intraplate shortening and underthrusting of a large basement ridge in the eastern nankai subduction zone abstract due to the collision of the izu bonin arc with central japan the subducting philippine sea oceanic plate is affected by lithospheric shortening seaward of the eastern nankai trough in the zenisu ridge area seismic reflection and refraction profiles across the ridge show a series of low angle thrusts cutting through the entire oceanic crust they also indicate that the formation of this ridge is related to a single lithospheric thrust with 10 km throw that inserts a corner of upper mantle within the lower crust we propose that the serpentinized mantle crust transition acts as a decoupling level above which distributed shortening results in the formation of a crustal prism although the amount of deformation is much smaller than in a mountain belt such as the pyrenees a parallelism can be proposed between both systems that correspond to intraplate shortening and have in common a single mantle shear zone and overlying distributed crustal deformation the underthrusting below the eastern nankai accretionary prism of a similar and now inactive tectonic structure the paleo zenisu ridge is evidenced by the deformation pattern of the margin this ridge is probably detached on top of the subduction decollement at least where volcanoes are impinging the continental margin part of the ridge may thus be accreted to the prism backstop we show that both zenisu and paleo zenisu systems accommodate the same n s compression related to convergence partitioning process this subduction kinematics can be understood in terms of a steady state model partially constrained by the trails of volcanoes that have impinged the margin n s shortening is accommodated by the thrusts seaward of the trough and right lateral motion is prevalent along the subduction system based on these kinematics we propose that the formation of the paleo zenisu and zenisu ridges are related to the successive collisions of volcanic massifs of the izu bonin arc with the japanese arc the zenisu system is a result of the collision of the izu block since 2 ma while the paleo zenisu system was the result of the earlier collision of the tanzawa massif between 5 and 3 ma

geodynamic implications of earthquake data in the southern tyrrhenian sea abstract data from short period seismic stations operating in calabria sicily and the aeolian islands have been used to investigate earthquake properties of the southern tyrrhenian lithosphere the results have been analyzed taking into account other geophysical and geological information available in the literature for the same region with the main purpose of contributing to the definition of local geodynamic processes space distribution and focal mechanisms of earthquakes support the hypothesis of southeastward migration of the southern tyrrhenian lithosphere shared by several authors in their tectonic modelling of the area we find that most seismicity appears to be related to two main faults crossing the aeolian island archipelago sisifo and vulcano and propose that these faults are important dislocation structures associated with the lithosphere migration process we also give a detailed picture of the seismic behaviour of the above mentioned fault systems by analyzing the earthquake depth distribution focal mechanisms seismic clustering and strain release some degree of activity was also detected at the eastern boundary of the southern tyrrhenian area calabrian arc in particular our results demonstrate that in this highly stressed zone of contact between different tectonic units tyrrhenian and ionian basins strain is released under seismic form up to depths of the order of 25 km in the crust and not just in the shallowest structures as proposed by other authors

segmentation of the nazca and south american plates along the ecuador subduction zone from wide angle seismic profiles we describe the deep structure of the south colombian northern ecuador convergent margin using travel time inversion of wide angle seismic data recently collected offshore the margin appears segmented into three contrasting zones in the north zone affected by four great subduction earthquakes during the 20th century normal oceanic crust subducts beneath the oceanic cretaceous substratum of the margin underlined by seismic velocities as high as 6 0 6 5 km s in the central zone the subducting oceanic crust is over thickened beneath the carnegie ridge a steeper slope and a well developed high velocity cretaceous oceanic basement characterizes the margin wedge this area coincides with a gap in significant subduction earthquake activity in the south zone the subducting oceanic crust is normal the fore arc is characterized by large sedimentary basins suggesting significant subsidence velocities in the margin wedge are significantly lower and denote a different nature or a higher degree of fracturing even if the distance between the three profiles exceeds 150 km the structural segmentation obtained along the ecuadorian margin correlates well with the distribution of seismic activity and the neotectonic zonation

deep crustal structure of the eastern nankai trough and zenisu ridge by dense airgun obs seismic profiling abstract an unprecedentedly extensive seismic refraction and wide angle reflection survey using 65 ocean bottom seismographs revealed detailed crustal structure around the eastern nankai trough a previously published crustal model shows an abrupt offset of the moho at the south of the zenisu ridge a prominent topographic high along the oceanward slope of the nankai trough our crustal model indicates that this offset of the moho extends southwestward continuously to 138 e decreasing its gap the survey area experienced the last two great earthquakes in 1854 and 1944 however the northeastern part of the survey area seems to have remained unruptured since the 1854 event factors controlling the size of the rupture area for great earthquakes are still a matter of debate there are several candidates for these factors in the survey area hypothetical tectonic boundaries that may or may not be oceanward prolongation of major on land tectonic lines estimated locations of slab disruption and the extent of moho offset along the strike of the zenisu ridge the main purpose of this survey is to clarify the relation between the crustal structure and these geophysical and geological features bounding the rupture area our crustal model from the trough axis to the continental slope is characterized by a well developed sedimentary wedge bounded by island arc crustal blocks consisting of upper and lower crust to the northwest furthermore the subducting oceanic crust which can be traced down to 25 km depth shows that the down dip angle steepens at 55 km landward from the trough axis on the basis of compilation of our crustal model with previously published models around the eastern nankai trough we derived an image of the entire subducting plate geometry for depths shallower than 20 km which is still poorly constrained by the land observation of microearthquakes significant lateral variations of the crustal structure and the slab geometry are recognized along one prominent canyon and the offset of the moho at the south of the zenisu ridge disappears to the southwest of the canyon moreover it seems that the slab disruption recognized at a depth greater than 20 km is connected to this canyon therefore the lateral variation of the crustal structure along the canyon may be one of the causes to stop rupture propagation of great earthquakes furthermore the crustal variation may also form a tectonic boundary that distinguishes the subduction pattern of the philippine sea plate including the influence of the izu ogasawara collision in the eastern nankai trough from the simple subduction pattern of the western nankai trough

deep seismic crustal structure beneath the bonin trough abstract the bonin trough and the bonin ridge are located in the fore arc region of the southern bonin arc system such a conspicuous trough ridge formation along an arc cannot be recognized in other trench island arc systems the southern bonin arc is characterized by other several unique geological and geophysical features the tectonics controlling their formation and evolution has attracted interest of many earth scientists but has not been understood well in 1987 we made a seismic refraction experiment across the bonin trough to clarify the structure of the crust and the upper mantle of this area which has not been known well to date high quality seismic data of airgun shooting and of explosive sources were acquired by ocean bottom seismograph and hydrophone obsh systems a detailed seismic velocity structure model was obtained by modeling of the observed travel times and amplitudes using a two dimensional ray tracing technique the structure is considerably variable across the bonin trough especially in the sedimentary layers in spite of its flat sea floor topography thinning of the upper crust is recognized in the trough suggesting that the bonin trough was formed by rifting of the island arc crust uplifting of the crust and the uppermost mantle beneath the bonin ridge is required in our structure model the depth to the moho boundary decreases from 18 km beneath the shichito ridge to 14 km beneath the bonin ridge this is in good agreement with the crustal structure expected from large positive gravity anomaly observed over the bonin ridge

pliocene pleistocene evolution of the tyrrhenian arc paleomagnetic determination of uplift and rotational deformation abstract the jaramillo subchron and the brunhes matuyama transition have been identified in the pelagic pleistocene marls located on the calabrian side of the messina straits italy polarity of the characteristic magnetization has been deduced from an analysis of multicomponent magnetization this magnetostratigraphic age determination combined with the paleobathymetry of the quaternary marls implies uplift rates of fault bounded blocks of 0 7 1 0 m 1000 yr a positive fold test on three sites from the trubi formation 4 3 ma shows a clockwise rotation of 36 an antipodal test on pliocene pleistocene micritic limestones 0 9 2 0 ma also indicates a rotation of 14 with the same trend both results suggest a rotation rate of about 9 ma these results are in agreement with a tectonic mechanism involving simultaneous rotation and uplifts a discussion of the possible mechanisms indicates that a compression phase oriented ene wsw could have played a dominant role

