inelastic off fault response and three dimensional dynamics of earthquake rupture on a strike slip fault 1 large dynamic stress off the fault incurs an inelastic response and energy loss which contributes to the fracture energy limiting the rupture and slip velocity using an explicit finite element method we model three dimensional dynamic ruptures on a vertical strike slip fault in a homogeneous half space the material is subjected to a pressure dependent drucker prager yield criterion initial stresses in the medium increase linearly with depth our simulations show that the inelastic response is confined narrowly to the fault at depth there the inelastic strain is induced by large dynamic stresses associated with the rupture front that overcome the effect of the high confining pressure the inelastic zone increases in size as it nears the surface for material with low cohesion 5 mpa the inelastic zone broadens dramatically near the surface forming a flowerlike structure the near surface inelastic strain occurs in both the extensional and the compressional regimes of the fault induced by seismic waves ahead of the rupture front under a low confining pressure when cohesion is large 10 mpa the inelastic strain is significantly reduced near the surface and confined mostly to depth cohesion however affects the inelastic zone at depth less significantly the induced shear microcracks show diverse orientations near the surface owing to the low confining pressure but exhibit mostly horizontal slip at depth the inferred rupture induced anisotropy at depth has the fast wave direction along the direction of the maximum compressive stress

pressure core analysis of geomechanical and fluid flow properties of seals associated with gas hydrate bearing reservoirs in the krishna godavari basin offshore india abstract physical properties of the sediment directly overlying a gas hydrate reservoir provide important controls on the effectiveness of depressurizing that reservoir to extract methane from gas hydrate as an energy resource the permeability of overlying sediment determines if a gas hydrate reservoir s upper contact will provide an effective seal that enables efficient reservoir depressurization compressibility stiffness and strength indicate how overlying sediment will deform as the in situ stress changes during production providing engineering data for well designs assessing these properties requires minimally disturbed sediment india s national gas hydrates program expedition 2 nghp 02 provided an opportunity to study these seal sediment properties reducing disturbance from gas exsolution and bubble growth by collecting a pressure core from the seal sediment just above the primary gas hydrate reservoir at site nghp 02 08 in area c of the krishna godavari basin the effective stress chamber esc and the direct shear chamber dsc devices in the suite of pressure core characterization tools pccts were used to measure permeability compressibility stiffness and shear strength at the in situ vertical stress geotechnical properties of the predominantly fine grained seal layer at in situ vertical stress are in typical clay sediment ranges with low measured permeability 0 02 md high compressibility c c 0 26 0 33 and low shear strength 404 kpa though pressure and temperature were maintained throughout the collection and measurement process to stabilize gas hydrate the lack of effective stress in the pressure core storage chamber and the chamber pressurization with methane free water caused core expansion and gas hydrate in a thin coarser grained layer to dissolve the pccts can reapply in situ stress with incremental loading steps during a consolidation test to account for sediment compaction gas hydrate dissolution can be limited by storing cores just above freezing temperatures and by using solid spacers to reduce the storage chamber s free volume

constitutive relationships and physical basis of fault strength due to flash heating 1 we develop a model of fault strength loss resulting from phase change at asperity contacts due to flash heating that considers a distribution of contact sizes and nonsteady state evolution of fault strength with displacement laboratory faulting experiments conducted at high sliding velocities which show dramatic strength reduction below the threshold for bulk melting are well fit by the model the predicted slip speed for the onset of weakening is in the range of 0 05 to 2 m s qualitatively consistent with the limited published observations for this model earthquake stress drops and effective shear fracture energy should be linearly pressure dependent whereas the onset speed may be pressure independent or weakly pressure dependent on the basis of the theory flash weakening is expected to produce large dynamic stress drops small effective shear fracture energy and undershoot estimates of the threshold slip speed stress drop and fracture energy are uncertain due to poor knowledge of the average contact dimension shear zone thickness and gouge particle size at seismogenic depths

numerical simulations of sand migration during gas production in hydrate bearing sands interbedded with thin mud layers at site nghp 02 16 abstract the geomechanical behavior of gas hydrate bearing sands during gas production is complex involving changes in hydrate dependent mechanical characteristics and also redistribution of stresses carried by the hydrate when an interbedded mud layer is present the complex response of the hydrate bearing sediments is more pronounced because hydrate dissociation tends to occur preferentially in the sands adjacent to the hydrate free mud layer which can act as a heat source this leads to non uniform sediment deformation and potentially to shearing deformation along the sand mud layers in addition since the hydrate bearing sand permeability becomes far greater than the mud permeability with hydrate dissociation a greater hydraulic gradient may exist along the interface between the sand mud layers these two behaviors may contribute to sand migration and thus a thorough study on the effect of a thin interbedded mud layer on the extent of sand migration is necessary this paper presents numerical sand migration modeling of sand mud interbedded hydrate bearing sediments found at site nghp 02 16 one of 25 sites drilled as a part of national gas hydrate program expedition 02 nghp 02 in the offshore of india subjected to depressurization for a comparison and better understanding simulation of homogeneous hydrate bearing sediments is also conducted it is found that greater sand migration occurs when the interbedded mud layer is present because of greater shearing deformation and greater pressure gradients at sand mud interface

