propagation characteristics of lg across the tibetan plateau abstract we present the results of a study on the propagation of lg within the tibetan plateau we analyze over 1200 seismograms for lg propagation from 185 local and regional earthquakes recorded at distances from 150 to 2000 km 52 of which are located within the tibetan plateau although the propagation of lg has been observed across most of asia lg has not been observed for paths crossing the tibetan plateau we analyze paths that cross the plateau boundaries and paths contained entirely within the tibetan plateau lg is generated within the tibetan plateau and can propagate to epicentral distances of at least 600 km for events located outside of the tibetan plateau lg is either absent or severely diminished for paths that cross both the himalayan and kunlun ranges confirming that the margins of the plateau weaken lg transmission based on the observations from our qualitative analysis we invert lg amplitudes for event station paths that are confined to the tibetan plateau to estimate the quality factor q the inversion yields a frequency dependent q function q f 366 37 f 0 45 0 06 0 5 f 16 h z similar observations in other areas indicate that frequency dependent apparent q within the tibetan plateau is well below that expected for a typical continental interior instead it has a frequency dependence that is characteristic of a tectonically active region

the southern caspian a compressional depression floored by a trapped modified oceanic crust the south caspian intracontinental depression floored by oceanic basement is a relatively stable block with minor deformation surrounded by active fold thrust belts of arcuate form talesh alborz and kopeh dagh mountains which have undergone intense late cainozoic crustal shortening the basin is interpreted as a neogene quaternary compressional depression bounded by multi role mountain bordering reverse faults and apparently floored by a late paleozoic triassic or late mesozoic early tertiary modified oceanic crust trapped along an old geosuture it may be a relic of an old paleozoic triassic ocean or else a marginal sea developed behind a mesozoic paleogene ocean and analysis of geological and geophysical data enables a scheme to be suggested the general arcuate shape of the alborz and the talesh bordering mountain belts follows the pattern of the supposed rigid and thickened ocean crust of south caspian depression a tectono sedimentary study of the south caspian region coupled w

correlations of volcanic ash texture with explosion earthquakes from vulcanian eruptions at sakurajima volcano japan abstract we compare the texture of volcanic ash with the maximum amplitude of explosion earthquakes a eq for vulcanian eruptions from sakurajima volcano we analyze the volcanic ash emitted by 17 vulcanian eruptions from 1974 to 1987 using a stereoscopic microscope we classify the glassy particles into smooth surface particles s type particles and non smooth surface particles ns type particles according to their surface conditions gloss or non gloss appearance as an indicator of the freshness of the particles s type particles are further classified into v type particles those including vesicles and nv type particles those without vesicles by means of examinations under a polarized microscopic of polished thin sections cross correlated examinations against seismological data show that 1 the number fraction of s type particles s fraction has a positive correlation with a eq 2 the number ratio of nv type particles to v type particles the n v number ratio has a positive correlation with a eq and 3 for explosions accompanied with bl type earthquake swarms the n v number ratio has a negative correlation with the duration of the bl swarms bl swarms refer to the phenomenon of numerous bl type earthquakes occurring within a few days prior to an increase in explosive activity kamo k 1978 some phenomena before the summit crater eruptions at sakura zima volcano bull volcanol soc japan 23 53 64 the positive correlation between the n v number ratio and a eq could indicate that a large amount of separated gas from fresh magma results in a large a eq plagioclase microlite textual analysis of nv type particles from five explosive events without bl swarms shows that the plagioclase microlite number density mnd and the l w length width ratio have a positive correlation with a eq a comparison between textural data mnd l w ratio crystallinity and the result of a decompression induced crystallization experiment couch s sparks r s j carroll m r 2003 the kinetics of degassing induced crystallization at soufriere hills volcano montserrat j petrol 44 1477 1502 suggests that a plagioclase microlite texture of volcanic ash from eruptions without bl swarms could be generated by a decompression of 100 160 mpa if the mnd is controlled by the water exsolution rate from melt the positive correlation between the mnd and a eq may suggest that a eq becomes large when the effective decompression is large and the water exsolution rate is high from 6 2 10 5 to 1 9 10 4 wt s the estimated magma ascent rate ranges from 0 11 to 0 35 m s which is one order of magnitude faster than that of an effusive eruption and one to three orders slower than those for a sub plinian eruption this suggests that the ascent rate of magma plays an important role in the occurrence of vulcanian eruptions we propose a simple model for vulcanian eruptions at sakurajima volcano that takes into account the correlation between the s fraction and a eq

textural heterogeneities in pumices from the climactic eruption of mount pinatubo 15 june 1991 and implications for magma ascent dynamics the climactic event of mount pinatubo represents one of the most thoroughly studied eruptions of the century and has provided important insights into the dynamics of explosive volcanism we have performed detailed textural analyses of the white and gray pumices of the plinian and pyroclastic flow deposits and found that differences in color and clast density reflect different crystal and vesicle amounts and size distributions white pumice has higher vesicularity deformed and highly coalesced vesicles with thin walls euhedral phenocrysts and microlite free groundmass gray pumice shows lower vesicularity wider ranges in vesicle number density limited coalescence vesicles with thick walls that are less deformed phenocrysts and microphenocrysts with abundant solution pitting and groundmass containing ubiquitous microlites and crystal fragments the presence of white and gray pumice varieties and the broad range in vesicularity and vesicle number density that characterizes both of them appear to record the complexities of conduit processes such as magma vesiculation and fragmentation and the development of conduit regions marked by different rheological behaviors in particular the results of this study suggest the likely importance of intense shear and viscous dissipation at the conduit walls a mechanism that may be responsible for the creation and discharge of the gray pumice of this eruption along with the dominant white variety

deep sea fragmentation style of havre revealed by dendrogrammatic analyses of particle morphometry in 2012 the eruption of deep sea volcano havre produced an abundance of fine ash at a depth of 1000 m below sea level in this study the 2d shapes of havre ash grains retrieved from the seafloor were compared quantitatively with those of particles generated in a suite of different fragmentation experiments which used remelted rhyolitic rock and pumice from the eruption site a new statistical data analysis technique denoted as dendrogrammatic analysis of particle morphology dapm is introduced it is designed to compare large numbers of morphometric data sets containing shape information for a set of ash particles to group them by morphological similarities and to visualize these clusters in a dendrogram further steps involve t tests and equivalence tests and reveal morphometric differences as well as matching features the dapm suggests that the majority of havre ash was thermohydraulically produced by induced fuel coolant interaction a subset of ash particles features an elongated tube morphology their morphometry matches that of particles that were experimentally produced by a combination of shearing and quenching and we infer that the natural particles were formed by synextrusive ash venting

surface wave dispersion crustal structure and sediment thickness variations across the barents shelf summary rayleigh and love wave group velocities were determined for 21 paths across the barents shelf those group velocities exhibit regional variations of 1 0 km or more at short periods depending upon the location of the path within the shelf only two different crustal shear velocity models beneath sedimentary layers are required however to explain all of the group velocity data one model pertains to most of the shelf from a longitude near the eastern coast of svalbard to novaya zemlya the other pertains to a 200 or 300km wide region at the western edge of the shelf shear velocities in the upper crust of the western region are significantly higher and the crust is much thinner than they are for the rest of the shelf that region is known to have moved to its present prosition from a point several hundred kilometres to the north during the caledonian orogeny surface wave group velocities within each of the two regions are strongly influenced by sediments which have accumulated in basins within the barents shelf some of these basins in the southern portion of the shelf may be 10 km or more in thickness