the mediterranean mare nostrum of earth sciences the mediterranean area is one of the most appealing natural laboratories in the world to study geodynamic and paleoclimatic processes on different scales consequently the mediterranean sea can be considered as the mare nostrum our sea of earth sciences its semi enclosed land locked configuration in a convergent setting between africa in europe in combination with its latitudinal position makes the mediterranean extremely suitable to study both fundamental plate tectonic processes and astronomically induced oscillations in climate for many years to come the mediterranean will certainly remain a fascinating area where an integrated and multi disciplinary approach will increasingly contribute to our understanding of geological geophysical geodetical and geochemical processes in an accurate and high resolution time frame

fault parameters in the messina strait southern italy and relations with the seismogenic source abstract the messina strait is characterized by strong deformations in pliocene and pleistocene times but considering the historical seismicity significant uncertainity exists as to whether any of the surface fault systems reflects the location geometry and mechanisms of the seismogenic source based on field data and structural analyses the parameters length l displacement d age of activity t minimum slip rate min and total geological moment m g have been derived for all the faults exposed on the sicilian and calabrian sides of the strait the analysis of these parameters shows that the highest rates of activity pertain to interconnected faults of different orientation and length which bound the uplifted crystalline basement relative to the pliocene pleistocene foundered zones of the calabrian side the comparison between the fault slip rates and the regional uplift rates shows that each single fault contributes for a limited fraction only to the whole deformation release in the area the contribution of the whole fault population expressed as cumulative m g release is still one order of magnitude less than the seismic moment release taking into account the underestimation of the geological parameters the lack of information on the offshore faults and the role of anelastic deformation by aseismic creep the data suggest that the long term recurrence of seismic breaks is partitioned at the surface in a complex array of interconnected faults some of these faults might represent basement inherited structures repeatedly and preferentially reactivated over geological time

strain partitioning in the zone of transition from nankai subduction to izu bonin collision central japan implications for an extensional tear within the subducting slab abstract the izu bonin ridge collision with central japan is a type example of collision of two island arcs in a subduction context based on the analysis of active faults paleomagnetic data and recent seismicity in the northern tip of the izu bonin arc and the adjacent oceanic plate we propose a tectonic schema of the izu collision system and its transition to the nankai subduction to the west the dense gps network deployed in central japan allows us to constrain the kinematics of the different blocks identified this new kinematic model demonstrates the essential role of strain partitioning there as the northward component of the izu bonin arc motion is locked in the collision zone 30 of the convergence between the philippine sea plate and central japan is accommodated within the philippine sea slab along the zenisu east izu deformation system we show that this pattern of deformation is related to an extensional tear of the philippine sea slab propagating southward along the volcanic arc we discuss the implications of this kinematic model on the seismogenic potential of the area and the dynamics of the central japan convergence zone

faults configuration around the eastern nankai trough deduced by multichannel seismic profiling abstract the japan marine science and technology center jamstec the ocean research institute university of tokyo and the laboratoire de geologie ecole normale superieure at paris carried out in collaboration a 120 channel reflection seismic survey using an airgun array with total 4200 cubic inch capacity around the eastern nankai trough in 1996 data obtained by r v hakuho university of tokyo were primarily processed by jamstec and were shared by the above three main bodies of this program to complete geoscientific interpretation the fault configuration around the eastern nankai trough is very complicated due to the collision of the izu peninsula against central japan the major remarkable faults around the eastern nankai area are the zenisu the kodaiba and the tokai thrusts the main objective of this paper is to clarify these fault configurations the zenisu thrust dips gently northward from the southern slope of the zenisu ridge and reaches the oceanic moho the location of the tokai thrust may jump at the tenryu canyon and appears to reach the top of the subducting oceanic plate the kodaiba fault penetrates a 5 5 5 6 km s layer which is the crust of the central japan forearc a large fault in the trough axis cuts the entire oceanic crust and has a flower structure which indicates that it has strike slip component its strike is nw se parallel to the plate motion and perpendicular to the strike of the zenisu ridge the top of the subducting oceanic crust of the philippine sea plate was defined beneath the continental slope of the central japan forearc the angle of subduction becomes abruptly steeper 50 km north of the deformation front we also observed at the southern roof of the zenis thrust a drape fold structure which is proof of subduction we suggest a subduction on the southern side of the zenisu ridge strongly deformed oceanic crust at the trough axis and some major faults in the forearc region reaching subducting oceanic crust

elevation of the last interglacial highstand in sicily italy a benchmark of coastal tectonics well preserved mis 5 5 terraces in sicily are identified primarily by the index fossil strombus bubonius and dated by amino acid racemization aar electron spin resonance esr uranium thorium u th and thermo luminescence tl methods this review of published data and new results for the island of sicily and neighbouring small islands of egadi ustica and lampedusa identifies areas of rapid uplift in the east up to 175 m elevation above sea level slower uplift in the north 29 m and relative stability in the northwest 2 18 m in contrast about 250 km of the southern coastline of sicily does not appear to contain mis 5 5 outcrops in eastern sicily correlation of mis 5 5 highstands is based on strombus bubonius discovered at 86 m and correlated with the inner margin terrace at 110 m in the taormina area a fossiliferous marine conglomerate on a terrace with an inner margin at 115 m occurs in an area with undated terrace morphology and elevation data based on esr methodology applied to fossils sampled at 105 m in taormina we attribute this terrace to mis 5 probably 5 5 this age allows us to constrain the date of one point along a very long coastline that is otherwise undated a newly discovered fossil beach between 7 and 9 m at cefalu north central sicily attributed to mis 5 1 5 3 using aar analysis permits correlation of mis 5 5 to a 29 m high tidal notch geomorphologically related to a terrace at the same elevation cefalu lies in an important position between the uplifted coastline of northeastern sicily and the more stable coastline of western sicily this compilation of mis 5 5 data for all of sicily reflects the active tectonics of eastern sicily in contrast to the rest of the island

deep structure of the u s atlantic continental margin offshore south carolina from coincident ocean bottom and multichannel seismic data we present the results of a combined multichannel seismic reflection mcs and wide angle ocean bottom seismic profile collected in 1988 across the carolina trough on the u s atlantic continental margin inversion of vertical incidence and wide angle travel time data has produced a velocity model of the entire crust across the continent ocean transition the margin consists of three structural elements 1 rifted continental crust comprising 1 4 km of post rift sedimentary rocks overlying a 30 34 km thick subsedimentary crust 2 transitional crust a 70 to 80 km wide zone comprising up to 12 km of postrift sedimentary rocks overlying a 10 to 24 km thick subsedimentary crust and 3 oceanic crust comprising 8 km of sedimentary rocks overlying an 8 km thick crystalline crust the boundary between rifted continental and transitional crust marked by the brunswick magnetic anomaly represents an abrupt change in physical properties with strong lateral increases in seismic velocity density and magnetic susceptibility the transitional crust contains mid crustal seaward dipping reflections observed on the mcs section and has seismic velocities of 6 5 6 9 km s in the midcrust and 7 2 7 5 km s in the lower crust modeling of potential field data shows that transitional crust also produces the prominent margin parallel gravity anomaly and the brunswick and east coast magnetic anomalies these observations support the interpretation that the transitional crust was formed by magmatism during continental breakup the prodigious thickness up to 24 km of igneous material rivals that interpreted on continental margins of the north atlantic e g hatton bank and voring plateau which formed in the vicinity of the iceland hotspot these observations when combined with other transects across the margin confirm previous suggestions that the u s atlantic margin is strongly volcanic and further imply that the magmatism was not the result of a long lived mantle plume

long term behaviour of the late quaternary normal faults in the straits of messina area calabrian arc structural and morphological constraints abstract the calabrian arc is located in the peri tyrrhenian orogenic belt of southern italy and is characterised by intense post collision dynamics this consists of rapid regional uplifting combined with active normal faulting affecting the tyrrhenian side of calabria and the ionian coast of sicily the siculo calabrian rift zone the effects of the two combined processes are well exposed in the straits of messina area that represent a major transfer zone located between two opposing sets of normal faults characterising the southern calabria and the northeastern sicily branches of the rift in the straits of messina region the distribution and elevation of the upper quaternary marine terraces suggest a southward propagation of the normal fault belt from calabria to sicily it occurred at about 125 ka after a long period of tectonic quiescence and was triggered by huge elastic releases in southern calabria the long term behaviour recorded on the distinct fault segments of the rift is to relate to the presence of a crustal barrier that influenced both the geometry and the fault propagation along the rift zone

