Fault (geology)

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

Surface faulting accompanying the Borah Peak earthquake and segmentation of the lost river fault, central Idaho

Abstract: On the morning of 28 October 1983, the Ms 7.3 Borah Peak earthquake struck central Idaho and formed a Y-shaped zone of surface faults that is divided into a southern, a western, and a northern section. The total length of the surface faults is 36.4 ± 3.1 km, and the maximum net throw is 2.5 to 2.7 m. The near-surface net slip direction, determined from the rakes of striations in colluvium, averaged 0.17 m of sinistral slip for 1.00 m of dip slip . The 20.8-km-long southern section is the main zone of surface faulting and coincides with the Thousand Springs segment of the Lost River fault. It has the largest amount of net throw, most complex rupture patterns, and best evidence of sinistral slip. The surface faults include zones of ground breakage as much as 140 m wide, en echelon scarps with synthetic and antithetic displacements, and individual scarps that are nearly 5 m high . The 14.2-km-long western section diverges away from the Lost River fault near the northern end of the southern section. The net throw on this section is generally less than 0.5 m but locally is as much as 1.6 m. The new ruptures are poorly developed across the crest and north flank of the Willow Creek hills; they are mostly downhill-facing, arcuate scars, perhaps incipient landslides, that may overlie a deeper zone of tectonic movement . The northern section, at least 7.9 km long, is on the Warm Spring segment of the Lost River fault and has a maximum net throw of about 1 m. The pattern of surface faulting on this section is simple compared to the other sections. A 4.7-km-long gap in 1983 surface faults separates the northern and southern sections but contains an older scarp of late Pleistocene age . Geologic, seismologic, and geodetic data from the earthquake suggest that barriers confined the primary coseismic rupture to the Thousand Springs segment of the fault. The rupture propagated unilaterally to the northwest from a hypocenter near the southeastern end of the segment. The southeastern boundary of the segment is marked by an abrupt bend in the range front, a 4-km-long gap in late Quaternary scarps, and transverse faults of Eocene age that intersect the Lost River fault . The northwestern boundary of the Thousand Springs segment is at the junction of the Willow Creek hills and the Lost River fault. Here, the southern and western sections of surface faults diverge and there is a gap in the 1983 scarps. During the first few weeks after the main shock, the large-magnitude and large stress-drop aftershocks clustered near this barrier. Later, aftershocks were mainly northwest of the barrier on the Warm Spring and Challis segments, and showed that strain adjustments eventually affected the entire northern part of the Lost River fault. Fault-scarp morphology and the bedrock geology suggest that the boundary between the Thousand Springs and Warm Spring segments has probably ruptured less frequently and had less net slip during much of the late Cenozoic than the interior of the adjacent segments. The 1983 faulting shows that although segment boundaries can stop or deflect primary ruptures, secondary surface faulting can occur on adjacent segments of the main fault. A late Pleistocene scarp in the 1983 gap suggests that infrequent earthquakes, perhaps larger than the 1983 event, might break through a segment boundary and thus release strain on two adjacent segments .

Very low frequency earthquakes excited by the 2004 off the Kii peninsula earthquakes : A dynamic deformation process in the large accretionary prism

Abstract: Anomalous seismic events were observed after the occurrence of the foreshock (Mw=7.2) and the main shock (Mw=7.5) of the 2004 off the Kii peninsula earthquakes. These anomalous events are characterized by very low-frequency energy of around 10 seconds with almost no higher-frequency energy and are considered the same as the very low-frequency (VLF) earthquakes discovered by Ishihara (2003) in some places along the Nankai trough, southwest Japan. The VLF seismic activity is mainly coincident with the aftershock area of the 2004 off the Kii peninsula earthquakes; however a minor activity was also excited in the southern Kii channel area. The VLF seismograms sometimes include higher-frequency wave trains with amplitudes much smaller than that of regular aftershocks. This indicates that VLF earthquakes have different source properties from the regular earthquakes. The centroid moment tensor analysis for one of the larger events suggests that the source depth is very shallow and the focal mechanism is the reverse faulting. These features suggest that the event occurs on the well-developed reverse fault system in the large accretionary prism near the Nankai trough. The swarm activity of VLF earthquakes might be considered as a chain-like occurrence of slips on the reverse fault system and thus the signature of a dynamic deformation process in the accretionary prism.

A fault-rupture model for seismic risk analysis

Abstract: A model for seismic risk analysis consistent with existing theories of earthquake mechanism and characteristics is developed. This model is based on the assumption that an earthquake originates as an intermittent series of fault ruptures in the Earth9s crust, and that the intensity of motion at a site is mainly contributed by the segment of the ruptured fault that is closest to the site. Since active faults in a region may be well-defined, partially defined, or completely unknown, various idealized source models are introduced in order to permit the modeling of all conceivable seismic sources. The significance of model parameters on the calculated seismic risk is studied: certain previous conclusions in this regard are critically reexamined. In particular, it is pointed out that previous seismic risk models, which implicitly assume that the energy is radiated from a point (the focus), can seriously underestimate the real risk, especially for high intensity motions. As an illustration of the model, the seismic risk analysis of a site in downtown San Francisco is presented.

Dynamical rupture process on a three‐dimensional fault with non‐uniform frictions and near‐field seismic waves

Abstract: Summary Dynamical rupture process on the fault is investigated in a quasithreedimensional faulting model with non-uniform distributions of static frictions or the fracture strength under a finite shearing pre-stress The displacement and stress time functions on the fault are obtained by solving numerically the equations of motion with a finite stress-fracture criterion, using the finite difference method If static frictions are homogeneous or weakly non-uniform, the rupture propagates nearly elliptically with a velocity close to that of P waves along the direction of pre-stress and with a nearly S wave velocity in the direction perpendicular to it The rise time of the source function and the final displacements are larger around the centre of the fault In the case when the static frictions are heavily non-uniform and depend on the location, the rupture propagation becomes quite irregular with appreciably decreased velocities, indicating remarkable stick-slip phenomena In some cases, there remain unruptured regions where fault slip does not take place, and high stresses remain concentrated up to the final stage These regions could be the source of aftershocks at a next stage The stick-slip faulting and irregular rupture propagation radiate highfrequency seismic waves, and the near-field spectral amplitudes tend to show an inversely linear frequency dependence over high frequencies for heavily non-uniform frictional faults