Fault (geology)

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

Internal architecture, permeability structure, and hydrologic significance of contrasting fault-zone types

Abstract: The Sand Hill fault is a steeply dipping, large-displacement normal fault that cuts poorly lithified Tertiary sediments of the Albuquerque basin, New Mexico, United States. The fault zone does not contain macroscopic fractures; the basic structural element is the deformation band. The fault core is composed of foliated clay flanked by structurally and lithologically heterogeneous mixed zones, in turn flanked by damage zones. Structures present within these fault-zone architectural elements are different from those in brittle faults formed in lithified sedimentary and crystalline rocks that do contain fractures. These differences are reflected in the permeability structure of the Sand Hill fault. Equivalent permeability calculations indicate that large-displacement faults in poorly lithified sediments have little potential to act as vertical-flow conduits and have a much greater effect on horizontal flow than faults with fractures.

Present tectonic motion across the Coast Ranges and San Andreas fault system in central California

Abstract: Geodetic results from very long baseline interferometry (VLBI), satellite laser ranging (SLR), and the Global Positioning System (GPS) are used to estimate angular velocities between the Sierran microplate, Pacific plate, and North American plate. The Sierra-Pacific pole of rotation lies nearer to the San Andreas fault than does the Pacific–North America pole of rotation and leads to different tectonic implications than if the latter is used. The angular velocities show that the San Andreas fault system and central California Coast Ranges accommodate motion of 39 ± 2 mm/yr, mainly by strike-slip faulting. (All confidence limits following ± signs in this paper are 95% confidence limits.) Fault-normal motion is small, is mainly convergent (at rates up to 3.3 ± 1.0 mm/yr), and varies along the coast, but is divergent (at 2.6 ± 1.2 mm/yr) across San Pablo Bay and associated topographic lows across which the Sierran and Central Valley watershed drains to the Pacific Ocean. The mountain ranges tend to be larger where the fault-normal convergence rates are larger. The low convergence rate (0.5 ± 1.8 mm/yr) normal to the San Andreas fault in the Carrizo Plain differs sharply from that previously inferred (8.2 ± 1.2 mm/yr and 4.9 ± 1.6 mm/yr) by Feigl et al. (1993). The difference is due to differences between their and our elastic strain accumulation models and between how their and our Pacific plate reference frames are defined. The ranges in most places require a minimum of 4 +2/–1 m.y. of fault-normal convergence at the present rate to attain their present cross-sectional area if erosion is neglected, more if it is not. The amount of convergence previously estimated from a balanced cross section across the Diablo Range in central California requires 10 +8/–3 m.y. of convergence at the present rate. The former is consistent with widely held views about the onset of the Coast Range orogeny, but the latter is not. Both are consistent, however, with the recent plate reconstructions by S. Cande, J. Stock, and colleagues, which indicate that Pacific plate motion relative to North America changed to a more convergent direction, 20°–25° clockwise of its prior direction, at ca. 8 to 6 Ma and not at 3.5 Ma, as had been previously inferred. The inferred change in direction of plate motion is large compared with the present angle of convergence across the straight and narrow segment of the San Andreas fault of 0.7°–4.7°, from which we infer that the Sierran microplate changed motion relative to North America at the same time (ca. 8 to 6 Ma) as did the Pacific plate. We further infer that the motion accommodated across the Great Basin must also have changed at the same time. We also examine the hypothesis that stable sliding occurs along the San Andreas fault and other northwest-striking strike- slip faults in central California where the fault-normal convergence rate is low or negative, and that these faults are unstable where the fault-normal convergence rate is high. Such a relationship appears to hold in general, but fails in detail. In particular, there are substantial sections of fault with small inferred rates of fault-normal convergence across which the San Andreas fault is locked. Moreover, the creeping section of the San Andreas fault (i.e., the section between Parkfield and the Calaveras junction) is the locus of greater fault-normal convergence (3.2 ± 1.4 mm/yr) than is the locked part of the fault (0.5 ± 1.8 mm/yr) south of Parkfield. Thus, this hypothesis is at best a partial explanation for the observed distribution of locked and nonlocked sections of the fault.

Earthquake Monitoring in Southern California for Seventy-Seven Years (1932-2008)

Abstract: The Southern California Seismic Network (SCSN) has produced the SCSN earthquake catalog from 1932 to the present, a period of more than 77 yrs. This catalog consists of phase picks, hypocenters, and magnitudes. We present the history of the SCSN and the evolution of the catalog, to facilitate user understanding of its limitations and strengths. Hypocenters and magnitudes have improved in quality with time, as the number of stations has increased gradually from 7 to ~400 and the data acquisition and measuring procedures have become more sophisticated. The magnitude of completeness (M_c) of the network has improved from M_c ~3.25 in the early years to M_c ~1.8 at present, or better in the most densely instrumented areas. Mainshock–aftershock and swarm sequences and scattered individual background earthquakes characterize the seismicity of more than 470,000 events. The earthquake frequency-size distribution has an average b-value of ~1.0, with M≥6.0 events occurring approximately every 3 yrs. The three largest earthquakes recorded were 1952 M_w 7.5 Kern County, 1992 M_w 7.3 Landers, and 1999 M_w 7.1 Hector Mine sequences, and the three most damaging earthquakes were the 1933 M_w 6.4 Long Beach, 1971 M_w 6.7 San Fernando, and 1994 M_w 6.7 Northridge earthquakes. All of these events ruptured slow-slipping faults, located away from the main plate boundary fault, the San Andreas fault. Their aftershock sequences constitute about a third of the events in the catalog. The fast slipping southern San Andreas fault is relatively quiet at the microseismic level and has not had an M>6 earthquake since 1932. In contrast, the slower San Jacinto fault has the highest level of seismicity, including several M>6 events. Thus, the spatial and temporal seismicity patterns exhibit a complex relationship with the plate tectonic crustal deformation.