micro x ray computed tomography imaging and ultrasonic velocity measurements in tetrahydrofuran hydrate bearing sediments naturally occurring gas hydrates contain significant amounts of natural gas that might be produced as an energy resource in the foreseeable future thus it is necessary to understand the pore space characteristics of hydrate reservoirs particularly the pore scale distribution of the hydrate and its interaction with the sediment four end member models for hydrate distribution in the pore space are pore filling sediment frame component envelope cementing and contact cementing the goal of this study is to compare the models with pore scale hydrate distributions obtained in laboratory formed hydrates our results verify hydrate pore scale distributions by direct visual observations that were previously implied by indirect elastic property measurements laboratory measurements were conducted using tetrahydrofuran as a guest molecule since tetrahydrofuran hydrate can be used as a proxy for naturally occurring hydrates we performed micro x ray computed tomography to obtain information about the distribution of hydrate in the pore space of synthetic sediment glass beads we also made ultrasonic velocity measurements on the same samples micro x ray computed tomography images and ultrasonic velocity measurements both indicate that the tetrahydrofuran hydrate forms in the pore space with a part of the hydrate bridging the grains without touching the grain surfaces these hydrate bearing sediments appear to follow a pore filling model with a portion of the hydrate becoming a load bearing part of the sediment frame

observations related to tetrahydrofuran and methane hydrates for laboratory studies of hydrate bearing sediments the interaction among water molecules guest gas molecules salts and mineral particles determines the nucleation and growth behavior of gas hydrates in natural sediments hydrate of tetrahydrofuran thf has long been used for laboratory studies of gas hydrate bearing sediments to provide close control on hydrate concentrations and to overcome the long formation history of methane hydrate from aqueous phase methane in sediments yet differences in the polarizability of thf polar molecule compared to methane nonpolar molecule raise questions about the suitability of thf as a proxy for methane in the study of hydrate bearing sediments from existing data and simple macroscale experiments we show that despite its polar nature thf s large molecular size results in low permittivity prevents it from dissolving precipitated salts and hinders the solvation of ions on dry mineral surfaces in addition the interfacial tension between water and thf hydrate is similar to that between water and methane hydrate the processes that researchers choose for forming hydrate in sediments in laboratory settings e g from gas liquid or ice and the pore scale distribution of the hydrate that is produced by each of these processes likely have a more pronounced effect on the measured macroscale properties of hydrate bearing sediments than do differences between thf and methane hydrates themselves

