earthquakes good tidings tidal stresses in the earth s crust don t seem to influence earthquakes water wells on the other hand seem strangely sensitive to seismic activity explanations are now proposed

the present day stress field of australia abstract the unusual present day crustal stress pattern of the australian continent has been the subject of scientific debate for over 25 years the orientation of maximum horizontal present day stress s hmax in continental australia is unlike all other major tectonic plates in that the stress pattern shows regional variability and it is not oriented sub parallel to the direction of absolute plate motion previous studies on the stress pattern of australia revealed that the complex stress pattern of the continent is controlled at a first order by the superposition of plate tectonic forces exerted at the plate boundaries however prior analysis of the contemporary australian crustal stress pattern have been unable to model or explain the stress pattern observed in most of eastern australia and has not extensively addressed the numerous smaller scale variations in stress orientation the recent development of unconventional reservoirs in australia has resulted in a greatly increased amount of new data for stress analysis in previously unstudied or poorly constrained areas in eastern australia in addition stress analysis in conventional hydrocarbon mineral and geothermal exploration in all other parts of the continent provides the opportunity to review and update the australian stress map asm this study presents the new release of the asm with a total of 2150 stress data records in australia increased from 594 data records in 2003 the 2016 asm contains 1359 data records determined from the interpretation of drilling related stress indicators 650 from earthquake focal mechanism solutions 139 from shallow engineering measurements and two from geological indicators the results reveal four distinct regional trends for the s hmax orientation in australia including a nne ssw s hmax trend in northern and northwestern australia which rotates to a prevailing e w orientation in most western and south australia the orientation of s hmax in eastern australia is primarily ene wsw and swings to nw se in southeastern australia a comparison between the new asm database and neotectonic features further confirms the role of contemporary stress in recent deformation of the australian crust the 2016 asm reveals significant discrepancies between newly observed s hmax orientations and predictions by published geomechanical numerical models for the australian continent forces generated at the boundary of indo australian plate remain the major control on the regional crustal stress pattern in continental australia however the increased data density particularly in eastern australia reveals numerous local perturbations of the stress field that were not previously clearly captured hence the key findings of the new release are that local intra plate stress sources are more significant than previously recognized particularly in eastern australian basins and cause substantial local scale rotations of the present day crustal stress pattern that have not been factored into existing australian stress models

present day stress field rotations associated with evaporites in the offshore nile delta evaporitic horizons are routinely interpreted to act as mechanical detachment sequences and thus signifi cantly infl uence the structural evolution of sedimentary basins and fold thrust belts however over 30 years of global in situ stress analysis have provided only poor evidence to support this widespread assumption this study examines present day stress orientations inferred from borehole breakout and drilling induced fractures in 44 boreholes in the offshore nile delta a total of 446 breakouts and 19 drilling induced fractures from 37 wells reveal sharply contrasting present day maximum horizontal stress shmax orientations in sequences above and below the extensive messinian evaporites of the eastern nile delta a typical deltaic margin parallel shmax e w in the central and ese wnw in the eastern nile delta is observed in parts of the nile delta that are below or do not contain evaporites however a scattered but largely margin normal nne ssw shmax is observed in sequences underlain by evaporites the 90 variation in present day shmax orientation above and below the messinian salts provides the fi rst convincing in situ evidence that evaporite sequences can act as major mechanical detachment horizons in addition the margin normal shmax orientation is subperpendicular to the strike of nearby active extensional faults indicating the existence of non andersonian faulting in the suprasalt region furthermore the evidence that the messinian evaporites act as an effective mechanical detachment suggests that suprasalt faulting in the eastern nile delta is not the result of basement related deformation and thus raises doubts about the often postulated extension of the suez fault zone into the eastern mediterranean

external modulation of volcanic activity summary time and again records of seismovolcanic activity that show a diurnal and semi diurnal modulation have been related to the earth s tidal stress field as an external triggering mechanism several seismic data sets from different volcanoes are examined including stromboli volcano italy mt ruapehu new zealand and merapi and batur indonesia where in some cases weather data are also incorporated a comparison with theoretical body tides in the spectral domain reveals strong evidence against a correlation between seismovolcanic activity and tidal stress however other parameters such as temperature and barometric pressure show the same diurnal and semi diurnal modulation and are therefore candidates for a possible external modulation of volcanic activity several mechanisms are discussed for stromboli a clear correlation between volcanic activity and barometric pressure is shown

influence of tidal effects on the periodicity of earthquake activity in diverse geological settings with particular emphasis on submarine hydrothermal systems abstract hydrothermal earthquakes may be defined as earthquakes caused by the hydrofracturing of the oceanic crust by submarine hydrothermal fluids it is suggested that they correspond to the cracking of the ocean crust at the brittle ductile transition that marks the downward limit of seawater circulation into the ocean crust semi diurnal variations in submarine hydrothermal discharge in backarc settings and at the crests of mid ocean ridges have been detected by hydrophone but no influence of tidal effects on the periodicity of microearthquakes around submarine hydrothermal vents has been observed so far although tidal stresses are too small to initiate earthquakes themselves they may trigger earthquake activity when imposed on slowly increasing tectonic stress these effects therefore tend to be observed in geologically active regions where stress levels in the earth s crust are high the influence of tides on the periodicity of microearthquakes tends to be complex because the effects of ocean and earth tides are not uniform at the surface of the earth and because the microearthquakes occur in a wide variety of geological settings earth tides cause compression and dilatation of the oceanic crust which may be the most significant factor in initiating changes in fluid flow rate and microseismicity in submarine hydrothermal systems much of the information now available on the tidal periodicity of microearthquakes is based on relatively short term monitoring which lacks statistical validity the development and installation of deep sea monitoring stations will permit rigorous long term monitoring programmes to be carried out which will lead to a better understanding of the nature and periodicity of microearthquakes around submarine hydrothermal vents

use of borehole imaging to improve understanding of the in situ stress orientation of central and northern england and its implications for unconventional hydrocarbon resources abstract new interest in the potential for shale gas in the united kingdom uk has led to renewed exploration for hydrocarbons in the carboniferous age bowland hodder shales under central and northern england following an incidence of induced seismicity from hydraulic fracturing during 2010 at preese hall lancashire the publically available databases quantifying the in situ stress orientation of the united kingdom have shown to be inadequate for safe planning and regulation of hydraulic fracturing this paper therefore reappraises the in situ stress orientation for central and northern england based wholly on new interpretations of high resolution borehole imaging for stress indicators including borehole breakouts and drilling induced tensile fractures these analyses confirm the expected north northwest south southeast orientation of maximum horizontal in situ stress identified from previous studies e g evans and brereton 1990 the dual caliper data generated by evans and brereton 1990 yields a mean shmax orientation of 149 87 with a circular standard deviation of 66 9 however the use of borehole imaging without incorporation of results from older dual caliper logging tools very significantly decreases the associated uncertainty with a mean shmax orientation of 150 9 with a circular standard deviation of 13 1 the use of high resolution borehole imaging is thus shown to produce a more reliable assessment of in situ stress orientation the authors therefore recommend that the higher resolution of such imaging tools should therefore be treated as a de facto standard for assessment of in situ stress orientation prior to rock testing use of borehole imaging should be formally instituted into best practice or future regulations for assessment of in situ stress orientation prior to any hydraulic fracturing operations in the uk

global crustal stress pattern based on the world stress map database release 2008 the world stress map wsm project is a global compilation of information on the contemporary crustal stress field from a wide range of stress indicators the wsm database release 2008 contains 21 750 stress data records that are quality ranked using an updated and refined quality ranking scheme almost 17 000 of these data records have a c quality and are considered to record the orientation of maximum horizontal compressional stress sh to within 25 as this is almost a triplication of data records compared with the first wsm database release in 1992 we reinvestigate the spatial wave length of the stress patterns with a statistical analysis on a global 0 5 grid the resulting smoothed global stress map displays both the mean sh orientation that follows from the maximum smoothing radius for which the standard deviation is b25 and a countour map that displays the wave length of the stress pattern this smoothed global map confirms that long wave length stress patterns n2000 km exist for example in north america and ne asia these have been used in earlier analyses to conclude that the global stress pattern is primarily controlled by plate boundary forces that are transmitted into the intraplate region however our analysis reveals that rather short wave length of the stress pattern b200 km are quite frequent too particularly in western europe alaska and the aleutians the southern rocky mountains basin and range province scandinavia caucasus most of the himalayas and indonesia this implies that local stress sources such as density contrasts and active fault systems in some areas have high impact in comparison to plate boundary forces and control the regional stress pattern

patterns of earthquakes and the effect of solid earth and ocean load tides at mount st helens prior to the may 18 1980 eruption seismographs near mount st helens volcano recorded an earthquake swarm lasting nearly 2 months prior to the catastrophic may 18 1980 eruption the earthquakes are divided into four classes based on station cpw 116 km seismogram characteristics 1 events with sv p amplitude ratio 3 and dominant frequency 3 hz 2 events with sv p ratio between 1 and 3 and dominant frequency 2 hz 3 events similar to characteristic 2 but with a strong probably surface wave phase just after the s phase and 4 events with frequencies between 1 and 2 hz lacking a clear s phase the seismicity pattern for each of the four classes is unique and we assume each group of events with similar wave forms represents a common physical process depth source area or fault orientation maximum likelihood b values range between 0 97 0 05 and 1 81 0 09 for the four groups we calculate solid earth stress and strain tides at the average hypocentral depth of 4 km we also calculate stress and strain tides induced by ocean loading their amplitudes are typically 20 40 those of the solid earth tides at the location of mount st helens a weak but significant correlation exists between the latter two classes of events and the tides for a time interval of about 5 days preceding the first onset of volcanic tremor and about 5 days thereafter the polarity of the correlation is opposite for the two classes of events in each case the phase of the correlation changes systematically with time the changes coinciding with the onset of tremor on march 31 and with a pronounced decrease in earthquake energy release rate on april 3 there are no significant correlations between the tides and the number of events or energy release of these two classes of earthquakes during any other interval between march 20 and may 18 1980 the first two classes of events show no evidence of significant tidal correlation at any time during the study period

