role of overpressures and in situ stresses in fault controlled hydrocarbon migration a case study abstract two wells no 1 and no 2 separated by 2 km and situated in the upper prospective zone of a tertiary basin in south east asiat cross the same mio pleistocene shaly sandy series from 1500 m downwards these series are subject to significant overpressures related to an active margin comparison of well data in terms of sonic compaction profiles anomalous pressures and temperatures has provided evidence of both a positive pressure and a temperature anomaly below a vertical depth of 1800 m in well 2 this anomaly is closely associated with the presence of liquid hydrocarbons in the 1900 2000 m reservoir interval the confirmed deep source of these fluids and the amounts that have migrated during a short time interval can be explained only by the transitory dynamics of active transfers along faults and fractures the episodic nature of this movement is the result of hydromechanical mechanisms in which the interactions between the high pressure regime and principal in situ stresses appear to be fundamental the processes which induce open faulting and fluid pressure build up in the direction of reservoirs with lower pressure regimes occur preferentially during phases of lateral tectonic stress relaxation as soon as the effective minimum stress component becomes negative the analysis of this case study gives a better understanding of the dynamic phenomena involving secondary hydrocarbon migration and caprock efficiency the results obtained will help to optimize the overpressure prediction approach in comparable geological contexts

quaternary pollen taphonomy examples of differential redeposition and differential preservation pollen assemblages are affected by a variety of processes between dehiscence and recovery here two examples from western canada illustrate two of the most important biases affecting pollen assemblages after production and initial dispersal a site in canada s yukon territory provides evidence of selective redeposition of penecontemporaneous pollen grains which is detectable only through paired coring a deposit on the floor of a kettle depression shows trends in the ratios of extra local pollen types which are opposed to the trends shown in a core taken from the slope of the same depression this is explained as a trend to reduced mobility of saccate grains with time due to increased permeability of the substrate on the kettle slope a study of the differential degradation of populus pollen illustrates the complexities of pollen preservation populus pollen is generally recognized as more susceptible to corrosion than are most other pollen types the poor preservation of this pollen type is attributed largely to its lack of harmomegathic adaptation and low sporopollenin content this real susceptibility is compounded by the pollen grain s lack of distinctive surface ornamentation inhibiting recognition of damaged grains at least part of the general impression of poor preservation of populus pollen may be due to a recent increase in the abundance of populus on the landscape in western canada early studies in this region may have sampled sediments from before the increase in populus and compared their pollen spectra with post populus increase vegetation