rupture process of a multiple main shock sequence analysis of teleseismic local and field observations of the tennant creek australia earthquakes of january 22 1988 on january 22 1988 three large intraplate earthquakes with ms 6 3 6 4 and 6 7 occurred within a 12 hour period near tennant creek australia these earthquakes which occurred over a small interval of time and within a small volume of space present a unique opportunity to study the rupture process of the class of intraplate earthquakes that occur as multiple main shocks broadband displacement and velocity records of body waves from teleseismically recorded data are analyzed to determine source mechanisms depths and complexity of rupture of each of the three main shocks hypocenters of an additional 150 foreshocks and aftershocks constrained by local arrival time data and field observations of surface rupture are used to complement the source characteristics of the main shocks in order to derive as complete a description of the rupture process as possible the interpretation of the combined data sets suggests that the overall rupture process involved unusually complicated stress release as locations of the main shock hypocenters progressively moved from west to east we infer that the first and third main shocks denoted as ms1 and ms3 produced the southeast northwest trending scarps observed at the western end the kunayungku fault and at the eastern end the east end of the lake surprise fault respectively of the rupture zone the epicenter of the only immediate foreshock was located in the gap between these two fault scarps ms1 nucleated near this epicenter and ruptured upward and to the northwest from a depth of 6 5 km ms3 ruptured predominantly to the se at a depth of 4 5 km the second main shock ms2 is inferred to have produced the deformation of the southwest trending central scarp segment the western end of the lake surprise fault from the sense of thrusting seen at the surface and from the distribution of aftershock hypocenters the south dipping nodal planes derived from waveform modeling are identified as the fault planes for earthquakes ms1 and ms3 in contrast the dip of the central fault scarp is reversed relative to the dips of the western and eastern fault scarps the rupture process of ms2 turns out to be commensurately complex and sufficiently explains the geological complexity ms2 consisted of three subevents the southeast dipping nodal plane of the first two subevents is coplanar with a southeast dipping plane implied by locations of aftershocks which did not break the surface choice of the north dipping plane as the rupture plane of the third subevent consistent with the surface deformation and coplanar with a second plane delineated by aftershocks would imply conjugate faulting the majority of the aftershocks are concentrated near the edges of the fault planes and there is an absence of activity in the center of the planes the areas of absent activity may represent the failed asperities of the main shocks in which substantial stress relief occurred the rupture process of each main shock is characterized by the rapid release of energy followed by a much slower release of moment and by aftershock zones whose dimensions exceed the inferred dimensions of the rupture these characteristics suggest that substantial slow slip occurred on each of the three fault interfaces that was not accompanied by major energy release the first main shock nucleated at the deepest part of its aftershock zone and ruptured upward in contrast ms2 and ms3 nucleated at depths that were one third to one half of the maximum depth of their aftershock zones this variation of focal depth and the strong increase of moment and radiated energy with each main shock imply that lateral variations of strength were more important than vertical gradients of shear stress in controlling the progression of rupture

postseismic stress and pore pressure readjustment and aftershock distributions abstract the time and spatial readjustment of stress and pore pressure due to a strike slip rupture are investigated by means of a model which treats the earth s crust as a linear poro elastic material infiltrated by fluids the analysis is based on a fundamental solution obtained by rice and cleary who treated the solid and fluid constituents as separately compressible the main shock is modeled as a sudden uniform stress drop and the resulting slip dislocation is maintained in time this suddenly introduced dislocation distribution sets up stress and pore pressure fields diffusion of fluid takes place from regions undergoing compression to regions undergoing dilation hence the stress and pore pressure fields evolve over time with time scales which depend on the flow parameters of the rock medium it is assumed that aftershocks occur when the coulomb stress defined as c n p where n and are the increase in normal stress and shear stress across a potential slip surface is a friction coefficient and p is the pore pressure increase exceeds the shear strength of the slip surface the coseismic elastic solution confirms the suggestion of spatial distribution of aftershocks by stein and lisowski the transient solution predicts a small rotation of the aftershock zone from the compression into the dilation region at the two ends of the main rupture whereas the aftershock zones for off fault aftershocks appear to expand and shift into the dilating regions a new plot is developed which provides a means to study the association between high coulomb stress increase with off fault aftershock locations the aftershock distributions of three earthquakes pasinler 1983 borrego mountain 1968 and haicheng 1975 are compared to the zones predicted by the model the earthquake aftershocks are skewed towards the dilatant regions and in some cases form distinct off fault clusters in the areas of high coulomb stress the ability of the model to provide a primary aftershock mechanism and the implications of this mechanism for material properties are discussed in the contexts of these sample earthquakes the model predicts the generally observed spatial expansion of the aftershock zones and suggests an explanation for the presence or absence of off fault aftershock clusters however the inaccuracy in location of aftershocks and the small magnitude change in predicted coulomb stress makes it difficult to confirm or disprove the theory that aftershock distributions are controlled by fluid flow

geodetic evidence for conjugate faulting during the 1988 tennant creek australia earthquake sequence summary repeat levelling measurements and detailed topographic profiles from the epicentral area of the 1988 january 22 tennant creek australia earthquakes are used to constrain the geometry of faulting associated with three m 6 earthquakes the observed elevation changes are modelled assuming elastic deformation and uniform slip on several faults the vertical deformation data are poorly fit by a single fault model and require at least three distinct faults in the preferred model two faults on either end of the zone of surface rupture have similar orientations but the central fault has an orientation conjugate to the flanking faults this interpretation is consistent with the identification of the fault planes with well defined dipping zones of aftershock hypocentres determined with data from portable seismograph arrays it is also consistent with the sense of surficial deformation documented by 75 topographic profiles across the scarps however a fourth fault associated with possible conjugate faulting in the central fault segment at the time of the second main shock is not required by the levelling data

