the reaction talc forsterite enstatite h2o new experimental results and petrological implications the reaction talc forsterite enstatite h 2 o has been investigated between 6 and 20 kbar previous high pressure experimental studies suggested various reaction positions mostly with positive p t slopes the new results show that the reaction has a negative slope in the pressure range studied it was bracketed between 640 and 680 degrees c at 10 kbar between 620 and 640 degrees c at 15 kbar and between 580 and 600 degrees c at 20 kbar the growth of antigorite at high pressures prevented the reaction from being determined above 20 kbar the reaction position is consistent with the previous low pressure data of chernosky et al 1985 is in very good agreement with the position calculated using the thermodynamic database of berman 1988 and is in reasonable agreement with the calculation using the thermodynamic database of holland and powell 1990 the high reaction temperatures suggested by previous high pressure experiments may reflect metastable or quench growth of talc in those experiments talc in equilibrium with forsterite is stable over a wide p t range in hydrothermally altered and metamorphosed ultramafic rocks and may be important for carrying h 2 o into subduction zones however talc dehydration occurs at too shallow a depth for the h 2 o to contribute directly to subduction zone volcanism

determination of venus winds by ground based radio tracking of the vega balloons a global array of 20 radio observatories was used to measure the three dimensional position and velocity of the two meteorological balloons that were injected into the equatorial region of the venus atmosphere near venus midnight by the vega spacecraft on 11 and 15 june 1985 initial analysis of only radial velocities indicates that each balloon was blown westward about 11 500 kilometers 8 000 kilometers on the night side by zonal winds with a mean speed of about 70 meters per second excursions of the data from a model of constant zonal velocity were generally less than 3 meters per second however a much larger variation was evident near the end of the flight of the second balloon consistent systematic trends in the residuals for both balloons indicate the possibility of a solar fixed atmospheric feature rapid variations in balloon velocity were often detected within a single transmission 330 seconds however they may represent not only atmospheric motions but also self induced aerodynamic motions of the balloon

plate tectonics at an awkward junction rules for the evolution of sovanco ridge area ne pacific summary the elegant geometrical rules of plate tectonics do not allow for a gradual shift in plate motion directions or the gradual as opposed to sudden cessation of subduction at the scale of the small plates in the ne pacific imperfections in boundary processes have a large effect on the net torque on the plates and heavily influence the evolution of the geometry in this area the rotation of the spreading directions and the diminution of true subduction along the southern canadian coast has not occurred by the sudden switching of plate motions from one stable condition to another instead it appears as if the dominant factor for the evolution is the resistance of the ocean floor to formation of new smoothly slipping transform faults compressive deformation of even young lithosphere is not only mechanically unlikely but is not helpful to the particular configurations found in this area instead a migrating shear zone and an episode of highly en echelon spreading along a new axis nearly perpendicular to the present juan de fuca ridge have resulted the present sovanco ridge was never a transform fault neither is the nootka fault a shear zone but the locus of stretching between plates whose motions are congruent at the juan de fuca ridge but diverge toward the continental margin

the galileo probe doppler wind experiment measurement of the deep zonal winds on jupiter during its descent into the upper atmosphere of jupiter the galileo probe transmitted data to the orbiter for 57 5 min accurate measurements of the probe radio frequency driven by an ultrastable oscillator allowed an accurate time history of the probe motions to be reconstructed removal from the probe radio frequency profile of known doppler contributions including the orbiter trajectory the probe descent velocity and the rotation of jupiter left a measurable frequency residual due to jupiter s zonal winds and microdynamical motion of the probe from spin swing under the parachute atmospheric turbulence and aerodynamic buffeting from the assumption of the dominance of the zonal horizontal winds the frequency residuals were inverted and resulted in the first in situ measurements of the vertical profile of jupiter s deep zonal winds a number of error sources with the capability of corrupting the frequency measurements or the interpretation of the frequency residuals were considered using reasonable assumptions and calibrations from prelaunch and in flight testing it is found that beneath the cloud tops about 700 mbar the winds are prograde and rise rapidly to 170 m s at 4 bars beyond 4 bars to the depth at which the link with the probe was lost nearly 21 bars the winds remain constant and strong corrections for the high temperatures encountered by the probe have recently been completed and provide no evidence of diminishing or strengthening of the zonal wind profile in the deeper regions explored by the galileo probe

variable winds on venus mapped in three dimensions 1 we present zonal and meridional wind measurements at three altitude levels within the cloud layers of venus from cloud tracking using images taken with the virtis instrument on board venus express at low latitudes zonal winds in the southern hemisphere are nearly constant with latitude with westward velocities of 105 ms 1 at cloud tops altitude 66 km and 60 70 ms 1 at the cloud base altitude 47 km at high latitudes zonal wind speeds decrease linearly with latitude with no detectable vertical wind shear values lower than 15 ms 1 indicating the possibility of a vertically coherent vortex structure meridional winds at the cloud tops are poleward with peak speed of 10 ms 1 at 55 s but below the cloud tops and averaged over the south hemisphere are found to be smaller than 5 ms 1 we also report the detection at subpolar latitudes of wind variability due to the solar tide

investigations of soil cracking and preferential flow in a weighing lysimeter filled with cracking clay soil an improved understanding of deep drainage processes in irrigated cracking soils is needed for sustainable irrigation management to investigate the effect of crack dynamics and macropore flow on drainage in cracking soils a series of irrigation experiments was carried out in a weighing lysimeter subsurface soil cracks of the initially very dry soil were investigated with a videoscope and changes in the surface expression of cracks in response to irrigation events were monitored by time lapse photography a bromide tracer was applied to one irrigation event variations in the combined soil and moisture mass and the volume of drainage out of the soil column was logged and the drainage ec and bromide content were determined no drainage occurred out of the soil column during the first 3 out of 6 irrigation events even though substantial surface runoff into the cracks occurred and at least initially soil cracks provided an uninterrupted flow path through the profile the breakthrough of the bromide tracer as well as an initially low ec of the drainage water indicate that preferential flow accounted for a substantial part of the first of the two drainage events even though the soil cracks were sealed on the surface at the onset of the irrigation causing the drainage the results show that lateral infiltration of macropore flow into the soil matrix can be substantial and should not be neglected while simulating macropore flow and deep drainage in cracking soils the results also indicate that soil cracks can remain pathways for preferential flow even after they are closed at the soil surface the type of water application appears to have an impact on the location of crack formation with flood irrigation favouring reappearance of cracks at previous crack locations and simulated rainfall resulting in shifting crack locations

initiation of strike slip faults serpentinization and methane the nootka fault zone the juan de fuca explorer plate boundary the nootka fault zone is a ridge trench trench transform fault that was initiated 4 ma when the explorer ridge became independent of the juan de fuca ridge multibeam data around the fault zone and a compilation of several seismic reflection surveys provide insight into initiation of strike slip faults previous interpretations assumed that the two faults seen cutting the seafloor are subparallel to shear between the explorer and juan de fuca plates and formed instantaneously at 4 ma increased data density shows that these faults are subparallel to seafloor magnetic anomalies and appear to have utilized extensional faults formed at the ridge they are surrounded by numerous buried steeply dipping small offset growth faults at least some of which are likely still active our observations corroborate analogue models of strike slip fault initiation that predict formation of riedel like shears within a zone of faulting and that displacement localizes over time the existence of several long subparallel faults and a very wide zone of faulting has been predicted by models of distributed shear at depth along the nootka fault zone basement has risen by several hundred meters and bright reversed polarity reflectors some of which are interpreted to be methane hydrate reflectors are common hydration likely as serpentinization of the upper mantle could explain both sets of observations serpentinization can result in a 30 50 volume expansion and methane is observed in vents driven by this process biogenic sources of methane are likely to be present and concentrated by currently active fluid flow in the faulted sediments