stress field constraints on intraplate seismicity in eastern north america focal mechanisms of 32 north american midplate earthquakes mo 3 8 6 5 were evaluated to determine if slip is compatible with a broad scale regional stress field derived from plate driving forces and if so under what conditions stress regime pore pressure and frictional coefficient using independent information on in situ stress orientations from well bore breakout and hydraulic fracturing data and assuming that the regional principal stresses are in approximately horizontal and vertical planes 10o the constraint that the slip vector represents the direction of maximum resolved shear stress on the fault plane was used to calculate relative stress magnitudes defined by the parameterb s2 s3 s s3 from the fault stress geometry as long as the focal mechanism has a component of oblique slip i e the b axis does not coincide with the intermediate principal stress direction this calculation identifies which of the two nodal planes is a geometrically possible slip plane gephart 1985 slip in a majority of the earthquakes 25 of 32 was found to be geometrically compatible with reactivation of favorably oriented preexisting fault planes in response to the broad scale uniform regional stress field slip in five events was clearly inconsistent with the regional stress field and appears to require a localized stress anomaly to explain the seismicity significantly all five of these events occurred prior to 1970 when many regional networks were installed and their focal mechanisms are inconsistent with more recent solutions of nearby smaller events the frictional likelihood of the geometrically possible slip on the selected fault planes was evaluated in the context of conventional frictional faulting theory the ratio of shear to normal stress on the fault planes at hypocentral depth was calculated relative to an assumed regional stress field regional stress magnitudes were determined from 1 s s3 ratios based on the frictional strength of optimally oriented faults the basis for the linear brittle portion of lithospheric strength profiles 2 the computed relative stress magnitude b values and 3 a vertical principal stress assumed equal to the lithostat two end member possibilities were examined to explain the observed slip in these less than optimally oriented fault planes first the frictional coefficient was held constant on all faults hydrostatic pore pressure was assumed regionally and the fault zone pore pressure was determined since pore pressure is a measurable quantity with real limits in the crust p0 s3 this end member case was used to determine which of the geometrically possible slip planes were frictionally likely slip planes alternately pore pressure was fixed at hydrostatic everywhere and the required relative lowered frictional coefficient of the fault zone was computed slip in 23 of the 25 geometrically compatible earthquakes was determined to also be frictionally likely in response to an approximately horizontal and vertical regional stress field derived from plate driving forces whose magnitudes are constrained by the frictional strength of optimally oriented faults assuming hydrostatic pore pressure regionally the conditions for slip on these 23 relatively well oriented earthquake faults were determined relative to this regional crustal strength model and require only moderate increases in pore pressure between about 0 4 0 8 of lithostatic hydrostatic is about 0 37 of lithostatic or alternately moderate lowering 50 of the frictional coefficient on the faults which slipped superlithostatic pore pressures are not required focal mechanisms for the two other earthquakes with slip vectors geometrically consistent with the regional stress field however did require pore pressures far exceeding the least principal stress or extremely low coefficients of friction these events may reflect either local stress rotations undetected with current sampling or poorly constrained focal mechanisms the analysis also confirmed a roughly north to south contrast in stress regime between the central eastern united states and southeastern canada previously inferred from a contrast in focal mecha nisms between the two areas most central eastern united states earthquakes occur in response to a strike slip stress regime whereas the southeastern canadian events require a thrust faulting stress regime this contrast in stress regime with a constant maximum horizontal stress orientation determined by far field plate driving forces requires a systematic lateral variation in relative stress magnitudes superposition of stresses due to simple flexural models of glacial rebound stresses are of the correct sense to explain the observed lateral variation but maximum computed rebound related stress magnitude changes are quite small about 10 mpa and do not appear large enough to account for the stress regime change if commonly assumed stress magnitudes determined from frictional strength apply to the crust at seismogenic depths

damped regional scale stress inversions methodology and examples for southern california and the coalinga aftershock sequence 1 we present a new focal mechanism stress inversion technique to produce regional scale models of stress orientation containing the minimum complexity necessary to fit the data current practice is to divide a region into small subareas and to independently fit a stress tensor to the focal mechanisms of each subarea this procedure may lead to apparent spatial variability that is actually an artifact of overfitting noisy data or nonuniquely fitting data that does not completely constrain the stress tensor to remove these artifacts while retaining any stress variations that are strongly required by the data we devise a damped inversion method to simultaneously invert for stress in all subareas while minimizing the difference in stress between adjacent subareas this method is conceptually similar to other geophysical inverse techniques that incorporate damping such as seismic tomography in checkerboard tests the damped inversion removes the stress rotation artifacts exhibited by an undamped inversion while resolving sharper true stress rotations than a simple smoothed model or a moving window inversion we show an example of a spatially damped stress field for southern california the methodology can also be used to study temporal stress changes and an example for the coalinga california aftershock sequence is shown we recommend use of the damped inversion technique for any study examining spatial or temporal variations in the stress field

aftershock sequence and statistics of the 2017 mw 5 5 pohang south korea earthquake implications of fault heterogeneity and postseismic relaxation

