tomographic imaging outside a seismic network application to the northeast japan arc the conventional local tomography can determine the 3d seismic velocity structure right beneath a seismic network but it cannot determine the 3d structure outside a seismic network in this study we show that such a limitation of tomography can be overcome if many earthquakes occur outside a seismic network in the northeast japan arc earthquakes occur actively from the japan trench to the pacific coast we detected and used sp depth phase to relocate the suboceanic earthquakes accurately and then used p and s wave arrival times from the relocated suboceanic events and the earthquakes under the northeast japan land area to determine the 3d p and s wave velocity and poisson s ratio structures of the entire northeast japan arc from the japan trench to the japan sea coast our results exhibit strong lateral heterogeneities under the forearc region the mainshock hypocenters of large interplate earthquakes m 7 0 8 2 mainly cluster near the boundaries of high and low velocity and poisson s ratio above the subducting pacific slab a few large earthquakes are located in areas with high velocity and low poisson s ratio these results suggest that lateral heterogeneities on the slab boundary can affect the rupture nucleation of large thrust earthquakes and the degree of interplate seismic coupling

variance estimate in frequency domain deconvolution for teleseismic receiver function computation summary in this study the problem of estimating the receiver function variance is discussed the receiver function computation is performed through a frequency domain deconvolution oldenburg 1981 of the vertical seismogram from the radial and tangential seismograms this technique requires a knowledge of the noise contaminating the seismic signals and provides an estimate of the receiver function variance oldenburg 1981 modelled this noise as additive amnion 1992 proposed to estimate the noise level from the power spectral density in a pre signal time window in this paper this approach is used jointly with an additional inversion which yields a sequence of spikes as a model for the true receiver function the resulting residual in a time window preceding the direct p pulse is used in order to evaluate the receiver function variance and the actual noise level involved in the frequency domain deconvolution this variance estimate is compared with the one provided by oldenburg s original method the application of this method to teleseismic recordings of earthquakes 5 1 mb 6 1 with similar backazimuths and distances shows that considering the additive noise only is not sufficient to explain the uncertainty measured from the pre signal in the receiver function moreover the estimated uncertainty seems to be independent of the seismic event magnitude suggesting a signal generated noise affecting the receiver function the proposed approach provides a variance estimate for a single receiver function allowing one to assess the statistical accuracy of its amplitudes in agreement with the uncertainty involved in a stacking procedure this is particularly useful when poor quality waveforms from relatively small earthquakes mb 5 1 are analysed and data from nearly co located events are not available for stacking

morphology of the subducting slab boundary in the northeastern japan arc abstract the detection and location of the subducting slab boundary is crucial for a better understanding of the mechanical coupling between the subducting oceanic plate and the overlying continental plate and of the occurrence of large and destructive earthquakes on the main thrust zone in the forearc region of subduction zones it is also important for the understanding of the dehydration of the subducting slab and the genesis of island arc magmatism and volcanism in this study we have estimated the morphology of the upper surface of the subducting pacific plate under northeastern ne japan by using 1372 sp converted waves at the slab boundary and 666 earthquakes beneath the pacific ocean that are well located with sp depth phases we found that the pacific slab beneath ne japan subducts at a small dip angle of less than 10 for the first descent to about 20 km depth and has a steeper dip angle of 15 25 at 30 50 km depth and about 30 at deeper areas the slab depth is approximately 50 km under the pacific coast 150 km under the japan sea coast and nearly 100 km beneath the volcanic front at 60 150 km depths earthquakes in the lower plane of the double seismic zone occur in the central part of the slab while the upper plane events occur within 10 15 km beneath the slab boundary there is a change in the slab surface geometry around 38 n which corresponds to the landward extension of an oceanic fracture zone the strike of the slab is nne in the north and becomes ne in the south the southern part of the slab has a steeper dip angle than the northern part we believe that these features are related to the different patterns of interplate seismicity and mechanical coupling between the northern and southern parts of ne japan many great m8 0 8 5 interplate earthquakes have occured in northern ne japan during recorded history 830 1995 and microearthquakes show a wide distribution in the forearc region from the japan trench to the pacific coast suggesting strong interplate seismic coupling in that region in contrast only a few m7 0 7 5 class earthquakes have occurred in southern ne japan in the same period and microearthquakes tend to cluster in many small areas suggesting weaker interplate coupling there the subducted slab may be segmented by the oceanic fracture zones and the degree of seismic coupling changes from segment to segment the behavior of the segment in southern ne japan might be affected by the subducted seamounts so that the slab subducts at a steeper dip angle this interpretation is supported by geomagnetic topographic and geological studies which show that many small sized seamounts were subducted in the past from the southern part of the japan trench but almost no seamounts were subducted beneath the northern part

the western mediterranean extensional basins and the alpine orogen the western mediterranean late oligocene miocene basins alboran valencia and provencal basins are a coherent system of interrelated troughs in all basins normal faults and thermal subsidence migrated toward the east progressively moving to the miocene to pleistocene algerian and tyrrhenian basins all those troughs appear elements of the back arc opening related to the eastward roll back of the w directed apennines maghrebides subduction zone similarly to western pacific back arc settings these late oligocene early miocene basins nucleated both within the betic cordillera e g alboran sea and in its foreland valencia and provencal troughs the n40 70 direction of grabens is oblique to the coexisting n60 80 trending orogen and shows its structural independence from the orogenic roots thus as the extension cross cuts the orogen and developed also well outside the thrust belt front the westernmost basins of the mediterranean had to develop independently from the alps betics orogen therefore the alboran extension considered a classic example of a basin generated by the collapse of an orogen cannot be ascribed to the detachment or annihilation of the lithospheric root in contrast with the eastward migrating extensional basins the betic balearic thrust front was migrating westward producing interference or inversion structures

crustal fabric in the tibetan plateau based on waveform inversions for seismic anisotropy parameters 1 the tibetan plateau has the thickest continental crust on earth and fabrics within the crust that are anisotropic to seismic waves may provide clues to how it reached such extreme proportions and how it is currently deforming waveform modeling using a global minimization inversion technique applied to receiver functions computed from 11 stations spanning the north south length of the eastern plateau has yielded a suite of crustal models that include anisotropy these models suggest that the tibetan crust contains 4 14 anisotropy at different depths that is likely a result of both fossil fabrics and more recent deformation all models contain anisotropy in the surface layer and for most stations the alignment of the slow symmetry axis suggests a relationship with crustal fabrics associated with e w trending thrust faults or sutures middle to lower crustal anisotropy is present at most stations with a fast axis trending n s to nw se in the south nearly e w in the central plateau and n s to ne sw in the northern plateau this pattern appears consistent with recent ductile deformation due to both topographically induced flow and to boundary forces from subducting lithosphere at the northern and southern margins of the plateau the orientations of crustal anisotropy determined for most stations in this study are significantly different from shear wave splitting fast polarization directions implying distinct deformation in the crust and mantle

crustal complexity from regional waveform tomography aftershocks of the 1992 landers earthquake california we construct a two dimensional velocity section sampling the mojave crustal block in southern california by modeling shear wave sh seismograms our approach uses individual generalized rays computed from a layered model the model is divided into blocks with variable velocity perturbations such that ray responses are allowed to shift relative to each other to maximize synthetic waveform fits to data an efficient simulated annealing algorithm is employed in this search the technique is applied to a collection of 25 aftershocks landers earthquake as recorded at two stations gsc and pfo separated by 200 km which bracket the event population along the landers fault system the events are assumed to have known mechanisms and epicenters but both their depths and origin times are allowed to vary the results indicate considerable variation especially in the top layer up to 13 which mirrors surface geology best fitting models contain a low velocity zone in the lower crust if we constrain the crustal thickness 29 km from receiver function analysis reduced lower crustal velocities imply crustal weakening which appears compatible with the shallow seismogenic zone found in the northern end of this section there is also evidence for a lateral jump in velocity of several percent across the san andreas with the faster velocities on the west

crustal structure of northern latium central italy from receiver functions analysis new evidences of a post collisional back arc margin evolution abstract the crustal velocity structure in a region of central apennines of italy at the hinge between the highly stretched portion of the monte argentario promontory and the magmatic province of the tolfa domes complex northern latium is discussed in this study s wave velocities at depth have been constrained by the modeling of p wave receiver functions rf from both temporary and permanent broadband seismic stations the computer 3d vs models show a thin crust 19 25 km made of a shallow and thin sedimentary cover a very high velocity and anisotropic layer related to a metamorphic basement and a low vs anisotropic layer in the middle lower crust above a shallow moho discontinuity modeled at about 20 km depth the volcano tectonic evolution of this portion of tyrrhenian back arc margin has been strongly influenced by its peculiar crustal architecture the low vs layer acted as a shear zone in the middle lower crust during the tyrrhenian extension also helping the development of plio quaternary magmatism our findings potentially give new constraints on the evolution of the area and to the general comprehension of back arc development in collisional regions

multiscale seismic tomography and mantle dynamics abstract in this article we first introduce the methodology of multiscale seismic tomography and the way to interpret the obtained tomographic images and then review the significant recent results of multiscale seismic tomography with emphasis on mantle plumes and subducting slabs global and regional tomography shows that most of the slab materials under western pacific and east asia are stagnant in the mantle transition zone before finally collapsing down to the core mantle boundary as a result of large gravitational instability from phase transitions local and teleseismic tomography studies have imaged clearly the subducting slabs and arc magma chambers in the upper mantle wedge indicating that geodynamic systems associated with arc magmatism and back arc spreading are related to deep processes such as convective circulation in the mantle wedge and dehydration reactions of the subducting slab because most hotspots are located in poorly instrumented continental and oceanic regions 3 d crust and upper mantle structure is determined for only a few hotspots such as iceland yellowstone and eifel which are covered by seismic networks and plume like slow anomalies are revealed under those hotspots global tomography has revealed deep mantle plumes under the major hotspots such as hawaii iceland kerguelen south pacific and africa strong lateral heterogeneities are revealed at the bottom of the mantle which are associated with the deeply subducted slabs and the birth of mantle plumes a thorough understanding of the deep earth structure will only be achieved by a combination of more effective seismic imaging techniques and dense coverage of global seismic networks particularly in the oceans

