Showing papers on "Fault (geology) published in 1995"

Introduction to Special Section: Mechanical Involvement of Fluids in Faulting

Abstract: A growing body of evidence suggests that fluids are intimately linked to a variety of faulting processes. These include the long term structural and compositional evolution of fault zones; fault creep; and the nucleation, propagation, arrest, and recurrence of earthquake ruptures. Besides the widely recognized physical role of fluid pressures in controlling the strength of crustal fault zones, it is also apparent that fluids can exert mechanical influence through a variety of chemical effects. The United States Geological Survey sponsored a Conference on the Mechanical Effects of Fluids in Faulting under the auspices of the National Earthquake Hazards Reduction Program at Fish Camp, California, from June 6 to 10, 1993. The purpose of the conference was to draw together and to evaluate the disparate evidence for the involvement of fluids in faulting; to establish communication on the importance of fluids in the mechanics of faulting between the different disciplines concerned with fault zone processes; and to help define future critical investigations, experiments, and observational procedures for evaluating the role of fluids in faulting. This conference drew together a diverse group of 45 scientists, with expertise in electrical and magnetic methods, geochemistry, hydrology, ore deposits, rock mechanics, seismology, and structural geology. Some of the outstanding questions addressed at this workshop included the following: 1. What are fluid pressures at different levels within seismically active fault zones? Do they remain hydrostatic throughout the full depth extent of the seismogenic regime, or are they generally superhydrostatic at depths in excess of a few kilometers? 2. Are fluid pressures at depth within fault zones constant through an earthquake cycle, or are they time-dependent? What is the spatial variability in fluid pressures? 3. What is the role of crustal fluids in the overall process of stress accumulation, release, and transfer during the earthquake cycle? Through what mechanisms might fluid pressure act to control the processes of rupture nucleation, propagation, and arrest? 4. What is the chemical role of fluids in facilitating fault creep, including their role in aiding solid-state creep and brittle fracture processes and in facilitating solution-transport deformation mechanisms? 5. What are the chemical effects of aqueous fluids on constitutive response, fractional stability, and long-term fault strength? 6. What are the compositions and physical properties of faultfluids at different crustal levels? 7. What are the mechanisms by which porosity and permeability are either created or destroyed in the middle to lower crust? What factors control the rates of these processes? How should these effects be incorporated into models of time-dependent fluid transport in fault zones? 8. What roles do faults play in distributing fluids in the crust and in altering pressure domains? In other words, when and by what mechanisms do faults aid in or inhibit fluid migration? What are the typical fluid/rock ratios, flow rates, and discharges for fault zones acting as fluid conduits? 9. Are fluids present in the subseismogenic crust, and by what transformation and/or transport processes are they incorporated into the shallower seismogenic portions of faults?

Fluid-rock reaction weakening of fault zones

Abstract: The presence of weak phyllosilicates may explain the low shear strengths of fault zones if they define well-developed fabrics. The growth of phyllosilicates is favored in meteoric water-dominated granitic fault systems, where mineral-aqueous fluid equilibria predict that modal phyllosilicate will increase via feldspar replacement reactions. In deeper, more alkaline, rock-dominated regimes, the reactions reverse, and feldspars tend to replace phyllosilicates. In Mg-rich mafic rocks, however, phyllosilicates (chlorite, biotite) can replace stronger framework and chain silicates in both shallower (<{approximately}10 km) meteoric H{sub 2}O-dominated and in deeper, alkaline, rock-dominated regimes. Where these phyllosilicates precipitate in active fault zones, they contribute directly to reaction softening. Because low-temperature deformation of phyllosilicates is not governed by frictional processes alone but can occur by pressure-independent dislocation glide, the strength of phyllosilicate-rich fault rocks can be low at all depths. Low strain rate creep during interseismic periods can align phyllosilicate grains in foliated gouge and phyllonites. Where preferred orientations are strong and contiguity of phyllosilicates is large, strengths of rocks within fault zones may approach minimum strengths defined by single phyllosilicate crystals. Fault zones containing localized high concentrations of phyllosilicates with strong preferred orientations in well-defined folia can exhibit aseismic slip, especially where mafic Mg-rich rocksmore » occur along the fault (like parts of the San Andreas Fault). 104 refs., 6 figs., 1 tab.« less

Fault overlap zones within developing normal fault systems

Abstract: Overlap zones between normal faults have been studied using a variety of 2D and 3D seismic reflection datasets. The overlaps are of two types, (i) relay zones in which displacement is transferred between the overlapping faults and (ii) non-relay overlaps in which displacement is not transferred. Overlap zones are continually formed and destroyed during the growth of a fault system. Overlap zones are formed either by interference between initially isolated faults or as a result of bifurcation of a single fault. The mode of overlap formation is reflected in the 3D geometry of the overlapping faults which may be either unconnected or linked at a branch-line or branch-point. Seismic reflection data from regions of growth faulting, and also sandbox analogue data, allow analysis of fault development through time. Reconstructions of the displacement distribution on some faults with sharp bends and associated hanging-wall splays, show that the bends originated as overlap zones which were later breached to form through-going faults. Depending on the displacements of relay-bounding faults, the effect of relay zones on hydrocarbon reservoirs may be to (a) provide structural closure, (b) form gaps in otherwise sealing faults or (c) increase reservoir connectivity.