appalachian stress study 1 a detailed description of in situ stress variations in devonian shales of the appalachian plateau we describe an experiment to measure variations in the state of stress within a horizontally bedded devonian shale sandstone limestone sequence in western new york a total of 75 stress measurements were made in three wells a kilometer or so apart using a wireline supported hydraulic fracturing system the stress profiles indicate that a major drop in horizontal stress level occurs in the generally massive shales this drop occurs principally across the lowermost member of a group of sand beds and corresponds to an offset in sh and sh of 3 5 and 9 mpa respectively above the sands thrust regime conditions prevail although the amount by which sh exceeds sv is undetermined since instantaneous shut in pressures isips were clipped at the level of sv due to fracture rotation below the sands the regime is strike slip with both horizontal stresses showing lateral uniformity despite substantial variations in topography the magnitude of sh in the sand beds themselves and a lower limestone remains at least as great as sv despite the decline in shale stress hence stress contrasts between these beds and neighboring shales become pronounced with depth the contrast in sh and sh between the lowermost sand and the immediately underlying shale is at least 6 and 14 5 mpa respectively sh levels in the lower strike slip regime are about 1 75 times greater than sh and are less than the value required to initiate slippage on favourably oriented frictional interfaces for the upper thrust regime and the sand and limestone beds however the inferred lower bound on sh is close to the slippage threshold for a coulomb friction coefficient of 0 6 the orientation of sh is ene with a standard deviation of 20 fracture traces were usually splayed occasionally spanning 30 of well bore no systematic correlation between mean orientation and lithology is evident significantly different orientations were obtained for adjacent tests in which almost identical islps were observed suggesting that the fractures quickly reorient themselves to propagate normal to the least principal stress direction similarly vertical traces were observed in those tests where isips apparently reflect sv suggesting that rotation to horizontal was rapid

seismic evidence for reverse activation of a paleo rifting system in the east sea sea of japan abstract the japanese islands were separated from the eurasian plate due to continental rifting during the oligocene to mid miocene which caused the opening of the east sea sea of japan such tectonic evolution in the east sea is important for understanding the evolution of back arc regions with active convergent margins to understand the evolution of the paleo rifted back arc region we investigate seismicity crustal seismic anisotropy focal mechanism solutions and ambient stress field around the korean peninsula the korean peninsula displays diffused seismicity with small and moderate earthquakes shallow earthquakes rarely occur in the central east sea the crustal fast shear wave polarization directions in the korean peninsula are observed to vary in azimuth between 40 and 90 the focal mechanism solutions are calculated by long period waveform inversions the ambient stress field is calculated from the focal mechanism solutions the compressional stress field in the korean peninsula is observed to be in ene wsw which is consistent with the fast shear wave polarization directions the compressional stress directions in the east sea progressively change from ene to se with increasing longitude the rapid change of compressional directions in the central east sea prohibits accumulation of stress causing rare shallow seismicity high seismicity of reverse faulting events is observed at the fringes of the east sea in particular around the east coast of the korean peninsula and the west coast of japanese islands which correspond to paleo rifted margins where compressional stresses are accumulated the compressional stress field and active thrustal events suggest reverse activation of paleo normal faults that were developed during the opening of the east sea

crustal structure of the continental margin of korea in the east sea japan sea from deep seismic sounding data evidence for rifting affected by the hotter than normal mantle abstract despite the various opening models of the southwestern part of the east sea japan sea between the korean peninsula and the japan arc the continental margin of the korean peninsula remains unknown in crustal structure as a result continental rifting and subsequent seafloor spreading processes to explain the opening of the east sea have not been adequately addressed we investigated crustal and sedimentary velocity structures across the korean margin into the adjacent ulleung basin from multichannel seismic mcs reflection and ocean bottom seismometer obs data the ulleung basin shows crustal velocity structure typical of oceanic although its crustal thickness of about 10 km is greater than normal the continental margin documents rapid transition from continental to oceanic crust exhibiting a remarkable decrease in crustal thickness accompanied by shallowing of moho over a distance of about 50 km the crustal model of the margin is characterized by a high velocity up to 7 4 km s lower crustal hvlc layer that is thicker than 10 km under the slope base and pinches out seawards the hvlc layer is interpreted as magmatic underplating emplaced during continental rifting in response to high upper mantle temperature the acoustic basement of the slope base shows an igneous stratigraphy developed by massive volcanic eruption these features suggest that the evolution of the korean margin can be explained by the processes occurring at volcanic rifted margins global earthquake tomography supports our interpretation by defining the abnormally hot upper mantle across the korean margin and in the ulleung basin