india national gas hydrate program expedition 02 operational and technical summary abstract the india national gas hydrate program is being steered by the government of india s ministry of petroleum and natural gas mopng with participation of directorate general of hydrocarbons dgh oil and natural gas corporation limited ongc and the national oil companies and research institutes of india the india national gas hydrate program expedition 01 nghp 01 established the presence of gas hydrate in the krishna godavari kg and mahanadi basins and in the offshore area of the andaman sea basin however the gas hydrates discovered during nghp 01 were mainly distributed as fracture filling material in fine grained clay rich sediments the india national gas hydrate program expedition 02 nghp 02 was carried out with an objective to discover gas hydrate in sand rich sediment along the eastern offshore margin of india ongc planned and executed nghp 02 on the behalf of the mopng nghp 02 started on march 3 2015 and was completed on july 28 2015 total 147 days using the japanese scientific drilling vessel chikyu d v chikyu during nghp 02 42 holes at 25 sites were drilled cored and or surveyed with downhole logging tools these sites were located in four areas along the eastern margin of india and formally named area a mahanadi basin three sites area b northern part of the kg basin twelve sites area c central part of the kg basin six sites and area e southern part to the kg basin four sites all 25 sites established during nghp 02 were first drilled and logged with logging while drilling lwd tools and an additional 17 holes were then drilled and or cored with conventional coring tools hpcs escs or pressure coring tools pctb wireline logging was conducted in 10 holes and formation tests using a dual packer modular formation dynamics tester mdt tool were carried out in two holes the onboard science team used the laboratory facilities on the d v chikyu to examine and analyse the physical properties geochemistry and sedimentology of all the cores collected during the expedition core samples were also analysed in additional post expedition shore based studies conducted in numerous domestic and international gas hydrate research laboratories the nghp 02 sediment cores were archived at the national gas hydrate core repository in mumbai india which is associated with the ongc gas hydrate research and technology centre ghrtc the necessary data for characterizing the occurrence of gas hydrate such as interstitial water chlorinities core derived gas chemistry physical and sedimentological properties thermal images of the recovered cores pressure core and downhole measured logging data lwd and or conventional wireline log data were obtained from most of the drill sites established during nghp 02 almost all the drill sites yielded evidence for the occurrence of gas hydrate however the inferred in situ concentration of gas hydrate varied substantially from site to site for the most part the interpretation of downhole logging data core thermal images interstitial water analyses and pressure core images from the sites established during nghp 02 indicate that the occurrence of concentrated gas hydrate is mostly associated with coarser grained sand rich sediments this paper presents the operational and technical summary of nghp 02

fault roughness at seismogenic depths from lidar and photogrammetric analysis fault surface roughness is a principal factor influencing earthquake mechanics and particularly rupture initiation propagation and arrest however little data currently exist on fault surfaces at seismogenic depths here we investigate the roughness of slip surfaces from the seismogenic strike slip gole larghe fault zone exhumed from ca 10 km depth the fault zone exploited pre existing joints and is hosted in granitoid rocks of the adamello batholith italian alps individual seismogenic slip surfaces generally show a first phase of cataclasite production and a second phase with beautifully preserved pseudotachylytes of variable thickness we determined the geometry of fault traces over almost five orders of magnitude using terrestrial laser scanning lidar ca 500 to 1 10 m is not anisotropic and less evolved than at smaller scales these observations are consistent with an evolution of roughness due to fault surface processes that takes place only at scales smaller or comparable to the observed net slip differences in roughness evolution between shallow and deeper faults the latter showing evidences of seismic activity are interpreted as the result of different weakening versus induration processes which also result in localization versus delocalization of deformation in the fault zone from a methodological point of view the technique used here is advantageous over direct measurements of exposed fault surfaces in that it preserves in cross section all of the structures which contribute to fault roughness and removes any subjectivity introduced by the need to distinguish roughness of original slip surfaces from roughness induced by secondary weathering processes moreover offsets can be measured by means of suitable markers and fault rocks are preserved hence their thickness composition and structural features can be characterised providing an integrated dataset which sheds new light on mechanisms of roughness evolution with slip and concomitant fault rock production

roughness of fault surfaces over nine decades of length scales 1 we report on the topographic roughness measurements of five exhumed faults and thirteen surface earthquake ruptures over a large range of scales from 50 m to 50 km we used three scanner devices lidar laser profilometer white light interferometer spanning complementary scale ranges from 50 m to 10 m to measure the 3 d topography of the same objects i e five exhumed slip surfaces vuache sillingy bolu corona heights dixie valley magnola a consistent geometrical property i e self affinity emerges as the morphology of the slip surfaces shows at first order a linear behavior on a log log plot where axes are fault roughness and spatial length scale covering five decades of length scales the observed fault roughness is scale dependent with an anisotropic self affine behavior described by four parameters two power law exponents h constant among all the faults studied but slightly anisotropic h 0 58 0 07 in the slip direction and h 0 81 0 04 perpendicular to it and two pre factors showing variability over the faults studied for larger scales between 200 m and 50 km we have analyzed the 2 d roughness of the surface rupture of thirteen major continental earthquakes these ruptures show geometrical properties consistent with the slip perpendicular behavior of the smaller scale measurements our analysis suggests that the inherent non alignment between the exposed traces and the along or normal slip direction results in sampling the slip perpendicular geometry although a data gap exists between the scanned fault scarps and rupture traces the measurements are consistent within the error bars with a single geometrical description i e consistent dimensionality over nine decades of length scales

