Fault-normal Stress Research Articles

California Creepmeters

Research Article| July 01, 2004 California Creepmeters Roger Bilham; Roger Bilham CIRES & Department of Geological Sciences University of Colorado Boulder, CO 80309-0399 roger.bitham@colorado.edu Search for other works by this author on: GSW Google Scholar Naia Suszek; Naia Suszek CIRES & Department of Geological Sciences University of Colorado Boulder, CO 80309-0399 roger.bitham@colorado.edu Search for other works by this author on: GSW Google Scholar Sean Pinkney Sean Pinkney CIRES & Department of Geological Sciences University of Colorado Boulder, CO 80309-0399 roger.bitham@colorado.edu Search for other works by this author on: GSW Google Scholar Seismological Research Letters (2004) 75 (4): 481–492. https://doi.org/10.1785/gssrl.75.4.481 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Roger Bilham, Naia Suszek, Sean Pinkney; California Creepmeters. Seismological Research Letters 2004;; 75 (4): 481–492. doi: https://doi.org/10.1785/gssrl.75.4.481 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search A handful of faults in California manifest some form of surface creep: afterslip, episodic slip, steady slip, or triggered slip. Creep can be considered a proxy for shear strain applied to a fault, although its rate is sensitive also to fault-normal stresses, variations in the frictional properties of the near-surface fault, and in some locations to variations in near-surface moisture content. Creepmeters afford a view of fault activity with micron precision, at rates of up to 5 mm/minute, though creep rates on most California faults are typically less than several millimeters per year. This article summarizes the current status of... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Fracture development within a stratovolcano: the Karaha-Telaga Bodas geothermal field, Java volcanic arc

Abstract Karaha-Telaga Bodas, a vapour-dominated geothermal system located in an active volcano in western Java, is penetrated by more than two dozen deep geothermal wells reaching depths of 3 km. Detailed paragenetic and fluid-inclusion studies from over 1000 natural fractures define the liquid-dominated, transitional and vapour-dominated stages in the evolution of this system. The liquid-dominated stage was initiated by a shallow magma intrusion into the base of the volcanic cone. Lava and pyroclastic flows capped a geothermal system. The uppermost andesite flows were only weakly fractured due to the insulating effect of the intervening altered pyroclastics, which absorbed the deformation. Shear and tensile fractures that developed were filled with carbonates at shallow depths, and by quartz, epidote and actinolite at depths and temperatures over 1 km and 300°C. The system underwent numerous cycles of overpressuring, documented by subhorizontal tensile fractures, anastomosing tensile fracture patterns and implosion breccias. The development of the liquid system was interrupted by a catastrophic drop in fluid pressures. As the fluids boiled in response to this pressure drop, chalcedony and quartz were selectively deposited in fractures that had the largest apertures and steep dips. The orientations of these fractures indicate that the escaping overpressured fluids used the shortest possible paths to the surface. Vapour-dominated conditions were initiated at this time within a vertical chimney overlying the still hot intrusion. As pressures declined, these conditions spread outward to form the marginal vapour-dominated region encountered in the drill holes. Downward migration of the chimney, accompanied by growth of the marginal vapour-dominated regime, occurred as the intrusion cooled and the brittle-ductile transition migrated to greater depths. As the liquids boiled off, condensate that formed at the top of the vapour-dominated zone percolated downward and low-salinity meteoric water entered the marginal parts of the system. Calcite, anhydrite and fluorite precipitated in fractures on heating. Progressive sealing of the fractures resulted in the downward migration of the cap rock. In response to decreased pore pressure in the expanding vapour zone, walls of the fracture system within the vapour-dominated reservoir progressively collapsed. It left only residual permeability in the remaining fracture volume, with apertures supported only by asperities or propping breccia. In places where normal stresses acting on the fracture walls exceeded the compressive strength of the wall rock, the fractures have completely collapsed. Fractures within the present-day cap rock include strike- and oblique-slip faults, normal faults and tensile fractures, all controlled by a strike-slip stress regime. The reservoir is characterized by normal faults and tensile fractures controlled by a normal-fault stress regime. The fractures show no evidence that the orientation of the stress field has changed since fracture propagation began. Fluid migration in the lava and pyroclastic flows is controlled by fractures. Matrix permeability controls fluid flow in the sedimentary sections of the reservoir. Productive fractures are typically roughly perpendicular to the minimum compressive stress, σ 3 , and are prone to slip and dilation within the modern stress regime.

