In Yokohama city limits, existence of active fault is not confirmed currently.
However, step is estimated in microearthquakes occurring frequently near city boundary with Kawasaki-shi and deep part of Yokohama-shi under the ground, and it is thought that that is located for extension of Tachikawa fault which is active fault.
Such data suggested possibility that Tachikawa fault extended to in Yokohama city limits.
Therefore we investigated presence and activity of active fault in Yokohama city limits.
Tachikawa fault is active fault spreading in southeastern direction from northwest from Musashimurayama-shi, Tokyo, Tachikawa-shi of Yokohama-shi Kitakata to Kunitachi-shi.
In past study, it is thought that Tachikawa fault produces earthquake of magnitude seven classes in ratio once in about 5,000 years. The south end of Tachikawa fault is considered to be the neighborhood of Tama River of Kunitachi-shi.
On seeing aerial photo, we find small steps of the topography to find active fault and estimate active fault from the continuity, but, in Yokohama-shi-jo Castle, the topography is modified by creation and cannot read the small topography.
On the other hand, after universities producing artificial earthquake before, and investigating geological feature structure of deep part under the ground from way of travelling of waves, it was reported to old stratum at around 5-7km in depth of Yokohama-shi that there was step. This step is located on the extension of Tachikawa fault.
Gravitational abnormality may be recognized around fault.
It is apt to be thought that gravity is the same anywhere on the earth, but delicately actually varies according to places. When there is heavy material from the outskirts underground, gravity on the surface of the earth grows big than normal and becomes small when there is light material. Therefore, we go up in shallow place, and, heavy stratum which there was in deep part under the ground, gravity level that is bigger than normal in case, the surface of the earth region is observed by fault exercise.
After arranging abnormality gravity (difference with standard value) measured on the surface of the earth around Yokohama-shi to clarify difference in grade of stratum in basement using this, and making the distribution map (figure -1), weight difference appears along Tachikawa fault clearly. Obi of this gravity difference extends to southeastern direction and seems to reach Yokohama-shi.
Figure -1 distribution map
Based on such data, we performed investigation to show in the next clause.
Method and result of investigation
We performed fault investigation in cooperation with Kawasaki-shi. Kawasaki-shi intended for underground deep part and performed earthquake exploration by anti-shooting method. Yokohama-shi performed *sohanshahosagurisa and gravity exploration targeting at slight parts. In addition, we show conduct position of each investigation in figure -2.
Figure -2 investigation position figure
(1) Gravity exploration
By analysis using existing gravity data, obi of gravity difference extended to Yokohama city limits on the extension of Tachikawa fault. In Yokohama-shi, we performed about 1,500 points of gravity measurement in the range of 5km in width, 20km in length to Asahi-ku, Yokohama-shi newly from Takatsu-ku, Kawasaki-shi. As a result, abnormal gravitational distribution similar to existing gravity data analysis result was detected even in (figure -3).
Figure -3 gravity measurement result
(gravity abnormality distribution map)
(2) Earthquake exploration by anti-shooting method
Kawasaki-shi performed this, and gives vibration from the surface of the earth and arrests vibration that reflected being hit by the underground stratum with seismometer and investigates underground geological feature structure. We show figure of summary of earthquake exploration by anti-shooting method in figure -4.
Figure of summary of earthquake exploration by figure -4 shooting method
We crossed Kawasaki-shi across the Tama River from Noge, Setagaya-ku, Tokyo, and anti-shooting method exploration was carried out with measuring line of about 15km to Kamikawaicho, Asahi-ku through Kohoku new subdivision.
Anti-shooting method exploration state of implementation
By anti-shooting method exploration result (figure -5), white and black striped pattern show reflector. It is thought that reflector expresses stratum and geologic difference.
Figure -5 shooting method exploration result
Figure -6 is result of anti-shooting method exploration that we performed in Nojima fault which was active by South Hyogo earthquake. Gap of reflector appears clearly. By result of Yokohama city limits, a series of reflectors is accepted in shallow place (place shallower than depth about 1,500m). Strata continue, and this shows that there is not fault. However, reflection wave is disturbed, and there are three places of places where continuity is indistinct. We can determine whether fault exists here with neither. Therefore, about these three places, we performed *sohanshahosagurisa with high precision more.
Anti-shooting method exploration result in figure -6 Nojima fault
Principle of *sohanshahosagurisa is the same as anti-shooting method exploration, but obtains good data of precision in superficial part by shortening wavelength of vibration. Position that we explored is inarticulate three places of data by anti-shooting method exploration. Reflectors continue in (figure -7) as a result of *sohanshahosagurisa, and there is no fault.
As a result of figure -7 *sohanshahosagurisa
In addition, *sohanshahosagurisa performed even Aoba Ward equal to the district of Yokohama-shi at the north end by length of 4km. In this, Tachikawa fault is to confirm what we do not reach in Yokohama-shi by place to investigate in the district of Yokohama-shi near Tachikawa fault at the north end.
We show the result in figure -8, but, after all, reflectors continue, and there is no fault.
*sohanshahosagurisaketsuka in the district at the north end of figure -8 Yokohama-shi
Does Tachikawa fault extend to Yokohama city limits?
From result of investigation by artificial earthquake carried out before and gravity exploration, Tachikawa fault might extend to Yokohama city limits. However, it was revealed that strata distributed over shallow place from anti-shooting method exploration and result of *sohanshahosagurisa continued. Because stratum called Kazusa formation that has begun to deposit about 1.5 million years ago does not slip off in findings by dislocation, there is not activity of fault after about 1.5 million years ago, and it is judged that Tachikawa fault which is active fault does not spread in Yokohama city limits.
We install 150 places of seismometers (strong-motion seismograph) in city limits to tell earthquake information to citizen more exactly more distantly in Yokohama-shi. There is plan to estimate structure of Yokohama city limits deep part under the ground from data of earthquake wave to be provided with this seismometer. In this way, we elucidate structure of inarticulate deep part under the ground under the ground so far and utilize as earthquake, the basics data about fault.
We offer my gratitude for teachers of committee which received instruction, cooperation from these duties deeply.
Teachers of committee (honorific title abbreviation)
- Kenichi Kojima (Yokohama City University Dean Satoshi)
- Masayuki Kikuchi (Earthquake Res. Inst., Univ. of Tokyo's professor) (Yokohama City University department of science guest professor)
- Yoshihiro Kinugasa (National Research Institutes geological survey place top study official)
Kazuhiro Seo (Tokyo Institute of Technology Graduate School synthesis science and technology studies graduate course professor)
Saburo Midorikawa (Tokyo Institute of Technology Graduate School synthesis science and technology studies graduate course professor)
Iware Matsuda (Kanto Gakuin University department of economics's professor)
Haruo Yamazaki (Tokyo Metropolitan University department of science's professor)