an investigation of the upper mantle beneath nw australia using a hybrid seismograph array summary a large array of short period portable seismographs was operated in the northern territory of australia for 3 months in 1986 in order to record earthquakes in the island arcs to the north and east of australia the array consisted of 18 digital recorders 28 analogue tape recorders and the 20 stations of the permanent warramunga wra seismic array an unusual aspect of this experiment was the variable station spacing and apertures of the different elements of the hybrid array the wra and portable digital arrays had station spacing of 2 2 km and 5 13 km respectively and allowed confident identification of phases returned from the upper mantle whilst the analogue array had station spacing of 40 km and allowed us to track mantle phases across its 500 km aperture seismograms from 17 shallow earthquakes mb 4 3 5 1 in the indonesian arc are used to investigate the p wave velocity structure of the upper mantle beneath nw australia we combine seismograms from these events into a composite record section covering ranges from lo00 to 2600km strong phases following the first p wave arrivals in this composite record section clearly indicate the presence of significant structure in the upper mantle lateral heterogeneity in the upper mantle causes the timing and amplitudes of mantle phases to vary across the array and among earthquakes in order to minimize the effects of lateral heterogeneity we filter and stack the data and concentrate on features in the data that are seen for many individual seismograms and for several earthquakes we calculate wkbj and reflectivity seismograms in order to construct a vertical velocity profile that fits the observed traveltimes and waveforms in an average sense our preferred model nwb 1 includes second order velocity discontinuities of 3 7 per cent near a depth of 200km 6 1 per cent near 400km and 3 7 per cent near 620km in order to reproduce the amplitudes of the later phases the low amplitudes of the first arrivals in the range from 1600 to 2200 km require either a low velocity zone below 230 km depth or a low velocity gradient between 230km and the 400 km discontinuity model nwb 1 is smoother than some models that have been previously proposed for this region which may have mapped lateral heterogeneity into vertical velocity profiles

seismic crustal structure of the proterozoic north australian craton between tennant creek and mount isa seismic recording along a 600 km traverse across the southern part of the north australian craton indicates that lateral velocity variations exist in the middle lower crust taken together with other data from the craton there is evidence for midcrustal velocities increasing from east to west and from south to north these trends could result from a greater proportion of high velocity granulites being emplaced at higher levels in the crust between tennant creek and mount isa the velocity structures near the surface are compatible with an assemblage of younger and older metamorphic rocks intruded by granites near surface 0 5 km p wave velocities on the tennant creek inlier increase from 5 47 km s at the surface to about 6 2 km s at 6 km depth and correspond with low grade metamorphics of the warramunga group interspersed with weathered granites overlying amphibolite facies rocks of older metamorphic domains in the mount isa geosyncline velocities of 6 03 6 15 km s correspond to the leichhart metamorphics interspersed with granites in the middle crust p wave velocities of 6 85 km s occur at a depth of 26 km near tennant creek whereas such velocities are not evident until depths of about 37 km near mount isa there is therefore a trend for midcrust velocities to increase from east to west the lower crust is characterized by velocities of 7 3 7 5 km s and the upper mantle velocities of 8 16 8 20 km s are reached at depths of 51 54 km exposed granulite facies rocks in central australia are low in heat producing elements and are compatible with the low surface heat flow generally measured in precambrian australia these data taken together with laboratory velocity measurements on worldwide samples reported elsewhere indicate that middle to lower crustal rocks could be an assemblage of granulite facies rocks varying from pyroxene granulite in the middle crust through hornblende granulite to garnet granulite in the lower crust

