time variation of b and qc at mt etna 1981 1987 abstract time variations of seismic attenuation qc and of the coefficient b in the frequency magnitude formula are presented for mt etna during 1981 1987 significant variations of both parameters were found suggesting long period variations of the physical state of the volcano

temporal variation of seismic b values beneath northeastern japan island arc 1 analysis of a high quality seismic catalog reveals that the average of seismic b values in the crust beneath most part of northeastern japan island arc decreased from 0 86 between 1984 and 1990 to 0 73 between 1991 and 1995 the two areas with the largest decrease are found to be in the same areas where the coupling between the north american and the pacific plates is the highest as suggested by a recent geodetic study in the same time period the annual seismic moment release increased by 10 times in addition there seems to be a corresponding increase in volcanic activities in the same area one of the most likely interpretations for the observations is an increase in the subduction rate starting from 1991 the timing of this possible increase in subduction rate is consistent with an apparent increase in global seismic activity

modelling twentieth century global ocean circulation and iceberg flux at 48 n implications for west greenland iceberg discharge abstract we have used a coupled ocean iceberg model to study the variation in global ocean circulation and north atlantic iceberg flux from 1900 to 2008 the latter component of the study focused particularly on greenland icebergs feeding into the labrador current and past newfoundland the model was forced with daily heat freshwater and wind fluxes from the twentieth century reanalysis the reanalysis heat fluxes were shown to be offset from the shorter ncep reanalysis and a grid point correction was applied to this component of the forcing the model produces a generally realistic ocean circulation although with an enhanced atlantic meridional overturning largely due to the forcing the modelled iceberg flux at 48 n is well correlated with the long term observed flux when using a modelled iceberg discharge that varies in a similar fashion to the highly variable observed flux at 48 n from this model we infer changes in the spatial and temporal variability of iceberg calving from western greenland during the first third of the twentieth century the majority of modelled icebergs reaching 48 n derive from southern greenland while only after 1930 is the traditional perspective of a majority of such icebergs originating from baffin bay consistent with model results decadal scale changes in the dominant regional sources are found with oscillations between western greenland and northern baffin bay the latter origin was modelled to be most important in the last third of the twentieth century although west greenland sources have increased in importance in recent years the model correctly reproduces the pronounced late spring peak in flux at 48 n for southern greenland icebergs but has an approximately six month offset for icebergs from baffin bay most likely due to resolution issues leading to model icebergs not being delayed in shallow coastal waters whereas in reality they may be grounded for some time or trapped in coastal sea ice

temporal variations of qc 1 and b values associated to the december 2000 january 2001 volcanic activity at the popocatepetl volcano mexico abstract we correlated temporal variations of coda attenuation q c 1 at 6 hz and b values with the 11 december 2000 23 january 2001 major eruptive activity at the popocatepetl volcano mexico we interpreted a total of 225 volcanic earthquakes with magnitudes between 1 9 and 3 3 located between 2 and 8 km from the summit of the volcano and registered from january 1999 to june 2001 q c 1 was estimated using a single scattering model and values of b were obtained applying the method of maximum likelihood our results indicate that from january 1999 to march 2000 period 1 q c 1 varied within the same range without significant changes then values of q c 1 steadily decreased during april mid december 2000 period 2 from about mid december 2000 onset of the major volcanic activity up to february 2001 period 3 q c 1 sharply increased reaching its peak value from march to june 2001 period 4 q c 1 decreased and increased in response to the post eruptive phase of dome formation and destruction process that took place during periods 2 and 3 average b values behaved opposite to the q c 1 trend within period 2 when q c 1 decreased b slowly increased at the start of period 3 b reached its peak value and returned to low values by the end of this time interval we associate the temporal changes of the two analyzed parameters to variations of pressure and or the variation in the degree of saturation of pore fluids that took place before during and after the major eruption during periods 2 and 3 q c 1 correlated negatively with b with a correlation coefficient of 0 61 and 0 83 respectively the significance of correlation cannot be rejected at the confidence level of 0 975 our observations may have important implications for future prediction studies of large eruptions at the popocatepetl volcano

statistical physics seismogenesis and seismic hazard the scaling properties of earthquake populations show remarkable similarities to those observed at or near the critical point of other composite systems in statistical physics this has led to the development of a variety of different physical models of seismogenesis as a critical phenomenon involving locally nonlinear dynamics with simplified rheologies exhibiting instability or avalanche type behavior in a material composed of a large number of discrete elements in particular it has been suggested that earthquakes are an example of a self organized critical phenomenon analogous to a sandpile that spontaneously evolves to a critical angle of repose in response to the steady supply of new grains at the summit in this stationary state of marginal stability the distribution of avalanche energies is a power law equivalent to the gutenberg richter frequency magnitude law and the behavior is relatively insensitive to the details of the dynamics here we review the results of some of the composite physical models that have been developed to simulate seismogenesis on different scales during 1 dynamic slip on a preexisting fault 2 fault growth and 3 fault nucleation the individual physical models share some generic features such as a dynamic energy flux applied by tectonic loading at a constant strain rate strong local interactions and fluctuations generated either dynamically or by fixed material heterogeneity but they differ significantly in the details of the assumed dynamics and in the methods of numerical solution however all exhibit critical or near critical behavior with behavior quantitatively consistent with many of the observed fractal or multifractal scaling laws of brittle faulting and earthquakes including the gutenberg richter law some of the results are sensitive to the details of the dynamics and hence are not strict examples of self organized criticality nevertheless the results of these different physical models share some generic statistical properties similar to the universal behavior seen in a wide variety of critical phenomena with significant implications for practical problems in probabilistic seismic hazard evaluation in particular the notion of self organized criticality or near criticality gives a scientific rationale for the a priori assumption of stationarity used as a first step in the prediction of the future level of hazard the gutenberg richter law a power law in energy or seismic moment is found to apply only within a finite scale range both in model and natural seismicity accordingly the frequency magnitude distribution can be generalized to a gamma distribution in energy or seismic moment a power law with an exponential tail this allows extrapolations of the frequency magnitude distribution and the maximum credible magnitude to be constrained by observed seismic or tectonic moment release rates the answers to other questions raised are less clear for example the effect of the a priori assumption of a poisson process in a system with strong local interactions and the impact of zoning a potentially multifractal distribution of epicentres with smooth polygons the results of some models show premonitory patterns of seismicity which could in principle be used as mainshock precursors however there remains no consensus on both theoretical and practical grounds on the possibility or otherwise of reliable intermediate term earthquake prediction

