model based analysis of δ34s signatures to trace sedimentary pyrite oxidation during managed aquifer recharge in a heterogeneous aquifer the oxidation of pyrite is often one of the main drivers affecting groundwater quality during managed aquifer recharge in deep aquifers data and techniques that allow detailed identification and quantification of pyrite oxidation are therefore crucial for assessing and predicting the adverse water quality changes that may be associated with this process in this study we explore the benefits of combining stable sulphur isotope analysis with reactive transport modelling to improve the identification and characterisation of pyrite oxidation during an aquifer storage and recovery experiment in a chemically and physically heterogeneous aquifer we characterise the stable sulphur isotope signal 34s in both the ambient groundwater and the injectant as well as its spatial distribution within the sedimentary sulphur species the identified stable sulphur isotope signal for pyrite was found to vary between 32 and 34 while the signal of the injectant ranged between 9 06 and 14 45 during the injection phase of the experiment both isotope and hydrochemical data together suggest a substantial contribution of pyrite oxidation to the observed temporally variable 34s signals the variability of the 34s signal in pyrite and the injectant were both found to complicate the analysis of the stable isotope data however the incorporation of the data into a numerical modelling approach allowed to successfully employ the 34s signatures as a valuable additional constraint for identifying and quantifying the contribution of pyrite oxidation to the redox transformations that occur in response to the injection of oxygenated water

multifrequency full waveform sonic logging in the screened interval of a large diameter production well abstracta set of field experiments using multiple transmitter center frequencies was completed to test the application potential of low frequency full waveform sonic logging in large diameter production wells wireline logs were acquired in a simple open drillhole and a high yield large diameter production well completed with wire wound sand screens at an aquifer storage and recovery site in perth western australia phase shift transform methods were applied to obtain phase velocity dispersion images for frequencies of up to 4 khz a 3d representation of phase velocity dispersion was developed to assist in the analysis of possible connections between low frequency wave propagation modes and the distribution of hydraulic properties for sandstone intervals in the test well the highest hydraulic conductivity intervals were typically correlated with the lowest phase velocities the main characteristics of dispersion images obtained from the sand screened well were highly comparable with those obtained at t

rise and fall of the southern santa cruz mountains california from fission tracks geomorphology and geodesy the santa cruz mountains are closely associated with a left bend along the right lateral san andreas fault the loma prieta area on the northeast side of the san andreas consists of fault bounded blocks that rise along active deeply rooted reverse and oblique slip faults six samples from a transect across this area yield concordant apatite fission track ages averaging 4 6 0 5 ma these ages date the time of cooling below 110 c and suggest that about 3 km of unroofing has occurred over the last 4 6 m y allowing for current elevations of about 1 km this suggests an average uplift rate of the order of 0 8 mm yr over the last 4 6 m y to further define the extent and distribution of this young uplift we used morphometric analyses of the youthful topography of the area steep drainage slopes and high local relief indicate that the area northeast of the san andreas forms a well defined zone of high uplift in contrast the region on the southwest side of the san andreas is characterized by broad upwarping and folding more subdued topography old fission track ages and mean quaternary uplift rates of 0 1 0 4 mm yr geodetic data 1906 san francisco earthquake and subsequent strain transients 20 years of interseismic deformation 1989 loma prieta earthquake and 2 years of post loma prieta earthquake strain show that the southern santa cruz mountains repeatedly rise and subside through a complex sequence of bay area deformation events an additional deformation element that involves reverse slip averaging 2 3 mm yr along the foothills thrust system must occur to explain the longer term uplift pattern in the loma prieta area since the late pliocene the asymmetry in deformation on opposite sides of the san andreas probably reflects the contrasting rock types on opposite sides of the fault the influence of preexisting structures and the interaction with neighboring faults of the san andreas system

effect of mechanical interaction on the development of strike slip faults with echelon patterns abstract the overlapping geometry of echelon strike slip faults is well known we have quantified this observation by measuring the amount of overlap and separation and plotting them against each other for over 120 examples although there is large scatter the data show a linear trend suggesting that the overlap increases proportionally with separation up to a limiting value we have analyzed this conspicuous relationship in terms of fault interaction by using a numerical model based on displacement discontinuity the results show that fault interaction is in fact an important factor in contributing to the overlapping geometry of echelon strike slip faults

shear fracture patterns and connectivity at geometric complexities along strike slip faults shear fracture patterns and connectivity at geometric complexities such as echelon fault steps and fault bends along strike slip faults are investigated by simulating shear fracture propagation paths using a displacement discontinuity boundary element method together with a maximum distortional strain energy density criterion it is shown that shear fracture patterns and connectivity depend on the specific geometric configurations the applied stress orientations and the coefficient of friction for a given initial configuration of the major faults the subsequent patterns of the associated shear fractures and their linkage can be predicted if the remote state of stress and the coefficient of friction are known the analyses of fault geometries commonly encountered in nature have important implications for evaluating the effect of faults on fluid flow and earthquake hazards associated with the geometric complexities along strike slip faults in the upper crust

tertiary uplift and shortening in the basin and range the echo hills southeastern nevada the echo hills is a distinctive area of tectonic uplift and shortening located at a right step in the left lateral bitter spring valley fault of the late tertiary lake mead fault system nevada the fundamental internal structure of the echo hills is an asymmetric anticline disrupted by reverse and oblique faulting an analysis of the stress distribution within the stepover indicates that the interaction between the two segments of the bitter spring valley fault is responsible for the compressive deformation and structural uplift in the echo hills given the number of strike slip faults in the basin and range similar examples of tertiary uplift and shortening probably occur elsewhere within the province

original forms and initial modifications of the galway lake road scarp formed along the emerson fault during the 28 june 1992 landers california earthquake a remarkable scarp formed southeast of the galway lake road during the mw 7 3 landers california earthquake over the first 11 months following the earthquake we repeated topographic surveys and ground stereoscopic photography and interpreted the observations of original forms and initial modifications of the faulted landforms the complexity of the surface rupture influenced the geomorphic responses the principal shear zone consisted of en echelon right and left lateral generally left stepping fault segments with cumulative offsets of 5 m right lateral and 2 5 m vertical typical fault scarp profiles have different slope angles above and below the surface rupture and a small free face the surface rupture formed steep irregular knickpoints in channels crossing the scarp the original channel surfaces were warped as they stepped down over tilted and rotated fracture bounded blocks the landforms were modified measurably by the above average winter 1992 1993 rainfall over surfaces controlled by hillslope processes only fault scarp erosion was controlled by rainsplash and slumping originally sharp corners were rounded and loose sediment collected in colluvial wedges surface flows over simple ramp steps in channel profiles caused incision that drove knick points back by as much as 50 cm over more complex portions of the principal shear zone flow was diverted into open fractures and little modification of the scarp occurred the pre earthquake topography at the site provided no clear evidence of the last similar slip event simple diffusion erosion models of the time required to bring the new scarp to its pre landers event configuration indicate that the last major rupture may have been several thousand years ago