Modeling Time-Dependent Pore Pressure Due to Capillary and Chemical Potential Effects and Resulting Wellbore Stability in Shales

During drilling operations, the mud filtrate interacts with the pore fluid around the wellbore and changes pore pressure by capillary and chemical potential effects. Thus the change in pore pressure around borehole becomes time-dependent, particularly in extremely low permeability shaley formations. In this paper, the change in pore pressure due to capillary and chemical potential effects are investigated experimentally. Analytical models are also developed based on the experimental results. A wellbore stability analysis model incorporating the time-dependent change in pore pressure is applied to a vertical well in a shale formation under normal fault stress regime.

Earthquakes and Associated Deformation in Northern Baluchistan 1892-2001

We examine a century of seismicity and earthquake-related deformation on the western margin of the Indian plate in the Pakistan region of northern Baluchistan. Several catalogs of earthquakes for this region currently exist, but early catalogs in particular suffer from errors, incompleteness, inhomogeneity, and location bias. We form a new catalog of more than 1000 earthquakes using original sources to confirm macroseismic locations and assign M S magnitudes to earthquakes since 1892. In doing so we reveal a systematic east-northeast bias in locations caused largely by the uneven global distribution of seismic stations used in their determination. An appendix provides narrative accounts and historical references to 34 significant earthquakes in the region. The pattern of seismicity in the past century shows activity over a 700-km-long, 200-km-wide segment of the plate boundary with predominantly strike-slip faulting to the west and thrust faulting to the east. At its narrowest near 29° N, transpression of the plate boundary is partitioned into reverse and strike-slip components separated by approximately 100 km. The M 7.3 1931 Mach earthquake (slip 1.1 m on a 40° east-southeast-dipping reverse fault) released fault-normal stresses that may have “unclamped” the subsequent M 7.7 left-lateral Quetta earthquake 4 years later. The northern Chaman fault system in the past century has been largely inactive, suggesting that this time period is not representative of long-term activity in the region and that up to 4 m of potential slip is currently available to drive one or more future M > 7 earthquakes. Despite triangulation installed in the late nineteenth and early twentieth centuries, no recent geodetic data are available to permit plate boundary velocities to be measured directly.

Nearly frictionless faulting by unclamping in long-term interaction models

In defiance of direct rock-friction observations, some transform faults appear to slide with little resistance. In this paper finite element models are used to show how strain energy is minimized by interacting faults that can cause long-term reduction in fault-normal stresses (unclamping). A model fault contained within a sheared elastic medium concentrates stress at its end points with increasing slip. If accommodating structures free up the ends, then the fault responds by rotating, lengthening, and unclamping. This concept is illustrated by a comparison between simple strike-slip faulting and a mid-ocean-ridge model with the same total transform length; calculations show that the more complex system unclamps the transforms and operates at lower energy. In another example, the overlapping San Andreas fault system in the San Francisco Bay region is modeled; this system is complicated by junctions and stepovers. A finite element model indicates that the normal stress along parts of the faults could be reduced to hydrostatic levels after ∼60–100 k.y. of system-wide slip. If this process occurs in the earth, then parts of major transform fault zones could appear nearly frictionless.

