There has been considerable research both on-fault and in adjacent wetland environments aimed at understanding the nature and timing of movements on the predominantly strike-slip Alpine Fault in southern Aotearoa New Zealand. Findings have documented significant horizontal displacement on its western side and uplift to the east but have as yet not reported any vertical land displacement to the west. Here, we report on a recent study of drowned forests, subsided soils, exposed tree stumps, and sedimentary evidence from Saltwater Lagoon, western South Island, which suggest multiple subsidence/compaction events. Radiocarbon dating of drowned trees revealed multiple tree ages with at least two buried soils overlain by apparent tsunami and/or slope failure deposits. The overall trend appears to be one of net lagoon subsidence/compaction. The recognition of co-seismic subsidence/compaction events in this area may help explain a geomorphological conundrum where coastal wetlands and lagoons are maintained in a region where substantial sediment supply to the coast should cause infilling and progradation. It is important to gain a far better understanding of the nature and extent of such co-seismic subsidence/compaction on the western side of the Alpine fault in order to determine the possible risks posed to coastal infrastructure. Equally, this applies internationally where there is potential to recognise seismic linkages between regional coastal geomorphologies such as lagoons, wetlands, drowned forests, and uplifted or isolated benches. This work highlights the significance of looking beyond the predominant nature of a fault's movement in order to fully understand a region's seismicity.

Megathrust earthquakes in subduction zones often go unreported because they are rare and the historical record is short. On the Ryukyu subduction zone of southwestern Japan, unlike neighboring Nankai Trough, the history and future potential of great interplate earthquakes are not well known. While the geodetic measurements on the islands suggest that the plate coupling is very weak, recent observations of slow seismic events as well as offshore geodetic measurements imply the presence of coupled patches along the megathrust. Furthermore, the historical and geological studies indicate evidence of great tsunamis. Here, we use fossil microatolls in Ishigaki island to reconstruct the relative sea level in the Holocene. The coral record reveals several relative emergence episodes clustering between 5-4 and 3-2 thousand years ago (ka). Elastic modeling shows that the observed motions can correspond to coseismic uplift associated with megathrust earthquakes. The clusters of megathrust events suggest possible supercycles of earthquakes with a recurrence interval of more than 2 ka. Such results imply a strong seismic hazard for the upcoming centuries. The devastating 1771 Meiwa earthquake and associated tsunami may mark the onset of the most recent seismic supercycle.

Roundness of gravel particles is widely used as an indicator of sediment transport, yet its potential for discriminating provenance between mainstem and tributary rivers has not been fully tested. In this study, we applied an image-analysis-based roundness measurement (Rgrains) to gravel samples from mainstem and tributary deposits in the Tama and Doushi river basins in central Japan. We used both modern riverbed sediments and terrace deposits across four grain-size fractions (2-4, 4-8, 8-16, and 16-32 mm) to examine suitable grain sizes for distinguishing mainstem and tributary deposits. The results suggest that terrace gravel clasts may be distinguished between mainstem and tributary sources by comparing their roundness distributions with those of modern riverbed sediments. Mainstem sediments consistently exhibited higher roundness than tributary sediments, whereas tributary deposits showed statistical characteristics biased toward specific roundness values, reflecting the restricted nature of their catchments and channels. The minimum grain size required for identification depended on the tributary gradient: in steep-gradient tributaries dominated by debris flows, clear differences were observed even in fine fractions (2-4 and 4-8 mm), whereas coarser fractions (16-32 mm) were generally required for low-gradient tributaries dominated by bedload transport. Furthermore, fluvial transport processes inferred from riverbed slopes appear to exert a stronger influence on clast roundness than lithology, which plays a relatively minor role. The roundness of terrace gravel clasts may preserve information on tributary gradients and transport modes at the time of deposition, and comparison with modern riverbed gravel clasts suggests that past tributary gradients may be reconstructed. These findings suggest that quantitative roundness analysis provides a useful approach for linking terrace gravel clasts to their provenance, offering insights into fluvial terrace development and sediment transport in mountainous rivers.