the nature of the landers mojave earthquake line the landers california earthquake of 28 june 1992 magnitude 7 3 is the latest of six significant earthquakes in the past 60 years whose epicenters and slip directions define a 100 kilometer alignment running approximately n15 w across the central mojave region this pattern may indicate a geologically young throughgoing fault that replaces numerous older strike slip faults by obliquely cutting across them these older faults and perhaps also the bend in the san andreas fault may be losing their ability to accommodate upper crustal deformation because they have become unfavorably oriented with respect to the regional stress field

evidence for paleoseismic slip on a continental low angle normal fault tectonic pseudotachylyte from the west salton detachment fault ca usa abstract the potential of continental low angle normal faults lanf to nucleate large 6 0 m w earthquakes at low angles remains unclear despite much focused research we document evidence for ancient seismicity along a continental lanf detachment fault that formed and slipped at low angles and produced tectonic pseudotachylyte these thick and laterally persistent pseudotachylyte accumulations along the west salton detachment fault wsdf salton trough usa preserve convincing evidence for a frictional melt origin including spherulitic microlites ductile flow structures preservation of high temperature phases as clasts and injection veins cumulative thickness of pseudotachylyte along the fault ranges from 0 1 to 1 5 m and pseudotachylyte cataclasite in the fault core and damage zone are exposed along 2 6 km length of the fault reworked fragments of pseudotachylyte in cataclasites and multiple generations of cataclasites provide evidence for the preservation of multiple earthquake cycles the limited exposure

energy constraints on kinematic models of oblique faulting loma prieta versus parkfield coalinga the loma prieta earthquake was a combination of right lateral strike slip and reverse faulting this oblique motion has been explained by a kinematic model that has the fault accommodate both the translation and convergence required by the pacific north american plate motions and a local bend in the san andreas fault however in other areas of california such as parkfield coalinga this combination of motions is taken up by decoupled strike slip and thrust faults in this paper i develop a simple physical model based on applying hamilton s principle to the energy released in earthquakes that allows us to determine if a region should undergo oblique or decoupled faulting this determination depends on the dip of available faults and the amount of convergence relative to the amount of translation these energy constraints correctly predict the different types of faulting at loma prieta and parkfield coalinga

geochemical reconstruction of the provenance weathering and deposition of detrital dominated sediments in the perth basin the cretaceous leederville formation south west australia article i nfo the deep n10 km permian to present day perth basin in southwest western australia is composed of sedimentary successions of continental to shallow marine origin this study examines the compositional variation weathering and provenance of the early cretaceous leederville formation within the perth basin over 200 sediment samples were retrieved from two deep boreholes of 350 to 530 m in depth encompassing the leederville formation and analysed for major and trace element geochemistry and mineralogy the leederville formation is composed of subarkosic sands predominantly composed of quartz 64 and k feldspars 27 silts and clays composed of kaolinite 24 54 k feldspars 20 29 and quartz 18 40 with their compositional variation reflecting weathering hydraulic sorting during transport mostly subaqueous deposition and subsequent diagenesis most trace elements are associated with fine grained lithologies due to their occurrence within clays sulphide or zircon major and trace element geochemistry suggests a typical passive margin origin with minimal intrabasinal sediment recycling the regionally extensive yilgarn craton appears to be the main sediment source for the leederville formation within the study area

development of pull apart basins a comparative study of well mapped active and ancient pull apart basins suggests a qualitative model for their continuous development pull aparts evolve from incipient to extremely developed basins through a sequence of closely related states new and compiled map data from several areas including the northern caribbean and turkey suggests the following stages in pull apart development 1 in rigid intracontinental strike slip plate boundary zones larger pull aparts nucleate at releasing fault bends along segments of the principal displacement strike slip fault zone which are oblique to the theoretical interplate slip lines 2 initial opening across releasing fault bends produces spindle shaped basins defined and often bisected by oblique slip faults connecting the discontinuous ends of the strike slip faults 3 increased strike slip offset produces basin shapes which we colloquially call lazy s shape for basins between sinistral faults and lazy z shape for basins between dextral faults 4

nitrate reduction in an unconfined sandy aquifer water chemistry reduction processes and geochemical modeling nitrate distribution and reduction processes were investigated in an unconfined sandy aquifer of quaternary age ground water chemistry was studied in a series of eight multilevel samplers along a flow line deriving water from both arable and forested land results show that plumes of nitrate contaminated groundwater emanate from the agricultural areas and spread through the aquifer the aquifer can be subdivided into an upper 10 to 15 m thick oxic zone that contains o2 and no3 and a lower anoxic zone characterized by fe2 rich waters the redox boundary is very sharp which suggests that reduction processes of o2 and no3 occur at rates that are fast compared to the rate of downward water transport nitrate contaminated groundwater contains total contents of dissolved ions that are two to four times higher than in groundwater derived from the forested area the persistence of the high content of total dissolved ions in the no3 free anoxic zone indicates the downward migration of contaminants and that active nitrate reduction is taking place nitrate is apparently reduced to n2 because both nitrite and ammonia are absent or found at very low concentrations possible electron donors in the reduced zone of the aquifer are organic matter present as reworked brown coal fragments from the underlying miocene and small amounts of pyrite at an average concentration of 3 6 mmol kg electron balances across the redoxcline based on concentrations of o2 no3 so42 and total inorganic carbon tic indicate that pyrite is by far the dominant electron donor even though organic matter is much more abundant groundwater transport and chemical reactions were modeled using the code phreeqm which combines a chemical equilibrium model with a one dimensional mixing cell transport model only the vertical component of the water transport was modeled since in contrast to rates along flow lines the vertical rates are close to constant as required by the one dimensional model average vertical transport rates of water in the saturated zone were obtained by tritium dating the modeling process is a two step procedure first the sediment column is initialized with natural water containing only oxygen as electron acceptor and subsequently agricultural waters containing both oxygen and nitrate are fed into the column the nitrate concentration of agricultural waters entering the saturated zone varies with time and an input function was therefore constructed by linear mixing of natural waters and agricultural waters this input function was fed into the column initialized with natural water and the model run forward in time to the year 1988 where field data are available comparison with field data shows that the variation in groundwater chemistry is well described by the model when reduction of oxygen and reduction of nitrate by pyrite oxidation are the only redox reactions occurring finally predictions are made for the distribution of water chemistry in the year 2003 downward progression of the redoxcline is accelerated by a factor of five due to nitrate pollution of the aquifer but absolute rates remain small of the order of a few centimeters per year the controlling factor for nitrate migration through the aquifer once it has reached the anoxic zone is the concentration and distribution of pyrite in the sediments

an apparent shear zone trending north northwest across the mojave desert into owens valley eastern california strain rates measured at four geodetic networks in eastern california situated between northern owens valley and transverse ranges along a small circle drawn about the pacific north america pole of rotation are remarkably consistent each exhibits 0 14 mu rad yr simple right lateral engineering shear strain accumulation across the local vertical plane tangent to the small circle local faults e g owens valley garlock helendale traversing these networks are not as closely aligned with the vertical planes of maximum shear strain accumulation as is the local tangent to the small circle a fifth network slightly east of the small circle shows no significant strain accumulation thus a shear zone trending n35 degree w from near the eastern end of the big bend of the san andreas fault to northern owens valley is indicated by these data this corresponds to the eastern california shear zone proposed on geological evidence by dokka and travis the shear zone carries approximately 8 mm yr of the pacific north america relative plate motion from the san andreas fault north northwest across the mojave desert into owens valley and the northern basin and range province the shear zone observed at the surface may be a manifestation of a through going subcrustal fault