crustal seismic imaging from multifold ocean bottom seismometer data by frequency domain full waveform tomography application to the eastern nankai trough 1 in offshore surveys the deep crust is generally investigated by traveltime tomography applied to sparse ocean bottom seismometer data the inferred velocity models are of limited resolution precluding a quantitative analysis of deep tectonic discontinuities if dense arrays of ocean bottom seismometers can be deployed the resulting data sets become amenable to full waveform processing which should lead to a significant improvement in the resolution of structures such acquisition and processing were achieved on the eastern nankai subduction zone full waveform inversion is entirely implemented in the frequency domain enabling efficient finite difference wave modeling and allowing to limit the inversion to a few discrete frequencies of increasing value such a hierarchical procedure defines a multiresolution approach to seismic imaging the data set was recorded by 93 instruments deployed along a 105 km long profile thirteen frequencies ranging from 3 to 15 hz were inverted sequentially wavelengths imaged by the full waveform inversion typically range between 0 5 and 8 km full waveform tomography reveals an intense compression in the upper prism and underlying backstop as evidenced by several thrusts the frontal ones still active corresponding to negative velocity anomalies the subducting paleo zenisu ridge is also reconstructed above which stands the decollement velocities in the upper mantle beneath the ridge are rather low 7 5 km s 1 suggesting partially serpentinized mantle beneath the ridge this paper demonstrates the feasibility of full waveform tomography based on dense ocean bottom seismometer data sets and its ability to quantitatively image the entire crust with a significantly improved resolution compared to what is usually achieved through traveltime tomography

holocene tectonic uplift patterns in northeastern sicily evidence from marine notches in coastal outcrops abstract marine notches are groove like features formed in rocky coastal outcrops in relation to past sea levels and are widely used as indicators of rates and patterns of tectonic activity examination of uplifted notches in the complex plate boundary setting of northeast sicily reveals the need for considerable care in their interpretation present notch forming processes are dominated by carbonate dissolution aided by microbial activity as waves surge and recede at the base of the cliffs and produce notches approximately 0 5 m above present biological mean sea level i e above an organic constructional rim of coralline algae used to define sea level at sea level dissolution is restricted through saturation and the bedrock protected by the outbuilding algal rim this assemblage varies in response to fetch exposure and uplift rate typically producing a complex band of uplifted notch features exposed above sea level correlations based on height of the shallow and discontinuous notch indentations within this band are unreliable however the upper margin of the band marked by the roof of the uppermost notch is a prominent and laterally persistent feature easily recognised as the boundary between subaerial and marine processes we propose that this is an approximately coeval datum formed when post glacial sea level rise decelerated in the mid holocene to match the rate of tectonic uplift a notion strengthened by the available dates of about 5 ka for this feature subsequent uplift outpaced later sea level rise taking the notches above sea level this proposed datum varies in height from about 2 m at capo milazzo on the north coast to about 5 m on the east coast bordered by the messina fault system the height differential is consistent with the respective elevations of nearby marine terraces dating from the last interglacial at about 125 ka but also exhibits shorter term variations suggesting holocene increases in activity rates on more localised structures affecting the localities involved regionally northeast sicily is characterised by long term differential uplift rather than behaving as a coherent block the highest uplift rate probably exceeding 2 mm a 1 calculated using the sea level curve from fairbanks r g 1989 a 17000 year glacio eustatic sea level record influence of glacial melting rates on the younger dryas event and deep ocean circulation nature 342 637 642 is suggested for a notch locality on the upthrown side of the malta escarpment rates of about 2 mm a 1 are indicated by the 5 m notch at localities bordered by the messina fault system and a rate close to 1 mm a 1 is suggested for the undated 2 m notch at capo milazzo

crustal structure across the middle ryukyu trench obtained from ocean bottom seismographic data abstract in 1988 an extensive geophysical investigation was performed at the middle ryukyu island arc s japan an aim of the investigation is to obtain a detailed crustal structure at the trench arc back arc system by various geophysical surveys in this paper we present fine crustal models obtained from ocean bottom seismographic obs data along two profiles one profile was acquired from the northwest edge of the philippine sea plate to the okinawa island profile 2 a and the other profile which is perpendicular to profile 2 a was located on the ryukyu arc profile 3 a crustal model of profile 2 a is characterized by a sedimentary wedge and subducting oceanic crust the sedimentary wedge consists of four layers whose velocities are 1 8 2 8 2 9 3 5 and 4 7 5 0 km s at the top of each layer the maximum thickness of the wedge is 9 km at 50 km landward from the trench underlying the sedimentary wedge the model shows subducting oceanic layer 2 and 3 5 3 and 6 8 km s a crustal model of profile 3 indicates three layers of sediment whose velocities are 1 7 2 0 2 4 2 8 and 3 7 km s above a 5 6 6 0 km s layer a low q value layer at the bottom of the sedimentary layer is required to explain an observed amplitude reduction versus offsets three deep crustal reflection phases are observed along profile 3 these phases are interpreted as the reflection phases from the oceanic crust subducting beneath the okinawa island the depth of the subducting oceanic moho is deduced to be 26 km at 120 km landward from the trench axis by the reflection phases

sea level change along the french mediterranean coast for the past 30 000 years abstract observational evidence for sea level change along the french mediterranean coast has been examined and compared with glacio hydro isostatic models to predict the spatial and temporal patterns of change for about the past 30 000 14c years these predictions are pertinent to discussions of changing ocean volumes during this interval the tectonic stability or otherwise of the coastal areas mantle rheology and the timing of possible human occupation of the now submerged coastal plain and caves such as cosquer cave near marseille the principal results from the analysis are i sea levels along this section of the coast have risen continually since the time of the last glacial maximum lgm and at no time during the holocene has the mean sea level been higher than that of today ii the coast has been tectonically stable between marseille and nice as well as further to the west in roussillon western corsica may have experienced a slow tectonic uplift of between 0 15 0 3 mm year for the past 3000 years but northernmost corsica appears to have been stable during this same interval iii during the lgm sea levels along the coast and immediate off shore areas stood at between 105 115 m below present level the range reflecting the importance of the isostatic contributions during oxygen isotope stage 3 sea levels do not appear to have risen locally above about 60 m iv the rebound parameters describing the mantle rheology and ice sheets required to match the limited observational evidence are consistent with the results of similar analyses carried out for other parts of europe because of its distance from the former northern ice sheets the isostatic factors are particularly sensitive to the value of the lower mantle viscosity v the model predictions for sea level change at the cosquer cave site and for its immediate environments indicate that the cave was last readily accessible before about 10 700 500 14c years about 12 500 500 cal years bp and that the cave entrance was completely flooded by 9000 200 radiocarbon years bp between about 9800 and 10 300 calibrated years bp the cave was above sea level throughout the oxygen isotope stage 3

zenisu ridge mechanical model of formation abstract seabeam seismic and submersible surveys took place during the kaiko project and revealed significant compressive deformation at the northeastern end of the philippine sea plate related to the recent collision of the izu ogasawara arc against central japan intraoceanic thrusting at the base of the zenisu ridge a linear topographic high running a few tens of kilometers south of the nankai trough is supported by tectonic magnetic and gravimetric data we investigate the formation of the zenisu ridge in terms of compressive mechanical failure of a thin elastic perfectly plastic plate subducting at a trench and subject to a regional compressive axial force the rheological envelope concept is used throughout the numerical calculations based on a detailed study of flexure of the present day bending far from the deformation zone we evaluate the bending forces involved the bulge is 120 to 150 m high and the compressive stress all along the nankai trough is about 100 mpa in the zenisu ridge area an additional compressive stress is superimposed due to the nearby collision at izu peninsula we compute the vertical distribution of the deviatoric stress before failure and find that the deviatoric stress is maximum at a depth of 20 25 km in the trench area and again at the surface 60 to 100 km seaward in the vicinity of the bulge the development of a thrust joining these two maxima through the entire thickness of the lithosphere is discussed the model predicts that the formation of the zenisu ridge did not occur before 4 ma and is caused by progressive tectonic uplift due to the redistribution of bending stresses as the ridge approaches the nankai trough

