variations of apparent stresses and stress drops prior to the earthquake of 6 may 1984 mb 5 8 in the adak seismic zone abstract on 6 may 1984 a m b 5 8 earthquake occurred within a subregion of the adak seismic zone that had shown clustering of m b 4 5 earthquakes during the last 3 yr this region has been examined for premonitory variations in stress drop and apparent stresses calculated from the s wave spectra of 69 microearthquakes m d 13 nm decrease with decreasing moment

seismic coda q and turbidity coefficient at the phlegraean fields volcanic area preliminary results digital recordings of microearthquake codas from shallow seismic events in the phlegraean fields region south central italy were used to calculate the attenuation factor qc

the north sea silverpit crater impact structure or pull apart basin the silverpit crater consists of a set of concentric faults up to 20 km in diameter in the southern north sea although formerly described as the first impact structure to be recognized in the uk its spatial association with local tectonic and stratigraphic features suggests that it is unlikely to have been caused by a random extra terrestrial event this paper proposes a terrestrial origin for the silverpit crater as a palaeogene pull apart basin linked to strike slip faulting in the carboniferous basement faulting and thinning of permian evaporites underlying the crater acted as a buffer to basement extension and caused both a flexural collapse of the overlying triassic strata and circumferential listric faulting in the cretaceous succession reactive diapirism in lower jurassic shales may have contributed to the formation of a central uplift at base cretaceous level the paper compares the tectonic setting of two contemporaneous strike slip related structures in southern england bovey basin in devon and the compton valence structure in dorset with that of the silverpit crater and concludes by briefly considering whether a pull apart model can help to explain the formation of upheaval dome a similar putative impact structure in the canyonlands national park of utah

high resolution fault displacement mapping from three dimensional seismic data evidence for dip linkage during fault growth abstract detailed mapping using high resolution three dimensional seismic data has revealed a number of sub horizontal anomalies in the distribution of vertical displacement throw on the planes of growth faults in the gulf of mexico recognition of these anomalies is highly sensitive to the interval at which the fault displacements are sampled because they represent local decreases in throw which are confined to small discrete parts of the fault planes the distribution of the anomalies is inconsistent with model displacement fields of quasi elliptical concentric contours and is therefore incompatible with models of fault growth by uniform slip distribution and radial tip line propagation an alternative model is proposed whereby the evolution of a fault plane is established by the propagation and linkage of precursor fault segments in the dip direction dip linkage overlap and linkage of fault tips in the dip direction results in relay structures that are sub parallel to fault strike and therefore displacement minima that are sub horizontal on normal and thrust faults however since they are orthogonal to the main slip direction on the fault these structures have a low preservation potential and are therefore unlikely to be well resolved on cross sectional seismic profiles

north sea rift system abstract the mesozoic north sea rift forms an integral part of the arctic north atlantic rift system rifting activity in the north sea commenced during the earliest triassic during the mid jurassic the central north sea underwent a temporary doming stage crustal extension peaked during the late jurassic earliest cretaceous abated subsequently and terminated altogether during the paleocene the saucer shaped cenozoic post rift basin straddles the axis of the viking and central grabens this speaks for the applicability of a depth dependent pure shear model rather than a simple shear model geophysical data indicate an important discrepancy between upper and lower crustal attenuation across the north sea rift suggesting that during rifting processes the mono discontinuity was seriously destabilized this requires a basic modification of currently favoured crustal stretching models

attenuation of coda waves in new england abstract seismic wave attenuation was measured in new england from the time domain decay of coda wave amplitudes on narrow bandpass filtered seismograms of local earthquakes the frequency band of interest was 0 75 to 10 hz q c f was found to increase with frequency across this band but there was also a difference between the q c f values for short t t 100 sec lapse times of coda wave propagation for the short lapse time data corresponding to wave paths primarily in the upper crust q c f increases from 400 at 3 hz to 1300 at 10 hz for the large lapse time data corresponding to wave paths in the lower crust and upper mantle q c f was found to vary from 660 at 1 hz to 1500 at 10 hz if we interpret this dataset in terms of a model incorporating both scattering and anelastic attenuation we find that the mean free path of seismic waves over this frequency band is 80 km for short lapse times and 400 km for long lapse times if we assume that scattering is entirely responsible for the observed attenuation we find that the minimum mean free path for short lapse times is about 75 km over all frequencies whereas for long lapse times the minimum mean free path decreases from 400 km at 0 75 hz to 90 km at 10 hz

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

the significance of normal faults in the exploration and production of north sea hydrocarbons abstract the mesozoic geological history of the northern north sea was dominated by extensional faulting in addition to forming the most important hydrocarbon traps such faults can be shown to exert a significant degree of control over the distribution of mature source rocks and the deposition and erosion of reservoir rocks as exploration and production have progressed the importance of small normal faults has been increasingly recognized although they do not totally offset reservoir formations such small faults may provide lateral seals thus disrupting hydrocarbon migration routes forming subtle traps and causing technical problems during production by unanticipated compartmentalization of fields consequently as the commercial size of fields is reduced by improving production technology and the north sea matures as a hydrocarbon province a thorough understanding of the origins and characteristics of normal faults as well as a command of the increasingly sophisticated techniques for analysing the behaviour of these faults will be vital to successful exploration programmes

geodynamic and hydrodynamic evolution of the broad fourteens basin the netherlands in relation to its petroleum systems abstract data analysis in combination with 2d basin modelling is used to study the hydrogeologic and hydrodynamic response of the basin fill of the broad fourteens basin to its geodynamic evolution and the significance of this response for the evolution of the petroleum systems in the basin data analysis resulted in the identification of the major mechanical processes involved in overpressure and fluid flow evolution in the basin sedimentary loading and unloading tectonic compression and extension topography of the groundwater table and provided a timeframework for the important permeability alterations in the basin and the distinct phases of fluid flow observed present day diagenetic features and fracture characteristics in the basin indicate permeability alterations and paleo fluid flow during distinct periods of tectonic activity in the basin e g during late jurassic rifting and late cretaceous inversion the basin boundary fault zones were repeatedly reactivated during the periods of tectonic activity the 2d basin modelling included sedimentary loading and unloading topography of the water table and petroleum generation as pressure and fluid flow influencing mechanisms in addition the influence of time dependent permeability of fault zones on the overpressure history and the evolution of petroleum systems was modelled and evaluated the favoured modelling scenario including a time dependent permeability of faults shows that syn rift and post rift sedimentation in combination with gas generation induced overpressured conditions in restricted parts of the basin that is mainly in the deeper low permeable carboniferous shales and zechstein evaporites the calculated pre inversion overpressured conditions disappear during subsequent inversion of the basin predicted present day pressures in the reservoir horizons are near hydrostatic and reasonably in accordance with observed pressures the favoured scenario also explains the known location of oil fields and the location and saturation characteristics of the gas occurrences the model results further show that the difference in timing of oil charging of the p9 and q1 lower cretaceous reservoirs explains the observed differences in geochemical composition of the accumulated oils the biodegraded and water washed nature of the late cretaceous charged q1 oil reservoir is consistent with the concentrated topography induced groundwater flow through lower cretaceous reservoir units during the late cretaceous inversion period late charging of the lower cretaceous oil reservoir in the p9 area explains the non biodegraded character of the accumulated oils in general this paper shows the importance of using data analysis to obtain an a priori characterization of the basin geofluid evolution and shows that paleohydrodynamic conditions and time dependent permeability of the hydrogeological framework have influenced significantly the petroleum systems in the basin

cluster and spectral characteristics of the aftershock activity of the kalamata september 13 1986 earthquake south greece abstract the spatial and temporal evolution of the aftershock sequence of the kalamata south greece 1986 earthquake is examined by the method of principal parameters ebbling and michelini 1986 the application of the method to 515 aftershocks of known location reveals a variety of orientations and dips for the subfaults which were activated during the aftershock sequence events which occurred in the northern area indicate the existence of two types of orientation which dip at four different angles while the southern area is characterized by almost uniform behavior activated later in the sequence displacement spectra obtained from hand digitized seismograms have been interpreted as evidence for a possible regional variation in the source parameters visual techniques were replaced by a numerical generalized inversion method which was applied to the spectra to evaluate spectral source parameters initial pulse widths measured on the seismograms were also used for the evaluation of source dimensions and average path attenuation theoretical seismograms were computed according to the model of sato and hirasawa 1973 and were then compared to the obtained initial pulses the results revealed that both regions are characterized by similar spectral properties typified by square or higher high frequency decay statistical methods showed no significant difference in the source parameters of the earthquakes between the two regions

discussion on an eocene age for the proposed silverpit impact crater journal vol 165 781 794 a recent paper in the jgs wall et al 2008 presented a reassessment of seismic data predicated on the assumption that the structure known as the silverpit crater was formed by a meteorite impact stewart allen 2002 this speculative hypothesis is merely based on a perceived similarity between the morphology of the silverpit crater and impact craters and has no scientific support stewart allen 2002 dated the geological event that formed the circular faults mapped at the top chalk level by taking the lowest seismic reflector not affected by the faulting apart from minor accommodation to represent the latest date before which the faulting must have occurred the unjustified assertion made now is that all coherent seismic events below the reflector cf wall et al 2008 fig 5 and top chalk are sea bed multiples in particular the package mu wall et al 2008 however the time interval between the possible sea bed multiple a strong peak just above the base of the sequence and the primary from which it may have been derived the trough picked as ha decreases up dip there is no indication from other published profiles of the area that the sea depth changes rapidly enough to cause such convergence nor could it be due to a migration effect because the actual dips about 4 5 are too shallow furthermore coherent reflectors on other profiles wall et al 2008 fig 8 to the ese of the crater fig 4 to the nw of the crater fig 5b to the right