effects of tidal stresses on volcanic activity at mount etna italy 1 here we model quantitatively the stress and strain induced by tidal forces as possible triggers of mount etna eruptions during 1989 2000 and 2001 by means of 2d numerical analysis by finite difference method applied to the volcano edifice spectral and vectorial analysis of tidal acceleration indicates that the quasi diurnal semi diurnal and ter diurnal constituents may have induced decompressions up to some kpa in the shallow i e 1 500 m gas saturated magma reservoir which controlled magma vesiculation and the timing of lava fountain activity from the south east crater sec we consider the geological heterogeneities in the volcanic edifice i e rock geometries and properties feeder conduit and vent systems volcanic landforms the magma properties i e bulk modulus volatile content and the ongoing volcano tectonic stresses i e volcano flank deformations which strongly influence the response of eruptive activity to tidal forces indeed the tidal factor may act on flank deformation concomitant to shallow magma inflation the implications of our model may help understand the eruptive behaviour and the disequilibrium state of the volcano and may be useful for hazard assessment at mount etna

non monotonic decrease of coal permeability with depth in highly compressional tectonic settings a case study in the bide santang basin sw china abstract coal permeability dependence on depth has been studied and discussed for a long time however the scattered distribution of coal permeability versus depth and specifically the non monotonic permeability decrease in highly compressional tectonic settings has not been fully understood this paper takes the bide santang basin sw china as a case study to discuss the non monotonic decrease of coal permeability with depth the results show that three regime intervals including the uppermost normal regime interval the middle mixed regime interval and the deep normal regime interval are identified in the bide santang basin a tri segmented linear model provides a better fit of coal permeability than a single linear model does coal permeability in the uppermost normal regime interval is characterized by exceptionally high values and quick decrease rate with increasing depth in comparison with that in other intervals the middle mixed regime interval is characterized by a permeability increasing trend with depth related primarily to the increase of horizontal differential stress in this interval in the deep normal regime interval the horizontal differential stress does not show any obvious trend and coal permeability resumes its decreasing trend with depth this study reveals the primary role of the horizontal differential stress besides the effective stress in controlling coal permeability in highly compressional tectonic settings characterized by stress regime transitions

the dynamics of a long lasting effusive eruption modulated by earth tides abstract deciphering eruptive dynamics in near real time is essential when dealing with hazard assessment and population evacuation recognition of specific patterns in time series measured during volcanic activity may help decipher distinctive behavior at active volcanoes providing insights into the underlying driving mechanisms multi parameter data sets usually agree on the overall trend characterizing the temporal evolution of an eruption providing insights into the first order eruptive dynamics however second order variations detected in different data sets remain often poorly understood the 2014 2015 holuhraun eruption iceland offers an excellent opportunity to investigate the dynamics of a long lasting effusive eruption we analyze the seismic tremor and the volcanic radiated power emitted by the lava field during the 6 months of the eruption using singular spectrum analysis ssa in both geophysical time series we identify periods from 5 to 32 days coinciding with periods of earth tides here we show that 50 of both signals are composed of tidal periods suggesting that magma movements follow frequencies imposed by lunisolar forces within the crust and at earth s surface

subsurface fractures in situ stress and permeability variations in the walloon coal measures eastern surat basin queensland australia abstract in situ stress pattern and subsurface fractures control the production of coal seam gas csg reservoirs and hence comprehensive analyses are required for better development of these reservoirs the surat basin is a prolific csg province with major economic impacts in eastern australia recent studies have reported significant stress variations in this basin due to local geological structures however a systematic analysis of localised stress perturbations within the basin and their impacts on reservoir permeability have not been presented thus far in the published literature in this paper we combine different sets of data including conventional log data wellbore image logs 2d seismic lines and interpreted permeability from well test data in order to characterise the role of geological structures in the subsurface fluid flow of the walloon coal measures in eastern surat basin analysis of 7 3 km of borehole image log across 33 vertical wells revealed a regional ene wsw orientation of the maximum horizontal stress shmax in the study area however there are massive variability in the shmax orientation spatially and with depth analysis of 2d seismic lines highlight the role of geological structures in stress perturbation and the interpretation of natural fractures using image logs show that most of the open fractures are oriented sub parallel to the acute angle of local shmax orientation interpreted well test data in the study wells demonstrates that most of the high permeability data points are located where in situ stress rotation is observed this indicates the influence of local shmax orientation responding to geological structures on the subsurface fluid flow in the eastern part of the surat basin

in situ stress natural fracture distribution and borehole elongation in the auburn geothermal well auburn new york hydraulic fracturing stress measurements and a borehole televiewer survey were conducted in a 1 6 km deep well at auburn new york this well which was drilled at the outer margin of the appalachian fold and thrust belt in the appalachian plateau penetrates approximately 1540 m of lower paleozoic sedimentary rocks and terminates 60 m into the precambrian marble basement analysis of the hydraulic fracturing tests indicates that the minimum horizontal principal stress increases in a nearly linear fashion from 9 9 0 2 mpa at 593 m to 30 6 0 4 mpa at 1482 m the magnitude of the maximum horizontal principal stress increases in a less regular fashion from 13 8 1 2 mpa to 49 0 2 0 mpa over the same depth range the magnitudes of the horizontal principal stresses relative to the calculated overburden stress are somewhat lower than is the norm for this region and are indicative of a strike slip faulting regime that at some depths is transitional to normal faulting as expected from the relative aseismicity of central new york state however analysis of the magnitudes of the horizontal principal stresses indicates at least to a depth of 1 5 km that frictional failure on favorably oriented preexisting fault planes is unlikely orientations of the hydraulic fractures at 593 and 919 m indicate that the azimuth of the maximum horizontal principal stress at auburn is n83 e 15 in agreement with other stress field indicators for this region the borehole televiewer log revealed a considerable number of planar features in the auburn well the great majority of which are subhorizontal dips 5 and are thought to be bedding plane washouts or drill bit scour marks in addition a smaller number of distinct natural fractures were observed on the borehole televiewer log of these the distinct steeply dipping natural fractures in the lower half of the sedimentary section at auburn tend to strike approximately east west while those in the upper part of the well and in the precambrian basement exhibit no strong preferred orientation the origin of this east west striking fracture set is uncertain as it is parallel both to the contemporary direction of maximum horizontal compression and to a late paleozoic fracture set that has been mapped to the south of auburn in addition to these planar features the borehole televiewer log indicates paired dark bands on diametrically opposite sides of the borehole throughout the auburn well processing of the borehole televiewer data in the time domain revealed these features to be irregular depressions in the borehole wall as these depressions were consistently oriented in a direction at right angles to the direction of maximum horizontal compression we interpret them to be the result of stress induced spalling of the borehole wall breakouts

a preliminary study of the present day in situ stress state in the ahe tight gas reservoir dibei gasfield kuqa depression abstract knowledge of the present day in situ stress state has significant applications in the exploration and development of tight gas reservoirs the ahe formation is an important tight gas reservoir in the dibei gasfield of kuqa depression however prior to this study little attention has been paid to the present day in situ stress field within the formation in the present study the in situ stress orientation and magnitudes were investigated based on well log calculations and geomechanical modeling the horizontal maximum principal stress shmax orientation was determined from interpretations of drilling induced tensile fractures ditfs and borehole breakouts in imaging logs which showed variations between nnw sse trending and nne ssw trending in the dibei gasfield the in situ stress magnitudes were calculated in four wells based on well logs the results indicated a normal faulting stress regime within the ahe tight gas reservoir numerical simulation of the present day in situ stresses showed that the magnitudes of vertical stress sv shmax and horizontal minimum principal stress shmin were 105 5 mpa 191 0 mpa 88 9 mpa 142 9 mpa and 79 1 mpa 127 7 mpa within the ahe formation respectively in addition considering the present day in situ stress state in the ahe formation of dibei gasfield natural fractures in directions parallel sub parallel to the shmax orientation with high fracture angles showed great contributions to subsurface fluid flow borehole instability may become a potentially significant problem when drilling vertical wells and horizontal wells deviated toward the shmax orientation in the ahe tight gas reservoir of dibei gasfield

