Fault (geology)

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

Present-day kinematics at the India-Asia collision zone

Abstract: The collision of the Indian subcontinent with Asia drives the growth and evolution of the greater Tibetan Plateau region. Fault slip rates resulting from the relative motion between crustal blocks can provide a kinematic description of the distribution of presentday deformation. I construct a three-dimensional, regional-scale elastic block model of the India-Asia collision zone that is consistent with geodetic observations of interseismic deforma tion, mapped fault system geometry, historical seismicity, and the mechanics of the earthquake cycle. This mechanical model of the elastic upper crust yields a set of kinematically consistent fault slip rates and block motions that may serve to constrain dynamic models of continental crustal dynamics.

Fractal analysis applied to characteristic segments of the San Andreas Fault

Abstract: Fractal theory is applied in a quantitative analysis of the San Andreas fault (SAF) geometry. The method, which directly measures the increase in total fault length with a decrease in ruler size, gives the fractal dimension D and scaling properties for the chosen length band 0.5–1000 km. A physical interpretation of D is that it measures the irregularity of the fault trace in the selected band. The fault is subdivided into six segments of distinctive seismic behavior. A “main” fault trace which shows either maximum coseismic slip or creeping was selected for analysis, with three alternative branches examined for the SAF system south of San Bernardino. Branches of the fault trace are not considered in this work. Fractal dimensions calculated for the different segments range from 1.0008 to 1.0191, and are different from 1.0 to the 95% confidence interval. These small changes reflect the overall smoothness of the main fault trace. Significant variations in D among segments indicate heterogeneities in the fault smoothness along strike. D also changes significantly from the short-length band to the long-length band where the demarcation point ranges from 1 to 2 km. The short-length band has larger D values. A slight correlation is obtained between the fractal dimension of the main trace and the extent of subparallel faulting. This indicates some correspondence between the main fault trace irregularity and the complexity of subsidiary fault traces in plan view.

Origin of “Reverse Drag” on the Downthrown Side of Normal Faults

Abstract: “Reverse drag,” also called “downbending” or “turnover,” was first recognized by Powell in the Colorado Plateau and subsequently was found to be an important structure associated with “down-to-basin” faulting in the Gulf Coast region. Many conflicting hypotheses have been proposed to explain this structure, most of which are based upon subsurface studies or observations in local areas. Detailed field studies of reverse drag made in the western Colorado Plateau reveal that the flexure extends practically the entire length of most normal faults in the region. It is characterized by a broad, asymmetrical arc on the downthrown block, approximately 1 mile wide, with maximum dips of more than 30 degrees near the fault plane. Normal drag is common adjacent to the fault on both the upthrown and downthrown blocks. Reverse drag has been formed repeatedly during recurrent movement along the Hurricane and Grand Wash faults, clearly indicating that it is genetically related to faulting. The magnitude of the flexure is roughly proportional to displacement, and the trend of the fold closely parallels the trend of the fault. In many places reverse drag passes both vertically and laterally into antithetic faults. Observations in the Grand Canyon reveal that the dip of the faults with which reverse drag is associated decreases with depth. It is concluded that reverse drag results from an alternate response to the same forces that produce antithetic faults and develops because of curvature of the fault plane at depth. Normal movement along a curved fault plane, in effect, tends to pull the blocks apart as well as to displace them vertically. Adjustments to fill the incipient gap by rupture produces antithetic faults, whereas failure by flexing develops reverse drag.

Fault transmissibility multipliers for flow simulation models

Abstract: Fault zone properties are incorporated in production flow simulators using transmissibility multipliers. These are a function of properties of the fault zone and of the grid-blocks to which they are assigned. Consideration of the geological factors influencing the content of fault zones allows construction of high resolution, geologically driven, fault transmissibility models. Median values of fault permeability and thickness are predicted empirically from petrophysical and geometrical details of the reservoir model. A simple analytical up-scaling scheme is used to incorporate the influence of likely small-scale fault zone heterogeneity. Fine-scale numerical modelling indicates that variability in fault zone permeability and thickness should not be considered separately, and that the most diagnostic measure of flow through a heterogeneous fault is the arithmetic average of the permeability to thickness ratio. The flow segregation through heterogeneous faults predicted analytically is closely, but not precisely, matched by numerical results. Identical faults have different equivalent permeabilities which depend, in part, on characteristics of the permeability field in which they are contained.

Three-dimensional velocity structure, seismicity, and fault structure in the Parkfield Region, central California

Abstract: This study examines the three-dimensional velocity structure in a 60- by 80-km region containing the Parkfield segment of the San Andreas fault. We use local earthquake and shot P arrival times in an iterative simultaneous inversion for velocity and hypocentral parameters. Using the three-dimensional model, we relocated 5251 events that occurred from 1969 to 1991, as well as the 1966 aftershocks, and computed 664 fault plane solutions. The San Andreas fault (SAF), characterized by a sharp across-fault velocity gradient, is the primary feature in the velocity solution. There is a 5–20% lateral change in velocity over a 4-km width, the contrast being sharper where there is better resolution. The model also shows significant variations in the velocity and in the complexity of the velocity patterns along the SAF. The largest across fault velocity difference is below Middle Mountain, where a large volume of low-velocity material impinges on the SAF from the northeast. This material is inferred to be overpressured and may be key to understanding the unusual behavior in the Parkfield preparation zone. A 20-km-long high-velocity slice is imaged northeast of the SAF near Gold Hill. Its along-fault length corresponds to the length of the maximum slip in 1966. The relocated seismicity shows that the San Andreas fault is a planar vertical fault zone at seismogenic depths. Ninety percent of the fault plane solutions that are on, or near, the SAF were right-lateral strike-slip on subvertical fault planes that parallel the SAF. Thus the surface fault complexities do not appear to extend to depth and therefore do not explain the rupture character at Parkfield. At Parkfield, variations in material properties play a key role in fault segmentation and deformation style. Our observations suggest that there may be a general relation between increasing velocity and increasing ability of the rocks to store strain energy and release it as brittle failure.