effects of source model variations on coulomb stress analyses of a multi fault intraplate earthquake sequence abstract fault models are quickly produced and iteratively improved over weeks to years following a major earthquake to characterise the dynamics of rupture evaluate the role of stress transfer and contribute to earthquake forecasting we model coulomb stress transfer cfs between the largest foreshock mw 5 4 1 year prior to first mainshock and three mw 6 1 to 6 5 earthquakes that occurred in a 12 hour period on january 22 1988 in central australia tennant creek earthquake sequence to investigate the role of static stress transfer in earthquake triggering relative to progressive source model development the effects of fault model variance are studied using cfs modelling of five different fault source model sequences 27 total models using different inputs from seismic and geospatial data some initial models do not yield positive cfs changes proximal to hypocentres but in all models preceding earthquakes generate positive cfs 0 1 bar on 10 to 30 of the forthcoming receiver fault rupture areas the most refined and data integrative model reveals cfs 0 7 to 13 bars within 2 km of impending hypocentres and large 30 to 99 areas of positive cfs when compared to global compilations of threshold cfs prior to impending ruptures average 3 71 bar median 1 bar this suggests that coulomb stress change theory adequately explains the tennant creek rupture sequence in the most refined model earthquake inter event times decrease as cfs increases suggesting that higher stress magnitudes may have more rapidly within hours triggered successive events thus accounting for some temporal aspects of this sequence cfs analyses provide a useful framework for understanding the spatiotemporal aspects of some intraplate earthquakes the progressive refinement of source models using emergent data may reduce epistemic uncertainties in the role of stress transfer that result from different model inputs approaches and results

mechanisms of seismic quiescences in the past decade there have been major advances in understanding the seismic cycle in terms of the recognition of characteristic patterns of seismicity over the entire tectonic loading cycle the most distinctive types of patterns are seismic quiescences of which three types can be recognized post seismic quiescence which occurs in the region of the rupture zone of an earthquake and persists for a substantial fraction of the recurrence time following the earthquake intermediate term quiescences which appear over a similar region and persist for several years prior to large plate rupturing earthquakes andshort term quiescences which are pronounced lulls in premonitory swarms that occur in the hypocentral region hours or days before an earthquake although the frequency with which intermediate term and short term quiescences precede earthquakes is not known and the statistical significance of some of the former has been challenged there is a need if this phenomena is to be considered a possibly real precursor to consider physical mechanisms that may be responsible for them the characteristic features of these quiescences are reviewed and possible mechanisms for their cause are discussed post seismic quiescence can be readily explained by any simple model of the tectonic loading cycle as due to the regional effect of the stress drop of the previous principal earthquake the other types of quiescence require significant modification to any such simple model of the possibilities considered only two seem viable in predicting the observed phenomena dilatancy hardening and slip weakening intermediate term quiescences typically occur over a region equal to or several times the size of the rupture zone of the later earthquake and exhibit a relationship between the quiescence duration and size of the earthquake they thus involve regional hardening or stress relaxation and agree with the predictions of the dilatancy diffusion theory short term quiescences on the other hand are more likely explained by fault zone dilatancy hardening and or slip weakening within a small nucleation zone because seismicity is a locally relaxing process seismicity should follow a behaviour known in rock mechanics as the kaiser effect in which only a very slight increase in strength due to dilatancy hardening or decrease in stress due to slip weakening is required to cause quiescence this is in contrast to other precursory phenomena predicted by dilatancy which require large dilatant strains and complete dilatancy hardening