mechanical properties of sand silt and clay containing tetrahydrofuran hydrate 1 the mechanical behavior of hydrate bearing sediments subjected to large strains has relevance for the stability of the seafloor and submarine slopes drilling and coring operations and the analysis of certain small strain properties of these sediments for example seismic velocities this study reports on the results of comprehensive axial compression triaxial tests conducted at up to 1 mpa confining pressure on sand crushed silt precipitated silt and clay specimens with closely controlled concentrations of synthetic hydrate the results show that the stress strain behavior of hydrate bearing sediments is a complex function of particle size confining pressure and hydrate concentration the mechanical properties of hydrate bearing sediments at low hydrate concentration probably 50 of pore space the behaviorbecomesmoreindependentofstressbecausethehydratescontrolbothstiffnessand strength and possibly the dilative tendency of sediments by effectively increasing interparticle coordination cementing particles together and filling the pore space the cementation contribution to the shear strength of hydrate bearing sediments decreases with increasing specific surface of soil minerals the lower the effective confining stress the greater the impact of hydrate formation on normalized strength

shakeout d ground motion estimates using an ensemble of large earthquakes on the southern san andreas fault with spontaneous rupture propagation 1 we simulate ground motion in southern california from an ensemble of 7 spontaneous rupture models of large mw7 8 northwest propagating earthquakes on the southern san andreas fault shakeout d compared to long period spectral accelerations from the next generation attenuation nga empirical relations shakeout d predicts similar average rock site values i e within roughly their epistemic uncertainty but significantly larger values in los angeles and ventura basins due to wave guide focusing effects the shakeout d ground motion predictions differ from those of a kinematically parameterized geometrically similar scenario rupture 1 the kinematic rock site predictions depart significantly from the common distance attenuation trend of the nga and shakeout d results and 2 shakeout d predictions of long period spectral acceleration within the basins of the greater los angeles area are lower by factors of 2 3 than the corresponding kinematic predictions we attribute these differences to a less coherent wavefield excited by the complex rupture paths of the shakeout d sources

earthquake ruptures with strongly rate weakening friction and off fault plasticity part 1 planar faults abstract observations demonstrate that faults are fractal surfaces with deviations from planarity at all scales we study dynamic rupture propagation on self similar faults having root mean square rms height fluctuations of order 10 3 to 10 2 times the profile length our 2d plane strain models feature strongly rate weakening fault friction and off fault drucker prager viscoplasticity the latter bounds otherwise unreasonably large stress concentrations in the vicinity of bends our choice of a cohesionless yield function prevents tensile stress states and thus fault opening a consequence of strongly rate weakening friction is the existence of a critical background stress level above which self sustaining rupture propagation in the form of self healing slip pulses first becomes possible around this level at which natural faults are expected to operate ruptures become extremely sensitive to fault roughness and exhibit substantial fluctuations in rupture velocity except for shallow inclinations of the maximum compressive stress to the fault less than about 20 the fluctuations are anticorrelated with the local fault slope these accelerations and decelerations of the rupture together with naturally emerging slip heterogeneity excite waves of all wavelengths and result in ground acceleration spectra that are flat at high frequency consistent with observed strong motion records

multi measurement approach for establishing the base of gas hydrate occurrence in the krishna godavari basin for sites cored during expedition nghp 02 in the offshore of india abstract the 2015 national gas hydrate program of india s second expedition nghp 02 acquired logging and coring datasets for constraining the base of the gas hydrate occurrence zone deepest gh and the theoretical base of gas hydrate stability zone bghs these data are used here for two primary goals to constrain the deepest occurrence of gas hydrate relative to predicted stability limits and the observed bsr and to characterize the nature of the contact between gas hydrate bearing sediment and the underlying gas hydrate free sediment a consensus depth for the deepest gh is derived for each nghp 02 coring site from downhole indicators of gas hydrate occurrence obtained from well log electrical resistivity and acoustic data pressure core compressional wave velocity measurements and conventional core measurements of anomalously low temperatures to establish the theoretical bghs models of gas hydrate phase stability with depth are compared with downhole temperature profiles derived from 1 assuming a constant geothermal gradient consistent with downhole temperature measurements and 2 assuming constant heat flow using a geotherm through the downhole temperature measurements and incorporating thermal conductivity calculated from borehole logging data although the deepest nghp 02 gh occurrences are controlled at several sites by a lithologic boundary most sites have deepest gh occurrences within a single coarse grained lithology cutoffs within a single coarse grained lithology which occur for the primary nghp 02 area b gas hydrate reservoir will inhibit pore pressure drawdowns used to extract methane from gas hydrate as an energy resource

an equivalent viscoelastic model for rock mass with parallel joints an equivalent viscoelastic medium model is proposed for rock mass with parallel joints a concept of virtual wave source vws is proposed to take into account the wave reflections between the joints the equivalent model can be effectively applied to analyze longitudinal wave propagation through discontinuous media with parallel joints parameters in the equivalent viscoelastic model are derived analytically based on longitudinal wave propagation across a single rock joint the proposed model is then verified by applying identical incident waves to the discontinuous and equivalent viscoelastic media at one end to compare the output waves at the other end when the wavelength of the incident wave is sufficiently long compared to the joint spacing the effect of the vws on wave propagation in rock mass is prominent the results from the equivalent viscoelastic medium model are very similar to those determined from the displacement discontinuity method frequency dependence and joint spacing effect on the equivalent viscoelastic model and the vws method are discussed

