self similar rupture implied by scaling properties of volcanic earthquakes occurring during the 2004 2008 eruption of mount st helens washington we analyze a group of 6073 low frequency earthquakes recorded during a week long temporary deployment of broadband seismometers at distances of less than 3 km from the crater at mount st helens in september of 2006 we estimate the seismic moment m0 and spectral corner frequency f0 using a spectral ratio approach for events with a high signal to noise snr ratio that have a cross correlation coefficient of 0 8 or greater with at least five other events a cluster analysis of cross correlation values indicates that the group of 421 events meeting the snr and cross correlation criteria forms eight event families that exhibit largely self similar scaling we estimate the m0 and f0 values of the 421 events and calculate their static stress drop and scaled energy er m0 values the estimated values suggest self similar scaling within families as well as between five of eight families i e m0 f0 3 and er m0 constant we speculate that differences in scaled energy values for the two families with variable scaling may result from a lack of resolution in the velocity model the observation of self similar scaling is the first of its kind for such a large group of low frequency volcanic tectonic events occurring during a single active dome extrusion eruption

stable carbon and oxygen isotopes in sphagnum fuscum peat from subarctic canada implications for palaeoclimate studies stable carbon and oxygen isotope ratios in single plant components in sphagnum peat have a good potential to reveal environmental changes in peat archives two peat profiles covering the past 600

detailed analysis of spatiotemporal variations of the stress field orientation along the izmit duzce rupture in nw turkey from inversion of first motion polarity data

inversion of controlled source audio frequency magnetotellurics data for a horizontally layered earth we present a technique for inverting controlled source audio frequency magnetotelluric csamt data to recover a 1 d conductivity structure the earth is modeled as a set of horizontal layers with constant conductivity and the data are apparent resistivities and phases computed from orthogonal electric and magnetic fields due to a finite dipole source the earth model has many layers compared to the number of data points and therefore the solution is nonunique among the possible solutions we seek a model with desired character by minimizing a particular model objective function traditionally csamt data are inverted either by using the far field data where magnetotelluric mt equations are valid or by correcting the near field data to an equivalent plane wave approximation here we invert both apparent resistivity and phase data from the near field transition zone and the far field regions in the full csamt inversion without any correction our inversion is compared with that obtained by inverting near field corrected data using an mt algorithm both synthetic and field data examples indicate that a full csamt inversion provides improved information about subsurface conductivity

water geochemistry controlled by carbonate dissolution a study of the river waters draining karst dominated terrain guizhou province china concentrations of major ions and sr and sr isotopic compositions of two major river systems wujiang river and yuanjiang river in karst dominated regions and during winter were measured the rivers and their tributaries have high total dissolved solid concentrations with tz ranging from 2 1 to 6 3 meq l which are significantly higher than that of the global river average calcium and hco3 followed by mg2 and so42 dominate the chemical composition of major ions in the river waters both na k and cl si account for 5 10 of the cations and anions respectively the waters of the wujiang river have high sr2 concentrations and lower 87sr 86sr ratios 0 7077 0 7110 while those of the qingshuijiang river one of two major tributaries of the yuanjiang river system show higher sr isotopic ratios 0 7090 0 7145 and low sr concentrations the lithology and geography of the river basins largely control the chemical and isotopic compositions of these rivers analyses of the stoichiometry of the river waters show that the water chemistry is controlled by carbonate dissolution by both carbonic and sulfuric acid especially of the wujiang river and suggest an origin of sulfate mainly from atmospheric input because of coal combustion and or oxidation of sulfide minerals in the catchments we identify three main weathering sources limestone dolomite and silicates chemical and sr isotopic composition of wujiang river waters are characterized mainly by limestone weathering while those of the qingshuijiang river waters mainly by dolomite and silicate weathering chemical weathering rates of carbonate and silicate are estimated and the results indicate that the chemical weathering rates of carbonate in the catchments are remarkably high significantly higher than most large rivers it is considered that presence of sulfuric acid in the rock weathering overuse of land and deterioration of vegetation are most likely the key factors responsible for the high weathering rates of carbonate in the catchments

seismicity triggered by fluid injection induced aseismic slip anthropogenic fluid injections are known to induce earthquakes the mechanisms involved are poorly understood and our ability to assess the seismic hazard associated with geothermal energy or unconventional hydrocarbon production remains limited we directly measure fault slip and seismicity induced by fluid injection into a natural fault we observe highly dilatant and slow 4 micrometers per second m s aseismic slip associated with a 20 fold increase of permeability which transitions to faster slip 10 m s associated with reduced dilatancy and micro earthquakes most aseismic slip occurs within the fluid pressurized zone and obeys a rate strengthening friction law 0 67 0 045ln v v with v 0 1 m s fluid injection primarily triggers aseismic slip in this experiment with micro earthquakes being an indirect effect mediated by aseismic creep

