Present‐day principal horizontal stress orientations in the Kumano forearc basin of the southwest Japan subduction zone determined from IODP NanTroSEIZE drilling Site C0009

Abstract

International audienceA 1.6 km riser borehole was drilled at site C0009 of the NanTroSEIZE, in the center of the Kumano forearc basin, as a landward extension of previous drilling in the southwest Japan Nankai subduction zone. We determined principal horizontal stress orientations from analyses of borehole breakouts and drilling-induced tensile fractures by using wireline logging formation microresistivity images and caliper data. The maximum horizontal stress orientation at C0009 is approximately parallel to the convergence vector between the Philippine Sea plate and Japan, showing a slight difference with the stress orientation which is perpendicular to the plate boundary at previous NanTroSEIZE sites C0001, C0004 and C0006 but orthogonal to the stress orientation at site C0002, which is also in the Kumano forearc basin. These data show that horizontal stress orientations are not uniform in the forearc basin within the surveyed depth range and suggest that oblique plate motion is being partitioned into strike-slip and thrusting. In addition, the stress orientations at site C0009 rotate clockwise from basin sediments into the underlying accretionary prism

IODP Expedition 319, NanTroSEIZE Stage 2: First IODP Riser Drilling Operations and Observatory Installation Towards Understanding Subduction Zone Seismogenesis

“…Wireline log Formation MicroImager (FMI) resistivity images and caliper data were used to identify the orientation of borehole enlargement, indicating the minimum horizontal stress orientation ("borehole breakouts"). These results allowed the in situ stress orientation to be determined (Lin et al, 2010), complementing measurements of in situ stress magnitude from other tools (see below) and in situ stress measurements at other sites across the margin (McNeill et al, 2004;Ienaga et al, 2006;Byrne et al, 2009). Minimum horizontal stress consistently trends NE-SW downhole (~700-1600 mbsf); therefore, the maximum horizontal stress trends NW-SE (Fig.…”

Stress state estimation by geophysical logs in NanTroSEIZE Expedition 319‐Site C0009, Kumano Basin, southwest Japan

“…[6] Saffer et al [2010] and Lin et al [2010] reported there is no breakout identified shallower than 1285 mbsf. However, the stick and slip motion of the tool due to the rugose or sticky borehole condition distorted and blurred the initial image which recorded incorrect depth and distorting images.…”

“…[4] Many drilling projects and expeditions such like Cajon Pass well; TCDP and NanTroSEIZE Expedition 314 have indicated that the borehole breakout and tensile fractures are formed by the acting principal stresses [Moos and Zoback, 1990;Wu et al, 2007;Lin et al, 2010]. High-resolution resistivity images measured from Formation MicroImager (FMI) are widely used for detection of such stress estimation.…”

Quantification of free gas in the Kumano fore-arc basin detected from borehole physical properties: IODP NanTroSEIZE drilling Site C0009

“…Within Unit III, both P and S wave velocities increase with depth, except in some regions where V p drops sharply, which we interpret as a function of the presence of free gas (discussed below). Within Unit IV, the borehole experienced stability problems, with frequent occurrence of drilling-induced compressive failure (i.e., borehole breakouts [Lin et al, 2010]), resulting in noisier sonic data. However, an increase in sonic velocity with depth is still distinguishable, especially in the P wave data (Figure 2).…”