Velocity‐Porosity Relations in Carbonate and Siliciclastic Subduction Zone Input Materials

Abstract

Subduction zones host the world's most devastating earthquakes and tsunamis. Earthquake generation, slip behavior, and seafloor/sub-seafloor deformation are tied to the physical properties and in situ stresses within the sediments and rocks that make up a subduction zone. However, these properties and conditions in situ remain poorly constrained due to limited measurements at discrete drilling locations. The porosity and pore pressure within subducting sediments directly influence the strength of subduction zones as the subduction process proceeds. Controlled-source seismic experiments are one method that can be used to estimate porosity and in situ pressure conditions, but this method requires calibration through direct measurement of porosity, seismic velocity, and consolidation in sediment and rock samples (

Effective Porosity Profile at IODP Site C0002 in the Heart of the Nankai Accretionary Prism, and Its Use for Predicting In Situ Seismic Velocities

“…The P wave velocity–effective porosity determined in this study for Site C0002 is also consistent with the V p –effective porosity data compiled by Jeppson and Kitajima (2022) for sediment inputs to the Sunda (IODP Expedition 362 Site U1480) and North Hikurangi (IODP Expedition 372/375 Site U1520) margins showing diverse lithologies (Figure S3 in Supporting Information ).…”

“…It was initially derived from the gamma ray log used as a proxy for clay volume, resulting in very high values, sometimes ≥1 (Erickson & Jarrard, 1998; Hoffman & Tobin, 2004; Tudge & Tobin, 2013). Recent studies use lower V sh values based on core data, such as the proportion of fine grains like clays and silts (Kitamura et al., 2021), or mineralogical data from XRD analyses (Jeppson & Kitajima, 2022).…”

“…The model acknowledges the effect of porosity and shale content for sediments showing normal or high consolidation. Following Jeppson and Kitajima (2022), we use a generic formulation of this model (Equations and , referred hereafter to as the Erickson‐Jarrard template, which will be further extended to apply to the model of Tudge and Tobin [2013]): $$${V}_{p}\ =A\ +\ B\,\phi +\frac{H}{{\left(\phi +C\right)}^{2}+D}\ +\ E\,\left({V}_{sh}-F\right)\,{X}_{m}$$$ where ϕ is porosity (either total porosity ϕ t or effective porosity ϕ eff ), V sh is the shale volume, A – H are empirical constants, and X m accounts for the abrupt change in behavior at a critical porosity ϕ c , above which stress is supported by the fluid rather than by the grain skeleton of the rock (Equation ): $$${X}_{m}\ =\ \mathrm{tanh}\left[G\left(\phi -{\phi }_{c}\right)\right]-\Vert \mathrm{tanh}\left[G\left(\phi -{\phi }_{c}\right)\right]\Vert $$$ where G is an empirical constant. In the initial paper by Erickson and Jarrard (1998), G = 40 for normally consolidated sediments and G = 20 for highly consolidated sediments.…”

“…The model acknowledges the effect of porosity and shale content for sediments showing normal or high consolidation. Following Jeppson and Kitajima (2022), we use a generic formulation of this model (Equations 2 and 3, referred hereafter to as the Erickson-Jarrard template, which will be further extended to apply to the model of Tudge and Tobin [2013]):…”

“…Initially, total porosity values were used (Hoffman & Tobin, 2004;Kitajima et al, 2017;Kitamura et al, 2021). Recent studies have shown that because of the effect of clay-bound water on total porosity, better relationships with V p can be obtained using effective porosity (Jeppson & Kitajima, 2022;Tudge & Tobin, 2013). In this study, if only total porosity was considered, the best fit option using the Erickson-Jarrard template (Equations 2 and 3) stays close to the solution of Kitamura et al (2021) (RSE = 233 m/s), that tends to predict high V p at low porosity.…”