‟Geologists have rarely enjoyed so good an opportunity as that afforded them by the convulsion in the Mino-Owari plain, of observing those great displacements of rocks called “faults”, which may only be brought about in the course of ages. Only a very few cases have ever been recorded in the annals of earthquakes, of the formation of throws of strata accompanied by subterranean shocks.” (Kotô 1993)
It is with these sentences that Bunjiro Kotô introduces his essential article entitled “On the Cause of the Great Earthquake in Central Japan, 1891.” In this article published in 1893, Kotô first makes an excellent review of what was known, unknown, and imagined at the time about the possible causes of earthquakes. Then, he presents what is probably the very first description of a coseismic fault rupture, that of the Nobi earthquake of magnitude ~Mw7.5, which happened in Central Japan in the morning of the 28th of October, 1891. With this article, Kotô was the first geologist to explicitly link earthquakes to tectonic faults. His seminal field description and article, as well as more recent studies (e.g., Kaneda and Okada 2008), have shown that the 1891 earthquake ruptured three NW-SE trending faults for a length of ~80km and with left-lateral coseismic offsets of several meters.
Roughly in the middle of these 80km, the earthquake scarp at Midori is spectacular. The black and white photograph, first published by Kotô in 1893, perfectly shows the scarp, the offset road and fields. It will become famous.
There is also a more artistic vision of the very same seismo-tectonic landscape: an anonymous fine art watercolour with delicate tones (source: Kyoto University digital archive), most probably drawn from the B&W photograph (note that there is the same person walking on the road).
The Midori site is located at a restraining bend between two strands of the earthquake rupture along the Neodani Fault zone. At this rather complicated site the coseismic offset is not only mostly left-lateral, as elsewhere along the fault, but also bears a strong vertical component.
Here is Kotô's exact description: “Amongst the extraordinary works done by the earthquake, the fault in Midori is the most remarkable. A fine new road had been obliquely cut into two, and the west half with the surrounding fields had sunk about 6 meters below the upper end. The eastern half had been pushed 4 meters northwards, as is well seen in the photograph by an abrupt change in the direction of the displaced road.”
Map and recent photograph of Midori site, from Kaneda and Okada 2008
As for many sites along the faults, modern geologists have made paleoseismological trenching to characterize the rupture in section, to search from older earthquakes and to date them. This is the case in Midori. Visible on the recent photograph above, the pyramid like building to the right of the road bears a museum preserving the paleoseimological trench. You'll find a detailed description of the trench observations in Sato et al. 1992, from which I extract the following figure. Christoph Grützner has posted a fine report about the visit of the Midori site and of the trench museum.
I will say more on Kotô's description of the 1891 earthquake rupture, and about his novel and seminal interpretation linking faults and earthquakes, in the next Tectoldies post.
Koto, B. (1893). On the cause of the great earthquake in central Japan, 1891,J. Coll. Sci. Imp. Univ. Jpn. 5, 295–353.
Kaneda, H., & Okada, A. (2008). Long-term seismic behavior of a fault involved in a multiple-fault rupture: Insights from tectonic geomorphology along the Neodani fault, central Japan. Bulletin of the Seismological Society of America, 98(5), 2170-2190. https://doi.org/10.1785/0120070204
Sato, H., A. Okada, T. Matsuda, and T. Kumamoto (1992). Geology of atrench across the Midori fault scarp, from the Nobi earthquake of 1891, central Japan, J. Geogr. 101, 556–572. Link to PDF (in japanese)
Link to the whatercolour of the Midori fault scarp at Kyoto University rare materials digital archive.
