Fault (geology)

About: Fault (geology) is a research topic. Over the lifetime, 26732 publications have been published within this topic receiving 744535 citations.

The relation between fault plane solutions for earthquakes and the directions of the principal stresses

Abstract: The stresses involved in shallow earthquakes and their occurrence along fault planes suggest that they occur by failure on weak planes, rather than by brittle fracture of a homogeneous material. Possible orientations of the stress tensor are examined to determine what limits fault plane solutions can place on the orientation of the greatest principal stress. For the general case of a triaxial stress, the only restriction is that this stress direction must lie in the quadrant containing P , but may be at right angles to the P direction. Thus shallow earthquakes impose a few limitations on the orientation of the stress tensor. In contrast the fault plane solutions from deep earthquakes are best explained by fracture of a homogeneous material, with the greatest principal stress directed down the dip of the earthquake zone.

Geometry of Shearing Stress and Relation to Faulting

Abstract: The distributions of intensity and orientation of maximum shearing stress in typical stress systems are plotted on stereographic projections in order to show the three-dimensional relationships. Mathematical expressions of these relationships as well as graphical methods of evaluation are given. Relationships of shearing stress to orientation of fault planes and orientation of net slip of faults are suggested. Methods of studying the relationships of faults and shearing stress are described.

Transfer Zones in the East African Rift System and Their Relevance to Hydrocarbon Exploration in Rifts (1)

Abstract: Transfer zones in extensional regions display a wide range of geometries from discrete fault zones to zones of broad warping. The classification of extensional fault displacement transfer zones developed in this paper includes three main criteria: (1) primary subdivision of transfer zones by relative attitude and direction of throw of the major faults (synthetic and conjugate); (2) secondary subdivision of conjugate transfer zones into transfer zones where the normal faults dip toward each other (convergent) and where the transfer zones occur between faults that dip away from each other (divergent); (3) the tertiary subdivision of conjugate relationships of transfer zones and secondary subdivision of synthetic transfer zones are by the fault terminations in plan view; fau t tips approach, or they overlap, completely overlap (termed "collateral"), or are in line (termed "collinear"). A high abundance of overlapping transfer zones occur in the East African rift where extension is low and large-scale cross faults (proto-transform faults?) are uncommon. Commonly, transfer zones in this region are relatively high areas with complex internal fault geometries flanked by deeps. Transfer zones in rifts contain complex but somewhat predictable structural geometries that make them optimum locations for structural hydrocarbon traps. The classification presented herein can help define and delineate those zones and to some degree predict their internal structural geometry.

Implications for mechanical properties of brittle faults from observations of the Punchbowl fault zone, California

Abstract: Field observations of the Punchbowl fault zone, an inactive trace of the San Andreas, are integrated with results from experimental deformation of naturally deformed Punchbowl fault rocks for a qualitative description of the mechanical properties of the fault and additional information for conceptual models of crustal faulting. The Punchbowl fault zone consists of a single, continuous gouge layer bounded by zones of extensively damaged host rock. Fault displacements were not only localized to the gouge layer, but also to discrete shear surfaces within the gouge. Deformation in the exposure studied probably occurred at depths of 2 to 4 km and was dominated by cataclastic mechanisms. Textural data also suggest that significant amounts of pore fluids were present during faulting, and that fluid-assisted mechanisms, such as dissolution, diffusion, and precipitation, were operative. The experimental data on specimens collected from the fault zone suggest that there is a gradual decrease in strength and elastic modulus and an increase in relative ductility and permeability toward the main gouge zone. The gouge layer has fairly uniform mechanical properites, and it has significantly lower strength, elastic modulus, and permeability than both the damaged and the undeformed host rock. For the Punchbowl fault and possibly other brittle faults, the variations in loading of the gouge zone with time are primarily governed by the morphology of the fault and the mechanical properties of the damaged host rock. In addition, the damaged zone acts as the permeable unit of the fault zone and surrounding rock. It appears that the gouge primarily governs whether displacements are localized, and it therefore may have a significant influence on the mode of slip.

Potentially induced earthquakes in Oklahoma, USA: Links between wastewater injection and the 2011 Mw 5.7 earthquake sequence

Abstract: Significant earthquakes are increasingly occurring within the continental interior of the United States, including five of moment magnitude (Mw) ≥ 5.0 in 2011 alone. Concurrently, the volume of fluid injected into the subsurface related to the production of unconventional resources continues to rise. Here we identify the largest earthquake potentially related to injection, an Mw 5.7 earthquake in November 2011 in Oklahoma. The earthquake was felt in at least 17 states and caused damage in the epicentral region. It occurred in a sequence, with 2 earthquakes of Mw 5.0 and a prolific sequence of aftershocks. We use the aftershocks to illuminate the faults that ruptured in the sequence, and show that the tip of the initial rupture plane is within ∼200 m of active injection wells and within ∼1 km of the surface; 30% of early aftershocks occur within the sedimentary section. Subsurface data indicate that fluid was injected into effectively sealed compartments, and we interpret that a net fluid volume increase after 18 yr of injection lowered effective stress on reservoir-bounding faults. Significantly, this case indicates that decades-long lags between the commencement of fluid injection and the onset of induced earthquakes are possible, and modifies our common criteria for fluid-induced events. The progressive rupture of three fault planes in this sequence suggests that stress changes from the initial rupture triggered the successive earthquakes, including one larger than the first.