利根川低地における「弥生の小海退」の検証

To clarify the Holocene uplift history of the Izu Islands, Japan, we analyze the elevations and 14 C ages of emerged sessile assemblages measured by accelerator mass spectrometry (AMS) on the islands of Niijima, Jinaijima, Shikinejima, and Kouzushima, on the northern Zenisu Ridge. The results suggest that uplift events took place after AD 1950 (uplift event 1), during AD 786-1891 (uplift event 2), during AD 600-1165 (uplift event 3), and during AD 161-686 (uplift event 4), although uplift events 3 and 4 are identified only at Kouzushima. The minimum amount of uplift was estimated to be 0.4-0.9 m in uplift event 1, 2.4-2.7 m in uplift event 2, 3.6 m in uplift event 3, and 3.3-8.1 m in uplift event 4. These events could have been caused by volcanic activity or strong earthquakes. There also remains the possibility that uplift event 2 was caused by the AD 1498 Meio earthquake; in contrast to the previous interpretation, the ages of uplift events are significantly older than the earthquake, based on conventional (non-AMS) methods.

Section: Discussionmentioning

confidence: 99%

Late Holocene uplift of the Izu Islands on the northern Zenisu Ridge off Central Japan

Kitamura

Imai

Mitsui

et al. 2017

“…The appearance of the paddy field weed Monochoria plantaginea suggests that the environmental change was due to human cultivation. Another possibility is the non-tectonic regional sea-level change known as the Yayoi Regression, which affected the Japanese coast between 2000 and 3000 yr BP (e.g., Ariake Bay Research Group 1965;Sakaguchi et al 1985;Umitsu 1991;Tanabe et al 2016). This regression may have led to coastal emergence and desalination at the study site.…”

Section: Origin Of Event Depositsmentioning

confidence: 99%

“…Based on the change in plant macrofossils, it is likely that after about 3000 cal yr BP, the study site was used for agriculture. Before the deposition of event deposit E1, plant macrofossils were dominated by Cladium chinense, a common species in salt marshes (Matsushita et al 2004;Tanabe et al 2016). However, above E1, it was replaced by Monochoria plantaginea, a well-known weed in paddy fields (Takeuchi et al 1995) (Fig.…”

Section: Paleoecologymentioning

confidence: 99%

Geological record of prehistoric tsunamis in Mugi town, facing the Nankai Trough, western Japan

Shimada

Fujino

Sawai

et al. 2019

Self Cite

Stratigraphic and paleontological investigations in Mugi Town, on the Pacific coast of Shikoku Island, revealed evidence of as many as five tsunami inundations from events along the Nankai Trough between 5581 and 3640 cal yr BP. Nine event deposits (E1-E9) were identified in cores ranging in length from 2 to 6 m, consisting of sandy and gravelly layers interbedded with organic-rich mud. Sedimentary structures in the event deposits observed by computed tomography included normal grading and sharp lower stratigraphic contacts. Event deposits E3, E6, E7, and E8 contained mainly brackish-marine diatom species, suggesting that they had been deposited during inundation by seawater. In addition, fossil diatom assemblages were markedly different above and below event deposits E3, E4, E6, and E8. For example, assemblages below event deposit E6 were dominated by a freshwater species (Ulnaria acus), whereas assemblages above it were predominantly brackish-marine (Diploneis smithii, Fallacia forcipata, and Fallacia tenera). We attributed these changes to the increase of marine influence due to coastal subsidence associated with subduction-zone earthquakes, as documented in the 1946 Showa-Nankai earthquakes. We conclude that event deposits E3, E6, and E8 and perhaps E4 and E7 were deposited by tsunamis generated by subduction zone earthquakes along the Nankai Trough. The ages of these event deposits, as constrained by ten radiocarbon ages, suggest that some of the tsunamis that impacted Mugi Town were correlated with those reported elsewhere along the Nankai Trough, thereby complementing the existing but still incomplete geological record for these events.

“…Radiocarbon dating shows that the sea-level rise took place between 1257 cal BC and 400 cal AD. Tanabe et al (2016) estimated about 2 m of relative sea-level rise during 1000-0 cal BC based on the analysis of sedimentary records around Kanto Plain, central Japan. However, eustatic sea level was likely stable during that time (Sloss et al 2007;Woodroffe et al 2012).…”

Section: Discussionmentioning

confidence: 99%

Geologic evidence for coseismic uplift at ~ AD 400 in coastal lowland deposits on the Shimizu Plain, central Japan

Kitamura

Ina²,

Suzuki³

et al. 2019

Megathrust earthquakes of magnitude~8 and ensuing large tsunamis occur along the Nankai and Suruga troughs, central Japan, with a recurrence interval of 90-270 years since an AD 684 Hakuho earthquake. Historical records show that the AD 1854 Ansei-Tokai earthquake caused coseismic uplift of~1 m in coastal areas west of Suruga Bay, including the Shimizu Plain, east Shizuoka Prefecture. The 1361 Shohei Tokai earthquake was accompanied by coseismic uplift, but no evidence has been found to indicate uplift before the earthquake. This study examined sediment cores recovered from the coastal beach ridge in the Shimizu Plain using sedimentological and paleontological analyses and radiocarbon dating. We infer from the results that a relative sea-level fall of 1.6 m took place at AD 398-428. This coincides with archeological evidence suggesting the occurrence of a large earthquake in west Shizuoka Prefecture in AD 400. This combination of evidence indicates that the drop in sea level was caused by coseismic uplift associated with a megathrust rupture along the Suruga Trough, with a recurrence interval of~500-1000 years. The results also indicate that a megathrust rupture occurred in the region from the Eastern Nankai Trough to the Suruga Trough at ca. AD 400.