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Overview of Holocene Tsunami Deposits along the Nankai, Suruga, and Sagami Troughs, Southwest Japan
Abstract and Figures
Tsunami deposits provide a basis for reconstructing Holocene histories of great earthquakes and tsunamis on the Pacific Coast
of southwest Japan. The deposits have been found in the past 15 years at lakes, lagoons, outcrops, and archaeological excavations.
The inferred tsunami histories span 3000 years for the Nankai and Suruga Troughs and nearly 10,000 years for the Sagami Trough.
The inferred histories contain recurrence intervals of variable length. The shortest of these —100–200 years for the Nankai
Trough, 150–300 years for the Sagami Trough — resemble those known from written history of the past 1000–1500 years. Longer
intervals inferred from the tsunami deposits probably reflect variability in rupture mode, incompleteness of geologic records,
and insufficient research.
The region's tsunami history could be clarified by improving the geologic distinction between tsunami and storm, dating the
inferred tsunamis more accurately and precisely, and using the deposits to help quantify the source areas and sizes of the
parent earthquakes.
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... Garrett et al., 2015;Meltzner et al., 2010;Natawidjaja et al., 2004;Nelson et al., 1996;Shennan et al., 2016), tsunami inundation (e.g. Kempf et al., 2017;Komatsubara & Fujiwara, 2007;Pinegina et al., 2017), and the identification of coseismic deposits and/or sedimentary structures in subaquatic settings (e.g. Avşar et al., 2016;Goldfinger et al., 2017;Howarth et al., 2014;Moernaut et al., 2014;Praet et al., 2017;St-Onge et al., 2012). ...
... Goldfinger et al., 2003;Kuehl et al., 2017) and the search for tsunami deposits (e.g. Kempf et al., 2017;Komatsubara & Fujiwara, 2007) or biological and sedimentological indicators for co-seismic elevation changes (e.g. Garrett et al., 2015;Philibosian et al., 2017) in coastal areas. ...
About 90% of the world’s earthquakes occur along the ‘Ring of Fire’, a horseshoe-shaped belt surrounding the Pacific Ocean. This seismically highly active zone consists of a quasi-continuous series of subduction trenches, formed at convergent plate boundaries. The process of subduction, where one lithospheric plate subducts below the other due to a density contrast, is associated with the largest and most destructive earthquakes on Earth. These are capable of reaching moment magnitudes (Mw) of 9.0 or higher, nucleating at the interface between the two convergent lithospheric plates (megathrust earthquake). Recent examples include the 2004 Sumatra-Andaman earthquake (Mw 9.1) and the 2011 Tohoku earthquake in Japan (Mw 9.0). These earthquakes are respectively the deadliest and costliest earthquakes of the 21st century, which includes the losses caused by the tsunami wave that is typically generated during these megathrust earthquakes. As a result, subduction zones are an important target for natural hazard studies, aiming at estimating and mitigating the impact of future earthquakes and potentially associated tsunamis on communities at risk.
A key aspect in seismic hazard assessment is understanding the earthquake recurrence mode and rate. For this purpose, numerous paleoseismic studies have explored the seismic history of subduction zones around the world by studying, for example, coastal marshes, coral microatolls as well as lakes, fjords and offshore environments. The sedimentary record of lakes and fjords in particular has proven exceptionally suitable for reconstructing the paleoseismological history of a specific region through the identification of multiple synchronous seismically-generated landslides and/or turbidites within the same basin. Most paleoseismic studies of subduction zones tend to focus on tsunamigenic, high-magnitude earthquakes in particular. Nevertheless, additional destructive earthquakes can occur in subduction-zone settings, i.e. within the subducting slab (intraslab earthquake) or overriding plate (crustal earthquake) rather than at the plate boundary. This is demonstrated by, for example, the 2017 Chiapas intraslab earthquake in Mexico (MW 8.2), with just under 100 casualties and an estimated economic cost that amounts to US$ 2 billion, or the 2006 Yogyakarta crustal earthquake in Indonesia (MW 6.4), resulting in over 5,700 casualties and US$ 3 billion of financial losses as a result of the high population density and poor construction practices in the affected area. This underscores the significant contribution of intraplate earthquakes to the imminent seismic hazard in subduction zones, which is often neglected in current paleoseismic research. This highlights the urgent need for more inclusive studies of the shaking history in these areas.
