pipe vesicles an alternate model for their origin most pipe vesicles occur near the base of basaltic flows where they are interpreted to be traces left by ascending gas bubbles some basaltic pillows also contain pipe vesicles but in these pillows pipe vesicles have a radial distribution indicating that buoyancy cannot be a factor in their formation pipe vesicles do not extend through chilled glassy margins at the base of flows or rims of pillows but rather they form only where there has been significant crystallization indicating that the gas forming these vesicles is exsolved from the lava and is not derived from an external source pipe vesicles in flows and pillows are proposed to form by the exsolution of gas onto bubbles that are attached to the zone of solidification as this zone advances into the cooling lava continued exsolution of gas causes the bubbles to grow normal to the solidification front as pipes gas tubes formed in a similar manner can be seen in most ice cubes lava near the advancing tip of a pipe vesicle cools more rapidly than elsewhere because heat is able to transfer along the pipe by radiation pipe vesicles therefore grow into the lava as cold fingers they cease growing when separate gas bubbles nucleate ahead of them in the ever broadening zone of solidification lava need contain no more than 0 01 wt h 2 o for pipe vesicles to form in this way

determination of fault planes at coalinga california by analysis of patterns in aftershock locations the may 2 1983 coalinga california earthquake was not anticipated because it took place along a previously unrecognized fault concealed beneath an anticline in a region that had little historic seismicity the main shock hypocenter was at 10 km depth and faulting did not break the surface there has been considerable controversy about which of the nodal planes from the main shock fault plane solution was the slip plane conflicting results from geodetic geologic and aftershock data imply that slip occurred along one or the other or both nodal planes the aftershock zone is large several planar features within the aftershock zone indicate that faulting was complex to delineate the planes of slip we applied the three point method a statistical technique by which event planes can be determined from aftershock locations we obtained several important results from our study first we identified a number of planes using locations of aftershocks some of which have not been previously identified from visual inspection of the aftershock locations second the method allows us to choose slip planes by associating three point planes with fault plane solutions based on proximity and similar orientation finally three of the planes identified by the method intersect near themore main shock hypocenter making it difficult to determine which plane is the main shock slip plane nevertheless aftershocks during the first 24 hours after the main shock clearly define the high angle ne dipping nodal plane whereas few aftershocks were located along the conjugate plane assuming that the aftershocks took place along the rupture zone we conclude that slip during the main shock occurred predominantly along the high angle ne dipping plane from the fault plane solution for this event copyright american geophysical union 1989 less

formation of lava tubes and extensive flow field during the 1991 1993 eruption of mount etna detailed mapping during the 1991 1993 eruption of mount etna has shown that there is a relationship between tumuli ephemeral vents lava tubes and their parent lava flows during this eruption many tubes formed in stationary inflated a a lava flows ephemeral vents at the fronts of these stationary flows and above lava tubes fed secondary lava flows many of which subsequently developed new tubes the resulting complex network of tubes ephemeral vents and secondary flows was responsible for most of the widening thickening and lengthening of the 1991 1993 etna lava flow field the supply of relatively uncooled lava via tubes to distal parts of this flow field allowed lava to flow 3 km farther from the vent than the longest channel fed lava flow our observations suggest that lava tubes play a more important role in the formation of extensive a a flow fields on etna than has previously been recognized

spatial correlation of seismic slip at the hdr soultz geothermal site qualitative approach one major goal of monitoring seismicity accompanying hydraulic frac turing of a reservoir is to obtain information about the pattern size and orientation of fractures the traditional method is to analyze the pattern of locations of the induced microearthquakes to investigate whether additional information about the fracture pattern may be obtained from induced microearthquake datasets we applied variogram analysis to investigate the spatial distribution of shear displacement at the source of microearthquakes variography is a method for analyzing spatial interde pendence of a variable the method was applied to a set of induced microearthquakes observed in the vicinity of the gpk1 injection well at soultz sou forets hot dry rock geothermal site alsace france variograms obtained for 20 vertical slices zones along the wellbore were compared with the orientation of the fractures de rived from a formation micro imager which is a resistivity log for measuring po sition and orientation of fractures in a borehole we found that variograms exhibiting spatial dependency correlated well with zones where fractures belong to one domi nant orientation set and variograms with no or low spatial dependency corresponded to zones with two or more fracture orientations sets we propose that this correlation results from the relation between the spatial variation of shear displacement the physical properties of fractures and their orientations