studies of stress and displacement distribution and the evolution law during rock failure process based on acoustic emission and microseismic monitoring abstract the evolutions of stress and deformation inside of rock is highly important in studies of the rock failure mechanism but is difficult to obtain via the traditional stress measurement methods due to limited measuring points thus studies of the stress and displacement evolution within rock were conducted based on acoustic emission ae monitoring in laboratory experiments the differences in the distributions and evolution characteristics of the stress field and deformation field before caving in a deep stope were also examined based on in situ microseismic ms monitoring the results show that the distributions of micro cracks apparent stress and deformation inside the rock are highly consistent and can reflect the spatial temporal evolution characteristics of the stress field and deformation field within the rock the accumulated apparent volume is more accurately than the strain to reflect the changes in inelastic deformation inside the rock based on in situ ms monitoring ms activities were found to be closely related to blasting disturbances during the caving process before a caving of the deep stope the distributions of stress and displacement showed obvious differences reflecting the loosening process of the rock mass the non uniformity and the differences in the stress and deformation can deepen the understanding of the rock mass failure process

the effects of attenuation and site response on the spectra of microearthquakes in the northeastern caribbean abstract the spectra of microearthquakes recorded by a seismic network in the northeastern caribbean were analyzed to determine the effects of attenuation and site response on these spectra the ratios of the spectral amplitudes at 5 and 20 hz were measured for several earthquakes at different distances from a fixed receiver to estimate q for p and s waves in the crust and upper mantle the average q p determined from three stations was 380 for epicentral distances between 40 and 200 km the average q s determined from two stations was estimated at 400 spectra of earthquakes closer than 40 km to one station indicate that a high q path q 800 exists for these short distances in the island arc platform the near equivalence of q p and q s observed in this study of the upper lithosphere contrasts with estimates of anderson et al 1965 that q p 9 4 q s in the asthenosphere this finding that q p is approximately equal to q s suggests that the attenuation mechanism in the upper lithosphere differs from that in the asthenosphere the spectra of nearby microearthquakes unaffected by crustal attenuation display corner frequencies which are uniform despite large differences in the seismic moments of the events the corner frequencies of these spectra appear to be characteristic of the receiver site and insensitive to the location and moment of the earthquakes at one station the s wave corner frequency of a nearby blast was identical to the corner frequency of a magnitude 4 8 earthquake derived from the record of a strong motion accelerograph at the same site

stratal architecture of a halokinetically controlled patch reef complex and implications for reservoir quality a case study from the aptian james limestone in the fairway field east texas basin abstract widespread carbonate platform growth in the cretaceous tethys is associated with expansive reef margin and interior patch reef development these reefs and their associated facies have the potential to be significant hydrocarbon reservoirs but characterization is challenging because of complex facies distributions inadequate understanding of biotic communities and processes impacting their development and difficulties delineating timelines in systems that build their own topography other factors that may impact underlying topography such as tectonics or halokinesis further complicate the story this study delves into these complexities in a dataset from the fairway field a large oil and gas field 115 km2 410 million barrels original oil in place that produces from the aptian james limestone in the east texas basin texas usa the field contains a patch reef complex here defined as an assemblage of patch reefs that grew near each other without necessarily being physically connected that has significant lateral and vertical facies heterogeneity previous attempts at characterization have failed to fully recognize the importance of its proximity to four salt domes which modified topography and influenced deposition prior to and during reef development here we reassess the stratal architecture of the fairway field from a sequence stratigraphic perspective with the goal of augmenting understanding of timing of reef development and facies distributions and ultimately building a better stratigraphic framework for a halokinetically influenced patch reef complex core descriptions and core calibrated wireline log correlation across the field yield a model that explains the relationship between halokinetic topography energy regime relative water depth and reef and reef associated facies development porosity and permeability measurements taken across the field are analyzed to determine best reservoir facies and their distribution the new stratigraphic model is useful for predicting facies architecture in other analogous reef complexes such as those elsewhere in the gulf of mexico and in the middle east its tie to reservoir facies distributions also improves its utility for exploration purposes furthermore this work demonstrates that halokinesis has a major impact on carbonate development and necessitates a new generalized model for halokinetically altered carbonate platforms

ice melt collapse pits and associated features in the 1991 lahar deposits of volcan hudson chile criteria to distinguish eruption induced glacier melt in subaerial volcaniclastic sequences structures formed by ice blocks can provide information about a volcano s history of lahar generation by glacier melt at volcan hudson in chile catastrophic lahars were initiated by eruption induced melting of glacier ice in august and october 1991 they transported large ice blocks 50 km down the rio de los huemules valley to the sea large current crescents with lee side lenses were formed where ice blocks were deposited during waning stages of the flood when stranded blocks of ice melted they left cone shaped and ring shaped heaps of ice rafted debris on the sediment surface several hundred ice blocks were completely buried within the aggrading lahar sediment and when these melted circular collapse pits formed in the sediment collapse types included subsided coherent blocks of sediment bounded by an outward dipping ring fracture trapdoor structures with horseshoe shaped fractures downsag pits with centroclinal dips locally up to 60 pits with peripheral graben and crevasses piecemeal highly fragmented collapse structures and funnel shaped pits containing disaggregated sediment a sequence of progressive collapse is inferred in which initial downsag and subsidence on an outward dipping ring fracture produces a small diameter pit this is followed by widening of the pit by progressive development of concentric ring fractures and downsag outside the early formed pit and by collapse of overhanging pit walls to produce vertical to inward dipping walls and aprons of collapse debris on the pit floor the various structures have potential for preservation even in regions prone to high rainfall and flooding and they can be used to indicate that former lahars contained abundant blocks of ice

source spectral properties of small earthquakes in the northern north sea abstract source parameters of 40 small shallow earthquakes m 2 5 3 5 in the northern north sea region were calculated using p wave spectra of digital records from the western norway network station hya the results show a difference in the source spectral properties of earthquakes generated in the north sea rift zone and the uplift zone of the norwegian coastal margins corner frequencies fo of p wave spectra are constant for the margin zone events for the seismic moment interval e12 e14 nm there is a tendency of decreasing fo with the seismic moment for rift zone events rift zone events have larger stress drops than margin zone events with the same magnitude increase of stress drop with moment was observed for both zones

structural and tectonic setting of the permo triassic basins of northwest europe abstract the permo triassic basins of nw europe formed by i late orogenic spreading of the variscan mountain belt eventually leading to ocean floor formation in the mid atlantic ii continental rifting propagating south from the arctic and iii thermal subsidence following carboniferous volcanic activity in the north sea spreading values in the northernmost north sea west of shetland and west and southwest of britain are relatively high but the sediments are dominantly continental the permo triassic rifting affected lithosphere thickened during caledonian and variscan orogenesis however subsidence associated with the arctic rifting together with global sea level changes allowed marine incursions into the north sea producing the large zechstein salt basins interference of the different basin formation mechanisms led to a complex three dimensional fault pattern and multidirectional extension across britain and the north sea this fault pattern formed the basic framework for subsequent jurassic and cretaceous basin formation

source spectral scaling and stress release estimates using strong motion records in greece a data set of 50 horizontal strong motion accelerograms from 18 main events from moderate and large earthquakes 5 2 mw 6 9 and distances 9 km r 116 km occurred at three different tectonic settings normal strike slip and thrust faulting were used to estimate stress parameters three different definitions of stress release were adopted the classical brune the dynamic rms root mean square and the apparent stress corner frequencies f0 based on the andrews 1986 methodology were computed and using these calculations seismic moments were estimated from independent sources teleseismic data the calculated corner fre quencies seem to be consistent based on seismic moments with previous estimates for the same area based on those source parameters a theoretical x square spectrum was derived from describing source radiation in terms of earthquake size frequency dependent site amplifications were included in the simulations separating the re cordings in three different categories b c and d corresponding to nehrp na tional earthquake hazards reduction program site classification the theoretical and observational spectra are in good agreement in a frequency range from 0 05 to 25 hz except for the cases where steep topography affects the recorded data a poor fit is also observed in cases where the distance of the recorded data is greater than 100 km and the attenuation model seems to be inadequate to describe the theoretical model the calculated parameters present a mean value of 55 16 bars for brune stress 72 26 bars for dynamic rms stress and 17 10 bars for apparent stress for all determined stresses the thrust faulting earthquakes exhibited higher values than the calculated normal and strike slip fault seismic events

on the origin of the southern permian basin central europe abstract a detailed study of the structural and stratigraphic evolution of the southern permian basin during latest carboniferous to early jurassic times supported by quantitative subsidence analyses and forward basin modelling for 25 wells leads us to modify the conventional model for the rotliegend zechstein development of this basin the late permian early jurassic tectonic subsidence curves are typical for a permian to early triassic extensional stage that is followed by thermal subsidence however a purely extensional model is extremely problematic because active faulting during this time is minor and generally hard to document using inverse techniques to model the subsidence curves we quantitatively show that a significant component of late permian and triassic tectonic subsidence can be explained by thermal relaxation of early permian lithospheric thinning and by delayed infilling of paleo topographic depressions that developed during the early permian in this interpretation stephanian autunian wrenching resulted in thermal destabilisation of the lithosphere deep fracturing of the crust disruption and erosion of its sedimentary cover and regional uplift of the area of the future southern permian basin upon termination of wrench tectonics and associated volcanism towards the end of the autunian the southern permian basin began to subside in response to thermal contraction of the lithosphere the evolving basin was isolated from the world oceans and had subsided possibly up to some 700 m below their level at the beginning of upper rotliegend sedimentation after catastrophic flooding of this paleo topographic depression at the beginning of the zechstein changing sea level sedimentation and subsidence rates remained essentially in balance although the effects of triassic rifting overprinted parts of the southern permian basin its overall subsidence pattern persisted well into the jurassic in contrast to the remainder of the southern permian basin permian and triassic crustal extension contributed significantly to the subsidence of the polish trough