recent developments in surface and subsurface karst geomorphology an introduction abstract where dissolution processes of bedrock dominate peculiar morphologies develop and overrule all landforms controlled by other surface processes pure karst landscapes are present in many parts of the world but most of the time landscapes are shaped by a multitude of processes the comprehension of the dissolution processes that act both at the surface and underground has developed rapidly in the last half century although major achievements had already been reached at the end of the xixth century this special issue gives an overview of some of the most recent developments in surface and subsurface karst geomorphology and reviews where further studies should be fostered

a numerical model for fluid injection induced seismicity at soultz sous forets abstract during fluid injection experiments at the geothermal site of soultz sous forets france more than 114 000 induced seismic events with magnitudes between 2 0 and 2 9 were detected by a local downhole monitoring network of these 35 039 events are sufficiently constrained to be located hypocenters align along a sub vertical planar structure with the apparent width being dominated by data scattering indicating that seismic activity predominantly occurs along a pre existing larger scale fault structure for this scenario we present a numerical model to simulate hydraulic overpressures and induced seismicity during hydraulic injection the numerical model is based on the physical processes of fluid pressure and stress diffusion with triggering of the induced seismicity being controlled by coulomb friction even in its simplest form of a fault zone without any structural heterogeneity the numerical model reproduces typical observations at soultz sous forets such as number and magnitude of induced events hypocenter locations including the kaiser effect occurrence of post injection seismicity and the largest magnitude event occurring several days after shut in

effects of long term fluid injection on induced seismicity parameters and maximum magnitude in northwestern part of the geysers geothermal field the long term temporal and spatial changes in statistical source and stress characteristics of one cluster of induced seismicity recorded at the geysers geothermal field u s are analyzed in relation to the field operations fluid migration and constraints on the maximum likely magnitude two injection wells prati 9 and prati 29 located in the northwestern part of the field and their associated seismicity composed of 1776 events recorded throughout a 7 year period were analyzed the seismicity catalog was relocated and the source characteristics including focal mechanisms and static source parameters were refined using first motion polarity spectral fitting and mesh spectral ratio analysis techniques the source characteristics together with statistical parameters b value and cluster dynamics were used to investigate and understand the details of fluid migration scheme in the vicinity of injection wells the observed temporal spatial and source characteristics were clearly attributed to fluid injection and fluid migration toward greater depths involving increasing pore pressure in the reservoir the seasonal changes of injection rates were found to directly impact the shape and spatial extent of the seismic cloud a tendency of larger seismic events to occur closer to injection wells and a correlation between the spatial extent of the seismic cloud and source sizes of the largest events was observed suggesting geometrical constraints on the maximum likely magnitude and its correlation to the average injection rate and volume of fluids present in the reservoir

evolution of pore fluid pressures in a stimulated geothermal reservoir inferred from earthquake focal mechanisms this paper proposes an inversion method to estimate the evolution of pore fluid pressure fields from earthquake focal mechanism solutions application of the method to induced seismicity in the basel enhanced geothermal system in switzerland shows the evolution of pore fluid pressure in response to fluid injection experiments the pore fluid pressure in the reservoir was less than the minimum principal stress at each depth indicating that hydraulic fracturing did not occur during the stimulation this suggests that seismic events may play an important role in promoting the development of permeable channels particularly southeast of the borehole where the largest seismic event mw 2 95 occurred this is not directly related to a drastic decrease in fault strength at the hypocenter but rather to positive feedback between permeability enhancement and elastic poroelastic stress loading from slipping interfaces

drivers of holocene palsa distribution in north america abstract palsas and peat plateaus are climatically sensitive landforms in permafrost peatlands climate envelope models have previously related palsa peat plateau distributions in europe to modern climate but similar bioclimatic modelling has not been attempted for north america recent climate change has rendered many palsas peat plateaus in this region and their valuable carbon stores vulnerable we fitted a binary logistic regression model to predict palsa peat plateau presence for north america by relating the distribution of 352 extant landforms to gridded modern climate data our model accurately classified 85 3 of grid cells that contain observed palsas peat plateaus and 77 1 of grid cells without observed palsas peat plateaus the model indicates that modern north american palsas peat plateaus are supported by cold dry climates with large seasonal temperature ranges and mild growing seasons we used palaeoclimate simulations from a general circulation model to simulate holocene distributions of palsas peat plateaus at 500 year intervals we constrained these outputs with timings of peat initiation deglaciation and postglacial drainage across the continent our palaeoclimate simulations indicate that this climate envelope remained stationary in western north america throughout the holocene but further east it migrated northwards during 11 5 6 0 ka bp however palsa extents in eastern north america were restricted from following this moving climate envelope by late deglaciation drainage and peat initiation we validated our holocene simulations against available palaeoecological records and whilst they agree that permafrost peatlands aggraded earliest in western north america our simulations contest previous suggestions that late permafrost aggradation in central canada was climatically driven

