upper ovetian trilobites from spain and their implications for the palaeobiogeography and correlation of the cambrian stage 3 in gondwana abstract the upper part of the la herreria formation in los barrios de luna leon province n spain has been revised from a palaeontological and biostratigraphical point of view two stratigraphic sections have been studied including their trilobite and ichnofossils contents the ichnofossil assemblages have a high diversity of species characterising the cruziana ichnofacies suggesting a shallow sublittoral environment for the upper part of the la herreria formation the trilobites species recognised are lunagraulos antiquus dolerolenus formosus dolerolenus longioculatus lunolenus lunae metadoxides richterorum metadoxides armatus and sardaspis sp from the upper ovetian lower cambrian stage 3 under discussion by the iscs the new trilobite assemblages make possible a good correlation between the lower cambrian formations of north spain sardinia south china and siberia analysis of the palaeobiogeographical meaning of all trilobite genera that have been identified in the upper ovetian of spain shows a strong connection between the northern peri gondwana margin and west gondwana with a low latitude distribution for the spanish trilobites at this time

a scaling law for slow earthquakes with the availability of the global positioning system and other technical advances a growing number of unusual earthquake phenomena occurring at relatively long periods have been recognized they include deep episodic tremor low frequency earthquakes slow slip events and silent earthquakes based on data chiefly from western japan ide et al report that these slow seismic events follow a unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes a diverse array of unusual earthquake phenomena occurring at relatively long periods slow events follow a unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes indicating that they comprise a new earthquake category recently a series of unusual earthquake phenomena have been discovered including deep episodic tremor1 low frequency earthquakes2 very low frequency earthquakes3 slow slip events4 and silent earthquakes5 6 7 8 9 each of these has been demonstrated to arise from shear slip just as do regular earthquakes but with longer characteristic durations and radiating much less seismic energy here we show that these slow events follow a simple unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes we find that their seismic moment is proportional to the characteristic duration and their moment rate function is constant with a spectral high frequency decay of f 1 this scaling and spectral behaviour demonstrates that they can be thought of as different manifestations of the same phenomena and that they comprise a new earthquake category the observed scale dependence of rupture velocity for these events can be explained by either a constant low stress drop model or a diffusional constant slip model this new scaling law unifies a diverse class of slow seismic events and may lead to a better understanding of the plate subduction process and large earthquake generation

long term changes of earthquake inter event times and low frequency earthquake recurrence in central california abstract the temporal evolution of earthquake inter event time iet may provide important clues for the timing of future events and underlying physical mechanisms of earthquake interaction in this study we examine 12 yr of local earthquake and low frequency earthquake lfe activity near parkfield ca from catalogs of 50 000 earthquakes and 730 000 lfes we focus on the long term evolution of iets after the 2003 mw6 5 san simeon and 2004 mw6 0 parkfield earthquakes the iets of local earthquakes along and to the southwest of the san andreas fault show clear decreases of several orders of magnitude after the parkfield and san simeon earthquakes followed by recoveries with time scales of 3 yr and 8 yr respectively we also observe decreases in recurrence times in some of lfe families followed by long term recoveries with time scales of 4 months to several years the long term recovery of the earthquake iet is a manifestation of the aftershock decay of the parkfield and san simeon earthquakes and the different recovery time scales likely reflect the different tectonic loading rates in the two regions the drop in the recurrence times of lfes after the parkfield earthquake is likely caused by static and dynamic stresses induced by the parkfield earthquake and the long term recovery in lfe recurrence time could be due to post seismic relaxation or gradual recovery of fault zone material properties the recovery time scales for general earthquake iet and lfe recurrence following the parkfield earthquake are similar to those estimated for repeating earthquake recurrence identified in previous studies indicating that they could be controlled by similar mechanisms

precipitation of dolomite using sulphate reducing bacteria from the coorong region south australia significance and implications dolomite was successfully precipitated in culture experiments that simulated microbiogeochemical conditions prevailing during late stages of evaporation in ephemeral hypersaline dolomitic lakes of the coorong region south australia analyses of lake and pore water samples document rapid geochemical changes with time and depth in both dolomitic and non dolomitic lakes extremely high sulphate and magnesium ion concentrations in lake waters decline rapidly with depth in pore waters throughout the sulphate reduction zone whereas carbonate concentrations in pore waters reach levels up to 100 times those of normal sea water ultimately sulphate is totally consumed and no solid sulphate is recorded in the dolomitic lake sediments most probable number calculations of lake sediment samples record the presence of large populations of sulphate reducing bacteria whereas sulphur isotope analyses of lake water samples indicate microbial fractionation in all the lakes studied viable populations of microbes from the lake sediments were cultured in anoxic conditions in the laboratory samples were then injected into vials containing sterilized clastic or carbonate grains or glass beads immersed in a solution that simulated the lake water falls in the levels of sulphate and rising ph in positive vials were interpreted as indicating active bacterial sulphate reduction accompanied by increased concentrations of carbonate within 2 months sub spherical sub micron size crystals of dolomite identical to those of lake sediments were precipitated it is concluded that bacterial sulphate reduction overcomes kinetic constraints on dolomite formation by removing sulphate and releasing magnesium and calcium ions from neutral ion pairs and by generating elevated carbonate concentrations in a hypersaline strongly electrolytic solution the results demonstrate that bacterial sulphate reduction controls dolomite precipitation in both the laboratory experiments and lake sediments it is proposed that dolomite formation through bacterial sulphate reduction provides a process analogue applicable to thick platformal dolostones of the past where benthic microbial communities were the sole or dominant colonizers of shallow marine environments

modeling low frequency earthquake recurrence patterns summary tectonic tremor and low frequency earthquakes lfe have been identified globally but the physical mechanisms responsible for different types of lfe recurrence patterns are still elusive here we use a brittle ductile friction model to study the frictional conditions leading to different lfe recurrence patterns observed in central california we do a comprehensive search of the friction parameters including the brittle contact failure length and ductile damping strength to generate synthetic lfe bursts that match three different occurrence patterns of lfe bursts we find that the output synthetic lfe burst recurrence type varies under different frictional conditions the chaotic recurrence lfe sources require a higher brittle to ductile friction ratio consistent with the observation that they are mainly at shallower part of the brittle ductile transition zone the tri modal recurrence lfe sources require a wide distribution of asperity sizes indicating that the segment of deep saf between monarch peak and parkfield is likely to be more fragmented and weaker than the surrounding segments

imaging surface contacts power law contact distributions and contact stresses in quartz calcite glass and acrylic plastic a procedure has been developed to obtain microscope images of regions of contact between roughened surfaces of transparent materials while the surfaces are subjected to static loads or undergoing frictional slip static loading experiments with quartz calcite soda lime glass and acrylic plastic at normal stresses to 30 mpa yield power law distributions of contact areas from the smallest contacts that can be resolved 3 5 m2 up to a limiting size that correlates with the grain size of the abrasive grit used to roughen the surfaces in each material increasing normal stress results in a roughly linear increase of the real area of contact mechanisms of contact area increase are by growth of existing contacts coalescence of contacts and appearance of new contacts mean contacts stresses are consistent with the indentation strength of each material contact size distributions are insensitive to normal stress indicating that the increase of contact area is approximately self similar the contact images and contact distributions are modeled using simulations of surfaces with random fractal topographies the contact process for model fractal surfaces is represented by the simple expedient of removing material at regions where surface irregularities overlap synthetic contact images created by this approach reproduce observed characteristics of the contacts and demonstrate that the exponent in the power law distributions depends on the scaling exponent used to generate the surface topography