Estimating coastal erosion by a tsunami is essential for land use planning, assessing hazards for current structures (e.g., coastal nuclear power plants), and for paleotsunami reconstruction. Such estimations are currently available only for sandy beaches, using sand sediment transport models, which are not applicable to gravel beaches, which are the most common beach type in high-latitude settings. This study extended the one-dimensional cross-shore XBeach-G model to account for two-dimensional gravel transport by a tsunami. First, this study confirmed that the extended XBeach-G model can simulate a time series of waveforms of solitary waves during laboratory experiments. The proposed model was then applied to gravel transport by the 2011 Tohoku-oki tsunami at Koyadori, Japan, and found that the simulation results were consistent with observations of gravel deposits in previous studies. It was revealed that infiltration and exfiltration have an impact on morphological change caused by a tsunami on gravel coasts. In the simulation, inundation depth over land by the tsunami increased due to groundwater exfiltration, which increased the onshore deposition volume of gravel tsunami deposits. The groundwater flow calculation has not been incorporated so far for tsunami modelling, but this is important for modelling tsunami inundation at gravel beaches and gravel sediment transport by a tsunami. However, choosing appropriate values for the sediment friction factor and multiplier in the equation for gravel transport is more critical to reproducing the deposition of gravel sediments by a tsunami because these parameters are more sensitive than the parameter of groundwater flow. Although the presented model has been developed for tsunami simulation on any gravel beach, further testing and validation are recommended.

The 2024 Noto Peninsula earthquake (Mw 7.5) caused significant tsunami inundation along the northeastern coast of the Noto Peninsula in Ishikawa Prefecture, Japan. To investigate the offshore slip distribution, we conducted field surveys and constructed a dataset of tsunami inundation depths. Using this dataset, we evaluated the performance of six previously proposed source fault models - developed based on crustal deformation and tsunami waveform data - in reproducing observed onshore tsunami inundation. Most of the models underestimated the tsunami inundation depth, likely due to insufficient constraints on the offshore slip amount resulting from limited geophysical observations. To address this issue, we introduced high-slip areas into offshore fault segments and tested multiple scenarios. This approach led to substantial improvements in model performance, with an appropriate scenario achieving a geometric mean (K) of 1.32 and a geometric standard deviation (kappa) of 1.95. The substantial improvements in model performance demonstrate that tsunami inundation depth data provide a valuable complementary constraint for modeling offshore slip. This constraint is particularly important in areas where direct geophysical observations are scarce and key features of the slip distribution may otherwise go undetected.

Field investigations and analyses of modern crevasse splay deposits can both elucidate the processes of levee breaching and help to identify past crevasse splay deposits from geologic strata, thereby estimating the magnitudes of ancient river floods. In this study, we studied levee breach processes and crevasse splay deposits in order to determine the distribution characteristics of the inundation area associated with the 2019 flooding of the Chikuma River, Central Japan. The crevasse splay formed by this event can be divided into three regions: proximal, medial, and distal splays. Behind the breached levee, sandy and gravelly sediment piles (proximal splay) formed at both sides of the crevasse channel, whereas sand and mud layers (medial and distal splays) were observed over a wide area within the inundation area, extending beyond the sediment piles. The upstream gravelly sediment pile (proximal splay) was characterized by clearly bounded lower sand and upper gravel layers, reflecting the process of levee breaching: the outer sandy soil of the artificial levee began to be scoured by external erosion, followed by the erosion of the inner gravelly soil. The sedimentary characteristics of the proximal splay deposits appear to have been strongly controlled by the local environment but are useful for inverse analysis of the progressive process of past and future levee breaches. Sandy crevasse splay deposits (medial splay) thinned rapidly away from the breached levee, whereas muddy crevasse splay deposits (distal splay) were thicker at lower elevations, indicating that they formed during the levee breach and stagnant stages, respectively. The distribution of the medial splay (35.7% of the inundation area) was restricted relative to that of the distal splay (~81.7% of the inundation area). This study indicates that it is important to determine the extent of muddy crevasse splay deposits from geologic strata in order to determine the inundation areas of past levee breaches.

