Fault (geology)

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

Fractal geometry in the San Andreas Fault System

Abstract: It has been noted that the spatial distribution of earthquakes and the mode of strain release in the San Andreas fault system is related to the complexity of fault geometry. Because of their rough appearance over many length scales, faults can be regarded as fractal surfaces. Direct estimates of fractal dimension D of portions of the San Andreas fault system between the northern Gabilan Range and the Salton Sea, including the postulated extent of the great 1857 Fort Tejon earthquake, are obtained from measured fault lengths, analogous to the lengths of coastlines as discussed by Mandelbrot. Regions characterized by complicated fault geometry are associated with larger values of D. Based on fault traces mapped at a scale of 1:750,000, D is 1.3 for this reach of the fault defined as a 30-km-wide band about a main fault trace. For that part near Parkfield which could be associated with the nucleation of the 1857 earthquake, D is 1.1; at this same scale, D is 1.4 for the San Andreas and related faults near San Bernardino where the 1857 rupture stopped, compared to 1.2 for the San Andreas-San Juan fault segments near the point of arrest of the 1966 Parkfield earthquake. At finer map scales (1:24,000 and 1:62,500) critical lengths of ∼ 500 m and 1 km are identified which might relate to the extent of off-San Andreas fault offsets. The critical lengths also suggest that fault geometry is not self-similar. If this fractal geometry persists through the seismic cycle, it may be possible to use a quantitative measure of complexity to explain the occurrence of great and characteristic earthquakes along a given reach of fault.

Cenozoic tectonics of the western United States

Abstract: The Cenozoic structures of the western United States are interpreted here as being products mostly of horizontal motion of the crust. The distribution of strike-slip faulting, tensional fragmentation of the brittle upper crust or rupturing of the entire continental crust, and compression define a pattern of northwestward motion increasing irregularly southwestward toward coastal California. Hans Becker, in 1934, and S. W. Carey, in 1958, are among those who have suggested such a tectonic system. The aggregate Cenozoic right-lateral displacement of Cretaceous and older rocks and structures by the northwest-trending strike-slip faults of coastal California is about 500 km. The greater part of this movement has occurred along the San Andreas fault, but many other faults share in it. At least six earthquakes within the past century have been accompanied by lateral displacements at the surface along faults of the San Andreas system. Successively greater offsets of successively older geologic terranes demonstrate continuing motion throughout Cenozoic time. Late Miocene materials have been displaced at least 160 km; Oligocene, at least 260 km. The present velocity of regional shear strain, about 6 cm/yr, demonstrated by geodetic resurveying in southern and central California, is about 8 times faster than the average needed to account for the total movement within the Cenozoic. The faults are in general associated with structures formed by oblique tension south of Los Angeles and with structures due to oblique compression north of that city. The opening of the Gulf of California and the Salton Trough by the oblique rifting of Baja California and the Peninsular Ranges away from mainland Mexico is the greatest of the tensional effects. The strike-slip faults may be confined to the crust. Earthquake foci extend no deeper than 16 km. The faults end to the south in the Gulf of California, whose crustal structure is oceanic. To the north, the San Andreas turns seaward as the north-facing Gorda scarp, west in line of which in deeper water is the south-facing Mendocino escarpment, produced apparently by an inactive left-lateral oceanic fault. The continental sliver of coastal and Baja California, west of the faults of the San Andreas system, may be drifting northwestward independently over the ocean floor and the mantle, and the leading point of the sliver may have been deflected westward when it hit the Mendocino scarp on the sea floor. East of this coastal movement system is the Basin and Range province, whose obvious Cenozoic structures are dominated by block faulting. The present ranges have formed mostly since early Miocene time, similar older ranges having been destroyed by erosion and deformation. The normal faulting, which is not associated within the region with any complementary tectonic compression, requires crustal extension as its basic cause. If the faults maintain their average 60° dips at depth, extension is half the dip-slip amount; but probably the major faults flatten downward, and the amount of extension about equals that of shallow dip-slip. Total Cenozoic extension in northern Nevada and Utah may have been 300 km. Concurrent volcanism much augmented the thinned and fragmented crust, and the volcanic terranes in turn have been fragmented by block faulting. Right-lateral strike-slip faults trend northwestward in lanes between normal-fault maintain blocks in the southwestern part of the Basin-Range province. Cenozoic displacements reach 50 km on the Las Vegas fault and 80 km on the Death Valley-Furnace Creek faults. Northeast of the strike-slip faults, ranges and basins trend north-northeastward in tension-gash orientation. Within the belt of lateral faulting, ranges undergoing active normal faulting mostly trend north-northwestward in oblique pull-apart orientation. The Sierra Nevada and Klamath Mountains have moved northwestward and rotated counterclockwise, thus moving away from the continental interior more in the north than in the south, and the extension distributed behind them has formed the Basin-Range province. The narrow block-fault Rio Grande valley system of New Mexico and southern Colorado is structurally and topographically similar to the rift valleys of East Africa and reflects localized crustal extension. The Idaho batholith, like the Sierra Nevada batholith, is drifting northwestward as an unbroken plate. Extension east of the Idaho batholith is taken up by normal-fault fragmentation in south-central Idaho and southwestern Montana, whereas extension south of the batholith has produced a rift through the continental crust, the Snake River Plain, filled deeply by lava. Seismic velocities indicate granitic crust to be lacking in at least the western part of the plain. Right-lateral faults of the Osburn system bound the batholithic plate on the north, and the motion they represent is taken up north of them by extension forming fault troughs. Integration of geologic and geophysical information shows that large regions of the Northwest are lava accumulations of continental crustal thickness, not old continental crust covered by lava. The volcanic terrane of northwestern Oregon and southwestern Washington forms new volcanic crust in a region which was oceanic before Cenozoic time. The volcanic terrane of southeastern Oregon, northeastern California, and northwestern Nevada fills an irregular tension rift through the Mesozoic continental crust. This rift resulted from the westward motion of the Klamath Mountains region, which was sundered from a position south of the Mesozoic terrane of northeastern Oregon and which was bent oroclinally as it moved westward in post-middle Eocene time. The Mesozoic terrane of northeastern Oregon pivoted away from the Idaho batholith to form a smaller orocline and left a triangular rift since filled by lava. Independent motion of continental crust over mantle and oceanic crust seems to be indicated. Inertial forces due to redistribution of rotational momentum among crustal fragments, mantle, and core may provide the motive power.

