Fluid transport properties in sediments and their role in large slip near the surface of the plate boundary fault in the Japan Trench

In situobservations of earthquake-driven fluid pulses within the Japan Trench plate boundary fault zone

“…The depths where advection signals are observed occur where the borehole intersects the fault damage zone, an ~100-m-wide region identified in geophysical logging data and core samples with open faults and fractures within lithified mudstone above the <4.87-m-thick clay-rich uppermost candidate plate boundary fault, and other mudstones, siliceous clays, chert, and basalt within the downgoing Pacific plate (Chester et al, 2013a;Rabinowitz et al, 2015;Keren and Kirkpatrick, 2016). Depths near the bottom of the observatory array, around the main plate boundary fault and where the largest part of the frictional heat signal is observed, are largely unaffected by advection, which is consistent with the previous inference that the plate boundary fault has much lower permeability than the damage zone due to its composition of low-porosity scaly clay (Fulton et al, 2013;Chester et al, 2013b, Tanikawa et al, 2013. The location and relatively small magnitude of borehole transients are consistent with the interpretation of the broad long-lasting 0.3 °C signal at ~819 mbsf as conductive frictional heat.…”

Pressure dependence of fluid transport properties of shallow fault systems in the Nankai subduction zone

“…The reduction rates of porosity in Nankai sediments (2.5% to 4% at 40 MPa from the initial porosity) seem quite small compared to those in Japan Trench sediments (9% to 15%; Tanikawa et al 2013), even though the reduction rates of permeability between the two sites were similar. The porosity tests in this study were performed under dry conditions, although the porosity of sediments in the Japan Trench was measured under water-saturated conditions.…”

“…This suggests that permeability reduction with depth is mainly explained by mechanical compaction with an increase in the effective pressure. Permeability variations at site C0004 and at site C0019 of The Japan Trench Fast Drilling Project (JFAST, IODP Expedition 343/343 T) (Tanikawa et al 2013) are larger than that at site C0007. The permeability variations observed in the JFAST sites were explained by the proportion of clay minerals, whereby the higher abundance of clay in the plate boundary fault materials resulted in extremely low permeability.…”

Multiple major faults at the Japan Trench: Chemostratigraphy of the plate boundary at IODP Exp. 343: JFAST

“…IODP Expedition 343 (JFAST) drilled through the accretionary wedge at the Japan Trench to investigate shallow, tsunamigenic earthquake slip (Chester et al, 2012). Several holes were drilled at the site; one hole was cored (C00019E) and has been studied extensively Kirkpatrick et al, 2015;Lin et al, 2014;Rowe et al, 2013;Sawai et al, 2014;Tanikawa et al, 2013;Ujiie et al, 2013;Yang et al, 2013). Samples for this study come from core C00019E, which is hereafter referred to as the "JFAST core".…”