regional source scaling of the 9 october 2006 underground nuclear explosion in north korea abstract the 9 october 2006 underground nuclear explosion une test in north korea was well monitored by dense regional seismic stations in south korea japan and china this observation allows extensive investigation of the regional source properties of the une the moment for isotropically radiated energy from the une is estimated to be 2 92 10 14 n m source spectra of major regional phases from the une are studied by inverting for apparent moments corner frequencies overshoot parameters attenuation factors and frequency power dependence parameters the overshoot parameters of p phases from the une are estimated to be high while those of s phases are estimated to be significantly low the inverted source spectra agree well with conventional models the low overshoot parameters of s phases suggest that their excitation sources may be different from those for pn and pg it is shown that pn lg and pg lg amplitude ratios are useful for discriminating between unes and natural earthquakes in the frequencies of 1 8 hz

venezuela basin crustal structure velocity depth profiles derived from six two ship expanding spread experiments in combination with other geophysical data define the characteristics of two distinct types of venezuela basin crust and the boundary between them each two ship common midpoint reflection refraction profile is automatically transformed into the p plane picked and interpreted to provide v z functions with appropriate confidence bounds the results together with gravity magnetic and near vertical incidence reflection data reveal a 50 000 km2 triangle of venezuela basin crust which resembles normal oceanic crust in a magnetic quiet zone north and west of this triangle lies the previously defined thick caribbean crust having two distinct layers above the m discontinuity acoustic basement there appears unusually smooth due to extensive basaltic sills and flows which were cored at deep sea drilling project sites 146 149 sills and 150 flows also depths to mantle are greater than normal interpretations of near vertical and wide angle reflection data show that the extra crustal thickness is due not only to the emplacement of the flows but also to the crust below being somewhat thicker than normal the boundary between the two crustal areas has a ne sw trend which parallels the dominant structural and magnetic lineations this boundary coincides in position though not in trend with the previously defined central venezuela basin fault zone further study is required to determine whether this boundary is of tectonic origin or if it represents a change in style of crustal production

paleomagnetism of jurassic volcanic rocks in the patagonia mountains southeastern arizona implications for the north american 170 ma reference pole paleomagnetic study of middle jurassic volcanic and volcaniclastic rocks in southeastern arizona yields a paleomagnetic pole located at 61 8 n 116 0 e k 49 6 95 6 2 these rocks from corral canyon represent an autochthonous segment of the jurassic cordilleran magmatic arc and were deposited directly on upper paleozoic carbonates of the cratonic miogeocline rock magnetic experiments demonstrate that the natural remanent magnetization of these rocks is dominantly a single component magnetization carried in both magnetite and hematite phases and is interpreted as a primary thermoremanent magnetism rb sr isotope geochemistry on whole rock samples from eight welded ash flow tuffs yields an isochron age of 172 2 5 8 2 ma the corral canyon pole falls between poles from newark trend group ii 179 3 ma and glance conglomerate 151 2 ma

crustal structure in southern korea from joint analysis of teleseismic receiver functions and surface wave dispersion we estimated crustal structures under 18 broadband stations in southern korea by combining receiver functions and surface wave dispersion with the genetic algorithm ga estimated crustal structures were analyzed together with previously determined structures under four stations gkp incn snu and tjn in chang et al 2004 the trend of moho depths estimated from the ga inversion generally coincides with the surface topography ranging from 26 km to 36 km in the inland however the moho depth distribution does not agree with the topography in the region around the chugaryeong fault which extends approximately north northeast south southwest in the central korean peninsula the shallow moho depth under this region may be related to consequential crustal thinning processes along the fault caused by extensional tectonic movements another discrepancy is found in the gyeongsang basin formed in a retroarc setting by the subduction of the izanagi plate in the early cretaceous a thick crust observed in the basin may be caused by two factors maturity of the basin and underplating of magma materials average crustal velocities vary from 6 02 km sec to 6 51 km sec in southern korea this variation indicates that crustal structures in southern korea involve diverse velocity profiles that change rapidly with distance remarkably a clear velocity discontinuity is observed at the depth range of 8 10 km under several stations