low frequency earthquakes and tomography in western japan insight into fluid and magmatic activity abstract low frequency lf microearthquakes are detected in the lower crust and uppermost mantle beneath western japan which may reflect fluid and magmatic activity in the subduction zone in this work we combine seismic tomography and the lf earthquakes to study the crustal and upper mantle structure and fluid and magmatic activity in western japan high resolution tomographic images under this region are determined using high quality arrival time data in kyushu the subducting philippine sea slab is detected clearly as a high velocity zone and the arc magma related low velocity low v anomalies under the active arc and back arc volcanoes are imaged clearly prominent low v zones are visible in the crust and uppermost mantle beneath the quaternary volcanoes along the japan sea coast and lf events occur actively under some of those volcanoes lf events are also detected in the source areas of the 1995 kobe earthquake m 7 2 and the 2000 tottori earthquake m 7 3 supporting the hypothesis that the large crustal earthquakes were triggered by the crustal fluids these results indicate that fluid and magmatic activities exist widely in western japan which are caused by dehydration of the subducting philippine sea slab and corner flow in the mantle wedge

the tyrrhenian apennines system structural setting and seismotectogenesis abstract the available geological geophysical and seismological data of the tyrrhenian apennines system are briefly reviewed and synthesized in a number of sketch maps integrating and comparing these data a large scale simple shear kinematic model is proposed for the tyrrhenian apennines system that is here interpreted as an oblique asymmetric passive continental margin developed during the progressive counterclockwise rotation of the adriatic foreland from the late miocene to the present in the frame of this working hypothesis the thinning and stretching of the tyrrhenian crust were accommodated by progressive eastward motion of extensional allochthonous slices bounded by low angle west dipping oblique shear zones rotating above a common detachment that penetrates into the mantle lithosphere at any given time the most external eastward moving slice is bounded by a transtensional shear zone at the top and by a transpressional one at the bottom the displacement within the hanging wall of the transtensional shear zone is accommodated by high angle normal faults and en echelon grabens the displacement within the hanging wall of the transpressional shear zone is instead accommodated by co axial en echelon sets of folds thrusts and high angle strike slip faults thus the development of the late miocene pliocene pleistocene contractional strain field of the apennines of italy might not be a consequence of complex subduction processes but subordinate to the deformation processes that lead to the progressive opening of the tyrrhenian sea the seismicity of the crustal apennines of italy is analyzed in the frame of the above history of progressive deformation and a seismotectogenetic model is outlined

3 d seismic structure of kii peninsula in southwest japan evidence for slab dehydration in the forearc abstract kii peninsula is located in the forearc region of southwest japan and has distinct structural and tectonic features characterized by high seismicity in the crust and the subducting philippine sea slab high surface heat flow high 3he 4he isotopic ratio and a local change in the geometry of the subducting slab we have tried to determine detailed 3 d p and s wave velocity structures of this region using a large number of high quality arrival time data recorded by dense seismic networks on the japan islands from the obtained seismic velocities we further estimated 3 d distributions of poisson ratio crack density saturation rate and porosity parameter in the study area our results show significant heterogeneities in the crust and upper mantle wedge characterized by low seismic velocities high poisson ratio high values of crack density saturation rate and porosity these results suggest the existence of fluids in the crust and mantle wedge resulting from the dehydration of the subducting philippine sea slab which can explain the observed geophysical and geochemical features in kii peninsula

water transportation from the subducting slab into the mantle transition zone using a recently developed petrogenetic grid for morb h2o we propose a new model for the transportation of water from the subducting slab into the mantle transition zone depending on the geothermal gradient two contrasting water transportation mechanisms operate at depth in a subduction zone if the geothermal gradient is low lawsonite carries h2o into mantle depths of 300 km with further subduction down to the mantle transition depth approximately 400 km lawsonite is no longer stable and thereafter h2o is once migrated upward to the mantle wedge then again carried down to the transition zone due to the induced convection at this depth hydrous phase olivine is stable and plays a role as a huge water reservoir in contrast if the geothermal gradient is high the subducted slab may melt at 700 900 c at depths shallower than 80 km to form felsic melt into which water is dissolved in this case h2o cannot be transported into the mantle below 80 km between these two endmember mechanisms two intermediate types are present in the high pressure intermediate type the hydrous phase a plays an important role to carry water into the mantle transition zone water liberated by the lawsonite consuming continuous reaction moves upward to form hydrous phase a in the hanging wall which transports water into deeper mantle this is due to a unique character of the reaction because phase a can become stable through the hydration reaction of olivine in the case of low pressure intermediate type the presence of a dry mantle wedge below 100 km acts as a barrier to prevent h2o from entering into deeper mantle 2006 published by elsevier b v on behalf of international association for gondwana research

new evidence for a low velocity layer on the subducting philippine sea plate in southwest japan abstract a distinct precursory phase is sometimes found prior to s arrival when deep focus and intermediate depth earthquakes are observed at seismic stations in the chugoku district southwest japan the phase from intermediate depth earthquakes has apparent velocity close to that of s wave in the philippine sea phs plate but at epicentral distance of 100 250 km it arrives earlier by about 2 5 s than the s wave propagated as a head wave along the upper boundary of the hps plate thus we interpreted the phase as sp wave into which the s wave refracted upward after traveling in the high velocity phs plate was converted at a boundary while propagating on a ray path between the plate and seismic station the precursory phase to s wave from deep focus earthquakes was also identified as sp wave that was caused by conversion of the direct s wave into p wave because the s sp times were independent of epicentral distance and focal depth and the direction of horizontal particle motion of the phase almost agreed with the azimuth direction of the epicenter the sp conversion interface was located at the upper boundary of the phs plate further it was demonstrated that a thin low velocity layer should overly just above the phs plate in order to explain both the observed difference in polarity of the s and sp waves and the amplitude ratio of the sp wave to the direct s wave

local earthquake reflection tomography of the landers aftershock area abstract a detailed tomographic image of the crust in the 1992 landers earthquake m 7 3 region in southern california is determined by using the first arriving s waves and the reflected s waves from the moho discontinuity sms ssms generated by the landers aftershocks these data were recorded by two seismic stations that are separated by about 200 km the tomographic image obtained has a lateral resolution of 25 35 km 1 6 to 1 8 of the station spacing this work indicates that crustal reflected waves are very useful to improve the spatial resolution of crustal tomography our results show a tendency for areas rich in seismicity to be associated with higher velocities suggesting that earthquake occurrence is closely related to the in situ structural heterogeneities of the fault zone

structural heterogeneity of the longmenshan fault zone and the mechanism of the 2008 wenchuan earthquake ms 8 0 we determined detailed 3 d images of p and s wave velocity vp vs and poisson s ratio in and around the longmenshan lms fault zone by using a large number of p and s wave arrival times from the aftershocks of the 2008 wenchuan earthquake ms 8 0 and other local events our results show that the structure of the lms fault zone north of the wenchuan main shock is very different from that south of the main shock the southern section of the lms fault zone contains a broad zone with low vp low vs and high anomalies while the northern segment exhibits more scattered heterogeneities corresponding to most of the aftershocks which occurred there a prominent low vp low vs and high anomaly exists directly beneath the wenchuan main shock hypocenter suggesting that in addition to compositional variations fluid filled fractured rock matrices may exist in the lms fault zone which may have influenced the generation of the large wenchuan earthquake our tomographic results provide sound seismic evidence for the hypothesis that an upward intrusion of lower crustal flow occurred along the lms fault zone in addition most small earthquakes before the 2008 wenchuan main shock occurred around the guanxian jiangyou fault while the wenchuan aftershocks were mainly concentrated on the yingxiu beichuan fault suggesting that the rupture process of the wenchuan earthquake may belong to an out of sequence thrusting event a suggestion which is in good agreement with the results from geological surveys and also quite similar to the rupture processes of the 1999 chi chi earthquake m 7 5 and the 2005 kashmir earthquake m 7 6 a few aftershocks occurred close to the blind guangyuan dayi fault in the sichuan basin suggesting that this blind fault was also ruptured by the wenchuan earthquake consistent with geological surveys

seismic detection of the untransformed basaltic oceanic crust subducting into the mantle summary subcrustal earthquakes occur in a slab like zone beneath sw japan in association with the subduction of the philippine sea plate they generate a distinct pair of p and s later phases with anomalously low apparent velocities of about 7 and 4 km s 1 which cannot be explained by a laterally homogeneous velocity structure these later phases are observed for particular source receiver geometries the sources at depths shallower than 50 60 km and the receivers beneath which the subcrustal seismic zone comes in contact with the bottom of the continental crust we examined the nature of these later phases using two methods one is to determine two types of apparent velocities an event common apparent velocity and a station common apparent velocity which may be different from one another for a laterally inhomogeneous velocity structure the other is to trace seismic rays numerically for a laterally heterogeneous velocity model an extensive analysis using these two methods leads to a conclusion that the slab like seismic zone constitutes a low velocity channel in the uppermost mantle with p and s velocities comparable to those of the lower continental crust beneath sw japan the waves trapped within the low velocity channel escape from it through its contact with the continental moho and are observed as a distinct pair of later phases the head waves guided by the bottom boundary of the low velocity channel are observed as weak arrivals of initial phases the thickness 10 km and the velocities vp 7 km s 1 vs 4 km s 1 of the subcrustal seismic zone indicates that it is made up of the untransformed basaltic oceanic crust subducted with the underlying lithospheric mantle and that the transformation of basalt to eclogite does not take place in the subducting slab at least down to depths of 50 60 km we present laterally heterogeneous structural models of the crust and the uppermost mantle along two profiles in sw japan the models are characterized by an apparent thickening of the lower continental crust in particular regions due to a direct contact of the subducting oceanic crust to the continental moho such features are qualitatively consistent with the results of explosion seismology and with the bouguer anomaly pattern in the relevant regions the oceanic lithosphere with the untransformed light oceanic crust at its top might still be buoyant after subduction a factor which must be taken into account in considering the driving mechanisms of plate tectonics the slab geometry and the earthquake related tectonic process

