postseismic relaxation following the 1989 ms7 1 loma prieta earthquake central california 1 the postseismic relaxation postseismic displacement less displacement that would have occurred at the preseismic rate measured by gps and leveling following the 1989 ms7 1 loma prieta earthquake is reexamined the temporal dependence of the relaxation over the first 1200 days postseismic is well described by 1 e t where 414 92 days quoted uncertainties are standard deviations that temporal dependence appears to be a linear function of the cumulative number of m 2 5 aftershocks that have occurred the relaxation is attributed to afterslip 1 56 0 20 m dextral strike slip and 0 60 0 04 m reverse slip on the downdip extension depth 16 21 km of the coseismic rupture plus a collapse 0 11 0 02 m fault normal displacement of the rupture zone depth 5 16 km because the postseismic uplift was determined by leveling over a route with relief in excess of 1000 m an allowance 18 2 ppm of height above the base elevation for excess beyond corrections already applied height dependent error in the measured uplift was estimated simultaneously with the afterslip and collapse parameters this new solution for afterslip and collapse on the plane of the rupture provides an alternative explanation to the suggestion by burgmann et al 1997 that the postseismic deformation was due to afterslip on the coseismic rupture and on a nearby shallow 30 dipping thrust

shallow fault zone dilatancy recovery after the 2003 bam earthquake in iran earthquakes radiate from slip on discrete faults but also commonly involve distributed deformation within a broader fault zone especially near the surface variations in rock strain during an earthquake are caused by heterogeneity in the elastic stress before the earthquake by variable material properties and geometry of the fault zones and by dynamic processes during the rupture stress changes due to the earthquake slip both dynamic and static have long been thought to cause dilatancy in the fault zone that recovers after the earthquake decreases in the velocity of seismic waves passing through the fault zone due to coseismic dilatancy have been observed followed by postseismic seismic velocity increases during healing dilatancy and its recovery have not previously been observed geodetically here we use interferometric analysis of synthetic aperture radar images to measure postseismic surface deformation after the 2003 bam iran earthquake and show reversal of coseismic dilatancy in the shallow fault zone that causes subsidence of the surface this compaction of the fault zone is directly above the patch of greatest coseismic slip at depth the dilatancy and compaction probably reflects distributed shear and damage to the material during the earthquake that heals afterwards coseismic and postseismic deformation spread through a fault zone volume may resolve the paradox of shallow slip deficits for some strike slip fault ruptures

clay mineral evolution changes in the mechanisms of formation and global distribution of phyllosilicate clay minerals through 4 567 ga of planetary evolution in our solar system reflect evolving tectonic geochemical and biological processes clay minerals were absent prior to planetesimal formation 4 6 billion years ago but today are abundant in all near surface earth environments new clay mineral species and modes of clay mineral paragenesis occurred as a consequence of major events in earth s evolution notably the formation of a mafic crust and oceans the emergence of granite rooted continents the initiation of plate tectonics and subduction the great oxidation event and the rise of the terrestrial biosphere the changing character of clay minerals through time is thus an important part of earth s mineralogical history and exemplifies the principles of mineral evolution

lower crustal structure in northern california implications from strain rate variations following the 1906 san francisco earthquake 1 it is well known that geodetic data from a single instant in time cannot uniquely characterize structure or rheology beneath active seismogenic zones nevertheless comparison of spatial and temporal variations in deformation rate with time dependent mechanical models can place valuable constraints on fault zone geometry and rheology we consider postseismic strain rate transients by comparing geodetic data from north of san francisco bay obtained between 1906 and 1995 to predictions from viscoelastic finite element models models include 1 an elastic plate over a viscoelastic half space 2 distributed shear within a viscoelastic layer 3 discrete shear zones within an otherwise elastic layer 4 discrete shear zones in combination with distributed viscoelastic shear and 5 midcrustal detachment surfaces we vary as applicable locking depth elastic thickness depth to the top and bottom of the distributed shear layer distributed shear relaxation time discrete shear zone relaxation time and discrete shear zone width the best fitting physically reasonable elastic plate over viscoelastic half space models 1 do a poor job simultaneously predicting spatial and temporal variations in the data the best fitting distributed shear models 2 do a poor job predicting spatial variations in the deformation rate although they fit the geodetic data recent findings from seismic reflection refraction studies in northern california argue against models with shallow subhorizontal detachments 5 models incorporating discrete shear zones 3 4 provide the best fit to the geodetic data and are consistent with seismic studies that argue for discrete fault zones extending through the entire crust

dynamics of izmit earthquake postseismic deformation and loading of the duzce earthquake hypocenter we have developed dynamic finite element models of izmit earthquake postseismic deformation to evaluate whether this deformation is better explained by afterslip via either velocity strengthening frictional slip or linear viscous creep or by distributed linear viscoelastic relaxation of the lower crust we find that velocity strengthening frictional afterslip driven by coseismic shear stress loading can reproduce time dependent global positioning system data better than either linear viscous creep on a vertical shear zone below the rupture or lower crustal viscoelastic relaxation our best frictional afterslip model fits the main features of postseismic slip inversions in particular high slip patches at and below the hypocenter and on the western karadere segment and limited afterslip west of the hersek delta burgmann et al 2002 the model requires a weakly velocity strengthening fault that is either low effective normal stress in the slipping regions or a smaller value for the parameter describing rate dependence of friction a b than is indicated by laboratory experiments our best afterslip model suggests that the coulomb stress at the duzce hypocenter increased by 0 14 mpa 1 4 bars during the izmit earthquake assuming right lateral slip on a surface dipping 50 to the north and by another 0 1 mpa during the 87 days between the izmit and duzce earthquakes in the marmara sea region within about 160 km of the izmit earthquake rupture this model indicates that the coulomb stresses increased by 15 25 of the coseismic amount during the first 300 days after the earthquake three hundred days after the earthquake postseismic contributions to coulomb stressing rate on the maramara region faults had fallen to values equal to or less than the inferred secular stress accumulation rate our estimates of postseismic coulomb stress are highly model dependent in the marmara region the linear viscous shear zone and viscoelastic lower crust models predict greater postseismic coulomb stresses than the frictional afterslip model near field stress and fault zone rheology estimates are sensitive to the earth9s elastic structure when a layered elastic structure is incorporated in our model it yields a coulomb stress of 0 24 mpa at the duzce hypocenter significantly more than the 0 14 mpa estimated from the uniform elastic model because of the higher near field coseismic stresses the layered elastic model requires a higher value of velocity strengthening parameter a b a b times effective normal stress r to produce comparable postseismic slip a b is estimated at 0 4 and 0 2 mpa respectively for the layered and uniform elastic models these results highlight the importance of understanding the earth9s elastic structure and the mechanism for postseismic deformation if we wish to accurately model coseismic and postseismic crustal stresses

coupling between geochemical reactions and multicomponent gas and solute transport in unsaturated media a reactive transport modeling study 1 the two way coupling that exists between biogeochemical reactions and vadose zone transport processes in particular gas phase transport determines the composition of soil gas to explore these feedback processes quantitatively multicomponent gas diffusion and advection are implemented into an existing reactive transport model that includes a full suite of geochemical reactions multicomponent gas diffusion is described on the basis of the dusty gas model which accounts for all relevant gas diffusion mechanisms the simulation of gas attenuation in partially saturated landfill soil covers methane production and oxidation in aquifers contaminated by organic compounds e g an oil spill site and pyrite oxidation in mine tailings demonstrate that both diffusive and advective gas transport can be affected by geochemical reactions methane oxidation in landfill covers reduces the existing upward pressure gradient thereby decreasing the contribution of advective methane emissions to the atmosphere and enhancing the net flux of atmospheric oxygen into the soil column at an oil spill site methane oxidation causes a reversal in the direction of gas advection which results in advective transport toward the zone of oxidation both from the ground surface and the deeper zone of methane production both diffusion and advection contribute to supply atmospheric oxygen into the subsurface and methane emissions to the atmosphere are averted during pyrite oxidation in mine tailings pressure reduction in the reaction zone drives advective gas flow into the sediment column enhancing the oxidation process in carbonate rich mine tailings calcite dissolution releases carbon dioxide which partly offsets the pressure reduction caused by o2 consumption

