Fault (geology)

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

Regional tectonic stress near the San Andreas fault in central and southern California

Abstract: [1] Throughout central and southern California, a uniform NNE-SSW direction of maximum horizontal compressive stress is observed that is remarkably consistent with the superposition of stresses arising from lateral variations in lithospheric buoyancy in the western United States, and far-field Pacific-North America plate interaction. In central California, the axis of maximum horizontal compressive stress lies at a high angle to the San Andreas fault (SAF). Despite relatively few observations near (±10 km) the fault, observations in the greater San Francisco Bay area indicate an angle of as much as 85°, implying extremely low fault strength. In southern California, observations of stress orientations near the SAF are rotated slightly counter-clockwise with respect to the regional field. Nevertheless, we observe an approximately constant angle between the SAF and the maximum horizontal stress direction of 68 ± 7° along ∼400 km of the fault, indicating that the SAF has moderately low frictional strength in southern California.

Fault textures in volcanic conduits: evidence for seismic trigger mechanisms during silicic eruptions

Abstract: It is proposed that fault textures in two dissected rhyolitic conduits in Iceland preserve evidence for shallow seismogenic faulting within rising magma during the emplacement of highly viscous lava flows. Detailed field and petrographic analysis of such textures may shed light on the origin of long-period and hybrid volcanic earthquakes at active volcanoes. There is evidence at each conduit investigated for multiple seismogenic cycles, each of which involved four distinct evolutionary phases. In phase 1, shear fracture of unrelaxed magma was triggered by shear stress accumulation during viscous flow, forming the angular fracture networks that initiated faulting cycles. Transient pressure gradients were generated as the fractures opened, which led to fluidisation and clastic deposition of fine-grained particles that were derived from the fracture walls by abrasion. Fracture networks then progressively coalesced and rotated during subsequent slip (phase 2), developing into cataclasite zones with evidence for multiple localised slip events, fluidisation and grain size reduction. Phase 2 textures closely resemble those formed on seismogenic tectonic faults characterised by friction-controlled stick-slip behaviour. Increasing cohesion of cataclasites then led to aseismic, distributed ductile deformation (phase 3) and generated deformed cataclasite zones, which are enriched in metallic oxide microlites and resemble glassy pseudotachylite. Continued annealing and deformation eventually erased all structures in the cataclasite and formed microlite-rich flow bands in obsidian (phase 4). Overall, the mixed brittle-ductile textures formed in the magma appear similar to those formed in lower crustal rocks close to the brittle-ductile transition, with the rheological response mediated by strain-rate variations and frictional heating. Fault processes in highly viscous magma are compared with those elsewhere in the crust, and this comparison is used to appraise existing models of volcano seismic activity. Based on the textures observed, it is suggested that patterns of long-period and hybrid earthquakes at silicic lava domes reflect friction-controlled stick-slip movement and eventual healing of fault zones in magma, which are an accelerated and smaller-scale analogue of tectonic faults.

Earthquake Failure Sequences Along a Cellular Fault Zone in a Three-Dimensional Elastic Solid Containing Asperity and Nonasperity Regions

Abstract: Numerical simulations of earthquake failure sequences along a discrete cellular fault zone are performed for a three-dimensional (3-D) model representing approximately the central San Andreas fault. The model consists of an upper crust overlying a lower crust and mantle region, together defining an elastic half-space with a vertical half-plane fault. The fault contains a region where slip is calculated on a uniform grid of cells governed by a static/kinetic friction law and regions where slip is prescribed so as to represent tectonic loading, aseismic fault creep, and adjacent great earthquakes. The computational region models a 70-km-long and 17.5-km-deep section of the San Andreas fault to the NW of the great 1857 rupture zone. Different distributions of stress drops on failing computational cells are used to model asperity (“Parkfield asperity”) and nonasperity fault regions. The model is “inherently discrete” and corresponds to a situation in which a characteristic size of geometric disorder within the fault (i.e., cell size, here a few hundreds of meters) is much larger than the “nucleation size” (of the order of tens of centimeters to tens of meters) based on slip weakening or state evolution slip distances. The computational grid is loaded by a constant plate motion imposed at the lower crust, upper mantle, and creeping fault regions and by a “staircase” slip history imposed at the 1857 and 1906 rupture zones. Stress transfer along and outside the fault due to the imposed loadings and failure episodes along the computational grid is calculated using 3-D elastic dislocation theory. The resulting displacement field in the computational region is compatible with geodetic and seismological observations only when the asperity and nonasperity regions are characterized by significantly different average stress drops. The frequency-magnitude statistics of the simulated failure episodes are approximately self-similar for small events, with b ≈ 1.2 (the b value of statistics based on rupture area bA is about 1) but are strongly enhanced with respect to self-similarity for events larger than a critical size. This is interpreted as a direct manifestation of our 3-D elastic stress transfer calculations; beyond certain rupture area and potency (seismic moment divided by rigidity) release values, the event is usually unstoppable, and it continues to grow to a size limited by a characteristic model dimension. This effect is not accounted for by cellular automata and block-spring models in which the adopted simplified stress transfer laws fail to scale properly with increasing rupture size. The simulations suggest that local maxima in observed frequency-magnitude statistics correspond to dimensions of coherent brittle zones, such as the width of the seismogenic layer or the length of a fault segment bounded by barriers. The analysis indicates that a single cell size, representing approximately a single scale of geometric disorder, cannot induce self-similarity in a 3-D elastic model over a broad range of magnitudes. A representation of geometric disorder covering a range of scales may thus be required to generate a wide domain of self-similar Gutenberg-Richter statistics. Our simulations show a great diversity in the mode of failure of the Parkfield asperity; the earthquakes themselves define an irregular sequence of events. The modeling, like many other discrete fault models, suggests that expectations for periodic Parkfield earthquakes and/or simple precursory patterns repeating from one event to the other are unrealistic.