Koichi Shimokawa

Unusually large earthquakes inferred from tsunami deposits along the Kuril trench

The Pacific plate converges with northeastern Eurasia at a rate of 8–9 m per century along the Kamchatka, Kuril and Japan trenches. Along the southern Kuril trench, which faces the Japanese island of Hokkaido, this fast subduction has recurrently generated earthquakes with magnitudes of up to ∼8 over the past two centuries. These historical events, on rupture segments 100–200 km long, have been considered characteristic of Hokkaidos plate-boundary earthquakes. But here we use deposits of prehistoric tsunamis to infer the infrequent occurrence of larger earthquakes generated from longer ruptures. Many of these tsunami deposits form sheets of sand that extend kilometres inland from the deposits of historical tsunamis. Stratigraphic series of extensive sand sheets, intercalated with dated volcanic-ash layers, show that such unusually large tsunamis occurred about every 500 years on average over the past 2,000–7,000 years, most recently ∼350 years ago. Numerical simulations of these tsunamis are best explained by earthquakes that individually rupture multiple segments along the southern Kuril trench. We infer that such multi-segment earthquakes persistently recur among a larger number of single-segment events.

ESR dating of fault movement using various defect centres in quartz; the case in the western South Fossa Magna, Japan

The dates of movements of the Itoigawa-Shizuoka Tectonic Line (I-S T.L.)and Minobu Fault in the western South Fossa Magna have been obtained by ESR dating. Measurements were made on various lattice defects of the OHC, Ge, Al and E′ centres in quartz grains of fault clay and quartz veins intruded at the time of movement. All the dates obtained from these centres in quartz grains from two localities of the I-S T.L. and IWO localities of the Minobu Fault fall in the range of 0.30–0.55 Ma; this suggests that all the centres were annihilated completely. and that the obtained age can be admitted as the actual data of the latest fault movement. Meanwhile, ESR dates of quartz vein and fault clay in other localities (locations 3, 6, 7) are divided into 0.30–0.55 Ma and 1.5 –2.0 Ma, which suggest incomplete resetting. The dates of 0.30–0.55 and 1.5–2.0 Ma might correspond to the clino- unconformities observed in Plio-Pleistocene sequence of this region.

Grouted sediment slices show signs of earthquake shaking

Sand and mud from Washington State, sampled with Japanese methods for identifying structure in unconsolidated deposits, have provided new evidence for earthquakes over the past 2000 years at the Cascadia subduction zone. Each sample was collected as a vertical slice, 0.5 m wide and up to 8 m long, in sheetpile driven into wet sand and mud beneath a tidal bank of the Columbia River (Figures 1 and 2). Painted with flexible, hydrophilic grout, the slices yielded full-size peels that reveal bedding and its disruption (Figure 3). Evidence for liquefaction is common, even where it is absent at the ground surface. Especially common are sills that imply lateral escape of water. These findings may affect ground-motion estimates for plate-boundary earthquakes in the northwestern United States and Canada.

ESR dating of quaternary tephra from Mt. Osore-zan using Al and Ti centres in quartz

Abstract Quaternary tephra of Mt. Osore-zan was dated by electron spin resonance. Quartz grains were separated from the tephra and ESR signals of Al and Ti centres were measured at 77 K. The signals of Al and Ti centres which have a complicated hyperfine structure were simplified by using wider field modulations width (5 gauss) than usually used (0.5–1 gauss). The influence of 5 gauss modulation width on the measurement of the signal intensity of Al and Ti centres was investigated. The intensity of the signal increases linearly with gamma irradiation and total doses obtained were 178 to 273 Gy and ESR ages were 0.19 to 0.32 Ma. These ages agree with the estimated age of about 0.3 Ma from the correlation of volcanic ash and terrace level.

ESR dating of fossil shells in the Middle to Upper Pleistocene strata in Japan

Abstract The accuracy of the ESR ages of fossil shells from the Boso Peninsula, Ooiso Hills and the Miyazaki Plain in Japan in the Middle to Upper Pleistocene were examined. The measurement test with a thermoluminescent dosimeter at the sampling point suggests that the dose rate of cosmic rays must be added to the calculated dose rate from chemical analysis. The results show that the ESR ages are in good agreement with the stratigraphically estimated ages. However, according to the chemical analysis of the mollusc shell itself, several shells give an abnormally high concentration of uranium. This may indicate that the shell has absorbed uranium from outside especially in sandy sediments.