initial investigation of the landers california earthquake of 28 june 1992 using terrascope the 1992 landers earthquake m s 7 5 m w 7 3 was recorded at six terrascope stations in southern california peak accelerations ranged from 0 16 g at svd 63 km to 0 0092 g at isa 245 km decreasing with distance away from the fault zone the peak velocity showed a different pattern reflecting the rupture directivity from south to north the largest peak velocity 19 cm sec was observed at gsc 125 km moment tensor inversion of long period surface waves yielded a mechanism with m 0 1 1 10 27 dyne cm m w 7 3 dip 74 rake 176 and strike 340 inversion of teleseismic p and s waves revealed two distinct sub events of 6 and 8 sec duration and about 10 sec apart the source parameters for the first and second events are m 0 1 9 10 26 dyne cm dip 83 rake 179 strike 359 and m 0 6 1 10 26 dyne cm dip 87 rake 178 strike 333 respectively the radiated wave energy e s was estimated as 4 3 10 23 ergs the ratio e s m 0 3 9 10 4 corresponds to a stress drop of 280 bars and suggests that the landers earthquake belongs to the group of high stress drop earthquakes and occurred on a fault with a long recurrence time the rupture directivity can be seen clearly in the records from pfo 68 km located to the south and gsc located to the north of the epicenter the maximum displacement at pfo is only 13 of that at gsc despite the shorter epicentral distance to pfo than to gsc the slip distribution determined with the empirical green s function method indicates that the landers earthquake consists of two distinct sub events about 30 km apart with the second sub event to the north being about twice as large as the first one this slip distribution is consistent with the teleseismic data and the surface offsets mapped in the field

evolution of the northern santa cruz mountains by advection of crust past a san andreas fault bend the late quaternary marine terraces near santa cruz california reflect uplift associated with the nearby restraining bend on the san andreas fault excellent correspondence of the coseismic vertical displacement field caused by the 17 october 1989 magnitude 7 1 loma prieta earthquake and the present elevations of these terraces allows calculation of maximum long term uplift rates 1 to 2 kilometers west of the san andreas fault of 0 8 millimeters per year over several million years this uplift in concert with the right lateral translation of the resulting topography and with continual attack by geomorphic processes can account for the general topography of the northern santa cruz mountains

the nature and origin of late proterozoic high grade gneisses of the leeuwin block western australia abstract the leeuwin block consists predominantly of felsic gneisses ranging in composition from granodiorite to alkali granite field petrographic and geochemical evidence indicates derivation from igneous parents with the presence of na fe pyroxene and arfvedsonitic amphibole suggesting a type anorogenic affinities and development within a continental rift environment narrow lenses and blocks of massive mafic gneiss are widely distributed throughout the felsic units and are interpreted as remnants of original tholeiitic dolerite dykes emplaced in a continental setting a small area of layered plagioclase hornblende gneiss near augusta has a dioritic composition and is associated with anorthosite and hornblendite it may represent part of an original layered basic intrusion metamorphic parageneses reveal a progressive northward increase in grade from amphibolite to granulite facies developed under low pressure conditions two pyroxene geothermometry indicates that peak metamorphic temperatures attained 690 c there are no major geochemical changes with metamorphic grade and there is no evidence of large ion lithophile element depletion in the granulites zircon fractions obtained from a felsic granulite near dunsborough give u pb ages of 550 570 ma these are interpreted as dating igneous emplacement and together with earlier geochronological data establish that the leeuwin block developed as a separate crustal entity during the late proterozoic pan african event the terrane formed at the western edge of an earlier proterozoic mobile zone indicating progressive westward cratonization away from the archaean yilgarn craton it also indicates a termination of mobile belt activity and a change from a compressional to tensional regime by at least 570 ma ago shear zones bounding the major tectonic units became reactivated as normal faults during the phanerozoic and continental break up occurred along the western margin of the pan african terrane leaving the leeuwin block as the only intact fragment of late proterozoic crust in western australia

the growth of geological structures by repeated earthquakes 2 field examples of continental dip slip faults a strong test of our understanding of the earthquake cycle is the ability to reproduce extant fault bounded geological structures such as basins and ranges which are built by repeated cycles of deformation along strike slip faults the coseismic and interseismic deformation can be nearly equal in magnitude and opposite in sign resulting in little permanent deformation except for the fault offset for dip slip faults portions of the crust are lifted and dropped and so buoyancy forces are exerted the seismic and interseismic deformations do not balance and structures grow and become subject to erosion and deposition we consider three examples for which the structure and fault geometry are well known the white wolf reverse fault in california site of the 1952 kern county m 7 3 earthquake the lost river normal fault in idaho site of the 1983 borah peak m 7 0 earthquake and the cricket mountain normal fault in utah site of quaternary slip events basin stratigraphy and seismic reflection records are used to profile the structure and coseismic deformation measured by leveling surveys is used to estimate the fault geometry to reproduce these structures we add the deformation associated with the earthquake cycle the coseismic slip and postseismic relaxation to the flexure caused by the observed sediment load treating the crust as a thin elastic plate overlying a fluid substrate the cumulative deformation is principally dependent on the elastic plate thickness modestly sensitive to the sediment substrate density difference and insensitive to the fluid viscosity for the 4 to 8 ma structures we deduce a longterm flexural rigidity of 2 15 1019 nm this is equivalent to an elastic plate thickness of 2 4 km for a young s modulus of 2 5 1010 nm 2 this value is found where independent estimates of the elastic thickness from the coherence between surface topography and gravity yield values of about 4 km but where coseismic fault slip extends to a depth of 10 15 km thus much of the seismogenic crust must weaken substantially during the life of active faults causing the fault bounded basins to narrow over time

carbonate mineral water interactions in sulfide rich mine tailings the chemical composition and mineralogy of coatings on carbonate minerals from mine tailings have been studied using aqueous geochemical methods time of flight laser ionization mass spectrometry tof lims and transmission electron microscopy tem the goal is to study major and trace element partitioning between the aqueous and solid phase and to infer mechanisms that control the concentrations of elements in the pore water of sulfide rich mine tailings pore water samples and carbonate mineral grains were collected from four geochemically distinct zones within the tailings oxidation of sulfide minerals near the surface results in a large range in pore water ph 3 85 to 6 98 and aqueous concentrations of metals and sulfate with increasing depth in the tailings mineral water interactions lead to increasing ph and decreasing concentrations of metals and sulfate calculated mineral saturation indices trends in the abundance of ca fe mg and mn in tof lims profiles through the secondary coatings and electron diffraction patterns obtained from the coatings suggest that precipitation dissolution of jarosite group minerals gypsum goethite akaganeite amorphous fe oxyhydroxides and siderite control the aqueous ca fe na k and so4 concentrations the occurrence of secondary coatings on primary minerals is widespread and reactions with the secondary minerals rather than the primary mineral substrate probably represent the principal controls on trace element distributions in the pore water the data indicate that adsorption surface complexation and co precipitation reactions are important controls on the concentrations of trace elements in the pore water the occurrence of siderite coatings on the surface of ankerite grains suggests that fe bearing dolomite structure carbonate minerals dissolve incongruently this corroborates inferences made by previous workers that solubility differences between calcite and siderite lead to calcite dissolution and siderite precipitation during acid neutralization the accumulation of secondary coatings will diminish the rate of mineral dissolution reactions involving the primary minerals consequently the use of kinetic rate constants obtained from the literature for simulating the dissolution of primary mineral phases such as the carbonates and alumino silicates will likely overestimate the actual rates

