crustal structure of a transform plate boundary san francisco bay and the central california continental margin wide angle seismic data collected during the bay area seismic imaging experiment provide new glimpses of the deep structure of the san francisco bay area block and across the offshore continental margin san francisco bay is underlain by a veneer 300 m of sediments beneath which p wave velocities increase rapidly from 5 2 km s to 6 0 km s at 7 km depth consistent with rocks of the franciscan subduction assemblage the base of the franciscan at 15 18 km depth is marked by a strong wide angle reflector beneath which lies an 8 to 10 km thick lower crust with an average velocity of 6 75 0 15 km s the lower crust of the bay area block may be oceanic in origin but its structure and reflectivity indicate that it has been modified by shearing and or magmatic intrusion wide angle reflections define two layers within the lower crust with velocities of 6 4 6 6 km s and 6 9 7 3 km s prominent subhorizontal reflectivity observed at near vertical incidence resides principally in the lowermost layer the top of which corresponds to the 6 s reflector of brocher et al 1994 rheological modeling suggests that the lower crust beneath the 6 s reflector is the weakest part of the lithosphere the horizontal shear zone suggested by furlong et al 1989 to link the san andreas and hayward calaveras fault systems may actually be a broad zone of shear deformation occupying the lowermost crust a transect across the continental margin from the paleotrench to the hayward fault shows a deep crustal structure that is more complex than previously realized strong lateral variability in seismic velocity and wide angle reflectivity suggests that crustal composition changes across major transcurrent fault systems pacific oceanic crust extends 40 50 km landward of the paleotrench but contrary to prior models probably does not continue beneath the salinian block a cretaceous arc complex that lies west of the san andreas fault in the bay area the thickness 10 km and high lower crustal velocity of pacific oceanic crust suggest that it was underplated by magmatism associated with the nearby pioneer seamount the salinian block consists of a 15 km thick layer of velocity 6 0 6 2 km s overlying a 5 km thick high velocity 7 0 km s lower crust that may be oceanic crust cretaceous arc derived lower crust or a magmatically underplated layer the strong structural variability across the margin attests to the activity of strike slip faulting prior to and during development of the transcurrent pacific north american plate boundary around 29 ma

structure and mechanics of the san andreas san gregorio fault junction san francisco california 1 the right lateral san gregorio and san andreas faults meet west of the golden gate near san francisco coincident seismic reflection and refraction profiling across the san gregorio and san andreas faults south of their junction shows the crust between them to have formed shallow extensional basins that are dissected by parallel strike slip faults we employ a regional finite element model to investigate the long term consequences of the fault geometry over the course of 2 3 m y of slip on the san andreas san gregorio fault system elongated extensional basins are predicted to form between the two faults an additional consequence of the fault geometry is that the san andreas fault is expected to have migrated eastward relative to the san gregorio fault we thus propose a model of eastward stepping right lateral fault formation to explain the observed multiple fault strands and depositional basins the current manifestation of this process might be the observed transfer of slip from the san andreas fault east to the golden gate fault

stress transfer by the 2008 mw 6 4 achaia earthquake to the western corinth gulf and its relation with the 2010 efpalio sequence central greece we investigate the interaction between transform faults and normal faults in western greece based on seismological analysis and static stress transfer calcula tions associated with the 8 june 2008 mw 6 4 achaia earthquake we present a re located earthquake catalog for the period between june 2008 and january 2010 when two normal faulting events on 18 mw 5 3 and 22 mw 5 2 january 2010 occurred at efpalio western corinth gulf they were located approximately 70 km northeast from the buried right lateral fault identified as the causative structure of the achaia earthquake the first efpalio event ruptured a mapped normal fault that trends east northeast west southwest dipping 55 60 to the south we estimate 2 fold seismic ity rate changes in the western corinth gulf region for the interseismic period june 2008 january 2010 and we find that inside this interval the monthly event rate re mained increased at a 2 significance level we calculate a coulomb stress increase 0 1 0 6 bar in the efpalio region using optimally oriented for failure planes and an 0 11 bar coulomb stress increase at the hypocenters of the january 2010 events when incorporating geologically defined receiver planes we conclude that the pos itive static stress changes following the achaia event promoted the observed spatio temporal clustering in the corinth gulf for this specific period we identify fault unclamping due to normal stress reduction as the physical mechanism in this case the high seismic hazard character of the target region 0 24g in the national building code emphasizes the importance of time dependent earthquake probabilities and stress mediated fault interaction studies

dipping san andreas and hayward faults revealed beneath san francisco bay california the san francisco bay area is crossed by several right lateral strike slip faults of the san andreas fault zone fault plane reflections reveal that two of these faults the san andreas and hayward dip toward each other below seismogenic depths at 60 and 70 respectively and persist to the base of the crust previously a horizontal detachment linking the two faults in the lower crust beneath san francisco bay was proposed the only near vertical incidence reflection data available prior to the most recent experiment in 1997 were recorded parallel to the major fault structures when the new reflection data recorded orthogonal to the faults are compared with the older data the highest amplitude reflections show clear variations in moveout with recording azimuth in addition reflection times consistently increase with distance from the faults if the reflectors were horizontal reflection moveout would be independent of azimuth and reflection times would be independent of distance from the faults the best fit solution from three dimensional traveltime modeling is a pair of high angle dipping surfaces the close correspondence of these dipping structures with the san andreas and hayward faults leads us to conclude that they are the faults beneath seismogenic depths if the faults retain their observed dips they would converge into a single zone in the upper mantle 45 km beneath the surface although we can only observe them in the crust

age and impacts of the caldera forming aniakchak ii eruption in western alaska the mid holocene eruption of aniakchak volcano aniakchak ii in southwest alaska was among the largest eruptions globally in the last 10 000 years vei 6 despite evidence for possible impacts on global climate the precise age of the eruption is not well constrained and little is known about regional environmental impacts a closely spaced sequence of radiocarbon dates at a peatland site over 1000 km from the volcano show that peat accumulation was greatly reduced with a hiatus of approximately 90 120 yr following tephra deposition during this inferred hiatus no paleoenvironmental data are available but once vegetation returned the flora changed from a cyperaceae dominated assemblage to a poaceae dominated vegetation cover suggesting a drier and or more nutrient rich ecosystem oribatid mites are extremely abundant in the peat at the depth of the ash and show a longer term increasingly wet peat surface across the tephra layer the radiocarbon sample immediately below the tephra gave a date of 1636 1446 cal yr bc suggesting that the eruption might be younger than previously thought our findings suggest that the eruption may have led to a widespread reduction in peatland carbon sequestration and that the impacts on ecosystem functioning were profound and long lasting