emplacement of the rocche rosse rhyolite lava flow lipari aeolian islands the rocche rosse lava flow marks the most recent rhyolitic extrusion on lipari island italy and preserves evidence for a multi stage emplacement history due to the viscous nature of the advancing lava 108 to 1010 pa s indicators of complex emplacement processes are preserved in the final flow this study focuses on structural mapping of the flow to highlight the interplay of cooling crust formation and underlying slope in the development of rhyolitic lavas the flow is made up of two prominent lobes small 0 2 m scale folding and a channelled geometry foliations dip at 2 4 over the flatter topography close to the vent and up to 30 50 over steeper mid flow topography brittle faults tension gashes and conjugate fractures are also evident across flow heterogeneous deformation is evident through increasing fold asymmetry from the vent due to downflow cooling and stagnation a steeper underlying topography mid flow led to development of a channelled morphology and compression at topographic breaks resulted in fold superimposition in the channel we propose an emplacement history that involved the evolution through five stages each associated with the following flow regimes 1 initial extrusion crustal development and small scale folding 2 extensional strain stretching lineations and channel development over steeper topography 3 compression at topographic break autobrecciation lobe development and medium scale folding 4 progressive deformation with stagnation large scale folding and re folding and 5 brittle deformation following flow termination the complex array of structural elements observed within the rocche rosse lava flow facilitates comparisons to be made with actively deforming rhyolitic lava flows at the chilean volcanoes of chaiten and cordon caulle offering a fluid dynamic and structural framework within which to evaluate our data

a new model for the emplacement of columbia river basalts as large inflated pahoehoe lava flow fields extensive flows of the columbia river basalt crb group in washington oregon and idaho are dominantly inflated compound pahoehoe sheet lavas early studies recognized that crb lavas are compound pahoehoe flows with textures suggesting low flow velocities but it was thought that the great thickness and extent of the major flows required very rapid emplacement as turbulent floods of lava over a period of days or weeks however small volume 1 km compound pahoehoe flows on kilauea hawai i demonstrate that such flows can thicken by at least an order of magnitude through gradual inflation and the same mechanism has been proposed for larger 10 20 km3 pahoehoe flows in iceland the vertical distribution of vesicles and other morphologic features within crb lava flows indicate that they grew similarly by inflation small pahoehoe lobes at the base and top of many crb pahoehoe lava flows indicate emplacement in a gradual piecemeal manner rather than as a single flood we propose that each thick crb sheet flow was active for months to years and that each group of flows produced by a single eruption a flow field was emplaced slowly over many years

toothpaste lava characteristics and origin of a lava structural type transitional between pahoehoe and aa toothpaste lava an important basalt structural type which illustrates the transition from pahoehoe to aa is particularly well displayed on the 1960 kapoho lava of kilauea volcano its transitional features stem from a viscosity higher than that of pahoehoe and a rate of flow slower than that of aa viscosity can be quantified by the limited settling of olivine phenocrysts and rate of flow by field observations related to the low angle slope on which the lava flowed much can be learned about the viscosity rheologic condition and flow velocity of lavas long after solidification by analyses of their structural characteristics and it is possible to make at least a semiquantitative assessment of the numerical values of these parameters