a 125 kyr long geological record of seismic source repeatability the messina straits southern italy and the 1908 earthquake ms7 2 the occurrence of characteristic earthquakes along segments of major active belts has been described as early as the beginning of the century the large 1908 messina straits blind normal faulting earthquake provides an opportunity to investigate the repetition of characteristic earthquakes and the persistency of the main features of the fault zone over a time scale in the order of 100 kyr we have compared seismological and geodetic descriptions of the 1908 rupture process with long term crustal deformation recorded at different stratigraphic levels by marine terraces marine sedimentary units and landscape features the wealth of elevated coastal features in and around a region affected by large normal faulting earthquakes indicates that coseismic strain is superimposed on much faster uplift on a regional scale we investigated in detail the pattern of deformation of the 125 kyr old lowest emergent terrace and found that it mimics the pattern of elevation changes observed following the earthquake this suggests 1 that the 1908 earthquake is characteristic 2 that its segment boundaries have been stationary during the past 125 kyr and 3 that even the locations of the regions of largest coseismic release do not change through many seismic cycles based on a comparison between observed heights of the 125 kyr terrace and 1908 displacements we estimated an average repeat time of 1000 yr for a 1908 type earthquake we also investigated the height position age and state of deformation of older terraces and found 1 that uplift of the region started about 1 ma and has since continued rather uniformly at 0 9 1 4 mm yr 1 2 that during this time the straits have narrowed at a rate of 10 20 mm yr 1 and 3 that all the main geomorphic features of the messina straits result from the interaction of fast regional uplift 1908 type coseismic subsidence and global sea level fluctuations

continental crust crustal underplating and low q upper mantle beneath an oceanic island arc a detailed structural model of the crust subducting slab and underlying upper mantle across the northern izu ogasawara bonin island arc system is derived from a marine seismic reflection and ocean bottom seismographic refraction survey and subsequent forward modeling combined with tomographic inversion the model indicates that the crust is thickest beneath the presently active rift zone and a granitic crust may have formed in the mid crust a highly attenuative mantle that is one with low quality q seems to be confined mainly beneath the presently active rift zone in contrast high p wave velocity persists in the lower crust between the forearc and eastern margin of the back arc basin suggesting a large scale magma input responsible for the arc formation

hypocenters in the nankai trough determined by using data from both ocean bottom and land seismic networks and a 3d velocity structure model implications for seismotectonic activity abstract we investigated the hypocenter distribution along the nankai trough off the kii peninsula japan by using data from both ocean bottom and land based seismic networks because seismic velocity structures differ greatly below land and ocean we used a 3d velocity model based on a structural model of the philippine sea plate phs which provides much more accurate estimations of source locations than 1d velocity models derived from either onshore or ocean bottom seismic data alone especially for sources between the coast and the trough axis we identified intensive seismic activity below the axis of the nankai trough which we interpreted to be aftershocks of the 2004 off the kii peninsula earthquakes m jma 7 1 7 4 and 6 5 but found much less seismic activity along the subducting phs the earthquake sources below the trough axis lie on dipping planes within the phs from which we inferred that a fracture zone is developing within the phs our estimated focal mechanisms for these earthquakes showed that their p axes were oriented north south to northeast southwest similar to those of the 2004 events which were rotated clockwise by 45 90 from the northwest southeast direction of convergence between the phs and the eurasian plates we consider that the fracture zone represents a boundary between the phs and the izu microplate which are converging in a north south direction and that the 2004 earthquakes and subsequent seismic activity were caused by the collision of these plates

crustal structure across the coseismic rupture zone of the 1944 tonankai earthquake the central nankai trough seismogenic zone 1 differences in the coseismic rupture process between the 1944 tonankai and the 1946 nankaido earthquakes have been studied by many fault models to understand what factors control coseismic rupture zones it is important to investigate differences in deep crustal structures of the rupture zones between the 1944 and 1946 earthquakes the previously published crustal structure of the rupture zone of the 1946 earthquake shows that the coseismic rupture extends to the neogene quaternary accretionary prism however little is known about the structure of the rupture zone of the 1944 earthquake to obtain a complete image of the seismogenic zone of the 1944 earthquake a wide angle seismic survey was performed across the presumed coseismic rupture zone of the 1944 earthquake from ocean to land our model for the crustal structure is based on ocean bottom seismographic data the crustal structure appears characteristic for subducting oceanic crust and a neogene quaternary accretionary prism bounded by an island arc crust the neogene quaternary accretionary prism reaches a maximum thickness of 7 km at 50 km distance landward from the deformation front the subducting oceanic crust can be traced down to 35 km the subduction angle becomes steeper landward reaching up to 11 beneath the island arc crust the depth of the top of subducting oceanic crust at the downdip limit of the rupture zone is 23 km while the updip limit is located beneath the island arc upper crust similar structures of the updip and downdip limits are also published for several other subduction zones

nankai trough and zenisu ridge a deep sea submersible survey abstract eight submersible dives between 3000 and 4200 m water depth were made off southern japan in the eastern nankai subduction zone benthic communities associated with chemosynthetic processes were discovered along the 800 m wide active tectonic zone at the toe of the accretionary prism a benthic community was also discovered along a zone of active compression at the foot of zenisu ridge 30 km south of nankai trough temperature measurements within the sediments below the benthic communities confirm that upward motion of interstitial water occurs there studies of water samples indicate advection of methane and light hydrocarbons specimens of the benthic community have been shown to have included in their shells carbonate resulting from methane consumption thus the benthic communities are related to overpressure driven fluid advection along tectonic zones with active surface deformation a 300 m high active scarp at the toe of the accretionary prism is related to relative motion in a 280 direction which is close to the 305 average direction of subduction in this area the dives establish further that compressive deformation is presently occurring at the foot of zenisu ridge the previous interpretation of the zenisu ridge as a zone of recent north south intraplate shortening 40 km south of the nankai trench is confirmed we conclude that tectonic evolution might well lead to future detachment of the zenisu ridge and overthrusting of this large piece of oceanic crust over the continental margin such a process might be an efficient one to emplace ophiolites over continents

seismicity at the eastern end of the 1944 tonankai earthquake rupture area the rupture areas of the large interplate thrust earthquakes along the nankai trough offshore southwestern japan are divided into several segments the 1944 tonankai earthquake ruptured one of the segments off of the kii peninsula in 2005 we conducted an ocean bottom seismograph experiment at the eastern end of the rupture area of the 1944 earthquake little seismic activity was observed on the plate interface most earthquake activity was within the subducting and overriding plates aftershocks of the 2004 off kii peninsula earthquake which was an intraplate earthquake in the subducting philippine sea plate were mainly located in the subducting oceanic crust and uppermost mantle however several earthquakes at the eastern end of the rupture area of the 1944 earthquake were in the sedimentary wedge with a focal mechanism indicating deformation by a subducting seamount the earthquakes and faults in the sedimentary wedge show deformation related to the irregular topography of the subducting oceanic crust to the east of the rupture area of the 1944 tonankai earthquake in contrast few earthquakes were observed in the sedimentary wedge in the rupture area of the 1944 earthquake difference in topography of the subducting oceanic crust in the two areas might have caused spatial variations of interplate coupling which in turn caused the observed differences in the deformation of the sedimentary wedge the spatial variation of interplate coupling may affect rupture propagation during large interplate earthquakes and cause the segmentation of large interplate earthquakes along the nankai trough

crustal structure around the eastern end of coseismic rupture zone of the 1944 tonankai earthquake abstract the high potential for a major earthquake to occur in the suruga trough tokai earthquake has been recognized since 1969 and is closely related to how far the coseismic rupture during the 1944 tonankai earthquake propagated eastward to understand what kind of structure might have interrupted the rupture of the 1944 event it is necessary to obtain the crustal structure around the eastern end of its rupture zone the crustal model deduced from a wide angle seismic survey shows characteristics of the subducting oceanic crust and the neogene quaternary accretionary sediments bounded by the crustal block of the island arc around the eastern end of the rupture zone of the 1944 event this model shows that the subduction oceanic crust has a particularly irregular geometry 40 km landward from the deformation front comparing our crustal models with a recently published coseismic rupture distribution of the 1944 event deduced from tsunami waveforms there is no slip in the east of the irregularity of the slab moreover it seems that coseismic rupture propagated northeastward avoiding this irregularity the spatial relationship between the irregularity of the slab found in this study and the rupture distribution is analogous to that between the subducted seamount found off the shikoku island and the rupture distribution of the 1946 nankai earthquake although the existence of a subducted ridge parallel to the nankai trough off the tokai district has been suggested based on published geomagnetic data and seafloor topography our crustal models may support the existence of a slab irregularity more suggestive of a subducted seamount at least at the eastern end of the coseismic rupture zone of the 1944 event such irregular geometry of the subducting oceanic crust in particular subducted seamounts may be one of the causes that control the rupture process of great earthquakes along the nankai trough