source parameter analysis from strong motion records of the friuli italy earthquake sequence 1976 1977 abstract time and frequency domain analyses are applied to strong motion data recorded in friuli italy during 1976 to 1977 an inversion procedure to estimate spectral parameters low frequency level corner frequency and high frequency decay has been applied to displacement spectra using a simple earthquake source model with a single corner frequency the data were digitized accelerograms from enea enel portable and permanent networks instrument corrected sh waves were selected from a set of 138 three component hand digitized records and 28 automatically digitized records thirty eight events with stations having 8 to 32 km epicentral distance were studied different stress drop estimates were performed showing high values 200 to 300 bars on the average with seismic moments ranging from 2 8 10 22 to 8 0 10 24 dyne cm the observation of systematic higher values of brune stress drop obtained from corner frequencies with respect to other time and frequency domain estimates of stress release and the evidence on time series of multiple rupture episodes suggest that the observed corner frequencies are most probably related to subevent ruptures rather than the overall fault size seven events recorded at more than one station show a good correlation between rms brune and dynamic stress drops and a constant scaling of this parameter as a function of the seismic moment when single station events are also considered a slight moment dependence of these three stress drop estimates is observed differently this may be explained by an inadequacy of the 2 high frequency decay of the source model or by high frequency attenuation due to propagation effects the high frequency cutoff of acceleration spectra indicates the presence of an f max in the range of 5 to 14 hz except for the stations where local site effects produce spectral peaks

impact of strong easterly trade winds on carbonate petroleum exploration relationships developed from caicos platform southeastern bahamas abstract brisk persistent easterly trade wind influences define holocene patterns of carbonate sedimentation across the caicos platform southeastern bahamas resultant predictive sedimentary facies models based on trade wind influences are more widely applicable to the exploration for subsurface carbonate plays than are existing models based on the northern bahamas facies patterns which are characterized by gentle trade winds and strong platform margin related oceanic processes swells and tidal currents the caicos platform relationships may be more applicable because many ancient shallow carbonate depositional environments were within the trade wind belts and commonly within broad intracratonic seas that were little influenced by oceanic processes the grainstone dominated caicos platform exhibits reservoir potential over much of its surface in contrast to northern bahamian platforms where oceanic tidal currents or swells and gentle easterly trade winds confine higher energy environments with reservoir potential to platform margins strong easterly trade winds across caicos platform promote widespread holocene platform interior oolitic skeletal and grapestone grainstone bodies on this platform orientations of ooid sand bodies vary depending on preexisting topography water depth and bottom energy shallow subtidal ooid sand shoals orient parallel to these winds ooid sands developed along older shorelines orient parallel to the shorelines but prograde perpendicular into these winds deeper platform interior oolitic sands exist as widespread sheet like deposits these trade winds allow reefs and ooids to coexist in many settings permit isolated linear reefs to flourish on certain leeward platform margins and promote effective off bank transport of carbonate sands that create onlapping grainstone wedges these relationships are very applicable to the rock record strong trade wind influences such as seen on caicos platform better explain the occurrence of cretaceous reef and or oolitic grainstone reservoirs developed well in from platform margins fairway field in east texas black lake field in louisiana than do existing northern bahamian models depositional models based on conditions in the northern bahamian models would predict low energy facies in these platform interior settings a trade wind driven depositional model characterized by strong persistent easterlies also better explains the origin of the onlapping wedge of skeletal grainstones at poza rica oilfield in mexico the caicos platform easterly trade wind driven depositional model should generally apply to any ancient shallow carbonate environment that developed 5 22 north or south of the paleoequator whether or not a platform margin was near by future carbonate exploration or exploitation should always factor in not only geological age but also physiographic latitudinal and climatic setting at a global and local scale

seismic imaging of the site response using microearthquake recordings part i method abstract microearthquake signals from widely distributed geographical regions have been noticed repeatedly to be strongly affected by local site effects the corresponding seismograms contain important information on the individual propagation path a single station inversion method is presented which allows the imaging of the subsurface impedance structure from locally recorded sh waves records the technique is based on the kunetz claerbout equation which in its original form states the relationship between the transmission response and the reflection response for p waves in a horizontally layered medium and vertical incidence claerbout 1968 by adapting claerbout9s formulation to the transmission problem for sh waves under oblique incidence microearthquake recordings can be used to calculate corresponding pseudo reflection seismograms which in turn can be inverted for the impedance structure by exploiting levinson recursion limitations in the high frequency content of the signals due to the combined effects of absorption recording system and bandlimited source signals as well as the influence of noise drastically reduce the resolutional power of the inversion procedure a number of different deconvolution techniques have been tested with synthetic seismograms for the applicability to reduce these effects good results have been obtained with the blind deconvolution technique of scherbaum and stoll 1985 utilizing the minimum phase property of the green9s function from the analysis of synthetic data it can be expected that for realistic source receiver situations and short period recording systems the main reflectors down to a depth of several hundreds of meters could be resolved

q c of three component seismograms of volcanic microearthquakes at campi flegrei volcanic area southern italy digital recordings of three component microearthquake codas from shallow seismic events in the volcanic region of campi flegrei southern italy were used with an automatic technique to calculate the attenuation factorq c codaq in the hypothesis of singles tos backscattering results show the same value ofq for each of the three components this result is interpreted as due to isotropics wave radiation pattern a check of the coda method was performed using a single station method based on simple assumptions on the direct sh wave spectrum single stationq was averaged over the stations and over the earthquakes results show that the two methods lead to comparable results a frequency dependence quite different from that evaluated in active tectonic regions was found for coda attenuation comparable to other volcanic areas throughout the world this is interpreted as due to the presence of magma that affects anelasticity and scattering

discovery of ring faults associated with salt withdrawal basins early cretaceous age in the east texas basin abstract the jurassic louann salt in the east texas basin has played a dominant role in influencing the structural and depositional history of the basin particularly during the jurassic and cretaceous periods salt tectonics is closely associated with sandstone distribution depositional facies and reef growth and consequently with petroleum traps in the basin salt withdrawal basins developed during the early cretaceous in response to salt movement and dissolution processes are characterized by the presence of an expanded section of lower cretaceous marine and deltaic sedimentary rocks recognition of fault sets associated with the evolution of these salt withdrawal basins has gone undetected until the recent coherence cubetm processing of a non proprietary 3 d seismic survey conducted by schlumberger in the la rue dome fairway field area henderson county texas images from coherence cube processing aptly exhibit extraordinary sets of concentric ring faults that comprise the periphery of two salt withdrawal basins the fairway oil field is located at the junction of these two sets of ring faults the structural style of these high angle ring faults creates a multitude of possible fault traps in a previously unattractive structural setting early cretaceous age of the ring faults establishes that these faults are a significant element in evaluating petroleum migration patterns and traps in the basin the discovery of early cretaceous ring faults in the east texas basin by coherence cube processing brings new perspectives to development and exploration drilling in this mature petroleum province

source parameters and scaling laws of the 1978 swabian jura southwest germany aftershocks the 3 september 1978 swabian jura southwest germany earthquake mwa 5 7 was followed by a large number of aftershocks which have been recorded with a local network of five portable seismic stations the seismic moments fault radii and the static stress drops have been determined from the sh displacement spectra using the brune 1970 source model the data set is consistent with the gutenberg richter energy magnitude relation below a wood anderson magnitude of about 4 the corner frequencies increase only slowly with decreasing magnitudes no corner frequency higher than 15 hz has been observed in the magnitude range down to 0 8 correspondingly the high frequency decay slopes show a strong increase when the corner frequencies are approaching the maximum frequency this prevents the use of slope data for q determinations in terms of the madariaga 1977 crack model the data show a strong influence of source complexities on the smaller events

spatial variations of seismic attenuation in the north west of iranian plateau from analysis of coda waves abstract in this study spatial variation of coda wave attenuation in the north western nw iranian plateau is explored using local earthquakes single backscattering method is used to map the spatial variations of coda q qc using 14 969 earthquakes which are recorded digitally by sixteen permanent stations observed spatial attenuation maps are well correlated with geological setting of study area north west of iranian plateau is dominated with high attenuation which is well correlated with quaternary deposits as well as the sahnad sabalan volcanic zone and tabriz fault these observation is in accordance with numerous 2d velocity tomograms which have shown very low velocity in the nw iran the region characterized by volcanic structures exhibits mainly higher seismic attenuation than the surrounding regions whereas paleo tethys basin in the eastern part of covered area are categorized by smaller attenuation than the other zone our observation shows good correlation with reported lower crustal low velocity region beneath the sahand and sabalan volcanos the sahand and sabalan volcanoes are very large structures and they dominate the pliocene quaternary magmatic landscape of nw iran in addition our observation has clearly shown existence of a probable fault zone trending in ne sw direction between tabriz city and urmia lake which is consistent with the trend of meizoseismal zone of the deh khareqan modern azarshahr earthquake m 6 8 in 1641

reply to discussion on an eocene age for the proposed silverpit impact crater journal vol 165 781 794 we welcome the opportunity to further clarify aspects of our paper and challenge key elements of owen s discussion and the inferences he makes regarding the rigour our interpretations and those of previous workers on the origin of the silverpit structure we deal with each of his comments in the order that he raises them owen refers to the interpretation of the silverpit structure as an impact crater as a speculative hypothesis this is consistent with an earlier statement he has made this feature termed the silverpit crater has earlier been interpreted as a meteor impact structure stewart allen 2002 without a shred of scientific justification thomson et al 2005 the inference that the hypothesis lacks any observational basis disregards a wealth of 3d seismic analysis by stewart allen 2002 2005 the statement is at best disingenuous as owen chooses to ignore the thorough work of earlier workers although all the observations for and against a impact origin are based upon 3d seismic data and there are as yet no other independent supportive lines of evidence we would highlight the vast body of work on impact cratering and the numerous impact craters on planetary bodies that have been identified purely from their surface morphology melosh 1989 in other words the absence of shocked quartz and other impact criteria does not invalidate the impact crater interpretation we have been very careful to term the silverpit structure a probable impact crater and will continue to do so until such unequivocal evidence is discovered in our paper we identify a key reflection that we term cf

overburden deformation related to the pre piercement development of salt structures in the north sea the nature of the stress regime imposed on a clastic overburden by a developing salt structure changes with time these changes are directly related to the evolving morphology of the salt structure in a sequence of events which is now quite well understood the stress pattern produced by initial almost purely vertical uplift changes rapidly into a more complex pattern as the flanks of the rising salt mass collapse progressively with flow of salt towards the crestal zone the two main stress elements involved derive from extension within the superincumbent overburden produced by the salt uplift and the effects of gravity that exercise an increasingly important influence as the structure develops vertically other factors also affecting the form taken by the overburden deformation are outlined the discussion is illustrated with examples of migrated seismic data