numerical modeling of thermal structure circulation of h2o and magmatism metamorphism in subduction zones implications for evolution of arcs abstract numerical models on thermal structure convective flow of solid generation and transportation of h2o rich fluid in subduction zones are consolidated to have a comprehensive view of the subduction zone processes heat balance circulation of h2o magmatism metamorphism growth of arcs and continental margins a large scale convection model with steady subduction of a cold old slab 130 myr old predicts rapid 100 myr cooling of subduction zones resulting in cessation of magmatism the model also predicts that the mantle temperature beneath arcs and continental margins is greatly affected by the effective temperature of the subducting slab i e the age of the subducting slab if subduction of a young hot slab including ridge subduction occurs every 60 to 120 myr as is suggested for eastern asia the average temperature beneath arcs is increased by about 300 c which may explain the long lasting magmatism in eastern asia associated with subduction of young slabs and ridges thermal structure and circulation of h2o are greatly modified to cause a transition from 1 normal arc magmatism 2 forearc mantle melting to 3 slab melting to produce a significant amount 100 km3 of granitic melts associated with both high p t and low p t type metamorphism the last stage of 3 can result in formation of a granitic batholith belt and a paired metamorphic belts synthesis of the numerical models and observations suggest that episodic subduction of young slabs and ridges can explain heat source for generating a large amount of granitic magmas of batholiths synchronous formation of batholith and regional metamorphic belts and p t conditions of the paired metamorphism even the high p t metamorphism requires an elevated geothermal structure in the forearc region associated with ridge subduction although the emplacement of the batholiths and the regional metamorphic belts and the mass balance in subduction zones are not well constrained at present the episodic event associated with ridge subduction is thought to be essential for net growth of arcs and continental margins as well as for the long term heat balance in subduction zones

seismic imaging of southwest japan using p and pmp data implications for arc magmatism and seismotectonics abstract to better understand the relationship between crustal heterogeneity and seismotectonics and arc magmatism we determined a detailed p wave velocity structure of the crust under a north south belt area in southwest japan by using the first arriving p waves and the reflected p waves from the moho discontinuity pmp from 60 crustal earthquakes recorded by 29 seismic stations our results revealed significant low velocity anomalies beneath the fault zone of the median tectonic line and the daisen volcano which suggest that daisen is potentially an active volcano in the lower crust we detected low velocity anomalies overlying the subducting philippine sea plate which may indicate the existence of fluids released by the slab dehydration the 1955 southern tokusima earthquake m 6 4 occurred between low and high high velocity zones the 2000 western tottori earthquake m 7 3 and other nearby strong earthquakes occurred around the daisen volcano which may be related to the upwelling magma and fluids

configuration of the subducting philippine sea slab in the eastern part of southwestern japan with seismic array and hi net data abstract it is important to know the shape of a subducting slab in order to understand the mechanisms of inter plate earthquakes and the process of subduction seismicity data and converted phases have been used to detect plate boundaries the configuration of the philippine sea slab has been obtained at the western part of southwestern japan at the eastern part of southwestern japan however the configuration of the philippine sea slab has not yet been confirmed a spatially high density seismic network makes it possible to detect the boundaries of the philippine sea slab we used a spatially high density temporal seismic array in the area the configuration of the philippine sea plate is obtained at the eastern part of southwestern japan using the temporal seismic array and permanent seismic network data and comparing the seismic structure obtained from the results of refraction surveys the configuration of the philippine sea plate obtained by this study does not bend sharply compared to previous models obtained from receiver function analyses we delineated the upper boundary of the slab to a depth of about 45 km the weak image of the boundary which corresponds to the upper mantle reflector beneath the source area of the 2000 western tottori earthquake was detected using the spatially dense array

subducting oceanic crust on the philippine sea plate in southwest japan abstract it is well known that subcrustal earthquakes occur in southwest japan owing to subduction of the philippine sea phs plate observing the earthquakes at a single station in the chugoku district we frequently find a distinct pair of later p and s phases after weak initial p and s phases respectively these later phases were observed only for earthquakes with particular source receiver geometries the events in the west seto inland sea epicentral distances from 120 km to 240 km and the source receiver direction almost parallel to the strike of the nankai trough the feature of initial and later phases was summarized as follows 1 the later phases are observed for subcrustal earthquakes occurring at depths between 40 km and 60 km but not for those below the depth 2 single station apparent velocities of initial p and s phases are 8 0 km s and 4 9 km s and they are somewhat higher than ordinary velocities of the uppermost mantle 3 single station apparent velocities of later p and s phases are on the other hand 6 6 km s and 3 8 km s and they are comparable to the velocities of the continental lower crust and the main layer of oceanic crust these features could not be explained unless the subcrustal earthquakes take place within the low velocity oceanic crust overlying the high velocity oceanic plate this suggests that the oceanic crust subducts beneath the west seto inland sea as a part of the phs plate

geochemical and petrological evidence of the subduction of delaminated adriatic continental lithosphere in the genesis of the neogene quaternary magmatism of central italy abstract serri g innocenti f and manetti p 1993 geochemical and petrological evidence of the subduction of delaminated adriatic continental lithosphere in the genesis of the neogene quaternary magmatism of central italy in m j r wortel u hansen and r sabadini editors relationships between mantle processes and geological processes at or near the earth s surface tectonophysics 223 117 147 the neogene quaternary magmatism of the northern apenninic arc took place in four phases separated in space and time which become progressively younger from west to east phase i 14 ma phase ii 7 3 6 0 ma phase iii 5 1 2 2 ma phase iv 1 3 0 1 ma this magmatism is the result of the activation of three physically separate sources 1 the adriatic continental crust extracted from the mantle in the late proterozoic 2 a strongly refractory recently k enriched harzburgitic mantle located in the mechanical boundary layer mbl of the lithosphere and 3 a recently metasomatized cpx rich mantle compositionally variable from iherzolite to wehrlite clinopyroxenite interpreted as an ephemerally k enriched asthenosphere the adriatic continental crust is the dominant source of the acid plutonic and volcanic rocks of the tuscan region the acid magmatism is mostly found inside an ellipsoidal area about 150 300 km centred on giglio island here defined as the tuscan crustal dome within this area mantle derived magmas unaffected by important crustal contamination processes and mixing with crustal anatectic melts have so far not been found pure crustal magmas are rare but are represented for example by some of the san vincenzo and roccastrada rhyolites virtually all the tuscan acid centres show evidence of mixing with potassic mantle derived magmas major and trace elements as well as 87 sr 86 sr and 143 nd 144 nd data on primitive rocks mg 65 reveal two groups of mantle derived magmas these define two distinct mantle enrichment trends both essentially due to the additions of k rich components which metasomatized separate compositionally diverse upper mantle sectors in both cases the most remarkable mineralogical effect of these enrichment processes is the production of variable amount of phlogopite through reaction between fluids and or melts with the mantle the rocks of group i ol hy and q normative lamproites ultrapotassic high mg latites ultrapotassic shoshonites and shoshonites saturated trend are considered to be derived by partial melting at low pressure 87 sr 86 sr 0 717 ce sr 0 3 and k 2 o na 2 o 6 7 and low ratios of 143 nd 144 nd 0 5121 0 5120 and ba la 87 sr 86 sr 0 712 and k 2 o na 2 o 8 9 values and low 143 nd 144 nd ba la 0 10 ratios these constraints do not allow to exclude a complete identity between the k rich components which metasomatized the mantle sources of the saturated and undersaturated trend magmas the geochemical and isotopic features of the components that metasomatized the mantle sources of the northern apenninic arc magmatism can be explained by a geodynamic process which causes a large amount of crustal materials to be incorporated within the upper mantle we propose that the delamination and subduction of the adriatic continental lithosphere related to the still ongoing northern apennine continental collision provide a viable mechanism to explain the genesis and eastward discontinuous migration of the magmatism in central italy the subduction of delaminated lithospheric mantle with lower crustal slivers would have exposed uppermost mantle adriatic mbl and crustal units previously imbricated in the apennine chain to the heating advected by the upwelling of a recently and ephemerally k enriched asthenospheric mantle wedge and by the underplating of magmas derived from it we consider that the diapiric uprising of a hot crustally contaminated asthenosphere occurs in the wake left above the sinking of the adriatic delaminated subducting continental lithosphere the delamination subduction process of the adriatic lithosphere has probably started in the early middle miocene but earlier than 15 14 ma ago as indicated by the age and petrologic characteristics of the first magmatic episode sisco lamproite of the northern apennine orogenesis

possible fault plane in a seismic gap area of the southern apennines italy revealed by receiver function analysis 1 unusual features in teleseismic receiver function computed at a single three component seismic station reveal the presence of a dipping interface in the shallow crust with a pronounced seismic velocity contrast in the campania lucania region southern apennines italy the locus of this finding is the pergola melandro basin situated between the maximum intensity areas of two of the most destructive earthquakes reported in the italian seismic catalogue the m 7 0 val d agri earthquake 1857 and the ms 6 9 irpinia earthquake 1980 this area is not associated with known historical events and for this reason is currently object of investigation as a potential seismic gap where the probability of future ruptures is higher than in surrounding regions also for the static stress increase caused by the two above mentioned earthquakes through an inversion procedure applied to the computed receiver functions we are able to satisfactorily constrain the strike dip depth and velocity contrast of this crustal discontinuity whose geometric characteristics closely match those proposed by some authors for the sources of the two neighboring earthquakes we argue that the detected interface is the seismogenic source which fills the gap the findings of this study could open new perspectives both in the study of seismic gap areas and in the identification of historical earthquake sources offering a new and effective tool to locate investigate and constrain buried or unknown faults

the 2007 niigata earthquake effect of arc magma and fluids to clarify the generation mechanism of large crustal earthquakes in the amur okhotsk plate boundary we determined high resolution 3d seismic velocity v p v s and poisson s ratio structures in niigata northeast japan using a large number of p and s wave arrival times from the aftershocks of the 2004 and 2007 niigata earthquakes as well as many intermediate depth earthquakes in this area our results show that the niigata mainshocks occurred around zones of low velocity and high poisson s ratio which suggest that fluids may have influenced the nucleation and rupture processes of the 2004 and 2007 niigata earthquakes a zone with pronounced low velocity and high poisson s ratio is revealed under the source areas of the 2004 and 2007 niigata earthquakes in the lower crust and uppermost mantle which may reflect the arc magma and fluids ascending from the upper mantle wedge these results indicate that the generation of the 2004 and 2007 niigata earthquakes was affected by the ascending magma and fluids released by the dehydration process of the subducting pacific slab c 2008 elsevier b v all rights reserved

crustal tomography under the median tectonic line in southwest japan using p and pmp data abstract the median tectonic line mtl is the largest active fault system on the japan islands and underlies many densely populated areas to understand the seismotectonics and to reduce seismic hazard in the mtl region we determined a detailed 3 d crustal structure under the region using a large number of arrival times of the first p waves and moho reflected waves pmp results of detailed resolution tests show that the addition of pmp data can significantly improve the resolution of the lower crustal structure and the entire crustal structure can be imaged better than that by using p wave data alone the obtained tomographic images show that prominent low velocity low v anomalies are visible in the upper crust along the mtl in addition some low v anomalies are visible in the lower crust on either side of the mtl these low v anomalies can be the weaker sections of the seismogenic crust and can be imprint of the existence and migration of aqueous fluids released by the dehydration process of the subducting philippine sea plate under southwest japan