effect of the sumatran mega earthquake on the global magnitude cut off and event rate the great sumatran earthquake of 2004 allows us to assess the statistics and statistical stability of the global earthquake catalogue from the digital era a key question is do such mega earthquakes continue to follow the gutenberg richter g r trend1 or is there an observable cut off2

spatial probability distribution of future volcanic eruptions at el hierro island canary islands spain abstract the 2011 submarine eruption that took place in the proximity of el hierro island canary islands spain has raised the need to identify the most likely future emission zones even on volcanoes characterized by low frequency activity here we propose a probabilistic method to build the susceptibility map of el hierro i e the spatial distribution of vent opening for future eruptions based on the probabilistic analysis of volcano structural data of the island collected through new fieldwork measurements bathymetric information as well as analysis of geological maps orthophotos and aerial photographs these data have been divided into different datasets and converted into separate and weighted probability density functions which were included in a non homogeneous poisson process to produce the volcanic susceptibility map the most likely area to host new eruptions in el hierro is in the south western part of the west rift high probability locations are also found in the northeast and south rifts and along the submarine parts of the rifts this map represents the first effort to deal with the volcanic hazard at el hierro and can be a support tool for decision makers in land planning emergency measures and civil defense actions

seismicity of the askja and barðarbunga volcanic systems of iceland 2009 2015 abstract a large seismic network deployed in the icelandic highlands recorded 100 000 earthquakes from 2009 to 2015 we develop a local magnitude scale appropriate for use in central iceland which is similar to the scale used by the iceland meteorological office using this large catalogue of earthquakes we analyze the spatial and temporal changes in seismicity rates and b values in microearthquakes recorded from the usually ductile lower crust we find that b values are high reflecting the presence of high thermal gradients and low stresses driving seismicity associated with the movement of melt in contrast b values in the upper crust are variable low b values indicative of a high stress environment are observed during seismic swarms such as those around mt her ubrei and around bar arbunga caldera a persistently seismically active area around a geothermal area within askja caldera has a b value around 1 but has a strong annual cycle of seismicity we attribute the annual cycle to varying load from the snow cover modulating the seismicity seismicity driven by the intrusion of a large dyke has a b value well above 1 driven by the high pore fluid pressures and thermal gradients around the dyke

three dimensional modeling of tidal currents and mixing quantities over the newfoundland shelf major semidiurnal m2 s2 and n2 and diurnal k1 and o1 barotropic tides over the newfoundland shelf are computed using a three dimensional nonlinear primitive equation model with the vertical eddy viscosity calculated from a level 2 5 turbulence closure scheme computed elevation cotidal charts for the five constituents are generally consistent with previous knowledge for this region comparisons based on a statistical analysis of the differences between the computed elevations and currents and in situ observations indicate good agreement while m2 tidal currents up to 20 30 cm s are dominant there are locally intensified diurnal currents up to 5 10 cm s in some outer shelf locations the diurnal current intensification is attributed to first mode continental shelf waves an examination is carried out for the vertical structure of the computed m2 current and for the temporal and spatial variability of model turbulent kinetic energy mixing length scale vertical eddy viscosity and bottom friction velocity the examination indicates that large vertical eddy viscosity magnitudes and bottom friction velocities are associated with strong currents in shallow regions where strong vertical shears produce large turbulent kinetic energy solutions with both specified and evolving vertical stratification indicate that the stratification has a significant influence on the vertical profile of tidal mixing parameters and currents in shallow areas tidally induced turbulence is substantially reduced in the bottom boundary layer and completely suppressed above it tidal currents decrease in the log layer increase significantly in the rest of the bottom boundary layer and decrease in the upper and middle water column

oceanic responses to hurricane igor over the grand banks a modeling study a three dimensional 3 d baroclinic finite volume ocean model fvcom was developed to examine the oceanic response to hurricane igor over the grand banks of newfoundland hurricane igor generated a storm surge of almost 1 m at st john s and about 0 8 m at three nearby coastal tide gauge stations bonavista argentia and st lawrence the surge magnitude from the 3 d baroclinic model agrees approximately with tide gauge observations at all four stations slightly better than that from an alternative 3 d barotropic case the sudden drop of sea surface temperature caused by the storm approximately 6oc as observed by buoys is well simulated by the baroclinic model with a k e turbulence closure a sensitivity simulation with the mellor yamada turbulence closure significantly underestimates sea surface cooling it is shown that the sea surface cooling is mainly associated with turbulent mixing and to a lesser degree with ekman upwelling the model solution shows that the largest surge occurred between bonavista and st john s further analysis suggests the generation of a free continental shelf wave after the storm made landfall with the peak surge propagating from st john s to st lawrence

modeling the mean circulation of the labrador sea and the adjacent shelves abstract a linear three dimensional diagnostic model is used to study several aspects of the mean circulation of the labrador sea and the adjacent shelves volume transport three dimensional structure of currents mesoscale features produced by jebar winter surface current derived from ice drift data difference between summer and winter circulation and effects of buoyancy and local wind on the circulation the summer circulation is forced by a sea surface elevation at the northern boundary tuned to produce a 35 sv sv 106 m3 s 1 total southward transport including the shelf the labrador current and the deep sea transports across the hamilton bank section a comparison of the model results with data collected over the shelves indicates that the model is able to reproduce the major features of the observations buoyancy effects on the circulation are found to be more important in the southern labrador sea than in the northern labrador sea the subbasin scale currents give rise to an alongshelf var

seismic hazard in western canada from gps strain rates versus earthquake catalog 1 probabilistic seismic hazard analyses psha are commonly based on frequency magnitude statistics from 50 100 yearlong earthquake catalogs assuming that these statistics are representative of the longer term frequency of large earthquakes we test an alternative psha approach in continental western canada including adjacent areas of northwestern u s a using regional strain rates derived from 179 global positioning system gps horizontal velocities gps strain rates are converted to earthquake statistics seismic moment rates and ground shaking probabilities in seismic source zones using a logic tree method for uncertainty propagation median gps based moment rates and shaking estimates agree well with those derived from earthquake catalogs in only two zones puget sound and mid vancouver island in most other zones median gps based moment rates are 6 150 times larger than those derived from earthquake catalogs shaking estimates 2 5 times larger although the gps based and catalog estimates commonly agree within their 67 uncertainties this discrepancy may represent an under sampling of long term moment rates and shaking by earthquake catalogs in some zones however a systematic under sampling is unlikely over our entire study area although not demonstrated with a high confidence level long term regional aseismic deformation may account for a significant part of the gps catalog discrepancy and in some areas represent as much as 90 of the total deformation budget in order to integrate gps strain rates in psha models seismic versus aseismic partitioning of long term deformation needs to be quantified and understood in terms of the underlying mechanical processes