mathematical model for calculating the horizontal principal stress of a faulted monoclinal structure in southwest qaidam basin china faulted monoclinal structures incorporate folds and faults their displacements cause stress release of differing degrees in various areas furthermore the faults commonly extend to the deep strata which may lead to the flow of boiling inclusions and localized thermal stresses a complex distribution of in situ stress appears in such areas which makes it challenging to drill wellbores in this study we first analyzed the geological structure and the factors influencing in situ stresses of a faulted monoclinal structure in southwest qaidam basin second based on huang s model a mathematical model for calculating the horizontal principal stress of the faulted monoclinal structure was developed considering the stress release from fault and fold displacements and the outflow from boiling inclusions third the magnitudes of the horizontal principal stresses of the study area were obtained using this new mathematical model and the predicted values had an average error of 2 63 according to a comparison with the results of the hydraulic fracturing field measurements fourth an in situ stress distribution in this area was established by combining the horizontal principal stresses magnitudes and the maximum horizontal principal stress direction finally based on the in situ stress distribution of the study area the mud density and trajectory for drilling the third stage of well z207 were determined the field application revealed that the design parameters met the engineering requirements satisfactorily which indicates that this mathematical model can predict the horizontal principal stress of the faulted monoclinal structure in southwest qaidam basin

earth tides and etnean volcanic eruptions an attempt at correlation of the two phenomena during the 1983 1985 and 1986 eruptions abstract an attempt has been made to correlate volcanic events on mt etna and moon sun attraction during the eruptions of 28 march 6 august 1983 12 march 13 july 1985 and 30 october 1986 the possibility of a trigger effect has been supposed for eruptive phenomena that originate by small and continuous external forces under particular stress conditions on the volcanic structure the parameters compared for the two phenomena are the hourly mean tremor amplitude recorded at serra la nave station and the hourly value of the vertical and of the two partial horizontal north south and east west components of the tidal acceleration induced by the moon sun system the relationship between two series of values the former for the tremor amplitude and the latter for one of the tidal acceleration components were then tested by statistical methods based on the information theoretic measure of interdependence between any pair of signal sequences the results show an effect of the horizontal tidal components on well known tectonic structures which may be widened or closed in contrast there is little evidence that the uprise of magma masses might depend on the vertical tidal attraction

present day stress field of southeast asia article i nfo it is now well established that ridge push forces provide a major control on the plate scale stress field in most of the earth s tectonic plates however the sunda plate that comprises much of southeast asia is one of only two plates not bounded by a major spreading centre and thus provides an opportunity to evaluate other forces that control the intraplate stress field the cenozoic tectonic evolution of the sunda plate is usually considered to be controlled by escape tectonics associated with india eurasia collision however the sunda plate is bounded by a poorly understood and complex range of convergent and strike slip zones and little is known about the effect of these other plate boundaries on the intraplate stress field in the region we compile the first extensive stress dataset for southeast asia containing 275 a d quality 177 a c horizontal stress orientations consisting of 72 stress indicators from earthquakes located mostly on the periphery of the plate 202 stress indicators from breakouts and drilling induced fractures and one hydraulic fracture test within 14 provinces in the plate interior this data reveals that a variable stress pattern exists throughout southeast asia that is largely inconsistent with the sunda plate s approximately ese absolute motion direction the present day maximum horizontal stress in thailand vietnam and the malay basin is predominately north south consistent with the radiating stress patterns arising from the eastern himalayan syntaxis however the present day maximum horizontal stress is primarily oriented nw se in borneo a direction that may reflect plate boundary forces or topographic stresses exerted by the central borneo highlands furthermore the south and central sumatra basins exhibit a ne sw maximum horizontal stress direction that is perpendicular to the indo australian subduction front hence the plate scale stress field in southeast asia appears to be controlled by a combination of himalayan orogeny related deformation forces related to subduction primarily trench suction and collision and intraplate sources of stress such as topography and basin geometry

mayon volcano philippines some insights into stress balance abstract mayon volcano is part of the bicol volcanic chain a peninsula in the south east of luzon island mayon s eruptive history during this century is fairly periodic with eruptions nearly every 10 years during the last two eruptions indications for tidal triggering of activities were found from statistical analyses hints for such a correlation exist for a number of volcanoes the statistical analysis of the seismicity points to a tidal triggering at mayon volcano in order to verify these indications the pressure balance between internal and external forces was computed the computations show that an interrelation between volcanic activities and tides especially ocean loading is probable for this purpose a global ocean model is improved by a regional model and a model of the local areas in order to consider the tidal distribution and the complicated geometry of the coasts the method of farrell w e 1972 deformation of the earth by surface loads rev geophys space physics 10 761 797 is used to calculate the ocean tidal loading the pressure increase in the vent due to vesiculation was tentatively modelled assumed a temperature decrease of 2 c per year the resulting stress is compared to the stress produced by the tidal forces body tides and ocean tidal loading it turns out that during a period of several days the small tidal forces may affect the stress balance considerably if the volcano is already in a critical state using fe modelling we estimate by neglecting all external forces how fast pressure can build up in the vent and to which extent the different physical parameters influence the pressure increase the maximum pressure resulting from the crystallization of the melt is compared with the maximum pressure yielding when in addition tidal forces are acting

implications of the pore pressure and in situ stress for the coalbed methane exploration in the southern junggar basin china abstract the southern junggar basin hosts abundant coalbed methane cbm resources and is expected to be an important cbm production base in china the pore pressure and in situ stress are key factors that influence the exploration and development of cbm however these factors are poorly understood in this area based on extended leak off test data mathematical models were established for pore pressure and in situ stress prediction using regression analysis subsequently the correlations between the pore pressure in situ stress and permeability were analyzed and the stress ratios were investigated finally the forming mechanism of stress zonation in the study area was discussed the results show that cbm reservoirs are generally underpressured and partly overpressured at depths greater than 800 m in the study area the study area is characterized by notable stress zoning the stress regime varies from reverse to strike slip to normal faulting with increasing depth significantly positive correlations exist between the pore pressure maximum principal stress minimum principal stress vertical stress and principal stress difference the minimum horizontal stress is approximately 70 of the vertical stress in the study area a mud weight window varying from the pore pressure to the most likely breakdown pressure is suggested at depths greater than 600 m a relatively high principal stress difference is observed which indicates that deep coals have a relatively high mechanical strength and their permeability can be enhanced by hydraulic fracturing the depth dependent stress zonation is mainly due to the influence of thrusting on the in situ stress which diminishes with increasing depth the compressional stress regime in the shallow area benefits the sealing of faults and the preservation of cbm resources in the study area the deep cbm resources have a development potential the outcomes of this study contribute to the cbm exploration and development in the study area

the present day state of tectonic stress in the darling basin australia implications for exploration and production knowledge of the full present day stress tensor and pore pressure has significant applications in the exploration and production of conventional and unconventional hydrocarbon reservoirs the darling basin of new south wales australia is an old sedimentary basin late cambrian silurian to early carboniferous in which there was limited information about the present day stress field prior to this study in this paper we evaluate the contemporary stress field of the darling basin using a dataset from recent exploration wells and perform a geomechanical risk assessment with respect to borehole stability fracture fault generation and reactivation our interpretations of borehole failures in borehole image logs reveal a prevailing east west orientation of the maximum horizontal stress throughout the darling basin the estimates of the magnitudes for the vertical minimum and maximum horizontal stress in the studied wells indicate a transition between thrust and strike slip faulting stress regime at 600 700 m depths where the magnitude of vertical stress and minimum horizontal stress are close to each other however the presence of borehole breakouts and drilling induced tensile fractures that we observe in the image logs at greater depths 900 2100 m indicate a transition into a strike slip tectonic stress regime below a depth range of approximately 700 900 m these findings are in agreement with overcoring stress measurements east and west of the investigated wells furthermore there are several neogene to recent geological structures in the study area that indicate thrust faulting with an east west oriented maximum horizontal stress orientation around this old sedimentary basin the consistency between the orientation of maximum horizontal stress determined from wellbore data and neotectonic structures is significant and implies that horizontal stress orientations derived from very recent geological features may be valuable inputs to geomechanical models in the absence of wellbore or other data however the recent surface geological structures suggest a thrust faulting stress regime that is in slight contrast to the transition between thrust and strike slip stress regime sh sh similar to sv indicated by petroleum data and highlights a potential pitfall of using neotectonic structures in geomechanical models in particular careful attention and verification should be made when using neotectonic structures for input calibration or confirmation of geomechanical models especially in intraplate tectonic settings such as australia c 2016 elsevier ltd all rights reserved

dupuit forchheimer solutions for radial flow with linearly varying hydraulic conductivity or thickness of aquifer 1 this paper presents new analytical solutions for steady state piezometric heads in phreatic and confined aquifers with radial flow the new solutions utilize two constant head boundaries and a hydraulic conductivity k or thickness of aquifer h that is a linear function of radius r previous solutions applied constant k and h and they were usually based on one constant head boundary which might be superimposed on a solution that allowed a constant discharge or recharge at the center of the aquifer the previous solutions may be difficult to apply to real aquifers because constant k and h is usually an oversimplification of nature the dupuit forchheimer assumption implies that vertical head gradients are ignored the magnitude of the error introduced by allowing h h r is evaluated numerically in this paper