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

the impact and significance of tephra deposition on a holocene forest environment in the north cascades washington usa high resolution palaeoecological analyses stratigraphy tephra geochemistry radiocarbon dating pollen and ordination were used to reconstruct a holocene vegetation history of a watershed in the pacific northwest of america to evaluate the effects and duration of tephra deposition on a forest environment and the significance of these effects compared to long term trends three tephra deposits were detected and evaluated mlf t158 and mlc t324 from the climactic eruption of mount mazama mlc t480 from a late pleistocene eruption of mount mazama and mlc t485 from a glacier peak eruption records were examined from both the centre and fringe of the basin to elucidate regional and local effects the significance of tephra impacts independent of underlying long term trends was confirmed using partial redundancy analysis tephra deposition from the climactic eruption of mount mazama approximately 7600 cal years bp caused a significant local impact reflected in the fringe location by changes to open habitat vegetation cyperaceae and poaceae and changes in aquatic macrophytes myriophyllum spicatum potamogeton equisetum and the alga pediastrum there was no significant impact of the climactic mazama tephra or other tephras detected on the pollen record of the central core changes in this core are potentially climate driven overall significant tephra fall was demonstrated through high resolution analyses indicating a local effect on the terrestrial and aquatic environment but there was no significant impact on the regional forest dependent of underlying environmental changes

timing and climate forcing of volcanic eruptions for the past 2 500 years volcanic eruptions contribute to climate variability but quantifying these contributions has been limited by inconsistencies in the timing of atmospheric volcanic aerosol loading determined from ice cores and subsequent cooling from climate proxies such as tree rings here we resolve these inconsistencies and show that large eruptions in the tropics and high latitudes were primary drivers of interannual to decadal temperature variability in the northern hemisphere during the past 2 500 years our results are based on new records of atmospheric aerosol loading developed from high resolution multi parameter measurements from an array of greenland and antarctic ice cores as well as distinctive age markers to constrain chronologies overall cooling was proportional to the magnitude of volcanic forcing and persisted for up to ten years after some of the largest eruptive episodes our revised timescale more firmly implicates volcanic eruptions as catalysts in the major sixth century pandemics famines and socioeconomic disruptions in eurasia and mesoamerica while allowing multi millennium quantification of climate response to volcanic forcing

major volcanic eruptions and climate a critical evaluation abstract this paper examines whether major volcanic eruptions of the past century have had a significant impact on surface land and ocean temperatures surface pressure and precipitation both multieruption composites and individual eruption time series are constructed and analyzed included in this work is an attempt to remove one source of interannual variability the el nino southern oscillation enso these exercises indicate that only the largest eruptions in terms of producing a stratospheric dust cloud are suggested in the climatic record removing the enso signal in the composite and individual eruption series enhances the apparent volcanic effect of the largest eruptions no volcanic signal is obvious in pressure and precipitation records

revising midlatitude summer temperatures back to a d 600 based on a wood density network annually resolved and millennium long reconstructions of large scale temperature variability are primarily composed of tree ring width trw chronologies changes in ring width however have recently been shown to bias the ratio between low and high frequency signals to overcome limitations in capturing the full spectrum of past temperature variability we present a network of 15 maximum latewood density mxd chronologies distributed across the northern hemisphere extratropics independent subsets of continental scale records consistently reveal high mxd before 1580 and after 1910 with below average values between these periods reconstructed extratropical summer temperatures reflect not only these long term trends but also distinct cooling pulses after large volcanic eruptions in contrast to trw dominated reconstructions this mxd based record indicates a delayed onset of the little ice age by almost two centuries the reduced memory inherent in mxd is likely responsible for the rapid recovery from volcanic induced cooling in the fourteenth century and the continuation of warmer temperatures until 1600

diatom inferred ecological responses of an oceanic lake system to volcanism and anthropogenic perturbations since 1290 ce abstract the impacts of natural and human induced processes on lake ecosystems in remote oceanic islands remain to be fully elucidated these lakes are excellent candidates to analyze the importance of anthropogenic vs natural forces driving lacustrine long term ecological evolution from previous pristine pre colonized conditions disentangling the effects of both is particularly relevant in highly active volcanic areas where catastrophic eruptions can act as an atypical natural driver altering the lake s long term ecological trajectories in this paper we study past ecological changes occurring in lake azul sao miguel island a crater lake from the remote azorean archipelago to address which were the main causes of its long term trophic history we analyzed diatom assemblages sedimentology and bulk organic matter of sediments deposited since ca 1290 ce when a huge local eruption occurred this episode drove the evolution of lake azul through six distinct phases commencing with a restart of ecological succession after tephra deposition disrupted biogeochemical cycling the alteration was so profound that the lake underwent a state of oligotrophic conditions for approx 650 yr nutrients were sourced by fish induced internal recycling and the overflow of the near lake verde during this period rather than by allochthonous nutrient inputs modulated by climate variability and or vegetation cover changes in the watershed after the official portuguese colonization it was only after recent artificial fertilization when the system overcame the volcanic induced long term resilience this over fertilization and a reduction in water turnover exacerbated the recent symptoms of eutrophication after 1990 ce contrary to other studies lake azul constitutes an uncommon case of long term resilience to trophic change induced by a cataclysmic volcanic eruption it brings new insights into the fate of lake ecosystems which might be affected by similar events in the future

the impact of the laacher see tephra on terrestrial and aquatic ecosystems in the black forest southern germany late glacial lake sediments containing the laacher see tephra lst 11 000 yr bp have been analysed for pollen and diatoms at two sites hirschenmoor rotmeer in the montane belt of the black forest southern germany in order to detect the possible impact of this major volcanic eruption on terrestrial and aquatic biota the pollen assemblages at both sites show a minor increase in grasses and sedges following the tephra deposition whereas the pollen of aquatic macrophytes do not show any consistent pattern partial redundancy analysis and monte carlo permutation tests suggest however that the lst had no statistically significant effect either on the terrestrial or aquatic pollen assemblages at either site when the effects of time and climatic change are allowed for statistically the diatom assemblages at both sites changed after the deposition of the lst and diatom inferred ph estimates suggest a short but non significant excursion towards lower values multivariate analyses of the diatom data indicate a statistically significant impact of the deposition of the lst and associated changes in sediment lithology on the diatom assemblages when the effects of climatic change and time are allowed for statistically however owing to the interaction between tephra and lithology it is not possible to conclude if the diatom changes were caused by the deposition of the lst or by lithological changes

a 3 d viscoelastoplastic model for simulating long term slip on non planar faults summary the geometrical complexity of non planar faults is known to affect not only fault slip rates but also crustal deformation and seismicity in the surrounding region but implementing non planar faults in numerical models has been challenging we have developed a 3 d viscoelastoplastic finite element model to simulate long term steady state fault slip along conceptual restraining bends and crustal deformation around them by incorporating plastic yielding in both fault zones and the surrounding crust this model can predict the quasi steady state stress field and strain partitioning while avoiding pathological stress build up encountered in traditional viscoelastic models here we detail the formulation of this model and explore major model parameters the model results show that restraining bends tend to impede fault slip increase shear stress in the surrounding crust and localize strain in a pair of belts off the fault zone these effects are enhanced by high viscosity in the lower crust and upper mantle high aspect ratio of bend width over fault length fast relative motion of fault blocks and high fault friction these model results may help explain the diffuse deformation surrounding the big bend of the san andreas fault and the more confined deformation around the lebanon bend of the dead sea fault

