Here I follow up on my previous post on the 1891 Japan earthquake and the seminal article by Bunjiro Kotô published in 1893.
‟The sudden elevations, depressions, or lateral shiftings of large tracts of country which take place at the time of destructive earthquakes are usually considered as the effects rather than the cause of subterranean commotions; but in my opinion, it can be confidently asserted that the sudden formation of the ‘great fault of Neo’ was the actual cause of the great earthquake of the 28th of October 1861 […] It is an established tenet of geology that a sudden faulting of the earth's crust will cause a shaking which is designated by the general term of tectonic earthquake.” (Kotô 1993, pages 352-353)
This is the conclusion of article by Bunjiro Kotô entitled “On the Cause of the Great Earthquake in Central Japan, 1891.” In this article, probably for the first time in the world, B. Kotô explicitly linked earthquakes to tectonic faults and to their sudden rupture.
Sketches from Kotô 1893 showing paddy fields and paths laterally offset by the earthquake rupture.
Kotô gave detailed descriptions of the surface rupture, and of the lateral and vertical offsets at many site (see sketches above). He rightly interpreted earthquake "mole-tracks" (a term which is now commonly used for some specific surface rupture geometries): “Amongst the extraordinary things done by the earthquake, one that always drew attention was the earth-rent. It strikes across hills and paddy-fields alike, cutting up the soft earth into enormous clods and raising them above the surface. It resembles the pathway of a gigantic mole more than anything else.” (Kotô, 1893, p.328)
He gave a perfect 3D cross-sectional sketch of these structures (see below) on which the link to the deeper tectonic fault is clearly established.
Earthquake rupture from Kotô 1893
I will end with these very clear words still taken from the article by Kotô: “The event of October 1891 seems to me to have been a renewed movement upon one of these pre-existing fissures, the Neo valley line of fault, by which the entire region lying to the right of it not only moved actually downwards but was shifted horizontally towards the north-west for from one to two metres along the plane of dislocation. This vertical movement and horizontal shifting seem to me to have been the sole cause of the late catastrophe.” (Kotô, 1893, page 329)
Koto, B. (1893). On the cause of the great earthquake in central Japan, 1891,J. Coll. Sci. Imp. Univ. Jpn. 5, 295–353.
