a subducting seamount beneath the nankai accretionary prism off shikoku southwestern japan a multi channel seismic mcs reflection survey was conducted to study the structure of the nankai convergent margin off shikoku island in july 1997 based on reflection characteristics we could identify three major seismic reflection units i e units a b and c the mcs data as well as swath bathymetric data reveal a buried circular seamount subducting beneath the nankai accretionary prism the subducting seamount is responsible for deformation of the accretionary wedge resulting in a compressed uplifted sediment knoll and many steep escarpments with ne sw strike a possible thrust fault was identified on the seaward flank of the seamount indicating compressive deformation a tectonic model for subduction of the seamount is presented based on the interpretation of the mcs data

the philippine sea new survey results reveal the structure and the history of the marginal basins the hydrographic department of japan initiated a long term geological and geophysical survey of the philippine sea in 1983 we here summarize the bathymetry and magnetic anomalies of this area focusing on the structure and the evolutionary process of the marginal basins the shikoku and parece vela basins developed as one backarc system in the later phase of their formation the significant difference between the two basins however is clearest near the extinct spreading centers the steep oki daito escarpment located in the northern west philippine basin was mapped for the first time during this survey the n s trending spreading fabric north of the escarpment is in striking contrast to the nw se to e w lineament in the south our data reveal a more complicated history of the western philippine sea than has been proposed in previous studies

shear zone thickness and the seismicity of chilean and marianas type subduction zones chilean type convergent margins have many large m 7 6 earthquakes whereas marianas type ones do not this dichotomy is enigmatic if the plate interface is viewed as a thin frictional decollement whereas it becomes understandable if it is viewed as a relatively thick sediment filled shear zone which thins or thickens arcward depending on subduction speed and sediment supply chilean type margins have thick trench fills and their shear zones generally thin arcward from inlets as much as several thousand metres high the most pronounced thinning being located near backstops tall up to several kilometres seamounts are subducted essentially intact to relatively great depths and confining pressures before jamming into the roof of the channel and becoming seismogenic asperities their near basal ruptures can generate large thrust type earthquakes mainly concentrated in seismic fronts near backstops marianas type margins in contrast have thin trench fills and their shear zones generally thicken arcward from inlets that can be as little as 300 m high seamounts are truncated near the inlet at low confining pressures and generate only small earthquakes after passing the inlet they do not touch the roof and therefore cannot generate large earthquakes a similar mechanism may explain seismic gaps at sediment poor regions of subduction zones

cooling rates of lunar orange glass beads abstract it is widely accepted that the apollo 17 orange glass beads are of volcanic origin the degree of degassing of the glass beads depends on their cooling rates so the estimation of volatile contents in the parental magmas of these lunar pyroclastic glasses also depends on the cooling rates the cooling rate can be estimated using the calorimetric properties of the glass across the glass transition in this study a series of heat capacity measurements were carried out on hand picked lunar volcanic orange glass beads during several cycles of heating to temperatures above their glass transition using a differential scanning calorimeter the cooling rate of orange glass beads sample 74220 867 was calculated to be 101 k min using the correlation between glass cooling rates and fictive temperatures estimated from their heat capacity temperature paths this cooling rate is close to the lower end of the range that best fits the diffusion profiles of several volatile species in the glass beads and at the upper end of the cooling rate range recorded in glasses quenched subaerially on earth the cooling rate is likely to be controlled by the cooling environment cooling medium such that the lunar volcanic glass beads could have been cooled in a gaseous medium released from volcanic eruptions on the moon but not during free flight in vacuum the existence of a gas medium suggests in turn that there may have been at least a short lived or episodic atmosphere on the early moon at around 3 5 ga

geometry and spatial variations of seismic reflection intensity of the upper surface of the philippine sea plate off the boso peninsula japan abstract in the region off the boso peninsula japan the pacific plate is subducting westward beneath both the honshu island arc and philippine sea plate while the philippine sea plate is subducting northwestward beneath the honshu island arc these complex tectonic interactions have caused numerous seismic events occurred in the past to better understand these seismic events it is important to determine the geometry of the plate boundary in particular the upper surface of the philippine sea plate we conducted an active source seismic refraction survey in july and august 2009 from which we obtained a 2 d p wave velocity structure model along a 216 km profile we used the velocity model and previously published data that indicate a p wave velocity of 5 0 km s for the upper surface of the subducting philippine sea plate to delineate its boundary with the overriding honshu island arc our isodepth contours of the upper surface of the philippine sea plate show that its dip is shallow at depths of 10 to 15 km far off the boso peninsula this shallow dip may be a result of interference from the pacific plate slab which is subducting westward under the philippine sea plate within our survey data we recognized numerous seismic reflections of variable intensity some of which came from the upper surface of the philippine sea plate an area of high seismic reflection intensity corresponds with the main slip area of the boso slow slip events our modeling indicates that those reflections can be explained by an inhomogeneous layer close to the upper surface of the philippine sea plate

a hadean to paleoarchean geodynamo recorded by single zircon crystals knowing when the geodynamo started is important for understanding the evolution of the core the atmosphere and life on earth we report full vector paleointensity measurements of archean to hadean zircons bearing magnetic inclusions from the jack hills conglomerate western australia to reconstruct the early geodynamo history data from zircons between 3 3 billion and 4 2 billion years old record magnetic fields varying between 1 0 and 0 12 times recent equatorial field strengths a hadean geomagnetic field requires a core mantle heat flow exceeding the adiabatic value and is suggestive of plate tectonics and or advective magmatic heat transport the existence of a terrestrial magnetic field before the late heavy bombardment is supported by terrestrial nitrogen isotopic evidence and implies that early atmospheric evolution on both earth and mars was regulated by dynamo behavior