Crevasse-splay deposits play an important role in the reconstruction of the magnitude of past flood events and in understanding the behavior of river systems. Despite the extensive studies conducted on the geometry and facies of crevasse-splay deposits, their spatiotemporal developmental processes have remained insufficiently understood. In this study, scaled flume experiments were conducted to study the relationship between the developmental processes of crevasse splays and their characteristics. An experimental flume was set up in a tank to simulate the 2019 Chikuma River flooding event in central Japan. To model the overbank flow, an opening was created on the side of the flume’s wall through which the flow flooded onto a horizontal acrylic plate. The sediment used in the experiments consisted of particles with grain sizes of approximately 0.3 and 0.1 mm, which were determined to be equivalent to bedload gravel and suspended sand in a real-scale river using dimensional analysis. The results of the experiments revealed three important findings: 1) Crevasse-splay deposits initially developed an asymmetric shape extending downstream of the main river channel but gradually showed a symmetric geometry. The river mainstream initially influenced the direction of the inundation flow, but channel bifurcations after the deposition of the sediment piles later changed the geometry of splays into a more symmetric shape. 2) Crevasse-splay deposits developed in two distinct regions (proximal and distal splay), corresponding to sediment transport by bedload and suspended load, respectively. These two regions are commonly observed in the actual field scale. 3) The original overbank flow was a sheet flow without channels, which caused coarse-grained sediments to be spread over a wide area. Subsequently, the accumulation of coarse sands in the developed channel interiors resulted in the buildup of finer-grained sediments upstream of the proximal splay. Thus, the proximal splay deposits became slightly coarse downstream, whereas they rapidly became fine at the boundary with the distal splay. These findings indicate that the characteristics of crevasse-splay deposits vary with the landform’s development stage, thus providing a basis for interpreting their depositional facies.

Massive boulders in landslide and tsunami deposits are prominent geomorphic features in various landscapes. Tracking their movement history is important for reconstructing past geologic dynamics; however, the reworking movements of massive boulders remain unresolved. The boulder field on the Ishigaki Island was formed by repeated tsunamis. Although the individual movement histories of boulders contribute to retrodict the history of different magnitude tsunamis, their radiocarbon ages only correspond to the tsunamis that detached boulders from the reef. Viscous remanent magnetization dating methods have been applied in reworking movements. These methods reveal signals associated with remanent magnetization that gradually grew since the reworking event, which helps to determine the passage of time. The methods were verified by comparison to the radiocarbon ages of un-reworked boulders detached by the recent Meiwa tsunami, while the estimated ages of such two boulders based on the classical relaxation theory contradicted the radiocarbon ages. Here, we show that a method based on the stretched exponential function addressed this contradiction. The reworking movement was estimated using an additional boulder, whose, using our method, radiocarbon age indicated that an older tsunami moved it, whereas the remanent magnetization age unveiled a reworking of the boulder attributed to the Meiwa tsunami.

A decade after the 2011 Tohoku-oki earthquake (Mw 9.0), geological surveys were conducted at multiple sites along the Pacific Coast of the tsunami-inundated Tohoku region in Japan, providing thousands of years of tsunami history. However, the challenges of correlation between historical records and geological tsunami deposits and identifying sources of historical and paleotsunamis have newly surfaced. Particularly the simultaneity and source of the 1611 Keicho tsunami in the Tohoku region and the seventeenth-century tsunami in the Hokkaido region are problematic. To solve such major issues, we conducted a tsunami-deposit survey at Sekinehama on the north coast of Shimokita Peninsula, near the junction of the Japan and Kuril trenches. We performed nondestructive analyses (X-ray computed tomography and micro-X-ray-fluorescence core scanning), grain-size analysis, tephra analysis, and radiocarbon dating of sediments from two coastal outcrops and inland drill cores. We identified five tsunami deposits (TD1–TD5) during the last 6 kyr and correlated them at a 200–400 m distance from the coast. They also correlate with previously identified tsunami deposits around the Shimokita Peninsula. From our study on tsunami deposits, we found other washover deposits in the coastal outcrops that are not represented in the inland cores. These indicate minor washover events related to small tsunamis and infrequent storm surges. The modeled age of the latest tsunami deposit is 500–300 cal yr BP (1450–1650 cal CE). This either correlates with two known tsunamis (the 1611 Keicho tsunami and another seventeenth-century tsunami) or is a previously unknown tsunami that occurred in the fifteenth–seventeenth centuries. If the latest tsunami deposit is to be accurately correlated with tsunami deposits previously identified within a 50-km distance from the study site, we need to consider an unknown fifteenth-century tsunami. Our investigation yields insights regarding the tsunami source in the vicinity of the junction of the Japan and Kuril trenches.

