a study of the regional variation of low frequency around the korean peninsula we studied regional graphic 1 at 1 hz graphic 2 around the korean peninsula based on broadband vertical component seismic records of six incorporated research institutions for seismology iris global seismographic network stations and 19 korean stations of the korea institute of geoscience and mineral resources kigam using 177 seismic events with magnitudes between 5 3 and 5 7 and depths less than 50 km the reversed two station method was applied and 94 high quality interstation paths were selected from 869 possible pairs these results show high graphic 3 paths around the sea of japan east sea reflecting the typical oceanic structure and low graphic 4 paths around northeastern china related to inactive seismicity assigning these path values into 193 cells around south korea with a size of 1 by 1 we observed that the regional graphic 5 decreased gradually from east to west between 2 and 1 10 3 1 embed inline graphic 2 gif 2 embed inline graphic 3 gif 3 embed inline graphic 4 gif 4 embed inline graphic 5 gif 5 embed inline graphic 6 gif

crustal characteristic variation in the central yamato basin japan sea back arc basin deduced from seismic survey results abstract the crustal structure of the yamato bank the central yamato basin and the continental shelf in the southern japan sea back arc basin is obtained based on a seismic survey using ocean bottom seismographs and seismic shot to elucidate the back arc basin formation processes the central yamato basin can be divided into three domains based on the crustal structure the deep basin the seamount and the transition domains in the deep basin domain the crust without the sedimentary layer is about 12 13 km thick very few units have p wave velocity of 5 4 6 0 km s which corresponds to the continental upper crust in the seamount and transition domains the crust without the sedimentary layer is about 12 16 km thick the p wave velocities of the upper and lower crusts differs among the deep basin the seamount and the transition domains these results indicate that the central yamato basin displays crustal variability in different domains the crust of the deep basin domain is oceanic in nature and suggests advanced back arc basin development the seamount domain might have been affected by volcanic activity after basin opening in the transition domain the crust comprises mixed characters of continental and oceanic crust this crustal variation might represent the influence of different processes in the central yamato basin suggesting that crustal development was influenced not only by back arc opening processes but also by later volcanic activity in the yamato bank and continental shelf the upper crust has thickness of about 17 18 km and p wave velocities of 3 3 4 1 to 6 6 km s the yamato bank and the continental shelf suggest a continental crustal character

implications of the north and south panama thrust belts for the origin of the panama orocline high resolution imaging of the north nptb and south panama thrust belts using the seamarc ii seafloor mapping system and digital seismic reflection profiling provides an excellent data base with which to determine the kinematics of oroclinal deformation of panama remarkable continuity of structural orientations over 300 km along the nptb argues for widespread uniformity in direction of thrusting between panama and the caribbean plate westward decrease in the width and height of the accretionary wedge indicates a westward decrease in the total amount of convergence two major cross faults cut the wedge in its eastern part forming two minor blocks with different relative motions with respect to the caribbean frontal thrust orientations change across these faults major faults have been proposed to cut the isthmus of panama but simple cross isthmus faulting alone cannot explain the orientations of the major thrusts of the nptb bending of a flexible beam can explain much of the observed structure both directions and rates though the scale of the orocline precludes considering this as an elastic phenomenon plastic deformation of panama may be a useful mechanical explanation at least for eastern panama but consideration of plastic slip lines must include the northern andes as well changing from more rigid blocklike offsets on faults in western panama to a continuum of strain in eastern panama may be the best explanation for observations at present

last stage of the japan sea back arc opening deduced from the seismic velocity structure using wide angle data the japan sea is one of the most well studied back arc basins in the northwestern pacific the seismic crustal model however has been inadequate to elucidate the detailed opening model of the japan sea in 2002 to clarify the late stage of the formation style of the japan sea opening a seismic experiment using 35 ocean bottom seismographs obss an air gun array and a multichannel hydrophone streamer was undertaken in the areas from the southwestern yamato basin the oki ridge and the southwestern oki trough to the coast of the southwestern japan island arc the crusts beneath the southwestern yamato basin and the oki ridge are estimated as having approximately 13 km and 19 5 km respectively the upper and lower crusts of the southwestern yamato basin are approximately 3 2 km and 8 km thick respectively those of the oki ridge are approximately 8 2 km and 10 5 km thick respectively the upper crust of the oki ridge thickens more steeply than that of the southwestern yamato basin however the lower crust thickens more gently the crustal structure of the southwestern yamato basin shows the extended continental crust accompanied with the opening of the japan sea a remarkable structural characteristic the upper crust being thinner than the lower crust caused by listric or complicated normal faults developed in the upper crust of the southwestern yamato basin this deformed upper crust is a common structural characteristic in the southern japan sea which includes the yamato basin the southern yamato basin including the southwestern yamato basin has the thinnest upper and lower crusts in the japan sea for that reason it is suggested that the southern yamato basin had the strongest deformation by a back arc opening and that the period of the opening in the southern yamato basin had been longest in the southern japan sea the formation process of the southern yamato basin is inferred to have two stages rifting and extension of continental crust separating the northeastern and southwestern japan island arcs from the asian continent and further the extension affected by the rotation of the southwestern japan island arc

origin of the absence of magnetic lineations in the yamato basin of the japan sea magnetic properties of mafic rocks from ocean drilling program hole 794 d we investigated magnetic properties of mafic rocks recovered from ocean drilling program hole 794 d in the yamato basin of the japan sea in contrast to other back arc basins in the western pacific lineated magnetic anomaly patterns have not been recognized in the yamato basin our objective is to clarify the reason for the absence of lineated magnetic anomalies by direct magnetic measurements of crustal rocks the dominant magnetic mineral of hole 794 d is titanomaghemite as commonly observed for typical oceanic basalts but the grain size is much larger greater than several tens of microns remanence properties i e low natural remanent magnetization intensities 1 5 a m low koenigsberger ratios 1 0 high viscous remanent magnetization and mixed magnetic polarities indicate that the mafic rocks cannot be a source of lineated magnetic anomalies while curie temperature saturation magnetization and susceptibility values are not significantly different from those of typical oceanic basalts magnetic properties related to stability are greatly reduced the large grain size of the mafic rocks from hole 794 d plays a primary role in differentiating the magnetic properties from those of typical oceanic basalts and is caused by the formation of sill sediment complexes beneath the yamato basin sill sediment complexes rather than extrusive basalts in typical oceanic crust are responsible for the absence of lineated magnetic anomalies in the yamato basin

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

evidence for active subduction below western panama widespread arc tholeiitic and calc alkaline volcanism characterized the geologic evolution of western panama and southeastern costa rica in middle to late miocene and quaternary time volcanism was and is clearly associated with subduction of nazca lithosphere geochemical data suggest that formation of mantle wedge derived melts and their differentiated products predominated during miocene time whereas slab derived melts were generated in the quaternary this difference in magma chemistry is attributed to a change in the thermal regime of the nazca lithosphere which may be related to emplacement of the galapagos plume

tectonic inheritance crustal architecture and contrasting structural styles in the venezuela andes surface and subsurface data image contrasting structural styles for the two flanks of the venezuela andes in the north a flexural basin developed in neogene times between the andes and the lake maracaibo north verging thrusts are mainly detached in the pre cretaceous substratum and form a deeply buried antiformal stack secondary decollement levels occur both in the upper cretaceous and tertiary strata accounting for the passive roof thrust of a conventional frontal triangle zone in the south the barinas basin hardly compares with a flexural basin it is largely dominated by both north and south verging basement involving structures paleogene normal faults are locally inverted and caribbean nappes are frequently reactivated or refolded by younger oblique neogene andean structures seismic profiles in this area also attest to the strong neogene structural inversion of upper jurassic lower cretaceous grabens hercynian or more likely early paleozoic structures account for local preexisting crustal heterogeneities reactivated during both the tethyan rifting and the andean deformations two trans andean regional profiles have been balanced and constrained by an inversion of the gravimetric data they imply a progressive deepening of the northern moho and a south dipping subduction of the infracontinental lithospheric mantle of the maracaibo block the shortening for both sections averages 60 km palinspastic restorations assume a relative cylindricity for the deep crustal architecture of the andes and minimize the possible effects of a progressive right lateral escape of the maracaibo block with respect to stable south america along the bocono fault strain partitioning during neogene oblique convergence induced surficial thrust fronts parallel to the plate boundary strike slip motion in the allochthon along the bocono fault and an asymmetric subduction wedging at depth