bulls eye pockmarks and polygonal faulting in the lower congo basin relative timing and implications for fluid expulsion during shallow burial abstract this study describes a new type of pockmark association from the lower congo basin offshore west africa consisting of up to 8 stacked paleopockmarks separated by intervals of drape and onlap fill the stacked paleopockmarks occur within the depocentres of polygonally faulted plio pleistocene sediments and are distributed evenly in the downslope parts of two salt mini basins the majority of the stacked pockmarks initiated synchronously in the late pliocene 3 ma with a subordinate initiation phase in the mid pliocene 4 ma the primary agents in pockmark formation are interpreted to be pore water expelled during early stage compaction together with biogenic methane bottom simulating reflections bsrs associated with free gas overlain by gas hydrates are currently found in the area it is speculated that biogenic methane accumulated within and below a clathrate cap which was repeatedly breached forming pockmarks at discrete horizons separated by intervals of draping sedimentation the mid and late pliocene pockmark initiations appear to coincide with sea level falls following periods of relatively stable highstand conditions several subsequent pockmark horizons may similarly correlate with subsequent sea level falls during the late pliocene and early pleistocene the stacked paleopockmarks are completely surrounded by polygonal faults and consistently occur within polygonal fault cells that crosscut the succession containing the stacked pockmarks early stage compaction and dewatering of the pliocene sediments thus preceded polygonal faulting providing a constraint on the conditions leading to polygonal faulting of the fine grained host sediments the relationship documented here is interpreted as due to the presence of a hydrate cap in the plio pleistocene mini basins which may have retarded the normal compaction processes and facilitated pockmark formation by allowing the build up of gas hydrate and free gas in the basin centres the relative timing and spatial relationships implies that fluids expelled due to polygonal faulting were not implicated in pockmark formation in this area

downsag and extension at calderas new perspectives on collapse geometries from ice melt mining and volcanic subsidence structures at calderas may form as a result of precursory tumescence subsidence due withdrawal of magmatic support resurgence and regional tectonism structural reactivation and overprinting are common to explore which types of structures may derive directly from subsidence without other factors evidence is reviewed from pits caused by the melting of buried ice blocks mining subsidence scaled subsidence models and from over 50 calderas this review suggests that complex patterns of peripheral deformation with multiple ring and arcuate fractures both inside and outside caldera rims topographic embayments arcuate graben and concentric zones of extension and compression may form as a direct result of subsidence and do not require a complex subsidence and inflation history downsag is a feature of many calderas and it does not indicate subsidence on an inward dipping ring fault as has been inferred previously where magmatic inflation is absent or slight initial arcuate faults formed during collapse are likely to be multiple and dip outwards to vertical associated downsag causes the peripheries of calderas undergo radial centripetal extension and this accounts for some of the complex peripheral fractures arcuate crevasses graben and some topographic moats the structural boundary of a caldera defined here as the outermost limits of subsidence and related deformation including downsag commonly lies outside ring faults and outside the embayed topographic wall it is likely to be funnel shaped i e inward dipping even though ring and arcuate fractures within it may dip outward inward dipping arcuate normal faults at shallow levels and steep inward dipping contacts between a caldera s fill and walls may both occur at a caldera that has initially subsided on outward dipping ring faults they arise due to peripheral surficial extension gravitational spreading and scarp collapse topographic enlargement at some calderas and the formation of embayments may reflect general progressive downsag and localized downsag respectively these processes may occur in addition to surficial degradation of oversteep ring fault scarps

an eocene age for the proposed silverpit impact crater three dimensional seismic analysis is used to describe the detailed morphology of the silverpit crater a proposed marine impact crater located in the southern north sea uk we examine and constrain for the first time the geometry extent and age of the post crater sediment units that define the crater fill a circular bowl shaped depression has been mapped and interpreted to represent the upper limit of impact deformation and the geometry of the crater at the end of cratering processes reflections exhibit a parallel onlap fill onto the bowl shaped crater floor indicating that accommodation space was created instantaneously without any contemporaneous sedimentation using regional lithostratigraphic markers constrained by microfossil and calcareous nannofossil analysis we have dated the first onlapping sediments onto the crater the age of the onlapping sediments allows us to constrain the crater formation to be middle eocene in age this date is c 10 15 ma younger than the previous ascribed age for the crater formation and rules out the silverpit crater as a k t boundary impact the age of crater formation considerably predates regional folding and accompanying salt flowage effectively ruling out a salt withdrawal origin for the silverpit crater

structure and emplacement of mud volcano systems in the south caspian basin the term mud volcano system is coined to describe the set of structures associated with a constructional edifice mud volcano and feeder complex that connects the volcano to its source stratigraphic unit three dimensional 3 d seismic data from the south caspian basin are used to investigate the structural elements and evolution of these systems mud volcano systems initiate via early kilometer scale biconic edifices termed pioneer cones these are fed by fluidization pipes tens of meters in width subsequent kilometer scale mud volcanoes grew via persistent extrusion fed by numerous additional fluidization pipes injected in the country rock this subvolcanic intrusion complex creates a densely intruded cylindrical zone similar in cross section to gryphon swarms observed at an outcrop onshore wall rock erosion and compaction of the intruded zone leads to the collapse of a downward tapering cone enveloping the cylindrical zone capped by ring faults that define a kilometer scale caldera that downthrows the overlying mud volcano mud volcanoes get buried during basin subsidence and can look like intrusive laccoliths at first glance on seismic data reactivation of mud flow through a conduit system generates a stack of superimposed mud volcanoes through time large volcanoes continue to dewater during burial and may locally remobilize this model of mud volcano evolution has similarities with igneous and salt tectonic systems to reduce drilling and geologic uncertainty mud volcano system extent and impacts on a reservoir can be assessed on 3 d seismic data simon stewart received his ph d from the imperial college london in 1992 in 1992 2000 he was seismic interpreter at amerada hess in support of exploration and appraisal drilling in the north sea in 2000 he moved to bp and has since worked on structural geology and well planning in the americas middle east and the former soviet union his special interest is in the integration of structural geology and seismic interpretation richard davies received his ph d from the university of edinburgh in 1995 in 1995 2003 he was with mobil and exxonmobil working on field development and exploration the north sea the west of shetlands and the west niger delta deep water slope in 2003 2005 he was a senior lecturer in earth sciences at cardiff university united kingdom currently he is professor and director of cerees center for research into earth energy systems durham university united kingdom his special interest is in seismic scale expression of fluid migration and diagenesis

source parameters of the 1980 mammoth lakes california earthquake sequence from the more than 1500 mammoth lakes earthquakes recorded on three component digital seismographs spudich et al 1981 150 were used in an analysis of the locations mechanism and source parameters a composite fault plane solution of nine earthquakes 3 9 m 5 1 defines a right lateral strike slip mechanism on a steeply dipping nearly east west plane striking s75 e or left lateral strike slip on a nearly north south plane striking n10 e vertical cross sections of well located aftershocks indicate possibly three east west planes that coincide with the locations of the four largest earthquakes with ml 6 0 using the spectral analysis of s waves brune 1970 source parameters for 67 earthquakes were determined forty eight had magnitudes greater than or equal to 3 0 seismic moments ranged from 9 20 1018 dyn cm to 2 33 1024 dyn cm earthquakes with seismic moment greater than about 1 0 1021 dyn cm had nearly constant stress drops 50 bars earthquakes with seismic moment less than about 1 0 1021 dyn cm had stress drops that apparently decrease as seismic moment decreases

attenuation of short period p waves at mount st helens an increase of seismicity occurred at mount st helens between may and july 1998 with more than 900 events md 2 2 recorded by the pacific north west seismograph network this article describes an attempt to estimate the temporal and spatial variation in seismic attenuation using 200 microearthquakes that occurred before january 1995 april 1998 during and after august december 1998 the period of higher seismicity and recorded on three different one component seismic stations epicentral distances of the studied events ranged between 0 5 and 15 km we performed the analysis distinguishing the deep depth 5 5 km from shallow depth 5 5 km events we used the frequency decay method to estimate the quality factor q and station site correction s for p waves in the frequency bands 2 7 and 18 30 hz the results show that the attenuation varies from site to site systematically and decreases with depth lower qp values are obtained for focal depths less than 5 5 km moreover the spatial variations of qp show minimum values 30 in the crater area that are interpreted as due to a low density mass distribution under the crater we find that qp was 30 higher before the period of high seismicity than after this change may be attributed to an increase of pressure in the magma chamber producing new cracks and or reopening of pre existing cracks which are the most viable mechanisms for increasing attenuation

3d seismic analysis of circular evaporite dissolution structures eastern mediterranean buried circular collapse structures above a tabular evaporitic body are recorded by recently acquired 3d seismic data on the levant basin and continental margin offshore israel eastern mediterranean the structures formed during the pliocene as buried messinian late miocene evaporites underwent extensive dissolution in a submarine deep water setting three dimensional seismic analysis is used to describe the detailed morphology of the structures and the associated overburden allowing the reconstruction of their origin and development it is proposed that evaporite dissolution led to the collapse of the weakly lithified overburden and this deformed with a series of concentric extensional faults from the structural analysis of the overburden the estimated maximum duration of the dissolution event is 0 75 1 ma the mechanism proposed for the creation of the circular collapse structures is subjacent dissolution of the more soluble evaporites in the messinian evaporites as a result of focused vertical fluid flow at the base of the evaporitic series rapid release of overpressured fluids as for example during an earthquake is thought to have initiated the focused fluid flow which impinged on the evaporitic seal to the point where dissolution occurred creating the localized circular collapse structures in the overburden

fluid flow during early compartmentalisation of rafts a north sea analogue for divergent continental margins high quality 3d seismic data tied to eighteen 18 boreholes are used to investigate the styles of faulting and associated fluid flow features in triassic early jurassic rafts of the broad fourteens basin southern north sea the study area is presented as an analogue for continental margins experiencing early stage gravitational gliding i e prior to complete separation and downslope translation of individual rafts in such a setting and for present day stress conditions fault slip data indicate that chasms and faults separating rafts in the broad fourteens basin comprise structures subject to dip slip and strike slip reactivation chasms and faults sub parallel to these latter chasms comprise the most significant bypass areas for fluid sourced from pre salt strata faults sub parallel to the main chasms show limited propagation into early cretaceous and cenozoic strata draping the rafts a character further stressed by the depth of occurrence of fluid pipes and dim spots this is an important observation and leads us to postulate that faults formed during early stage rafting control fluid flow in regions where gravitational gliding is limited such as west and equatorial africa southeast brazil and parts of the gulf of mexico