determination of stress from slip data faults and folds a new technique is derived to invert slickenside data for the stress field that caused the faulting episode this inversion is simplified by the assumption that the magnitude of the tangential traction on the various fault planes at the time of rupture is similar study of three normal faulting regimes shows that the inversion derived with this assumption yields results that closely match older inversions that did not include the assumption hence the assumption may be valid and is shown to be justified by analyzing a simple fracture criterion application of slip data inversions is extended from faulting regimes to the slip on bedding plane faults in folding regimes comparison of the inversion results with the geometry of the folds shows this application to be successful greatly increasing the number of data sets that can be used to find the paleostress field

body wave analysis for shallow intraplate earthquakes in the korean peninsula and yellow sea abstract four medium size shallow intraplate earthquakes from the korean peninsula and yellow sea have been studied in order to find their source mechanisms focal depths source time functions and seismic moments focal depths obtained by matching short period synthetic seismograms to the observations are found to be very shallow between 6 and 9 km to determine the source mechanisms source time functions and seismic moments we inverted teleseismic long period p and sh waves from gdsn data although the inversions are performed on only a few teleseismic body waves the radiation patterns of the moment tensors are consistent with most of the p wave first motion polarities observed at regional and teleseismic distances the overall agreement in relative amplitudes and waveforms between observations and synthetics indicates that the earthquake source parameters are reasonably well determined for two earthquakes from the yellow sea and one from the north korean peninsula the mechanisms are predominantly of strike slip style while the earthquake in the central korean peninsula shows a dip slip faulting consistent ene wsw trending compression p axis and nnw sse tension t axis are found for all studied earthquakes

the butterfly delta system of qiongzhou strait morphology seismic stratigraphy and sedimentation abstract analysis of high resolution sub bottom profiles topographical data and sediment samples reveals that the tidal delta system of qiongzhou strait is morphologically a butterfly delta system consisting of western mouth tidal delta wmtd and eastern mouth tidal delta emtd wmtd covers an area of about 11 000 km 2 and is more than two times larger than emtd 4000 km 2 wmtd is fan shaped with a tributary branch extending southwards along hainan island while emtd is typically fan shaped seismic architectures of wmtd and emtd are both characterized by a thick progradational unit overlying a thin transgression unit both wmtd and emtd start to develop at about 8 7 cal ka bp when qiongzhou strait completely opened and have the same three sedimentary facies tidal delta top tidal delta slope and tidal delta toe the formation of wmtd and emtd is attributed to qiongzhou strait s hydrodynamic regime which is dominated by the strong east west reciprocal tidal currents and influenced by the year round westward mean flow modified by the wind driven seasonal currents

focal mechanisms of recent earthquakes in the southern korean peninsula summary we evaluate the stress field in and around the southern korean peninsula with focal mechanism solutions using the data collected from 71 earthquakes ml 1 9 5 2 between 1999 and 2004 for this the hypocentres were relocated and well constrained fault plane solutions were obtained from the data set of 1270 clear p wave polarities and 46 sh p amplitude ratios the focal mechanism solutions indicate that the prevailing faulting types in south korea are strike slip dominant oblique slip faultings with minor reverse slip component the maximum principal stresses 1 estimated from fault slip inversion analysis of the focal mechanism solutions show a similar orientation with e w trend 269 275 and low angle plunge 10 25 for all tectonic provinces in south korea consistent with the e w trending maximum horizontal stress hmax of the amurian microplate reported from in situ stress measurements and earthquake focal mechanisms the directions of the intermediate 2 and minimum 3 principal stresses of the gyeongsang basin are however about 90 deg off from those of the other tectonic provinces on a common 2 3 plane suggesting a permutation of 2 and 3 our results incorporated with those from the kinematic studies of the quaternary faults imply that nnw to ne striking faults dextral strike slip or oblique slip with a reverse slip component are highly likely to generate earthquakes in south korea