controlled source audiomagnetotellurics in geothermal exploration theoretical and field tests indicate that the controlled source audiomagnetotelluric csamt method provides an efficient means of delineating the shallow resistivity pattern above a hydrothermal system utilizing a transmitter overcomes the main limitation of conventional audiomagnetotellurics variable and unreliable natural source fields reliable csamt measurements can be made with a simple scalar receiver our calculations for a half space show that the plane wave assumption is valid when the transmitter is more than 3 skin depths away in the broadside configuration and more than 5 skin depths away in the collinear configuration three dimensional 3 d numerical modeling results for a bipole source 5 skin depths away compare well with those for a plane wave source showing that the method is valid a csamt survey at the roosevelt hot springs geothermal area in utah produced apparent resistivity contour maps at four frequencies 32 98 977 and 5208 hz these maps show the same features as those of a

control of hazard due to seismicity induced by a hot fractured rock geothermal project in 2003 hydraulic stimulations were carried out in a geothermal field in eastern el salvador central america as part of a project to explore the feasibility of commercial hot fractured rock energy generation a key requisite for this environmentally friendly energy source is that the fracturing of the hot rocks at depths of 1 2 km must not produce levels of ground shaking at the surface that would present a serious disturbance or threat to the local population thresholds of tolerable ground motion were inferred from guidelines and regulations on tolerable levels of vibration and from correlations between instrumental strong motion parameters and intensity considering the vulnerability of the exposed housing stock the thresholds were defined in terms of peak ground velocity pgv and incorporated into a traffic light system that also took account of the frequency of occurrence of the induced earthquakes the system was implemented through a dedicated seismograph array and locally derived predictive equations for pgv the traffic light was used as a decision making tool regarding the duration and intensity of pumping levels during the hydraulic stimulations the system was supplemented by a small number of accelerographs and re calibrated using records obtained during the rock fracturing

comparison of stable carbon and oxygen isotopes in picea glauca tree rings and sphagnum fuscum moss remains from subarctic canada stable isotope ratios from tree rings and peatland mosses have become important proxies of past climate variations we here compare recent stable carbon and oxygen isotope ratios in cellulose of tr

effects of permafrost aggradation on peat properties as determined from a pan arctic synthesis of plant macrofossils permafrost dynamics play an important role in high latitude peatland carbon balance and are key to understanding the future response of soil carbon stocks permafrost aggradation can control the magnitude of the carbon feedback in peatlands through effects on peat properties we compiled peatland plant macrofossil records for the northern permafrost zone 515 cores from 280 sites and classified samples by vegetation type and environmental class fen bog tundra and boreal permafrost and thawed permafrost we examined differences in peat properties bulk density carbon c nitrogen n and organic matter content and c n ratio and c accumulation rates among vegetation types and environmental classes consequences of permafrost aggradation differed between boreal and tundra biomes including differences in vegetation composition c n ratios and n content the vegetation composition of tundra permafrost peatlands was similar to permafrost free fens while boreal permafrost peatlands more closely resembled permafrost free bogs nitrogen content in boreal permafrost and thawed permafrost peatlands was significantly lower than in permafrost free bogs despite similar vegetation types 0 9 versus 1 5 n median long term c accumulation rates were higher in fens 23g c m 2 yr 1 than in permafrost free bogs 18g c m 2 yr 1 and were lowest in boreal permafrost peatlands 14g c m 2 yr 1 the plant macrofossil record demonstrated transitions from fens to bogs to permafrost peatlands bogs to fens permafrost aggradation within fens and permafrost thaw and reaggradation using data synthesis we have identified predominant peatland successional pathways changes in vegetation type peat properties and c accumulation rates associated with permafrost aggradation

onshore migration of a deep water brachiopod fauna from the lower ordovician tonggao formation jiangnan slope southeastern guizhou province south chinathis article is one of a series of papers published in this special issue on the theme the dynamic reef and shelly communities of the paleozoic this special is in honour of our colleague and friend paul copper brachiopods from the lower tonggao formation floian late early ordovician of sandu southeastern guizhou province represent a deep water benthic shelly fauna from the jiangnan slope facies of the south china paleoplate the fauna contains 16 brachiopod taxa with orthides lingulates and pentamerides being the common groups concurrent graptolites confine the brachiopod fauna to the tetragraptus approximatus and the acrograptus filiformis biozones early floian the first appearance datum fad of several brachiopod genera such as paralenorthis protoskenidioides and nereidella in the sandu slope facies is one or two graptolitic biozones lower than their fad in shallower water facies on the yangtze platform later in the late floian these genera became major components of the sinorthis fauna that flourished over much of the yangtze platform suggesting a migration of the benthic shelly fauna from slope to platform facies during the floian radiation in south china compared to the platform fauna

