[S-SS07] 地震波伝播:理論と応用

[SSS07-P06] Antipodal Observations of Earth’s Middle Inner Core

Butler Rhett²、*坪井 誠司¹、中川 剛史¹ (1.海洋研究開発機構、2.ハワイ大学)

キーワード：地球内核、地震対蹠点、スペクトル要素法、大規模数値計算

Earth's inner core structure holds influence over the outer core convection, its thermal history and exchange of heat with the outer core, the magnetic field and its generation, its iron crystalline growth and evolution from its beginnings, its composition with lighter elements in the iron alloy and slow shear wave speeds, its anisotropic seismic velocities, and heterogeneity within its spherical form. Many seismological studies have been brought to bear upon this tiny fraction (<1%) of Earth's volume. Our approach has been to use antipodal (>179 degree) data to constrain seismic structures and properties illuminated by the 360 degree azimuthal convergence of seismic energy and its concomitant amplification. Shallow earthquakes (<55 km depth) are followed by surface reflections (i.e., pPKIKP) and their timing permits study of the upper 250 km of the inner core, but which mask the middle inner core. To see deeper into the middle inner core, we must employ intermediate depth earthquakes (>100 km) wherein surface reflections follow and do not interfere with potential middle inner core phases sampling its deeper structure. To obtain a more complete antipodal data set we reviewed all events > Mw 5.7 and > 179 degree distant from permanent and temporary stations between 1980 and 2022, resulting in 605 candidates. Of these, 62 event-station pairs were from intermediate depth earthquakes-reviewing this data set we found 7 good candidates with sources and receivers antipodal between China-Malaysia-Indonesia and western South America. These antipodal pathways are neighboring the event-stations studied by Butler and Tsuboi (2021), which were compatible with `PREM-like' structure in the upper 250 km of the inner core. Using the JAMSTEC Earth Simulator-4 and the SPECFEM3D_GLOBE software, synthetic seismograms were generated from the intermediate depth, earthquake centroid moment tensors (CMT) and the PREM radial earth model with ellipticity. For most cases, we use 18816 cores of the Earth Simulator 4 with NEX=1792 and the minimum period of the synthetics is about 2.4 sec and also we use 41,334 cores with NEX=2656 and the minimum period is about 1.6 sec for limited cases. This initial analysis shows that PREM fits the primary features of the antipodal PKIKP data within the middle inner core, which is commensurate with the structure of the top 250 km of the inner core. Further analysis will be presented examining the finer structure in the data to resolve additional features. In the Butler and Tsuboi (2021) analysis of shallow earthquakes, the antipodal diameters orthogonal to Chile-China were found to be highly heterogenous compared to the `PREM-like' diameters between Chile-China. However, none of these orthogonal pathways through the middle inner core are part of the 1980–2022 intermediate-depth data set for Earth. Future sites for seismic stations that would be optimal for sensing Earth’s middle inner core are in Africa opposite the intermediate depth and deep Tonga-Kermadec subduction zone, or in southern Spain opposite the New Zealand Bay of Plenty.