spatial heterogeneity of tectonic stress and friction in the crust the complex geometry of faults seismicity and diversity of earthquake mechanisms suggest that the stress and strength in earth s crust are spatially heterogeneous we investigated the degree of heterogeneity using the following two end member models in one end member model we assumed that the orientation of stress is uniform in the crust as is assumed in many stress inversion studies in this model the variability of earthquake mechanisms means that friction during faulting must vary for each event we computed friction from the ratio of the resolved shear stress to the effective normal stress on the fault plane with the assumption of hydrostatic pore pressure the values of vary over a large range from 0 to 1 5 in the other extreme model we assumed optimally oriented slip and a constant 0 6 as is suggested by byerlee s law for all the earthquakes and determined the local stress orientation for each earthquake the orientation of the stress changes drastically from one earthquake to another and the assumption of uniform stress field commonly used in stress inversion is not warranted an important conclusion is that a regionally uniform stress field and constant friction on optimally oriented faults are mutually exclusive the actual situation in the crust is most likely to be intermediate between these two end member models from the existing data alone we cannot determine the degree of heterogeneity uniquely but both and the local stress field near earthquake faults are likely to vary substantially and studies on earthquake rupture dynamics must take these heterogeneities into consideration

the 20 january 2007 odaesan korea earthquake sequence reactivation of a buried strike slip fault the m w 4 6 odaesan korea earthquake of 20 january 2007 11 hr 56 min had nine locatable foreshocks in the previous 67 hours and nine aftershocks during the 11 hours after the main event we obtained accurate locations of this sequence by employing the double difference earthquake relocation method with differential travel time measurements using waveform cross correlation we found that 19 accurately relocated foreshocks mainshock and aftershocks aligned along a 1 2 km long lineation with an azimuth of west northwest east southeast 115 which coincides with a nodal plane of the mainshock focal mechanism that strikes 114 regional waveform modeling of the mainshock constrained a focal depth of 11 1 km and the focal mechanism is a pure strike slip faulting along the vertical nodal planes with a horizontal p axis plunge 1 trending east northeast 69 the pulse rise time of the mainshock source time function estimated from an empirical green s function analysis ranges from 0 1 to 0 16 sec which yields a source radius of 0 6 km and is consistent with the spatial distribution of the foreshock and aftershock sequences the stress drop of graphic 1 is obtained for the mainshock using a radius of 0 6 km for a circular rupture and seismic moment graphic 2 the 20 january 2007 odaesan earthquake may be the first significant earthquake in southern korea to date where the fault plane is constrained by accurate locations of the foreshock and aftershock sequences its source mechanism vertical strike slip fault at depth suggests that the event represents reactivation of a buried high angle fault in the precambrian basement by the contemporary east east northeast trending regional horizontal compressive stress 1 embed inline graphic 1 gif 2 embed inline graphic 2 gif

spatial heterogeneity of tectonic stress and friction in the crust new evidence from earthquake focal mechanisms in taiwan we performed inversions of earthquake focal mechanisms in central taiwan to investigate the heterogeneity of the stress field and fault strength and temporal variations of stress parameters friction and pore pressure associated with the 1999 chi chi earthquake we divided the focal mechanism data into two groups before and after the chi chi earthquake and analysed them separately with the assumption of a uniform stress field the friction coefficient is mostly within a range of 0 2 0 4 in central taiwan which is lower than the commonly quoted laboratory result 0 6 0 85 the low friction coefficient is also inferred by the rotation of principal stress axes after the chi chi earthquake by contrast if we assume that the friction is constant and failures occur on optimally oriented planes we find that the resulting stress orientations must be spatially variable however a large dispersion of stress orientations is not seen in borehole breakouts and fault slip data implying a constant friction model might be ruled out our analysis suggests that either the distribution of the coefficient of friction or pore pressure changed during the 1999 chi chi earthquake we infer that the pore pressure probably rose in the chi chi rupture area and northern longitudinal valley and dropped in the areas south of the coseismic rupture area after the main shock

