[M-GI31] 法地質学への招待

[MGI31-P03] Forensic research of beach sand collected from Aomori in Japan: an application of rock magnetic and chemical analyses

*川村 紀子¹、松下 拓哉¹、板宮 裕実²、杉田 律子² (1.海上保安大学校　基礎教育講座、2.科学警察研究所)

キーワード：法地質学、磁気測定、海岸堆積物

When concealments and smugglings have occurred on beaches, for a successful criminal conviction to occur, it is essential to physically and chemically investigate beach sediments. Previous studies suggest that the microtextural analysis of quartz is useful for characterizing beach sediments (Itamiya et al., 2019; 2020; 2022). In order to suggest the newly method to characterize the particle of beach sediments, we applied chemical and magnetic analyses to the samples collected at nine sites (from 1M-1 to 1M-9) from Shimokita peninsula in Aomori; the northernmost part of the main island in Japan. Sites 1M-1, 1M-2, 1M-3, 1M-4, 1M-5, and 1M-6 are located at the eastern coast of peninsula; the Pacific Ocean side. While the other sites: 1M-7, 1M-8, and 1M-9 are located in the western coast which faces toward Mutsu Bay. The sedimentary rock which was formed from Pleistocene to Holocene have been distributed, while Osore and Towada volcanoes are located near the sampling sites. Rock magnetic and chemical analyses of the beach sediment samples were performed. Results indicate that the beach sediments vary according to the lithology. Concentration-dependent magnetic parameters increase southward. The direction of the coastal drift is from the south to the north, suggesting that volcanic ash which is rich in magnetic minerals from Towada (basaltic) volcano are supplied to the sampling sites. The sediment collected at 1M-4 where are round Sabishiro Mine indicates the maximum value of anhysteresis remanent magnetization in the samples. Magnetic grain size parameters: Mrs/Ms and Hcr/Hc values are also reflected the lithology. Sites 1M-1, 1M-2, and 1M-7 (which dacite are distributed) are plotted in the pseudo-single domain (PSD) region for (titano-) magnetite on the Day plot. While the other samples are plotted on other regions. This indicates that fine-grained magnetic minerals are assembled at the sites. Thermo-magnetometry results suggest that goethite (FeOOH), pyrrhotite (Fe₇S₈), magnetite (Fe₃O₄), hematite (αFe₂O₃), and (titano) maghemite (rFe₂O₃) are common in the sediment samples at the Pacific side sites. Chemical analysis results of quartz inclusions indicate that the element distributions are different with the sites. Results of magnetic properties and chemical analyses of the beach sediments indicate that the origin of sand particles can be characterized andesite and/or basalt. We can suggest as the useful methods for forensic investigations.