numerical study of gas production from marine hydrate formations considering soil compression and hydrate dissociation due to depressurization abstract depressurization is considered a key method for extracting natural gas from marine gas hydrates in the petroleum industry permeability is one of the significant factors impacting gas production low permeability formations tend to be related to low gas production the effective permeability of hydrate bearing sediments depends on both porosity and hydrate saturation because the sediments bear external loads during depressurization analyses of gas production from hydrate should consider the volume change in the formation in response to earth stress in this paper several parametric analyses were conducted using a fully coupled thermo hydro chemo mechanical thcm model to analyze gas production during depressurization when considering soil compression the simulation results show that boundary effects exist when small models are used for long term simulations compression may result in a lower effective permeability in the hydrate dissociated zone than that in the hydrate undissociated zone the gas flow rate is affected by the difference in the effective permeability between the hydrate dissociated zone and the hydrate undissociated zone meanwhile lower downhole pressure leads to more serious soil compression which may not benefit gas production

terashake2 spontaneous rupture simulations of mw 7 7 earthquakes on the southern san andreas fault abstract previous numerical simulations terashake1 of large m w 7 7 southern san andreas fault earthquakes predicted localized areas of strong amplification in the los angeles area associated with directivity and wave guide effects from northwestward propagating rupture scenarios the terashake1 source was derived from inversions of the 2002 m w 7 9 denali alaska earthquake that source was relatively smooth in its slip distribution and rupture characteristics owing both to resolution limits of the inversions and simplifications imposed by the kinematic parameterization new simulations terashake2 with a more complex source derived from spontaneous rupture modeling with small scale stress drop heterogeneity predict a similar spatial pattern of peak ground velocity pgv but with the pgv extremes decreased by factors of 2 3 relative to terashake1 the terashake2 source excites a less coherent wave field with reduced along strike directivity accompanied by streaks of elevated ground motion extending away from the fault trace the source complexity entails abrupt changes in the direction and speed of rupture correlated to changes in slip velocity amplitude and waveform features that might prove challenging to capture in a purely kinematic parameterization despite the reduced pgv extremes northwest rupturing terashake2 simulations still predict entrainment by basin structure of a strong directivity pulse with pgvs in los angeles and san gabriel basins that are much higher than predicted by empirical methods significant areas of those basins have predicted pgv above the 2 probability of exceedance poe level relative to current attenuation relationships even when the latter includes a site term to account for local sediment depth and wave guide focusing produces localized areas with pgv at roughly 0 1 0 2 poe about a factor of 4 5 above the median in contrast at rock sites in the 0 100 km distance range the median terashake2 pgvs are in very close agreement with the median empirical prediction and extremes nowhere reach the 2 poe level the rock site agreement lends credibility to some of our source modeling assumptions including overall stress drop level and the manner in which we assigned dynamic parameters to represent the mechanical weakness of near surface material future efforts should focus on validating and refining these findings assessing their probabilities of occurrence relative to alternative rupture scenarios for the southern san andreas fault and incorporating them into seismic hazard estimation for southern california

dynamic rupture on a material interface with spontaneous generation of plastic strain in the bulk abstract we discuss 2d simulations of in plane ruptures on a fault between different solids in a model that includes dynamic generation of plastic strain off the fault the fault is governed by a regularized friction with a gradual response to abrupt changes of normal stress while plastic strain in the bulk is governed by a coulomb yielding criterion the occurrence of off fault damage stabilizes the slip velocity on the fault stable self sustaining ruptures propagate on the material interface as narrow unidirectional pulses in the direction of slip on the more compliant side of the fault plastic strain is generated only on the stiffer side of the fault the tensional quadrant in the direction of rupture propagation in a strip of approximately constant width that is correlated with the slip velocity on the fault simulations for various values of confining normal stress material contrast and cohesion in the bulk suggest that significant generation of off fault damage is limited to conditions that correspond to the top few kilometers of the crust the results are compatible with field observations of shallow asymmetric damage patterns in the structure of several large faults

strong velocity weakening and powder lubrication of simulated carbonate faults at seismic slip rates 1 high velocity friction tests were conducted on solid and hollow cylinders of carrara calcite marble dolomite marble silicate bearing calcite marble and calcite gouge to investigate the strength of carbonate faults during seismic slip the experiments performed at normal stresses of 0 6 14 7 mpa slip rates of 0 03 1 60 m s and room temperature in a rotary shear friction testing machine yielded an extraordinarily low steady state friction coefficient 0 1 at slip rates of 1 1 1 2 m s the slip weakening distance of 4 28 m became shorter at higher normal stress or frictional work rate strong velocity weakening was observed not only in steady state but also in nonsteady state friction while the slip rate was changing thus slip deceleration was accompanied by fault strength recovery large rapid temperature rises in narrow shear localization zones less than a few micrometers induced carbonate decomposition such as the breakdown of calcite into aggregates of cao nanograins and co2 in carrara marble scanning electron microscope observation revealed that the shear localization zone in the highly porous decomposition product was a layer of scattered small grains mostly 1 m in diameter these microstructures and the measured high permeability 10 14 m2 of the decomposed marble indicate that the dominant weakening mechanism in our experiments was possibly powder lubrication powder rheology at high slip rates is not yet well understood but the frictional behavior of nanograins appears to be strongly velocity dependent if decarbonation occurs during seismic slip in natural carbonate faults powder lubrication may make the faults slippery even under fluid drained conditions