lunar volcanism produced a transient atmosphere around the ancient moon abstract studies of the lunar atmosphere have shown it to be a stable low density surface boundary exosphere for the last 3 billion years however substantial volcanic activity on the moon prior to 3 ga may have released sufficient volatiles to form a transient more prominent atmosphere here we calculate the volume of mare basalt emplaced as a function of time then estimate the corresponding production of volatiles released during the mare basalt forming eruptions results indicate that during peak mare emplacement and volatile release 3 5 ga the maximum atmospheric pressure at the lunar surface could have reached 1 kpa or 1 5 times higher than mars current atmospheric surface pressure this lunar atmosphere may have taken 70 million years to fully dissipate most of the volatiles released by mare basalts would have been lost to space but some may have been sequestered in permanently shadowed regions on the lunar surface if only 0 1 of the mare water vented during these eruptions remains in the polar regions of the moon volcanically derived volatiles could account for all hydrogen deposits suspected to be water currently observed in the moon s permanently shadowed regions future missions to such locations may encounter evidence of not only asteroidal cometary and solar wind derived volatiles but also volatiles vented from the interior of the moon

a fault model of the 1946 nankaido earthquake derived from tsunami data abstract a fault model of the 1946 nankaido earthquake m 8 2 is determined by the use of tsunami records of uwajima shimotsu and hososhima which were located within or near the area of major coseismic crustal deformation synthetic tsunamis computed for various fault models are matched with the observed tsunamis to determine the fault parameters a low angle thrust model slightly revised from a previous model by ando is consistent with the observed tsunamis the duration of faulting is constrained as less than 10 min based upon the tsunami the fault is divided into an eastern and a western segment corresponding to areas associated with and without aftershocks respectively the fault area and dislocation for the western segment are 150 70 km2 and 6 m and those for the eastern segment are 150 70 km2 and 3 m respectively the total seismic moment is 4 7 1028 dyn cm significantly smaller than that obtained from a geodetic model by fitch and scholz but still larger than that of the seismic model by kanamori the discrepancy in seismic moment between the seismic and the present models ran2 could be interpreted in terms of a slow dislocation on the fault but this interpretation does not match the seismic intensity distribution and damage pattern and the slow slip model for the nankaido earthquake is rejected the discrepancy between the two seismic moments is considered insignificant within error involved in data and modeling assumptions if the revised geodetic model ran2 is modified the seismic moment required to explain the observed tsunamis would be reduced further by 30 if we consider the uncertainties involved in the fault model of kanamori and the fault finiteness effect affecting the amplitude of seismic waves the seismic moment required to interpret the seismic wave data could be increased possibly being more than twice that of kanamori thus the two seismic moments from the different data sets could be close to each other within allowable tolerance this implies that the rise time of the nankaido earthquake was short enough to generate short period seismic waves from both the western and the eastern fault segments

water chemistry and control of weathering of pichola lake udaipur district rajasthan india the water chemistry of pichola lake revealed that it is dominated by na and hco3 the lake water chemistry strongly reflects the dominance of continental weathering aided by anthropogenic activity such as tourist influx developmental activities in the catchment area and disposal of untreated municipal and domestic sewage into the lake basin the supply of major ions is mainly through weathering of the silicate rock exposed in the drainage basin and the contribution from saline and alkaline soils groundwater because of the semiarid conditions of the region the increase in phosphate loading and consequent depletion of silica suggests biological consumption of the latter the observed chemical data of pichola lake was used to predict the mineral assemblages in the carbonate and aluminosilicate system it demonstrates that calcite and dolomite are the possible minerals that are in equilibrium with the lake water system and that the lake water chemistry is in the range of stability of kaolinite

structure of the jan mayen microcontinent and implications for its evolution an extensive seismic survey using ocean bottom seismographs obs was performed in the area across the jan mayen basin north atlantic from the jan mayen ridge to the iceland plateau the jan mayen ridge and surrounding area are considered to be a fragment of a continent which was separated from greenland just prior to magnetic anomaly 6 this study presents the crustal structure of the jan mayen microcontinent and the ocean continent transition to the west of the jan mayen ridge the crustal structures from the centre of the jan mayen ridge to the jan mayen basin are characterized by a deep sedimentary basin a thin basaltic layer within the sedimentary section and extreme thinning of the continental crust towards the iceland plateau the obs data indicate that a continental upper crust v p 5 8 6 1 km s 1 and lower crust v p 6 7 6 8 km s 1 underlie the deep sedimentary basin the thickness of the continental lower crust varies significantly from 12 km beneath the jan mayen ridge to almost zero thickness beneath the northwestern part of the jan mayen basin an ocean continent transition zone is found at the western edge of the jan mayen basin within the 10 km wide transition zone crustal velocities increase towards the iceland plateau and approach the velocities of the oceanic crust obtained at the iceland plateau that is 3 8 5 1 km s 1 oceanic layer 2a 5 9 6 5 km s 1 oceanic layer 2b and 6 8 7 3 km s 1 oceanic layer 3 the crustal model indicates very thin oceanic crust 5 km immediately oceanwards of the ocean continent transition zone beneath the iceland plateau the oceanic crust is thicker 9 km than the typical thickness of normal oceanic crust this might imply that the oceanic crust at the iceland plateau has been generated by asthenospheric material slightly hotter than normal from the crustal structure obtained by the present study it is proposed that the western part of the jan mayen ridge may be referred to as a non volcanic continental margin generated by a long duration of rifting even if the asthenospheric material upwelling along the margin were hotter than normal only small amounts of magmatic intrusions and extrusions would have been generated because of significant conductive cooling under the long duration of rifting