mapping of crustal heterogeneity in the north sea basin via the propagation of lg waves summary the propagation of lg waves within the continental area in and around the north sea basin shows strong dependence on the path between source and receiver paths lying within the british isles and norway show very clear lg phases but paths which cross the graben zone lying in the middle of the north sea basin have very weak lg over 150 paths have now been studied across the region and the character of the lg wave has been described by comparison with the size of the sn phase for shallow events this gives a stable measure of the efficiency of lg propagation the regions which appear to block the transmission of lg are quite localized in the middle of the north sea in the region with graben structures and extend on into the netherlands a weak zone of poor transmission appears to be associated with the oslo graben with the relatively dense areal coverage provided by paths crossing the basin with a wide range of azimuths it is possible to attempt to invert for the pattern of crustal heterogeneity which gives rise to the observed character of the lg propagation an interative scheme has been devised to find the propagation properties in cells of a 1 x 1 grid and a well defined map of the heterogeneity is obtained the strongest heterogeneities correlate very well with the major tectonic features of the viking and central grabens running north south through the basin where the path coverage is greatest the position of the heterogeneity can be tightly constrained and lies within a rather narrow zone around 100 km wide despite the number of paths employed it is not possible to obtain this level of resolution over the whole region seen in the light of theoretical studies of lg wave transmission the heterogeneity map gives an indication of the strength of horizontal gradients in seismic properties in the crust these are most likely to be associated with crustal thinning beneath the graben structures

petrologic constraints on the decompression history of magma prior to vulcanian explosions at the soufriere hills volcano montserrat abstract a series of 88 vulcanian explosions occurred at the soufriere hills volcano montserrat between august and october 1997 conduit conditions conducive to creating these and other vulcanian explosions were explored via analysis of eruptive products and one dimensional numerical modeling of magma ascent through a cylindrical conduit the number densities and textures of plagioclase microlites were documented for twenty three samples from the events the natural samples all show very high number densities of microlites and 50 by number of microlites have areas 2 pre explosion conduit conditions and decompression history have been inferred from these data by comparison with experimental decompressions of similar groundmass compositions our comparisons suggest quench pressures 13 75 mpa each during ascent from chamber to surface values are consistent with field studies of the same events and statistical analysis of explosion time series data the microlite volume number density trend with depth reveals an apparent transition from growth dominated crystallization to nucleation dominated crystallization at pressures of 7 mpa and lower a concurrent sharp increase in bulk density marks the onset of significant open system degassing apparently due to a large increase in system permeability above 70 vesicularity this open system degassing results in a dense plug which eventually seals the conduit and forms conditions favorable to vulcanian explosions the corresponding inferred depth of overpressure at 250 700 m near the base of the dense plug is consistent with depth to center of pressure estimated from deformation measurements here we also illustrate that one dimensional models representing ascent of a degassing crystal rich magma are broadly consistent with conduit profiles constructed via our petrologic analysis the comparison between models and petrologic data suggests that the dense conduit plug forms as a result of high overpressure and open system degassing through conduit walls

paleoclimatic changes in southern africa during the intensification of northern hemisphere glaciation evidence from odp leg 175 site 1085 abstract pliocene age sediments from ocean drilling program leg 175 site 1085 a and b in the cape basin were analyzed to investigate the impact of the intensification of northern hemisphere glaciation inhg on the south atlantic benguela current system from 4 to 2 ma proxies for productivity concentrations and mass accumulation rates of total organic carbon carbon to nitrogen ratios percent calcium carbonate and percent biogenic silica as well as weight percent sand a proxy for preservation or winnowing peak at 3 2 3 0 2 4 and 2 25 ma normative calculations of allied trace and major elemental determinations indicate synchronous increases in productivity peaks as well as high concentrations and accumulations of terrigenous sediments coeval increases in hemipelagic sedimentation and productivity indicators could be the result of enhanced eolian sedimentation resulting from strengthened winds leading to elevated rates of upwelling and enhanced productivity however rapid burial as indicated by high sedimentation rates could also enhance preservation the very high concentrations 30 and accumulations up to 60 g cm 2 kyr limit the likelihood that eolian sedimentation was the only transport mechanism invoking an additional fluvial source rapid burial by either eolian or fluvial transport links these intervals of enhanced preservation and productivity with continental climate changes resulting from 1 increased winds and or dust availability due to higher aridity in the namibia northern south africa region 2 lowered sea level related to increased ice volume 3 increased sediment load due to wetter conditions in the continental interior or 4 some combination peaks at 3 2 2 4 and 2 25 ma are coincident with maximum precession suggesting a link between hemipelagic sedimentation and enhanced monsoonal circulation over southern africa the site 1085 sedimentary record during the inhg seems to be controlled by low latitude processes linked to precession rather than hig latitude processes

towards fast and routine analyses of volcanic ash morphometry for eruption surveillance applications the morphometry of volcanic ash produced by explosive eruptions yields ample information on fragmentation processes e g magmatic vs magma water interactions and on transport and sedimentation mechanisms most previous works on volcanic clast morphometry focused on the apparent 2d projected shape of ash grains here termed apash to infer processes and eruptive styles however textural analyses of ash grains has remained a long and tedious task that made such approaches inappropriate for eruption surveillance duties in this work we show that new technological advances on automated dispersion of granular materials imaged with a camera coupled microscope and enhanced computer capabilities enable fast and high resolution image acquisition of thousands of ash grains that resolve this limitation with a morpho grainsizer designed for such fast and routine measurements we perform a series of apash analyses on selected ash fractions of tephra deposits from known eruptive styles we record the size aspect ratio circularity and convexity of apash images and assess resolution reproducibility minimum population size and total analytical duration and offer recommendations for the reporting of apash data for inter laboratory comparisons to avoid fractal geometry concerns our analyses are carried out at constant size range 250 300 m and optical magnification 5 on 3000 grains per samples collected from homogenized samples results from the andesitic 1999 ongoing eruption of tungurahua volcano ecuador show that ash particles from the moderate 2001 phase are relatively equant and convex in shape while the stronger 2006 subplinian phase produced ash grains with more elongated less circular and less convex apash signatures ash grains from a basaltic scoria cone forming eruption show even more ragged apash characteristics overall our protocol allows obtaining accurate and reproducible morphometric measurements that reveal subtle variations of the morphological signature and the short duration 1 8 hours of the whole analytical process renders high resolution analyses of ash shape achievable for volcano surveillance applications this research ultimately aims to set up a morphometric database of apash results for well defined eruptive styles in order to interpret on a short term basis any apash data from active volcanoes for monitoring purposes

regional seismic waveform discriminants and case based event identification using regional arrays abstract in this study we have investigated regional phase spectra spectral ratios and amplitude ratios as discriminants for case based regional seismic event identification using western norway explosions and earthquakes recorded at the noress array all events were in a comparable local magnitude range 2 to 3 and distance range 300 to 500 km from the noress array thus the propagation paths from the earthquakes and explosions were similar but not identical many of the events previously not identified as blasts had time independent spectral modulations indicative of ripple firing these include a number of offshore events which appear to be underwater explosions the modulations for the offshore events may have been caused by bubble pulse interference effects and or underwater reverberations amplitude ratios of pn sn and pn lg measured off of incoherent beams were examined on a region by region basis for a group of confirmed blasts events strongly suspected to be earthquakes and other events that had time independent spectral modulations the blast and earthquake groups were well separated by both ratios with the blasts producing more p wave energy relative to s wave energy than the earthquakes however the lg spectral ratio provided no separation this result agrees with others in shield regions that amplitude ratios between compressional and shear waves discriminate better than low frequency to high frequency spectral ratios for individual phases the converse holds for events in orogenic regions such as the basin and range tectonic region of the western united states for reasons that are not well understood but may relate to differences in crustal attenuation this fact argues in favor of using a case based approach to event identification where events are identified by comparing them to previous events or cases that occur in the same or similar tectonic environments in the absence of a well understood unifying theory for regional event identification characterizing events on the basis of geophysical and signal similarities to previous cases may be the only way of providing meaningful event identifications

lg coda q variation across eurasia and its relation to crustal evolution an extensive set of broadband and short period seismograms from regional seismic events has been used to derive a tomographic image of broad scale variations of lg coda q for most of eurasia the image indicates that lg coda q at a frequency of 1 hz varies between about 200 and 1000 lowest q values 800 are found in four regions the east european shield the siberian shield the indian shield and the southwestern and oldest portion of the altaid belt the pattern of lg coda q variation across eurasia suggests that crustal q in any region is directly proportional to the length of time that has elapsed since the most recent episode of large scale compressional tectonic activity or upper mantle heating that occurred there we interpret the low q as resulting largely from hydrothermal fluids generated by tectonic activity or heating that now reside in cracks and permeable portions of the eurasian crust crustal q increases with time after that activity as fluids are lost to the surface or absorbed by retrograde metamorphism

