Magnitude information is an essential measure when assessing risks of large-scale earthquakes and tsunamis. Tsunami deposit characteristics, inundation characteristics, tsunami hydrodynamics, and sedimentation processes are the keys to obtaining magnitude information. Research on the 2011 off the Pacific coast of Tohoku Earthquake and Tsunami covering both terrestrial and subaqueous environments offers a basis for drawing a comprehensive picture of tsunami sedimentology, and increasing our ability to extract magnitude information. Issues related to estimating magnitude information related to a tsunami from its deposits are discussed in the context of some recent and earlier research on the 2011 and 869 Jogan earthquakes and tsunamis, and implications for future research are presented.
We present geologic evidence for middle-late Holocene paleo-tsunamis and coseismic uplift based on analyses of coastal sediments from the Shizuoka Plain at the west coast of Suruga Bay and Minami Izu at the southern Izu Peninsula, Japan. We also discuss tsunami deposits from a coastal lowland area at Shimoda city, southern Izu Peninsula, Japan, as identified in sediment cores (8–10 m long) from eight sites. The results are summarized as follows. 1. Three possible tsunami deposits detected from the Shizuoka Plain are dated at ca. AD 1000, 3565–3486 cal BP, and 4080–3900 cal BP (Kitamura et al., 2013a). These deposits are not found at the southern Izu Peninsula. 2. In the coastal lowland area of downtown Shimoda, at least four washover sand beds occur in back-marsh deposits that are younger than 3500 cal BP. 3. Emerged marine sessile assemblages indicate that three episodes of coastal uplift have occurred at the southern Izu Peninsula, at AD 570–820 (uplift of 0.9–2.0 m), AD 1000–1270 (0.3–0.8 m), and AD 1430–1660 (1.9–2.2 m)(Kitamura et al., 2014). These paleo-earthquakes may have occurred on an unknown, reverse fault located offshore from Shimoda.
Tsunami deposits in Northeastern Japan produced by the 2011 off the Pacific coast of Tohoku Earthquake and tsunami contain relatively high concentrations of arsenic and heavy metals. Because the impacts of tsunamis on the distribution and behavior of arsenic and heavy metals should be considered carefully in surface soils and the underground environment after a major disaster, standards are needed for seawater leaching tests to identify arsenic and heavy metals in tsunami deposits. For that purpose, tsunami deposits from the 2011 off the Pacific coast of Tohoku Earthquake were sampled at Kesen-numa city, Minamisanriku town, Ishinomaki city, and Shiogama city in Miyagi Prefecture, Northeastern Japan. Seventeen inorganic elements including As and heavy metals (Li, Sc, V, Cr, Co, Ni, Cu, Zn, As, Rb, Sr, Mo, Cd, Cs, Ba, Pb, U) were measured in a solution leached from tsunami deposits with pure water, artificial seawater, and natural seawater using inductively coupled plasma-mass spectrometry (ICP-MS) and hydride generation-atomic absorption spectrometry (HG-AAS). In the leachate obtained using pure water, concentrations of As and heavy metals were quantified successfully with the ICP-MS method. Concentrations of V, Cr, Co, Ni, Cu, and Zn did not differ from background levels in artificial seawater and natural seawater. When measuring As in seawater leaching tests, HG-AAS is a useful method for performing high-sensitivity analyses. In particular, HG-AAS is superior to ICP-MS for detecting low As concentrations (less than 30 μg/l) in a seawater leaching test.
A total of 129 samples of tsunami deposits were collected immediately after the 2011 off the Pacific coast of Tohoku Earthquake at 5 km intervals over the 250 km length of the tsunami inundation zone. They were analyzed for 10 major rock-forming elements, arsenic, and base metals (Cu, Zn, Pb, and Cd) with EDXRF. Statistical analysis indicates that the populations of arsenic and heavy metals—with the exception of Cd, which had values below the detection limit—comprise two log-normal distributions divided by a threshold value: 89 ppm Cu, 245 ppm Zn, 48 ppm Pb, and 21 ppm As. Spider diagrams of their anomalous values normalized by average marine sediments in Sendai bay indicate a characteristic pattern in each mineralization province. They are coincident to river sediment anomalies detected before the tsunami and are also compatible with the production of disused mines located in the background area. We conclude that almost all base metals and arsenic anomalies in the tsunami deposits distributed on the Sanriku coast and Sendai Plain by the 2011 off the Pacific coast of Tohoku Earthquake are affected by specific geological and mineralogical properties.