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

evolution of the sea of japan back arc and some unsolved issues abstract the sea of japan back arc system provides an exceptional opportunity to study a virtually intact continent ocean back arc system in which recent tectonic inversion has exposed entire sequences of back arc structure on land moreover japan s dense seismic geodetic monitoring networks deployed country wide as well as moveable pools of ocean bottom seismometers provide a rich data set through which to investigate deep back arc structure earlier investigations have produced an initial understanding of back arc opening timing structural evolution temporal spatial patterns of magmatic activity many questions remain among them the mechanism of back arc opening pull apart or trench rollback the dynamics of interacting plates location of the philippine sea plate with time the origin of the anomalously thick japan sea ocean crust and possible influences of far field forces india asia collision given existing high resolution geophysical data sets and extensive on land exposures of back arc sequences and structures the sea of japan back arc is a promising context in which to address both local and more universal questions of how back arc systems evolve here we review the tectonic setting and geological evolution of the sea of japan based our own and others work and briefly discuss outstanding questions that invite further investigation

crustal structure of a nonvolcanic rifted margin east of newfoundland a detailed seismic refraction study was undertaken of the rifted continental margin southeast of the grand banks of newfoundland along a preexisting deep multichannel seismic reflection profile the primary objectives were to determine the topography of moho across the margin to detect any evidence for crustal underplating and to determine the position of the continent ocean boundary data were obtained from nine ocean bottom seismometers deployed along the profile with an array of six 1000 inch3 air guns as the source two additional refraction lines were shot parallel to structure over the continental slope the resultant velocity model is well constrained and consistent with the observed gravity the crust thins abruptly beneath the continental slope to a major structural transition identified as the continent ocean boundary there is no significant magmatic underplating of the thinned continental crust seaward of the transition the seismic velocity structure is characterized by a 50 km wide zone of intermediate velocity 7 2 7 6 km s beyond this is a thin 3 5 km crust with low velocity 4 5 5 km s and high gradient over mantle with a velocity of 7 7 km s increasing to 8 0 km s the zone of intermediate velocity is interpreted to be highly serpentinized mantle material beneath a very thin crust suggesting little or no magmatic activity during and immediately after rifting this interpretation is consistent with results from adjacent and conjugate margins

pn travel time tomography of the paleo continental collision and rifting zone around korea and japan abstract a recent dense deployment of seismic stations in south korea and japan allows regional seismic imaging of the far east asian region that experienced continental collisions and riftings we perform seismic imaging based on a mantle lid p wave pn travel time tomography to exploit the tectonic imprints in the lithosphere the average pn velocity in the region is estimated to be 7 95 0 03 km sec the inverted pn velocities illuminate the tectonic structures high velocities of 8 15 km sec are observed in the precambrian massif regions while low velocities of 7 8 km sec are associated with the fold belt and sedimentary basin regions in the southern korean peninsula the pn velocity is estimated to be low in the backarc basins including the ulleung and the yamato basins while it is estimated to be higher on the continental fragments including the south korea plateau and the oki bank the high velocity structures along the japanese islands support the separation of the japanese province from the eurasia plate the high velocity anomaly along the east coast of the korean peninsula around the hupo bank suggests intrusion and solidification of high density material in the lower crust and mantle lid

vp vs ratios in the upper crust of the southern korean peninsula and their correlations with seismic and geophysical properties abstract the p and s velocity ratio v p v s is a seismic property that is convertible to poisson s ratio a widely analyzed physical property of elastic materials the lateral variation in the v p v s ratios of the upper crust of the southern korean peninsula is investigated based on the travel times of p and s waves of local events with epicentral distances of 50 km or less and focal depths of 25 km or less the dense seismic networks in the korean peninsula allow us to investigate the regional variation of v p v s ratios these v p v s ratios are estimated to be 1 60 1 91 with an average of 1 69 the v p v s ratios are as high as 1 73 1 91 in the gyeongsang basin jeju island and offshore region of the eastern okcheon belt but have relatively low values of 1 64 1 72 in precambrian massif regions the stability of the v p v s ratios is tested with randomly resampled travel time data the results of which suggest nearly constant v p v s ratios with increasing depth in the upper crust the influences of plausible errors on origin times and arrival times are quantified to verify the v p v s estimates the v p v s ratios are correlated with geological and tectonic structures comparisons with known seismic and geophysical properties suggest that structural features observed on the surface may extend at least to the mantle lid