continental signature of a ridge trench triple junction northern vancouver island south of the juan de fuca pacific north america triple junction located off northern vancouver island there is subduction and to the north there is oblique convergence along the queen charlotte transform fault in the mid tertiary periods of extension including crustal thinning and volcanism formed queen charlotte basin heat flow on northern vancouver island 67 mw m2 is the same as in queen charlotte basin to the north but it decreases abruptly to the south over a distance of 70 km to an average value of 46 mw m2 compared to the rest of the island the northern island has more subdued topography a lower mean elevation slightly higher bouguer gravity values and lower seismicity late tertiary volcanics which occur only on the northern part of the island geochemically resemble the tertiary masset formation which formed during periods of extension in queen charlotte basin these data are consistent with a tertiary history including periods of extension for northern vancouver island similar to queen charlotte basin and distinguish it from the rest of vancouver island the southern boundary of this northern block extends northeastward from brooks peninsula approximately above the inferred northern edge of the subducted juan de fuca plate and parallel to a series of linear magnetic anomalies associated with a northeasterly trending fault zone the abrupt transition in observed heat flow is most likely associated with the edge of the subducted plate which produces large temperature differences at depths of 30 40 km the time constant for these thermal effects to reach the surface conductively indicates that this boundary has been stable over tens of millions of years and therefore the location of the subducting plate s northern edge must have been stationary or moving very slowly over this period consequently it is likely that the triple junction has been located on average for the last 40 myr off brooks peninsula

seismic structure across the active subduction zone of western canada the vancouver island seismic project was conducted in 1980 to study the structure of the subducting oceanic juan de fuca plate and the overriding continental america plate the principal seismic refraction line line i was a 350 km onshore offshore profile perpendicular to the continental margin an array of 32 receivers was located on the america plate on the mainland and across vancouver island and extended offshore with three ocean bottom seismometers obs s two shots were fired at the eastern end of the line and 17 shots were located along the westernmost 100 km of the profile control for the interpretation of the onshore offshore profile was provided by a reversed refraction profile along the length of vancouver island and by a marine refraction profile recorded on the obs s modeling of the seismic structure of this complex region utilized an iterative inversion method for travel times from explosions in which shots at several locations are recorded on the same set of receivers and utilized an algorithim based on asymptotic ray theory for the calculation of synthetic seismograms through two dimensional media the major features of the refraction structural model are that 1 the oceanic lithosphere dips at 3 or less beneath the continental slope so the bend in the subducting slab occurs landward of the foot of the slope 2 the oceanic lithosphere dips at 14 16 beneath the continental shelf until it passes beneath the continental moho at 37 km depth below western vancouver island 3 an upper mantle reflector may correspond to the base of the subducting lithosphere and 4 a segment of high velocity material above the downgoing crust with velocity 7 7 km s and depth range 20 25 km may represent a remnant of subducted lithosphere perhaps detached when the subduction zone jumped westward to its present position

influence of venus topography on the zonal wind and uv albedo at cloud top level the role of stationary gravity waves based on the analysis of uv images at 365 nm of venus cloud top altitude 67 2 km collected with vmc venus monitoring camera on board venus express vex it is found that the zonal wind speed south of the equator from 5 s to 15 s shows a conspicuous variation from 101 to 83 m s with geographic longitude of venus correlated with the underlying relief of aphrodite terra we interpret this pattern as the result of stationary gravity waves produced at ground level by the up lift of air when the horizontal wind encounters a mountain slope these waves can propagate up to the cloud top level break there and transfer their momentum to the zonal flow such upward propagation of gravity waves and influence on the wind speed vertical profile was shown to play an important role in the middle atmosphere of the earth by lindzen 1981 but is not reproduced in the current gcm of venus atmosphere from lmd in the equatorial regions the uv albedo at 365 nm varies also with longitude we argue that this variation may be simply explained by the divergence of the horizontal wind field in the longitude region from 60 to 10 where the horizontal wind speed is increasing in magnitude stretch it triggers air upwelling which brings the uv absorber at cloud top level and decreases the albedo and vice versa when the wind is decreasing in magnitude compression this picture is fully consistent with the classical view of venus meridional circulation with upwelling at equator revealed by horizontal air motions away from equator the longitude effect is only an additional but important modulation of this effect this interpretation is comforted by a recent map of cloud top h2o fedorova et al 2015 showing that near the equator the lower uv albedo longitude region is correlated with increased h2o we argue that h2o enhancement is the sign of upwelling suggesting that the uv absorber is also brought to cloud top by upwelling

earthquake activity in northern cascadia subduction zone off vancouver island revealed by ocean bottom seismograph observations abstract offshore seismicity at the cascadia margin is poorly constrained because nearly all previous recordings of earthquakes were made using land based networks we conducted earthquake monitoring off vancouver island in northern cascadia using ocean bottom seismographs our results show that most of the offshore seismicity is concentrated along the nootka fault zone otherwise seismicity is extremely low with no earthquakes located along the shallow seismogenic part of the megathrust the lack of interplate seismicity may indicate complete healing and locking of the megathrust over three centuries after the great earthquake of 1700 and a somewhat lower degree of structure heterogeneity such as subducting seamounts events along the nootka fault zone occur over a 10 15 km depth range this wide distribution and the previously reported overall moment release rate suggest that a significant part of deformation of this fault zone is aseismic several earthquakes beneath the continental shelf may be related to faults dividing tectonic terrains within the overriding plate

active deformation of the gorda plate constraining deformation models with new geophysical data the gorda plate the southernmost fragment of the larger juan de fuca plate system is an example of a nonrigidly deforming tectonic accommodation zone or buffer plate absorbing deformation and allowing the surrounding larger plates to act in a more rigid fashion here we present a new structural analysis of the plate based on full plate bathy metric coverage augmented by seismic reflection data and earthquake moment tensors and locations we interpret internal deformation of the gorda plate as an asymmetrical flexural slip buckle with a vertical axis utilizing reactivation of spreading ridge fabric normal faults as strike slip faults newly formed second generation faults crosscutting the structural grain overprint the reactivated structures the spreading fabric faults finally begin a second phase of extension as the plate approaches the subduction zone this model based on fault constraints has allowed investigation of ridge plate subduction interac tions and suggests that spreading rate variations along the gorda ridge may be con trolled by internal deformation of the plate rather than the reverse as previously hypothesized

effect of pore geometry on vp vs from equilibrium geometry to crack seismic wave velocities of melt or aqueous fluid containing systems are studied over a wide range of pore shapes including oblate spheroids tubes cracks and an equilibrium geometry controlled by a dihedral angle the relative role of liquid compressibility and pore geometry on the v p v s velocity ratio is clarified the result clearly indicates that p and s velocity structures determined by seismic tomography can be used to verify whether interfacial energy controlled melt or fluid geometry equilibrium geometry is achieved relationships between the diverse models are clearly established by relating each model to the oblate spheroid model in terms of the equivalent aspect ratio as a function of the aspect ratio a significant effect of pore geometry on d in v s d in v p the ratio of the fractional changes in v s and v p is shown equilibrium geometry of the partially molten rocks characterized by a dihedral angle of 20 40 corresponds to an aspect ratio of 0 1 0 15 the value of d in v s d in v p expected for the texturally equilibrated partially molten rocks is shown to be 1 1 5 which is much smaller than that expected for cracks and dikes with an aspect ratio of 10 2 10 3 in the upper mantle low velocity regions the seismologically obtained value of d in v s d in v p is within this range beneath the bolivian andes 1 1 1 4 but is as high as 2 beneath iceland 1 7 2 3 and beneath northeastern japan 2 0 the former region can be regarded as a region where equilibrium geometry is achieved and the latter regions can be regarded as regions where dikes and veins typical of a system far from the textural equilibrium dominate

the moho in subduction zones abstract the moho in subduction zones exists in two distinct forms one associated with the subducting oceanic plate and second with the overriding plate the seismic expression of both forms is linked to the nature of a landward dipping low velocity zone lvz that has been detected in a majority of subduction zones about the globe and that approximately coincides with wadati benioff seismicity we review seismic studies that constrain the properties of the lvz in cascadia where it has been extensively studied for over a quarter century a model in which the lvz is identified with hydrated pillow basalts and sheeted dikes of oceanic crustal layer 2 is consistent with available geological and geophysical data and reconciles previously conflicting interpretations in this model the upper oceanic crust is hydrated through intense circulation at the ridge and becomes overpressured upon subduction as a result of metamorphic dehydration reactions combined with an impermeable plate boundary above and a low porosity gabbroic layer 3 below the resulting seismic velocity contrast approaching 50 for s waves significantly overwhelms that of a weaker underlying oceanic moho at greater depths oceanic crust undergoes eclogitization in a top down sense leading to gradual disappearance of the lvz the large volume change accompanying eclogitization is postulated to rupture the plate boundary allowing fluids to penetrate the cooled forearc mantle wedge pervasive serpentinization and free fluids reduce velocities within the wedge thereby diminishing erasing or even inverting the seismic contrast associated with the moho of the overriding plate this model is tested against observations of lvzs and forearc mantle structure worldwide

