a thermodynamic model for the plagioclase liquid hygrometer thermometer a new thermodynamic model for the plagioclase liquid exchange reaction between the albite naalsi 3 o 8 and anorthite caal 2 si 2 o 8 components is presented which can be used as a plagioclaseliquid hygrometer or thermometer the model incorporates calorimetric and volumetric data for the pure liquid and crystalline components which permits the effect of temperature and pressure on the exchange reaction to be calculated independently from the effect of composition this allows a more accurate assessment of the effect of melt composition including dissolved water concentration on the exchange reaction from plagioclase liquid equilibrium experiments activity composition relations for the plagioclase solid solution are taken from holland and powell 1992 the new hygrometer is calibrated on 71 plagioclase liquid experiments of which 45 are hydrous and 26 are anhydrous three filters were applied to the phase equilibrium data 1 crystallanities 30 2 pure h 2 o fluidsaturated and 3 compositional totals including h 2 o component of 97 101 for hydrous quenched glasses the final data set spans a wide range of liquid compositions 46 74 wt sio 2 plagioclase compositions an 93 an 37 temperatures 825 1230 c pressures 0 300 mpa and dissolved melt water concentrations 0 7 wt h 2 o the standard error of estimate see for the model is 0 32 wt h 2 o and all liquid compositions are fitted equally well when the model is used as a thermometer all measured temperatures are recovered equally well within 14 c on average the model is only recommended for applications that fall within the compositional bounds of the calibration data set i e metaluminous basalts through rhyolites in equilibrium with an 95 an 35 it is not yet calibrated for rhyolites crystallizing plagioclase more sodic than an 30 owing to an absence of phase equilibrium experiments on rhyolites that pass the required filters the new plagioclase liquid hygrometer thermometer is available as a visual basic program that runs on excel 2004

relationship between cyclone tracks anticyclone tracks and baroclinic waveguides abstract this study presents observational results concerning the structure and evolution of high frequency fluctuations with periods shorter than 6 days in the geopotential height field the results are based on a statistical analysis of 1000 500 and 250 mb height fields derived from twice daily nmc final analyses over the northern hemisphere for the 20 winters 1964 65 through 1983 84 the disturbances assume the form of waves elongated in the meridional direction with a mean wavelength of 4000 km a westward tilt with height and a mean eastward phase propagation of 12 1 5 m s 1 these results support an interpretation in terms of finite amplitude baroclinic waves whose structure and evolution varies with geographical location over the continents and especially along the eastern slopes of the rockies and the tibetan plateau the waves show marked departures in structure and evolution from those over the oceans in particular we find evidence of a systematic influence of the terrain in steering the

origin of andesite in the deep crust and eruption rates in the tancitaro nueva italia region of the central mexican arc the tancitaro nueva italia region 4400 km 2 is located within the west central mexican volcanic belt and spans 80 km of arc length along the volcanic front on the basis of 65 40 ar 39 ar ages it is shown that a total volume of 326 57 km 3 of magma has erupted from 200 vents since 1 2 ma nearly a third of this volume 103 5 km 3 was erupted as crystal rich andesite from a large stratovolcano volcan tancitaro whereas the remaining two thirds 223 52 km 3 span the complete range from 51 to 63 wt sio 2 are relatively crystal poor and were erupted from peripheral vents the total proportions of erupted lava types are 6 7 basalt 18 basaltic andesite and 74 76 andesite there is no rhyolite and only a trace of dacite there is no evidence that any significant magma differentiation occurred in the upper crust along this arc segment based on phenocryst abundances and modes major and trace element data as well as phase equilibrium experiments from the literature the following mechanisms for formation of andesite in this region can be ruled out 1 crystal fractionation of basalt basaltic andesite in the upper crust 2 magma mixing of basalt basaltic andesite with dacite rhyolite and 3 assimilation of granitoid by basalt basaltic andesite instead the same body of evidence indicates that the andesites were formed primarily in the deep crust through partial melting 20 of hornblende rich 40 gabbronorite the basaltic andesites are required owing to their elevated ni concentrations to be hybrids of the andesitic partial melts with basaltic residual liquids that have undergone variable two pyroxene fractionation in the deep crust

simulated ground motion in santa clara valley california and vicinity from m 6 7 scenario earthquakes abstract models of the santa clara valley scv 3d velocity structure and 3d finite difference software are used to predict ground motions from scenario earthquakes on the san andreas saf monte vista shannon south hayward and calaveras faults twenty different scenario ruptures are considered that explore different source models with alternative hypocenters fault dimensions and rupture velocities and three different velocity models ground motion from the full wave field up to 1 hz is exhibited as maps of peak horizontal velocity and pseudospectral acceleration at periods of 1 3 and 5 sec basin edge effects and amplification in sedimentary basins of the scv are observed that exhibit effects from shallow sediments with relatively low shear wave velocity 330 m sec scenario earthquakes have been simulated for events with the following magnitudes 1 m 6 8 7 4 calaveras sources 2 m 6 7 6 9 south hayward sources 3 m 6 7 monte vista shannon sources and 4 m 7 1 7 2 peninsula segment of the saf sources ground motions are strongly influenced by source parameters such as rupture velocity rise time maximum depth of rupture hypocenter and source directivity cenozoic basins also exert a strong influence on ground motion for example the evergreen basin on the northeastern side of the scv is especially responsive to 3 5 sec energy from most scenario earthquakes the cupertino basin on the southwestern edge of the scv tends to be highly excited by many peninsula and monte vista fault scenarios sites over the interior of the evergreen basin can have long duration coda that reflect the trapping of seismic energy within this basin plausible scenarios produce predominantly 5 sec wave trains with greater than 30 cm sec sustained ground motion amplitude with greater than 30 sec duration within the evergreen basin

modified topographic amplification factors for a single faced slope due to kinematic soil structure interaction in this paper we evaluate the additive effects of topography soil nonlinearity and soil structure interaction ssi along the crest of an idealized 40 m high cliff type topographic feature with slope inclination 300 where excessive damage was observed during the athens 1999 earthquake the objective of this paper is to investigate the relative contribution of topographic amplification and kinematic ssi as a function of the incident motion frequency content and geotechnical site conditions for a surface and an embedded structure located at the cliff crest for this purpose we perform elastic parametric and nonlinear site specific two dimensional finite element simulations using three profiles and six input motions we illustrate the role of ssi in altering the response at the location of peak topographic amplification potential behind the crest the effects of incident motion incoherency on the transient structural response and the beneficial contribution of structural embedment we finally suggest that empirical models for base slab averaging of shallow foundations developed as a function of site conditions structural dimensions and center line location could be combined with topographic amplification factors to predict realistic design spectra for structures located on irregular topographic features