temporal and three dimensional spatial analyses of the frequency magnitude distribution near long valley caldera california summary the 3 d distribution of the b value of the frequency magnitude distribution is analysed in the seismically active parts of the crust near long valley caldera california the seismicity is sampled in spherical volumes containing n 150 earthquakes and centred at nodes of a grid separated by 0 3 km significant variations in the b value are detected with b ranging from b 0 6 to b 2 0 high b value volumes are located near the resurgent dome and at depths below 5 km at mammoth mountain b values are found to be much lower south of the long valley caldera we interpret this to indicate that an active magma body has advanced from depths below 8 km to depths of 4 to 5 km beneath mammoth mountain in 1989 and that anomalous crust either highly fractured or containing unusually high pore pressure such as is the case in the vicinity of active magma bodies exists north of the seismically active area beneath the resurgent dome at all depths we also investigate the spatial distribution of temporal variations of the frequency magnitude distribution by introducing diverential b value maps b values increased from b 0 8 to b 1 5 underneath mammoth mountain at the onset of the 1989 earthquake swarm and remained high thereafter this suggests that an intrusion permanently altered the average distribution of cracks at 5 10 km depth or that the pore pressure permanently increased we propose that high b values are a necessary but not suycient condition near a magmatic body and therefore spatial b value mapping can be used to aid in the identification of active magma bodies

variations in the frequency magnitude distribution with depth in two volcanic areas mount st helens washington and mt spurr alaska the frequency magnitude distribution of earthquakes characterized using the b value is examined as a function of space beneath mount st helens 1988 1996 and mt spurr 1991 1995 at mount st helens two volumes of anomalously high b b 1 3 can be observed at depths of 2 6 3 6 km below the crater floor and below 6 4 km these anomalies coincide with 1 the depth of vesiculation of ascending magma and 2 the suggested location of a magma chamber at mount st helens study of mt spurr reveals an area of high b value b 1 3 at a depth of about 2 3 4 5 km below the crater floor of the active vent crater peak we propose that the higher material heterogeneity in the vicinity of a magma chamber or conduit due to vesiculation of the ascending magma is the main cause of the increased b value at shallow depths alternatively interaction of magma with groundwater may have increased pore pressure and lowered the effective stress the deeper anomaly at mount st helens is likely caused by high thermal stress gradients in the vicinity of the magma chamber our results indicate that detailed mapping of the frequency magnitude distribution can be used as a tool to trace vesiculation and locate active magma chambers

anomalously high b values in the south flank of kilauea volcano hawaii evidence for the distribution of magma below kilauea s east rift zone abstract the pattern of b value of the frequency magnitude relation or mean magnitude varies little in the kaoiki hilea area of hawaii and the b values are normal with b 0 8 in the top 10 km and somewhat lower values below that depth we interpret the kaoiki hilea area as relatively stable normal hawaiian crust in contrast the b values beneath kilauea s south flank are anomalously high b 1 3 1 7 at depths between 4 and 8 km with the highest values near the east rift zone but extending 5 8 km away from the rift also the anomalously high b values vary along strike parallel to the rift zone the highest b values are observed near hiiaka and pauahi craters at the bend in the rift the next highest are near makaopuhi and also near puu kaliu the mildest anomalies occur adjacent to the central section of the rift the locations of the three major and two minor b value anomalies correspond to places where shallow magma reservoirs have been proposed based on analyses of seismicity geodetic data and differentiated lava chemistry the existence of the magma reservoirs is also supported by magnetic anomalies which may be areas of dike concentration and self potential anomalies which are areas of thermal upwelling above a hot source the simplest explanation of these anomalously high b values is that they are due to the presence of active magma bodies beneath the east rift zone at depths down to 8 km in other volcanoes anomalously high b values correlate with volumes adjacent to active magma chambers this supports a model of a magma body beneath the east rift zone which may widen and thin along strike and which may reach 8 km depth and extend from kilauea s summit to a distance of at least 40 km down rift the anomalously high b values at the center of the south flank several kilometers away from the rift may be explained by unusually high pore pressure throughout the south flank or by anomalously strong heterogeneity due to extensive cracking or by both phenomena the major b value anomalies are located sse of their parent reservoirs in the direction of motion of the flank suggesting that magma reservoirs leave an imprint in the mobile flank we hypothesize that the extensive cracking may have been acquired when the anomalous parts of the south flank now several kilometers distant from the rift zone were generated at the rift zone near persistent reservoirs since their generation these volumes may have moved seaward away from the rift but earthquakes occurring in them still use the preexisting complex crack distribution along the decollement plane at 10 km depth the b values are exceptionally low b 0 5 suggesting faulting in a more homogeneous medium

spatial variations in the frequency magnitude distribution of earthquakes at mount pinatubo volcano the frequency magnitude distribution of earthquakes measured by the b value is mapped in two and three dimensions at mount pinatubo philippines to a depth of 14 km below the summit we analyzed 1406 well located earthquakes with magnitudes m d 0 73 recorded from late june through august 1991 using the maximum likelihood method we found that b values are higher than normal b 1 0 and range between b 1 0 and b 1 8 the computed b values are lower in the areas adjacent to and west southwest of the vent whereas two prominent regions of anomalously high b values b 1 7 are resolved one located 2 km northeast of the vent between 0 and 4 km depth and a second located 5 km southeast of the vent below 8 km depth the statistical differences between selected regions of low and high b values are established at the 99 confidence level the high b value anomalies are spatially well correlated with low velocity anomalies derived from earlier p wave travel time tomography studies our dataset was not suitable for analyzing changes in b values as a function of time we infer that the high b value anomalies around mount pinatubo are regions of increased crack density and or high pore pressure related to the presence of nearby magma bodies manuscript received 16 december 2002

