Fault (geology)

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

Fault trace complexity, cumulative slip, and the shape of the magnitude-frequency distribution for strike-slip faults : a global survey

Abstract: SUMMARY We examine whether the shape of the magnitude-frequency distribution for strike-slip faults is described by the Gutenburg-Richter relationship (log n = a - bM) or by the characteristic earthquake model, by analysing a data set of faults from California, Mexico, Japan, New Zealand, China and Turkey. For faults within regional seismic networks, curves of the form log n yrpl= a - bM, where n yr-' is the number of events per year equal to magnitude M, are fit to the instrumental record of seismicity, and geological data are used to estimate independently the size and recurrence rate of the largest expected earthquakes that would rupture the total length of the fault. Extrapolation of instrumentally derived curves to larger magnitudes agrees with geological estimates of the recurrence rate of the largest earthquakes for only four of the 22 faults if uncertainties in curve slope are considered, and significantly underestimates the geological recurrence rates in the remaining cases. Also, if we predict the seismicity of the faults as a function of fault length and slip rate, and the predicted seismicity is distributed in accord with the Gutenburg-Richter relationship, we find the predicted recurrence rate to be greater than the observed recurrence rates of smaller earthquakes along most faults. If individual fault zones satisfy the Gutenburg-Richter relationship over the long term, our observations imply that, during the recurrence interval of the largest expected earthquakes, the recurrence of lesser-sized events is not steady but, rather, strongly clustered in time. However, if the instrumental records provide an estimate of the long-term rate of small to moderate earthquakes along the faults, our observations imply that the faults generally exhibit a magnitude-frequency distribution consistent with the characteristic earthquake model. Also, we observe that the geometrical complexity of strike-slip faults is a decreasing function of cumulative strike-slip offset. The four faults we observe to be consistent with the GutenburgRichter relationship are among those characterized by the least amount of cumulative slip and greatest fault-trace complexity. We therefore suggest that the ratio of the recurrence rate of small to large earthquakes along a fault zone may decrease as slip accumulates and the fault becomes smoother.

Submarine fault scarps in the Sea of Marmara pull-apart (North Anatolian Fault): Implications for seismic hazard in Istanbul

Abstract: Earthquake scarps associated with recent historical events have been found on the floor of the Sea of Marmara, along the North Anatolian Fault (NAF). The MARMARASCARPS cruise using an unmanned submersible (ROV) provides direct observations to study the fine-scale morphology and geology of those scarps, their distribution, and geometry. The observations are consistent with the diversity of fault mechanisms and the fault segmentation within the north Marmara extensional step-over, between the strike-slip Ganos and Izmit faults. Smaller strike-slip segments and pull-apart basins alternate within the main step-over, commonly combining strike-slip and extension. Rapid sedimentation rates of 1?3 mm/yr appear to compete with normal faulting components of up to 6 mm/yr at the pull-apart margins. In spite of the fast sedimentation rates the submarine scarps are preserved and accumulate relief. Sets of youthful earthquake scarps extend offshore from the Ganos and Izmit faults on land into the Sea of Marmara. Our observations suggest that they correspond to the submarine ruptures of the 1999 Izmit (Mw 7.4) and the 1912 Ganos (Ms 7.4) earthquakes. While the 1999 rupture ends at the immediate eastern entrance of the extensional Cinarcik Basin, the 1912 rupture appears to have crossed the Ganos restraining bend into the Sea of Marmara floor for 60 km with a right-lateral slip of 5 m, ending in the Central Basin step-over. From the Gulf of Saros to Marmara the total 1912 rupture length is probably about 140 km, not 50 km as previously thought. The direct observations of submarine scarps in Marmara are critical to defining barriers that have arrested past earthquakes as well as defining a possible segmentation of the contemporary state of loading. Incorporating the submarine scarp evidence modifies substantially our understanding of the current state of loading along the NAF next to Istanbul. Coulomb stress modeling shows a zone of maximum loading with at least 4?5 m of slip deficit encompassing the strike-slip segment 70 km long between the Cinarcik and Central Basins. That segment alone would be capable of generating a large-magnitude earthquake (Mw 7.2). Other segments in Marmara appear less loaded.

Seismic hazard in the Marmara Sea region following the 17 August 1999 Izmit earthquake

Abstract: On 17 August 1999, a destructive magnitude 7.4 earthquake occurred 100 km east of Istanbul, near the city of Izmit, on the North Anatolian fault. This 1,600-km-long plate boundary(1,2) slips at an average rate of 2-3 cm yr(-1) (refs 3-5), and historically has been the site of many devastating earthquakes(6,7). This century alone it has ruptured over 900 km of its length(6). Models of earthquake-induced stress change(8) combined with active fault maps(9) had been used to forecast that the epicentral area of the 1999 Izmit event was indeed a likely location for the occurrence of a large earthquake(9,10). Here we show that the 1999 event itself significantly modifies the stress distribution resulting from previous fault interactions(9,10). Our new stress models take into account all events in the region with magnitudes greater than 6 having occurred since 1700 (ref. 7) as well as secular interseismic stress change, constrained by GPS data(11). These models provide a consistent picture of the long term spatio-temporal behaviour of the North Anatolian fault and indicate that two events of magnitude equal to, or greater than, the Izmit earthquake are likely to occur within the next decades beneath the Marmara Sea, south of Istanbul.