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

seafloor subsidence induced by gas recovery from a hydrate bearing sediment using multiple well system abstract the response behavior of the methane exploitation from natural gas hydrate ngh using multiple well system is complex and needs to be investigated as gas production from a single vertical well generally cannot meet commercial demand this study numerically investigates the production performance and geomechanical response of an unconfined hydrate deposit in shenhu area south china sea under single and multiple vertical well conditions for a single vertical well with a mild constant bottom hole pressure gas production is relatively stable however seafloor subsidence exhibits an initial rapid drop and a subsequent mild drop stage the vertical displacement is highly developed at the top and bottom of the production zone the results from doublet and triplet vertical wells indicate that both the gas and water production and seafloor subsidence increase with the increase in number of production wells the superimposition of subsidence leads to a deterioration in subsidence and the change in location for the largest subsidence which may affect the risk location of well instability the interference of pore pressure and subsidence increases with the decrease in well spacing however gas production decreases and water production changes insignificantly furthermore a same subsidence at seafloor cannot indicate the same evolution of subsidence in the vertical and lateral direction the results presented in this study help in balancing the production and subsidence of the ngh exploitation in complex well configurations

laboratory formation of noncementing hydrates in sandy sediments natural hydrate bearing sediment hbs predominantly exists in noncementing habit and its limited availability for use in laboratory studies demands a time effective and repeatable laboratory process for forming representative samples with natural accumulation habit this study reports on a three step laboratory process for forming noncementing methane hydrate in sandy sediments 1 initial hbs formation under excess gas conditions 2 slow saline water 5 wt cacl2 injection under strictly controlled pressure temperature p t conditions and 3 a temperature warming cooling cycle changes in compressional wave velocity vp of sediment as well as p t condition were monitored throughout the tests the evolution of vp in good agreement with rock physics model calculations suggested that the transition from cementing hydrate into noncementing hydrate occurs during saline injection as well as temperature warming cooling cycle the proposed process appeared to be an efficient and consistent substitute for the existing methods to form noncementing hydrate habit in sandy sediments

evidence for an earthquake barrier model from mw 7 8 kokoxili tibet earthquake slip distribution abstract the slip distribution of the mw 7 8 kokoxili tibet 2001 earthquake has been measured at high resolution using optical correlation of satellite images and provides both the parallel and perpendicular components of the horizontal co seismic slip this reveals a variation of the horizontal slip at a scale of 20 km along strike anti correlation of slip parallel and perpendicular to the fault indicates transfer of slip from the horizontal to the vertical component at the ends of segments these features suggest a rupture model with segments separated by strong persistent geometric barriers the unexpected ending of the rupture south of the main fault can be explain by such a structure but bears important implications for the initiation and rupture directivity of the next earthquake

off fault plasticity favors the arrest of dynamic ruptures on strength heterogeneity two dimensional cases we study the effects of a plastic behavior of the volume around the fault on in plane and anti plane 2d rupture dynamics both rupture modes exhibit similar answer to off fault yielding in terms of modification of the kinematics of the rupture front and in terms of energy lost outside the fault plane we then compare the ability of the rupture to propagate through a barrier on the interface the plastic behavior responsible for a linear increase of the global fracture energy during dynamic crack growth enhances the rupture front sensitivity to a static resistance increase on the fault consequently the rupture arrest is more easily provoked in heterogeneous models that include a plastic yielding even with relatively small variations of frictional resistance along the fault plane

southern australian rainfall and the subtropical ridge variations interrelationships and trends 1 monthly values of the latitude and intensity mean maximum pressure of the subtropical ridge str averaged across australian longitudes 105 155 e have been related to variations and trends in rainfall over southern australia south of 30 s for 1958 2005 there is an abrupt shift of latitude of the subtropical ridge over australia at the onset of winter in the cooler part of the year the intensity of the str is more closely related to rainfall variations and correlations between latitude and rainfall are a by product of the relationship of intensity with both latitude and rainfall whereas in summer the latitude of the str is more closely related to rainfall the decline in southern australian rainfall in recent decades which has occurred principally in autumn appears related to a trend towards a more intense str rather than a trend in the latitude of the str

effects of dissociation on the shear strength and deformation behavior of methane hydrate bearing sediments abstract a series of tests were conducted in order to investigate the shear strength and deformation behavior of methane hydrate bearing sediments during dissociation using the thermal recovery method or depressurization method an innovative temperature controlled high pressure triaxial apparatus which can reproduce the in situ conditions of hydrate reservoirs was used the results indicate that 1 the failure strength of isotropically consolidated methane hydrate bearing sediments which dissociated completely using the thermal recovery method is less than that of pure toyoura sand however the initial stiffness and volumetric strain are higher than that of pure toyoura sand 2 the thermal recovery method will cause the failure of methane hydrate bearing sediments when the axial load is higher than the strength of methane hydrate bearing sediments after dissociation 3 the depressurization method will not cause collapse of methane hydrate bearing sediments during depressurization however water pressure recovery will lead to failure when the axial load is larger than the strength of the methane hydrate bearing sediments after dissociation 4 the depressurization rate shows little effect on the ultimate deformation of methane hydrate bearing sediments while the initial deformation rate increases with increasing depressurization rate 5 the larger the reduction of pore pressure the larger axial strain and volumetric strain