an analysis of the nonlinear magma edifice coupling at grimsvotn volcano iceland continuous monitoring of seismicity and surface displacement of active volcanoes can reveal important features of the eruptive cycle here high quality gps and earthquake data recorded at grimsvotn volcano by the icelandic meteorological office during the 2004 2011 inter eruptive period are analyzed these showed a characteristic pattern with an initial 2 year long exponential decay followed by 3 year long constant surface displacement inflation rate we model it by using a one magma reservoir model in an elastic damaging edifice with incompressible magma and constant pressure at the base of the magma conduit seismicity rate and damage were first modeled and simple analytical expressions were derived for the magma reservoir overpressure and surface displacement as functions of time very good fits of the seismicity and surface displacement data were obtained by fitting only three phenomenological parameters characteristic time and power strain show maxima from which reference times were inferred that split the inter eruptive period into five periods after the pressurization periods damage occurring in the third period induced weakly nonlinear variations in magma overpressure and flow and surface displacement during the fourth period the damage dominated and variations became more strongly nonlinear the reservoir overpressure decreased and magma flow increased this process lasted until the power strain reached its second maximum where instability was generalized this maximum is a physical limit the occurrence of which shortly precedes rupture and eventually eruption this analysis allows characterization of the state of the volcanic edifice during the inter eruptive period and supports medium term prediction of rupture and eruption

the 15 march 2007 explosive crisis at stromboli volcano italy assessing physical parameters through a multidisciplinary approach basaltic volcanoes are dominated by lava emission and mild explosive activity nevertheless many basaltic systems exhibit from time to time poorly documented and little understood violent explosions a short lived multiblast explosive crisis paroxysmal explosion occurred on 15 march 2007 during an effusive eruptive crisis at stromboli italy the explosive crisis which started at 20 38 14 ut had a total duration of 5 min the combined use of multiparametric data collected by the permanent instrumental networks seismic acoustic and thermal records and a field survey carried out immediately after the event enabled us to constrain the eruptive dynamics and quantify physical parameters the eruption consisted of three major pulses in the first lithic blocks and ash were ejected at speeds of 100 155 m s and 130 210 m s respectively the high solid load of the eruptive jet resulted in the partial collapse of the column with the formation of a small volume pyroclastic density current the second 12 s long pulse emitted 2 2 2 7 107 kg of tephra mass discharge rate 1 9 2 3 106 kg s forming a 3 km high convective plume dispersing tephra up to the west coast and a dilute density current with limited dispersal downslope of the craters a final 30 s long phase formed a scoria flow with a volume of 1 5 1 7 104 m3 mass discharge rate 5 9 6 7 105 kg s a total runout of 200 m and a velocity of 45 m s the total gas volume involved in the explosion was 1 3 1 9 104 m3 with an initial overpressure of 7 9 0 4 mpa we compared the 15 march 2007 event with historical paroxysms in particular with that of 5 april 2003 which was remarkably similar

variations in earthquake size distribution across different stress regimes the earthquake size distribution follows in most instances a power law1 2 with the slope of this power law the b value commonly used to describe the relative occurrence of large and small events a high b value indicates a larger proportion of small earthquakes and vice versa statistically significant variations of b values have been measured in laboratory experiments mines and various tectonic regimes such as subducting slabs near magma chambers along fault zones and in aftershock zones3 however it has remained uncertain whether these differences are due to differing stress regimes as it was questionable that samples in small volumes such as in laboratory specimens mines and the shallow earth s crust are representative of earthquakes in general given the lack of physical understanding of these differences the observation that b values approach the constant 1 if large volumes are sampled4 was interpreted to indicate that b 1 is a universal constant for earthquakes in general5 here we show that the b value varies systematically for different styles of faulting we find that normal faulting events have the highest b values thrust events the lowest and strike slip events intermediate values given that thrust faults tend to be under higher stress than normal faults we infer that the b value acts as a stress meter that depends inversely on differential stress

mapping active magma chambers by b values beneath the off ito volcano japan the b value of the frequency magnitude relation and thus the mean magnitude in the off ito volcanic area is not uniform based on detailed b value tomography using about 10 000 events in an area of 10 km radius and in the upper 15 km of the crust a high b value anomaly b 15 below 7 km depth and with a radius of about 2 km located below the coast southeast of ito contrasts with lower values of typically b 0 7 north of it and at shallower depths on the basis of surface deformations a tensile crack dislocation source was located above 7 km depth thus we surmise that the magma chamber in the off ito area was located below 7 km depth the correlation of a high b value anomaly with this model supports our hypothesis that active magma chambers may be mapped by high b value anomalies the magma body may be described as consisting of two parts a lower well established chamber below 11 km depth and a shallower part 11 7 km depth the b value in the volume surrounding the magma chamber increased from 0 7 during the 1980s to 1 5 during the earthquake swarms of the 1990s this may reflect a trend of increasing crack density due to the intrusion activity

statistical evidence of transitioning open vent activity towards a paroxysmal period at volcan santiaguito guatemala during 2014 2018 abstract long term eruptive activity at the santiaguito lava dome complex guatemala is characterised by the regular occurrence of small to moderate size explosions from the active caliente dome between november 2014 and december 2018 we deployed a seismo acoustic network at the volcano which recorded several changes in the style of eruption including a period of elevated explosive activity in 2016 here we use a new catalogue of explosions to characterise changes in the eruptive regime during the study period we identify four different phases of activity based on changes in the frequency and magnitude of explosions at the two ends of the spectrum of repose times we find pairs of explosions with near identical seismic and acoustic waveforms recorded within 1 10 min of one another and larger explosions with recurrence times on the order of days to weeks the magnitude frequency relationship for explosions at santiaguito is well described by a power law we show that changes in b value between eruptive regimes reflect temporal and spatial changes in rupture mechanisms likely controlled by variable magma properties we also demonstrate that the distribution of inter explosion repose times between and within phases is well represented by a poissonian process the poissonian distribution describing repose times changes between and within phases as the source dynamics evolve we find that changes in source properties restrict the extrapolation of explosive behaviour to within a given eruptive phase limiting the potential for long term assessments of anticipated eruptive behaviour at santiaguito

spatial variations in the frequency magnitude distribution of earthquakes at soufriere hills volcano montserrat west indies the frequency magnitude distribution of earthquakes measured by the b value is determined as a function of space beneath soufriere hills volcano montserrat from data recorded between august 1 1995 and march 31 1996 a volume of anomalously high b values b 3 0 with a 1 5 km radius is imaged at depths of 0 and 1 5 km beneath english s crater and chance s peak this high b value anomaly extends southwest to gage s soufriere at depths greater than 2 5 km volumes of comparatively low b values b 1 are found beneath st george s hill windy hill and below 2 5 km depth and to the south of english s crater we speculate the depth of high b value anomalies under volcanoes may be a function of silica content modified by some additional factors with the most siliceous having these volumes that are highly fractured or contain high pore pressure at the shallowest depths