the tectonic evolution of the fennoscandian border zone in denmark abstract the fennoscandian border zone is a complex tectonic zone defining the border between the stable fennoscandian baltic shield and the metastable danish basin and polish trough it forms the northwestern part of the tornquist teisseyre lineament one of the major european tectonic sutures intricate tectonic movements took place within the fennoscandian border zone from early paleozoic to recent time it is postulated that its evolution was dominated during the early palaeozoic by tensional tectonics and during the late paleozoic by dextral wrench movements the latter led to the development of curvilinear faults and the subsidence of pull apart basins transtensional stresses governing the triassic to early cretaceous evolution of the fennoscandian border zone induced the differential subsidence of rift basins during the late cretaceous and early cenozoic dextral transpressional stresses induced the partial inversion of mesozoic basins associated with the fennoscandian border zone this is reflected by the uplift of a major anticlinorium over which cretaceous strata became progressively truncated whilst adjacent areas developed into late cretaceous early tertiary depocentres basin inversion diminishes to the northwest along the fennoscandian border zone and disappears in the skagerrak area

modeling dynamic triggering of tectonic tremor using a brittle ductile friction model 1 we study the physics of dynamically triggered tectonic tremor by applying a brittle ductile friction model in which we conceptualize the tremor source as a rigid block subject to driving and frictional forces to simulate dynamic triggering of tremor we apply a stress perturbation that mimics the surface waves of remote earthquakes the tectonic and wave perturbation stresses define a phase space that demonstrates that both the timing and amplitude of the dynamic perturbations control the fundamental characteristics of triggered tremor tremor can be triggered instantaneously or with a delayed onset if the dynamic perturbation significantly alters the frictional state of the tremor source

the lower middle cambrian stages and stratotypes from the iberian peninsula a revision lower middle cambrian regional stages of the iberian peninsula cordubian ovetian marianian bilbilian acadoparadoxides and solenopleuropsidae stages are now reconsidered in the light of new stratigraphic and palaeontological trace fossils trilobites and archaeocyaths studies these stages are re described with selected stratotypes and chronostratigraphic boundaries the names leonian and caesaraugustian are formally proposed to replace the provisional stages of acadoparadoxides and solenopleuropsidae respectively a correlation chart of the lower middle cambrian lithostratigraphic units from the iberian peninsula is also presented two regessive transgressive cycles are proposed to explain the lithological and palaeontological peculiarities through the lower middle cambrian sequence in the iberian peninsula

the brittle plastic transition and the depth of seismic faulting a simple rheological model of shearing of the lithosphere that has gained wide acceptance is a two layer model with an upper brittle zone in which deformation takes place by frictional sliding on discrete fault surfaces and a lower plastic zone in which deformation takes place by bulk plastic flow the two are separated by an abrupt brittle plastic transition which is assumed to be indicated by the lower limit of seismicity experimental studies however as well as the deformation structures of mylonites indicate that a broad transitional field of semi brittle behavior lies between these extremes this is a field of mixed mode deformation with a strength that can be expected to be considerably higher than that predicted from the extrapolation of high temperature flow laws for quartzofeldspathic rocks the semi brittle field lies between t1 the onset of quartz plasticity at about 300 c and t2 feldspar plasticity at about 450 c a model is presented in which the transition t1 does not correspond to a transition to bulk flow but to a change from unstable velocity weakening friction to stable velocity strengthening friction t1 thus marks the depth limit of earthquake nucleation but large earthquakes can propagate to a greater depth t3 t3 t2 which corresponds to the lower limit of dynamic frictional behavior in the semi brittle field and approximately to the peak in strength the zone between t1 and t3 is one of alternating behavior with flow occurring m the interseismic period and with co seismic dynamic slip occurring during large earthquakes this zone is characterized by mylonites interlaced by pseudotachylytes and other signs of dynamic faulting the transition t1 is also marked by a change in the generation mechanism of fault rocks from abrasive wear above which produces cataclastites to adhesive wear below which is proposed as an important generation mechanism of mylonites in the upper part of the semi brittle field

diagenesis of silica rich mound bedded chalk the coniacian arnager limestone denmark abstract the coniacian arnager limestone formation is exposed on the danish island of bornholm in the baltic sea it is composed of mound bedded siliceous chalk and x ray diffraction and scanning electron microscopy indicate a content of 30 70 insoluble minerals including authigenic opal ct quartz clinoptilolite feldspars calcite dolomite and barite opal ct and clinoptilolite are the most common and constitute 16 53 and 2 9 respectively the content of insoluble minerals varies laterally both within the mounds and in planar beds and the opal ct content varies by up to 10 vertically the mounds consist of two microfacies spiculitic wackestone and bioturbated spiculitic wackestone containing 10 22 and 7 12 moulds after spicules respectively subsequent to deposition and shallow burial dissolution of siliceous sponge spicules increased the silica activity of the pore water and initiated precipitation of opal ct the opal ct formed at temperatures around 17 c the precipitation lowered the silica activity and the si al ratio of the pore water resulting in precipitation of clinoptilolite feldspar and smectite calcite formed synchronously with the latest clinoptilolite minor amounts of quartz precipitated in pore water with low silica activity during maximum burial probably to depths of 200 250 m the dissolution of sponge spicules and decomposition of the sponge tissue also resulted in the release of ba2 sr2 mg2 ca2 and co32 facilitating precipitation of barite and dolomite precipitation of especially opal ct reduced the porosity to an average of 40 and cemented the limestone the study highlights the diagenetic pathways of bio siliceous chalk and the effects on preservation of porosity and permeability

mineralogical characterization of protolith and fault rocks from the safod main hole 1 washed cuttings provide a continuous record of the rocks encountered during drilling of the main hole of the san andreas fault observatory at depth safod both protolith and fault rocks exhibit a wide variety of mineral assemblages that reflect variations in some combination of lithology p t conditions deformation mechanisms and fluid composition and abundance regions of distinct neomineralization bounded by faults may record alteration associated with fluid reservoirs confined by faults in addition both smectites occurring as mixed layer phases and serpentine minerals are found in association with active strands of the san andreas fault that were intersected during drilling although their rheological influence is not yet fully known faults containing these mineralogical phases are prime candidates for continuous coring during phase 3 of safod drilling in the summer of 2007

silica diagenesis i solubility controls abstract the diagenetic sequence of silica is examined in terms of several thermodynamic and kinetic concepts the relationship between solubility and surface area or particle size is sufficient to explain simple opal a opal ct quartz transformations suggesting that the role of temperature and other physical parameters may not be as critical as previously thought complexation and adsorption in an impure system when added to the model allow for possibilities of opal a transformation directly to quartz of clay and zeolite formation from an opaline silica source and of variations in silica transformation rates due to presence of impurities in the system

the middle lower cambrian ovetian lunagraulos n gen from spain and the oldest trilobite records the type material of agraulos antiquus sdzuy 1961 from the la herreria formation northern spain is revised together with additional material and included in the new genus lunagraulos the stratigraphical range of lunagraulos antiquus sdzuy 1961 occurring below that of the trilobite species of the genera lunolenus metadoxides and dolerolenus in the type locality of los barrios de luna in the province of leon northern spain and the accompanying ichnofossil assemblage demonstrate an ovetian age lower part of cambrian stage 3 currently being discussed by the international subcommission on cambrian stratigraphy for this species moreover the trilobite lunagraulos tamamensis n gen n sp is found in the tamames sandstone near the village of la rinconada in the province of salamanca central spain the biostratigraphical position of this new taxon and its accompanying ichnoassemblage is also analysed and assigned to the lowermost ovetian stage the genus lunagraulos is therefore the oldest agraulid found in the fossil record the exceptional presence of lunagraulos in a marine coarse siliciclastic succession a facies rather typical for the ichnofossils cruziana and rusophycus some of the oldest signs of trilobite activity suggests that first trilobite representatives may have inhabited high to middle energy marine environments this hypothesis may also explain both the taxonomic and biostratigraphic heterogeneity of the first trilobite genera appearing across the world due to preservation problems in this type of facies comparison of the lunagraulo s biostratigraphy with other coeval spanish fossil assemblages allows us to propose its intercontinental correlation with the oldest records of currently known trilobites