Heat flow, stress, and rate of slip along the San Andreas Fault, California

Abstract: The absence of a heat flow anomaly greater than ∼0.3 µcal/cm2/sec associated with the San Andreas fault is used to estimate the upper limit for the steady state or initial shear stress. Under the assumption that the long-term rate of motion along the fault is 5 cm/yr and occurs primarily in the form of creep, this upper limit is about 100 bars. If the motion is primarily accomplished by faulting during large earthquakes and if the frictional stress is equal to the final stress as suggested by E. Orowan (1960), the upper limit is estimated to be about 200 bars. Without Orowan's assumption, the estimation of the upper limit is about 250 bars, based on earthquake energy-magnitude-moment relations. If the long-term rate of motion along the San Andreas fault is only ∼2 cm/yr, these results are increased to 250, 350, and 400 bars, respectively.

Fault growth and fault scaling laws: Preliminary results

Abstract: We report progress made in the last few years on the general problem of the mechanism of fault growth and the scaling laws that result Results are now conclusive that fault growth is a self-similar process in which fault displacement d scales linearly with fault length L Both this result and the overall nature of along-strike fault displacement profiles are consistent with the Dugdale-Barenblatt elastic-plastic fracture mechanics model In this model there is a region of inelastic deformation near the crack tip in which there is a breakdown from the yield strength of the unfractured rock to the residual frictional strength of the fault over a breakdown length S and displacement d0 Limited data also indicate that S and d0 also scale linearly with L, which implies that fracture energy G increases linearly with L The scaling parameters in these relationships depend on rock properties and are therefore not universal In our prime field locality, the Volcanic Tableland of eastern California, we have collected data over 2 orders of magnitude in scale range that show that faults obey a power law size distribution in which the exponent C in the cumulative distribution is ∼13 If the fault is growing within the brittle field, the zone of inelastic deformation consists of a brittle process zone which leaves a wake of fractured rock adjacent to the fault Preliminary results of modeling the process zone are consistent with observations now in hand both in predicting the preferred orientation of cracks in the process zone wake and the rate of falloff of crack density as a function of distance from the fault The preferred orientation of these cracks may be used to infer the mode and direction of propagation of the fault tip past the point in question According to the model, the width of the process zone wake may be used to infer the length of the fault at the time its tip passed the measurement point, but data have not yet been collected to verify this prediction If the fault displacement has been accumulated by repeated seismic slips, each of these will sweep the fault with a crack tip stress field of a smaller spatial extent than that of the fault tip stress field, producing an inner, more intensely fractured, process zone wake This may be the mechanism that creates the cataclasite zone, rather than simple frictional wear, as has been previously supposed