minimum magnitude of completeness in earthquake catalogs examples from alaska the western united states and japan we mapped the minimum magnitude of complete reporting m c for alaska the western united states and for the junec earthquake catalog of japan mc was estimated based on its departure from the linear frequency magnitude relation of the 250 closest earthquakes to grid nodes spaced 10 km apart in all catalogs studied mc was strongly heterogeneous in offshore areas the mc was typically one unit of magnitude higher than onshore on land also mc can vary by one order of magnitude over distance less than 50 km we recommend that seismicity studies that depend on complete sets of small earthquakes should be limited to areas with similar mc or the minimum magnitude for the analysis has to be raised to the highest com mon value of mc we believe that the data quality as reflected by the mc level should be used to define the spatial extent of seismicity studies where mc plays a role the method we use calculates the goodness of fit between a power law fit to the data and the observed frequency magnitude distribution as a function of a lower cutoff of the magnitude data mc is defined as the magnitude at which 90 of the data can be modeled by a power law fit mc in the 1990s is approximately 1 2 0 4 in most parts of california 1 8 0 4 in most of alaska aleutians and panhandle excluded and at a higher level in the junec catalog for japan various sources such as ex plosions and earthquake families beneath volcanoes can lead to distributions that cannot be fit well by power laws for the hokkaido region we demonstrate how neglecting the spatial variability of m c can lead to erroneous assumptions about deviations from self similarity of earthquake scaling

role of pore fluids in the generation of seismic precursors to shear fracture a systematic study of temporal changes in seismic b values defined as the log linear slope of the earthquake frequency magnitude distribution has shown that large earthquakes are often preceded by an intermediate term increase in b followed by a decrease in the months to weeks before the earthquake1 the onset of the b value increase can precede earthquake occurrence by as much as 7 years a recently proposed fracture mechanics model of the earthquake source2 explains these temporal fluctuations in b in terms of the underlying physical processes of time varying applied stress and crack growth the model predicts two minima in b separated by a short lived maximum here we report the results of controlled laboratory deformation experiments done in simulated upper crustal conditions on both air dried and water saturated rock specimens as found in previous experiments3 5 shear fracture in dry specimens is characterized by a decline in b during anelastic deformation to a single minimum reached just before failure but in water saturated specimens when pore fluid volume is kept constant by servo control we also observe a second intermediate term b value minimum so reproducing the double b value anomaly predicted by the model2

interface locking along the subduction megathrust from b value mapping near nicoya peninsula costa rica 1 to understand controlling factors for seismogenesis and strain accumulation in subduction megathrusts we examine seismicity patterns across nicoya costa rica to determine the overall and spatial variability in the earthquake frequency magnitude distribution along the interface the mean reduction in earthquake activity with magnitude b value is higher 1 06 than global subduction zone averages 0 6 0 8 suggesting the interface here is weakly coupled strong spatial variations in b are anticorrelated 0 53 with geodetic estimates of interface locking of e norabuena et al 2004 high b prevails in two regions including the subducted fisher seamount chain a broad zone of reduced b is observed at and offshore the central nicoya coast extending towards an imaged locked patch these results suggest b value studies may be useful in identifying regions of increased interface locking in subduction zones which may indicate regions capable of large slip in future large earthquakes

interannual variations of volume transport in the western labrador sea based on topex poseidon and woce data abstract the interannual variations of volume transport in the western labrador sea are estimated using six years of topex poseidon altimeter data and hydrographic data from a woce repeat section and a method based on the linear momentum equation in which the sea surface is the level of known motion the interannual variation of the total transport in spring summer has a range of 6 2 sv sv 106 m3 s 1 and is positively correlated with the fall winter north atlantic oscillation nao index and wind patterns in the northwest north atlantic the total transport anomaly is decomposed into a barotropic and a baroclinic component the interannual change of the barotropic transport is similar to that of the total transport and is positively correlated with the fall winter nao index the baroclinic transport anomaly in comparison has a smaller magnitude and the opposite sign the authors conjecture that the deepened icelandic low in high index years generates a strong cyclonic wind stress curl which in tur

seasonal variability of the labrador current and shelf circulation off newfoundland 1 three dimensional finite element models were established for the newfoundland and labrador shelf to investigate climatological monthly mean wind and density driven circulation the model was forced using wind stresses from the national center for environmental prediction national center for atmospheric research reanalysis data prescribed at the sea surface large scale remote forcing determined from a north atlantic model monthly mean temperature and salinity climatology and m2 tide on the open boundary the model results were examined against various in situ observations moored current meter tide gauge and vessel mounted acoustic doppler current profiler data and satellite drift measurements and discussed together with literature information the seasonal mean circulation solutions were investigated in terms of relative importance of wind to density forcing for the labrador current the model results indicate significant seasonal and spatial variations consistent generally with previous study results and in approximate agreement with observations for the major currents the region is dominated by the equatorward flowing labrador current along the shelf edge and along the labrador and newfoundland coasts the labrador current is strong in the fall winter and weak in the spring summer the mean transport of the shelf edge labrador current is 7 5 sv at the seal island transect and 5 5 sv through the flemish pass the seasonal ranges are 4 5 and 5 2 sv at the two sections respectively density and wind driven components are both important in the inshore labrador current the density driven component dominates the mean component of the shelf edge labrador current while the large scale wind forcing contributes significantly to its seasonal variability

a poisson model for earthquake frequency uncertainties in seismic hazard analysis 1 frequency magnitude distributions and their associated uncertainties are of key importance in statistical seismology when fitting these distributions the assumption of gaussian residuals is invalid since event numbers are both discrete and of unequal variance in general the observed number in any given magnitude range is described by a binomial distribution which given a large total number of events of all magnitudes approximates to a poisson distribution for a sufficiently small probability associated with that range in this paper we examine four earthquake catalogues new zealand institute of geological and nuclear sciences southern california southern california earthquake center the preliminary determination of epicentres and the harvard centroid moment tensor both held by the united states geological survey using independent poisson distributions to model the observations we demonstrate a simple way of estimating the uncertainty on the total number of events occurring in a fixed time period