a slow thrust slip event following the two 1996 hyuganada earthquakes beneath the bungo channel southwest japan we report a slow thrust slip event that occurred beneath the bungo channel region southwestern japan on oct and dec 1996 two hyuganada earthquakes both mw 6 7 followed by afterslips occurred in addition a crustal movement characterized by an extremely slow rise was observed around the bungo channel about 200 km north from the epicenters and continued for about 300 days long assuming a slow slip on the plate boundary we estimate its duration and surface displacements from gps time series data by curve fitting and then determine the fault slip distribution we found that a slow slip without any earthquakes continued for nearly one year and released the seismic moment comparable to that of the hyuganada earthquakes occurrence of the slow thrust slip event suggests that this kind of event may be a characteristic mode of stress release at a transition region of interplate coupling

precise location of san andreas fault tremors near cholame california using seismometer clusters slip on the deep extension of the fault 1 we examine a 24 hour period of active san andreas fault saf tremor and show that this tremor is largely composed of repeated similar events utilizing this similarity we locate the subset of the tremor with waveforms similar to an identified low frequency earthquake lfe master template located using p and s wave arrivals to be 26 km deep to compensate for low signal to noise we estimate event pair differential times at clusters of nearby stations rather than at single stations we find that the locations form a near linear structure in map view striking parallel to the saf and near the surface trace therefore we suggest that at least a portion of the tremor occurs on the deep extension of the fault likely reflecting shear slip similar to subduction zone tremor if so the saf may extend to the base of the crust 10 km below the deepest regular earthquakes on the fault

authigenic zeolites in late pleistocene sediments of the south atlantic angola basin two types of authigenic zeolites from late pleistocene hemipelagic sediments of the south atlantic were examined by xrd and sem analysis the xrd characteristics and the thermal behaviour of the zeolites suggest that type 1 is composed of clinoptilolite type 2 was identified as mordenite based on morphological features and qualitative elemental composition their authigenic nature is suggested from the shape of the crystals and their growth on and within foraminifera tests volcanic ash material acting as precursor minerals to zeolite formation could not be found nor were the zeolites directly related to siliceous remains an authigenesis from pore waters must be assumed mineral stabilities for amorphous sio2 and zeolites were calculated with the computer program solmineq88 interstitial waters proved to be oversaturated throughout the core in respect to all zeolites considered biogenic opal represents the limiting solid phase for si concentrations in pore waters therefore the dissolution of opal is regarded as the main source of si for interstitial waters below 9 m core depth where opal is only present in traces detrital feldspars and the zeolites themselves are affected by dissolution this may indicate the metastable nature of the zeolites also corroborated by the presence of two almost isochemical members of the same zeolite group

channelling versus inversion origin of condensed upper cretaceous chalks eastern isle of wight uk evidence from regional stratigraphical patterns in santonian campanian chalk is used to infer the presence of a very broad channel system 5 km across with a depth of at least 50 m running nnw sse across the eastern isle of wight only the western part of the channel wall and fill is exposed within this channel were smaller erosional structures

barium peaks at glacial terminations in sediments of the equatorial atlantic ocean relicts of deglacial productivity pulses in sediment cores recovered from oligotrophic settings of the western and eastern north equatorial atlantic ocean ceara rise and sierra leone rise distinct peaks of excess barium were detected at glacial interglacial transitions these ba maxima are unrelated to any other potential productivity proxy e g organic carbon carbonate or opal moreover they coincide with minima in organic carbon contents despite the lack of correlation between excess barium and organic carbon we ascribe these ba spikes to pulses of productivity that occurred during glacial interglacial transitions the discrepancy between barium and organic carbon in these transitional sediment intervals is attributed to the action of downward progressing oxidation fronts during deglacial nonsteady state depositional conditions the oxidation fronts were initiated due to the overall low sedimentation rates prevailing in the study areas and an increase in bottom water oxygen concentrations at the onset of enhanced nadw production during interglacial periods the fronts led to a very efficient oxidation of the organic carbon initially present leaving peaks in solid phase barium as relicts of these short time productivity events this assumption is supported by the depth arrangement of the solid phase peaks of ba fe and mn along the 6 5 oxygen isotope stage boundary in sediments of the ceara rise which show striking similarities to the distribution of these elements in oxidized sapropel intervals in eastern mediterranean sediments it remains unclear whether productivity was proportional to the magnitude of the baexcess maxima possibly higher dissolved ba concentrations in intermediate and deep waters during glacial interglacial transitions caused the precipitation of barite to increase out of proportion of the postulated deglacial productivity pulses

possible deep fault slip preceding the 2004 parkfield earthquake inferred from detailed observations of tectonic tremor 1 earthquake predictability depends in part on the degree to which sudden slip is preceded by slow aseismic slip recently observations of deep tremor have enabled inferences of deep slow slip even when detection by other means is not possible but these data are limited to certain areas and mostly the last decade the region near parkfield california provides a unique convergence of several years of high quality tremor data bracketing a moderate earthquake the 2004 magnitude 6 0 event here i present detailed observations of tectonic tremor from mid 2001 through 2008 that indicate deep fault slip both before and after the parkfield earthquake that cannot be detected with surface geodetic instruments while there is no obvious short term precursor i find unidirectional tremor migration accompanied by elevated tremor rates in the 3 months prior to the earthquake which suggests accelerated creep on the fault 16 km beneath the eventual earthquake hypocenter

periodic chaotic and doubled earthquake recurrence intervals on the deep san andreas fault earthquake recurrence histories may provide clues to the timing of future events but long intervals between large events obscure full recurrence variability in contrast small earthquakes occur frequently and recurrence intervals are quantifiable on a much shorter time scale in this work i examine an 8 5 year sequence of more than 900 recurring low frequency earthquake bursts composing tremor beneath the san andreas fault near parkfield california these events exhibit tightly clustered recurrence intervals that at times oscillate between 3 and 6 days but the patterns sometimes change abruptly although the environments of large and low frequency earthquakes are different these observations suggest that similar complexity might underlie sequences of large earthquakes