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

plate tectonics and crustal deformation around the japanese islands we analyze over a century of geodetic data to study crustal deformation and plate motion around the japanese islands using the block fault model for crustal deformation developed by matsu ura et al 1986 in this model crustal deformation corresponds to the sum of relative movement among designated blocks and displacement due to dislocations along the locked parts of designated faults we model the area including the japanese islands with 19 crustal blocks and 104 faults based on the distribution of active faults and seismicity three kinds of data are used rates of change of angles and lengths and average velocities of very long baseline interferometry stations with the inversion of these geodetic data we obtain block motions and average slip rates of faults this geodetic model predicts that the pacific plate moves n69 2 w at about 80 3 mm yr relative to the eurasian plate which is much lower than that predicted in geologic models substantial aseismic slip occurs on the subduction boundaries the block containing the izu peninsula may be separated from the rigid part of the philippine sea plate the faults on the coast of japan sea and the western part of the median tectonic line have slip rates exceeding 4 mm yr while the fossa magna does not play an important role in the tectonics of the central japan the geodetic model requires the division of northeastern japan contrary to the hypothesis that northeastern japan is a part of the north american plate owing to rapid convergence the seismic risk in the nankai trough may be larger than that of the tokai gap

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

the lunar dynamo background it is unknown whether the moon has a fully differentiated and melted structure with a metallic core or retains a partially primordial unmelted interior the differentiation history of the moon is manifested by its record of past magnetism paleomagnetism although the moon today does not have a global magnetic field the discovery of remanent magnetization in lunar rocks and in the lunar crust demonstrated that there was a substantial lunar surface field billions of years ago however the origin intensity and lifetime of this field have been uncertain as a result it has been unclear whether this magnetization was produced by a dynamo in the moon s advecting metallic core or by fields generated externally to the moon establishing whether the moon formed a core dynamo would have major implications for understanding its interior structure thermal history and mechanism of formation as well for our understanding of the physics of planetary magnetic field generation the interior structure of the moon and the lunar dynamo new magnetic measurements of lunar rocks have demonstrated that the ancient moon generated a dynamo magnetic field in its advecting liquid metallic core innermost red shell this dynamo may have been driven by convection possibly powered by crystallization of the core innermost red sphere and or stirring from the solid mantle thick green shell the magnetic field was recorded as magnetization by rocks on the lunar surface image created by hernan canellas the interior structure of the moon and the lunar dynamo new magnetic measurements of lunar rocks have demonstrated that the ancient moon generated a dynamo magnetic field in its advecting liquid metallic core innermost red shell this dynamo may have been driven by convection possibly powered by crystallization of the core innermost red sphere and or stirring from the solid mantle thick green shell the magnetic field was recorded as magnetization by rocks on the lunar surface image created by hernan canellas advances a new generation of laboratory magnetic studies of lunar rocks and spacecraft measurements of lunar crustal magnetic fields have produced major advances in our understanding of the evolution of ancient magnetic fields on the moon it has now been established that a dynamo magnetic field likely existed on the moon from at least 4 5 billion to 3 56 billion years ago with an intensity similar to that at the surface of earth today the field then declined by at least an order of magnitude by 3 3 billion years ago the early epoch of high field intensities may require an exceptionally energetic power source such as mechanical stirring from mantle precession the extended history of the lunar dynamo appears to demand long lived power sources such as mantle precession and core crystallization outlook measurements of the intensity of the ancient lunar dynamo have shown that it was surprisingly intense and long lived the next phase of lunar magnetic exploration will be to obtain more accurate measurements of field paleointensities and to determine when the dynamo initiated and finally disappeared this will be coupled with the continued development of magnetohydrodynamic models for characterizing mechanical and other unusual dynamo mechanisms and further investigations into the thermal structural and geodynamical history of the lunar core and mantle the eventual availability of absolutely oriented samples and in situ spacecraft measurements of bedrock should enable the first measurements of the paleo orientation of lunar magnetic fields such directional data could determine the lunar field s geometry and reversal frequency as well as constrain ancient local and global scale tectonic events

long term evolution of the geomagnetic dipole moment abstract the geomagnetic field intensity measured at the surface of the planet is a potential indicator of the dynamo activity in the conducting liquid core of the earth rapid field variations must be generated by the rapid fluid motions within the core whereas long term changes could be associated with other processes such as changes in boundary conditions at the core mantle interface and or at the outer liquid inner solid core boundary for about 50 years paleomagnetists gathered records of absolute paleointensity using the magnetization of lava flows distributed over the globe in order to reconstruct the field intensity changes during the past 3000 ma despite recent acquisition of paleointensity records the temporal distribution of the results remains limited except within a few time intervals to extract coherent features of the time averaged field intensity over periods of a few million years however significant informations can be gained from the recent updated paleointensity database by averaging the virtual dipole moment over very long time intervals the results suggest the existence of a long term evolution of the dipole field intensity during the past 3 billion years from 3 1022 a m2 at 1000 2000 ma to 8 1022 a m2 at present times