structure and emplacement of a rhyolitic obsidian flow little glass mountain medicine lake highland northern california many rhyolitic obsidian flows show consistent stratigraphic relations among textural units exposed in the flow fronts of undissected flows and in cross sections of older flows the stratigraphy of the holocene little glass mountain rhyolitic obsidian flow consists of from bottom to top air fall tephra deposits basal breccia coarsely vesicular pumice obsidian finely vesicular pumice and surface breccia slightly crystalline rhyolite occurs near the vent areas a model of obsidian flow emplacement based on these textural relations is presented this stratigraphy may reflect the distribution of volatiles within the magma source region with the interlayered contact between coarsely vesicular pumice and overlying obsidian indicating stratification of volatiles in the magma body the distribution of pumiceous and glassy zones along with the orientations of flow banding can be used to map the surface structure of rhyolitic obsidian flows this complex structure as mapped on the little glass mountain flow reflects both the initial flow stratigraphy and subsequent deformational processes during emplacement of the lava three processes disrupt the initial flow configuration the rise of coarse pumice diapirs from the base of the flow the inward propagation of fractures in areas of extension and surface folding in sites of flow parallel compression subvertical flow banding in vent areas indicates that fracturing accompanies the emergence of lava most of the observed upper surface of a dome originates as a fracture plane the structure of domes that form over vent areas may reflect the orientation of dike like conduits as well as the local state of stress during extrusion

the fold illusion the origins and implications of ogives on silicic lavas abstract folds on the surfaces of mafic lavas are among the most readily recognized geological structures and are used as first order criteria for identifying ancient lavas on earth and other planetary bodies however the presence of surface folds on the surface of silicic lavas is contested in this study and we challenge the widely accepted interpretation that silicic lava surfaces contain folds using examples from the western united states and sardinia italy we interpret the ridges and troughs on their upper surfaces typically referred to as ogives or pressure ridges as fracture bound structures rather than folds we report on the absence of large scale buckle style folds and note instead the ubiquitous presence of multiple generations and scales of tensile fractures comparable to crevasses in glaciers and formed in ways similar to already recognized crease structures we report viscosity data and results of stress analyses that preclude folding ductile deformation in compression of the upper surface of silicic lavas at timescales of emplacement weeks to months therefore analysis of fold geometry wavelength amplitude etc is erroneous and instead the signal produced reflects the strength and thickness of the brittle upper surface stretching over a ductile interior the presence of ogives on the surfaces of lavas on other planetary bodies may help to elucidate their rheological properties and crustal thicknesses but relating to their tensile strength not viscosity

ductile brittle transition structures in the basal shear zone of a rhyolite lava flow eastern australia abstract structural and textural evidence indicates that the basal breccia of the tertiary minyon falls rhyolite of tweed volcano eastern australia formed mainly by fragmentation within the basal shear zone of the lava flow cooling at the flow base led to lava behaviour passing through the ductile brittle transition while subjected to progressive deformation the intact lavas of the basal zone record intense ductile shearing in multiple folds rotated phenocrysts strong alignment of crystallites and micro folding of these crystal alignments minor brittle structures including faults on fold limbs and microfaults occur in the intact lava immediately above the basal breccia these structures represent incipient brecciation and complete brecciation has occurred by intense faulting of previously intact lava brittle deformation migrated upward into overlying intact lava as cooling occurred this process is distinctly different from overriding and incorporation of surface breccias which is commonly invoked as the primary process of basal breccia formation

the roza member columbia river basalt group a gigantic pahoehoe lava flow field formed by endogenous processes we present studies on the physical volcanology of the 15 ma roza member of the wanapum formation in the columbia river basalt group the roza member represents a compound pahoehoe flood basalt lava flow field with an area of 40 300 km 2 and a volume of 1300 km 3 it consists of 4 major lava flows each composed of numerous decimeter to kilometer long pahoehoe lobes roza lavas feature a wide range of pahoehoe surface structures such as lava rise plateaus tumuli and surface breakouts and we illustrate that the lava morphology is inconsistent with previous proposals of rapid emplacement for these lavas an integral component of the roza flow field is the sheet lobe with internal structures identical to those of inflated pahoehoe sheet lobes from hawaii and iceland both at the same scale and at much larger scales we identify a three part division of the sheet lobes into basal crust lava core and lava crust which are interpreted as the equivalent to the bottom crust the liquid lava core and the surface crust of an active inflating pahoehoe lobe the upper lava crust grows continuously during lava emplacement and its growth rate can be approximated by conductive cooling this relationship is used to calculate the emplacement time for individual roza sheet lobes and to derive a first order estimate on the duration of the roza eruption the results indicate that the emplacement of individual roza lobes lasted for months to years and that the lava flow field was constructed over a period of at least 14 years we propose that the roza flows achieved great areal dimensions and thicknesses by inflation and endogenous growth as the lava flowed from vent to flow front it traveled under an insulating crust which maintained cooling rates of 0 1 c km and allowed for efficient transport of lava over distances up to 300 km

