Examination of the largest-possible tsunamis (Level 2) generated along the Nankai and Suruga troughs during the past 4000 years based on studies of tsunami deposits from the 2011 Tohoku-oki tsunami

Huge tsunami waves have repeatedly bombarded the southern end of the Ryukyu Islands (Miyako and Yaeyama Islands, southwestern Japan) at several-hundred-year intervals. Therefore, clarifying the islands’ paleotsunami history is important for risk assessment. Nevertheless, discrepancies of paleotsunami histories exist among regional studies of tsunami boulders and sandy tsunami deposits. Radiocarbon ages of tsunami boulders indicate that tsunami events of the last 2400 years have occurred every 150–400 years, most recently the historical 1771 Meiwa tsunami. Sandy tsunami deposits at Yaeyama Islands show that four tsunami events of the last 2000 years struck the islands at approximately 600-year intervals. Sandy tsunami deposits of the Miyako Islands have been studied only rarely. Therefore, studying sandy tsunami deposits in the Miyako Islands is crucially important for clarifying the paleotsunami history of this region. We conducted a trench survey on Minna Island, located among the westernmost Miyako Islands, which revealed two sandy tsunami deposits under a coral tsunami boulder transported by the 1771 tsunami. The upper tsunami deposit was likely deposited by the 1771 tsunami, as inferred from stratigraphic correlation to the tsunami boulder. However, the lower tsunami deposit was probably deposited 700–1000 years ago, which is consistent with the age range of the paleotsunami reported for Yaeyama Islands. Because sandy tsunami deposits found in this and earlier studies are thick and deposited at high elevation and far inland, these are useful markers of large tsunami events similar to the 1771 event. However, the reported tsunami boulders of various sizes are deposited along the coast and reefs: they can be formed not only by large tsunami events but also by small ones. It is noteworthy that each tsunami deposit is coarse and thick (40–48 cm) relative to the island elevation (about 12 m maximum, 7 m above the mean sea level at the study site). By assuming that tsunamis have affected this region repeatedly during the past few thousand years at around 600-year intervals, tsunamis might have been important geomorphic agents for building up small reef-surrounded islands such as Minna Island.

Section: Introductionmentioning

confidence: 99%

Millennial paleotsunami history at Minna Island, southern Ryukyu Islands, Japan

Fujita

Goto

Iryu

et al. 2020

“…No marine mollusc shells were detected in most areas (Szczuciński et al 2012;Takashimizu et al 2012;Kitamura 2016), except in one area on the Sendai Plain (Fujiwara et al 2014). Sugawara et al (2014) suggested that the absence of marine fossils can be explained if the tsunami caused a significant amount of erosion on the beach and in the coastal forest areas, but the erosion was minor on the offshore seafloor.…”

Section: Previous Studies Of Molluscan Assemblages Within Tsunami Depmentioning

confidence: 99%

“…Paleotsunami research based on coastal geological records is a useful approach to the detection of high-magnitude, low-frequency tsunami events along subduction zones (e.g., Atwater 1987;Minoura et al 2001;Nanayama et al 2003;Cisternas et al 2005;Monecke et al 2008;Shishikura et al 2010;Ishimura and Miyauchi 2015;Nelson et al 2015;Kitamura 2016;Inoue et al 2017). Although previous paleotsunami research along the Ryukyu Trench has examined coralline boulders (herein, "tsunami boulders") (Kawana and Nakata 1994;Goto et al 2010;Araoka et al 2013), Ando et al (2018) recently identified three sandy tsunami deposits (T-I, T-II, and T-IV) and a layer of buried tsunami boulders (T-III) in a trench on Ishigaki Island (Figs.…”

Section: Introductionmentioning

confidence: 99%

Using molluscan assemblages from paleotsunami deposits to evaluate the influence of topography on the magnitude of late Holocene mega-tsunamis on Ishigaki Island, Japan

Kitamura

Ito

Ikuta

et al. 2018

Self Cite

Four ancient tsunami deposits were identified in a trench excavated on Ishigaki Island, Okinawa, Japan. Three of the tsunami deposits (T-I, T-II, and T-IV) consist of calcareous sand beds, whereas the other (T-III, located stratigraphically between T-II and T-IV) consists of boulders. Deposit T-I was caused by a tsunami in 1771.14 C dating, together with the elevations of the landward margins of these sandy tsunami deposits, suggests that tsunamis II and IV were similar in size to the 1771 tsunami, although the influence of local topographic features on the magnitudes of tsunamis has not yet been examined. This study reconstructs the local topographic features by comparing the molluscan assemblages incorporated within the tsunami deposits with those in recent beach deposits. The presence of species that inhabit the intertidal zone in lagoonal settings in all the assemblages indicates that the present-day shallow lagoon has been present off the study area since the occurrence of tsunami T-IV, which supports the previous hypothesis that the magnitudes of the 1771 tsunami and tsunamis II and IV were similar. These molluscan assemblages also suggest that a high relative abundance of large, heavy mollusc shells is a feature of the paleotsunami deposits in the coastal lowlands found along the shallow coral lagoons.

“…1). Many studies have examined the earthquakes and tsunami events along subduction zones (e.g., Atwater 1987;Minoura et al 2001;Nanayama et al 2003;Monecke et al 2008;Shishikura et al 2010;Ishimura and Miyauchi 2015;Kitamura 2016;Inoue et al 2017;Ando et al 2018;Kitamura et al, 2018a, b;Goto et al 2018). The Philippine Sea Plate is subducting beneath the Eurasian Plate at these troughs, and megathrust earthquakes of magnitude~8 and ensuing large tsunamis have occurred since an AD 684 Hakuho earthquake, with a recurrence interval of 90-270 years (Ando 1975b;Ishibashi 2004).…”

Section: Introductionmentioning

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

Self Cite

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.