Faulting processes at high fluid pressures: An example of fault valve behavior from the Wattle Gully Fault, Victoria, Australia

Abstract: The internal structures of the Wattle Gully Fault provide insights about the mechanics and dynamics of fault systems exhibiting fault valve behavior in high fluid pressure regimes. This small, high-angle reverse fault zone developed at temperatures near 300°C in the upper crust, late during mid-Devonian regional crustal shortening in central Victoria, Australia. The Wattle Gully Fault forms part of a network of faults that focused upward migration of fluids generated by metamorphism and devolatilisation at deeper crustal levels. The fault has a length of around 800 m and a maximum displacement of 50 m and was oriented at 60° to 80° to the maximum principal stress during faulting. The structure was therefore severely misoriented for frictional reactivation. This factor, together with the widespread development of steeply dipping fault fill quartz veins and associated subhorizontal extension veins within the fault zone, indicates that faulting occurred at low shear stresses and in a near-lithostatic fluid pressure regime. The internal structures of these veins, and overprinting relationships between veins and faults, indicate that vein development was intimately associated with faulting and involved numerous episodes of fault dilatation and hydrothermal sealing and slip, together with repeated hydraulic extension fracturing adjacent to slip surfaces. The geometries, distribution and internal structures of veins in the Wattle Gully Fault Zone are related to variations in shear stress, fluid pressure, and near-field principal stress orientations during faulting. Vein opening is interpreted to have been controlled by repeated fluid pressure fluctuations associated with cyclic, deformation-induced changes in fault permeability during fault valve behavior. Rates of recovery of shear stress and fluid pressure after rupture events are interpreted to be important factors controlling time dependence of fault shear strength and slip recurrence. Fluctuations in shear stress and transient rotations of near-field principal stresses, indicated by vein geometries, are interpreted to indicate at least local near-total relief of shear stress during some rupture events. Fault valve behavior has important effects on the dynamics of fluid migration around active faults that are sites of focused fluid migration. In particular, fault valve action is expected to lead to distinctly different fluid migration patterns adjacent to faults before, and immediately after, rupture. These fluid migration patterns have important differences with those predicted by models for dilatancy-diffusion effects and for poroelastic responses around reverse faults.

The rupture zone of Cascadia great earthquakes from current deformation and the thermal regime

Abstract: An important but poorly -known part of the earthquake hazard at near-coastal cities of western North America from southern British Columbia to northern California is from great thrust earthquakes on the Cascadia subduction zone. Although there have been no such events in the historical record, there is good geological evidence that they have occurred in the past. The downdip landward limit of the seismogenic or seismic rupture zone on the subduction thrust fault has been estimated for the whole Cascadia margin from (1) the locked zone from dislocation modeling of current deformation data, and (2) the thermal regime, taking the downdip limit of seismic behavior on the fault to be controlled by temperature. The geodetic data include ten leveling lines, tide gauges at six locations along the coast, one high precision gravity line, seven horizontal strain arrays, and a continuously recording Global Positioning System (GPS) network. There is present uplitfor most of the coast at a rate of a few millimeters per year, decreasing inland, and shortening across the coastal region at about 0.1 gstrain/yr (i.e., 10 mm/yr over a distance of 100 krn). The present interseismic uplift is consistent with the great earthquake coseismic subsidence inferred from buried coastal salt marshes and other paleoseismicity data. The modeled width of the locked zone that is taken to be accumulating elastic strain averages 60 krn fully locked, plus 60 krn transition (90 km fully locked with no transition gives similar interseismic deformation). It is wider off the Olympic Peninsula of northern Washington and narrower off central Oregon to northern California. This unusually narrow downdip extent compared to many other subduction zones is a consequence of high temperatures associated with the young oceanic plate and the thick blanket of insulating sediments on the incoming crust. The variations in the modeled locked zone from geodetic data correspond well to variations along the margin of downdip temperatures on the fault as estimated from nmnerical thermal models, taking the maximran temperature for the fully locked seismogenic zone to be 350oC with a transition zone to 450oC. The temperatures on the subduction thrust fault and thus the downdip extent of the seismogenic zone depend on five local subduction parameters: (1) the age of the subducting plate, (2) the plate convergence rate, (3) the thickness of insulating sediments on the incoming crust, (4) the dip angle profile of fle fault, and (5) the themal properties of the overlying material. The landward limit to the seismogenic zone, extending little if at all beneath the coast, limits the ground motion from great subduction earthquakes at the larger Cascadia cities that lie 100-200 km inland. The narrow width also limits the earthquake size but events of magnitude well over 8 are still possible; the maximum depends on the along-margin length. If the whole Cascadia margin seismogenic zone fails in a single event, empirical fault area versus magnitude relations give earthquakes as large as M w =9.