regional seismic observations of the 9 october 2006 underground nuclear explosion in north korea and the influence of crustal structure on regional phases 1 the crustally guided shear wave lg is typically the most prominent phase of a nuclear explosion at regional distance this lg phase is analyzed often to discriminate a nuclear explosion from a natural earthquake in addition the lg phase allows us to determine the size of the detonation a nuclear explosion test in north korea was conducted on 9 october 2006 the epicenter was located close to the eastern shore of the korean peninsula resulting in raypaths that vary significantly according to the azimuths in particular rays radiated in the southern direction experience lateral variation of crustal structures at the continental margin we examine the influence of raypaths on regional seismic phases by comparing the spectra and waveforms from different raypaths three natural earthquakes in north korea are also examined to determine the raypath effect we find that the lg from the nuclear explosion dissipated significantly as result of energy leakage into the mantle resulting from variations in crustal thickness along the portion of the raypath traversing the western tip of the sea of japan east sea some of the leaked energy develops into mantle lid waves sn causing a large energy increase to sn a similar feature is observed in the records of natural earthquakes this feature is confirmed by seismic waveform modeling the raypath effect also causes underestimation of magnitude the lg body wave magnitude mb lg is estimated to be 3 8 4 2 for records from pure continental paths and 2 6 3 4 for records from paths crossing continental margins this result illustrates the need to consider raypath effects for the correct estimation of magnitudes of regional events including a nuclear explosion

influence of continental margin on regional seismic wavefield abstract regional seismic analysis is an only way to investigate the source properties of small or moderate sized seismic events that occur in physically unaccessible regions such as oceanic regions and politically prohibited regions the ground motions by earthquakes need to be understood for better mitigation of seismic hazards particularly in continental margins that often incorporate high seismicity it has not been fully understood how seismic waves interact with complex crustal structures in continental margins responses of regional waves to complex crustal structures need to be quantified analysis of regional waveforms from controlled sources is desirable for investigation of influence of crustal structures the influence of continental margin around the korean peninsula is investigated in terms of spectral contents horizontal to vertical h v spectral ratios and quality factors using regional waveforms for the 2009 north korean underground nuclear explosion test that was well recorded by stations in the southern korean peninsula regional waveforms and spectral amplitudes vary significantly by path spectral contents of regional phases are different among stations in common great circle directions all regional phases modulate highly in continental margins path dependent seismic attenuation is strong in low frequencies 3 hz and weak in high frequencies 3 hz continental margins cause directional energy partition of regional waves depending on the path regional waves attenuate highly in passage across continental margin and then regrow on continental paths the growth rate is stronger than inherent attenuation rate causing seismic amplification the shapes of h v ratios are similar among various regional phases at common stations on the other hand the h v ratios for common phases vary by station in the same azimuths characteristic differences in h v ratios between two horizontal components suggest directional partition of seismic energy by crustal structures in the paths

japan sea opening history and mechanism a synthesis the respective tectonic effects of back arc spreading and continental collision in asia are considered either as two independent processes or as closely interrelated extrusion tectonics assumes that the opening of the south china sea and the left lateral motion along the red river fault are geometrically linked in a pull apart manner this model is not accepted by several workers because the structural link between the two processes is not clearly demonstrated in the case of the japan sea we can show without ambiguity that back arc opening was controlled by large intracontinental strike slip faults which can be easily understood as effects of the india asia collision far from the indenter the japan sea opened in the early miocene in a broad pull apart zone between two major dextral strike slip shear zones the first one extends from north sakhalin to central japan along 2000 km it has accommodated about 400 km of finite displacement deformation along it varies from dextral transpression in the north to dextral transtension in the south the second is between korea and sw japan and has accommodated a smaller displacement of about 200 km the extensional domain in between lies in the back arc region of japan distributed stretching of the arc crust resulted in the formation of most of the japan sea while localized oceanic spreading at the southern termination of the eastern transpressional shear zone shaped the japan basin the first oceanic crust was formed in a small triangle based on the eastern shear zone and spreading propagated westward inside the pull apart region timing of oceanic crust formation of formation of the dextral shear zones and of block rotation in between as well as the internal structure of the basins and the geometry of deformation along the master shear zones are used to reconstruct the opening history this evolution is discussed by comparison to other manifestations of the arc and back arc activity such as the history of sedimentation and volcanism the paper then suggests that the collision of india can have tectonic consequences as far north as japan and sakhalin and describes the geometrical relation of back arc opening there and diffuse extrusion