comparative analysis of teleseismic p coda and lg waves from underground nuclear explosions in eurasia this study has investigated the characteristics of p coda and lg waves recorded at the norsar and graefenberg arrays from presumed nuclear explosions in western russia lg waves were found to be more energetic relative to the preceding coda in the mid band frequency range 0 2 to 1 0 hz recorded on the broadband graefenberg instruments than in the high frequency band 0 6 to 3 0 hz at either array comparison of the log rms coda envelopes at the two arrays revealed that the early high frequency p coda at norsar is stronger relative to lg than at graefenberg also the amplitudes of p waves and early p coda waves within 55 sec after p recorded at the centers of the norsar subarrays are positively correlated indicating the effect of focusing defocusing on the p and coda waves however for subarrays which cluster spatially there is an inverse correlation which seems to be due to scattering attenuation suggesting that much of the early p coda is generated by local p wave scattering beneath the norsar array also the norsar coda envelopes flatten between about 219 and 352 sec after p whereas the graefenberg coda decays monitonically from the p wave out to the sn and lg arrival times the standard deviation across the norsar array of the log rms amplitudes of lg and s coda phases are higher by about 0 05 to 0 06 magnitude units than those of p coda phases these differences are probably related to differential site and propagation path effects on p s and lg waves in the p coda the flattening of the norsar coda envelopes appears to be a burst of energy not associated with any known regional phases the burst may be caused by forward scattering of lg to p waves along the path between the source and norsar based on a regional travel time curve for western russia the start and end time of the coda flattening is consistent with lg to p scattering in the distance range of 23 to 29 from norsar which brackets the ural mountains the lg propagation path from the source region to graefenberg does not appear to traverse the ural mountains which may explain why the coda flattening is not observed at graefenberg

furthering the investigation of eruption styles through quantitative shape analyses of volcanic ash particles abstract volcanic ash morphology has been quantitatively investigated for various aims such as studying the settling velocity of ash for modelling purposes and understanding the fragmentation processes at the origin of explosive eruptions in an attempt to investigate the usefulness of ash morphometry for monitoring purposes we analyzed the shape of volcanic ash particles through a combination of 1 traditional shape descriptors such as solidity convexity axial ratio and form factor and 2 fractal analysis using the euclidean distance transform edt method we compare ash samples from the hydrothermal eruptions of iwodake japan in 2013 tangkuban perahu indonesia in 2013 and marapi sumatra indonesia in 2015 the dome explosions of merapi java indonesia in 2013 the vulcanian eruptions of merapi in 2010 and tavurvur rabaul papaua new guinea in 2014 and the plinian eruption of kelud indonesia in 2014 particle size and shape measurements were acquired from a particle size analyzer with a microscope camera attached to the instrument clear differences between dense blocky particles from hydrothermal or dome explosions and vesicular particles produced by the fragmentation of gas bearing molten magma are well highlighted by conventional shape descriptors and the fractal method in addition subtle differences between dense blocky particles produced by hydrothermal explosions dome explosions or quench granulation during phreatomagmatic eruptions can be evidenced with the fractal method the combination of shape descriptors and fractal analysis is therefore potentially able to distinguish between juvenile and non juvenile magma which is of importance for eruption monitoring

preeruptive magma viscosity an important measure of magma eruptibility 1 using a compilation of melt compositions meltwater contents temperatures and phenocryst contents the preeruptive viscosities under magma reservoir conditions are calculated for 83 erupted magmas the basaltic to rhyolitic magmas have preeruptive viscosities over the range 101 to 108 pa s although bulk sio2 content has often been used as a qualitative measure of preeruptive magma viscosity the bulk sio2 content shows a weak correlation with magma viscosity correlation coefficient r 0 5 because of a wide range of phenocryst contents from 0 to 50 vol andesitic magmas have viscosities ranging from 102 to 107 pa s which are lower or higher than those of phenocryst poor rhyolitic magmas with 105 to 106 pa s focusing on andesitic to rhyolitic magmas the r between bulk sio2 contents and magma viscosities changes to 0 1 in contrast the melt only sio2 content from a basaltic to rhyolitic melt shows a good linear correlation with melt only viscosity r 0 9 although most of the calculated viscosities of erupted magmas fall below 106 pa s as consistent with the previous compilation study this paper describes 20 examples of highly viscous magmas with 106 pa s in most cases composed of mixtures of high silica rhyolitic melt 75 79 wt sio2 and abundant phenocrysts 30 55 vol in these highly viscous magmas 9 examples have erupted following the precursory eruption of less viscous magma suggesting that precursory dike propagation and conduit formation by the less viscous magma with 106 pa s induced the following eruption of less eruptible highly viscous magmas

breadcrust bombs as indicators of vulcanian eruption dynamics at guagua pichincha volcano ecuador vulcanian eruptions are common at many volcanoes around the world vulcanian activity occurs as either isolated sequences of eruptions or as precursors to sustained explosive events and is interpreted as clearing of shallow plugs from volcanic conduits breadcrust bombs characteristic of vulcanian eruptions represent samples of different parts of these plugs and preserve information that can be used to infer parameters of pre eruption magma ascent the morphology and preserved volatile contents of breadcrust bombs erupted in 1999 from guagua pichincha volcano ecuador thus allow us to constrain the physical processes responsible for vulcanian eruption sequences of this volcano morphologically breadcrust bombs differ in the thickness of glassy surface rinds and in the orientation and density of crack networks thick rinds fracture to create deep widely spaced cracks that form large rectangular domains of surface crust in contrast thin rinds form polygonal networks of closely spaced shallow cracks rind thickness in turn is inversely correlated with matrix glass water content in the rind assuming that all rinds cooled at the same rate this correlation suggests increasing bubble nucleation delay times with decreasing pre fragmentation water content of the melt a critical bubble nucleation threshold of 0 4 0 9 wt water exists below which bubble nucleation does not occur and resultant bombs are dense at pre fragmentation melt h2o contents of 0 9 wt only glassy rinds are dense and bomb interiors vesiculate after fragmentation for matrix glass h2o contents of 1 4 wt rinds are thin and vesicular instead of thick and non vesicular a maximum measured h2o content of 3 1 wt establishes the maxi mum pressure 63 mpa and depth 2 5 km of magma that may have been tapped during a single eruptive event more common h2o contents of 1 5 wt suggest that most eruptions involved evacuation of 1 5 km of the conduit as we expect that substantial overpressures existed in the conduit prior to eruption these depth estimates based on magmastatic pressure are maxima moreover the presence of measurable co2 17 ppm in quenched glass of highly degassed magma is inconsistent with simple models of either open or closed system degassing and leads us instead to suggest re equilibration of the melt with gas derived from a deeper magmatic source together these observations suggest a model for the repeated vulcanian eruptions that includes 1 evacuation of the shallow conduit during an individual eruption 2 depressurization of magma remaining in the conduit accompanied by open system degassing through permeable bubble networks 3 rapid conduit re filling and 4 dome formation prior to the subsequent explosion an important part of this process is densification of upper conduit magma to allow repressur ization between explosions at a critical overpressure trapped pressurized gas fragments the nascent impermeable cap to repeat the process

role of precursory less viscous mixed magma in the eruption of phenocryst rich magma evidence from the hokkaido komagatake 1929 eruption during the 1929 activity of hokkaido komagatake volcano the plinian eruption of a phenocryst rich andesite was preceded by a small eruption of more mafic magma formed by magma mixing a similar eruption sequence has been reported for some other eruptions pallister et al 1996 venezky and rutherford 1997 suggesting that eruption of a mixed magma is a precursor of phenocryst rich magmas for the purpose of understanding the tapping processes of the phenocryst rich magma chamber we investigated the temporal variation in the erupted magma and estimated the viscosity and density of the end member and mixed magmas with constraints drawn from petrography for the precursory mixed magma we estimate 33 4 vol phenocrysts andesitic dacitic melt composition 3 wt h2o content and temperature of 1040 c in comparison for the climactic silicic end member magma we estimate 48 3 vol phenocryst high silica rhyolitic melt 3 wt h2o and temperature of 950 c respectively the mafic end member magma which was not erupted is thought to be an almost aphyric basaltic andesitic magma based on mass balance calculation of the phenocryst content the proportion of the mafic end member magma component in the mixed magma was calculated to be 20 40 wt on the basis of these data we estimate magma viscosities of 103 9 106 9 and 102 0 pa s for the mixed silicic end member and mafic end member magmas respectively the calculated density differences among these magmas are inconsequential when possible errors are considered we calculate the minimum excess pressure required for dike propagation to be 31 mpa for the silicic end member magma and 8 mpa for the mixed magma using the estimated viscosity and dike propagation model of rubin 1995 if we assume that excess pressure is limited by the wall rock strength of the magma chamber excess pressure retainable in the magma chamber is less than ca 20 mpa this suggests that the mixed magma was able to ascend to the surface without freezing whereas the viscous silicic end member magma could not the formation and precursory eruption of the mixed magma are therefore effective and necessary initiation processes for the phenocryst rich viscous magma eruption