geological controls on focused fluid flow associated with seafloor seeps in the lower congo basin a synthesis of backscatter imagery coupled with a large 3d seismic dataset in the lower congo basin lcb reveals a patchy distribution of features interpreted to be associated with fluid seepage from 300 m to 2500 m water depth with the exception of one region of anomalous backscatter positive relief mounds all inferred seep sites occur in negative relief pockmarks the extensive 3d seismic dataset in the lcb offers a unique opportunity to study the plumbing system that is feeding surface cold seep systems and in general to reconstruct the relationship between tectonics and fluid flow in continental margins the fluid seeps in the lcb are associated with morphologically stratigraphically or tectonically controlled focused fluid flow the integration of the geophysical datasets backscatter imagery coupled to 3d seismic clearly indicates that fluid seeps are not randomly distributed but their seabed organization reflects 1 the location of the underlying structure reservoir or trap where the fluids are coming from 2 the geometry and morphology of the reservoir trap and 3 the discontinuities in the sedimentary column along which fluids have migrated in the lcb seafloor pockmarks are always associated with underlying tectonic structures fault zones salt diapirs polygonal faults or buried sedimentary bodies turbiditic channels erosional surfaces whereas they never occur above sub horizontal parallel stratified fine grained sediments even if triggering processes can not be clearly defined here we propose a model of seafloor fluid seep organization which represents a new tool for identifying the geometry of flow pathways and the underlying buried bodies where the fluids are originating from this qualitative 3d model provides insight into the geohydrologic processes of continental margins

implications of passive salt diapir kinematics for reservoir segmentation by radial and concentric faults abstract reservoirs associated with salt diapirs that are circular or elliptical in plan view can be segmented by radial faults concentric faults both or neither explanations for this variation are offered based on integration of fault kinematics published studies and seismic examples strata around diapirs can be divided into three structural domains with characteristic fault styles 1 roof zone between the present day depositional surface and crest of a buried inactive diapir this zone is characterised by doming radial faults and occasionally conic graben doming is usually caused by differential compaction and increases with depth 2 a steeply dipping sheath of strata surrounds the diapir out to a distance of less than one diapir radius recording near surface diapir growth in poorly lithified sediments these contain early gravity slumps and late concentric extensional faults regional layer bound polygonal fault systems form radial faults within approximately one diapir radius the only significant radial faulting mechanism below the roof zone 3 diapir root zone of strata whose structural architecture records the diapirs initiating mechanism typically rim synclines or graben that detach on the salt source layer concentric faults at radial distances of a kilometre or more have been found only in root zone rim synclines and sheared sheath strata these models and observations can assist seismic interpretation and structural model building in areas of poor geophysical imaging for instance below overhanging salt

physical modeling of structures formed by salt withdrawal implications for deformation caused by salt dissolution by creating 15 physical models we investigated deformation above subsiding tabular salt salt walls and salt stocks dry quartz sand simulated a brittle sedimentary roof above viscous silicone representing salt the modeled diapiric walls had linear planforms and rectangular semicircular triangular or leaning cross sectional shapes the stock was cylindrical in models where the source layer or allochthonous salt sheet was initially tabular a gentle flat bottomed syncline bounded by monoclinal flexures formed above a linear zone where the silicone was locally removed above all subsiding diapirs the deformed roof was bounded by an inner zone of steep convex upward reverse faults and an outer zone of normal faults above subsiding diapiric walls extensional and contractional zones were balanced above the subsiding salt stock conical concentric fault zones comprised inner reverse faults and outer normal faults sediments were added both before prekinematic and during synkinematic salt withdrawal in entirely prekinematic roofs reverse fault zones and normal fault zones both widened with time reverse faults propagated upward from the corners of the withdrawing diapirs new reverse faults formed in the footwalls of reverse faults each nearer the center of the deepening roof trough conversely new normal faults formed successively outward from the sagging trough synkinematic deposition retarded faulting but the pattern of inner reverse and outer normal faults was repeated however reverse faults formed successively outward whereas normal faults formed inward new conceptual models suggest that salt dissolution forms similar structures to those physically modeled for salt withdrawal the appropriate physical models resemble natural dissolution structures above tabular salt extension alone above diapirs is not caused merely by salt withdrawal or dissolution but by regional extension or active diapirism

seismicity in subduction zones from local and regional network observations the data provided by local and regional seismograph networks are essential for the solution of many problems of subduction zone seismology the capabilities of such networks are limited by the instrumentation currently in common use and the unfavorable source station geometry often imposed by the regional geography nevertheless important contributions have come from the data gathered in many of the earth s subduction zones the accuracy of hypocenter locations based on regional data is affected by the complex velocity structures characteristic of subduction zones but the problems are now well understood examples of numerous studies of the spatial configurations of the seismicity in subduction zones and consequent interpretations of seismogenesis and subduction processes are reviewed studies of the distributions of earthquakes in time and with magnitude for events down to the microearthquake level have the potential for clarifying the earthquake generating processes and possibly a basis for earthquake prediction other uses of local and regional network data have been for investigations of coda s and the identification of asperities in subduction zones

loci and maximum size of thrust earthquakes and the mechanics of the shallow region of subduction zones earthquakes do not extend updip along the plate interface at most subduction zones all of the way to the plate boundary at the trench axis or deformation front rather the shallowest part of that interface moves mainly through stable aseismic slip that part of the plate boundary referred to here as the aseismic zone occurs along the base of an accreted wedge of young sediments the probable primary cause for the existence of this aseismic zone is the stable slip properties of the unconsolidated and semiconsolidated sediments in that zone subducted sediment is progressively dewatered and underplated to the base of the overriding plate through this process more consolidated rocks eventually come into contact at depth across both sides of the plate boundary from the point where sufficiently hard rock is found across the plate interface that interface will change its slip behavior to unstable stick slip sliding which is characteristic of consolidated material under most conditions this type of motion is accommodated seismically as episodic slip in large earthquakes the location of this transition to seismic behavior referred to as the seismic front marks the deep end of the aseismic zone and the top of the seismogenic zone i e that part of the plate interface that moves primarily in thrust earthquakes several convergent plate margins are discussed to illustrate the seismic front and the aseismic zone we find that the seismic front defined by smaller earthquakes that occur during the interval between large events is nearly the same as that for large events as inferred from the locations of their aftershocks the location of the seismic front is important for making estimates of the maximum possible size of thrust earthquakes along the plate boundary because it delimits the trenchward limit of the potential rupture area of these events the size of large thrust earthquakes is proportional to rupture area and to at least the cube of the downdip width w of that area thus by defining the location of the seismic front for convergent margins it is possible to make better estimates of w and hence to deduce the maximum size of future interplate earthquakes the average repeat time of such events is also related to w and so better estimates of average repeat time are possible from improved knowledge of w the hypothesis that the location of the seismic front is related to the maximum depth of subduction of unconsolidated sediment has implications for forearc mechanics we use a mechanical analysis laboratory modeling and multichannel seismic information to develop a simple model explaining the growth of forearcs the outer arc high or trench slope break that occurs arcward of most accretionary wedges represents an abrupt change in the critical taper of the accretionary wedge we argue that that change is caused by a large arcward increase in the strength of the material within the overriding plate that stronger material is called the backstop our laboratory modeling experiments indicate that a backstop with a trenchward dipping upper surface results in the development of an overlying structure that includes all of the primary morphologic features observed in modern forearcs in our modeling an accretionary wedge develops trenchward of the backstop an outer arc high develops above the trenchward toe of the backstop and farther arcward a passive forearc basin forms above the stronger material of the backstop this model is consistent with the observation that earthquakes do not extend updip along the plate interface all the way to the trench axis we hypothesize that plate motion is accommodated seismically along the base of the consolidated backstop and mostly or entirely aseismically trenchward of the toe of the backstop along the base of the accretionary wedge at several margins the seismic front is approximately coincident with the outer arc high supporting this interpretation

seismic source parameters for microearthquakes of the granada basin southern spain abstract ninety five microearthquakes of the granada basin southern spain with duration magnitudes ranging from 1 3 to 3 5 moment magnitude from 0 9 to 2 5 have been spectrally analyzed with digital recordings from ten stations of the andalusian seismic network the coda q quality factor qc assumed to be frequency dependent was used instead of q for the path correction and the q adopted was equal to 9 4 qc the k parameter for site attenuation correction had values in the range of 0 02 0 05 fourier displacement spectra were calculated for p and s waves and analyzed with respect to the source model of brune 1970 1971 spectral parameters were automatically determined by the method of snoke 1987 the seismic moment ranged from 1017 to 1020 dyne cm with average m0 p to m0 s ratio of 1 02 and the source radii spanned from 0 14 to 0 40 km with average r p to r s ratio of 1 3 the seismic energy values ranged from 1 103 to 9 107 j with p to s energy ratio from 1 to 10 for about 85 of the events whereas the static stress drop varied from 0 02 to 2 6 bar the scaling relation between the seismic moment and the stress drop indicates a decrease in stress drop with decreasing seismic moment apparent stress is linearly correlated with stress drop and have on average similar values and this points out to a partial stress drop