computer simulations of california tectonics confirm very low strength of major faults recent improvements in the technique of modeling fault networks with thin plate finite elements yield full convergence of the solutions without any compromise in the representation of the frictional rheology of the upper crust or the transition to dislocation creep in the lower crust we apply these techniques to model the california region incorporating all faults with estimated slip rate over 1 mm yr as well as variations in elevation heat flow and crustal thickness velocity boundary conditions on the model sides are based on the nuvel 1 plate model and an approximation of deformation in the great basin the frictional and dislocation creep rheologic constants of the crust are calibrated to reproduce the observed variations in the maximum depth of seismicity which occurs at model temperatures of 350 410 c this leaves two free parameters the time averaged coefficient of friction on faults and the apparent activation energy for creep in the lower crust these parameters are systematically varied in three sets of 10 81 and 64 experiments respectively the predictions of each model are tested against three published data sets a set of 79 geologic limits on average slip rate on faults a set of 221 principal stress directions and a set of 841 secular rates of geodetic baselines both trilateration and vlbi very long baseline interferometry the patterns of model scores indicate that the time averaged friction coefficient of major faults is only 0 17 0 25 or only 20 30 of the value 0 85 that is assumed for the friction in the intervening blocks the laser trilateration data taken alone would indicate a preference for a high stress model but this model is one with very little fault slip and seismicity which can be rejected on other grounds in the final three parameter set of 64 models we test whether the anomalous weakness of faults is uniform or proportional to net slip with slip dependent weakening there is a general reduction in prediction errors which supports the hypothesis the geologic data and geodetic data agree in implying that fault weakness that is friction of 0 17 is general with an additional 30 slip dependent weakening of the san andreas to friction of 0 12 or 14 of normal the stress data however are fit best if all weakness is slip dependent effectively only the san andreas is weak thus the hypothesis that all weakening is slip dependent should not be rejected the best models predict all available data with rms root mean square mismatch of as little as 3 mm yr 6 of the relative plate velocity so their predictions may be useful for seismic hazard estimation at least along faults where no data are available this study extends the finding of very low friction from subduction zones where it was previously documented to a dominantly strike slip system in which underthrusting of wet sediment is not widespread return of previously subducted water from the mantle and its chemical or physical binding in fine grained gouge are possible explanations for the very low friction

block modeling of crustal deformation of the northern walker lane and basin and range from gps velocities 1 we infer rates of crustal deformation in the northern walker lane nwl and western basin and range using data from the mobile array of gps for nevada transtension and other continuous gps networks including the earthscope plate boundary observatory we present 224 new gps velocities correct them for the effects of viscoelastic postseismic relaxation and use them to constrain a block model to estimate fault slip rates the data segregate the nwl into domains based on differences in deformation rate pattern and style deformation is transtensional with highest rates near the western and eastern edges of the nwl some basins e g tahoe experience shear deformation and extension normal slip is distributed throughout the nwl and basin and range where 11 subparallel range bounding normal fault systems have an average horizontal extension rate of 0 1 mm yr comparison between geologic and geodetic slip rates indicates that out of 12 published geologic rates 10 agree with geodetic rates to within uncertainties this suggests that smaller crustal blocks move steadily similar to larger lithospheric plates and that geodetic measurements of slip rates are reliable in zones of complex crustal deformation for the two slip rates that disagree geologic rates are greater the vertical axis rotation rate of the carson domain is 1 3 0 1 my clockwise lower than the 3 to 6 my obtained in paleomagnetic measurements this suggests that vertical axis rotation rates may have decreased over the last 9 13 my as the role of faulting has increased at the expense of rigid rotations

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

spatially heterogeneous diapycnal mixing in the abyssal ocean a comparison of two parameterizations to observations 1 the spatial distributions of the diapycnal diffusivity predicted by two abyssal mixing schemes are compared to each other and to observational estimates based on microstructure surveys and large scale hydrographic inversions the parameterizations considered are the tidal mixing scheme by jayne st laurent and co authors jsl01 and the roughness diffusivity model rdm by decloedt and luther comparison to microstructure surveys shows that both parameterizations are conservative in estimating the vertical extent to which bottom intensified mixing penetrates into the stratified water column in particular the jsl01 exponential vertical structure function with fixed scale height decays to background values much nearer topography than observed jsl01 and rdm yield dramatically different horizontal spatial distributions of diapycnal diffusivity which would lead to quite different circulations in ogcms yet they produce similar basin averaged diffusivity profiles both parameterizations are shown to yield smaller basin mean diffusivity profiles than hydrographic inverse estimates for the major ocean basins by factors ranging from 3 up to over an order of magnitude the canonical 10 4 m2 s 1abyssal diffusivity is reached by the parameterizations only at depths below 3 km power consumption by diapycnal mixing below 1 km of depth between roughly 32 s and 48 n for the rdm and jsl01 parameterizations is 0 40 tw 0 28 tw respectively the results presented here suggest that present day mixing parameterizations significantly underestimate abyssal mixing in conjunction with other recently published studies a plausible interpretation is that parameterizing the dissipation of bottom generated internal waves is not sufficient to approximate the global spatial distribution of diapycnal mixing in the abyssal ocean

spatial variability of turbulent mixing in the abyssal ocean ocean microstructure data show that turbulent mixing in the deep brazil basin of the south atlantic ocean is weak at all depths above smooth abyssal plains and the south american continental rise the diapycnal diffusivity there was estimated to be less than or approximately equal to 0 1 x 10 4 meters squared per second in contrast mixing rates are large throughout the water column above the rough mid atlantic ridge and the diffusivity deduced for the bottom most 150 meters exceeds 5 x 10 4 meters squared per second such patterns in vertical mixing imply that abyssal circulations have complex spatial structures that are linked to the underlying bathymetry

on a simple empirical parameterization of topography catalyzed diapycnal mixing in the abyssal ocean abstract the global spatial distribution of the turbulent diapycnal diffusivity in the abyssal ocean is reexamined in light of the growing body of microstructure data revealing bottom intensified turbulent mixing in regions of rough topography a direct and nontrivial implication of the observed intensification is that the diapycnal diffusivity k is depth dependent and patchily distributed horizontally across the world s oceans theoretical and observational studies show that bottom intensified mixing is dependent upon a variety of energy sources and processes whose contributions to mixing are sufficiently complex that their physical parameterization is premature only rudimentary parameterizations of tidally induced mixing have been attempted although the tides likely provide no more than half of the mechanical energy available for diapycnal mixing in the abyssal ocean here an empirical and still rudimentary parameterization of the spatially variable mean diffusivity k based on a large collection

mount mazama eruption calendrical age verified and atmospheric impact assessed geochemical identification of mount mazama ash in the greenland ice sheet project 2 gisp2 ice core gives a calendrical age of 7627 150 cal yr b p 5677 150 b c for the eruption thus providing a more accurate early holocene stratigraphic time line than previously available the gisp2 record of volcanically derived sulfate suggests a total stratospheric aerosol loading between 88 and 224 mt spread over an 6 yr period following the eruption of mount mazama taking into account the likelihood of some tropospheric aerosol transport to greenland realistic estimates of the resulting atmospheric optical depth range from 0 6 to 1 5 these values may have produced a temperature depression of 0 6 to 0 7 c at mid to high northern latitudes for 1 3 yr after the eruption these results indicate that the 5677 b c eruption of mount mazama was one of the most climatically significant volcanic events of the holocene in the northern hemisphere we also calculate a maximum stratospheric cl release of 8 1 mt by the eruption which may have led to substantial stratospheric ozone depletion

