Evaluation of Geochemical Characteristics of Tsunami Deposits by the 2011 off the Pacific Cost of Tohoku Earthquake Using Dimensionality Reduction with a Principal Component Analysis

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.