monitoring the volcanic unrest of el hierro canary islands before the onset of the 2011 2012 submarine eruption 1 on 10 october 2011 a submarine volcanic eruption started 2 km south from el hierro island spain since july 2011 a dense multiparametric monitoring network was deployed all over the island by instituto geografico nacional ign by the time the eruption started almost 10000 earthquakes had been located and the deformation analyses showed a maximum deformation of more than 5 cm earthquake migration from the north to the south of the island and acceleration of seismicity are in good correlation with changes in the deformation pattern as well as with some anomalies in geochemical and geomagnetic parameters an earthquake of local magnitude 4 3 at 12 km depth 8 october 2011 and shallower seismicity a day after preceded the onset of the eruption this is the first time that a volcanic eruption is fully monitored in the canary islands data recorded during this unrest episode at el hierro will contribute to understand reawakening of volcanic activity in this region and others of similar characteristics

a characteristic earthquake model of the seismicity preceding the eruption of mount st helens on 18 may 1980 abstract the two classes of earthquakes which preceded the explosive eruption of mount st helens washington on 18 may 1980 can be related to two different forms of energy release this is manifest both in the predominant periods of the two classes of seismic energy release at 1 0 and 0 55 s and in the ratio of their source dimensions in the range 2 1 to 6 1 inferred from characteristic magnitudes of the two possible classes of earthquakes apparent in the discrete frequency magnitude distribution the observed increase in the ratio of long to short period amplitudes of surface waves recorded on mso a local wwssn station can be numerically related to the growth of a magma chamber or conduit within which are generated the transient oscillations thought to be responsible for volcanic tremor these calculations predict an increase in the chamber s diameter at a rate of 2 7 m day 1 at depth in the 2 months preceding the eruption and this is consistent with an observed growth rate of 2 m day 1 of the precursory bulge which appeared at the surface on the mountain s north flank at the time the frequency magnitude distribution is analysed in two parts corresponding to a power law at low magnitudes and a gaussian distribution of typical fault lengths at high magnitudes an extension of the characteristic earthquake model first developed for individual fault zones the power law can in general have a non integer exponent which can be equated to a fractal self similarity dimension d temporal changes in d which is proportional to the seismic b value are proposed as an alternative and complementary description of the way seismic energy is released to the more commonly used viewpoints of changes in stress and heterogeneity despite a reassessment of the completeness of the magnitude catalogue no clear seismic precursor to the explosive eruption is observed in the b value although an earlier phase of phreatic eruptions was strongly associated with a b value anomaly

are volcanic seismic b values high and if so when abstract the gutenberg richter exponent b is a measure of the relative proportion of large and small earthquakes it is commonly used to infer material properties such as heterogeneity or mechanical properties such as the state of stress from earthquake populations it is well known that the b value tends to be high or very high for volcanic earthquake populations relative to b 1 for those of tectonic earthquakes and that b varies significantly with time during periods of unrest we first review the supporting evidence from 34 case studies and identify weaknesses in this argument due predominantly to small sample size the narrow bandwidth of magnitude scales available variability in the methods used to assess the minimum or cutoff magnitude m c and to infer b informed by this we use synthetic realisations to quantify the effect of choice of the cutoff magnitude on maximum likelihood estimates of b and suggest a new work flow for this choice we present the first quantitative estimate of the error in b introduced by uncertainties in estimating m c as a function of the number of events and the b value itself this error can significantly exceed the commonly quoted statistical error in the estimated b value especially for the case that the underlying b value is high we apply the new methods to data sets from recent periods of unrest in el hierro and mount etna for el hierro we confirm significantly high b values of 1 5 2 5 prior to the 10 october 2011 eruption for mount etna the b values are indistinguishable from b 1 within error except during the flank eruptions at mount etna in 2001 2003 when 1 5 b b 1 our results confirm that these volcano tectonic earthquake populations can have systematically high b values especially when associated with eruptions at other times they can be indistinguishable from those of tectonic earthquakes within the total error the results have significant implications for operational forecasting informed by b value variability in particular in assessing the significance of b value variations identified by sample sizes with fewer than 200 events above the completeness threshold

spatial variation of seismic b values beneath makushin volcano unalaska island alaska abstract the frequency magnitude distribution was spatially mapped beneath makushin volcano unalaska island alaska using an earthquake catalog of 491 events that occurred between july 2001 and april 2005 an area of high seismic b values 2 0 is found 4 km east of makushin s main vent at a depth between 4 and 7 km the anomaly is statistically significant based on utsu s p test t utsu on seismicity in report of the joint research institute for statistical mathematics tokyo 1992 139 157 and is not data processing method or parameter dependent interestingly a recent insar interferometric study z lu j a power v s mcconnell c wicks jr d dzurism preeruptive inflation and surface interferometric coherence characteristics by satellite radar interferometry at makushin volcano alaska 1993 2000 j geophys res 107 2002 2266 doi 10 1029 2001jb000970 inferred a surface uplift of about 7 cm during the two year period prior to october 1995 centered approximately in the area with the observed anomalous b values the uplift was caused by the volume increase of an inferred magma chamber at a depth of about 7 km the close correspondence of the seismic and insar observations suggests that the heterogeneous area associated with the observed high b values is most likely the result of increased crack density associated with the magma chamber this study demonstrates the effectiveness of combining insar and seismological observations in locating magma chambers and areas of high heterogeneity in the crust

seismicity at the volcanoes of katmai national park alaska july 1995 december 1997 abstract upper crustal seismicity located within katmai national park occurs mainly within four distinct clusters originating near martin mageik mm subnet volcanoes trident volcano katmai caldera and snowy volcano analyses of earthquake frequency magnitude distributions reveal high b values beneath mm subnet 1 5 normal b values at trident volcano 1 0 and intermediate b values at katmai caldera 1 3 for all seismicity occurring between july 1995 and december 1997 detailed analyses of subsets of b values and hypocenter locations at mm subnet reveal a temporal increase in b value associated with an increase in the maximum depth of seismicity the changes occurred during a shallow earthquake swarm beneath mageik volcano between october 16 and 25 1996 and again in november december 1997 before the swarm the weighted least squares b value was 1 01 at mm subnet increased to 1 59 during the swarm and remained anomalously high until april 1997 the corresponding maximum depth of seismicity is generally less than 5 km for well located earthquakes occurring after september 18 1996 but shifted to 10 km during both the october 1996 swarm and the november december 1997 period the november december 1997 event is not associated with an increase in the rate of seismicity or the b value the october 1996 swarm had a cumulative moment release of 5 0 10 20 dyn cm and decayed from a peak rate of 110 events per day with a modified omori law p value of 1 06 0 11 modelling by the flow law with a 10 km depth limit for seismicity suggests that temperature gradients are on the order of 20 40 c km 1 in agreement with the p value estimate we infer that a simple pressurizing intrusion was not associated with the october swarm because higher stresses should increase seismicity and lower the b value the opposite of what we observed alternatively an actively degassing intrusion would reduce the effective stress and increase the b value at shallow depths while the increased stress would induce seismicity at depth surface temperature measurements taken at the mageik crater lake in july 1995 and again in july 1997 revealed an increase of about 10 c in the lake water temperature consistent with a degassing intrusive event