fine ash content of explosive eruptions abstract in explosive eruptions the mass proportion of ash that is aerodynamically fine enough to cause problems with jet aircraft or human lungs 50

distal volcanic impacts on peatlands palaeoecological evidence from alaska abstract despite the fact that volcanic ash tephra layers are found preserved in peat deposits around the world comparatively little research has investigated the impacts of distal volcanic emissions on peatlands this study investigates the impacts of several late holocene volcanic eruptions on five peatlands in southern alaska using a palaeoecological approach testate amoebae analysis peat humification analysis and a basic analysis of plant macrofossil components were applied across 11 tephra layers changes in macrofossil and testate amoebae assemblages occur across several of the tephra layers the humification results were considered unreliable because of a methodological problem a finding which may have implications for other studies using this technique redundancy analyses on testate amoebae data show statistically significant changes associated with two tephras the most likely causes of the impacts are volcanic gases acidic precipitation or tephra derived leachates the finding that some tephras are associated with impacts whereas others are not may relate to the season of the eruption or meteorological conditions at the time of ash fall these results suggest the sensitivity of peatlands and peatland microbial communities to distal volcanic products and imply that changes in key palaeoclimatic proxies may be caused by a mechanism independent of climate change implications of the results for peat based palaeoclimatic studies are discussed as are possible directions for future research

an earthquake instability model based on faults containing high fluid pressure compartments it has been proposed that large strike slip faults such as the san andreas contain water in seal bounded compartments arguments based on heat flow and stress orientation suggest that in most of the compartments the water pressure is so high that the average shear strength of the fault is less than 20 mpa we propose a variation of this basic model in which most of the shear stress on the fault is supported by a small number of compartments where the pore pressure is relatively low as a result the fault gouge in these compartments is compacted and lithified and has a high undisturbed strength when one of these locked regions fails the system made up of the neighboring high and low pressure compartments can become unstable material in the high fluid pressure compartments is initially underconsolidated since the low effective confining pressure has retarded compaction as these compartments are deformed fluid pressure remains nearly unchanged so that they offer little resistance to shear the low pore pressure compartments however are overconsolidated and dilate as they are sheared decompression of the pore fluid in these compartments lowers fluid pressure increasing effective normal stress and shear strength while this effect tends to stabilize the fault it can be shown that this dilatancy hardening can be more than offset by displacement weakening of the fault i e the drop from peak to residual strength if the surrounding rock mass is sufficiently compliant to produce an instability slip will propagate along the fault until the shear fracture runs into a low stress region frictional heating and the accompanying increase in fluid pressure that are suggested to occur during shearing of the fault zone will act as additional destabilizers however significant heating occurs only after a finite amount of slip and therefore is more likely to contribute to the energetics of rupture propagation than to the initiation of the instability

how geometrical constraints contribute to the weakness of mature faults increasing evidence that the san andreas fault has low shear strength1 has fuelled considerable discussion regarding the role of fluid pressure in controlling fault strength byerlee2 3 and rice4 have shown how fluid pressure gradients within a fault zone can produce a fault with low strength while avoiding hydraulic fracture of the surrounding rock due to excessive fluid pressure it may not be widely realised however that the same analysis2 4 shows that even in the absence of fluids the presence of a relatively soft gouge layer surrounded by harder country rock can also reduce the effective shear strength of the fault as shown most recently by byerlee and savage5 as the shear stress across a fault increases the stress state within the fault zone evolves to a limiting condition in which the maximum shear stress within the fault zone is parallel to the fault which then slips with a lower apparent coefficient of friction than the same material unconstrained by the fault here we confirm the importance of fault geometry in determining the apparent weakness of fault zones by showing that the apparent friction on a sawcut granite surface can be predicted from the friction measured in intact rock given only the geometrical constraints introduced by the fault surfaces this link between the sliding friction of faults and the internal friction of intact rock suggests a new approach to understanding the microphysical processes that underlie friction in brittle materials

physical models of rifting and transform faulting due to ridge push in a wedge shaped oceanic lithosphere by scaled physical modelling we have investigated the mechanical response to gravitational forces in an oceanic lithosphere overlying a less dense asthenosphere in the models an upper wedge shaped layer of sand represented an oceanic lithosphere 0 35 ma old with a half spreading velocity of 3 cm yr and a lower layer of polydimethylsiloxane pdms mixed with dense wolframite powder represented the asthenosphere in the models as in nature isostatic compensation resulted in uplift of ridges and subsidence on their flanks the resulting relief was responsible for ridge push we tested two main configurations straight ridges and offset ridges in all the models ridge push was sufficient to cause plate motion underlying advection and symmetrical rifting at the ridge axis there was no need to impose plate motions through external pistons and motors in models of straight ridges the style of normal faults in the axial rift zone depended on the local thickness of the brittle sand layer for thick layers normal faults rafted out from the active zone of rifting creating a fossil topography of tilted blocks between faults dipping toward the ridge in a model of an offset ridge with thin lithosphere at the ridge crest and no embedded weakness ridge push was responsible for a short transform fault linking en echelon rifts in a similar model but with thick lithosphere an oblique rift formed at about 20 to the offset trace we conclude that ridge push was not adequate to create an ideal transform fault in a model of an offset ridge with an embedded thin vertical layer of pure pdms at 90 to the ridge transform motion concentrated along this weak layer and the resulting structural style was very similar to that in nature on the basis of these results we infer that in nature 1 ridge push can indeed drive plate motion and 2 ridge push can drive strike slip motion on transform faults provided that these are weaker than the adjacent oceanic lithosphere and that they form early in the history of spreading

the sensitivity of a tanzanian crater lake to catastrophic tephra input and four millennia of climate change diatom genera in many large east african lakes change little throughout the holocene period suggesting relatively stable ecological conditions and some resilience to environmental change ecosystem stability is less common in smaller more sensitive lakes such as those within volcanic craters where external impacts can cause abrupt and rapid fluctuations a 4100 year diatom and cyanobacteria pigment record from lake massoko a volcanic crater lake in southern tanzania is used to illustrate important switches in resource ratios following tephra deposition 1190 years ago it is hypothesized that the tephra reduced the rate of p diffusion from the sediments and increased the si p ratio in the lake a period of acute change in planktonic diatom communities resulted from the tephra impact and lasted c 110 years the magnitude of the change shown by the diatoms and their slow recovery from the tephra may be due in part to a coincident fall in lake level caused by a reduction in regional rainfall the statistical significance of the tephra impact relative to that of catchment and climate change has been tested using variance partitioning and rate of change analysis multiproxy indicators show an important period of positive water balance 1700 ago and a relatively dry episode persisting between 1000 and 400 years ago the lake ecosystem is shown to be highly sensitive to both climate change and tephra deposition