sulfur diffusion in rhyolite melts diffusion rates for sulfur in rhyolite melt have been measured at temperatures of 800 1100 c water contents of 0 7 3 wt and oxygen fugacities from the quartz fayalite magnetite buffer to air experiments involved dissolution of anhydrite or pyrrhotite into rhyolite melt over time scales of hours to days electron microprobe analysis was used to measure sulfur concentration profiles in the quenched glasses regression of the diffusion data in dry rhyolite melt gives dsulfur 0 05 exp 221 80rt which is one to two orders of magnitude slower than diffusion of other common magmatic volatiles such as h2o co2 and cl diffusion of sulfur in melt with 7 wt dissolved water is 1 5 to 2 orders of magnitude faster than diffusion in the anhydrous melt depending on temperature sulfur is known to dissolve in silicate melts as at least two different species s2 and s6 the proportions of which vary with oxygen fugacity despite this oxygen fugacity does not appear to affect sulfur diffusivity except under extremely oxidizing conditions this result suggests that diffusion of sulfur is controlled by one species over a large range in oxygen fugacity the most likely candidate for the diffusing species is the sulfide ion s2 re equilibration between s2 and s6 in oxidized melts must generally be slow compared to s2 diffusion in order to explain the observed results in a silicic melt undergoing degassing sulfur will tend to be fractionated from other volatile species which diffuse more rapidly this is consistent with analyses of tephra from the 1991 eruption of mount pinatubo philippines and from other high silica volcanic eruptions

relocation of teleseismically recorded earthquakes near tennant creek australia implications for midplate seismogenesis the three ms 6 tennant creek australia earthquakes of january 22 1988 were preceded by small and moderate earthquakes in 1986 and 1987 we have used the method of joint hypocenter determination to estimate the positions of teleseismically recorded 1986 1987 earthquakes the 1988 main shocks and aftershocks with respect to a system of surface fault scarps the 1986 1987 shocks and the 1988 main shocks nucleated near the center of the zone of surface scarps where fault segment boundaries at depth are implied by complexities in the distribution of scarps at the surface this suggests that the fault segments that ruptured in 1988 were already in existence in 1986 1987 which is consistent with the hypothesis that strong midplate earthquakes occur on preexisting faults the redetermined 1986 1987 hypocenters are however also consistent with the hypothesis that midplate seismicity is localized by stress concentration due to bulk rheological heterogeneity of the crust because they are situated on a regionally prominent gravity anomaly the teleseismically recorded seismicity of the tennant creek region prior to the 1988 main shocks has the temporal pattern of a swarm followed by a lull the concentration of swarm earthquakes between two scarps is consistent with models in which precursory swarm earthquakes correspond to faulting that is spatially distinct from the site of primary main shock faulting the time interval between swarm and main shocks is similar to intervals between intermediate term precursory swarms and main shocks in regions that have much higher rates of tectonic loading this similarity suggests that the time intervals between precursory swarms and subsequent main shocks are not strongly influenced by the rate of tectonic loading but are determined primarily by time dependence of the failure process the spatial distribution of teleseismically recorded aftershocks is in most respects like that of aftershocks recorded by a local network of portable stations in the half year following the main shocks the set of teleseismically recorded aftershocks like the set of locally recorded aftershocks includes some events that occurred well away from the causative faults of the main shocks at the length scale of the 1988 main shock rupture the distribution of aftershocks occurring more than one year after the main shocks is not representative of the distribution of earlier aftershocks

