Yokohama City Area active fault Basic Survey Results Report

The presence of active fault has not yet been confirmed in Yokohama.
However, small earthquakes occur frequently near the border between Kawasaki City and Kawasaki City, and steps are estimated to be deep underground in Yokohama,faultactive fault.
Such data suggested the possibility that Tachikawa fault was extended to Yokohama.
For this reason, we conducted a survey on the presence or absence of active fault in Yokohama.

Tachikawa fault is the active fault extending from northwest to southeast from Musashimurayama, Tokyo, Tachikawa and Kunitachi City in Kitakata, Yokohama.
Previous studies have shown that Tachikawa fault will cause magnitude 7 class earthquakes approximately once every 5,000 years. The southern tip of Tachikawa fault is said to be near the Tama River in Kunitachi City.
To find active fault, you can look at aerial photographs, find small steps in the terrain, and estimate active fault from its continuity.
On the other hand, universities and others generated an artificial earthquake before and searched the geological structure deep underground from the way the waves were transmitted, and it was reported that there was a step in the old stratum around 5 to 7 km depth of Yokohama City. Was. This step is located on the extension of Tachikawa fault.
Abnormality abnormalities may be observed around fault.
On Earth, gravity is often thought to be the same everywhere, but in reality it varies slightly depending on the location. If there is a substance that is heavier than the surrounding area, the gravity on the ground surface will be larger than normal, and if there is a light material, it will be smaller. Therefore, when a heavy stratum deep underground rises to a shallow part due to the fault movement, a gravity value larger than normal is observed on the surface.
The difference in weight is clearly shown along the Tachikawa fault, where the abnormal gravity (difference from the standard values) measured on the surface of Yokohama, and a distribution map (Fig. 1) was created. This gravity difference band extends in the southeast direction and appears to reach Yokohama City.

Figure 1 Distribution Diagram

Based on these data, we conducted the survey as shown in the next section.

The fault survey was conducted in cooperation with Kawasaki City. Kawasaki City conducted seismic surveys using a rebellion method for deep underground areas. Yokohama City conducted shallow reflections and gravity exploration in shallow areas. Figure 2 shows the location of each survey.

Fig. 2 Survey Location Map

（１） Gravity exploration
Analysis using existing gravity data showed that the gravity difference band extended to Yokohama city area on the extension of Tachikawa fault. In Yokohama City, approximately 1500 new gravity measurements were taken in a range of 5 km wide and 20 km long, from Takatsu Ward, Kawasaki City to Asahi Ward, Yokohama City. As a result (Fig. 3), the distribution of abnormal gravity is similar to the existing gravity data analysis results.

Figure 3 Gravity Measurement Results

(Gravity abnormal distribution map)

（２） Earthquake exploration by anti-injection
This was done by Kawasaki City, where vibration is applied from the ground surface and captures the vibrations that have been reflected in the underground stratum with a seismometer to explore the underground geological structure. Figure 4 shows an overview of the seismic survey using the rebellion method.

Fig. 4 Overview of Earthquake Exploration Using Reflecting Method

The rebellion exploration was carried out on a side line of about 15 km from Noge, Setagaya-ku, Tokyo, across the Tama River, and through Kohoku New Town to Kamikawa-cho, Asahi-ku.

Status of anti-injection exploration

The white and black stripes show the reflective surfaces in the refraction (Fig. 5), the white and black stripes show the reflected surface. The reflected surface is thought to represent differences in geological formations and geological structures.

Fig. 5 Reflectivity Survey Results

Figure 6 shows the results of a rebellion survey conducted at Nojima fault Nojima, which was active in the Hyogoken Nanbu Earthquake. The deviation of the reflected surface is clearly visible. According to the results of Yokohama city area, continuous reflective surfaces is observed in shallow places (less than 1500 m deep). This indicates that the stratum is continuous and there is no fault. However, there are three places where the reflected wave is disturbed and the continuity is unclear. It is not possible to judge whether or not fault exists. For this reason, more accurate shallow reflections were conducted for these three locations.

Figure 6 Results of Reaction Investigation at Nojima fault Nojima

（３） Shallow reflexplosion
The principle of shallow reflection is the same as that of anti-reflective exploration, but by shortening the wavelength of the vibration, we obtain accurate data in shallow parts. There were three locations where the data was unclear in the rebellion exploration. As a result of the shallow reflection (Fig. 7), the reflective surfaces are continuous and no fault.

Fig. 7 Results of shallow Reflective Exploration

In addition, shallow reflections were carried out at a length of 4 km in Aoba Ward, the northern end of Yokohama. This is to confirm that Tachikawa fault has not reached Yokohama by conducting a survey at the northern end of Yokohama City near Tachikawa fault.

Investigation status of shallow reflections

The results are shown in Figure 8 shows that the reflection surface is continuous and there is no fault.

Figure 8 Results of shallow reflections at the northern end of Yokohama City

Based on previous surveys of artificial earthquakes and gravity exploration, the Tachikawa fault could be extended to Yokohama city area. However, the results of anti-reflection and shallow reflections revealed that the stratums distributed in shallow areas are continuous. According to the survey results, since the formation of the Kazusa Group, which began to accumulate about 1.5 million years ago, has not been shifted by faults, the fault has not been active since about 1.5 million years ago, and Tachikawa fault, the active fault, has not been extended to Yokohama.

I would like to express my deepest gratitude to the committee members for their guidance and cooperation in this work.

• Chairperson

Kenichi Kojima (Dean Satoshi, Dean, Yokohama City University)

• Vice Chair

Masayuki Kikuchi (Professor, Earthquake Research Institute, The University of Tokyo) (Visiting Professor, Faculty of Science, Yokohama City University)

• Committee members

Yoshihiro Kinugasa (Chief Researcher, Geological Survey Institutes, National Research Institutes)
Seo Kazuhiro (Professor, Graduate School of Science and Engineering, Tokyo Institute of Technology)
Saburo Midorikawa (Professor, Graduate School of Science and Engineering, Tokyo Institute of Technology)
Matsuda Iwayo (Professor, Faculty of Economics, Kanto Gakuin University)
Haruo Yamazaki (Professor, Faculty of Science, Tokyo Metropolitan University)