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

early diagenesis and its relationship to depositional environment and relative sea level fluctuations upper cretaceous marshybank formation alberta and british columbia early diagenesis of the upper cretaceous late coniacian to early santonian marshybank formation was controlled by depositional environment composition of depositional water fe and organic content of the sediment sedimentation rate proximity to the shoreline and influx of meteoric water related to relative sea level fall five depositional environments each characterized by a distinct early diagenetic mineral assemblage have been recognized offshore shelf sediments that were deposited in a dysaerobic environment are characterized by abundant framboidal pyrite and rare septarian concretions composed of early calcite and siderite intense sulphate reduction promoted by the dysaerobic depositional water was the primary influence on early diagenesis offshore shelf sediments deposited under aerobic conditions are characterized by abundant concretions composed of two generations of siderite s1 and s2 in this environment methanogenesis rather than sulphate reduction was more important early diagenesis of the inner shelf sands was generally limited however in sands deposited proximal to the shoreline mixing of marine and meteoric waters promoted crystallization of fe rich chlorite and siderite the shoreface was characterized by dissolution of detrital minerals in the upper portion and precipitation of kaolinite or illite smectite in the lower portion in the coastal plain environment brackish water and early reducing conditions resulted in formation of abundant euhedral pyrite ankerite rather than siderite is the typical early diagenetic carbonate the 18o values of the earliest cements i e early calcite siderite s1 inner shelf siderite indicate crystallization from a low 18o marine derived porewater assuming crystallization at 25 c a 18o value of about 7 smow can be estimated for the seaway during marshybank formation time similar calculations for the overlying dowling member puskwaskau formation suggest that the 18o value of the seaway increased to about 4 smow consistent with its transgressive nature very low 18o values are exhibited by siderite s2 these results indicate crystallization during intermediate diagenesis 60 c from meteoric water 15 smow that entered the marshybank formation during sea level lowstand

crustal models of the geothermal areas of southern tuscany italy abstract geological and geophysical data gathered in the geothermal areas of southern tuscany located in the inner part of the northern apennines indicate the presence of granite intrusions the most relevant data are 1 regional asthenospheric ascent to a depth of 40 50 km 2 granitic dykes as old as 3 8 2 9 ma and a widespread contact aureole found in several deep geothermal wells 3 very high heat flow values interpreted as the result of convective circulation of hot fluids above shallow magmatic bodies 4 the presence of low density bodies inferred from low gravity values often correlated with positive heat flow anomalies 5 the occurrence of a highly reflective horizon named k in a depth range of 3 12 km in the geothermal areas as shown by seismic reflection data 6 a 30 40 km wide low velocity body below the k horizon whose existence is supported by local earthquake tomography and teleseismic studies 7 the presence of a conductive body which correlates well with the anomalous tomographic and teleseismic body as revealed by magnetotelluric surveys the goal of the paper is to test different alternative crustal models by using these data the models are 1 granite intrusion 2 geopressurized fluids below impervious layers 3 contact metamorphic aureole 4 mylonite layers at the upper lower crust transition and 5 some combination of the four the occurrence of shallow intrusive bodies still partially molten and associated with saline brines is considered the most tenable model

upper crustal structure of the larderello geothermal field as a feature of post collisional extensional tectonics southern tuscany italy abstract reflection seismic profiles crossing the larderello geothermal field permit the distinction of three different seismic facies units in the upper continental crust the upper 3 4 km of the crust is characterized by low reflectivity its base corresponding to a reflector referred to as the k horizon and recognized in many other areas of southern tuscany beneath this reflector a seismic facies of 0 8 1 0 twt thickness is characterized by strong convergent and divergent reflections forming a lozange shaped geometry the lowermost seismic facies is characterized by almost horizontal reflections based on the distribution of the hypocentral microearthquakes temperature data recorded in deep boreholes and the geometry of normal faults the k horizon is explained here as a kinematically active rheological boundary which separates a brittle upper part from a ductile lower part according to this hypothesis the k horizon and the lozange reflections could originate from a mylonite band located at the contact between two geological bodies characterized by different rheological behavior it is inferred that this horizon acts as a decoupling plane in the upper continental crust it developed as a consequence of the extensional tectonics and the high heat flow which characterize southern tuscany

mantle dynamics of western pacific and east asia insight from seismic tomography and mineral physics abstract recent results of high resolution seismic tomography and mineral physics experiments are used to study mantle dynamics of western pacific and east asia the most important processes in subduction zones are the shallow and deep slab dehydration and the convective circulation corner flow processes in the mantle wedge the combination of the two processes may have caused the back arc spreading in the lau basin affected the morphology of the subducting philippine sea slab and its seismicity under southwest japan and contributed to the formation of the continental rift system and intraplate volcanism in northeast asia which are clearly visible in our tomographic images slow anomalies are also found in the mantle under the subducting pacific slab which may represent a small mantle plumes b upwellings associated with the slab collapsing down to the lower mantle or c sub slab dehydration associated with deep earthquakes caused by the reactivation of large faults preserved in the slab combining tomographic images and earthquake hypocenters with phase diagrams in the systems of peridotite water we proposed a petrologic model for arc volcanism arc magmas are caused by the dehydration reactions of hydrated slab peridotite that supply water rich fluids to the mantle wedge and cause partial melting of the convecting mantle wedge a large amount of fluids can be released from hydrated morb at depths shallower than 55 km which move upwards to hydrate the wedge corner under the fore arc and never drag down to the deeper mantle along the slab surface slab dehydration reactions at 120 km depth are the antigorite related 5 reactions which supply water rich fluids for forming the volcanic front phase a and mg surssasite breakdown reactions at 200 and 300 km depths below 700 c cause the second and third arcs respectively moreover the dehydration reactions of super hydrous phase b phases d and e at 500 660 km depths cause the fluid transportation to the mantle boundary layer mbl 410 660 km depth the stagnant slabs extend from japan to beijing china for over 1000 km long indicating that the arc trench system covers the entire region from the japan trench to east asia we propose a big mantle wedge bmw model herein where hydrous plumes originating from 410 km depth cause a series of intra continental hot regions fluids derived from mbl accumulated by the double sided subduction zones rather than the india asia collision and the subsequent indentation into asia are the major cause for the active tectonics and mantle dynamics in this broad region

age and temperature constraints on metamorphism and exhumation of the syn orogenic metamorphic complexes of northern apennines italy abstract the apennine range is a young convergent orogen that formed over a retreating subduction zone syn orogenic metamorphic rocks are exposed in the mid tuscan ridge tuscan coast and tuscan archipelago and provide information about processes of continental subduction and exhumation associated with accretion and retreat apatite and zircon fission track analyses were applied to some of these syn orogenic metamorphic rocks the new low temperature thermochronological data helped to constrain the timing of the metamorphic peak and the age and modality of exhumation of these rocks that were structurally buried at 20 to 40 km depth during the main tectonogenic phase of the northern appenine belt in the giglio and elba islands and in the monti dell uccellina along the coast minimum ages of partially reset zircon samples yielded a first time constraint on the late oligocene age of the metamorphic peak the ca 27 ma obtained at the monti dell uccellina is coeval to that found in the alpi apuane located at the northern termination of the mid tuscan ridge the data from the tuscan archipelago point to a slightly older age in the monti pisani south of the alpi apuane and in the argentario promontary along the coast south of the monti dell uccellina completely reset zircon samples but composed by different populations were behaving as multiple thermochronometers and confirm the already proposed isothermal nature of the exhumation occurred after the metamorphic peak zircon p1 peak indicated that at 13 10 ma rocks were already at shallow crustal levels

lower slab boundary in the japan subduction zone abstract we have successfully detected the lower boundary of a subducting slab the successive imaging of the lower slab boundary beneath northeastern ne japan is attained by receiver function rf depth conversion analysis using a recent 3d tomographic velocity model we use waveforms from 249 teleseismic events collected by hi net and j array short period stations in ne japan rfs are calculated through frequency domain division of radial components by vertical ones with a water level of 0 001 and a 1 0 hz low pass gaussian filter assuming that all later phases in the radial rfs are due to ps phases converted at discontinuities beneath stations we calculate depth converted rfs mapped onto the cross section with the ccp common conversion point stacking in a cross section the slab surface and the oceanic moho can be imaged down to 120 km depth for the greater depths the rf amplitudes corresponding to them cannot be seen because in the oceanic crust basalt would be completely metamorphosed to eclogite below this depth the lower boundary of the pacific slab can also be traced down to 200 km depth or more it is parallel to the slab surface and the oceanic moho and the thickness between the slab surface and the lower boundary is 80 km finally we estimate a top to bottom slab velocity model that explains the rfs observed at broadband stations with the synthetic rfs this model exhibits a 13 velocity reduction downwards the lower slab boundary which would relatively sharp for the base of the thermal boundary layer therefore this sharp discontinuity is presumably considered to be the subducting g gutenberg discontinuity that is formed by the change of the amount of h 2 o water meaning that the g discontinuity is the chemical boundary at the bottom of the oceanic lithosphere the g discontinuity depth is controlled by the potential temperature of the asthenospheric mantle beneath the mid ocean ridge and hence the observed thickness of 80 km i e the g discontinuity depth corresponds to 1375 c in potential temperature when this boundary is formed