dispersion of the volcanic sulfate cloud from a mount pinatubo like eruption 1 we use the geos 5 general circulation model to simulate the transport of the volcanic cloud from an eruption similar to the 1991 eruption of mount pinatubo the simulated aerosol optical thickness and transport of the volcanic cloud are in good agreement with observations of the actual pinatubo eruption from the stratospheric aerosol and gas experiment ii sage ii and the advanced very high resolution radiometer avhrr and with vertical profiles of sulfur dioxide observed by the microwave limb sounder mls we tested the importance of initial conditions corresponding to the specific meteorological situation at the time of the eruption by comparing results when geos 5 is initialized using modern era retrospective analyses for research and applications merra reanalysis fields with results when it is initialized from an existing model run we found no significant difference in the transport of the cloud we show how the inclusion of the interaction between volcanic sulfate aerosol and radiation is essential for a reliable simulation of the transport of the volcanic cloud the absorption of longwave radiation by the volcanic sulfate largely induces the rising of the volcanic cloud up to the middle stratosphere and the divergent motion from the latitude of the eruption to the tropics our simulations indicate that the cloud is transported to the northern hemisphere through a lower stratospheric pathway and to middle and high latitudes of the southern hemisphere through a middle stratospheric pathway centered at about 30 hpa the direction of the middle stratospheric pathway depends on the season of the eruption

high precision ultra distal holocene tephrochronology in north america far travelled volcanic ashes tephras from holocene eruptions in alaska and the pacific northwest have been traced to the easternmost extent of north america providing the basis for a new high precision geochronological framework throughout the continent through tephrochronology the dating and correlation of tephra isochrons in sedimentary records the reported isochrons are geochemically distinct with seven correlated to documented sources in alaska and the cascades including the mazama ash from oregon w7600 years old and the eastern lobe of the white river ash from alaska w1150 years old these findings mark the beginning of a tephrochronological framework of enhanced precision across north america with applications in palaeoclimate surface process and archaeological studies the particle travel distances involved up to w7000 km also demonstrate the potential for continent wide or trans atlantic socio economic disruption from similar future eruptions

oceanic isopycnal mixing by coordinate rotation abstract current numerical models of oceanic circulation differentiate between the eddy diffusion and viscosity transport along the geopotential horizontal and vertical directions only in order to model the effect of anisotropic turbulence as diffusive transport along and across density surfaces the isopycnal mixing tensor has been transformed from a diagonal second rank tensor in the isopycnal coordinate system to a tensor containing off diagonal elements in the geopotential coordinate system

diatom inferred aquatic impacts of the mid holocene eruption of mount mazama oregon high resolution diatom stratigraphies from mid holocene sediments taken from fringe and central locations in moss lake a small lake in the foothills of the cascade range washington have been analyzed to investigate the impacts and duration of tephra deposition on the aquatic ecosystem up to 50 mm of tephra was deposited from the climactic eruption of mount mazama 7958 7795 cal yr bp with coincident changes in the aquatic ecosystem the diatom response from both cores indicates a change in habitat type following blanket tephra deposition with a decline in tychoplanktonic fragilaria brevistriata and staurosira venter and epiphytic diatom taxa indicating a reduction in aquatic macrophyte abundance additionally the central core shows an increase in tychoplanktonic aulacoseira taxa interpreted as a response to increased silica availability following tephra deposition partial redundancy analysis however provides only limited evidence of direct effects from the tephra deposition and only from the central core but significant effects from underlying environmental changes associated with climatic and lake development processes the analyses highlight the importance of duplicate analyses fringe and central cores and vigorous statistical analyses for the robust evaluation of aquatic ecosystem change

stress near geometrically complex strike slip faults application to the san andreas fault at cajon pass southern california slip on an undulatory strike slip fault induces predictable residual stresses in the adjacent crust elastic analytic and finite element models are developed to quantify these stresses for arbitrary fault geometries across fault parallel planes domains of reverse normal right lateral and left lateral residual stresses are induced near the bends in the fault these residual stresses increase in magnitude from one slip event to the next typically reaching failure level after several events a two dimensional analytic elastic plate model for slip on a sinusoidal fault provides general results on the scales and patterns of slip induced stresses and fourier synthesis allows for the solution with arbitrary but small fault distortions maximum fault normal residual stress is proportional to the square of the wavenumber of the sinusoidal trace and decays exponentially away from the fault reduced to 2 e times the maximum value at a distance equal to 1 wavenumber this fault parallel residual shear stress oscillates between dextral and sinistral along the fault with maximum magnitude adjacent to the maximum excursions in the fault trace fault parallel residual shear stress is maximum at a distance from the fault of 1 wavenumber where its magnitude is 1 e times the maximum normal stress on the fault two and three dimensional finite element analyses extend the analytic model they account for depth decaying fault perturbation amplitude and slip deficit and are valid for large fault undulations application of the model to the san andreas fault in the cajon pass region produces the complex distribution of fault parallel normal reverse right lateral and left lateral structures recognized near the main trace left lateral stresses on planes subparallel to the san andreas fault at the cajon pass well are predicted by this model

earthquakes as a coupled shear stress high pore pressure dynamical system the migration coalescence and localization of slip seismicity and zones of high pore pressure are modeled using a porosity reduction mechanism to drive pore pressure within a fault zone in excess of hydrostatic increased pore pressure in discrete cells creates zones of low effective stress which induces slip that may propagate to surrounding cells depending on the local state of stress at slip stress is transferred using the solution for a rectangular dislocation in an elastic half space and pore pressures are redistributed by conserving fluid mass using simple assumptions about fault rheology and permeability it is shown that the interaction between shear stress and effective stress evolves to a state of earthquake clustering with repeated events locked zones and large variations in fault strength the model evolves from a uniform shear stress state on a strong fault to a heterogeneous shear stress state on a weak fault

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

laboratory results indicating complex and potentially unstable frictional behavior of smectite clay a central problem in explaining the apparent weakness of the san andreas and other plate boundary faults has been identifying candidate fault zone materials that are both weak and capable of hosting earthquake like unstable rupture our results demonstrate that smectite clay can be both weak and velocity weakening at low normal stress 30 mpa our data are consistent with previous work which has focused on higher normal stress conditions 50 mpa and greater and found only velocity strengthening if natural fault zones contain significant smectite one key implication of our results is that localized zones of high pore pressure which reduce effective normal stress could be important in controlling potential sites of earthquake nucleation our experiments indicate that friction of smectite is complex and depends upon both sliding velocity and normal stress this complexity highlights the need for detailed experiments that reflect in situ conditions for fault gouges

climatic and volcanic forcing revealed in a 50 000 year diatom record from lake massoko tanzania the interactions between climatic and volcanic forcing on diatom communities contained in a 50 000 year sedimentary sequence from lake massoko tanzania were examined at the century scale 19 discrete tephra inputs to the lake isolated the sedimentary nutrient supply and shifted the diatom communities to those tolerant of low phosphorus levels whereas at the millennial scale diatom inferred shifts in precipitation evaporation based on conductivity optima and diatom life form ratios were broadly similar to lake level reconstructions from lake rukwa lake malawi and others in the region some fluctuations of lake massoko are consistent with the precession driven changes in insolation but the major climate shifts do not relate directly to orbital forcing of summer insolation south of the equator and show more consistency with records from the equatorial and northern tropics that receive rainfall from the passing of the intertropical convergence zone sea surface temperatures are strongly correlated to multimillennial scale climate patterns over this region of africa