infrasound and gravity waves over the andes observed by a pressure sensor on board a stratospheric balloon the study of infrasound acoustic and gravity waves sources and propagation in the atmosphere of a planet gives us precious insight on atmosphere dynamics climate and even internal structure the implementation of modern pressure sensors with high rate sampling on stratospheric balloons is improving their study we analyzed the data from the national aeronautics and space administration ultra long duration balloon mission 16 may to 30 june 2016 here we focus on the balloon s transit of the andes mountains we detected gravity waves that are associated to troposphere convective activity and mountain waves an increase of the horizontal wavelengths from 50 to 70 km with increasing distance to the mountains is favoring the presence of mountain waves we also report on the detection of infrasounds generated by the mountains in the 0 01 0 1 hz range with a pressure amplitude increase by a factor 2 relative background signal besides we characterized the decrease of microbaroms power when the balloon was flying away from the ocean coast these observations suggest in a way similar to microseisms for seismometers that microbaroms are the main background noise sources recorded in the stratosphere even far from the ocean sources finally we observed a broadband signal above the andes between 0 45 and 2 hz probably associated with a thunderstorm the diversity of geophysical phenomena captured in less than a day of observation stresses the interest of high rate pressure sensors on board long duration balloon missions

low frequency earthquakes at the southern cascadia margin we use seismic waveform data from the mendocino experiment to detect low frequency earthquakes lfes beneath northern california during the april 2008 tremor and slip episode in southern cascadia 59 templates were generated using iterative network cross correlation and stacking and grouped into 34 distinct lfe families the main front of tremor epicenters migrates along strike at 9 km d 1 we also find one instance of rapid tremor reversal observed to propagate in the opposite direction at 10 20 km h 1 as in other regions of cascadia lfe hypocenters from this study lie several kilometers above a recent plate interface model south of cascadia lfes were discovered on the maacama and bucknell creek faults the bucknell creek fault may be the youngest fault yet observed to host lfes these fault zones also host shallow earthquake swarms with repeating events that are distinct from lfes in their spectral and recurrence characteristics

seismicity in cascadia abstract we examine spatio geometric patterns in the density of seismicity within the cascadia forearc to gain insight into controls on seismogenesis tremor epicenters exhibit the most regular distribution defining a 40 80 km wide band extending along almost the entire convergent margin from the southern terminus of gorda plate subduction in northern california to the central explorer plate below northern vancouver island based on prior characterization of constituent low frequency earthquakes the up and down dip limits of tremor are assumed to represent slab isodepth contours at nominal values of 28 and 45 km between which fluids at near lithostatic overpressures are trapped within the subducting slab epicenters of earthquakes within the north american crust are anticorrelated with those of tremor and concentrated in washington and northern california where they are sandwiched between tremor and the cascade volcanic arc seismicity within the subducting plate possesses the most limited epicentral distribution seismicity is confined to shallow depths off vancouver island and in northern california and projects to greater depths beneath washington and southern british columbia comparison of seismicity patterns with long wavelength slab geometry and thermo petrologic constraints suggests that seismicity occurrence in cascadia is governed by an interplay between slab strain metamorphic dehydration within the subducting oceanic plate and a plate boundary seal that controls where fluids enter the overriding plate the inferred fluid evolution model harbors interesting implications for mantle wedge hydrology forearc crustal composition and volcanism in cascadia and other warm subduction zones

recent movements of the juan de fuca plate system analysis of the magnetic anomalies of the juan de fuca plate system allows instantaneous poles of rotation relative to the pacific plate to be calculated from 7 ma to the present by combining these with global solutions for pacific america and absolute relative to hot spot motions a plate motion sequence can be constructed this sequence shows that both absolute motions and motions relative to america are characterized by slower velocities where younger and more buoyant material enters the convergence zone pivoting subduction the resistance provided by the youngest portion of the juan de fuca plate apparently resulted in its detachment at 4 ma as the independent explorer plate in relation to the hot spot framework this plate almost immediately began to rotate clockwise around a pole close to itself such that its translational movement into the mantle virtually ceased after 4 ma the remainder of the juan de fuca plate adjusted its motion in response to the fact that the youngest material entering the subduction zone was now to the south differences in seismicity and recent uplift between northern and southern vancouver island may reflect a distinction in tectonic style between the normal subduction of the juan de fuca plate to the south and a complex underplating occurring as the explorer plate is overridden by the continent the history of the explorer plate may exemplify the conditions under which the self driving forces of small subducting plates are overcome by the influence of larger adjacent plates the recent rapid migration of the absolute pole of rotation of the juan de fuca plate toward the plate suggests that it too may be nearing this condition

earth based radio tracking of the galileo probe for jupiter wind estimation although the galileo probe was designed to communicate only to the orbiter the probe radio signal was detected at two earth based radio observatories where the signal was a billion times weaker the measured signal frequency was used to derive a vertical profile of the jovian zonal wind speed due to the mission geometry the earth based wind estimates are less sensitive to descent trajectory errors than estimates based on probe orbiter doppler measurements the two estimates of wind profiles agree qualitatively both show high wind speeds at all depths sampled

a reanalysis of venus winds at two cloud levels from galileo ssi images abstract the global circulation of the venus atmosphere is characterized at cloud level by a zonal super rotation studied over the years with data from a battery of spacecrafts orbiters balloons and probes among them the galileo spacecraft monitored the venus atmosphere in a flyby in february 1990 in its route toward jupiter since the flyby was almost equatorial published analysis of zonal winds obtained from displacements of cloud elements on images obtained by the ssi camera belton m j s and 20 colleagues 1991 science 253 1531 1536 stop at latitudes 50 north and south in this paper we present new results on venus winds based on a reanalysis of an extended set of images obtained at two wavelengths 418 nm violet and 986 nm near infrared that sense different altitude levels in the upper cloud our main result is that we have been able to extend the zonal wind profile up to the polar latitudes 70 n and 70 s at 418 nm and 70 n at 986 nm binned and smoothed profiles are given in tabular form we show that the zonal winds drop in their velocity poleward of latitudes 45 n and 50 s where an intense meridional wind shear develops at the two cloud levels our data confirm the magnitude of this shear retrieved previously from radio occultation data but disagrees with it in the latitudinal location of the sheared region the new wind data can be used to recalibrate the zonal winds retrieved from the previous measurements of the temperature field and the cyclostrophic balance assumption the meridional profiles of the zonal winds at the two cloud levels are used to assess the vertical wind shear in the upper cloud layer as a function of latitude and locate the most unstable region

wind streaks on venus clues to atmospheric circulation magellan images reveal surface features on venus attributed to wind processes sand dunes wind sculpted hills and more than 5830 wind streaks have been identified the streaks serve as local wind vanes representing wind direction at the time of streak formation and allowing the first global mapping of near surface wind patterns on venus wind streaks are oriented both toward the equator and toward the west when streaks associated with local transient events such as impact cratering are deleted the westward component is mostly lost but the equatorward component remains this pattern is consistent with a hadley circulation of the lower atmosphere

double difference tomography the method and its application to the hayward fault california we have developed a double difference dd seismic tomography method that makes use of both absolute and relative arrival times by reducing systematic errors using the more accurate relative arrival times the method produces an improved velocity model simultaneously it yields event locations of a quality equivalent to those of the dd location method while avoiding simplifying assumptions of that method we test this method on a synthetic dataset and find that it produces a more accurate velocity model and event locations than standard tomography we also test this method on a hayward fault california earthquake dataset spanning 1984 1998 the earthquakes relocated by this method collapse to a thin line along the fault trace consistent with previous results the dd velocity model has sharper velocity contrasts near the source region than the standard tomography model

subduction zone magnetic anomalies and implications for hydrated forearc mantle continental mantle in subduction zones is hydrated by release of water from the underlying oceanic plate magnetite is a significant byproduct of mantle hydration and forearc mantle cooled by subduction should contribute to long wavelength magnetic anomalies above subduction zones we test this hypothesis with a quantitative model of the cascadia convergent margin based on gravity and aeromagnetic anomalies and constrained by seismic velocities and find that hydrated mantle explains an important disparity in potential field anomalies of cascadia a comparison with aeromagnetic data thermal models and earthquakes of cascadia japan and southern alaska suggests that magnetic mantle may be common in forearc settings and thus magnetic anomalies may be useful in mapping hydrated mantle in convergent margins worldwide