trapped waves in the edge region of stratospheric polar vortices 1 a global distribution and seasonal variability of short period 2 days disturbances in the lower and middle stratosphere are investigated using 6 hourly european centre for medium range weather forecasts reanalysis data over 15 years 1979 1993 two dimensional spectral analysis and a recently developed lagrangian mean diagnostic are used for the analysis it is shown that the short period disturbances are most active around the polar night jet in each winter hemisphere they have a wavelike structure in the longitudinal direction with typical zonal wavelengths of 1 700 2 000 km i e zonal wave number 10 12 and a latitudinally evanescent structure with a half width of about 1 500 km suggesting that they are waves trapped in the edge region of the polar vortex where the latitudinal gradient of isentropic potential vorticity is maximized the trapped waves have a nearly barotropic structure over a depth greater than 10 km because the polar vortex in the northern hemisphere is much deformed and not symmetric around the pole dynamical characteristics of the trapped waves in the southern hemisphere alone are investigated by applying a lag correlation analysis to the reanalysis data the characteristics of trapped waves are fairly dependent on the state of the polar vortex such as zonal wind speed and its vertical shear they have a significant westward intrinsic phase velocity from june through october when the polar vortex is most stable this indicates that the trapped waves during this period have their own dynamics contrary to the filaments being passively advected by the large scale flow

seismic response of a hill the example of tarzana california the northridge california earthquake that strongly shook the city of los angeles in january 1994 produced one of the highest ground accelerations ever recorded in an earthquake at a site located on top of a small hill in tarzana about 6 km south of the epicenter the subsequent study of aftershock recordings obtained by a dense seismic array deployed on the hill a few days after the earthquake showed the existence of a strong amplification at stations located at the top of the hill relative to stations near the base spudich et al 1996 resonances and polarization rotations were also observed we investigate in this study the role that the topography of the site played on the observed ground motions and accelerations to this aim we perform numerical simulations and study the response of the three dimensional topography of the site to incident shear waves polarized in different directions the method used is a boundary integral equation scheme in which the green s functions are calculated by the discrete wavenumber method the results obtained show that the topography of the site though quite gentle the hill is less than 20 m high strongly affects the ground motions in the frequency range between 2 and 15 hz many of the observed characteristics of the seismic response at tarzana are explained in part by its topography the consistent amplification of ground motion at and near the top of the hill the directional seismic response of the hill that results in a strong amplification of the ground motion transverse to the direction of elongation of the hill the existence of a fundamental transverse oscillatory resonance mode of the hill at 3 to 5 hz the rotation of the polarization of ground motion and the spatial variation of amplification over the hill at the fundamental resonance mode the seismic response of the topography however does not fully explain the amplitude of the effects observed the three dimensional geological structure of the site must in some way amplify the effect of the topography to produce the observed seismic response in spite of not being as strong as the observed effect the topographic effect of the site is considerable the ground motion is amplified by factors ranging from 30 to 100 at some locations in the frequency range from 2 to 15 hz rapid spatial variations of ground shaking intensities can take place over distance scales of a few tens of meters at high frequency finally the results of the simulation indicate that the topography of the site amplified the large east west accelerations recorded there during the northridge mainshock by 30 to 40

no effect of h2o degassing on the oxidation state of magmatic liquids abstract the underlying cause for why subduction zone magmas are systematically more oxidized than those formed at mid ocean spreading ridges is a topic of vigorous debate it is either a primary feature inherited from the subduction of oxidized oceanic crust into the mantle or a secondary feature that develops because of h 2 o degassing and or magma differentiation low total iron contents and high melt h 2 o contents render rhyolites sensitive to any effect of h 2 o degassing on ferric ferrous ratios here pre eruptive magmatic fe 2 concentrations measured using fe ti oxides that co crystallized with silicate phenocrysts under hydrous conditions are compared with fe 2 post eruptive concentrations in ten crystal poor fully degassed obsidian samples five are microlite free no effect of h 2 o degassing on the ferric ferrous ratio is found in addition fe ti oxide data from this study and the literature show that arc magmas are systematically more oxidized than both basalts and hydrous silicic melts from iceland and yellowstone prior to extensive degassing nor is there any evidence that differentiation i e crystal fractionation crustal assimilation is the cause of the higher redox state of arc magmas relative to those of iceland yellowstone rhyolites instead the evidence points to subduction of oxidized crust and the release of an h 2 o rich fluid and or melt with a high oxygen fugacity f o 2 which plays a role during h 2 o flux melting of the mantle in creating basalts that are relatively oxidized

an exact solution for effects of topography on free rayleigh waves an exact solution for the effects of topography on rayleigh wave amplification is presented the solution is obtained by incorporating conformal mapping into complex variable stress functions developed for free rayleigh wave propagation in an elastic half space with a flat upper surface results are presented for free rayleigh wave propagation across isolated symmetric ridges and valleys it is found for wavelengths that are comparable to ridge widths that horizontal rayleigh wave amplitudes are amplified at ridge crests and that vertical amplitudes are strongly reduced near ridge crests relative to horizontal and vertical amplitudes of free rayleigh waves in the flat case horizontal amplitudes are strongly deamplified at valley bottoms relative to those for the flat case for rayleigh wavelengths comparable to valley widths wave amplitudes in the symmetric ridges and valleys asymptotically approach those for the flat case with increased wavelengths increased ridge and valley widths and with horizontal distance from and depth below the isolated ridges and valleys also prograde particle motion is predicted near crests of narrow ridges and near the bottoms of narrow valleys finally application of the theory at two sites known for topographic wave amplification gives a predicted surface wave amplification ratio of 3 80 at the ridge center for a frequency of 1 0 hz at robinwood ridge in northern california and a predicted surface wave amplification ratio of 1 67 at the ridge center for the same frequency at the cedar hill nursery site at tarzana in southern california

