Study of deep mantle convection by investigating seismic scattering

2020 Fiscal Year Final Research Report

• Project/Area Number: 18K03784
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Review Section: Basic Section 17040:Solid earth sciences-related
• Research Institution: Kyushu University
• Principal Investigator: Kaneshima Satoshi 九州大学, 理学研究院, 教授 (80202018)
• Co-Investigator(Kenkyū-buntansha): ヘルフリック ジョージ 東京工業大学, 地球生命研究所, 研究員 (80747163)
• Project Period (FY): 2018-04-01 – 2021-03-31
• Keywords: 下部マントル / 不均質構造 / 化学組成 / 地震波アレイ解析 / SP散乱波 / 玄武岩物質

Outline of Final Research Achievements

We investigate global distribution of S-to-P scatterers in shallow to mid-lower mantle beneath subduction zones, where deep seismicity extends down to the bottom of the upper mantle. By array processing broadband and short period wave form data obtained at seismic networks, we seek anomalous later phases in the P codas. The later phases usually arrive along off-great circle paths and significantly later than S-to-P conversion at the "660 km" discontinuity and are adequately regarded as scattered waves. The S-to-P scattered waves often show amplitudes comparable to the "S660P" waves, which indicates that a spatial change in elastic properties by several percent occurs at the scatterers as abruptly as the post-spinel transformation and should arise from compositional heterogeneity. Nearly half of S-to-P scatterers are shallower than 1000 km, and the number of scatterers decreases with depth.

Free Research Field

固体地球物理学

Academic Significance and Societal Importance of the Research Achievements

この研究課題はそれ自体としては特段の社会的意義を持つものではないが、地質学的な時間スケールにおけるマントル全体の物質循環の変遷を明らかにする際に鍵となる観測事実を与えることを目指している。マントル深部と地球表層の間に起きる物質循環の問題は、地球表層の環境や生命進化と深く関わる地球科学の最重要課題の一つである。