reactive transport controls on sandy acid sulfate soils and impacts on shallow groundwater quality disturbance or drainage of potential acid sulfate soils pass can result in the release of acidity and degradation of infrastructure water resources and the environment soil processes affecting shallow groundwater quality have been investigated using a numerical code that integrates bio geochemical processes with water solute and gas transport the patterns of severe and persistent acidification ph 4 in the sandy carbonate depleted podzols of a coastal plain could be reproduced without calibration based on oxidation of microcrystalline pyrite after groundwater level decrease and or residual groundwater acidity due to slow vertical solute transport rates the rate of acidification was limited by gas phase diffusion of oxygen and hence was sensitive to soil water retention properties and in some cases also to oxygen consumption by organic matter mineralization despite diffusion limitation the rate of oxidation in sandy soils was rapid once pyrite bearing horizons were exposed even to a depth of 7 5 m groundwater level movement was thus identified as an important control on acidification as well as the initial pyrite content increase in the rate of fe ii oxidation lead to slightly lower ph and greater accumulation of fe iii phases but had little effect on the overall amount of pyrite oxidized aluminosilicate kaolinite dissolution had a small ph buffering effect but lead to the release of al and associated acidity simulated dewatering scenarios highlighted the potential of the model for risk assessment of bio geochemical impacts on soil and groundwater over a range of temporal and spatial scales

multicycle dynamics of nonplanar strike slip faults 1 we perform two dimensional dynamic models of strike slip faults with a change in strike a bend over multiple earthquake cycles to examine the long term effects of nonplanar fault geometry a viscoelastic model a proxy for off fault deformation and tectonic loading is introduced for the interseismic process to avoid pathological stress buildup around the bend a finite element method with an elastodynamic model is used to simulate dynamic earthquake ruptures we find that stresses near the bend differ strongly from the regional stress field and that the fault develops a relatively steady state in which the stress level and the event pattern on the fault are stable reduced normal stress on the dilatational side and increased normal stress on the compressive side of the bend during dynamic ruptures result in the bend serving as an initiation and or a termination point s for rupture typical events on such a fault consist of two classes unilateral events that rupture only the favorable segment and bilateral events that rupture the favorable segment and part of or the entire unfavorable segment in the latter class of events a time delay in rupture around the bend results from a high yield stress on the compressive side of the bend other effects of the bent fault geometry include higher displacement on the inward wall than on the outward wall higher slip on the more favorable segment than on the less favorable segment and a large slip velocity on the compressive side of the bend

interactions between the landers and hector mine california earthquakes from space geodesy boundary element modeling and time dependent friction the m w 7 1 hector mine earthquake happened 7 years after the 1992 m w 7 3 landers earthquake within a system of right lateral strike slip faults comprising the eastern california shear zone because estimates of recurrence intervals of m 7 earthquakes on these faults range anywhere from 1500 to 50 000 years the close temporal spacing of these two earthquakes suggests that there is some interaction between the two current models of simple coulomb static stress interactions between the earthquake ruptures do not predict an obvious cause and effect relationship the interaction between the computed normal and right lateral shear stress reductions induced by the landers earthquake at the hector mine hypocenter is unclear we use a combination of space geodesy boundary element modeling and computer modeling of time dependent fault friction to investigate the interaction between the two earthquakes we compare stress changes on the hector mine rupture plane induced by the landers earthquake with the detailed slip distribution inferred using a combination of gps and insar data the slip distributions of both earthquakes are also used to infer the magnitude of the shear stress drop on each earthquake rupture and the orientation of the remote background stress consistent with these two recent events for each earthquake the azimuth of the remote maximum compressive stress was approximately 17 6 and the magnitude of the shear stress drop was 8 1 mpa on the landers rupture and 10 2 mpa on the hector mine rupture using a simple spring and slider model as a proxy for the rate and state frictional response of the hector mine faults to landers earthquake imposed stress steps we find that while a decrease in normal stress at the hector mine hypocenter would by itself have caused the hector mine earthquake to nucleate immediately the simultaneous decrease in shear stress may have caused a delay in the peak shear loading consideration of an acceptable range of rate and state friction parameters secular strain rates and crustal stiffnesses leads us to conclude that this delay could have been 0 to 40 years

the growth of geological structures by repeated earthquakes 1 conceptual framework in many places earthquakes with similar characteristics have been shown to recur if this is common then relatively small deformations associated with individual earthquake cycles should accumulate over time to create geological structures following this paradigm we show that existing models developed to describe leveling line changes associated with the seismic cycle can be adapted to explain geological features associated with a fault in these models an elastic layer containing the fault overlies a viscous half space with a different density fault motion associated with an earthquake results in immediate deformation followed by a long period of readjustment as stresses relax in the viscous layer and isostatic equilibrium is restored deformation is also caused as a result of the loading and unloading due to sediment deposition and erosion in this paper the parameters that control the growth of dip slip structures are identified we find that the flexural rigidity of the crust or the apparent elastic thickness provides the main control of the width of a structure the loading due to erosion and deposition of sediment determines the ratio of uplift to subsidence between the two sides of the fault the flexure due to sediment load is much more important in this respect than whether the fault is normal or reverse in character we find that in general real structures are associated with apparent elastic thicknesses of 4 km or less and thus with very low flexural rigidities

geochemical controls on sediment reactivity and buffering processes in a heterogeneous aquifer the injection and recovery of oxic water into deep anoxic aquifers may help to alleviate short and longterm imbalance between freshwater supply and demand the extent and structure of physical and geochemical heterogeneity of the aquifer will impact the water quality evolution during injection storage and recovery water sediment interactions within the most permeable parts of the aquifer where the bulk of the injectant will penetrate may dominate however water quality may also be impacted by interactions within the finer grained less permeable but potentially highly reactive media in this study the heterogeneity of the reductive capacity of an aquifer selected for water reuse projects was characterised the amount type and reactivity of the sedimentary reductants present determined and the relationship between reductive capacity and sedimentary lithologies quantified the average potential reductive capacities prctot based on total organic c and pyrite concentrations of the sediment were quantified for sands 382 lmol o2 g 1 clays 1522 lmol o2 g