paleostress field evolution of the australian continent since the eocene 1 although the low order present stress field of most continents is fairly well established information on paleostress fields is generally sparse knowledge of paleostresses is crucial for understanding brittle tectonic reactivation through time the indian australian plate lends itself well to a reconstruction of paleostresses as it has undergone enormous changes in plate driving forces through the tertiary and there is a rich record of fault reactivation from sedimentary basins we reconstruct the plate boundary configuration and age area distribution of ocean crust around australia through time to obtain estimates for ridge push slab pull and collisional forces acting on the indian australian plate since the eocene other model parameters we explore are the effects of the australian antarctic discordance and the mechanical strength of the australian continental margin we apply these constraints to model the orientation of the maximum horizontal compressive stress shmax regime for the present early miocene and early eocene using the commercial software abaqus along with the optimization software nimrod o we use an elastic two dimensional plane stress finite element model with a resolution of 0 2 in both longitude and latitude realistic elastic parameters representing different rock types and geologic provinces for the australian continent have been included to model the stress field of a heterogeneous plate we show that spatially significant rotations of shmax directions can be modeled as a consequence of perturbations of shmax in areas of juxtaposed rigid and compliant rheologies the absence of the collisional papua new guinea boundary in the miocene and reduced ridge push force from the south result in stress directions considerably different from the present stress directions over the northern australian continent in the early miocene in particular show large disparity with present stress directions stress orientations for the australian plate during the early eocene are controlled predominantly by ridge push forces arising from spreading in the wharton basin in the indian ocean and vary substantially with stress directions in the early miocene and the present because of the drastically different plate geometry and boundary configurations fault reactivation histories observed on the northwest shelf of australia and in the bass strait region are consistent with modeled changes in stress directions through time

the australian stress map knowledge of the in situ stress field of the australian continent has increased greatly since compilation of the world stress map in 1992 principally by analysis of borehole breakouts and drilling induced tensile fractures in petroleum wells stress orientations are variable across the australian continent as a whole however within 15 of 16 individual stress provinces defined in the australian continent of one to a few hundred kilometres scale mean stress orientations are statistically significant the stress provinces and stress trajectory mapping reveal that there are systematic continental scale rotations of stress orientation within australia unlike many other continental areas stress orientations do not parallel the direction of absolute plate motion nonetheless the regional pattern of stress orientation is consistent with control by plate boundary forces if the complex nature of the convergent northeastern boundary of the indo australian plate and stress focusing by collisional segments of the boundary is recognized

relationships among in situ stress fractures and faults and fluid flow monterey formation santa maria basin california we used borehole televiewer bhtv data from four wells within the onshore and offshore santa maria basin california to investigate the relationships among fracture distribution orientation and variation with depth and in situ stress our analysis of stress induced well bore breakouts shows a uniform northeast maximum horizontal stress sh max orientation in each well this direction is consistent with the sh max direction determined from well bore breakouts in other wells in this region the northwest trend of active fold axes and kinematic inversion of nearby earthquake focal plane mechanisms in contrast to the uniformity of the stress field fracture orientation dip and frequency vary considerably from well to well and within each well with depth fractures can be divided into distinct subsets on the basis of fracture frequency and orientation which correlate with changes of lithology and physical properties although factors such as tectonic history diagenesis and structural variations obviously have influenced fracture distribution integration of the in situ stress and fracture data sets indicates that many of the fractures faults and bedding planes are active small scale strike slip and reverse faults in the current northeast trending transpressive stress field in fact we observed local breakout rotations in the wells providing kinematic evidence for recent shear motion along fracture copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received april 15 1996 revised manuscript received december 27 1996 final acceptance july 15 1997 2department of geophysics stanford university stanford california 94305 2215 we wish to thank tom zalan of the chevron u s a production company for providing the offshore well data unocal corporation for providing the data on the onshore well and marcia mclaren from pacific gas and electric company for providing the earthquake focal mechanisms used in the stress inversion analysis the data used as background seismicity in figure 1 were extracted from the world wide web of the southern california seismic network scsn catalog operated jointly by the seismological laboratory at caltech and the u s geological survey both in pasadena california we appreciate the comments and helpful discussions from daniel moos steve graham and lev vernik

onset of the persistent activity at stromboli volcano italy stromboli known worldwide as the lighthouse of the mediterranean is commonly believed to have been in a state of persistent activity for the past 2000 2500 years however historical sources older than 1000 a d are not accurate enough to assess if the activity of the volcano was exactly the same as we see at present in order to attempt to identify the onset of the present eruptive regime and assess if it has been maintained with the same characteristics through time stratigraphic and radiometric studies of the recent tephra deposits were undertaken up to 4 m deep stratigraphic trenches dug at a height of approximately 500 m on the ne flank of the volcano exposed a conformable tephra pile containing charcoal fragments one of the most interesting finds was the discovery of a 7 cm thick weathered bed rich in organic matter thin palaeosol approximately 3 m below the surface the sequence underneath the palaeosol consists of decimetre thick lapilli fallout beds alternating with ash deposits bearing small charcoals with calibrated ages of between the fourth century b c and the first century a d the sequence above the palaeosol is charcoal free and consists of coarse ash deposits with discrete centimetre thick lapilli fallout beds composed of crystal poor golden pumice and subordinate crystal rich black scoriae similar to scoria pumice pairs emitted during the more energetic explosions of the present day activity the data collected indicate that between the third and seventh centuries a d after a period of quiescence the activity resumed with an eruptive style identical to the present one we conclude that the lighthouse of the mediterranean actually began its activity in a period much later than previously thought

plate boundary forces are not enough second and third order stress patterns highlighted in the world stress map database 1 the world stress map project compiles a global database of contemporary tectonic stress information of the earth s crust early releases of the world stress map project demonstrated the existence of first order plate scale stress fields controlled by plate boundary forces and second order regional stress fields controlled by major intraplate stress sources such as mountain belts and zones of widespread glacial rebound the 2005 release of the world stress map project database provides for some areas high data density that enables us to investigate third order local stress field variations and the forces controlling them such as active faults local inclusions detachment horizons and density contrasts these forces act as major controls on the stress field orientations when the magnitudes of the horizontal stresses are close to isotropic we present and discuss examples for venezuela australia romania brunei western europe and southern italy where a substantial increase of data records demonstrates some of the additional factors controlling regional and local stress patterns

stress induced borehole elongation a comparison between the four arm dipmeter and the borehole televiewer in the auburn geothermal well the nature and origin of borehole elongation recorded by the four arm dipmeter calipers is studied utilizing information obtained from hydraulic fracturing stress measurements and borehole televiewer data taken in a well located in auburn new york a preferred orientation n10 w s10 e 10 and a less prominant e w orientation of borehole elongation was observed on two runs of the dipmeter comparisons of borehole geometry determined using the televiewer and the dipmeter show that both tools give the same orientation of borehole elongtion provided that the zone of elongation is longer than 30 cm comparisons of dipmeter caliper data with orientation of in situ stress and natural fractures obtained from hydrofracturing tests and televiewer data show that the n10 w s10 e borehole elongations 1 are axisymmetric 2 are aligned with the minimum horizontal stress sh and 3 are not associated with natural fractures intersecting the well these elongations are interpreted as stress induced well bore breakouts the e w elongation direction is characterized by an asymmetric borehole cross section in thinly bedded rocks and is not caused by breakouts this asymmetric geometry can be discriminated from breakouts using the oriented electric measurements provided by the dipmeter this study demonstrates that the dipmeter can be used to determine the orientation of sh by mapping breakouts confirming the results of earlier less detailed studies and provides a firm basis for mapping regional stress patterns using existing dipmeter data

triggering of volcanic eruptions although earthquakes and volcanic eruptions are each manifestations of large scale tectonic plate and mantle motions it is usually thought that the occurrences of these events are not directly related there have been some studies however in which triggering of volcanic eruptions by earthquakes remote from the volcano has been proposed1 2 the 1992 landers southern california earthquake caused triggered seismicity at very large distances3 including the magmatically active4 long valley caldera region which also experienced a significant coincident deformation transient5 motivated by this demonstration of the ability of a distant earthquake to disturb a volcanic system and the earlier studies of specific cases of eruption triggering we examine here the historical record of eruptions and earthquakes to see if there are indeed significantly more eruptions immediately following large earthquakes we find that within a day or two of large earthquakes there are many more eruptions within a range of 750 km than would otherwise be expected additionally it is well known6 that volcanoes separated by hundreds of kilometres frequently erupt in unison the characteristics of such eruption pairs are also consistent with the hypothesis that the second eruption is triggered by earthquakes associated with the first

a formulation for vertically integrated groundwater flow in a stratified coastal aquifer we present the comprehensive discharge potential for steady three dimensional flow in horizontally stratified coastal aquifers with a horizontal base and a vertical coastline the gradient of this comprehensive potential gives the vertically integrated discharge throughout the aquifer i e the specific discharge vector as a function of three dimensional space integrated over the saturated portion of the aquifer the boundary values of the comprehensive potential along the coast can be computed precisely given the geometry of the aquifer the hydraulic conductivities of the strata the elevations of the horizontal planes that separate the strata and the elevation of the impermeable base of the aquifer relative to sea level boundary conditions of the comprehensive potential may either be given in terms of its gradient or computed from given heads along the boundaries the governing equation of the comprehensive potential is the poisson equation in areas of infiltration and the laplace equation elsewhere the computation of interface elevations piezometric heads and the vertical distribution of flow requires that an assumption be made regarding the relation between the comprehensive potential and piezometric heads we adopt the dupuit forchheimer approximation for this purpose and make use of the ghyben herzberg equation we present several applications of the approach and find that the stratification may have a significant effect on the boundary value of the comprehensive potential and thus on the flow rates in the aquifer this article is protected by copyright all rights reserved

