the mount st helens volcanic eruption of 18 may 1980 large short term surface temperature effects the surface temperature effects of the 18 may 1980 eruption of mount st helens volcano were examinedfor 1 day immediately after the eruption 24 hour temperature differences and model output statistics errors as well as the detailed temporal evolution of surface temperature at selected stations were used during the daytime hours immediately after the eruption the temperature was suppressed by the volcanic plume by as much as 8 c that night low level volcanic dust produced temperature enhancements of up to 8 c these effects quickly diminished the next day as the volcanic dust cloud dissipated and moved toward the east the net local effect of the eruption appears to be warming in contrast to cooling which might be expected over climatic time scales

simulation of historic glacier variations with a simple climate glacier model glacier variations during the last few centuries have shown a marked coherence over the globe characteristic features are the maximum stand somewhere in the middle of the nineteenth century and the steady retreat afterwards with some minor interrruptions depending on the particular region in many papers qualitative statements have been made about the causes of these fluctuations lower temperatures associated with solar variability and or volcanic activity are the most popular explanations in particular the statistical relation between glacier activity and major volcanic eruptions appears to be strong in this paper an attempt is made to simulate recent glacier fluctations with a physics based model a simple climate model calculating perturbations of surface radiation balance and air temperature not necessarily in phase is coupled to a schematic time dependent glacier model the climate model is forced by volcanic activity greenland acidity and or lamb s dust veil index and greenhouse warming solar variability was not considered because its effect on climate has still not been demonstrated in a convincing way the output is translated into variations in equilibrium line altitude driving the glacier model the simulated variations in glacier length show good agreement with the observed record but the amplitude is too small this is improved when mass balance gradients are assumed to be larger in warmer climates compared to recently published modelling studies of particular glaciers in which series of local parameters e g tree ring width and temperature were used as forcing the present simulation is better this suggests that the radiation balance is a decisive factor with regard to glacier variations on longer time scales the model experiments lend support to the results of porter 1986 who concluded from a more qualitative study that a strong relation exists between periods of increased volcanic activity and glacier advances a comparison of some selected runs shows that according to the present model the greenhouse warming would be responsible for about 50 of the glacier retreat observed over the last 100 years

the depth of trenches as a function of the subduction rate and age of the lithosphere abstract the trench is one of the most characteristic features of the subduction zones for a given arc or for all the subduction zones put together relationships between their depth variations the variations of the subduction rate and the age of the sinking lithosphere can be established certain other parameters have been examined with those mentioned above but without success it will be demonstrated that the observed relationships can be interpreted in terms of an elastic model and of a viscous model

three dimensional numerical modeling of temperature and mantle flow fields associated with subduction of the philippine sea plate southwest japan we investigated temperature and mantle flow distributions associated with subduction of the philippine sea phs plate beneath southwest japan by constructing a three dimensional parallelepiped model incorporating a past clockwise rotation the bathymetry of the philippine sea plate and distribution of the subducting velocity within its slab the geometry of the subducting plate was inferred from contemporary seismic studies and was used as a slab guide integrated with historical plate rotation into the 3 d simulation using the model we estimated a realistic and high resolution temperature field on the subduction plate interface which was constrained by a large number of heat flow data and attempted to clarify its relationship with occurrences of megathrust earthquakes long term slow slip events l sses and nonvolcanic low frequency earthquakes lfes results showed that the oblique subduction coupled with the 3 d geometry of subducting phs plate was a key factor affecting the interplate and intraplate temperature distributions leading to a cold anomaly in the plate interface beneath western shikoku the bungo channel and the kii peninsula temperatures in the slab core in these regions at a depth near the continental moho were nearly 200 c lower than that in eastern shikoku indicating a high thermal lateral heterogeneity within the subducting plate the geothermal control of the lfes beneath western shikoku was estimated to be within a range from 400 to 700 c and the interplate temperature for the l sses with a slip larger than 15 cm beneath the bungo channel was estimated to be approximately 350 500 c a large horizontal temperature gradient of 2 5 c km was present where the lfes occurred repeatedly the steep temperature change was likely to be related to the metamorphic phase transformation from lawsonite or blueschist to amphibolite of hydrous minerals of the mid ocean ridge basalt of the subducting phs plate

mechanism of deep low frequency earthquakes further evidence that deep non volcanic tremor is generated by shear slip on the plate interface 1 we study the mechanism of low frequency earthquakes lfes in the nankai trough in western shikoku japan precise locations have previously suggested that they represent shear slip on the plate boundary in this paper we examine the mechanism of these events due to the low signal to noise ratio we analyze stacked lfe waveforms and compare them with the waveforms of nearby earthquakes of known mechanism within the subducting philippine sea plate analysis of both the focal mechanism using p wave first motions and the moment tensor using s waveforms indicates that lfes represent shear slip on a low angle thrust fault dipping to the northwest namely the plate interface together with reports that deep tremor consists of a swarm of lfes our results suggest that deep tremor is generated directly by shear slip on the plate interface and as such represents a seismic signature of the accompanying slow slip events

strength of fracture zones from their bathymetric and gravitational evolution fracture zone evolution is investigated using dynamic models that allow the fault zones to freely slip this is an improvement over past formulations where bathymetric offsets were imposed kinematically the models use a viscoelastoplastic rheology that incorporates the influence of fault friction on fracture zone slip history using viscoelastic plates we assess the role of small scale convection on removal of the lowermost thermal lithosphere beneath fracture zones through a comparison of synthetic gravity to free air gravity across fracture zones we find that the amplitude of the gravity jump across fracture zones is best fit by models with weak faults that have depth averaged yield strengths 10 mpa fracture zones with such low strengths can convert to subduction zones with 100 km of convergence many fracture zones do not fit plate subsidence models with locked or slipping faults but are better fit by systems that are tectonically deformed by modest amounts of extension

deep low frequency microearthquakes in or around seismic low velocity zones beneath active volcanoes in northeastern japan abstract precise hypocenter relocations have been made for shallow intraplate microearthquakes beneath northeastern japan by applying source region station corrections the relocated hypocenter distribution shows that most of the events are confined to the upper 15 km of the crust which forms the brittle seismogenic zone in this volcanic arc it also shows that exceptionally deep microearthquakes well below the base of the seismic aseismic boundary occur at twelve locations of northeastern japan they occur in the lowermost crust or in the uppermost mantle where rheological properties of rocks are presumed to be in the ductile regime all the 153 events presently detected in the lowermost crust or in the uppermost mantle have anomalously low predominant frequencies 1 5 5 hz both for p and s waves in contrast to those 8 20 hz of normal shallow events in the brittle seismogenic zone the magnitudes of these events are small m 2 2 moreover all of these events are located beneath active volcanoes and or near p wave low velocity zones in the uppermost mantle the proximity of these events to the active volcanoes and other anomalous features suggest that these deep low frequency events are generated by magmatic activity of mantle diapirs in the uppermost mantle