Activation of the Nyainqentanghla Shear Zone: Implications for uplift of the southern Tibetan Plateau

Abstract: Neogene extension of the Tibetan plateau is manifested as a series of north-south trending graben, the most prominent of which is the Yadong-Gulu rift. The Nyainqen- tanghla Shan, a NE-SW trending mountain range -100 km NW of Lhasa, bounds the western margin of the Yangbajian graben, the central segment of the Yadong-Gulu rift. The eastern edge of the Nyainqentanghla massif is marked by a low angle (--25 o) detachment fault shear zone of amphibolite grade mylonites. The noAr/3Ar thermal history results from samples collected along two deeply incised valleys within the massif reveal that a rapid cooling event propagated from --8 Ma in the core of the range to --4 Ma within the high strain zone at the eastern boundary. Assuming that faulting initiated at high angle (-60o), thermal histories were fit to a numerical simulation of slip on a normal fault to yield estimates of both the age of fault initiation and the slip rate history. The form of the isotopically derived thermal histories are similar to general form predicted by the thermal model and suggests that significant movement began at 8+1 Ma in the southern valley (Goring-la) and proceeded at an average slip rate of-3 mm/yr between -8 and 3 Ma. A more complex history is required to fit the data from the northern valley (Balum Chun), but the timing of initiation and average slip rate are similar to the Goring-la result. Numerical simulations in which the fault angle is varied indicate that the isotopically derived temperature histories are inconsistent with slip occurring at low angle (<40o). Because the extension direction of the Yangbajian graben is representative of most rifts on the southern Tibetan plateau, our data suggest that crustal thickness and elevation reached close to their present values by 8+1 Ma. A carbon isotopic shift in pedogenic carbonates from the Siwalik Formation at about 7.5 Ma appears to reflect intensification of the Asian monsoon and, by inference, that the plateau had attained an important threshold elevation by that time. Formation of a diffuse plate boundary in the Indian oceanic lithosphere beginning at 7.5-8.0 Ma is also consistent with this history. We suggest that the plateau had attained a threshold area and elevation by 8+1 Ma sufficient to trigger these three independent manifestations.

Uplift and denudation of the central Alaska Range: A case study in the use of apatite fission track thermochronology to determine absolute uplift parameters

Abstract: Apatite fission track thermochronology (AFTT) on granitic samples collected in the central Alaska Range in conjunction with geologic constraints from basins to the north (Nenana Basin) and south (Cook Inlet) of the range is used to constrain the timing, amount, rate, and pattern of surface uplift, rock uplift, and denudation since the late Miocene. The conversion from a thermal frame of reference (apatite fission track data) to an absolute frame of reference (with respect to mean sea level), which requires constraining the paleoland surface elevation, the paleomean annual temperature, and the paleogeothermal gradient, is evaluated and shown to be viable in the context of an exhumed apatite partial annealing zone (PAZ). Apatite ages at Denali (Mount McKinley) range from 16 Ma near the summit (∼6 km elevation) to 4 Ma at ∼2 km elevation. A distinctive break in slope in the apatite age profile at an elevation of 4.5 km, also marked by a change in confined track length distributions, marks the base of an exhumed apatite PAZ. Rock uplift and denudation are greatest at Denali, decreasing southward away from the McKinley strand of the Denali fault system as shown by progressively older apatite ages (7–35 Ma) from a suite of samples along the Kahiltna Glacier. A correlative decrease in topography occurs southward from the fault. The central Alaska Range lies within an arc defined by the Denali fault, with the highest peaks (including Denali) concentrated at the arc apex. Patterns of rock uplift and denudation within the central Alaska Range mimic topography. Between early and late Miocene, and possibly earlier, the central Alaska Range was most likely an area of relative tectonic and thermal stability. Rock uplift, denudation, and mean surface uplift of the Denali region began by the Late Miocene (∼5–6 Ma), being ∼8.5 km, ∼5.7 km, and ∼2.8 km, respectively, at average rates of ∼1.5 km/m.y., ∼1 km/m.y., and ∼0.5 km/m.y. The amount of rock uplift, denudation, and surface uplift decreases to ∼3 km, ∼2 km, and ∼1 km at Little Switzerland, some 45 km south of the Denali fault. We conclude that the topographic and rock uplift patterns of the central Alaska Range, the shape and proximity of the McKinley strand of the Denali Fault to these patterns, the timing of the onset of rock uplift and denudation at ∼5–6 Ma, and a significant change in relative plate motion between North America and the Pacific plates circa 5.6 Ma are all inherently related.