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

new geological and geochronological data from the colombian andes continental growth by multiple accretion the regional geology of colombia consists of a composite palaeozoic to cenozoic sedimentary basin overlying precambrian shield rocks which are exposed in the eastern cordillera magdalena valley and at least part of the extensive llanos plains a palaeozoic metamorphic schist belt with abundant post metamorphic igneous rocks comprises the central cordillera a mesozoic volcanic suite extends from the western flank of the central cordillera and accounts for the bulk of the western cordillera a cenozoic sedimentary sequence probably overlying a late cretaceous oceanic crust and fore arc complex forms the pacific coastal plain the continental edge of central and southern colombia is interpreted as a composite margin made up of successively accreted oceanic island arc related sequences of palaeozoic early mesozoic and late cretaceous ages the present day cycle which began in the miocene is not associated with island arc volcanism

tracking stress and hydrothermal activity along the eastern lau spreading center using seismic anisotropy abstract using p wave refraction data from the l scan controlled source seismic experiment the three dimensional anisotropy structure of the upper crust is determined along the eastern lau spreading center elsc the tomographic image of anisotropy magnitude and orientation is constructed from 197 000 travel time measurements of p wave arrivals recorded on 83 ocean bottom seismometers providing the most detailed and extensive view of upper crustal spreading center anisotropy to date p wave speeds vary with the azimuth of the seismic ray path a type of anisotropy produced by stress aligned lithospheric cracks and microcracks across the study area the fast axes of anisotropy are generally oriented parallel to the trend of the spreading center as expected for ridge parallel cracks that form in association with seafloor spreading the seismic study encompassed four individual spreading segments of the elsc separated by three overlapping spreading centers osc the two larger oscs exhibit high anisotropy that penetrates deep into the upper crust surrounding these oscs and in the wake of the largest are regions where the fast axes of anisotropy are misaligned relative to the general trend in these areas the observations agree with numerical models and seafloor morphology that suggest tensile stress concentrations and a high degree of brittle crack formation the tomographic images indicate that the location and dynamics of persistent ridge offsets can be tracked back through time via their anomalous anisotropy along the spreading centers and away from ridge tips the anisotropy is greater where the melt supply is inferred to be greater and smaller where the melt supply is inferred to be smaller this is opposite to the trend expected if simple tectonic stress models govern anisotropy alternatively hydrothermal activity is expected to increase with magma supply and can explain higher anisotropy via hydrofracturing this study provides the first evidence that seismic anisotropy tracks variations in hydrologic activity along the crests of oceanic spreading centers

deep crustal structure off akita eastern margin of the japan sea deduced from ocean bottom seismographic measurements abstract a seismic exploration was carried out to detect the heterogeneity of the seismic wavespeed structure associated with the plate convergence at the eastern margin of the japan sea two airgun obs ocean bottom seismograph profiles were shot off akita japan where a seismic gap seems to exist but the location of the plate boundary has not been confirmed one of the profiles was 60 km long trending nne ssw named line obs 9 at the northeastern end of the yamato basin and the other was 170 km long trending wnw ese line nt95 1 parallel to the direction of the supposed plate convergence the crustal structure beneath line obs 9 consists of six layers the uppermost layer is sediment three layers are identified beneath the top sedimentary layer and their p wavespeeds are estimated to be 3 3 3 4 km s 5 1 5 4 km s and 5 8 6 3 km s which corresponds to the upper crust underneath these layers a layer with p wavespeed ranges of 6 3 7 2 km s comprises the middle and lower crust the depth of moho is inferred to be 19 km these wavespeed values are comparable with those of the present japanese island arc while the thickness of the crust is one half of that of the japanese arc the crustal model supports the scenario that the yamato basin is formed by extension of the island arc the crustal model for line nt95 1 shows a transition from the extended island arc structure beneath the yamato basin to a thicker crust similar to the japanese arc p wavespeed heterogeneity related to the plate boundary is not detected however a significant change in the structural model along the profile is found around the region where the largest change in the seafloor topography exists in that region the wavespeeds in the middle crust have lower values than those of the neighboring area and the moho begins to deepen towards the japanese island arc from comparison with the relationship between p wavespeed structure and aftershock distribution of the 1993 hokkaido nansei oki earthquake southwest of hokkaido m jma 7 8 the region may correspond to a possible fault position of the pending earthquake

variation in deformation of the south panama accretionary prism response to oblique subduction and trench sediment variation migrated single channel seismic lines and sea marc ii side scan and bathymetry data document an active accretionary prism along the obliquely convergent margin of south panama a region previously believed to be a transform margin the eastern flank of the coiba ridge is being subducted in this region creating the requisite geometry for along strike variation in trench sediment thickness and type the regional east dip of the downgoing plate causes the depth of the oceanic crust along the trench to drop approximately 1800 m from west to east in the western region of the study area the incoming sedimentary section consists of approximately 600 m of pelagic and hemipelagic sediments a wedge of trench turbidites that overlies the incoming hemipelagic sediments thickens from less than 100 m in the western region to more than 900 m in the eastern region of the study area the eastward increase in sediment thickness correlates with the following changes in the accretionary prism 1 decrease in initial surface slope 2 broadening of the inner trench slope 3 increase in thrust spacing 4 steepening of frontal thrusts and 5 fold development each of the responses reflects in varying degrees the increase in volume and changing physical properties of the accreted material as well as the changing shear strength of the decollement the increase in overburden pressure and compaction in the deeper sediments together with the change in lithology from oceanic to trenchfill sediments should produce an eastward increase in sediment shear strength both within the wedge and along its base because the decollement will tend to form where the pore fluid pressure overburden ratio is at a maximum the shear strength within the wedge should increase relative to basal shear strength decreased surface slopes will result from the increasing contrast in shear strength between the wedge and its base

tectonic implications of tomographic images of subducted lithosphere beneath northwestern south america we used seismic tomography to investigate the complex structure of the upper mantle below northwestern south america images of slab structure not delineated by previous seismicity studies help us to refine existing tectonic models of subducted caribbean pacific lithosphere beneath the study area beneath western venezuela and colombia we distinguish two slabs a maracaibo and a redefined bucaramanga slab the maracaibo slab coinciding with most of the bucaramanga slab previously defined by w d pennington dips in a direction of 150 at an angle of 17 to a depth of 275 km and correlates to the subducted late cretaceous oceanic plateau of the caribbean plate farther south a second slab dips at an angle of 50 in a direction of 125 to a depth of at least 500 km and correlates to the subducted oceanic crust of the nazca plate and the downdip extension of the panama island arc we refer to this slab as the redefined bucaramanga slab because it is different from the bucaramanga slab segment defined by pennington the area of the south american plate overriding both slabs is characterized by the absence of an active volcanic arc an anomalously wide topographically uplifted and tectonically active area and the northward escape of the maracaibo block along active strike slip faults in support of earlier studies we attribute this to the underthrusting of the caribbean oceanic plateau our shallowly dipping maracaibo slab along the base of the south american lithosphere and to the recent collision of the panama island arc rafted in on more steeply dipping crust of the nazca plate our redefined bucaramanga slab

deep structure of the continental margin and basin off greater kabylia algeria new insights from wide angle seismic data modeling and multichannel seismic interpretation abstract during the algerian french spiral survey aimed at investigating the deep structure of the algerian margin and basin two coincident wide angle and reflection seismic profiles were acquired in central algeria offshore greater kabylia together with gravimetric bathymetric and magnetic data this 260 km long offshore onshore profile spans the balearic basin the central algerian margin and the greater kabylia block up to the southward limit of the internal zones onshore results are obtained from modeling and interpretation of the combined data sets the algerian basin offshore greater kabylia is floored by a thin oceanic crust 4 km with p wave velocities ranging between 5 2 and 6 8 km s in the northern hannibal high region the atypical 3 layer crustal structure is interpreted as volcanic products stacked over a thin crust similar to that bordering the margin and related to miocene post accretion volcanism these results support a two step back arc opening of the west algerian basin comprising oceanic crust accretion during the first southward stage and a magmatic and probably tectonic reworking of this young oceanic basement during the second westward opening phase the structure of the central algerian margin is that of a narrow 70 km magma poor rifted margin with a wider zone of distal thinned continental crust than on the other margin segments there is no evidence for mantle exhumation in the sharp ocean continent transition but transcurrent movements during the second opening phase may have changed its initial geometry the plio quaternary inversion of the margin related to ongoing convergence between africa and eurasia is expressed by a blind thrust system under the margin rising toward the surface at the slope toe and by an isostatic disequilibrium resulting from opposite flexures of two plates decoupled at the continental slope this disequilibrium is likely responsible for the peculiar asymmetrical shape of the crustal neck that may thus be a characteristic feature of inverted rifted margins