clinoptilolite as a new proxy of enhanced biogenic silica productivity in lower miocene carbonate sediments of the bahamas platform isotopic and thermodynamic evidence abstract massive clinoptilolite authigenesis was observed at about 1105 meters below sea floor mbsf in lower miocene well compacted carbonate periplatform sediments from the great bahama bank ocean drilling program odp leg 166 site 1007 the diagenetic assemblage comprises abundant zeolite crystallized within foraminifer tests and sedimentary matrix as well as mg smectites in carbonate rich deposits the formation of the zeolite requires a supply of silica thus the objective of the study is to determine the origin of the silica supply its diagenetic evolution and consequently the related implications on interpretation of the sedimentary record in terms of local or global paleoceanographic change for lack of evidence for any volcaniclastic input or traces of si enriched deep fluids circulation an in situ biogenic source of silica is validated by isotopic data and chemical modeling for the formation of such secondary minerals in shallow water carbonate sequences geochemical and strontium isotopic data clearly establish the marine signature of the diagenetic zeolite as well as its contemporaneous formation with the carbonate deposition sr model ages of 19 6 23 2 ma the test of saturation for the pore fluids specifies the equilibrium state of the present mineralogical assemblage seawater rock modeling specifies that clinoptilolite precipitates from the dissolution of biogenic silica which reacts with clay minerals the amount of silica opal a involved in the reaction has to be significant enough at least 10 wt to account for the observed content of clinoptilolite occurring at the most zeolite rich level modeling also shows that the observed amount of clinoptilolite 19 reflects an in situ and short term reaction due to the high reactivity of primary biogenic silica opal a until its complete depletion the episodic occurrence of these well lithified zeolite rich levels is consistent with the occurrence of seismic reflectors particularly the p2 seismic sequence boundary located at 1115 mbsf depth and dated as 23 2 ma the age range of most zeolitic sedimentary levels biostratigraphic ages of 21 5 22 ma correlates well with that of the early miocene glaciation mi 1 and mi 1a global events thus the clinoptilolite occurrence in the shallow carbonate platform environment far from volcanogenic supply or in other sensitive marine areas is potentially a significant new proxy for paleoproductivity and oceanic global events such as the miocene events which are usually recognized in deep sea pelagic sediments and high latitude deposits

the effect of grain coating microquartz on preservation of reservoir porosity clay coatings have been widely accepted by many workers as an explanation for preserving high porosity in deeply buried sandstones but few workers have realized that similar effects can be produced by microcrystalline quartz coatings this phenomenon can be expected only under special circumstances but in such cases it can have profound consequences for exploration in the central graben area of the southern north sea unusually high porosity 20 27 and permeability 100 1000 md are found in certain zones in upper jurassic sandstones at depths of 3 4 4 4 km the porosity in these zones is 5 15 higher than expected based on average porosity depth trends from brent and haltenbanken sandstones we propose that the high porosity is due to continuous grain coatings of euhedral microcrystalline quartz crystals that are 0 1 2 m thick the distribution of microcrystalline quartz coatings is controlled by the presence of siliceous sponge spicules rhaxella which implies a sedimentological control on the reservoir quality we present a thermodynamic model showing how continuous microcrystalline quartz coatings inhibit development of no mal macrocrystalline quartz overgrowths sourced mainly from stylolites high porosities in parts of various upper jurassic oil fields ula and gyda have previously been explained by inhibition of quartz cementation by early hydrocarbon charge we suggest that the microcrystalline quartz coatings provide a more plausible explanation

propagation dynamics of seismic and aseismic slip governed by fault heterogeneity and newtonian rheology 1 slow earthquakes called episodic tremor and slip ets events propagate over 100 km at low average velocities 10 km per day along several plate interfaces accompanying seismic and aseismic slip these low propagation velocities differentiate slow earthquakes from ordinary earthquakes and thus understanding their propagation processes is fundamental to understanding the poorly constrained physics governing the diversity and universality of earthquakes we show that rheological heterogeneity on faults primarily governs ets propagation on the basis of comprehensive modeling and observations that correlate migration patterns with the energetics of tremor on a plate bounding fault in southwest japan the fault has persistent small scale segmentation in which ets events started propagating energetically in relatively brittle sections and decelerated generally with a parabolic pattern in relatively ductile sections simulated spontaneous ruptures that are based on these parabolic tremor migration patterns constrain the cause of ductility to newtonian plastic flow or perhaps dilatant strengthening but reject large scale fluid flows we discuss possible elementary processes underlying the newtonian rheology this model is also consistent with the observed seismicity migration pattern before the 2011mw9 0 tohoku oki earthquake suggesting delayed triggering by themw 7 3 foreshock

porosity reduction during diagenesis of diatomaceous rocks with lithification and burial diatomaceous rock transforms from opal a to opal ct and then quartz this study documents two different patterns of porosity reduction and their associated mineralogical and density changes using samples from the monterey formation california samples that have one set of properties are called group 1 and those showing other trends are assigned to group 2

brittle and ductile friction and the physics of tectonic tremor 1 observations of nonvolcanic tremor provide a unique window into the mechanisms of deformation and failure in the lower crust at increasing depths rock deformation gradually transitions from brittle where earthquakes occur to ductile with tremor occurring in the transitional region the physics of deformation in the transition region remain poorly constrained limiting our basic understanding of tremor and its relation to earthquakes we combine field and laboratory observations with a physical friction model comprised of brittle and ductile components and use the model to provide constraints on the friction and stress state in the lower crust a phase diagram is constructed that characterizes under what conditions all faulting behaviors occur including earthquakes tremor silent transient slip and steady sliding our results show that tremor occurs over a range of ductile and brittle frictional strengths and advances our understanding of the physical conditions at which tremor and earthquakes take place

cambrian biostratigraphy of the tal group lesser himalaya india and early tsanglangpuan late early cambrian trilobites from the nigali dhar syncline precise biostratigraphic constraints on the age of the tal group are restricted to 1 a basal level correlative with the anabarites trisulcatus protohertzina anabarica assemblage zone of southwest china 2 a level near the boundary of the lower and upper parts of the tal group correlative with the early tsanglangpuan stage drepanuroides zone and 3 an interval low in the upper part of the tal group correlative with later in the tsanglangpuan stage palaeolenus zone these correlations are based on small shelly fossil and trilobite taxa other chronostratigraphic constraints include the marked negative 13 c isotopic excursion coincident with the transition from the krol group to the tal group this excursion is used as a proxy for the precambrian cambrian boundary in several sections worldwide and if applied to the lesser himalaya indicates that the boundary is at or just above the base of the tal group the upper parts of the tal group may be of middle or late cambrian age and might form proximal equivalents of sections in the zanskar spiti region of the tethyan himalaya both faunal content and lithological succession are comparable to southwest china furthering recent arguments for close geographic proximity between the himalaya and the yangtze block during late neoproterozoic and early cambrian time trilobites from the uppermost parts of the sankholi formation from the nigali dhar syncline are described and referred to three taxa one of which drepanopyge gopeni is a new species they are the oldest trilobites yet described from the himalaya

a dolomitized diatomite in an oligocene miocene deep sea fan succession gonfolite lombarda group northern italy abstract an unique 5 10 cm thick layer of a dolomitized diatomite which contains traces of volcanic material occurs intercalated between terrigenous mudstones thin bedded turbidites and pebbly mudstones of the montana member burdigalian of the oligocene miocene gonfolite lombarda group south alpine molasse dolomite formation took place during early diagenesis under conditions of sulfate reduction as suggested by the strongly negative 13 c values 13 pdb stereoscan electron microscope observations show that the dolomite grew as a pore filling cement enveloping deeply corroded diatom frustules the dolomitized diatomite can be correlated with coeval occurrences of biosiliceous tripoli and cineritic deposits in the northern apennines and the southern alps its preservation as a unique and distinct layer in the deep sea fan facies of the gonfolite lombarda group appears to reflect a particular event and suggests that it was deposited rapidly rather than during a longer time interval of slow pelagic sedimentation