venus summary and review of science research activities 1983 87 in the past four years the first high resolution synoptic images of the surface of venus have been obtained and we have learned more about the nature of its surface and atmosphere the major accomplishments of the u s s r in venus exploration have been observed with great interest by u s investigators some of whom have begun joint research efforts with soviet scientists venera 15 and 16 radar imaging experiments and new data obtained at the arecibo observatory have provided answers to some important questions conjectured volcanic features have been resolved into unmistakable calderas or distinct lava flow morphologies the question of whether venus like the earth may have high erosion rates is answered and from the observed number of presumed impact craters it appears that resurfacing rates on venus are much smaller than on earth no evidence of water or wind activity has been seen

forearc structure beneath southwestern british columbia a three dimensional tomographic velocity model 1 this paper presents a three dimensional compressional wave velocity model of the forearc crust and upper mantle and the subducting juan de fuca plate beneath southwestern british columbia and the adjoining straits of georgia and juan de fuca the velocity model was constructed through joint tomographic inversion of 50 000 first arrival times from earthquakes and active seismic sources wrangellia rocks of the accreted paleozoic and mesozoic island arc assemblage underlying southern vancouver island in the cascadia forearc are imaged at some locations with higher than average lower crustal velocities of 6 5 7 2 km s similar to observations at other island arc terranes the mafic eocene crescent terrane thrust landward beneath southern vancouver island exhibits crustal velocities in the range of 6 0 6 7 km s and is inferred to extend to a depth of more than 20 km the cenozoic olympic subduction complex an accretionary prism thrust beneath the crescent terrane in the olympic peninsula is imaged as a low velocity wedge to depths of at least 20 km three zones with velocities of 7 0 7 5 km s inferred to be mafic and or ultramafic units lie above the subducting juan de fuca plate at depths of 25 35 km the forearc upper mantle wedge beneath southeastern vancouver island and the strait of georgia exhibits low velocities of 7 2 7 5 km s inferred to correspond to 20 serpentinization of mantle peridotites and consistent with similar observations in other warm subduction zones estimated dip of the juan de fuca plate beneath southern vancouver island is 11 16 and 27 at depths of 30 40 and 50 km respectively

northern cascadia episodic tremor and slip a decade of tremor observations from 1997 to 2007 1 we analyze continuous seismic and gps records collected in the last decade 1997 2007 to establish the most comprehensive observational basis for northern cascadia episodic tremor and slip ets events a simple ets scale system using a combination of a letter and a digit is proposed to quantitatively characterize the spatial and temporal dimensions of ets events clear correlation between gps and tremor signals is observed for all a b class episodes but the gps signature is less obvious for minor ones regular ets recurrence can be established only for a b class episodes in southern vancouver island halting and jumping are very common in ets migration patterns and along strike migration can happen in both directions a prominent tremor gap is observed in midisland around 49 5 n this gap coincides with the epicenters of the only two large earthquakes beneath vancouver island ets tremors also tend to occur in places where the local seismicity is relatively sparse the tremor depth distribution shows a peak in the 25 35 km range where strong seismic reflectors i e the e layer are documented detailed waveform analysis confirms the existence of shallow tremors above the currently interpreted plate interface our results suggest that a significant portion of the tremor activity and perhaps associated shearing are taking place along well developed structures such as the e layer while fewer tremor bursts are generated elsewhere in response to the induced stress variation throughout the source volume

overview of the coordinated ground based observations of titan during the huygens mission coordinated ground based observations of titan were performed around or during the huygens atmospheric probe mission at titan on 14 january 2005 connecting the momentary in situ observations by the probe with the synoptic coverage provided by continuing ground based programs these observations consisted of three different categories 1 radio telescope tracking of the huygens signal at 2040 mhz 2 observations of the atmosphere and surface of titan and 3 attempts to observe radiation emitted during the huygens probe entry into titan s atmosphere the probe radio signal was successfully acquired by a network of terrestrial telescopes recovering a vertical profile of wind speed in titan s atmosphere from 140 km altitude down to the surface ground based observations brought new information on atmosphere and surface properties of the largest saturnian moon no positive detection of phenomena associated with the probe entry was reported this paper reviews all these measurements and highlights the achieved results the ground based observations both radio and optical are of fundamental importance for the interpretation of results from the huygens mission

water movement and soil swelling in a dry cracked vertisol abstract vertisols are widely used for irrigated rice in the senegal river basin where farmers are increasingly reporting problems with soil salinization water movement at the onset of the growing season when soils are dry and cracked may have a large impact on soil salinity and water economy water movement and soil swelling processes were studied in a dry cracked vertisol in the senegal river valley surface irrigation and simulated rainfall intensity 88 mm h 1 on cracked 2 25 m 2 plots crack width 0 01 0 02 m crack depth 0 3 m resulted in crack closure within 4 5 h starting at the soil surface soil swelling was separated into two components 1 swelling of the 0 01 0 02 m border zone of a soil island soil mass distinctively separated by cracks and 2 swelling of the rest of the soil island as a whole soil swelling was heterogeneous with a very rapid expansion of the border zone at the moment of crack closure the relative contributions of these two components to crack closure were respectively 80 90 and 10 20 24 h later these percentages changed slightly only maximum bulk linear shrinkage determined at the field level was only 7 results indicate the importance of rapid local heterogeneous swelling processes to water flow into cracked vertisol resulting in limited bypass flow use of bulk shrinkage curves and assumptions on isotropy of shrinkage and swelling processes when modelling water and solute flow in cracked vertisols should therefore be done with great care

significant geometric variation of the subducted plate beneath the northernmost cascadia subduction zone and its tectonic implications as revealed by the 2014 mw 6 4 earthquake sequence abstract at the northernmost extent of the cascadia subduction zone the explorer plate subducts at approximately 2 cm yr less than half the rate of the juan de fuca plate to the south the boundary between these two plates is known as the nootka fault zone which is one of the focuses of the seafloor earthquake array japan canada cascadia experiment seajade during this survey an m w 6 4 earthquake occurred on 24 april 2014 this event and the subsequent aftershocks referred to as the nootka sequence reveal an approximately 40 km long subducted fault within the explorer plate to the north of the nootka fault zone we infer that the fault is a subducted conjugate fault because of its nearly identical orientation to those seaward of the subduction front within the nootka fault zone the depth distribution and focal mechanisms of the aftershocks indicate significant margin parallel deformation of the subducting plate the subduction interface at the nootka sequence fault has been deflected downward to the northwest from a depth of approximately 15 25 km over a distance of 25 km we propose two possible scenarios that are modified from previously suggested slab tear model with induced margin parallel mantle flow to explain the significant deformation of the young warm subducting explorer plate to the northwest of this change in slab geometry a lack of seismic activity above the plate interface indicates that the explorer plate has partially decoupled from the overriding north america plate we conclude that the geometric variation separating the southern explorer plate from the north along with decoupling and a possible intraslab tear may be a significant combination to resist the propagation of a megathrust rupture across this boundary

do microplates in subduction zones leave a geological record active microplate boundaries in ocean continent subduction zones may induce deformation of the overlying plate and spatial or geochemical variations in the volcanic arc we discuss two modern cases the first is the south gorda juan de fuca plate boundary in the cascadia subduction zone where there is little or no effect on the overriding plate and the oceanic plate takes up much of the deformation the second case is the cocos rivera plate boundary in the middle america trench where the overlying colima graben contains substantial deformation in a zone extending from the trench to the volcanic arc and the sub duction related volcanism is spatially and geochemically complex we apply these observations to boundaries of the arguello monterey guadalupe and magdalena microplates which existed in the subduction zone west of baja california at various times from 20 to 12 5 ma the past positions of these boundaries relative to baja california are constrained by global plate reconstructions closure of the gulf of california and an estimate of extension in the mexican basin and range province existing regional mapping and our additional reconnaissance mapping show that paleocene to eocene fluvial and marine sedimentary rocks south of ensenada along the western baja california peninsula and eastward to the mid miocene volcanic arc are undeformed limited available data reveal no major spatial or geochemical variations in the mid miocene volcanic arc that might correlate with the past positions of the microplate boundaries thus these microplate boundaries had little to no effect on the overriding continental plate the nature of guadalupe and magdalena interactions with north america may have been closer to the south gorda juan de fuca example with possible internal deformation of the microplates the monterey and arguello microplates may have behaved like the modern explorer plate with largely strike slip motion relative to north america during their last stages of existence tectonic patterns similar to these examples may be expected from other plate boundaries where a plate is fragmenting as it enters a subduction zone e g the aluk plate in the trench beneath west antarctica in early tertiary time whether these microplates subsequently become attached to the overriding continental plate or to a larger oceanic plate and whether this causes deformation in the region of the former subduction zone may depend on the velocities of the nearby major plates and the relative orientations of the microplate boundaries