in situ stress orientations in the xiagou tight oil reservoir of qingxi oilfield jiuxi basin northwestern china abstract knowledge of the present day in situ stress orientation is important for borehole stability fluid flow in fractured reservoirs and hydraulic fracture stimulation etc the qingxi oilfield of jiuxi basin northwestern china is an old oilfield however within which there were no systematic investigations into the present day in situ stress orientation prior this study in the present study the orientation of horizontal maximum stress shmax in the qingxi oilfield was interpreted and analyzed based on 149 borehole breakouts bos and 131 drilling induced fractures difs from electrical borehole imaging logs in 23 wells our interpretations revealed a prevailing ne sw trending 14 94 n 52 74 n shmax orientation in the xiagou tight oil reservoir of qingxi oilfield however the shmax orientation indicated various tendencies both laterally and with burial depth within single wells which were influenced by well developed faults natural fractures and bedding planes the presence of these structures caused great contrasts of rock mechanical properties influencing stress orientation variations in addition the effects of present day in situ stress orientation on natural fractures hydraulic fracture stimulation and borehole stability were discussed in the qingxi oilfield ne sw trending natural fractures indicated favorable contributions to subsurface fluid flow hydraulic fractures would propagate vertically following ne sw trending wells were more likely to experience borehole instability issues if they were deviated towards ne sw trending the results can provide geological references for subsequent tight oil production in the qingxi oilfield of jiuxi basin

experimental observations of the mechanics of borehole failure in porous sandstone abstract to investigate borehole failure under hydrostatic stress in sandstone of varying porosity experiments were conducted on three sandstone varieties with 2 5 mm bores tennessee darley dale and penrith sandstone types were selected to represent most of the spectrum of sandstone porosity variations respectively 0 07 0 12 and 0 28 confining pressures ranged up to 500 mpa and pore fluid volumometry was used to detect bore failure failed samples were studied microstructurally using optical and scanning electron microscopy each rock type failed with the development of oriented breakout features normally seen to develop around boreholes loaded under non hydrostatic stress and in tennessee sandstone at least these developed in a consistent orientation with respect to an external reference frame dip direction of foreset beds all of the rock types showed strength and or elastic anisotropy thus the formation of oriented breakouts under hydrostatic loading is attributed to the effects of anisotropy two modes of breakout development were observed in darley dale and penrith sandstones a combination of intergranular shear and extensional fracture produced broad and shallow features with breakout width unaltered during growth in tennessee sandstone a purely extensional mode of intragranular fracture was observed leading to deeper breakout features the samples tested showed higher failure pressures for smaller bore sizes when failure pressure is normalized with respect to grain crushing pressure p i and bore diameter with the product of porosity with grain size all data lie on a single master curve with bore failure pressure approximately 0 15 p i in the regime of bore size independent behaviour

in situ stress orientations in the uk southern north sea regional trends deviations and detachment of the post zechstein stress field the orientation of the maximum horizontal compressive stress shmax in the uk southern north sea has been determined using data derived from borehole breakout analysis of four arm caliper logs the results agree with existing stress models for nw europe confirming that horizontal stresses in the region have an approximately nw se orientation of shmax this is interpreted as being a result of plate boundary convergence local deviations in the shmax orientations are observed spatially and also vertically within some wells some of these deviations are attributed to rotations of the stress field adjacent to faults or between different fault blocks the data also suggest detachment of the stress regime in the post permian cover rocks caused by the presence of a thick underlying permian aged evaporite sequence and associated halokinesis analyses of borehole resistivity image logs have been used to verify the shmax orientations in some wells these image logs validate some of the stress indicators whilst highlighting a number of deficiencies in the use of four arm caliper data to characterise borehole breakouts from the available data it is difficult to unambiguously define the nature of variations from the mean shmax orientations observed further analyses of image log data over greater depth ranges are therefore required in order to investigate more fully the effects of stress rotations near faults and apparent stress detachment above salt cored anticlinal structures

systematic along axis tidal triggering of microearthquakes observed at 9 50 n east pacific rise 1 hydrothermal fluid circulation at mid ocean ridges facilitates the exchange of heat and chemicals between the oceans and the solid earth and supports chemosynthetic microbial and macro faunal communities the structure and evolution of newly formed oceanic crust plays a dominant role in controlling the character and longevity of hydrothermal systems however direct measurements of subsurface processes remain technologically challenging to obtain previous studies have shown that tidally induced stresses within the subseafloor modulate both fluid flow and microearthquake origin times in this study we observe systematic along axis variations between peak microearthquake activity and maximum predicted tidal extension beneath the hydrothermal vent site at 9 50 n east pacific rise we interpret this systematic triggering to result from pore pressure perturbations propagating laterally through the hydrothermal system based on our observations and a one dimensional pore pressure perturbation model we estimate bulk permeability at 10 13 to 10 12 m2 within layer 2b over a calculated diffusive lengthscale of 2 0 km

numerical modelling of coal seam depressurization during coal seam gas production and its effect on the geomechanical stability of faults and coal beds abstract this coupled geomechanical fluid flow numerical modelling study investigates the geomechanical impact of depressurization of coal beds on fault and coal seam geomechanical stability during coal seam gas production using a simplified 3d reservoir structure based on the gloucester basin eastern australia the model examines transient stress pore pressure and fluid flow patterns during gas production under three different background regional stress scenarios with the aim of understanding possible geomechanical failure states in a fault and coal beds the model results show that depressurization leads to shear stress increase in normal faulting stress regimes but decreases in reverse faulting dominant in eastern australia and strike slip faulting stress regimes this is in combination with an overall effective stress increase associated with pore pressure reduction depressurization resultant fault failure appears to be possible only under normal stress regimes in addition a critically stressed fault segment is likely to be a precursor requirement for fault reactivation to occur as a result of depressurization fluid depletion within normal faulting stress conditions results further show fluid flow is dominated by lateral flow along coal beds converging towards depressurization locations wells fault permeability governs fluid transport along across the fault and development of pore pressure compartmentalisation across the fault under all three background stress scenarios flow velocities in coal beds are governed by coal permeability

ridge forces absolute plate motions and the intraplate stress field torque poles are calculated for a variety of possible forces acting on the plates including ridge push slab pull and collisional resistance these torque poles are then compared to the directions of absolute plate motions there is a strong correlation between ridge torque poles and the azimuth of absolute plate motions for the north american south american pacific cocos and eurasian plates simple slab pull torques correlate well with absolute motion azimuths for the pacific nazca and cocos plates and moderately well with the absolute motion azimuth of the indoaustralian plate collisional resistance torque poles correlate with the absolute motion azimuth of the eurasian plate only the correlations are presented as further evidence that the absolute reference frame for plate motion is determined by the surface plates themselves torque poles for various forces are also compared with several long wavelength features of the global intraplate stress field that also tend to be aligned with absolute motion directions in general ridge torque directions agree well with the orientations of maximum horizontal stresses for stable north america western europe and south america and provide an alternative explanation for the alignment in terms of ridge push forces rather than basal drag collisional resistance forces can also explain the alignment of stresses in western europe for the indoaustralian plate the torque pole for collisional resistance forces is consistent with the general pattern of stresses in at least the western half of the plate but is not a good predictor of the entire data set for the plate other processes in addition to collisional resistance must be important for the indo australian plate ridge push forces may account for a significant portion of the long wavelength features of the intraplate stress field especially away from continental collisions such a conclusion is consistent with negative buoyancy of the slab being an important component of the driving mechanism as previously suggested slab forces may be largely balanced within the subducted slab itself and thus have limited effect on deformation of the surface plates

stress state differences between sedimentary cover and basement of the songliao basin ne china in situ stress measurements at 6 7 km depth of an icdp scientific drilling borehole sk ii abstract the stress state down to the basement within sedimentary basins yet remains poorly understood the anelastic strain recovery asr method was employed to measure the stress state in the sk ii borehole in the songliao basin northeast china the results show that the stress state differs significantly between the sedimentary cover and basement in the sedimentary cover 6296 6335 m the maximum principal stress 1 is nearly vertical which is in the normal faulting stress regime dominated by gravitation in accordance with the normal faults observed on seismic reflection profiles within the sedimentary cover in contrast the basement 6646 6846 m shows that 1 is nearly horizontal suggesting a strike slip or reverse faulting regime the stress state of the basement is close to the reasonable stress condition for the eurasian plate far from a western pacific plate subduction zone and consistent with the focal mechanisms of earthquakes 7 15 km in the vicinity of icdp sk ii borehole the difference of stress state may imply that the far field stress generated by the western pacific plate has limited effects on the sedimentary cover an increase in thickness correlates with an increasing influence of tectonic stress with depth

strombolian explosions 2 eruption dynamics determined from acoustic measurements strombolian activity consists of a series of explosions due to the breaking of a large overpressurized bubble at the surface of the magma column acoustic pressure due to sound waves has been measured and analyzed at stromboli for more than 50 explosions three parts can be distinguished in the acoustic pressure waveform which are related to the behavior of the bubble before during and after its bursting before the sharp rise in acoustic pressure the signal is dominated by waves with a frequency of 2 hz which develop on the nose of the bubble they produce sound in air by imposing a rapid motion to the interface and one could detect a bubble travelling in the uppermost 30 m of the magma column when the bubble reaches the air magma interface its strong vibration driven by a large overpressure inside the gas generates the main event with a frequency around 9 hz after the bubble has burst kinematic waves of frequency around 4 5 hz are the main source of sound they develop at the surface of the magma left on the conduit side the three types of motion although determined independently give consistent results furthermore combining the results obtained for the two types of kinematic waves the magma viscosity is estimated to be of 300 65 pa s which is in good agreement with petrological constraints and corroborates the validity of our analysis this suggests that acoustic measurements constitute a powerful tool in the understanding of eruption dynamics