a proposal for the kinematic modelling of w dipping subductions possible applications to the tyrrhenian apennines system in w dipping subduction zones there is a general eastward progression of the back arc basin accretionary wedge foredeep complex with the forward progression early stages of the complex are revealed by slices of upper crust and sedimentary cover abandoned to the west left floating above a new section of mantle a major shear zone should form at the new moho separating upper crust slices of earlier accretionary stages and the eastward flowing mantle the mantle wedging at the top of the subduction plane could be responsible for the uplift of the central parts of the belt the retreating of the subduction hinge is interpreted as due to the push generated by the eastward mantle flow detected in the hot spot reference frame the foredeep depth is mainly a function of the radius of curvature of the subduction hinge the frontal wedge is constructed from the stacking of the upper layers of the subducting plate and the syntectonic clastics that fill the foredeep which are progressively involved in thrusting and later by extension in order to preserve volume balance the lithosphere of the eastern plate before subduction has to be the same size as that which has been subducted due to the longer length of the arc with respect to the original length of the linear margin between the two converging plates laterally stretched subducted lithosphere is predicted at depth w dipping subductions usually have a short life probably due to their inherent capability to produce new lateral heterogeneities of the lithosphere the thin back arc which are a key factor in controlling and generating new subductions both e and w dipping this model is applied to the apennines tyrrhenian sea system

a detailed subduction structure in the kuril trench deduced from ocean bottom seismographic refraction studies abstract in 1983 we conducted an ocean bottom seismograph obs experiment in the southernmost part of the kuril trench beneath which the pacific plate is subducting under hokkaido island japan the aim of this experiment was to determine crust and upper mantle structure from the oceanic basin to the continental slope by dense seismic refraction profiling using explosives and airguns the total length of the profiles was 560 km along which ten obs s were deployed the observed data were of good quality which enabled us to obtain a detailed velocity structure of the active margin down to 20 30 km we constructed a velocity structure model by ray tracing and amplitude modeling in the oceanic basin the crust has a typical oceanic structure characterized by three layers with p wave velocities of 1 8 3 8 6 5 and 6 5 7 0 km s the velocity gradient in layer 3 increases downward from 0 075 to 0 10 s 1 the moho discontinuity was well constrained by clearly observed p n and p m p phases the total thickness of the oceanic crust was determined to be 8 km and almost constant in the oceanic basin the p wave velocity is 7 9 km s beneath the moho discontinuity which increases downward with a rather small velocity gradient 0 015 0 03 s 1 beneath the continental slope we found relatively low velocity 2 5 to 5 5 5 8 km s material the oceanic moho discontinuity associated with the subducting plate was traced down to a depth of 25 km our seismic data strongly suggest that oceanic layer 2 is smoothly subducting and does not break up to form a wedge structure this result is in remarkable contrast with the velocity structure in the active margin of the ryukyu trench area about 1000 km south of the present experimental area where we found a 12 km thick prominent low velocity wedge is situated 50 150 km landward from the trench axis on the seaward side of the wedge the surface of the igneous basement undulates severely the wedge was probably formed by materials of oceanic origin such a difference in velocity structure suggests that the subduction mechanism in the trench area differs from region to region in the northwestern pacific

western nankai trough seismogenic zone results from a wide angle ocean bottom seismic survey the nankai trough southwestern japan is recognized as a vigorous seismogenic zone with well studied historic earthquakes this paper presents results of a wide angle ocean bottom seismographs obs study at the western nankai trough seismogenic zone the obs data used were acquired on a profile 250 km long across the presumed coseismic slip zone of the 1946 nankaido earthquake ms 8 2 the main purpose of the seismic study is to obtain an entire crustal cross section of the seismogenic zone for the 1946 earthquake the crustal model is characterized by a gentle sloping of subducting oceanic crust and thick overlying sedimentary wedge p wave seismic velocities of the subducting oceanic crust show normal oceanic crustal velocities vp 5 0 5 6 km s and 6 6 6 8 km s in oceanic layers 2 and 3 respectively the maximum thickness of the sedimentary wedge is 9 km at 70 km from the trough axis with vp 3 4 4 6 km s in the deeper part the subducting oceanic crust traced down to 25 km depth shows that the subduction angle becomes steeper landward 3 2 and 7 2 at 0 50 km and 50 100 km from the trough axis respectively the oceanic crust is smooth to the hypocenter zone down to 40 km depth beneath shikoku island our crustal model shows that the downdip limit of the coseismic slip area does not extend to the deep end of the oceanic crust island arc crust contact zone even though there is large uncertainty about the seaward limit of the coseismic slip zone the crustal model clearly indicates that the updip limit of the coseismic slip zone extends beneath the young accretionary prism

three dimensional structure from feathered two dimensional marine seismic reflection data the eastern nankai trough 1 for a typical two dimensional 2 d marine multichannel seismic reflection survey cross currents cause receiver cable side drift and data collection becomes a limited swath 3 d survey processing of swath marine survey data using standard 2 d imaging procedures can create spurious discontinuities and wipeouts in the images of reflection events these anomalies are commonly attributed to geology and tectonics during interpretation we show how discontinuous reflections and wipeouts are created wherever subsurface structures have significant cross profile dip we also show that more accurate imaging can be achieved by cross dip processing which takes into account the 3 d character of the data and of the geology the final products are a much improved stack determinations of structural dips at right angles to the survey line and an improved velocity model we apply this cross dip processing to data from the eastern nankai trough sw japan improved imaging combined with the new cross dip information allows us to constrain better the 3 d character of the deformation in the accretionary prism and oceanic crust in particular the extracted structural cross dip values provide evidence that supports two hypotheses related to the underthrusting of a large oceanic ridge paleo zenisu ridge underneath the accretionary wedge 1 parts of the accretionary wedge basins have been uplifted and deformed due to the subduction of a volcano on the ridge 2 the early phase of oceanic lithosphere shortening which produced the underthrusted paleo zenisu ridge was related to n s compression oblique to the general plate motion in this region

strain partitioning between the suruga trough and the zenisu thrust neogene to present tectonic evolution in the onland and offshore tokai district japan abstract the tokai district in central japan is located close to the convergent boundary between the philippine sea and eurasian plates and has experienced not only repeated large interplate earthquakes but also intense aseismic movement in this paper the spatial and temporal tectonic evolution of the tokai district particularly around the omaezaki area is discussed to assess whether the district has been and will be active or inactive according to a geological survey the horizontal crustal shortening strain can imply the hypothetical tectonic model that the area has been getting less active and the strain rate since the neogene can be calculated as 12 and 2 10 6 year respectively the present interseismic horizontal crustal strain and strain rate around the omaezaki area are approximately 4 10 7 and 4 10 9 year by comparing these rates the decrease since neogene can imply the hypothetical tectonic model that the area has been getting less active influenced by the strain partitioning between the suruga trough and the zenisu thrust

structural features of the bonin arc implications for its tectonic history abstract the bonin arc from the shikoku basin to the trench axis is broken into four crustal blocks each separated by major right lateral strike slip faults from north to south they are termed the sumisu jima the sofu gan and the central bonin dislocations these dislocations are delineated by 1 1 changes in trends of en echelon ridge and trough structures of the main arc 2 2 right lateral offsets of the bathymetry and structure of the arc s outer ridge 3 3 right lateral offsets in the free air gravity contours the most likely time of formation of the en echelon structures and fracturing of the arc into the four crustal blocks is between 27 and 17 m y b p miocene during this time japan was moving southeastward relative to asia and the philippine sea plate including the bonin arc was moving northward subducting beneath southwest japan also convergence and subduction of the pacific plate was taking place along the east side of the bonin arc which involved the collision of thick buoyant crustal blocks of the marcus necker ridge near the south end of the arc these movements resulted in compressive forces acting on the bonin arc from the northwest and produced a clockwise torque within the arc the response to these forces included development of ne nne structural trends within the arc and fracturing of the arc into four crustal blocks offset from each other by ne nne right lateral displacements