Low-angle normal faults and seismicity: A review

Abstract: Although large, low-angle normal faults in the continental crust are widely recognized, doubts persist that they either initiate or slip at shallow dips (<30°), because (1) global compilations of normal fault focal mechanisms show only a small fraction of events with either nodal plane dipping less than 30° and (2) Andersonian fault mechanics predict that normal faults dipping less than 30° cannot slip. Geological reconstructions, thermochronology, paleomagnetic studies, and seismic reflection profiles, mainly published in the last 5 years, reinforce the view that active low-angle normal faulting in the brittle crust is widespread, underscoring the paradox of the seismicity data. For dip-slip faults large enough to break the entire brittle layer during earthquakes (M_w ∼ 6.5), consideration of their surface area and efficiency in accommodating extension as a function of dip θ suggests average recurrence intervals of earthquakes R' ∝ tan θ, assuming stress drop, rigidity modulus, and thickness of the seismogenic layer do not vary systematically with dip. If the global distribution of fault dip, normalized to total fault length, is uniform, the global recurrence of earthquakes as a function of dip is shown to be R ∝ tan θ sin θ. This relationship predicts that the frequency of earthquakes with nodal planes dipping between 30° and 60° will exceed those with planes shallower than 30° by a factor of 10, in good agreement with continental seismicity, assuming major normal faults dipping more than 60° are relatively uncommon. Revision of Andersonian fault mechanics to include rotation of the stress axes with depth, perhaps as a result of deep crustal shear against the brittle layer, would explain both the common occurrence of low-angle faults and the lack of large faults dipping more than 60°. If correct, this resolution of the paradox may indicate significant seismic hazard from large, low-angle normal faults.

The relationship between late Cenozoic tectonic events and stress field and basin development in northeast Japan

Abstract: The regional tectonic stress field, basin development, and crustal deformation of the NE Japan arc in the interval between 32 Ma to the Quaternary can be synthesized based on dike, vein, and fault orientation data, as well as on the compilation of the regional geology. An extensional stress field became prevalent from 32 Ma, and major normal faulting started at 25–20 Ma, which resulted from back arc rifting. Normal faulting, trending nearly parallel to the arc, propagated from the present Japan Sea coast to the forearc side, following the trenchward migration of the main volcanic field. From 20 to 15 Ma, normal faults with an oblique trend to the arc developed due to the counterclockwise rotation of NE Japan. The rapid clockwise rotation of SW Japan since 16 Ma produced a NW-SE directed transtensional stress regime in the NE Japan arc. Due to crustal stretching associated with this pull-apart movement, the back arc side of the NE Japan arc subsided rapidly to middle bathyal environments. After the termination of the opening of the Japan Sea at about 14 Ma, a neutral stress regime prevailed, which included phases of both weak extension and compression. Lithospheric cooling eventually led to thermal subsidence of the back arc region, and igneous underplating caused uplift of the axial zone of the volcanic arc. The increase in velocity of the westward motion of the Pacific plate at around 4 Ma produced strong compression across the arc, reactivating most of the Miocene normal faults, and uplifting the volcanic arc. The greatest crustal shortening occurred in areas that were stretched the most in the Miocene within the volcanic arc, which implies tectonic inversion.

Evolution of Salt-Related Structures in Compressional Settings

Abstract: Sandbox experiments analyzed by computerized X-ray tomography provide relevant models of salt-related contractional structures and improve understanding of the relative importance of the many parameters influencing structural style. In front of thin-skinned fold and thrust belts, the salt layers provide decollement surfaces, which allow the horizontal strain to propagate far toward the edge of the foreland. As shortening increases, older structures forming in front of the system can be overtaken by out-of-sequence faulting and folding. The very low friction coefficient of salt layers induces a symmetric stress system. This promotes pop-up structures rather than asymmetric thrust faults. Salt extrusions are related to former salt ridges or salt walls squeezed by compression and dragged along thrust planes or to local low-pressure zones along crestal tear faults during folding. The salt that spreads out from the fault is rapidly dissolved. The resultant surface collapse structures are progressively filled by a mixture of Recent sediments and reprecipitated evaporites. Salt pinch-outs, either depositional or structural in origin, are a major controlling factor of the deformation geometry in fold and thrust belts. They trigger, either locally or regionally, contractional structures, including folds and thrusts, in rapidly prograding passive margins deforming by gravity gliding. In this structural context, salt pinch-outs also thicken due to differential loading and gravity spreading. The structural complexity in inverted grabens or in basement-involved orogenic belts where salt is present is the outcome of many factors. The salt thickness, the preexisting extensional structures, the synsalt and postsalt rifting, and the related distribution of older salt structures and sediments all localize folds and thrusts during later contraction. The relative orientation of the former extensional structures to the younger shortening structures largely controls the style of inversion (fault reactivation versus forced folding and short-cuts). Salt is the main detachment level between the folded cover rocks and the underlying faulted basement. However, secondary detachments, which are common in the overburden, add further complexities--triangle zones in the cores of anticlines and fish-tailed periclinal terminations.

