Reconstruction of paleo coastal environment and tsunami deposits from the Kanto earthquakes in Ena bay, Miura Peninsula, Japan
Abstract
Interplate earthquakes and subsequent tsunamis leave geological evidence such as tsunami deposits and environmental changes due to the crustal movement in coastal regions, therefore studies of coastal paleo-environment changes and tsunami deposits from paleo-earthquake by geological and microfossil analysis are important for long-term earthquake forecast. Recurrence interval of the great interplate earthquakes, called Taisho-type Kanto earthquake, along the Sagami Trough was estimated to be 200-400 years. In contrast, Genroku-type was estimated to be 2000-2700 years. However, earthquake histories prior to the 1703 Genroku Kanto earthquake are little known, while some candidates were proposed from historical literature (Ishibashi, 1991, 1994; Kaneko, 2011). On the other hand, Shimazaki et al. (2011) conducted Geo-slicer surveys in Koajiro Bay of Miura Peninsula and suggested that the 1293 earthquake was the Kanto earthquake prior to the 1703 Genroku earthquake. However, paleo-seismological data of past Kanto earthquakes are still limited. Moreover, there are hardly natural coastal wetlands that would preserve tsunami deposits, because the natural environments were collapsed by revetments in Kanto. In order to reveal the histories of Kanto earthquakes, we conducted coring surveys using 3m handy Geo-slicers at Ena bay in Miura Peninsula, and obtained nine event layers of coarse sediments including shell fragments and gravels in tidal and subtidal muddy sediments. Each of these event deposits have sharp lower contact indicating that they accompanied with a strong current. Changes of diatom assemblages show a fluctuation of relative abundance of marine species, suggesting changes in a paleo-sea depth. Namely, marine benthic species gradually decrease prior to the deposition of tsunami deposits indicating coastal subsidence before tsunami, and benthic species increase above event deposits indicating coastal uplift after the tsunami. The tide gauge in the nearby bay indicates that Miura Peninsula uplifted during the 1923 Kanto earthquake and now gradually subsides. Consequently, the diatoms suggest environmental changes corresponding to similar coseismic uplift and interseismic subsidence. Based on the above features, we conclude that these event deposits, named as T1-T9 units from the top to the bottom, are tsunami deposits without T3 from the past Kanto earthquakes. The diatoms in muddy sediment between T3 and T4 showed no clear tendency to change of paleo-sea depth because the muddy sediment is very thin. Therefore the origin of T3 unit is unknown at this stage. The T1 unit corresponds to the 1923 Taisho Kanto earthquake based on Pb-210 dating. Radiocarbon ages indicate that the T2, T3, T4, T5, T6, T7, T8 and T9 unit deposited about 300, 500, 700, 2000, 3000, 3300, 3700, and 4000 cal. yBP, respectively.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMNH31A1583C
- Keywords:
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- 9320 GEOGRAPHIC LOCATION Asia;
- 7221 SEISMOLOGY Paleoseismology;
- 3030 MARINE GEOLOGY AND GEOPHYSICS Micropaleontology;
- 4300 NATURAL HAZARDS