Shear strength of sediments approaching subduction in the Nankai Trough, Japan as constraints on forearc mechanics

Abstract

[1] The mechanical behavior of the plate boundary fault zone is of paramount importance in subduction zones, because it controls megathrust earthquake nucleation and propagation as well as the structural style of the forearc. In the Nankai area along the NanTroSEIZE (Kumano) drilling transect offshore SW Japan, a heterogeneous sedimentary sequence overlying the oceanic crust enters the subduction zone. In order to predict how variations in lithology, and thus mechanical properties, affect the formation and evolution of the plate boundary fault, we conducted laboratory tests measuring the shear strengths of sediments approaching the trench covering each major lithological sedimentary unit. We observe that shear strength increases nonlinearly with depth, such that the (apparent) coefficient of friction decreases. In combination with a critical taper analysis, the results imply that the plate boundary position is located on the main frontal thrust. Further landward, the plate boundary is expected to step down into progressively lower stratigraphic units, assisted by moderately elevated pore pressures. As seismogenic depths are approached, the decollement may further step down to lower volcaniclastic or pelagic strata but this requires specific overpressure conditions. High-taper angle and elevated strengths in the toe region may be local features restricted to the Kumano transect.Components: 9,385 words, 7 figures.

Lithologic control of frictional strength variations in subduction zone sediment inputs

“…An intact sample of the siliceous mudstone from between the décollement and pelagic clay at Site C0019, which correlates with the diatomaceous claystone at Site 436, exhibited a cohesion value of 220 kPa . In comparison, measured cohesion of mudstones from Sites C0011 and C0012 along the Kumano transect at the Nankai Trough ranges from 13 to 160 kPa (Ikari et al, 2013a), consistent with calculated cohesion values of 35 and 41 kPa for two samples from these sites (Stipp et al, 2013). There is a large amount of amorphous silica in the Japan Trench mudstone unit, up to ~60% at both Sites C0019 and 436 (Kameda et al, 2015a), which might indicate advanced cementation.…”

“…However, these low friction values cannot be entirely explained by mineral assemblage; total clay content is generally high (50%-80%) in the weak samples from all three transects, but the smectite content is not necessarily high, and at Site 1173 is <~30%. Smectite contents on the Kumano transect are comparable with those of the Ashizuri sites, however a Coulomb wedge analysis considering the relatively steep taper angle at the toe suggests that the plate boundary occurs in the stronger, ash-bearing USB facies (Ikari et al, 2013a).…”

“…For this study, we include the results of experiments on samples from Muroto Site 1173 following this same procedure. Downhole profiles of frictional strength measurements were conducted on intact samples from Sites C0011 and C0012 at in situ effective normal stresses (≤5 MPa) and 10 µm/s shearing velocity in a single-direct shear apparatus by Ikari et al (2013a). Intact samples preserve in situ effects such as fabric or cementation; however, for other boreholes in the Nankai Trough, some studies have found that these effects on friction are small (Ikari and Saffer, 2011;Saffer et al, 2012).…”

“…This effect has led to the formation of a Nankai margin-wide zone of anomalously high porosity, which was suggested to be caused by diagenetic cementation (e.g., Spinelli et al, 2007;Raimbourg et al, 2011;White et al, 2011). However, low measured values of cohesion in intact samples from Sites C0011 and C0012 indicate a lack of cementation (Ikari et al, 2013a). It has been therefore suggested that the origin of the high-porosity zone is not cementation of the bulk sediment, but rather the formation of locally confined aggregates from patchy precipitation of amorphous silica (Hüpers et al, 2015).…”

Subduction zone megathrust earthquakes

“…Many studies have focused on the frictional stability of various clay-rich and carbonate sedi ments observed entering subduction zones to identify the conditions under which velocity weakening behavior is possible (e.g., den Hartog et al, 2012;den Hartog and Spiers, 2013;Ikari et al, 2013;Kurzawski et al, 2016). Geersen et al (2013) inferred that the clay-rich sediment composition in the 2004 Sumatra earthquake rupture zone was as important as the thickness in controlling the shallow rupture.…”