To meet the underrepresentation of intraplate earthquakes in paleoseismic research, this dissertation is focused on two particular areas along the ‘Ring of Fire’ (southern Chile and Sumatra), where inter- as well as intraplate earthquakes are known to occur. More specifically, these are located in a context of oblique subduction, resulting in clustering of crustal earthquakes along extensive strike-slip fault systems that accommodate the trench-parallel component of plate convergence. To disentangle the contribution of each of the different types of earthquake sources that results in coseismic movement (i.e. deformation and/or shaking) of the considered area, we combine a series of seismotectonic and sedimentological approaches. In this way, we aim to advance our current knowledge on the interplay between megathrust, crustal and intraslab earthquakes and improve future seismic hazard assessments.
... Tsunamis following two M9-class earthquakes that occurred in close succession in the 21st century, the 2004 CE Indian Ocean tsunami and 2011 CE Tōhoku tsunami, have provided a clear incentive to study the record of past tsunamis and incorporate paleotsunami data into forecasts of future events. Results of paleotsunami studies along the Nankai and Sagami Troughs up to 2006 CE are summarized in Komatsubara and Fujiwara (2007). Garrett et al. (2016) provides a comprehensive review of the recurrence history of great Nankai Trough earthquakes in the Middle to Late Holocene, including not only tsunami deposits and liquefaction features on land, but also marine and lacustrine turbidites. ...
... The two older deposits formed between 4000 and 3000 cal BP; storm surges and tidal channel migration also remain possible explanations for them. The three youngest deposits are linked with tsunami inundation in 1707, 1605 and 1498 CE by Tsuji et al. (1998) and Komatsubara and Fujiwara et al. (2007); however, more recent recalibration of radiocarbon data highlights difficulties with ascribing particular historical tsunamis to these deposits (Garrett et al., 2016). Sato et al. (2016b) identified a marine incursion event around 4790-4420 cal. ...
Located near Japan's most densely populated and industrially active region, the Nankai Trough subduction zone has long been highlighted as a high-risk area for damaging earthquakes and tsunamis. In contrast, less attention has been paid the adjacent Ryukyu Trench because historical and geological records are scarce. In order to develop better quantitative estimates of the timing and size of the earthquakes and tsunamis generated along these subduction zones, comprehensive studies using geological, seismological and historical methods have been conducted. Since the 1990s, studies of tsunami deposits in this region have contributed to our current understanding of the history of tsunamis over the last 6000 years. Following the 2011 Tōhoku earthquake and tsunami, paleotsunami research has especially focused on guiding and enhancing tsunami disaster management and mitigation measures. The last nine years have seen a rapid increase in paleotsunami data from the Nankai Trough and Ryukyu Trench coasts. These recent studies reveal that there are significant differences in the size and recurrence pattern of earthquakes and tsunamis along these subduction zones. For instance, large earthquakes have repeatedly occurred along the Nankai Trough every 100–200 years. On the other hand, large earthquakes and tsunamis may not have occurred over the last few thousand years at the junction between these subduction zones. There is also no evidence for the occurrence of large earthquakes along the northern and central Ryukyu Trench over this time period. Large earthquakes and tsunamis may have occurred every few hundred years at the southern end of the Ryukyu Trench. Nevertheless, data on the occurrence of these earthquakes are still insufficient, both in quantity and quality, to estimate the maximum size or recurrence pattern.