the immediate environmental effects of tephra emission the earth s history is punctuated by large explosive eruptions that eject large quantities of magma and silicate rock fragments into the atmosphere these tephra particles can sometimes be dispersed across millions of square kilometres or even entire continents the interaction of tephra with or in receiving environments may induce an array of physical chemical and biological effects the consequences for affected systems and any dependent communities may be chronic and localised in the event of frequent small eruptions while larger and rarer events may have acute regional scale impacts it is therefore necessary to document the range of possible impacts that tephra may induce in receiving environments and any resulting effects in interconnected systems we collate results from many studies to offer a detailed multidisciplinary and interdisciplinary review of the immediate post eruptive effects of tephra emission into the atmosphere onto vegetation soil or ice snow surfaces and in aquatic systems we further consider the repercussions that may be induced in the weeks to years afterwards in the atmosphere tephra can influence cloud properties and air chemistry by acting as ice nuclei in or by offering sites for heterogeneous reactions respectively tephra on vegetation causes physical damage and sustained coverage may elicit longer term physiological responses tephra deposits on soils may alter their capacity to exchange gas water and heat with the atmosphere or may have a specific chemical effect such as nutrient input or acidification on sensitive soils tephra deposition onto snow or ice may affect ablation rates rivers and lakes may experience turbidity increases and changes in their morphology as a result of fallout and prolonged months or years erosion from the tephra covered catchment in the first weeks after deposition tephra leaching may affect river chemistry the abundance and speciation of phytoplankton populations in lakes may be altered by tephra induced changes in water chemistry or sediment water nutrient cycling in the oceans tephra deposition may fertilise fe limited waters with potential impacts on the global carbon cycle embracing the full complexity of environmental effects caused by tephra fall demands a renewed investigative effort drawing on interdisciplinary field and laboratory studies combined with consideration of the interconnectivity of induced impacts within and between different receiving environments

estimates of volcanic induced cooling in the northern hemisphere over the past 1 500 years explosive volcanism can alter global climate and hence trigger economic political and demographic change1 2 the climatic impact of the largest volcanic events has been assessed in numerous modelling studies and tree ring based hemispheric temperature reconstructions3 4 5 6 however volcanic surface cooling derived from climate model simulations is systematically much stronger than the cooling seen in tree ring based proxies suggesting that the proxies underestimate cooling7 8 and or the modelled forcing is unrealistically high9 here we present summer temperature reconstructions for the northern hemisphere from tree ring width and maximum latewood density over the past 1 500 years we also simulate the climate effects of two large eruptions in ad 1257 and 1815 using a climate model that accounts explicitly for self limiting aerosol microphysical processes3 10 our tree ring reconstructions show greater cooling than reconstructions with lower spatial coverage and based on tree ring width alone whereas our simulations show less cooling than previous simulations relying on poorly constrained eruption seasons and excluding nonlinear aerosol microphysics our tree ring reconstructions and climate simulations are in agreement with a mean northern hemisphere extra tropical summer cooling over land of 0 8 to 1 3 c for these eruptions this reconciliation of proxy and model evidence paves the way to improved assessment of the role of both past and future volcanism in climate forcing

radiocarbon age constraints on rates of advance and retreat of the puget lobe of the cordilleran ice sheet during the last glaciation abstract calibrated radiocarbon dates of organic matter below and above till of the last fraser glaciation provide limiting ages that constrain the chronology and duration of the last advance retreat cycle of the puget lobe in the central and southeastern puget lowland seven dates for wood near the top of a thick proglacial delta have a weighted mean age of 17 420 90 cal yr b p which is the closest limiting age for arrival of the glacier near the latitude of seattle a time distance curve constructed along a flowline extending south from southwestern british columbia to the central puget lowland implies an average glacier advance rate of ca 135 m yr the glacier terminus reached its southernmost limit ca 16 950 yr ago and likely remained there for ca 100 yr in the vicinity of seattle where the glacier reached a maximum thickness of 1000 m ice covered the landscape for ca 1020 yr postglacial dates constraining the timing of ice retreat in the central lowland are as old as 16 420 cal yr b p and show that the terminus had retreated to the northern limit of the lowland within three to four centuries after the glacial maximum the average rate of retreat was about twice the rate of advance and was enhanced by rapid calving recession along flowline sectors where the glacier front crossed deep proglacial lakes

an improved simulation of the deep pacific ocean using optimally estimated vertical diffusivity based on the green s function method an improved vertical diffusivity scheme is introduced into an ocean general circulation model to better reproduce the observed features of water property distribution inherent in the deep pacific ocean structure the scheme incorporates a a horizontally uniform background profile b a parameterization depending on the local static stability and c a parameterization depending on the bottom topography weighting factors for these parameterizations are optimally estimated based on the green s function method the optimized values indicate an important role of both the intense vertical diffusivity near rough topography and the background vertical diffusivity this is consistent with recent reports that indicate the presence of significant vertical mixing associated with finite amplitude internal wave breaking along the bottom slope and its remote effect the robust simulation with less artificial trend of water properties in the deep pacific ocean illustrates that our approach offers a better modeling analysis for the deep ocean variability

a high precision age estimate of the holocene plinian eruption of mount mazama oregon usa the climactic eruption of mount mazama in oregon north america resulted in the deposition of the most widespread holocene tephra deposit in the conterminous united states and south western canada the tephra forms an isochronous marker horizon for palaeoenvironmental sedimentary and archaeological reconstructions despite the current lack of a precise age estimate for the source eruption previous radiocarbon age estimates for the eruption have varied and greenland ice core ages are in disagreement for the mazama tephra to be fully utilised in tephrochronology and palaeoenvironmental research a refined precise and accurate age for the eruption is required here we apply a meta analysis of all previously published radiocarbon age estimations n 81 and perform bayesian statistical modelling to this data set to provide a refined age of 7682 7584 cal yr bp 95 4 probability range although the depositional histories of the published ages vary this estimate is consistent with that estimated from the gisp2 ice core of 7627 150 yr bp zdanowicz et al 1999

sensitivity of the gfdl ocean general circulation model to a parameterization of vertical diffusion abstract a coarse resolution primitive equation general circulation model with idealized geometry and forcing is used to explore sensitivity to the assumption that vertical diffusion depends upon local stability a case with constant diffusivity is compared with a case in which the diffusivity is inversely proportional to the local brunt v frequency the stability dependent parameterization of vertical diffusivity yields a poleward heat flux similar to that of a small constant diffusivity however this parameterization increases the mean temperature in the deep ocean by about 0 8 c and the strength of the meridional circulation by over 40 in addition the stability dependent diffusivity is found to increase the stratification in the deep ocean the experiments suggest that it may be possible to calibrate the rate of deep water formation of general circulation models without affecting the poleward heat transport by varying the magnitude of the vertical diffusivity below the thermocline the explicit