venus atmospheric structure and dynamics from the vega lander and balloons new results and pds archive abstract the longest lived in situ measurement platforms at venus have been the soviet vega balloons in 1985 and the only high quality pressure temperature profile in the lowest 10 km of the atmosphere is that from the vega 2 lander here we review the mission and the resultant literature and report the archival of numerical data from these investigations on the nasa planetary data system atmospheres node to facilitate their access to the community we additionally report some new results including the striking absence of a signature of the planetary boundary layer in the near surface potential temperature profile from the vega 2 lander in contrast to the well defined boundaries seen in a comparable profile at titan

cracking patterns bypass flow and nitrate leaching in greek irrigated soils abstract bypass flow of water during irrigation by flooding and associated leaching of nitrates were investigated in sixty three 40 cm high undisturbed cores of nineteen calcareous alluvial greek soils cracks conducting bypass water formed a network of polygons on the soil surface the sizes of the polygons increased curvilinearly with clay content large polygons were associated with deeper cracking of the soil mass than the small polygons did the size of the polygons decreased with depth in the profile upon irrigation cracks developed in dry soil were closed by swelling and by soil material falling from their walls the closing was either complete or incomplete the latter leaving a shallow u shaped trace upon drying cracks reformed along the u shaped traces elsewhere cracks developed at new places the volume of bypass water showed a sharp initial decrease with time because of filling of the cracks and swelling but remained significant even after 2 hours the usual irrigation time suggesting that a significant amount of the applied water may move to the subsoil without wetting the surface soil bypass flow showed a marked decrease with increasing initial soil moisture the amounts of nitrate leached by bypass water were rather small ranging from 0 01 to 1 8 of nitrates present in the soil cores

double seismic zone of the nazca plate in northern chile high resolution velocity structure petrological implications and thermomechanical modeling this paper presents an interdisciplinary study of the northern chile double seismic zone first a high resolution velocity structure of the subducting nazca plate has been obtained by the tomodd double difference tomography method the double seismic zone dsz is observed between 80 and 140 km depth and the two seismic planes is 20 km apart then the chemical and petrologic characteristics of the oceanic lithosphere associated with this dsz are deduced by using current thermal petrological seismological models and are compared to pressure temperature conditions provided by a numerical thermomechanical model our results agree with the common hypothesis that seismicity in both upper and lower planes is related to fluid releases associated with metamorphic dehydration reactions in the seismic upper plane located within the upper crust these reactions would affect material of basaltic morb composition and document different metamorphic reactions occurring within high p 2 4 gpa and low t 130 km lawsonite amphibole eclogite conditions the lower plane lying in the oceanic mantle can be associated with serpentinite dehydration reactions the vp and vs characteristics of the region in between both planes are consistent with a partially 25 30 vol antigorite 0 10 vol brucite and 4 10 vol chlorite hydrated harzburgitic material discrepancies persist that we attribute to complexities inherent to heterogeneous structural compositions while various geophysical indicators evidence particularly cold conditions in both the descending nazca plate and the continental fore arc thermomechanical models indicate that both seismic planes delimit the inner slab compressional zone around the 400 c 50 c isotherm lower plane earthquakes are predicted to occur in the slab s flexural neutral plane where fluids released from surrounding metamorphic reactions could accumulate and trigger seismicity fluids migrating upward from the tensile zone below could be blocked in their ascension by the compressive zone above this plane thus producing a sheeted layer of free fluids or a serpentinized layer therefore earthquakes may present either downdip compression and downdip tensile characteristics numerical tests indicate that the slab s thermal structure is not the only factor that controls the occurrence of inner slab compression 1 a weak ductile subduction channel and 2 a cold mantle fore arc both favor inner slab compression by facilitating transmission of compressional stresses from the continental lithosphere into the slab 3 decreasing the radius of curvature of the slab broadens the depth of inner slab compression whereas 4 decreasing upper plate convergence diminishes its intensity all these factors indicate that if dszs indeed contour inner slab compression they cannot be linked only to slab unbending but also to the transmission of high compressional stresses from the upper plate into the slab

seismotectonics of the explorer region 1 the explorer region offshore western canada is a tectonically complex area surrounded by the pacific north america and juan de fuca plates existing tectonic models for the region differ fundamentally proposed plate configurations range from multiple independent plate fragments to an explorer plate now fused to north america along the continental margin and cut by pacific north america transform faults in the west we present new seismological data constraining the region s current tectonics we use three component regional waveforms to determine the source parameters of 84 earthquakes with magnitude greater than 4 combined with 34 harvard centroid moment tensor solutions they represent the region s largest earthquake source parameter data set obtained by robust waveform modeling techniques in addition we perform joint epicenter determination to relocate larger earthquakes recorded since 1918 the source parameters and improved locations provide a consistent tectonic picture earthquake slip vector azimuths along the pacific plate boundary change smoothly and are significantly less northerly oriented than the pacific north america plate motion direction requiring an independent explorer plate the present day pacific explorer boundary is formed by transform faults subparallel to the revere dellwood wilson fault plate motion vectors indicate that the winona block is part of the explorer plate current explorer motion is more northerly than indicated by magnetic anomalies prior to 2 ma implying a recent change possibly coinciding with a northwestward ridge jump near explorer plate s northern end transferring the winona block from the pacific to the explorer plate in response to these plate motion changes the region north of the western sovanco fracture zone was assimilated into the pacific plate the region around the eastern sovanco fracture zone characterized by broadly distributed seismicity is composed of well defined sets of conjugate faults bounding rotating crustal blocks earthquake fault strikes agree with the dominant northwest southeast fault sets however the conjugate sets must be also active to fully accommodate present day explorer plate motion the sw portion of the strike slip nootka fault zone the explorer juan de fuca plate boundary is well defined by focused seismicity however its full extent under nootka island remains unresolved the explorer north america boundary shows sporadic low magnitude seismicity our explorer north america rotation pole predicts convergence varying from negligible at the boundary s northwest end to 2 cm yr at the se end this convergence can be accommodated either by subduction or by crustal thickening extending to the north american continent we favor subduction based on low deformation rates observed by onshore gps sites the present explorer plate system configuration is a result of stepwise reorientation of the explorer ridge system each step successively reducing the subduction rate relative to north america

detection and location of low frequency earthquakes using cross station correlation source geometry and simple propagation of s waves generated by low frequency earthquakes lfes in northern cascadia results in strong waveform coherence on horizontal channels for stations at smaller epicentral distances as recognized by previous workers this cross station similarity can be exploited for detection of lfes in tremor we develop a cross station correlation approach based on waveform coherence and travel time consistency that exploits a full complement of network stations and we demonstrate its application to separate arrays on vancouver island and washington state our approach yields thousands of impulsive lfe detections per slow slip episode and hundreds of events between episodes lfe epicentral distributions reveal the presence of well defined regions of high asperity density with those regions farther downdip that are active during interepisodic tremor and slow slip periods the lfe epicenters also display pronounced spatiotemporal clustering that compares favorably with independent tremor and lfe template catalogs and that defines rapid tremor reversals with complex propagation patterns the lfe source depths can be estimated for some detections in which p waves are identified on the vertical channel through correlation with the horizontal component s waveforms lfe hypocenters cluster between two recently developed plate interface models below southern vancouver island our approach enables the identification of high signal to noise ratio impulsive lfe detections from targeted regions on the plate boundary for generation of lfe templates to be used in studies of structure and seismogenesis in cascadia online material additional maps of station density for southern vancouver island epicenter locations for northern washington and additional time distance plots for southern vancouver island

