Rough Fault Research Articles

The role of disorder and stress concentration in nonconservative fault systems

The aim of this paper is to understand the origin of the deviations from the Gutenberg–Richter law observed for individual earthquake faults. The Gutenberg–Richter law can be reproduced by slider-block fault models showing in its quasi-static limit self-organized criticality. However, in this model limit the earthquake ruptures are described by propagating narrow slip pulses leading to unrealistic low stress concentrations at the rupture front. To overcome such unrealistic rupture behavior, we introduce a new state-dependent stress distribution rule accounting for broader slip pulses up to crack-like behavior. Our systematic analysis of the generalized model shows that the earthquake characteristics can be described in terms of critical point behavior, resulting in subcritical, critical, and supercritical system states. We can explain the realized state of self-organized systems by the effect of individual ruptures on the stress field. This effect depends strongly on the fault roughness. For spatially smooth systems, more realistic rupture characteristics lead to supercritical behavior, equivalent to characteristic earthquake distributions empirically observed for several individual faults. For rough faults, earthquakes cannot rupture the whole system and seismic energy is released by small events only. The transition between both regimes occurs at an intermediate degree of heterogeneity, where the earthquake activity is reminiscent of self-organized criticality. Thus, our results predict that for individual faults one should in general observe systematic deviations from the Gutenberg–Richter law for large earthquake sizes.

Heterogeneity and the earthquake magnitude‐frequency distribution

Regional earthquake populations are widely observed to have power‐law magnitude‐frequency statistics. Whether earthquake distributions on individual faults are similarly power‐law is presently the subject of considerable research effort. Recent careful observations suggest that event statistics on rough faults are power‐law whereas earthquake distributions on smooth faults are better described by a characteristic model in which there are more large earthquakes than would be expected by extrapolation from the distribution of small events. Here the relation between heterogeneity and the magnitude‐frequency distribution is investigated in a cellular automaton that includes new nearest‐neighbor stress transfer rules which produce realistic stress concentrations. The results agree with the suggestion that smooth faults have more characteristic earthquake distributions and support the view that an earthquake's size is controlled by rupture arrest.

Exploration Strategies and Possible Submarine Fan Complexes in the Rough Creek Graben, Western Kentucky: ABSTRACT

The Rough Creek Graben is a deep, east-west-oriented rift basin more than 160 km long and 40 to 70 km wide in western Kentucky. It is a north-dipping half graben bounded on the north by the Rough Creek Fault Zone and on the east by a presumed tectonic inversion structure associated with reactivation of the East Continent Rift Basin. The half graben is filled with up to 5.5 km of dominantly marine sediments of the Cambrian pre-Knox Group that thin away from the Rough Creek Fault Zone. Most of the pre-Knox oil and gas exploration in the graben has taken place near the Rough Creek Fault Zone. The boundary fault has a polyphase movement history and is characterized by extensive fracturing in and near the fault zone. Both the mechanical drilling problems and the lack of exploration success that have marked past exploration attempts are likely related to the high degree of fracturing associated with the northern edge of the graben. The pre-Knox rocks south of the boundary-fault zone are virtually untested and include viable exploration targets. Recent investigations using limited seismic reflection data provide strong evidence for the presence of basin-floor submarine fan deposition at least in the western,more » basal part of the graben. Several fan complexes up to 2 km in aggregate thickness have been interpreted in the northern part of the basin. To the south, thinner, less continuous fan deposits are inferred. If confirmed by drilling, these fan complexes could represent important future hydrocarbon reservoirs in many areas of the graben.« less

Coal Rank Trends in Western Kentucky Coal Field and Relationship to Hydrocarbon Occurrence: ABSTRACT

