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

Abstract

Abstract The relationship between P‐wave velocity (Vp) and unconfined compressive strength (C0) can be used to estimate in situ rock strength and stresses at depth in subduction zones. However, few reliable data on the Vp‐C0 relationship in deep accretionary prism sediments are available. Here, we determined the Vp‐C0 relationship down to ∼3.2 km in the Nankai accretionary prism from Vp measurements and indentation tests of drill cuttings retrieved from International Ocean Discovery Program Site C0002. In our results, the Nankai accretionary prism sediments tended to be weaker compared with estimates obtained by using other empirically determined Vp‐C0 relationships. We also examined the relationship between Vp and porosity in the accretionary prism. The Vp‐porosity relationship obtained by using only data of Nankai accretionary prism sediments showed that Vp generally increased with decreasing porosity; this result is consistent with published Vp‐porosity relationships for Nankai Trough sediments. The empirical relationships derived in this study should improve estimates of stress state, pore pressure, and fault strength in the Nankai accretionary prism.