Paleotsunami deposit investigations and numerical tsunami computations have been performed to elucidate the source and size of large tsunamis along the Kuril to Japan Trenches, particularly for unusual tsunamis that occurred in the seventeenth century, the 1611 CE Keicho tsunami (M 8.1) along the Japan Trench and seventeenth-century tsunami (> Mw 8.8) along the Kuril Trench, which caused serious damages on the coastal residents and environments. Moreover, several paleotsunami deposits dating from the thirteenth to eighteenth centuries have been reported along the area between the Kuril and Japan subduction zones, but their sources have not been clarified. In this study, we estimated the tsunami sources from numerical simulations using the distribution of fifteenth- to seventeenth-century tsunami deposits at Sekinehama along the coast of the Shimokita Peninsula. Based on numerical simulations with previously proposed fault models, the tsunami deposits showing similar ages at Sekinehama and another site on the coast of Shimokita Peninsula, which are within 50 km apart, could not be explained except with the huge earthquake models (> Mw 9.1), whose rupture zones extend to not only the Kuril or Japan Trenches but also their flexural area. Thus, we modified or newly proposed twelve fault models located in the flexural area between the two trenches to explain tsunami deposits possibly around the seventeenth century at the above-mentioned two sites on the coast of Shimokita Peninsula. Simulations using these models elucidated that the rupture in the shallow or deep plate boundaries with > 14–32 m slip (> Mw 8.55–8.76) is necessary. If the tsunami deposits around the seventeenth century along the Iburi–Hidaka coast in Hokkaido and those at the two sites mentioned above might be left by an identical event, an interplate earthquake with > 18–40 m slip (> Mw 8.62–9.2) in the flexural area is needed. Moreover, this interplate earthquake might have occurred in the deep plate boundary than in the shallower plate boundary based on slip deficit and slow earthquake distribution data. Our results offer significant insights into a large earthquake (> M 8) along the Kuril and Japan Trenches in the fifteenth to seventeenth century.

This study examines the sedimentary characteristics, provenance, and depositional processes of the storm deposits from 2007 Cyclone Sidr that have been identified on the southern coast of Bangladesh. Three sedimentary units up to 70 cm thick have been identified in fifteen geological core collected at a distance of 135-277 m from the coast. Laboratory analyses of grain size, TOC, TN, delta C-13, and diatom assemblages were carried out to characterize the deposits. Massive to parallel laminated bluish gray mud underlies the storm overwash deposits. White to light gray, massive to parallel laminated, normal graded 1-17 cm thick sand overlies the mud unit with sharp to erosional contact. The mean grain size and thickness of this sand unit decrease landward with increasing sorting value, where grain size distribution of sand is comparable with modern beach sand. The unimodal sand that dominated the base of the storm deposits grades into bimodal olive-gray sandy silt in the upper part of the deposits. The grain size implies that the sand carried from the beaches and mud likely sourced from the suspended, nearshore sediment of the bay and adjacent rivers. TOC/TN, delta C-13 values and the presence of brackish mudflat origin diatoms with silty sand laminae in bluish gray mud indicate a tide-influenced environment. Post-storm sand and sandy silt sediments unit contain freshwater and marine-brackish diatoms that have likely been deposited from flooding caused by storm surge water from the bay and overbank flooded river water due to subsequent heavy rainfall. These sediments were sorted according to storm waves and settled over the study area. This study indicates that the low-lying delta coast gets the influences of high water levels both from the bay and the river during a storm. The modern storm deposit will help to characterize the paleo/prehistoric tropical cyclone in geological time for future studies in this area.