numerical simulations of deformation and dynamics of horizontally lying slabs summary recent results of seismic tomography have revealed that configurations of subducted slabs are varied some lie horizontally on the boundary between the upper and the lower mantle and others penetrate into the lower mantle in this study to understand the physical mechanism of deformation and dynamics of subducted slabs in the mantle a 2 d numerical model was constructed for this purpose the slabs were treated so as to move freely in an incompressible fluid model by removing a guide right after its bending at a depth of 660 km and at a depth of 400 km in the upper mantle incorporating density and viscosity jumps and a clapeyron slope for a pv mw phase transformation of olivine obtained from high pressure and hightemperature experiments at a depth of 660 km into the model we investigate temperature and velocity distributions state of stress and viscosity distribution in relation to the formation of horizontally lying slabs at a depth of 660 km when the slabs are treated to move freely just after their bending at the 660 km phase boundary the slabs temporarily appear to lie horizontally on the phase boundary however the slabs penetrate the phase boundary and eventually sink into the lower mantle the length of the horizontally lying portion of the slab becomes longer as descending velocity increases the lengths are about 1500 km 1200 km and 800 km for subducting velocities of 8 cm yr 1 4 cm yr 1 a nd 2c m yr 1 respectively when a viscosity jump of 10 to 100 times the viscosity for the upper mantle is given at the 660 km phase boundary the length of the horizontally lying portion of the slab becomes shorter because of friction between the slab and the underlying lower mantle for all the models in which the guide was removed at a depth of 400 km the slabs penetrate into the lower mantle without lying horizontally on the phase boundary these results indicate that it is difficult for the slabs to bend at the pv mw phase boundary and to form the configuration of the horizontally lying slabs even if the slabs descend with a small dip angle the calculated maximum principal compressive stress axes are oriented horizontally within the horizontally lying portion of the slabs which is inconsistent with seismologically observed vertical directions of the p axes there in order to explain the observed configurations and state of stress within the horizontally lying slabs it would be necessary to investigate the effects of a complicated viscosity and composition structure in the mantle in more detail

lateral shifts of apenninic foredeep depocentres reflecting detachment of subducted lithosphere abstract the hypothesis of lateral migration of slab detachment was formulated on the basis of seismic tomography results on the 3 d structure of subduction zones in the mediterranean region m j r wortel and w spakman structure and dynamics of subducted lithosphere in the mediterranenan region proc k ned akad wet 95 1992 325 347 the redistribution of slab pull forces associated with the tearing process is taken to affect foredeep development by adding a lateral component to internal external depocentre migrations in the case of the apenninic arc the direction of detachment and associated depocentre migration is expected to be southeast this prediction has been confronted with the tectonostratigraphy of oligocene to recent apenninic foredeeps results show there are two episodes marked by important lateral shifts of foredeep depocentres during the early late miocene tortonian the depocentre of the northern apennines foredeep shifted towards the southeast this is evidenced by the relative positions of the depocenters of the burdigalian serravallian marnoso arenacea and the tortonian umbrian marchean and lazian minor basins a further stepwise southeastward migration can be inferred from the plio pleistocene records of the central and southern apennines in a more general sense our results indicate that slab detachment may play a significant role in the evolution of foredeeps along convergent plate boundaries

porphyrin geochemistry of atlantic jurassic cretaceous black shales abstract late jurassic early cretaceous black shales and an overlying sequence of albian campanian zeolitic claystones from the falkland plateau dsdp ipod leg 71 site 511 were analyzed for tetrapyrrole pigment type and abundance the black shale sequence was found to be rich in dpep series dominated free base nickel ni and to a lesser extent vanadyl v 0 porphyrins a low level of organic maturity i e precatagenesis is indicated for these strata as nickel chelation by free base porphyrins is only 50 75 complete proceeding down hole to 627 meters sub bottom electronic and mass spectral data reveal that the proposed benzo dpep bd and tetrahydrobenzo dpep thbd series are present in the free base and ni species as well as the more usual occurrence in v 0 porphyrin arrays highly reducing conditions are suggested by an abundance of the pah perylene substantial amounts of the thbd bd series and a redox equilibrium between free base dpep and 7 8 dihydro dpep series which exist in a 7 1 molar ratio the albian campanian claystone strata were found to be tetrapyrrolepoor and those pigments present were typed as cu ni highly dealkylated c26 max etioporphyrins thought to be derived via redeposition and oxidation of terrestrial organic matter om results from the present study are correlated to our past analyses of jurassic cretaceous sediments from atlantic margins in an effort to relate tetrapyrrole quality and quantity to basin evolution and om sources in the proto atlantic

copper and copper ii porphyrins of the cretaceous tertiary boundary at stevns klint denmark high levels of copper ii cu 2a were found with the major part s 90 of the total cu located in the smectite cu 175 ppm and kerogen cu up to 1000 ppm of the basal black marl of the cretaceous tertiary kt boundary informal type sedimentary rock the fish clay at stevns klint denmark anomalous abundance 4000 ppm of the kerogen cu 2a porphyrins in this marl was detected by electron spin resonance a model is proposed in which the enormous acid rains caused by the kt asteroid impact washed out the humics already enriched with cu 2a cu 2a porphyrins of the top horizon of the nearby oxic soil into the fish clay basin during the kt event fl 2000 elsevier science b v all rights reserved

the mount st helens volcanic eruption of 18 may 1980 minimal climatic effect an energy balance numerical climate model was used to simulate the effects of the mount st helens volcanic eruption of 18 may 1980 the resulting surface temperature depression is a maximum of 0 1 c in the winter in the polar region but is an order of magnitude smaller than the observed natural variability from other effects and will therefore be undetectable

on the trigger of el nino southern oscillation by the forcing of early el chichon volcanic aerosols after the volcanic eruption of el chichon in 1982 the distribution of volcanic aerosols in the upper atmosphere was zonally very inhomogeneous for about a month or more and was located in a fairly uniform zonal band around the earth in the northern subtropical region for the ensuing several months the impact of those aerosols absorbing radiation on the atmospheric motion is discussed in the model of gill 1980 it is shown that the mean westerly wind induced near the central pacific equatorial region would be able to trigger or amplify the el nino southern oscillation several observed results related to the simulation are discussed in some detail the location of the initial center of most aerosols relative to the central pacific is important in the model and the impact of the forcing at the agung event of 1963 is estimated to be smaller than that of the present event which seems to agree with observed results

tephrochronology of the siple dome ice core west antarctica correlations and sources abstract a total of 24 tephra bearing volcanic layers have been recognized between 550 and 987 m depth in the siple dome a sdm a ice core in addition to a number already recognized tephra in the upper 550 m dunbar et al 2003 kurbatov et al 2006 the uniform composition and distinctive morphological of the particles composing these tephra layers suggest deposition as a result of explosive volcanic eruptions and that the layers therefore represent time stratigraphic markers in the ice core despite the very fine grain size of these tephra mostly less than 20 microns robust geochemical compositions were determined by electron microprobe analysis the source volcanoes for these tephra layers are largely found within the antarctic plate statistical geochemical correlations tie nine of the tephra layers to known eruptions from mt berlin a west antarctic volcano that has been very active for the past 100 000 years previous correlations were made to an eruption of mt takahe another west antarctic volcano and one to mt hudson located in south america kurbatov et al 2006 the lowest tephra layer in the ice core located at 986 21 m depth is correlated to a source eruption with an age of 118 1 1 3 ka suggesting a chronological pinning point for the lower ice an episode of anomalously high volcanic activity in the ice in the sdm a core between 18 and 35 ka gow and meese 2007 appears to be related to eruptive activity of mt berlin volcano at least some of the tephra layers found in the sdm a core appear to be the result of very explosive eruptions that spread ash across large parts of west antarctica off the west antarctic coast as well as also being recognized in east antarctica basile et al 2001 narcisi et al 2005 narcisi et al 2006 some of these layers would be expected to should be found in other deep antarctic ice cores particularly ones drilled in west antarctica providing correlative markers between different cores the analysis of the tephra layers in the siple dome core along with other antarctic cores provides a timing framework for the relatively proximal antarctic and south american volcanic eruptive events allowing these to be distinguished from the tropical eruptions that may play a greater role in climate forcing