global mapping of diapycnal diffusivity in the deep ocean based on the results of expendable current profiler xcp surveys 1 based on the results of the extensive xcp surveys in the north pacific we first examine the empirical relationship between the estimated diapycnal diffusivity in the thermocline and the numerically predicted available energy density of the semidiurnal internal tide major energy source for diapycnal diffusivity at each location the diapycnal diffusivity is estimated by applying the parameterization of gregg 1989 to the fine scale vertical shear of horizontal velocity measured by xcp next the numerically predicted energy density of the semidiurnal internal tide at each longitude and latitude in the world s oceans is incorporated into the resulting empirical relationship to obtain the global distribution of diapycnal diffusivity in the thermocline this model predicts that strong diapycnal mixing mixing hotspot is limited to the prominent topographic features at latitudes between 20 and 30

seismic evidence for a lower crustal detachment beneath san francisco bay california results from the san francisco bay area seismic imaging experiment basix reveal the presence of a prominent lower crustal reflector at a depth of 15 kilometers beneath san francisco and san pablo bays velocity analyses indicate that this reflector marks the base of franciscan assemblage rocks and the top of a mafic lower crust because this compositional contrast would imply a strong rheological contrast this interface may correspond to a lower crustal detachment surface if so it may represent a subhorizontal segment of the north america and pacific plate boundary proposed by earlier thermo mechanical and geological models

lacustrine responses to tephra deposition examples from mexico one of the major problems with palaeoclimate investigations in volcanic regions is that tephra inputs to lakes can cause changes in proxies analogous to those of climate forcing we review the range of impacts thought to be associated with tephra deposition distinguishing between direct effects on lake ecosystems and indirect changes to catchment nutrient cycles to achieve better understanding of these complex responses we have used high resolution diatom analysis from around 17 tephra layers in three mexican lakes a positive response to the tephra inputs has been identified for over half of these layers the most common response was for diatom concentrations to increase after tephra deposition also in plankton dominated systems fragilaria spp replaced stephanodiscus spp and aulacoseira spp an increase in the supply of silica to the lakes through their catchments is probably the cause of the diatom changes in the lakes studied direct effects of the tephra inputs to these lakes can be excluded as they would not generate changes of sufficient longevity the impact of these tephras which lasted for several decades was insufficient to perturb the underlying long term climate forcing of these lake systems c 2004 elsevier ltd all rights reserved

strength of chrysotile serpentinite gouge under hydrothermal conditions can it explain a weak san andreas fault chrysotile bearing serpentinite is a constituent of the san andreas fault zone in central and northern california at room temperature chrysotile gouge has a very low coefficient of friction 0 2 raising the possibility that under hydrothermal conditions might be reduced sufficiently to 0 1 to explain the apparent weakness of the fault to test this hypothesis we measured the frictional strength of a pure chrysotile gouge at temperatures to 290 c and axial shortening velocities as low as 0 001 m s as temperature increases to 100 c the strength of the chrysotile gouge decreases slightly at low velocities but at temperatures 200 c it is substantially stronger and essentially independent of velocity at the lowest velocities tested we estimate that pure chrysotile gouge at hydrostatic fluid pressure and appropriate temperatures would have shear strength averaged over a depth of 14 km of 50 mpa thus on the sole basis of its strength chrysotile cannot be the cause of a weak san andreas fault however chrysotile may also contribute to low fault strength by forming mineral seals that promote the development of high fluid pressures

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

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

use of paleo records in determining variability within the volcanism climate system abstract volcanic eruptions that inject large quantities of sulfur rich gases into the stratosphere have the capability of cooling global climate by 0 2 0 3 c for several years after the eruption equatorial eruptions will impact global climate whereas mid latitude eruptions can cool climate in the hemisphere of origin magnitude of cooling varies by latitude and it is possible for warming to occur in certain regions primarily during the winter although instrumental records have been used to quantify the volcanic impact on climate they are limited in their temporal and spatial coverage and the style and magnitude of eruptions occurring over the two centuries of instrumental records is limited a thorough evaluation of the range of variability in the volcanism climate system requires a multidisciplinary approach that includes the analysis of ice core records geological data atmospheric measurements and visual phenomena tree ring records and other proxy data evaluation of these longer time series indicates that multiple volcanic eruptions have the potential to force climate over decadal to multi decadal time frames especially when these eruptions enhance or extend pre existing cool conditions on the other hand a lack of climatically significant eruptions may result in warmer average temperatures over decadal time frames because the volcanic cooling component within the climate system is absent mega eruptions like the toba eruption of 71 000 73 000 yr ago may impact climate on centennial time frames through positive feedback mechanisms evidence exists which indicates that environmental changes associated with rapid climatic fluctuations such as crustal loading unloading with glaciation deglaciation and variability in glacial meltwater loading on ocean basins may cause an increase in volcanic activity

geodetic estimates of fault slip rates in the san francisco bay area bourne et al 1998 have suggested that the interseismic velocity profile at the surface across a transform plate boundary is a replica of the secular velocity profile at depth in the plastosphere on the other hand in the viscoelastic coupling model the shape of the interseismic surface velocity profile is a consequence of plastosphere relaxation following the previous rupture of the faults that make up the plate boundary and is not directly related to the secular flow in the plastosphere the two models appear to be incompatible if the plate boundary is composed of several subparallel faults and the interseismic surface velocity profile across the boundary known each model predicts the secular slip rates on the faults which make up the boundary as suggested by bourne et al the models can then be tested by comparing the predicted secular slip rates to those estimated from long term offsets inferred from geology here we apply that test to the secular slip rates predicted for the principal faults san andreas san gregorio hayward calaveras rodgers creek green valley and greenville faults in the san andreas fault system in the san francisco bay area the estimates from the two models generally agree with one another and to a lesser extent with the geologic estimate because the viscoelastic coupling model has been equally successful in estimating secular slip rates on the various fault strands at a diffuse plate boundary the success of the model of bourne et al 1998 in doing the same thing should not be taken as proof that the interseismic velocity profile across the plate boundary at the surface is a replica of the velocity profile at depth in the plastosphere

mechanical analysis of controls on strain partitioning in the himalayas of central nepal we present a mechanical analysis of the problem of slip partitioning between the major thrust systems in a collisional range we focus on two structures in the himalayas of central nepal the main himalayan thrust mht and the main central thrust mct we use finite element modeling to test the influence of various parameters such as friction coefficients and surface processes and we investigate how they affect the distribution of deformation between these two faults we observe that reproduction of the late quaternary kinematic pattern across the range with our model requires strict conditions on the friction coefficients such that the mht is very weak whereas the mct is significantly stronger the most important parameter that controls slip partitioning appears to be the dip angle of the mct with a gentler or steeper mct promoting or inhibiting slip respectively we also show that transient loading and unloading through focused glacial erosion in the higher part of the range can unclamp the mct and allow a significant increase in slip rates the results of this mechanical sensitivity investigation have important implications for the dynamics of the himalayan wedge and point toward along strike structural variations as a first order control on slip partitioning