evolution of the eastern margin of korea constraints on the opening of the east sea japan sea abstract we interpreted marine seismic profiles in conjunction with swath bathymetric and magnetic data to investigate rifting to breakup processes at the eastern korean margin that led to the separation of the southwestern japan arc the eastern korean margin is rimmed by fundamental elements of rift architecture comprising a seaward succession of a rift basin and an uplifted rift flank passing into the slope typical of a passive continental margin in the northern part rifting occurred in the korea plateau that is a continental fragment extended and partially segmented from the korean peninsula two distinguished rift basins onnuri and bandal basins in the korea plateau are bounded by major synthetic and smaller antithetic faults creating wide and considerably symmetric profiles the large offset border fault zones of these basins have convex dip slopes and demonstrate a zig zag arrangement along strike in contrast the southern margin is engraved along its length with a single narrow rift basin hupo basin that is an elongated asymmetric half graben analysis of rift fault patterns suggests that rifting at the korean margin was primarily controlled by normal faulting resulting from extension rather than strike slip deformation two extension directions for rifting are recognized the onnuri and hupo basins were rifted in the east west direction the bandal basin in the east west and northwest southeast directions suggesting two rift stages we interpret that the east west direction represents initial rifting at the inner margin while the japan basin widened rifting propagated southeastward repeatedly from the japan basin toward the korean margin but could not penetrate the strong continental lithosphere of the korean shield and changed the direction to the south resulting in east west extension to create the rift basins at the korean margin the northwest southeast direction probably represents the direction of rifting orthogonal to the inferred line of breakup along the base of the slope of the korea plateau after breakup the southwestern japan arc separated in the southeast direction indicating a response to tensional tectonics associated with the subduction of the pacific plate in the northwest direction no significant volcanism was involved in initial rifting in contrast the inception of sea floor spreading documents a pronounced volcanic phase which appears to reflect asthenospheric upwelling as well as rift induced convection particularly in the narrow southern margin we suggest that structural and igneous evolution of the korean margin although it is in a back arc setting can be explained by the processes occurring at the passive continental margin with magmatism influenced by asthenospheric upwelling

pore fluid chemistry from the second gas hydrate drilling expedition in the ulleung basin ubgh2 source mechanisms and consequences of fluid freshening in the central part of the ulleung basin east sea during the second gas hydrate drilling expedition in the ulleung basin ubgh2 pore fluids were collected from sites that sampled sediments from the continental slope to the basin center and included locations within and outside seismic identified chimney like features at five non chimney sites established during the ubgh2 discrete excursions to low cl concentrations within the gas hydrate occurrence zone ghoz are associated with water enriched in d and 18o as expected from gas hydrate dissociation during core recovery estimates of gas hydrate saturation values sh based on cl and d values are in good agreement and show that when gas hydrate occurs in discrete horizons or within strata bound fractures throughout the ghoz it occupies 5 65 of the pore space fluids recovered from three sites characterized by seismic inferred chimneys show massive hydrate accumulation near the seafloor accompanied by the formation of shallow cl rich brines with d and 18o values lower than seawater chemical and isotopic data from fluids sampled at all drilled sites reveal significant fluid freshening below the gas hydrate stability zone ghsz the distribution of cl k and d indicate that the observed freshening is predominantly caused by clay mineral dehydration illitization previously reported heat flow values in the center of the basin indicate that the sediments deeper than 800 mbsf meter below seafloor are warm enough to expect clay diagenesis with the onset of smectite to illite transformation source fluid temperatures 70 c are confirmed through the use of na li and mg2 li geothermometry based on these observations we propose the following scenario for the evolution of formation fluids in the ulleung basin where high formation temperatures and rapid sedimentation at the basin center led to sediment compaction and clay dehydration reactions that result in fluid overpressures and associated fracturing of the formation sediment fractures and fluid possible gas migration in the chimney features imaged in seismic records are consistent with our postulated upward transport of fluids from 800 m depth though the ghsz and to the seafloor