microseismicity evidence for subduction of the caribbean plate beneath the south american plate in northwestern venezuela over 1100 microearthquakes with body wave magnitude mb 4 have been located in western venezuela and the southwestern caribbean region since the installation in 1980 of the venezuelan seismological array together with 120 events of mb 4 one of them with surface wave magnitude ms 6 this tectonically complex region is part of the boundary between the caribbean and the south american plates the main seismically active feature inland in western venezuela is the northeast striking 600 km long 100 km wide right lateral strike slip bocono fault zone along the venezuelan andes about 80 of the earthquakes located in the entire region in the period 1980 mid 1995 have occurred on this fault zone at focal depths 20 km microearthquake activity at lower rates also occurs northwest of the venezuelan andes both in the continental and caribbean sea regions part of this activity takes place at depths down to 150 km northwest oriented seismicity depth profiles show the existence of a benioff zone dipping to the southeast beneath northwestern venezuela and northern colombia this indicates the presence of a northeast striking southeast dipping subducted slab of the caribbean plate beneath the south american plate hypocentral locations show that the northeastern end of this subduction occurs northwest of the curacao aruba region in the vicinity of a northwest trending right lateral strike slip fault zone that joins up with the northeastern end of the bocono fault zone this latter place turns out to be the western end of the east west striking san sebastian fault along the venezuelan coast

double saloon door tectonics in the japan sea fossa magna and the japanese island arc abstract a number of criteria considered diagnostic of double saloon door rifting and seafloor spreading are matched by data from the japanese arc these include i a pair of terranes sw and ne honshu which rotated in opposite directions from 22 21 ma to 14 11 ma ii rotated terranes which comprise a retro arc fold thrust belt attached to an accretionary wedge intruded by a magmatic arc iii contemporaneous backarc extension from 24 to 21 ma which is brought to a halt by progressive collision of the izu bonin and japan arcs from 15 to 5 ma iv isolation of blocks of thicker continental crust by areas of thin continental or oceanic crust during backarc rifting v such isolation may be due to simultaneous rifting or to progressively seaward rifts associated with ridge jumps towards the subduction zone vi opposite rotations are accommodated by subduction rollback demonstrated by seaward migration of the volcanic front from 30 26 ma to 16 15 ma vii concurrent development of a major arc orthogonal rift the fossa magna from 23 18 ma to 14 ma which was thereafter inverted from 15 ma to the recent viii a northeast propagating rift in the northern japan basin demonstrated by the relationship of linear magnetic anomalies to the mapped continent ocean boundary driving mechanisms for double saloon door tectonics are discussed in relation to various reconstructions of the northwest pacific opposite rotational torques leading to opposite terrane rotations may be caused by rollback of a curved trench hingeline or by the divergent slab sinking forces of the pacific and philippine sea plates

seismic constraints of the formation process on the back arc basin in the southeastern japan sea to clarify the formation process of the back arc basin in the japan sea which is located next to the northwestern pacific a seismic survey using ocean bottom seismographs and an air gun array was undertaken in areas from the northern yamato basin to the coast of the northeastern japan island arc off awa shima island the crust beneath the northern yamato basin off awa shima island is approximately 16 km thick the upper and lower crusts are respectively about 5 km thick with a steep velocity gradient and about 10 km thick with a gentle velocity gradient in the basin there are very few units with p wave velocity of 5 4 6 0 km s corresponding to the continental upper crust the crustal structure of the northern yamato basin has characteristics of thicker oceanic crust the high velocity lowermost crust in the northern yamato basin with 7 2 7 4 km s might show melt formed by a slightly high mantle temperature during the opening of the basin however the crust beneath the areas from the sado ridge to the coast which is approximately 25 26 km thick is slightly thinner than that of the continental crust and island arc crust the crustal structure beneath this area is inferred to be a rifted continental and or a rifted island arc crust

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

back arc rifting in the korea plateau in the east sea japan sea and the separation of the southwestern japan arc from the korean margin abstract the south korea plateau is a remnant of continental crust in the middle of the eastern korean margin that was deformed by extension in association with back arc rifting and separation of the southwestern japan arc in the cenozoic multichannel seismic profiles show that the south korea plateau preserves structure of rift basins flanked by uplifted footwall blocks rift basins in the plateau are filled with distinct syn and post rift sequences separated by a prominent breakup unconformity resulting from uplift the crustal and upper mantle structures computed by ambient noise tomography indicate that the south korea plateau defines the seaward limit of rifted continental crust in addition rifting occurred with vigorous asthenospheric upwelling induced along the korean margin and depth dependent stretching therefore the south korea plateau underwent the entire sequence of tectonic events typical of a passive continental margin encompassing rifting with subsidence uplift erosion and breakup prior to the separation of the southwestern japan arc breakup at the korean margin may substantiate successive episodes of back arc spreading which migrated toward the arc in response to trench retreat we suggest that the southwestern japan arc moved to its present location from the southern part of the korean margin with a significant amount of clockwise rotation during back arc spreading

oceanic crust in the japan basin of the japan sea by the 1990 japan ussr expedition in september of 1990 a seismic refraction and reflection survey was conducted in the japan basin in the northeastern part of the japan sea as a part of the japan ussr joint expedition twenty six ocean bottom seismometers obss were deployed on two 200 km long lines explosives and an airgun were fired as controlled seismic sources on the two mutually perpendicular lines airguns were also used as the source for the multi channel reflection profiles the crustal structure deduced is that of a typical oceanic basin the crustal thickness is about 8 5 km including 2 km of sediment we obtained a more detailed crustal structure than that obtained previously from the dense airgun shooting data the crustal structure is well resolved to show layer 1a layer 1b layer 2a 2b 2c layer 3 and the mantle the crust basically consists of laterally homogeneous layers but the moho deepens slightly westward

big mantle wedge anisotropy slabs and earthquakes beneath the japan sea abstract the japan sea is a part of the western pacific trench arc backarc system and has a complex bathymetry and intense seismic activities in the crust and upper mantle local seismic tomography revealed strong lateral heterogeneities in the crust and uppermost mantle beneath the eastern margin of the japan sea which was determined using p and s wave arrival times of suboceanic earthquakes relocated precisely with sp depth phases ambient noise tomography revealed a thin crust and a thin lithosphere beneath the japan sea and significant low velocity low v anomalies in the shallow mantle beneath the western and eastern margins of the japan sea observations with ocean bottom seismometers and electromagnetometers revealed low v and high conductivity anomalies at depths of 200 300 km in the big mantle wedge bmw above the subducting pacific slab and the anomalies are connected with the low v zone in the normal mantle wedge beneath ne japan suggesting that both shallow and deep slab dehydrations occur and contribute to the arc and back arc magmatism the pacific slab has a simple geometry beneath the japan sea and earthquakes occur actively in the slab down to a depth of 600 km beneath the ne asian margin teleseismic p and s wave tomography has revealed that the philippine sea plate has subducted aseismically down to the mantle transition zone mtz 410 660 km depths beneath the southern japan sea and the tsushima strait and a slab window is revealed within the aseismic philippine sea slab seismic anisotropy tomography revealed a nw se fast velocity direction in the bmw which reflects corner flows induced by the fast deep subduction of the pacific slab large deep earthquakes m 7 0 depth 500 km occur frequently beneath the japan sea western margin which may be related to the formation of the changbai and ulleung intraplate volcanoes a metastable olivine wedge is revealed within the cold core of the pacific slab at the mtz depth which may be related to the deep seismicity however many of these results are still preliminary due to the lack of seismic stations in the japan sea the key to resolving these critical geoscientific issues is seismic instrumentation in the japan sea for which international cooperation of geoscience communities in the east asian countries is necessary

plate motions in the north andean region repeated geodetic measurements with the global positioning system gps provide direct measurements of displacements due to plate motions and active crustal deformation in central america and northern south america an area of complex interaction of the nazca cocos caribbean and south american plates the displacement rates for the period 1988 1991 obtained from the results of the first three central and south america casa gps campaigns are in general agreement with the predictions of the nuvel 1 plate motion model but there are differences in detail between the observations and the model the nazca north andes convergence rate vector measured by gps is different from the nuvel 1 vector at 95 confidence the difference implies that the north andes are moving northward relative to south america the measured convergence between the caribbean plate and the north andes suggests that the southern margin of the caribbean plate is located in the south caribbean deformed belt the april 1991 costa rica earthquake and the cocos caribbean convergence rate determined by gps suggest the possibility of significant ongoing deformation between central america and the stable interior of the caribbean plate our gps results are consistent with deformation of the overriding plates at the convergent margins of central and south america and confirm that active convergence is occurring around much of the southern margin of the caribbean plate from colombia west to costa rica costa rica and panama are not part of the stable caribbean plate instead the south caribbean deformed belt and the north panama fold belt probably represent the southern margin of the caribbean plate