Geochemical analyses (total nitrogen: TN, total organic carbon: TOC, total sulfur: TS, and stable sulfur isotope ratio: δ34S) were performed on tsunami deposits by the 2011 off the Pacific coast of Tohoku Earthquake obtained from Iwate, Minamisanriku, Kesen-numa, and Sendai Bay areas for a total of 133 samples. Although TOC/TN ratios ranged widely (0.7–90), most of the samples (more than 80% of samples) ranged from 10–30, which were similar results to those for marine surface sediment in the Pacific coast area. Low δ34S values (from -29.6‰ to -11.3‰) from Iwate, Minami Sanriku, and Sendai Bay tsunami deposits were observed. Generally, such low δ34S values result from large isotope fractionation caused by the activity of sulfate-reducing bacteria in marine sediment under a sulfate (SO42-) rich environment. These results suggest that the tsunami deposits were mainly derived from marine sediment. High correlations between TS and As contents (r=0.958) were observed in tsunami deposits from the Kesen-numa area. Furthermore, the high δ34S values (from -3.1‰ to -0.7‰) in the tsunami deposits from the Kesen-numa area correspond well with those of hydrothermal ore samples from the Kesen-numa area. These results indicate that sulfur and arsenic in tsunami deposits from the Kesen-numa area originated in slag from the mines.
The great tsunami that followed the 2011 off the Pacific coast of Tohoku Earthquake caused derangement of superficial sediments in coastal areas of Sendai Plain and Sendai Bay. Tsunami deposits were transported with flooded seawater in the form of mud, sand, or granules, and were deposited in the coastal region. These deposits record important information that facilitate determining the age and frequency of past tsunami events. The origins of tsunami deposits and the derangement of marine sediments are studied based on a chemical composition analysis of tsunami deposits, marine sediments, and samples of subsurface soil collected before the earthquake. The solubility of chemical components, such as of chromium, iron, nickel, copper, zinc, arsenic, and lead, in 1 M HCl was much higher for tsunami deposits than subsurface soils. The metal speciation and correlation of chemical composition indicate that the main origins of tsunami deposits are coastal marine sediments. The transformation of marine sediments to the coastal side, except Arahama offing, is also confirmed from active regional shifts of bacteria. These imply that the tsunami had a large effect on mobilizing marine sediments in the northern area of Sendai Bay, Nanakita River offing, Arahama offing, and Hiroura offing based on the extent of bacterial activity shift and chemical compositions of organic materials.
Ancient tsunami deposits are distributed in paddy fields throughout the Sendai plain, on the Pacific coast of the Tohoku area in northeastern Japan. These deposits contain gravel, mud, shells, and microfossils originating from the seafloor, sandy beaches, and coastal soils. Paleotsunami deposits offer valuable clues for identifying past tsunami inundation areas, but previous studies had difficulty dating tsunami deposits for correlation with historic disaster events. This study collects age data from continuous soil profiles that include tsunami deposits. Nine continuous soil sediment samples are collected using a Handy Geoslicer (Fukken Co. Ltd.) near the coast of Sendai Bay in northeastern Japan. The soil slices (HS1–9) are 84–183 cm long and consist of cultivated surface soils, peaty clay, silt, and fine to medium sands. Slices HS2–7 are subsampled at 1-cm intervals and water contents of 689 discrete samples are measured. Plant residues are extracted from the samples for radiocarbon (14C) dating with a Tandetron accelerator mass spectrometry system (Model 4130 AMS, HVEE) at the Center for Chronological Research, Nagoya University. Water contents range from 6.2 to 83.0 wt.%, and the lowest values are observed in layers of well-rounded medium sand. Variations in water content agree well with changes in sedimentary facies. Total organic carbon contents of plant residues range from 45.9 to 54.5 wt.% (50.4 wt.% on average) and their stable carbon isotope ratios range from -25.1 to -30.1 permil (vs. PDB), which are consistent with the ratios of modern terrestrial C3 plants. The calibrated ages of plant residues from these tsunami deposits are about 1000–1300 cal BP, which agree well with the date of the Jogan Earthquake and tsunami on the Sendai plain (869 AD).