techno economic forecasting of a hypothetical gas hydrate field in the offshore of india abstract the national gas hydrate program 01 expedition 2006 and the national gas hydrate program 02 expedition 2015 have established the presence of large deposits of gas hydrates in deep water areas offshore india this paper presents the results of a techno economic study carried out for the development of a hypothetical gas hydrate field in the krishna godavari kg basin located along the eastern margin of india standard industry approaches have been used to carry out capital cost and operational cost estimates based on initial reservoir simulation modelling results three cases have been considered to encompass the possible range of reservoir production responses in the first case 18 mmscmd of gas is envisaged to be produced from 120 wells for a production period of 10 years in the second case 12 mmscmd of gas is envisaged to be produced from the 120 wells for 16 years in the third case 6 mmscmd of gas is envisaged to be produced from the 120 wells for 30 years the production rates assumed in case 3 best represents what is believed to be the most reasonable production rates as modeled for a gas hydrate accumulation nghp 02 area b site nghp 02 16 drilled during nghp 02 for case 1 an initial determination is that field development is economically viable for prevailing gas price of usd 7 67 mmbtu with npv of 1921 mmusd and irr of 27 87 with a cost of production of usd 4 74 mmbtu for case 2 the study indicates that field development is viable with npv of usd 986 mmusd and irr of 20 27 and cost of production of usd 5 8 mmbtu for case 3 the cost of production is usd 9 mmbtu no production data is available for the gas hydrate accumulations discovered during nghp 02 in the offshore of india therefore these techno economic results generated in this study can only be validated after matching the results from future pilot production testing under actual field conditions

critical state soil constitutive model for methane hydrate soil 1 this paper presents a new constitutive model that simulates the mechanical behavior of methane hydrate bearing soil based on the concept of critical state soil mechanics referred to as the methane hydrate critical state mhcs model methane hydrate bearing soil is under certain geological conditions known to exhibit greater stiffness strength and dilatancy which are often observed in dense soils and also in bonded soils such as cemented soil and unsaturated soil those soils tend to show greater resistance to compressive deformation but the tendency disappears when the soil is excessively compressed or the bonds are destroyed due to shearing the proposed model represents these features by introducing five extra model parameters to the conventional critical state model it is found that for an accurate prediction of ground settlement volumetric yielding plays an important role when hydrate soil undergoes a significant change in effective stresses and hydrate saturation which are expected during depressurization for methane gas recovery

physical property characteristics of gas hydrate bearing reservoir and associated seal sediments collected during nghp 02 in the krishna godavari basin in the offshore of india abstract india s national gas hydrate program expedition 02 nghp 02 was conducted to better understand geologic controls on gas hydrate occurrence and morphology targeting coarse grained sediments along the lower continental slope offshore eastern india this study combines seismic logging while drilling data and a petroleum system approach to provide a regional geologic and lithologic context for 1 gas hydrate morphology and distribution and 2 effects of fine grained sediments clays and other grains smaller than 63 m on gas production in nghp 02 area b in the krishna godavari basin area b seismic data show a buried anticline syncline structure with strong reflectors r1 and r2 that delineate two of the five lithologic units unit i shallowest ii r1 iii iv r2 and v below the bas of gas hydrate stability throughout area b gas hydrate morphology depends on its placement within these units specifically core and grain scale measurements indicate fines content exerts a primary control on the gas hydrate distribution and morphology units i ii and iii are generally fine grained on the anticline crest these units host primarily grain displacing gas hydrate veins linked to pore occupying gas hydrate in thin localized coarser grained deposits diatoms in unit iii increase porosity with depth reaching 70 where it contacts unit iv the gas hydrate reservoir associated with reflector r2 the unit iii lithology and porosity allow fluid and dissolved phase methane to escape unit iv and form gas hydrate in the fine grained overburden within unit iv fine grained layers are interbedded with coarser grained gas hydrate reservoir sands and the fines content even in the sands is high enough to impact hydraulic and mechanical properties during gas production fluid motion during gas production can mobilize fines which can then clog pore throats limiting production rates pore water freshening during gas hydrate dissociation can increase fines mobilization particularly given the smectite identified in the fine grained interbeds

euclidean and fractal models for the description of rock surface roughness accurate description of the topography of rock surfaces is essential because surface roughness affects frictional strength the flow of fluids in joints and fractures the seismic behavior of faults and the formation of gouge and breccia in fault zones real rock surfaces can be described using self similar and self affine fractal models of surface roughness if a surface is self similar a small portion of the surface when magnified isotropically will appear statistically identical to the entire surface if a surface is self affine a magnified portion of the surface will only appear statistically identical to the entire surface if different magnifications are used for the directions parallel and perpendicular to the surface at least two parameters are required to describe a fractal model one parameter typically describes how roughness changes with scale while the other specifies the variance or surface slope at a reference scale the divider method and the spectral method are in common use to determine the best fit fractal model from surface profile data power spectra from self similar surfaces have slopes of 3 on log log plots of power spectral density versus spatial frequency while spectra from self affine surfaces have slopes other than 3 power spectra can be interpreted with greater facility if dimensionless amplitude to wavelength ratios are contoured on plots of power spectral density versus frequency the topography of many natural rock surfaces including both fractures and faults is approximately self similar within the 6 5 order of magnitude wide wavelength band of 10 m to 40 m within smaller wavelength bands natural rock surfaces may exhibit self affine behavior