an inverse problem in microtectonics for the determination of stress tensors from fault striation analysis abstract a method is proposed to compute some characteristics of a possible stress tensor related to striations measured in a given faulted area if several tectonic phases are responsible for the striations the method separates and determines the successive stress tensors the iterative algorithm consists of alternatively a sorting the data attributed to a tectonic phase and b computing the characteristics of the stress tensor of this phase the method is tested using an application to synthetic and actual data

multiphased tectonic evolution of the central algerian margin from combined wide angle and reflection seismic data off tipaza algeria 1 the origin of the algerian margin remains one of the key questions still discussed in the western mediterranean sea due to the imprecise nature and kinematics of the associated basin during the neogene for the first time the deep structure of the maghrebian margin was explored during the spiral seismic survey in this work we present a n s transect off tipaza west of algiers a place where the margin broadens due to a topographic high khayr al din bank new deep penetration seismic profiles allow us to image the sedimentary sequence in the algerian basin and the crustal structure at the continent ocean boundary modeling of the wide angle data shows thinning of the basement from more than 15km in the continental upper margin to only 5 6km of oceanic type basement in the algerian basin and reveals a very narrow or absent transitional zone analysis of the deep structure of the margin indicates features inherited from its complex evolution 1 an oceanic type crust in the deep basin 2 similarities with margins formed in a transform type setting 3 a progressive deepening of the whole sedimentary cover and the thickening of the plio quaternary sediments at the margin foot coeval with 4 a downward flexure of the basement in the basin these features argue for a multiphased evolution of the margin including 1 an early stage of rifting and or spreading 2 a late transcurrent episode related to the westward migration of the alboran domain and 3 a diffuse plio quaternary compressional reactivation of the margin

two modes of back arc spreading back arc spreading is initiated with rifting along the volcanic zone of an island arc under extensional tectonics the width of the volcanic zone of the arc which is closely related to the dip angle of the subducting slab should cause a variation in the style of initial rifting and subsequent back arc spreading two modes of back arc spreading single rift type and multirift type are introduced from the comparative study of the bonin arc and the japan sea divergent tectonics in an island arc above a steeply dipping slab will cause a single rift system in a narrow volcanic zone but will cause a multirift system in a broad volcanic zone which is related to a shallow dipping angle of subduction

one off deep crustal earthquake swarm in a stable intracontinental region of the southwestern korean peninsula abstract a deep crustal strike slip earthquake swarm that occurred in the intracontinental region of the southwestern korean peninsula is investigated the earthquakes are detected using matched filter analysis the hypocenters are then refined showing more than 500 micro to small strike slip earthquakes concentrated on a fault plane with dimensions of 500 m by 300 m at a depth of 21 km the low temperature in the source depth facilitates the seismicity the earthquake swarm was transiently active for 15 days from 25 april 2020 to 12 may 2020 presenting a high seismicity rate with a gutenberg richter b value of 2 01 0 05 the earthquake swarm began in the central area and expanded outward with time the spatiotemporal seismicity distribution reflects bilateral rupture the focal mechanism solutions suggest a left lateral strike slip fault with a strike of 103 6 3 2 and a dip of 73 3 11 7 the spatial distribution of seismicity generally agrees with the focal mechanism solutions the orientation of the inverted stress field agrees with the ambient stress field suggesting that the earthquake swarm occurred as a consequence of ambient lithostatic stress loading coulomb stress changes suggest that the stress increased in the directions of wnw ese and nne ssw from the fault the transiently active earthquake swarm began to occur after the 2011 mw9 0 tohoku oki megathrust earthquake suggesting a rapid release of accumulated stress in response to changes in the properties of the medium caused by the megathrust earthquake

latest pleistocene holocene tectonics of the ecuadorian andes abstract a structural study was carried out in the most tectonically active areas of the ecuadorian andes including part of the interandean valley the cordillera real and the subandean zone since the pleistocene the region has undergone complex wrench tectonics which are still active apparently two different stress states have coexisted within different parts of the andean range during the latest pleistocene and holocene 1 1 the external part of the chain including the cordillera real and the westernmost part of the subandean zone has been subject to a east northeast west southwest direction of greatest principal stress associated motions are partitioned between right lateral oblique thrusting and pure right lateral strike slip faulting along north south to north northeast trending structures as a whole these define a right lateral transpressive shear zone 2 2 in the interandean valley which lies above a major tertiary suture zone deformations are consistent with a north south direction of greatest horizontal principal stress associated with recent development of a left lateral transtensional shear zone in the first zone fault displacements are at least an order of magnitude larger than in the second zone the present tectonics of the area is explained by a kinematic model in which the wedge shaped cordillera real crustal block bounded to the east by a right lateral shear zone and to the west by a left lateral shear zone is being uplifted and extruded northward with respect to the cordillera occidental and to the stable south american foreland this motion is the result of oblique convergence between the nazca and south american plates

age and spreading history of the cayman trough as determined from depth heat flow and magnetic anomalies the opening of the cayman trough records aspects of relative motion between the north american and caribbean plates essential to accurate reconstruction of caribbean evolution assessment of opening rates based upon basement subsidence analysis as well as magnetic anomaly interpretation shows that the trough may have opened at one third to one half previously proposed rates depths to basement show that the crust has subsided away from the present axis in two stages a younger stage characterized by substantial and asymmetric greater to the east subsidence preceded by an older stage of slight increases of depth with distance depth to age relations calculated from subsidence curves which accommodate lateral as well as vertical cooling of the accreting slab indicate that the trough has opened at an overall rate of less than 15 mm yr since 25 30 ma and at 20 30 mm yr prior to 30 ma marine magnetic anomalies show lineations perpendicular to the trough including a distinct central anomaly but the pattern is neither well defined nor symmetrical about the present axis however partial anomaly sequences can be identified on both sides of the axis and the distribution of these sequences indicates that spreading may have involved frequent short ridge jumps mostly eastward the overall rate of opening as indicated by this interpretation of the magnetics is about 15 and 30 mm yr since and prior to 26 ma both subsidence and magnetics indicate that the trough opened by at least 45 50 ma

basin development and tectonic history of the llanos basin eastern cordillera and middle magdalena valley colombia the middle magdalena valley eastern cordillera and llanos basin constituted a major regional sedimentary basin from the triassic to the middle miocene basin development began during the triassic to the earliest cretaceous with a synrift megasequence related to the separation of north and south america in the proto caribbean the synrift megasequence began with deposition in a continental environment that became paralic and shallow marine in the early cretaceous basin development continued into the cretaceous in a back arc setting east of the andean subduction zone the back arc megasequence was dominated by shallow marine sedimentation and produced an excellent regional source rock during the turonian coniacian marine deposition was abruptly terminated during the ear y maastrichtian due to the final accretion of the western cordillera accretion of the western cordillera created the early pre andean foreland basin megasequence of late maastrichtian to early eocene age this depositional episode consists of coal rich alluvial plain coastal plain and estuarine deposits throughout the middle magdalena valley eastern cordillera and eastern llanos basin the megasequence was terminated by middle eocene deformation in the magdalena valley which ended sediment deposition throughout colombia loading effects of this deformation reestablished the basin in which the late pre andean foreland basin megasequence was deposited until the early miocene this megasequence also consists of alluvial plain coastal plain and estuarine deposits including the primary reservoir in the llanos foothills the upper eocene mirador fo mation the megasequence also includes a series of four major grossly coarsening upward cycles in the llanos basin these cycles correspond to changes in sea level sediment supply and foreland basin loading the mudstone in the lowermost of these cycles is the regional seal in the llanos basin and foothills the middle miocene onset of andean deformation in the eastern cordillera isolated the middle magdalena valley from the llanos basin the deformation was dominated by inversion of the basin controlling faults the resultant loading of the lithosphere created the accommodation space for the andean foreland basin megasequence a major transgression into the llanos basin coeval with this deformation caused deposition of marine mudstones in the lower part of the megasequence however the majority of the andean foreland basin megasequence consists of the guayabo formation a classic molasse sequence deposited in a high energy coarse grained bed load dominated fluvial system that was supplied by the developing mountains of the eastern cordillera