integration of surface slip and aftershocks to constrain the 3d structure of faults involved in the m 7 3 landers earthquake southern california investigation of the mechanics of complex earthquakes requires insight into fault geometry we use linear elastic quasi static mechanical models of faults involved in the m 7 3 landers earthquake to test the influence of fault continuity dip and strike on fault slip and the perturbed stress field in regions of complex surface deformation and dense aftershock activity subsurface fault structure is constrained with both geological and geophysical data by comparing observed right lateral surface slip with slip along model faults and by comparing focal mechanisms with model coulomb planes at the locations of large aftershocks the refined structure of major faults at landers includes a southward extension of the johnson valley fault jvf at depth beyond the mapped fault trace a 75 w dip along the central johnson valley fault and a near surface discontinuity along the southern homestead valley fault hvf the final model better captures observed slip deficits along the johnson valley and homestead valley faults improves quantitative fit to the observed offset data and captures the local stress state in 9 of 12 locations studied relating aftershock orientations to local stresses resulting from mainshock fault slip provides a mechanical basis for deciphering between the two nodal planes provided by each focal mechanism slip along nonplanar faults with friction of 0 6 produces model failure planes in a variety of orientations indicating that heterogeneous aftershocks do not necessarily result from low fault friction mechanical selection of nodal planes suggests that aftershocks do not consistently reflect the orientation of their cluster or the orientation and slip direction of mainshock faults

shear zones formed along long straight traces of fault zones during the 28 june 1992 landers california earthquake surface rupturing during the 28 june 1992 landers california earthquake east of los angeles accommodated right lateral offsets up to about 6 m along segments of distinct en echelon fault zones with a total length of 80 km the offsets were accommodated generally not by faults distinct slip sur faces but rather by shear zones tabular bands of localized shearing along simple stretches of fault zones at landers the rupture is characterized by telescoping of shear zones and intensification of shearing broad shear zones of mild sheafing containing narrow shear zones of more intense sheafing containing even nar rower shear zones of very intense sheafing which may contain a fault thus the ground ruptured across broad belts of shearing with clearly defined subparallel walls oriented nw each broad belt consists of a broad zone of mild sheafing extending across its entire width 50 to 200 m and much nan ower a few meters wide shear zones that accommodate most of the offset of the belt and are por trayed by en echelon tension cracks in response to right lateral shearing the slices of ground bounded by the tension cracks rotated in a clockwise sense producing left lateral sheafing and the slices were forced against the walls of the shear zone producing thrusting even narrower shear zones formed within the narrow shear zones although these probably are guides to right lateral fault segments below the surface rupturing during the earthquake is characterized not by faulting but by the formation of shear zones at various scales

geomorphological hierarchies for complex mega dunes and their implications for mega dune evolution in the badain jaran desert abstract the evolution of mega dunes is sometimes attributed to factors other than the wind but evidence for this is lacking it is assumed that the dominance of wind in maintaining the evolution of mega dunes should be characterized by regular height spacing relationships that have been found for simple dunes or wind ripples which are dominantly formed by the wind in this context we studied the height spacing relationship for the complex reversing mega dunes in the badain jaran desert which features the tallest mega dunes in the world the complex mega dunes were divided into three hierarchical orders according to the cumulative probability plots of dune height and spacing measurements and the coefficients of variability of dune heights and spacings were in accordance with values reported for other deserts the relationship between dune spacing and height for all the three orders of dunes could be expressed reasonably well by a uniform linear function that was also applied to the height wavelength relationship for wind ripples in other deserts this relationship was found to be similar to those for several other deserts and subaqueous bedforms this implies that there should be few unique factors in maintaining the evolution of complex mega dunes in the badain jaran desert compared with the superimposed simple dunes and dunes in other deserts dune fields and subaqueous bedforms and that the tallest mega dunes on the earth can be maintained by the wind

fault steps and the dynamic rupture process 2 d numerical simulations of a spontaneously propagating shear fracture fault steps may have controlled the sizes of the 1966 parkfield 1968 borrego mountain 1979 imperial valley 1979 coyote lake and the 1987 superstition hills earthquakes this project investigates the effect of fault steps of various geometries on the dynamic rupture process we have used a finite difference code to simulate spontaneous rupture propagation in two dimensions we employ a slip weakening fracture criterion as the condition for rupture propagation and examine how rupture on one plane initiates rupture on parallel fault planes the geometry of the two parallel fault planes allows for stepover widths of 0 5 to 10 0 km and overlaps of 5 to 5 km our results demonstrate that the spontaneous rupture on the first fault segment continues to propagate onto the second fault segment for a range of geometries for both compressional and dilational fault steps a major difference between the compressional and dilational cases is that a dilational step requires a longer time delay between the rupture front reaching the end of the first fault segment and initiating rupture on the second segment therefore our dynamic study implies that a compressional step will be jumped quickly whereas a dilational step will cause a time delay leading to a lower apparent rupture velocity we also find that the rupture is capable of jumping a wider dilational step than compressional step

a comparison of the geochemical response to different managed aquifer recharge operations for injection of urban stormwater in a carbonate aquifer abstract managed aquifer recharge mar is increasingly being considered as a means of reusing urban stormwater and wastewater to supplement the available water resources subsurface storage is advantageous as it does not impact on the area available for urban development while the aquifer also provides natural treatment however subsurface storage can also have deleterious effects on the recovered water quality through water rock interactions which can also impact on the integrity of the aquifer matrix a recent investigation into the potential for stormwater recycling via aquifer storage transfer and recovery astr in a carbonate aquifer is used to determine the important hydrogeochemical processes that impact on the recovered water quality an extensive period of aquifer flushing allows observation of water quality changes under two operating scenarios 1 separate wells for injection and recovery representing astr and 2 a single well for injection and recovery representing aquifer storage and recovery asr calcite dissolution produces the dominant inorganic chemical change to the quality of stormwater following storage in the carbonate aquifer independent of the mode of mar operation astr or asr the magnitude of calcite dissolution in response to the injection of urban stormwater exceeds that previously reported within the same aquifer using both stormwater and reclaimed wastewater while some dissolution is induced in response to redox processes the primary influence on dissolution is the reactivity of the source water itself as indicated by the sub saturation with respect to calcite cation exchange is evident through the astr mode of operation producing a marginal increase in the na concentration during freshening of the storage zone this increase in na concentration is not evident in the asr mode as cation exchange is limited to the initial pore flushes of the storage zone aquifer storage provides some treatment through nutrient removal mainly through removal of 35 of the dissolved organic c doc doc removal is greatest when the mar operation involves separate injection and recovery wells as astr allows enhanced removal by sorption asr can lead to nutrient recycling around the injection well which can produce water quality that is atypical of the quality in the bulk of the storage zone water recovered from asr shows some removal of doc through microbial oxidation coincident with removal of o2 from the source water which is considered to be a sustainable process during subsurface treatment during astr enhanced doc removal is attributed to adsorption but as with cation exchange this removal may be limited to the initial period of aquifer conditioning oxidation of pyrite is evident during the initial stages of injection until the pool of reactive pyrite within the storage zone is consumed affecting the quality of water recovered via the astr mode only