Abstract Vitrinite maximum reflectance trends for the Middle Pennsylvanian Springfield (No. 9) coal bed in the Western Kentucky coalfield indicate that coal rank is high volatile A bituminous (0.70-0.85% Rmax) in portions of the Webster syncline in Webster and Union counties (south of the Rough Creek fault zone) and is generally high volatile C bituminous (<0.60% Rmax) in the remainder of the coalfield. Within the Webster syncline, the reflectance of the Springfield coal does not vary significantly despite a difference in elevation of nearly 300 m. Reflectance gradients in the Webster syncline are not well defined; reflectances are generally highest in the Baker (No. 13) coal bed and are lower in the underlying Herrin (No. 11) and Briar Hill (No. 10) coals. Rank trends from the Springfield data alone suggest pre-tectonic coalification, but the lack of well-defined reflectance gradients complicates this interpretation. Hydrothermal alteration may have been a significant cause of the coalification in this region. Previous investigations of coals in fault blocks have considered the role of hydrothermal mineralization in coalification. Southern Union and Webster counties are adjacent to the Fluorspar District, a region of extensive hydrothermal metamorphism. It is possible that convective heat transfer related to Fluorspar District mineralization led to vertical heterogeneity in the temperature profile and consequent metamorphic history. In Henderson County, north of the Rough Creek fault zone, coal rank is significantly lower, as low as 0.42% Rmax for the Springfield coal. Reflectance gradients are, again, poorly defined. Extensive oil and gas development has occurred in the high volatile C bituminous region to the north of the Rough Creek fault zone, whereas fewer pools, with lower production, are known within the Webster syncline to the south of the fault zone. The rank of the Middle Pennsylvanian coals can be used to estimate the level of maturation of the Devonian New Albany shale, a likely source rock for much of the oil and gas in the coalfield. Previous studies on the maturation of the New Albany shale, which lies about 1 km below the Springfield coal, indicate an equivalent medium volatile bituminous (1.0-1.2% Rm) rank in the Fluorspar District. New Albany rank decreases to an equivalent high volatile B/C (0.60% Rm) north of the Rough Creek fault zone. The significance of the New Albany reflectances is dependent on the level of suppression of vitrinite reflectance in organic-rich shales. The existence of reflectance suppression would imply that the shales could be more mature than studies to date have indicated. It is possible that the episodes of basin dewatering which influenced the coal rank south of the Rough Creek fault zone also flushed the potential oil reservoirs in the same region.

Interpretation of Recent Seismic Data from a Frontier Hydrocarbon Province: Western Rough Creek Graben, Southern Illinois and Western Kentucky: ABSTRACT

The northern basement fault of the Rough Creek graben is seismically discernible and has surface expression in the Rough Creek fault zone. The southern basement fault is not clearly defined seismically, but can be inferred from shallow faulting and gravity data. This fault is roughly coincident with the Pennyrile fault zone. Extensional faults that formed the rift boundaries were the sites of late-stage compressional and extensional tectonics. Flower structures observed along the graben boundaries probably indicate post-Pennsylvanian wrench faulting. The basement within the graben plunges north-northwest, with the lowest point occurring south of the Rough Creek fault zone. Pre-Knox sediments thicken to approximately 12,000 in this area. The Knox Megagroup thickens toward the Mississippi Embayment, ranging from 4800 ft (southeastern graben area) to more than 7000 ft (west end of graben). Upper Ordovician to Devonian units also display westward thickening. The top of the Meramecian, New Albany, Maquoketa, and the base of the Knox generate continuous, high-amplitude seismic reflections due to large impedance contrasts between clastic and carbonate units. Shallow oil and gas production (Mississippian and Pennsylvanian) is present in this area. However, deep horizons (Knox, Lower Cambrian) remain relatively untested. Potential hydrocarbon traps in the pre-Knox sequence observed onmore » seismic include fault blocks and updip pinch-outs.« less

Effects of physical fault properties on frictional instabilities produced on simulated faults

Laboratory studies of large‐scale simulated faults show that physical properties of the fault, specifically normal stress and fault roughness, significantly influence the unstable shear failure behavior of the fault. In addition, the experiments provide insights into important length‐scaling effects that are useful for assessing concepts such as critical crack length or rupture nucleation dimension. Stick‐slip shear failures have been generated along a 2‐m‐long simulated fault in a block of Sierra white granite. Experiments were conducted at normal stresses between 0.6 and 4 MPa with two different prepared roughnesses of the simulated fault: a smooth fault with a profilometer measured roughness of 0.2 μm and a rough fault with a measured roughness of ∼80 μm. High‐speed records of shear strains and slip velocities clearly show both the propagation of the stick‐slip failures and details of the local deformations and motions from which slip weakening of the fault at the onset of sliding can be resolved. Both the dynamic stress drops of the stick‐slip events and the apparent fracture energies of the events determined from the records of the fault slip weakening increase with increasing normal stress. Although critical slip‐weakening displacements are insensitive to normal stress, they increase with increased fault roughness such that the apparent fracture energies are also larger on the rough fault than on the smooth fault. Slip velocities in these experiments are influenced by roughness and are proportional to stress drop. Although the onset of slip is sharply defined, the termination of slip is more gradual without a clear stopping phase. Measured rupture velocities on the rough fault are lower than those on the smooth fault. When a length scale based on rupture nucleation dimensions, directly proportional to critical displacements, is introduced, the data suggest that at fault lengths close to the nucleation lengths, rupture velocity and slip velocity increase with increasing fault length.