neogene stratigraphy paleoceanography and paleobiogeography in northwest south america and the evolution of the panama seaway abstract the atrato basin in the pacific coastal region of nw south america colombia exhibits stratigraphic and biostratigraphic characteristics similar to other neogene pacific coastal basins of southern central america and northern south america an evaluation of the neogene stratigraphy and foraminiferal biostratigraphy with emphasis on paleoceanographic and paleobiogeographic phenomena indicate the following 1 previous to the middle miocene well aerated deep and open oceanic conditions and free and active water circulation prevailed along the steep continental margins of nw south america 2 during the early middle miocene right after the neogene hiatus nh 2 15 2 ma tectonic disturbances triggered the initial uplift of the panama sill bottom water circulation foraminiferal and sedimentation changes are observed in both the pacific and caribbean coastal areas of nw south america 3 during the middle miocene coinciding with the neogene hiatus nh 3 12 9 11 8 ma an abrupt paleobathymetric change from lower to middle bathyal depths indicates a major uplift of the panama sill to about 1000 m reflecting the middle miocene tectonic disturbances in nw south america 4 during the late middle miocene immediately following this uplift a distinctive pacific benthic foraminiferal fauna abruptly appeared and extended from ecuador to california because these assemblages do not occur in the adjacent caribbean region a circulation barrier between the atlantic and pacific oceans as a result of intensification of the cool marginal california current is proposed anoxic middle to upper bathyal environments prevailed under well aerated cool surface waters in the pacific coastal areas of nw south america 5 during the latest miocene immediately following the neogene hiatus nh 6 7 0 6 3 ma surface water circulation between the caribbean and the pacific was re established and the previous influence of the california current disappeared a rapid filling and shallowing of the basin to mostly neritic depths is indicated along the pacific and caribbean coastal basins of colombia as well as a progressive increase in similarity between the coastal pacific and caribbean benthic biotas this shallowing in the early late miocene also appears to have been associated with the earliest terrestrial interchange of ground sloths to north america and raccoons and their allies to south america 6 immediately following early pliocene time the panamanian isthmus became completely emergent providing a terrestrial environment favourable for intermingling of terrestrial faunas and floras between north and south america i correlate this pliocene uplift with the neogene hiatus nh 8 3 7 3 1 ma four regional unconformities associated with tectonic disturbances in the nw corner of south america have also been recognized and correlated with neogene hiatuses recorded from the oceans early middle miocene middle miocene late miocene and early pliocene

accurate measurements of residual topography from the oceanic realm in the oceans our understanding of plate subsidence as a function of age permits residual depth anomalies to be identified and mapped these anomalies may reflect dynamic topography and could be an important means for constraining convective circulation of the sublithospheric mantle here we analyze a global database of seismic reflection and wide angle profiles from heavily sedimented oceanic crust which abuts continental lithosphere at 449 locations we calculated water loaded subsidence compared it with a reference age depth relationship and determined residual depth we then combined these spot measurements of residual depth with observations from mid oceanic ridges and from selected ship track bathymetry to construct a global map of residual depth our results suggest that the amplitude of residual depth varies by up to 1 km with wavelengths of order 103 km we compare our residual depths with free air gravity and seismic tomographic anomalies our results show that residual depths correlate with long wavelength gravity anomalies in contrast correlations between residual depths and vertically averaged shear velocity anomalies within the upper and or the lower mantle are weaker the largest discrepancies occur at short 1000 km wavelengths these combined observations suggest that residual depth anomalies could be generate by density variations within a thin 102 km low viscosity layer beneath the lithosphere our global compilation should play a significant role in helping to refine predictive geodynamical models

deep slab dehydration and large scale upwelling flow in the upper mantle beneath the japan sea we present a three dimensional p and s wave tomography model beneath the central japan sea using arrival time data obtained by long term ocean bottom seismometer observations the tomographic model is determined down to over 300 km the resulting tomographic image has two features in the mantle wedge first a remarkable low velocity region is spread within the shallow part of the mantle wedge beneath the japan arc with a slight extension beneath the japan sea second an inclined low velocity anomaly exists in the deeper part beneath the japan sea and converges to the slab at 300 km depth moreover its anomaly amplitude is distinctly small compared to the shallow part of the low velocity region these low velocity anomalies are interpreted to represent fluids that were dehydrated from the pacific and philippine sea slabs and melt production affected by the fluid our observations suggest that the deep dehydration from the pacific slab is found at a depth of approximately 300 km and large scale upwelling flow mechanically induced by plate subduction exists beneath the japan sea our results indicate the importance of the deep heterogeneous mantle wedge structure beneath the japan sea with regards to better understanding the magmatism of the subduction dynamics in japan and the evolution of the opening of the japan sea

the southern part of the armorican orogeny a result of crustal shortening related to reactivation of a pre hercynian mafic belt during carboniferous time abstract this synthesis of detailed geological and geophysical information relates deep crustal structure to deformation and the emplacement of granites in the south armorican massif and the adjacent continental shelf near the continental margin a zone of geophysical anomalies is regarded as a nearly complete line of suture between a southern continent and america a major mafic body in the trace of the supposed suture in interpreted as a relic of probable oceanic crust which has apparently acted as a deflector of regional strain it is proposed that late in the hercynian collision history during late carboniferous time compressional strain acted across the mafic body and can be directly related to generation of younger regional structures in the metasediments and to localization of granite emplacement

late quaternary kinematics of the pallatanga strike slip fault central ecuador from topographic measurements of displaced morphological features the northeast trending pallatanga right lateral strike slip fault runs across the western cordillera connecting n50 e n70 e trending normal faults in the gulf of guayaquil with n s reverse faults in the interandean depression over most of its length the fault trace has been partly obscured by erosional processes and can be inferred in the topography only at the large scale only the northern fault segment which follows the upper rio pangor valley at elevations above 3600 m is prominent in the morphology valleys and ridges cut and offset by the fault provide an outstanding record of right lateral cumulative fault displacement the fault geometry and kinematics of this particular fault segment can be determined from detailed topographic levellings the fault strikes n30 e and dips 75 to the nw depending on their size and nature transverse morphological features such as tributaries of the rio pangor and intervening ridges reveal right lateral offsets which cluster around 27 11m 41 5 4 m 590 65 m and 960 70 m the slip vector deduced from the short term offsets shows a slight reverse component with a pitch of about 11 5 sw the 41 5 4 m displacements are assumed to be coeval with the last glacial termination yielding a mean holocene slip rate of 2 9 4 6 mm yr 1 assuming a uniform slip rate on the fault in the long term the 27 m offset appears to correlate with an identified middle holocene morphoclimatic event and the long term offsets of 590 m and 960 m coincide with the glacial terminations at the beginning of the last two interglacial periods

p wave velocity structure of the margin of the southeastern tsushima basin in the japan sea using ocean bottom seismometers and airguns abstract the tsushima basin is located in the southwestern japan sea which is a back arc basin in the northwestern pacific although some geophysical surveys had been conducted to investigate the formation process of the tsushima basin it remains unclear in 2000 to clarify the formation process of the tsushima basin the seismic velocity structure survey with ocean bottom seismometers and airguns was carried out at the southeastern tsushima basin and its margin which are presumed to be the transition zone of the crustal structure of the southwestern japan island arc the crustal thickness under the southeastern tsushima basin is about 17 km including a 5 km thick sedimentary layer and 20 km including a 1 5 km thick sedimentary layer under its margin the whole crustal thickness and thickness of the upper part of the crust increase towards the southwestern japan island arc on the other hand thickness of the lower part of the crust seems more uniform than that of the upper part the crust in the southeastern tsushima basin has about 6 km s layer with the large velocity gradient shallow structures of the continental bank show that the accumulation of the sediments started from lower miocene in the southeastern tsushima basin the crustal structure in southeastern tsushima basin is not the oceanic crust which is formed ocean floor spreading or affected by mantle plume but the rifted extended island arc crust because magnitudes of the whole crustal and the upper part of the crustal thickening are larger than that of the lower part of the crustal thickening towards the southwestern japan island arc in the margin of the southeastern tsushima basin high velocity material does not exist in the lowermost crust for that reason the margin is inferred to be a non volcanic rifted margin the asymmetric structure in the both margins of the southeastern and korean peninsula of the tsushima basin indicates that the formation process of the tsushima basin may be simple shear style rather than pure shear style