We studied onshore sediment cores obtained from a coastal marsh on the south coast of Beppu Bay, northern Kyushu Island, Japan, to ascertain whether prehistoric intraplate earthquakes in the bay had generated tsunamis, and to establish a chronology and recurrence intervals for these earthquakes. The latter had been inferred from investigations on shallow offshore faults, but the timing of the fault movements has large uncertainties. We identified five sand layers in the up to 8.8 m long sedimentary sequence, which is mainly composed of non-marine organic-rich mud and marine organic-poor mud. Most sand layers, 1-20 cm thick, exhibit sharp upper and lower contacts with the surrounding muds, implying that they were deposited by sudden events. The sand layers are geochemically distinct, with peaks in Si, K, Ca, Ti, Mn, Fe, and Sr, and diatom assemblages dominated by brackish-marine and marine species. This suggests that the sand came from the sea bottom and/or shore rather than from the freshwater marsh. Based on radiocarbon age modeling, the depositional ages of these sand layers are: 3270-3450 cal yr BP, 4250-4510 cal yr BP, 4970-5280 cal yr BP, 5750-6750 cal yr BP, and 6430 -7070 cal yr BP. The oldest deposit is inferred to record a tsunami associated with the 7.3 ka calderaforming eruption of the Kikai volcano, southern Japan. Thus, we exclude it from the calculation of the recurrence interval of tsunamigenic intraplate earthquakes in the bay. Recurrence is approximately 460 -1850 years, including the 1596 CE Keicho Bungo earthquake and a fault rupture 1700-2200 years ago, which have been reported in studies on ruptures of shallow offshore strike-slip and normal faults. Brackish-marine and marine species dominate the fossil diatom assemblages in the upper muddy sand layer (younger than 2750-2870 cal yr BP) above the non-marine organic-rich mud, implying that the muddy sand was deposited in an inner bay or tidal flat environment. The facies change from non-marine to marine sediments after 2750-2870 cal yr BP cannot be explained by a change in the regional sea level because the latter has fallen in the last 6000 years. Therefore, the most likely interpretation for this environmental change is a local coseismic subsidence of the marsh, probably attributed to a rupture of the active fault located south of the marsh. This study is the first to report on recurrent tsunamigenic intraplate earthquakes based on research of onshore tsunami deposits, and suggests that investigating tsunami deposits can contribute to an assessment of tsunami risks for intraplate earthquakes elsewhere. (c) 2021 Elsevier Ltd. All rights reserved.

The Eastern Mediterranean Basin (EMB) is under the threat of tsunami events triggered by various causes including earthquakes and landslides. We propose a deployment of Offshore Bottom Pressure Gauges (OBPGs) around Crete Island, which would enable tsunami early warning by data assimilation for disaster mitigation. Our OBPG network consists of 12 gauges distributed around Crete Island, with a 100‐km interval, based on three criteria to select the locations. The station network must have a good azimuthal coverage and have enough (>50 km) distance from the coast, and the OBPGs are placed at the locations where the most energetic wave dynamics occur, which is confirmed by Empirical Orthogonal Function (EOF) analysis of pre‐calculated tsunami scenarios. We demonstrate three test cases comprising a hypothetical seismogenic tsunami in east Sicily, a hypothetical landslide tsunami in the Aegean Sea, and the real tsunami event of the May 2020 off the Crete earthquake. Our designed OBPG network achieves an accuracy of 88.5% for the hypothetical seismogenic tsunami and 87.3% for the hypothetical landslide tsunami with regard to the forecasting of first tsunami peak. For the real event of May 2020, it predicts the tsunami arrival at tide gauge NOA‐04 accurately; the observed and forecasted amplitudes of the first wave are 5.0 cm and 4.5 cm, respectively. The warning lead time for the May 2020 event was ~10 min. Therefore, our results reveal that the assimilation of OBPG data can satisfactorily forecast the amplitudes and arrival times for tsunamis in the EMB.

Numerous studies have attempted to use boulder deposits in coastal zones for assessing the hazard due to tsunamis and/or storms. One critical problem is still related to the determination of boulder ages. Although the age of wave-emplaced boulders can often be obtained through 14C and U/Th dating, marine organisms that were killed during transport or shortly afterward are required for these dating methods to provide accurate ages. In addition, these approaches cannot determine ages of multiple movements of single boulders with complex transport histories. Paleomagnetic dating offers the potential to overcome current challenges for dating the dislocation coastal boulders. After dislocation, wave-emplaced boulders successively acquire a viscous remanent magnetization (VRM) that is parallel to the Earth's geomagnetic field. Since the formation of the VRM is a function of site-specific ambient temperature and time, VRM can be used to determine ages for the emplacement of tsunami and storm boulders. This chapter summarizes state of the art sampling, measurement, and analysis strategies for VRM dating of coastal boulders. First case studies from Ishigaki Island, Beppu Bay, and the Sanriku coast (all Japan) illustrate the potential and current limitations of the method when applied to date the dislocation of tsunami and storm boulders.