thermal constraints on the seismogenic portion of the southwestern japan subduction thrust for coastal cities an important factor in earthquake hazard from subduction zone earthquakes is the landward extent of the seismogenic portion of the subduction thrust fault in this study we test the hypothesis that the maximum downdip extent is defined by a critical temperature we have developed a transient thermal model for the nankai subduction zone of southwest japan to allow comparison of the thermally estimated downdip extent of the seismogenic zone with that from 1 seismicity and tsunami data for two great subduction earthquakes 2 the coseismic faulting extent of these events estimated from geodetic deformation data and 3 the interseismic locked zone determined from interseismic geodetic data the nankai margin has extensive heat flow and heat production data to control thermal models and thus crustal temperatures it has earthquake tsunami and geodetic data that constrain the coseismic rupture portion of the subduction thrust fault for past great earthquakes and the portion of the thrust fault that is locked and storing interseismic elastic strain on the nankaido margin off shikoku island the thermal model indicates that a temperature of 350 c taken to be the limit for seismic initiation from laboratory and field data is reached on the subduction thrust fault 150 km from the trench a transition zone into which rupture may extend with decreasing offset taken to be 450 c extends an additional 45 km downdip the thermal model results are in excellent agreement with the maximum downdip extent of coseismic displacement for the 1946 nankaido ms 8 2 earthquake off shikoku island and with the downdip extent of the present locked zone in the region of the 1944 tonankai ms 8 2 earthquake to the northeast the subduction angle is much steeper and the thermal models indicate a narrower downdip seismogenic extent the seismogenic locked zone from earthquake and geodetic data is also narrower thus our analysis of the southwest japan margin indicates that all three constraints on the downdip extent of the seismogenic zone thermal coseismic and interseismic geodetic data are in general agreement the study also supports the hypothesis that the seismogenic portion of subduction thrust faults is limited primarily by temperature the thermal control implies that subduction thrust faults with shallow dip have wider seismogenic zones compared to those with steep dip the subducting plate age and thus heat flow and the thickness of the insulating sediments on the incoming plate are also very important to the thermal regime and thus to the seismogenic width the relation of the maximum seismic rupture area to the interseismic locked zone is particularly important for earthquake hazard estimation on subduction margins such as cascadia where there have been no great historical events

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

subducted seamount imaged in the rupture zone of the 1946 nankaido earthquake the nankai trough is a vigorous subduction zone where large earthquakes have been recorded since the seventh century with a recurrence time of 100 to 200 years the 1946 nankaido earthquake was unusual with a rupture zone estimated from long period geodetic data that was more than twice as large as that derived from shorter period seismic data in the center of this earthquake rupture zone we used densely deployed ocean bottom seismographs to detect a subducted seamount 13 kilometers thick by 50 kilometers wide at a depth of 10 kilometers we propose that this seamount might work as a barrier inhibiting brittle seismogenic rupture

crustal structure of the central part of the voring basin mid norway margin from ocean bottom seismographs abstract regional ocean bottom seismograph obs data acquired in the central and northern part of the voring basin mid norway margin have been modelled by use of two dimensional 2 d ray tracing the regional dataset comprises thirty obss deployed along seven 100 170 km long profiles the deeper part of the voring basin is difficult to map using multichannel reflection data due to the presence of sills at intermediate sedimentary levels 2 5 km below sea floor but the modelling of the obs data reveals that this technique provides a reliable estimate of structures and seismic velocities from the sea floor to the upper mantle the shallow and intermediate sediments to 5 km below sea floor are characterized by a vertical increase in velocity due to increased confining pressure there is also considerable lateral variation in the velocities within sedimentary layers at all levels the obs data confirm that intrusions of sills are important at intermediate and deep sedimentary levels 2 10 km below sea floor in most parts of the area the sills seem to vary in lateral extent from about 20 km to more than 100 km and their thickness is generally inferred to be about 200 m the low velocity in the upper crystalline crust 6 2 km s confirms that the crust in the voring basin is of continental origin in most parts of the area the velocity of the lower crust is as high as 7 3 7 6 km s this high velocity layer is interpreted as a magmatic underplated body with strong lateral variations in thickness the base of the 7 3 km s layer is interpreted as the moho and the upper mantle velocity is estimated to 8 2 km s

the effect of seamount subduction on seismic coupling it has long been speculated that the subduction of seamounts may have profound mechanical consequences particularly on the generation of large subduction zone earthquakes we consider this problem as a special case of the seismic coupling model of scholz and campos the model predicts that the subduction of a large seamount will increase the normal stress across the subduction interface and hence will enhance seismic coupling in the case of coupled arcs this increased coupling will result in a large increase in the recurrence intervals of earthquakes such events will therefore appear rare in the instrumental record as is observed for decoupled arcs the subduction of a large seamount should cause them to become locally coupled three of the latter localities have been found with satellite altimetry data two on the tonga kermadec trench and one on the izu bonin trench the diagnostic feature is a shallowing or obstruction of the trench axis adjacent to a chain or group of large seamounts on the subducting plate in each case the small to moderate plate interface seismicity typical of decoupled seismic zones is almost completely absent in a 100 200 km arc segment indicating that the subduction zone is locally locked as predicted by the model the occurrence of large interplate earthquakes at both tonga sites confirms that they are seismically coupled

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

structure of the nankai trough accretionary zone from multichannel seismic reflection data new multichannel seismic reflection data collected over the nankai trough image the accretionary complex in two areas the international program of ocean drilling leg 87 transect area western area and the region of upcoming ocean drilling program leg 131 eastern area the incoming shikoku basin sedimentary section consists of hemipelagic clays and thin terrigenous turbidites the basin section is overlain by a trench wedge that is 12 16 km wide and 350 750 m thick at the thrust front accretionary deformation begins in a protothrust zone that is characterized by thickening and seaward tilting of the trench wedge the zone in the western area is 4 5 km wide and is characterized by kink folds the zone in the eastern area is only 2 5 km wide and does not exhibit such folds the frontal thrusts in each area are imaged as fault plane reflections and ramp upward from within the basin hemipelagic section the overthrusting sediments form fault bend folds over these ramps thrust spacing at the toe of the slope is 1 5 2 5 km the second thrust cuts up from an inferred decollement within the shikoku basin sedimentary section in the eastern area a reflection marking the top of the basin pelagic sediment section changes from normal to reversed polarity about 6 3 km seaward of the thrust front and underlies the entire protothrust zone this reflector continues with reversed polarity under the accretionary complex and is at the level of the basal decollement the underlying basin pelagic section is apparently thrust undisturbed beneath the accretionary prism the reversal of polarity indicates a change in reflection coefficient that is due to a combination of decreasing seismic velocity and density across the interface this decrease in velocity and density may indicate that the decollement is a zone of high porosity due to fluid expulsion from deeper within the accretionary prism the reflections from the first and second thrusts are also reversed polarity possibly indicating that they also are pathways of fluid expulsion the critical wedge taper of the western area is greater than that of the eastern area an observation that is consistent with the existence of an overpressured decollement in the eastern area