seasonal variations in the southern hemisphere storm tracks and jet streams as revealed in a reanalysis dataset abstract regional characteristics of the climatological seasonal variations in southern hemisphere sh storm tracks are examined based on a reanalysis dataset when differences in vertical structure between the subpolar and subtropical jet streams spj and stj respectively are considered the regional characteristics can be interpreted reasonably well from a potential vorticity perspective of baroclinic eddy growth and downstream development of a baroclinic wave packet eddy activity in the upper and lower tropospheres is strongest in the core region of the spj over the eastern south atlantic and indian ocean throughout the year even in austral winter when the intense stj forms over the indian ocean and south pacific showing its eddy driven nature the spj accompanies the strong surface westerlies along a well defined baroclinic zone above an intense oceanic frontal zone in this core region of the storm track low level eddy activity is strongly correlated with the local near surface baroclinicity

ferric ferrous ratios h 2 o contents and d h ratios of phlogopite and biotite from lavas of different tectonic regimes complete chemical analyses including ferric and ferrous iron h2o contents and dd values for 16 phlogopite and biotite and 2 hornblende separates are presented samples were obtained from volcanic rocks from four localities 1 phlogopite phenocrysts from minette lavas from the western mexico continental arc 2 biotite and hornblende phenocrysts from andesite lavas from mono basin california 3 phlogopite and biotite from clinopyroxenite nodules entrained in potas sic lavas from the east african rift uganda and 4 phlogopite phenocrysts from a wyomingite lava in the leucite hills wyoming the fe 2 o 3 contents in the mi cas range from 0 8 to 10 5 wt corresponding to 0 09 to 1 15 fe 31 per formula unit pfu water contents vary from 1 6 to 3 0 wt corresponding to 1 58 to 3 04 oh pfu significantly less than would be expected for a site fully occupied by hydroxyl cation and anion based normalization procedures provide accurate mineral for mulae with respect to most cations and anions but are unable to generate accurate estimates of fe 31 yfe t a nd overestimate oh at the expense of o on the hydroxyl site these inaccuracies are present despite acceptable adjust ed totals and stoichiometric calculated site occupancies the phlogopite and biotite phenocrysts in arc related lavas from western mexico and eastern california have the highest fe 31 yfe t ratios 56 87 reflecting high magmatic oxygen fugacities dnno 51 2t o 15 in contrast to those from uganda 25 40 and the leucite hills 23 there is no correlation between the oh content and the fe 31 yfe t ratio in the micas values of k mgyfe 21 d 2s errors were calculated for three phlogo pite olivine pairs 0 12 0 12 0 26 0 14 0 09 0 12 two biotite hornblende pairs 0 73 0 08 and 1 22 0 10 and a single phlogopite augite pair 1 15 0 12 values of k fyoh d for two biotite and hornblende pairs could not be determined without significant error because of the extremely low f contents 0 2 wt of the four phases the dd values obtained in this study encompass a large range 2137 to 243 the phlogo pite and biotite separates from uganda have dd values of 270 to 249 which overlap those believed to represent primary mantle there is a larger range in dd values 2137 to 243 for phlogopite phenocrysts from west ern mexico minette lavas although their range in d 18 o values 5 2 6 2 is consistent with normal mantle it is unlikely therefore that the variable dd values reflect heterogeneity in the mantle source region of the minette magmas nor can the extremely low dd values reflect degassing of h2 or h2o since almost 100 loss of dis solved water in the magma is required an unrealistic scenario given the stability of the hydrous phenocrysts the very low dd values of the mascota minette phlogo pites require that the hydrogen be introduced from an external source e g meteoric water whatever the pro cess responsible for the observed hydrogen isotope com position it had no effect on the d 18 o value fo2 ah2o or bulk composition of the host magmas

volcan tancitaro michoacan mexico 40ar 39ar constraints on its history of sector collapse abstract volcan tancitaro is a 97 3 km3 stratovolcano located in the michoacan guanajuato volcanic field mgvf part of the trans mexican volcanic belt prior to this study there was only one k ar date 530 60 ka ban m hasenaka t delgado granados h takaoka n 1992 k ar ages of lavas from shield volcanoes in the michoacan guanajuato volcanic field mexico geofisica internacional 31 4 467 473 and one sector collapse event reported for this volcano in the literature garduno monroy v h corona chavez p israde alcantara i mennella l arreygue e bigioggero b chiesa s 1999 carta geologica de michoacan scale 1 250 000 universidad michoacana de san nicolas de hidalgo capra l macias j l scott k m abrams m garduno monroy v h 2002 debris avalanches and debris flows transformed from collapses in the trans mexican volcanic belt mexico behavior and implications for hazard assessment journal of volcanology and geothermal research 113 81 110 twenty six new 40ar 39ar ages indicate that volcan tancitaro became active 793 22 ka and that the most recent effusive activity occurred at 237 34 ka two catastrophic sector collapse events are identified and dated the first one occurred on the west side between 694 and 571 ka whereas the second one occurred on the east side between 261 and 238 ka the older collapse produced a 2 3 3 4 km3 debris avalanche and laharic deposit spread over 567 km2 whereas the more recent collapse left a 3 km wide horseshoe shaped scar on the eastern flank and produced a 3 6 7 0 km3 debris avalanche and laharic deposit that covers 654 km2 reconstruction of the main edifice of volcan tancitaro using arcgis software and digital elevation models indicates that the volume removed during the eastern sector collapse was 4 7 km3

the effect of glaciation on the intensity of seismic ground motion seismicity is known to contribute to landscape denudation through its role in earthquake triggered slope failure but little is known about how the intensity of seismic ground motions and therefore triggering of slope failures may change through time topography influences the intensity of seismic shaking generally steep slopes amplify shaking more than flatter slopes and because glacial erosion typically steepens and enlarges slopes glaciation may increase the intensity of seismic shaking of some landforms however the effect of this may be limited until after glaciers retreat because valley ice or ice caps may damp seismic ground motions two dimensional numerical models flac 6 0 were used to explore how edifice shape rock stiffness and various levels of ice inundation affect edifice shaking intensity the modelling confirmed that earthquake shaking is enhanced with steeper topography and at ridge crests but it showed for the first time that total inundation by ice may reduce shaking intensity at hill crests to about 20 50 of that experienced when no ice is present the effect is diminished to about 80 95 if glacier ice level reduces to half of the mountain slope height in general ice cover reduced shaking most for the steepest sided edifices for wave frequencies higher than 3 hz and when ice was thickest and the rock had shear stiffness well in excess of the stiffness of ice if rock stiffness is low and shear wave velocity is similar to that of ice the presence of ice may amplify the shaking of rock protruding above the ice surface the modelling supports the idea that topographic amplification of earthquake shaking increases as a result of glacial erosion and deglaciation it is possible that the effect of this is sufficient to have influenced the distribution of post glacial slope failures in glaciated seismically active areas copyright 2012 john wiley sons ltd