gps measurements of crustal deformation in the baikal mongolia area 1994 2002 implications for current kinematics of asia 1 we present new geodetic results of crustal velocities over a large part of northern asia based on gps measurements in the baikal rift zone and mongolia spanning the 1994 2002 period we combine our results with the gps velocity field for china of wang et al 2001 and derive a consistent velocity field for most of asia we find contrasted kinematic and strain regimes in mongolia with northward velocities and n s shortening in westernmost mongolia but eastward to southeastward motion and left lateral shear for central and eastern mongolia this eastward to southeastward motion of central and eastern mongolia is accommodated by left lateral slip on the e w trending tunka bolnay and gobi altay faults 2 1 2 mm yr 1 2 6 1 0 mm yr 1 and 1 2 mm yr 1 respectively and by about 4 mm yr 1 of extension across the baikal rift zone consequently 15 of the india eurasia convergence is accommodated north of the tien shan by n s shortening combined with dextral shear in the mongolian altay and by eastward displacements along major left lateral strike slip faults in central and eastern mongolia we find a counterclockwise rotation of north and south china as a quasi rigid block around a pole north of the stanovoy belt which rules out the existence of an amurian plate as previously defined and implies 2 mm yr 1 of left lateral slip on the qinling shan fault zone

fault plane solutions of earthquakes and active tectonics of the tibetan plateau and its margins summary fault plane solutions of earthquakes within and on the margins of the tibetan plateau show diverse styles of faulting and deformation with thrust faulting and crustal shortening normal to the margins of the plateau and with normal and strike slip faulting resulting in roughly east west crustal extension within the plateau the direction of overthrusting of the himalaya onto the indian shield is radially outward varying from southwest in the western himalaya to south southeast in the east assuming that the indian shield behaves rigidly this requires a west northwest divergence of western tibet from southeastern tibet at a rate of 18 9 mm yr 1 comparable with the rate of convergence at the himalaya fault plane solutions of earthquakes in the southern portion of the tibet plateau consistently show large components of normal faulting on roughly north striking planes and corroborate such extension within the high plateau where elevations exceed 5000 m normal and strike slip faulting occur so that an overall east southeast west northwest extension of the region at about 10 mm yr 1 is partitioned into roughly equal parts of crustal thinning and north northeast south southwest crustal shortening about 5 mm yr 1 in general strike slip faulting characterizes solutions for earthquakes within eastern tibet where mean elevations drop below 4500 5000 m but the orientations of the strike slip faults vary across the region in central tibet left lateral slip occurs on planes trending roughly northeast but for earthquakes farther east the orientations of that plane become progressively east west and then southeast this variation in orientation implies a rotation of material along curved left lateral shear zones thus the eastward extrusion of tibet appears to be facilitated not only by rapid left lateral shear but also by large clockwise rotations of the material in eastern tibet the rate of eastward extrusion of material in eastern tibet relative to the tarim basin to its north is roughly 30 40 mm yr 1 fault plane solutions of earthquakes in the northern and eastern margins of tibet show large components of thrust faulting with the p axes oriented radially outward from the plateau and approximately perpendicular to the regional topographic contours of the plateau the orientation of this crustal shortening is northeast southwest on the northeastern margin east west on the eastern margin and northwest southeast in the longmenshan on the southeastern margin thus at least some of the extrusion of eastern tibet out of india s northward path into asia is absorbed by crustal shortening on the margins of the plateau the variation from normal faulting in the high tibetan plateau where elevations exceed 5000 m to dominantly strike slip faulting farther east where elevations are lower and then to thrust faulting on the margins of the plateau where elevations drop below 3000 m surely results at least in part from a decrease in the value of the vertical stress the magnitude of the east west compressive stress need not vary across the plateau