Effect of Faulting on Fluid Flow in Porous Sandstones: Geometry and Spatial Distribution

Abstract: We present a methodology to describe fault geometry at different scales and to characterize the distribution of these scales on the flanks of a salt intrusion in the Colorado Plateau (Arches National Park, United States). This methodology is based on the recognition of the physical processes of faulting and on the quantitative characterization of the structural and petrophysical properties of faults in porous sandstones. The methods used include a variety of mapping techniques (photography, aerial photography, string mapping, theodolite surveys, etc.), as well as techniques for determining fluid flow properties. The resulting study is a prototype for understanding seismic and subseismic scales of heterogeneity related to faulting and fracturing in subsurface reservoirs. > Faulting in porous sandstones on the flanks of the salt intrusion is developed at different stages, from simple deformation bands (1-20 mm shear offset) to slip planes (>1 m shear offset) and complex fault zones. We document that deformation-band outcrop geometry is characterized by a sinuous anastomosing pattern resulting from the linkage of quasitabular segments via ramp or "eye" structures. These connecting structures recur at different scales and provide lateral continuity of the deformation bands; therefore, deformation bands have good geometric sealing characteristics. Slip planes, which are not interconnected, may have poor geometric sealing characteristics. In the hanging wall of a major normal fault, the quantitative spatial distribution of the faults can be correlated with bending of the strata, probably associated with the salt intrusion. The number of deformation bands, the most ubiquitous element, is proportional to the amount of slip on a single major fault. Deformation bands also have a very high density (>100 m-1) in stepovers between slip planes. In these areas we find the largest anomalies in permeability. In zones of high strata curvature, the average layer-parallel permeability can drop one to two orders of magnitude with respect to the host rock; if complex fault zones are present, the average permeability can drop more than four orders of magnitude in the direction normal to the faults. Finally, by using outcrop and laboratory data that describe the effect of distinctive structural units on fluid flow, we quantify the three-dimensional distribution of permeability in a reservoir analog at any scale, and we show that such permeability distribution could be implemented in a geology-based reservoir simulator.

Miocene NNE-directed extensional unroofing in the Menderes Massif, southwestern Turkey

Abstract: Structural investigations in the central part of the Menderes Massif (Odemio-Kiraz submas-sif) reveal the presence of a large-scale, low-angle extensional shear zone with a top-to-the-N-NE shear sense. Regional ductile deformation was accompanied by the intrusion of two syntectonic granodiorites that have been dated with the 40 Ar/ 39 Ar method. One hornblende isochron age of 19.5 ± 1.4 Ma and two biotite plateau ages of 13.1 ± 0.2 and 12.2 ± 0.4 Ma, respectively, constrain that extension was already active in the early Miocene. Successive tectonic denudation of the Odemio-Kiraz submassif resulted in the formation of a N-dipping detachment fault, in which ductile fabrics were severely reworked by cataclasis under decreasing temperature. Syntectonic Neogene sediments, currently exposed along the southern margin of the Gediz Graben, were deposited in the hanging wall of the extensional fault system and were tectonically emplaced onto the cataclasites during progressive exhumation. Minor rotation ( c . 15° caused southward tilting of the sediments. Ongoing NNE-directed extension created a steep normal fault that truncates the detachment fault and constitutes the southern boundary fault of the Gediz Graben.

Gold‐quartz veins in metamorphic terranes and their bearing on the role of fluids in faulting

Abstract: Gold-quartz vein fields in metamorphic terranes such as greenstone belts provide evidence for the involvement of large volumes of fluids during faulting and may be products of seismic processes near the base of the seismogenic regime. In the Val d'Or district of the Abitibi greenstone belt, Canada, quartz-tourmaline-carbonate veins form a vein field (30 × 15 km) in the hanging wall of a crustal-scale fault zone, which was the main channelway for upward migration of the deeply generated fluids. The veins occur in small high-angle reverse faults and in adjacent horizontal extensional fractures extending up to 75 m in intact rocks. They have formed incrementally during active reverse faulting in response to crustal shortening, at depths corresponding to those at the base of the seismogenic zone in actively deforming crust. Detailed structural and fluid inclusion studies provide evidence for generally lithostatic but fluctuating fluid pressures (ΔPƒ of the order of 200 MPa) and for cyclic stress reversals during vein formation and provide good support for the fault valve model. A comparison of vein characteristics with “standard” earthquake rupture parameters suggests that each slip increment along veins in reverse faults was accompanied by a small earthquake (4 > M > 3 or less). The large vein field thus represents both the extent of fluid dispersion in the hanging wall of a crustal-scale channelway and the distribution of small earthquakes integrated over the lifetime of the hydrothermal system. It is proposed that such small earthquakes along veins in reverse faults are related to large earthquakes (M > 6) nucleating near the base of the seismogenic regime along the nearby crustal-scale fault, either as aftershocks or as a precursory smarm.