a diagnostic study of jet streaks kinematic signatures and relationship to coherent tropopause disturbances abstract a diagnostic study is conducted of the kinematics and evolution of upper level jet streaks representative of three of the four phases in the shapiro conceptual model of a jet streak progressing through a synoptic scale baroclinic wave over north america the three phases selected for consideration apply to those segments of the wave pattern where jet streaks are relatively straight the 1200 utc 2 december 1991 case trough over ridge considers a strong jet streak located over eastern north america and constitutes the bulk of the study the other two cases which also concern jet streaks over north america are from 0000 utc 11 november 1995 northwesterly flow and 0000 utc 28 october 1995 southwesterly flow kinematic signatures consistent with the classic four quadrant conceptual model for a straight jet streak are evident in all three cases although flow curvature and thermal advection lead to significant departures from this conceptual model the position of the jet streak within the s

the little red hill seismic experimental study topographic effects on ground motion at a bedrock dominated mountain edifice the kinematic response of a bedrock dominated mountain edifice to seismic shaking near lake coleridge in the southern alps of new zealand was examined a small elongated and bedrock dominated mountain ridge little red hill was equipped with a seismic array seven earss instruments mark l 4 3d seismometers were installed along the ridge crest the flank and at the base of the 210 m high 500 m wide and 800 m long mountain edifice from february to july 2006 seismic records of local and regional earthquakes were used to provide information on amplification and deamplification effects as well as on the frequency response at different parts on the edifice the ground motion records were analyzed using three different methods comparisons of peak ground accelerations power spectral density analysis and standard spectral ratio analysis time and frequency domain analyses show that site amplification is concentrated along the elongated crest of the edifice where amplifications of up to 1100 were measured relative to the motion at the flat base the field experiment also highlighted the extreme variations in amplification with regard to closely located sites but at contrasting topographic locations theoretical calculations and frequency analyses of field data indicate a maximum response along the ridge crest of little red hill for frequencies of about 5 hz which correlate to wavelengths approximately equal to the half width or height of the edifice the consequence of amplification effects on the stability and degradation of rock masses can be seen areas showing high amplification effects overlap with the spatial distribution of seismically generated block fields at little red hill it is concluded that topography geometry and distance to the seismic source play key roles in causing amplification effects of seismic ground motion and supporting dilation and degradation of rock mass across bedrock dominated mountain edifices

explosion coupling in frozen and unfrozen rock experimental data collection and analysis nuclear monitoring agencies often use seismic amplitudes to estimate the yields of underground nuclear tests any emplacement phenomena that can alter those amplitudes and lead to bias in estimated yields must be considered in the analysis one condition that might cause such a bias is detonation in frozen rock laboratory analyses mellor 1971 miller and florence 1991 have shown that frozen rock has faster seismic velocity and greater compressive strength than unfrozen rock this increased strength is hypothesized to reduce the seismically estimated yield of an explosion in frozen rock to test this hypothesis we conducted the frozen rock experiment fre a series of explosions in frozen and unfrozen rock in central alaska during august 2006 over 120 seismic instruments were deployed to record six detonations three in frozen and three in unfrozen dry media at a wide range of distances and azimuths the data acquired show that the frozen test site explosions had significantly larger amplitudes for all phases p s and surface waves above 8 10 hz these data confirm that the frozen rock medium was stronger and resulted in a smaller seismic source radius for the explosions thus increasing the corner frequency when compared to the unfrozen rock explosions between 3 and 9 hz the unfrozen shots produced slightly larger s and surface waves resulting in different p s spectral ratio plots for the frozen and unfrozen shots possibly affecting regional phase discrimination we show that the observed amplitude differences for these shots can be effectively modeled using the mueller murphy 1971 explosion source and the in situ p and s wave velocities for the two test sites differences in the velocities at the frozen and unfrozen rock test sites are caused by minor metamorphic facies changes saturated versus dry conditions and the presence of ice in the pores and fractures at the frozen test site extrapolation of the results of this study to synthetic nuclear explosions suggests there may not be significant coupling differences between explosions in frozen and unfrozen hard rock

life history of mobile troughs in the upper westerlies abstract increasing evidence indicates that surface cyclogenesis is predominantly a response to the approach of a preexisting trough at upper levels a question then arises about the origin of the upper level predecessor as an initial approach to this question mobile troughs in the major band of westerlies were crudely tracked in daily northern hemispheric 500 mb analysis during nine recent cold seasons these troughs were identified only in the 552 dam height contour between 8 and 15 of them were present on a given day study of a particular cold season showed a median duration of 12 days and a mode of 5 days average zonal phase speed was 13 m s 1 locations or origin and termination of individual troughs were distributed over all longitudes but births greatly exceeded deaths over and east of the rocky mountains in north america and the highlands of central asia trough terminations dominated over the eastern portions of the oceans within the quasi steady planetary waves origins and terminations o

characteristics of the southern hemisphere winter storm track with filtered and unfiltered data abstract the structure and evolution of the fluctuations in synoptic scales in the southern hemisphere sh during winter are discussed using six years of european centre for medium range weather forecasts analyses it is shown that patterns from unfiltered meridional wind series in the sh display all the features needed to represent the synoptic scale waves typical periods and wavelengths are similar to those observed in the northern hemisphere 4 days 4000 km although over the pacific ocean they can be as high as 7 8 days and 4700 km respectively as in the northern hemisphere tilts are not geographically fixed but change with the stage of the evolution of the wave the phase speed of the waves agrees with the low level winds in extensive areas of the middle latitudes and ranges from 12 m s 1 in the indian ocean to 6 m s 1 in the pacific ocean the estimated group velocities achieve maximum values of about 38 m s 1 also in the indian ocean and agree with the upper level maximum winds in accor

