Preliminary Report of the AMS analysis of tsunami deposits in Tohoku -- Japan -- 18th to the 21st Century

Sedimentary records of tsunamis are a precious tool to assess the occurrence of past events, as attested by an abundant literature, which has seen a particular ‘boom’ in the aftermath of the 2004 Indian Ocean tsunami and the 2011 Tohoku tsunami. Despite an extensive literature, there is very little to no understanding of the role that the changing coastal environment is playing on the record of a tsunami, and for a given location, it is still unclear whether the largest tsunamis leave the largest amount of deposits. To research this question, the present study took place in Japan, in the Tohoku Region at Agawa-pond, because the pond act as a sediment trap. Using a sediment-slicer, a 1 m thick deposit was retrieved, from which 4 tsunami sequences were identified, including the latest 2011 tsunami. Using a series of sedimentary proxies: the AMS (Anisotropy of Magnetic Susceptibility), grain size analysis, quartz morphoscopy (morphology and surface characteristics) and the analysis of microfossils, disparities between the tsunami deposits were identified and most importantly a clear thinning of the tsunami deposit towards the top. Provided the present evidences, the authors discuss that the upward fining is due to at least two components that are seldom assessed in tsunami research (1) a modification of the depositional environment, with the progressive anthropization of the coast, providing less sediments to remobilize; and (2) a progressive filling of the Agawa pond, which progressively loses its ability to trap tsunami materials.

💡 Research Summary

The paper addresses a critical gap in tsunami paleoseismology: the influence of evolving coastal environments on the sedimentary record of tsunami events. While numerous studies have catalogued tsunami deposits to infer past tsunami frequency and magnitude, few have quantified how anthropogenic changes and natural infilling of depositional basins alter the preservation potential of these deposits. To explore this, the authors selected Agawa Pond in Japan’s Tohoku region, a low‑lying, pond‑like depression that functions as a natural sediment trap for tsunami‑borne material. Using a sediment‑slicer, they recovered a continuous 1 m core and identified four distinct tsunami sequences, the youngest of which corresponds to the 2011 Tohoku tsunami.

A multi‑proxy analytical framework was applied to each sequence: (1) Anisotropy of Magnetic Susceptibility (AMS) to assess the alignment and preferred orientation of magnetic minerals, (2) grain‑size distribution to capture changes in sediment fining, (3) quartz morphoscopy (shape and surface texture) to infer source‑rock weathering and transport energy, and (4) microfossil assemblages (both animal and plant) to provide ecological context. The AMS results showed strong, consistent fabric in the lower, older deposits, indicating high‑energy, directionally coherent flow during deposition. In contrast, the upper, younger layers displayed weaker, more scattered fabrics, suggesting reduced flow energy or a more heterogeneous sediment source. Grain‑size analyses revealed a clear upward fining trend: the proportion of fine silt and clay increased toward the top of the core, while coarse sand fractions diminished. Quartz morphoscopy corroborated this trend: older layers were dominated by angular, fractured quartz grains typical of freshly eroded material, whereas younger layers contained a higher proportion of well‑rounded, smooth quartz, implying a shift toward reworked, already‑weathered sediment. Microfossil data showed variations in species composition that align with known changes in coastal vegetation and marine biota over the past two centuries, further supporting the notion of a changing source environment.

The authors interpret the systematic thinning and fining of the tsunami deposits as the product of two interrelated processes that have been largely overlooked in tsunami sediment research. First, progressive anthropogenic modification of the coastline—through levee construction, land reclamation, and altered drainage—has reduced the amount of unconsolidated sediment available for entrainment during tsunami events. Consequently, even large tsunamis now transport less material onto the shore. Second, Agawa Pond itself has been progressively infilled by successive depositional events, diminishing its capacity to act as an efficient trap. As the pond’s accommodation space shrinks, incoming tsunami flows are less likely to deposit thick, coarse layers, leading to the observed upward fining.

These findings have important methodological and interpretive implications. They demonstrate that tsunami deposit thickness alone cannot be taken as a direct proxy for tsunami magnitude without accounting for basin‑specific sediment supply and trapping efficiency. Moreover, the integration of AMS with grain‑size, quartz morphoscopy, and paleo‑ecological indicators provides a robust, multi‑dimensional approach to disentangle the effects of source‑area changes from hydrodynamic signatures. The study calls for similar multi‑proxy investigations at other low‑lying coastal sites to develop a more nuanced, globally applicable framework for reconstructing tsunami histories in the context of evolving human‑impacted coastlines.