a 15 year catalog of more than 1 million low frequency earthquakes tracking tremor and slip along the deep san andreas fault low frequency earthquakes lfes are small rapidly recurring slip events that occur on the deep extensions of some major faults their collective activation is often observed as a semicontinuous signal known as tectonic or nonvolcanic tremor this manuscript presents a catalog of more than 1 million lfes detected along the central san andreas fault from 2001 to 2016 these events have been detected via a multichannel matched filter search cross correlating waveform templates representing 88 different lfe families with continuous seismic data together these source locations span nearly 150 km along the central san andreas fault ranging in depth from 16 to 30 km this accumulating catalog has been the source for numerous studies examining the behavior of these lfe sources and the inferred slip behavior of the deep fault the relatively high temporal and spatial resolutions of the catalog have provided new insights into properties such as tremor migration recurrence and triggering by static and dynamic stress perturbations collectively these characteristics are inferred to reflect a very weak fault likely under near lithostatic fluid pressure yet the physical processes controlling the stuttering rupture observed as tremor and lfe signals remain poorly understood this paper aims to document the lfe catalog assembly process and associated caveats while also updating earlier observations and inferred physical constraints the catalog itself accompanies this manuscript as part of the electronic supplement with the goal of providing a useful resource for continued future investigations

a comparison between excess barium and barite as indicators of carbon export 1 since dymond et al 1992 proposed the paleoproductivity algorithm based on bio ba which relies on a strong correlation between ba and organic carbon fluxes in sediment traps this proxy has been applied in many paleoproductivity studies barite the main carrier of particulate barium in the water column and the phase associated with carbon export has also been suggested as a reliable paleoproductivity proxy in some locations we demonstrate that baexcess total barium minus the fraction associated with terrigenous material frequently overestimates babarite barium associated with the mineral barite most likely due to the inclusion of barium from phases other than barite and terrigenous silicates e g carbonate organic matter opal fe mn oxides and hydroxides a comparison between overlying oceanic carbon export and carbon export derived from baexcess shows that the dymond et al 1992 algorithm frequently underestimates carbon export but is still a useful carbon export indicator if all caveats are considered before the algorithm is applied babarite accumulation rates from a wide range of core top sediments from different oceanic settings are highly correlated to surface ocean 14 c and chlorophyll a measurements of primary production this relationship varies by ocean basin but with the application of the appropriate f ratio to 14 c and chlorophyll a primary production estimates the plot of babarite accumulation and carbon export for the equatorial pacific atlantic and southern ocean converges to a global relationship that can be used to reconstruct paleo carbon export index terms 3022 marine geology and geophysics marine sediments processes and transport 4267 oceanography general paleoceanography 4825 oceanography biological and chemical geochemistry keywords paleoproductivity barite export production excess barium marine sediments citation eagle m a paytan k r arrigo g van dijken and r w murray a comparison between excess barium and barite as indicators of carbon export paleoceanography 18 1 1021 doi 10 1029 2002pa000793 2003

frictional slip of granite at hydrothermal conditions sliding on faults in much of the continental crust likely occurs at hydrothermal conditions i e at elevated temperature and elevated pressure of aqueous pore fluids yet there have been few relevant laboratory studies to measure the strength sliding behavior and friction constitutive properties of faults at hydrothermal conditions we slid laboratory granite faults containing a layer of granite powder simulated gouge velocity stepping experiments were performed at temperatures of 23 to 600 c pore fluid pressures ph2o of 0 dry and 100 mpa wet effective normal stress of 400 mpa and sliding velocities v of 0 01 to 1 m s 0 32 to 32 m yr conditions were similar to those in earlier tests on dry granite to 845 c by lockner et al 1986 the mechanical results define two regimes the first regime includes dry granite up to at least 845 and wet granite below 250 c in this regime the coefficient of friction is high 0 7 to 0 8 and depends only modestly on temperature slip rate and ph2o the second regime includes wet granite above 350 c in this regime friction decreases considerably with increasing temperature temperature weakening and with decreasing slip rate velocity strengthening these regimes correspond well to those identified in sliding tests on ultrafine quartz we infer that one or more fluid assisted deformation mechanisms are activated in the second hydrothermal regime and operate concurrently with cataclastic flow slip in the first cool and or dry regime is characterized by pervasive shearing and particle size reduction slip in the second hot and wet regime is localized primarily onto narrow shear bands adjacent to the gouge rock interfaces weakness of these boundary shears may result either from an abundance of phyllosilicates preferentially aligned for easy dislocation glide or from a dependence of strength on gouge particle size major features of the granite data set can be fit reasonably well by a rate and temperature dependent three regime friction constitutive model chester this issue we extrapolate the experimental data and model fit in order to estimate steady state shear strength versus depth along natural slipping faults for sliding rates as low as 31 mm yr we do this for two end member cases in the first case pore pressure is assumed hydrostatic at all depths shallow crustal strength in this case is similar to that calculated in previous work from room temperature friction data while at depths below about 9 13 km depending on slip rate strength becomes less sensitive to depth but sensitive to slip rate in the second case pore pressure is assumed to be near lithostatic at depths below 5 km strength is low at all depths in this case 20 mpa in agreement with observations of weak faults such as the san andreas the predicted depth of transition from velocity weakening to velocity strengthening lies at about 13 km depth for a slip rate of 31 mm yr in rough agreement with the seismic aseismic transition depth observed on mature continental faults these results highlight the importance of fluid assisted deformation processes active in faults at depth and the need for laboratory studies on the roles of additional factors such as fluid chemistry large displacements higher concentrations of phyllosilicates and time dependent fault healing

a slip pulse model with fault heterogeneity for low frequency earthquakes and tremor along plate interfaces 1 deep low frequency earthquakes lfes and nonvolcanic tremor have distinctive characteristics unlike those of regular earthquakes including strong anisotropy in their migration velocity and source spectra displaying 1 f decay we show that a physical model can explain these features in a simple framework with slip pulses originating on fault heterogeneity and triggered by slow slip events lfe tremor source areas in the model consist of unstable patches sparsely and heterogeneously distributed following a gaussian distribution the difference in their migration speeds along dip and along strike was reproduced without anisotropic rheological properties by introducing alignments of their sources similar to observed streaks of lfes tremor the key to reproducing inverse linear spectral decay is that the slip pulse has a constant mean moment rate this model provides new insights into the physical source process of lfes and tremor and should find practical use in assessing properties of deep plate interfaces

activity diagrams for clinoptilolite susceptibility of this zeolite to further diagenetic reactions clinoptilolite is the predominant zeolite in diagenetically altered volcanic rocks at yucca mountain nevada having formed by posteruptive reactions of ground water with vitric tuffs in the pyroclastic deposits compositional variations of clinoptilolites in the fractured and zeolitized tuffs not presently in contact with ground water and the vulnerability of zeolites to burial diagenesis raise questions about the long term stability of clinoptilolite equilibrium activity diagrams were calculated for clinoptilolite solid solutions in the seven component system ca na k mg fe al si plus h sub 2 o employing available thermodynamic data for related minerals aqueous species and water stability fields are portrayed graphically assuming the presence of potassium feldspar saponite and hematite and using ranges of activities for sio sub 2 and al sup 3 defined by the saturation limits for several silica polymorphs gibbsite kaolinite and pyrophyllite the clinoptilolite stability field broadens with increasing atomic substitution of ca for na and k for ca reaches a maximum for intermediate activities of dissolved al and decreases with increasing temperature the thermodynamic calculations show that ground water of the sodium bicarbonate type is approximately in equilibrium at 25c with calcite and several zeolites including heulandite and calcic clinoptilolite mg rich clinoptilolites are stabilized in ground water depleted in ca sup 2 the activity diagramsmore indicate that prolonged diagenetic reactions with ground water depleted in al enriched in na or ca and heated by the thermal envelope surrounding buried nuclear waste may eliminate sorptive calcic clinoptilolites in fractured tuffs and underlying basal vitrophyre less