characterization of fault zones there are currently three major competing views on the essential geometrical mechanical and mathematical nature of faults the standard view is that faults are possibly segmented and heterogeneous euclidean zones in a continuum solid the continuum euclidean view is supported by seismic gravity and electromagnetic imaging studies by successful modeling of observed seismic radiation geodetic data and changes in seismicity patterns by detailed field studies of earthquake rupture zones and exhumed faults and by recent high resolution hypocenter distributions along several faults the second view focuses on granular aspects of fault structures and deformation fields the granular view is supported by observations of rock particles in fault zone gouge by studies of block rotations and the mosaic structure of the lithosphere which includes the overall geometry of plate tectonics by concentration of deformation signals along block boundaries by correlation of seismicity patterns on scales several times larger than those compatible with a continuum framework and by strongly heterogeneous wave propagation effects on the earth s surface the third view is that faults are fractal objects with rough surfaces and branching geometry the fractal view is supported by some statistical analysis of regional hypocenter locations by long range correlation of various measurements in geophysical boreholes by the fact that observed power law statistics of earthquakes are compatible with an underlying scale invariant geometrical structure by geometrical analysis of fault traces at the earth s surface and by measurements of joint and fault surfaces topography

palaeoecology of a palsa and a filled thermokarst pond in a permafrost peatland subarctic quebec canada radiocarbon dating and macrofossil data from a palsa and a filled thermokarst pond within a subarctic permafrost peatland were used to reconstruct their evolution and to distinguish between allogenic and autogenic processes that had been involved in their development peat began to accumulate in the peatland basin shortly after 6000 cal bp the initial stage was a shallow bay or a salty marsh followed by a wet marsh triggered by a relatively rapid drop in sea level related to a rapid isostatic uplift of land by 5640 cal bp the site had transformed into a rich fen a high rate of peat accumulation led to the establishment of a short lived intermediate fen by 4610 cal bp and to a poor fen from 4200 cal bp until 1760 cal bp low water tables associated with decreased precipitation occurred between 5170 and 4610 cal bp and 3100 and 1760 cal bp between 1760 and the little ice age there was ombrotrophication of the site largely as a result of a thick peat accumulation during this period autogenic processes had controlled the general hydrosere while allogenic processes mainly precipitation had influenced species composition permafrost established during the little ice age leading to palsa formation and it has been melting in response to recent climate warming and precipitation increases macrofossil results from the filled thermokarst pond show that plant succession used the followed hydrosere calliergon giganteum and sphagnum riparium when the pond s depth was at a maximum s riparium and c giganteum when the pond was partly filled in and s lindbergii and s riparium since the pond has been completely filled in

shear and tensile earthquakes caused by fluid injection 1 we apply rock mechanics concepts to the seismological observations in order to explain why during hydraulic injection some events display tensile and some shear deformation the presence of non shear components depends on the differential stress and the fracture orientation with respect to the 1 direction provided the slip vector is parallel to the traction we define four types of earthquakes according to the ratio of the shear and tensile components assuming a griffith failure envelope hybrid events containing both shear and tensile components can occur for fractures striking within 22 5 of 1 we argue that pure tensile fractures striking parallel to 1 are unlikely in the presence of natural fractures the low shear traction of tensile events also implies their small stress drops by applying the analysis to two different data sets soultz sous forets and cotton valley we show that different orientations of natural fractures and differential stress in the targeted formations made each region favorable for different non dc components in the injection induced seismicity

a comparison of em induction and gpr methods in areas of karst abstract electromagnetic em induction and ground penetrating radar gpr methods have been used extensively in areas of karst to detect and delineate major structural and solution features and to improve site assessments these methods do not work equally well in all soils and the amount of practical information derived from surveys has varied the choice of the most appropriate method often depends on the electrical properties of the soils this study characterized the use of gpr and em at two contrasting sites in florida and pennsylvania the florida site is underlain by relatively thick layers of electrically resistive sands overlying loamy marine sediments and limestone bedrock gpr was effectively used to determine the thickness of surface layers and the locations of buried solution features multiple contrasting soil horizons and layers weakened relationships and created non unique interpretations for em the pennsylvania site is was underlain by clayey residuum and limestone bedrock the clayey residuum is opaque to gpr and observation depths are severely restricted em induction effectively characterized the depths to bedrock within the pennsylvania site the integrated use of both methods supported by commensurate ground truth verifications helped to increase confidence in interpretations

long term stability of permafrost in subarctic peat plateaus west central canada long term vegetation succession and permafrost dynamics in subarctic peat plateaus of west central canada have been studied through detailed plant macrofossil analysis and extensive ams radiocarbon dating of two peat profiles peatland inception at these sites occurred around 5800 5100 yr bp 6600 5900 cal bp as a result of paludification of upland forests at the northern peat plateau site located in the continuous permafrost zone palaeobotanical evidence suggests that permafrost was already present under the forested upland prior to peatland development paludification was initiated by permafrost collapse but re aggradation of permafrost occurred soon after peatland inception at the southern site located in the discontinuous permafrost zone the aggradation of permafrost occurred soon after peatland inception in the peat plateaus permafrost conditions have remained very stable until present sphagnum fuscum dominated stages have alternated with more xerophytic communities characterized by erica