thermo mechanical model of the mantle wedge in central mexican subduction zone and a blob tracing approach for the magma transport the origin of the central mexican volcanic belt cmvb and the influence of the subducting cocos plate on the cmvb volcanism are still controversial in this study the temperature and mantle wedge flow models for the mexican subduction zone are developed using the finite element method to investigate the thermal structure below cmvb the numerical scheme solves a system of 2d stokes equations and 2d steady state heat transfer equation two models are considered for the mantle wedge the first one with an isoviscous mantle wedge and the second one with strong temperature dependent viscosity the first model reveals a maximum temperature of 830 c in the mantle wedge which is not sufficient for melting of wet peridotite also the geotherm of the subducting plate upper surface does not intersect the dehydration melting solidus for mafic minerals the second model predicts temperatures of more than 1200 c beneath the cmvb for a wide range of rheological parameters reference viscosity and activation energy up to 0 6 wt h2o can be released down to 60 km depth through metamorphic changes in the oceanic crust of the subducting slab the melting of this oceanic crust apparently occurs in a narrow depth range of 50 60 km and also melting of the mantle wedge hydrated peridotite is now expected to take place beneath cmvb considering that the melting processes on and in the vicinity of the subducting plate surface generate the most of the volcanic material a dynamic model for the blob tracers is developed using stokes flow at infinite prandtl number the blobs of 0 2 10 0 km in diameter migrate along very different trajectories only at low wrapping viscosities w 1 0 14 5 10 17 pa s the modeling results show that the fast trajectories terminate at the same focus location at the base of the continental crust while the arrival points of slow trajectories which are common for the blobs of smaller size 0 4 0 5 km are scattered away from the average focus location this observation may give us a hint on a possible mechanism of strato and mono volcanoes genesis the rise

volcanic forcing of climate over the past 1500 years an improved ice core based index for climate models 1 understanding natural causes of climate change is vital to evaluate the relative impacts of human pollution and land surface modification on climate we have investigated one of the most important natural causes of climate change volcanic eruptions by using 54 ice core records from both the arctic and antarctica our recently collected suite of ice core data more than double the number of cores ever used before reduces errors inherent in reconstructions based on a single or small number of cores which enables us to obtain much higher accuracy in both detection of events and quantification of the radiative effects we extracted volcanic deposition signals from each ice core record by applying a high pass loess filter to the time series and examining peaks that exceed twice the 31 year running median absolute deviation we then studied the spatial pattern of volcanic sulfate deposition on greenland and antarctica and combined this knowledge with a new understanding of stratospheric transport of volcanic aerosols to produce a forcing data set as a function of month latitude and altitude for the past 1500 years we estimated the uncertainties associated with the choice of volcanic signal extraction criteria ice core sulfate deposition to stratospheric loading calibration factor and the season for the eruptions without a recorded month we forced an energy balance climate model with this new volcanic forcing data set together with solar and anthropogenic forcing to simulate the large scale temperature response the results agree well with instrumental observations for the past 150 years and with proxy records for the entire period through better characterization of the natural causes of climate change this new data set will lead to improved prediction of anthropogenic impacts on climate the new data set of stratospheric sulfate injections from volcanic eruptions for the past 1500 years as a function of latitude altitude and month is available for download in a format suitable for forcing general circulation models of the climate system

a new high precision furnace for paleomagnetic and paleointensity studies minimizing magnetic noise generated by heater currents inside traditional thermal demagnetizers although magnetic noise fields generated by heater currents in thermal demagnetizers have been noticed for a long time no satisfactory tests have been conducted to quantify their effects toward this end we have developed a new high precision thermal demagnetizer that greatly reduces the magnetic noise field we show the data quality generated by the new oven and the comparative results on several real samples that demonstrate the effects of the magnetic noise field due to heater currents the properties of the spurious magnetization emanating from the heater currents critically depend on the decay rate of amplitude and its waveform of electric power which is delivered to oven coils at the end of the heating stage of thermal demagnetization these results also illustrate the potential applications of this new instrument in paleomagnetism and paleointensity studies

thermal regime of the southwest japan subduction zone effects of age history of the subducting plate abstract during a mid miocene at about 15 ma tectonic reorganization an ocean spreading ridge perpendicular to the strike of the nankai trough subduction zone stopped spreading since that time there has been subduction of the cooling fossil spreading ridge in the present study we have developed a time dependent thermal subduction model for this region using the finite element method there are good seismological and geological constraints for the plate geometry and for the subduction history of the past 15 ma boundary conditions are specified so that the oceanic plate subducting at the nankai trough becomes cooler as it gets older the model results agree with the present heat flow trend that decreases landward and explain the paleothermal regime of high mid miocene temperatures inferred from land geological studies the tectonics of the region prior to 15 ma has some uncertainties assuming subduction of an active spreading ridge or an 80 ma old lithosphere prior to 15 ma give the same results for the present thermal regime of the seaward portion of the forearc but different results for the most landward region 250 km the thermal history of the forearc since 15 ma can be summarized as a rapid warming period as a consequence of ridge subduction followed by a cooling trend to the present as a result of the aging of the subducting plate the results illustrate the thermal consequences of one type of ridge subduction they also demonstrate that the thermal regime of a subduction zone depends critically on the age history of the subducting oceanic lithosphere especially if it is young as well as such parameters as the subducting plate dip angle and thickness of insulating sediments on the incoming oceanic crust this dependence is especially important when the thermal regime is used to constrain the seismogenic behaviour of the subduction thrust fault

fracturing of volcanic systems experimental insights into pre eruptive conditions conditions for fracturing are a primary control on the behaviour of volcanic systems especially during the approach to eruption we here present the results of deformation experiments under simulated volcanic conditions on a porhyritic andesite from ancestral mount shasta andesite was chosen as a representative material because it is common at subduction zone volcanoes among both erupted products and country rock we deformed the lava in tension and triaxial compression tests at strain rates of 10 5 s 1 confining pressures from 0 to 50 mpa and temperatures up to 900 degrees c we also concurrently recorded acoustic emissions ae in order to monitor cracking activity the results show that deformation behaviour changes significantly in the temperature range 600 750 degrees c thus as temperatures increased across this interval the tensile fracture toughness increased from 2 5 0 5 mpa m 1 2 to 3 5 1 mpa m 1 2 the compressive strength decreased from 110 30 mpa to 55 35 mpa at 900 degrees c and the corresponding young s modulus decreased from 20 4 gpa to 6 4 gpa the changes occur when the deformation of the sample changes from elastic brittle to brittle ductile behaviour which we attribute to the blunting of crack tips due to melting of the glass phase and enhanced crystal plasticity at high temperature ae activity was observed in all experiments indicating that earthquakes can be generated not only in country rock but also in hot magma such as may be found in lava domes and at the margins of magma conduits in addition the trends in accelerating ae event rates before sample failure were comparable to those seen in earthquakes before some volcanic eruptions and a minimum in the seismic b value coincided with sample failure applied to volcanic systems the results suggest that 1 andesite strength and elasticity will not be affected by temperature or pressure beyond similar to 10 100 m from active magma 2 before eruptions fractures propagate preferentially through weaker horizons in a mechanically heterogeneous volcano and 3 volcanic rocks have characteristic seismic b values that are perturbed during the approach to bulk failure each of these conditions provides quantitative constraints on models for seismic precursors to eruption or intrusion c 2009 elsevier b v all rights reserved