crustal structure of the north iberian continental margin from seismic refraction wide angle reflection profiles abstract the structure and geodynamics of the southern margin of the bay of biscay have been investigated from a set of 11 multichannel seismic reflection profiles recorded also at wide angle offsets in an onshore offshore network of 24 obs obh and 46 land sites this contribution focuses on the analysis of the wide angle reflection refraction data along representative profiles the results document strong lateral variations of the crustal structure along the margin and provide an extensive test of the crustal models previously proposed for the northern part of the iberian peninsula offshore the crust has a typical continental structure in the eastern tip of the bay which disappears smoothly towards the nw to reach crustal thickness close to 10 km at the edge of the studied area 45 n 6 w the analysis of the velocity depth profiles altogether with additional information provided by the multichannel seismic data and magnetic surveys led to the conclusion that the crust in this part of the bay should be interpreted as transitional from continental to oceanic typical oceanic crust has not been imaged in the investigated area onshore the new results are in good agreement with previous results and document the indentation of the bay of biscay crust into the iberian crust forcing its subduction to the north the interpreted profiles show that the extent of the southward indentation is not uniform with an alpine root less developed in the central and western sector of the basque cantabrian basin n s to ne sw transfer structures seem to control those variations in the indentation degree

the transition from a volcanic to a nonvolcanic rifted margin off eastern canada three multichannel seismic reflection profiles were collected on the rifted continental margin southeast of nova scotia eastern canada the profiles cross the east coast magnetic anomaly ecma which parallels much of the margin of eastern north america south of the grand banks and which is usually associated with the transition from continental to oceanic crust studies to the south of the work reported here suggest that the ecma may be related to the emplacement of large thicknesses of late rift stage or early drift stage igneous material which is characterized by seaward dipping reflections in basement and a high velocity lower crustal layer the seismic data show that seaward dipping reflections sdr continue northward into the study area and support the correlation between the sdr unit and the presence of a well developed ecma magnetic modeling confirms this association although it does not rule out an additional contribution to the magnetic anomaly from an edge effect or suture just north of the study area the ecma diminishes and is no longer well developed the sdr unit also terminates and it is not observed over most of the nova scotian margin if our understanding of the origin of these features is correct then their disappearance marks a transition from a volcanic margin in the south to a nonvolcanic margin in the north the association of the transition with significant changes in the prerift fabric of the adjacent continental crust in the trend of synrift extensional structures and in the width of the zone of thinned continental crust below the margins must be clues to the deeper processes controlling the amount of volcanism produced we suggest that these clues are consistent with small scale convection as a mechanism for delivering large melt volumes to crustal depths during rifting

seismic indicators of gas hydrate and associated gas in the ulleung basin east sea japan sea and implications of heat flows derived from depths of the bottom simulating reflector abstract analysis of multi channel seismic reflection data from the ulleung basin east sea japan sea reveals various seismic indicators of gas hydrate and associated gas including the bottom simulating reflector bsr enhanced reflections below the bsr and seismic chimneys the recent recovery of massive gas hydrate by drilling together with these seismic indicators strongly suggests favorable conditions for the formation of gas hydrate in the area the bsr is most common and of a wide range of amplitude and continuity seismic chimneys characterized by columnar zones of amplitude reduction terminate mostly within the gas hydrate stability zone ghsz but a few extend to pockmarks or mounds on the seafloor seismic chimneys with seafloor expressions probably represent vertical vents for gas originating from below the ghsz the increase in the degree of amplitude reduction in seismic chimneys with increasing seismic frequency may be due to seismic attenuation by gas bubbles trapped in hydrate filled coated fractures in the ghsz bsr derived heat flows range from about 65 to over 115 mw m2 comparable to those from direct measurements heat flows in the northern part of the study area are very high for the age of the basin suggesting hotter than normal mantle temperature the area of high 105 mw m2 heat flows also coincides largely with the distribution of the presumed incipient oceanic crust formed during the earliest stage of seafloor spreading

three dimensional seismic refraction tomography a comparison of two methods applied to data from the faeroe basin this paper presents a comparison of two tomographic methods for determining three dimensional 3 d velocity structure from first arrival travel time data the first method is backprojection in which travel time residuals are distributed along their ray paths independently of all other rays the second method is regularized inversion in which a combination of data misfit and model roughness is minimized to provide the smoothest model appropriate for the data errors both methods are nonlinear in that a starting model is required and new ray paths are calculated at each iteration travel times are calculated using an efficient implementation of an existing method for solving the eikonal equation by finite differencing both inverse methods are applied to 3 d ocean bottom seismometer obs data collected in 1993 over the faeroe basin consisting of 53 479 travel times recorded at 29 obss this is one of the most densely spaced large scale 3 d seismic refraction experiments to date different starting models and values for the free parameters of each tomographic method are tested a new form of backprojection that converges more rapidly than similar methods compares favorably with regularized inversion but the latter method provides a simpler model for little additional computational expense when applied to the faeroe basin data bounds on two model features are assessed using regularized inversion with combined smoothness and flatness constraints an inversion of synthetic data corresponding to 100 data recovery from the real experiment shows a marked improvement in lateral resolution at deeper depths and demonstrates the potential of currently feasible 3 d refraction experiments to provide well resolved long wavelength velocity models the similarity of the final models derived from the two tomographic methods suggests that the results from the new form of backprojection can be relied on when limited computational resources rule out regularized inversion

