Results of Underground Step Structure Survey in northern Yokohama

About result of underground step structure investigation of northern Yokohama-shi
－ It turns out that the active fault will cause an earthquake in the near future -

Yokohama City collects seismic observation data in Yokohama City and conducts research and research on the mechanism of earthquake occurrence in order to implement scientifically based on earthquake disaster countermeasures.
As part of this, in order to predict the shaking of earthquakes in each region with high accuracy, the aim of grasping the underground structure up to the hard rock in the deep underground, called the earthquake base, from 1998 to 2000, We carried out underground structure investigation (PDF: 704KB) in the city. Based on the results of this survey, it was confirmed that the earthquake base at a depth of about 3 km underground in the northern part of the city is about 1 km deep to the north.
Aoba Ward and Tsuzuki Ward  , where the step structure is expected to pass through with specialized and technical guidance, the Ministry of Education, Culture, Sports, Science and Technology (Chairperson: Kenichi Kojima, Professor, Yokohama City University) was established in 2001 to confirm the direction and length of this step structureactive fault.

Based on the results of the survey so far, the following was confirmed.
(1)　This step structure is fault, which extends east-west through the Aoba Ward, and the deflection of the stratum becomes smaller as it moves eastward.
(2)　The fault has been suspended since about 120,000 years ago and is not the active fault that will cause earthquakes in the near future.

※Location map of underground step structure survey (PDF: 148KB)

[On-site survey (Reflective seismic survey)]
Seismological waves were artificially generated by a seismic vehicle, and the reflected waves reflected from the surface at the boundary of each stratum were measured with a seismometer installed densely on the surface of the ground, and the structure of the underground was investigated. From the survey results, we confirmed that this step structure is fault, which extends east-west through Aoba Ward.

Analysis of boring materials:
Based on past boring materials, we created a cross-sectional view of the area around which fault passes, and investigated the displacement of the stratum accumulated by fault's activities. From the survey results, it was confirmed that the activity of the fault was stopped because no displacement of the stratum about 120,000 years ago.

◆What is active fault?
The stratum and topography record traces of earthquakes long ago are recorded. If there is a discrepancy between the top, bottom, left and right in the stratum and topography, which should have been a series of seismic activity in the past. Such stratum and topography deviations are called fault, and fault, which has been repeatedly active during the recent geological age and isactive fault.
The Japanese Islands are said to have about 2,000 active fault. There are about 30 active fault in Kanagawa Prefecture, but there is no active fault in Yokohama. (An active fault survey was conducted in the two years of 2007 and 8, and as a result, it was confirmed that Tachikawa fault had not extended to Yokohama city area.)