the effect of water and fo2 on the ferric ferrous ratio of silicic melts new experiments on the effect of dissolved water on the ferric ferrous ratio of silicic melts have been performed at 200 mpa between 800 c and 1000 c and for fo2 between nno 1 35 and nno 6 6 water saturated conditions were investigated compositions studied include six metaluminous synthetic melts with feotot progressively increasing from 0 47 to 4 25 wt two natural obsidian glasses one peraluminous and the other peralkaline and a synthetic rhyolitic glass having the composition of the matrix glass of the june 15 1991 pinatubo dacite ferrous iron was analyzed by titration and feotot by electron microprobe variation of quench rate was found to have no detectable effect on the ferric ferrous ratio of the hydrous silicic melts investigated at nno no dependence of the ferric ferrous ratio with temperature is observed at fo2 nno 1 the experimental ferric ferrous ratios are equal or lower than calculated the peralkaline samples show the same type of behaviour a non linear relationship between xfe2o3 xfeo and fo2 implies that a term for dissolved water must be added to the kc equation if it is to be applied to the calculation of ferric ferrous ratios of hydrous silicic melts above nno 1 the ferric ferrous ratio is essentially controlled by the anhydrous melt composition and fo2 however differences exist between measured and calculated ferric ferrous ratios of silicic melts that are not all attributable to the effect of dissolved water additional work is needed to describe more precisely the dependence of the ferric ferrous ratio on anhydrous melt composition the oxidizing effect of water is restricted to relatively reduced magmatic liquids in oxidized calk alkaline magma series the presence of dissolved water will not largely influence melt ferric ferrous ratios

the aurora volcanic field california nevada oxygen fugacity constraints on the development of andesitic magma the aurora volcanic field located along the northeastern margin of mono lake in the western great basin has erupted a diverse suite of high k and shoshonitic lava types with 48 to 76 wt sio2 o ver the last 3 6 million years there is no correlation between the age and composition of the lavas three quarters of the volcanic field consists of evolved 4 wt mgo basaltic andesite and andesite lava cones and flows the majority of which contain sparse euhedral phenocrysts that are normally zoned there is no evidence of mixed hybrid magmas the average eruption rate over this time period was 200 m 3 ykm 2 yyear which is typical of con tinental arcs and an order of magnitude lower than that for the slow spreading mid atlantic ridge all of the au rora lavas display a trace element signature common to subduction related magmas as exemplified by baynb ratios between 52 and 151 pre eruptive water contents ranged from 1 5 wt in plagioclase rich two pyroxene andesites to 6 wt in a single hornblende lamprophyre and several biotite hornblende andesites calculated oxygen fugacities fall within 0 4 and 12 4 log units of the ni nio buffer the aurora potassic suite follows a classic calc alkaline trend in a plot of feo t ymgo vs sio 2 and displays linear decreasing trends in feo t and tio2 with sio2 content suggesting a prominent role for fe ti oxides during differentiation however develop ment of the calc alkaline trend through fractional crys tallization of titanomagnetite would have caused the residual liquid to become so depleted in ferric iron that its oxygen fugacity would have fallen several log units below that of the ni nio buffer nor can fractionation of hornblende be invoked since it has the same effect as titanomagnetite in depleting the residual liquid in ferric iron together with a thermal stability limit that is lower than the eruption temperatures of several andesites 1040 10808c derived from two pyroxene ther mometry unless some progressive oxidation process occurs fractionation of titanomagnetite or hornblende cannot explain a calc alkaline trend in which all erupted lavas have oxygen fugacites the ni nio buffer in con trast to fractional crystallization closed system equi librium crystallization will produce residual liquids with an oxygen fugacity that is similar to that of the initial melt however the eruption of nearly aphryic lavas ar gues against tapping from a magma chamber during equilibrium crystallization a process that requires crys tals to remain in contact with the liquid a preferred model involves the accumulation of basaltic magmas at the mantle crust interface which solidify and are later remelted during repeated intrusion of basalt as an end member case closed system equilibrium crystallization of a basalt followed by equilibrium partial melting of the gabbro will produce a calc alkaline evolved liquid namely high sio 2 and low feo t ymgo with a relative fo2 corrected for the effect of changing temperature that is similar to that of the initial basalt differentiation of the aurora magmas by repeated partial melting of previous underplates in the lower crust rather than by crystal fractionation in large stable magma chambers is consistent with the low eruption rate at the aurora vol canic field

experimental determination of coexisting iron titanium oxides in the systems fetialo fetialmgo fetialmno and fetialmgmno at 800 and 900 c 1 4 kbar and relatively high oxygen fugacity a synthetic low melting rhyolite composition containing tio2 and iron oxide with further separate additions of mgo mno and mgo mno was used in hydrothermal experiments to crystallize ilm hem and usp mt solid solutions at 800 and 900 c under redox conditions slightly below nickel nickel oxide nno to approx 3 log 10 f text o 2 units above the nno oxygen buffer these experiments provide calibration of the feti oxide thermometer oxygen barometer at conditions of temperature and oxygen fugacity poorly covered by previous equilibrium experiments isotherms for our data in roozeboom diagrams of projected usp vs ilm show a change in slope at 60 ilm consistent with the second order transition from feti ordered ilm to feti disordered ilm hem this feature of the system accounts for some but not all of the differences from earlier thermodynamic calibrations of the thermobarometer in rhyolite containing 1 0 wt mgo 0 8 wt mno or mgo mno usp mt crystallized with up to 14 of aluminate components and ilm hem crystallized with up to 13 geikielite component and 17 pyrophanite component relative to the fetialo system these components displace the ferrite components in usp mt and the hematite component in ilm hem as a result projected contents of ulvospinel and ilmenite are increased these changes are attributed to increased non ideality along joins from end member hematite and magnetite to their respective mg and mn bearing titanate and aluminate end members the compositional shifts are most pronounced in ilm hem in the range ilm50 80 a solvus region where the chemical potentials of the hematite and ilmenite components are nearly independent of composition the solvus gap widens with addition of mg and even further with mn the bacon hirschmann correlation of mg mn in usp mt and coexisting ilm hem is displaced toward increasing mg mn in ilmenite with passage from ordered ilmenite to disordered hematite orthopyroxene and biotite crystallized in experiments with added mgo and mgo mno their x fe varies with log 10 f text o 2 and t consistent with equilibria among ferrosilite annite and ferrite components and the chemical potentials of sio2 and orthoclase in the liquid experimental equilibration rates increased in the order opx bt ilm hem usp mag