discussion on the north sea silverpit crater impact structure or pull apart basin ken thomson writes the suggestion that the silverpit crater is a pull apart basin smith 2004 and not an impact structure stewart allen 2002 demonstrates that its origin requires further investigation as noted by underhill 2004 the coincidence of the structure with a salt withdrawal basin suggests that halokinesis may provide an alternative mechanism although the model provided by smith involves salt movement it is overly complex as strike slip detachment tectonics are not required instead gravitational collapse of the post salt cover provides a simpler mechanism the southern north sea is dominated by the development of large linear salt swells jenyon 1988 stewart coward 1995 elevation and tilting of the swell flanks results in gravitational instability and a range of deformation styles can develop jenyon 1988 stewart coward 1995 thomson 2004 the instability commonly leads to listric faulting of the post salt cover down slope towards the adjacent salt low jenyon 1988 stewart coward 1995 thomson 2004 detachment levels for the listric faults occur at a number of stratigraphic levels the top of the upper permian zechstein group salt which forms the swells provides a detachment at the base of the post salt cover whilst detachment within the post salt cover in both the triassic rot halite and within the upper cretaceous chalk have also been recognized jenyon 1988 stewart coward 1995 thomson 2004 the majority of the flank collapse faults dip in the same direction as the cover rocks they displace i e towards the salt withdrawal basin and away from the swell but occasionally listric faults can be found which dip away from the salt withdrawal basin towards the swell jenyon 1988 associated with the listric faulting rollover anticlines can develop in the hanging walls stewart coward 1995 whilst slump folds

a practical model for organic richness from porosity and resistivity logs a practical method the delta log r technique for identifying and calculating total organic carbon in organic rich rocks has been developed using well logs the method employs the overlaying of a properly scaled porosity log generally the sonic transit time curve on a resistivity curve preferably from a deep reading tool in water saturated organic lean rocks the two curves parallel each other and can be overlain since both curves respond to variations in formation porosity however in either hydrocarbon reservoir rocks or organic rich non reservoir rocks a separation between the curves occurs using the gamma ray curve reservoir intervals can be identified and eliminated from the analysis the separation in organic rich intervals results from two effects the orosity curve responds to the presence of low density low velocity kerogen and the resistivity curve responds to the formation fluid in an immature organic rich rock where no hydrocarbons have been generated the observed curve separation is due solely to the porosity curve response in mature source rocks in addition to the porosity curve response the resistivity increases because of the presence of generated hydrocarbons the magnitude of the curve separation in non reservoirs is calibrated to total organic carbon and maturity and allows for depth profiling of organic richness in the absence of sample data this method allows organic richness to be accurately assessed in a wide variety of lithologies and maturities using common well logs

seismic characteristics of fluid escape pipes in sedimentary basins implications for pipe genesis abstract fluid escape pipes were first documented from 3d seismic data over a decade ago and have subsequently been identified in many petroliferous basins worldwide they are characterized on seismic data by vertical to sub vertical zones of reduced reflection continuity that have a columnar geometry in three dimensions the upper terminations of these pipes commonly coincide with pockmarks or palaeo pockmarks signifying a close connection of pipe formation with a high flux fluid expulsion process dimensions range from tens to hundreds of metres in diameter and hundreds to over a thousand metres in height and the slenderness ratio defined as height diameter ranges from 0 8 to over 20 pipes are frequently associated with sub vertical clustering of amplitude anomalies on seismic data related either to the presence of free gas or to cementation linked to the passage of hydrocarbons three mechanisms have been suggested to explain pipe genesis 1 hydraulic fracturing 2 erosional fluidisation and 3 capillary invasion we suggest a further two possible mechanisms in the form of localised collapse by volume loss and synsedimentary flow localisation we review all five mechanisms and conclude that it is unlikely that a single mechanism applies but that combinations of these processes may all occur in particular contexts fluid escape pipes may be far more widespread that currently appreciated and they may play a critical role in secondary hydrocarbon migration and in providing leakage pathways for trapped hydrocarbons through overlying seals

the earthquake sequence of june 1992 near saba west indies abstract during june 1992 a minor earthquake swarm occurred close to the island of saba a small composite volcano which is the northernmost member of the inner volcanically active lesser antilles arc the initial phase of the seismic activity was recorded at some permanent eastern caribbean seismic stations the nearest of which was located 55 km away on 14 june 1992 a seismograph was installed on saba to improve the detection and location capacities of the permanent network with regard to this swarm sixty events were detected of which only fifteen could be located the largest earthquake of the sequence occurred on 11 june 1992 had a duration magnitude of m d 4 5 and was felt at modified mercalli intensities iv v in saba and iii in st kitts the epicentres exhibit a sw ne trend and focal depths are mainly clustered in the range 10 16 km a t value of 0 73 was obtained for the sequence spectral analysis yielded various source parameters moments in the range 0 38 10 13 23 66 10 13 nm source radii from 87 to 126 m and stress drops from 1 3 to 79 9 bar moment increases with increasing magnitude while stress drop appears to decrease with decreasing seismic moment similar to many other studies although the evidence as to the nature of the origin of this swarm is not conclusive it seems to suggest a tectonic rather than volcanic origin

stress transfer and nonlinear stress accumulation at subduction type plate boundaries application to the aleutians a study of stress accumulation in seismic gaps and of stress transfer along linear plate boundaries is presented time dependent reloading of plate boundaries following seismic ruptures is modeled by a modified elsasser model of a coupled lithosphere asthenosphere plate system this model is applied to study a series of large earthquakes in the aleutian islands and the alaska peninsula in 1938 1965 it is found that the rat island earthquake and the 1948 earthquake in the central aleutians are likely to have been triggered by adjacent ruptures in the sense that their occurrence would have come at a later time had their neighboring segments not been ruptured stresses in the unalaska gap and the shumagin gap are at a relatively high level and these segments of the plate boundary may be expected to rupture in the near future in general in the ten years about 16 of the earthquake cycle for the aleutians following an earthquake the stress recovery in the rupture zone is highly nonlinear resulting in a much more rapid stress accumulation than the linear case even at a later stage of an earthquake cycle adjacent ruptures can cause an acceleration of loading rate in addition to the coseismic stress jump a good example is the influence of the 1964 alaska earthquake on the 1938 rupture zone a general conclusion of this work is that long term earthquake prediction models must take into account the nonlinear stress accumulation behavior in seismic gaps also we have shown the interaction of adjacent plate boundary segments which suggests that some large earthquakes may have been triggered by nearby ruptures

synthesis of salt tectonics in the southern north sea uk abstract a structural model encompassing the southern north sea basin west of the central graben has been developed this model consists of a rift system affecting the post salt section around the basin margin and a large area of detached compressional buckle folds within the basin this pattern is initially a response to gravity sliding of the post salt section on the salt within the basin during the late triassic to late jurassic a close relationship between the location and trend of the peripheral graben system and basement structures in the pre salt is noted pre jurassic extension across the peripheral graben systems was balanced by the sum of fault heaves at the pre salt rotliegend level and shortening across salt cored buckle folds in the post salt section salt pillows and swells passively infilled the cores of these gravity induced buckle folds cretaceous and tertiary inversion involved basin tilt and renewed movement on basin bounding basement faults notably reverse movements did not propagate from basement structures up into the peripheral graben systems the post salt sedimentary section experienced gravity spreading in response to inversion related uplift resulting in syn inversion extensional faulting in the sole pit high where the mesozoic section was thickest this extension combined with a loss of fault heave in the pre salt section was balanced by amplification of salt cored buckle folds in the centre of the basin in the context of the model described here salt pillows represent passive infill of thin skinned compressional buckle folds which later amplified during thick skinned basement shortening crestal collapse of such folds occurs via normal faulting accompanied by reactive diapirism such reactive diapirs establish conduits through which salt may leak leading to pillow deflation and ultimately conduit preservation as a salt wall flanked by rim synclines in areas where the buckle folds were emergent the salt structures described here are related to cover folds and faults which in turn reflect episodes of basin extension tilting and inversion hence individual salt structures can be said to be only remotely connected with regional intraplate stresses

p wave spectra for ml 5 foreshocks aftershocks and isolated earthquakes near parkfield california abstract wood anderson seismograms recorded at mount hamilton mhc 185 km 327 santa barbara sbc 180 km 158 and tinemaha tin 240 km 56 provide data for comparing p wave spectra for two immediate 17 min foreshocks one early 55 hr foreshock two aftershocks and two isolated parkfield earthquakes all are m l 5 0 shocks with epicenters within 7 km of the common epicenter of the 1934 and 1966 parkfield main shocks the set of events is well suited for testing the hypothesis that foreshocks are high stress drop sources calculated stress drops are controlled by source directivity at azimuths aligned with the fault break at mhc and sbc p wave radiation from the three foreshocks is focused along one fault trace azimuth suggesting that foreshock sources are characterized by pronounced unilateral rupture expansion at tin broadside to the fault where directivity has minimum effect on calculated relative stress drop the two immediate foreshocks are higher stress drop sources the early foreshock is a low to average stress drop source indicating the possibility that stress concentration is a rapidly occurring phenomenon in rupture nucleation alternatively the stress field is highly variable on the scale of 2 to 3 km in the focal region of an impending earthquake with a rupture length of 20 to 30 km

reservoir leakage along concentric faults in the southern north sea implications for the deployment of ccs and eor techniques high quality 3d seismic and borehole data in the broad fourteens basin southern north sea is used to investigate newly recognised concentric faults formed in salt withdrawal basins flanking reactivated salt structures throw depth and throw distance plots were used to understand the growth histories of individual faults as a result three families of concentric faults are identified a intra seal faults within a salt withdrawal basin b faults connecting the seal and the reservoir on the crest of an inverted anticline c raft bounding faults propagating into reservoir units they have moved obliquely and show normal throws even though they formed during a period of regional compression faults in the salt withdrawal basin and on the inverted anticline are highly segmented increasing the chances of compartmentalisation or localised fluid flow through fault linkages slip tendency analysis was carried out on the distinct fault families to compare the likelihood of slip along a fault at different pore fluid pressures and within different lithologies our results show that sections of the faults are optimally oriented with regards to maximum horizontal stresses hmax increasing the slip tendency the identified faults cut through a variety of lithologies allowing different values of pore fluid pressures to build up before faults reactivate within the vlieland sandstones pore fluid pressures of 30 mpa are not sufficient to reactivate pre existing faults whereas in the deeper posidonia shales faults might reactivate at pore fluid pressures of 25 mpa fluid flow features preferentially occur near fault segments close to failure heterogeneity in slip tendency along concentric faults and high degrees of fault segmentation present serious hazards when injecting co2 into the subsurface this study stresses the importance of high quality 3d seismic data and the need to evaluate individual fault systems when investigating potential reservoirs for carbon capture and storage and enhanced oil recovery