mineral stimulation of subsurface microorganisms release of limiting nutrients from silicates abstract microorganisms play an important role in the weathering of silicate minerals in many subsurface environments but an unanswered question is whether the mineral plays an important role in the microbial ecology silicate minerals often contain nutrients necessary for microbial growth but whether the microbial community benefits from their release during weathering is unclear in this study we used field and laboratory approaches to investigate microbial interactions with minerals and glasses containing beneficial nutrients and metals field experiments from a petroleum contaminated aquifer where silicate weathering is substantially accelerated in the contaminated zone revealed that phosphorus p and iron fe bearing silicate glasses were preferentially colonized and weathered while glasses without these elements were typically barren of colonizing microorganisms corroborating previous studies using feldspars in laboratory studies we investigated microbial weathering of silicates and the release of nutrients using a model ligand promoted pathway a metal chelating organic ligand 3 4 dihydroxybenzoic acid 3 4 dhba was used as a source of chelated ferric iron and a carbon source to investigate mineral weathering rate and microbial metabolism in the investigated aquifer we hypothesize that microbes produce organic ligands to chelate metals particularly fe for metabolic processes and also form stable complexes with al and occasionally with si further the concentration of these ligands is apparently sufficient near an attached microorganism to destroy the silicate framework while releasing the nutrient of interest in microcosms containing silicates and glasses with trace phosphate mineral inclusions microbial biomass increased indicating that the microbial community can use silicate bound phosphate inclusions the addition of a native microbial consortium to microcosms containing silicates or glasses with iron oxide inclusions correlated to accelerated weathering and release of si into solution as well as the accelerated degradation of the model substrate 3 4 dhba we propose that silicate bound p and fe inclusions are bioavailable and microorganisms may use organic ligands to dissolve the silicate matrix and access these otherwise limiting nutrients

metallic ion binding by bacillus subtilis implications for the fossilization of microorganisms the silicification of bacterial cells has been followed in a laboratory simulation by electron microscopy and energy dispersive x ray spectroscopy structural degradation of the bacteria was not influenced by silica availability but the remains of degraded cells did promote the precipitation of silica in contrast cells reacted with ferric iron before aging remained intact and structures conforming to the original morphology of the bacteria were preserved by silicification the binding of metallic ions by microbial cells in particular the retention of iron is therefore considered to be an important contributing factor to the fossilization of microorganisms

the velocity field along the san andreas fault in central and southern california the velocity field within a 100 km broad zone centered on the san andreas fault between the mexican border and san francisco bay has been inferred from repeated surveys of trilateration networks in the 1973 1989 interval the velocity field has the appearance of a shear flow that remains parallel to the local strike of the fault even through such major deflections as the big bend of the san andreas fault in the transverse ranges of southern california across strike profiles of the fault parallel component of velocity exhibit the expected sigmoidal shape whereas across strike profiles of the fault normal component of velocity are flat and featureless no significant convergence upon the fault is observed even along the big bend sector of the fault simple dislocation models can explain most of the features of the observed velocity field but those explanations are not unique about 35 mm yr of relative plate motion is accounted for within the span of the trilateration networks geologic studies indicate that the secular slip rate on the san andreas fault is about 35 mm yr the agreement between these two estimates implies that most of the strain accumulation is elastic and will be recovered in subsequent earthquakes the relative motion observed across the san andreas fault 35 mm yr plus that observed across the eastern california shear zone 8 mm yr accounts for most 43 mm yr of the observed north america pacific relative plate motion 47 mm yr

the 1989 loma prieta earthquake imaged from inversion of geodetic data we invert geodetic measurements of coseismic deformation from the 1989 ms7 1 loma prieta earthquake to determine the geometry of the fault and the distribution of slip on the fault plane the data include electronic distance measurements global positioning system and very long baseline interferometry vectors and elevation changes derived from spirit leveling the fault is modeled as a rectangular dislocation surface in a homogeneous elastic half space first we assume that the slip on the fault is uniform and estimate the position orientation and size of the fault plane using a nonlinear quasi newton algorithm the best fitting dislocation strikes n48 4 w and dips 76 9 sw consistent with the trend of the aftershock zone and moment tensor solutions bootstrap resampling of the data is used to graphically illustrate the uncertainty in the location of the rupture plane the 95 confidence envelope overlaps the aftershock zone arguing that there is not a significant discrepancy between the geodetic data and the aftershock locations second we estimate the slip distribution using the best fitting uniform slip fault orientation but increase the fault length to 40 km and the downdip width to 18 km the fault is divided into 162 subfaults 18 along strike and 9 along dip each subfault is allowed to have constant right lateral and reverse components of slip we then solve for the slip on each subfault that minimizes a linear combination of the norm of the weighted data residual and the roughness of the slip distribution the smoothing parameter which determines the relative weight put on fitting the data versus smoothing the slip distribution is chosen by cross validation simulations indicate that cross validation estimates of the smoothing parameter are nearly optimal the preferred slip distribution is very heterogeneous with maximum strike slip and dip slip of about 5 and 8 m respectively located roughly 10 km north of the hypocenter there is insignificant dip slip in the southeastern most part of the fault causing the rake to vary from nearly pure right lateral in the southeast to oblique right reverse in the northwest the change in rake is consistent with a uniform stress field if the fault dip increases by about 10 toward the southeast as indicated by the aftershock locations there was little slip above 4 km depth consistent with the observation that there was little if any surface rupture

creep and quakes on the northern transition zone of the san andreas fault from gps and insar data 1 the san juan bautista sjb segment forms the northern transition zone of the creeping section of the san andreas fault it is an area of moderate seismicity the largest instrumentally recorded earthquakes have been m5 5 however historic records suggest six m 6 earthquakes occurred near the sjb segment between 1840 and 1899 it is also an area that has experienced several slow earthquakes we perform a joint inversion of gps and insar data to determine the current rate and distribution of interseismic creep we find two low slip locked segments at mid seismogenic depths that may represent source regions for the 19th century earthquakes we find that the sjb segment is currently accumulating strain energy at the rate of one m6 3 6 7 earthquake per century at this rate the 1840 1899 sequence released 135 510 years of accumulated moment deficit

long term creep rate changes and their causes 1 we discuss measurements of the long term creep rate from the san juan bautista section of the san andreas fault these observations of aseismic slip span a period of approximately half a century making it the longest creep record ever measured along an active fault zone the records show systematic changes in creep rate over time scales of tens of years and we address potential causes of these changes in principle they may be caused either by changes in resistive forces within the fault zone or by variations in plate tectonic driving forces such driving force variations have recently been suggested to have occurred in the same area western north america and within the same time window these should give rise to observable effects in the creep data but we find no evidence for temporal changes in plate tectonic forcing at these time scales a natural explanation for the observed long term changes is elastic reloading of adjacent locked portions of the san andreas fault after large earthquakes and possibly aseismic strain release that occurs before the earthquakes

deformation in the lower crust of the san andreas fault system in northern california a continuous seismic velocity and reflectivity cross section of the san andreas fault system in northern california shows offsets in the lower crust and the mohorovicic discontinuity near the san andreas and maacama strike slip faults these faults may cut through the crust to the upper mantle in a zone less than 10 kilometers wide the northern california continental margin to the eastern edge of the coast ranges is underlain by a high velocity lowermost crustal layer that may have been emplaced within 2 million years following the removal of the gorda plate slab the rapid emplacement and structure within this layer are difficult to reconcile with existing tectonic models

time dependent land uplift and subsidence in the santa clara valley california from a large interferometric synthetic aperture radar data set 1 we invert 115 differential interferograms derived from 47 synthetic aperture radar sar scenes for a time dependent deformation signal in the santa clara valley california the time dependent deformation is calculated by performing a linear inversion that solves for the incremental range change between sar scene acquisitions a nonlinear range change signal is extracted from the ers insar data without imposing a model of the expected deformation in the santa clara valley cumulative land uplift is observed during the period from 1992 to 2000 with a maximum uplift of 41 18 mm centered north of sunnyvale uplift is also observed east of san jose seasonal uplift and subsidence dominate west of the silver creek fault near san jose with a maximum peak to trough amplitude of 35 mm the pattern of seasonal versus long term uplift provides constraints on the spatial and temporal characteristics of water bearing units within the aquifer the silver creek fault partitions the uplift behavior of the basin suggesting that it acts as a hydrologic barrier to groundwater flow while no tectonic creep is observed along the fault the development of a low permeability barrier that bisects the alluvium suggests that the fault has been active since the deposition of quaternary units

postseismic rebound in fault step overs caused by pore fluid flow near field strain induced by large crustal earthquakes results in changes in pore fluid pressure that dissipate with time and produce surface deformation synthetic aperture radar sar interferometry revealed several centimeters of postseismic uplift in pull apart structures and subsidence in a compressive jog along the landers california 1992 earthquake surface rupture with a relaxation time of 270 45 days such a postseismic rebound may be explained by the transition of the poisson s ratio of the deformed volumes of rock from undrained to drained conditions as pore fluid flow allows pore pressure to return to hydrostatic equilibrium

temporal geophysical signatures from contaminant mass remediation we have previously documented changes in bulk electrical conductivity self potential sp and ground penetrating radar gpr reflections in a field setting caused by biogeochemical transformations of hydrocarbon contaminated media these transformations are associated with hydrocarbon biodegradation the results of surface geophysical surveys acquired in 1996 2003 and 2007 document changes in geophysical signatures associated with removing hydrocarbon mass in the contaminated zone initial investigations in 1996 showed that relative to background the contaminated area was characterized by higher bulk electrical conductivity positive sp anomaly and attenuated gpr reflections repeated surveys in 2003 and 2007 over the contaminated area showed that in 2007 the bulk electrical conductivityhad reverted to near background conditions the positive sp anomaly became more negative and the zone of attenuated gpr reflections showed increased signal strength removal of hydrocarbon mass in the vadose zone o