the evolution of aqueous vapor from silicate melts effect on oxygen fugacity abstract the simplest possible aqueous vapor evolved from an iron bearing magma under normal conditions is an h 2 o h 2 mixture hydrogen is produced by the oxidation of fe ii to fe iii by water in the magma yielding an increase in f o 2 equations are derived which model this mass transfer process the resulting differential equation expresses the change in magmatic f o 2 as a function of a change in n 1 h 2 o the vapor evolution progress variable the calculations demonstrate that in this simple scheme very iron poor magmas h 2 o feo are required to produce significant increases in magmatic f o 2 the incremental increase in o 2 during vapor evolution becomes progressively smaller as o 2 increases with this process becoming highly inefficient as the hematite stability field is approached other more complete processes involving meteoric water assimilation of altered rocks and subsolidus processes may produce the observed increase in f o 2 inferred to have occurred in systems of normal iron contents in low iron systems such as the high silica rhyolitic magmas associated with climax type porphyry molybdenum systems high oxygen fugacities may be obtained by this mechanism

an observational study of medium scale wave dynamics in the southern hemisphere summer part i wave structure and energetics abstract medium scale waves zonal wavenumbers 4 7 frequently dominate southern hemisphere sh summer midlatitude circulation patterns this work is an observational study that focuses on their temporal and spatial characteristics along with detailing the forcing mechanisms responsible for their formation medium scale wave characteristics for three sh summers 1978 79 through 1980 81 are discussed it is shown that the time mean medium scale wave structure is consistent with the basic state linear wave propagation characteristics the energetics of the medium scale waves are studied using the transformed eulerian mean formalism of plumb it is found that wave zonal mean exchange is a valid concept for describing the sh summer atmospheric circulation and that the flow vacillates between periods of highly perturbed and zonally symmetric states with a time wale on the order of 10 20 days these vacillations result from nonlinear baroclinic instabilities and the medium scale waves exhibit well defined

initial investigation of site and topographic effects at robinwood ridge california abstract following the 1989 loma prieta earthquake a dense array of seven digitally recorded three component seismograph stations was deployed on robinwood ridge 7 3 km northwest of the epicenter the purpose of this array was to investigate the cause of high levels of structural damage and ground cracking observed on the ridge crest aftershocks recorded by the array allow a comparison of ground motion up the slope of the ridge from the base to the crest the data present an extremely complicated pattern of ground motion that demonstrates the importance of the three dimensionality of the problem slowness analysis of p wave trains show initial arrivals propagating away from the source with small angles of incidence and large apparent velocities consistent with direct arrivals after 0 5 sec propagation azimuths become more random and apparent velocities drop indicating nearly horizontal wave propagation and multiply reflected and diffracted phases within the ridge slowness analysis and particle motion diagrams of horizontal components of motion show dramatic variations in ground motion with changes in azimuth of the source and a complicated interaction between body waves and rayleigh and love waves results suggest that the larger amplitude more coherent arrivals at the array stations favor a propagation direction parallel to the ridge axis an amplification factor of from 1 5 to 4 5 is seen for frequencies from 1 0 to 3 0 hz with wavelengths comparable to the base of the ridge part of which may be caused by local site effects and part by topographic amplification in addition amplifications of up to a factor of 5 are seen at higher frequencies and are attributed to local site effects these effects are most notable from 4 to 8 hz on the vertical components and from 6 to 9 hz on the horizontal components the entire robinwood ridge area may also have been situated in a region of heightened mainshock ground motion due to source directivity and radiation pattern effects

an evaluation of the effect of degassing on the oxidation state of hydrous andesite and dacite magmas a comparison of pre and post eruptive fe 2 concentrations the bulk post eruptive wt feo concentration in each of 11 phenocryst poor 5 andesite and dacite 60 69 wt sio2 lavas from different monogenetic vents in the mexican arc has been measured by titration in duplicate the results match within analytical error the wt feo content of the magmas during phenocryst growth pre euptive which were calculated on the basis of oxygen fugacity and temperature results from fe ti two oxide oxygen barometry the average deviation between the pre and post eruptive feo concentrations is 0 15 wt application of the plagioclase liquid hygrometer shows that at the time of phenocryst growth these 11 magmas contained from 3 8 wt h2o which was extensively degassed upon eruption there is no evidence that degassing of 8 wt h2o changed the oxidation state of these magmas calculations of pre eruptive and post eruptive oxygen fugacity values relative to the ni nio buffer in terms of log10 units for the 11 samples span a similar range pre eruptive nno 0 9 to 0 7 and post eruptive nno 0 4 to 0 8 the data further show that extensive groundmass closed system crystallization had no affect on bulk fe3 fe2 ratios finally there is no systematic variation in the range of pre eruptive fe3 fet values of the samples as a function of sio2 concentration i e differentiation therefore the results of this study indicate that the elevated fe3 fet ratios of arc andesites and dacites compared with magmas erupted in other tectonic settings cannot be attributed to the effects of 1 degassing of h2o 2 closed system crystallization and or 3 differentiation effects but instead must be inherited from their parental source rocks i e mantle derived arc basalts

an amplification mechanism of medium scale tropopausal waves abstract the mechanism for the amplification of medium scale tropopausal waves with horizontal wavelengths of 2000 3000 km and wave periods of 1 2 days that are basically neutral is investigated using a three dimensional wave activity flux distinctive upward wave activity fluxes are found in the midtroposphere 400 hpa upstream of the region where the medium scale waves are dominant that is where the horizontal components of the wave activity flux are large these upward fluxes converge in the upper troposphere 300 hpa this convergence is a major source of medium scale waves producing large values of the horizontal components of the wave activity flux in the active regions examination of the spatial structure of the heat flux shows that the upward fluxes are due to occasional baroclinic interactions between upper level medium scale waves and lower tropospheric disturbances with the same temporal scales the seasonal and geophysical variations of the waves depend mostly on the amount of baroclin

site effects in the city of lourdes france from h v measurements implications for seismic risk evaluation abstract the pilgrimage city of lourdes in the french pyrenees has been heavily damaged by earthquakes in the seventeenth and eighteenth centuries assessments of local seismic hazard have been performed through a seismic microzonation of the city a campaign of urban noise recording has been conducted at about 180 points to determine seismic wave amplification by means of spectral ratios of the horizontal to vertical h v components a comparison of this ratio obtained from ambient noise measurements with spectral ratios obtained from earthquake s wave recording h href reveals that the fundamental resonance frequency is well estimated but that the site amplification is generally underestimated resonance frequencies and amplitudes exhibit coherent patterns that relate to the geological structure bedrock sites are generally characterized by flat spectra whatever the geological nature of bedrock unless topography or karst are present along the river in particular at the pilgrimage site thick sediments induce low resonance frequencies with large amplifications along and across valleys north and east of the city h v spectra show systematic variations according to sediment filling properties finally on the hill flanks east of the city h v spectra rather relate to topography an additional issue with h v spectral ratios based on ambient noise is whether this indicator reflects meaningful geological variations this question is tested on sites where the s wave ground velocity structure is known a first test consists of estimating h v spectral ratios by means of simulation of urban noise through the summation of synthetic seismograms with random properties a second test consists of estimating h href spectral ratios by propagating an earthquake record through a 1d soil profile these two modelings successfully predict most of the observed spectrum characteristics in addition they give some information on the nature of the urban noise which may include a significant amount of shear energy

