subsurface geology and disaster risk in urban areas, so that the citizen can share them.
In the disaster mitigation, it is of central importance to understand the generation mechanisms of disaster, because we cannot fully take measures against it without knowing them. For example, in the homepage of Ministry of Land, Infrastructure, Transport and Tourism, a figure showing the tendency of liquefaction relating to the divisions of geomorphology is attached. In this figure, it is shown that liquefaction more tends to occur in natural levee than back marsh. However, to understand this fact, it is necessary to understand the transport and sedimentation by flooding, and the differences in mechanical properties between soils consisting of sand and mud. Similarly, for other subsurface geology and disaster risk in urban areas, it is indispensable to understand the generation mechanisms of disaster to mitigate it. In this presentation, I will talk about the generation mechanisms of earthquake-related disaster, collapse of roads and debris flow.
Since ground disaster in urban areas is greatly affected by subsurface geology, it is first of all necessary to know it. In contrast to the strata constituting mountains formed at old geological ages, river terraces and alluvial plains, on which cities were built, consist of unconsolidated Quaternary sediments. For the 1995 Southern Hyogo Prefecture Earthquake, the disaster belt with the width of 1-2 km and length of 20 km was generated along the coast of Kobe, where a seismic intensity of 7 has been reported. This belt has been now interpreted to have been caused by the very-thick Quaternary sediments having been shaken by the earthquake, and the seismic waves amplified. Generally, such weak ground consists of alluvial sediments. If one looks at seismic intensities as a function of grain size of alluvial sediments, mud sediments are weaker (N-value<5) and thus give rise to higher seismic intensities than sand and gravel sediments (Ishikawa et al., 2000, The Quaternary Research). Inversely, liquefaction more tends to occur in sand layers than mud layers, because the former is more permeable than the latter. On the other hand, diluvial deposits are stronger, and thus less affected by earthquake disasters. Therefore, they can be suitable subsurface ground for residential areas. Road collapse which has recently occurred in Yashio city, Saitama prefecture could be a similar disaster to liquefaction caused by earthquake. The road collapse was more directly related to the corrosion of a sewer pipe, and its resultant breakage and formation of holes, from which sediments outflew. However, it is important to note that alluvial sediments deposited under the sea which invaded the area during the period of the Jomon transgression made the outflow of sediments easier, because the sediments consisting of very-fine sand mixed with silt grains are not only weak (N-value<5), but also relatively permeable. Finally, with the increasing populations in urban areas, residential land development is performed on the foot of the mountains. These areas are susceptible to not only ground disasters if cut and fill of the lands is performed, but also debris flow because of the proximity to mountains.
In the disaster mitigation, it is of central importance to understand the generation mechanisms of disaster, because we cannot fully take measures against it without knowing them. For example, in the homepage of Ministry of Land, Infrastructure, Transport and Tourism, a figure showing the tendency of liquefaction relating to the divisions of geomorphology is attached. In this figure, it is shown that liquefaction more tends to occur in natural levee than back marsh. However, to understand this fact, it is necessary to understand the transport and sedimentation by flooding, and the differences in mechanical properties between soils consisting of sand and mud. Similarly, for other subsurface geology and disaster risk in urban areas, it is indispensable to understand the generation mechanisms of disaster to mitigate it. In this presentation, I will talk about the generation mechanisms of earthquake-related disaster, collapse of roads and debris flow.
Since ground disaster in urban areas is greatly affected by subsurface geology, it is first of all necessary to know it. In contrast to the strata constituting mountains formed at old geological ages, river terraces and alluvial plains, on which cities were built, consist of unconsolidated Quaternary sediments. For the 1995 Southern Hyogo Prefecture Earthquake, the disaster belt with the width of 1-2 km and length of 20 km was generated along the coast of Kobe, where a seismic intensity of 7 has been reported. This belt has been now interpreted to have been caused by the very-thick Quaternary sediments having been shaken by the earthquake, and the seismic waves amplified. Generally, such weak ground consists of alluvial sediments. If one looks at seismic intensities as a function of grain size of alluvial sediments, mud sediments are weaker (N-value<5) and thus give rise to higher seismic intensities than sand and gravel sediments (Ishikawa et al., 2000, The Quaternary Research). Inversely, liquefaction more tends to occur in sand layers than mud layers, because the former is more permeable than the latter. On the other hand, diluvial deposits are stronger, and thus less affected by earthquake disasters. Therefore, they can be suitable subsurface ground for residential areas. Road collapse which has recently occurred in Yashio city, Saitama prefecture could be a similar disaster to liquefaction caused by earthquake. The road collapse was more directly related to the corrosion of a sewer pipe, and its resultant breakage and formation of holes, from which sediments outflew. However, it is important to note that alluvial sediments deposited under the sea which invaded the area during the period of the Jomon transgression made the outflow of sediments easier, because the sediments consisting of very-fine sand mixed with silt grains are not only weak (N-value<5), but also relatively permeable. Finally, with the increasing populations in urban areas, residential land development is performed on the foot of the mountains. These areas are susceptible to not only ground disasters if cut and fill of the lands is performed, but also debris flow because of the proximity to mountains.