Grain-size distributions of the sand layers and reference sands of potential sources.

Grain-size distributions of the sand layers and reference sands of potential sources.</p

Biplots of sorting and mean diameter on grain size distribution.

The left and right figures indicate the west area and the east area, respectively.</p

Geological columnar sections.

Arrows indicate calibrated radiocarbon dates (ka). The gray numbers are the dates using the bulk sample. Circles indicate sand layers targeted for grain size analysis. The inserted map is the same as Fig 1C and shows the location of the boring sites and survey lines.</p

Maps of the study area.

(a) Overall view and earthquake rupture zone. Triangles indicate volcanoes that are the source of tephras. The solid gray box and the gray shading indicate the rupture zones of the historical and reconstructed paleo-earthquakes, respectively. The dotted lines show the distribution of active faults. The star indicates the epicenter of the 1982 Urakawa-oki earthquake. The solid black box marks the range shown in Fig 1B. (b) DEM data for coastal areas (Geospatial Information Authority of Japan: https://fgd.gsi.go.jp/download/menu.php) and tsunami deposit study sites. (c) Topographic classifications and coring sites observed tephra layers in the Kabari area. The circles with grayscale indicate the layer thickness of Sand KS1. Triangles indicate sites where Sand KS1 was not identified. Stars indicate coring sites reported by Takashimizu et al. [11]. The red double line shows the distribution limits of Sand KS2. Black and green arrows indicate sites where modern reference samples of the grain size and diatom analysis were collected, respectively. The reference sample legends are B: beach, R: river, Br: beach ridge, M: river mouth, and D1–D4 corresponding to the diatom samples shown in Table 2.</p

Depositional ages of the sand layers based on radiocarbon dating.

(a) Age-depth model in Site W2. The light and dark gray histograms show the calibrated and modeled probability density functions, respectively. The blue solid lines and blue shade indicate the ranges of the 2σ and 1σ modeled ages, respectively. The red bands indicate the ages of the tsunami events reported [19] and the historical records. The dotted lines show the sedimentation rate trends of the upper (Us-b–Sand KS2_L) and lower strata (Sand KS3_U–B-Tm) based on the average values. The solid black lines show sedimentation rates of 0.016 cm/year and 0.020 cm/year as a reference. (b) The summary of the calibrated ages measured in the peat layers above and below Sand KA and Sand KB each boring site along survey line E. The thick and thin black lines indicate the ranges of the 2σ and 1σ calibrated ages, respectively. The gray bars show the estimated depositional ages of the sand layers.</p

Core photographs and X-ray CT images at Sites W2 and E2.

The CT mode profiles are plotted in correspondence with the CT images. For Site W2, a CT side image is shown, and its cross-sectional location is indicated by the arrows. The sketches display the interpretation of sedimentary structures of the sand layers and tephra layers.</p

Wide-area map of northern Hidaka and aerial photograph in 1944.

Wide-area map of northern Hidaka and aerial photograph in 1944.</p

Results of radiocarbon dating.

Geological evidence, such as tsunami deposits, is crucial for studying the largest rupture zone of the Kuril Trench in Hokkaido, Japan, due to its poor historical record. Although 17th-century tsunami deposits are widely distributed across Hokkaido, the presence of multiple wave sources during that period, including the collapse of Mt. Komagatake, complicates the correlation with their wave sources. Understanding the regional distribution of these tsunami deposits can provide valuable data to estimate the magnitude of megathrust earthquakes in the Kuril Trench. The northern part of Hidaka, Hokkaido, where tsunamis from multiple wave sources are expected to overlap, is distant from the Kuril Trench. To clarify the depositional history of tsunami deposits in such distal areas, evaluating the influence of the depositional environments on the event layer preservation becomes even more critical. We conducted field surveys in Kabari, located in the northern Hidaka region, identifying three sand layers from the 10th to the 17th century and two layers dating beyond 2.3 thousand years ago. The depositional ages of most sand layers potentially correlate with tsunami deposits resulting from the Kuril Trench earthquakes. Utilizing reconstructed paleo-sea level data, we estimated that most sand layers reached approximately 2 m in height. However, it is noteworthy that the latest sand layer from the 17th century exhibited an unusual distribution, more than 3 m in height. This suggests a different wave source as the Mt. Komagatake collapse. The discovery of multiple sand layers and their distributions is crucial to constraining the maximum magnitude of giant earthquakes in the Kuril Trench and understanding the volcanic tsunami events related to Mt. Komagatake.</div

Diatom assemblages and close-up photographs near Sands KA and KB.

Dark gray and light gray boxes indicate sand layers and silt layers, respectively.</p

Reconstructed depositional environments and relative sea-level curves in the Kabari area.

(a) Interpretation of the depositional environments along Line E is summarized from diatom assemblage analysis. (b) The results of SLIPs obtained in the Kabari area are summarized. The width and height of the T-shape symbols indicate the dating error and the altitude difference between SLIPs and mean sea level as tidal change range, respectively. The solid gray lines show the relative sea-level change curves, which are a combination of the local crustal deformation [26] and the GIA model obtained for the Shimokita Peninsula [58].</p