13:45 〜 14:00
[SIT20-01] 豪州大陸下のX不連続面: レシーバ関数とマルチモード表面波の同時ベイズ推定によるマッピング
★招待講演
キーワード：X不連続面、レーマン面、レシーバ関数、表面波、ベイズ推定、上部マントル
Upper mantle discontinuities (X-discontinuities; X-Ds) below the Lithosphere-Asthenosphere Boundary (LAB) in Australia are mapped using the joint Bayesian inversions with P-wave receiver functions (P-RFs) and multimode surface-wave dispersions (SWDs). The X-D is a seismic interface beneath LAB, accompanying the velocity increase, which can be linked to the base of the asthenosphere (e.g., Hua et al., 2023), Lehmann discontinuity (L-D) (e.g., Deuss et al., 2004; Caló et al., 2016), and phase transition of pyroxene (e.g., Williams & Revenaugh, 2005; Akashi et al., 2009). This discontinuity was investigated by many seismologists using regional or teleseismic body waves (e.g., Lehmann, 1961; Deuss & Woodhouse, 2004; Pugh et al., 2021). Several previous studies in the Australian continent detected seismic discontinuities around 200–320 km depths (Hales et al., 1980; Drummond et al., 1982; Revenaugh & Jordan, 1991; Taira & Yoshizawa, 2020). Gaherty & Jordan (1995) and Taira & Yoshizawa (2020) suggested that one of the X-Ds, which may be equivalent to the L-D, is accompanied by the weakening of radial anisotropy. However, many earlier seismological studies were limited to localized investigations, leaving the spatial distributions of X-Ds across Australia unresolved.
In this study, we applied the trans-dimensional Bayesian inversion jointly using P-RFs and SWDs to permanent and temporary broadband seismic stations in Australia. Compiling the inversion results for all the employed stations across Australia, we mapped the Ps conversion points (CPs) from X-Ds associated with the positive S-wave speed jumps (> 0.04 km/s) beneath the LAB. We identified X-D CPs at multiple depths (~170 km, 220–280 km, and ~310 km) beneath the Australian continent, below which the radial anisotropy generally weakens.
The shallowest X-D (~170 km: 170km-D) is predominantly found beneath eastern Australia, whose spatial distribution is consistent with an interface with a positive velocity gradient around 150 km (PVG-150) found by an earlier global S-wave RF study (Hua et al., 2023). They suggested the PVG-150 reflects the base of a partially molten layer in the asthenospheric mantle. In eastern Phanerozoic Australia, where the lithosphere is thinner (70–100 km) than cratonic Australia, our inversion results indicate a very low isotropic S-wave speed of approximately 4.3–4.4 km/s beneath the shallow LAB, which can be attributed to the high-temperature asthenosphere (~1300℃) (Tesauro et al., 2020). Although we do not find clear evidence of the partial melt, the 170km-D, associated with the clear velocity jump beneath the shallow LAB, likely represents the base of the low seismic velocity zone due to the thermal effects in the asthenosphere.
The deeper X-Ds around 200–300 km are distributed across the entire continental regions in Australia, which may correspond to the L-D or phase transitions of pyroxene. These depths are consistent with the phase transition depths from the orthopyroxene to high-pressure clinopyroxene (Akashi et al., 2009; Jacobsen et al., 2010) and/or the coesite to stishovite (Akaogi et al., 1995; Ono et al., 2017), roughly matching the thermal condition of the Australian continent (Tesauro et al., 2020). Thus, the deeper X-Ds may partially originate from these phase transitions.
In addition, the X-Ds identified in this study can also be associated with the weakening of radial anisotropy. Karato (1992) suggested that the L-D reflects a transition in the rheological behavior of constituent materials from dislocation to diffusion creep, based on laboratory experiments. This transition can be seismologically observed as a change in anisotropic properties, i.e., from radial anisotropy to isotropy across the L-D, as reported by Gaherty & Jordan (1995). Therefore, one or more of the detected X-Ds in this study may correspond to the Lehmann discontinuity.
In this study, we applied the trans-dimensional Bayesian inversion jointly using P-RFs and SWDs to permanent and temporary broadband seismic stations in Australia. Compiling the inversion results for all the employed stations across Australia, we mapped the Ps conversion points (CPs) from X-Ds associated with the positive S-wave speed jumps (> 0.04 km/s) beneath the LAB. We identified X-D CPs at multiple depths (~170 km, 220–280 km, and ~310 km) beneath the Australian continent, below which the radial anisotropy generally weakens.
The shallowest X-D (~170 km: 170km-D) is predominantly found beneath eastern Australia, whose spatial distribution is consistent with an interface with a positive velocity gradient around 150 km (PVG-150) found by an earlier global S-wave RF study (Hua et al., 2023). They suggested the PVG-150 reflects the base of a partially molten layer in the asthenospheric mantle. In eastern Phanerozoic Australia, where the lithosphere is thinner (70–100 km) than cratonic Australia, our inversion results indicate a very low isotropic S-wave speed of approximately 4.3–4.4 km/s beneath the shallow LAB, which can be attributed to the high-temperature asthenosphere (~1300℃) (Tesauro et al., 2020). Although we do not find clear evidence of the partial melt, the 170km-D, associated with the clear velocity jump beneath the shallow LAB, likely represents the base of the low seismic velocity zone due to the thermal effects in the asthenosphere.
The deeper X-Ds around 200–300 km are distributed across the entire continental regions in Australia, which may correspond to the L-D or phase transitions of pyroxene. These depths are consistent with the phase transition depths from the orthopyroxene to high-pressure clinopyroxene (Akashi et al., 2009; Jacobsen et al., 2010) and/or the coesite to stishovite (Akaogi et al., 1995; Ono et al., 2017), roughly matching the thermal condition of the Australian continent (Tesauro et al., 2020). Thus, the deeper X-Ds may partially originate from these phase transitions.
In addition, the X-Ds identified in this study can also be associated with the weakening of radial anisotropy. Karato (1992) suggested that the L-D reflects a transition in the rheological behavior of constituent materials from dislocation to diffusion creep, based on laboratory experiments. This transition can be seismologically observed as a change in anisotropic properties, i.e., from radial anisotropy to isotropy across the L-D, as reported by Gaherty & Jordan (1995). Therefore, one or more of the detected X-Ds in this study may correspond to the Lehmann discontinuity.