Hung-Yu Wu

The Post-Earthquake Stress State on the Tohoku Megathrust as Constrained by Reanalysis of the JFAST Breakout Data

The JFAST drilling project endeavored to establish the stress state on the shallow subduction megathrust that slipped during the M9 Tohoku earthquake. Borehole breakout data from the drillhole can constrain both the orientation and magnitude of the principal stresses. Here we reanalyze that data to refine our understanding of the stress state on the fault. In particular, we: (1) Improve the identification of breakouts, (2) Consider a fuller range of stress states consistent with the data, and (3) Incorporate new and more robust laboratory constraints on rock strength. The original conclusion that the region is in a normal faulting regime after the earthquake is strengthened by the new analysis. The combined analysis suggests the earthquake released sufficient elastic strain energy to reset the local stress field.

Stress state and its anomaly observations in the vicinity of a fault in NanTroSEIZE Expedition 322

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 (Sv = 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.

Stress State in the Kumano Basin and in Slope Sediment Determined From Anelastic Strain Recovery: Results From IODP Expedition 338 to the Nankai Trough

Three-dimensional, in situ stresses in the Kumano Basin and slope sediment (IODP Sites C0002 and C0022) in the Nankai Trough, southwest Japan, have been determined using the anelastic strain recovery (ASR) of core samples. Two samples taken from Hole C0002J, located in the bottom of the Kumano Basin, indicate that the maximum principal stress, σ1, is vertical. The intermediate principal stress, σ2, is oriented ENE–WSW, parallel to the trench axis. These stress orientations are similar to those obtained using ASR and borehole breakout methods in previous expeditions. In contrast, a sample from the lower section of the slope sediment (Hole C0022B), located beneath the megasplay fault, is characterized by σ1 plunging moderately to the ESE and σ3 oriented near-horizontally, trending NNE–SSW. The direction of maximum horizontal stress obtained from ASR (WNW–ESE) is similar to that inferred from borehole breakouts in an adjacent hole (NW–SE). Trench-normal compression and a near-vertical σ2 are also inferred from focal mechanisms of very-low-frequency earthquakes within the Nankai accretionary prism, and from borehole breakouts in the hanging wall of the megasplay fault. These observations suggest that the horizontal compressional regime extends to a shallower level than previously thought, likely due to the shallow portion of the megasplay fault accumulating tectonic stress in response to plate convergence.