constraints on cooling rates and permeabilities of pumice in an explosive eruption jet from colour and magnetic mineralogy abstract in explosive volcanic eruptions vesicular magma droplets produced by fragmentation are propelled into the atmosphere where they are chilled to form pumices the thermal history of droplets and the permeability of their internal bubble networks determine how much they are deformed in the eruption jet and hence what information pumices record about the state of the magma at fragmentation we study these aspects of the minoan plinian eruption of santorini volcano by quantifying the rate of oxidation reactions that took place when air entered the hot magma fragments in our experiments white minoan pumices were heated for minutes to hours between 600 and 850 c either in air or in an atmosphere with an oxygen fugacity at the ni nio buffer pumices were unchanged by heating at ni nio those heated in air often became pink to dark pink depending on heating time and their curie temperatures as determined by magnetic susceptibility measurements increased we use oxidation rates deduced from these experiments in conjunction with calculations of the rate of conductive cooling and of the rate at which air can enter a pumice to constrain the conditions experienced by pumices during the eruption natural minoan pumices less than about 5 cm in radius are white whereas larger ones often have white rims and pink interiors with curie temperatures higher than those of white material we infer that small pumices were cooled before being oxidized and that oxidation of the interiors of large clasts mostly took place during flight at temperatures within a few tens of degrees of magmatic values white rims of large pumices despite being permeable were cooled before oxidation could occur permeability developed in the liquid state but did not develop early enough with respect to cooling or was not large enough to allow extreme oxidation we give measurements of pumice permeabilities that should be close to magmatic values

thermal effects on the physical properties of limestones from the yucatan peninsula abstract the effect of thermal degradation on the compression strength ultimate compression strain color and mass loss of four limestones extracted from the yucatan peninsula was studied the samples sets were independently subjected to 25 100 200 300 400 500 and 600 c during one hour for each temperature the thermal degradation of the investigated rocks and their chemical composition microstructure and porosity is also investigated the results show a correlation between the mechanical properties thermal behavior the presence of minerals and porosity in each type of limestone rock the variation of the limestones color with temperature was described in terms of standard space color parameters and their diffuse reflectance

effect of low q sediments and crustal q on lg attenuation in the united states q values for lg and its coda exhibit significant lateral variations across the united states some of which correlate with known variations in the thickness of shallow sedimentary layers the possibility that all variations in lg attenuation are caused by changes in the thickness of shallow sediments is investigated by computing multi mode synthetic seismograms for crustal models with various thicknesses of low q sediments using crustal velocity and q models obtained in previous studies and reasonable values for the thicknesses and intrinsic q values of sediments in several regions we find that the variation of lg q values between the appalachian and rocky mountains can easily be produced by varying accumulations of sandstone and shale of mesozoic age and younger by contrast throughout most of the western united states neither the low lg q values which have been observed nor the regional variation of those values can be explained by accumulations of low q sediments instead their explanation appears to require low and laterally varying values of q in the crystalline crust

on the nature of the teleseismic pn phase observed on the ultralong range profile quartz russia the most prominent secondary phase observed in the records from the ultralong profile quartz crossing northern eurasia is the high frequency long range teleseismic pn this phase propagating with the group velocity of about 8 1 to 8 2 km sec to over 3000 km distances dominates the records within the frequency range above about 5 hz within this teleseismic pn we distinguish onsets of several branches having higher apparent velocities between 8 5 and 8 6 km sec using travel time modeling techniques we associate the teleseismic pn with whispering gallery modes traveling within the top 160 km of the mantle the long incoherent coda of this phase results from scattering and from reverberations of seismic waves within the crust the contrast in frequency contents between the teleseismic pn and deeper refracted and reflected phases is explained by the increase of inelastic attenuation within the prominent low velocity zone lvz below the depth of about 150 km

propagation characteristics of short period sn and lg in the middle east abstract we present propagation characteristics of the short period regional seismic phases sn and lg for the middle east digital waveform data from the iranian long period array ilpa and the global seismic network gsn stations abkt alibek turkmenistan anto ankara turkey keg kottamya egypt and kiv kislovodsk georgia were used in this study three component rotated seismograms were bandpass filtered 0 5 to 5 0 hz and examined for the presence and strength amplitude of sn and lg relative to the pn and pg phases the short period sn and lg waveforms show great variability sn propagates efficiently for paths crossing the mediterranean black and caspian seas as well as for paths passing north of the greater caucasus alborz and kopet dag mountains generally sn is not observed or propagates inefficiently for paths with segments beneath the turkish plateau and northern iranian plateau consistent with a previous study by kadinsky cade et al 1981 this study finds that the zone of inefficient sn propagation coincides with a zone of low pn velocities pn tomography lg is consistently not observed for paths crossing the mediterranean black and caspian seas lg propagates efficiently for paths crossing the northern arabian shield generally lg propagates efficiently for paths contained within the turkish iranian plateau paths in the middle east greater than about 2000 km consistently show no or little lg energy the correspondence of zones of inefficient sn propagation and low pn velocities suggests that high temperatures and partial melt lie at shallow depths in the upper mantle beneath the turkish plateau and northern iranian plateau the presence of water presumably due to subduction of hydrated oceanic crust lowers the solidus temperature and enhances partial melt lg is blocked by oceanic crustal structures as well as by structural and topographic variations associated with tectonic boundaries

the 1759 earthquake in the bekaa valley implications for earthquake hazard assessment in the eastern mediterranean region analysis of macroseismic data based on primary sources for large though infrequent historical earthquakes ms 6 5 that occurred along an approximately 350 km long segment of the northern part of the dead sea fault system primarily in lebanon and syria for the period 1100 1988 reveals the following 1 ten events occurred in three relatively short periods tens of years with repeat times of 200 350 years 2 the events most probably broke this north segment of the dead sea fault system possibly including the westernmost segment of the east anatolian fault system near the border between syria and turkey 3 the lack of such large events during the past 100 years should not be interpreted to minimize potential earthquake hazard in this region and 4 the ms 7 plus earthquake on november 25 1759 almost certainly produced surface faulting probably along the yammouneh fault in the bekaa valley and caused heavy destruction with great loss of life in numerous villages and towns including safad damascus beirut and baalbek this main event was preceded by a ms 6 plus foreshock on october 30 1759 in the southern part of the epicentral area of the main shock near the towns of safad and qunaitra which were almost totally destroyed with considerable loss of life

regional seismic phases across the ligurian sea lg blockage and oceanic propagation abstract the ligurian sea west mediterranean is an example of a narrow oceanic basin which stops lg wave propagation locating the exact point of lg extinction was one of the goals of the sisbalig ii seismological experiment which deployed both inland stations in provence and corsica and ocean bottom seismometers obs in the ligurian sea a precise analysis of all available regional seismological data using energy diagrams reveals the role of the provence margin in the lg phase extinction for seismic events occurring in the southwestern alps the extinction is not a progressive phenomenon along oceanic travel path but occurs in a narrow zone of about 20 km width in the vicinity of the provence margin the extinction is accompanied by the generation of two types of diffracted waves the mantle phase sn v s 3 5 km s 1 and very low velocity s waves appearing both near the provence margin and in the middle of the ligurian basin we performed numerical simulation of sh wave propagation in two dimensional 2d media to investigate the mechanisms of lg extinction the simulations confirm that simple crustal thinning at the continent ocean boundary alone cannot cause the observed extinction of lg waves although the change in moho depth does induce the diffraction of a part of the incident lg wavetrain into mantle waves giving rise to the sn phase on the other side of the basin the existence of soft oceanic sediments is a very important factor in explaining the extinction as a large part of the lg energy is transferred into slow s waves trapped in the sediments moreover comparison of synthetic seismograms and observed data shows that the observed two packets of low velocity waves are an indication of the asymmetry of the ligurian basin with the existence of two shallow basins with very low s velocity 1 0 km s 1 near the provence margin and in the middle of the ligurian sea however the attenuation of lg waves in the vicinity of the provence margin is more abrupt in the data than in the simulations this discrepancy could result from the complex geometry of the continent ocean transition zone and its possibly very diffractive nature two factors that could add to lg extinction at the margin and are impossible to take into account in models