pervasive remagnetization of detrital zircon host rocks in the jack hills western australia and implications for records of the early geodynamo it currently is unknown when earth s dynamo magnetic field originated paleomagnetic studies indicate that a field with an intensity similar to that of the present day existed 3 5 billion years ago ga detrital zircon crystals found in the jack hills of western australia are some of the very few samples known to substantially predate this time with crystallization ages ranging from 3 0 4 38 ga these zircons might preserve a record of the missing first billion years of earth s magnetic field history however a key unknown is the age and origin of magnetization in the jack hills zircons the identification of 3 9 ga i e hadean field records requires first establishing that the zircons have avoided remagnetization since being deposited in quartz rich conglomerates at 2 65 3 05 ga to address this issue we have conducted paleomagnetic conglomerate baked contact and fold tests in combination with u pb geochronology to establish the timing of the metamorphic and alteration events and the peak temperatures experienced by the zircon host rocks these tests include the first conglomerate test directly on the hadean zircon bearing conglomerate at erawandoo hill although we observed little evidence for remagnetization by recent lightning strikes we found that the hadean zircon bearing rocks and surrounding region have been pervasively remagnetized with the final major overprinting likely due to thermal and or aqueous effects from the emplacement of the warakurna large igneous province at 1070 million years ago ma although localized regions of the jack hills might have escaped complete remagnetization there currently is no robust evidence for pre depositional 3 0 ga magnetization in the jack hills detrital zircons

structural and magmatic responses to spreading ridge subduction an example from southwest japan subduction of a spreading ridge is expected to leave a geological signature upon the overriding accretionary prism distinct from that of conventional convergent plate boundaries the late oligocene early miocene rocks of the shimanto accretionary prism at cape muroto shikoku island record a tectonomagmatic fabric that is anomalous with respect to typical accretionary prisms these accreted strata have been imprinted with an unusual late stage event that involved 1 regional cusp like flexing of structural trends in the prism 2 near trench magmatism and 3 pervasive faulting this event overlaps in time with the opening of the shikoku back arc basin circa 26 14 ma the shikoku basin spreading ridge trends perpendicular to the strike of the accretionary prism and is now located immediately offshore of cape muroto the regional flexing of structural trends in the prism affects rocks as old as cretaceous although the flexure is most tightly developed in the late oligocene early miocene portion of the prism where it is steeply plunging kinematic analysis of the flexure and the pervasive fault system indicates that the flexure most likely resulted from indentation of the prism by a rigid feature at cape muroto the core of the flexure forms a locus for mafic dikes that are petrochemically equivalent to some mid ocean ridge basalts morb from the shikoku back arc basin preliminary paleomagnetic data suggest that the dikes were intruded during the later stages of flexure development these data combined with plate reconstructions for the area at 15 ma indicate that indentation of the prism most likely resulted from early miocene orthogonal subduction of the topographically elevated shikoku basin spreading ridge in the muroto area morb magmatic rocks in the core of the flexure appear to have emanated from the active spreading ridge during subduction

coseismic slip resolution along a plate boundary megathrust the nankai trough southwest japan geodetic survey measurements are used to estimate the coseismic slip distribution in the 1944 tonankai mw 8 1 and 1946 nankaido mw 8 3 earthquakes and to assess quantitatively the degree to which this slip is resolved on the plate boundary megathrust data used include 798 angle changes from triangulation surveys 328 leveling section differences and 5 coseismic tidal gage offsets many of the nominally coseismic triangulation data span 50 years nearly half the earthquake cycle and correction for interseismic deformation using post 1950 observations is applied microseismicity is used to define the configuration of the plate boundary interface and approximate it with a continuous multisegment fault model because the onshore geodetic data have very limited resolving power for offshore fault segments offshore coseismic slip was constrained by satake s 1993 estimation based on tsunami data the majority of the coseismic slip occurs between 15 and 25 km depth although resolution declines toward the trench axis it is sufficiently good to define two distinct high slip regions one off southeastern shikoku island 11 m maximum and the other offshore of kii peninsula 3 m maximum the slip magnitude off southeastern shikoku coupled with the plate convergence rate would imply an recurrence interval of about 270 years much longer than the average repeat time of 120 years for historical great earthquakes on the nankai trough however the maximum coseismic slip is sensitive to the assumed fault geometry and slippage on trough parallel splay faults could significantly decrease the maximum slip to about 6 m

enhanced atmospheric oxygen outflow on earth and mars driven by a corotating interaction region 1 solar wind controls nonthermal escape of planetary atmospheric volatiles regardless of the strength of planetary magnetic fields for both earth with a strong dipole and mars with weak remnant fields the oxygen ion o outflow has been separately found to be enhanced during corotating interaction region cir passage here we compared the enhancements of o outflow on earth and mars driven by a cir in january 2008 when sun earth and mars were approximately aligned the cir propagation was recorded by stereo ace cluster and mars express mex during the cir passage cluster observed enhanced flux of upwelling oxygen ions above the earth s polar region while mex detected an increased escape flux of oxygen ions in the martian magnetosphere we found that 1 under a solar wind dynamic pressure increase of 2 3 npa the rate of increase in martian o outflow flux was 1 order higher than those on earth and 2 as a response to the same part of the cir body the rate of increase in martian o outflow flux was on the same order as for earth the comparison results imply that the dipole effectively prevents coupling of solar wind kinetic energy to planetary ions and the distance to the sun is also crucially important for planetary volatile loss in our inner solar system