seismic images under the beijing region inferred from p and pmp data abstract in this study a new tomographic method is applied to over 1500 high quality pmp moho reflected wave travel time data as well as over 38 500 high quality first p wave arrivals to determine a detailed 3d crustal velocity structure under beijing and adjacent areas results of detailed resolution analyses show that the pmp data can significantly improve the resolution of the model in the middle and lower crust after the pmp data are included in the tomographic inversion our new model not only displays the tectonic feature appeared in the previous studies but also reveals some new features the zhangjiakou bohai sea fault zone zhang bo zone is imaged as prominent and continuous low velocity low v anomalies in the shallower crust while in the middle and lower crust it shows intermittent low v anomalies extending down to the uppermost mantle furthermore the pattern of low v anomalies is different along the zhang bo zone from the southeast to the northwest indicating that there exist large differences in the dynamic evolution of taihangshan and yanshan uplifts and north china depression basin prominent low v anomalies are visible under the source area of the 4 july 2006 wen an earthquake m 5 1 suggesting that the occurrence of the wen an earthquake is possibly related to the effect of the crustal fluids probably caused by the upwelling of the hot and wet asthenospheric materials due to the deep dehydration of the stagnant pacific slab in the mantle transition zone the fluids in the lower crust may cause the weakening of the seismogenic layer in the upper and middle crust and thus contribute to the initiation of the wen an earthquake this is somewhat similar to the cause of the 1695 sanhe pinggu earthquake and the 1976 tangshan earthquake in the region as well as the 1995 kobe earthquake in japan and the 2001 bhuj earthquake in india

three dimensional seismic structure beneath southwest japan the subducting philippine sea plate abstract a three dimensional seismic structure beneath southwest japan has been investigated in detail by applying an inversion method this analysis clearly reveals the present and possibly the past subduction of the philippine sea plate beneath southwest japan beneath the izu peninsula the philippine sea plate does not appear to subduct but collides against the continental asian plate here and its thickness appears thinnest at most about 30 km from the tokai to the shikoku regions the high velocity zone corresponds well to the zone of subcrustal earthquakes down to a depth of about 60 70 km indicating that the subduction of the present philippine sea plate does not reach below this depth here however a small high velocity portion appears at depths between 100 km and 150 km beneath a part of the kinki region and probably extends to the chugoku region where no corresponding seismic activities exist these high velocities might be an indication of the remnants of the past subduction of the philippine sea plate in the kyushu region the high velocity zone continues down to a depth of about 150 km or even to 200 km in the southern part corresponding to a steeply inclined seismic zone the philippine sea plate in this region appears to be thicker than in regions east of the kyushu palau ridge it was also found that there exist narrow low velocity zones intervening within the philippine sea plate where two aseismic ridges the kyushu palau and shichito iwojima ridges extend northwards and the nankai trough shows warping these low velocity zones give support to the buoyant subduction hypothesis it also appears that lateral heterogeneities existing within and around the subducting plate are closely related to the mode of rupture of large earthquakes occurring around there

an attempt to detect temporal variations of crustal structure in the source area of the 2006 wen an earthquake in north china abstract lei et al 2008 revealed a low velocity low v anomaly in the lower crust under the source area of the 4 july 2006 wen an earthquake m 5 1 in north china in this work we tried to investigate the temporal variations of the crustal structure by using a number of p and pmp moho reflected wave arrival times recorded by 107 digital seismic stations from earthquakes that occurred separately in 2002 2003 2004 and 2005 2006 to determine p wave velocity structures in and around the source area of the wen an earthquake in different periods our results show that tomographic images inferred from the data sets in different years are all dominated by a low v anomaly in the lower crust under the wen an source area however there exist some differences in the p wave velocity image in the wen an source area inferred from the p and p pmp data sets suggesting that the pmp data have improved the tomographic images in the middle and lower crust but the results from the 2005 2006 p and p pmp data sets all show a relatively lager increase of the low v anomaly under the wen an source area in the amplitude and extent as compared with those from the 2003 and 2004 p and p pmp data sets incorporating the previous results if this low v anomaly may indicate the existence of fluids then our results suggest that the occurrence of the wen an earthquake is not only related to the long term influence of fluids that decrease the effective normal stress on the fault plane but also closely associated with the drastic increase of such influence however this study is just an experimental work and the results are still preliminary because the resolution scale of the present tomographic model is much larger than the wen an source area and our extensive tests show that different samplings of seismic rays from different data sets have affected the details of the tomographic images suggesting that the present sparse data coverage can hardly detect reliably any temporal variations of the velocity anomalies in the wen an source area in future studies it is necessary to improve the resolution of crustal tomography to the size of the rupture zone and utilize identical seismic ray paths from the same pairs of sources and receivers in order to detect any temporal variations of the crust structure in the source area of a large earthquake

seismic velocity discontinuities in the crust and uppermost mantle beneath the kanto district central japan identified from receiver function imaging and repeating earthquake activity abstract we constructed vertical cross sections of depth converted receiver function images to estimate the seismic velocity structure of the crust and uppermost mantle beneath the kanto district central japan repeating earthquake data for the plate boundary were also used to estimate geometries of the subducting philippine sea plate and the subducting pacific plate as a result we present images of some major seismic discontinuities the upper boundary of the pacific plate dips to the northwest in northern kanto and to the west southwest in southern kanto with some undulations on the other hand the upper boundary of the philippine sea plate as a whole dips to the northwest however it is concave to the northeast in the southern boso peninsula we suggest that the low velocity mantle wedge may be indicated on the top of both subducting plates plate thickness gradually decreases to the northeast the northeastern end of the philippine sea plate is interpreted to be at depths of 45 90 km the moho discontinuity in the overriding plate is deeper than 25 km in the northern kanto it contacts the subducting philippine sea plate in the southwestern part near 35 8 n

east asia seismotectonics magmatism and mantle dynamics abstract in this article we review the significant recent results of geophysical studies and discuss their implications on seismotectonics magmatism and mantle dynamics in east asia high resolution geophysical imaging revealed structural heterogeneities in the source areas of large crustal earthquakes which may reflect magma and fluids that affected the rupture nucleation of large earthquakes in subduction zone regions the crustal fluids originate from the dehydration of the subducting slab magmatism in arc and back arc areas is caused by the corner flow in the mantle wedge and dehydration of the subducting slab the intraplate magmatism has different origins the continental volcanoes in northeast asia such as changbai and wudalianchi seem to be caused by the corner flow in the big mantle wedge bmw above the stagnant slab in the mantle transition zone and the deep dehydration of the stagnant slab as well the tengchong volcano in southwest china is possibly caused by a similar process in bmw above the subducting burma microplate or indian plate the hainan volcano in southernmost china seems to be a hotspot fed by a lower mantle plume associated with the pacific and philippine sea slabs deep subduction in the east and the indian slab s deep subduction in the west down to the lower mantle the occurrence of deep earthquakes under the japan sea and the east asia margin may be related to a metastable olivine wedge in the subducting pacific slab the stagnant slab finally collapses down to the bottom of the mantle which may trigger upwelling of hot mantle materials from the lower mantle to the shallow mantle beneath the subducting slabs and cause the slab plume interactions some of these issues such as the origin of intraplate magmatism are still controversial and so further detailed studies are needed from now

moho depth variation in the central part of northeastern japan estimated from reflected and converted waves abstract we detected reflected p and s waves at the moho in seismograms of shallow crustal earthquakes in the central part of northeastern ne japan p to s and s to p mode converted waves at the moho are also detected in seismograms of intermediate depth earthquakes in the subducted pacific slab the depth distribution of the moho in the central part of ne japan is estimated by using travel time differences between these later phases and the first p and s arrivals we collected travel time data of 496 reflected waves from 67 shallow earthquakes and 200 converted waves from 65 intermediate depth earthquakes the depth to the moho estimated is deep beneath the land and becomes shallower toward the eastern and western coastlines the moho depth is about 34 km in the middle of the land area about 30 km under the coastline of the japan sea and 27 29 km under the coastline of the pacific ocean

influence of interaction between small asperities on various types of slow earthquakes in a 3 d simulation for a subduction plate boundary recently the occurrence of slow earthquakes such as low frequency earthquakes and very low frequency earthquakes have been recognized at depths of about 30 km in southwest japan and cascadia these slow earthquakes occur sometimes in isolation and sometimes break into chain reaction producing tremor that migrates at a speed of about 5 15 km day and suggesting a strong interaction among nearby small asperities in this study we formulate a 3 d subduction plate boundary model with two types of small asperities chained along the trench at the depth of 30 km our simulation succeeds in representing various types of slow earthquakes including low frequency earthquakes and rapid slip velocity in the same asperity and indicates that interaction between asperities may cause the very low frequency earthquakes our simulation also shows chain reaction along trench with propagation speed that can be made consistent with observations by adjusting model parameters which suggests that the interactions also explain the observed migration of slow earthquakes

seismic velocity structure of the crust beneath the japan islands abstract more than 13 000 arrival times of 562 local shallow earthquakes selected from the japan university network earthquake catalog are used to investigate the seismic velocity structure of the crust beneath the japan islands we simultaneously determined the hypocenter parameters p and s wave station corrections and depth distributions of the conrad and moho discontinuities beneath the whole of the japan islands by applying an inversion method the p and s wave station corrections show similar distribution patterns they are negative relatively late arrivals of seismic waves in hokkaido and positive relatively early arrivals of seismic waves in western japan in eastern honshu they are positive along the coast of the pacific ocean and negative in the land area and along the coast of the japan sea the conrad discontinuity is located at a depth range from 12 to 22 km in hokkaido the conrad is shallow in the eastern pan and becomes deep toward the west in ne honshu the conrad is deep beneath the land area and becomes shallow toward the surrounding seas in western japan the conrad is shallow in the northern part and becomes deep toward the south the moho is located in a depth range from 25 to 40 km in hokkaido the moho has the shape of a circular cone with a maximum depth of 36 km in most parts of the japan islands the moho is deep beneath the land area and becomes shallow toward the surrounding seas the moho is deepest up to 40 km beneath the central part of the chubu district