... The shorelines of Japan experience extreme coastal flooding from both seasonal typhoons and episodic tsunami events (Komatsubara and Fujiwara 2007;Sasaki and Yamakawa 2007;Suzuki et al. 2008). Quantifying the spatially-varying risk associated with typhoons and tsunamis across Japan is an essential part of hazard preparation and planning (e.g. ...
... For example, overwash deposits from high-energy flood events that are preserved within the sedimentary sequences of coastal environments, such as those from back-barrier lakes and marshes, can be used to reconstruct pre-instrumental tsunami and typhoon frequency and intensity (e.g. Komatsubara and Fujiwara 2007;Komatsubara et al. 2008;Chagué-Goff et al. 2012;Wallace et al. 2014;Brandon et al. 2015; Baranes et al. 2016Baranes et al. , 2018Chaumillon et al. 2017). The sandy overwash deposits left by tsunamis and typhoons can often have very similar characteristics including being composed of coarse-grained beach material, exhibiting a lateral thinning landward, and containing elemental signatures consistent with marine-derived sediments. ...
Previous studies in western Kyushu revealed prominent marine-derived flood deposits that date to the late thirteenth-century and are interpreted to be a result of two legendary typhoons linked to the failed Mongol invasions of Japan in 1274 and 1281. The regional persistence and prominence of sediments dating to these “Kamikaze” typhoon events (meaning divine wind) raise questions about the origins of these late thirteenth-century deposits. This is due in part to uncertainty in distinguishing between tsunami and storm-induced deposition. To provide additional insight into the true cause of prominent late thirteenth-century flood deposits in western Kyushu, we present a detailed assessment of an additional event deposit dating to the late thirteenth-century from Lake Kawahara near Nagasaki, Japan. This particular deposit thickens landward towards the primary river flowing into Lake Kawahara and exhibits anomalously low Sr/Ti ratios that are consistent with a fluvial rather than a marine sediment source. When combined with previous flood reconstructions, results support the occurrence of an extreme, late thirteenth-century event that was associated with both intense marine- and river-derived flooding. Results therefore contribute to a growing line of evidence for the Kamikaze typhoons resulting in widespread flooding in the region, rather than the late thirteenth-century deposit being associated with a significant tsunami impact to western Kyushu.
... Multiple studies have attempted to reveal past earthquake events in these seismically active regions by studying a variety of natural archives such as lakes, fjords, coastal marshes, and continental margins. Common examples are studies of turbidites and/or mass-transport deposits (MTDs) present in both lacustrine (e.g., Moernaut et al., 2018;Praet et al., 2017) and offshore environments (e.g., Goldfinger et al., 2003;Kuehl et al., 2017) and the search for tsunami deposits (e.g., Kempf et al., 2017;Komatsubara & Fujiwara, 2007) or biological and sedimentological indicators for coseismic elevation changes (e.g., Garrett et al., 2015;Philibosian et al., 2017) in coastal areas. ...
Grain‐size analysis and end‐member modelling of a long sediment core from Aysén Fjord (southern Chile) allows to identify over 25 seismo‐turbidites in the last 9000 years. Considering the shaking intensities required to trigger these turbidites (V½‐VI½), the majority can be related to megathrust earthquakes. Multiple studies in south‐central Chile have aimed at finding traces of giant, tsunamigenic megathrust earthquakes leading to the current 5500‐year‐long paleoseismological record of the Valdivia Segment. However, none of these cover the southern third of the segment. Aysén Fjord allows to fill this data gap and presents the first, crucial paleoseismic data to demonstrate that the 1960 event was not unique for the Valdivia Segment, yielding a recurrence rate of 321 ± 116 years in the last two millennia. Moreover, the oldest identified events in Aysén Fjord date back to 9000 cal years BP, and thus also extend the regional paleoseismological record in time. We infer a large temporal variability in rupture modes, with successions of full‐segment ruptures alternating with partial and cascading ruptures. The latter seem to significantly postpone the occurrence of another full rupture when consecutively occurring in different parts of the segment. Additionally, one outstanding period of seismic quiescence – during which no megathrust earthquake evidence has been found at any paleoseismic site – occurred after a full rupture in AD ~745 that presents an unusual uplift/subsidence pattern. Such variability makes it highly speculative to anticipate the rupture mode of the next megathrust earthquake along the Valdivia Segment.