olivine pyroxene h2o system as a practical analogue for estimating the elastic properties of fluid bearing mantle rocks at high pressures and temperatures 1 by using gruneisen theory and third order finite strain theory elastic wave velocities in olivine pyroxene assemblages are accurately determined as a function of pressure and temperature equilibrium distributions of h2o fluid in mantle rocks have been determined at high pressure and temperature and are used to estimate vp and vs in the olivine pyroxene h2o system importantly velocities calculated for the peridotite h2o system successively reproduce the absolute vp and vs values measured in the laboratory for dehydrated serpentinite known equilibrium distributions of h2o co2 fluid in peridotite liquid systems allow one to estimate elastic properties of fluid bearing mantle rocks from mineral and liquid elasticity data

a comparative study of low frequency earthquake templates in northern cascadia abstract using network correlation methods we generate low frequency earthquake templates for a set of 4 composite arrays on vancouver island and washington state that employ data from earthscope transportable and flexible arrays plate boundary observatory polaris and permanent network canadian national seismograph network pacific northwest seismic network sources on the basis of empirical and semi analytical arguments the templates can be viewed as green s function sections corresponding to moment tensor point sources with step function time dependence in displacement low frequency earthquake hypocentres follow the general epicentral distribution of tremor and occur along tightly defined surfaces in depth with washington locations averaging slightly deeper than those on vancouver island we invert template waveforms for moment tensor mechanisms and find that data are well modelled by double couple sources for southern vancouver island with the highest quality templates the majority of mechanisms are consistent with shallow thrusting in the direction of plate motion the three other data sets with lower signal to noise levels show predominantly thrust mechanisms with more variable orientations taken together with other constraints our observations support the hypothesis that low frequency earthquakes manifest shear slip on a relatively thin plate boundary

hydraulic fracturing of soil during laboratory experiments part 2 propagation the propagation of hydraulic fractures in soil was studied in the laboratory by glycerine injected at a constant rate into rectangular samples of silty clay confined in a triaxial cell during propagation the trace of a hydraulic fracture appeared on the side of the cell as a nearly straight narrow line that tapered from 0 05 mm to negligible aperture at the tip pressure of the glycerine was recorded as a function of time and is characterized by a period of nearly linear increase a break in slope followed by a period of increasing pressure but decreasing slope a maximum and then a period of decreasing pressure tests terminated early in that sequence of events indicated that the break in slope marks the onset of hydraulic fracturing the pressure at the onset of fracturing was predicted from a determination of the fracture toughness of the silty clay the forms of the pressure records were markedly affected by the water content of the soil increasing water content decreased the pressure required to

tomographic imaging of hydrated crust and mantle in the subducting pacific slab beneath hokkaido japan evidence for dehydration embrittlement as a cause of intraslab earthquakes abstract we estimate detailed three dimensional seismic velocity structures in the subducting pacific slab beneath hokkaido japan using a large number of arrival time data from 6902 local earthquakes a remarkable low velocity layer with a thickness of 10 km is imaged at the uppermost part of the slab and is interpreted as hydrated oceanic crust the layer gradually disappears at depths of 70 80 km suggesting the breakdown of hydrous minerals there we find prominent low velocity anomalies along the lower plane of the double seismic zone and above the aftershock area of the 1993 kushiro oki earthquake m7 8 since seismic velocities of unmetamorphosed peridotite are much higher than the observations hydrous minerals are expected to exist in the lower plane as well as the hypocentral area of the 1993 earthquake on the other hand regions between the upper and lower planes where seismic activity is not so high compared to the both planes show relatively high velocities comparable to those of unmetamorphosed peridotite our observations suggest that intermediate depth earthquakes occur mainly in regions with hydrous minerals which support dehydration embrittlement hypothesis as a cause of earthquake in the subducting slab

confidence intervals for motion and deformation of the juan de fuca plate finite rotation parameters for 17 isochrons from anomalies 1 4a for the juan de fuca pacific plate pair are reported comparison of confidence intervals for plate rotations derived by different techniques indicates that a single plate misfit criterion significantly underestimates the confidence region defining the confidence interval by a threshold level of misfit determined by an f statistic gives similar results to the technique of bootstrap resampling for internally consistent data bootstrap resampling offers far greater robustness in the case of inconsistent data precision of about 1 km in the direction normal to isochrons is available for the younger anomalies this precision allows confident recognition of 2 5 km of eastward deformation of the pacific plate 50 100 km south of the active sovanco transform slight deformation of the northern juan de fuca plate adjacent to the nootka fault zone is also apparent east of the northern gorda ridge younger 3 ma anomalies do not show measurable deformation near the blanco fracture zone but older anomalies are up to 30 km east of their position predicted by rigid behavior of the juan de fuca plate reconstruction of the overlap geometry of past propagators shows no evidence that the extent of overlap has ever exceeded 70 km even with offset of 75 km in one case and 140 km in another new techniques for testing for intervals of constant plate motion require at least two changes in instantaneous motion at 99 confidence a clockwise change in motion direction at about 5 ma and an increase in the gradient of spreading rate since 2 ma two other changes in gradient since 5 ma are required at 95 confidence

deformation of the so called gorda plate a new compilation of magnetic anomaly identifications recent side scan sonar data and other data provide constraints on the mechanism of deformation of the area commonly known as the gorda plate a sharp boundary between the rigid juan de fuca plate and deformed material trends southeast from the northern part of the gorda ridge in material older than 2 3 ma the boundary is gradational and trends more to the east previous observations that the anomalies east of the ridge are shorter than their counterparts on the pacific plate are confirmed there is no evidence for subduction or obduction along the mendocino transform adjacent to the ridge the deformation is interpreted to be distributed nw se right lateral simple shear possibly controlled by ductile deformation of the lower crust deformation of this sense is capable of producing the observed shortening the anomalies away from the ridge apparently in crust of age 2 3 ma there must be a transition to deformation on observed ne sw left lateral faults parallel to the original spreading fabric

high resolution subducting slab structure beneath northern honshu japan revealed by double difference tomography the high resolution seismic velocity structure of the subducting slab beneath northern honshu japan has been obtained by double difference tomography capitalizing on the existence of two planes of seismicity the upper plane lies in the region with average to high vp vs ratios 1 72 1 85 which may correspond to the transformations of metabasalt and metagabbro to blueschist the lower plane is associated with very low vp vs ratios 1 6 1 7 in sharp contrast with high vp vs ratios 1 8 1 85 in the region between the two planes these features may be explained by forsterite enstatite h2o formation from serpentine dehydration in the lower plane and partial hydration of the region between the two planes our results provide direct constraints for petrologic modeling of the subducting slab

are the lower planes of double seismic zones caused by serpentine dehydration in subducting oceanic mantle in a number of subduction zones earthquakes at 50 200 km depth define two dipping planes separated by 20 40 km that appear to merge downdip upper plane earthquakes are inferred to occur within the subducting oceanic crust whereas lower plane earthquakes occur in the subducting oceanic mantle beneath northeast japan the results of a new finite element heat transfer model suggest that the lower seismic plane cuts across isotherms at a shallow angle lower plane earthquakes occur at 550 800 c at 100 km depth and at 350 600 c at 160 km depth these conditions coincide with the dehydration reaction antigorite serpentine forsterite enstatite h 2 o which suggests that lower plane earthquakes may be triggered by dehydration embrittlement which in turn suggests that the subducting oceanic mantle is partially hydrated serpentinization may occur in the trench outer rise region where faulting may promote infiltration of seawater several tens of kilometers into the oceanic lithosphere if this hypothesis is correct current subduction zone h 2 o budgets may significantly underestimate the amount of bound h 2 o entering the subduction factory

the vega venus balloon experiment in june 1985 two instrumented balloons were placed in the atmosphere of venus as part of the vega mission each balloon traveled about 30 percent of the way around the planet at a float altitude near 54 kilometers in situ sensors measured pressure temperature vertical wind velocity cloud particle backscatter ambient light level and frequency of lightning a ground based network of 20 radio antennas tracked the balloons by very long baseline interferometry vlbi techniques to monitor the venus winds the history organization and principal characteristics of this international balloon experiment are described