Late Palaeozoic basins of the southern U.S. continental interior

The timing and geometry of late Palaeozoic inhomogenous deformation in the southern U.S. continental interior from the Appalachian foreland of Kentucky-Tennessee through Oklahoma and Texas to the Ancestral Rockies of Colorado-Wyoming-Utah can be definitively linked with a discrete sequence of collisional events in the Appalachian-Ouachita-Marathon orogenic belt to the south (Permian coordinates). A progressive staged collisional sequence beginning in late Mississippian times in the southern Appalachians and culminating in early Permian times in the Marathons led to a progressive deformation of the adjacent craton in a wide swath dominated by right-lateral shear (e.g. Rough Creek Fault zone) by which a Mexican promontory of North America was displaced towards a Pacific ‘free face’. While the deformation in the Appalachian-Marathon belt was dominated by vertical plane strain leading to crustal thickening, the associated continental interior deformation can be averaged as a horizontal plane strain at or near sea level with localized deep extensions (Delaware) and flexural (Arkoma) basins and compressions uplifts (Amarillo-Wichita) giving local source areas.

Geophysical Evidence for Deep Basin in Western Kentucky

The Rough Creek fault zone is a major element of the 38th Parallel lineament in western Kentucky and southern Illinois near the head of the Mississippi embayment. Gravity, magnetic, and subsurface data suggest that this fault zone marks the northern boundary of a large graben in the Precambrian basement for which we propose the name Rough Creek graben. This graben is a major structural feature which probably formed initially in late Precambrian to early Paleozoic time and has been reactivated (perhaps several times) during the late Paleozoic and possibly the Mesozoic. The graben is as much as 5.5 km deep and the large volume of deeply buried sediments favors further exploration.

THE STRUCTURE AND CARNARVON TERRACE STRATIGRAPHY OF THE AND WALLABY PLATEAU

The Carnarvon Terrace, Wallaby Plateau and adjacent slopes cover about 350 000 km2 beyond the continental shelf of central Western Australia. Water depths within the region range from about 600 m on the Carnarvon Terrace to 5000 m over the Cuvier Abyssal Plain.The dominant structural grain of the Carnarvon Terrace varies from north-northeast in the north to north-northwest in the south. A Palaeozoic structural high (Gascoyne Sub-basin) extends across the central Carnarvon Terrace. This feature is controlled by a Precambrian basement high. which is onlapped by sedimentary rocks of Ordovician to Carboniferous age. The high is bounded in the north by a major northeast trending fault, which may be an extension of the Rough Range Fault. Troughs trending parallel to the fault are part of the southern Exmouth Sub-basin; the largest trough contains up to 2000 m of possible Triassic and Jurassic sediments. A north-northwest trending half-graben 600 km long and up to 100 km wide, bounds the Palaeozoic high to the south. Normal faults, downthrown to the west, are common within this half-graben, which appears to connect with the Abrolhos Sub-basin and thus probably contains Permian, Triassic and Jurassic section.Northwest and possibly north-northeast trends occur on the Wallaby Plateau. Much of the Plateau consists of folded (?)Palaeozoic rocks and possibly shallow Proterozoic basement, with igneous bodies abundant on its western and southern margins.Rifting of the margin probably commenced in the Permian and culminated in a final phase from the Late Jurassic to Neocomian. This was followed by a period of intense erosion and peneplanation during the Neocomian. Formation of oceanic crust which now underlies the Cuvier and Perth Abyssal Plains, began in the Neocomian and was accompanied by a marine transgression which resulted in deposition of Aptian to Cenomanian shallow-marine clastic sediments over both the Carnarvon Terrace and Cuvier Abyssal Plain. After a hiatus, deposition of a post-Senonian shelf carbonate sequence commenced and has continued until the present day with only minor interruptions.The existence of a large trough of Mesozoic sediments that is probably a northwestern extension of the Abrolhos Sub-basin considerably upgrades the hydrocarbon potential of the southern Carnarvon Terrace. Traps along the possible offshore extension of the Rough Range Fault and Devonian reef complexes within the Palaeozoic high may also be prospective for hydrocarbons.