postearthquake relaxation evidence for laterally variable viscoelastic structure and water content in the southern california mantle i reexamine the lower crust and mantle relaxation following two large events in the mojave desert the 1992 m7 3 landers and 1999 m7 1 hector mine california earthquakes time series from continuous gps sites out to 300 km from the ruptures are used to constrain models of postseismic relaxation crustal motions in the mojave desert region are elevated above background for several years following each event to account for broadscale relaxation of the lower crust and mantle the burgers body model is employed involving transient and steady state viscosities joint afterslip postseismic relaxation modeling of the gps time series up to one decade following the hector mine earthquake reveals a significant rheological contrast between a northwest trending southwest domain that envelopes the san andreas fault system and western mojave desert and an adjacent northeast domain that envelopes the landers and hector mine rupture areas in the central mojave desert the steady state viscosity of the northeast domain mantle asthenosphere is inferred to be 4 times greater than that of the southwest domain this pattern is counter to that expected for regional heat flow which is higher in the northeast domain but it is explicable by means of a nonlinear rheology that includes dependence on both strain rate and water concentration i infer that the southwest domain mantle has a relatively low steady state viscosity because of its high strain rate and water content the relatively low mantle water content of the northeast domain is interpreted to result from the continual extraction of water through igneous and volcanic activity over the past 20 myr

postseismic strain following the 1989 loma prieta earthquake from gps and leveling measurements postseismic deformation in the 5 years following the 1989 loma prieta earthquake has been measured with the global positioning system and precise leveling postearthquake velocities at distances greater than 20 km from the coseismic rupture are not significantly different from those observed in the 20 years prior to the earthquake however velocities at stations within 20 km of the rupture exceed preearthquake rates and exhibit unanticipated contraction normal to the strike of the san andreas fault system a combination of forward modeling and nonlinear optimization suggests that the observed postseismic deformations were caused by aseismic oblique reverse slip averaging 2 9 cm yr on the san andreas fault and or the loma prieta rupture zone and 2 4 cm yr reverse slip along a buried fault within the foothills thrust belt the best fitting sources of postseismic deformation are all located at depths of less than 15 km we find no evidence for accelerated flow or shear below the loma prieta rupture in the first 5 years following the earthquake the inferred postseismic slip is likely to have been caused by the coseismic stress change updip of the 1989 rupture

a physical model for strain accumulation in the san francisco bay region stress evolution since 1838 1 understanding of the behavior of plate boundary zones has progressed to the point where reasonably comprehensive physical models can predict their evolution the san andreas fault system in the san francisco bay region sfbr is dominated by a few major faults whose behavior over about one earthquake cycle is fairly well understood by combining the past history of large ruptures on sfbr faults with a recently proposed physical model of strain accumulation in the sfbr we derive the evolution of regional stress from 1838 until the present this effort depends on 1 an existing compilation of the source properties of historic and contemporary sfbr earthquakes based on documented shaking geodetic data and seismic data bakun 1999 and 2 a few key parameters of a simple regional viscoelastic coupling model constrained by recent gps data pollitz and nyst 2004 although uncertainties abound in the location magnitude and fault geometries of historic ruptures and the physical model relies on gross simplifications the resulting stress evolution model is sufficiently detailed to provide a useful window into the past stress history in the framework of coulomb failure stress we find that virtually all m 5 8 earthquakes prior to 1906 and m 5 5 earthquakes after 1906 are consistent with stress triggering from previous earthquakes these events systematically lie in zones of predicted stress concentration elevated 5 10 bars above the regional average the sfbr is predicted to have emerged from the 1906 shadow in about 1980 consistent with the acceleration in regional seismicity at that time the stress evolution model may be a reliable indicator of the most likely areas to experience m 5 5 shocks in the future index terms 1206 geodesy and gravity crustal movements interplate 8155 1236 geodesy and gravity rheology of the lithosphere and mantle 8160 1243 geodesy and gravity space geodetic surveys keywords crustal deformation plate boundary zones viscoelastic relaxation san francisco bay region

postseismic deformation following the 2010 m 7 2 el mayor cucapah earthquake observations kinematic inversions and dynamic models due to its location on a transtensional section of the pacific north american plate boundary the salton trough is a region featuring large strike slip earthquakes within a regime of shallow asthenosphere high heat flow and complex faulting and so postseismic deformation there may feature enhanced viscoelastic relaxation and afterslip that is particularly detectable at the surface the 2010 m 7 2 el mayor cucapah earthquake was the largest shock in the salton trough since 1892 and occurred close to the us mexico border and so the postseismic deformation recorded by the continuous gps network of southern california provides an opportunity to study the rheology of this region three year postseismic transients extracted from gps displacement time series show four key features 1 1 2 cm of cumulative uplift in the imperial valley and sim 1 cm of subsidence in the peninsular ranges 2 relatively large cumulative horizontal displacements 150 km from the rupture in the peninsular ranges 3 rapidly decaying horizontal displacement rates in the first few months after the earthquake in the imperial valley and 4 sustained horizontal velocities following the rapid early motions that were still visibly ongoing 3 years after the earthquake kinematic inversions show that the cumulative 3 year postseismic displacement field can be well fit by afterslip on and below the coseismic rupture though these solutions require afterslip with a total moment equivalent to at least a m 7 2 earthquake and higher slip magnitudes than those predicted by coseismic stress changes forward modeling shows that stress driven afterslip and viscoelastic relaxation in various configurations within the lithosphere can reproduce the early and later horizontal velocities in the imperial valley while newtonian viscoelastic relaxation in the asthenosphere can reproduce the uplift in the imperial valley and the subsidence and large westward displacements in the peninsular ranges we present two forward models of dynamically coupled deformation mechanisms that fit the postseismic transient well a model combining afterslip in the lower crust newtonian viscoelastic relaxation in a localized zone in the lower crust beneath areas of high heat flow and geothermal activity and newtonian viscoelastic relaxation in the asthenosphere and a second model that replaces the afterslip in the first model with viscoelastic relaxation with a stress dependent viscosity in the mantle the rheology of this high heat flow high strain rate region may incorporate elements of both these models and may well be more complex than either of them

interaction of the san andreas fault creeping segment with adjacent great rupture zones and earthquake recurrence at parkfield three dimensional finite element calculations are employed to study interactions in space and time between the creeping segment of the san andreas fault in central california and the adjacent currently locked zones of the 1857 and 1906 great earthquakes vertically the model consists of an elastic upper crust over a maxwell viscoelastic region representing the entire lower crust or a narrower horizontal detachment layer and a stiffer and more viscous upper mantle the crust has a single vertical fault extending to the top of the mantle at 25 km depth in zones along strike corresponding to the 1857 and 1906 events the top 12 5 km of the fault is locked against slip except in great earthquakes below the locked zones and everywhere along the creeping region between them the fault is freely slipping the model parameters are compatible with seismological and geological observations and with a ratio of maxwell relaxation time to the relaxing layer thickness in the range 1 to 2 yr km as established by li and rice 1987 and fares and rice 1988 based on fits to geodetic data along the san andreas fault an imposed constant far field shear motion and periodic 1857 and 1906 type earthquakes generate slip rates along the creeping fault segment that evolve in time throughout the entire earthquake cycle shortly after an adjacent great earthquake slip rates in the creeping zone are higher than the far field velocity while later in the cycle they are lower hence time dependency should be accounted for when measurements of fault slip are used to estimate the plate motion if parkfield earthquakes are a response to a time dependent loading of the type simulated here their recurrence interval would tend to lengthen with time since the 1857 event thus the hypothesis of characteristic periodic earthquakes at parkfield may not provide the best estimate of the occurrence time of the next event using for example the statistics of past events and assuming that parkfield earthquakes are a response to a slip deficit near middle mountain and that the elastic crustal layer is 17 5 km thick we find that the next event is predicted for about 1992 9 years if the lower crust is a 7 5 km thick layer having a material relaxation time of 15 years and 1995 11 years if the 7 5 km thick lower crust is characterized by a relaxation time of 7 5 years these values may be compared to the 1988 7 years estimate based on periodicity in time the modeling results also indicate that the interaction between the 1857 and 1906 rupture zones is small

in situ conditions and interactions between microbes and minerals in fine grained marine sediments a tem microfabric perspective microbes their exocellular secretions and their impact on the mineralogy and microfabric of fine grained continental margin sediments were investigated by transmission electron microscopy techniques were used that retained the in situ spatial relations of both bio organic and mineralogical constituents photomicrographs were taken of characteristic mineral microbe associations in the first meter of burial at conditions ranging from aerobic to anaerobic single celled prokaryotes prokaryotic colonies and eukaryotic organisms were observed as were motile sessile and predatory species bacterial cells dominate the assemblage the most commonly observed mineral biological interaction was the surrounding or close association of isolated heterotrophic bacterial cells by clay minerals almost without exception the external surfaces of the bacteria were covered with secreted exocellular slimes composed of cross linked polysaccharide fibrils these fibrils act to bind sediment grains into relatively robust microaggregates roughly le 25 micro m in diameter these exocellular polymers can significantly impact the interaction between microbes and minerals as well as the chemical and physical transport of fluids and dissolved aqueous species through the sediment although pore water chemical profiles from the field sites studied have dissolved fe and mn no close association was found between the microbes imaged and precipitated metal oxyhydroxides or other authigenic minerals such asmore is commonly reported from laboratory cultures less