influence of fault bends on ruptures data on slip along the san andreas fault have been examined to understand the influence of a bend in the fault system on the extent of slip release a correlation of slip along the fault in the great 1906 and 1857 earthquakes with fault geometry shows that maximum slip occurred near a major change in the fault strike and may be a function of this change the data appear to suggest that in general it takes about 10 years of strain accumulation to negotiate a 1 change in fault strike assuming a slip rate of about 3 cm yr seismic hazard along the san andreas fault may therefore also be a function of fault geometry

numerical simulation of spontaneous rupture processes on twonon coplanar faults the effect of geometry on fault interaction summary analyses of earthquake sources have revealed that the earthquake rupture process is complex and that the rupture does not occur on a single plane earthquake faults are often composed of several subfaults and rupture propagation tends to decelerate or terminate at places where the fault strike changes these observations imply that fault geometry including fault steps and fault strike change plays an important role in earthquake rupture complexity in this paper we calculate the spontaneous rupture processes of two non coplanar faults in 2 d in plane problems attempting to clarify the eiect of fault geometry we consider two simple modelsmodels in which two faults are either parallel or perpendicular to each other we calculate spontaneous rupture propagation on the faults bya nite diierence method andwe then compare the results in our simulations rupture initially grows on the main fault and stress perturbation from the main rupture then triggers rupture on the secondary fault propagation of the main fault rupture controls a spatio temporal pattern ofstress diierence in the uniform elastic medium which determines the rupture process of the secondary fault the rupture propagation and termination of the secondary fault are signi cantly diierent between the two models the diierence is obvious when rupture of the main fault is arrested and the secondary fault is located near the arrested end of the main fault when the secondary fault is parallel to the main fault rupture can propagate ahead on the secondary fault however when the secondary fault is perpendicular to the main fault rupture is either not triggered on the secondary fault or soon terminates if triggered this variation of the rupture process implies that fault interaction depending on geometry can explain the termination and change of rupture at places where the fault strike varies this shows the importance of the fault geometry in studying spontaneous dynamic rupture processes

reactivity of organic matter in aquifer sediments geological and geochemical controls reduction rates in aquifers are commonly carbon limited but little is known about the molecular composition and degradability of sedimentary organic matter som in aquifer sediments the composition source and degradation status of som in aquifer sediments of fluvio glacial pleistocene and shallow marine pliocene origin were determined using flash pyrolysis gas chromatography mass spectrometry incubation experiments 106 d were used to assess the reactivity of som towards molecular oxygen a dominance of lignin derived components and long chain odd over even predominant alkanes indicate that terrestrial higher land plants were the main source of som even in the shallow marine sediments while bacterial lipid derived hopanoids and iso and anteiso c15 and c17 fatty acids indicate a minor contribution of microbial biomass no compositional difference was observed between som present in the fine 63 m and coarse fraction 63 2000 m a significant part of som was not present as low molecular weight compounds but was macromolecularly bound for the fluvio glacial sediments a relatively higher abundance of resistant macro molecular compounds was in agreement with stronger signs of aerobic lignin alkane and hopanoid oxidation the more degraded status of som in the fluvio glacial sediments was consistent with their significantly lower som mineralization 2 6 during incubation as compared with the shallow marine sediments 9 14 the reactivity towards oxygen of som was controlled by the extent of past aerobic oxidation not the age of som but the extent of oxygen exposure during syn and postdepositional conditions seems most important in affecting the degradation status of som in aquifer sediments and thus their ability to reduce oxidants copyright 2004 elsevier ltd

characterization of the reactivity of riverine heterogeneous sediments using a facies based approach the rhine meuse delta the netherlands large groundwater resources are found in densely populated lowland areas which consist often of young unconsolidated and reduced sediments when anthropogenic activities lead to oxygenation of the aquifer breakdown of the main reduced fractions i e sedimentary organic matter som and pyrite could lead to severe groundwater deterioration such as acidification heavy metal mobilization and increased hardness the characterization of the reactive properties of these sediments is important in predicting groundwater deterioration but is often complicated by the high degree of heterogeneity of these sediments in this study the potential reduction capacity prc based on som and pyrite content the potential buffer capacity pbc based on carbonate content potential acidification capacity pac based on the potential acid production by sulfide oxidation and the measured reduction capacity mrc of five facies which are typical of the riverine sediments in the rhine meuse delta the netherlands were determined a universal facies classification model was used to classify the deposits into more homogeneous sub units based on lithologic and geogenic properties with a further sub division into oxic or anoxic redox environment based upon groundwater data and field observations the bulk chemical data show strong variation across facies for the median values of prc 186 9093 mmol o2 kg 1 pbc 17 132 mmol o2 kg 1 and pac 36 1530 mmol h kg 1 the mrc was measured as reactivity to molecular o2 exposure and was 0 5 567 3 mmol o2 kg 1 steady state oxidation rates were in the wide range of 0 001 10 355 mmol o2 kg 1 day 1 but were typically about 3 8 times faster in fine facies than in coarse facies both the prc and mrc depend strongly on grain size but also on the syn post depositional environment and redox conditions the main part of the prc consists of som but pyrite reactivity is higher than som reactivity as shown by the relative depletion of pyrite in oxic subfacies and the preferential oxidation during the oxidation experiments some facies are very prone to acidification because the pac is higher than the pbc but the oxidation experiments also show that acidification could already start before the prc is fully exhausted this study is one of the few that combines bulk chemical data groundwater data and reactivity measurements and shows that a facies based approach is a practical tool in characterizing the reactivity of heterogeneous deposits 2007 elsevier ltd all rights reserved

off fault ground ruptures in the santa cruz mountains california ridge top spreading versus tectonic extension during the 1989 loma prieta earthquake the ms 7 1 loma prieta earthquake of 18 october 1989 produced abundant ground ruptures in an 8 by 4 km area along summit road and skyland ridge in the santa cruz mountains predominantly extensional fissures formed a left stepping crudely en echelon pattern along ridges of the hanging wall block southwest of the san andreas fault about 12 km northwest of the epicenter the fissures are subparallel to the san andreas fault and appear to be controlled by bedding planes faults joints and other weak zones in the underlying tertiary sedimentary strata of the hanging wall block the pattern of extensional fissures is generally consistent with tectonic extension across the crest of the uplifted hanging wall block also many displacements in laurel creek canyon and along the san andreas and sargent faults are consistent with right lateral reverse faulting inferred for the mainshock additional small tensile failures along the axis of the laurel anticline may reflect growth of the fold during deep seated compression however the larger ridge top fissures commonly have displacements that are parallel to the north northeast regional slope directions and appear inconsistent with east northeast extension expected from this earthquake measured cumulative displacements across the ridge crests are at least 35 times larger than that predicted by the geodetically determined surface deformation these fissures also occur in association with ubiquitous landslide complexes that were reactivated by the earthquake to produce the largest concentration of co seismic slope failures in the epicentral region the anomalously large displacements and the apparent slope control of the geometry and displacement of many co seismic surface ruptures lead us to conclude that gravity is an important driving force in the formation of the ridge top fissures shaking induced gravitational spreading of ridges and downslope movement may account for 90 or more of the observed displacements on the linear fissures similar fissures occurred in the same area and elsewhere near the san andreas fault during the predominantly right lateral 1906 san francisco earthquake and suggest that the loma prieta ground ruptures may in large part be independent of fault kinematics