tectonic deformation in the new madrid seismic zone inferences from map view and cross sectional boundary element models the lack of instrumental recordings and obvious fault scarps associated with the 1811 1812 new madrid earthquakes necessitates examination of more subtle indicators of the geometry and type of faulting associated with these events morphologic and geologic features and the distribution of modern seismicity are used to infer the slip distribution and type of faulting strike or dip slip number strike length and width of the major faults in the new madrid seismic zone nmsz this is accomplished through two dimensional boundary element modeling of the strain field arising from slip on hypothetical faults that is driven by either coseismic or uniform regional strains tectonic deformation is reflected in 1 the seismicity 2 the lake county uplift 3 reelfoot lake 4 the fractured rocks of the blytheville arch and 5 the st francis sunk lands many of these features can be qualitatively explained as resulting from tectonic deformation due to slip on two left stepping right lateral strike slip faults that are coincident with the northeast trending zones of seismicity and the blytheville arch the morphology appears to be at least in part a consequence of major earthquakes that rupture these faults the locations of the 1811 1812 and largest post 1812 earthquakes and the models are consistent with a process in which the 1811 1812 earthquakes relieved accumulated regional shear strain causing the greatest post 1812 shear strains to exist at the ends of the fault zone modeling results also suggest that the numerous small earthquakes in the nmsz are not aftershocks of the 1811 1812 earthquakes but instead represent continuous localized adjustments to a uniform regional strain field the bootheel lineament does not appear to be significant in shaping the morphology geologic structure and pattern of seismicity in the nmsz the absence of fault scarps and coherence of shallow reflectors overlying the blytheville arch suggests that the major nmsz faults may not have ruptured to the surface during the 1811 1812 earthquakes the inferred lengths of the 1811 1812 earthquake ruptures also suggest that their sizes may have been overestimated model predicted subsidence within the st francis sunk lands indicates that tectonic deformation also may have influenced alluvial processes in the nmsz

constraints on locations of large intraplate earthquakes in the northern territory australia from observations at the warramunga seismic array three earthquakes of ms 6 3 6 4 and 6 7 near tennant creek northern territory australia on january 22 1988 produced 35 km of surface rupture in the precambrian shield the surface deformation consists of low angle 20 30 thrust faulting on 2 main scarps separated by a 7 km gap the kunayungku western fault and the eastern end of the lake surprise eastern fault dip to the south while the western end of the lake surprise fault dips to the north although these earthquakes are among the largest ever recorded on the australian continent there is no record of earthquakes larger than magnitude 5 within 500 km prior to 1987 data from the warramunga wra seismic array located 30 km east of the rupture show no local activity between 1965 and 1986 two small events in the fault area were observed in february 1986 and were followed by a series of 6 mb 4 to 5 2 earthquakes in january 1987 and their aftershocks the 1986 1987 and january 22 1988 mainshocks all nucleated within a zone about 5 km across from north to south but elongated radially away from wra by up to 30 km although the distribution of these events is in the direction of poorest hypocentral resolution we can place meaningful constraints on the epicenters by examining the range of solutions that fit the p arrival time data the 1986 and largest 1987 events were restricted to the western part of the fault zone the three mainshocks progressed from west to east suggesting that the kunayungku scarp was produced by the first mainshock and the west and east ends of the lake surprise scarp by the second and third respectively

teleseismic investigation of the january 22 1988 tennant creek australia earthquakes the three january 22 1988 earthquakes are the largest to occur within the australian continent in 20 years and produced two colinear but discontinuous wnw trending scarps enclosing a shorter ene trending scarp teleseismic long period p and sh and short period p waves are used to constrain the locations and source parameters for the three events the third earthquake has a seismic moment 1019nm equal to that of the 1968 meckering event and most likely ruptured the southern of the wnw trending faults relative relocations suggest that the two smaller earthquakes initiated north or nw of the third event centroid depths are less than 6 km but most likely 3 to 4 km and source durations are 4 to 6s so it is probable that each event broke the surface all three earthquakes have predominantly thrust mechanisms and nodal planes that strike west to wnw consistent with faults dipping southward at 30 to 40 only the first has a nodal plane that could support a north dip for the ene trending fault as proposed by other researchers on the basis of ground deformation and preliminary aftershock locations however source directivity evident in the short period p waves and relocations are interpreted better if this event like the others ruptured a sw dipping fault plane the ene trending scarp is interpreted to be a tear fault in the hanging wall that may have acted as a barrier to slip on a single continuous sw dipping fault resulting in this unusual at least for australia sequence of three large similar earthquakes