measuring dynamic topography an analysis of southeast asia 1 theoretical models of dynamic topography generated by subducting slabs and consistent with the observed geoid predict 1 2 km of subsidence on wavelengths of 102 103 km the demonstrable existence of such subsidence has important implications for understanding sedimentary basin formation relative sea level changes as well as the earth s viscosity and density structure here we present the results of a combined analysis of subsidence data from oceanic marginal basins and their fringing continental shelves we have chosen to analyze a large area of southeast asia where dynamical predictions are believed to be well constrained by the history of subduction our joint analysis constrains the maximum amplitude of dynamic subsidence to be 300 m with a range of 0 500 m significantly less than predicted actively spreading marginal basins have no resolvable dynamic subsidence but after removing the effects of crustal thickness variation a few inactive marginal basins are consistently 200 500 m deeper than normal adjacent continental sedimentary basins record a possible dynamic subsidence of 0 500 m although wavelengths are shorter than expected 102 km there is also disagreement between the predicted and observed distribution of anomalous subsidence as well as inconsistencies in initiation and duration these results suggest that mass excess within the mantle represented by subducted slabs has little discernible effect on surface topography instead subsidence and marine inundations throughout southeast asia are predominantly controlled by lithospheric extension and or by foreland loading the spatial and temporal distribution of anomalous subsidence suggests that it might not be caused by dynamic topography associated with subducting slabs a large discrepancy between predicted and observed dynamic topography could be reconciled by changing the density structure of subducted slabs by supporting mass excess with a marked increase in viscosity at depth or by decoupling slabs from the surface with a very low viscosity layer

seismic structure of the source region of the 2007 chuetsu oki earthquake revealed by offshore onshore seismic survey asperity zone of intraplate earthquake delimited by crustal inhomogeneity abstract the 2007 niigata ken chuetsu oki earthquake occurred on july 16 2007 with a magnitude of 6 8 immediately after the mainshock a dense network of ocean bottom seismometers and temporary land seismic stations were deployed to obtain the accurate aftershock location a seismic survey using ocean bottom seismometers land stations and controlled sources at sea and on land was also conducted along three profiles to estimate the detailed velocity structure of the source region a thick sedimentary layer covers the crust and this layer is thickest near the coast the upper crust has a p wave velocity of 6 km s and a large lateral heterogeneity with respect to thickness and velocity the lower crust with p wave velocity of 7 km s has a thickness of approximately 10 km the thickness of the crust is estimated to be approximately 24 km the precise aftershock distribution was obtained by using the high resolution velocity structure the aftershocks in the upper crust form a plane dipping to the southeast most of the aftershocks are located in the upper crust in addition a small number of aftershocks in the lower crust seem to be positioned on the same plane formed by the aftershocks in the upper crust the mainshock fault estimated from the aftershock distribution is positioned in the high velocity body of the upper crust a large deformation of the sediments above the epicentral region is interpreted to be due to the repeated occurrence of large earthquakes the lower crust immediately below the fault has low velocity the source region of the 2007 event in the upper crust is sandwiched by a weakening structure we suggest that the ductile deformation of both the sediments and the lower crust causes stress accumulation in the source region

seismic evidence for stretched continental crust in the japan sea the japan sea is a major marginal sea in the western pacific comprised of basins and bathymetric highs technically these features were created by multiple rifting and spreading processes as exemplified in the creation of the japan basin and yamato basin however there are smaller scale features for which seismic crustal models have been inadequate to infer or verify their origin and evolution recent advances in marine seismological surveys enable us to obtain heterogeneous seismic crustal structures in sufficient detail to characterize these features in september of 1992 a seismic experiment was conducted to obtain the detailed seismic crustal structure in the southwestern part of the japan sea including the basin and bank areas seventeen ocean bottom seismometers obss were deployed on two lines explosives and airguns were fired as controlled seismic sources the model of the crust obtained in the northern tsushima basin is about 13 km thick including a 1 5 km thick sedimentary layer beneath the bank area the crust is 22 km thick and the upper crust is relatively homogeneous with a p wave velocity of about 6 km s the bank is interpreted to be a stretched continental crust fragment

a seismic experiment in the ulleung basin tsushima basin southwestern japan sea east sea of korea understanding the crustal structure of the ulleung basin tsushima basin in the southwestern japan sea east sea of korea is important for reconstructing the opening tectonics of the japan sea a korea and russia collaborative seismic experiment was carried out in 1991 to investigate the crustal structure of this basin using ocean bottom seismometers obss and large capacity air guns the results of the experiment reveal that the ulleung basin is floored by distinctive upper and lower crustal layers with velocities typical of layers 2 and 3 the lower crust however is unusually thick 7 8 km and the moho lies flat at 15 16 km depths though similar to the yamato basin in crustal thickness the ulleung basin has a much thicker sedimentary sequence and a slightly thinner lower crust in the center geophysical evidence thus shows that the crust under the ulleung basin has velocity range and crustal layering falling closer to those of normal oceanic crust than the yamato basin the formation mechanism of the thicker than normal lower crust should be clarified to better understand the complex opening process in the southern japan sea

the geology and mesozoic collisional history of the cordillera real ecuador abstract the geology of the metamorphic rocks of the cordillera real of ecuador is described in terms of five informal lithotectonic divisions we deduce that during the mesozoic repeated accretionary events occurred and that dextral transpression has been of fundamental importance in determining the tectonic evolution of this part of the northern andes the oldest event recognised of probable late triassic age may be related to the break up of western gondwana and generated a regional belt of s type plutons during the jurassic major calc alkaline batholiths were intruded following this in latest jurassic to early cretaceous time a volcano sedimentary terrane of possible oceanic or marginal basin origin the alao division and the most westerly gneissic chaucha arenillas terrane were accreted to continental south america the accretion of the oceanic western cordillera took place in latest cretaceous to earliest tertiary time this latter event coincided with widespread thermal disturbance as evidenced by the large number of young k ar mineral ages recorded from the cordillera real

model for late neogene deformation in panama previous workers mostly using earthquake data have postulated both discrete faults and complex diffuse zones for the caribbean nazca plate boundary in panama we have better resolved the location and character of late neogene plate boundary structures by using satellite imagery and a compilation of existing geologic map data on the basis of these results we propose that panama is moving northwestward away from a zone of active convergence along the continental margin of south america toward more easily subducted areas of the colombian basin movement is accommodated across a diffuse zone by a complex interaction of oroclinal bending left lateral strike slip faulting and subduction

flexural isostasy and loading sequence of the dokdo seamounts on the ulleung basin in the east sea sea of japan abstract the isostatic compensation and loading sequence of the dokdo seamounts on the ulleung basin in the east sea are studied by a flexure model using gravity and bathymetric data the dokdo seamounts comprise dokdo dok island lying as a cluster of emerged small islets surmounting a large submerged volcanic edifice and two voluminous tablemounts simheungtaek and isabu in spite of their similarity in topographic expression a large difference of about 50 mgal between the observed gravity anomaly peaks over dokdo and the isabu tablemount implies different amounts of compensation a comparison between the observed and calculated gravity anomalies of the seamounts shows that the flexural compensation investigated in this study predicts well the observed data the flexure model suggests that the dokdo seamounts might be composed of high density volcanic rocks modeling results show that the flexural rigidity or effective elastic thickness of the lithosphere under dokdo is stronger or thicker than that under the isabu tablemount it implies that the age of lithosphere at the time of loading for isabu was younger than that for dokdo the effective elastic thicknesses associated with the dokdo seamounts approximately follow 200 400 c isotherms in the cooling plate model the results of this study and the former age dating of dokdo suggest that the isabu tablemount was formed first followed by dokdo after the opening of the east sea ceased

closing the gap between regional and global travel time tomography recent global travel time tomography studies by zhou 1996 and van der hilst et al 1997 have been performed with cell parameterizations of the order of those frequently used in regional tomography studies i e with cell sizes of 1 2 these new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations 5 cells the inferred structures are however of larger scale than is usually obtained in regional models and it is not clear where and if individual cells are actually resolved this study aims at resolving lateral heterogeneity on scales as small as 0 6 in the upper mantle and 1 2 3 in the lower mantle this allows for the adequate mapping of expected small scale structures induced by for example lithosphere subduction deep mantle upwellings and mid ocean ridges there are three major contributions that allow for this advancement first we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the earth s mantle by seismic waves spakman and bijwaard 1998 second we exploit the global data set of engdahl et al 1998 which is a reprocessed version of the global data set of the international seismological centre their reprocessing included hypocenter redetermination and phase reidentification finally we combine all data used p pp and pwp phases into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal to noise ratio is improved in the approximate solution of the huge inverse problem we obtain a variance reduction of 57 1 synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells 0 6 or 1 2 in the shallow parts of subduction zones decreasing to approximately 2 3 resolution in well sampled regions in the lower mantle vertical resolution can be worse up to several hundreds of kilometers in subduction zones with rays predominantly pointing along dip important features of the solution are as follows 100 200 km thick high velocity slabs beneath all major subduction zones sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle large high velocity anomalies in the lower mantle that have been attributed to subduction of the tethys ocean and the farallon plate and low velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under iceland east africa the canary islands yellowstone and the society islands our findings corroborate that the 660 km boundary may resist but not prevent present day large scale mass transfer from upper to lower mantle or vice versa this observation confirms the results of previous global mantle studies that employed coarser parameterizations