sedimentary basins and the blockage of lg wave propagation in the continents the phenomenon of lg blockage where lg is strongly attenuated by crustal heterogeneities poses a serious problem to ctbt monitoring because lg is an important seismic phase for discrimination this paper examines blockage in three continental regions where the lg blockages may be caused by large enclosed sedimentary basins along the propagation path the barents sea basin blocks lg propagation across the barents sea from the russian nuclear test sites at novaya zemlya to scandinavian stations also early lg waves are observed in sn codas on norsar noress and arcess recordings of novaya zemlya explosions where direct lg is blocked early lg waves may have resulted from sn to lg mode conversion at the contact between the barents basin and the kola peninsula the northern and southern caspian sea basins also block lg waves from pnes and earthquakes perhaps due to thick low velocity low q sediments replacing the granitic layer rocks in the crust lg blockage has also been observed in the western mediterranean levantine basin due to low q sediments and crustal thinning a basin capture model is proposed to explain lg blockage in sedimentary basins in this model shear waves that reverberate in the crust and constitute the lg wave train are captured delayed and attenuated by thick low velocity sediments that replace the granitic layer rocks of the upper crust along part of the propagation path sn waves which propagate below the basin would not be blocked and in fact the blocked lg waves may be diverted downward into sn waves by the low velocity sediments in the basin

color development of iron oxides during rhyolite weathering over 52 000 years abstract to evaluate the behavior of iron during weathering colors of four rhyolites with different degrees of weathering eruption ages 1 1 1 8 26 and 52 ka were studied because the initial chemical compositions of the four lavas were similar to one another it is considered that the four lavas had similar whitish color at the time of eruption however at present more weathered lavas have more yellow brownish color indicating the formation of iron hydr oxides or fe bearing clays in order to characterize those weathering products spectro colorimetry and differential dissolution method were combined the colors of 1 1 and 1 8 ka lavas are close to the color of ferrihydrite whereas those of 26 and 52 ka lavas approach to the color of goethite this indicates that the initial stage of weathering is dominated by the formation of ferrihydrite and with increasing weathering the transformation from ferrihydrite to goethite proceeds it is estimated from the change of rock color that the half life for the transformation reaction of ferrihydrite to goethite in the field is 1100 years which is at least 80 times longer than previously reported values based on laboratory experiments the presence of impurities such as si and al in ferrihydrite can be a possible origin for retarding the transformation rate in the field

conduit process in vulcanian eruptions at sakurajima volcano japan inference from comparison of volcanic ash with pressure wave and seismic data to elucidate the conduit processes controlling the amplitude of air pressure waves apw from vulcanian eruptions at the sakurajima volcano japan we examine ash particles emitted by eruptions preceded by swarms of low frequency b type earthquakes bl swarms we measure the water content of glassy ash an indicator of shallow magma storage pressure and vesicle textures such as vesicle number density vnd these data allow us to reconstruct the shallow conduit by comparing vesicularity with inferred pressure and therefore depth of magma storage the results show that vnd increases with depth implying formation of a dense outgassed magma cap underlain by more vesicular less outgassed magma the vnd and water content in the glassy ash positively correlate with the duration of bl swarms suggesting that such seismic signals reflect upward migration of deep gas and vesicle rich magma finally it is determined that apw positively correlates with vnd suggesting that the amplitude of the air pressure waves is controlled by the amount of accumulated gas and bubble rich magma below the dense magma cap

measurements of dielectric constant of volcanic ash erupted from five volcanoes in japan the dielectric properties of volcanic ash from five volcanoes in japan were measured by using the transmission reflection coaxial line method of a network analyzer in a frequency range from 3 to 13 ghz the measured permittivity of powdered ash was then converted to that of solid ash dense rock equivalent with the aid of effective medium theory the values of solid ash relative permittivities for the five volcanoes revealed that they were in a relatively narrow range their real part was generally in a range from five to six in magnitude while their imaginary part was in a range from 0 08 to 0 18

color change processes of a plinian pumice and experimental constraints of color change kinetics in air of an obsidian colors of plinian pumices were measured by spectrocolorimetry and their quantitative color parameters in the l a b color space were determined a series of heating experiments of obsidian was conducted to simulate the color change processes of rhyolitic glasses in these experiments following three stages of color change processes were observed stage i showed a rapid b yellowishness increase associated with fast dehydration controlled by water diffusivity dwater in stage ii a reddishness increase was accompanied by fe2 decrease both a increase and fe2 decrease can be simulated by a diffusion model obtained diffusivity doxidation were about two orders of magnitude smaller than dwater the a value increase after the oxidation in stage iii appeared to be quasi linear with time indicating the zeroth order reaction corresponding to the formation of hematite like structures in rhyolitic glasses the diffusion limited a increase model in stage ii was applied to a natural plinian pumice fall unit to evaluate time periods of color change processes through oxidation by air of fragmented rhyolitic materials

temporal variation in volcanic ash texture during a vulcanian eruption at the sakurajima volcano japan abstract we discuss the process of vulcanian eruptions on the basis of temporal variation of the texture of andesitic volcanic ash ejected from the sakurajima volcano japan falling ash samples were collected every 2 50 min for petrological analysis these ash samples contained juvenile particles altered rock fragments and fragments of crystals here we subdivide the juvenile particles into blocky vesicular and fluidal particles because the crystallinities of the vesicular and fluidal particles are lower than those of the blocky particles we refer to the vesicular and fluidal particles as low crystallinity particles lcps and the blocky particles as high crystallinity particles hcps the temporal variation observed in ash texture reveals that the ratio of lcps to hcps lcp hcp was small at the initiation of the eruption and increased as the eruption progressed the higher microlite crystallinity in the hcps indicates crystallization under lower pressures for longer durations relative to the lcp magma the increase of the lcp hcp ratio during a single vulcanian eruption suggests that the emission of less viscous gas rich magma under a cap rock increases as the eruption progresses such temporal variation in ash texture reflects the transition from a sudden explosion to continuous ash emission during a vulcanian eruption

influence of particle aggregation on deposition of distal tephra from the may 18 1980 eruption of mount st helens volcano the may 18 1980 eruption of mount st helens msh produced an extensive ashfall deposit in washington idaho and montana with a minumum volume of 0 55 km3 tephra an unusual feature of the deposit is the occurrence of a second thickness maximum 325 km ene of msh near ritzville washington grain size and component abundance analysis of samples along the main dispersal axis indicates that ash in this region is very fine grained mean size 22 m poorly sorted polymodal and rich in glass shards and pumice fragments a computer simulation of ash fallout from an atmospherically dispersed eruption plume was developed to evaluate various hypotheses for the origin of the distal ash characteristics particularly the thickness versus distance relationship the model was constrained by observations of the eruption column height elevation of major ash transport lateral spreading of the eruption plume and atmospheric wind structure in the vicinity of msh results of different simulations indicate that the second thickness maximum cannot be attributed to either decreased wind velocities over central washington or injection of fine ash above the horizontal wind velocity maximum near the tropopause for the model to fit the observed characteristics of the deposit significant particle aggregation of ash finer than 63 m must be invoked the best fit occurs when ash less than 63 m is aggregated into particles several hundred microns in diameter with a settling velocity of 0 35 m s support for this process comes from the observation and collection of fragile ash clusters of similar size which fell at pullman washington during the may 18 eruption sorem 1982 the premature fallout of fine ash as particle aggregates is a fundamental process in the origin of the grain size characteristics variations in component abundances and thickness versus distance relationship of the may 18 msh ash fall deposit