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

nickel porphyrins in the bedding planes of a colorado lean oil shale abstract a colorado lean oil shale was found to have red colored material on the faces of bedding planes and fractures this red material is almost entirely the nickel chelate of a single c 29 porphyrin of deoxophylloerythroetioporphyrin type the molecular weight of the chelate is 490 or 492 depending on which of the two major isotopes of nickel is chelated a small amount of a methylene homologue 14 mass units higher in molecular weight is also present porphyrins extracted from the powdered shale are an extended homologous series of deoxophylloerythroetioporphyrin type and possibly etio type nickel porphyrins as determined by mass spectrometry the origin of the red porphyrins coating the shale fractures and bedding planes therefore was exogenous to these particular shale samples

three dimensionality of slab detachment due to ridge trench collision laterally simultaneous boudinage versus tear propagation the detachment of subducted tectonic plates is a process that has been increasingly associated with collisional scenarios and the end of subduction in various locations worldwide in particular the propagation of slab detachment tearing of a subducting plate has been described in conceptual models as a cause for spatially and temporally progressing surface effects such as slab gap volcanism and uplift however there is little understanding of the causes and dynamics associated with three dimensional 3 d slab tearing especially in the case of ridge trench collision here we show using fully dynamic 3 d numerical models that the process of detachment due to ridge trench collision depends on the geometry of the ridge segments approaching the trench for a finite laterally symmetric slab the 3 d detachment process occurs nearly simultaneously along strike by way of boudinage type necking and opening of holes central to the slab for a case involving the approach of two offset ridge segments to the trench slab tearing occurs in the form of 1 a vertical propagating separation along the age offset boundary within the slab that was previously weakened by a transform weak zone and 2 horizontal propagating detachment controlled by lateral transfer of slab pull to adjacent surface plate segments however lateral decoupling between offset adjacent plate segments and the propagating nature of the vertical and horizontal tearing are dependent upon fracture zones remaining weak through the subduction zone whether detachment occurs simultaneously along strike or propagates laterally the process is controlled by plastic yielding of the slab interior when young lithosphere entering the trench can no longer support slab pull

a latitudinally dependent volcanic dust veil index and its effect on climate simulations abstract a latitudinally dependent annual average volcanic dust veil index is developed based on indices of lamb and mitchell this index is then used to force a climate model to determine if an improved volcanic index could improve the correlations previously found when forcing the model with hemispherically averaged dust it does not this suggests that natural variability may also be an important cause of climate change during the past 400 years

striations duration migration and tidal response in deep tremor deep tremor in subduction zones is thought to be caused by small repeating shear slip events on the plate interface with significant slow components it occurs at a depth of about 30 kilometres and provides valuable information on deep plate motion and shallow stress accumulation on the fault plane of megathrust earthquakes tremor has been suggested to repeat at a regular interval migrate at various velocities and be modulated by tidal stress here i show that some time invariant interface property controls tremor behaviour using precise location of tremor sources with event duration in western shikoku in the nankai subduction zone japan in areas where tremor duration is short tremor is more strongly affected by tidal stress and migration is inhibited where tremor lasts longer diffusive migration occurs with a constant diffusivity of 10 4 m 2 s 1 the control property may be the ratio of brittle to ductile areas perhaps determined by the influence of mantle wedge serpentinization on the plate interface the spatial variation of the controlling property seems to be characterized by striations in tremor source distribution which follows either the current or previous plate subduction directions this suggests that the striations and corresponding interface properties are formed through the subduction of inhomogeneous structure such as seamounts for periods as long as ten million years

slab dehydration in warm subduction zones at depths of episodic slip and tremor abstract non volcanic tremor nvt and episodic slow slip events sses have been observed below the seismogenic zone of relatively warm subduction zones for the past 20 years geophysical and geologic observations show that this portion of the subduction interface is fluid rich and many models for these slip behaviors necessitate high pore fluid pressures however whether these fluids are sourced from local dehydration reactions in particular lithologies or require up dip transport from greater depths is not known we present thermodynamic models of the petrologic evolution of four lithologies typical of the plate interface average morb seafloor altered morb hydrated depleted morb mantle and metapelite along predicted plate boundary pressure temperature p t paths at several warm subduction segments where nvt and sses are observed at depths between 25 65 km the models suggest that 1 2 wt h2o is released at the depths of nvt sses in jalisco colima guerrero cascadia and shikoku due to punctuated dehydration reactions within morb primarily through chlorite and or lawsonite breakdown these reactions produce sufficient in situ fluid across a narrow p t range to cause high pore fluid pressures at nvt sse depths dehydration of hydrated peridotite is minimal at these depths for most margins and metapelite releases h2o

integration of transient strain events with models of plate coupling and areas of great earthquakes in southwest japan we model the crustal deformation caused by two long term subduction slip transients in southwest japan which we refer to as the 2000 2004 tokai and the 2002 2004 bungo channel slow slip events sses we use re analysed geonet position time series and a kalman filter based network inversion method to image the spatiotemporal slip variation of the two events on the plate interface during the period of 1998 2004 67 and 2000 2005 both events are found to have complex slip histories with multiple subevents in addition to a newly identified slip subevent in 2002 2003 we find that the major event in the bungo channel sse initiated in early 2003 beneath the northeastern corner of the region and expanded southwestward in contrast to the slip characteristics suggested by other studies the re analysed gps data in the tokai region shows a renewed slip activity for the tokai sse in early 2003 2004 at a similar location as in the period of 2001 2002 the equivalent m w for both the tokai and bungo channel sses are about 7 0 our results show that the tokai sse appears to start before the miyaki kozu seismovolcanic event integrating plate coupling and sses shows that the transient slip zones are located in a region between the locked zones and the epicentres of the low frequency earthquakes lfes at least part of the interseismic slip deficit is released by episodic sses beneath the bungo channel region we find excellent temporal correspondence between transient slip and adjacent lfes for both sse suggesting that they are closely related and possibly reflect that long term slow slip may modulate the occurrence of lfes

determination of the pore fluid pressure ratio at seismogenic megathrusts in subduction zones implications for strength of asperities and andean type mountain building 1 we construct the differential stress profile across the fore arc in a subduction zone from the force balance between the shear stress at seismogenic megathrust and the lithostatic pressure we assume that is written by 1 n where is the pore fluid pressure ratio is the coefficient of static friction and n is the normal stress given a density structure of the fore arc wedge we determine by comparing calculated fore arc stresses with observed ones as 0 95 0 98 in shikoku miyagi peru north chile and south chile and 0 90 0 93 in south vancouver island and washington the parameter averaged over the seismogenic megathrust is of the order of 10 mpa stress drops of great earthquakes in these zones occupy 14 87 and not a constant fraction of they on the other hand increase linearly with 1 we propose a simple fault model in which the area of asperities as a fraction of the total fault area is proportional to 1 variation of fractional area of asperities thus may explain the observed correlation and the regional variation of assuming that the differential stress at summit of the andean mountains is zero not at the coast as observed at present we determine to be 0 84 in north chile in the mountain building stage such a smaller value of along with 0 9 in subduction zones would suggest that variation of controls the tectonic style of the earth