biochemical evidence for minimal vegetation change in peatlands of the west siberian lowland during the medieval climate anomaly and little ice age peatland vegetation is controlled primarily by the depth of the water table making peat paleovegetation a useful climate archive we applied a biochemical approach to quantitatively estimate the plant sources of peat carbon based on 1 neutral sugar compositions of sphagnum vascular plants and lichens and 2 lignin phenol compositions of vascular plants we used these biochemical indices to characterize vegetation change over the last 2000 years in four peat cores from the west siberian lowland russia to investigate climate change during the medieval climate anomaly and little ice age the vegetation was dominated by sphagnum in all four cores but was punctuated by several rapid but transient transitions to vascular plant dominance in the two cores from the southern west siberian lowland 60 n possibly indicating permafrost development however there was no evidence for sustained vegetation change in response to either climatic event in cores from southern west siberian lowland this suggests that these climatic events were relatively mild in the southern west siberian lowland although the sensitivity of bog plant communities to climate change remains poorly understood

icelandic permafrost dynamics since the last glacial maximum model results and geomorphological implications abstract iceland s periglacial realm is one of the most dynamic on the planet with active geomorphological processes and high weathering rates of young bedrock resulting in high sediment yields and ongoing mass movement permafrost is discontinuous in iceland s highlands and mountains over c 800 m a s l and sporadic in palsa mires in the central highlands during the late pleistocene and holocene iceland s periglacial environment varied considerably in time and space dominated by glacial fluctuations and periglacial processes to evaluate the dynamics of permafrost in iceland since the last deglaciation we use the output of a coupled climate ice sheet model to force a transient permafrost model cryogrid 2 from the last glacial maximum lgm through to the present we find that permafrost was widespread across the deglaciated areas of western northern and eastern iceland after the lgm and that up to 20 of iceland s terrestrial area was underlain by permafrost throughout the late pleistocene this influenced geomorphological processes and landform generation the early collapse of the marine based ice sheet together with the aggradation of permafrost in these zones initiated the formation of abundant and now relict rock glaciers across coastal margins permafrost degraded rapidly after the younger dryas with a marked impact on slope stability permafrost that formed during the little ice age is again thawing rapidly and an escalation in slope failure and mass movement might be currently underway our study demonstrates that large regions of iceland have been underlain by permafrost for millennia facilitating landform development and influencing the stability of steep slopes

results of a controlled source audiofrequency magnetotelluric survey at the puhimau thermal area kilauea volcano hawaii the puhimau thermal area in the volcanoes national park hawaii has attracted interest because it represents an area with anomalously high heat flow and the heat source may be a shallow magma body at puhimau a variety of geophysical data including heat flow measurements have been acquired the self potential data suggest that the heat source is a result of a magma intrusion which plunges steeply northward the heat flow data can be explained in terms of a shallow relatively thick magma intrusion with a solidification rate sufficient to supply the surficial heat flow for the period of time since the appearance of the thermal area in 1936 in february of 1984 we performed a controlled source audiofrequency magnetotelluric csamt electromagnetic geophysical survey at the puhimau thermal area to investigate the electrical nature of the thermal anomaly and estimate the depth to any hot water and or remaining molten magma the csamt survey consisted of several lines of stations where two orthogonal prima

the application of high resolution 3d seismic acquisition techniques for carbonate reservoir characterization in china this paper describes major challenges to seismic acquisition in china s tarim basin figure 1 and how high resolution 3d seismic surveys can improve reservoir imaging in this area

carbon accumulation in permafrost peatlands in the northwest territories and nunavut canada average long term apparent rates of carbon c accumulation larca were estimated for four peat cores from arctic and subarctic canada detailed analyses of dry bulk density and c content were used to determine variations in c accumulation rates throughout the cores larca range from 12 5 to 16 5 g c m 2 yr 1 over the past 6700 10000 years rates are lower for the surface layers of arctic high centred peat polygons at 5 3 to 7 1 g c m 2 yr 1 for the last 3500 4500 years by comparison the rate for the near surface peat from a sphagnum fuscum hummock in the high subarctic was considerably higher at 24 1 g c m 2 yr 1 the highest carbon accumulation rates were from core segments older than 4500 bp which represent fen stages according to palaeoecological analysis the average larca in our study are considerably lower than recent estimates of average carbon accumulation in boreal peatlands this difference is attributable partly to lower carbon percentages in our cores compared to the mean or estimated v

long term climate variability in continental subarctic canada a 6200 year record derived from stable isotopes in peat the rapid warming of arctic regions during recent decades has been recorded by instrumental monitoring but the natural climate variability in the past is still sparsely reconstructed across many a