palaeoclimate science causes and effects of antarctic ice some 34 million years ago there was a rapid growth of ice on antarctica a modelling study indicates that the ultimate cause of this glaciation was a decrease in the concentration of atmospheric carbon dioxide see letter p 574 antarctica developed an ice sheet about 34 million years ago following a period of ice free conditions known as the super greenhouse climate previous research has suggested that ice sheet growth was driven by either the thermal isolation of antarctica arising from the opening of ocean gateways or by progressively lower atmospheric co2 content matt huber and colleagues now use a climate model to show that the growth of the ice sheet forced by changes in co2 drove the changes in ocean circulation in contrast the opening of the gateways had relatively little impact on ocean circulation

a support operator method for 3 d rupture dynamics summary we present a numerical method to simulate spontaneous shear crack propagation within a heterogeneous 3 d viscoelastic medium wave motions are computed on a logically rectangular hexahedral mesh using the generalized finite difference method of support operators som this approach enables modelling of non planar surfaces and non planar fault ruptures our implementation the support operator rupture dynamics sord code is highly scalable enabling large scale multiprocessors calculations the fault surface is modelled by coupled double nodes where rupture occurs as dictated by the local stress conditions and a frictional failure law the method successfully performs test problems developed for the southern california earthquake center scec u s geological survey usgs dynamic earthquake rupture code validation exercise showing good agreement with semi analytical boundary integral method results we undertake further dynamic rupture tests to quantify numerical errors introduced by shear deformations to the hexahedral mesh we generate a family of meshes distorted by simple shearing in the along strike direction up to a maximum of 73 for scec usgs validation problem number 3 grid induced errors increase with mesh shear angle with the logarithm of error approximately proportional to angle over the range tested at 73 rms misfits are about 10 per cent for peak slip rate and 0 5 per cent for both rupture time and total slip indicating that the method which up to now we have applied mainly to near vertical strike slip faulting is also capable of handling geometries appropriate to low angle surface rupturing thrust earthquakes additionally we demonstrate non planar rupture effects by modifying the test geometry to include respectively cylindrical curvature and sharp kinks

triaxial compressive properties of artificial methane hydrate bearing sediment 1 knowledge of the mechanical properties of gas hydrate bearing sediments is essential for simulating the geomechanical response to gas extraction from a gas hydrate reservoir in this study drained triaxial compression tests were conducted on artificial methane hydrate bearing sediment samples under hydrate stable temperature pressure conditions toyoura sand average particle size d50 0 230 mm number 7 silica sand d50 0 205 mm and number 8 silica sand d50 0 130 mm were used as the skeleton of each specimen axial loading was conducted at an axial strain rate of 0 1 min 1 at a constant temperature of 278 k the cell and pore pressures were kept constant during axial loading we found that the strength and stiffness of the hydrate sand specimens increased with methane hydrate saturation and with the effective confining pressure and the secant poisson s ratio decreased with the effective confining pressure the stiffness depends on the type of sand forming the skeleton of the specimens although the strength has little dependence on the type of sand according to an earlier work hydrate sand specimens are thought to contract in the early stage of axial loading before starting to expand owing to the dilatancy effect as is the case for many other geological materials the test results in this study are discussed in relation to the deformation mechanism proposed in an earlier work

high velocity frictional properties of gabbro high velocity friction experiments have been performed on a pair of hollow cylindrical specimens of gabbro initially at room temperature at slip rates from 7 5 mm s to 1 8 m s with total circumferential displacements of 125 to 174 m and at normal stresses to 5 mpa using a rotary shear high speed friction testing machine steady state friction increases slightly with increasing slip rate at slip rates to about 100 mm s velocity strengthening and it decreases markedly with increasing slip rate at higher velocities velocity weakening steady state friction in the velocity weakening regime is lower for the non melting case than the frictional melting case due perhaps to severe thermal fracturing a very large peak friction is always recognized upon the initiation of visible frictional melting presumably owing to the welding of fault surfaces upon the solidification of melt patches frictional properties thus change dramatically with increasing displacement at high velocities and such a non linear effect must be incorporated into the analysis of earthquake initiation processes

characterization of fault roughness at various scales implications of three dimensional high resolution topography measurements accurate description of the topography of active fault surfaces represents an important geophysical issue because this topography is strongly related to the stress distribution along fault planes and therefore to processes implicated in earthquake nucleation propagation and arrest to date due to technical limitations studies of natural fault roughness either performed using laboratory of field profilometers were obtained mainly from 1 d profiles with the recent development of light detection and ranging lidar apparatus it is now possible to measure accurately the 3 d topography of rough surfaces with a comparable resolution in all directions both at field and laboratory scales in the present study we have investigated the scaling properties including possible anisotropy properties of several outcrops of two natural fault surfaces vuache strike slip fault france and magnola normal fault italy in limestones at the field scale digital elevation models of the fault roughness were obtained over surfaces of 0 25 m2 to 600 m2 with a height resolution ranging from 0 5 mm to 20 mm at the laboratory scale the 3 d geometry was measured on two slip planes using a laser profilometer with a spatial resolution of 20 m and a height resolution less than 1 m