cyclic ridge subduction at an inter plate locked zone off central japan 1 there are significant variations in the size especially lateral extent of the co seismic rupture zones and inter plate locked zones for an inter plate mega thrust earthquake obvious factors controlling those variations have been still unsolved questions we designed integrated seismic profiles including a wide angle seismic profile and a multichannel reflection seismic profile at the eastern nankai trough in central japan refraction tomography and pre stack depth migration of wide angle seismic data successfully image a trough parallel cyclic ridge subduction with thickness of 13 20 km and wavelength of 35 50 km at a geodetically determined inter plate locked zone in a non rupture zone during the 1944 tonankai mega thrust earthquake this result suggests a first direct evidence for the subducted seamount strongly locked hypothesis

deformable backstop as seaward end of coseismic slip in the nankai trough seismogenic zone abstract wide angle seismic surveys performed in the last decade have clarified the 3 d crustal structure along the nankai trough the geometry and velocity structure of the southwestern japan subduction zone are now well constrained comparing these observations with the rupture distribution of historic great thrust earthquakes it appears that the coseismic rupture occurred along plate boundaries deeper than the wedge backstop boundary the boundary between the neogene quaternary accretionary wedge and the crust forming the backstop from the view of spatial relationship both rupture distributions of the last two large events and the crust forming the backstop are considerably retreated from the trough axis in the west and east off the kii peninsula in both areas seamount or ridge subduction is apparent in seismic results geomorphological data and geomagnetic data the landward indentation of the deformable backstop which corresponds to the crustal block of old accreted sediments may be formed by seamount subduction according to published results of sandbox modeling in particular the subducted seamount may be a structural factor affecting the recession of the crustal block forming the backstop

structure of the costa rica convergent margin offshore nicoya peninsula we present the results of a seismic refraction survey conducted offshore costa rica near the nicoya peninsula a dip profile and three strike profiles were carried out over 22 ocean bottom hydrophones and seismographs and were also recorded by land receivers these data are used to construct a crustal structure model of the convergent margin from 20 km seaward of the middle america trench onto the nicoya peninsula the best constrained portion of our model is the velocity at the top of the margin wedge immediately below the slope apron velocities increase from 3 5 to 4 2 to 4 6 km s at distances of 10 20 and 30 50 km landward of the trench these velocities are higher than observed within margin wedges at other well studied convergent margins but lower than the velocities within the adjacent nicoya complex which are 5 5 km s at similar depths below the surface we interpret the margin wedge velocities as indicating that material similar to the nicoya complex extends seaward to near the lower slope but that fracturing alteration or accretion processes have lowered the velocity of the margin wedge with respect to the nicoya complex the seismic refraction data cannot constrain the exact thickness or velocity of a possible low velocity zone lvz overlying the subducting plate however geologically reasonable structures are only produced with a lvz 400 m thick velocities in the upper part of oceanic layer 2 are 3 5 4 0 km s within the subducting slab these velocities are unusually low for oceanic crust of this age and may correlate with a proposed highly permeable zone at the top of the subducting crust the top of the subducted slab is well resolved and deepens from 5 km depth at the trench to 15 16 km depth at the nicoya peninsula coastline the dip angle of the subducting plate increases from 6 to 13 at a distance of 30 km from the trench interplate seismicity appears to become common 55 km from the trench where the plate boundary is at 14 km depth

seismogenic zone temperatures and heat flow anomalies in the to nankai margin segment based on temperature data from iodp expedition 333 and thermal model the location of the seismogenic zone across the to nankai margin segment has been widely investigated using geodetic tsunami seismologic and heat flow data as well as thermal models seafloor heat flow measurements show large scatter raising questions about the thermal state of the incoming and overriding plates in this study new temperature and conductivity measurements recorded in the philippine sea plate psp and in the accretionary prism during iodp expedition 333 are integrated with recent data on the margin structure and seismicity to improve constraints on the thermal structure of the incoming plate and the subduction zone as well as the thermal conditions within the seismogenic zone iodp expedition 333 measurements provide heat flow values that are lower than the average of nearby seafloor measurements thermal modeling for the psp suggests that hydrothermal warming is insignificant in the trench thus an observed widespread positive thermal anomaly in the shikoku basin that peaks at the kashinozaki knoll more likely results from vigorous thermal convection in the upper mantle in back arc context the modeled thermal structure of the margin from the trench to the mantle shows a significantly colder interplate contact than in previous studies the seismogenic zone is likely to be 30 170 km from the deformation front corresponding to the 100 340 c temperature range the 1944 co seismic slip zone extends farther southward where temperatures are about 60 c slow slip earthquakes are located in the downdip transition zone between 340 and 440 c across the intersection of the moho and the megathrust fault this improved thermal structure from the trench to the mantle results mainly from a better estimate of oceanic plate age slab dip and sedimentation rate variations in the trench this last parameter cools the interplate contact until a depth of 35 km shifting the thermally defined seismogenic zone landward by 20 km and significantly diminishing its influence near the corner flow area

seismic attenuation in the costa rica margin wedge amplitude modeling of ocean bottom hydrophone data abstract travel time and amplitude modeling were carried out on data recorded by three ocean bottom seismometers along two strike profiles located 10 20 km landward of the middle america trench offshore costa rica these two profiles bracket a region of rapid change in physical properties and structural styles the travel time modeling indicates little structural change along the profiles and therefore the data are suitable for amplitude modeling with the reflectivity method for the profile 10 km from the trench amplitude modeling indicates a margin wedge with velocities of 4 4 4 km s and qp 25 50 for the profile 20 km from the trench the amplitude modeling indicates a margin wedge with velocities of 4 4 4 8 km s and qp 50 75 the primary new result of this analysis is the constraint that qp 25 75 within the margin wedge at both locations these high attenuation values are consistent with fracturing of the material making up the margin wedge either ophiolitic nicoya complex rocks dewatered and cemented sediments or a tectonic mixture of both

propagation mechanism of long period ground motions for offshore earthquakes along the nankai trough effects of the accretionary wedge a thick accretionary wedge with low seismic velocity overlying a subducting plate is an important factor affecting the amplitudes and durations of long period ground motions for great offshore earthquakes we performed a series of 3d long period ground motion simulations to obtain a better understanding of the effects of the accretionary wedge along the nankai trough japan the simulation results demonstrate that the accretionary wedge has the effect of decreasing the peak amplitude and velocity response but amplifying and elongating later phases of long period ground motions in the land area these effects depend significantly on the focal depth and are pronounced for shallow seismic sources the amplified and elongated later phases originate mainly from prominent basin induced surface waves excited near the trough axis we also identified hard sediments with intermediate s wave velocity in the accretionary wedge as a key layer that enhances the propagation of long period ground motions from offshore to onshore in the case of the nankai trough the hard sediments allow long period ground motions developed inside the accretionary wedge to be efficiently transmitted outward in the longer period than 10 s comparisons between the observed and simulated records suggest that for more accurate predictions of long period ground motions velocity structure models of not only the accretionary wedge but also the surrounding upper crust should be validated and revised using seafloor observations

tertiary evolution of the shimanto belt japan a large scale collision in early miocene to decipher the miocene evolution of the shimanto belt of southwestern japan structural and paleo thermal studies we carried out in the western area of shikoku island all units constituting the belt both in its cretaceous and tertiary domains are in average strongly dipping to the nw or se while shortening directions deduced from fault kinematics are consistently orientated nnw sse peak paleo temperatures estimated with raman spectra of organic matter increase strongly across the southern tertiary portion of the belt in tandem with the development of a steeply dipping metamorphic cleavage near the southern tip of ashizuri peninsula the unconformity between accreted strata and fore arc basin present along the whole belt corresponds to a large paleo temperature gap supporting the occurrence of a major collision in early miocene this tectonic event occurred before the magmatic event that affected the whole belt at 15 ma the associated shortening was accommodated in two opposite modes either localized on regional scale faults such as the nobeoka tectonic line in kyushu or distributed through the whole belt as in shikoku the reappraisal of this collision leads to reinterpret large scale seismic refraction profiles of the margins where the unit underlying the modern accretionary prism is now attributed to an older package of deformed and accreted sedimentary units belonging to the shimanto belt when integrated into reconstructions of philippine sea plate motion the collision corresponds to the oblique collision of a paleo izu bonin mariana arc with japan in early miocene