relationships among temperature dehydration of the subducting philippine sea plate and the occurrence of a megathrust earthquake low frequency earthquakes and a slow slip event in the tokai district central japan abstract in this study we performed two dimensional numerical simulations of temperature distribution associated with subduction of the philippine sea phs plate and estimated the dehydration process of hydrous mid ocean ridge basalt morb in the oceanic crust in the tokai district central japan we discuss the relationship among temperature dehydration and a seismogenic zone of an expected megathrust tokai earthquake and the occurrence of a slow slip event sse and deep low frequency earthquakes lfes the depth range of the seismogenic zone for the megathrust earthquake was estimated to be 8 22 km narrowing toward the east the most suitable value of the pore pressure ratio on the plate interface was estimated to be 0 97 indicating minimal frictional heating there the temperatures of the upper surface of the phs plate where the tokai sse occurred from 2000 to 2005 were estimated to be 350 450 c therefore the tokai sse is considered to have occurred at the transition zone between unstable and stable sliding in addition hydrous morb was transformed from blueschist into greenschist near the region where the tokai sse occurred the temperatures of the upper surface of the phs plate where lfes occur were estimated to be 450 500 c therefore lfes are considered to occur near the down dip limit of the transition zone the amount of dehydration from the oceanic crust of the subducting phs plate near the region where lfes are distributed in a belt like form decreases toward the east with deepening of the hypocenters of the lfes thus the mechanisms of generation of lfes in the tokai district might differ from those in southwest and central japan where lfes generally occur at depths between 27 and 35 km in summary we consider that the occurrence of lfes is related to both temperature conditions and dehydration process however there is an exceptional area in the eastern part of the tokai district where the occurrence of lfes is restricted by temperature conditions rather than dehydration from the oceanic crust because the hypocenter depth is greater than that in southwest and central japan

the structure and development of the baikal rift depression abstract the baikal rift depression located in southeastern siberia is one of the best investigated intra continental rift structures in the world in addition to being the site of comprehensive studies by russian geo scientists for decades the baikal depression is now the subject of new international investigations using advanced technical means and technologies the present paper summarizes the principal results concerning the morphostructure of the baikal rift the stratigraphy of the cenozoic deposits and the stages of the baikal depression development based on cross sections which exhibit the impact of both tectonic and climatic processes the development of the baikal depression occurred through two stages i the pre crypto rift stage of the late cretaceous eocene and ii active rifting of the oligocene recent the second stage is divided into several steps and substeps separated by tectonic phases formation of the baikal rift zone appears to be related to the indo eurasian plate collision related to a model of passive rifting the uplifted shoulders of the depression have a complex morphostructure overlain by cenozoic deposits which are thin and restricted in area the near marginal zones of the rift are largely defined by the system of longitudinal blocks of different width surfaces of the blocks are inclined either to the uplifted shoulder of the rift bound by listric faults typical for western edges of the depression or to the axis of the depression cenozoic deposits on the blocks consist of accumulative plains composed primarily of thick lacustrine sequences

variations of fluid pressure within the subducting oceanic crust and slow earthquakes 1 we show fine scale variations of seismic velocities and converted teleseismic waves that reveal the presence of zones of high pressure fluids released by progressive metamorphic dehydration reactions in the subducting philippine sea plate in tokai district japan these zones have a strong correlation with the distribution of slow earthquakes including long term slow slip ltss and low frequency earthquakes lfes overpressured fluids in the ltss region appear to be trapped within the oceanic crust by an impermeable cap rock in the fore arc and impede intraslab earthquakes therein in contrast fluid pressures are reduced in the lfe zone which is deeper than the centroid of the ltss because there fluids are able to infiltrate into the narrow corner of the mantle wedge leading to mantle serpentinization the combination of fluids released from the subducting oceanic crust with heterogeneous fluid transport properties in the hanging wall generates variations of fluid pressures along the downgoing plate boundary which in turn control the occurrence of slow earthquakes

tetrapyrrole pigments in united states humic coals abstract analyses of forty two united states humic coals have revealed a striking divergence between chlorophyll diagenesis in coals when compared to the more widely studied marine sediments shales asphalts and petroleums porphyrins of humic coals have been found to be dominated by the etio series to lack members above c 32 and in lower ranked samples e g sub bituminous b high volatile c to exhibit mass spectral envelopes with unique even carbon number predominances the weighted average mass as well as the carbon number maximum viz mode of these coal porphyrin homologies has been found to decrease as rank increases the generation of porphyrins of the etio series is suggested as occurring both during early coal formation including oxidative scission of the isocyclic ring at the phorbide stage and later during the catagenic alteration of surviving dpep series porphyrins a preliminary study of chlorophyll diagenesis in a south florida peat partially substantiates this suggestion and has further shown that the coal porphyrins can be derived from bacterial as well as higher plant chlorophylls

slow earthquakes coincident with episodic tremors and slow slip events we report on the very low frequency earthquakes occurring in the transition zone of the subducting plate interface along the nankai subduction zone in southwest japan seismic waves generated by very low frequency earthquakes with seismic moment magnitudes of 3 1 to 3 5 predominantly show a long period of about 20 seconds the seismicity of very low frequency earthquakes accompanies and migrates with the activity of deep low frequency tremors and slow slip events the coincidence of these three phenomena improves the detection and characterization of slow earthquakes which are thought to increase the stress on updip megathrust earthquake rupture zones

ice cores as an index of global volcanism from 1850 to the present to evaluate an important cause of past climate change climate researchers need reliable estimates of volcanic aerosol loading in the atmosphere previous indices the dust veil index volcanic explosivity index and those by mitchell 1970 sato et al 1993 and khmelevtsov et al unpublished manuscript 1993 all have drawbacks ice core acidity and sulfate records because they contain physical evidence of atmospheric loading are a promising source of information on past volcanic aerosols but these records contain large nonvolcanic signals as well we have compared northern hemisphere nh and southern hemisphere sh annual average versions of five indices with eight nh and six sh ice core records for the period 1850 to the present in an attempt to identify the volcanic signal common to all records the indices are all highly correlated with each other for the nh although the individual ice core records are in general not well correlated with each other or with any of the indices a composite derived from averaging the cores the ice core volcano index ivi shows promise as a new index of volcanic aerosol loading this new index correlates well with the existing non ice core volcanic indices and with high frequency temperature records for the sh the individual ice cores and indices are better correlated the shivi is again highly correlated with all indices and individual ice cores but not with high frequency temperature records for both hemispheres the southern oscillation index shows no significant correlation with the volcanic indices ice cores or the ivi thus providing no evidence for the impact of volcanic eruptions on el nino southern oscillation events

porphyrin distributions in crude oils from the jianghan and biyang basins china abstract a number of oils tar sands and extracts of suspected source rocks from the jianghan and biyang basins of china have been studied to investigate their porphyrin content the results obtained suggest that these distributions are controlled to a large extent by the nature of the depositional environment and the maturity levels of the sediments it is proposed that the predominance of ni porphyrins in the sediments of these basins is caused mainly by the high ni content of the groundwater the relatively high content of a c32 porphyrin isomer with a five membered ring with a methyl substituent is indicative of environments with very high salinity the porphyrin distributions have been affected through thermal maturation with the major changes being dealkylation alkylation within etio porphyrins the instability of ni dpep porphyrins is suggested as a major reason for the rarity of dpep porphyrins in chinese oils the ratios of dpep etio or dpep etio dpep c 31 etio a c 28 etio b and etio a c 31 c 30 are useful as thermal maturity parameters and the results also indicate that the percentage of dpep porphyrins decreases through migration