influence of basement structures on in situ stresses over the surat basin southeast queensland the jurassic to cretaceous sedimentary rocks of the surat basin in southeast queensland host a significant volume of coal seam gas resources consequently knowledge of the in situ stress is important for coal permeability enhancement and wellbore stability using wireline log data and direct stress measurements we have calculated stress orientations from 36 wells and stress magnitudes from 7 wells across the surat basin our results reveal a relationship between high tectonic stress and proximity to structures within the underlying basement rocks the influence of tectonic stresses is diminished with depth in areas with thicker sedimentary cover that are relatively far from the basement structures we suggest that this relationship is due to the redistribution of in situ stresses around areas where basement is shallower and where basement structures such as the leichhardt burunga fault system are present this behavior is explained by a lower rigidity in the thickest basin cover which reduces the ability to maintain higher tectonic stress over the entire surat basin a significant amount of variability in in situ stress orientation is observed the authors attribute this stress variability to complex plate boundary interactions on the northern and eastern margins of the indo australian plate

variation of in situ stress regime in coal reservoirs eastern yunnan region south china implications for coalbed methane production knowledge of in situ stress distribution is fundamental for coalbed methane production however it is poorly understood in the eastern yunnan region south china in this study the horizontal maximum shmax and minimum shmin principal stress and vertical stress sv were systematically analyzed for the first time the results indicated that the magnitudes of shmax shmin and sv showed positive correlations with burial depth in general three types of in situ stress fields were determined 1 shmax sv shmin in shallow layers with burial depths less than approximately 600 m 1970 ft below ground level bgl indicating a dominant strike slip faulting stress regime 2 in medium layers approximately 600 800 m 1970 2625 ft bgl the in situ stress state followed multiple relationships suggesting that the in situ stress regime was transformed and 3 sv shmax shmin in deep layers with burial depths greater than approximately 800 m 2625 ft bgl indicating a dominant normal faulting stress regime coal permeabilities obtained from injection falloff well tests showed that they were widely distributed and no obvious relationships were found between coal permeability and effective in situ stress magnitude in the study area the development and orientation of previously generated natural fractures combined with the present day in situ stress distribution controlled the permeability in coal reservoirs differential stress and presence of natural fractures significantly affected the geometry and pattern of hydraulic fractures in addition in the eastern yunnan region locations with relatively deep depths in vertical wells and approximately west northwest east southeast trending horizontal wells suffered high potential of borehole instability because of the high differential stress

subsurface fracture analysis and determination of in situ stress direction using fmi logs an example from the santonian carbonates ilam formation in the abadan plain iran abstract the relationship between the present day stress field and natural fractures can have significant implications for subsurface fluid flow in particular fractures that are aligned in orientations favourable for reactivation by either shear or tensile failure in the in situ stress field often exhibit higher hydraulic conductivities the ilam formation of southwestern iran is an important hydrocarbon reservoir containing numerous natural fractures however little is known about the state of stress in this region or any of iran s petroleum provinces we conducted analysis of the present day maximum horizontal stress orientation and the density orientation and hydraulic conductivity of natural fractures in the ilam carbonates using high resolution formation micro imager resistivity logs in two wells a total of 51 breakouts with an overall length of 215 m were observed in the two wells indicating a maximum horizontal stress orientation of 68 n 7 6 in well a and 58 n 6 3 in well b furthermore the wellbore derived stress orientations determined herein are consistent with those inferred from nearby earthquake focal mechanism solutions indicating that stresses in the sedimentary cover are linked to the resistance forces generated by arabia eurasia collision furthermore the correlation between stress orientations estimated from earthquake focal mechanism solutions and breakouts indicates that focal mechanism solution data which is often considered to be unreliable for stress field analysis near transform margins may provide reliable information on the stress orientation near continental collision zones the image log data also reveals three sets of open and presumably hydraulically conductive fractures with strikes of i 160 170 n ii 110 140 n and iii 070 080 n fracture set iii is consistent with being formed and open in the present day stress field however fracture sets i and ii strike at a high angle to the present day maximum horizontal stress and are interpreted herein to be the result of either pre or syn folding related forces the observation that different sets of open fractures in the field can be either sensitive or insensitive to the present day stress is critical for improving hydrocarbon recovery

a review of controls on lava lake level insights from halema uma u crater kīlauea volcano the height of the lava column is a fundamental measure of open vent volcanic activity but little continuous long term data exist to understand this parameter the recent 2008 2018 lava lake activity at the summit of k lauea volcano provides an opportunity to track and understand the processes that control lava level over timescales ranging from minutes to years we review recently published data as well as analyze the long term record of lava level at k lauea s summit during 2009 2017 longer timescale fluctuations over days to months have a strong positive linear correlation with ground deformation suggesting they reflect pressure changes in the summit magma reservoir shorter timescale fluctuations over minutes to hours have an inverse relationship with spattering i e outgassing intensity at the lake surface increased outgassing lowers the lake level and vice versa our analysis of the long term lava level data thus confirms that lava level at halema uma u is controlled by these two types of processes 1 those related to magma reservoir pressure changes such as magma supply rate and 2 shallow outgassing fluctuations such as gas pistoning frequency filtering can isolate pressure and outgassing driven components of lava level change time series analysis indicates that there was no large persistent periodicity in the lava level a minor fortnightly oscillation might be related to earth tides but was not continuously present the correlation between lava level and deformation of the summit indicates that the lava lake acts as a reliable piezometer tracking lava level over time may thus provide an indication of flank eruptive potential we show that long term interdisciplinary monitoring is necessary to discriminate the processes that control lava level

the present day in situ stress field within coalbed methane reservoirs yuwang block laochang basin south china abstract understanding in situ stress variation is extremely important for coalbed methane cbm exploration and development however precise information on the present day in situ stress field in the yuwang block of laochang basin is unclear in the present study the horizontal maximum shmax and minimum shmin principal stress and vertical stress sv in coal seams of yuwang block were systematically analyzed based on measured injection falloff data and geomechanical models the results indicated that in the yuwang block vertically shallow coal seams less than 800 m below ground level bgl mainly experienced the strike slip faulting stress regime shmax sv shmin whereas deep layers more than 800 m bgl were characterized by the dominant normal faulting stress regime sv shmax shmin laterally with the no 9 coal seam case study relatively lower stress magnitudes were mainly located around yuwang and well l 4 regions coal permeability obtained from injection falloff well tests decreased exponentially with the increased effective stress suggesting that coal permeability in the yuwang block was in situ stress controlled in addition differential stress and presence of natural fractures within coal seams significantly affected the geometry and pattern of hydraulic fractures in coal seams of yuwang block the present day in situ stress field contributed to generate complex hydraulic fracture networks the study results provided geological references for cbm development in the yuwang block of laochang basin

apparent tidal influence on magmatic activity at oldoinyo lengai volcano tanzania as observed in moderate resolution imaging spectroradiometer modis data this paper investigates the relative timing of solid earth tides and thermal anomalies associated with volcanic activity at oldoinyo lengai ol tanzania from 2000 to 2008 the low viscosity of ol s carbonatite magmas may make it particularly susceptible to tidal stresses thermal data from the moderate resolution imaging spectroradiometer modis were filtered using the modlen algorithm and earth tides were modeled using tsoft application of the schuster and chi squared tests resulted in apparent correlations between times of thermal anomalies and the phase of the semi diurnal and biannual solid earth tides however for semi diurnal and biannual tides limited acquisition times of the modis data account for the apparent correlations re examining the data while accounting for the bias introduced by the limited acquisition times correlations are no longer found onset times of eruptive events and times of thermal anomalies show no statistically significant correlation with the fortnightly tide indicating this is not an influence on activity at ol tidal influences on magmatic activity if at all present cannot be observed in coarse resolution thermal data on semi diurnal and fortnightly time scales based on the available data correlations on biannual timescales cannot be ruled out

paleotectonic stress field modeling and prediction of natural fractures in the lower silurian longmaxi shale reservoirs nanchuan region south china abstract natural fractures serve as significant storage spaces for hydrocarbons and are favorable for fluid flow in shale gas reservoirs therefore predicting the intensity and distribution of natural fractures within reservoirs are of extreme significance for shale gas exploration and development natural fractures in the nanchuan region are generally in high dip angles and the dominant fracture strikes are in the ne sw and nw se directions most natural fractures within the longmaxi formation are formed in the late yanshanian period therefore in this study the late yanshanian paleotectonic stress field was numerically investigated the comprehensive rupture rate crr was defined and calculated as an indicator to quantitatively predict the intensity and distribution of natural fractures within the longmaxi formation the results indicated that regions with well developed fractures were mainly located in around fault zones and fold zones among fault fold orientations change and at fault tips the intensity and distribution of natural fractures in the longmaxi formation of nanchuan region were influenced by many factors including tectonic activities mineral compositions etc the present study provides preliminary insights into natural fractures within the longmaxi formation of nanchuan region south china which can guide and support shale gas exploration and development