A present-day tectonic stress map for eastern Asia region

Based on the data of earthquake centroid moment tensor (CMT) solution, P-wave first motion focal mechanism solution and deep hole breakouts, a present-day tectonic stress map for eastern Asia region is compiled. The original stress data are smoothed for every 200 km × 200 km area by taking the average of all stress indicators within each sub-region. The stress map shows the spatial distribution of the orientation of principal stress axes and the stress regimes. An earthquake focal mechanism map for the eastern Asia is also given. The maps of orientation of principal stress axes show that, apart from the strong influence of the collision between the Indian Ocean plate and Eurasian plate, the present-day tectonic stress in eastern Asia is significantly affected by the back-arc extension of the subduction zones. The joint effect of the continental collision at the Himalaya arc and back-arc extension in the Burma arc region may be responsible for the remarkable rotation of the principal stress orientations in southeastern part of the Tibet plateau. The joint action of the collision between the Philippine Sea plate and Eurasian plate at Taiwan Island and the back-arc extension of the Ryukyu arc affect the stress field in eastern part of China. There are no strong earthquakes in the present day in the vast back-arc region of the Java trench subduction zone. The back-arc extension there may create a condition favorable to the southward flow of the lithosphere material in southeastern Asia. In the inner part of the Tibet plateau region, roughly demarcated by the Kunlun mountain, the northern and northeastern part is a broad intracontinental compressive zone, while the southern and southwestern part is generally in a normal-faulting stress state.

Poroelastic coupling between the 1992 Landers and Big Bear Earthquakes

A three‐dimensional finite element model was constructed to investigate the significance of poroelastic coupling between the 1992 Landers and Big Bear earthquakes in southern California. The homogeneous poroelastic model predicted a maximum increase in left lateral slip potential (change in shear stress less the change in effective fault normal stress scaled by a coefficient of friction) along the southwest part of the Big Bear fault, consistent with the epicentral location. In contrast, slip potential calculated for a weak fault zone in a state of isotropic stress for drained conditions, indicated a maximum increase along the northeast part of the Big Bear fault.

Dilational processes accompanying earthquakes in the long valley caldera

Regional distance seismic moment tensor determinations and broadband waveforms of moment magnitude 4.6 to 4.9 earthquakes from a November 1997 Long Valley Caldera swarm, during an inflation episode, display evidence of anomalous seismic radiation characterized by non-double couple (NDC) moment tensors with significant volumetric components. Observed coseismic dilation suggests that hydrothermal or magmatic processes are directly triggering some of the seismicity in the region. Similarity in the NDC solutions implies a common source process, and the anomalous events may have been triggered by net fault-normal stress reduction due to high-pressure fluid injection or pressurization of fluid-saturated faults due to magmatic heating.

The influence of fault compaction on fault zone evolution

Abstract A numerical simulation of coupled fluid flow and failure in a fault zone is presented, that shows the impact of the dynamics of fault growth on the relationship between seismic and sub-seismic fault scaling. A tabular fault zone is modelled which undergoes non-uniform compaction, leading to development of pressure gradients in the fault zone. Increased pore fluid pressures result in a reduction of the fault-normal stresses, and hence frictional failure. The model suggests a possible physical mechanism for the development of non-power-law scaling of fault displacements, implying that observations of such behaviour are real, rather than the result of poor sampling. The fault displacement scaling law appears to result from the dynamics of the fault evolution, which depend upon the relative rates of compaction of fault material and fluid flow within the fault zone. When fault compaction is slow, event sizes show a power-law scaling relationship. When compaction is fast, pressure gradients are unable to dissipate and the distribution of these event sizes is non-power-law. It is suggested that the dynamics of fault evolution may be inferred from geological evidence, with the relics of repeated small events (e.g. aligned calcite fibres) implying a relatively high compaction rate, whereas cataclasites would suggest relatively low compaction rates.

Laboratory study of fault healing and lithification in simulated fault gouge under hydrothermal conditions