near surface permafrost aggradation in northern hemisphere peatlands shows regional and global trends during the past 6000 years the history of permafrost aggradation and thaw in northern peatlands can serve as an indicator of regional climatic history in regions where records are sparse we infer regional trends in the timing of permafrost aggradation and thaw in north american and eurasian peatland ecosystems based on plant macrofossils and peat properties using existing peat core records from more than 250 cores results indicate that permafrost was continuously present in peatlands during the last 6000 years in some present day continuous permafrost zones and formed after 6000 bp in peatlands in the isolated to discontinuous permafrost regions rates of permafrost aggradation in peatlands generally increased after 3000 bp and were greatest between 750 and 0 bp corresponding with neoglacial cooling and the little ice age lia respectively peak periods of permafrost thaw occurred after 250 bp when permafrost aggradation in peatlands reached its maximum extent and as temperatures began warming after the lia suggesting that p

audiomagnetotelluric characterization of range front faults snake range nevada two controlled source audio magnetotelluric csamt profiles were collected on the eastern flank of the snake range in eastern nevada across geologically complex terrain to investigate the suspected presence of faults along the range front the location of the range bounding faults is not easily determined on geologic grounds because of the presence of extensive young sedimentary cover and overall geologic complexity characterization of the presence location and structural style of the range front faults is critical to assessment of connectivity of groundwater aquifers near the mountain front and in adjacent alluvial basins a total of 48 csamt soundings were recorded along two lines that were chosen to maximize subsurface geologic information two generations of faults were interpreted based on the csamt data an older low angle fault that is cut by a younger more steeply dipping fault lack of deep boreholes in the region required that the subsurface interpretation rely on analogy from surface outcrops within and adjacent to the study area the success of the csamt method as applied in this study hinged on near ideal collection conditions the relatively high contrast in electrical resistivity provided by the rock types involved and well developed geologic conceptual models of the region

mapping chromite deposits with audio magnetotellurics in the luobusa ophiolite of southern tibet the exploration of podiform chromites in the indus yarlong zangbo suture zone of southern tibet has proved difficult in recent decades because most known deposits pinch out and then reappear in the same direction a number of ground based geophysical approaches including gravity magnetic and controlled source audio frequency magnetotelluric methods have been applied to explore for these chromite deposits but have mostly failed to delineate prospective areas we present a successful podiform chromite exploration case history that is based on audio magnetotellurics amt more than 8 000 amt stations were utilized in this study within a 10 km2 area of the ophiolite belt line separations were 80 m or 40 m and station separation was 20 m we implemented bostick conversion and non linear conjugate gradient inversions were implemented for data interpretation while two dimensional 2d resistivity sections and three dimensional 3d resistivity imaging were used to elucidate the inner structure and distribu

mapping deeply buried karst cavities using controlled source audio magnetotellurics a case history of a tunnel investigation in southwest china abstractkarst cavity mapping is attracting great interest from engineering geologists because of its relation to the dangerous geohazards faced during engineering construction ground based geophysical methods still face challenges in karst mapping and concealed karst cavities potentially pose threats to tunnel construction in southwest china given the significant contrast in electrical resistivity between karst cavities and their host rocks geoelectrical methods are widely used for mapping these cavities we have developed a successful case history of mapping karst cavities on a planned railway route using controlled source audio magnetotellurics csamt scalar csamt with frequencies ranging from 0 5 to 8192 hz was used for field data acquisition a full frequency domain apparent resistivity correction method was used for near field corrections electromagnetic array profiling emap filtering was used for topographic and static shift corrections and the bostick conversion was used for data interp

change of stress regime during geothermal reservoir stimulation earthquakes are induced by man made changes of the stress field by injection or withdrawal of fluids in hydrocarbon production geothermal exploitation and wastewater disposal however the actual perturbation of the stress field and stress release by injection induced seismicity remains largely unknown we provide evidence for currently not understood hydromechanical processes after shut in of the well we invert earthquake focal mechanisms from a massive stimulation to invert for stress resolved in time and depth to obtain changes of the stress orientation and magnitude prior information about fracture orientations from well logs is taken into account comparison with independent stress measures reveals that stresses obtained from inversion of fluid induced seismicity are highly perturbed and not representative of the initial stress field the horizontal stresses change by tens of megapascals turning the stress regime from transitional normal faulting strike slip faulting to pure normal faulting the observed stress changes are attributed to large scale aseismic deformation