subduction of the eastern panama basin and seismotectonics of northwestern south america spreading along the cocos nazca plate boundary since the breakup of the farallon plate in the miocene has resulted in the formation of the panama basin and a complex interaction of plates in and near northwestern south america current plate boundaries have been defined and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes considering only well located events and through focal mechanism determinations the existence of relict plate boundaries bathymetric features and the panamanian isthmus has affected the subduction process of the nazca plate beneath south america and determined the present day configuration of the subducting lithospheric plate there is no single triple junction separating the caribbean south american and nazca plates instead the panamanian isthmus and surrounding areas are accommodating east west compression and a lesser degree of north south compression along a series of thrust faults striking nw to ne and the andean ranges of ecuador colombia and venezeula are moving as a block nne relative to the rest of the south american plates along a system of faults following the front of the eastern cordillera the subducted portions of the panama basin and old farallon plate have become segmented into three pieces recognized in this study from north to south they are 1 a bucaramanga segment continuous with the caribbean seafloor northwest of colombia 2 a cauca segment continuous with oceanic crust nazca plate currently being subducted beneath south america at the colombia ecuador trench and 3 an ecuador segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern peru the segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted

quantifying crustal extension and shortening in the back arc region of northeast japan 1 the northeast japan nej arc has experienced strong extension in the early to middle miocene time and contractive deformation since pliocene time we determine the amounts of the miocene extension and pliocene contraction along four transects across the nej arc by area balancing restoration using seismic reflection gravity and surface geologic data we find that the miocene extension is as much as 31 56 km while the pliocene quaternary contraction is only 10 15 km the style of the miocene extension in the back arc region of nej is highly asymmetric the zone of concentrated extension is characterized by a breakaway fault on the west a rollover basement anticline on the east and abnormally deep 10 km basins and strongly rotated fault blocks in between suggesting the existence of a large scale detachment fault at a mid crustal level beneath the extended zone we reveal that the pliocene quaternary contractive surface deformation in the back arc region of nej resulted principally from positive tectonic inversion of the underlying detachment fault previous works have suggested that andean type orogens have often experienced negative and positive inversions alternately in their back arc regions although the former has been well documented for example for the cenozoic north american cordillera the latter has rarely been reported because subsequent contraction has obscured the older structures since the nej arc is still at an incipient stage and hence its older structures are well preserved our study provides an important example of positive tectonic inversion of andean type orogens

cenozoic global plate motions cenozoic global plate motions relative to the hot spots are investigated and compared to plate motions in a mean lithosphere reference frame plate motions were analyzed over six time intervals divided by ages 10 25 43 48 and 56 ma chosen as much as possible to coincide with key plate reorganizations alternative motion circuits and rotational parameters were considered and evaluated with paleomagnetic data from the pacific and north american plates the circuit found to be in best agreement with the paleomagnetic data is one in which the hot spots in the atlantic region are assumed to be fixed relative to the hot spots in the pacific region throughout the cenozoic the hot spot and mean lithosphere reference frames have been in continual slow relative motion the rate of motion is nonuniform however most of the motion having occurred during the middle cenozoic the net cenozoic rotation of the lithosphere relative to the hot spots is described by a right handed rotation of 7 about a euler pole at 46 s 87 e which yields a 5 displacement of the north poles of the two reference frames this motion is small enough that inferences drawn about plate speeds in one reference frame should be valid in the other analysis of the global motions resulting from our preferred model showed that many characteristics of current plate motions have persisted throughout the cenozoic plate speeds correlate with latitude plates moving faster near the equator than near the poles throughout the cenozoic as at present continental plates except for the indian plate moved slower than oceanic plates throughout the cenozoic even the structure of the velocity fields as revealed in a contour of root mean square velocities in equatorial bands persists throughout the cenozoic the migration of the paleomagnetic axis over time is also compared to the hot spot and mean lithosphere reference frames the paleomagnetic axis has shifted 5 10 relative to the hot spot frame and a lesser amount relative to the mean lithosphere frame

an anomalously thick layering of the crust of the yamato basin southeastern sea of japan the final stage of back arc spreading abstract the yamato basin is believed to have been formed by back arc spreading of the japanese islands a seismic velocity structure beneath the basin was modeled on the basis of seismic data acquired during the delp 85 wakashio cruise conducted by the japanese research group of the international lithosphere project ilp the dataset consists of multichannel reflection profiles airgun shot refraction profiles recorded by ocean bottom seismometers obss and explosion refraction profiles also recorded by the obss the model indicates that the crustal layering is twice as thick as in a normal oceanic basin but the velocity in each layer and that of the underlying uppermost mantle is very similar to that of oceanic basin crust the crust is 14 km thick at the middle of the oblong basin along its strike and thinner towards the rim of the basin tending toward 11 km stratigraphy of the sedimentary layer and the acoustic basement suggest that volcanism during the late stage of spreading is responsible for the thick layering and the seamount chain located along the spreading axis of the basin

seismotectonic properties and zonation of the far eastern eurasian plate around the korean peninsula regional seismotectonics provides crucial information for seismic hazard analysis which is difficult to address with short term earthquake records the far eastern eurasian plate around the korean peninsula presents a stable intraplate environment with diffuse seismicity of which responsible tectonics and active faults are difficult to identify combined analysis of instrumental and historical earthquake records is required for assessment of long term seismicity properties seismotectonic provinces are identified from the spatial distribution of seismicity properties controlled by the medium properties and stress field the boundaries of the seismotectonic provinces are defined considering the medium properties that can be inferred from geological geophysical and tectonic features the gutenberg richter frequency magnitude relationships and maximum magnitudes for the seismotectonic provinces are determined using instrumental and historical earthquake records the validity of maximum magnitude estimation is tested with synthetic data a parametric method the tate pisarenko method produces more accurate estimates than non parametric methods a modified tate pisarenko method is proposed for estimation of maximum magnitudes for incomplete short term earthquake catalogs the maximum magnitude of events for the whole region is approximately the same as the average of the maximum magnitudes of events for subdivided provinces causing apparent variation in maximum magnitudes depending on the number of seismotectonic provinces consideration of a reasonable number of seismotectonic provinces may be needed for proper assessment of seismic hazard potentials is recommended the combined analysis of historical and instrumental earthquake records suggests maximum magnitudes greater than 7 around the peninsula

analysis of gravity anomalies in the ulleung basin east sea sea of japan and its implications for the crustal structure of rift dominated back arc basin abstract the ulleung basin ub one of three major basins in the east sea sea of japan is considered to represent a continental rifting end member of back arc basin system but is much less understood compared to the nearby yamato basin yb and japan basin jb this study examines the gravity anomalies of the ub since the variation in crustal thickness can provide important insights on the mode of extension during basin opening our analysis shows that the moho depth from the sea surface varies from 16 km at the basin center to 22 km at the edges however within the central part of the basin the crustal thickness not including sediment is more or less the same 10 12 km by varying only about 10 20 of the total thickness contrary to the previous suggestions our finding of anomalous but uniformly thick crust is consistent with the recent seismic results from the yb 14 km on average a mantle residual gravity anomaly high 20 mgal exists in the northeastern part of the ub this feature is interpreted as the location of maximum extension slightly thinner crust by 1 km together with another moderate gravity high to the southwest the two anomalies form a nne ssw line which corresponds to the direction of the major tectonic structures of the korean peninsula we argue that the a massive magmatic emplacement took place extensively in the lower crust of the ub during the opening significantly increasing its overall thickness to almost twice as that of the jb where a mid ocean ridge style seafloor spreading occurred two important post opening processes took place after the formation of uniformly thick crust post rift volcanic intrusions in the north especially in its northeast sections but had little effect on the residual gravity anomaly itself and the deflection of crust in response to differential sediment loading towards the south producing the median high in the basement in response to the flexural bending we also conducted a simple test to examine what effect the variations in the mantle potential temperature and degree of extension may have on the gravity anomaly according to our model the latter case is much more likely to cause the variations in gravity anomaly than the former