modeling of carbon cycling and biogeochemical changes during injection and recovery of reclaimed water at bolivar south australia 1 managed aquifer recharge is an increasingly popular technique to secure and enhance water supplies among a range of recharging techniques single well aquifer storage and recovery asr is becoming a common option to either augment drinking water supplies or facilitate reuse of reclaimed water for the present study a conceptual biogeochemical model for reclaimed water asr was developed and incorporated into an existing reactive multicomponent transport model the conceptual and numerical model for carbon cycling includes various forms of organic and inorganic carbon and several reactive processes that transfer carbon within and across different phases the major geochemical processes considered in the model were microbially mediated redox reactions driven by the mineralization of organic carbon mineral dissolution precipitation and ion exchange the numerical model was tested and applied for the analysis of observed data collected during an asr field experiment at bolivar south australia the model simulation of this experiment provides a consistent interpretation of the observed hydrochemical changes the results suggest that during the storage phase dynamic changes in bacterial mass have a significant influence on the local geochemistry in the vicinity of the injection extraction well farther away from the injection extraction well breakthrough of cations is shown to be strongly affected by exchange reactions and in the case of calcium by calcite dissolution

surficial slip distribution on the central emerson fault during the june 28 1992 landers earthquake california we present the results of our mapping of a 5 6 km length of the central emerson fault that ruptured during the 1992 landers earthquake in the southwestern mojave desert california the right lateral slip along this portion of the rupture varied from about 150 to 530 cm along the main rupture zone in some locations a total of up to 110 cm of additional right lateral slip occurred on secondary faults up to 1 7 km away from the main rupture zone other secondary faults carried up to several tens of centimeters of left lateral or thrust displacement the maximum net vertical displacement was 175 cm east side up the sense of vertical slip across the main fault zone varied along strike but in most cases it was consistent with the sense of vertical slip in previous earthquakes as indicated by the locations of areas of older uplifted and abandoned alluvial fan surfaces although variations in surficial slip have been reported along previous strike slip ruptures our closely spaced slip measurements allow a much more detailed study of slip variability than was possible previously we document variations in slip as large as 1 m or more over distances ranging from 1 2 km to a few tens of meters suggesting that strains of the order of 10 1 may have occurred locally within the surficial sediments the long wavelength kilometer scale variations in surficial slip may be influenced by fault geometry and perhaps by the thickness of unconsolidated sediments the slip variations over shorter length scales tens of meters may be caused by variations in the proportion of the total shear that occurs on visible brittle fractures versus that which occurs as distributed shear warping or rotation the variability of slip along the ruptures associated with the landers earthquake calls for caution in interpreting geomorphic offsets along prehistoric fault ruptures

mechanics of fault junctions a sharp bend in a fault must be part of a triple junction and slip on the three fault segments at the junction must have a vector sum of zero in order to avoid an unphysical 1 r stress singularity accompanying the slip a volume change occurs at the junction either a void opens or intense local deformation is required to avoid material overlap the energy absorbed due to the volume change is proportional to the slip increment times the total past slip accumulated at the junction at a new junction the energy absorbed is a small fraction of the energy released by slip on the fault system but after a number of earthquakes proportional to shear modulus over confining stress the junction becomes a strong barrier to further slip although slip occurs more easily on old rupture surfaces than on fresh fractures the growing barrier strength of junctions requires that there be some fresh fracture in earthquakes perhaps a small fraction of the surface that slips in any earthquake is fresh fracture which could provide the instability needed to explain earthquakes a numerical model in two dimensional static plane strain shows even without accounting for energy absorbed at the junction that a bend in a fault acts as a barrier if slip is impeded on the associated fault spur the stress concentration at the bend will tend to induce slip on the spur slip occurring on the spur unstably with a drop in the coefficient of friction can induce increased slip on the main fault segments with no change in the coefficient of friction a fault junction provides a natural realization of barrier and asperity models without appealing to arbitrary variations of fault strength the location of the fresh fracture that occurs after a junction becomes a strong barrier remains an unanswered question it is likely to be initiated near the stress concentration at the old junction a model simulating the effect of fresh fracture near old junctions might explain earthquakes without appealing to an unstable friction law anywhere

speculations on the geometry of the initiation and termination processes of earthquake rupture and its relation to morphology and geological structure earthquake initiation and termination processes are commonly described in terms of barriers and asperities barriers fall into two classes geometric barriers are associated with places where the orientation of a failure surface changes and relaxation barriers where stress is low because aseismic creep processes outpace tectonic loading geometric barriers fall into conservative and nonconservative subgroups according to whether finite fault motion can proceed without the creation of new structures or whether it demands the creation of new faulting or void space the multiple faulting or fragmentation associated with some nonconservative barriers can disrupt fault planes and form asperities by means of selected examples it is shown that a description in terms of these barriers can help one to visualise the processes of earthquake rupture and its relation to the geological environment

the accommodation of large strains in the upper lithosphere of the earth and other solids by self similar fault systems the geometrical origin of b value plate tectonics has provided a method of visualizing the geometry of the deformation of the earth s lithosphere on a large scale the description is so concise that for many purposes it provides an explanation of geological processes that overshadows the need to understand the driving processes the mechanics of the zones between the plates are less well understood particularly in continental regions where large areas are subject to deformation both continuous and discontinuous models have been tried but both have obvious drawbacks

surface rupture at a fault bend the 28 june 1992 landers california earthquake mapping of surface rupture associated with the 28 june 1992 landers earthquake along a portion of the north central emerson fault permits us to characterize the fractures and to interpret them in terms of a contractional bend in the fault trace the main rupture exhibits right lateral and reverse slip and is localized in a zone about 50 m wide along the present day trace of the emerson fault the surface deformation within its northeastern block the hanging wall is accommodated by a dome structure about 2 km long and 0 5 km wide and a complex assemblage of fractures the complexity of the fracture pattern around the fault bend results in part from 1 interaction between the main rupture and many shorter subparallel faults 2 the initiation and propagation of fractures at geometric complexities such as fault segment tips and fault bends and 3 bedding plane faulting both geological data and rupture kinematics indicate that uplift and shortening are associated with the fault bend the state of stress around the emerson fault bend is determined for co seismic conditions fault slip boundary conditions and interseismic conditions the remote applied stress and frictional fault boundary conditions using both displacement discontinuity boundary element and dislocation methods respectively we also investigate a possible fault propagation path at an idealized bend using the maximum distortional strain energy density criterion results of the theoretical analyses provide support for an understanding of the location and the asymmetry of the dome structure and the mechanisms for some of the observed fracture systems in terms of the state of stress and fault propagation paths around the fault bend and other smaller scale complexities along the fault trace