probing slow earthquakes with deep learning slow earthquakes may trigger failure on neighboring locked faults that are stressed sufficiently to break and slow slip patterns may evolve before a nearby great earthquake however even in the clearest cases such as cascadia slow earthquakes and associated tremor have only been observed in intermittent and discrete bursts by training a convolutional neural network to detect known tremor on a single seismic station in cascadia we isolate and identify tremor and slip preceding and following known larger slow events the deep neural network can be used for the detection of quasi continuous tremor providing a proxy that quantifies the slow slip rate furthermore the model trained in cascadia recognizes tremor in other subduction zones and also along the san andreas fault at parkfield suggesting a universality of waveform characteristics and source processes as posited from experiments and theory

lower cambrian acritarchs from northern spain the precambrian cambrian boundary and biostratigraphic implications acritarchs are reported from basal cambrian rock units inthe cantabrian region of northern spain that are known to contain archaeocyathan and trilobite faunas biostratigraphic correlation of the iberian sequences with other regions has been hampered by the strong provincialism of these faunas however this report of evidently cosmopolitan acritarch taxaestablishes the time equivalence of early cambrian trilobite faunas from iberia baltoscandia and the east european platform eep our data suggest that the detrital deposition of the lower cambrian herreria formation embraces at least three and possibly four lower cambrian acritarch zones previously identified in the eep eastern siberia baltoscandia scotland greenland svalbard and western north america the early cambrian transgression in northern spain was probably initiated in talsy times schmidtiellus mickwitzi trilobite zone in baltoscandia and the eep in part corresponding to the dokidocyathus regularis archaeocyathian zone of the middle tommotian in siberia

early cambrian continental reconstructions climatically related sedimentary facies and faunal distributions have been combined with palaeomagnetic data to make provisional reconstructions of the early cambrian world laurentia baltica and siberia appear to have formed a continental group which rifted apart from each other prior to 600 ma the consolidation of much of gondwana probably occurred in the pan african orogeny well before the cambrian though the assembly of several east asian terranes is still speculative the archaeocyathan reefs of siberia southern europe and morocco with their bigotinid trilobite fauna suggest that these areas were adjacent to each other and at low latitudes avalonia had close faunal links with western gondwana but lacked bigotinids and archaeocyathans and may have been situated off west africa and florida during the early cambrian there was a northward migration of laurentia towards the equator as its separation from baltica and siberia increased and a southward movement of gondwana

precise tremor source locations and amplitude variations along the lower crustal central san andreas fault 1 we precisely locate 88 tremor families along the central san andreas fault using a 3d velocity model and numerous p and s wave arrival times estimated from seismogram stacks of up to 400 events per tremor family maximum tremor amplitudes vary along the fault by at least a factor of 7 with by far the strongest sources along a 25 km section of the fault southeast of parkfield we also identify many weaker tremor families which have largely escaped prior detection together these sources extend 150 km along the fault beneath creeping transitional and locked sections of the upper crustal fault depths are mostly between 18 and 28 km in the lower crust epicenters are concentrated within 3 km of the surface trace implying a nearly vertical fault a prominent gap in detectible activity is located directly beneath the region of maximum slip in the 2004 magnitude 6 0 parkfield earthquake

stratigraphically controlled silicification in danian chalk and its implications for reservoir properties southern danish central graben abstract the effect of silica on reservoir quality in chalk is subject to continuing debate in particular the stratigraphic variations in silica content within the chalk matrix have not been addressed sufficiently in this study a new approach is applied to constrain silica variability in reservoir chalk utilizing hand held x ray fluorescence hh xrf on samples from four maastrichtian danian cores from hydrocarbon producing fields in the southern danish central graben the results indicate a significant lateral and stratigraphic variability in silica concentrations across the study area data display a common stratigraphic signature in all wells however with maximum silica concentrations and minimum porosities occurring in the lower part of the danian ekofisk formation correlation between hh xrf quantified silica concentrations and biostratigraphic data indicates the synchronous onset of an at least semi regional silica event during the mid danian nannofossil zones nntp2f g to nntp4a d whereas the upper boundary of the peak silica unit is poorly constrained generally stratigraphic trends in silica concentrations appear to be negatively correlated with porosity trends however comparison between the hh xrf quantified silica content and reservoir data porosity permeability estimated pore radius and calculated specific surface area shows poor correlation indicating that silica content within the ekofisk reservoir is not the only factor controlling the reservoir quality unknown complex diagenetic interactions between calcite silica and clay probably influence the reservoir properties by promoting calcite cementation in stratigraphically controlled zones making the ekofisk reservoir much less predictable than the underlying maastrichtian tor formation

geologic heterogeneity can produce aseismic slip transients 1 slow slip and tectonic tremor in subduction zones take place at depths where there is abundant evidence for distributed shear over broad zones 10 103 m composed of rocks with marked differences in mechanical properties here we model quasi dynamic rupture along faults composed of material mixtures characterized by different rate and state dependent frictional properties to determine the parameter regime capable of producing slow slip in an idealized subduction zone setting keeping other parameters fixed the relative proportions of velocity weakening vw and velocity strengthening vs materials control the sliding character stable slow or dynamic along the fault the stability boundary between slow and dynamic is accurately described by linear analysis of a double spring slider system with vw and vs blocks our results place bounds on the volume fractions of vw material present in heterogeneous geological assemblages that host slow slip and tremor in subduction zones

a small scale oceanic eddy off the coast of west africa studied by multi sensor satellite and surface drifter data a small scale oceanic eddy which was generated in autumn 2011 at the headland of cap vert off the coast of senegal west africa and then propagated westward into the open north atlantic ocean is studied by multi sensor satellite and surface drifter data the eddy was generated after a sudden increase of the trade winds causing an enhanced southward flow and upwelling at the coast of senegal after this wind burst event an extremely nonlinear cyclonic eddy with a radius of about 10 to 20 km evolved downstream of cap vert with rossby number larger than one our analysis suggests that the eddy was generated by flow separation at the headland of cap vert the eddy was tracked on its way into the open north atlantic ocean from satellites over 31 days via its sea surface temperature and chlorophyll a chl signature and by a satellite tracked surface drifter the satellite images show that this small scale eddy transported nutrients from the upwelling region westward into the oligotrophic north atlantic thus giving rise to enhanced chl concentration there maximum chl concentration was encountered few days after vortex generation which is consistent with a delayed plankton growth following nutrient supply into the euphotic zone within the eddy furthermore the eddy was imaged by the synthetic aperture radar sar onboard the envisat satellite it is shown that the radar signatures of cold eddies result from damping of short surface waves by biogenic surface films which arise from surface active material secreted by the biota in the cold eddy as well as by the change of the stability of the air sea interface highlights processes during the onset of coastal upwelling off west africa multi sensor remote sensing and in situ observations highly nonlinear small scale eddy generation at cap vert nrcs reduction in parts of the eddy due to biogenic surface films