crustal structure volcanism and opening mode of the ulleung basin east sea sea of japan abstract a study of the ulleung basin in the southwestern east sea sea of japan based on single and multi channel seismic reflection profiles and ocean bottom seismometer obs data suggests that the basin formed largely by pull apart opening and the deep northern basin is underlain by thicker than normal oceanic crust 10 km thick probably formed during the earliest stage of seafloor spreading volcanic activity in association with basin opening occurred over much of the basin but it ceased first in the south while it continued in the north until recently the principal evidence for pull apart opening includes 1 an overall rhomboidal shape of the basin consisting of two basement lows and a median high known to be characteristic of pull apart basins 2 the location and orientation of the major volcanic sources suggesting more or less n s opening and 3 the strike slip fault system along the western basin margin which appears to have guided the pull apart opening the lack of extensional deformation along the western basin margin which would have been prevalent if sw japan had rotated away from the korean peninsula as suggested by paleomagnetic data is also not in favor of the alternative rotational opening of the basin the absence of graben and rift topography in the acoustic basement in the deep northern ulleung basin is indicative of the non brittle or ductile nature of the crust tau p analysis of the obs data reveals that the igneous crust consists of upper and lower layers with velocities typical of oceanic layers the velocity depth profiles with a two gradient velocity structure is also characteristic of oceanic crust however the lower layer is much thicker than oceanic layer 3 and the change of velocity gradient which occurs at around 6 5 km s in typical oceanic crust occurs at about 6 km s suggesting that the crust is not a typical oceanic crust the meager basin size and short opening period may suggest that the basin opening did not lead to the formation of full fledged oceanic crust but instead resulted in embryonic or incipient oceanic crust thickened by upper mantle thermal perturbations evoked by rifting

tectonic sedimentary and volcanic evolution of a back arc basin in the east sea sea of japan abstract this paper focuses on the tectonic sedimentary and volcanic evolution of a unique back arc basin ulleung basin in the southwestern part of the east sea sea of japan the basin consists of thick extended continental or transitional crust and an overlying sedimentary succession 4 8 km thick with interlayered volcanic flows and sills defining a number of seismic units of variable reflection characteristics the northern margin is bounded by faulted continental blocks south korea plateau with isolated basement lows and sub basins with intruded and extruded volcanics whereas the southern margin is underlain by deep seated basement with a thick 8 km thick sedimentary succession the western margin is bounded by a series of strike slip and normal faults produced by nnw sse directed dextral movements these structural features suggest a south southeastward drift of the southwestern japanese arc away from the south korea plateau during the early to middle miocene involving large scale right lateral strike slip deformation along the western margin akin to pull apart basin formation during the back arc opening the thinned continental crust was largely modified by intrusive volcanics under tensional stress regime the volcanics also extruded across the axis of extension as well as along the boundary faults on the west in the middle to late miocene major faults in the southern and western margins were inverted forming partial closure of the basin especially in the southern part although the plateau in the northern margin experienced continuous subsidence the ulleung basin thus provides an example of an immature back arc basin which experienced a rather brief episode of rifting and extension followed by closure due to tectonic inversion

lithospheric folding based understanding on the origin of the back arc basaltic magmatism beneath jeju volcanic island 1 compression near collisional plate boundaries causes lithospheric folding which results in the decrease of the pressure beneath the ridge of the fold while the pressure beneath the trough increases the decompression beneath the lithosphere is likely to bring on basaltic magmatism along and below the ridge we investigate the subsurface structure beneath jeju cheju volcanic island and its vicinity and propose an alternative hypothesis that the basaltic magma beneath the island could be caused by episodic lithospheric folding unlike the prevailing hypothesis of the intraplate basaltic magmatism that requires extending lithosphere our hypothesis can explain how the basaltic magma could be generated at the back arc regions without the extension