lizardite versus antigorite serpentinite magnetite hydrogen and life the serpentinization of peridotite operates according to one or the other or a combination of two end member mechanisms in low temperature environments 50 300 c where lizardite is the predominant serpentine mineral olivine is consumed by reaction with h2o but its composition mg remains unchanged mg rich lizardite magnetite and dihydrogen gas brucite are products of the reaction at higher temperatures 400 600 c rates of mgfe diffusion in olivine are orders of magnitude faster with the result that the growth of mg rich antigorite can be accommodated by a compositional adjustment of olivine eliminating the need to precipitate magnetite and evolve hydrogen this latter end member mechanism probably best reflects the situation in the forearc mantle wedge

juan de fuca slab geometry and its relation to wadati benioff zone seismicity 1 a new model of the subducted juan de fuca plate beneath western north america allows first order correlations between the occurrence of wadati benioff zone earthquakes and slab geometry temperature and hydration state the geo referenced 3d model constructed from weighted control points integrates depth information from earthquake locations and regional seismic velocity studies we use the model to separate earthquakes that occur in the cascadia forearc from those that occur within the underlying juan de fuca plate and thereby reveal previously obscured details regarding the spatial distribution of earthquakes seismicity within the slab is most prevalent where the slab is warped beneath northwestern california and western washington suggesting that slab flexure in addition to expected metamorphic dehydration processes promotes earthquake occurrence within the subducted oceanic plate earthquake patterns beneath western vancouver island are consistent with slab dehydration processes conversely the lack of slab earthquakes beneath western oregon is consistent with an anhydrous slab double differenced relocated seismicity resolves a double seismic zone within the slab beneath northwestern california that strongly constrains the location of the plate interface and delineates a cluster of seismicity 10 km above the surface that includes the 1992 m7 1 mendocino earthquake we infer that this earthquake ruptured a surface within the cascadia accretionary margin above the juan de fuca plate we further speculate that this earthquake is associated with a detached fragment of former farallon plate other subsurface tectonic elements within the forearc may have the potential to generate similar damaging earthquakes

earthquakes and plastic deformation of anhydrous slab mantle in double wadati benioff zones double wadati benioff seismic zones dsz with two parallel planes of seismicity separated by 15 30 km are a global feature of subduction zones in the 50 200 km depth range upper plane seismicity is generally attributed to dehydration of the oceanic crust but the origin of the lower seismicity plane is debated serpentine or hydrous phase dehydration embrittlement is a commonly advocated mechanism that implies significant slab mantle hydration high resolution seismic tomography revealed low seismic velocities in the lower seismicity plane that are better explained by seismic anisotropy of anhydrous deformed peridotites than by serpentinization earthquakes correlate with anisotropic planar shear zones and favor a shear instability mechanism as the cause of lower plane seismicity without requiring the presence of water in the center of subducting slabs the contribution of the subducted lithospheric mantle to the water budget of subduction zones is thus likely limited to the first 2 3 kilometers beneath oceanic crust

explorer deformation zone evidence of a large shear zone and reorganization of the pacific juan de fuca north american triple junction recently collected multibeam bathymetry and hydroacoustic earthquake data are used to investigate the recent tectonics of the explorer juan de fuca ridge systems the boundaries between the pacific and northern juan de fuca plates the bathymetric and seismic evidence presented is consistent with a zone of shear extending well south of the sovanco fracture zone to include the heck and heckle seamounts and potentially as far south as the springfield seamounts and cobb offset along the juan de fuca ridge this indicates that the triple junction between the pacific juan de fuca north american plates may be reorganizing southward to establish at the cobb offset

modeling of water balance cracking and subsidence of clay soils abstract a general procedure has been developed to model water balance cracking and subsidence of clay soils the main feature of this procedure is the introduction of the shrinkage characteristic in addition to the water retention and hydraulic conductivity curves into simulation models the proposed procedure enables direct calculation of volume changes in dependance on moisture transport with appropriate assumptions for geometry of swelling and shrinkage these volume changes are converted into cracking and subsidence taking into account the calculated area of shrinkage cracks at the soil surface together with the maximum infiltration rate of the soil matrix and the rainfall intensity rainfall is dynamically partitioned in matrix and crack infiltration in this way bypass flow and resulting rapid rises of groundwater levels in cracked soils can be simulated the procedure was validated by adapting a model for calculation of transient moisture flow in soils flowex into a version applicable on clay soils flocr computations with flocr of subsidence cracking groundwater level and top layer wetness of a dutch basin clay soil during 1985 were in good agreement with field observations moreover the proposed method of bypass calculation is supported by good correspondence between measured and simulated rapid rises of groundwater levels

implications of the vega balloon results for venus atmospheric dynamics both vega balloons encountered vertical winds with typical velocities of 1 to 2 meters per second these values are consistent with those estimated from mixing length theory of thermal convection however small scale temperature fluctuations for each balloon were sometimes larger than predicted the approximate 6 5 kelvin difference in temperature consistently seen between vega 1 and vega 2 is probably due to synoptic or planetary scale nonaxisymmetric disturbances that propagate westward with respect to the planet there is also evidence from doppler data for the existence of solar fixed nonaxisymmetric motions that may be thermal tides surface topography may influence atmospheric motions experienced by the vega 2 balloon

galileo doppler measurements of the deep zonal winds at jupiter changes in the speed of the galileo probe caused by zonal winds created a small but measurable doppler effect in the probe relay carrier frequency analysis of the probe relay link frequency allows direct measurements of the speed of jupiter s zonal winds beneath the cloud tops the deep winds were prograde and strong reaching a sustained 190 to 200 meters per second at an altitude marked by a pressure of 24 bars the depth and strength of the zonal winds severely constrain dynamic modeling of the deeper layers and begin to rule out many shallow weather theories

the neogene alert bay volcanic belt of northern vancouver island canada descending plate edge volcanism in the arc trench gap abstract the alert bay volcanic belt trends northeasterly across northern vancouver island coincident with the trace of the subducted juan de fuca explorer plate edge volcanism began in the west at brooks peninsula about 8 ma ago but occurred in most centers 3 5 1 ma ago there is a suggestion of eastward migration of activity and shift from basalt to dacite or rhyolite with time most of the volcanism was coincident with a time of rapid changes in the geometry of subduction as inferred from offshore magnetic patterns and with a hiatus in mainland cascade volcanic arc activity geometry and chronometry suggest this is a descending plate edge volcanic belt where disruption of steady state plate consumption patterns triggered magma genesis chemically the rocks are quite variable with divergent fractionation trends one trend resembles that of mull hebrides with a plagiophyric basalt of transitional alkaline subalkaline mildly tholeiitic and aluminous character which differentiated to clinopyroxene andesite and eventually to tholeiitic rhyolite and mildly tholeiitic calc alkaline dacite both of k poor magma type the other trend is like the cascades with aluminous aphyric calc alkaline basalt hornblende and or hypersthene andesite and k poor dacite this divergent character is also evident in ba rb nb and zr fractionation trends major and trace element discriminant diagrams generally identify the basalts as within plate types the 87 sr 86 sr isotope ratio is relatively low averaging 0 70325 and shows no trend with rock type or differentiation series oxygen in the entire suite is relatively heavy 18 o averaging 7 1 even the basalts are 18 o enriched oxygen shows no trend with degree of hydration rock type or series these isotopic and chemical data are compatible with minor crustal contamination of mafic primary magmas followed by fractional crystallization under different oxidation and hydration conditions

the northern limit of the subducted juan de fuca plate system analysis of data recorded at an array of three component broadband seismograph stations deployed on northern vancouver island and the adjacent british columbia mainland at the northern end of the cascadia subduction zone provides the first constraints on the s wave velocity structure of this region and permits us to define the northern limit of the subducted juan de fuca plate system during a 2 year period more than 80 teleseisms were recorded at our five stations the method of receiver function analysis was used to constrain the s velocity structure to upper mantle depths beneath the northern three stations a relatively simple continental crust is interpreted with a well defined moho near 37 39 km depth an upper crustal s velocity discontinuity at these stations is interpreted as the top of the high velocity rocks of the wrangellia terrane in contrast more complicated structure dominated by pronounced low velocity zones dipping to the ne are interpreted beneath our southern two stations the shallower low velocity zone is 6 8 km thick has an s velocity contrast of 0 6 1 1 km s and lies within the continental crust this feature is similar to a pronounced low velocity layer the e zone imaged beneath southern vancouver island the deeper low velocity zone is interpreted as the subducted oceanic crust we interpret the pronounced change in s velocity structure that we observe as the northern limit of the subducted oceanic plate beneath vancouver island this change coincides with significant changes in topography heat flow gravity and geochemistry