depositional and paleohydrogeological controls on the distribution of organic matter and other reactive reductants in aquifer sediments abstract the reactivity of sedimentary reductants is the main control for the natural attenuation of common groundwater contaminants such as nitrate or chlorinated hydrocarbons middle miocene to late pleistocene 0 01 20 my old aquifer sands of marine fluvial fluvio glacial and aeolian origin were characterized to determine the control of sediment depositional environment sediment age and paleohydrology on the distribution and reactivity of sedimentary reductants in addition to the variability in the molecular composition of sedimentary organic matter som the reduction reactivity of these sediments was determined oxygen consumption and carbon dioxide production during sediment incubations indicated that som pyrite and ferrous iron bearing carbonates were the main reductants bulk 13 c org values 25 indicated that terrestrial higher land plants were the main precursor of som regardless of sediment origin however pyrolysis gc ms analysis showed that som in the marine tertiary sands contained lignin with preserved side chains while in the pleistocene fluvial and fluvio aeolian sediments highly degraded lignin and recalcitrant macromolecular aliphatic structures dominated som indicative of aerobic degradation the higher dynamics of these terrestrial depositional environments as compared with their marine counterpart likely allows for increased oxygen exposure times and thus more intense aerobic degradation of som the aquifer sediment that fills a large erosional valley created during the saale ice age likely consists of fluvio glacially reworked marine tertiary sediments as supported by carbonate isotopic evidence the more degraded status and decreased reactivity of som in this sediment than in its precursor is likely due to the increased oxygen exposure during dynamic fluvio glacial reworking despite the highly degraded nature of som and the absence of pyrite in the pleistocene fluvial and fluvio aeolian sediments oxygen consumption rates were high during the incubation of sediments from the shallowest part of the aquifer a reactive ferroan carbonate phase was inferred as the main source of oxidant demand in shallow fluvial and aeolian sands depleted oxygen and carbon isotopes indicated that this phase was groundwater derived aerobic degradation during sediment deposition appears to control the molecular composition and reactivity of som in aquifer sediments and to affect the potential of subsequent diagenetic pyrite formation in addition paleo hydrological conditions may result in the accumulation of a ferrous iron bearing carbonate phase precipitated during the exfiltration of fe ii containing anoxic groundwater

the morphology of strike slip faults examples from the san andreas fault california fault zones are seldom straight nor are fractures within them continuous permanent strain fields are imprinted near a fault of complex geometry as a result of continued slip these strain fields may be manifest as vertical deformation near the fault strain fields that increase area result in subsidence and strain fields that reduce area result in uplift although these effects are modified by erosion and deposition the general features of fault zone morphology are a consequence of the geometry and slip distribution along strike graphical views of strain fields associated with simple vertical strike slip faults are derived using boundary element methods if the fault is straight and parallel to the applied shear strain the hills and valleys resulting from finite fault rupture are separated by the fault trace this is rarely observed in nature instead strike slip faults cut through hills and valleys apparently of their own making we show this to be a consequence of segments of the fault being nonparallel to each other and to the overall applied shear strain coseismic strains are large but transient since they are substantially annulled by rupture on contiguous fault segments we test our theory with examples from parts of the san andreas fault system in california the morphologies of the parkfield region and the southern san andreas fault in the coachella valley are shown to be consistent with plausible distributions of slip on mapped surface faults the location of the salton sea appears to be a consequence of a reduction of slip on the san andreas fault toward the brawley seismic zone

the black mountain asperity seismic hazard of the southern san francisco peninsula california black mountain a 860 m high wedge of franciscan formation mostly comprising basic ultrabasic rock sw of palo alto and just ne of the san andreas fault marks an abrupt bend in the fault at the northern end of a 100 km long segment of the fault that strikes 9 more e w than the fault to the north or south it also bounds a marked change in the physiographic setting of the fault which to the north follows a well developed linear fault valley and to the south follows a poorly defined topographic trace that traverses the rugged santa cruz mountains the bend at black mountain had a profound effect on the 1906 rupture the 75 km section to the se of this point slipped only 1 1 4 m as compared to the 2 5 4 m typical of the rupture on the san francisco peninsula to the nw this 75 km long slip deficit region from black mountain to san juan bautista if ruptured in its entirety would produce a ms 6 9 earthquake the conditional probability of this rupturing in the near future is the highest of any section of the san andreas fault except parkfield this earthquake would rupture 30 km farther northwest and be about 3 times larger than that previously proposed by others and this constitutes a greater risk to the southern san francisco peninsula than previously expected

rupture progression along discontinuous oblique fault sets implications for the karadere rupture segment of the 1999 izmit earthquake and future rupture in the sea of marmara abstract large earthquakes in strike slip regimes commonly rupture fault segments that are oblique to each other in both strike and dip this was the case during the 1999 izmit earthquake which mainly ruptured e w striking right lateral faults but also ruptured the n60 e striking karadere fault at the eastern end of the main rupture it will also likely be so for any future large fault rupture in the adjacent sea of marmara our aim here is to characterize the effects of regional stress direction stress triggering due to rupture and mechanical slip interaction on the composite rupture process we examine the failure tendency and slip mechanism on secondary faults that are oblique in strike and dip to a vertical strike slip fault or master fault for a regional stress field well oriented for slip on a vertical right lateral strike slip fault we determine that oblique normal faulting is most favored on dipping faults with two different strikes both of which are oriented clockwise from the strike slip fault the orientation closer in strike to the master fault is predicted to slip with right lateral oblique normal slip the other one with left lateral oblique normal slip the most favored secondary fault orientations depend on the effective coefficient of friction on the faults and the ratio of the vertical stress to the maximum horizontal stress if the regional stress instead causes left lateral slip on the vertical master fault the most favored secondary faults would be oriented counterclockwise from the master fault for secondary faults striking 30 oblique to the master fault right lateral slip on the master fault brings both these secondary fault orientations closer to the coulomb condition for shear failure with oblique right lateral slip for a secondary fault striking 30 counterclockwise the predicted stress change and the component of reverse slip both increase for shallower angle dips of the secondary fault for a secondary fault striking 30 clockwise the predicted stress change decreases but the predicted component of normal slip increases for shallower angle dips of the secondary fault when both the vertical master fault and the dipping secondary fault are allowed to slip mechanical interaction produces sharp gradients or discontinuities in slip across their intersection lines this can effectively constrain rupture to limited portions of larger faults depending on the locations of fault intersections across the fault intersection line predicted rakes can vary by 40 and the sense of lateral slip can reverse application of these results provides a potential explanation for why only a limited portion of the karadere fault ruptured during the izmit earthquake our results also suggest that the geometries of fault intersection within the sea of marmara favor composite rupture of multiple oblique fault segments

near field investigations of the landers earthquake sequence april to july 1992 the landers earthquake which had a moment magnitude m w of 7 3 was the largest earthquake to strike the contiguous united states in 40 years this earthquake resulted from the rupture of five major and many minor right lateral faults near the southern end of the eastern california shear zone just north of the san andreas fault its m w 6 1 preshock and m w 6 2 aftershock had their own aftershocks and foreshocks surficial geological observations are consistent with local and far field seismologic observations of the earthquake large surficial offsets as great as 6 meters and a relatively short rupture length 85 kilometers are consistent with seismological calculations of a high stress drop 200 bars which is in turn consistent with an apparently long recurrence interval for these faults