Fault (geology)

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

Induced earthquake magnitudes are as large as (statistically) expected

Abstract: A major question for the hazard posed by injection-induced seismicity is how large induced earthquakes can be. Are their maximum magnitudes determined by injection parameters or by tectonics? Deterministic limits on induced earthquake magnitudes have been proposed based on the size of the reservoir or the volume of fluid injected. However, if induced earthquakes occur on tectonic faults oriented favorably with respect to the tectonic stress field, then they may be limited only by the regional tectonics and connectivity of the fault network. In this study, we show that the largest magnitudes observed at fluid injection sites are consistent with the sampling statistics of the Gutenberg-Richter distribution for tectonic earthquakes, assuming no upper magnitude bound. The data pass three specific tests: (1) the largest observed earthquake at each site scales with the log of the total number of induced earthquakes, (2) the order of occurrence of the largest event is random within the induced sequence, and (3) the injected volume controls the total number of earthquakes rather than the total seismic moment. All three tests point to an injection control on earthquake nucleation but a tectonic control on earthquake magnitude. Given that the largest observed earthquakes are exactly as large as expected from the sampling statistics, we should not conclude that these are the largest earthquakes possible. Instead, the results imply that induced earthquake magnitudes should be treated with the same maximum magnitude bound that is currently used to treat seismic hazard from tectonic earthquakes.

Sea-Floor Spreading and Structural Evolution of Southern Red Sea

Abstract: The Red Sea has entered the early stages of continental dispersal, and its structural evolution fundamentally has been the rifting and breaching of continental lithosphere by normal faulting attendant on the process of sea-floor spreading. In Oligocene time, the continental lithosphere of the Red Sea area was bowed into a large regional arch with normal faults across the crest. Subsequently, rifting by normal faults that propagated upward through the brittle part of the lithosphere caused strong subsidence on horst and graben and tilted blocks; this rifting led to an extensive marine incursion, and a thick evaporite sequence was deposited in the restricted, hot, arid, low-latitude setting of the Miocene Red Sea trough. A second rift west of the earlier central or axial Mi cene rift originated in the southern Red Sea area in Pliocene time, and the two features now are evolving concurrently over a distance of at least 400 km. An evaporite section of very shallow marine origin accumulated in the western rift during the Quaternary; it constitutes a modern example of salt accumulating in a narrow, restricted, rifted trough which is forming as a consequence of continental breakup. Although considerable separation has occurred in the Red Sea, the present opposing coastlines were never in contact, because the fragmentation of continental lithosphere was very largely attained by normal faulting. Only if a vertical fault cuts the entire thickness of lithosphere could points on opposite coastlines ever have been contiguous.

Extension on the Tibetan plateau: recent normal faulting measured by InSAR and body wave seismology

Abstract: We use InSAR and body wave modelling to determine the faulting parameters for a series of five Mw 5.9–7.1 normal faulting earthquakes that occurred during 2008, including the March 20 Yutian earthquake (Mw 7.1), one of the largest normal faulting events to have occurred recently on the continents. We also study three earlier normal faulting earthquakes that occurred in southern Tibet between 1992 and 2005. Coseismic deformation for each of these eight events is measured with ascending and descending interferograms from ENVISAT, ERS and ALOS SAR data. Elastic dislocation modelling of the line-of-sight InSAR displacements and body wave seismological modelling of P and SH waves are used to estimate fault parameters and are found to be in good agreement for all the events studied. The use of InSAR to measure deformation allows a relatively precise determination of the fault location in addition to resolving the focal plane ambiguity. Only five of the eight events are associated with a clear surface topographic feature, suggesting that an underestimation of the amount of extension would result from using the surface expressions of normal faulting alone. The observations, in all cases, are consistent with slip on planar surfaces, with dips in the range 40–50°, that penetrates the uppermost crust to a depth of 10–15 km. We find no evidence for active low-angle (dip less than 30°) normal faulting. The contribution of the normal faulting to overall extension estimated by summing seismic moments over earthquakes for the past 43 yr is 3–4 mm yr−1, or 15–20 per cent of the rates of extension measured across the plateau using GPS. 85 per cent of the moment release in normal faulting over the past 43 yr has occurred in regions whose surface height exceeds 5 km. This observation adds weight to the suggestions that the widespread normal faulting on the plateau is the result of variations in the gravitational potential energy of the lithosphere.

Diverse Pliocene-Quaternary tectonics in a transform environment, San Francisco Bay region, California

Abstract: The San Francisco Bay region occupies part of the diffuse transform boundary zone between the Pacific and North American plates. Although dextral strike-slip faulting is dominant, the plate motion is expressed in a variety of ways. Some strike-slip faults are parallel with the plate boundary, but some are slightly oblique. The major strike-slip faults are zones in which an interplay occurs between strands, some of which are en echelon. This interplay may be responsible for some apparent pull-apart basins and presumed normal faults along the Calaveras fault zone. In extensive areas between strike-slip fault zones, there are many compressional structures—folds and reverse faults—that are Pliocene-Quaternary in age, hence largely coeval with the strike-slip faults. Some of the compressional structures are oblique to, and others are parallel with, the major strike-slip faults. We have divided the San Francisco Bay region into domains on the basis of the types, orientations, and relationships of structures, or, in one case, lack of strong deformation. Each domain has responded in its own way to the regional northwest-southeast dextral shear imposed by the relative plate motion. In an attempt to understand the origin of various structures and the interrelationships among them, we compared established models with our observations. Some observed geometric relationships agree quite well with classical Coulomb-Anderson and simple shear models, but others require further explanation. Models based on fault interaction seem to apply to certain cases. For example, the East Bay Hills domain, which is between the left-stepping Calaveras and Hayward–Rodgers Creek fault zones, is under compression resulting from interaction between the two strike-slip zones.

Mechanisms of extension at nonvolcanic margins: Evidence from the Galicia interior basin, west of Iberia

Abstract: [1] We have studied a nonvolcanic margin, the West Iberia margin, to understand how the mechanisms of thinning evolve with increasing extension. We present a coincident prestack depth-migrated seismic section and a wide-angle profile across a Mesozoic abandoned rift, the Galicia Interior Basin (GIB). The data show that the basin is asymmetric, with major faults dipping to the east. The velocity structure at both basin flanks is different, suggesting that the basin formed along a Paleozoic terrain boundary. The ratios of upper to lower crustal thickness and tectonic structure are used to infer the mechanisms of extension. At the rift flanks (stretching factor, β ≤ 2) the ratio is fairly constant, indicating that stretching of upper and lower crust was uniform. Toward the center of the basin (β ∼ 3.5–5.5), fault-block size decreases as the crust thins and faults reach progressively deeper crustal levels, indicating a switch from ductile to brittle behavior of the lower crust. At β ≥ 3.5, faults exhume lower crustal rocks to shallow levels, creating an excess of lower crust within their footwalls. We infer that initially, extension occurred by large-scale uniform pure shear but as extension increased, it switched to simple shear along deep penetrating faults as most of the crust was brittle. The predominant brittle deformation might have driven small-scale flow (≤40 km) of the deepest crust to accommodate fault offsets, resulting in a smooth Moho topography. The GIB might provide a type example of nonvolcanic rifting of cold and thin crust.