rapid three dimensional hypocentral determination using a master station method a master station ms method is presented in this paper to rapidly determine hypocenters in three dimensional 3 d heterogeneous velocities an equal differential time edt surface is defined as the collection of all spatial points that satisfy the time difference between two arrivals which can be two picks at two stations or two different phase picks at one station the edt surface is independent of the origin time and will contain the hypocenter for an event with j arrivals there are j 1 independent edt surfaces the ms method determines the hypocenter that satisfies two types of constraints to be traversed by most edt surfaces and to yield minimum travel time residual statistics the statistics include both the residual variance and the amplitude of the origin time error the combined use of the edt surfaces and residual statistics allows for a unique determination of the hypocenter and origin time using different types of phase arrivals in principle only three arrivals are minimally required to constrain a unique solution if three different stations are used for a 3 d velocity model the edt surfaces and the residual statistics can be computed efficiently using a reference file created by moser s 1991 ray tracing method an illustration of the ms method is given for 27 small events that occurred in southern california using a p wave velocity model modified from that of magistrale et al 1992 the average misfit between the bulletin hypocenters and the new solutions is 3 8 km if a 3 d velocity model is accurate the ms method can be a viable means of hypocenter determination

source parameters of picoseismicity recorded at mponeng deep gold mine south africa implications for scaling relations abstract we investigate the source parameters of picoseismic and nanoseismic events m w 4 1 recorded with a high sensitivity seismic network at the mponeng gold mine in south africa to gain new insights into the scaling of small seismic events the japanese german underground acoustic emission research in south africa jaguars network composed of one three component 3c accelerometer sensitivity 50 hz to 25 khz and 8 acoustic emission ae sensors sensitivity 1 khz to 180 khz is located at a depth of 3268 m and covers the limited volume of approximately 300 300 300 m the ae sensors are calibrated with respect to the 3c accelerometer in the frequency band 400 hz 17 khz the waveform data of two datasets are analyzed 1 the aftershock sequence of an m w 1 9 event that occurred approximately 30 m from our network and 2 the postblasting activity recorded during working days located at a distance 90 m from the network at the exploitation level for the analysis we applied spectral fitting and spectral ratio methods the calculated values of m w range from 0 8 down to 4 1 with corner frequencies 0 8 khz 13 6 khz source sizes from 8 cm to 1 3 m we observe static stress drops ranging from 1 mpa to 10 0 mpa with apparent stresses of 0 01 mpa 1 00 mpa stress drops are independent of the moment suggesting self similar behavior in the analyzed magnitude range 4 1 m w

holocene thermal maximum in the western arctic 0 180 w the spatio temporal pattern of peak holocene warmth holocene thermal maximum htm is traced over 140 sites across the western hemisphere of the arctic 0 180 w north of 60 n paleoclimate inferences based on a wide variety of proxy indicators provide clear evidence for warmer than present conditions at 120 of these sites at the 16 terrestrial sites where quantitative estimates have been obtained local htm temperatures primarily summer estimates were on average 1 6 0 8 c higher than present approximate average of the 20th century but the warming was time transgressive across the western arctic as the precession driven summer insolation anomaly peaked 12 10 ka thousands of calendar years ago warming was concentrated in northwest north america while cool conditions lingered in the northeast alaska and northwest canada experienced the htm between ca 11 and 9 ka about 4000 yr prior to the htm in northeast canada the delayed warming in quebec and labrador was linked to the residual laurentide ice sheet which chilled the region through its impact on surface energy balance and ocean circulation the lingering ice also attests to the inherent asymmetry of atmospheric and oceanic circulation that predisposes the region to glaciation and modulates the pattern of climatic change the spatial asymmetry of warming during the htm resembles the pattern of warming observed in the arctic over the last several decades although the two warmings are described at different temporal scales and the htm was additionally affected by the residual laurentide ice the similarities suggest there might be a preferred mode of variability in the atmospheric circulation that generates a recurrent pattern of warming under positive radiative forcing unlike the htm however future warming will not be counterbalanced by the cooling effect of a residual north american ice sheet

direct inversion of earthquake first motions for both the stress tensor and focal mechanisms and application to southern california in inverting populations of focal mechanisms to estimate stress misfits may arise owing to 1 errors in determining the fault plane solutions and 2 real variations in the state of stress among a number of techniques that invert for stress most effectively regard only the latter and only indirectly describe the former through statistical means so make it difficult to differentiate real stress variations from noisy data ambiguity arises because realistic methods for determining focal mechanisms for small earthquakes depend on p wave first motions in a highly nonlinear manner which is difficult to propagate into stress inversions to address these issues we present a new method for constraining stress tensors in which first motion observations are directly inverted for stress without assuming that focal mechanisms are known the technique produces estimates of four stress parameters a suite of focal mechanisms consistent with both the stress tensor and the first motions and estimates of the uncertainty in stress it also provides a natural test of the stress homogeneity hypothesis and a means to identify those earthquakes whose first motions are not consistent with a homogeneous stress tensor when tested against artificial data the method correctly recovers input stresses and yields uncertainties that depend upon both data distribution and reliability of first motion picks as expected utilizing first motions from the southern california catalog the method is applied to several clusters and aftershock sets from 1981 to 1992 typical uncertainties in stress orientation exceed 20 30 at the 95 confidence level resulting from 5 mispick rates typical of even the cleanest data these uncertainties exceed some previous estimates for similar populations presumably because they account for focal mechanism un certainties and perhaps indicating that some previously reported stress variations are not statistically significant most populations are found to be internally consistent with a homogeneous stress field the most significant exceptions being aftershock sequences of some major earthquakes in at least one case the 1992 landers sequence aftershocks at the north end of the rupture zone appear to reflect exceedingly heterogeneous stresses overall the method provides a useful and robust approach for evaluating stress constraints provided by p wave first motions

