10:15 〜 10:30
[SIT18-06] Simultaneous Analysis of Seismic Velocity and Electrical Conductivity for Subsurface Imaging of Geofluids in the Crust and the Uppermost Mantle
★Invited Papers
キーワード：地震波速度、電気伝導度、固液混相、水溶液、メルト、ジョイントインバージョン
Aqueous fluids and melts in the Earth’s crust and upper mantle (hereafter referred to as geofluids) play crucial roles not only in the hydrothermal and magmatic processes but also in various geodynamic processes and the Earth’s evolution. For instance, aqueous fluids are presumed to reduce frictional strength to induce earthquakes within the crust and along the plate boundary in NE Japan (Sibson, 2009; Hasegawa et al., 2012). Such fluids present at depth may chemically react with the rocks and reduce rock viscosity or form hydrous minerals and act as a lubricant in case they develop along the subducting slab. Thus, this enhances plate motion and subduction (Seno and Kirby, 2014; Nakao et al., 2016). A small amount of melt may exist at the bottom of the plates, which may also act as a lubricant at the lithosphere–asthenosphere boundary (Kawakatsu et al., 2009) and affect plate motion. Geofluids are also key in accounting for geochemical differentiation of the solid Earth’s system, including large-scale heterogeneity represented by mantle geochemical hemispheres (Iwamori and Nakamura, 2015) because they act as effective transport agents of volatile and incompatible elements.
Various studies attempted to resolve distribution of the geofluids in the Earth’s interior, based on seismic and magnetotelluric (MT) structures and phenomena. In particular, low seismic velocity and/or high electrical conductivity (σ ) have been frequently highlighted to indicate the presence of fluids (e.g., Eberhart-Phillips et al., 1995), which is also examined through variations in the P-wave/S-wave velocity ratio (VP/VS ), seismic reflection, and attenuation (e.g., Nakajima et al., 2013; Okada et al., 2014). This can potentially be used to constrain the fluid parameters, including the fluid fraction, composition of the fluid phase (aqueous fluid or melt), and geometry of the fluid distribution (e.g., vein or grain boundary network) (Takei, 2002).
In this context, Iwamori et al. (2021) attempted to improve the subsurface imaging of potentially variable lithologies with geofluids by combining the quantitative estimates of seismic velocity and electrical conductivity, based on the existing experimental/theoretical knowledge of the physical properties of fluids, solids, and their mixtures, which will be discussed in this presentation. First, the physical properties of rocks and liquid phases (aqueous fluid and silicate melt) are described based on previous experimental and theoretical studies, which are used to estimate the bulk properties as a mixture of these phases for a given set of pressure, temperature, lithology, liquid fraction, and geometry. We term this as “forward model,” and it can be utilized for the “inversion model”, which estimates the parameter values for lithology and liquid based on the observed seismic velocity and electrical conductivity.
A total of 78 lithologies, an aqueous fluid with NaCl (~0 to 10 wt.%), and mafic to felsic melt appropriate for the crust and the uppermost mantle conditions were described in terms of Vp, Vs and σ, as per previous experimental measurements and molecular dynamics simulation. This forward model generates synthetic Vp, Vs, and σ, referring to the seismic velocities and electrical conductivity observed in the northeast Japan arc. After generation of the synthetic Vp, Vs, and σ, the original lithology and liquid parameters (phase, fraction, aspect ratio, and connectivity) were searched by implementing the grid search algorithm to map the misfit over the broad parameter space. The mapping shows the presence of a global misfit minimum around the optimized solution and the possibility of resolving the lithology and the liquid phase parameters based on the observed Vp, Vs, and σ by using the forward model.
Various studies attempted to resolve distribution of the geofluids in the Earth’s interior, based on seismic and magnetotelluric (MT) structures and phenomena. In particular, low seismic velocity and/or high electrical conductivity (σ ) have been frequently highlighted to indicate the presence of fluids (e.g., Eberhart-Phillips et al., 1995), which is also examined through variations in the P-wave/S-wave velocity ratio (VP/VS ), seismic reflection, and attenuation (e.g., Nakajima et al., 2013; Okada et al., 2014). This can potentially be used to constrain the fluid parameters, including the fluid fraction, composition of the fluid phase (aqueous fluid or melt), and geometry of the fluid distribution (e.g., vein or grain boundary network) (Takei, 2002).
In this context, Iwamori et al. (2021) attempted to improve the subsurface imaging of potentially variable lithologies with geofluids by combining the quantitative estimates of seismic velocity and electrical conductivity, based on the existing experimental/theoretical knowledge of the physical properties of fluids, solids, and their mixtures, which will be discussed in this presentation. First, the physical properties of rocks and liquid phases (aqueous fluid and silicate melt) are described based on previous experimental and theoretical studies, which are used to estimate the bulk properties as a mixture of these phases for a given set of pressure, temperature, lithology, liquid fraction, and geometry. We term this as “forward model,” and it can be utilized for the “inversion model”, which estimates the parameter values for lithology and liquid based on the observed seismic velocity and electrical conductivity.
A total of 78 lithologies, an aqueous fluid with NaCl (~0 to 10 wt.%), and mafic to felsic melt appropriate for the crust and the uppermost mantle conditions were described in terms of Vp, Vs and σ, as per previous experimental measurements and molecular dynamics simulation. This forward model generates synthetic Vp, Vs, and σ, referring to the seismic velocities and electrical conductivity observed in the northeast Japan arc. After generation of the synthetic Vp, Vs, and σ, the original lithology and liquid parameters (phase, fraction, aspect ratio, and connectivity) were searched by implementing the grid search algorithm to map the misfit over the broad parameter space. The mapping shows the presence of a global misfit minimum around the optimized solution and the possibility of resolving the lithology and the liquid phase parameters based on the observed Vp, Vs, and σ by using the forward model.