crustal heterogeneity in the 2000 western tottori earthquake region effect of fluids from slab dehydration abstract in order to understand the generating mechanism of the western tottori earthquake m 7 3 that occurred on 6 october 2000 in southwest japan we determined high resolution 3d p and s wave velocity and poisson s ratio structures in the epicentral area we used 17 542 p and 13 831 s wave high quality arrival times from 721 tottori aftershocks and other local microearthquakes recorded by the dense high sensitivity seismic network hi net installed recently on the japan islands significant variations of up to 4 for seismic velocity and 9 for poisson s ratio are revealed in the aftershock area areas with large coseismic slips and high aftershock activity are associated with high p wave velocity v p high poisson s ratio and high electrical conductivity which may represent strong and competent parts of the fault zone containing fluids the western tottori mainshock hypocenter is located in a boundary zone where both velocity and poisson s ratio change drastically both p and s wave velocities are slower in the lower crust under the mainshock hypocenter and low frequency microearthquakes were detected within the slow anomalies around the moho discontinuity before and after the occurrence of the 2000 western tottori earthquake these results and other related evidence suggest that the strong crustal heterogeneity in the western tottori source area is associated with fluids and arc magma under nearby the daisen volcano which may have influenced the nucleation and rupture process of the western tottori earthquake the arc magma and fluids are thought to result from the dehydration reactions of the subducting philippine sea slab in this region these results suggest that the nucleation of shallow earthquakes could be controlled by a deep structure and regime and so it is important to study the deeper crust and upper mantle structure and processes for clarifying the generating mechanism of large crustal earthquakes

fine structure beneath the tohoku district northeastern japan arc as derived by an inversion of p wave arrival times from local earthquakes abstract three dimensional p wave velocity structure beneath the tohoku district northeastern japan arc is investigated by an inversion of arrival times from local earthquakes using the method originally due to aki and lee 1976 in the crust 0 32 km depth a low velocity region is found along the volcanic front and its vicinity velocities at depths of 32 65 km are low beneath the regions where many quaternary volcanoes and geothermal areas are distributed in the region deeper than 65 km the subduction of the pacific plate is clearly revealed and the mantle structure above the descending plate is rather uniform these features suggest that volcanic activities have relation to the upper mantle structure the results obtained in this study will be helpful in investigating the mechanism of magma generation in a subduction zone

mapping the crustal thickness in southwest japan using moho reflected waves in this work we have tried to detect and collect later phases associated with the moho discontinuity and use them to study the lateral variations of the crustal thickness in southwest sw japan we first compute synthetic seismograms for local earthquakes taking into account the focal mechanism solutions estimated from first motion polarity data then we compare the synthetics with the observed waveforms to detect the major later phases in seismograms of shallow crustal earthquakes in sw japan taking the advantage of the high quality and great quantity of hi net waveform data now available throughout the entire japan islands we have detected 1659 moho reflected phases pmp and sms from shallow events in sw japan we estimated the crustal thickness in the study area using travel time differences between these later phases and the first p and s arrivals our results show that the moho is deep in chubu district and becomes shallower along the coastlines of the japan sea and the pacific ocean a thinner crust appears in osaka bay and awajii island where the 1995 kobe earthquake m7 2 occurred

zonal structure of cenozoic basalts related to mantle upwelling in southwest japan cenozoic olivine bearing basaltic rocks in the central chugoku district form eight provinces each 20 to 30 km in diameter these provinces can be grouped into three zones almost parallel to the elongation of the sw japan arc based on geological and petrological characteristics the san yo zone the closest to the nankai trough is characterized by a small amount of alkaline basalt in the sekiryo zone intermediate zone transitional basalt is dominant the san in zone along the japan sea coast is characterized by subalkaline basalt and a large amount of andesitic rocks the eruption volume of these volcanics exponentially increases toward the back arc side less than 1 km3 to more than 100 km3 chemical compositions of relatively undifferentiated basalts show systematic across arc variations sio2 and al2o3increase and feo mgo and cao decrease toward the back arc side 44 to 54 13 to 18 11 to 6 13 to 7 and 12 to 8 wt at feo mgo 0 85 respectively melting experiments on the basalt h2o co2 systems show that the pressure and temperature of magma segregation decrease toward the back arc side 17 19 kbar and 1340 1320 c to 8 kbar and 1250 c and the coexisting phases change from a spinel lherzolite assemblage to a harzburgite assemblage these pt conditions define a trend which can be interpreted as a pt path for adiabatic upwelling of the mantle material consequently the observed volcanism and zonation may have been caused by upwelling of a plume from a deep part of the mantle

significance of secondary porosity in interpreting sandstone composition secondary porosity resulting from dissolution of unstable framework grains such as feldspar and rock fragments including chert is common in sandstones extensive dissolution of framework grains may result in misinterpreting the original composition of a sandstone and hence in misinterpreting its provenance to avoid these problems secondary porosity caused by grain dissolution must be recognized sandstone composition may be properly evaluated by including the dissolved portion of a framework grain as a grain rather than as porosity while point counting this should be useful in interpreting original composition of sandstones and in paleogeographic reconstruction of prospect areas

evidence for the location and cause of large crustal earthquakes in japan large crustal earthquakes magnitude 5 7 8 0 depth 0 20 km during a period of 115 years from 1885 to 1999 in japan occurred in or around zones of low seismic velocity revealed by seismic tomography the low velocity zones may represent weak sections of the seismogenic crust the crustal weakening is considered to be closely related to the subduction of the oceanic pacific and philippine sea plates in this region along the volcanic front and in back arc areas the crustal weakening may be caused by active volcanoes and magma chambers in the forearc areas fluids are detected in the earthquake source zones which may have contributed to the crustal weakening and the rupture nucleation the fluids may be related to the dehydration of the subducting philippine sea slab these results indicate that large crustal earthquakes do not strike anywhere but only anomalous areas that may be detected with geophysical methods

structure of the upper part of the philippine sea plate estimated by later phases of upper mantle earthquakes in and around shikoku japan abstract in the chugoku kinki and shikoku districts southwest japan a distinct pair of later p and s phases is frequently observed after p and s phases for earthquakes at depths less than 60 km occurring in the vicinity of the west seto inland sea caused by the subduction of the philippine sea phs plate the observed phases are dominant in all three components with an apparent velocity almost equal to the p and s velocity of the lower crust if the upper mantle earthquakes take place within a low velocity oceanic crust overlying the high velocity phs plate and if this oceanic crust is in contact with the continental lower crust between the sources and the receivers waves traveling through the oceanic and the lower crust can be observed as later phases travel times and synthetic waveforms were calculated using a three dimensional ray tracing method and a three dimensional gaussian beam method the calculated travel times and amplitudes of the later phases are in good accordance with the observed values these indicate that in the subducting oceanic crust a gabbroic phase remains without transformation to eclogitic rocks to depths of about 60 km similar later phases are observed in the chugoku district from the upper mantle earthquakes beneath the shikoku district these indicate that the continental lower crust is in contact with the subducting oceanic crust beneath shikoku and that no mantle wedge exists

metastable olivine wedge in the subducting pacific slab and its relation to deep earthquakes abstract a metastable olivine wedge mow is detected in the subducting pacific slab under the japan sea by applying a forward modeling approach with 3 d ray tracing to high quality arrival time data from 78 well located deep earthquakes the data were recorded by the dense and high quality seismic networks on the japan islands the double difference earthquake location technique is improved and used to relocate the 78 deep earthquakes precisely our results show that the mow does exist in the pacific slab and it has a p wave velocity 3 lower than the normal mantle or 4 lower than the surrounding slab velocity the top and bottom depths of the mow are 400 and 570 km respectively and the mow is located in the central portion of the slab after precise relocation all the 78 deep earthquakes are found to occur within the mow or along its edges within the slab the cut off depth of deep seismicity under the japan sea is consistent with the estimated mow bottom depth our results favor the transformational faulting as the mechanism of deep earthquakes in the pacific slab under the japan sea

subducting plate boundary beneath the northeastern japan arc estimated from sp converted waves abstract in the tohoku district the northeastern part of honshu japan a conspicuous phase is frequently observed between p and s phases in seismograms for events in the lower seismic plane of the double planed deep seismic zone this phase is dominant in the vertical component and has an apparent velocity almost equal to or slightly smaller than the p phase we have never found this phase in seismograms for events in the upper seismic plane from these characteristics the phase is interpreted as sp converted from s to p at the upper boundary of the descending oceanic plate ubp using the travel times of the sp waves we estimated the location of the ubp beneath the tohoku district to be just above the upper seismic plane of the double planed deep seismic zone this position is consistent with those determined from the analyses of the travel times of the scsp and ps waves which are respectively the seismic wave converted from scs to p and that from p to s at the ubp

authigenic mineralizations at unconformities implication for reservoir characteristics abstract scanning electron microscopy coupled with petrographic and reservoir characteristic measurements were carried out on 447 core samples from seven different boreholes penetrating concealed unconformities they include cretaceous devonian lower lias rhaeto liassic unknown age carboniferous limestone series triassic lower carboniferous triassic upper coal measures triassic permo trias permian and multiple unconformities from the middle east western desert iraq reservoir characteristics at unconformities have been adversely affected by the development of authigenic minerals formed by the mixing of formation waters with infiltrated acidic waters into the paleo surfaces followed by extensive marine incursions and invasion of alkaline waters the result is the formation of complex authigenic minerals in the form of overgrowths of quartz and rhombic feldspars accompanied by clay mineral neoformations as pore fillings and coatings in the form of kaolinite illite mixed layered illite smectites and to a lesser extent chlorites the final elimination of porosity was achieved by late diagenetic calcite colomite and sometimes gypsum cementation

modes of stepwise eastward migration of the northern tyrrhenian sea back arc extension evidences from the northern latium offshore italy the structural architecture of a portion of the eastern side of the tyrrhenian sea back arc basin has been defined reinterpreting a public data set made up by seismic reflection profiles a detailed reconstruction of the extensional front migration trough time across the area has been achieved migration has been defined by identifying the generation of unconformities in the graben like basins developed in the area as due to the activation and deactivation of normal faults such phenomena are part of a tectonic process that also involved blocks rotation crustal thinning and stretching syn rift extension affected the area since the early to middle miocene our estimate principally interesting the north westernmost sectors during late miocene and throughout the plio quaternary the extensional front moved stepwise toward the southeastern and eastern sectors widespread postrift conditions established over the whole area since the late pliocene to early pleistocene since the quaternary the locus of active extension further migrated toward the present onshore area where a pronounced volcanic phase developed in this evolutionary framework the rates of observed migration could be correlated with the acceleration of calabrian slab rollback since pliocene furthermore direct evidences of magmatism were not recognized in the offshore although the observed crustal thinning the absence of volcanic bodies in the study area could be related to normal faults which were unable to sufficiently extend the crust in association with lower crust and mantle processes unable to produce melts