mapping of the b value anomalies beneath mt etna italy during july august 2001 lateral eruption 1 the pattern of b value of the frequency magnitude relation or mean magnitude has been analyzed as function of space beneath mt etna during the intense lateral eruption of july august 2001 the data used august 1999 august 2001 subdivided in the pre eruptive and eruption period show significant space variations in the b value for each of the two periods a volume of anomalously high b value can be observed in the southeastern flank of the volcano the anomalies located at 3 1 km b 1 84 0 20 and at about 1 1 km b 2 11 0 29 depth are interpreted as two different steps of a magma migration process ended with the july august 2001 eruption all the available geophysical evidences such as geodetic deformation measurements focal mechanism and tomographic studies support this interpretation

spatial mapping of the b value at mount etna italy using earthquake data recorded from 1999 to 2005 1 the spatial pattern of the b value of the frequency magnitude relation has been analyzed using gridding techniques beneath mount etna italy a regional data set of 2900 events with md duration magnitude 1 5 up to 15 km depth occurring between august 1999 and december 2005 has been used two regions with an abnormally high b value have been found one centered beneath the southern part of the valle del bove above the 6 km below sea level bsl deep basement and the other beneath the summit region 2 km bsl east of the central craters we can infer that these high b value anomalies are regions of increased crack density and or high pore pressure related to the presence of nearby magma storage this interpretation is supported by all the available geophysical evidence such as tomographic studies and geodetic deformation measurements the data set has also been subdivided into five periods corresponding to different phases of volcanic activity 2001 preeruption 2001 eruptive 2002 2003 preeruption 2002 2003 eruptive and 2002 2003 posteruption the minimum magnitude of completeness mc and the b value were computed for each period a volume of anomalously high b values can be observed in each of these periods except for the 2002 2003 preeruption interval this approach has allowed the detection of the transient presence of magmatic intrusions during the various periods evaluated

depth dependence of earthquake frequency magnitude distributions in california implications for rupture initiation statistics of earthquakes in california show linear frequency magnitude relationships in the range of m2 0 to m5 5 for various data sets assuming gutenberg richter distributions there is a systematic decrease in b value with increasing depth of earthquakes we find consistent results for various data sets from northern and southern california that both include and exclude the larger aftershock sequences we suggest that at shallow depth 0 to 6 km conditions with more heterogeneous material properties and lower lithospheric stress prevail rupture initiations are more likely to stop before growing into large earthquakes producing relatively more smaller earthquakes and consequently higher b values these ideas help to explain the depth dependent observations of foreshocks in the western united states the higher occurrence rate of foreshocks preceding shallow earthquakes can be interpreted in terms of rupture initiations that are stopped before growing into the mainshock at greater depth 9 15 km any rupture initiation is more likely to continue growing into a larger event so there are fewer foreshocks if one assumes that frequency magnitude statistics can be used to estimate probabilities of a small rupture initiation growing into a larger earthquake then a small m2 rupture initiation at 9 to 12 km depth is 18 times more likely to grow into a m5 5 or larger event compared to the same small rupture initiation at 0 to 3 km

mode switching in volcanic seismicity el hierro 2011 2013 the gutenberg richter b value is commonly used in volcanic eruption forecasting to infer material or mechanical properties from earthquake distributions such studies typically analyze discrete time windows or phases but the choice of such windows is subjective and can introduce significant bias here we minimize this sample bias by iteratively sampling catalogs with randomly chosen windows and then stack the resulting probability density functions for the estimated b value to determine a net probability density function we examine data from the el hierro seismic catalog during a period of unrest in 2011 2013 and demonstrate clear multimodal behavior individual modes are relatively stable in time but the most probable b value intermittently switches between modes one of which is similar to that of tectonic seismicity multimodality is primarily associated with intermittent activation and cessation of activity in different parts of the volcanic system rather than with respect to any systematic inferred underlying process

temperature and salinity variability on the scotian shelf and in the gulf of maine 1945 1990 examination of temperature and salinity data from the scotian shelf gulf of maine gulf of st lawrence and the adjacent continental slope has shown that the dominant low frequency event over the last 45 years was a cooling and subsurface freshening of the water masses from 1952 to 1967 followed by a rapid reversal of these trends the largest temperature and salinity changes 1952 1967 were 4 6 c and 0 7 respectively and occurred at about 100 m over the slope exchanges with shelf waters and vertical mixing gave rise to the surface manifestation of this variability the westward transport of the labrador current was found to have similar variability increasing from about 1 106 m3 s 1 in the early 1950s to about 4 106 m3 s 1 in the mid 1960s a simple model that accounts for this variation of transport and has a constant entrainment of north atlantic water indicates that changes of the westward flow of the labrador current could contribute significantly to the t s fluctuations

insights into the 2011 2012 submarine eruption off the coast of el hierro canary islands spain from statistical analyses of earthquake activity summary the purpose of this work is to gain insights into the 2011 2012 eruption of el hierro canary islands by mapping the evolution of the seismic b value the el hierro seismic sequence offers a rather unique opportunity to investigate the process of reawakening of an oceanic intraplate volcano after a long period of repose the 2011 2012 eruption is a submarine volcanic event that took place about 2 km off of the southern coast of el hierro the eruption was accompanied by an intense seismic swarm and surface manifestations of activity the earthquake catalogue during the period of unrest includes over 12 000 events the largest with magnitude 4 6 the seismic sequence can be grouped into three distinct phases which correspond to well separated spatial clusters and distinct earthquake regimes the estimated b value is of 1 18 0 03 and a magnitude of completeness of 1 3 for the entire catalogue b is very close to 1 0 which indicates completeness of the earthquake catalogue with only minor departures from the linearity of gutenberg richter frequency magnitude distribution the most straightforward interpretation of this result is that the seismic swarm reached its final stages and no additional large magnitude events should be anticipated similarly to what one would expect for non volcanic earthquake sequences the results dividing the activity in different phases illustrate remarkable differences in the estimate of b value during the early and late stages of the eruption the early pre eruptive activity was characterized by a b value of 2 25 in contrast the b value was 1 25 during the eruptive phase based on our analyses and the results of other studies we propose a scenario that may account for the observations reported in this work we infer that the earthquakes that occurred in the first phase reflect magma migration from the upper mantle to crustal depths the area where magma initially intruded into the crust because of its transitional nature is characterized by high fracturing thus favours anomalously high b values the larger magnitude earthquakes recorded in the second phase may reflect relaxation around the magma reservoir that had fed the eruption and thus lower b values

