Late Quaternary Faulting and Seismic Hazard in Southeastern Arizona and Adjacent Portions of New Mexico and Sonora, Mexico
KeywordsArizona Geological Survey Open File Reports
Basin and Range Province
Santa Rita Fault
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CitationPearthree, P.A., 1986, Late Quaternary Faulting and Seismic Hazard in Southeastern Arizona and Adjacent Portions of New Mexico and Sonora, Mexico. Arizona Geological Survey Open File Report, OFR-86-08, 22 p.
DescriptionGeomorphic and Quaternary geologic studies provide data with which to assess seismic hazard in southeastern Arizona and adjacent New Mexico and Sonora, Mexico, where one large (M ~ 7 1/4) historic earthquake has occurred against a background of very low seismicity. Conclusions regarding the distribution and timing of late Quaternary faulting are based on (1) estimated ages of soils based on correlation with soils near Las Cruces in southern New Mexico; (2) use of surface age-fault offset relationships to constrain the age of most-recent fault movement and to estimate the frequency of movement along individual faults; and (3) morphologic analyses of fault scarps to estimate their ages. Individual late Quaternary faults in the region have surface rupture recurrence intervals on the order of 105 years. However, the major earthquake that occurred in 1887 in northeastern Sonora is evidently part of a series of 5 or 6 surface-rupturing earthquakes that have occurred since 20 ka in a N-S-trending zone straddling the Arizona-New Mexico border. Surface ruptures during the late Pleistocene (about 20-120 ka) occurred from near Tucson east to the border area, but the rate of surface rupture occurrence was evidently 4-25 times lower than during the past 20 kyo The rate of Holocenelatest Pleistocene surface-rupturing, while much lower than some portions of the northern Basin and Range province, evidently represents a burst of activity relative to the average long-term rate of faulting in southern Arizona.
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Numerical modeling of fault formation and the dynamics of existing faults.Williams, Charles Addison, Jr. (The University of Arizona., 1990)This research is an investigation into two different aspects of the faulting process. The first part of the study focuses on the initial stages of fault formation, while the second analyzes the deformation produced by an existing fault. The section on fault formation is an attempt to determine whether slip on an existing fault has a significant effect on the formation of subsequent faults. A two-dimensional elastic finite element technique is used to examine the system of stresses produced by slip on an initial fault, assuming that deformation occurs either elastically or by brittle failure. A Mohr-Coulomb failure criterion is used to determine the most likely region of secondary fault initiation. A strain energy criterion is then used to find the preferred direction of fault propagation. The study on fault formation is subdivided into two sections representing two idealized tectonic environments: purely extensional and purely compressional. The section on extensional fault formation explains the prevalence of grabens in extensional tectonic regimes as a consequence of the stress perturbations due to slip on an initial normal fault. Slip on the initial fault produces a region of high proximity to failure at the surface of the downthrown block. A secondary fault would be expected to initiate in this region. The direction of propagation of this fault that most effectively relieves the shear stress (and therefore minimizes the total strain energy) is toward the initial fault, resulting in an antithetic orientation, or graben. The width of the graben is found to be controlled by the depth of the initial normal fault, rather than the depth to a change in material properties. The study of compressional fault formation indicates that, except for steeply-dipping faults, the presence of an initial thrust fault tends to suppress the formation of other faults in its vicinity. However, if a secondary fault initiates near an initial thrust fault, the direction in which it propagates will be influenced by the presence of the initial fault. The way in which it is influenced is dependent on the fault dip. The final part of this study examines the deformation produced by repeated earthquake cycles on the San Andreas fault in southern California. A three-dimensional, time-dependent kinematic finite element model is used to investigate the influence of slip distribution and rheological parameters on the predicted horizontal and vertical deformation. The models include depth-varying rheological properties and power-law viscoelastic behavior. The predicted deformation patterns are fairly sensitive to the parameters used in this study. Of particular importance is the calculation of vertical uplift rate since, in many cases, models that cannot be distinguished from each other on the basis of horizontal deformation may produce distinctive vertical uplift patterns.
Geomorphic analyses of young faulting and fault behavior in central Nevada.Pearthree, Philip Arnim. (The University of Arizona., 1990)This dissertation research assesses the behavior of young faults in central Nevada through analyses of landforms associated with these faults. Four large earthquakes have occurred since 1915 in a striking N-S belt in central Nevada; no comparable earthquakes have occurred elsewhere in the Great Basin. The frequency of large-earthquake occurrence, and temporal and spatial patterns and rates of faulting in central Nevada during the Holocene were assessed through geomorphic and geologic studies of young fault scarps. Ages of paleoseismic events were estimated primarily through analyses of fault scarp morphologies and characterization and quantification of soil development associated with alluvial surfaces. Rates of fault scarp degradation were explored through diffusion-based modeling of latest Pleistocene pluvial shoreline scarps. Morphologic scarp age depends strongly on scarp size; modest variations in local climate, particle size, and aspect are less important. Incorporating a factor that depends on scarp size almost always decreases the scatter in scarp age estimates, and is critical if only small scarps exist along a fault zone. An average of ±30% uncertainty about the mean scarp age estimate remains after these analyses. Soil development indices were calibrated using 14 Holocene to latest Pleistocene soil profiles in central Nevada whose maximum ages are constrained. Soil development indices were used to estimate ages of faulted and unfaulted alluvial surfaces along fault scarps. Soils and morphologic fault scarp age estimates for paleoseismic events are generally consistent. Temporal and spatial patterns and rates of faulting during the Holocene were evaluated using age estimates for paleoseismic events. The long-term rate of faulting is about 10 times lower than the historical rate. There were no other N-S belts of faulting during the Holocene, although scarp ages suggest that there may have been other temporal clusters of faulting. There have been spatial clusters of faulting during portions of the Holocene. The extensional deformation rate across central Nevada during the Holocene is about 0.5-0.75 mm/yr. Integrating this rate with fault-slip data from other portions of the northern Great Basin, the Holocene extensional deformation rate is 3.5-6.5 mm/yr, substantially lower than the historical deformation rate.
Thermochronometric and textural evidence for seismicity via asperity flash heating on exhumed hematite fault mirrors, Wasatch fault zone, UT, USAMcDermott, Robert G.; Ault, Alexis K.; Evans, James P.; Reiners, Peter W.; Univ Arizona, Dept Geosci (ELSEVIER SCIENCE BV, 2017-08)Exhumed faults record the temperatures produced by earthquakes. We show that transient elevated fault surface temperatures preserved in the rock record are quantifiable through microtextural analysis, fault-rock thermochronometry, and thermomechanical modeling. We apply this approach to a network of mirrored, minor, hematite-coated fault surfaces in the exhumed, seismogenic Wasatch fault zone, UT, USA. Polygonal and lobate hematite crystal morphologies, coupled with hematite (U-Th)/He data patterns from these surfaces and host rock apatite (U-Th)The data, are best explained by friction-generated heat at slip interface geometric asperities. These observations inform thermomechanical simulations of flash heating at frictional contacts and resulting fractional He loss over generated fault surface time temperature histories. Temperatures of >similar to 700-1200 degrees C, depending on asperity size, are sufficient to induce 85-100% He loss from hematite within 200 pm of the fault surface. Spatially-isolated, high temperature microtextures imply spatially -variable heat generation and decay. Our results reveal that flash heating of asperities and associated frictional weakening likely promote small earthquakes (M-w approximate to -3 to 3) on Wasatch hematite fault mirrors. We suggest that similar thermal processes and resultant dynamic weakening may facilitate larger earthquakes. (C) 2017 Elsevier B.V. All rights reserved.