Case for very low coupling stress on the Cascadia Ssubduction Fault

Abstract: A fundamental problem in plate tectonics is the shear strength of major plate boundary faults. This translates to the question whether the generally observed small earthquake stress drops of 3–10 MPa on major faults release most of the accumulated stress or only a small fraction of it. There is strong evidence that the San Andreas fault, a major transform plate boundary, is weak (<20 MPa shear resistance). It is not yet clear whether subduction thrust faults are also weak. We present two types of evidence from the northern Cascadia subduction zone that indicate very low coupling shear stress on that plate interface and hence very low strength of the subduction thrust fault, comparable to that estimated for the San Andreas fault. First, the well-defined surface heat flow and heat generation allow negligible frictional heating on the plate interface. The average shear stress on the fault must thus be very low over a time scale of a few million years. Second, focal mechanism solutions for small crustal earthquakes in the southern Vancouver Island area indicate that the horizontal stress in the direction of plate convergence has a similar magnitude to the vertical stress. This inferred stress state requires the present tectonic stress coupled across the subduction thrust fault to be very low. One explanation for the weakness of the fault is the presence of near-lithostatic pore fluid pressure in the region of the fault zone for which there is independent evidence. The conclusion of a weak subduction thrust fault does not conflict with geodetic observations of contemporary surface deformation which indicate that the fault is currently locked, accumulating strain energy toward a future great earthquake. The surface deformation responds to the small (<20 MPa) temporal changes of the stress field associated with the subduction earthquake cycle. This transient stress is superimposed on the larger background regional stress field in which the maximum compression is parallel to the margin. The weakness of the Cascadia subduction thrust fault and the unusual stress state of the forearc region have important implications for earthquake hazards. For example, a subduction earthquake may induce large strike-slip earthquakes in the forearc that affect a large area.

Slip patterns and earthquake populations along different classes of faults in elastic solids

Abstract: Numerical simulations of slip instabilities on a vertical strike-slip fault in an elastic half-space are performed for various models belonging to two different categories. The first category consists of inherently discrete cellular fault models. Such are used to represent fault systems made of segments (modeled by numerical cells) that can fail independently of one another. Their quasi-independence is assumed to provide an approximate representation of strong fault heterogeneity, due to geometric or material property disorder, that can arrest ruptures at segment boundaries. The second category consists of models having a well-defined continuum limit. These involve a fault governed by rate- and state-dependent friction and are used to evaluate what types of property heterogeneity could lead to the quasi-independent behavior of neighboring fault segments assumed in the first category. The cases examined include models of a cellular fault subjected to various complex spatial distributions of static to kinetic strength drops, and models incorporating rate- and state-dependent friction subjected to various spatial distributions of effective stress (normal stress minus pore pressure). The results indicate that gradual effective stress variations do not provide a sufficient mechanism for the generation of observed seismic response. Strong and abrupt fault heterogeneity, as envisioned in the inherently discrete category, is required for the generation of complex slip patterns and a wide spectrum of event sizes. Strong fault heterogeneity also facilitates the generation of rough rupture fronts capable of radiating high-frequency seismic waves. The large earthquakes in both categories of models occur on a quasi-periodic basis; the degree of periodicity increases with event size and decreases with model complexity. However, in all discrete segmented cases the models generate nonrepeating sequences of earthquakes, and the nature of the large (quasi-periodic) events is highly variable. The results indicate that expectations for regular sequences of earthquakes and/or simple repetitive precursory slip patterns are unrealistic. The frequency-size (FS) statistics of the small failure episodes simulated by the cellular fault models are approximately self-similar with b ≈ 1.2 and bA ≈ 1, where b and bA are b values based on magnitude and rupture area, respectively. For failure episodes larger than a critical size, however, the simulated statistics are strongly enhanced with respect to self-similar distributions defined by the small events. This is due to the fact that the stress concentrated at the edge of a rupture expanding in an elastic solid grows with the rupture size. When the fault properties (e.g., geometric irregularities) are characterized by a narrow range of size scales, the scaling of stress concentrations with the size of the failure zone creates a critical rupture area terminating the self-similar earthquake statistics. In such systems, events reaching the critical size become (on the average) unstoppable, and they continue to grow to a size limited by a characteristic model dimension. When, however, the system is characterized by a broad spectrum of size scales, the above phenomena are suppressed and the range of (apparent) self-similar FS statistics is broad and characterized by average b and bA values of about 1. The simulations indicate that power law extrapolations of low-magnitude seismicity will often underestimate the rate of occurrence of moderate and large earthquakes. The models establish connections between features of FS statistics of earthquakes (range of self-similar regimes, local maxima) and structural properties of faults (dominant size scales of heterogeneities, dimensions of coherent brittle zones). The results suggest that observed FS statistics can be used to obtain information on crustal thickness and fault zone structure.

