Paleoseismology of the Itoigawa-Shizuoka Tectonic Line
K. OKUMURA, K. SHIMOKAWA, H. YAMAZAKI, and E. TSUKUDA  Recent surface faulting events along the middle section of the Itoigawa-Shizuoka Tectonic Line -trenching survey of the Gofukuji fault near Matsumoto, central Japan-. Zisin (The Journal of the Seismological Society of Japan), 46, 425-438.
[abstract] The middle section of the Itoigawa-Shizuoka Tectonic Line (Middle ISTL) is an active fault system that extends NW-SE for 50 km from Matsumoto to Kobuchizawa, in central Japan. The Middle ISTL is characterized by high average slip-rate reaching 8 to 10 mm/yr during the Late Pleistocene and Holocene.
[Figure of the faults and trenches]
This is one of the highest slip-rate reported from active faults on land in Japan. Empirical relation between slip-rate and recurrence time indicates that the Middle ISTL may rupture more than once a thousand year. The previously known recurrence time estimates of 3500 to 5000 years were significantly longer than the expected recurrence time. The last faulting event on the Middle ISTL most likely occurred in 841 A.D. according to historic record and paleoseismological works. Since the elapsed time is about 1152 years, the estimation of recurrence time is critical to evaluate the potential of next earthquake. In order to know the history of recent faulting events, we excavated the Gofukuji fault, the northernmost segment of the Middle ISTL at Namiyanagi, south of Matsumoto. Investigation of 6 trenches, 3 test pits and topography around the trenches brought following results. The ages of three most recent faulting events are 445-1386 A.D., 150-334 A.D., and 839-189 B.C. The recurrence time is between 111 and 1236 years. Assuming the correlation of the last event with the 841A.D. event, the recurrence time is estimated to be 339 to 1173 years. Average recurrence interval in this case ranges 515 to 840 years.
[Figure on time-series and forecast]
The elapsed time of 711 to 1152 years is probably longer than the recurrence time. The average slip rate of the fault since c.a. 6000 B.C. is 4.9 to 13.9 mm/yr left-lateral. The left-lateral coseismic slip during the last event is estimated as 6 to 9 m.
Photo: Fault scarplet of the Gofukuji fault south of Matsumoto
Photo: The Gofukuji fault exposed in Namiyanagi Trench. Three faulting events in 2500 years were recognized on this wall.
Photo: Timing of faulting events on the Gofukuji fault. Is next one coming?
K. OKUMURA and R. IMURA  Paleoseismology and Segmentation of the Itoigawa-Shizuoka Tectonic Line Active Fault System (ISTL), Central Japan. EOS, vol. 76, no46. supplement, F363.
[abstract] The ISTL is one of the most remarkable active faults on land in Japan for its very high slip rate as an intraplate active fault reaching around 1 cm/yr. Though the high slip rate indicates short recurrence intervals of large earthquakes less than 1,000 years, historic seismicity is quite low during these 1150 years. The ISTL consists of northern [55 km], middle [60 km], and southern [35 km] sections. The estimates of respective average slip rate are 0.3, 1.0, and 0.2 cm/yr. At Matsumoto, three recent events were dated 445-1,386 A.D., 150-334 A.D., and 839-189 B.C. Kobuchizawa trench exposed the last event younger than 678-852 A.D. and two probable Holocene events. The recurrence time of the 40 km portion of the Middle section excluding the Matsumoto area is 3,000 to 5,000 years and the age of the last event coincides with the 841 A.D. historic event. The activity in Matsumoto area is much higher than the rest. Omachi and Hakuba trenches in the northern section exposed an event between 700 and 1000 A.D., and the recurrence time is expected to be longer than Matsumoto.
[Figure: Locality map]
[Figure: Summary of the results]
Photo: Exposure of the Kamishiro fault in Hakuba Trench. The concentric fold at lower right consists of 8 to 10 ka lacustrine deposits. The fold is truncated by horizontal faults near the bottom of the trench. Deformation of the overlying peat and black clay layers of 7 to 0 ka indicates recent fault activity. The angular unconformity in the middle of the peat and clay is a probable horizon of a faulting event, which age is around 3 ka.
Photo: Oblique view into the trench Faults are indicated by yellow dots.
Photo: Reverse faulting exposed in the Omachi Trench. A fragment of A.D. 5th to 6th century pottery was found at red pin in the black soil of the footwall. The snowy peaks behind are around 10,000 feet high in the Hida Mountains.
Photo: Japanese style--incline-walled and labor-intensive--trenching at Kobuchizawa. Debris-flow deposits of around 10 ka are faulted a few times during the Holocene.
K. OKUMURA, R. IMURA, T. IMAIZUMI, H. SAWA, and M. TOGO  Recent surface faulting and segmentation of the Itoigawa-Shizuoka tectonic line active fault system. Abstracts, 1996 Japan Earth and Planetary Science Joint Meeting, 147.
K. OKUMURA, R. IMURA, T. IMAIZUMI, H. SAWA, and M. TOGO  Recent Surface Faulting along the Northern Part of the Itoigawa-Shizuoka Tectonic Line. Programme and Abstracts, Japan Association for Quaternary Research, 26, 110-111.
[Figure: Summary of the results]