thermal and magmatic evolution of the moon as with all science our continually developing concepts of lunar evolution are firmly tied to both new types of observations and the integration of these observations to the known pool of data this process invigorates the intellectual foundation on which old models are tested and new concepts are built just as the application of new observational tools to lunar science in 1610 galileo s telescope and 1840 photography yielded breakthroughs concerning the true nature of the lunar surface the computational and technological advances highlighted by the apollo and post apollo missions and associated scientific investigations provided a new view of the thermal and magmatic evolution of the moon 1 1 pre apollo view of the thermal and magmatic evolution of the moon many of the early views of the moon manifested in mythology and art throughout the world were primarily tied to lunar and terrestrial cycles and the relationships between the sun and the moon prophetically myths involving the lunar deities mwuetsi from zimbabwe and coyolxauhqui from mexico told of rather violent or catastrophic events in which the moon was expunged from the earth numerous ancient scientific observations were made about the nature of the moon ranging from those uncovered in early neolithic sites that correctly identified mare crisium and mare humorum to the insights made by greek philosophers such as anaxagoras ca 500 428 b c and democritus ca 460 370 b c who attached terrestrial analogues to its character stone mountains with the advent of the telescope 1610 and photography 1840 as scientific tools for lunar exploration semiquantitative data could be collected that would provide an intellectual foundation for scientific interpretation initially modern terrestrial geological analogs were extended to the moon lunar highlands volcanic craters seas combined with the rigors of computational modeling these observational data were extended to predict the original thermal state of the moon and its thermal and magmatic history its proximity to the earth

a magnetotelluric sounding across vancouver island detects the subducting juan de fuca plate magnetotelluric1 mt and geomagnetic2 depth sounding measurements have been made at 18 sites 7 with remote reference3 across vancouver island fig 1 beneath which the juan de fuca plate is underthrusting4 5 vancouver island is part of the old accretionary wrangellia terrane6 and is located in the central portion of a 350 km forearc region that extends from the sediment filled trench7 to the garibaldi volcanic belt8 gravity9 and seismic refraction studies10 suggest that the descending plate has a shallow northeasterly dip of 8 16 with perhaps higher dips beneath northeastern vancouver island a high resolution seismic reflection survey11 along lines 1 and 3 fig 1 located a zone of strong acoustic reflections the e horizon11 near the top of the subducting juan de fuca plate at depths of 23 34 km our model of the broad band mt data defines a sloping highly conducting zone also near the top of the subducting plate at the same depths as the seismic e horizon the high electrical conductivity in this zone the e conductor is the result of saline fluid within the pore spaces of sedimentary and mafic materials of the upper oceanic crust that have been subducted to those depths beneath vancouver island these are the first data which clearly define a dipping conductive layer associated with the boundary region between converging plates they have significant implications for thrust earthquakes and metamorphic reactions that occur in subduction zones

life of an oceanic plate cooling time and assimilation time an oceanic plate undergoes two different thermal stages in its life from its creation at the spreading center to its assimilation under the subduction zone first the cooling stage and secondly the heating stage introducing the terms cooling time and assimilation time as measures of intervals of the former and latter stages we discussed the quantitative correlation between the two the cooling time ac is defined as the time required for the oceanic plate to move from a spreading center to a trench and is equivalent to the age of the oceanic plate when it just reached the trench the assimilation time tm is defined as the time required for the subducting plate to lose the ability to generate earthquakes since it began to subduct we estimated the values tm and ac at 50 sites in major subduction zones of the world the result indicates a positive linear relation between ac and tm simply expressed by tm ac9 2 ma while mckenzie s subduction model and the thickening plate model theoretically expect the relationship to be expressed as tm ac the discrepancy between our result and the theoretical model suggests the delay of effective heating of the subducting plate under the subduction zone and or the rapid increase of the plate thickness around the spreading center

seismic structure of western end of the nankai trough seismogenic zone 1 in 1998 a seismic experiment along a 320 km long profile using an air gun array and ocean bottom seismographs obss was collected in the on shore offshore forearc region of the western nankai trough seismogenic zone southwestern japan this area is outside the coseismic rupture area of the 1946 nankaido earthquake ms 8 2 the main objectives of this paper are to clarify the p wave and the poisson s ratio profile around the forearc region of southwestern japan and estimate differences between this area and the nankaido coseismic rupture area the p wave velocity model estimated by two dimensional 2 d ray tracing and travel time inversion indicates three units with distinctive velocity patterns the lens shaped unit has a maximum thickness of 5 km and velocity of 3 4 4 0 km s the underlying unit has velocity of 5 4 5 6 km s and its p wave velocity increases on land vp 6 2 6 4 km s the relative homogeneous unit with p wave velocity of 6 6 6 9 km s exists just beneath secondary unit with p velocity of 5 4 5 6 km s the angle of subduction is 7 we also obtained a poisson s ratio of the crustal structure of the forearc region the poisson s ratio of the southern part of secondary unit is larger than that of the northern part we conclude that these data indicate clay rich metamorphic rocks plus oceanic basaltic rocks rather than high porosity materials comprise the majority of the layer we suggest that the region of high poisson s ratio probably corresponds to underplating and the metamorphism of the clay rich sediments and possibly the oceanic basalt just below the decollement