Three-Dimensional Simulation of a Magnitude 7.75 Earthquake on the San Andreas Fault

Abstract: Simulation of 2 minutes of long-period ground motion in the Los Angeles area with the use of a three-dimensional finite-difference method on a parallel supercomputer provides an estimate of the seismic hazard from a magnitude 7.75 earthquake along the 170-kilometer section of the San Andreas fault between Tejon Pass and San Bernardino. Maximum ground velocities are predicted to occur near the fault (2.5 meters per second) and in the Los Angeles basin (1.4 meters per second) where large amplitude surface waves prolong shaking for more than 60 seconds. Simulated spectral amplitudes for some regions within the Los Angeles basin are up to 10 times larger than those at sites outside the basin at similar distances from the San Andreas fault.

Morphology of pyroclastic cones and tectonics

Abstract: The relationships between morphology and spatial distribution of 1315 Quaternary pyroclastic cones and coeval faulting of the volcanic substrate are analyzed in the following regions with different structural settings: Tepic Rift (Mexico), Ethiopian Rift, Mexican Volcanic Belt, Canary Archipelago, and Mount Etna. Field data and analog experiments of tephra cone emplacement and collapse enable the definition of a number of parameters which can be used to infer the geometry of the fracture feeding the magma to a pyroclastic cone. The strike of the feeding plane is directly related to: (1) the elongation of cone base and crater, (2) the location of depressions on the crater rim, and (3) the alignment of pyroclastic cones in relation to a given vent spacing. In addition, the strike and dip of faults affect the direction of cone breaching. These relationships are valid for volcanic substrate topographic surfaces with an inclination of less than 9° and are especially sensitive to fault escarpment and cone height, lava and cone density, and fault orientation with respect to the dip of the volcanic substrate topography. Relations 1 and 2 become more pronounced for regions undergoing extensional tectonics, where edifices also have a larger dimension. Whereas breaching in the direction of the fault dip is more widespread in regions under extension, breaching along the fault strike as well as the coincidence between fault strike and vent alignment are more frequent in regions with transcurrent or transtensional tectonics.

Multifractal scaling properties of a growing fault population

Abstract: SUMMARY A numerical rupture model, introduced in Cowie, Vanneste & Sornette (1993), is used to simulate the growth of faults in a tectonic plate driven by a constant plate boundary velocity. We find that the plate initially deforms by uncorrelated nucleation of small faults reflecting the distribution of material properties. With increasing strain, growth and coalescence of existing faults dominate over nucleation, a power-law distribution of fault sizes appears, and the fault pattern is fractal. Furthermore, the combined effect of fault clustering and the correlation between fault displacement and fault size leads to a strongly multifractal deformation pattern. We show theoretically that the multifractal spectrum depends explicitly on the exponent c, which defines the size distribution of the faults, as size and displacement are correlated. For different realizations of the numerical model, we calculate the exponent c, and fractal structure of the deformation through time as strain accumulates. We explore in detail the time evolution of the capacity (D0), information (D1), and correlation (D2) fractal dimensions. We relate these scaling parameters to the physical mechanisms of fault nucleation, growth and linkage during different phases of the deformation and discuss the factors that determine the values of the exponents. A consistently observed systematic decrease in the values of c, D1 and D2 through time indicates that the relative strain contribution of the smallest faults decreases as the total strain increases, a signature of the localization of faulting.

Regional strike slip in the eastern continental margin of Korea and its tectonic implications for the evolution of Ulleung Basin, East Sea (Sea of Japan)

Abstract: Air-gun profiles delineate a large-scale strike-slip fault system along the eastern continental margin of Korea. This system comprises two right-stepping curvilinear master faults (Hupo and Yangsan faults) that are closely associated with en echelon nominal faults, and reverse faults and folds. These structural features indicate two phases of regional deformation. In the earlier phase (late Oligocene to early Miocene), the margin experienced extensional shear strain with dextral strike-slip movements along the Hupo and Yangsan faults that guided a pull-apart opening of the PohangYoungduk Basin. The later phase deformation (late Miocene to early Pliocene) was governed by shortening strain that induced a convergent strike-slip reactivation of the Hupo fault as well as widespread contractile faulting and folding. The regional deformation in the eastern Korean continental margin reflects three stages of tectonic evolution of the Ulleung Basin: (1) pull-apart opening stage from late Oligocene to early Miocene, (2) rotational opening stage with a differential rotation of the Japanese Arc in middle Miocene, and (3) back-arc closing stage from the end of middle Miocene to present.

