Mechanical Weakness of the Nankai Accretionary Prism: Insights From V_p Measurements of Drill Cuttings

Abstract

Knowledge of the stress state in subduction zones is crucial for understanding the mechanics of geological and geophysical processes occurring in subduction zones, such as earthquake generation (e.g., Moore & Saffer, 2001), stress changes during the earthquake cycle (e.g., Wang & Hu, 2006) and accretionary wedge formation (e.g., Dahlen, 1990). Rock strength, which must be known to determine the stress state, is often determined by combining a failure criterion with the relationship between P-wave velocity (V p ) and unconfined compressive strength (C 0 ) (e.g., Chang et al., 2010;Song & Chang, 2017;Zoback, 2007). For example, to estimate the possible ranges of in situ horizontal principal stresses, measurements of breakout width, an indicator of rock failure, are commonly combined with estimates of rock strength (C 0 ) derived from V p measurements. However, because relatively few C 0 data are available from laboratory experiments on drill cores, C 0 is estimated instead from logging-while-drilling (LWD) V p data and laboratory-derived V p -C 0 relationships (e.g., Chang et al., 2010). Thus, when LWD V p data are available, the V p -C 0 relationship can be used for estimating rock strength and stresses at depth in subduction zones (e.g., Chang et al., 2006). Among published V p -C 0 relationships, those derived from measurements of sedimentary rocks, in particular, vary depending on the tectonic field and the rock type and age (e.g.,

Multiple Types of Porosity – P‐Wave Velocity Relationships for the Nankai Trough

“…Kitamura et al. (2021) also reported the porosity ‐ V p relationships consistent with the high‐ V p model of Erickson and Jarrard (1998) in the accretionary prism at Site C0002 based on laboratory measurements of cuttings samples. At Site C0009, porosity is almost constant throughout the entire depth range, whereas V p steeply increases at the boundary between the LKB and inner prism.…”

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 empirical models of Erickson and Jarrard (1998) (modified by Kitajima et al., 2017), Hoffman and Tobin (2004), and Kitamura et al. (2021) are also reported. A new inversion fit is also proposed (thick black line), based on Equation , with the coefficients of Table 1.…”

“…A third issue was the type of porosity considered. 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).…”

“…The inversion was operated with the optimize module of the Python SciPy package, using the parameters of Kitamura et al. (2021) as initial parameters and a L 2 cost function, using the classical Euclidian norm. Quality of the fit was estimated via residual standard errors (RSEs).…”

“…Crossplot between P wave velocity and (a) total porosity, and (b) effective porosity.Crossplots can be built either from the sample data for which the P wave velocity of logs are interpolated (large circled colored points), or from the LWD data for which the corresponding porosity has been derived from the exponential trend of Equation7. The empirical models ofErickson and Jarrard (1998) (modified byKitajima et al, 2017),Hoffman and Tobin (2004), andKitamura et al (2021) are also reported. A new inversion fit is also proposed (thick black line), based on Equation2, with the coefficients of Table1.…”