pahoehoe and aa in hawaii volumetric flow rate controls the lava structure the historical records of kilauea and mauna loa volcanoes reveal that the rough surfaced variety of basalt lava called aa forms when lava flows at a high volumetric rate 5 10 m3 s and the smooth surfaced variety called pahoehoe forms at a low volumetric rate 5 10 m3 s this relationship is well illustrated by the 1983 1990 and 1969 1974 eruptions of kilauea and the recent eruptions of mauna loa it is also illustrated by the eruptions that produced the remarkable paired flows of mauna loa in which aa formed during an initial short period of high discharge rate associated with high fountaining and was followed by the eruption of pahoehoe over a sustained period at a low discharge rate with little or no fountaining the finest examples of paired lava flows are those of 1859 and 1880 1881 we attribute aa formation to rapid and concentrated flow in open channels there rapid heat loss causes an increase in viscosity to a threshold value that varies depending on the actual flow velocity at which when surface crust is torn by differential flow the underlying lava is unable to move sufficiently fast to heal the tear we attribute pahoehoe formation to the flowage of lava at a low volumetric rate commonly in tubes that minimize heat loss flow units of pahoehoe are small usually 1 m thick move slowly develop a chilled skin and become virtually static before the viscosity has risen to the threshold value we infer that the high discharge rate eruptions that generate aa flows result from the rapid emptying of major or subsidiary magma chambers rapid near surface vesiculation of gas rich magma leads to eruptions with high discharge rates high lava fountains and fast moving channelized flows we also infer that long periods of sustained flow at a low discharge rate which favor pahoehoe result from the development of a free and unimpeded pathway from the deep plumbing system of the volcano and the separation of gases from the magma before eruption achievement of this condition requires one or more episodes of rapid magma excursion through the rift zone to establish a stable magma pathway

spatial and temporal analysis of a seismic series using a new version of the three point method application to the 1989 antequera spain earthquakes abstract the three point method tpm has been successfully applied to several seismic series and has provided information about the spatial characteristics azimuth and dip of the fault planes activated in the rupture process a new development of the tpm to determine temporal characteristics is presented to obtain the evolution of the fracturing process of an active fault system for the analysis of the 158 microearthquakes and earthquakes that took place in the seismic series of antequera in june 1989 the choice of a threshold magnitude m u 2 5 has permitted the events related to the most relevant fractures to be distinguished secondly only the events between two concentric spheres here named spatial crown with respect to a given earthquake have been used in order to avoid taking into account earthquakes that are too close to each other together with the very distant events that have little relation to the event analysed the spatial crown has permitted some clear results in the antequera series where we have found that the fracturing process began fundamentally with n 80 e planes and evolved to n 65 w planes finally an error analysis enables an estimation of the uncertainty in the results to be obtained from the errors in the data

aftershocks and temporal clustering of deep earthquakes this paper evaluates how the focal depth the geographic location and magnitude affect the incidence of aftershocks and the occurrence of temporal clustering of nearby events for selected deep earthquakes h 70 km in the catalog of the international seismological centre for comparison we also evaluate shallow h 70 km and deep earthquakes for which locations and moment tensors are reported by the harvard group the primary conclusions are as follows 1 the incidence of aftershocks depends strongly on focal depth with aftershocks being less common for events between 100 and 450 km depth than for shallower or deeper events events with focal depth shallower than 40 km have a higher incidence of aftershocks and more pronounced temporal clustering than do deeper events 2 for the data available at present the incidence of aftershocks and amount of temporal clustering for deep events are independent of geographic region except for dependence caused by regional differences in the focal depths of events 3 deep earthquakes with large magnitudes are more likely to have aftershocks than are smaller magnitude events in accordance with models where the number of aftershocks is a poisson random variable that is a function of magnitude however several large events are anomalous in that they have many more aftershocks or many fewer aftershocks in the catalog than predicted by the model