Seismic data, geologic observations and laboratory friction studies suggest that faults lithify and strengthen (heal) during the interseismic period of the earthquake cycle. We report on experiments to investigate the influence of healing duration and temperature on the strength and healing rate of simulated faults. Layers of μm-sized quartz powder were used to simulate granular fault gouge. Gouge layers were sheared within Sioux quartzite in a triaxial pressure vessel at elevated pressures ( P c = 250 MPa), elevated temperatures (230–636°C), and in the presence of water ( P H 2O = 75 MPa). We performed ‘hold-slide’ experiments in which samples were subjected to a period of healing under hydrostatic load, followed by shear deformation. Healing times ranged from a few hundred s to 10 5 s. To isolate the effects of lithification and temperature on friction, we ran two types of hold-slide experiments: (1) samples were subjected to healing and shear deformation at elevated temperature; and (2) following healing at elevated temperature, temperature was reduced prior to shear. We also ran slide-hold-slide experiments in which samples were healed under shear load at elevated temperature (636°C). Experiments in which deformation was carried out at a lower temperature than healing show that both the static and sliding coefficient of friction increase with heal time. Samples healed and deformed at higher temperature showed lower peak strengths, and within the scatter of our data, did not exhibit time-dependent strengthening. These data are interpreted to result from enhanced lithification rates at elevated temperature. Samples healed under shear load exhibited a log-linear decrease in friction with hold time. Modeling using rate- and state-dependent friction laws indicates velocity-strengthening behavior, with friction a - b values of 0.05 to 0.08. Our data indicate that at high temperature, the effective state-evolution term b is negative. Moreover, the data indicate the rate of frictional strengthening varies significantly for hydrostatic and non-hydrostatic load conditions. Our measurements of healing are consistent with seismic estimates of fault-healing rates, if stress drop is a fraction of the total shear stress and effective fault normal stress is 60 to 100 MPa.

Present-day stress field and active tectonics in southern peninsular Italy

SUMMARY A systematic collection of present-day stress directions has allowed us to gain new insights into the recent tectonic evolution of the Italian Peninsula. Horizontal stress directions from breakouts in 40 deep wells drilled for oil exploration have been compared to compressional and tensional axes of 17 earthquake fault-plane solutions (4.2 < M < 7) and 17 stress axes obtained from structural analysis on Quaternary rocks. Notwithstanding the different scales and depths of the data considered in this worknear-surface for structural data, 0-6 km depth for breakouts, GZ 3-20 km for earthquake focal mechanisms-the three distributions are surprisingly similar, all showing a predominant northeast-directed minimum horizontal stress in a broad region from the Tyrrhenian backarc through the Apenninic belt and up to the Apulian foreland. The belt, which corresponds to a region of high elevation, contemporary uplift and strong seismic release, with earthquakes as large as M=7, is characterized by a normalfaulting stress regime. The breakout data demonstrate that the NE-oriented extension associated with the seismogenic normal faults of the southern Apennines is determined by a regional stress with o,=NE, in agreement with the seismological evidence that these faults are young ( post-Middle Pleistocene). The breakout data show that even the foredeep and the most internal foreland region, where the ( Apulian) foreland crops out, are dominated by northeast-oriented extension, even if we cannot establish the stress regime of this region due to the lack of seismicity. This suggests that a model in which the underthrusting Adriatic plate is in compression and in flexure, generating extension in the upper (Apenninic) plate, is not applicable, unless what we observe (NE-directed Shmin) is limited to the upper part of the Adriatic plate, while its deeper portion might be in compression. Compared to the recent compressional tectonics that affected parts of the study region in the Upper Pliocene and Lower-Middle Pleistocene, it seems likely that an important change in the stress field occurred in this area during the Quaternary, most probably around 0.7 Ma. Considering the main tectonic processes acting in southern Italy-NNW convergence between Africa and Eurasia and subduction of Ionian and Adriatic lithosphere with associated slab pull and back-arc riftingwe speculate that the extension observed in southern peninsular Italy can be explained either by a slab window or by a buoyant subducted continental lithosphere in the upper mantle between the two active arcs, the northern Apennines and the Calabrian arc.

