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M. Zoback

Publications and source records attributed to M. Zoback.

5 recordsLinked to original sources
USGS2007

Stress orientations of Taiwan Chelungpu-Fault Drilling Project (TCDP) hole-A as observed from geophysical logs

[1] The Taiwan Chelungpu-fault Drilling Project (TCDP) drilled a 2-km-deep research borehole to investigate the structure and mechanics of the Chelungpu Fault that ruptured in the 1999 M w 7.6 Chi-Chi earthquake. Geophysical logs of the TCDP were carried out over depths of 500–1900 m, including Dipole Sonic Imager (DSI) logs and Formation Micro Imager (FMI) logs in order to identify bedding planes, fractures and shear zones. From the continuous core obtained from the borehole, a shear zone at a depth of 1110 meters is interpreted to be the Chelungpu fault, located within the Chinshui Shale, which extends from 1013 to 1300 meters depth. Stress-induced borehole breakouts were observed over nearly the entire length of the wellbore. These data show an overall stress direction (∼N115°E) that is essentially parallel to the regional stress field and parallel to the convergence direction of the Philippine Sea plate with respect to the Eurasian plate. Variability in the average stress direction is seen at various depths. In particular there is a major stress orientation anomaly in the vicinity of the Chelungpu fault. Abrupt stress rotations at depths of 1000 m and 1310 m are close to the Chinshui Shale's upper and lower boundaries, suggesting the possibility that bedding plane slip occurred during the Chi-Chi earthquake.

Geophysical Research Letters↗
USGS2007

Lithosphere stress and deformation

After several decades of investigation, a surprisingly simple view of the lithospheric state of stress has emerged. Broad regions of the lithosphere (scales up to thousands of kilometers) are characterized by remarkably uniform stress fields, both in terms of orientations and relative magnitudes. There is a strong correlation between observed intraplate stress orientations and orientations predicted by models of the plate-driving forces acting on the plate geometry, suggesting that the same forces that drive the plates also stress their interiors. Lateral variations in density and thickness of the crust and lithosphere give rise to gravitational potential energy differences that locally induce forces comparable in magnitude to plate-driving forces. In addition, mantle density inhomogeneities may also influence lithospheric stresses through the linkage of mantle flow. Direct measurements of stress magnitudes at depth indicate that the brittle crust is in a state of ‘frictional faulting equilibrium’, that is, the stress differences are close to, and limited by, the stress levels required to induce slip on the most well-oriented pre-exising planes. This characteristic of ‘critically stressed crust’ stress applies equally well to active plate boundary regions as well as ‘stable’ intraplate regions, indicating that stress differences at depth are comparable in these very distinct tectonic provinces. The difference in deformation between the two provinces is simply the rate of deformation, primarily controlled by the integrated strength of the lithosphere. Warmer, weaker lithosphere (such as along plate boundaries) will deform about eight orders of magnitude more rapidly than old, cold lithosphere in the interior of plates.

Book chapter↗
USGS2004

Stress orientations and magnitudes in the SAFOD pilot hole

Borehole breakouts and drilling-induced tensile fractures in the 2.2-km-deep SAFOD pilot hole at Parkfield, CA, indicate significant local variations in the direction of the maximum horizontal compressive stress, SHmax, but show a generalized increase in the angle between SHmax and the San Andreas Fault with depth. This angle ranges from a minimum of 25 ?? 10?? at 1000-1150 m to a maximum of 69 ?? 14?? at 2050-2200 m. The simultaneous occurrence of tensile fractures and borehole breakouts indicates a transitional strike-slip to reverse faulting stress regime with high horizontal differential stress, although there is considerable uncertainty in our estimates of horizontal stress magnitudes. If stress observations near the bottom of the pilot hole are representative of stresses acting at greater depth, then they are consistent with regional stress field indicators and an anomalously weak San Andreas Fault in an otherwise strong crust. Copyright 2004 by the American Geophysical Union.

Geophysical Research Letters↗