the eruptive history of the tequila volcanic field western mexico ages volumes and relative proportions of lava types the eruptive history of the tequila volcanic field 1600 km 2 in the western trans mexican volcanic belt is based on 40 ar 39 ar chronology and volume esti mates for eruptive units younger than 1 ma ages are reported for 49 volcanic units including volcn tequila an andesitic stratovolcano and peripheral domes flows and scoria cones volumes of volcanic units 1 ma were obtained with the aid of field mapping ortho aerial pho tographs digital elevation models dems and arcgis software between 1120 and 200 kyrs ago a bimodal distribution of rhyolite 35 km 3 and high ti basalt 39 km 3 dominated the volcanic field between 685 and 225 kyrs ago less than 3 km 3 of andesite and dacite erupted from more than 15 isolated vents these lavas are crystal poor and show little evidence of storage in an upper crustal chamber approximately 200 kyr ago 31 km 3 of andesite erupted to form the stratocone of volcn tequila the phenocryst assemblage of these la vas suggests storage within a chamber at 2 3 km depth after a hiatus of 110 kyrs 15 km 3 of andesite erupted along the w and se flanks of volcn tequila at 90 ka most likely from a second discrete magma chamber lo cated at 5 6 km depth the youngest volcanic feature 60 ka is the small andesitic volcano cerro tomasillo 2 km 3 over the last 1 myr a total of 128 22 km 3 of

a diagnostic study of waves on the tropopause the spatial structure and phase velocity of tropopause disturbances localized around the subpolar jet in the southern hemisphere are investigated using 6 hourly european centre for medium range weather forecasts reanalysis data covering 15 yr 1979 93 the phase velocity and phase structure of the tropopause disturbances are in good agreement with those of an edge wave vertically trapped at the tropopause however the vertical distribution of the ratio of potential to kinetic energy exhibits maxima above and below the tropopause and a minimum around the tropopause in contradiction to edge wave theory for which the ratio is unity throughout the troposphere and stratosphere this difference in vertical structure between the observed tropopause disturbances and edge wave theory is attributed to the effects of a finite depth tropopause together with the next order corrections in rossby number to quasigeostrophic dynamics

the fina nagu volcanic complex unusual submarine arc volcanism in the rapidly deforming southern mariana margin in the mariana convergent margin large arc volcanoes disappear south of guam even though the pacific plate continues to subduct and instead small cones scatter on the seafloor these small cones could form either due to decompression melting accompanying back arc extension or flux melting as expected for arc volcanoes or as a result of both processes here we report the major trace and volatile element compositions as well as the oxidation state of fe in recently dredged fresh pillow lavas from the fina nagu volcanic chain an unusual alignment of small closely spaced submarine calderas and cones southwest of guam we show that fina nagu magmas are the consequence of mantle melting due to infiltrating aqueous fluids and sediment melts sourced from the subducting pacific plate into a depleted mantle wedge similar in extent of melting to accepted models for arc melts fina nagu magmas are not as oxidized as magmas elsewhere along the mariana arc suggesting that the subduction component responsible for producing arc magmas is either different or not present in the zone of melt generation for fina nagu and that amphibole or serpentine mineral destabilization reactions are key in producing oxidized arc magmas individual fina nagu volcanic structures are smaller in volume than mariana arc volcanoes although the estimated cumulative volume of the volcanic chain is similar to nearby submarine arc volcanoes we conclude that melt generation under the fina nagu chain occurs by similar mechanisms as under mariana arc volcanoes but that complex lithospheric deformation in the region distributes the melts among several small edifices that get younger to the northeast

the effect of topography on earthquake ground motion a review and new results a brief review of experimental and theoretical results about the effect of topography on seismic motion shows that they are consistent only on a qualitative basis amplification at mountain tops for wavelengths comparable with mountain widths is predicted and observed but the numerical simulations often underestimate the actual observed effects we propose that this disagreement is because current models are not complex enough we therefore computed the response to incident sh waves of a set of different complex configurations that include two dimensional surface topography with or without periodic ridges and subsurface layering the results show that 1 the topographic effect in itself is difficult to isolate from other effects like surface layering and therefore the amplification on top of geomorphologically complex sites cannot be predicted by a priori estimations based solely on topography and 2 the high crest base amplification ratio observed in the field cannot usually be matched even with complex two dimensional structures with incident plane sh waves which suggests that more complex models are needed to incorporate more complex wave fields e g sv surface and three dimensional geologic configurations

global characteristics of medium scale tropopausal waves observed in ecmwf operational data global characteristics and seasonal variation of medium scale zonal wavelengths of 2000 3000 km waves observed around the midlatitude tropopause are examined using 6 hourly european centre for medium range weather forecasts operational data over four years 1990 93 covering both hemispheres medium scale waves and synoptic scale waves are extracted using time filters and their characteristics are compared hovmoller diagrams indicate the existence of medium scale waves in the southern hemisphere as well as in the northern hemisphere the zonal wavelengths of medium scale waves are slightly larger in the southern hemisphere than in the northern hemisphere medium scale waves are mostly active in three regions the north atlantic in winter dec jan feb the north pacific in spring mar apr may mam and the south indian ocean in autumn mam these regions are roughly corresponding to and slightly downstream of storm tracks where synoptic waves are dominant significant differences in seasonal variation of the intensity between the two kinds of waves are also found the maximum of wave amplitudes is seen around the tropopause at latitudes slightly higher than the jet stream axis where the meridional gradient of the quasigeostrophic potential vorticity qpv is maximized the positive large qpv gradient is attributed to the atmospheric structure around the midlatitude tropopause that is located in vertical westerly shear of the jet stream this fact suggests that the medium scale waves are waves trapped around the midlatitude tropopause

a diagnostic equation for tendency of lapse rate tropopause heights and its application abstractthe tropopause is the boundary between the troposphere and stratosphere and is normally defined by the temperature lapse rate previous studies have noted that synoptic scale and planetary