major optical depth perturbations to the stratosphere from volcanic eruptions pyrheliometric period 1881 1960 a detailed chronology of major stratospheric dust veils from 1881 to 1960 has been constructed by searching the primary literature for relevant observational data of various kinds especially pyrheliometry data from 23 observing sites in both hemispheres have been reduced in a rigorous fashion to yield a table of pyrheliometric optical depth perturbations as a function of year month and latitude band to convert to visual reference wavelengths the tabular entries should be multiplied by a time dependent factor that is somewhat greater than unity ten measurable volcanic dust veils have been established in this manner and have been analyzed as to their generation transport decay and total mass these clouds arose from eruptions of krakatau 1883 an unidentified volcano 1890 soufriere and pelee 1902 santa maria 1902 ksudach 1907 katmai 1912 puyehue 1921 paluweh 1928 komagatake 1929 and quizapu 1932 other turbidity indicators have also been used in the analysis including starlight extinction purple twilight glows color of sun and moon polarization of blue sky light bishop s ring around the sun and dark lunar eclipses global stratospheric aerosol loadings have been computed from the peak turbidities agreement with the aerosol masses derived from polar ice cores is good in the important cases of krakatau santa maria and after a major correction of the ice core value katmai during the long period 1881 1992 about 80 of all stratospheric aerosols generated by the largest sulfur producing eruptions were emplaced during the two short time intervals 1883 1902 and 1982 1991 the long term 1881 1992 average annual production rate of stratospheric so2 from the largest eruptions was 0 8 tg yr 1 about half the average rate since 1981 implications of the principal results are discussed

temperature distribution of the upper surface of the subducted philippine sea plate along the nankai trough southwest japan from a three dimensional subduction model relation to large interplate and low frequency earthquakes summary the nankai subduction fault in southwest japan is a plate boundary along which large megathrust earthquakes have repeatedly occurred a previous 2 d thermal model suggested that the updip and downdip limits of the seismogenic zone are closely related to the temperature of the plate interface we estimated a new temperature distribution for the plate interface based on a 3 d thermal convection model that incorporates revised governing equations temporal changes in relative plate motion the shape of the subducted slab temperature variations related to basin evolution and more heat flow data and model regions than have been used in previous models the margin parallel spatial variation in the temperature of the plate interface was caused by spatio temporal changes associated with the cooling of the shikoku basin the updip limit of the thermally delineated seismogenic zone thermal seismogenic zone was approximately parallel to the trough axis due to a spreading rate that increased with distance from a fossil ridge although the margin parallel age dependency of the subducting plate was taken into account the downdip limit of the thermal seismogenic zone was determined by temperature except in eastern kyushu where it was determined by a boundary between the continental moho depth and the upper surface of the subducting plate the maximum coseismic slip associated with the 1944 tonankai m 7 9 and the 1946 nankai m 8 0 earthquakes and a strongly coupled region estimated from gps data inversion obtained by previous studies were both located in the thermal seismogenic zone on the plate interface low frequency earthquakes located at the downdip of the thermal seismogenic zone took place at temperatures of about 400 500 c this may be related to the phase transformation of hydrous minerals in oceanic crust from lawsonite blueschist jadeite to amphibole eclogite and the associated decreases in water content of approximately 2 4 wt the thermal seismogenic zone is narrowest off the kii peninsula where the hypocentres of the tonankai and nankai earthquakes were located the narrow seismogenic zone may have determined the rupture initiation points for these earthquakes as was previously demonstrated by numerical simulations of the earthquake cycle

three dimensional visualization of a near vertical slab tear beneath the southern mariana arc the use of a three dimensional ray tracing inversion algorithm has greatly enhanced the resolution of gradients and strong variations in wave speeds to create improved p wave tomographic images of the mariana arc region the images obtained from the mariana arc region show relatively low amplitudes of heterogeneity due to the limited number of seismic stations in the area despite these limitations detailed interpretations of the three dimensional geometry and morphology of the pacific plate subducting beneath the philippine sea plate have provided a three dimensional model of the steep dip of the pacific plate and the curvature of the slab beneath the mariana arc in unprecedented detail the new p wave tomography and seismicity depict a previously unidentified e w trending near vertical tear in the subducting plate at the southern end of the mariana arc that divides the arc into two distinct segments a steeply dipping curved slab penetrating the lower mantle and a short 250 km depth slab along the challenger deep segment of the arc the slab tear is likely to be the result of the need to accommodate the reduced volume the slab must occupy as it is subducted beneath the philippine sea plate along a curved arc

thermal structure coupling and metamorphism in the mexican subduction zone beneath guerrero summary temperature is one of the most important factors that controls the extent and location of the seismogenic coupled and transition partially coupled segments of the subduction interplate fault the width of the coupled fault inferred from the continuous gps observations for the steady interseismic period and the transient width of the last slow aseismic slip event m w 7 5 that occurred in the guerrero subduction zone in 2001 2002 extends up to 180 220 km from the trench previous thermal models do not consider this extremely wide coupled interface in guerrero subduction zone that is characterized by shallow subhorizontal plate contact in this study a finite element model is applied to examine the temperature constraints on the width of the coupled area the numerical scheme solves a system of 2 d stokes equation and 2 d steady state heat transfer equations the updip limit of the coupling zone is taken between 100 and 150 c while the downdip limit is accepted at 450 c as the transition from partial coupling to stable sliding from the entire coupled zone the seismogenic zone extends only up to 82 km from the trench inferred from the rupture width of large subduction thrust earthquakes corresponding to the 250 c isotherm only a small amount of frictional heating is needed to fit the intersection of the 450 c isotherm and the subducting plate surface at 180 205 km from the trench the calculated geotherms in the subducting slab and the phase diagram for morb are used to estimate the metamorphic sequences within the oceanic subducting crust a certain correlation exists between the metamorphic sequences and the variation of the coupling along the interplate fault

use of paleo records in determining variability within the volcanism climate system abstract volcanic eruptions that inject large quantities of sulfur rich gases into the stratosphere have the capability of cooling global climate by 0 2 0 3 c for several years after the eruption equatorial eruptions will impact global climate whereas mid latitude eruptions can cool climate in the hemisphere of origin magnitude of cooling varies by latitude and it is possible for warming to occur in certain regions primarily during the winter although instrumental records have been used to quantify the volcanic impact on climate they are limited in their temporal and spatial coverage and the style and magnitude of eruptions occurring over the two centuries of instrumental records is limited a thorough evaluation of the range of variability in the volcanism climate system requires a multidisciplinary approach that includes the analysis of ice core records geological data atmospheric measurements and visual phenomena tree ring records and other proxy data evaluation of these longer time series indicates that multiple volcanic eruptions have the potential to force climate over decadal to multi decadal time frames especially when these eruptions enhance or extend pre existing cool conditions on the other hand a lack of climatically significant eruptions may result in warmer average temperatures over decadal time frames because the volcanic cooling component within the climate system is absent mega eruptions like the toba eruption of 71 000 73 000 yr ago may impact climate on centennial time frames through positive feedback mechanisms evidence exists which indicates that environmental changes associated with rapid climatic fluctuations such as crustal loading unloading with glaciation deglaciation and variability in glacial meltwater loading on ocean basins may cause an increase in volcanic activity

low frequency tremors intraslab and interplate earthquakes in southwest japan from a viewpoint of slab dehydration 1 low frequency tremors lft recently found in the southwest japan fore arc likely occur due to hydro fracturing from fluids leaving the dehydrating philippine sea slab however there are some places without such lft they are n izu kanto e shikoku and s kyushu these are the places where island arc type crust is subducting we propose that dehydration of the subducted crust does not occur beneath these regions because it is composed mainly of tonalite lacking the quantity of hydrous minerals seen in normal subducting oceanic crust we show that almost no earthquake occurs within the subducted crust in such regions consistent with dehydration embrittlement hypothesis for intraslab seismicity the lack of dehydration from the crust would also affect the mode of occurrence of interplate earthquakes because pore fluid pressure is more difficult to rise at the thrust zone this might be reflected in the irregular occurrence of great earthquakes in these regions