methane hydrate formation in partially water saturated ottawa sand bulk properties of gas hydrate bearing sediment strongly depend on whether hydrate forms primarily in the pore fluid becomes a load bearing member of the sediment matrix or cements sediment grains our compressional wave speed measurements through partially water saturated methane hydrate bearing ottawa sands suggest hydrate surrounds and cements sediment grains the three ottawa sand packs tested in the gas hydrate and sediment test laboratory instrument ghastli contain 38 1 porosity initially with distilled water saturating 58 31 and 16 of that pore space respectively from the volume of methane gas produced during hydrate dissociation we calculated the hydrate concentration in the pore space to be 70 37 and 20 respectively based on these hydrate concentrations and our measured compressional wave speeds we used a rock physics model to differentiate between potential pore space hydrate distributions model results suggest methane hydrate cements unconsolidated sediment when forming in systems containing an abundant gas phase

the velocity structure of the australian shield from seismic travel times abstract travel times between sources and stations in the australian shield are used to develop p and s wave velocity models for the precambrian shield of western and central australia errors in travel times are minimized by using sources for which locations and origin times are constrained by local data or fault scarps apparent velocities between 1500 and 2200 km for p 8 8 km sec and s 4 8 km sec require a high velocity lid and the maximum range of observation of this branch 2200 km requires that the lid extend to a depth of 205 20 km because of the distribution of sources and receivers there are no observations from the shield at ranges greater than 2300 km so we use times from stations in phanerozoic eastern australia and published global summary times as a guide for interpretation at greater ranges allowing for known differences in teleseismic travel time residuals between the precambrian shield and eastern australian the p times are best fit by a model that has a small velocity inversion at the base of the lid and the s times are best fit by a model with a more substantial reduction in velocities however both p and s wave travel times can be satisfied with models that have velocities below 210 km that are concordant with global averages such as iasp91 thus the velocities inferred beneath the lid are low relative to those of the lid but not necessarily low relative to global averages

seismic moment assessment of earthquakes in stable continental regions ii historical seismicity summary this part i study in conjunction with part 11 develops a method to determine within specified uncertainty bounds the seismic moment and thus moment magnitude of all earthquakes of stable continental regions scr for which instrumental or intensity data exist its basis is polynomial regression analysis using a database of scr earthquakes with direct seismic moment determination the independent variables include modern teleseismic magnitudes and regional magnitudes part i and isoseismal areas or number of recording stations part 11 part i11 is an application of the methodology of parts i and i1 to several major historical earthquakes all data used in the regressions are assigned individual uncertainties estimated from the literature or from experience formal confidence limits 68 per cent or 95 per cent on both the regression formulas and the predicted seismic moment values are then possible via error propagation analysis the most complete development is for the teleseismic magnitudes m and mb for both the final regression for log m is a quadratic formula that closely emulates the relationship between amplitude magnitudes and log m expected from dislocation theory and source scaling arguments regressions are also derived for the regional magnitudes mlg and m because there are many scr events mainly pre 1964 that have no teleseismic magnitudes prediction uncertainties from teleseismic magnitudes in moment magnitude units are in the ao 18 0 28 range and from regional magnitudes in the ko 23 0 38 range over a wide magnitude band finally the methodology developed here is generic even though the database is specific application to plate boundary oceanic intraplate or active continental intraplate regions should be straightforward

the accommodation of large strains in the upper lithosphere of the earth and other solids by self similar fault systems the geometrical origin of b value plate tectonics has provided a method of visualizing the geometry of the deformation of the earth s lithosphere on a large scale the description is so concise that for many purposes it provides an explanation of geological processes that overshadows the need to understand the driving processes the mechanics of the zones between the plates are less well understood particularly in continental regions where large areas are subject to deformation both continuous and discontinuous models have been tried but both have obvious drawbacks