revised focal depths and velocity model for local earthquakes in the adak seismic zone a technique suggested by hales et al 1981 for determining hypocentral depths that are relatively independent of the velocity model originally used to locate the earthquakes is applied to a carefully selected set of earthquakes located with the adak seismic network the origin times are taken from the wadati diagram for each event and the epicentral distances from the standard hypocenter solution are assumed to be correct a straight ray interpretation based on the use of a single average velocity from the hypocenter to the surface is then used to determine the average velocity and the depth from the slope and intercept of the linear x 2 t 2 plot from the adak catalog for august 1974 to may 1983 8046 located events 579 earthquakes with good p data and 347 with good s data were selected on the basis of excellent time picks good quality solutions and reliable wadati and x 2 t 2 plots the revised depths show that the sharp cutoff of the shallower main thrust zone activity at 26 km seen in the adak data set is an artifact of the routine location procedure routine depths are found to be systematically too shallow by a mean amount of 4 35 km with origin times 0 20 sec too early the velocities derived in this study agree with those in the standard model for depths below 80 km but are higher than the standard values at shallower depths the two models can be reconciled by invoking small gradients of velocity in the uppermost three layers but this result is not unique the results indicate local variations of the shallower velocities within the adak seismic zone one of which low velocities beneath the volcanic arc has been found in an earlier independent study

the tectonic evolution of the southern dutch north sea during the palaeogene basin inversion in distinct pulses abstract in this paper we discuss the response of a sedimentary basin to compressional reactivation of extensional faults in distinct pulses the inversion of the broad fourteens basin in the southern dutch north sea is used as a case study the first pulse of inversion occurred during the late mesozoic as a result of the sub hercynean phase a second pulse occurred during the mid palaeocene laramide phase a final inversion pulse occurred around the eocene oligocene boundary coinciding with the pyrenean tectonic event we reconstructed the stratigraphic and structural development of the area during the palaeogene in detail basic data are two 2d seismic surveys supported by 74 geophysical wireline logs on which newly constructed depth and thickness maps are based the reconstruction is aided by a quantitative subsidence analysis cessation of compression resulted in relaxation and tensional reactivation of faults caused differential subsidence inheritance of geological structures is a key element in the tectonic activity in the broad fourteens basin during the palaeogene tectonic movement during the palaeogene was always accommodated by reactivation of palaeozoic mesozoic faults the intensity of inversion in the southern north sea basin during the pyrenean phase appears much larger than previously noticed moreover the area affected by the inversion is much wider than during previous phases minor differences in the direction of the local stress field between the tectonic phases and by the deposition of a thick sedimentary succession during the periods of tectonic quiescence between the inversion phases can account for this

a 20 km diameter multi ringed impact structure in the north sea most craters found on earth are highly eroded poorly preserved and only exposed on land here we describe a multi ringed impact structure discovered in the north sea from the analysis of three dimensional seismic reflection data the structure is 20 km in diameter and has at least ten distinctive concentric rings located between 2 and 10 km from the crater centre the structure affects cretaceous chalk and jurassic shales and is well preserved below several hundred metres of post impact tertiary strata which constrains its age to be 60 65 myr old the formation of concentric ringed impact structures at this relatively small scale had not previously been thought possible especially on the terrestrial planets we have mapped the ring structures at a resolution of tens of metres both laterally and in depth and show that the rings are fault bounded graben structures similar to fault arrays formed in low strain rate detachment tectonic settings strata deeper than 500 m palaeodepth appear unfaulted and we infer that the concentric ring structures may have accommodated post impact extension towards the excavated crater through detachment on weak layers within the chalk

large enigmatic crater structures offshore southern california summary digital mosaics of swath and conventional bathymetry data reveal large distinct near circular crater structures in the inner continental borderland offshore southern california two have maximum crater diameters that exceed 30 km and a third has a crater diameter of approximately 12 km all three features exhibit the morphology of large complex craters raised outer rim ring moat and central uplift yet their exact origin remains a mystery preliminary analyses of available seismic gravity and magnetic data over these structures reveal both similarities and distinct differences in geometry structure and geophysical signature to known impact sites all three crater structures however occur within the catalina terrane a highly extended volcanic and metamorphic province floored by catalina schist basement a likely alternative origin may thus involve explosive volcanism caldera collapse and resurgent magmatism and or possibly plutonism and schist remobilization associated with the catalina terrane no single model for crater formation whether impact caldera or pluton fully accounts for all of the present observations regarding the morphology internal structure and known geology of these near circular features timing of crater formation post dates the initial rifting and rotation of the western transverse ranges and appears to predate major right slip along the san clemente and san diego trough fault systems or approximately 18 to 16 ma regardless of their origin these complex craters represent some of the largest structures of their kind in western north america and provide a unique opportunity to better understand the development of unusual crater structures in a submarine environment

an experiment in earthquake prediction and the 7 may 1986 andreanof islands earthquake abstract the occurrence of the 7 may 1986 andreanof islands earthquake provides the opportunity to evaluate the earthquake forecast and specific prediction that had been formulated and promulgated starting more than 2 yr before the prediction experiment was based on the observation of seismic quiescence in the part of the aleutian subduction zone monitored by the central aleutians adak seismic network the magnitude estimate m s 7 to 7 5 was based on the size of the quiescent zone and the seismic history of the adak region since 1900 the time of occurrence was predicted as a test of the discovery by habermann of a quiescence lasting about 3 yr prior to the 2 may 1971 adak canyon event and the hypothesis that a characteristic precursor time is a valid concept the place was originally forecast as near adak island then specified with epicenter at the eastern margin of adak canyon rupturing to the west under the canyon the 7 may 1986 earthquake had magnitude m s 7 7 m w 8 0 with epicenter 140 km east of the predicted place rupturing east of adak canyon and stopping in the area predicted as the nucleation point the time of this event was 6 months later than predicted thus the specifically predicted event has not happened the largest part of the rupture did go through the segment that had been most active before the onset of quiescence and which experienced the sharpest and most persistent quiescence a cause and effect relation between preparation for this earthquake and the quiescence is still to be firmly established but the distribution of the pattern of quiescence relative to the epicentral location and the site of the greatest moment release suggest a physical connection between earthquake and seismicity patterns the predictions of location and time were acknowledged throughout as highly speculative no firm basis exists for quantitative estimation of the uncertainties to be assigned to the elements of an intermediate term prediction this experiment lends support to the validity of quiescence as a precursor at least for some earthquakes and demonstrates the need for local network data to provide the high resolution of seismicity required for prediction efforts based on seismicity patterns

emplacement of giant mud volcanoes in the south caspian basin 3d seismic reflection imaging of their root zones exceptional quality 3d data for the largest mud volcano yet described provide the first detailed imaging of the plumbing architecture that connects a major volcanic edifice to its source layer at depth the volcano is in the south caspian basin and consists of an extruded submarine mud bicone 10 km wide and 1 4 km thick overlying an oval caldera 1 2 1 6 km in width and 0 5 km in depth the caldera narrows downwards into a zone of collapsed country rock forming a downward tapering cone 1 km in height the vertex of which is located close to the top of the mud source layer the imaged structural elements lead to an evolutionary model a narrow steep fluidization pipe fed the oldest pioneer cone we propose that numerous additional fluidization pipes injected the country rock forming a densely intruded cylindrical zone similar to gryphon swarms observed at outcrop onshore wall rock erosion and compaction of the intruded zone led to collapse of the downward tapering cone that linked upwards into ring faults that define the caldera margins later mud flowage focused on the conical sheared margins volumetric contraction of the extruded volcanic cone led to an unusual concentric system of minor outward facing normal faults this model has many similarities to syntheses of igneous maar diatreme caldera systems for which it may be analogous

spectral characteristics and source parameters of microearthquakes from the mt cameroon volcanic region west africa summary estimates of source parameters seismic moment source radius and stress drop for a representative sample of 33 mt cameroon region microearthquakes with magnitudes in the range md 0 8 2 8 have been obtained from the analysis of their far field displacement amplitude spectra the spectral character of these events seems to be characterized by a lack of high frequencies the computed seismic moments lie between 10 and 10l6 nm and vary linearly with magnitude albeit with some scatter source radii are relatively large in the range 135 703m the stress drops are small 0 4 19 bar and roughly constant independent of seismic moment thus suggesting that the property of self similarity seems to hold over this moment range for this area

a scattering model to explain seismic q observations in the lithosphere between 1 and 30 hz observations of lithospheric q for shear waves in the range 1 30 hz reported in the literature are examined using a scattering model consisting of randomly distributed spheres for the single scattering approximation i have assumed 1 q 1 q sub i gv omega where q sub i is the intrinsic q due to anelasticity v is the velocity of shear waves omega is angular frequency and g is the turbidity g is taken as a constant in this paper since g integral n a sigmada where n a da is the number of scattering spheres of radius a unit volume and sigma is the scattering cross section for a sphere this suggests that geometrical scattering dominates this will be true for ka 1 where k is wavenumber based on this criterion and the observation that q is large i e g small at a frequency of 0 05 hz a is of the order of magnitude 1 10 km fits of observations of shear wave q from japan and central asia indicate that q sub i 2000 500 for the lithosphere of both regions g 0 01 km for the japan observations and g 0 005 km sup 1 for central asia

local earthquakes of the mexico basin in mexico city κ q source spectra and stress drop abstract we analyzed 14 local earthquakes of the mexico basin 2 3 m w 3 8 recorded on hill sites at local distances 8 0 r 60 5 km to study the spectral attenuation of seismic waves spectral decay parameter kappa quality factor q source spectra and brune stress drop we found an average 0 value of 0 0457 s in hill sites which is similar to values reported in other regions assuming a 1 r geometrical spreading we obtain a quality factor given by q f 72 f 0 83 source spectra of all studied events are in good agreement with the 2 model from the synthetic modeling of ground motions at local distances we explain an anomalous behavior of the corner frequency versus seismic moment and also explain the relatively low values observed for stress drop 5 mpa the resulting high spectral attenuation values obtained may be responsible for the low acceleration values of ground motions observed for crustal earthquakes occurring inside and near to the mexico basin