Understanding the geochemical characteristics of tsunami deposits assists in revealing the mechanisms of tsunami inundation, assessing environmental risks, and using tsunami deposits industrially. Understanding geochemical characteristics is complex because geochemical compositions result from an accumulation of various known and unknown processes, such as original compositions of the hinterland, weathering, and transport. This study applies the principal component analysis (PCA) to the bulk chemistry of tsunami deposits generated by the 2011 off the Pacific coast of Tohoku Earthquake sampled from coastal areas ranging from Iwate prefecture to Fukushima prefecture. The PCA creates a set of uncorrelated synthetic variables called principal components (PCs) from a set of correlated observational data using an orthogonal transformation. By ignoring principal components that have small statistical variances, an important low-dimensional subspace can be extracted from a high-dimensional dataset. In addition, synthetic variables that have large statistical variances are considered to reflect important factors that control variations of datasets. The PCA was performed on whole-rock compositions of 18 major elements and heavy-metal elements. The first principal component (PC1) shows a clear inverse correlation between Si and other elements, which is considered to reflect the amount of sand composed of quartz and other silicate minerals. The second principal component (PC2) is characterized by a clear inverse correlation between minor elements such as lithophile and siderophile elements and chalcophile elements. The PC2 is considered to reflect the degree of enrichment of chalcophile elements caused by the interaction between original sediments and seawater. The third principal component (PC3) is characterized by an inverse correlation between Na and other major elements. The PC3 is considered to reflect the interaction between original sediments and seawater. Geological implications are not identified for other minor PCs, which have small variances (< 10%), probably due to observational noise and a combination of several known and unknown factors. The above three factors—amount of sand (PC1), enrichment of the chalcophile elements (PC2), and interaction of seawater (PC3) —are important processes that contribute to chemical variations of tsunami deposits generated by the 2011 off the Pacific coast of Tohoku Earthquake. These results and other geological evidence might be useful for understanding the mechanisms of tsunami inundation.
Long-term dissolution experiments from waste dumps that includes marine sediment under both anoxic and aerobic conditions were performed to elucidate the As dissolution mechanism and assess As dissolution risk. Core samples drilled at a waste disposal site in Sendai were collected for the experiment. Almost completely oxidized and partially oxidized portions of a drilling core were both characterized by naked-eye observation and were used for the elution experiments. XANES analyses reveal that the chemical state of As in the partially oxidized samples is As (V) and As (III) or sulfide form, and acidic solutions are formed by a reaction with a hydrogen peroxide solution, indicating the existence of sulfide minerals. The arsenic state in the almost completely oxidized sample is mainly As (V). There is no significant difference in As concentrations in both anoxic (maximum concentration: 4.30 μg/l) and aerobic extracts (4.16 μg/l) from the almost completely oxidized sample, and the predominant As species in both solutions are arsenate. On the other hand, As concentrations in extracts from partially oxidized samples under an anoxic condition (maximum concentrations of two samples: 25.9 and 24.7 μg/l) tend to be higher than those from aerobic samples (10.9 and 4.96 μg/l). Experiments to separate aqueous arsenic speciation show that arsenate is the predominant As species in the extracts under an anoxic condition, whereas both arsenate and arsenite coexist in an aerobic solution. Water samples from wells and pond at the waste disposal site were also analyzed to clarify water quality (pH and toxic element concentrations) before release into a natural river near the waste disposal site. These water samples are, in general, characterized by low dissolved oxygen (less than 1 mg/l), and experimental results are similar to those under an anoxic condition. Arsenate is the predominant aqueous As species. Dissolution of As is enhanced under more alkaline conditions (greater than pH 8.5), indicating that arsenate sorbed onto a mineral surface is possibly released.