liquidus temperatures of the skaergaard magma the liquidus temperatures of the skaergaard intrusion can be estimated for the layered series from plagioclase compositions plagioclase saturation in one atmosphere melting experiments on evolved north atlantic basalts is a function of an content mol and can be described by an empirical linear relationship t c 899 3 6 an 1 20 c this relationship predicts a total crystallization interval for the intrusion of 150 c plagioclase crystallized in the hidden zone an 71 at 1155 c at the base of the lower zone an 66 at 1137 c and finally in the sandwich horizon an 30 at 1007 c these temperatures are in good agreement with previous estimates based on melting studies of suitable chilled margin rocks and gabbros from the skaergaard intrusion and liquid line of descent modeling our temperature estimates however are markedly lower by up to 60 c than recent temperature estimates based on an extrapolation of high pressure experimental data for the kiglapait intrusion to skaergaard emplacement conditions proposed variations in magmatic pressure during skaergaard evolution and reasonable estimates of magmatic water contents do not alter these conclusions

using short term postseismic displacements to infer the ambient deformation conditions of the upper mantle 1 to interpret short term postseismic surface displacements in the context of key ambient conditions e g temperature pressure background strain rate water content creep mechanism we combined steady state and transient flow into a single constitutive relation that can explain the response of a viscoelastic material to a change in stress the flow law is then used to investigate mantle deformation beneath the eastern california shear zone following the 1999 m7 1 hector mine earthquake the flow law parameters are determined using finite element models of relaxation processes constrained by surface displacement time series recorded by 55 continuous gps stations for 7 years following the earthquake results suggest that postseismic flow following the hector mine earthquake occurs below a depth of 50 km and is controlled by dislocation creep of wet olivine diffusion creep models can also explain the data but require a grain size 3 5 mm that is smaller than the inferred grain size 10 20 mm based on the mantle conditions at these depths in addition laboratory flow laws predict dislocation creep would dominate at the stress grain size conditions that provide the best fit to diffusion creep models model results suggest a transient creep phase that lasts 1 year and has a viscosity 10 times lower than subsequent steady state flow in general agreement with laboratory observations the postseismic response is best explained as occurring within a relatively hot upper mantle e g 1200 1300 c at 50 km depth with a long term background mantle strain rate of 0 1 0 2 mstrain yr consistent with the observed surface strain rate long term background shear stresses at the top of the mantle are 4 mpa then decrease with depth to a minimum of 0 1 0 2 mpa at 70 km depth before increasing slowly with depth due to the pressure dependence of viscosity these conditions correspond to a background viscosity of 10 21 pa s within a thin mantle lid that decreases to 5 10 19 pa s within the underlying asthenosphere this study shows the utility of using short term postseismic observations to infer long term mantle conditions that are not readily observable by other means

active detachment faulting in the san francisco bay area measurements of crustal deformation may help determine if mid crustal reflectors imaged at 15 km depth below the san francisco bay area california are active detachment horizons that accommodate strike slip shear and or convergence across the region elastic dislocation models predict distinguishable secular surface displacements for fault geometries involving vertical shear zones or horizontal detachments only where faults step bend or branch along their trace in the southern san francisco bay area the san andreas hayward and calaveras fault zones splay from a single trace of the creeping central segment of the san andreas fault whereas measured horizontal surface displacements in the central and northern bay area are equally well predicted by models of strike slip faults below the major faults and by models involving a detachment a zone of fault normal motion observed in the southern bay area is not well explained by models involving only vertical strike slip faults a model involving detachment slip parallel to the san andreas fault appears more consistent with the observations we find no evidence for significant san andreas fault normal regional contraction or detachment faulting however additional data and better models are required to more confidently constrain the three dimensional active fault architecture

crustal earthquake instability in relation to the depth variation of frictional slip properties recent stability studies using constitutive relations of the type found by dieterich ruina and others to describe frictional slip of rocks in the laboratory have provided a new explanation of the depth cutoff of shallow crustal earthquakes the class of friction laws discussed has the property that the sliding stress depends on normal stress temperature slip rate and slip history for sliding at a fixed slip rate v and fixed environment e g normal stress temperature etc the shear strength evolves toward a steady state value ss v stability analysis show that for d ss v dv 0 velocity strengthening implies stable steady state sliding at least for a broad class of constitutive laws more fully described in the paper experiments by dieterich and by tullis and weeks on westerly granite with mature sliding surfaces indicate that d ss v dv is negative at room temperature whereas higher temperature experiments by stesky show that d ss v dv becomes positive above approximately 300 c therefore in the case of the earth where temperature increases with depth the above observations seem to suggest that the depth cutoff of crustal earthquake activity can be understood in terms of the variation of the frictional response with depth from a regime with d ss v dv 0 this is not inconsistent with but rather refines the suggestion by sibson and others that the depth cutoff is due to a transition from brittle friction to ductile flow further our results show definitively that the shallow depth confinement of seismicity is compatible with a model in which deformation is localized to a fault zone extending well below the seismogenic depth the depth confinement does not require a model showing transition to a zone of broadly distributed creep flow beneath the seismogenic zone following mavko a two dimensional quasi static strike slip fault model is analyzed but using different numerical procedures and a depth variation of frictional properties based on the laboratory data mentioned above and the lachenbruch sass depth variation of temperature for the san andreas fault the resulting predictions of such features as the confinement of crustal earthquakes to shallow depths the development of locked patches the recurrence time for the seismic cycle the seismic stress drop and displacement etc are generally in agreement with the observed characteristics of large scale strike slip earthquakes along the san andreas fault calculations based on the laboratory data predict for example that stick slip nucleates around 5 7 km that large seismic motion occurs around the nucleation depth and above but diminishes gradually to zero at 13 15 km depths that rapid postseismic creep occurs over another 3 4 km depth and that at greater depths steady slip consistent with the average plate velocity is only modestly perturbed by the earthquakes occurring above the most uncertain parameter is the slip weakening distance l for evolution of fault surface state we illustrate how features of the predicted earthquake cycles vary with l over the range for which calculations are feasible

when faults communicate viscoelastic coupling and earthquake clustering in a simple two fault system 1 3 d finite element models of a simplified northern and southern san andreas type fault system are presented with the goal of better understanding how great earthquakes m 7 5 on one major segment of a fault can affect the earthquake cycle on another colinear fault segment separated from the first by an asesimically creeping segment we find that the earthquake cycles of the two seismogenic fault segments become coupled as the lower crustal viscosity and or the fault separation distance are decreased further models with a 10 30 difference in relative fault breaking strengths exhibit a bi modal distribution of repeat times for each fault resulting in earthquakes that appear clustered in time

transient rheology of the uppermost mantle beneath the mojave desert california abstract geodetic data indicate that the m7 1 hector mine california earthquake was followed by a brief period a few weeks of rapid deformation preceding a prolonged phase of slower deformation we find that the signal contained in continuous and campaign global positioning system data for 2 5 years after the earthquake may be explained with a transient rheology quantitative modeling of these data with allowance for transient linear biviscous rheology in the lower crust and upper mantle demonstrates that transient rheology in the upper mantle is dominant its material properties being typified by two characteristic relaxation times 0 07 and 2 years the inferred mantle rheology is a jeffreys solid in which the transient and steady state shear moduli are equal consideration of a simpler viscoelastic model with a linear univiscous rheology 2 fewer parameters than a biviscous model shows that it consistently underpredicts the amplitude of the first 3 months signal and allowance for a biviscous rheology is significant at the 99 0 confidence level another alternative model deep postseismic afterslip beneath the coseismic rupture predicts a vertical velocity pattern opposite to the observed pattern at all time periods considered despite its plausibility the advocated biviscous rheology model is non unique and should be regarded as a viable alternative to the non linear mantle rheology model for governing postseismic flow beneath the mojave desert

chondrules in the qingzhen type 3 enstatite chondrite possible precursor components and comparison to ordinary chondrite chondrules abstract the qingzhen eh3 fall appears to be the least equilibrated enstatite chondrite mineral compositions vary widely from chondrule to chondrule and in some cases from grain to grain within individual chondrules as in ordinary chondrite oc chondrules most elements show appreciable ranges in concentration porphyritic olivine pyroxene pop chondrules are richer than porphyritic pyroxene pp and radial pyroxene rp chondrules in refractory lithophiles and siderophiles unlike oc chondrules qingzhen s rp and pp chondrules have similar bulk compositions elements in each of the following sets intercorrelate significantly probably because these elements were present in the same nebular components 1 1 fe co ni ir au 2 2 ca eu se 3 3 al sc hf 4 4 na ree 5 5 cl br several additional elements show no correlations set 1 elements were probably derived from a metal component as in oc chondrules set 3 elements which occur in higher concentrations in olivine bearing pop chondrules suggest the existence of a refractory lithophile rich olivine rich component similar to the one previously inferred for oc chondrules set 2 elements suggest a precursor component rich in oldhamite and set 4 elements suggest another refractory bearing component components 2 and 4 were not precursors of oc chondrules for some chondrules interelement ratios of refractory lithophiles such as ca al ti sc and ree are similar to ci ratios this indicates that these elements were probably derived from silicates we suggest that the earliest solid phases were refractory silicates that were sulfurized before chondrule formation some of these sulfurized components e g one rich in oldhamite were abundant in the region where the qingzhen chondrules formed