Heat flow from Cajon Pass, fault strength, and tectonic implications

Abstract: Measured heat flow at Cajon Pass is consistent with predictions based on local site conditions and regional heat flow. With observations now ranging to a depth of 3½ km, there is still no evidence for significant frictional heating anywhere on the San Andreas fault. The result supports the view, long suggested from heat flow studies, that the fault is weak in spite of estimates based on Byerlee's law, isotropic strength, and hydrostatic fluid pressure that suggest a strength several times larger. Recent evidence (Zoback et al., 1987; Mount and Suppe, 1987) that the maximum principal stress might be almost normal to the San Andreas fault would support the weak-fault model and add constraints over and above those imposed by heat flow; e.g., local friction coefficients μ ≲ 0.1 or fluid pressures along the fault greater than lithostatic (λ > 1), compared to μ ≲ 0.2 or fluid pressure greater than twice hydrostatic (λ > 0.74) for the heat flow constraint alone. These constraints are a challenge to existing models of faulting, and they are stimulating promising new points of view. The balance of plate boundary forces around a weak fault depends on the basal traction coupling the seismic layer to the rest of the system; heat flow limits the coupling force across the fault to an insignificant ∼ 1011 N/m. The weak fault also precludes significant near-field basal driving tractions, but it permits a large basal drag force which could result in a highly stressed seismic layer offering appreciable resistance to plate motion through its base. Such tractions could develop progressively if the fault surface weakens as it evolves; if they exist, they should cause an observable reduction in shear stress resolved in the fault direction and a rotation of principal axes as the fault is approached; if they do not exist, the seismic layer rides passively on the lower crust. Heat flow measurements should detect whether such basal tractions might be associated with basal decoupling and flow. Coupling at the base of the seismic layer is controlled by the rheological profile, the usual representation of which raises three questions in applications to the San Andreas fault zone. First, the linear frictional portion through the seismic layer implies a resisting force on the fault much greater than the heat flow limit permits. Second, the large stresses implied for the temperature-sensitive ductile layer might be unsustainable; they could lead to shear heating and weakening at plate boundary strain rates. Third, in the ductile layer the stress is sensitive to whether deformation is concentrated in narrow vertical mylonite zones, as sometimes assumed in models of the earthquake cycle, or more broadly distributed by bulk flow in a deep-crustal “asthenosphere.” Horizontal basal shear stresses are of the same order as vertical strike-slip stresses near the base of the seismic layer; they could result in bulk flow or horizontal detachment leading to a different pattern of long-term stress, strain rate, and dissipation and a requirement for decoupling and basal drag on the seismic layer in the near field. Results from the San Andreas fault taken with long-standing speculation about the orthogonal relation between oceanic transform faults and extensional spreading centers suggest that strike-slip transform faults might be anomalously weak in both continental and oceanic settings.

Structural, petrological and thermal evolution of a tertiary ductile strike-slip shear zone, Diancang Shan, Yunnan

Abstract: The Diancang Shan, a horst massif within the Red River fault zone in Yunnan, People's Republic of China, preserves a structural, petrological, and thermal record of two distinct phases of tectonic activity: a left-lateral ductile shear that terminated between 20 and 17 Ma and a ductile-to-brittle phase of normal faulting which began at 4.7 Ma and remains active. Mylonitic rocks in the core of the range display an early, steep, high-temperature (HT), schistosity and a horizontal stretching lineation that are both parallel to the trend of the belt. Kinematic indicators indicate that shear was left-lateral. The complex shape of the HT schistosity at the southern termination of the massif likely results from a large-scale, oblique, left-lateral C' shear plane that dismembered the shear zone and separated the Ailao Shan and the Diancang Shan as left-lateral deformation terminated. Thermochronological and thermobarometric results suggest that the gneisses were partially unroofed during this event. Along the eastern edge of the Diancang Shan, the HT fabrics were overprinted by low-temperature structures during activation of east dipping normal faults. Cooling associated with this normal/right-lateral faulting along the Diancang Shan (and perhaps activation of the right-lateral/normal movement on the Range Front fault farther south along the Ailao Shan) began at 4.7±0.1 Ma. These results tend to support the view that extrusion of Indochina occurred along the left-lateral Red River shear zone between 35 and 19–17 Ma. Initiation of right-lateral/normal slip during the late Miocene may relate to eastward extensional collapse of the thickened Tibetan crust or, more probably, to initiation of the second phase of extrusion.