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

the continental margin off oregon from seismic investigations in april and may 1996 a geophysical study of the cascadia continental margin off oregon and washington was carried out aboard the german rv sonne as a cooperative experiment between geomar the usgs and coas offshore central oregon which is the subject of this study the experiment involved the collection of wide angle refraction and reflection data along three profiles across the continental margin using ocean bottom seismometers obs and hydrophones obh as well as land recorders two dimensional modelling of the travel times provides a detailed velocity structure beneath these profiles the subducting oceanic crust of the juan de fuca plate can be traced from the trench to its position some 10 km landward of the coastline at the coastline the moho has a depth of 30 km the dip of the plate changes from 1 5 westward of the trench to about 6 5 below the accretionary complex and to about 16 further eastward below the coast the backstop forming western edge of the siletz terrane an oceanic plateau that was accreted to north america about 50 ma ago is well defined by the observations it is located about 60 km to the east of the deformation front and has a seaward dip of 40 at its seaward edge the base of the siletz terrane seems to be in contact with the subducting oceanic crust implying that sediments are unlikely to be subducted to greater depths the upper oceanic crust is thinner to the east of this contact than to the west at depths greater than 18 km the top of the oceanic crust is the origin of pre critical reflections observable in several land recordings and in the data of one ocean bottom instrument these reflections are most likely caused by fluids that are released from the oceanic crust by metamorphic facies transition

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

a deep strong reflector in the nankai accretionary wedge from multichannel seismic data implications for underplating and interseismic shear stress release 1 we present the results of structural and stratigraphic interpretations for the multichannel seismic profiles to reveal the structure of the nankai accretionary wedge off shikoku island on the basis of reflection characteristics we identify three major seismic units that correspond to a unit consisting of both oligocene miocene accretionary prism and the sedimentary cover unit a a unit of underthrust miocene pliocene hemipelagic sediments unit b and a unit of subducting pre miocene oceanic crustal rocks unit c according to relative strength of the topmost reflector of unit b the nankai accretionary wedge can be divided into three different structural zones along the cross strike direction apparent decollement reflector zone low amplitude reflector lar zone and deep strong reflector dsr zone despite the velocity uncertainty involved in the depth conversion and therefore thickness estimation error the decollement step down and the possible duplex structures in the lar and dsr zones support that the landward thickness variation of unit b is caused by the structural thickening such as the underplating process structural features and reverse polarity suggest that the dsr as an upper interface of the underplated layer unit b corresponds to a shear plane or a fault zone with high pore fluid pressures we found that most of the dsr was observed in a region which has not suffered from the 1946 coseismic dislocation even though it was locked during the interseismic period the dsr as a shear plane or a fault zone constrains a regional variety of the coseismic dislocation around the updip limit of the nankai seismogenic zone considering all of the dsr characteristics we propose a hypothesis for the dsr s development and role the dsr may play an important role as a possible releaser of the shear stress energy stored at the plate boundary around the updip limit of the nankai seismogenic zone a good correlation between the theoretical vertical displacement and seafloor topographic feature support the possible slip along the dsr as a shear plane

source mechanisms of the 1944 tonankai and 1946 nankaido earthquakes spatial heterogeneity of rise times the most recent great earthquakes that occurred along the nankai trough are the 1944 tonankai and 1946 nankaido earthquakes the aftershock areas of both events are smaller than the tsunami source areas and the seismic moments estimated from geodetic or tsunami data are much larger than those estimated from seismic data until now these discrepancies have not been reconciled we propose a model based on analysis of tsunami waveform data we find that the rise time is very short on the portion of the fault plane corresponding to the aftershock area while rupture lasts for 3 9 min on the rest of the fault where aftershocks were absent since the easternmost portion of the 1944 fault slipped slowly and acted as a relaxation barrier the 1944 rupture might have stopped here the seismic moments of the 1944 and 1946 events on the basis of both tsunami and geodetic data are 2 8 1021 nm and 4 0 1021 n m respectively the moments of the portions with short rise time are consistent with the previous seismic estimates by kanamori 1972

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

interplate coupling in southwest japan deduced from inversion analysis of gps data abstract recently the geographical survey institute of japan completed the installation of a gps continuous observation network in japan which has enabled us to investigate real time crustal movements in this study we attempt to obtain spatial distribution of interplate coupling and relative plate motion between subducting and overriding plates in southwest japan using horizontal and vertical deformation rates which were observed at 247 gps observation stations during the period from april 6 1996 to march 20 1998 for this purpose we carried out an inversion analysis of geodetic data incorporating akaike s bayesian information criterion abic as a result strong interplate coupling was found off shikoku and kumanonada regions which corresponds well with the fault regions of the 1946 nankai m 8 1 and the 1944 tonankai m 8 0 earthquakes respectively we also found that interplate coupling becomes weak at depths deeper than about 30 to 40 km beneath the shikoku and kii peninsula the recurrence time of great trench type earthquakes was roughly estimated as 107 years which is consistent with previous research the direction of relative plate motion is oriented n53 w which is close to the direction predicted from the plate motion model on the other hand a large forward slip was found in the hyuganada region off southeast of kyushu since the coseismic displacements associated with the two 1996 hyuganada earthquakes m 6 6 m 6 6 are removed from the gps data this suggests that after slip occurred near the source region and or that kyushu moves southeastward stationarily due to other tectonic forces