... The coast of Honshu is mainly modeled in Neogene and Quaternary volcanic rocks to the northeast and in Paleogene and Cretaceous intrusive rocks to the southwest (Sunamura et al., 2014;Taira, 2001). The island is seismically active, historically struck by several great (Mw *8) and giant (Mw *9) earthquakes, which occurred along the offshore subduction zones, including the Japan Trench (Satake, 2015;Satake and Fujii, 2014), and is hit by recurrent tsunamis (Komatsubara and Fujiwara, 2007;Sawai et al., 2008). ...
Tsunamis are one of several processes that contribute to coastal remodeling. This research interpreted geomorphic features over time to better understand if a relationship exists between the flash remodeling of coasts, expressed by bidimensional-fractal dimensions, and known tsunami energy. Fractal geometry analysis, through the box-counting and correlation integral methods, was applied to the physiography of four oceanic coastlands: three in Sumatra, Indonesia, and one in Japan, hit by tsunamis in the last 14 years. Their shoreline fractal dimensions before and after the events and the present-day ones were calculated and compared. Results highlighted any difference or convergence of calculated fractal dimensions. Significant numerical variations of fractal dimensions of the shores before and after each tsunami were registered, and those values gradually decreased post-tsunami. These shorelines, considered stable before tsunamis, increased in physiographic irregularity up to 5–11% immediately after the phenomena; this slowly diminished to 2–5% about 8 to 15 years later; and finally to 3–6% present-day, compared to the pre-event shorelines. Considering these changes of the fractal dimension and the hydrodynamic energy of the tsunami, responsible for the abrupt coastal remodeling, a simple empirical expression and evaluation of the residual resilience is proposed. As a first step, a real physical meaning, in terms of energy, is attributed to the (dimensionless) fractal dimension.
... Many palaeotsunami studies have been conducted throughout the region of south-western Japan facing the Nankai Trough (e.g. Komatsubara & Fujiwara, 2007;Komatsubara et al., 2008;Garrett et al., 2016;Sawai et al., 2016;Fujiwara & Tanigawa, 2017;Tanigawa et al., 2018). In contrast though, there have been few studies in eastern Kyushu, with most focusing on the northern part of the island (for example, Ryujin Lake; Fig. 1B). ...
The Hyuga‐nada Sea, south‐eastern Kyushu, Japan, is located between a strong (Nankai Trough) and a weak interplate coupling zone (Ryukyu Trench). Over the past 400 years this area has only experienced Magnitude 7.5 earthquakes or smaller and associated small‐scale tsunamis. However, this short historical record most likely does not include the full range of high magnitude, low frequency giant earthquakes that might have occurred in the region. Thus, it is still unclear whether giant earthquakes and their associated tsunamis have occurred in this region. This paper reports on a prehistoric tsunami deposit discovered in a coastal lowland in south‐eastern Kyushu facing the Hyuga‐nada Sea. There is a reddish‐brown pumiceous layer preserved in a non‐marine, organic‐rich mud sequence obtained from onshore sediment cores. This layer is recognized as the ca 4600 year old Kr‐M tephra (that is now placed around 4500 years ago) sourced from Mount Kirishima, southern Kyushu. Another whitish pumiceous layer is evident below the Kr‐M tephra in almost all of the sediment cores. A relatively high percentage of marine and brackish diatoms is recorded within this lower pumiceous layer (but not in the surrounding muds or in the overlying Kr‐M tephra), indicating a marine or beach sediment source. Plant material obtained from organic‐rich mud immediately below the event layer was dated to approximately 4430 to 4710 cal. yr BP, providing a limiting‐maximum age for this marine incursion event. The presence of marine diatoms below the event layer is probably explained by pre‐seismic subsidence. An absence of the resting spore of the planktonic brackish diatom Cheatoceros and the appearance of the freshwater diatom Eunotia serra immediately above the event layer probably represents a marked change to a relatively low‐salinity environment. Assuming that there were no significant local geomorphological changes, such as drainage obstruction caused by formation of a new barrier spit, it is considered that co‐seismic or immediate post‐seismic uplift are the most likely explanation for this notable environmental change. Based on the crustal movements noted before and after the marine incursion this event is interpreted here as an earthquake‐generated tsunami. Moreover, because of these notable seismic crustal movements the tsunamigenic earthquake probably occurred immediately offshore of the study site.
