Results of an underground step structure survey in the northern part of Yokohama City

About result of underground step structure investigation in the northern part of Yokohama-shi
－ It turns out that active fault is not causing earthquakes in the near future -

In Yokohama City, we collect seismic observation data in the Yokohama city area and conduct surveys and research on the mechanism of earthquake occurrence in order to implement scientifically grounded earthquake countermeasures.
As part of this, from 1998 to 2000, with the aim of grasping the underground structure up to the hard rock deep underground called the earthquake base in order to predict the shaking of earthquakes in each region with high accuracy, We carried out underground structure investigation (PDF: 704KB) in the city. Based on the results of this survey, it was confirmed that the seismic base at a depth of about 3 km underground in the northern part of the city was about 1 km deeper to the north.
In order to confirm the direction and length of this step structure, as well as whether it is a active fault that will cause earthquakes in the future, we established the Yokohama Underground Step Structure Investigation Committee (Chairman: Kenichi Kojima, Professor of Yokohama City University), which is expected to pass the step structure while receiving specialized and technical guidance from the Aoba Ward and Tsuzuki Ward  , where the step structure is expected to pass.

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

※Ground step structure survey location map (PDF: 148KB)

[On-site survey (rejection method earthquake exploration)]
The reflected waves that artificially generate seismic waves with a seismic vehicle and reflect from the ground surface at the boundary of each stratum were measured with a seismometer that was closely installed on the ground surface, and the structure of the underground was investigated. Based on the results of the survey, we confirmed that this step structure is a fault that extends east-west through Aoba Ward.

[Analysis of bowling materials]
Based on the previous boring materials, we created a cross-sectional view of the area where the fault passes, and investigated the displacement of the stratum accumulated through the activities of the fault. Based on the results of the survey, we confirmed that the activities of the fault have been suspended since 120,000 years ago were not observed.

◆What is active fault?
The strata and topography record traces of earthquakes from a long time ago. If there is a discrepancy between the top and bottom and left and right in the strata and topography that should have been a continuation, there may have been earthquake activity in the past. Such gaps in strata and topography are called fault, and fault, which is estimated to be active repeatedly in recent geological eras and will continue in the future, is called active fault.
It is said that there are about 2,000 active faults in the Japanese Islands. There are about 30 active faults in Kanagawa Prefecture, but there is no active fault in Yokohama. (An active fault survey was conducted in the two years of 1995 and 8, and as a result, it was confirmed that Tachikawa fault did not extend to the Yokohama city area.)