The group of researchers found out possible active fault underneath the center of Tokyo. There are numerous buildings and highways above the active fault. The fault has been active for 200,000 years.
Mr. Toyokura, the former engineering chief of Dia consultants CO., Ltd. and his fellows collected more than 1,000 strata data to analyze. The strata data was from historical boring survey for past building construction and engineering works.
The fault runs from JR Iidabashi station for 7km in the direction of NNE-SSW.
The data taken from Bunkyoku showed 2m gap of the loamy layer to contain volcanic ash from 70,000 ~ 80,000 years ago.
The sand layer from over 90,000 years ago had 3.5m gap, “Tokyo layer” from 200,000 years ago had 12m gap.
Mr. Toyokura comments,
“The deeper the layer is, the bigger gap it has, which shows the faut moved several times. Probably it moved 4 times from 90,000 to 200,000 years ago. The type of fault is the normal fault.”
(*A normal fault is one where the fault dips toward the downthrown block. Normal faults occur in rifted terranes, such as Mid-Ocean Ridges, the African Rift, and the Basin and Range Province of western North America. It is shown on geologic maps as a black line with either a block pattern on the downthrown side, or the letters U/D showing the upthrown and downthrown sides. [Link] In Japan, normal fault is supposed to be less active.)
Prof. Yamazaki from Tokyo Metropolitan University is a little more conservative. He comments, “This kind of fault can be made by other causes but earthquake. We need more data to judge if it’s really from earthquakes.”
Assistant professor, Ikeda from Tokyo university talks, “Normal fault is in Boso peninsula, Chiba too, and the cause is not known. The normal fault in Fukushima prefecture was actually moved because of 311, a great earthquake in an ocean trench may move those normal fault too. ”