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... Paleoseismic evidence along the Nankai-Suruga Trough were summarized in Komatsubara and Fujiwara (2007) and Garrett et al. (2016). Geological evidence of tsunamis has been studied at coastal ponds in Kochi Prefecture. ...
This study investigates the Holocene sedimentary history of a small coastal lowland in Nankoku, Kochi Prefecture, on the coast of southern Japan facing the Nankai Trough. The sedimentary fill of the lowland area consists mainly of marine-brackish clay overlain by beds of freshwater clay and peat. We found four laterally extensive sand sheets, one directly underlying the freshwater deposits and the other three interbedded with them. Radiocarbon dates show that these sand sheets were deposited between 5970 and 2440 cal. BP. Although the sand sheets contained few marine-brackish diatoms, they were concentrated in the seaward part of the study site, suggesting that they were deposited by marine inundations. These sand sheets were formed as a result of tsunamis or unusually large storm surges. The apparent frequency of marine inundations during 5970–2440 cal. BP was much lower than that of megathrust earthquakes along the Nankai Trough recorded during the last 1300 years. Event deposits were absent between 2440 and 960 cal. BP, a gap that we attribute to the development of beach ridges. The new marine inundation records reported here will aid efforts to reconstruct the timing and recurrence intervals of megathrust earthquakes in the western Nankai Trough.
Paleotsunami studies within the Nankai Trough region started in the late 1990s and have contributed to our understanding of the tsunami history of this region over the past 6000 yr. Notably, the discovery of unusually thick tsunami deposits on the western Nankai coast suggests that exceptionally large tsunamis have occurred in this area. This has led to a hypothesis that there is a “hyperearthquake cycle” of 300-500 yr, which is distinct from the ordinary Nankai Trough earthquake cycle of 100-150 yr. However, the number of reliably documented tsunami deposits is still too small to accurately reconstruct the size and source area of each paleotsunami. In this review, we propose two main fields as future research directions for tsunami deposit research in the Nankai Trough region. The first involves verifying the presence or absence of great earthquakes during the historical period for which there is no written record of Tokai or Nankai earthquakes. Although the last three Tokai and Nankai earthquakes (i.e., after the 18th century) occurred in pairs with a short time interval between them (<2 yr), it remains unknown if the older Tokai and Nankai earthquakes also occurred in pairs. This is fundamental for improving our understanding of the fault rupture pattern along the Nankai Trough subduction zone. The second field of research involves quantification of the size of past tsunamis. Quantification of paleotsunami inundation height and area is needed to verify the “hyperearthquake cycle”, and to delineate the maximum tsunami size that should be adopted in hazard mitigation measures.
