Open AccessStress state and its anomaly observations in the vicinity of a fault in N an T ro SEIZE E xpedition 322
Author(s) -
Wu HungYu,
Saito Saneatsu,
Kinoshita Masataka
Publication year - 2015
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1002/2015gc006072
Subject(s) - geology , borehole , breakout , coring , fault (geology) , azimuth , seismology , differential stress , stress (linguistics) , petrology , drilling , geotechnical engineering , geometry , deformation (meteorology) , mechanical engineering , linguistics , philosophy , mathematics , finance , engineering , economics , oceanography
To better understand the stress state and geological properties within the shallow Shikoku Basin, southwest of Japan, two sites, C0011A and C0011B, were drilled in open‐ocean sediments using Logging While Drilling (LWD) and coring, respectively. Resistivity image logging was performed at C0011A from sea floor to 950 m below sea floor (mbsf). At C0011B, the serial coring was obtained in order to determine physical properties from 340 to 880 mbsf. For the LWD images, a notable breakout anomaly was observed at a depth of 615 m. Using resistivity images and a stress polygon, the potential horizontal principal stress azimuth and its magnitude within the 500–750 mbsf section of the C0011A borehole were constrained. Borehole breakout azimuths were observed for the variation by the existence of a fault zone at a depth of 615 mbsf. Out of this fracture zone, the breakout azimuth was located at approximately 109° ± 12°, subparallel to the Nankai Trough convergence vector (300–315°). Our calculations describe a stress drop was determined based on the fracture geometry. A close 90° (73° ± 12°) rotation implied a 100% stress drop, defined as a maximum shear stress drop equal to 1 MPa. The magnitude of the horizontal principal stresses near the fracture stress anomaly ranged between 49 and 52 MPa, and the bearing to the vertical stress ( S v = 52 MPa) was found to be within the normal‐faulting stress regime. Low rock strength and a low stress level are necessary to satisfy the observations.