Neogene volcanism and Holocene earthquakes in the Tanlu fault zone, eastern China

In this paper, we study the relationship between Neogene volcanism and Holocene earthquakes in the Tanlu fault, eastern China. We find that fault segments through which Neogene and Quaternary magma have extruded do not show Holocene slip because they are covered by unfaulted basalts. In contrast, fault branches that are away from the Quaternary volcanic centers display Quaternary faulting and are responsible for earthquakes as recorded both historically and geomorphologically. Therefore, magma intrusion appears to modify fault activity in two different ways: (1) within the faults, the cooling magma serves as a cohesion or barrier, welding the faults so that they become stronger in resisting slip; (2) beneath the faults, the upwelling magma promotes slip of faults above the magma body, and hence generation of earthquakes. Physically, the first case results from contraction of the cooling magma, which causes a relative increase in fault-normal stress so that the fault failure resistance is enhanced. The second case results from the upward dynamic force and the heat brought in by the magma body, both of which cause the effective fault-normal stress to decrease so that the fault failure resistance is reduced. That could explain why earthquakes occur on faults bypassing the volcanic centers as typified in the Yishu fault zone and in the regions where heat flows are relatively high as shown in the Bohai Bay area.

Earthquake recurrence on the southern San Andreas modulated by fault-normal stress : R. Palmer, R. Weldon, E. Humpheeys & F. Saucier, Geophysical Research Letters, 22(5), 1995, pp 535–538

Earthquake recurrence on the southern San Andreas modulated by fault-normal stress : R. Palmer, R. Weldon, E. Humpheeys & F. Saucier, Geophysical Research Letters, 22(5), 1995, pp 535–538

Dilatancy, compaction, and slip instability of a fluid‐infiltrated fault

We analyze the conditions for unstable slip of a fluid infiltrated fault using a rate and state dependent friction model including the effects of dilatancy and pore compaction. We postulate the existence of a steady state drained porosity of the fault gouge which depends on slip velocity as ϕss = ϕ0 + εln(v/v0) over the range considered, where v is sliding velocity and ε and v0 are constants. Porosity evolves toward steady state over the same distance scale, dc, as “state.” This constitutive model predicts changes in porosity upon step changes in sliding velocity that are consistent with the drained experiments of Marone et al. (1990). For undrained loading, the effect of dilatancy is to increase (strengthen) ∂τss/∂lnv by μssε/(σ – p)β where μss is steady state friction, σ and p are fault normal stress and pore pressure, and β is a combination of fluid and pore compressibilities. Assuming ε ∼ 1.7×10−4 from fitting the Marone et al. data, we find the “dilatancy strengthening” effect to be reasonably consistent with undrained tests conducted by Lockner and Byerlee (1994). Linearized perturbation analysis of a single degree of freedom model in steady sliding shows that unstable slip occurs if the spring stiffness is less than a critical value given by kcrit = (σ‐p)(b‐a)/dc ‐ εμssF(c*)/βdc where a and b are coefficients in the friction law and F(C*) is a function of the model hydraulic diffusivity c* (diffusivity/diffusion length2). In the limit c* →∞ F(c*) → 0, recovering the drained result of Ruina (1983). In the undrained limit, c* → 0, F(c*) → 1, so that for sufficiently large ε slip is always stable to small perturbations. Under undrained conditions (σ – p) must exceed εμss/β(b ‐ a) for instabilities to nucleate, even for arbitrarily reduced stiffness. This places constraints on how high the fault zone pore pressure can be, to rationalize the absence of a heat flow anomaly on the San Andreas fault, and still allow earthquakes to nucleate without concommitant fluid transport. For the dilatancy constitutive laws examined here, numerical simulations do not exhibit large interseismic increases in fault zone pore pressure. The simulations do, however, exhibit a wide range of interesting behavior including: sustained finite amplitude oscillations near steady state and repeating stick slip events in which the stress drop decreases with decreasing diffusivity, a result of dilatancy strengthening. For some parameter values we observe “aftershock” like events that follow the principal stick‐slip event. These aftershocks are noteworthy in that they involve rerupture of the surface due to the interaction of the dilatancy and slip weakening effects rather than to interaction with neighboring portions of the fault. This mechanism may explain aftershocks that appear to be located within zones of high mainshock slip, although poor resolution in mainshock slip distributions can not be ruled out.