dynamic fault interaction during a fluid injection induced earthquake the 2017 mw 5 5 pohang event the november 15th 2017 mw 5 5 pohang earthquake south korea has been linked to hydraulic stimulation and fluid injections making this the largest induced seismic event associated with an enhanced geothermal system egs to understand its source dynamics and fault interactions we conduct the first 3d high resolution spontaneous dynamic rupture simulations of an induced earthquake we account for topography off fault plastic deformation under depth dependent bulk cohesion rapid velocity weakening friction and 1d subsurface structure a guided fault reconstruction approach that clusters spatio temporal aftershock locations including their uncertainties is used to identify a main and a secondary fault plane that intersects under a shallow angle of 15 based on simple mohr coulomb failure analysis and 180 dynamic rupture experiments in which we vary local stress loading conditions fluid pressure and relative fault strength we identify preferred two fault plane scenarios that well reproduce observations we find that the regional far field tectonic stress regime promotes pure strike slip faulting while local stress conditions constrained by borehole logging generate the observed thrust faulting component our preferred model is characterized by overpressurized pore fluids non optimally oriented but dynamically weak faults and a close to critical local stress state in our model earthquake rupture jumps to the secondary fault by dynamic triggering generating a measurable non double couple component our simulations suggest that complex dynamic fault interaction may occur during fluid injection induced earthquakes and that local stress perturbations dominate over the regional stress conditions these findings therefore have important implications for seismic hazard in active geo reservoir

a refined methodology for stress inversions of earthquake focal mechanisms we develop an improved methodology for reliable high resolution inversions of focal mechanisms to background stress field orientation and stress ratio r in two or three dimensions the earthquake catalog is declustered to remove events likely affected strongly by local stress interactions rather than reflecting the large scale background stress field the declustered data are discretized with the k means algorithm into groups containing a number of focal mechanisms between a minimum number nmin and 2nmin synthetic tests indicate that nmin 30 provides stable inversion results under different stress regimes and noise conditions when r 0 5 while nmin 45 is needed for r near 0 or 1 additional synthetic tests compare the performance of selecting the fault plane of each focal mechanism using a the plane with lowest misfit angle between the slip vector from the focal mechanism and shear traction from the stress tensor and b the plane with highest instability coefficient representing proximity to the optimally oriented fault for given stress field and friction coefficient the instability criterion is found to provide more accurate inversion results under all tested stress regimes stress ratios and noise conditions the refined inversion methodology combines selecting fault planes using the instability criterion iteratively with a damped simultaneous inversion of different focal mechanism groups results characterizing neighborhoods of discretized domains merged during the damped inversion provide high resolution information independent of the discretization some aspects of the methodology are illustrated with focal mechanism data from the san jacinto fault zone in southern california

long term evolution of intraplate seismicity in stress shadows after a megathrust abstract megathrusts produce large permanent lithospheric displacements as well as strong transient ground shaking up to regional distances the influence of the 2011 m9 0 tohoku oki earthquake on the seismicity in stable intraplate regions around the korean peninsula is investigated the differential lateral displacements by the megathrust build transient radial tension field over the backarc lithosphere constructing stress shadows for all types of faults in optimal orientation over the wide backarc region including the southern korean peninsula a characteristic seismic velocity decrease after the megathrust supports the medium relaxation the number of earthquakes with magnitudes greater than or equal to 2 5 was increased by 61 after the megathrust the earthquakes occurred episodically a series of unusual earthquake swarms and a temporal cluster of moderate size earthquakes were observed in the yellow sea region the significant seismicity increase since the megathrust may be due to the fluid diffusion during the transient tension field and the pore fluid pressure increase during the ambient compressional stress field recovery by tectonic loading the induced seismicity may continue until the ambient stress field is fully recovered

an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence the orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and r 2 1 3 1 a measure of relative stress magnitudes under the assumption of uniform stress in the source region this analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation in the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions the method allows for errors in orientations of both the fault planes and slip directions we have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind by using a grid search of stress models rather than a linearization scheme we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses the method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population the technique has been applied to 76 events from the san fernando earthquake sequence for which we have found best fit stresses plunge and azimuth 1 7 187 2 27 281 3 62 84 and r 0 65 the average misfit between this stress model and all the data is about 8 and all but eight of the aftershocks have misfits of less than 20 these values are considerably less than the uncertainty of the focal mechanism determinations therefore significant stress inhomogeneities are not required by the data our analysis does not support the suggestion of a change in stresses during the aftershock sequence as proposed by others on the basis of an apparent change in focal mechanisms