finite difference modelling of lg blockage summary a p sv finite difference method is used to calculate seismic wave propagation in the crust and upper mantle the anelastic effect is excluded from the calculation and therefore any wavefield changes can be attributed to either geometrical or structural effects snapshots of displacement distrihutions within and synthetic seismograms at the free surface of different models revealed that the inclusion of a water column over an appropriate section of the crust is necessary to explain the severe lg attenuation widely observed in many parts of the world in particular for an oceanic propagation path they clearly show that the effect of a water column over the crust is an important factor in blocking lg piupagation a propagation path of more than 100 km over an oceanic crust can completely block the transmission of lg waves originating in a neighbouring continental crust the energy of the lg wavetrain is redistributed at continental margins most of the incoming energy is either converted to waves which propagate into the mantle or is backscattered to form waves which propagatc in the continental crust in the opposite direction very little energy is presented at the expected arrival times at the other side of the oceanic crust which is consistent with the lg blocking phenomena simplified 2 d models of the north sea central graben are used to simulate lg propagation in a laterally varying continental path the inclusion of a water column over the crust enhances the attenuation of lg the deeper the water column the greater the attenuation the undulation of sea bottom can further enhance the attenuation of lg

decompression rate of magma at fragmentation inference from broken crystals in pumice of vulcanian eruption abstract decompression rate of magma at fragmentation immediately before or after fragmentation was determined using broken crystals found in the pumices ejected during the vulcanian explosion of sakurajima volcano japan the combined analysis of textural data obtained from the natural pumice samples and a simple model for the crystal fracturing in vesiculating viscous magma indicated a decompression rate 7 0 10 3 7 8 10 4 pa s for the ejecta produced during the later phase of vulcanian explosion this result suggests that the short duration of the vulcanian explosion is controlled by the rapid decrease of the magma ascent rate to a vent an understanding of the control mechanism of the termination of an eruption by precise prediction of the eruption process is essential for both volcanology and hazard mitigation

discrimination of earthquakes and explosions in southern russia using regional high frequency three component data from the iris jsp caucasus network high frequency regional records from small earthquakes magnitude 4 5 and comparable magnitude chemical explosions are analyzed to find a reliable seismic discriminant in southern russia near kislovodsk the digital three compo nent seismograms recorded during 1992 by the caucasus network operated by la mont doherty earth observatory since 1991 in the distance ranges 15 to 233 km are used mean vertical component pg lg spectral amplitude ratios in the band 8 to 18 hz are about 1 3 and 3 2 for earthquakes and explosions respectively in this region we find that the vertical component pg lg spectral ratio in the frequency band 8 to 18 hz serves quite well for classifying these events a linear discriminant function analysis indicates that the pg lg spectral ratio method provides discrimination power with a total misclassification probability of about 7 the pg lg spectral ratios of rotated three component regional records improve the discrimination power of the spectral ratio method over the vertical component pg lg ratios preliminary analysis indicates that distance corrected vertical component pg lg ratios improve the dis crimination power by about 4 over uncorrected ratios but we find that an even better discriminant is the pg lg spectral ratio of the three component regional records corrected for the free surface effect in the frequency band 8 to 18 hz the free surface corrected three component pg lg spectral ratio provides discrimination power with a total misclassification probability of only 2 6 free surface corrected and network averaged pg lg spectral ratios provide transportability of the spectral ratio method to various regions worldwide

color measurements of volcanic ash deposits from three different styles of summit activity at sakurajima volcano japan conduit processes recorded in color of volcanic ash abstract we measured quantitatively colors of volcanic ash deposits erupted from three different styles of summit activity strombolian activity vulcanian explosions and continuous ash venting activity at sakurajima volcano from 1974 to 1985 colors of strombolian ash samples have larger yellow components of their visible spectra b values than those of explosion and continuous venting ash samples colors of explosion ash samples show larger variation in both red and yellow components of their visible spectra a and b values respectively while colors of continuous venting ash samples are in the narrow ranges within colors of explosion ash samples colors of components with lower densities than 3 1 g cm 3 groundmass and phenocrystic plagioclase obtained by magnetic and heavy liquid separation methods are similar to the unseparated bulk ash samples this result suggests that the color variations of ash deposits are mainly originated from the particles composed of groundmass the particles can be classified into three different types of particles with different vesicularity and crystallinity vesicular particle vp dense particle with vesicles dpv and dense particle without vesicles dp analytical results of component proportions chemical compositions of groundmass glasses ferrous iron contents and surface ferric materials show that 1 vp has larger yellow components of the visible spectrum b values and high ferrous iron content and is less crystallized than the dp and dpv 2 dp has larger red and yellow components of its visible spectrum a and b values respectively and involves ferric materials on the surfaces produced by oxidation process and 3 dpv has smaller red and yellow components of its visible spectrum a and b values respectively and involves less ferric materials on the ash surfaces color differences of ash deposits from three different activity styles can be explained by the different mixing ratios of vp dpv and dp during the strombolian activity the vp is a main component in the ash which is formed from relatively less degassed and crystallized magma in the vulcanian explosion and continuous ash venting activity the proportions of dpv and dp in ash are larger than that in the strombolian activity the highly crystallized dp may correspond to a vent cap and dpv to a magma below the cap the color measurements of ash deposits provide information on the pre eruptive processes at the shallower levels of a conduit

frequency dependent attenuation in the crust beneath central france from lg waves data analysis and numerical modeling abstract the quality factor of shear waves in the crust beneath central france is evaluated by the combined use of data analysis and theoretical modeling the measurement is made from lg wave records of 18 earthquakes at 22 short period vertical seismic stations of the french seismic network the geometrical attenuation of lg waves has been calculated through a numerical simulation in a perfectly elastic model the station responses including site effects have been evaluated from lg waves and from records of the seismic noise q has been calculated for a set of 11 frequencies between 0 5 and 10 hz and has been found to be in the form q 290 f 0 52 where f is the frequency the numerical simulation leads to the interpretation of q of lg waves as the mean value of q within the crust theoretical and experimental decay of energy with distance are in very good agreement the lg waves present a different sensitivity to a low q zone whether this zone is associated with the sediments or with scattering in the lower crust the lower crust beneath central france appears from our experimental results to be associated with high values of q

investigation of teleseismic lg blockage and scattering using regional arrays abstract incoherent beamforming and continuous frequency wavenumber polar scan analysis have been applied to study the characteristics of p coda and lg waves recorded at the regional arrays noress and arcess from presumed explosions on the northwestern russian platform and at test sites of novaya zemlya and eastern kazakh also recordings at the graefenburg array near the new geress regional array of novaya zemlya events and an eastern european platform white sea event have been compared with the norwegian array recordings of the same events the white sea explosion at distances of 14 and 22 from noress and graefenburg respectively exhibits lg rms amplitudes on incoherent beams that exceed those of sn however for explosions at novaya zemlya at 20 from noress and eastern kazakh at 30 from arcess the lg waves have much smaller amplitudes than sn in the case of novaya zemlya the lg wave is almost completely blocked at both noress and arcess but is well recorded at graefenburg this reduction of the lg amplitudes relative to those from events at longer distances cannot be accounted for by anelastic attenuation and spreading comparison of propagation paths suggests that the reduction in amplitudes may be due to blockage and scattering of lg in the barents sea analysis of sn coda waves on incoherent beams and continuous frequency wavenumber analysis polar scans suggests that they can result from lg to sn scattering from the coastline between the barents sea and scandinavia recent geologic studies have revealed significant variations in crustal structure in the barents sea basin which may cause the blockage of lg waves from novaya zemlya in the case of eastern kazakh explosions arcess incoherent beams show much reduced lg amplitudes relative to the p coda waves compared with noress even though the distance to arcess 31 is less than that to noress 38 this reduction in lg amplitude at both noress and arcess is accompanied by enhancement of p and sn coda the timing of the coda waves and the propagation paths suggest that lg waves may be partially blocked and scattered by the ural mountains and that the p and sn coda waves may result from lg to pn pn to sn sn to lg and lg to sn conversions from lateral heterogeneities in the crust beneath the urals the blockage is greater for the eastern kazakh to arcess path because it crosses the middle part of the urals whereas the eastern kazakh to noress path crosses the southern part of the urals chain these blockage effects seem to result from sudden lateral variations in shallow crustal structure such as variations in sediment thicknesses in the sedimentary basins of the russian platform and adjacent to the urals these blockage effects can be predicted from geologic studies and need to be accounted for when using lg amplitudes to estimate absolute yields of explosions and for event identification