Earthquake recurrence on the southern San Andreas modulated by fault‐normal stress

Earthquake recurrence data from the Pallett Creek and Wrightwood paleoseismic sites on the San Andreas fault appear to show temporal variations in repeat interval. These sites are located near Cajon Pass, southern California, where detailed mapping has revealed geomorphically and structurally expressed domains of alternating extension and contraction respectively associated with releasing and restraining bends of the San Andreas fault. We investigate the interaction between strike‐slip faults and auxiliary reverse and normal faults as a physical mechanism capable of producing such variations. Under the assumption that fault strength is a function of fault‐normal stress (e.g. Byerlee's Law), failure of an auxiliary fault modifies the strength of the strike‐slip fault, thereby modulating the recurrence interval for earthquakes. In our finite element model, auxiliary faults are driven by stress accumulation near restraining and releasing bends of a strike‐slip fault. Earthquakes occur when fault strength is exceeded and are incorporated as a stress drop which is dependent on fault‐normal stress. The model is driven by a velocity boundary condition over many earthquake cycles. Resulting synthetic strike‐slip earthquake recurrence data display temporal variations similar to observed paleoseismic data within time windows surrounding auxiliary fault failures. Although observed recurrence data for the two paleoseismic sites are too short to be definitive about the temporal variations or the physical mechanism responsible for it, our simple model supports the idea that interaction between a strike‐slip fault and auxiliary reverse or normal faults can modulate the recurrence interval of events on the strike‐slip fault, possibly producing short term variations in earthquake recurrence interval.

1983年鳥取県中部地震と断層面に垂直な応力により発生した余震

1983年鳥取県中部地震と断層面に垂直な応力により発生した余震

Structural controls of Holocene reactivation of the Meers fault, southwestern Oklahoma, from magnetic studies

Holocene reactivation of the aseismic Meers fault in southwestern Oklahoma illustrates the limitation of using the historical seismic record for identifying hazardous faults in the central United States. The 26- to 37-km-long fault scarp is one of the few known scamps recording Holocene movement in the central and eastern United States. Two documented late Holocene slip events, each with about 2.5 m of net slip and estimated M s ranging from 6¾ to 7¼, identify the Meers fault as a potentially hazardous fault. During Carboniferous and Early Permian tectonism, the Meers fault displaced rocks of sharply contrasting magnetic properties. Analysis of aeromagnetic data and twelve ground-magnetic profiles provides a detailed look at the fault within the magnetic basement. Because subsequent reactivation has been minor and of an opposite sense, the pronounced magnetic anomaly associated with the Meers fault reflects Paleozoic structures in the magnetic basement. The location of the Holocene fault scarp corresponds to the strong horizontal magnetic gradient caused by Paleozoic offset of magnetic basement, indicating that the Paleozoic fault controlled Holocene displacement. Two features apparent in both sets of magnetic data are splays of the Meers fault northwest of the Holocene scarp and dikelike bodies immediately south of the fault. Magnetic susceptibility measurements and rock magnetic data from unoriented core penetrating a dikelike body were incorporated into models of the ground-magnetic profiles. In most cases, secondary faults mapped or visible on low-sun-angle photographs correspond to faults modeled from magnetic data. This correlation shows that preexisting structures probably controlled secondary faulting. However, secondary faults at the southeastern end of the 26-km long continuous fault scarp, previously interpreted from low-sun-angle photography, are not apparent in the magnetic data. Of importance to seismic hazard evaluation, the magnetic models show that the northwestern splays probably begin at the northwestern end of the reactivated segment and may indicate a persistent rupture propagation barrier to the west. In addition, the models show the dip of the Meers fault to be nearly vertical to about 0.5 km depth. This dip is consistent with the nearly straight fault trace, results of trenching studies, interpretation of shallow seismicreflection data, and regional gravity and aeromagnetic models. In the present-day strike-slip regional stress field, the observed up-to-the-north Holocene displacement suggests that either the fault continues to dip steeply at depth or the regional stress field is approaching a normal-faulting stress regime. If the former is true, the scarcity of near-vertical faults with similar orientation within the area of the southern Oklahoma aulacogen implies that few are likely candidates for reactivation.