interaction between smectite and bacteria implications for bentonite as backfill material in the disposal of nuclear waste abstract this study presents experimental results on the interaction between smectite clays nontronite and mx80 bentonite and the facultative anaerobic heterotrophic shewanella putrefaciens in two types of conditions i batch experiments with low solid to liquid ratios and agitated oxygenated conditions and ii reaction cell experiments with high solid to liquid ratios in anaerobic confined volume conditions the former was chosen to study the ability of s putrefaciens to live on smectite as the only substrate and the latter was designed to simulate more compacted subsurface environments of underground repository waste sites bacterial cell counts in the batch experiments reveal the prolonged survival of s putrefaciens in the smectite suspension compared to standard laboratory culture media in the case of nontronite variations in solution chemistry indicate bacterial consumption and or partial binding of cations microscopic investigations show associated biofilm smectite aggregates and si rich gels produced by the partial dissolution of clay mineral grains in contrast the mx80 bentonite was not seen to be chemically affected by bacterial activity in batch cultures however the confined volume experiments using reaction cell x ray diffraction combined with peak calculations calcmix do indicate that s putrefaciens has a pronounced effect on the water content of compacted mx80 bentonite the presence of these bacteria enhances both the amount of adsorbed interlayer water and the available pore space the anaerobic conditions were also favourable for accessory phase dissolution notably calcite and synchronous precipitation of lepidocrocite related to bacterially induced changes in ph and eh the varied response of the two studied clays to the presence of bacteria is attributed largely to the materials composition the interlayer ca of nontronite facilitates bacterial attachment to surfaces and fe iii provokes the production of chelators that enhance mineral dissolution although mx80 bentonite is less affected by bacterially enhanced dissolution it is more sensitive to microstructural changes mechanisms involve aggregation of na smectite particles in voids created by cell lyses the initial production of biofilm and the ph and eh dependent dissolution and precipitation of accessory minerals this investigation highlights the importance of including bacteria mineral studies in assessing the safety issue of underground disposal of nuclear waste material

microbial response to surface microtopography the role of metabolism in localized mineral dissolution abstract we examine the role of microtopographical surface features on sulfide minerals in localizing and aligning bacterial adhesion experimental data shows strong correlation between bacterial cell alignment and principal crystallographic axes of pyrite 100 and 110 while bacteria often adhere to visible surface imperfections such as scratches in many cases no associated surface features are visible additionally the size of the surface imperfection does not unambiguously determine its effect in localizing and aligning bacterial cells we theoretically model bacterial adhesion we find that the depth of a surface feature such as a scratch is less important than its cross sectional shape surface features that conform to the bacterial shape can strongly alter local bacterial adhesion energies even with heights of only 10 nm hence small local surface alterations due to bacterial metabolism could strongly affect local adhesion parameters and may account for the observed bacterial distributions on mineral surfaces

parental liquids of the skaergaard intrusion cumulates petrogenetic relationships between compositions advocated as possible liquids parental to the skaergaard layered series cumulates have been investigated using equations for the prediction of liquidus temperature and liquid path followed during fractional crystallization given a suitably defined sequence of primocryst mineral assemblages a variation of the fractionation scheme provides a new method for the calculation of the parent liquid composition

microbial colonization and weathering of silicates in a petroleum contaminated groundwater abstract the influence of native microorganisms on mineral dissolution and precipitation was examined in a petroleum contaminated aquifer near bemidji minnesota in situ microcosms containing clean silicate fragments were established in the contaminated microbially active groundwater over one year periods the recovered minerals were then examined by sem and esem for microbial colonization patterns and weathering features these experiments reveal distinct patterns of colonization and weathering associated with microbial attachment and growth microcline anorthoclase and oligoclase were widely colonized and the colonized surfaces deeply weathered with secondary clays precipitated on some uncolonized surfaces other feldspars in contrast were uncolonized and unweathered specific mineral removal rate was estimated from etch pit depth while bulk weathering rate was estimated from the rate of change of silica concentration in the groundwater in this system the primary control of silicate dissolution is apparently microbial colonization and metabolic activity

postseismic deformation following the 1999 chi chi earthquake taiwan implication for lower crust rheology on 1999 september 21 the mw 7 6 chi chi earthquake ruptured a segment of the chelungpu fault a frontal thrust fault of the western foothills of taiwan the stress perturbation induced by the rupture triggered a transient deformation across the island which was well recorded by a wide network of continuously operating gps stations the analysis of more than ten years of these data reveals a heterogeneous pattern of postseismic displacements with relaxation times varying by a factor of more than ten and large cumulative displacements at great distances in particular along the longitudinal valley in eastern taiwan where relaxation times are also longer we show that while afterslip is the dominant relaxation process in the epicentral area viscoelastic relaxation is needed to explain the pattern and time evolution of displacements at the larger scale we model the spatiotemporal behavior of the transient deformation as the result of afterslip on the decollement that extends downdip of the chelungpu thrust and viscoelastic flow in the lower crust and in the mid crust below the central range we construct a model of deformation driven by coseismic stress change where afterslip and viscoelastic flow are fully coupled the model is compatible with the shorter relaxation times observed in the near field which are due to continued fault slip and the longer characteristic relaxation times and the reversed polarity of vertical displacements observed east of the central range our preferred model shows a viscosity of 0 5 1 x 10 19 pa s at lower crustal depths and 5 x 10 17 pa s in the mid crust below the central range between 10 and 30 km depth the low viscosity zone at mid crustal depth below the central range coincides with a region of low seismicity where rapid advection of heat due to surface erosion coupled with underplating maintain high temperatures estimated to be between 300 c and 600 c from the modeling of thermo chronology and surface heat flow data

frictional properties on the san andreas fault near parkfield california inferred from models of afterslip following the 2004 earthquake the abundance of geodetic and seismic data recording postseismic deformation following the 2004 parkfield earthquake provides an unprecedented opportunity to resolve frictional properties on the parkfield section of the san andreas fault the parkfield segment is a transition between the locked section to the southeast that last ruptured in the 1857 fort tejon earthquake and the creeping section to the northwest we develop three dimensional rate and state dependent friction models of afterslip following the 2004 earthquake to investigate the frictional behavior of the fault it is assumed that the coseismic rupture occurred on an area of the fault surrounded by aseismic creep that accelerated after the earthquake we estimate the distribution of coseismic slip afterslip and rate state frictional parameters by inverting a two step slip model in the model we 1 estimate the coseismic slip distribution from 1 hz global positioning system gps data and 2 use the corresponding coseismic shear stress change on the fault as input into a numerical afterslip model governed by rate state friction we find the rate state frictional parameter a b an indicator of frictional stability is in the range 10 4 10 3 at 50 mpa normal stress which is near the transition from potentially unstable negative a b to nominally stable positive a b friction the estimate of a b values falls within a wide range of experimental values reported for serpentinite which crops out along the san andreas fault zone the critical slip distance d c which characterizes the distance over which strength breaks down during a slip event is in the range 0 01 0 1 m consistent with seismic estimates and a fault gouge thickness of 1 10 m the afterslip model reproduces most features observed in the gps time series data including high surface velocities in the first few months after the earthquake and lower rates at later times as well as the cumulative postseismic displacement the model tends to underpredict the displacement data at later times suggesting that perhaps the modeled afterslip period ends too quickly or an unmodeled deformation process dominates the signal at later times