seismic structure of the extended continental crust in the yamato basin japan sea from ocean bottom seismometer survey abstract we present the result of a seismic experiment conducted using ocean bottom seismometers and an airgun in the yamato basin of the japan sea the japan sea is one of the most well studied back arc basins in the western pacific the japan sea is believed to have been formed by back arc opening however the timing and formation processes of the opening of individual basins in and around the japan sea are not clear to reveal the crustal structure of the yamato basin it is important to consider the formation process of the japan sea therefore we conducted a seismic survey and estimated the p wave seismic velocity structure beneath the 170 km profile using a 2 d ray tracing method a layer with a p wave velocity of 3 4 4 0 km s underlies the sedimentary sections which is thought to consist of a sill and sediment complex the upper crust below the profile varies greatly in thickness the thickness of the upper crust is 3 5 km in the thinnest part and 7 km in the thickest part the thickness of the lower crust is approximately 8 km and is relatively constant over the profile the total thickness of the crust is approximately 15 km including the sedimentary layer the distribution of p wave velocities and the thickness indicate that the crust in the yamato basin is neither a typical continental nor a typical oceanic crust from the point of view of seismic velocity the obtained structure is more similar to a continental crust than to an oceanic crust the large lateral thickness variation in the upper crust and the uniform thickness of the lower crust suggest that the crust in the study area was formed by rifting extension of continental crust during the opening of the japan sea the margins of the continent or of island arcs can be divided into two types volcanic rifted margins and non volcanic rifted margins volcanic rifted margins are normally classified by the presence of a high velocity body in the lower crust at the volcanic rifted margin the high velocity body is formed in the lower crust we did not observe such a high velocity layer at depth on our profile in contrast the acoustic basement consists of a sill and sediment complex therefore we infer that the study area was affected by volcanism during its formation however the magmatism was not significantly massive

geodynamics of the northern andes subductions and intracontinental deformation colombia new regional seismological data acquired in colombia during 1993 to 1996 and tectonic field data from the eastern cordillera ec permit a reexamination of the complex geodynamics of northwestern south america the effect of the accretion of the baudo panama oceanic arc which began 12 myr ago is highlighted in connection with mountain building in the ec the istmina and ibague faults in the south and the santa marta bucaramanga fault to the northeast limit an e se moving continental wedge progressive indentation of the wedge is absorbed along reverse faults located in the foothills of the cordilleras northward of 5 n and transpressive deformation in the santander massif crustal seismicity in colombia is accurately correlated with active faults showing neotectonic morphological evidences intermediate seismicity allows to identify a n ne trending subduction segment beneath the ec which plunges toward the e se this subduction is interpreted as a remnant of the paleo caribbean plateau pcp as suggested by geological and tomographic profiles the pcp shows a low angle subduction northward of 5 2 n and is limited southward by a major e w transpressive shear zone normal oceanic subduction of the nazca plate np ends abruptly at the southern limit of the baudo range northward the np subducts beneath the choco block overlapping the southern part of the pcp cenozoic shortening in the ec estimated from a balanced section is 120 km stress analysis of fault slip data in the ec northward of 4 n indicates an e se orientation of 1 in agreement with the pcp subduction direction northward near bucaramanga two stress solutions were observed 1 a late andean n80 e compression and 2 an early andean nw se compression

tectonic setting of western pacific marginal basins abstract global kinematics as well as magnetic anomalies of marginal basins with continental geology and paleomagnetic data as additional constraints are used to present a set of reconstructions of the western pacific marginal basins between 56 ma and the present at key periods 56 43 32 20 12 and 3 ma our model accounts for the rapid motion of exotic terranes along the whole of the western pacific convergent zone marginal basins appear to open in a great variety of tectonic settings the two extreme examples being the mariana trough where trench suction may be the predominant driving force and the south china sea where intracontinental deformation appears to be the major mechanism the study of marginal basins is a 3 d problem which must take into account the whole tectonic context subduction related tectonics in cross section and upper plate deformation in map view and not only a 2 d problem the classical trench arc back arc problem

spatiotemporal seismicity evolution and seismic hazard potentials in the western east sea sea of japan the earthquakes in the western east sea sea of japan mostly occur in the continental margin off the east coast of the korean peninsula the seismic hazard potentials in and around the western ease sea are studied based on analyses of tectonic structures seismicity features earthquake source properties coulomb stress changes and strong ground motions the earthquake source mechanisms suggest that paleo rifting structures in the western east sea were activated by the current stress field a low stress cumulation rate results in the occurrence of earthquakes with long recurrence intervals the background seismicity suggests that earthquakes with magnitudes m w 5 0 6 0 and 7 0 may occur within every sim 44 sim 336 and sim 2550 years at 95 confidence level the spatial distribution of earthquakes changes with time most earthquakes are clustered within sim 60 km from the coast the seismicity analysis indicates an apparent increase of moderate size m w 3 5 earthquakes since the 2011 m w 9 0 tohoku oki megathrust earthquake static stress changes by moderate size inland earthquakes induce offshore events the seismicity and coulomb stress changes suggest high seismic potentials around the western margin of the ulleung basin earthquakes with magnitudes m w 6 0 7 0 in the western east sea may produce peak ground accelerations of 0 2 g within the distance of sim 40 80 km which includes the coastal regions

the source fault of the 1983 nihonkai chubu earthquake revealed by seismic imaging abstract from 2011 to 2012 we conducted marine seismic surveys using a multichannel seismic reflection mcs system and ocean bottom seismometers obss in and around the hypocentral region of the 1983 nihonkai chubu earthquake the survey covered the area from the japan coast of the japan sea to the yamato basin and the japan basin we investigated the relationship between the nihonkai chubu earthquake and the crustal structure obtained from seismic reflection imaging and the p wave velocity structure near the hypocentral region the focus in the present paper is on seismic data that were acquired using both the mcs system and obss based on the results the crustal structure of the japan sea around the hypocentral region of the 1983 nihonkai chubu earthquake is divided into three types island arc crust thick oceanic crust and oceanic crust the nihonkai chubu earthquake occurred near the boundary between the thick oceanic crust and the island arc crust there is a strong possibility that the east dipping reflector detected in the crust was the source fault of the nihonkai chubu earthquake in addition the location of anticlines formed by the east dipping reverse fault occurring in the boundary of the crustal structure associated with the source fault of the nihonkai chubu earthquake was confirmed in the range of latitude from 39 20 to 41 15 n although they were not found south of this segment there is also a concentrated area of reverse faults and folds approximately 100 km west of the source region of the nihonkai chubu earthquake this area is located near the boundary between the thick oceanic crust and the oceanic crust

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

emeralds in the eastern cordillera of colombia two tectonic settings for one mineralization colombian emeralds are formed through a hydrothermal sedimentary process on the western side of the eastern cordillera the deposits are linked by tear faults and associated thrusts developed during a compressive tectonic phase that occurred at the time of the eocene oligocene boundary prior to the major uplift of the cordillera during the andean phase middle miocene on the eastern side of the cordillera emerald mineralization occurred earlier at the time of the cretaceous tertiary boundary during a thin skinned extensional tectonic event linked to evaporite dissolution this event predates the andean phase during which this part of the chain was folded and thrust over the llanos foreland

seismic stratigraphy and structure of the caribbean igneous province multichannel seismic reflection profiles reveal the tectonic and stratigraphic complexity of the caribbean province a smooth reflector b forms the top of widespread cretaceous volcanic flows drilled during dsdp leg 15 and odp leg 165 in addition we also show other seismic evidence of volcanism sills and dipping horizons the southern parts of the venezuela and colombia basins are underlain by rough basement that shows many characteristics of oceanic crust several thick volcanic plateaus separated by deep basins with thin crust comprise the caribbean volcanic province in the venezuela basin only normal oceanic crust is present but in other basins this crust is underlain by underplated volcanic material the crust of the venezuela basin is less than 5 km thick and was probably formed in the pacific during the jurassic refraction data indicate that a thin layer 2v is formed by original oceanic crust overlain by a thin volcanic layer an intra basement reflection sub b marks the top of original oceanic crust that is sandwiched between an upper volcanic layer and lower underplated material this underplated layer forms a very thick layer 3v beneath the beata and nicaragua volcanic plateaus the upper part of layer 3v is gabbroic and outcrops on the beata ridge the thickening occurs mainly in the high velocity lower part of layer 3v and is attributed to the presence of magnesian rocks picrites or ultramafic cumulates a highly reflective horizon r is located at the top of this layer we present a model where the galapagos plume is 2500 km wide but has narrow heads that formed the caribbean volcanic plateaus separated by deep basins the caribbean igneous province is compared to similar volcanic features in the indian ocean kerguelen plateau and in the western pacific ocean ontong java plateau nauru and pigafetta basins the thick plateaus of the caribbean province resemble the kerguelen and ontong java plateaus whereas the deep basins present some similarities with the nauru and pigafetta basins the caribbean igneous province is younger 90 75 ma than the indian and pacific plateaus 110 130 ma however several geological evidences from land and geophysical offshore observations indicate that the onset of volcanic activity in the caribbean province could be as old as 115 ma