low frequency continuous tremor around the moho discontinuity away from volcanoes in the southwest japan 1 recent enhancement of seismic networks in the japan islands revealed occurrence of low frequency continuous tremors of a beltlike distribution in the southwest japan where the subducting philippine sea plate reaches depths of 30 40 km source depth of the tremor is estimated by selecting tremor segments with relatively clear p wave onsets the source region of the tremor is assumed to correspond to lowermost parts of crust close to a triple boundary of the crust mantle wedge and the subducting slab the long duration of the phenomenon indicates existence of fluid relating to the generation of the tremor the most probable fluid is considered to be water produced by dehydration process of chlorite and amphibole in the slab on the basis of data from high pressure and temperature experiments the northern border of the beltlike distribution is possibly rimmed by the edge of the mantle wedge because serpentine formation absorbs fluid water

subduction of the philippine sea plate beneath southwestern japan slab geometry and its relationship to arc magmatism 1 we carry out high resolution three dimensional seismic tomography of the crust and upper mantle beneath southwestern sw japan using arrival time data obtained from the nationwide seismograph network the tomographic images provide new insights into the configuration of the philippine sea slab and arc magmatism the results confirm the existence of an aseismic portion of the philippine sea slab at greater depths beyond the seismic portion the philippine sea slab is subducting aseismically down to at least a depth of 200 km in chubu 60 80 km in kinki and 60 km in chugoku and is subducting seismically to depths of 150 200 km in kyushu in the chubu district it is subducting seismically at a shallow dip and then bends downward beneath the volcanic area whereas in the kinki district it is subducting subhorizontally as far as the japan sea the differences in the slab geometry between adjacent regions correlate with the differences in volcanic activity suggesting the importance of the geometry of the philippine sea slab on arc magmatism our tomographic images further imply subduction of the philippine sea slab down to a depth of around 70 km northwest of the izu peninsula the philippine sea slab in that region might have split into western and eastern parts separated by a slab tear and collided with the pacific slab at depths of 150 200 km

dehydration induced porosity waves and episodic tremor and slip episodic tremor and slip ets along the subduction interface takes place where there is abundant evidence for elevated near lithostatic pore pressures at sufficiently great depths 30 45 km that chemical dehydration reactions must act as their dominant source we simulate fluid and heat flow while tracking the location of a vertically oriented one dimensional column of material as it subducts through the slow slip and tremor zone the material in the column is transformed through a pressure dependent and temperature dependent dehydration reaction that we describe with a generalized nonlinear kinetic rate law column deformation is largely dominated by viscous creep with a closure rate that depends linearly on porosity this behavior causes the dehydration reaction to generate traveling porosity waves that transport increased fluid pressures within the slow slip region to explore the possibility that the observed periodicity of slow slip and tremor in subduction zones can be explained by the migration of such porosity waves we derive a dispersion relation that accurately describes our numerical results we also obtain an expression for how the thickness of the dehydrating layer is expected to vary as a function of the parameters in the reaction rate law although the amplitudes of pore pressure perturbations rival those that are produced by known external forcings e g tides or passing surface waves our analysis suggests that given reasonable estimates of rock viscosity permeabilities in the range 6 5 10 15 to 5 10 10 m2 are required for porosity wave trains to form at periods comparable to those of slow slip and tremor

rheologic controls on slab dynamics several models have been proposed to relate slab geometry to parameters such as plate velocity or plate age however studies on the observed relationships between slab geometry and a wide range of subduction parameters show that there is not a simple global relationship between slab geometry and any one of these other subduction parameters for all subduction zones numerical and laboratory models of subduction provide a method to explore the relative importance of different physical processes in determining subduction dynamics employing 2 d numerical models with a viscosity structure constrained by laboratory experiments for the deformation of olivine we show that the observed range in slab dip and the observed trends between slab dip and convergence velocity subducting plate age and subduction duration can be reproduced without trench motion i e slab roll back for locations away from slab edges successful models include a stiff slab that is 100 1000 times more viscous than previous estimates from models of plate bending the geoid and global plate motions we find that slab dip in the upper mantle depends primarily on slab strength and plate boundary coupling with a small dependence on subducting plate age once the slab sinks into the lower mantle the primary processes controlling slab evolution are 1 the ability of the stiff slab to transmit stresses up dip 2 resistance to slab descent into the higher viscosity lower mantle and 3 subduction induced flow in the mantle wedge corner

three dimensional seismic velocity structure and configuration of the philippine sea slab in southwestern japan estimated by double difference tomography 1 three dimensional seismic velocity structure in and around the philippine sea plate subducting beneath southwestern sw japan is determined by applying double difference tomography method to arrival time data for earthquakes obtained by a dense nationwide seismic network in japan a region of low s wave velocity and high vp vs of several kilometers in thickness is recognized immediately above the region of intraslab seismicity in a wide area from tokai to kyushu this characteristic layer dips shallowly in the direction of slab subduction compared with the upper surface of the philippine sea slab based on seismic reflection and refraction surveys on seven survey lines we interpret that the low vs and high vp vs layer corresponds to the oceanic crust of the philippine sea slab on the basis of the position of the low vs and high vp vs layer and the precisely relocated hypocenter distribution of intraslab earthquakes the upper surface of the philippine sea slab is reliably determined for the entire area of sw japan nonvolcanic deep low frequency earthquakes that occurred associated with the subduction of the philippine sea slab are distributed along the isodepth contour of 30 km in sw japan except for the tokai district where the depth of deep low frequency earthquakes becomes gradually deeper toward northeast

two dimensional thermal modeling of subduction of the philippine sea plate beneath southwest japan abstract in this study we constructed a two dimensional thermal convection model associated with subduction of the philippine sea phs plate beneath southwest japan to estimate the thermal state to evaluate the validity of the calculated temperature distribution we compared the calculated heat flows with observed heat flow data such as the high sensitivity seismograph network japan hi net borehole land borehole marine probe and bottom simulating reflectors hi net heat flow data which have never been used for thermal modeling have spatially high resolutions on land and enable detailed estimation of the thermal state beneath the japanese islands we considered the spatio temporal change in the age of the phs plate change in its plate motion direction at 3 ma and the geometry of its upper surface calculated heat flows associated solely with subduction of the phs plate passing through central and eastern shikoku and the kii peninsula were not consistent with observations they tended to be lower on the seaward side of the corner of the mantle wedge and to have much longer spatial wavelengths on the continental side to explain heat flow data in southwest japan frictional heating at the plate interface on the seaward side of the corner of the mantle wedge and temperature changes due to surface erosion and sedimentation on land associated with crustal deformation during the quaternary must be incorporated into the model the most suitable pore pressure ratio to explain the heat flow data is estimated to be 0 95 temperatures on the upper surface of the phs plate obtained in this study are lower at shallow depths and higher at greater depths than those obtained by hyndman et al 1995 indicating that the seismogenic zone inferred from this study is narrower than that of their model microearthquakes within the subducting phs plate occur at temperatures below 700 800 c