deep structure of japan subduction zone as derived from local regional and teleseismic events we have determined a detailed three dimensional p wave velocity structure of the japan subduction zone to 500 km depth by inverting local regional and teleseismic data simultaneously we used 45 318 p wave arrivals from 1241 shallow and deep earthquakes which occurred in and around the japan islands the arrival times are recorded by the japan university seismic network which covers the entire japan islands densely and uniformly we also used 4211 travel time residuals from 100 teleseismic events which are read from seismograms recorded by seismic stations in northeastern japan in comparison with the previous results obtained from only local and regional events the present result for the area around the lower plate boundary and the mantle below the plate is determined more reliably because of the addition of 7035 data from 100 teleseismic events and 41 very deep earthquakes in the crust and uppermost mantle low velocity zones are clearly visible beneath active volcanoes in the mantle wedge the low velocity zones generally parallel with the slab and exist continuously to a depth of about 200 km which is consistent with the petrological geochemical and geodynamic studies we consider that the existence of volcanism related low velocity anomalies in the mantle wedge is a general seismological characteristic of subduction zones in light of all the available tomographic results for many subduction zones in the world the pacific slab beneath japan is imaged more clearly than in previous studies as a high velocity zone with a thickness of 80 90 km and a p wave velocity 4 6 higher than the normal mantle lower velocity anomalies are found in the mantle below the slab

vp and vs tomography of kyushu japan new insight into arc magmatism and forearc seismotectonics high resolution 3d tomographic images of the crust and upper mantle are determined under the entire arc region of kyushu by inverting a large number of arrival time data of p wave s wave and sp depth phases the hypocenters of the offshore earthquakes are accurately located by applying a combined method that makes use of sp depth phases and double difference of the p wave and s wave arrival times our tomographic results confirmed the major features delineated by previous studies and revealed some new features of structural heterogeneity related to arc magmatism beneath the kyushu arc high velocity anomalies correspond to the subducting philippine sea slab whilst low velocity zones in the mantle wedge are clearly imaged coinciding well with the velocity heterogeneities determined by previous studies strong low velocity anomalies are distributed extensively along the volcanic front and extend to the back arc side in the crust and upper mantle our present seismic images together with the previous results suggest that the fluids supplied by the dehydration processes of the descending philippine sea slab and the hot upwelling materials related to the opening of the okinawa trough might have contributed to the unzen volcanism under the forearc region of the kyushu subduction zone prominent low velocity anomalies are visible at depths of 10 40 km along the upper boundary of the subducting philippine sea slab and large interplate earthquakes are located around the low velocity zones which is similar to the features revealed in the northeast japan subduction zone the spatial distribution of large thrust earthquakes below the offshore region reflects the existence of weak coupled or decoupled portions on the interface of the subducting philippine sea slab

inhomogeneous distribution of deep slow earthquake activity along the strike of the subducting philippine sea plate abstract the spatial distribution of deep slow earthquake activity along the strike of the subducting philippine sea plate in southwest japan is investigated these events usually occur simultaneously between the megathrust seismogenic zone and the deeper free slip zone on the plate interface at depths of about 30 km deep low frequency tremors are weak prolonged vibrations with dominant frequencies of 1 5 5 hz whereas low frequency earthquakes correspond to isolated pulses included within the tremors deep very low frequency earthquakes have long period 20 s seismic signals and short term slow slip events are crustal deformations lasting for several days slow earthquake activity is not spatially homogeneous but is separated into segments some of which are bounded by gaps in activity the spatial distribution of each phase of slow earthquake activity is usually coincident although there are some inconsistencies very low frequency earthquakes occur mainly at edges of segments low frequency earthquakes corresponding to tremors of relatively large amplitude are concentrated at spots where tremors are densely distributed within segments the separation of segments by gaps suggests large differences in stick slip and stable sliding caused by frictional properties of the plate interface within each segment variations in the spatial distribution of slow earthquakes reflected inhomogeneities corresponding to the characteristic scales of events

contemporary deformation and seismicity analysis in southwest japan during 2010 2018 based on gnss measurements a significant amount of work has been conducted in the japanese islands on high precision positioning surveying natural disaster mitigation reference frame and datum definitions however there is substantial scope for exploring the current seismotectonic activity of japan which would provide further awareness of disaster management the short recurrence time of great inter plate earthquakes and well recorded geodetic observations provide one of the best examples to analyze the combination of contemporary deformation and seismic intensity in southwest sw japan in the current study we analyzed three districts of sw japan which include mainly kyushu district and parts of chugoku and shikoku districts we predicted global navigation satellite system gnss measurements based on amurian fixed reference frame aref velocities with the help of singular spectrum analysis ssa method and examined the regional deformation from april 01 2010 to december 31 2018 in addition we analyzed the earthquakes clustered calculated by the nearest neighborhood distance method which showed prominently bimodal seismicity of sw japan and revealed the existence of two statistically distinct earthquake populations the events inside a cluster were normally close to their nearest neighbors whereas events from distinct clusters were comparatively aloof from each other finally the paper provides the spatial strain and rotational rate variation for selected seismotectonic districts that are in relatively good agreement with the seismic deformation inferred from earthquake focal mechanisms we believe that our integrated approaches would provide a better insight into the active deformation of sw japan

upper mantle properties of the tuscan tyrrhenian area a framework for its recent tectonic evolution abstract within the alpine mediterranean deformational system several interesting features in the physical properties of the upper mantle have been recognized through the regional analysis of surface wave dispersion a large upper mantle anomaly uma here called the tuscan uma has been identified in the area northeast of corsica which includes part of the northern tyrrhenian sea and part of the north central apennines this anomalous upper mantle area together with the upper mantle characteristics of the tyrrhenian sea located further to the south provides a natural framework for comprehensive interpretation of the large data set of geological and geophysical observations such as land and marine geology volcanism gravity magnetic data seismic data and seismology deep temperatures and surface heat flow

tomography of the source area of the 1995 kobe earthquake evidence for fluids at the hypocenter seismic tomography revealed a low seismic velocity 5 and high poisson s ratio 6 anomaly covering about 300 square kilometers at the hypocenter of the 17 january 1995 magnitude 7 2 kobe earthquake in japan this anomaly may be due to an overpressurized fluid filled fractured rock matrix that contributed to the initiation of the kobe earthquake

structural factors controlling the rupture process of a megathrust earthquake at the nankai trough seismogenic zone summary the nankai trough is a vigorous subduction zone where large earthquakes have been recorded with a recurrence time of 100 200 yr the 1946 nankaido earthquake is well known as an unusual event among these earthquakes because the rupture zone estimated from long period geodetic data is more than twice as large as that derived from seismic wave data in the summer of 1999 an onshore offshore deep seismic survey was performed along a 355 km long profile in the western nankai trough seismogenic zone seismic signals both from an airgun array 207 l and land explosions maximum of 500 kg were recorded simultaneously by 98 oceanbottom seismographs and 93 land seismic stations conventional 2 d seismic reflection data were also acquired along part of the offshore profile from the wide angle seismic data we found a subducting seamount at the centre of the proposed rupture zone with dimensions of 13 km thick by 50 km wide at 10 km depth the seismic velocity image also shows that the seamount is now colliding with the japanese island arc crust from this significant structure this paper proposes that the subducted seamount functioned as a barrier at least during the 1946 earthquake i e the rupture of the 1946 earthquake extended over the entire locked zone to the east of the subducted seamount and then the rupture was deflected around the subducted seamount at the down dip end of the locked zone between cape muroto and cape ashizuri another significant structure a highly reflective layer is obtained beneath shikoku island a very slow p wave velocity 3 km s 1 is necessary in a thin layer at the base of the island arc crust in order to explain the observed high amplitude reflection phases an area of low resistivity obtained by a previous magnetotelluric study corresponds to the highly reflective layer this suggests a possible water layer at the base of the island arc crust the water may be generated by dehydration of the downgoing probably partially serpentinized mantle which is implied by a low p wave velocity 7 5 km s 1 beneath the subducted seamount a locally observed non slip region during the 1946 earthquake at the eastern part of shikoku island is interpreted as a result of weak coupling at the possible water layer

crustal structure fault segmentation and activity of the median tectonic line in shikoku japan abstract we conducted a seismic tomographic analysis to estimate the crustal structure beneath the shikoku and chugoku regions in japan the philippine sea slab phs slab subducts continuously in a se nw direction beneath this region and the crustal structure is complex furthermore the median tectonic line mtl one of the longest and most active arc parallel fault systems in japan hereafter the mtl active fault system is located in this area and the right lateral strike slip movement of this fault system is related to the oblique subduction of the phs slab the mtl active fault system has ruptured repeatedly during the last 10 000 years and has high seismic potential our tomographic analysis clarified the heterogeneous crustal structure along the mtl active fault system this fault system in shikoku can be divided into two segments an east segment and a west segment on the basis of the velocity structure this segmentation model is consistent with other such models that have been determined from geological and geomorphological data such as fault geometry slip rate and faulting history this consistency suggests that the surface characteristics of the mtl active fault system are related to structural properties of the crust in particular a prominent low velocity low v zone is present in the lower crust beneath the east segment our tomographic images show that the lower crust structure beneath the east segment is obviously different from that of the other segment furthermore this low v zone may indicate the presence of fluid possibly related to dehydration of the phs slab as the presence of fluid in the lower crust affects the activity of the fault stress accumulation and the fault failure mechanism may differ between the two segments of the mtl active fault system

spatiotemporal distribution of low frequency earthquakes in southwest japan evidence for fluid migration and magmatic activity abstract low frequency earthquakes lfes in the lower crust and uppermost mantle are widely observed in southwest japan and they occur not only along the subducting philippine sea phs slab interface but also beneath active arc volcanoes the volcanic lfes are still not well understood because of their limited quantities and less reliable hypocenter locations in this work seismic tomography is used to determine detailed three dimensional 3 d p and s wave velocity vp and vs models of the crust and upper mantle beneath southwest japan and then the obtained 3 d vp and vs models are used to relocate the volcanic lfes precisely the results show that the volcanic lfes can be classified into two types pipe like and swarm like lfes and both of them are located in or around zones of low velocity and high poisson s ratio anomalies in the crust and uppermost mantle beneath the active volcanoes the pipe like lfes may be related to the fluid migration from the lower crust or the uppermost mantle whereas the swarm like lfes may be related to local magmatic activities or small magma chambers the number of lfes sometimes increases sharply before or after a nearby large crustal earthquake which may cause cracks and fluid migration the spatiotemporal distribution of the lfes may indicate the track of migrating fluids as compared with the tectonic lfes along the phs slab interface the volcanic lfes are more sensitive to fluid migration and local magmatic activities high pore pressures play an important role in triggering both types of lfes in southwest japan