characterization of the luminance and shape of ash particles at sakurajima volcano japan using ccd camera images we develop a new method for characterizing the properties of volcanic ash at the sakurajima volcano japan based on automatic processing of ccd camera images volcanic ash is studied in terms of both luminance and particle shape a monochromatic ccd camera coupled with a stereomicroscope is used to acquire digital images through three filters that pass red green or blue light on single ash particles we measure the apparent luminance corresponding to 256 tones for each color red green and blue for each pixel occupied by ash particles in the image and the average and standard deviation of the luminance the outline of each ash particle is captured from a digital image taken under transmitted light through a polarizing plate also we define a new quasi fractal dimension d qf to quantify the complexity of the ash particle outlines we examine two ash samples each including about 1000 particles which were erupted from the showa crater of the sakurajima volcano japan on february 09 2009 and january 13 2010 the apparent luminance of each ash particle shows a lognormal distribution the average luminance of the ash particles erupted in 2009 is higher than that of those erupted in 2010 which is in good agreement with the results obtained from component analysis under a binocular microscope i e the number fraction of dark juvenile particles is lower for the 2009 sample the standard deviations of apparent luminance have two peaks in the histogram and the quasi fractal dimensions show different frequency distributions between the two samples these features are not recognized in the results of conventional qualitative classification criteria or the sphericity of the particle outlines our method can characterize and distinguish ash samples even for ash particles that have gradual property changes and is complementary to component analysis this method also enables the relatively fast and systematic analysis of ash samples that is required for petrologic monitoring of ongoing activity such as at the sakurajima volcano

permeability reduction of fractured rhyolite in volcanic conduits and its control on eruption cyclicity repetitive explosions of silicic volcanoes are thought to be triggered by volcanic gases in the shallow parts of the conduit to trap the volcanic gases in the conduit the lava dome or plug must be impermeable however silicic magma undergoes brittle fracturing during ascent because its viscosity increases via dehydration and groundmass crystallization since fractured magma has high permeability the reduction of permeability is a key process that allows gases to accumulate leading to an explosion we performed uniaxial compression experiments on rhyolite fragments at temperatures of 700 900 c to investigate the permeability reduction of fractured rhyolite experimental results show that permeability is controlled by porosity and permeability reduction can be caused by compaction of fractured magma based on the observed relationship between permeability and porosity we calculate the time scale of permeability reduction i e the transition from permeable to impermeable magma the calculated time scale 100 1000 s is much shorter than the eruption repose 1 10 h observed at sakurajima volcano japan this result implies that an impermeable lava dome and plug formed during the eruption repose in contrast the repose time 20 40 min of santiaguito volcano guatemala is shorter than the time scale of permeability reduction 10 h this indicates that gases could not have accumulated before volcanic explosions and that other mechanisms could control eruption cyclicity

sn to sg conversion and focusing along the atlantic margin morocco implications for earthquake hazard evaluation digital data from a telemetered short period seismic network in morocco provide a new perspective for understanding the cause of severe shaking and macroseismic reports in morocco produced by large offshore earthquakes located along the azores gibraltar seismic zone even though the earthquake epicenters are 500 1000 km away from the moroccan coast historical records show that such events are capable of producing considerable damage in inland areas we analyze 15 earthquakes that occurred in this region the records show multiple s phases with varying frequencies and amplitudes the s phase with the largest amplitude usually misinterpreted as sn has a phase velocity of 4 2 4 4 km s 1 we show that these s arrivals can best be explained as sn to sg converted phases calculated locations of the conversion points for these phases exhibit two distinct zones almost parallel to the atlantic coastline one is located along the passive continental margin and the other is located about 100 km inland from the coastline we interpret these two zones to be regions where a sudden change in crustal thickness occurs such zones act to focus and magnify the amplitudes of seismic phases this interpretation explains the unusually strong felt reports within morocco from such distant offshore events and it has a significant effect on earthquake hazard evaluation and mitigation studies

ascent driven crystallisation of dacite magmas at mount st helens 1980 1986 we introduce a novel scheme that enables natural silicic glasses to be projected into the synthetic system qz ab or h2o in order to relate variations in volcanic glass chemistry to changing pressure p and temperature t conditions in the sub volcanic magma system by this means an important distinction can be made between ascent driven and cooling driven crystallisation under water saturated or undersaturated conditions in samples containing feldspar and a silica phase quartz or tridymite quantitative p t estimates of the conditions of last equilibrium between crystals and melt can be made formation of highly silicic melts i e 77 wt sio2 is a simple consequence of the contraction of the silica phase volume with decreasing pressure such that high silica glasses can only form by crystallisation at low pressure resorption of quartz crystals appears to be a further diagnostic feature of decompression crystallisation groundmass and inclusion glasses in dacites from the 1980 1986 eruption of mount st helens volcano wa span a wide range in sio2 68 80 wt anhydrous the compositions of the least evolved sio2 poor inclusions in amphibole phenocrysts record entrapment of silicic liquids with 5 4 wt water corresponding to a water saturation pressure of 200 mpa at 900 c the compositions of more evolved higher sio2 plagioclase hosted inclusions and groundmass glasses are consistent with extensive ascent driven fractional crystallisation of plagioclase oxide and orthopyroxene phenocrysts and microlites to low pressures during this polybaric crystallisation plagioclase phenocrysts trapped melts with a wide range of dissolved water contents 3 5 5 7 wt magmas erupted during the plinian phase of the 18 may 1980 eruption were derived from a large reservoir at depths of 6 km subsequent magmas ascended to varying depths within the sub volcanic system prior to extraction from glass chemistry and groundmass texture two arrest levels have been identified at depths of 0 5 1 and 2 4 km a single dome sample from february 1983 contains groundmass plagioclase tridymite and quartz testifying to temperatures of at least 885 c at 11 mpa these shallow storage conditions are comparable to those in the cryptodome formed during spring 1980 the corresponding thermal gradient 0 2 c mpa 1 is consistent with near adiabatic magma ascent from 8 km we argue that the crystallisation history of mount st helens dacite magma was largely a consequence of decompression crystallisation of hot magma beyond the point of water saturation this challenges the conventional view that phenocryst crystallisation occurred by cooling in a large magma chamber prior to the 1980 1986 eruption because the crystallisation process is both polybaric and fractional it cannot be simulated directly using isobaric equilibrium crystallisation experiments however calculation of the phase proportions in water saturated 910 15 c experiments by rutherford et al 1985 over the pressure range 220 125 mpa reproduces the crystallisation sequence and phenocryst modes of mount st helens dacites from 18 may 1980 by allowing for the effects of fractional versus equilibrium crystallisation entrained residual source material and small temperature differences between nature and experiment phase compositions can also be matched to the natural samples we conclude that decompression of water saturated magma may be the dominant driving force for crystallisation at many other silicic volcanic centres

displaced terranes and crustal evolution of the levant and the eastern mediterranean geophysical data from the levant and the eastern mediterranean suggest that this area can be divided into distinct crustal units of different origins for example a marked difference in crustal structure and structural style exists in the levant between samaria and judea south of the carmel structure and the galilee and lebanon north of it on the other hand several microcontinental fragments such as cyprus and the eratosthenes and anaximander seamounts are embedded within the oceanic crust of the eastern mediterranean the present location of these units has resulted from complex tectonic processes which were active during the mid paleozoic mesozoic evolution of this area during this period several crustal units were accreted to the northern margin of africa while others were detached away from it the crustal evolution of the eastern mediterranean and the levant is intimately linked with the closure of the paleo tethys the formation of the neo tethys and its subsequent closure