genesis and geomorphologic and ecological evolution of the ganzirri salt marsh messina italy the results of geological and geomorphologic surveys on the salt marsh of ganzirri pantano grande combined with geophysical researches and historiographical data are reported here to define the genesis of the marsh and to evaluate the physical factors that influenced its recent evolution the genesis of the pantano grande may be due to a state of equilibrium reached between differential lowering of the coastal plain confined by normal faults and generalized chain uplift in particular two normal faults are considered the first borders the northern shore of the pantano grande and the second bounds the ionian coastal plain towards the south concerning the recent evolution of the pantano grande the importance of the sterile conglomerate outcrop which borders the ganzirri coastal plain is stressed the conglomerate is interposed between sediments that define the pantano grande basin and the ionian sea and influenced the water exchange between the marsh and the sea before the excavation of two canals that link up the pantano grande with the sea the conglomerate limiting the spread of benthic species has definitively affected the ecological structure of the original marsh

silician magnetite from the dales gorge member of the brockman iron formation hamersley group western australia we report silician magnetite from banded iron formation bif in the dales gorge member of the brockman iron formation hamersley group western australia magnetite mesobands typically consisting of individual 100 m microlaminae are revealed to be composed of silician magnetite overgrowths on magnetite silician magnetite overgrowths contain from 1 to 3 wt sio 2 whereas low si magnetite domains contain less than 1 wt sio 2 silicon solid solution is present in the magnetite crystal lattice as determined by in situ micro x ray diffraction and high resolution transmission electron microscopy three textures are distinguished in magnetite mesobands 1 magnetite sub microlaminae with silician magnetite overgrowths 2 recrystallized magnetite fragments with silician magnetite overgrowths and 3 a complex intergrowth of magnetite and silician magnetite all three textures are found in magnetite mesobands from the bif4 5 and bif12 16 macrobands of the dales gorge type section drill core ddh 47a from wittenoom western australia magnetite domains contain numerous submicrometer to micrometer inclusions of quartz carbonate stilpnomelane and apatite whereas silician magnetite overgrowths are devoid of mineral inclusions the presence of mineral inclusions in magnetite indicates the bif oxide precipitate was not chemically pure iron oxyhydroxide oxide magnetite domains display textures formed during soft sediment deformation that are the earliest and best preserved relict sedimentary structures in this bif silician magnetite is the dominant iron oxide in the dales gorge bif and is present in many other sub greenschist facies bifs worldwide we suggest the former presence of organic matter creates reducing conditions necessary to stabilize silician magnetite thus silician magnetite is a potential biosignature in bifs

regional variation of magnetization of oceanic crust subducting beneath the nankai trough 1 we present the characteristics of marine magnetic anomalies in the nankai trough and the northern shikoku basin the marine magnetic anomalies are clearly visible over the shikoku basin but gradually disappear across the nankai trough the expected reason for the decrease in the marine magnetic anomaly amplitudes across the trough is the increase in depth of the magnetic source the subducting oceanic crust forward and inverse magnetic modeling results which take into account the slab depth distribution show variations in magnetization that cannot be explained solely by increasing depth of the slab we identify five zones along the trough axis on the basis of the pattern of the magnetic amplitude profile two ridges a seamount chain and two basement zones the magnetic inversion analysis indicates cyclic intensity variations along the ridge zones the largest contrast in magnetization along the n s lineation before the deformation front occurs off ashizuri as low temperature chemical oxidation effects are most likely the main cause of the reduction in magnetization there are considerable differences in the original carrier of magnetic intensity in the as and kii peninsula kp zones and the original carrier in as is characterized by much higher original magnetic intensities this indicates several possibilities the oceanic crust originated from asymmetric spreading there was magma injection during the late stage volcanism or the as zone was affected by an anomalous heat source it would be concluded from the current data that the basement in as originally had a much stronger magnetic carrier and suffered stronger low temperature oxidation effects than the eastern part of the shikoku basin the regional differences in intensity strongly suggest a variety of magnetic minerals and phase changes associated with temperature and mechanical dislocation

strain modes within the forearc arc and back arc domains in the izu japan and taiwan arc continent collisional settings in this study i examine the strain modes of the forearc arc and back arc domains in arc continent collisional settings leading to arc material subduction delamination and or accretion the study focusses on two well documented colliding island arcs the izu bonin mariana ibm arc in japan and the luzon arc in taiwan both carried by the philippine sea plate firstly there is a body of evidence that both the ibm and the luzon arcs were built on the same late jurassic to early cretaceous proto philippine sea plate crust their internal structure is thus more heterogeneous than expected from paleogene or neogene supposedly intra oceanic island arcs secondly those arc systems and proximal back arcs have similar seismic characteristics attesting either for the presence of a middle crust with continental velocities and or serpentinized uppermost mantle that facilitate crustal shortening slivering and subsequent decoupling from the rest of the subducting plate it is shown that the proximal back arc domain called rear arc in case of paleoarc activity overlying the mantle wedge and the subducting slab may lose its strength if slab derived hydration occur decoupling then occurs below the moho arc delamination likely occurs in mid crustal levels because middle crust heated by nearby magmatism becomes weak accretion of arc material onto the upper plate depends on the characteristics of the arc itself and the geodynamic configuration most of the accreted material is probably underplated rather than frontally accreted

high precision slip distribution of the 1944 tonankai earthquake inferred from tsunami waveforms possible slip on a splay fault abstract inversion of tsunami waveforms is a well established technique for estimating the slip distributions of subduction zone earthquakes with some of the most detailed results having been obtained for earthquakes in the nankai trough sw japan the present study although it uses a method and tsunami waveform data set almost identical to previous study aims to improve on previous work by using a more precise specification of initial conditions for the calculation of tsunami green s functions specifically we incorporated four improvements in the present study 1 we used a realistic plate model based only on seismic survey results and assumed it to be the fault plane of the 1944 tonankai earthquake 2 the smallest subfaults consistent with the long wavelength approximation were used in the tsunami inversion analysis 3 we included the effect of horizontal displacement of the ocean bottom on tsunami generation and 4 we performed a checkerboard resolution test as obtained in previous studies a zone of high slip 2 0 m was resolved off the shima peninsula however the more precise calculation of tsunami green s functions has revealed additional detail that was not evident in previous studies which we demonstrate is resolvable and correlates with the position of known faults in the accretionary prism while there was little or no slip near the trench axis in the eastern part of the rupture zone there was up to 1 5 m of slip resolved within 30 km of the trough axis in the western part along the coast of the kii peninsula this troughward slip zone coincides with the position of a large splay fault mapped in multichannel reflection surveys furthermore it is also clear that the upper edge of the enshu fault off shima and atsumi peninsulas is consistent with the up dip limit of slip in the eastern part of our model we tested the possibility that slip occurred on the former splay fault instead of on the plate interface during the 1944 tonankai earthquake and find that slip on this splay fault is also consistent with the data although we cannot distinguish whether slip was dominant on the splay fault or on the plate interface we further suggest that the position of the enshu fault may be determined by the subduction of topographic highs and that such faults may have an important influence on the up dip rupture limit of the 1944 tonankai and potentially other subduction zone earthquakes

