Constraints on in situ stress across the shallow megasplay fault offshore the Kii Peninsula, SW Japan from borehole breakouts

Author(s): Olcott, K. A.; Saffer, D. M.
Author Affiliation(s): Primary:
Pennsylvania State University, Department of Geosciences, State College, PA, United States
Volume Title: AGU 2012 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2012; American Geophysical Union 2012 fall meeting, San Francisco, CA, Dec. 3-7, 2012. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: The orientation and magnitude of stress in active tectonic settings, and in the vicinity of active faults, are important toward understanding faulting and earthquake mechanics. In areas where boreholes have been drilled, stress magnitude can be quantified by combining observations of compressional borehole breakout (BO) widths with estimates or measurements of unconfined compressive rock strength (UCS). Here, we estimate stress at Integrated Ocean Drilling Program (IODP) Sites C0004 and Site C0010, drilled into the accretionary prism offshore the Kii Peninsula, SW Japan, as part of the IODP NanTroSEIZE project. These sites penetrated the shallow portion of the megasplay fault, a major out of sequence thrust fault that cuts the accretionary prism and branches from the master décollement ≈55 km from the trench. Drilling at Site C0004 included logging while drilling (LWD) and coring to a total depth (TD) of ≈399 m. Drilling at Site C0010, located ≈3.5 km along strike, included LWD to a TD of 555 mbsf. The megasplay fault is marked by a 60-m wide fractured zone at Site C0004, whereas at Site C0010 it is defined by a sharp contact between the overriding thrust wedge and footwall sediments. At Site C0010, the hole was temporarily suspended for 3 days due to weather conditions, and ≈70 m of the hanging wall and the upper portion of the footwall were relogged after re-entry and prior to continued drilling. We measure BO width from LWD resistivity-at-the-bit (RAB) images, and estimate a range of UCS values from existing experimentally-derived relations between P-wave velocity (Vp) and UCS. Under the assumption that stresses at the edge of BO are in equilibrium with the rock strength, we quantify far field horizontal stresses SHmax and Shmin as a function of depth at each drillsite. Our results indicate that stresses at both Site C0004 and Site C0010 define a thrust-faulting regime, in both the hanging wall and footwall of the fault. This implies that stress is not decoupled across the shallow megasplay. Computed horizontal stress magnitudes are generally similar at both Sites, with SHmax referenced to the seafloor ranging from 7.4 - 8.5 MPa at ≈250 mbsf and 11.5-11.9 MPa at ≈350 mbsf. There is a distinct shift to lower estimated stress magnitudes over the 150 m below the megasplay at Site C0010, but not at Site C0004, based on narrower observed breakouts in the footwall at Site C0010. The relogged portion of Site C0010 also presents a unique opportunity to explore the possibility that BO grow through time as formation pore pressure and thus rock strength equalize after drilling. BO appear to have grown by as much as 60° between the first and second logging runs. This growth corresponds to an increase in computed horizontal stress of ≈2 MPa. If this process is common, it implies that stresses computed from BO width in low-permeability sediments may under-estimate the true in situ stress magnitude.
Year of Publication: 2012
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 16 Structural Geology; Asia; Far East; Faults; Honshu; IODP Site C0004; IODP Site C0010; Integrated Ocean Drilling Program; Japan; Kii Peninsula; NanTroSEIZE; North Pacific; Northwest Pacific; Pacific Ocean; Tectonics; West Pacific
Coordinates: N331236 N331236 E1364112 E1364112
N331300 N331300 E1364300 E1364300
Record ID: 2014082004
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