Four paleotsunamis deposits are exposed in a trench on the coastal lowland north of the southern Ryukyu subduction zone trench. Radiocarbon ages on coral and bivalve shells show that the four deposits record tsunamis date from the last 2000 yrs., including a historical tsunami with a maximum run-up of 30 m in 1771, for an average recurrence interval of approximately 600 yrs. Ground fissures in a soil beneath the 1771 tsunami deposit may have been generated by stronger shaking than recorded by historical documents. The repeated occurrence of the paleotsunami deposits supports a tectonic source model on the plate boundary rather than a nontectonic source model, such as submarine landslides. Assuming a thrust model at the subduction zone, the seismic coupling ratio may be as low as 20%.
Hamanako is an economically and culturally important tidal lake on the Pacific coast of Honshu, Japan. Between 1954 and 1973, entrance works were constructed to regulate the channel to the lake. In a previous paper, the tidal records from Maisaka just inside the lake, prior to regulation, were analyzed to explain the responses of the lake to the ocean tide and other water level changes. The present paper compares water level responses with those following regulation of the entrance. The principal differences are that at Maisaka the tidal range increased up to 1965, following the first stages of regulation, but it has been constant since then. In the inner part of the lake, regulation has increased the tidal range but reduced its variability and has reduced the mean water level in the lake. The southern sand shoal divides the lake into two tidal basins, with Maisaka lying in the smaller outer basin. Studies of the inner basin have shown that its tidal range continues to increase. A hydrodynamic model has been used to show that this increase is primarily due to scouring of the inner channel through the southern shoal. The model has also shown that the constancy of the Maisaka tidal amplitude since 1965 is not a static equilibrium, but is a balance between the effects of the outer entrance channel scouring and increasing the Maisaka tide, offset by the inner channel scouring and reducing the Maisaka tide. © 2017 © World Scientific Publishing Company and Japan Society of Civil Engineers
Onshore tsunami deposits from recent, historical, and prehistorical tsunamis have been reported from many parts of the world. First, we review the existing English and Japanese literature on onshore tsunami deposits in coastal lowland. Then we identify the significance of their general and defining characteristics. We classify the significance of the tsunami deposits sedimentologically, paleoseismologically, and their relevance to civil engineering. We then present our review of tsunami deposits associated with the 1993 Hokkaido-Nansei-oki Earthquake on the estuary of the Usubetsu River, Hokkaido, Japan. Finally we consider that the sedimentary characteristics such as facies and grain size distribution of these modern deposits can be used to recognize past tsunami deposits and hence past tsunamis.
A tectonic geomorphological study is one of the best methods of evaluating the timing and the crustal deformation of pre-historic earthquake. To reveal the cycle of interplate earthquakes along the Sagami Trough, I investigated the emerged shoreline topography and the fossilized sessile assemblages in the Miura Peninsula and the Boso Peninsula. The distribution pattern of coseismic vertical displacement during the +1*-Genroku Kanto Earthquake inferred from the height distribu-tion of the paleo-shoreline suggests that the fault source model consists of a dual fault system of the Fault A and B. Fault A is also the source of the +3,-Taisho Kanto Earthquake. The geometry and ages of the emerged shoreline topography divided into several levels indicate that the characteristic earthquake generated from Fault A has occurred about every .** years. One of several events, it is accompanied with a slip of Fault B, which has a recurrence interval of ,***,1** years.
Possible tsunami deposits were found at two archeological sites along the Pacific coast near Hamana Lake in central Japan. This region has frequently suffered heavy damage from large earthquakes along the Nankai trough. Three sand deposits covering layers that include archeological remains were found at one of the sites. The ages of the sand deposits were constrained by remains just below the sand deposits, which showed good correspondence to the ages of three major historic earthquakes that seriously damaged this area, i.e., A.D. 1707, 1605, and 1498. These features strongly suggest that the sand deposits were caused by tsunami from the historic earthquakes. Core samples obtained near this site showed that a similar sequence continues into a deeper part, suggesting the existence of tsunami deposits of events older than A.D. 1498, before which few historical records are available in the Tokaido side. Further investigation of tsunami deposits in this area would yield useful information on the reconstruction of large earthquakes along the Nankai trough.