EXPERIMENTAL STUDIES OF STICK-SLIP AND THEIR APPLICATION TO THE EARTHQUAKE SOURCE MECHANISM

Detailed laboratory experiments have been performed on the stick-slip characteristics of friction of granite, particularly in relation to such environmental conditions as the stiffness of loading system, the velocity of ram advance in a testing machine and the normal load across sliding surfaces. It is found that the stick-slip amplitude Δμ=ΔF/N(ΔF: force drop and N: normal load) decreases at high stiffness, at high velocity and at high nomal load, and that there are the interre1ated effects of the stiffness, the velocity and the normal load on stick-slip. The somewhat conflicting observations so far on the effect of stiffness on stick-slip amplitude are explained system-atically considering the interrelated effects. Stiffness influences not only the slip amplitude, but the time of slipping, which corresponds to the rise time of the source time function of an earthquake. Frictional behavior is also investigated for various rock types in relation to roughness and bulk hardness of sliding surfaces. Stick-slip diminishes on rough fault surfaces. Hard rocks tend to increase stick-slip amplitude. The in situ stiffness k for the unilaterally propagating fault of an earthquake was estimated in a previous paper as k=μ(υ/β)2W=ρυ2W, where μ is rigidity, υ is the propagating velocity of dislocation, β is shear wave velocity, W is fault width, and ρ is density. The in situ stiffness obtained from the above formula for a large shallow focus earthquake is 6 or 7 orders of magnitude more than the stiffness of a typical testing machine in a laboratory. It has been pointed out that the stress drop associated with earthquakes is remarkably small compared with that accompaning fracture or stick-slip in laboratory experiments. This difference may be partly due to the different stiffnesses of the two systems.

Geology of Basement in Midwestern United States

A thick sedimentary cover over the midwestern United States has restricted geologic investigation of the basement complex in the past to a slow and fragmentary study based on a few deep wells. Recent basement studies have been stimulated by State-wide geophysical surveys now available for regional interpretation. Modern investigations have included mapping of basement configuration, lithology, orogenic trends, and geochronology. A map showing basement configuration reflects the basin and arch provinces recognized in the Paleozoic rocks of the Midwest, and thus supports the usual premise that the basement forms a regional structural framework for the overlying sedimentary strata. Cross sections based upon seismic surveys and extrapolations from deep well data show that Paleozoic structures, such as the La Salle anticline and the Rough Creek and Kentucky fault systems, are underlain by basement ridges and scarps. Thickening and thinning of Lower Ordovician and Cambrian strata over basement scarps and ridges demonstrate basement influence on early Paleozoic sedimentation. The relation of basement relief to oil accumulation is a possibility that has not yet been demonstrated fully. Lithologic studies of well samples reveal that the basement in the Midwest is predominantly granitic. Linear gravity and magnetic anomalies in Michigan, Indiana, Ohio, and Kentucky may be caused by Keweenawan-type basalts superposed on the granite. A study of basement lithologies and isotopic age determinations helps in the recognition of extensions of structural provinces of the Canadian shield into this region. A geochronological map based on sparse age data shows, for example, a southwestern extension of the Grenville front through Michigan and Indiana.

Relation of Rough Creek Fault of Kentucky to Ouachita Deformation

The Ouachita structures west of the Mississippi Embayment form a belt extending from the Mexican border to Arkansas. The Rough Creek fault of Kentucky and Illinois is part of a system of faults and steep folds which is similar to the Ouachita structures in its general trend and in the occurrence of thrusting from the south. The Ouachita structures show no sign of diminishing where they pass beneath the Mississippi Embayment on its west side, although the structures of the Rough Creek fault system increase toward the west in the amount of thrusting and in the intensity of deformation. The Rough Creek fault system is considerably north of a projection of the Ouachita deformation across the Mississippi Embayment, but this may be explained by assuming that the belt of deforma ion bends around the Ozark uplift as it bends around the Llano uplift of Texas. Additional evidence is obtained from northwestern Tennessee, where wells in the Mississippi Embayment between the Ouachita and Rough Creek structures have found deformed and altered Paleozoic rocks.