neogene strike slip faulting in sakhalin and the japan sea opening we describe structural data from a 2000 km n s dextral strike slip zone extending from northern sakhalin to the southeast corner of the japan sea satellite images field data and focal mechanisms of earthquakes in sakhalin are included in the interpretation since miocene time the deformation in sakhalin has been taken up by n s dextral strike slip faults with a reverse component and associated en echelon folds narrow en echelon neogene basins were formed along strike sup faults and were later folded in a second stage of deformation we propose a model of basin formation along extension al faults delimitating dominos between two major strike slip faults and subsequent counterclockwise rotation of the dominos in a dextral transpressional regime basins becoming progressively oblique to the direction of maximum horizontal compression and undergoing shortening the association of both dextral and compressional focal mechanisms of earthquakes indicates that the same transpressional regime still prevails today in sakhalin we present fault set measurements undertaken in noto peninsula and yatsuo basin at the southern end of the sakhalin east japan sea strike slip zone early and middle miocene formations recorded the same transtensional regime as observed along the west coast of ne honshu during the early and middle miocene the strike slip regime was transpressional to the north in sakhalin and hokkaido and transtensional to the south along the west coast of ne honshu as far as noto peninsula and yatsuo basin dextral motion accommodated the opening of the japan sea as a pull apart basin with the tsushima fault to the west the opening of the japan sea ceased at the end of the middle miocene when transtension started to change to e w compression in the japan arc subduction of the japan sea lithosphere under the japan arc started 1 8 ma ago the evolution of the stress regime from transtensional to compressional in the southern part of the strike slip zone is related to the inception of the subduction of the young philippine sea plate lithosphere under the japan arc during the late miocene subduction related extension is a necessary condition for the opening of the japan sea two possible mechanisms can account for dextral shear in this area 1 counterclockwise rotation of crustal blocks due to the collision of india with asia 2 extrusion of the okhotsk sea block squeezed between the north america and eurasia plates

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

amount of clockwise rotation of southwest japan fan shape opening of the southwestern part of the japan sea abstract paleomagnetic measurements have been carried out on welded tuffs ranging in age between 58 ma and 112 ma from yamaguchi and go river areas in the central part of southwest japan the new data together with those of younger igneous rocks published previously define the change of paleomagnetic field direction during the late mesozoic cenozoic period for southwest japan the paleomagnetic direction from this area has pointed 56 3 clockwise from the expected field direction estimated from apwp apparent polar wandering path of the whole of eurasia during the period between 100 and 20 ma in comparison with the expected one from the eastern margin of eurasia korea china siberia the cretaceous field direction of southwest japan shows the clockwise deflection by 44 49 these results establish that while the eastern margin of eurasia including southwest japan was rotated more or less with respect to the main part of eurasia during last 100 ma southwest japan was rotated clockwise through more than 40 with respect to the eastern margin of eurasia since 20 ma the large amount of rotation for southwest japan implies that it is rotated by an opening of the southwestern part of the japan sea which widens northeastward fan shape opening the tectonic feature of southwest japan and the japan sea is analogous to that of corso sardinia and the ligurian sea in the mediterranean indicating that the fan shape opening is a specific feature of the rifting of the continental sliver at the continental rim

gulf stream excursions and sectional detachments generate the decadal pulses in the atlantic multidecadal oscillation abstractdecadal pulses within the lower frequency atlantic multidecadal oscillation amo are a prominent but underappreciated amo feature representing decadal variability of the subpolar gyre e g the great salinity anomaly of the 1970s and wielding notable influence on the hydroclimate of the african and american continents here clues are sought into their origin in the spatiotemporal development of the gulf stream s gs meridional excursions and sectional detachments apparent in the 1954 2012 record of ocean surface and subsurface salinity and temperature observations the gs excursions are tracked via meridional displacement of the 15 c isotherm at 200 m depth the gs index whereas the amo s decadal pulses are targeted through the amo tendency which implicitly highlights the shorter time scales of the amo index the gs s northward shift is shown to be preceded by the positive phase of the low frequency north atlantic oscillation lf nao and followed by a positive amo tendency by 1 25 and 2 5 yea