dissolution of well and poorly ordered kaolinites by an aerobic bacterium abstract previous research by our group e g chem geol 132 1996 25 geochim cosmochim acta 64 2000 1363 has shown that an aerobic pseudomonas mendocina bacterium enhances fe hydr oxide dissolution in order to obtain fe under fe limited conditions the p mendocina is incapable of utilizing fe as a terminal electron acceptor and requires several orders of magnitude lower fe concentrations than do dissimilatory fe reducing bacteria the research reported here compared the effects of the p mendocina on dissolution of well and poorly ordered clay minerals society source clay kaolinites kga 1b and kga 2 respectively under fe limited conditions kga 1b and kga 2 contain 0 04 and 0 94 bulk wt fe respectively and their surface fe si atomic ratios 0 008 and 0 012 following strong cleaning of the kaolinites in 5 8 m hcl at 85 c the surface fe si atomic ratios decreased to 0 004 and 0 008 respectively both kaolinites also developed a si enriched surface precipitate upon strong cleaning because the p mendocina take up fe we could not measure fe release from the kaolinite directly but rather had to monitor it indirectly by comparing microbial populations sizes under fe limited growth conditions we found that microbial growth on uncleaned weakly cleaned and strongly cleaned kaolinites increased with the amount of fe readily available to organic ligands as estimated by dissolution in 0 001 m oxalate ph 3 this suggests that it is the amount of readily accessible fe that controls fe acquisition and hence microbial growth the trend is based on only a relatively small range of kaolinite fe contents and the research thus needs to be expanded to include kaolinites with a broader range of bulk and surface fe concentrations significant enhancement of al release was observed in the presence of the bacteria along with generally some enhancement of si release this enhancement of kaolinite dissolution could be related to an observed ph increase from 7 8 to 9 in the presence of the bacteria and or to production of al chelating agents the p mendocina produce a variety of organic exudates including siderophores chem geol 132 1996 25 geomicrobiology 2001b and further studies into the effects of the siderophores on al complexation and on kaolinite dissolution are ongoing

biogeophysics a new frontier in earth science research 1 biogeophysics is a rapidly evolving earth science discipline concerned with the geophysical signatures of microbial interactions with geologic media it spans the established disciplines of geomicrobiology biogeoscience and geophysics biogeophysics research in the last decade has confirmed the potential for geophysical techniques to measure not simply the physical and chemical properties of the subsurface as already well established but also to detect microbes microbial growth and microbe mineral interactions thus representing a major paradigm shift in geophysical thinking in this review we begin by defining biogeophysics and provide a historical perspective we then consider microbial alterations of petrophysical properties as such alteration is the source of most biogeophysical signals our review then focuses on geophysical interrogation of microbial processes including the direct detection of microbial cells and biofilm formation microbial metabolic by products microbe mediated redox processes and biogeochemical and microbe mineral transformations we conclude by discussing challenges opportunities and potential new applications of biogeophysics to the exploration of life in extreme environments e g the deep biosphere the cryosphere and other planets we find that published biogeophysics studies to date are mostly observation based presenting only empirical relationships between microbial and geophysical variables future research endeavors must focus on developing theoretical and or numerical models for predicting geophysical signals arising from microbial activity

the differentiation of the skaergaard intrusion mcbirney and naslund have missed the two central points that we reiterate for clarity first the composition of the cotectic cumulate extract for the lzc best estimated from the average composition of adcumulate rocks had a substantially lower silica content than any conceivable basaltic liquid so silica enrichment is unavoidable this is true of all the dyke compositions reported by brooks and nielsen even the anomalous rock e with sio2 46 second it is the normal behaviour of tholeiite basaltic magma to produce increasingly iron rich cumulate extracts with little change in silica until feti oxide precipitation occurs when silica enrichment and compositional divergence begins such divergence between equivalent cumulate plutonic and fractionated volcanic rocks hads been widely documented skaergaard magmas do have some uncommon features however similar assemblages exist in volcanic rocks as in the andesitic to rhyolitic glasses of the loch ba ring dyke mull sparks 1988 it is also a basic principle that elements enriched in cumulate rocks often become depleted in the corresponding liquids iron enrichment in the skaergaard cumulates is complementary to iron depletion in liquids

field evidence for geophysical detection of subsurface zones of enhanced microbial activity 1 geochemical data from closely spaced vertical intervals in a hydrocarbon impacted aquifer were used to assess the relationship between bulk conductivity and zones of enhanced microbial activity the bulk conductivity was measured using in situ vertical resistivity probes microbial activity was verified using terminal electron acceptors nitrate sulfate iron and manganese dissolved inorganic carbon dic and major ion chemistry peaks in bulk conductivity in the aquifer overlapped with zones where nitrates and sulfates were depleted total petroleum hydrocarbon iron manganese dissolved ions and dic were elevated suggesting a link between higher electrical conductivity and zones of enhanced microbial activity stimulated by the presence of hydrocarbon thus the subsurface expression of microbial activity is apparently recorded in the bulk conductivity measurements our results argue for combining geophysics with biogeochemistry studies to delineate subsurface zones of enhanced microbial activity

evidence of power law flow in the mojave desert mantle studies of the earth s response to large earthquakes can be viewed as large rock deformation experiments in which sudden stress changes induce viscous flow in the lower crust and upper mantle that lead to observable postseismic surface deformation laboratory experiments suggest that viscous flow of deforming hot lithospheric rocks is characterized by a power law in which strain rate is proportional to stress raised to a power n refs 2 3 most geodynamic models of flow in the lower crust and upper mantle however resort to newtonian linear stress strain rate relations here we show that a power law model of viscous flow in the mantle with n 3 5 successfully explains the spatial and temporal evolution of transient surface deformation following the 1992 landers and 1999 hector mine earthquakes in southern california a power law rheology implies that viscosity varies spatially with stress causing localization of strain and varies temporally as stress evolves rendering newtonian models untenable our findings are consistent with laboratory derived flow law parameters for hot and wet olivine the most abundant mineral in the upper mantle and support the contention that at least beneath the mojave desert the upper mantle is weaker than the lower crust

lithospheric rheology constrained from twenty five years of postseismic deformation following the 1989 mw 6 9 loma prieta earthquake abstract the october 17 1989 m w 6 9 loma prieta earthquake provides the first opportunity of probing the crustal and upper mantle rheology in the san francisco bay area since the 1906 m w 7 9 san francisco earthquake here we use geodetic observations including gps and insar to characterize the loma prieta earthquake postseismic displacements from 1989 to 2013 pre earthquake deformation rates are constrained by nearly 20 yr of usgs trilateration measurements and removed from the postseismic measurements prior to the analysis we observe gps horizontal displacements at mean rates of 1 4 mm yr toward loma prieta mountain until 2000 and 2 mm yr surface subsidence of the northern santa cruz mountains between 1992 and 2002 shown by insar which is not associated with the seasonal and longer term hydrological deformation in the adjoining santa clara valley previous work indicates afterslip dominated in the early 1989 1994 postseismic period so we focus on modeling the postseismic viscoelastic relaxation constrained by the geodetic observations after 1994 the best fitting model shows an elastic 19 km thick upper crust above an 11 km thick viscoelastic lower crust with viscosity of 6 10 18 pa s underlain by a viscous upper mantle with viscosity between 3 10 18 and 2 10 19 pa s the millimeter scale postseismic deformation does not resolve the viscosity in the different layers very well and the lower crustal relaxation may be localized in a narrow shear zone however the inferred lithospheric rheology is consistent with previous estimates based on post 1906 san francisco earthquake measurements along the san andreas fault system the viscoelastic relaxation may also contribute to the enduring increase of aseismic slip and repeating earthquake activity on the san andreas fault near san juan bautista which continued for at least a decade after the loma prieta event

crude oil in a shallow sand and gravel aquifer iii biogeochemical reactions and mass balance modeling in anoxic groundwater abstract crude oil floating on the water table in a sand and gravel aquifer provides a constant source of hydrocarbons to the groundwater at a site near bemidji minnesota the degradation of hydrocarbons affects the concentrations of oxidized and reduced aqueous species in the anoxic part of the contaminant plume that developed downgradient from the oil body the concentrations of fe2 mn2 and ch4 eh measurements and the 13c ratios of the total inorganic c indicate that the plume became more reducing ver a 5 a period however the size of the contaminant plume remained stable during this time field data coupled with laboratory microcosm experiments indicate that benzene and the alkylbenzenes are degraded in an anoxic environment in anaerobic microcosm experiments conducted under field conditions almost complete degradation 98 was observed for benzene in 125 d and for toluene in 45 d concentrations of aqueous fe2 and mn2 increased in these experiments indicating that the primary reactions were hydrocarbon degradation coupled with fe and mn reduction mass transfer calculations on a 40 m flowpath in the anoxic zone downgradient from the oil body indicated that the primary reactions in the anoxic zone are oxidation of organic compounds precipitation of siderite and a ferroan calcite dissolution of iron oxide and outgassing of ch4 and co2 the major difference in the two models presented is the ratio of co2 and ch4 that outgasses both models indicate quantitatively that large amounts of fe are dissolved and reprecipitated as ferrous iron in the anoxic zone of the contaminant plume

direct observations of aluminosilicate weathering in the hyporheic zone of an antarctic dry valley stream abstract this study focused on chemical weathering and bacterial ecology in the hyporheic zone of green creek a mcmurdo dry valley antarctica stream an in situ microcosm approach was used to observe dissolution features on the basal plane surface of muscovite mica four mica chips were buried in december 1999 and dug up 39 d later atomic force microscopy afm of the basal plane surfaces revealed small anhedral 10 a deep etch pits covering 4 of the surfaces from which an approximate basal plane dissolution rate of 8 3 10 18 mol muscovite cm 2 s 1 was calculated on the basis of the geometric surface area for the study period this is an integrated initial dissolution rate on a fresh surface exposed for a relatively brief period over the austral summer and should not be compared directly to other long term field rates the observation of weathering features on mica agrees with previous stream and watershed scale studies in the dry valleys which have demonstrated that weathering occurs where liquid water is present despite the cold temperatures afm imaging of mica surfaces revealed biofilms including numerous small