experimental evidence for the alteration of the fe3 σfe of silicate melt caused by the degassing of chlorine bearing aqueous volatiles the temporal evolution of the redox conditions undergone by degassing magmas has petrologic implications for magmatic phase equilibria we present new experimental data that demonstrate that a cl bearing magmatic volatile phase may affect the concentration and activity of ferrous iron in silicate liquid via the preferential mass transfer of fe 2 into the volatile phase the data are consistent with a model wherein the exsolution of a cl bearing volatile phase from the melt modifies the mgo feo ratio of the residual degassed melt increasing the fe 3 fe ratio of the melt degassing induced alteration of the melt may cryptically affect the phases that constitute the hypersolidus crystallizing assemblage the new results indicate that a low to moderate salinity magmatic volatile phase may contain several weight percent iron present as fecl 2 as such passive degassing over geologically plausible protracted periods has the potential to extract significant quantities of fe 2 resulting in a residual melt depleted in feo relative to fe 2 o 3 notably preferential mass transfer of fe 2 from the melt to the fluid results in self oxidation of the melt driving the oxygen fugacity of the melt across the phase boundary that demarcates the stability fields of sulfide and sulfate phases stabilizing sulfate at the expense of sulfide such a process has significant implications for the remobilization of chalcophile metals during the evolution of magmatic hydrothermal systems

role of shape and numbers of ridges and valleys in the insulating effects of topography on the rayleigh wave characteristics this research work is inspired by the recently accepted concept that high frequency rayleigh waves are generated in the epicentral zone of shallow earthquakes such high frequency rayleigh waves with large amplitude may develop much of spatial variability in ground motion which in turn may cause unexpected damage to long span structures like bridges underground pipelines dams etc in the hilly regions further it has been reported that topography acts as an insulator for the rayleigh waves ma et al bssa 97 2066 2079 2007 the above mentioned scientific developments stimulated to quantify the role of shape and number of ridges and valleys falling in the path of rayleigh wave in the insulating effect of topography on the rayleigh waves the simulated results reveals very large amplification of the horizontal component of rayleigh wave near the top of a triangular ridge which may cause intensive landslides under favorable condition the computed snapshots of the wave field of rayleigh wave reveals that the interaction of rayleigh wave with the topography causes reflection splitting and diffraction of rayleigh wave in the form of body waves which in turn provides the insulating capacity to the topography insulating effects of single valley is more than that of single ridge further this effect was more in case of elliptical ridge valley than triangular ridge valley the insulating effect of topography was proportional to the frequency of rayleigh wave and the number of ridges and valleys in the string the obtained level of insulation effects of topography on the rayleigh wave energy of rayleigh wave reduced to less than 4 after crossing a topography of span 4 5 km calls for the consideration of role of hills and valleys in seismic hazard prediction particularly in case of shallow earthquakes

a pliocene ignimbrite flare up along the tepic zacoalco rift evidence for the initial stages of rifting between the jalisco block mexico and north america the tepic zacoalco rift a nw trending corridor 50 250 km is one arm of a triple rift system in western mexico together with the colima rift and the middle america trench it bounds the jalisco block a portion of western mexico that may be moving independently of north america the predominant basement rock types in the tepiczacoalco rift are rhyolitic ash fl ow tuffs and lavas which were previously assumed to be oligocene miocene in age related to the sierra madre occidental volcanic province or older new 40 ar 39 ar dates on 41 volcanic samples reveal a previously unrecognized voluminous fl are up of rhyolitic ignimbrites between 5 and 3 ma throughout the entire corridor of the tepic zacoalco rift they are often associated with pliocene high ti basalts the eruption rate during this pliocene time period was an order of magnitude higher hundreds of m m y than that documented in the tepic zacoalco rift over the last 1 m y the pliocene ash fl ow tuffs have been faulted along nw trending lineaments producing vertical offsets up to at least 500 m the voluminous ignimbrite fl are up in the tepiczacoalco rift at 5 3 ma may refl ect the initial stages of rifting of the jalisco block away from north america analogous to what occurred in the proto gulf region at 12 6 ma prior to the transfer of baja california from north america to the pacifi c plate additionally new 40 ar 39 ar dates show that the sierra madre occidental volcanic province extends across the entire width of the tepic zacoalco rift and terminates abruptly at the northern boundary of the jalisco block near the rio ameca in contrast paleoceneeocene basement from the jalisco block extends northward into the tepic zacoalco rift where it is locally overlain by sierra madre occidental rhyolites

balanced asymmetries of waves on the tropopause tropopause disturbances have long been recognized as important features for extratropical weather since they produce organized vertical motion in the troposphere observations of cyclonic tropopause disturbances show localized depressions of the tropopause with stratospheric values of potential vorticity extending to lower altitudes anticyclonic disturbances are associated with comparatively smaller upward deflections of the tropopause analytical solutions for nonlinear interfacial wave motions are derived for an intermediate balanced dynamics based on small rossby number asymptotics beyond quasigeostrophy traveling edge wave solutions reveal realistic asymmetries such that cyclones are associated with greater deflections of the interface as well as larger anomalies in pressure and vertical motion compared to anticyclones

observed cyclone anticyclone tropopause vortex asymmetries relatively little is known about coherent vortices near the extratropical tropopause even with regard to basic facts about their frequency of occurrence longevity and structure this study addresses these issues through an objective census of observed tropopause vortices the authors test a hypothesis regarding vortex merger asymmetry where cyclone pairs are repelled and anticyclone pairs are attracted by divergent flow due to frontogenesis emphasis is placed on arctic vortices where jet stream influences are weaker in order to facilitate comparisons with earlier idealized numerical simulations results show that arctic cyclones are more numerous persistent and stronger than arctic anticyclones an average of 15 cyclonic vortices and 11 anticyclonic vortices are observed per month with maximum frequency of occurrence for cyclones anticyclones during winter summer there are are about 47 more cyclones than anticyclones that survive at least 4 days and for longer lifetimes 1 day survival probabilities are nearly constant at 65 for cyclones and 55 for anticyclones mean tropopause potential temperature amplitude is 13 k for cyclones and 11 k for anticyclones with cyclones exhibiting a greater tail toward larger values an analysis of close proximity vortex pairs reveals divergence between cyclones and convergence between anticyclones this result agrees qualitatively with previous idealized numerical simulations although it is unclear to what extent the divergent circulations regulate vortex asymmetries