dipping seismic reflectors electrically conductive zones and trapped water in the crust over a subducting plate the lithoprobe program across the northern cascadia subduction zone at vancouver island has obtained geophysical data that suggest a dipping zone of trapped free pore water in the overlying continental crust land and marine multichannel seismic reflection profiles show several bands of reflectors the primary one at a depth of about 30 km is about 5 km thick dips inland and appears to coincide with an electrically conductive layer defined by magnetotelluric data the downgoing oceanic crust is imaged as a thin reflector dipping at about 15 and 10 km deeper than the primary reflector a location that is consistent with seismicity and seismic refraction data both the reflectors and the conductor have been modeled by near horizontal lamellae with 1 4 saline water porosity detailed geothermal measurements defining heat flow that decreases inland indicate that the dipping bands are nearly isothermal at a temperature of 400 500 c it is suggested that free water generated by dehydration of the downgoing oceanic plate migrates upward until reduced temperature results in hydration reactions and mineral precipitation that forms an impermeable barrier low vertical permeability is required to keep the water from separating to the top of the layer this can be reconciled with the low electrical resistivity if pore interconnection is through thin grain boundary tubes as predicted by theoretical equilibrium pore geometry although very small effective grain size is necessary an abrupt transition from high permeability to very low permeability is predicted at a several percent porosity in agreement with the modeled porosity such pore geometry with low permeability may be restricted to temperatures above about 400 c i e in greenschist to amphibolite facies conditions where grains will deform to equilibrium geometry and cracks are annealed the region above the reflective and conductive band is estimated to be within blueschist conditions consistent with the inferred high seismic velocity from refraction data and high density from gravity data

contrasting subduction structures within the philippine sea plate hydrous oceanic crust and anhydrous volcanic arc crust we show contrasting subduction structures within the philippine sea plate inferred from active source wide angle reflection data previous studies showed that large amplitude reflections from the slab are observed in southwest japan and indicated that a thin low velocity layer with a high fluid content is formed along the top of the subducting oceanic crust on the contrary we found that the slab reflections have smaller amplitudes in the izu collision zone central japan where the izu bonin volcanic arc has been colliding subducting suggesting that such a low velocity layer does not exist beneath the collision zone this structural difference is also supported by p wave and s wave velocity anomalies by passive source tomography and electrical conductivity and correlates with the regional distribution of deep tremors and intraslab earthquakes both of which are induced by dehydration processes within the downgoing slab based on these comparisons we suggest that the original structure of the incoming plate controls the contrasting subducting systems typical oceanic plate absorbs water by hydrothermal circulation at spreading centers and or seawater infiltration at outer rises whereas volcanic arc crust consumes a large amount of hydrous minerals for melt production and metamorphoses to more stable anhydrous forms before subduction

oxygen escape from the earth during geomagnetic reversals implications to mass extinction abstract the evolution of life is affected by variations of atmospheric oxygen level and geomagnetic field intensity oxygen can escape into interplanetary space as ions after gaining momentum from solar wind but earth s strong dipole field reduces the momentum transfer efficiency and the ion outflow rate except for the time of geomagnetic polarity reversals when the field is significantly weakened in strength and becomes mars like in morphology the newest databases available for the phanerozoic era illustrate that the reversal rate increased and the atmospheric oxygen level decreased when the marine diversity showed a gradual pattern of mass extinctions lasting millions of years we propose that accumulated oxygen escape during an interval of increased reversal rate could have led to the catastrophic drop of oxygen level which is known to be a cause of mass extinction we simulated the oxygen ion escape rate for the triassic jurassic event using a modified martian ion escape model with an input of quiet solar wind inferred from sun like stars the results show that geomagnetic reversal could enhance the oxygen escape rate by 3 4 orders only if the magnetic field was extremely weak even without consideration of space weather effects this suggests that our hypothesis could be a possible explanation of a correlation between geomagnetic reversals and mass extinction therefore if this causal relation indeed exists it should be a many to one scenario rather the previously considered one to one and planetary magnetic field should be much more important than previously thought for planetary habitability

tectonics alteration and the fractal distribution of hydrothermal veins in the lower ocean crust abstract gabbros from hole 735b preserve the igneous structural and metamorphic history beneath the southwest indian ridge down to middle amphibolite facies conditions the gabbro was isolated from the zone of lithospheric necking by detachment faulting unroofing and block uplift at the inside corner high of the atlantis ii transform lower crustal accretion as preserved in the core is a complex integration of igneous hydrothermal and tectonic processes alteration began at anhydrous granulite conditions as surface faulting penetrated the brittle ductile transition to form brittle ductile shear zones in the amphibolite facies hydrous alteration around these zones was extensive while undeformed sections remained virtually unaltered amphibole veins formed during the brittle ductile deformation have a high fractal dimension reflecting an unclustered distribution consistent with high strain and cracking rates in the zone of lithospheric necking beneath the ridge however the fractal dimensions of the two major gneissic amphibolite zones in hole 735b are different and suggest that they represent discrete fault zones formed at different times below middle amphibolite facies a dramatic drop in the extent of alteration reflects cooling cracking and alteration under static conditions similar to layered intrusions initially fracture and vein formation exploited undeformed sections where elastic strain accumulated during cooling from high temperatures was unrelieved by recrystallization plagioclase diopside veins formed at this time have the same orientation as higher temperature veins but a mineralogy reflecting much lower rock permeabilities and a low fractal dimension reflecting a tight clustering of veins as would be anticipated for crack formation at low strain rates late zeolite and carbonate veins and irregular smectite lined near vertical cracks also have a very low fractal dimension reflecting clustering appropriate to low strain near static conditions these however show no preference for undeformed sections of the core and are oriented with subvertical and subhorizontal maxima this indicates crack formation under the present day stress field thus elastic strain due to cooling from high temperatures and extension during lithospheric necking dissipated after plagioclase diopside vein formation

seismic consequences of warm versus cool subduction metamorphism examples from southwest and northeast japan warm and cool subduction zones exhibit differences in seismicity seismic structure and arc magmatism which reflect differences in metamorphic reactions occurring in subducting oceanic crust in southwest japan arc volcanism is sparse and intraslab earthquakes extend to 65 kilometers depth in northeast japan arc volcanism is more common and intraslab earthquakes reach 200 kilometers depth thermal petrologic models predict that oceanic crust subducting beneath southwest japan is 300 to 500 c warmer than beneath northeast japan resulting in shallower eclogite transformation and slab dehydration reactions and possible slab melting