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As a result of step structure investigation in northern Yokohama-shi under the ground

Last update date March 12, 2020

About result of step structure investigation in northern Yokohama-shi under the ground
- When is not active fault which will cause earthquake in the near future; revelation -

In Yokohama-shi, we conduct research about collection of seismometry data in Yokohama city limits and earthquake occurrence mechanism to carry out earthquake disaster measures with grounds scientifically.
For the purpose of grasping basement structure to firm bedrock in basement deep part called earthquake base, we hung in 2000 from 1998 to predict shaking of earthquake every area with high precision as the part and carried out basement structure investigation (PDF: 704KB) in the city. Step structure that earthquake base in depth of about 3km below ground was deepened about 1km from this findings to the north side in the city northern part was confirmed.
In Aoba Ward predicted that step structure passed and the Tsuzuki Ward  , we utilized grant of Ministry of Education, Culture, Sports, Science and Technology while installing Yokohama-shi basement step structure investigation committee (chairperson: Kenichi Kojima Yokohama City University professor) consisting of people of learning and experience for the purpose of what this step structure intended to spread and we confirmed whether it was length, active fault which would cause earthquake in the future again in 2001, and receiving specialized technique-like instruction and carried out field work or analysis of bowling information.

From past findings, next was confirmed.
(1) This step structure is fault extending to East-West directions through the Aoba Ward, and bend of stratum becomes small as we go to the east direction.
(2) Activity stops after 120,000 years ago, and this fault is not almost active fault which will cause earthquake in the near future.

※Step structure investigation position figure (PDF: 148KB) under the ground

Summary of investigation

[field work (anti-shooting method earthquake exploration)]
We produced earthquake wave with vibromotive car artificially and we measured with seismometer which installed reflection wave that it reflected on boundary surface of each stratum from the surface of the earth in the surface of the earth thickly and investigated underground structure. From findings, this step structure confirmed that it was fault which extended to East-West directions through Aoba Ward.

[analysis of bowling material]
Based on bowling material of the past, we made stratum cross section around the area that fault passed and investigated displacement of stratum which accumulated by activity of fault. Because displacement of stratum after 120,000 years ago was not almost accepted by findings, we confirmed when activity of this fault stopped.

◆With active fault
In stratum and the topography, sign of earthquake from old days is recorded far. In stratum and the topography which should have been continuity, is originally past if there are any discrepancy to top and bottom and right and left; of seismic activity there might be. We call gap of such stratum and topography fault and work repeatedly for the recent geological age and call estimated fault with active fault when we will work in the future.
It is said that there is about 2,000 active fault in the Japanese Islands. There is about 30 active fault in Kanagawa, but there is no active fault in Yokohama city limits.

◆Conduct (from 1995 to 1996) of active fault research
Because step was estimated in microearthquakes not to feel for the human body occurring frequently and deep part of city limits under the ground near Kawasaki-shi boundary in northern Yokohama-shi, and it was thought that that was located for extension of Tachikawa fault which was active fault, we performed active fault research in two years in 1995, 1996. As a result, it was confirmed that Tachikawa fault did not extend to Yokohama city limits.

◆Constitution of Investigation Committee for Yokohama-shi basement step structure

Committee member constitution
PostBelongingFull name
ChairpersonProfessor Yokohama City University Graduate School synthesis science graduate courseKenichi Kojima
Vice chairpersonProfessor Yokohama City University Graduate School synthesis science graduate courseMasanori Saito
Vice chairpersonEarthquake Res. Inst., Univ. of Tokyo's professorMasayuki Kikuchi
Committee memberProfessor Tokyo Institute of Technology Graduate School synthesis department of science and engineering graduate courseYoshihiro Kinugasa
Committee memberProfessor Tokyo Institute of Technology Graduate School synthesis department of science and engineering graduate courseKazuhiro Seo
Committee memberYokohama City University Graduate School synthesis science graduate course assistant professorSeiji Tsuboi
Committee memberManager Kanto Gakuin University economicsIware Matsuda
Committee memberProfessor Tokyo Institute of Technology Graduate School synthesis department of science and engineering graduate courseSaburo Midorikawa
Committee memberTokyo Metropolitan University Graduate School science graduate course professorHaruo Yamazaki
Committee memberTokyo Institute of Technology Graduate School synthesis department of science and engineering graduate course assistant professorHiroaki Yamanaka

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