african stress pattern from formal inversion of focal mechanism data abstract the kinematic models and the associated orientation of extensional stress of the east african rift system have been subjected to much debate since a long time in the past decades the proposed models relied on the interpretation of the overall rift geometry geological fault slip data and the few focal mechanisms available these models generally suffer of a poor time control and an underestimation of the possible changes in the stress field and geodynamic regime with time and space in the recent years there has been a significant increase in the number of focal mechanisms available for the entire rift system and it is now possible to estimate the present day stress field in relative detail based on seismotectonic data alone we compile 347 focal mechanism data from the global harvard cmt catalogue and various other sources and grouped 332 of them in 24 distinct regions boxes on the basis of their geographical proximity kinematic homogeneity and tectonic setting for each box and for the same data set reduced stress tensors have been obtained by formal stress inversion using both the tensor program delvaux sperner 2003 and the slick method michael 1984 1987 both inversion methods show in comparable results in terms of horizontal stress axes orientations and tectonic stress regimes which stem for the robustness of the approach the obtained stress pattern reflects a complex interaction between 1st order effects as different driving forces including plate boundary forces and 2nd and 3rd order effects as gravitational potential of topography intra lithospheric processes and the influence of structural heterogeneities of the rift structures the evidence present in the stress orientations of the 2nd and 3rd order stress pattern as the variations in the horizontal stress axes along the axis of the rift are of particular interest as they were not yet captured or shown in earlier numerical models of the stress field additional sources of tectonic forces in supplement to the gravitational potential energy forces as considered in the earlier models are necessary to explain the observed patterns

transition from rifted continental to oceanic crust at the southeastern korean margin in the east sea japan sea 1 the southeastern korean margin documents the processes of continental rifting and seafloor spreading that eventually led to the opening of the southern part of the east sea japan sea two dimensional crustal structure of the southeastern korean margin was computed from ocean bottom seismometer data by tomographic inversion and iterative forward modeling the crustal structure shows the emplacement of high velocity 7 km s lower crust under the continental shelf and slope area associated with a rapid transition from rifted continental to oceanic crust the high velocity lower crust is interpreted as magmatic underplating formed by voluminous igneous activity during rifting magnetic modeling confirms its primary correlation with a prominent magnetic anomaly along the edge of the korean peninsula we suggest that the rifting and subsequent seafloor spreading at the korean margin was significantly controlled by the supply of magma in a region of hotter than normal mantle temperature

stress tensor and fault plane solutions for a population of earthquakes an algorithm for the simultaneous estimation of the orientation and shape of the stress tensor and the individual fault plane solutions for a population of earthquakes is studied it corresponds to a synthesis of the methods used by brillinger et al 1980 to obtain focal mechanisms and by armijo and cisternas 1978 for stress tensor analysis in microtectonics the input data are the polarities of the p arrival and take off angles for the set of source station pairs the method distinguishes in general which one of the nodal planes corresponds to the fault and gives the direction of the slip the application to the aftershock sequence of the 1980 arudy earthquake western pyrenees shows that the observations may be explained by a single stress tensor producing a n32 e extension with a likelihood of 95 per cent

seismological constraints on the collision belt between the north and south china blocks in the yellow sea abstract the korean peninsula eastern china and the yellow sea comprise the eastern eurasian plate and are believed to share considerable tectonic evolution history the tectonic structures in the yellow sea are poorly understood raising difficulty in reconstruction of tectonic evolution history in the eastern eurasian plate the tectonic structures in the yellow sea are constrained by seismicity and fault plane solutions of earthquakes the fault plane solutions are determined by waveform inversions and seismic phase polarity analyses the ambient stress fields are deduced from the fault plane solutions the primary stress field around the yellow sea is composed of ene wsw directional compression and nnw sse directional tension normal faulting earthquakes with ene wsw directional strikes are observed in the central yellow sea between the shandong peninsula and the central korean peninsula the normal faulting region is interpreted to be a northern margin of collision belt between the north and south china blocks the normal faulting system suggests post collisional lithospheric delamination causing reverse activation of paleo thrustal faults that were developed by the collision between the north and south china blocks in the early jurassic period