5:15 PM - 7:15 PM
[HDS11-P03] Interpretation and significance of ground structure using high-density microtremor array observation for hazard risk assessment
Keywords:Hazard Risk Assessment, Subsurface structure, Microtrmor array observation
Introduction
Velocity structure models of the ground from the seismic base to the surface are used to predict earthquake ground motions for earthquake disaster prevention. Most of these models are constructed on a 250 m mesh, but the actual ground structure, especially below the engineering base, often changes within a range smaller than this scale, and a more accurate model is needed. In this report, we describes the detailed irregular structure of the S-wave velocity structure equivalent to the basement surface of the Alluvium and the associated vibration characteristics of the ground.
Interpretation and significance of ground structure inferred from high-density microtremor array observations
1.Interpolation and refinement of topographic and geological information
The top depth of the velocity layer of the S-wave velocity structure (300 m/s, etc.) obtained by microtremor survey consists of shallow flat areas and deep trough-like areas. AVS30 is large in the former and small in the latter. Corresponding to the borehole data scattered around the area, the Alluvial basement surface is shallow in the former and deep in the latter. It is difficult in principle to infer the structure of such a buried topographic surface from discrete borehole data alone.
2.Contribution to the elucidation of the formation process of the ground on the plateau and lowlands
The buried valleys were formed during the peak of the glacial sea-level lowering period, and the buried flat surfaces were formed during the preceding period when the sea level was lowering. The present study indicates that similar buried topographic surfaces are distributed in the subsurface of the valley floor lowlands around and within the plateaus in the southern Kanto region, except for the Tokyo Lowland and the Nakagawa Lowland. This is an important finding for elucidating the late Quaternary geologic history of this area. Further studies on the regional contrast of buried topographic surfaces and the timing of their formation are desirable.
3.Contribution to improving the accuracy of earthquake motion prediction
Identification of buried valleys where thick alluvium is distributed and amplification of shaking in the frequency band that causes damage to houses is large, and the distribution range of buried flats where amplification of seismic shaking in this frequency band is not large, is essential for accurate prediction of seismic motions in urbanized alluvial lowlands where population and structures are densely built up. The tight-lattice microtremor survey can obtain this information more efficiently and at a lower budget than other methods.
Future Development
Microtremor observation is a method with low epistemic uncertainty that does not depend on the skill of the surveyor and analyst. Microtremor observation is a method with low epistemic uncertainty that does not depend on the skill of the surveyor or analyst. Microtremor observation at many sites is useful not only for earthquake disaster prevention but also for understanding fundamental geotechnical structures.
Velocity structure models of the ground from the seismic base to the surface are used to predict earthquake ground motions for earthquake disaster prevention. Most of these models are constructed on a 250 m mesh, but the actual ground structure, especially below the engineering base, often changes within a range smaller than this scale, and a more accurate model is needed. In this report, we describes the detailed irregular structure of the S-wave velocity structure equivalent to the basement surface of the Alluvium and the associated vibration characteristics of the ground.
Interpretation and significance of ground structure inferred from high-density microtremor array observations
1.Interpolation and refinement of topographic and geological information
The top depth of the velocity layer of the S-wave velocity structure (300 m/s, etc.) obtained by microtremor survey consists of shallow flat areas and deep trough-like areas. AVS30 is large in the former and small in the latter. Corresponding to the borehole data scattered around the area, the Alluvial basement surface is shallow in the former and deep in the latter. It is difficult in principle to infer the structure of such a buried topographic surface from discrete borehole data alone.
2.Contribution to the elucidation of the formation process of the ground on the plateau and lowlands
The buried valleys were formed during the peak of the glacial sea-level lowering period, and the buried flat surfaces were formed during the preceding period when the sea level was lowering. The present study indicates that similar buried topographic surfaces are distributed in the subsurface of the valley floor lowlands around and within the plateaus in the southern Kanto region, except for the Tokyo Lowland and the Nakagawa Lowland. This is an important finding for elucidating the late Quaternary geologic history of this area. Further studies on the regional contrast of buried topographic surfaces and the timing of their formation are desirable.
3.Contribution to improving the accuracy of earthquake motion prediction
Identification of buried valleys where thick alluvium is distributed and amplification of shaking in the frequency band that causes damage to houses is large, and the distribution range of buried flats where amplification of seismic shaking in this frequency band is not large, is essential for accurate prediction of seismic motions in urbanized alluvial lowlands where population and structures are densely built up. The tight-lattice microtremor survey can obtain this information more efficiently and at a lower budget than other methods.
Future Development
Microtremor observation is a method with low epistemic uncertainty that does not depend on the skill of the surveyor and analyst. Microtremor observation is a method with low epistemic uncertainty that does not depend on the skill of the surveyor or analyst. Microtremor observation at many sites is useful not only for earthquake disaster prevention but also for understanding fundamental geotechnical structures.