crustal anisotropy and state of stress at uturuncu volcano bolivia from shear wave splitting measurements and magnitude frequency distributions in seismicity abstract the physical signatures of unrest in large silicic magma systems are commonly observed in geophysical surveys yet the interactions between magmatic processes and crustal stresses are often left unconstrained stresses in the mid and upper crust exert a strong control on the propagation and stalling of magma and magma ascent can in turn change the magnitude and orientation of these stresses including those associated with hydrothermal systems this study assesses the state of stress at the restless uturuncu volcano in the bolivian andes with space depth and time using observations of seismic anisotropy and the magnitude frequency distributions of local earthquakes shear wave splitting measurements are made for 677 events in the upper crust 1 25 km below sea level between june 1 2009 and march 10 2012 and b values are calculated using the aki maximum likelihood method for a range of catalog subsets in the entire crust 5 to 65 km below sea level the b value of the crustal events is unusually low b 0 66 0 09 indicating that the seismogenic region features strong material with high stresses that are released with limited influence from hydrothermal fluids the 410 good quality shear wave splitting results have an average delay time of 0 06 0 002 s and an average percent anisotropy ranging from 0 25 0 04 to 6 2 0 94 with a mean of 1 70 0 32 fast shear wave polarization directions are highly variable and appear to reflect a combination of tectonic and magmatic stresses that overprint the regional e w compressive stress associated with the convergence of the nazca and south american plates the shear wave splitting results and b values suggest that the upper crust beneath uturuncu 0 7 km below the summit is characterized by a weak and localized hydrothermal system in a poorly developed fracture network we conclude that stresses imposed by crustal flexure due to magmatic unrest above the altiplano puna magma body activate crack opening on a pre existing fault beneath the volcano generating seismicity and a spatially variable 1 10 anisotropy above the brittle ductile transition zone these results suggest that strong stresses in relatively unfractured upper crustal rocks may locally inhibit fluid migration in large silicic magma systems leading to pluton emplacement and effusive volcanism rather than explosive eruptions

seasonal variation of the three dimensional mean circulation over the scotian shelf the seasonal mean circulation over the scotian shelf is studied numerically by computing mean and tidal current fields for winter spring and summer using a three dimensional nonlinear diagnostic model the mean current fields are forced by seasonal mean baroclinic pressure gradients tidal rectification uniform wind stresses and associated barotropic pressure gradients a historical hydrographic database is used to determine the climatological mean baroclinic forcing upstream open boundary conditions are estimated from the density fields to give no normal geostrophic bottom flow and are specified as either along boundary elevation gradients or depth integrated normal velocities the numerical solutions for nominal bimonthly periods january february april may and july august reveal the dominant southwestward nearshore and shelf break flows of relatively cool and fresh shelf water from the gulf of st lawrence and newfoundland shelf with speeds up to about 20 cm s the seasonal intensification of the southwestward flows is reproduced by the model with the transport increasing from 0 3 sv in summer to 0 9 sv in winter on the inner halifax section there are also pronounced topographic scale influences of submarine banks basins and cross shelf channels on the circulation such as anticyclonic gyres over banks and cyclonic gyres over basins baroclinicity is the dominant forcing throughout the domain but tidal rectification is comparable on the southwestern scotian shelf e g about 0 2 sv recirculating transport around browns bank for all the periods the mean wind stress generates offshore surface drift in winter the solutions are in approximate agreement with observed currents and transports over the scotian shelf although there are local discrepancies

the mean barotropic circulation on the newfoundland shelf and slope we present results of a numerical model calculation of the barotropic mean circulation in the grand banks region of the continental shelf off eastern canada the modeled circulation is compared with available current data and shows qualitative and some quantitative agreement the model accounts for the general flow of the labrador current along the shelf edge its splitting into two branches north of flemish pass and south of cabot strait and the circulation over the grand banks on the other hand it underestimates the strength of the clockwise circulation over flemish cap and the width of the labrador current it is also possible that neglect of the influence of the gulf stream and north atlantic current gives rise to local discrepancies between observed and modeled flows

the process of serpentinization in dunite from new caledonia abstract dunite from new caledonia displays three types of serpentine dominated veins the earliest type 1 veins are narrow 50 100 m wide and rarely extend across more than a single olivine grain they are lizardite contain abundant brucite and never contain magnetite type 2 veins are 0 01 to 0 1 mm wide extend across several olivine grains and cut across the type 1 veins they are lizardite contain magnetite often in vein interiors and contain less brucite than type 1 veins type 3 veins are dominantly chrysotile cm scale have a magnetite rich core and extend for meters or more analyses of two representative samples indicate that the type 1 veins have relatively fe rich serpentine xmg 0 92 and brucite xmg 0 82 these minerals are less magnesian than those in the type 2 veins serpentine has xmg 0 93 0 94 and brucite has xmg 0 84 in the magnetite rich core to the type 3 vein both serpentine xmg 0 94 0 97 and one of the two brucite populations xmg 0 94 are mg rich opx in harzburgite layers in these samples is cut by serpentine veins that are on the order of 0 05 mm wide the serpentine veins after opx lack talc or magnetite and as with veins cutting olivine the older veins are more fe rich xmg 0 84 than the younger veins xmg 0 90 we conclude that the formation of magnetite was accompanied by the extraction of iron from the early formed serpentine and brucite thermodynamic calculations suggest that the type 1 veins formed in a rock dominated system where the activities of feo mgo and sio2 were dictated by the compositions of olivine and orthopyroxene in contrast the type 2 veins were formed in a more fluid dominated system where the infiltrating fluid was relatively oxidizing and out of equilibrium with the original brucite serpentine assemblage reduction of this fluid was accompanied by reaction of brucite and serpentine to magnetite and hydrogen by producing magnetite this reaction extracted iron from brucite and serpentine making them both more magnesian this would drive the brucite serpentine magnetite assemblage to higher oxygen fugacity progressively decreasing the efficiency of the magnetite forming reactions