variation in vertical stress in the baram basin brunei tectonic and geomechanical implications the vertical or lithostatic stress is an important factor in tectonic and geomechanical studies and is commonly used in the prediction of pore pressures and fracture gradients however the vertical stress is not always calculated in situ and the approximation of 1 0 psi ft 22 63 mpa km is often used for the vertical stress gradient vertical stress has been determined in 24 fields in the baram basin brunei using density log and checkshot velocity survey data the baram basin shows a variation in vertical stress gradient between 18 3 and 24 3 mpa km at 1500 m depth below the surface this variation has a significant effect on in situ stress related issues in field development such as wellbore stability and fracture stimulation the variation is caused by a bulk rock density change of 2 48 2 07 g cm 3 from the hinterland of the delta to its front differential uplift and erosion of the delta hinterland and undercompaction associated with overpressure are the interpreted causes of the density and hence vertical stress variation q 2004 elsevier ltd all rights reserved

well bore breakouts and in situ stress the detailed cross sectional shape of stress induced well bore breakouts has been studied using specially processed ultrasonic borehole televiewer data we show breakout shapes for a variety of rock types and introduce a simple elastic failure model which explains many features of the observations both the observations and calculations indicate that the breakouts define relatively broad and flat curvilinear surfaces which enlarge the borehole in the direction of minimum horizontal compression this work supports the hypothesis that breakouts result from shear failure of the rock where the compressive stress concentration around the well bore is greatest and that breakouts can be used to determine the orientation of the horizontal principal stresses in situ

topography boundary forces and the indo australian intraplate stress field the relative contribution of topographic e g ridge push continental margins and elevated continental crust and plate boundary e g subduction and collisional forces to the intraplate stress field in the indo australian plate iap is evaluated through a finite element analysis two important aspects of the iap intraplate stress field are highlighted in the present study 1 if substantial focusing of the ridge push torque occurs along the collisional boundaries i e himalaya new guinea and new zealand many of the first order features of the observed stress field can be explained without appealing to either subduction or basal drag forces and 2 it is possible to fit the observed shmax maximum horizontal stress orientation and stress regime information with a set of boundary conditions that results in low tectonic stress magnitudes e g tens of megapascals averaged over the thickness of the lithosphere throughout the plate this study therefore presents a plausible alternative to previous studies of the iap intraplate stress field which predicted very large tectonic stress magnitudes hundreds of megapascals in some parts of the plate in addition topographic forces due to continental margins and elevated continental material were found to play an important role in the predicted stress fields of continental india and australia and the inclusion of these forces in the modeling produced a significant improvement in the fit of the predicted intraplate stresses to the available observed stress information in these continental regions a central focus of this study is the relative importance of the boundary conditions used to represent forces acting along the northern plate margin we note that a wide range of boundary conditions can be configured to match the large portion of the observed intraplate stress field and this nonuniqueness continues to make modeling the iap stress field problematic while our study is an important step forward in understanding the sources of the iap intraplate stress field a more complete understanding awaits a better understanding of the relative magnitude of the boundary forces acting along the northern plate margin

reduction of saltwater intrusion by modifying hydraulic conductivity we present an approach for reducing saltwater intrusion in coastal aquifers by artificially reducing the hydraulic conductivity in the upper part of selected areas by using a precipitate we apply a previously presented analytical approach to develop formulas useful for the design of artificial barriers equations for the location of the tip of the saltwater wedge are presented and verified through a sand tank experiment the analysis is capable of computing discharges exactly but requires the dupuit forchheimer approximation to compute points of the interface between flowing fresh and stationary saltwater we consider a vertical coastline and boundaries in the fresh water zone of either given discharge or given head we demonstrate in the paper that reduction of the hydraulic conductivity in the upper part of a coastal aquifer will result in a decrease of saltwater intrusion and present analytic expressions that can be used for design purposes the previously presented analytical approach can be applied to design systems to reduce salt water intrusion caused by pumping inland from the zone that contains salt groundwater this article is protected by copyright all rights reserved

reservoir characterization of an organic rich dolomitic tight oil reservoir the lower cretaceous xiagou formation in the qingxi sag jiuquan basin nw china abstract this study presents the reservoir characterization of the unconventional organic rich dolomitic tight oil reservoir of the lower cretaceous xiagou formation in the qingxi sag jiuquan basin to better describe pore characteristics and understand contributors to pore development of the mixed siliciclastic carbonate reservoirs samples were examined by thin section x ray diffraction xrd analysis total organic carbon toc and rock eval pyrolysis porosity and permeability measurement field emission scanning electron microscopy fesem and nuclear magnetic resonance nmr four types of laminations including carbonate lamination siliceous lamination argillaceous lamination and organic matter lamination are developed the average toc of 1 67 and calculated ro of 0 72 0 96 indicate that the xiagou source rocks are qualifed oil source various types of pores with different sizes and micro fractures are observed in such tight mixed siliciclastic carbonate reservoirs compared to mudstone shale and tight sandstone the tight and mixed siliciclastic carbonate reservoirs are characterized by 1 widely developed and multiple types of lamina structure 2 various pore types without secondary om pores generated by organic matter conversion 3 wide pore size distributions and 4 abundant and multiple genetic types of micro fractures the difference in mineral composition is the primary cause of different pore structure characteristics dolomitic mudstone is dominated by interlayer pore of clay mineral while argillaceous dolomite is characterized by interparticle fractures interparticle pores and micro fractures including shrinked micro fracture in dolomitic siltstone the residual intergranular pore and intergranular dissolved pore are the most common pores the nanopore occurs widely in all lithologies but the proportion of micrometers scale pores and macro scale nanopore increases from dolomitic mudstone to argillaceous dolomite and dolomitic siltstone the positive correlation between porosity permeability and toc indicate the existence of pores or micro fractures within or along the edge of organic matters the similar thermal maturity level ro 0 72 0 96 eliminates maturation as a variable in porosity development in the studying area the lithofacies associations with high quality source rocks and reservoirs relatively high porosity and permeability or rich in brittle minerals are the favorable targets

effect of borehole deviation on breakout orientations well bore breakouts are zones of spalling and fracture that form on opposite sides of a well bore and tend to change the cross sectional shape of the borehole from circular to roughly elliptical in vertical holes drilled in areas where one principal stress sv is vertical breakouts tend to form at opposite ends of the borehole diameter parallel to the least compressive horizontal principal stress direction sh this paper uses an analytical elastic solution for stress at the wall of a borehole to analyze the rotation of breakout orientations away from the direction of sh as the borehole deviates from the direction of the vertical principal stress the calculated orientations of breakouts in deviated boreholes depend on the type of faulting regime in which the well was drilled i e normal strike slip or thrust on the deviation angle o of the borehole axis from vertical on the angle between the horizontal projection of the borehole axis and the direction of sh and on the relative magnitudes of the three principal stresses sh sh the greatest compressive horizontal stress and sv in the strike slip faulting regime regardless of the values of sv sh and sh a borehole must deviate at least 35 from vertical before the horizontal projection of the breakout orientation differs by more than 10 from sh in the normal and thrust faulting regimes however the borehole deviation angle ocrit required to rotate projected breakout orientations by 10 from sh approaches zero as the value of sh approaches that of sh in the thrust faulting regime only about 3 of all combinations of sv sh and sh will produce ocrit values less than 10 while about 12 of all such combinations will yield ocrit 10 in the normal faulting regime about 12 and 33 of such combinations will yield ocrit 20 in the thrust and normal faulting regimes respectively careful study of changes in breakout orientation as a function of borehole deviation may improve the resolution of inferred stress directions when studying breakouts in deviated boreholes in these two faulting regimes

radial flow in vertically graded hydraulic conductivity aquifers type curve expressions are derived analytically for drawdowns around a fully penetrating well in vertically graded hydraulic conductivity aquifers a linear increase in the hydraulic conductivity with depth is adopted in this paper solution of the relevant groundwater movement equations are achieved by the application of boltzmann transformation individually for the continuity and laminar flow law equations necessary type curves are presented for determining the aquifer parameters from an aquifer test performed in the field the analytical results show that the conventional theis and jacob methods are not applicable in the cases of vertically graded hydraulic conductivity aquifers in addition a straight line method is proposed for the late time drawdown data however both type curve and straight line procedures developed in the paper yield the theis and jacob method equivalents respectively the application of the methodology is given for fissured and fractured hard rock aquifer test data obtained i

modeling the effects of eruptive and seismic activities on flank instability at mount etna italy 1 the identification and evaluation of trigger mechanisms for volcano flank instabilities and or collapse represent a key issue for risk assessment in densely populated volcanic areas as well as in long distance settings particularly in case of island or coastal volcanoes here we address quantitatively the effects of external seismic and inner magmatic forcing on the stress strain state associated to flank instabilities at mount etna sicily southern italy by means of a 2 d finite difference method numerical modeling modeled seismic actions include strong near field strong far field and low magnitude near field earthquakes magmatic actions consider the inner pressure changes induced by energetic lava fountains in the summit crater area and subvertical and oblique dike ascent below the summit area model results are validated in light of available monitoring data and recent eruptive activity numerical results show that the main strain effects are produced by high magnitude near field earthquakes expected return time of 103 yrs and by vertical uprise of a magma dike below the volcano summit area maximum displacements in the order of tens of centimeters may involve the summit area up to some 106 m3 m over some kilometers laterally stress releases up to 107 pa may affect a limited portion of the magmatic conduit thus favoring major effusive flank eruptions major catastrophic events such as volcano flank collapse should not be expected by applying either individually or combined the aforementioned actions