apparent stresses stress drops and amplitude ratios of earthquakes preceding and following the 1975 hawaii ms 7 2 main shock abstract s wave spectra for approximately 300 earthquake signals 2 8 m l 3 9 were obtained from hand digitized wood anderson records from hilo hawaii all sources were located within part of the rupture volume of the 1975 hawaii m s 7 2 earthquake the epicentral distance to hilo was about 45 3 km the backazimuth was 180 10 south and the sources were located at depths of 9 1 4 km the apparent stress and brune stress drop based on 3 sec windows of the s waves were examined for possible temporal and spatial dependence most of the data showed a remarkable constancy of the average apparent stress and stress drop per event as a function of time no measurable change in average signal content took place at the time of the main shock a statistically significant difference in the average stress parameters per event was found between neighboring subvolumes radius 5 km of the 1975 aftershock zone a volume previously proposed as asperity based on seismicity pattern interpretation contained earthquakes with higher apparent stresses of 9 0 8 8 bars compared to the neighboring volume with 4 2 6 1 bars seventy eight spectra obtained from a second station showed the same difference between the two volumes as that found at station hilo suggesting that the signal differences should be interpreted in terms of source differences in addition amplitude ratios of sv p waves measured at hilo showed that a systematic difference of 10 in fault plane orientation existed during years before the 1975 main shock this agrees with the rotation of the fault plane during the main rupture reported elsewhere we hypothesize that heterogeneities along the fault plane with dimensions of about 5 km cause changes in fault plane dip differences in average stress release per earthquake strong contrasts in seismicity patterns and differences in aseismic strain release by fault creep these same heterogeneities in the fault plane apparently also control the location of major earthquakes and the complexity of their ruptures

geomechanical analysis of a welding salt layer and its effects on adjacent sediments abstract we simulate welding of the source layer of a salt diapir with a forward finite element model and study stresses and deformation in the salt layer and the diapir as well as in their adjacent sediments welded salt layers are abundant in mature salt basins where most or all of the salt has withdrawn into diapirs however there is little understanding of the stress field in these layers and their adjacent sediments we show that salt flow along the source layer leads to significant stress anomalies inside the layer and in adjacent sediments in the source layer salt pressure becomes higher than overburden stress in nearly welded areas and becomes lower than overburden stress in adjacent thicker areas when the source layer welds stresses increase significantly in sediments near the weld tip which helps compaction of these sediments and possibly their fracturing and faulting our model illustrates that all sediments overlying the weld experience this stress increase and the associated material changes as the weld tip propagates along the weld we present natural examples fitting our predictions and discuss the importance of our results for the exploration characterization and production of reservoirs near welded salt layers

salt dissolution and salt related deformation of the forth approaches basin uk north sea abstract this paper addresses the potential role of regional scale salt dissolution as a mechanism responsible for salt related deformation this analysis is based on an investigation of the late permian to triassic succession of the forth approaches basin on the nw margin of the north permian basin and makes use of a regional 2d seismic grid tied to five key exploration wells this basin is ideal for this study because it consists of a deformed central region with a ne sw trending axis and an undeformed periphery which provides a template and key datums for reconstruction of the adjacent zone of deformation structural styles in this zone include simple salt cored anticlines salt walls salt diapirs detachment faults and unusually for the north sea intra zechstein salt cored folds pre triassic detailed mapping was carried out in uk block 26 10 and this shows that the dominant structural trend is ne sw suggesting that basement tilting influenced structural development the stratal geometry of the numerous triassic mini basins is used to constrain the way in which accommodation space in the mini basins was created by depletion of mobile salt the spatial pattern of salt depletion can be accounted for by two contrasting mechanisms or some combination thereof 1 deformation through downbuilding during a phase of regional subsidence or 2 flow induced by regional salt dissolution volumetric arguments necessary to discriminate between these two mechanisms are inconclusive due to later deformation of the top zechstein datum selective preservation of z3 and z4 cycles of the zechstein over large parts of the block 26 10 study area suggests that much of the salt depletion process preferentially removed z2 halite this observation is compatible with a process of subjacent dissolution possibly resulting from low salinity groundwater circulating in the pre zechstein aquifers

apparent stress of earthquakes within the shallow subduction zone near kamchatka peninsula apparent stresses of 105 moderate sized magnitude mb from 4 5 to 6 0 shallow earthquakes from the subduction zone near kamchatka peninsula nw pacific were analyzed our results allowed us to formulate the following properties of the earthquake apparent stress for the seismic moment range 1015 to 1018 n m and the focal depth range from 0 to 60 km 1 apparent stresses depend on the fault nature of earthquakes they are greater for thrusts and smaller for normal faults a difference of a factor of 3 2 average apparent stresses were greater in the coastal zone near kamchatka where subducting oceanic and overriding continental plates interact 2 5 mpa and just under the trench axis 2 9 mpa but lower in near trench zones on the western slope of the trench 0 4 mpa this difference is a factor of 6

rietveld analysis of x ray powder diffraction patterns as a potential tool for the identification of impact deformed carbonate rocks previous x ray powder diffraction xrd studies revealed that shock deformed carbonates and quartz have broader xrd patterns than those of unshocked samples entire xrd patterns single peak profiles and rietveld refined parameters of carbonate samples from the sierra madera impact crater west texas unshocked equivalent samples from 95 miles north of the crater and the mission canyon formation of southwest montana and western wyoming were used to evaluate the use of x ray powder diffraction as a potential tool for distinguishing impact deformed rocks from unshocked and tectonically deformed rocks at sierra madera dolostone and limestone samples were collected from the crater rim lower shock intensity and the central uplift higher shock intensity unshocked equivalent dolostone samples were collected from well cores drilled outside of the impact crater carbonate rocks of the mission canyon formation were sampled along a transect across the tectonic front of the sevier and laramide orogenic belts whereas calcite subjected to significant shock intensities at the sierra madera impact crater can be differentiated from tectonically deformed calcite from the mission canyon formation using rietveld refined peak profiles weakly shocked calcite from the crater rim appears to be indistinguishable from the tectonically deformed calcite in contrast rietveld analysis readily distinguishes shocked sierra madera dolomite from unshocked equivalent dolostone samples from outside the crater and tectonically deformed mission canyon formation dolomite

3d seismic analysis of the structure and evolution of a salt influenced normal fault zone a test of competing fault growth models abstract in this paper we determine the structure and evolution of a normal fault system by applying qualitative and quantitative fault analysis techniques to a 3d seismic reflection dataset from the suez rift egypt our analysis indicates that the october fault zone is composed of two fault systems that are locally decoupled across a salt bearing interval of late miocene messinian age the sub salt system offsets pre rift crystalline basement and was active during the late oligocene early middle miocene it is composed of four planar nw se striking segments that are hard linked by n s striking segments and up to 2 km of displacement occurs at top basement suggesting that this fault system nucleated at or more likely below this structural level the supra salt system was active during the pliocene holocene and is composed of four nw se striking listric fault segments which are soft linked by unbreached relay zones segments in the supra salt fault system nucleated within pliocene strata and have maximum throws of up to 482 m locally the segments of the supra salt fault system breach the messinian salt to hard link downwards with the underlying sub salt fault system thus forming the upper part of a fault zone composed of i a single amalgamated fault system below the salt and ii a fault system composed of multiple soft linked segments above the salt analysis of throw distance t x and throw depth t z plots for the supra salt fault system isopach maps of the associated growth strata and backstripping of intervening relay zones indicates that these faults rapidly established their lengths during the early stages of their slip history the fault tips were then effectively pinned and the faults accumulated displacement via predominantly downward propagation we interpret that the october fault zone had the following evolutionary trend i growth of the sub salt fault system during the oligocene to early middle miocene ii cessation of activity on the sub salt fault system during the middle miocene to early pliocene iii stretching of the sub and supra salt intervals during pliocene regional extension which resulted in mild reactivation of the sub salt fault system and nucleation of the segmented supra salt fault system which at this time was geometrically decoupled from the sub salt fault system and iv pliocene to holocene growth of the supra salt fault system by downwards vertical tip line propagation which resulted in downward breaching of the salt and dip linkage with the sub salt fault system the structure of the october fault zone and the rapid establishment of supra salt fault lengths are compatible with the predictions of the coherent fault model although we note that individual segments in the supra salt array grew in accordance with the isolated fault model our study thereby indicates that both coherent and isolated fault models may be applicable to the growth of kilometre scale basin bounding faults furthermore we highlight the role that fault reactivation and dip linkage in mechanically layered sequences can play in controlling the three dimensional geometry of normal faults

orebody geometry in lode gold deposits from zimbabwe implications for fluid flow deformation and mineralization the geometry of some orebodies can be described simply and accurately by three orthogonal axes u v w the ratios between these axes can be expressed as a parameter j u v 1 v w 1 and represented by a graph of u v plotted against v w analogous to the treatment of strain ellipsoids the orientations of orebodies can be plotted simply on projections using the uvw axes measurements of ore bodies from two examples of lode gold deposits from the zimbabwe craton show that most of these orebodies are oblate however orebodies can have significant u v ratios implying a component of pipe like fluid flow during mineralization pipe flow is demonstrated to be orders of magnitude more conductive than flow in planar veins and faults there are significant variations in orebody geometry between deposits and within different sections of a single deposit w values appear to be influenced by host rock more permeable rocks have higher w a negative trend of j value with orebody volume indicates that orebodies do not evolve in a self similar way but tend to more oblate shapes with increasing volume

cenozoic evolution of the northwestern salar de atacama basin northern chile 1 since 90 ma the nonmarine salar de atacama basin has been the largest deepest and most persistent sedimentary basin of northern chile integration of 200 km of two dimensional seismic reflection data with surface geological data clarifies oligocene and neogene evolution of the northern part of the basin a normal fault with 6 1 km of vertical separation controlled the western boundary of the basin during the accumulation of the oligocene lower miocene paciencia group the combination of this structure a similar one in the calama basin and regional structural data suggests that localized extension played an important role within a tectonic environment dominated by margin perpendicular compression and margin parallel strike slip deformation seismic data substantiate the surface interpretation that much of the cordillera de la sal ridge resulted from diapiric flow of the paciencia group diapiric flow initiated during the late early miocene or middle miocene associated with a deep reverse fault