microbe mineral interactions the impact of surface attachment on mineral weathering and element selectivity by microorganisms one of the major gaps within the field of biogeochemistry is the lack of a detailed and deep understanding of the mechanismbehind the microbial inducement of mineral dissolution the association of

postseismic relaxation theory on the spherical earth abstract postseismic displacements and strains observed at the earth9s surface can be explained through the relaxation of a viscoelastic asthenosphere underlying a purely elastic crust a theory is presented for modeling the effects of postseismic relaxation on a spherically symmetric earth for an earthquake point source located in the upper elastic layer the displacement field is decomposed into its toroidal and spheroidal components a linear maxwell rheology in the viscoelastic layers is assumed enabling the use of the correspondence principle for the solutions of the equations of static equilibrium displacement and strain fields are then calculated using normal mode summation computational results are presented for two simple earthquake sources a strike slip fault and a uniaxial thrust fault the patterns of postseismic displacements and strains are found to depend strongly on both the earth model and earthquake source geometry in particular the elastic plate thickness asthenosphere thickness fault type and fault length each play a major role in determining the spatial pattern of postseismic relaxation effects asthenospheric viscosity controls the temporal pattern of relaxation

postseismic deformation following the 1989 m 7 1 loma prieta california earthquake postseismic deformation along a 90 km profile bisecting the projected surface trace of the coseismic rupture of the 1989 loma prieta earthquake has been monitored by frequent gps surveys for 3 3 years following the earthquake in addition to the expected deformation associated with secular strain accumulation on the san andreas and calaveras faults deformation associated with postseismic readjustment has been detected most of that deformation can be attributed to 1 5 m right lateral and 0 9 m reverse postseismic slip on a 5 km wide downdip extension of the loma prieta rupture in addition there seems to be a 0 1 m postseismic collapse of the loma prieta rupture zone in the direction perpendicular to the plane of the rupture the fault normal n48 e surface displacements plotted as a function of time exhibit a curvature suggesting a relaxation time of about 1 4 years similar plots of the fault parallel n42 w displacement components do not exhibit significant curvature presumably the deformation shown in those plots is dominated by secular strain accumulation along the san andreas and calaveras faults rather than postseismic relaxation

aseismic slip and fault interaction from repeating earthquakes in the loma prieta aftershock zone 1 along creeping sections of the san andreas and other faults small asperities in the fault zone load and fail in characteristic repeating earthquake sequences which can be used as subsurface creepmeters here we use these virtual creepmeters to examine and compare deep slip rates on both the northwestern end of the creeping section of the san andreas fault near san juan bautista and on the nearby subparallel sargent fault while creep on the san andreas increased dramatically due to static stress changes in response to the 1989 loma prieta earthquake the sargent showed very little immediate response consistent with loma prieta finite slip models that put this section of the fault in a region of less than 3 bar coulomb stress increase after about 10 years the san andreas creep rate fell back close to the interseismic rate and variations in creep became coherent in time with the sargent indicating a mutual driving force in the system

time dependent triggered afterslip following the 1989 loma prieta earthquake we employ a modified version of the network inversion filter to investigate time dependent slip following the 1989 loma prieta earthquake previous analysis of global positioning system gps and leveling data suggests afterslip on the loma prieta rupture as well as aseismic slip on a thrust fault northeast of the san andreas fault which we identify with the foothills thrust belt we analyzed 173 daily gps solution files at 62 stations collected from 1989 8 to 1998 3 a total of 1 134 three dimensional relative baseline determinations the observed position changes are assumed to result from secular deformation random benchmark motions and temporally varying fault slip the data reveal temporal variations in slip rate but poorly resolve spatial variations in fault slip the amount of temporal smoothing is estimated by maximum likelihood conditional on this estimate reverse slip on the foothills thrust decays from 45 12 mm yr immediately after the earthquake to zero by 1992 reverse slip on the loma prieta rupture surface decays from 57 11 mm yr to zero by 1994 right lateral slip on the loma prieta rupture surface decays monotonically from 30 10 mm yr to zero by 1994 these results suggest that 1 triggered afterslip can occur off the main rupture zone on adjacent faults 2 shallow afterslip dominated the postseismic deformation for the 8 years following the earthquake and 3 postseismic slip on the foothills thrust may account for a significant portion of its total slip budget

reassessing the 2006 guerrero slow slip event mexico implications for large earthquakes in the guerrero gap in guerrero mexico slow slip events have been observed in a seismic gap where no earthquakes have occurred since 1911 a rupture of the entire gap today could result in a mw 8 2 8 4 earthquake however it remains unclear how slow slip events change the stress field in the guerrero seismic region and what their implications are for devastating earthquakes most earlier studies have relied on a sparse network of global navigation satellite systems measurements here we show that interferometric synthetic aperture radar can be used to improve the spatial resolution we find that slip due to the 2006 slow slip event enters the seismogenic zone and the guerrero gap with 5 cm slip reaching depths as shallow as 12 km we show that slow slip is correlated with a highly coupled region and estimate that slow slip events have decreased the total accumulated moment since the end of the 2001 2002 slow slip event 4 7 years by 50 nevertheless even accounting for slow slip the moment deficit in the guerrero gap increases each year by mw 6 8 the guerrero gap therefore still has the potential for a large earthquake with a slip deficit equivalent to mw 8 15 accumulated over the last century correlation between the slow slip region and nonvolcanic tremor and between slow slip and an ultraslow velocity layer supports the hypothesis of a common source potentially related to high pore pressures

crude oil in a shallow sand and gravel aquifer ii organic geochemistry abstract crude oil spilled from a pipeline break in a remote area of north central minnesota has contaminated a shallow glacial outwash aquifer part of the oil was sprayed over a large area to the west of the pipeline and part of it accumulated in an oil body that floats at the water table to the east of the point of discharge total dissolved organic carbon tdoc concentrations in shallow groundwater collected in the oil spray area reach 16 mg l this is nearly an order of magnitude higher than the tdoc concentrations of native groundwater 2 3mg l the additional tdoc derives from the partial degradation of petroleum residues deposited at the land surface and transported to the aquifer by vertical recharge in the vicinity of the oil body tdoc concentrations in groundwater are 48 mg l 58 of the tdoc being composed of non volatile organic c the majority of the volatile doc 63 is a mixture of low molecular weight saturated aromatic and alicyclic hydrocarbons derived from the oil downgradient from the oil body along the direction of groundwater flow concentrations of all measured constituents of the tdoc pool decrease concentrations begin to decline most rapidly however in the zone where dissolved o 2 concentrations begin to increase 50m downgradient from the leading edge of the oil within the anoxic zone near the oil body removal rates of isometric monoaromatic hydrocarbons vary widely this indicates that the removal processes are mediated mainly by microbiological activity molecular and spectroscopic characterization of the tdoc and its spatial and temporal variation provide evidence of the importance of biogeochemical processes in attenuating petroleum contaminants in this perturbed subsurface environment

feldspars as a source of nutrients for microorganisms phosphorus and nitrogen are essential macronutrients necessary for the survival of virtually all living organisms in groundwater systems these nutrients can be quite scarce and can represent limiting elements for growth of subsurface microorganisms in this study we examined silicate sources of these elements by characterizing the colonization and weathering of feldspars in situ using field microcosms we found that in carbon rich anoxic groundwaters where p and n are scarce feldspars that contain inclusions of p minerals such as apatite are preferentially colonized over similar feldspars without p a microcline from s dakota which contains 0 24 p 2 o 5 but 1 mu mol g nh 4 super was heavily colonized and deeply weathered a similar microcline from ontario which has no detectable p or nh 4 super was barren of attached organisms and completely unweathered after one year anorthoclase 0 28 p 2 o 5 approximately 1 mu mol g nh 4 super was very heavily colonized and weathered whereas plagioclase specimens 0 01 p 1 mu mmol g nh 4 super were uncolonized and unweathered in addition the observed weathering rates are faster than expected based on laboratory rates we propose that this system is particularly sensitive to the availability of p and the native subsurface microorganisms have developed biochemical strategies to aggressively scavenge p or some other essential nutrient such as fe super 3 from resistant feldspars the result of this interaction is that only minerals containing p will be significantly colonized and these feldspars will be preferentially destroyed as the subsurface microbial community scavenges a limiting nutrient

