Evolution of fault systems at a strike-slip plate boundary: A viscoelastic model (original) (raw)
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Crustal rheology and faulting at strike-slip plate boundaries: 2. Effects of lower crustal flow
Journal of Geophysical Research: Solid Earth, 2000
We present a numerical model of deformation at a strike-slip plate boundary within a linear viscoelastic crest, which is driven by far-field plate motions and basal mantle velocities. The crest is assumed to have uniform elastic properties but continuously varying viscosity as a function of depth. Brittle faulting is represented by static elastic dislocations that are imposed when stresses exceed a critical threshold for fracture or frictional sliding. The locations and depth extents of faults in this model are not prespecified but, instead, are governed by stress evolution within the crest. We find that when a primarily elastic upper crest is underlain by a low-viscosity lower crustal layer, the deformation zone broadens in time to encompass many parallel strike-slip faults in an interacting network. In contrast, when the entire crest behaves elastically, the deformation zone remains narrow and focused on a single plate-bounding fault, reflecting imposed mantle motions. Surface strain rate patterns within the interacting fault network are complex and reflect significant faultingrelated strain rate perturbations that decay over timescales of postseismic relaxation in the lower crest (10-100 years). The fault network has a characteristic spacing, with complex fault interactions and with the depth extents of faults increasing with time to a maximum depth governed by crustal rheology. The maximum depth of faults is limited by stress relaxation and large-scale viscous flow in the lower crest, which confines brittle failure to shallow and midcrustal levels.
Strike-slip fault structure and fault-system evolution: a numerical study applying damage rheology
2010
In seismically active regions, faults nucleate, propagate, and form networks that evolve over time. Progressive strain localization and periodic fault pattern re-configuration induce the accumulation and healing of fault zone damage. The damage zones are characterized by distributed fractures, veins, and secondary faults, and may act as barriers for propagating earthquake ruptures, or as nucleation sites for earthquakes. They interact with seismic waves, promoting strong surface motions during earthquakes, and can focus fluid flow and enhance mineralization. In spite of their great scientific, social, and economic significance, interactions between these evolving damage zones and crustal deformation remain unresolved. Indeed, geodynamic models generally treat active faults as surfaces embedded in a medium with nonevolving material properties.
8. Structure of Large-Displacement, Strike-Slip Fault Zones in the Brittle Continental Crust
Rheology and Deformation of the Lithosphere at Continental Margins, 2004
Characterizing the structure of fault zones is necessary to understand the mechanical, fluid flow and geophysical properties of the lithosphere. This paper provides a detailed characterization of two large-displacement, strike-slip fault zones of the San Andreas system in southern California, the Punchbowl and North Branch San Gabriel faults. The faults cut crystalline and well-lithified sedimentary rocks, and consist of broad zones of fractured and faulted rock (damage zone) containing one, or more, narrow, tabular zones of highly sheared rock (fault core). Subsidiary faults and fractures of the damage zone formed in response to stress cycling associated with slip. The fault core is composed of very fine-grained, altered fault-rocks that reflect high shear strain, extreme comminution, and enhanced fluid-rock reactions. The characteristic and relatively ordered structure of these large-displacement faults is consistent with progressive damage accumulation over the lifetime of the fault. Layers of ultracataclasite containing mesoscopic slip surfaces within fault cores record extreme localization of slip at the macroscopic and mesoscopic scale. Progressive accumulation of ultracataclasite from abrasive wear along a slip surface throughout faulting history can explain the particle size distribution, layered structure, and sharp boundaries of the ultracataclasite layer. The longevity and relative stability of the slip surface is evidenced by the accumulation of progressively younger ultracataclasite towards the slip surface. Although not a common feature, the incorporation of slivers of wall rock into the ultracataclasite layer document occasional branching of the slip surface within fault cores. The damage-zone and fault-core characterization may be used to describe the geophysical, mechanical, and fluid-flow properties of brittle fault zones in crystalline and well-lithified sedimentary rocks of the continental crust.
GeoHazards
As an earthquake is capable of causing significant losses, a strain buildup and release model following an earthquake is of importance for mitigation purposes. In this study, we aim to model strain buildup and release on a strike-slip fault which consists of elastic–brittle (upper crust) and elastic–viscous (lower crust and upper mantle) layers using a finite element model. The fault strength during strain buildup is controlled by the friction coefficient and cohesion, in addition to the viscoelastic parameter, as shown in the deformation model using Maxwell’s material. In the strain buildup model, we found that the differential stress on the elastic layer is larger than that on the viscoelastic layer and that the differential stress increases with the thickness of the elastic layer. When the viscoelastic layer is thinner, the deformation observed on the surface is larger. However, the differential of stress in the strain release model on the elastic layer is smaller than that on th...