the role of stratification dependent mixing for the stability of the atlantic overturning in a global climate model using the climber 3 coupled climate model of intermediate complexity the effect of a stratification dependent vertical diffusivity on the sensitivity of the atlantic ocean meridional overturning circulation to perturbations in freshwater forcing is investigated the vertical diffusivity is calculated as n where n is the local buoyancy frequency and the parameter is a measure of the sensitivity of the vertical diffusivity to changes in stratification independent of the stratification of the deep ocean is weakly increased as a response to an anomalous freshwater flux in the north atlantic in these experiments in the region of freshwater forcing and north of it this is caused by the freshwater anomaly itself but farther south it is associated with anomalously warm surface waters caused by a reduction of the northward oceanic heat transport subsequently and in opposition to results from previous studies the overturning is reduced by the anomalous freshwater flux independent of the choice of however the amount of reduction in overturning following a freshwater perturbation is found to depend critically on the choice of the mixing sensitivity i f cr the response is similar to the model s response using constant vertical diffusivity 0 for cr a sharp increase of the sensitivity is found the value of cr is found to be between 0 5 and 1 a general feedback is proposed explaining this threshold behavior if is large both positive and negative perturbations of stratification are amplified by associated changes in diffusivity in the experiments presented here this enhances the initial positive stratification anomaly in northern high latitudes which is created by the anomalous freshwater flux as a result convection is strongly reduced and the overturning is significantly weakened

stress and deformation along wavy frictional faults locally inhomogeneous stress states are expected along faults owing to slip on geometrically irregular fault surfaces we use an analytical model of elastic deformation along a wavy frictional fault to evaluate the variation in local stress state as a function of surface roughness elastic modulus slip coefficient of friction and far field stress the total stress state along the fault may be described by the sum of a basic stress component resulting from frictional slip on a planar fault surface and a perturbed stress component resulting from the presence of roughness roughness produces a variation in normal stress across the fault surface and assuming roughness and modulus appropriate to crustal faults the normal stress should be reduced to a near zero magnitude locally such that separation of fault walls is likely the large variation in normal stress along the fault surface resulting from fault roughness may be responsible in part for complexity in moment release during large earthquakes and for lateral variation in seismic coupling along faults the variation in principal stress orientations and magnitudes along a fault increases with a decrease in the coefficient of friction of the fault the location and size of regions with a high likelihood for brittle failure depend on the orientation of the far field principal stress and fault friction the average orientation of the principal stresses in the region of likely failure is not the same as the far field principal stress orientation although inversion of earthquake and fabric data for stress orientation along a fault may be possible the model results suggest that inversion results are insufficient to determine far field stress states and fault friction without additional independent data

field evidences for the role of static friction on fracture orientation in extensional relays along strike slip faults comparison with photoelasticity and 3 d numerical modeling fault friction is a parameter that is difficult to assess along fault zones since its determination depends on the knowledge of any factor controlling the state of stress around faults in brittle homogeneous rocks a limited number of these factors such as the shape of the fault surface the vicinity of fault tips or the remote stress ratio are crucial to constrain for this determination in this paper we propose to analyse a field example in which all these properties are met and where the nature of the slipped structure suggest differences in static friction we compare the orientations of branching fractures at strike slip relay zones between en echelon stylolites and en echelon joints both reactivated in shear the field data are compared with both photoelastic and 3 d numerical models that consider the remote stress conditions and the role of the geometry of the strike slip segments based on field observations these analyses quantitatively demonstrate the significant role of fault friction on the local stress field orientation and subsequent fracture formation this work points out that estimations of fault friction based on analyses of fracture patterns or in situ stresses must be accompanied with a thorough investigation of the 3 d fault shape its segmentation and the remote stress state

slip rates on san francisco bay area faults from anelastic deformation of the continental lithosphere long term slip rates on major faults in the san francisco bay area are predicted by modeling the anelastic deformation of the continental lithosphere in response to regional relative plate motion the model developed by bird and kong 1994 is used to simulate lithospheric deformation according to a coulomb frictional rheology of the upper crust and a dislocation creep rheology at depth the focus of this study is the long term motion of faults in a region extending from the creeping section of the san andreas fault to the south up to the latitude of cape mendocino to the north boundary conditions are specified by the relative motion between the pacific plate and the sierra nevada great valley microplate argus and gordon 2000 rheologic frictional parameters are specified as independent variables and prediction errors are calculated with respect to geologic estimates of slip rates and maximum compressive stress directions the model that best explains the region wide observations is one in which the coefficient of friction on all of the major faults is less than 0 15 with the coefficient of friction for the san andreas fault being approximately 0 09 consistent with previous inferences of san andreas fault friction prediction error increases with lower fault friction on the san andreas indicating a lower bound of saf 0 08 discrepancies with respect to previous slip rate estimates include a higher than expected slip rate along the peninsula segment of the san andreas fault and a slightly lower than expected slip rate along the san gregorio fault

parameter sensitivity of primitive equation ocean general circulation models abstract experiments with a low resolution primitive equation ocean general circulation model with idealized basin geometry and surface forcing have been carried out in order to identify the processes controlling the climatically important aspects of the circulation emphasis was placed on the sensitivity of the model solutions to the magnitude of the vertical diffusivity scaling arguments suggest and the numerical experiments confirm that the solutions are most sensitive to the magnitudes of the wind stress curl and the vertical diffusivity for small vertical diffusivity wind forcing dominates the solution the vertical scale of the thermocline is set by the strength of the ekman pumping and there is a multiple gyre circulation in the upper layers for large vertical diffusivity diabatic surface forcing dominates the solution vertical diffusion controls the vertical scale of the thermocline and there is a single large anticyclonic gyre in the upper layers both the meridionally and zonally inte

three dimensional seismic velocity structure of the san francisco bay area seismic travel times from the northern california earthquake catalogue and from the 1991 bay area seismic imaging experiment basix refraction survey were used to obtain a three dimensional model of the seismic velocity structure of the san francisco bay area nonlinear tomography was used to simultaneously invert for both velocity and hypocenters the new hypocenter inversion algorithm uses finite difference travel times and is an extension of an existing velocity tomography algorithm numerous inversions were performed with different parameters to test the reliability of the resulting velocity model most hypocenters were relocated 12 km under the sacramento river delta 6 km beneath livermore valley 5 km beneath the santa clara valley and 4 km beneath eastern san pablo bay the great valley sequence east of san francisco bay is 4 6 km thick a relatively high velocity body exists in the upper 10 km beneath the sonoma volcanic field but no evidence for a large intrusion or magma chamber exists in the crust under the geysers or the clear lake volcanic center lateral velocity contrasts indicate that the major strike slip faults extend sub vertically beneath their surface locations through most of the crust strong lateral velocity contrasts of 0 3 0 6 km s are observed across the san andreas fault in the middle crust and across the hayward rogers creek calaveras and greenville faults at shallow depth weaker velocity contrasts 0 1 0 3 km s exist across the san andreas hayward and rogers creek faults at all other depths low spatial resolution evidence in the lower crust suggests that the top of high velocity mafic rocks gets deeper from west to east and may be offset under the major faults the data suggest that the major strike slip faults extend sub vertically through the middle and perhaps the lower crust and juxtapose differing lithology due to accumulated strike slip motion the extent and physical properties of the major geologic units as constrained by the model should be used to improve studies of seismicity strong ground motion and regional stress