Trenching study of the Miyazono fault of the Minoh fault system, at Miyazono, Kurume, Fukuoka prefecture in 1996

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The Minoh fault is an east-west trending fault zone on the northern foot of the Minoh mountains, in the northern Kyushu, extending from Kurume City to Ukiha Town. The fault zone consists of several normal faults in parallel or echelon. Recently we made a trench at Miyazono, Kurume City in the west-central part of the zone. The observations of the trench wall and drilling cores lead the following conclusion. 1. A normal fault with the northern side downthrown was expose d on the trench wall, by which the Pleistocene slope-deposits were brought into fault contact with the pre-Tertiary metamorphic rocks. Another normal fault was inferred to exist about 20m north of it from boring data, which indicate also the northern, lowland side is downthrown. 2. The fault is covered with undisturbe d strata of the later half of the 7th century to the 8th, indicating the latest movement of the fault was prior to the 8th century. This fault zone is known to generate the A. D.679 large earthquake in historical time in the west area. But in the present study it is not be abled to confirmed that the faults in this site moved or not at that time. 3. Long-term average vertical slip rates in the present area are estimated at about 0.1∼0.2m/1000years, based on the following observations: the weathered red gavel beds estimated at 300ky in age is displaced about 35m. And the AT tephra layer of about 25ky in age is displaced about 4.3m. 4. Total displacement since the Pleistocene bed began to deposit is about 50m a l o ng the above two fault in this area. The lowland-side fault ceased the movement earlier than the mountainside fault. This may indicate that the active segment in this fault zone migrated toward the mountainside.

Trenching study of the Miyazono fault of the Minoh fault system, at Miyazono, Kurume, Fukuoka prefecture in 1996

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