the parkfield tremors reveal slow and fast ruptures on the same asperity a tremor source on the san andreas fault produced an unusual sequence of low frequency earthquakes until it was disrupted by the 2004 parkfield earthquake the peculiar recurrence pattern has now been modelled showing that such slip behaviour occurs when the tremor asperity size is close to the critical nucleation size of earthquakes an isolated tremor source on the san andreas fault near parkfield california produces an unusual sequence of low frequency earthquakes which regularly oscillated between recurrence intervals of three and six days until it was disrupted by the 2004 magnitude 6 0 parkfield earthquake after that event the source ruptured every three days only gradually returning to its initial alternating recurrence pattern after two years deepa mele veedu and sylvain barbot have now modelled the mechanism driving the tremor source to form this peculiar recurrence pattern they demonstrate that such slip behaviour can occur when the tremor asperity size is close to the critical nucleation size of earthquakes they also show that changes in pore pressure following the 2004 parkfield earthquake can explain the sudden change and gradual recovery of the recurrence intervals the deep extension of the san andreas fault is believed to be creeping but the recent observations of tectonic tremors from these depths indicate a complex deformation style1 in particular an isolated tremor source near parkfield has been producing a sequence of low frequency earthquakes2 that indicates an uncommon mechanism of stress accumulation and release the tremor pattern regularly oscillated between three and six days from mid 2003 until it was disrupted by the 2004 magnitude 6 0 parkfield earthquake after that event the tremor source ruptured only about every three days but over the next two years it gradually returned to its initial alternating recurrence pattern the mechanism that drives this recurrence pattern is unknown here we use physics based models to show that the same tremor asperity the region from which the low frequency earthquakes radiate can regularly slip in slow and fast ruptures naturally resulting in recurrence intervals alternating between three and six days this unusual slip behaviour occurs when the tremor asperity size is close to the critical nucleation size of earthquakes we also show that changes in pore pressure following the parkfield earthquake can explain the sudden change and gradual recovery of the recurrence intervals our findings suggest a framework for fault deformation in which the same asperity can release tectonic stress through both slow and fast ruptures

dehydration induced porosity waves and episodic tremor and slip episodic tremor and slip ets along the subduction interface takes place where there is abundant evidence for elevated near lithostatic pore pressures at sufficiently great depths 30 45 km that chemical dehydration reactions must act as their dominant source we simulate fluid and heat flow while tracking the location of a vertically oriented one dimensional column of material as it subducts through the slow slip and tremor zone the material in the column is transformed through a pressure dependent and temperature dependent dehydration reaction that we describe with a generalized nonlinear kinetic rate law column deformation is largely dominated by viscous creep with a closure rate that depends linearly on porosity this behavior causes the dehydration reaction to generate traveling porosity waves that transport increased fluid pressures within the slow slip region to explore the possibility that the observed periodicity of slow slip and tremor in subduction zones can be explained by the migration of such porosity waves we derive a dispersion relation that accurately describes our numerical results we also obtain an expression for how the thickness of the dehydrating layer is expected to vary as a function of the parameters in the reaction rate law although the amplitudes of pore pressure perturbations rival those that are produced by known external forcings e g tides or passing surface waves our analysis suggests that given reasonable estimates of rock viscosity permeabilities in the range 6 5 10 15 to 5 10 10 m2 are required for porosity wave trains to form at periods comparable to those of slow slip and tremor

rift related volcanism predating the birth of the rheic ocean ossa morena zone sw iberia abstract two very different periods of magma emplacement in the crust of the ossa morena zone early and main events in sw iberia have been previously interpreted to record a cambrian early ordovician rifting event that is thought to have culminated in the opening of the rheic ocean during the early ordovician new stratigraphic petrographic geochemical and sm nd isotope data from cambrian volcanic rocks included in six key low grade sections in both portugal and spain considerably improve our understanding of these events these data 1 confirm the existence of two rift related magmatic events in the cambrian of the ossa morena zone 2 demonstrate that the early rift related event was associated with migmatite and core complex formation in the mid upper crust and is represented by felsic peraluminous rocks the parent magmas of which were derived mainly from crustal sources and 3 show the main rift related event to be represented by a bimodal association of felsic and mafic rocks with minor amounts of intermediate rocks some of the mafic rocks show n morb affinity whereas others have oib or e morb affinities suggesting different heterogeneous mantle sources depleted and enriched asthenospheric and lithospheric plume like and non plume like the acid and intermediate rocks appear to represent hybrid mixtures of crust and mantle derived magmas this new data supports the hypothesis that the onset of rifting was associated with a process of oblique ridge trench collision we interpret the significant differences between the early and main events as reflecting the evolution from a wide rift stage with passive extension mainly accommodated by lower crust flow in a high heat flow setting to a narrow rift stage with active extension characterized by extension rates that outpaced thermal diffusion rates

spontaneous and triggered aseismic deformation transients in a subduction fault model 1 aseismic deformation transients can emerge as a natural outcome of the rate and state friction processes revealed in laboratory fault sliding experiments when that constitutive formulation is applied to model subduction earthquake sequences transients can arise spontaneously for certain effective stress variations with depth we show that if interstitial fluids are present and pore pressure is near lithostatic around and downdip from the frictional stability transition transients with recurrence intervals of 1 year are predicted on the basis of laboratory friction parameters and their temperature hence depth variations the recurrence interval decreases with and reaches 14 months when is 2 3 mpa dimensional analysis and numerical studies show that the fault response primarily depends on a parameter w h here the high pore pressure zone extends distance w updip from the stability transition and h is the stable patch size for steady sliding evidence that such fluid conditions may actually be present is independently provided by the occurrence of nonvolcanic tremors as apparent responses to extremely small stress changes and by petrological constraints on expected regions of dehydration for the shallow dipping subduction zones where transients are observed transient sequences can also be triggered by a modest one time step like interseismic stress perturbation on the subduction fault due to nearby earthquakes or to pore pressure changes e g during episodes of metamorphic fluid release properties of triggered transients and future thrust earthquakes depend on the interseismic time when the perturbation is introduced its relative location along the subduction fault and its magnitude

localized ductile shear below the seismogenic zone structural analysis of an exhumed strike slip fault austrian alps 1 the miocene salzachtal ennstal mariazell puchberg semp strike slip fault in austria allows study of the internal structure of a fault zone from the near surface to 30 km depth as it enters the tauern window along the rinderkarsee shear zone the semp fault passes from a dominantly brittle to a dominantly ductile structure the shear zone consists of three 1 to 100 m wide zones of brittle ductile and ductile deformation separated by 500 m wide zones of less deformed rocks the southern shear zone is mylonitic with ductile amphibole and plagioclase weak crystal preferred orientations imply that the main deformation mechanism was dislocation accommodated grain boundary sliding the northern and central shear zones are characterized by discrete millimeter wide shear zones with ductile quartz muscovite and biotite and brittle feldspar shear zone nucleation at the grain scale involved dislocation creep and the transformation of plagioclase to muscovite strain then localized in muscovite rich grain boundary shear zones that linked to form throughgoing shear zones

elastodynamic analysis for slow tectonic loading with spontaneous rupture episodes on faults with rate and state dependent friction we present an efficient and rigorous numerical procedure for calculating the elastodynamic response of a fault subjected to slow tectonic loading processes of long duration within which there are episodes of rapid earthquake failure this is done for a general class of rate and state dependent friction laws with positive direct velocity effect the algorithm allows us to treat accurately within a single computational procedure loading intervals of thousands of years and to calculate for each earthquake episode initially aseismic accelerating slip prior to dynamic rupture the rupture propagation itself rapid post seismic deformation which follows and also ongoing creep slippage throughout the loading period in velocity strengthening fault regions the methodology is presented using the two dimensional 2 d antiplane spectral formulation and can be readily extended to the 2 d in plane and 3 d spectral formulations and with certain modifications to the space time boundary integral formulations as well as to their discretized development using finite difference or finite element methods the methodology can be used to address a number of important issues such as fault operation under low overall stress interaction of dynamic rupture propagation with pore pressure development patterns of rupture propagation in events nucleated naturally as a part of a sequence the earthquake nucleation process earthquake sequences on faults with heterogeneous frictional properties and or normal stress and others the procedure is illustrated for a 2 d crustal strike slip fault model with depth variable properties for lower values of the state evolution distance of the friction law small events appear the nucleation phases of the small and large events are very similar suggesting that the size of an event is determined by the conditions on the fault segments the event is propagating into rather than by the nucleation process itself we demonstrate the importance of incorporating slow tectonic loading with elastodynamics by evaluating two simplified approaches one with the slow tectonic loading but no wave effects and the other with all dynamic effects included but much higher loading rate