aftershocks of the 1988 january 22 tennant creek australia intraplate earthquakes evidence for a complex thrust fault geometry summary on 1988 january 22 three earthquakes of ms 6 3 6 7 occurred in a 12 hr period near tennant creek northern territory in the proterozoic north australian craton and produced 32 km of surface rupture on two main scarps the lake surprise scarp is shaped like a boomerang with the east arm trending ese and the west arm wsw the kunayungku scarp lies 7 km to the northwest and trends ese locations of 211 aftershocks determined using data from portable seismograph arrays define a 10 km by 40 km aftershock zone elongated parallel to the trend of the surface ruptures in the western and eastern part of the fault zone aftershocks occur only south of the kunayungku and eastern lake surprise scarps respectively in the central section in contrast aftershocks lie primarily to the north of the western arm of the lake surprise scarp focal depths range from the surface to 8km in all sections the shallowest earthquakes lie closest to the scarps whereas deeper events lie further away in cross section the aftershocks lie in three distinct inclined zones which delineate fault surfaces ruptured by the main shocks aftershocks are concentrated near the edges of the inferred fault surfaces suggesting that stress was relieved in the centre of the faults by rupture during the main shocks the inferred fault plane dips are 45 to the ssw for the kunayungku fault 55 to the nnw for the western lake surprise fault and 35 to the ssw for the eastern lake surprise fault these dip directions are consistent with the sense of thrust movement inferred from surface deformation the reversal of fault dip and sense of movement between the central and end segments requires a complicated fault geometry the aftershock data would allow a second south dipping plane in the central section that does not intersect the surface this blind fault may be responsible for much of the moment release interpreted from the modelling of broad band teleseismic waveforms choy bowman 1989 rupture of the north dipping plane may have been controlled by a pre existing fault marked at the surface by a quartz ridge 5 km long and may be of secondary importance although the quartz ridge suggests previous faulting the absence of significant topographic relief together with estimates of erosion rates suggest an average repeat time in excess of 104yr preliminary interpretation of water withdrawal histories and water level data from a borefield adjacent to the eastern lake surprise scarp does not support a causal relationship between the borefield and the occurrence of the earthquakes the nearby warramunga wra seismic array provides a record of local earthquake activity from 1965 to the present the fault area was essentially aseismic until two small earthquakes in 1986 february and a series of six magnitude 4 5 earthquakes in 1987 january which were followed by more than 1000 aftershocks the frequency of occurrence of aftershocks declined very slowly in 1987 with p 0 2 in the modified omori formula whereas for the 1988 aftershock sequence p 1 the low p during 1987 may have been diagnostic of high stress in the source and water authority provided partial financial support for j r b and g g

instrumental seismicity of central idaho hypocenters of regionally recorded earthquakes that occurred in central idaho from 1944 through october 1985 have been recomputed with travel times calibrated by locally recorded aftershocks of the borah peak idaho earthquake of 1983 the effect of the relocation is to define more sharply the sizes and relative positions of seismic source zones in central idaho and to move epicenters systematically south often by more than 10 km from their previously cataloged positions the distribution of epicenters of the borah peak main shock and early aftershocks suggests that the fault segment that ruptured in the main shock was approximately a parallelogram with one pair of sides parallel to the zone of surface fault scarps associated with the earthquake and with the other pair of sides parallel to the slip vector of the earthquake such a rupture shape would be expected on geometrical grounds if the 1983 rupture zone were terminated on the north and south by intersection with adjacent segments of the lost river fault that have the same slip vector as the 1983 rupture epicenters of earthquakes occurring before 1983 define a seismic zone the white cloud peaks zone that is approximately parallel to the borah peak aftershock zone but situated from 30 to 40 km to the west the relocated epicenters may also be interpreted as defining a north northeast trending seismic zone near seafoam and a north northeast trending seismic zone that is situated north and west of challis here called the twin peaks myers cove zone the seafoam zone includes the epicenter of the largest instrumentally recorded central idaho earthquake prior to the borah peak earthquake the magnitude 6 1 shock of 12 july 1944 the region within 25 km of the epicenter of the borah peak main shock was quiescent for at least two decades before the main shock for magnitudes mb lg of 3 5 and greater