Models of recurrent strike-slip earthquake cycles and the state of crustal stress
Journal of Geophysical Research, 1991
Numerical models of the strike-slip earthquake cycle, assuming a viscoelastic asthenosphere coupling model, are examined. The time-dependent simulations incorporate a stress-driven fault, which leads to tectonic stress fields and earthquake recurrence histories that are mutually consistent. Single-fault simulations with constant far-field plate motion lead to a nearly periodic earthquake cycle and a distinctive spatial distribution of crustal shear stress. The predicted stress distribution includes a local minimum in stress at depths less than typical seismogenic depths. The width of this stress "trough" depends on the magnitude of crustal stress relative to asthenospheric drag stresses. The models further predict a local near-fault stress maximum at greater depths, sustained by the cyclic transfer of strain from the elastic crust to the ductile asthenosphere. Models incorporating both low-stress and high-stress fault strength assumptions are examined, under Newtonian and non-Newtonian rheology assumptions. Model results suggest a preference for low-stress a shear stress level of---10 MPa) fault models, in agreement with previous estimates based on heat flow measurements and other stress indicators. 10 7 years and the episodic near-fault motions that are variable on time scales as short as years or months. This transition is largely mediated by the crustal stress field, which in concert with deeper mantle convection drives both the brittle frictional behavior of faulted rock and the broadscale movement of the plates.
Mechanics, Structure and …, 2010
We present results on evolving geometrical and material properties of large strike-slip fault zones and associated deformation fields, using 3-D numerical simulations in a rheologically-layered model with a seismogenic upper crust governed by a continuum brittle damage framework over a viscoelastic substrate. The damage healing parameters we employ are constrained using results of test models and geophysical observations of healing along active faults. The model simulations exhibit several results that are likely to have general applicability. The fault zones form initially as complex segmented structures and evolve overall with continuing deformation toward contiguous, simpler structures. Along relatively-straight mature segments, the models produce flower structures with depth consisting of a broad damage zone in the top few kilometers of the crust and highly localized damage at depth. The flower structures form during an early evolutionary stage of the fault system (before a total offset of about 0.05 to 0.1 km has accumulated), and persist as continued deformation localizes further along narrow slip zones. The tectonic strain at seismogenic depths is concentrated along the highly damaged cores of the main fault zones, although at shallow depths a small portion of the strain is accommodated over a broader region. This broader domain corresponds to shallow damage (or compliant) zones which have been identified in several seismic and geodetic studies of active faults. The models produce releasing stepovers between fault zone segments that are locations of ongoing interseismic deformation. Material within the fault stepovers remains damaged during the entire earthquake cycle (with significantly reduced rigidity and shearwave velocity) to depths of 10 to 15 km. These persistent damage zones should be detectable by geophysical imaging studies and could have important implications for earthquake dynamics and seismic hazard.
Viscous roots of active seismogenic faults revealed by geologic slip rate variations
2013
Viscous flow in the deep crust and uppermost mantle can contribute to the accumulation of strain along seismogenic faults in the shallower crust 1 . It is difficult to evaluate this contribution to fault loading because it is unclear whether the viscous deformation occurs in localized shear zones or is more broadly distributed 2 . Furthermore, the rate of strain accumulation by viscous flow has a power law dependence on the stress applied, yet there are few direct estimates of what the power law exponent is, over the long term, for active faults. Here we measure topography and the offset along fault surfaces created during successive episodes of slip on seismically active extensional faults in the Italian Apennines during the Holocene epoch. We show that these data can be used to derive a relationship between the stress driving deformation and the fault strain rate, averaged over about 15 thousand years (kyr). We find that this relationship follows a well-defined power law with an exponent in the range of 3.0-3.3 (1σ). This exponent is consistent with nonlinear viscous deformation in the deep crust and, crucially, strain localization promoted by seismogenic faulting at shallower depths. Although we cannot rule out some distributed deformation, we suggest that fault strain and thus earthquake recurrence in the Apennines is largely controlled by viscous flow in deep, localized shear zones, over many earthquake cycles.
Crustal transpressional fault geometry influenced by viscous lower crustal flow
Geology
The San Andreas fault (California, USA) is near vertical at shallow (<10 km) depth. Geophysical surveys along the San Andreas fault reveal that, at depths of 10–20 km, it dips ~50–70° to the southwest near the Western Transverse Ranges and dips northeast in the San Gorgonio region. We investigate the possible origin of along-strike geometric variations of the fault using a three-dimensional thermomechanical model. For two blocks separated by transpressional faults, our model shows that viscous lower crustal material moves from the high-viscosity block into the low-viscosity block. Fault plane-normal flow in the viscous lower crust rotates the fault plane due to the simple shear flow at the brittle-ductile transition depth. This occurs irrespective of initial fault dip direction. Rheological variations used to model the lower crust of Southern California are verified by independent observations. Block extrusion due to lower crustal viscosity variation facilitates the formation of ...
American Journal of Mathematics and Mathematical Sciences , 2012
Fault systems in seismically active regions often consist of a number of interacting faults with different inclinations to the vertical. A movement across one of them may have significant influence on the accumulation of strain and stress on the others and thereby affect the possibilities of movement across them. For a better understanding of this interaction between faults a theoretical model has been developed consisting of two interacting strike-slip faults, both buried in a Maxwell viscoelastic half-space representing the lithosphere-asthenosphere system. Green's functions and integral transforms are used to obtain exact solutions for displacements, stresses and strains in the model in three different cases-the case where no fault creeping, the case when one fault is creeping and the other is locked and the case when both the faults are creeping. The effect of creep across one fault on the shear stress near the other is found to depend significantly on the dimensions (e.g., width of the faults), nature of creeping velocity across the faults and the relative positions of the faults as well as their inclinations to the vertical.