Constraints on present‐day Basin and Range deformation from space geodesy

Abstract: We use new space geodetic data from very long baseline interferometry and satellite laser ranging combined with other geodetic and geologic data to study contemporary deformation in the Basin and Range province of the western United States. Northwest motion of the central Sierra Nevada block relative to stable North America, a measure of integrated Basin and Range deformation, is 12.1±1.2 mm/yr oriented N38°W±5° (one standard error), in agreement with previous geological estimates within uncertainties. This velocity reflects both east-west extension concentrated in the eastern Basin and Range and north-northwest directed right lateral shear concentrated in the western Basin and Range. Ely, Nevada is moving west at 4.9±1.3 mm/yr relative to stable North America, consistent with dip-slip motion on the north striking Wasatch fault and other north striking normal faults. Comparison with ground-based geodetic data suggests that most of this motion is accommodated within ∼50 km of the Wasatch fault zone. Paleoseismic data for the Wasatch fault zone and slip rates based on seismic energy release in the region both suggest much lower slip rates. The discrepancy may be explained by some combination of additional deformation away from the Wasatch fault itself, aseismic slip, or a seismic rate that is anomalously low with respect to longer time averages. Deformation in the western Basin and Range province is also largely confined to a relatively narrow boundary zone and in our study area is partitioned into the eastern California shear zone, accommodating 10.7±1.6 mm/yr of north-northwest directed right-lateral shear, and a small component (∼1 mm/yr) of west-southwest - east-northeast extension. A slip rate budget for major strike-slip faults in our study area based on a combination of local geodetic or late Quaternary geologic data and the regional space geodetic data suggests the following rates of right-lateral slip: Owens Valley fault zone, 3.9±1.1 mm/yr; Death Valley-Furnace Creek fault zone, 3.3±2.2 mm/yr; White Mountains fault zone in northern Owens Valley, 3.4±1.2 mm/yr; Fish Lake Valley fault zone, 6.2±2.3 mm/yr. In the last few million years the locus of right-lateral shear in the region has shifted west and become more north trending as slip on the northwest striking Death Valley-Furnace Creek fault zone has decreased and is increasingly accommodated on the north-northwest striking Owens Valley fault zone.

Fluid Flow in Fault Zones: Analysis of the Interplay of Convective Circulation and Topographically Driven Groundwater Flow

Abstract: High-permeability faults, acting as preferential pathways for fluid migration, are important geological structures for fluid, energy, and solute transport. This paper examines the interaction of thermally driven convective circulation in a steeply dipping fault zone and groundwater flow through the surrounding country rock that is driven by a regional topographic gradient. We consider a geometry where a fault zone with a homogeneous, isotropic permeability is located beneath a narrow valley in a region with substantial topographic relief. System behavior is best characterized in terms of the large-scale permeabilities of the country rock and the fault zone. Using three-dimensional numerical simulations, we map in permeability space four fluid flow and heat transfer regimes within a fault zone: conductive, advective, steady convective, and unsteady convective. The patterns of fluid flow and/or heat transfer are substantially different in each of these regimes. Maximum discharge temperatures can also be plotted in permeability space; the maximum discharge temperature in the advective regime is in general lower than that in the steady convective regime. A higher basal heat flux expands the convective regime in permeability space, as does a greater fault depth. Higher topographic relief on the regional water table compresses the convective regime, with the advective regime suppressing convective circulation at lower country rock permeabilities. If convective cells with aspect ratios close to 1 cannot form, the steady convective regime is smaller in permeability space, and the boundary between steady and unsteady convection occurs at lower values of fault zone permeability. At low country rock permeabilities a water table gradient along the surface trace of the fault of approximately 0.3% suppresses convective cells; at higher country rock permeabilities, convection can be suppressed by smaller gradients on the water table.

A rheologic model for wet crust applied to strike-slip faults

Abstract: The strength and stability of crustal faults are not adequately addressed by the widely used two-mechanism rheologic models of the crust based on Byerlee's law and power law creep. The models neglect the fluid-assisted mechanisms of deformation that are important to strength of the crust and do not describe the variation in rate dependence of friction that governs the stability of fault slip. Several distinct mechanisms of frictional slip are defined on the basis of variations in frictional behavior and microfabric of simulated fault gouge in laboratory experiments. State variable constitutive relations are used in a multiple-mechanism formulation to describe the rate and temperature dependence of three friction mechanisms in wet quartz gouge at elevated temperatures. This multimechanism description of friction is substituted for Byerlee's law in the two-mechanism models to generate a multimechanism rheologic model for the crust. The multimechanism model is used to predict the rate and temperature dependence of crustal strength and the slip rate and depth (temperature) conditions over which each mechanism dominates. Application of the model to strike-slip faults in the crust illustrates that the thickness of the actively shearing zone within faults is a critical parameter governing fault strength and stability. The model predicts that frictional strength at midcrustal depths is significantly reduced relative to Byerlee's law only for thick fault zones. The reduction in strength is due to the operation of a strain rate sensitive friction mechanism involving combined cataclasis and solution transfer. The rheologic model predicts that only very thin faults display the rate dependent characteristics necessary for initiation of seismic slip to any significant depth in the crust.