new methods for estimating the spatial distribution of locked asperities and stress driven interseismic creep on faults with application to the san francisco bay area california 1 we introduce new forward and inverse methods for inferring long term fault slip rates interseismic fault creep rates and distribution of locked and creeping patches on faults using geodetic and geologic data the forward model consists of fault bounded blocks in an elastic crust overlying a maxwell viscoelastic mantle interseismic elastic distortion of the blocks is modeled due to periodic locking and unlocking of faults throughout the earthquake cycle patches on the fault are assumed to be either locked during the interseismic period or creeping at constant shear stress we utilize a bayesian probabilistic inversion method to infer the posterior probability distribution of long term interseismic fault slip rates distribution of locked and creeping patches and relative weighting of multiple data sets we illustrate the method with an inversion of a synthetic data set we apply the method to estimate fault slip rates and the distribution of interseismic creep on faults in the san francisco bay area ca using gps derived velocities and geologic measurements of fault slip rates we show that the inferred fault slip rates and areas of the locked regions of faults are sensitive to the assumed viscosity of the upper mantle and the timing of past earthquakes and can be significantly different from values inferred from elastic models that do not include viscous flow considering models with different viscosities inferred fault slip rates on major bay area faults can differ by factors of 1 5 4 0 and the inferred moment accumulation rate can differ by factors of 2 13

evidence for postseismic deformation of the lower crust following the 2004 mw6 0 parkfield earthquake previous studies have shown that postseismic relaxation following the 2004 mw6 0 parkfield ca earthquake is dominated by afterslip however we show that some fraction of the afterslip inferred from kinematic inversion to have occurred immediately below the seismically ruptured area may in fact be a substitute for viscous postseismic deformation of the lower crust using continuous gps and synthetic aperture radar interferometry we estimate the relative contribution of shallow afterslip at depth less than 20km and deeper seated deformation required to account for observed postseismic surface displacements exploiting the possible separation in space and time of the time series of displacements predicted from viscoelastic relaxation we devise a linear inversion scheme that allows inverting jointly for the contribution of afterslip and viscoelastic flow as a function of time we find that a wide range of models involving variable amounts of viscoelastic deformation can fit the observations equally well provided that they allow some fraction of deep seated deformation at depth larger than 20 km these models require that the moment released by postseismic relaxation over 5 years following the earthquake reached nearly as much as 200 of the coseismic moment all the models show a remarkable complementarity of coseismic and shallow afterslip distributions some significant deformation at lower crustal depth 20 26 km is required to fit the geodetic data the condition that postseismic deformation cannot exceed complete relaxation places a constraint on the amount of deep seated deformation the analysis requires an effective viscosity of at least 10 18 pa s of the lower crust assuming a semi infinite homogeneous viscous domain this deep seated deformation is consistent with the depth range of tremors which also show a transient postseismic response and could explain as much as 50 of the total postseismic geodetic moment the remaining fraction being due to afterslip at depth shallower than 20 km lower crustal postseismic deformation could reflect a combination of localized ductile deformation and aseismic frictional sliding

increased mg release rates and related mg isotopic signatures during bacteria phlogopite interactions abstract phlogopite weathering experiments were carried out under aerobic conditions in a closed system to study mg isotopic fractionation during the dissolution and processes involved in the presence of bacterial strains four different bacterial strains with different metabolisms were chosen biotic experiments were performed in batch reactors at 24 c for 12 days in parallel abiotic phlogopite weathering experiments were also performed using organic and nitric acids citric and gluconic acids were used to model the effects of chelating agents produced by bacteria and nitric acid was used to model the effects of acidifying agents the results indicate that both decreases in ph and the production of metabolites during heterotrophic bacterial activities significantly accelerate the release of mg and si in solution also at a given ph the fraction of elements released in solution is greater in the presence of bacteria or citric acid compared to that released in the presence of nitric and gluconic acids magnesium isotopic analyses indicate that the solutions obtained using nitric acid and acidifying bacterial strains display on average 26mg values that are close to or slightly heavier by 0 4 than those of fresh phlogopite 1 2 0 08 in contrast in experiments performed with citric acid the mg leached into solution is slightly enriched in light isotopes by 0 3 relative to the initial phlogopite while its binding to organics is expected to be related to its preferential enrichment in heavy isotopes despite its small range of variations the 26mg values of solutions vary inversely with ph thus suggesting that secondary phases preferentially enriched in 26mg at the highest ph values may play a key role

slicing up the san francisco bay area block kinematics and fault slip rates from gps derived surface velocities received 26 october 2004 revised 11 february 2005 accepted 10 march 2005 published 16 june 2005 i observations of surface deformation allow us to determine the kinematics of faults in the san francisco bay area we present the bay area velocity unification bavu bay view a compilation of over 200 horizontal surface velocities computed from campaign style and continuous global positioning system gps observations from 1993 to 2003 we interpret this interseismic velocity field using a three dimensional block model to determine the relative contributions of block motion elastic strain accumulation and shallow aseismic creep the total relative motion between the pacific plate and the rigid sierra nevada great valley sngv microplate is 37 9 0 6 mm yr 1 directed toward n30 4 w 0 8 at san francisco 2 fault slip rates from our preferred model are typically within the error bounds of geologic estimates but provide a better fit to geodetic data notable right lateral slip rates in mm yr 1 san gregorio fault 2 4 1 0 west napa fault 4 0 3 0 zone of faulting along the eastern margin of the coast range 5 4 1 0 and mount diablo thrust 3 9 1 0 of reverse slip and 4 0 0 2 of right lateral strike slip slip on the northern calaveras is partitioned between both the west napa and concord green valley fault systems the total convergence across the bay area is negligible poles of rotation for bay area blocks progress systematically from the north america pacific to north america sngv poles the resulting present day relative motion cannot explain the strike of most bay area faults but fault strike does loosely correlate with inferred plate motions at the time each fault initiated

joint estimation of afterslip rate and postseismic relaxation following the 1989 loma prieta earthquake global positioning system gps data from campaigns carried out over the 5 years following the 1989 loma prieta earthquake and leveling data measured in 1990 and 1992 define the postseismic velocity field around the loma prieta rupture zone subtraction of a background velocity field yields a residual velocity pattern which we interpret as the product of two physical processes 1 slow afterslip along distinct planes in the upper cru st and 2 viscoelastic relaxation of the lower crust and upper mantle biirgmann et al 1997 previously derived an afterslip model involving uniform afterslip on two optimally determined planes in cluding oblique reverse slip on the coseismic rupture and reverse slip on a shallow thrust fault to the northeast of the san andreas fault we further consider models of distributed slip on these two fault planes plus a viscoelastic relaxation pattern which depends on a suitable cose ismic rupture model and crust and mantle viscosities several fault models from the literature were considered for the 1989 coseismic rupture with nearly identical impact on the results simultaneous maximum likelihood inversion of the gps and leveling data for afterslip distri bution and viscosity yields the following results 1 a good fit to the data is obtained by smooth afterslip distributions without any viscoelastic relaxation being required 2 tangible broad scale viscoelastic relaxation of the lower crust and upper mantle are present in this data set at 97 confidence and a lower crustal viscosity of 1029 pa s is obtained however the viscosity of both the lower crust and upper mantle are poorly constrained 3 for a given misfit 20 less integrated afterslip and smoother afterslip distributions result when viscoelas tic relaxation is included 4 maximum slip rates on the slip distributed models are 3 5 crn yr the dominant patches estimated on the two planes fill in the entire depth range 4 13 km without significant overlap and deeper afterslip is not required the afterslip distribution on the coseismic rupture plane is strongly dominated by reverse slip immediately southeast of the main center of coseismic reverse slip

etch pit formation on iron silicate surfaces during siderophore promoted dissolution abstract understanding the effects of microbiota on mineral alteration requires the ability to recognize evidence of bacteria promoted dissolution on mineral surfaces although siderophores are known to promote mineral dissolution their effects on mineral surfaces are not well known we have utilized atomic force microscopy afm x ray photoelectron spectroscopy xps and mirau vertical scanning interferometry vsi to investigate surfaces after incubation with the siderophore desferrioxamine b mesylate dfam and under colonies of bacteria iron silicate glass planchets chemically similar to hornblende were incubated in buffered growth medium with siderophore producing bacteria bacillus sp for 46 days with parallel abiotic experiments conducted with and without 240 m dfam with and without 0 01 g l 1 of microbially produced extracellular polysaccharides eps alginate or xanthan gum some glass planchets were protected by dialysis tubing from direct contact with the eps weekly sampling and analysis of all filtered sample solutions showed negligible fe and al release in the control experiments and significant release of fe and al in the presence of dfam with negligible changes in ph concentration of fe in the filtered solutions after incubation with bacteria was below detection consistent with uptake of fe by cells release of fe al and si in control xanthan only and alginate only experiments was negligible release of these elements was enhanced in all experiments containing dfam and greatest in alginate dfam experiments afm and vsi analyses reveal widespread small etch pits and greater root mean squared roughness on siderophore exposed surfaces and fewer localized larger etch pits on bacteria exposed surfaces this is the first documented case of etch pit development during siderophore promoted dissolution roughness was not affected by the growth medium alginate or xanthan gum alone the roughness trends among samples correlate with trends in fe depletion documented by xps enhanced dissolution and roughness cannot be attributed to direct contact with eps because no significant chemical or physical differences were observed between surfaces directly exposed to eps and those protected by dialysis tubing acetate released from the eps may have enhanced the siderophore promoted dissolution siderophores produced by bacillus sp may be responsible for some of the biopits the difference in size and distribution of the biopits may be related to colonization