lg attenuation in a region with both continental and oceanic environments abstract the crustally guided shear wave lg is typically the strongest phase at regional distances lg phases are analyzed often for estimation of magnitudes of regional events the variation of lg in regions with both continental and oceanic environments has been rarely investigated we investigate lg attenuation in a plate margin area around korea and japan that is encompassed by seas and oceans the mean quality factor of lg at 1 hz q 0 is 498 the geometrical spreading exponent term is estimated as 0 75 these observations characterize the study region as a high attenuation region compared to typical intraplate continental regions low q regions are widely developed over the japanese islands with some spotted high q regions relatively high q is observed in most regions of the korean peninsula except a southeastern part where a cretaceous volcanic sediment basin exists the high q values are close to the typical q value in continental crust significantly low q of 100 or less is observed in most oceanic regions including the east sea sea of japan the pacific ocean and the south china sea the high attenuation of lg in oceanic regions suggests possible underestimation of magnitudes of oceanic events thus it appears that proper correction of lg amplitude is highly desired for accurate estimation of magnitudes of regional oceanic events

crustal structure of the southern korean peninsula from seismic waves generated by large explosions in 2002 and 2004 in order to investigate the velocity structure of the southern part of the korean peninsula seismic refraction profiles were obtained along a 294 km wnw ese line and a 335 km nnw sse line in 2002 and 2004 respectively seismic waves were generated by detonating 500 1000 kg explosives in drill holes at depths of 80 150 m the seismic signals were recorded by portable seismometers at nominal intervals of 1 5 1 7 km separate velocity tomograms were derived from first arrival times using a series expansion method of travel time inversion the raypaths indicate several mid crust interfaces including those at approximate depths of 2 3 15 17 and 22 km the moho discontinuity with refraction velocity of 7 8 to 8 4 km s has a maximum depth of 37 39 km under the southern central portion of the peninsula the moho becomes shallower as the yellow sea and the east sea are approached on the west and east coasts of the peninsula respectively the depth of the 7 6 km s velocity contour varies from 29 4 km to 36 5 km the discrepancy in depth between the seismological moho and the interpreted critically refracting interface may result from the presence of a gradual transition between the crust and mantle the velocity tomograms show particular crustal structures including 1 the existence of an over 70 km wide low velocity zone centered at 6 7 km depth under the okchon fold belt and ryeongnam massif 2 existence of high velocity materials under the gyeongsang basin and 3 the downward extension of the yeongdong fault to depths greater than 10 km

reconstruction of subduction and back arc spreading in the nw pacific and aleutian basin clues to causes of cretaceous and eocene plate reorganizations the eocene 50 45 ma major absolute plate motion change of the pacific plate forming the hawaii emperor bend is thought to result from inception of pacific plate subduction along one of its modern western trenches subduction is suggested to have started either spontaneously or result from subduction of the izanagi pacific mid ocean ridge or from subduction polarity reversal after collision of the olyutorsky arc that was built on the pacific plate with ne asia here we provide a detailed plate kinematic reconstruction of back arc basins and accreted terranes in the northwest pacific region from japan to the bering sea since the late cretaceous we present a new tectonic reconstruction of the intraoceanic olyutorsky and kronotsky arcs which formed above two adjacent oppositely dipping subduction zones at 85 ma within the north pacific region during another pacific wide plate reorganization we use our reconstruction to explain the formation of the submarine shirshov and bowers ridges and show that if marine magnetic anomalies reported from the aleutian basin represent magnetic polarity reversals its crust most likely formed in an 85 to 60 ma back arc basin behind the olyutorsky arc the olyutorsky arc was then separated from the pacific plate by a spreading ridge so that the 55 to 50 ma subduction polarity reversal that followed upon olyutorsky ne asia collision initiated subduction of a plate that was not the pacific hence this polarity reversal may not be a straightforward driver of the eocene pacific plate motion change whose causes remain enigmatic

crustal structure of the southeast georgia embayment carolina trough preliminary results of a composite seismic image of a continental suture and a volcanic passive margin new deep penetration multichannel seismic reflection data combined with refraction results and magnetics modeling support a hypothesis that the carolina trough is a mesozoic volcanic passive margin exhibiting a seaward dipping wedge and associated underplating the structure of carolina platform continental crust is consistent with the late paleozoic continental collision that produced the appalachians but imbrication has had no obvious effect on shallower structures produced by mesozoic extension and volcanism the origin of prominent magnetic anomalies crossing the southeast georgia embayment can be explained by processes attending mesozoic separation of africa and north america and is not related to a paleozoic continental suture as previously postulated