ephemeral plate tectonics at the queen charlotte triple junction three plate boundaries the queen charlotte transform the cascadia subduction zone and the juan de fuca ridge meet in a complex triple junction offshore vancouver island some interpretations of the plate tectonics of the region have included the explorer ridge the dellwood knolls and the tuzo wilson volcanic field as part of the spreading system requiring up to three triple junctions in the region new seabeam bathymetry interpreted with existing regional data sets indicate that the dellwood knolls and tuzo wilson volcanic field are not independent plate boundaries but the result of leaky transform tectonics between two overlapping transform faults seismicity shows that the explorer plate is being deformed by pacific north american relative motion as a new transform plate boundary forms and cuts off the explorer ridge the system is evolving to a single triple junction at the northern terminus of the juan de fuca ridge where it meets the nootka deformation zone thus the explorer microplate which was spawned 5 ma is an ephemeral adjustment to mechanical difficulties at the triple junction this new model implies that the explorer subduction zone is no longer active ephemeral oceanic microplates have also existed at pacific north american triple junctions off southern california and baja california

anomalous lithospheric structure of northern juan de fuca plate a consequence of oceanic rift propagation abstract anomalous crustal and upper mantle structure of northern juan de fuca plate is revealed from wide angle seismic and gravity modelling a 2 d velocity model is produced for refraction line ii of the 1980 vancouver island seismic project visp80 the refraction data were recorded on three ocean bottom seismometers obss deployed at the ends and middle of a 110 km line oriented parallel to the north american continental margin the velocity model is constructed via ray tracing and conforms to first arrival amplitude observations and travel time picks of direct converted and reflected phases between sub sediment depths of 3 to 11 km depths normally associated with the lower crust and upper oceanic mantle the final model shows that compressional wave velocities decrease significantly from southeast to northwest along the profile at sub sediment depths of 11 km at the northwestern end of the profile p wave velocities are as low as 7 2 km s a complementary 2 d gravity model using the geometry of the velocity model and velocity density relationships characteristic of oceanic crust is produced the high densities required to match the gravity field indicate the presence of peridotites containing 25 30 serpentine by volume rather than excess gabbroic crust within the deep low velocity zone anomalous travel time delays and unusual reflection characteristics observed from proximal seismic refraction and reflection experiments suggest a broader zone of partially serpentinized peridotites coincident with the trace of a pseudofault we propose that partial serpentinization of the upper mantle is a consequence of slow spreading at the tip of a propagating rift

ab initio elastic properties of talc from 0 to 12 gpa interpretation of seismic velocities at mantle pressures and prediction of auxetic behaviour at low pressure talc is a hydrous magnesium rich layered silicate that is widely disseminated in the earth from the seafloor to over 100 km depth in ultra high pressure metamorphism of oceanic crust in this paper we determine the single crystal elastic constants at pressures from 0 to 12 gpa of talc triclinic c 1 and monoclinic c2 c polytypes using ab initio methods we find that talc has an extraordinarily high elastic anisotropy at zero pressure that reduces with increasing pressure the exceptional anisotropy is complemented by a negative poisson s ratio for many directions in crystal space calculations show that talc is not only one of very few common minerals to exhibit auxetic behaviour but the magnitude of this effect may be the largest reported so far for a mineral the compression vp and shear vs wave velocity anisotropy is 80 and 85 for the triclinic polytype at pressures where talc is known be stable in the earth up to 5 gpa the vp and vs anisotropy is reduced to about 40 for both velocities which is still a very high value vp is slow parallel to the c axis and fast perpendicular to it this remains unchanged with increasing pressure and is observed in both polytypes the shear wave splitting difference between fast and slow s wave velocities at low pressure has high values in the plane normal to the c axis with a maximum near the a axis in the triclinic and the b axis in the monoclinic polytype the c axis is the direction of minimum splitting the pattern of shear wave splitting does not change significantly with pressure the volume fraction of talc varies between 11 and 41 for hydrated mantle rocks but the lack of data on the crystallographic preferred orientation cpo precludes a detailed analysis of the impact of talc on seismic anisotropy in subduction zones however it is highly likely that cpo can easily develop in zones of deformation due to the platy habit of talc crystals for random aggregates of talc the isotropic vp vs and vp vs ratio have significantly lower values than those of antigorite and may explain low velocity regions in the mantle wedge vp vs ratios are more complex in anisotropic media because there are fast and slow s waves resulting in vp vs1 and vp vs2 ratios for every propagation direction making interpretation difficult in deformed polycrystalline talc with a cpo talc on the subduction plate boundary can strongly influence guided wave velocity as cpo would develop in this region of intense shearing the very low coefficient of friction 0 1 of talc above 100 c could also explain silent earthquakes at shallow depths ca 30 km along the subduction plate boundaries frequently responsible for tsunami

morphology of the explorer juan de fuca slab edge in northern cascadia imaging plate capture at a ridge trench transform triple junction the explorer plate is a young oceanic microplate that accommodates relative motion between the pacific juan de fuca and north america plates near northern vancouver island canada the northern limit of explorer plate juan de fuca subduction and the fate of the slab in northern cascadia are poorly understood we use passive teleseismic recordings from an array of polaris broadband seismic stations to image crustal and upper mantle structure beneath northern vancouver island into the interior of british columbia a clear signature of subducted material extends northeast from the brooks peninsula at crustal levels beneath georgia strait and the mainland deep into the mantle to 300 km depth complexity in slab morphology results from juan de fuca ridge subduction and toroidal flow around the slab edge in agreement with geophysical and geological data we propose a tectonic model for the explorer plate in which its separation from the juan de fuca plate is caused by the thermomechanical erosion of the slab edge and slab thinning at shallow levels both of which slow convergence with north america and lead eventually to plate capture

thermal state of the explorer segment of the cascadia subduction zone implications for seismic and tsunami hazards the explorer segment of northernmost cascadia is an end member warm subduction zone with very young incoming plate and slow convergence rate understanding the megathrust earthquake potential of this type of subduction zone is of both geodynamic and societal importance available geodetic observations indicate that the subduction megathrust of the explorer segment is currently locked to some degree but the downdip extent of the fault area that is potentially seismogenic is not known here we construct finite element models to estimate the thermally allowed megathrust seismogenic zone using available knowledge of regional plate kinematics structural data and heat flow observations as constraints despite ambiguities in plate interface geometry constrained by hypocenter locations of low frequency earthquakes beneath vancouver island the thermal models suggest a potential rupture zone of 60 km downdip width located fully offshore using dislocation modeling we further illustrate that a rupture zone of this size even with a conservative assumption of 100 km strike length can cause significant tsunami genic deformation future seismic and tsunami hazard assessment in northern cascadia must take the explorer segment into account

vega balloon dynamics and vertical winds in the venus middle cloud region the vega balloons provided a long term record of vertical wind fluctuations in a planetary atmosphere other than earth s the vertical winds were calculated from the observed displacement of the balloon relative to its equilibrium float altitude the winds were intermittent a large burst lasted several hours and the peak velocity was 3 meters per second

evidence for trench parallel mantle flow in the northern cascade arc from basalt geochemistry a b s t r a c t geochemical data for basalts from the garibaldi volcanic belt northern segment of the cascade arc define arc parallel gradients in trace elements and isotope ratios that extend at least 150 km into the arc from the northern margin of the subducting juan de fuca plate southerly increases in zr nb ba nb th la pb ce 208 pb 204 pb and 176 hf 177 hf indicate greater mantle depletion and higher slab derived contributions with distance from the slab edge temperatures and pressures of mantle melt segregation also decrease to the south the gradients are most plausibly explained as a consequence of slab rollback induced toroidal flow at the northern slab edge nootka fault zone whereby enriched oib type ne pacific asthenospheric mantle from beneath the slab is drawn into the mantle wedge in a trench parallel southerly flow pattern melts of the enriched asthenosphere are progressively diluted to the south by melts of the slab modified depleted mantle wedge arc parallel changes in slab thermal conditions cannot account for these gradients trench parallel geochemical gradients in the northern cascade arc are consistent with shear wave splitting data numerical modeling and experimental studies showing that trench parallel mantle flow may be a common phenomenon near slab edges and slab gaps