deep structure of the japan subduction zone abstract we determined 3 d p wave velocity structure down to 700 km depth under the japan islands using a large number of arrival time data from local and teleseismic events simultaneously we collected 207 000 arrival times from 7743 shallow and deep earthquakes occurred in and around japan and 34 148 data from 333 teleseismic events which were recorded by over 1000 seismic stations on the japan islands our tomographic model revealed some new features the philippine sea slab is found to subduct down to 500 km depth under southwest japan though the seismicity within the slab ends at 150 200 km depth significant low velocity anomalies are found to exist in the deep portion of the mantle wedge above the pacific slab which may be caused by the deep dehydration process of the slab slow anomalies are detected in the mantle beneath the pacific slab which may be caused by a mantle plume or upwelling of hot mantle materials associated with the deep subduction of the pacific slab and its collapsing down to the lower mantle

crustal extension in the northern apennines the transition from compression to extension in the alpi apuane core complex an interpretation of northern apennine geology is presented which relates the temporal and spatial occurrence of both compressional and extensional deformation features in terms of the changing dynamic evolution within an accretionary wedge after a model proposed by platt 1986 followed by the initiation and development of continental rifting during cretaceous to eocene time an accretionary wedge formed as remnant tethyan oceanic crust subducted beneath the rotating corsica sardinia microplate microplate collision during the oligocene was characterized by the rapid imbrication of buoyant continental crust of the italian continental margin the record of which is preserved within the duplex structure geometry of the alpi apuane region the overthickened wedge geometry returned to a more stable configuration by developing extensional features during the miocene both listric normal faults at upper crustal levels and shear zones indicating evidence of distributed ductile extensional strain at mid crustal levels are recorded it is proposed that large scale regional extension with associated volcanism beginning in the messinian was represented by the intrusion of asthenospheric material from the subducted plate into the already attenuated accretionary complex further rifting perhaps aided by subduction and back arc processes in the southern apennines led to the formation of the tyrrhenian sea as an oceanic basin both the apennines and north american core complexes record evidence of crustal thickening followed by crustal thinning and finally of continental rifting this suggests that the similar histories of these regions with vastly different plate tectonic settings may both be explained by processes linked to the changing internal dynamics of accretionary wedges

porosity enhancement from chert dissolution beneath neocomian unconformity ivishak formation north slope alaska secondary porosity caused by chert dissolution is common in the hydrocarbon producing fluvial facies of the ivishak formation triassic north slope alaska petrographic observations suggest that macroporosity caused by chert dissolution tends to increase toward the neocomian unconformity in the prudhoe bay field a lateral increase in core porosity from 15 at about 30 km from the unconformity to 30 near the unconformity and in permeability from 50 md at about 30 km from the unconformity to 800 md near the unconformity is evident toward the unconformity this increase occurs within the fluvial facies zone 4 of nearly uniform grain size and framework composition chert litharenite major chert dissolution probably took place during the neocomian uplift when the ivishak formation was exposed to acidic meteoric waters in the near surface environment

distribution of crustal fluids in northeast japan as inferred from resistivity surveys abstract we carried out magnetotelluric mt surveys in central northeastern japan two dimensional resistivity profiles along three survey lines show similar features each other by comparing the resistivity distribution to the distribution of seismic velocities we inferred the distribution and flow of crustal fluids three fluid flow paths were detected based on the distribution of regions of low resistivity the first path ascends from the top of the upper mantle passes through the lower crust and reaches the surface forming a fluid chamber within the lower or middle crust this path is related to the volcanic activity in the backbone range the second path rises from the first fluid chamber and has produced small fluid reservoirs on both sides of the backbone range these small reservoirs are considered to be related to the seismicity of the region the third path is located to the east of the volcanic front and represents another fluid flow path from the uppermost mantle to the lower crust that may have formed a small fluid reservoir to the east of the volcanic front

deep structure of the northeastern japan arc and its implications for crustal deformation and shallow seismic activity abstract seismic tomography studies in the northeastern japan arc have revealed the existence of an inclined sheet like seismic low velocity and high attenuation zone in the mantle wedge at depths shallower than about 150 km this sheet like low velocity high attenuation zone is oriented sub parallel to the subducted slab and is considered to correspond to the upwelling flow portion of the subduction induced convection the low velocity high attenuation zone reaches the moho immediately beneath the volcanic front or the ou backbone range running through the middle of the arc nearly parallel to the trench axis which suggests that the volcanic front is formed by this hot upwelling flow aqueous fluids supplied by the subducted slab are probably transported upward through this upwelling flow to reach shallow levels beneath the backbone range where they are expelled from solidified magma and migrate further upward the existence of aqueous fluids may weaken the surrounding crustal rocks resulting in local contractive deformation and uplift along the backbone range under the compressional stress field of the volcanic arc a strain rate distribution map generated from gps data reveals a notable concentration of east west contraction along the backbone range consistent with this interpretation shallow inland earthquakes are also concentrated in the upper crust of this locally large contraction deformation zone based on these observations a simple model is proposed to explain the deformation pattern of the crust and the characteristic shallow seismic activity beneath the northeastern japan arc

styles of back arc extension in the central mediterranean in the central mediterranean two back arc basins the liguro provencal lpb and the tyrrhenian basin tb opened progressively and consecutively from the late eocene oligocene to the present evolution in space and time of rifting and drifting processes along three different transects across these basins shows differences in the style of extension lpb opened with the formation of a narrow single rift while in the tb deformation and magmatism is spread over a wide area moreover at the northern end of the tb the locus of extension progressively migrated towards the east whereas in the southern tb the locus of extension and magmatism migrated inside the basin inducing continental break up and drifting of the previously formed older conjugate basins we propose that these different styles of back arc extension depend upon internal conditions such as prerift rheology linked with its geological heritage and external conditions e g the style of subduction

mapping the crustal structure under active volcanoes in central tohoku japan using p and pmp data 1 we present high resolution tomographic images of the crust under the active arc volcanoes in the central part of northeast japan tohoku determined by using arrival times of first p waves and post critically reflected waves from the moho discontinuity pmp results of detailed resolution analyses show that the addition of pmp data can improve significantly the resolution of the lower crustal structure after the pmp data are included in the tomographic inversion the low velocity low v anomalies in the lower crust under the active volcanoes can be better imaged the low v zones are clearly visible in the entire crust beneath the volcanoes extending from the surface down to the moho discontinuity arc magma related deep low frequency microearthquakes are located around the low v zones in the lower crust under the volcanoes which occurrence is probably associated with the movement of fluid magma under the volcanoes

tomographic low velocity anomalies in the uppermost mantle around the northeastern edge of okinawa trough the backarc of kyushu a simultaneous inversion for three dimensional velocity structure and hypocentral parameters is used to determine the tomographic images of the backarc of kyushu southwestern japan the data used in the imaging are the first arrival times of 2736 local earthquakes recorded at 305 stations in a wide region from western japan to eastern china additional data are provided by including arrival times of 357 teleseismic earthquakes we got the high resolution of the uppermost mantle images for p waves at least in the depth range to 150 km the latitude range of 30 34 n and the longitude range of 128 132 e results of inversion reveal a major zone of strong low velocity under west off kyushu island in the depth from 40 km to 100 km this feature may result from partially melting in relation to the upwelling of hot mantle around the northeastern edge of the extending okinawa trough

deep structural heterogeneities and the tectonic evolution of the abruzzi region central apennines italy revealed by microseismicity seismic tomography and teleseismic receiver functions abstract the crustal structure of central apennines italy is still poorly defined leaving uncertainties on the tectonic style thin or thick skinned responsible for the development of the thrust and fold belt the today active extension which replaced compression since early quaternary is presumably influenced by the pre existing structure that yields location and segmentation of the fault system to focus on such issues we computed p and s wave velocity models of the crust by using the independent methodologies of local earthquakes tomography and teleseismic receiver function we document strong lateral and vertical heterogeneities that define shallow imbricate sheets of the mesozoic cover that overlay exceptionally high vp and high vs bodies these bodies can be interpreted as either dolomitic or partially hydrated mafic rocks the two alternative interpretations respectively imply an ultra thick deposition of dolomitic rocks in the hanging wall of triassic normal fault or a deep exhumation of the pre mesozoic basement during the early mesozoic sin rift tectonic in both cases these bodies influenced the evolution of the thrust and fold belt very remarkably active normal faults like those ruptured during the still ongoing 2009 l aquila sequence concentrate at the border of these bodies suggesting that they have an active role in the segmentation of the normal fault system the rheological behavior of such high vp high vs bodies weak or strong is still uncertain but of utmost importance to understand the risk of future normal faulting earthquakes

deep slab subduction and dehydration and their geodynamic consequences evidence from seismology and mineral physics abstract we present new pieces of evidence from seismology and mineral physics for the existence of low velocity zones in the deep part of the upper mantle wedge and the mantle transition zone that are caused by fluids from the deep subduction and deep dehydration of the pacific and philippine sea slabs under western pacific and east asia the pacific slab is subducting beneath the japan islands and japan sea with intermediate depth and deep earthquakes down to 600 km depth under the east asia margin and the slab becomes stagnant in the mantle transition zone under east china the western edge of the stagnant pacific slab is roughly coincident with the ne sw daxing anling taihangshan gravity lineament located west of beijing approximately 2000 km away from the japan trench the upper mantle above the stagnant slab under east asia forms a big mantle wedge bmw corner flow in the bmw and deep slab dehydration may have caused asthenospheric upwelling lithospheric thinning continental rift systems and intraplate volcanism in northeast asia the philippine sea slab has subducted down to the mantle transition zone depth under western japan and ryukyu back arc though the seismicity within the slab occurs only down to 200 300 km depths combining with the corner flow in the mantle wedge deep dehydration of the subducting pacific slab has affected the morphology of the subducting philippine sea slab and its seismicity under southwest japan slow anomalies are also found in the mantle under the subducting pacific slab which may represent small mantle plumes or hot upwelling associated with the deep slab subduction slab dehydration may also take place after a continental plate subducts into the mantle