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

assessing the geomechanical stability of interbedded hydrate bearing sediments under gas production by depressurization at nghp 02 site 16 abstract establishing the geomechanical stability of marine sediments in the vicinity of a production well is one of the key design considerations in planning offshore gas production from marine hydrate reservoirs this paper presents an assessment of the sediment stability at india s national gas hydrate program expedition 2 nghp 02 site 16 area b offshore eastern india for which gas production is to be carried out by depressurization one important feature of the study is that extensive calibration of constitutive model parameters has been conducted based on laboratory test data from pressured core samples from analysis perspective the site is challenging because the hydrate reservoir consists of thin layers of hydrate bearing sands interbedded with mud moreover depressurization at the depth of a reservoir more than 2750 m below sea surface will lead to a pore pressure drop and accordingly an effective confining stress increase as high as 25 mpa in dealing with thin interbedded hydrate bearing strata meshing requirements for flow and geomechanical analysis are quite different from those for reservoirs with thicker massive layers an axisymmetric model and one way coupling simulations were thus adopted for this study in which the geomechanical study utilizes pore pressure and hydrate saturation output from the flow study but the flow study does not takes the porosity changes from the geomechanical analysis instead the reduction of porosity due to sediment deformation in the flow study is based on a pressure dependent pore compressibility relationship derived from geomechanical modeling the rationality is validated through back computing the pore compressibility from the geomechanical deformation results the study shows that large compression in the reservoir will result in movement of the sediments from above and below as well as laterally in smaller magnitudes and the sediment is deemed stable during the gas production period

physical properties of hydrate bearing sediments 1 methane gas hydrates crystalline inclusion compounds formed from methane and water are found in marine continental margin and permafrost sediments worldwide this article reviews the current understanding of phenomena involved in gas hydrate formation and the physical properties of hydrate bearing sediments formation phenomena include pore scale habit solubility spatial variability and host sediment aggregate properties physical properties include thermal properties permeability electrical conductivity and permittivity small strain elastic p and s wave velocities shear strength and volume changes resulting from hydrate dissociation the magnitudes and interdependencies of these properties are critically important for predicting and quantifying macroscale responses of hydrate bearing sediments to changes in mechanical thermal or chemical boundary conditions these predictions are vital for mitigating borehole local and regional slope stability hazards optimizing recovery techniques for extracting methane from hydrate bearing sediments or sequestering carbon dioxide in gas hydrate and evaluating the role of gas hydrate in the global carbon cycle

rupture dynamics and ground motion from 3 d rough fault simulations 1 we perform three dimensional 3 d numerical calculations of dynamic rupture along non planar faults to study the effects of fault roughness on rupture propagation and resultant ground motion the fault roughness model follows a self similar fractal distribution over length scales spanning three orders of magnitude from 102 to 105 m the fault is governed by a strongly rate weakening friction and the bulk material is subject to drucker prager viscoplasticity fault roughness promotes the development of self healing rupture pulses and a heterogeneous distribution of fault slip at the free surface and at depth the inelastic deformation generated by the large dynamic stress near rupture fronts occurs in a narrow volume around the fault with heterogeneous thickness correlated to local roughness slopes inelastic deformation near the free surface however is induced by the stress waves originated from dynamic rupture at depth and spreads to large distances 10 km away from the fault the present simulations model seismic wave excitation up to 10 hz with rupture lengths of 100 km permitting comparisons with empirical studies of ground motion intensity measures of engineering interest characteristics of site averaged synthetic response spectra including the distance and period dependence of the median values absolute level and intra event standard deviation are comparable to appropriate empirical estimates throughout the period range 0 1 3 0 s this class of model may provide a viable representation of the ground motion excitation process over a wide frequency range in a large spatial domain with potential applications to the numerical prediction of source and path specific effects on earthquake ground motion

rough faults distributed weakening and off fault deformation 1 we report systematic spatial variations in fault rocks along nonplanar strike slip faults cross cutting the lake edison granodiorite sierra nevada california sierran wavy fault and lobbia outcrops of the adamello batholith in the italian alps lobbia wavy fault in the case of the sierran fault pseudotachylyte formed at contractional fault bends where it is found as thin 1 2 mm fault parallel veins epidote and chlorite developed in the same seismic context as the pseudotachylyte and are especially abundant in extensional fault bends we argue that the presence of fluids as illustrated by this example does not necessarily preclude the development of frictional melt in the case of the lobbia fault pseudotachylyte thickness varies along the length of the fault but the pseudotachylyte veins thicken and pool in extensional bends we conduct a quantitative analysis of fault roughness microcrack distribution stress and friction along the lobbia fault numerical modeling results show that opening in extensional bends and localized thermal weakening in contractional bends counteract resistance encountered by fault waviness resulting in an overall weaker fault than suggested by the corresponding static friction coefficient the models also predict static stress redistribution around bends in the faults which is consistent with distribution of microcracks indicating significant elastic and inelastic strain energy is dissipated into the wall rocks due to nonplanar fault geometry together these observations suggest that damage and energy dissipation occurs along the entire nonplanar fault during slip rather than being confined to the region close to the dynamically propagating crack tip

nonplanar faults mechanics of slip and off fault damage stress interactions and sliding characteristics of faults with random fractal waviness in a purely elastic medium differ both qualitatively and quantitatively from those of faults with planar surfaces with nonplanar fault models solutions for slip diverge as resolution of the fractal features increases and the scaling of fault slip with fault rupture dimension becomes nonlinear we show that the nonlinear scaling of slip and divergence of solutions arise because stresses from geometric interactions at irregularities along nonplanar faults grow with increasing slip and produce backstresses that progressively impede slip however in real materials with finite strength yielding will halt the growth of the interaction stresses which will profoundly affect slip of nonplanar faults we infer that in the brittle seismogenic portion of the earth s crust off fault yielding occurs on pervasive secondary faults predicted rates of stress relaxation with distance from major faults with random fractal roughness follow a power law relationship that is consistent with reported clustering of background seismicity up to 15 kilometers from faults