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

archaeological and historiographical implications of recent uplift of the peloro peninsula ne sicily abstract in ancient times the name peloro was used to indicate an anthropic area that gradually developed around the first known human settlement on the sicilian shore near the straits of messina since the 5th century bc historians have documented that numerous naval armadas landed for long periods at peloro however the present day morphology of the peloro cape peninsula does not have any protected inlet that would offer a location to repair hundreds of ships as has been documented by historical sources to address this discrepancy geomorphologic data were collected and analyzed to verify whether historical documents were consistent with the palaeotopography of the area this approach is based on the analysis of the morphotectonic evolution of the coastal lowland that resulted from regional uplift over the quaternary and holocene the results indicate that the harbour was located in the basin of the pantano piccolo salt marsh and was large and deep enough to have sheltered up to 320 ships

crustal and upper mantle structure in the ryukyu island arc deduced from deep seismic sounding summary in 1984 an extensive geophysical investigation was conducted in the northernmost part of the ryukyu island arc south of kyushu island japan the aim of this investigation was to obtain new information on the nature of crust and upper mantle at the continental margin through various geophysical measurements seismic profiling gravity magnetism sea beam and heat flow this paper presents the crust and upper mantle structure deduced from ocean bottom seismographic profiling conducted along two lines one line 190 km long was taken behind the northernmost part of the ryukyu island arc parallel to the major tectonic units i e the okinawa trough ryukyu arc and ryukyu trench the other 295 km long was perpendicular to the tectonic units along these profiles we deployed ocean bottom seismograms spaced 10 20km apart as a controlled source we used both explosives and an airgun array the experiment was successful and provided quite important information on the tectonics of a trenchisland arc back arc system the velocity structure obtained for the profile behind the ryukyu islands has continental properties the sediment thickness is 3 4 km although it shows lateral variations due to basement undulation the velocities of the upper and lower crust are 5 8 6 2 and 6 6 6 8 km s respectively the total crustal thickness decreases southwestward from 27 30 to 23 24 km this is direct evidence for crustal thinning associated with the process of back arc spreading the velocity structure along the profile from the trench to the island arc clearly shows the subduction accretion and deformation at this margin the crust beneath the trench has oceanic properties the sediment thickness is 1 3 3 0 km beneath which the igneous basement shows severe undulation the total crustal thickness is 7 8 km the p velocity is slightly less than 8 0 km s our seismic data revealed a huge sedimentary wedge located 50 150 km landward of the trench the maximum thickness of this wedge exceeds 12 km the origin of this wedge may be oceanic because the p wave velocity in its eastern half is almost comparable to that obtained for the trench area the velocity structure landward of the wedge is continental and similar to that obtained for the profile behind the island arc

geometry and relative motion of the philippine sea plate and pacific plate beneath the kanto tokai district japan the recent acquisition of high density and improved seismic data by the kanto tokai k t observational network of the national research institute of earth science and disaster prevention requires the revision of conventional plate configuration models for the k t district we propose a new interpretation of the configuration and relative motion of the philippine sea phs plate pacific pac plate and eurasian eur plate on the basis of the distribution of hypocenters velocity structure and focal mechanisms in the k t district the new model for the configuration of the phs and pac slabs clearly delineates the phs slab subducting northwestward beneath the k t district from the sagami and suruga troughs and the pac slab subducting westward beneath the phs slab from the japan trench the thickness of the descending phs slab is estimated to be 30 5 km the proposed configuration of the phs slab is significantly different from those obtained by other researchers including our own previous study the new model is fully consistent with global plate motion data the new model leads to an interpretation of the tectonic processes taking place beneath the k t district for the sagami trough wing of the phs slab the northeastern portion of the slab is dipping northwestward in direct contact with the upper surface of the pac slab the southwestern portion carrying the izu peninsula has been bent upward because of its buoyancy producing the observed contorted shape for the suruga trough wing the dip direction of the phs slab gradually changes from northwestward at the eastern portion to northward at the western portion the new model also indicates the presence of offset within the phs slab between the sagami and suruga trough wings this area is characterized by a low level of seismic activity the absence of a high velocity structure and the presence of quaternary volcanoes it seems reasonable to speculate that the distribution of quaternary volcanic edifice construction is controlled by the location of the offset within the phs slab in view of this new model the tectonic processes of large earthquakes which occurred recently in the studied area are also reexamined

zenisu ridge a deep intraoceanic thrust related to subduction off southwest japan abstract during the french japanese kaiko project seabeam seismic and submersible observations were made in the eastern part of the nankai subduction zone close to the area of collision between the izu bonin island arc and the japan margin the most prominent feature is the zenisu ridge an elongated relief of the philippine sea plate running parallel to the trench magnetic anomalies indicate that the crust of the zenisu ridge is a part of the shikoku oceanic basin formed in the early miocene 23 ma ago and presumably uplifted at a later stage structural analysis of seismic data and diving observations lead us to interpret the superficial structure as being due to compressive tectonics mapping the acoustic basement reveals that the southeastern flank of the ridge is bounded by a double thrust both segments being of equal magnitude vertical offset about 1 to 1 5 km geophysical data support the hypothesis of a main thrust cutting through most of the lithosphere and flattening at depth the overall structure of the surrounding area reveals a compressive deformation zone widening toward the east the magnitude of the compressive deformation decreasing westward as well as southward of the zenisu ridge

detailed subduction structure across the eastern nankai trough obtained from ocean bottom seismographic profiles to investigate the deep crustal structure of the philippine sea plate at its northern margin we performed a seismic refraction and wide angle reflection survey around the eastern nankai trough by using an ocean bottom seismograph array with an air gun source we derived a crustal structure model across the trough from the shikoku basin to the continental slope that explains not only our seismic data but also previously published gravity data the origin of the zenisu ridge a conspicuous topographic high along the oceanward slope of the nankai trough is still a matter of argument there has been some controversy as to whether the igneous activity of the izu ogasawara arc or the seafloor spreading of the shikoku basin is responsible for the formation of the ridge although the crustal thickness beneath the ridge is between 8 and 11 km slightly thicker than typical oceanic crust our structure model clearly indicates that the zenisu ridge has an oceanic crust and its structure is very similar to that of the shikoku basin beneath the south flank of the zenisu ridge the moho shows an offset of 5 km in depth this offset may correlate with the recently proposed nascent subduction boundary and subduction collision tectonics of this area the velocity structure beneath the continental slope appears characteristic for a well developed accretionary wedge bounded by the continental upper crust of the japan island arc to the northwest and subducting oceanic crust which can be traced beneath the accretionary wedge and the continental upper crust

the slip distribution of the 1946 nankai earthquake estimated from tsunami inversion using a new plate model abstract a new model for the shape of the philippine sea plate psp subducting beneath southwest japan was constructed by combining marine seismic survey results and seismicity data this resulted in a plate model that is generally 3 5 km shallower than previous estimates and contains new details of plate boundary structure near the nankai trough axis we estimated the slip distribution of the 1946 nankai earthquake by inverting tsunami waveforms using the new plate boundary model and found that the large slip off cape muroto and seismic moment were reduced by about 10 from the previous estimate of tanioka and satake 2001a however we also found reason to suspect the tide gauge closest to the epicenter at shimotsu and this resulted in a much more substantial reduction in the amount of slip in the eastern segment of the rupture area we also find that our new rupture model correlates well with features of the subducting plate boundary such as a large subducting seamount off cape muroto kodaira et al 2000a this suggests that such features have an important effect on either earthquake rupture propagation or the interseismic accumulation of elastic strain energy or both

deep seismic imaging of the eastern nankai trough japan from multifold ocean bottom seismometer data by combined travel time tomography and prestack depth migration 1 the easternmost segment of the nankai trough remained unruptured during the 1944 and 1946 earthquakes that affected the rest of the trench it is therefore a zone of seismic gap that also undergoes a peculiar tectonic regime due to its proximity with the izu collision zone we investigate this area with densely sampled active seismic data one hundred ocean bottom seismometers spaced only 1 km apart were deployed along a line perpendicular to the trough a two step tomographic approach designed for this type of acquisition is applied first a long wavelength velocity model is built by a linearized inversion of first arrival travel times its resolution is assessed and its reliability is confirmed through a semiglobal approach in which a large range of starting models is tested second this tomographic model is used to exploit late reflected arrivals with a prestack depth migration the result of which is validated by classically modeling refraction and reflection travel times in the derived structural model thus both velocities and discontinuities are mapped in our model with a minimum of data interpretation and a priori knowledge our results bring new insights on the deep crustal zones of the margin and its evolution the existence of presently inactivated major thrusts is very clearly evidenced in the backstop a general timing for its evolution and deformation is proposed farther seaward the presence of a subducted ridge beneath the accretionary wedge is confirmed its compressive structure and origin are demonstrated it exhibits a thickened lower crust which nature is discussed besides strong analogies between this paleo zenisu subducting ridge and the zenisu ridge observed farther south are also emphasized bringing evidences for a steady state crustal deformation pattern