highly dealkylated copper and nickel etioporphyrins in marine sediments abstract copper porphyrins have been recognized as natural constituents of marine sediments only within the past 5 years palmer and baker 1978 science201 49 51 in that report it was suggested that these pigments may derive from and be markers for oxidized terrestrial organic matter redeposited in the marine environment in the present study we describe the distribution of copper porphyrins in sediments from several north pacific and gulf of california dsdp ipqd sites legs 56 63 64 these allochthonous pigments have now been found to be accompanied by identical arrays of highly dealkylated nickel etioporphyrins evaluation of data from this and past studies clearly reveals that there is a strong carbon number distribution similarity betweeen coincident cu and ni etioporphyrins this homology match is taken as reflecting a common source for the tetrapyrrole ligands of this population of cu and ni chelates predepositional generation of these highly dealkylated etioporphyrins is concluded from the occurrence of these pigments in sediments continuing essentially all stages of in situ chlorophyll diagenesis cf baker and louda 1983a advances in organic geochemistry 1981 pp 401 421 that is their presence is not regulated by the in situ diagenetic continuum thus the highly dealkylated cu and ni etioporphyrins represent an allochthonous background over which autochthonous viz marine produced chlorophyll derivatives are deposited and are undergoing in situ diagenesis

velocity anomalies and spatial distributions of physical properties in horizontally lying slabs beneath the northwestern pacific region abstract we calculated velocity anomalies by constructing thermal structural models of horizontally lying slabs beneath kuriles central japan northern izu bonin and central izu bonin arcs the values of v p and v s anomalies become maximum in the depth range of 100 to 200 km within the slabs reaching 5 to 6 4 for v p anomalies and 7 to 8 5 for v s anomalies the calculated v s anomalies are larger than the v p anomalies and the ratio of the former to the latter is 1 2 to 1 5 within the slab this suggests that v s anomalies are more sensitive to temperature variation than v p anomalies indicating that the existence of the horizontally lying slabs is detectable in more detail by using s wave we also calculated spatial distributions of physical properties in the horizontally lying slabs beneath the four regions in the calculation we assumed that the material of the slabs consists of olivine mg 0 89 fe 0 11 2 sio 4 the estimated values of physical properties within the slabs are larger than those in the surrounding upper mantle except for poisson s ratio while the values within the metastable wedge of olivine become smaller this is most remarkable within the slab beneath the central izu bonin arc due to the lowest temperature among the four regions the horizontally lying slab above the 660 km seismic discontinuity beneath the central izu bonin arc is denser up to 60 kg m 3 and the slab within the metastable wedge of olivine is lighter up to 120 kg m 3 the values of the physical properties except for poisson s ratio decrease with increasing the horizontal distances of the horizontally lying portions of the slabs this is due to thermal assimilation which is largely dependent on moving velocity of the slabs

reassessment of crete greenland ice core acidity volcanism link to climate change previous comparisons of the crete greenland ice core acidity record with climate variations of the last 1 400 years suggested that changes in volcanism may have significantly influenced climate on this time scale however the ice core acidity record consists of volcanically induced spikes superimposed on a background acidity of nonvolcanic origin that varied significantly in amplitude herein the authors produce a record of crete volcanism based solely on an objective definition of individual volcanic events in the ice core record there are 92 acidity peaks in the 1 420 year record since at least 26 of the post 1 600 eruptions are of local icelandic origin they estimate a mean recurrence interval of 20 8 years for hemispheric scale eruptions rich in sulphate on decadal time scales volcanism has varied by a factor of three four over the last 1 400 years however the climate volcanism correlation is considerably less impressive r minus 0 23 than previously concluded r minus 0 52 the good correspondence between volcanism and climate previously reported are due to background acidity levels that show a significant little ice age increase the background increase may reflect changes in ocean productivity 24 refs 5 figs 1 tab

laboratory simulations of tensile fracture development in a volcanic conduit via cyclic magma pressurisation during volcanic unrest high magma pressure induces cracking and faulting of the country rock providing conduits for the transport of magma and other fluids these conduits known as dykes are fundamental structures for the transport of magma to the surface in volcanically active regions the mechanics of dyke propagation is not yet fully understood but is crucial to better model dyke emplacement and eruption in volcanoes central to this need is a greater understanding of the mechanical properties of the magma country rock interaction as a function of known magmatic pressure temperature and stress here we report data from a series of experiments in which we cyclically compress viscoelastic rhyolitic magma at 828 c 892 c and 918 c inside a cylindrical conduit like shell of basalt from mt etna italy until fracture occurs the compression is performed under strain rates cyclically varying between 5 10 6 and 5 10 5 s 1 the resultant monitored axial loading and relaxation illustrates how the presence of a visco elastic fluid magma controls the stress induced at the conduit margin boundary this is achieved by analysing the viscoelastic relaxation through time to calculate an apparent modulus which is found to decrease with both increasing temperature and time in the 4 cycles before failure we find that the apparent modulus decreases from 180 to 40 gpa 80 to 20 gpa and 8 to 1 gpa for imposed stress cycles at 828 c 892 c and 918 c respectively we theoretically estimate a tensile strength at failure of approximately 7 11 mpa consistent with recent field data and in agreement with a model derived from the sample geometry and basic material parameters post experimental neutron computed tomography and microscopic analyses further reveal the fragmentation of the melt and generation of tuffisite veins inside the conduit due to spontaneous crack nucleation associated with conduit wall fracture the geometry of the rupture area inside the melt is akin to a mach cone associated with supershear fractures we discuss our findings in terms of magma rock interaction leading to dykes tuffisite veins and magma fragmentation

middle miocene swift migration of the ttt triple junction and rapid crustal growth in southwest japan a review we review recent progress in geological and geophysical investigation in sw japan nankai trough and philippine sea plate psp and propose a hypothesis for the miocene tectonics in sw japan driven by middle miocene swift migration of the ttt trench trench trench triple junction the new hypothesis is based on the new interpretations as follows near trench magmatism in the outer zone of sw japan is ascribed to a collision of proto izu arc in addition to the previous model of an oceanic ridge of the shikoku basin and hot psp subduction the indentation structures at capes ashizuri muroto in shikoku and shiono on the kii peninsula were previously explained by kink folding due to recent e w compression we alternatively suppose the collision of the active arc or topographic peaks such as seamounts inferred from geological and experimental observations the main crustal component in sw japan is suggested to be of igneous plutonic rocks rather than the previous interpretation of cretaceous to tertiary accretionary complexes this is typically illustrated in the outer zone to the north of capes ashizuri muroto and shiono from geophysical observation of gravity anomalies velocity and resistivity together with geological estimations of caldera age and the size of its root pluton episodic crustal growth due to intrusion of igneous rock and subduction of the psp may have stopped after approximately 12 ma and restarted at approximately 6 ma our emphasis for this gap is a cessation and resurgence of subduction rather than the previous interpretation i e decreasing of subduction rate

seismic tomography of taiwan improved constraints from a dense network of strong motion stations in this study a large collection of 41 141 s p times from the untapped records of the taiwan strong motion instrumentation program tsmip network is combined with the p and s wave arrival times from the taiwan central weather bureau seismic network cwbsn to image the v p and v p v s structures beneath taiwan the records from the 680 tsmip stations throughout taiwan in the past 15 years enhance the path coverage and the resolution in the tomography inversions tremendously our result for the v p structure largely confirms previous studies but brings better constraint on the v p v s structure the colliding luzon volcanic arc is characterized by a belt of high v p and high v p v s with high seismicity that includes the offshore islands of lutao and lanyu and the coastal ranges in eastern taiwan at the depth between about 13 and 25 km this high v p v s belt can be traced to the subduction zone in the region between hualian and ilan in the deeper portion the shallow portions of the southwestern coastal plain and the pingtung region are also characterized by a belt of high v p v s with lower seismicity most of the events occurred at the base of the high v p v s zones we suggest that material strength in those regions may be too low to accumulate stress which may indicate water saturated young sediments finally the central range region is characterized by a low v p v s belt