a preliminary study of regional seismic discrimination in central asia with emphasis on western china abstract in support of a comprehensive test ban treaty we have started to evaluate regional seismic event discrimination capabilities for central asia emphasizing western china we have measured noise and signal levels of over 260 earthquakes and 26 underground nuclear explosions recorded at the broadband digital station wmq in western china and over 120 earthquakes and five nuclear explosions at station aak in kyrgyzstan the explosions are from the kazakh test site kts and lop nor china the earthquakes are mostly from northwest china event magnitudes measured as either m b or m l range between 2 5 and 6 5 and maximum event station distance is about 1700 km we formed phase spectral cross spectral short period long period and long period ratios to test many possible event discriminants all ratios were corrected for distance effects before forming ratio versus magnitude discrimination plots the most consistent discriminants for separating earthquakes from explosions are p s phase ratios for frequencies above 3 to 4 hz p wave f 1 hz to rayleigh wave 0 03 hz f p s ratios separation between earthquakes and explosions increases with frequency and for the p r ratios separation increases with increasing p frequency the p 3 to 6 hz s 0 75 to 1 5 hz cross spectral ratios also separate earthquakes and explosions these cross spectral ratios may prove quite useful because due to weak high frequency l g signals from explosions the high frequency p l g ratio sometimes cannot be evaluated but the p 3 to 6 hz l g 0 75 to 1 5 hz can be evaluated successfully p spectral ratios 3 to 6 hz 0 75 to 1 5 hz also separate earthquakes and explosions but not quite as consistently as the p s ratios however p spectral ratios may prove useful in cases where path effects block or highly attenuate s n and l g waves rendering the p s ratios unavailable in contrast to nevada test site nts events the l g 3 to 6 hz 0 75 to 1 5 hz spectral ratio does not separate earthquakes and nuclear explosions in central asia this observation may be a source effect as asian explosions are thought to be detonated in highly lithified rocks below the water table while most of the smaller m b for station aak the short period spectral and cross spectral ratios identify a few earthquakes not recorded at wmq from the north pamir region that plot with the lop nor explosions the waveforms from these earthquakes lack surface waves and a distinct l g phase these events which we interpret as having occurred in a subducted slab do separate from the explosions on high frequency f 4 hz p s discrimination plots for station wmq long period r l rayleigh wave love wave ratios separate kts explosions and asian earthquakes however r l ratios fail to identify the large lop nor explosions recorded at station aak because the love waves are as large as the rayleigh waves suggesting that strong tectonic release occurs with these explosions because we have examined only one path at a single station for these explosions source radiation pattern may be influencing this observation

lateral variations of high frequency seismic wave propagation at regional distances across the turkish and iranian plateaus in this study we investigate crustal and uppermost mantle physical properties that characterize some of the continental plateaus of the middle east this is done as part of a larger effort to map and compare high frequency wave propagation at regional distances across the earth s continental plateaus thousands of short period wwssn seismograms recorded at stations located in the middle east and produced by earthquakes with epicentral distances less than about 20 were examined visually in an effort to study lateral variations of high frequency 0 5 2 hz seismic wave propagation across this area particularly to the north of the zone of continental collision between the africa arabian and eurasian plates variations of frequencies and amplitudes of sn and lg relative to p are examined and mapped throughout the region and this work is supplemented by a study of velocities of pn sn and lg sn amplitude variations are very striking in this area an important observation of this study is that sn propagates efficiently beneath a major part of the turkish and iranian plateaus sn is strongly attenuated however in the northernmost portion of the plateaus south of the black and caspian seas and in an area between the two seas these regions are characterized in general by active tectonism including volcanism faulting and folding however this active tectonism is not restricted to the areas of high sn attenuation but appears to extend beneath other parts of the iranian and turkish plateaus patterns of lateral variations in the propagation of lg are not as consistent as those for sn lg propagates efficiently across turkey iran and adjacent regions but the lg waves that cross the turkish and iranian plateaus are weak and have relatively long predominant periods of about 2 5 s the lg phase is not observed when the path of propagation crosses the southern caspian and the black seas consistent with the evidence of oceanic type crustal structure beneath these seas lg is also not observed from subcrustal events located in the hindu kush region the velocity of pn beneath most of the plateau areas and particularly in the regions of sn attenuation ranges between 8 0 and 8 2 km s velocities for sn and lg throughout the middle east are about 4 5 and 3 4 km s respectively it is possible to interpret the efficient sn propagation and the relatively normal uppermost mantle p and s velocities beneath a major part of the turkish iranian plateau as indicating a partial underthrusting of the arabian continental plate beneath the iranian and turkish continental blocks alternatively the mapped regions of efficient sn propagation and high sn attenuation may result from a differential cooling of a past thermal anomaly in this case the isostatic compensation of the plateaus could be due to an anomalously hot uppermost mantle that developed behind the mid to upper tertiary subduction zone s in this region and to a subsequent crustal shortening after the collision of the arabian and eurasian plates our results cannot uniquely confirm the validity of the above proposed models

sakurajima volcano a physico chemical study of the health consequences of long term exposure to volcanic ash regular eruptions from sakurajima volcano japan repeatedly cover local urban areas with volcanic ash the frequency of exposure of local populations to the ash led to substantial concerns about possible respiratory health hazards resulting in many epidemiological and toxicological studies being carried out in the 1980s however very few mineralogical data were available for determination of whether the ash was sufficiently fine to present a respiratory hazard in this study we review the existing studies and carry out mineralogical geochemical and toxicological analyses to address whether the ash from sakurajima has the potential to cause respiratory health problems the results show that the amount of respirable 4 m material produced by the volcano is highly variable in different eruptions 1 1 18 8 vol the finest samples derive from historical plinian eruptions but considerable amounts of respirable material were also produced from the most recent vulcanian eruptive phase since 1955 the amount of cristobalite a crystalline silica polymorph which has the potential to cause chronic respiratory diseases is 3 5 wt in the bulk ash scanning electron microscope and transmission electron microscope imaging showed no fibrous particles similar to asbestos particles surface reactivity tests showed that the ash did not produce significant amounts of highly reactive hydroxyl radicals 0 09 1 35 mol m 2 at 30 min in comparison to other volcanic ash types a basic toxicology assay to assess the ability of ash to rupture the membrane of red blood cells showed low propensity for haemolysis the findings suggest that the potential health hazard of the ash is low but exposure and respiratory conditions should still be monitored given the high frequency and durations of exposure

high frequency seismic wave propagation beneath the indian shield himalayan arc tibetan plateau and surrounding regions high uppermost mantle velocities and efficient sn propagation beneath tibet summary this paper reports the regional propagation characteristics of high frequhcy p p s and l waves 0 5 to 2hz and p pg and s velocities for several regions in central asia tibet and india an important result is that s waves propagate efficiently in the uppermost mantle beneath most of the tibetan plateau also s transmission in tibet is not affected by crossing many physiographic and tectonic boundaries such as the indus tsangpo suture these observations are similar to observations of s propagation in stable continental and shield regions efficient s propagation is also observed across the stable tarim basin along the tien shan and himalayan mountains and across the indian shield s is strongly attenuated across the northern tibetan plateau chang thang terrane such strong attenuation suggests the existence of a low q zone in the uppermost mantle beneath that region this interpretation is supported by recent geological observations of widespread quaternary to recent basaltic volcanism in the chang thang terrane l propagates efficiently across the tarim basin the indian shield and along the strike of the himalayan mountains l is also observed although less pronounced for paths crossing large portions of the complicated structures of tien shan karakoram and pamir l is not observed for paths crossing the tibetan plateau the southern boundary of the region of inefficient l propagation in tibet lies in the vicinity of the indus tsangpo suture zone the lack of l can be attributed to attenuation within the tibetan crust however other possibilities including scattering by structural discontinuities at the margins of tibet are equally plausible pg and s are the most prominent features on most short period wwssn seismograms at epicentral distances less than about 4 apparent pg velocity determined for the area of the himalayan mountains is 6 0 0 2 km s l apparent velocities of p and s waves that have at least 50 per cent of their paths in the uppermost mantle beneath the tibetan plateau are 8 42 it 0 10 and 4 73 f 0 06 km s respectively these velocities are quite similar to those determined for the himalayan mountains p 8 45 f 0 08 km s i s 4 74 0 05 km s and the indian shield p 8 40 0 08 km s s 4 69 k 0 04 km s p