large scale relocation of two decades of northern california seismicity using cross correlation and double difference methods 1 we simultaneously reanalyzed two decades 1984 2003 of the digital seismic archive of northern california using waveform cross correlation cc and double difference dd methods to improve the resolution in hypocenter locations in the existing earthquake catalog generated at the northern california seismic network ncsn by up to three orders of magnitude we used a combination of 3 billion cc differential times measured from all correlated pairs of events that are separated by less than 5 km and 7 million p wave arrival time picks listed in the ncsn bulletin data were inverted for precise relative locations of 311 273 events using the dd method the relocated catalog is able to image the fine scale structure of seismicity associated with active faults and revealed characteristic spatiotemporal structures such as streaks and repeating earthquakes we found that 90 of the earthquakes have correlated p wave and s wave trains at common stations and that 12 are colocated repeating events an analysis of the repeating events indicates that uncertainties at the 95 confidence level in the existing network locations are on average 0 7 km laterally and 2 km vertically correlation characteristics and relative location improvement are remarkably similar across most of northern california implying the general applicability of these techniques to image high resolution seismicity caused by a variety of plate tectonic and anthropogenic processes we show that consistent long term seismic monitoring and data archiving practices are key to increase resolution in existing hypocenter catalogs and to estimate the precise location of future events on a routine basis

a silent slip event on the deeper cascadia subduction interface continuous global positioning system sites in southwestern british columbia canada and northwestern washington state usa have been moving landward as a result of the locked state of the cascadia subduction fault offshore in the summer of 1999 a cluster of seven sites briefly reversed their direction of motion no seismicity was associated with this event the sudden displacements are best explained by 2 centimeters of aseismic slip over a 50 kilometer by 300 kilometer area on the subduction interface downdip from the seismogenic zone a rupture equivalent to an earthquake of moment magnitude 6 7 this provides evidence that slip of the hotter plastic part of the subduction interface and hence stress loading of the megathrust earthquake zone can occur in discrete pulses

slip on the san andreas fault at parkfield california over two earthquake cycles and the implications for seismic hazard parkfield california which experienced m 6 0 earthquakes in 1934 1966 and 2004 is one of the few locales for which geodetic observations span multiple earthquake cycles we undertake a comprehensive study of deformation over the most recent earthquake cycle and explore the results in the context of geodetic data collected prior to the 1966 event through joint inversion of the variety of parkfield geodetic measurements trilateration two color laser and global positioning system including previously unpublished two color data we estimate the spatial distribution of slip and slip rate along the san andreas using a fault geometry based on precisely relocated seismicity although the three most recent parkfield earthquakes appear complementary in their along strike distributions of slip they do not produce uniform strain release along strike over multiple seismic cycles since the 1934 earthquake more than 1 m of slip deficit has accumulated on portions of the fault that slipped in the 1966 and 2004 earthquakes and an average of 2 m of slip deficit exists on the 33 km of the fault southeast of gold hill to be released in a future perhaps larger earthquake it appears that the fault is capable of partially releasing stored strain in moderate earthquakes maintaining a disequilibrium through multiple earthquake cycles this complicates the application of simple earthquake recurrence models that assume only the strain accumulated since the most recent event is relevant to the size or timing of an upcoming earthquake our findings further emphasize that accumulated slip deficit is not sufficient for earthquake nucleation online material model fault geometry fit to the data for the inversions and model resolution

stylolites porosity depositional texture and silicates in chalk facies sediments ontong java plateau gorm and tyra fields north sea comparison of chalk on the ontong java plateau and chalk in the central north sea indicates that whereas pressure dissolution is controlled by effective burial stress pore filling cementation is controlled by temperature effective burial stress is caused by the weight of all overlying water and sediments as counteracted by the pressure in the pore fluid so the regional overpressure in the central north sea is one reason why the two localities have different relationships between temperature and effective burial stress in the chalk of the ontong java plateau the onset of calcite silicate pressure dissolution around 490 m below sea floor bsf corresponds to an interval of waning porosity decline and even the occurrence of proper stylolites from 830 m bsf is accompanied by only minor porosity reduction because opal is present the pore water is relatively rich in si which through the formation of ca silica complexes causes an apparent super saturation of ca and retards cementation the onset of massive pore filling cementation at 1100 m bsf may be controlled by the temperature dependent transition from opal ct to quartz in the stylolite bearing chalk of two wells in the gorm and tyra fields the nannofossil matrix shows recrystallization but only minor pore filling cement whereas microfossils are cemented cementation in gorm and tyra is thus partial and has apparently not been retarded by opal controlled pore water a possible explanation is that due to the relatively high temperature silica has equilibrated to quartz before the onset of pressure dissolution and thus in this case dissolution and precipitation of calcite have no lag this temperature versus effective burial stress induced difference in diagenetic history is of particular relevance when exploring for hydrocarbons in normally pressured chalk while most experience has been accumulated in the over pressured chalk of the central north sea

diagenetic clinoptilolite and opal ct from the middle eocene wittering formation isle of wight u k abstract thinly interbedded estuarine sandstones and mudstones in the middle eocene wittering formation bracklesham group isle of wight contain authigenic clinoptilolite opal ct and possibly authigenic smectite over a thickness of 8 5 m the clinoptilolite and opal ct cements have resulted in these beds being the only ones tightly cemented in the wittering formation the rest being soft sands and muds the unit contains abundant siliceous sponge spicules many of which are preserved as voids and were the likely source of the opal ct the presence of an organic rich rooted paleosol histosol immediately overlying the unit is believed to have contributed significantly to the early diagenetic history of the estuarine sediments in particular the formation of zeolite although the ph and dissolved silica of unmodified seawater are too low for zeolite formation high concentrations of dissolved silica are believed to have resulted from dissolution of siliceous sponge spicules opal a and formation of silica organic acid complexes organic acid dissolution of feldspars absent from the zeolite bearing interval is the most probable source of alumina and cations for zeolite formation ree data show that volcanic ash was never present and cannot therefore have been the source of the zeolite this investigation therefore demonstrates a novel route by which the pore fluid chemistry required for zeolite formation can be achieved

silica activity and the smectite illite reaction early silicate diagenesis is characterized by the precipitation of minerals with high silica activity such as opaline silica smectite and clinoptilolite and by pore water silica activities exceeding quartz saturation with burial this early high silica activity diagenesic facies is succeeded by a low silica activity facies distinguished by quartz precipitation and progressive development of illite as the nonexpandable component of mixed layer clays we suggest that the smectite illite reaction occurs as a consequence of the reduction of silica activity at the onset of quartz precipitation data from oceanic and sedimentary basins confirm that the smectite illite reaction does not control aqueous silica activities where the smectite illite reaction occurs at higher temperatures aqueous silica activities likewise reach quartz saturation at higher temperatures