Curved slip zones in an elastic half-plane

Abstract Slip on a curved fault loaded by far-field compressive stress induces changes in normal stress along the fault surface. A two-dimensional crack model is developed to quantify these stresses for curved fault geometries. Under plane strain (or plane stress), the curved slipping region is replaced by continuously distributed dislocations, and the corresponding singular integral equation for the dislocation density is derived and solved numerically. Because our solution scheme incorporates the exact asymptotic form of slip near the end of the slipping region, results for the stress intensity factors are accurate. To determine the effects of fault curvature on faulting, the changes in fault-normal stress accompanying dip-slip are calculated for typical faulting events. The changes in normal stress accompanying normal faulting increase the frictional resistance to slip advance near the free surface, and those accompanying reverse faulting reduce the frictional resistance to slip advance near the free surface. Under compressional tectonic loading, slip is retarded on the planar slip zones of higher dips, but enhanced if the dip decreases with depth. On the other hand, under extensional tectonic loading, slip is retarded on the planar slip zones of smaller dips, but enhanced if the dip increases with depth. Curvature of a concave-downward slip zone enhances compressive normal stress at the shallow end of the slip zone under compressional tectonic loading, tending to prevent propagation of the slip zones to the free surface. On the other hand, the curvature of a concave-upward slip zone reduces the compressive normal stress at the shallow end under the uniform extensional tectonic loading, thus assisting propagation of slip zones to the free surface. By contrast, for deep slip zones, the changes in normal stress are practically not affected by the fault curvature. To illustrate the local stress field near a sharp bend, we examine a sharply curved slip zone subjected to frictional sliding. Slip on such a fault induces significant compressional fault-normal stress near the bend. These changes add compression across the slip zone, thus increasing the frictional resistance to slip over the curved bend. Our results show that the magnitude of induced normal stresses is sharply localized near the bend.

A two-layer model for aseismic slip on the Superstition Hills fault, California

Abstract A new differential creepmeter on the Superstition Hills fault reveals that afterslip at approximately 28 mm/year consists of episodic creep events super-imposed on a slow stable-slip component of 2.4 mm/year. The relative contribution from this background slip (8.5% of the total aseismic surface slip) has remained approximately constant for the past 2 years. To account for this observed behavior, we proposed a two-layer model for aseismic slip in which stable-sliding occurs from the surface to a transition depth, below which episodic creep events are initiated. These creep events propagate to the surface through the stable-sliding layer. From the ratio of background slip velocity to total afterslip velocity, we estimate the ratio of depths of stable-sliding to episodic-slip regions to be approximately 1:10. Thus, assuming that episodic slip extends to 3 km, we infer that background surface creep is moderated by processes in the uppermost 300 m. We speculate that the transition depth is sensitive to applied fault-normal stresses and suggest that the ratio of stable-sliding to episodic-slip velocities may provide an indication of secular variations in tectonic stress. Since the background creep is evidently continuous and little affected by episodic events, its velocity may provide a sensitive indication of applied tectonic strain in the period range days to years.

Time dependence and trigger mechanisms for the Kariba (Rhodesia) earthquakes

Correlations between time-dependent parameters of incremental stress and induced seismicity during the filling of Lake Kariba (Rhodesia/Zambia) support the hypothesis, originally advanced in 1970, that failure was at that time triggered by increment to the existing tectonic stress field. About three years after the mainshock of September, 1963, these correlations disappeared and it is suggested that the principal triggering mechanism has been increase of pressure in the water in pores and fractures of the rock, from mid-1966 through 1974. The Mohr failure criterion with Coulomb friction is used to show that both types of triggering are possible in a normal-fault régime. Triggering by incremental solid stress requires a friction coefficient < 0.577 in a normal-fault stress field. If the Okavango Delta, the Middle Zambezi Rift and the Luangwa Rift form a single member of the African Rift system, the only two flooded parts of this rift are now the main centres of seismic activity, namely the Okavango Delta and Lake Kariba.