in situ stress state in the linxing region eastern ordos basin china implications for unconventional gas exploration and production abstract the carboniferous and permian sedimentary rocks mainly the shanxi and taiyuan formations in the linxing region eastern ordos basin china host a significant volume of unconventional gas resources coalbed methane shale gas and tight sandstone gas currently the in situ stress state is poorly understood but knowledge of this is extremely important for a range of applications such as gas exploration and production fracture stimulation and wellbore stability the maximum horizontal stress s hmax minimum horizontal stress s hmin and vertical stress s v magnitudes and the s hmax orientation in the linxing region were systematically analyzed for the first time in the present study which can provide a reference for subsequent numerical simulation and hydraulic fracturing design based on borehole breakouts and drilling induced tensile fractures interpreted from borehole imaging logs the s hmax orientation rotates from nee sww trending in the southern part to nww see trending in the northern part of the linxing region both conventional logs and extended leak off tests were used for stress magnitude determination the results revealed three types of in situ stress fields s v s hmax s hmin s hmax s v s hmin and s hmax s v s hmin and a dominant strike slip stress regime s hmax s v s hmin was found for the entire well section in the target shanxi formation and taiyuan formation in the linxing region in addition differential stress increased with depth in the linxing region which indicates that wellbore instability might be a potentially significant problem when drilling wells that are vertical or n s trending

tidal modulation of eruptive activity at open vent volcanoes evidence from stromboli italy terra nova 24 233 237 2012 abstract the role of earth tides as a relevant controlling factor of a variety of geological processes including seismicity and volcanism is widely debated stromboli volcano italy is well known for its persistent explosive activity and represents an ideal setting to investigate tidal effects on the energy release and eruptive behaviour of volcanoes herein we report new evidence that tidal stresses may modulate the timing of eruptive activity at volcanoes in an open vent state we report statistically significant correlations of the peak eruption frequencies to fortnightly tidal maxima i e full and new moon during a 17 month long period of activity in 2010 2011 in fact the number of explosive events per hour increases by 85 during tidal maxima with respect to the average values we suggest that tidally induced cycles of compression decompression of country rocks controls fluctuating magma supply rate from the shallow plumbing system and thus eruption frequency

recycling and re hydration of degassed magma inducing transient dissolution crystallization events at stromboli italy abstract intrusive degassing and recycling of degassed and dense magma at depth have been proposed for a long time at stromboli the brief explosive event that occurred at the summit craters on 9 january 2005 threw out bombs and lapilli that could be good candidates to illustrate recycling of shallow degassed magma at depth we present an extensive data set on both the textures and the mineral bulk rock and glassy matrix chemistry of the 9 jan products the latter have the common shoshonitic basaltic bulk composition of lavas and scoriae issued from typical strombolian activity in contrast they differ by the heterogeneous chemistry of their matrix glasses and their crystal textures that testify to crystal dissolution event s just prior magma crystallization upon ascent and eruption comparison between mineral paragenesis of the natural products and experimental phase equilibria suggest water induced magma re equilibration we propose that mineral dissolution is related to water enrichment of the recycled degassed magma via differential gas bubble transfer and to some extents its physical mixing with volatile rich magma blobs however all these features illustrate transient processes even though evidence of mineral dissolution is ubiquitous at stromboli its effect on the bulk magma chemistry is minor because of the subtle interplay between mineral dissolution and crystallization in magmas having comparable bulk chemistry

earth s spin and volcanic eruptions evidence for mutual cause and effect interactions the angular velocity of earth s rotation shows decadal oscillations due to the lunisolar gravitational torque as well as inter or intra annual changes arising from the angular momentum exchange between the atmosphere and the solid earth the energies involved in the length of day lod variations may affect the crustal deformation rate and seismic energy release on a global scale we found significant correlation between the occurrences of major volcanic eruptions and the lod pattern since ad 1750 on a multiyear scale eruption frequency worldwide increases with lod changes moreover the injection of sulphur gases into the atmosphere during major eruptions is accompanied by significant inter annual lod variations this provides evidence of complex mutual cause and effect interactions stress changes induced by multiyear variations in earth s spin may affect climactic volcanic activity also the atmosphere s dynamic response to volcanic plumes may result in global changes of wind circulation and climate with consequent lod variations

cause and evolution of intraplate orogeny in australia abstractintraplate orogeny remains an enigmatic process in terms of plate tectonics regional continental stress elds agree well with stress orientations predicted by the main forces driv ing and resisting plate motion however regional continental stress elds fail to explain major localized compressive plate deformation in continental interiors far away from plate bound aries global or plate wide intraplate stress models typically assume a laterally homogeneous plate rheology and are con ned to modeling the present without addressing the antiquity of the current stress eld an understanding of intraplate stress and deformation requires the linking of spatial variations in continental rheology with time dependent plate geometry and driving forces here we demonstrate how the complex interplay between juxtaposed weak and strong geological provinces and changes in far eld plate boundary forces has caused intra plate orogenesis and tectonic reactivation in southeastern australia during the tertiary our ndings are contrary to the opinion that continental interiors are insensitive to compression at plate margins and help to explain the mechanisms causing intraplate orogenesis keywords

utilization of observations of well bore failure to constrain the orientation and magnitude of crustal stresses application to continental deep sea drilling project and ocean drilling program boreholes

the origins of the intraplate stress field in continental australia the ridge push force acting on the indo australian plate exerts a significant torque 8 5 1025n m about a pole at 30 3 n 34 5 e the angular difference between this torque pole and the observed pole of rotation for the plate 19 2 n 35 6 e is less than 12 and suggests that the ridge push force plays an important role in the dynamics of the indo australian plate we have used an elastic finite element analysis to study the predicted intraplate stress field in continental australia for four models which employ different boundary conditions to balance the ridge push torque acting on the plate the modeling indicates that a number of important features of the observed stress field within the australian continent can be explained in terms of balancing the ridge push torque with resistance imposed along the himalaya papua new guinea and new zealand collisional boundaries segments these features include n s to ne sw oriented compression in the northern australia and e w oriented compression in southern australia our analysis also shows that subduction processes along the northern and eastern boundaries provide only second order controls on the intraplate stress field in continental australia

a stress province boundary and tractions on the north american plate stress provinces characterized by nearly constant directions and relative magnitudes of horizontal principal stresses are potentially informative of the forces acting on a lithosphere plate this applies most strongly to stress provinces which occupy a large part of a plate such as the mid continent stress province of north america identified by zoback and zoback1 we have used fractures known as breakouts in oil wells which are controlled by the in situ stress to show that this stress province extends through the western canadian sedimentary basin to the arctic coast there is a strong possibility that a uniform stress orientation with greatest compression ne sw is co extensive with the north american continent east of the rocky mountains excluding the appalachian and gulf of mexico stress provinces we further report new breakout orientations in the far northwest of canada which locate the boundary between the mid continent stress province and an alaskan stress province with largest compression nnw sse probably controlled by the subduction of the pacific plate under alaska

stress orientations from borehole wall fractures with examples from colorado east texas and northern canada azimuthally aligned breakouts in oil wells are explained as shear fractures in the zone of amplified stress difference near the borehole in a stress field having unequal horizontal principal stres

breakouts in alberta and stress in the north american plate the paper reports a detailed statistical study of breakout azimuths in 48 oil wells widely distributed in the alberta sedimentary basin bringing the number of oil wells contributing azimuths to 94 for the western canadian basin the azimuths show significant regional variation between the northern central and southern parts of the basin twenty one wells in which breakouts cover depth ranges greater than 600 m were used to investigate regression of breakout azimuths on depth ten wells give positive regression coefficients 11 are negative and no coefficient is significant at 95 confidence level if the dominant northwest southeast orientation of the long axes of breakouts gives the orientation of the lesser horizontal principal stress sh following the hypothesis of bell and gough the inferred stress orientations indicate that throughout the whole basin the direction of the maximum horizontal stress sh is northeast southwest stress measurements by strain relief techniques in a mine and from hyd

drilling induced tensile wall fractures implications for determination of in situ stress orientation and magnitude abstract detailed investigation of failure of the borehole wall in two scientific drilling projects the german ktb kontinentales tiefbohrprogramm der bundesrepublik deutschland and the european geothermal research project at soultz sous forets france has lead to new insight in the phenomena of tensile fractures induced in the wellbore wall during drilling comparison of the orientation of the fractures with the orientation of the horizontal principal stress known from breakout and hydraulic fracturing analysis demonstrates that these fractures are reliable indicators of the orientation of the maximum horizontal principal stress s h a model for the initiation of the fractures is presented which points out the important influences of a the tectonic stress state b increased mud pressures during drilling operation and c thermal stresses induced by circulation of relatively cold drilling mud analysis of drilling induced fractures in the gpk1 borehole at soultz sous forets where the magnitude of s h is known from hydraulic fracturing experiments demonstrates the validity of this model for the initiation of the fractures further a new method is proposed to estimate the magnitude of s h from the occurrence of drilling induced fractures and knowledge of thermally induced stress and pumping pressure during drilling the method is successfully applied to both ktb boreholes an independent method to estimate the magnitude of s h based on the analytical calculation of the stress intensity factor for drilling induced fractures taking into consideration both increased mud pressure and thermal stress is also presented application of this method confirms the results derived with the analysis described above additionally the evaluation of the orientation of the fractures with respect to the wellbore axis indicates that over major depth sections of the investigated wells the vertical stress is a principal stress