long period magnetotelluric measurements near the central california coast a land locked view of the conductivity structure under the pacific ocean summary telluric data from hollister california were combined with magnetic data from the fresno california magnetic observatory to determine the mt response at long periods the shifted eigenstate analysis of latorraca et al 1986 was used to determine the maximum apparent resistivities and corresponding eih phases for periods from 1 to 30 h the apparent resistivity and phase at hollister are greatly influenced by the ocean with the mantle branch beginning at a period of about 12 h in typical continental areas the mantle branch usually begins at periods of 10s or less a maximum likelihood inversion algorithm was used to invert the observed data and to determine the sensitivity of the model parameters the data required a resistivity thickness product of approximately 1 x lo9 q m2 1 x lo6 q m km for the oceanic lower crust and upper mantle the oceanic mantle shows a rather small gradient of conductivity from approximately 200 to 640 km in depth and a rapid conductivity increase below 640km the sensitivity analysis shows the data are most sensitive to the oceanic parameters except for the electric field amplitudes which are also sensitive to the resistivities under the observation site

holocene climate effects on the development of a peatland on the tuktoyaktuk peninsula northwest territories abstract six cores were collected from a small peatland on the tuktoyaktuk peninsula yielding up to 1 8 m of organic material and 1 2 m of ice rich material stratigraphic relationships radiocarbon dating and pollen plant macrofossil and isotopic analyses are used to reconstruct the developmental history of the peatland organic matter began to accumulate in the basin about 7200 yr b p during a period when the climate is thought to have been warmer than present initially the permafrost table was probably below the bottom of the basin and an open water mineral wetland with emergent and submergent aquatic vegetation occupied the site enriched 18 o values from ice samples confirm the existence of an open water body with a water balance strongly affected by evaporation transformation to a graminoid fen peatland was underway by 6300 yr b p permafrost and ice began to form in the peatland at this time low centered polygons probably began to develop by 4700 yr b p and subsequently evolved into the high centered polygons which characterise the southwestern part of the peatland today peat accumulation in the polygons has been extremely slow for the last 4000 years the eastern wetter part of the basin has largely remained unchanged since its inception about 7000 yr b p development of the peatland may be linked to permafrost formation that was controlled by regional climate changes during the holocene

holocene development of the rogovaya river peat plateau european russian arctic in this study the holocene development of a peat plateau area in the east european russian arctic is reconstructed based on detailed macrofossil physico chemical and radiocarbon analyses from two peat sequences basal dates from these two c 2 m long peat profiles are c 9420 bp and c 9250 bp from another six peat sequences gross stratigraphic descriptions and additional radiocarbon dates are available basal dates from two short 1 m peat profiles indicate further peatland expansion at c 3635 bp and c 1285 bp the oldest macrofossils of tree birch are dated to c 9500 bp and those of conifers presumably spruce to c 8000 bp tree stands became rare in the study area after c 2800 bp but occasionally occur until present peatlands formed through terrestrialization of ponds or paludification of forested uplands between 9000 and 3100 bp the peatlands were wet rich fens beginning from c 3100 bp there are marked changes in their surface hydrology connected with climatic cooling and permafrost

high fluid pressure and triggered earthquakes in the enhanced geothermal system in basel switzerland 1 we analyzed 118 well constrained focal mechanisms to estimate the pore fluid pressure field of the stimulated region during the fluid injection experiment in basel switzerland this technique termed focal mechanism tomography fmt uses the orientations of slip planes within the prevailing regional stress field as an indicator of the fluid pressure along the plane at the time of slip the maximum value and temporal change of excess pore fluid pressures are consistent with the known history of the wellhead pressure applied at the borehole elevated pore fluid pressures were concentrated within 500 m of the open hole section which are consistent with the spatiotemporal evolution of the induced microseismicity our results demonstrate that fmt is a robust approach being validated at the meso scale of the basel stimulation experiment we found average earthquake triggering excess pore fluid pressures of about 10 mpa above hydrostatic overpressured fluids induced many small events m 3 along faults unfavorably oriented relative to the tectonic stress pattern while the larger events tended to occur along optimally oriented faults this suggests that small scale hydraulic networks developed from the high pressure stimulation interact to load hydraulically isolated high strength bridges that produce the larger events the triggering pore fluid pressures are substantially higher than that predicted from a linear pressure diffusion process from the source boundary and shows that the system is highly permeable along flow paths that allow fast pressure diffusion to the boundaries of the stimulated region