statistical methods for comparing directions to the orientations of focal mechanisms and wadati benioff zones we propose simple graphical methods for evaluating whether a set of direction vectors are clustered with respect to particular directions axes or planes as examples of these techniques we present data concerning the relationship between 1 aftershock initial event geometry and nodal planes of focal mechanisms 2 aftershock initial event geometry and the plane of the wadati benioff zone and 3 b axes of focal mechanisms and the plane of the wadati benioff zone

thick lava flows of karisimbi volcano rwanda insights from sir c interferometric topography we use a digital elevation model dem derived from interferometrically processed sir c radar data to estimate the thickness of massive trachyte lava flows on the east flank of karisimbi volcano rwanda the flows are as long as 12 km and average 40 60 m up to 140 m in thickness by calculating and subtracting a reference surface from the dem we derived a map of flow thickness which we used to calculate the volume up to 1 km3 for an individual flow and 1 8 km3 for all the identified flows and yield strength of several flows 23 124 kpa using the dem we estimated apparent viscosity based on the spacing of large folds 1 2 1012 to 5 5 1012 pa s for surface viscosity and 7 5 1010 to 5 2 1011 pa s for interior viscosity for a strain interval of 24 h we use shaded relief images of the dem to map basic flow structures such as channels shear zones and surface folds as well as flow boundaries the flow thickness map also proves invaluable in mapping flows where flow boundaries are indistinct and poorly expressed in the radar backscatter and shaded relief images

the thermal stealth flows of santiaguito dome guatemala implications for the cooling and emplacement of dacitic block lava flows thick slow moving block lava flows are associated with extrusive activity in dacitic systems where lava core depressurization during flow front collapse generates devas tating block and ash flows dimensional and rare thermal data collected during jan uary 2000 for an active dacitic block flow at santiaguito guatemala provide insight into cooling and emplacement mechanisms flow velocity was low 12 5 m d 21 in spite ginal shear zones the axial part of the flow front was thicker than the marginal zones and was oversteepened this geometry can be explained by a higher vertical velocity gradient in the axial zone causing more frequent and larger volume flow front col lapses axial zone collapses also penetrate farther up flow but not sufficiently to de pressurize the flow core and generate a block and ash flow for such a block and ash flow to occur we calculate that an in crease in velocity and or thickness due to increased slope or topographic confine ment must occur whereas low surface temperatures make block flows invisible to short wave infrared sensors the low veloc ity also contributes to the stealthy behavior of these flows their stealthy nature how ever masks the fact that they can extend many kilometers moving block and ash flow sources closer to vulnerable communities

measuring ground displacements from sar amplitude images application to the landers earthquake ers sar amplitude images are utilized to map ground displacements from a sub pixel correlation method it yields a ground two dimensional displacement field with independent measurements about every 128m in azimuth and 250m in range the accuracy depends on the characteristics of the images for the landers test case the 1 uncertainty is 0 8m in range and 0 4m in azimuth we show that this measurement provides a map of major surface fault ruptures accurate to better than 1km and information on coseismic deformation comparable to the 92 gps measurements available although less accurate this technique is more robust than sar interferometry and provides complementary information since interferograms are only sensitive to the displacement in range

pipe vesicles in basalt trails left by dense immiscible melt droplets sinking through a viscous basal thermal boundary layer abstract pipe vesicles tubular cavities found in the basal parts of p hoehoe lava flows worldwide constitute one of their fundamental primary structures like other vesicles pipe vesicles have been ascribed to gas bubbles whether rising buoyantly through viscous lava or trapped in an inward moving solidification front these models involve special and unrealistic conditions and still poorly explain characteristic features of pipe vesicles in a new model proposed here pipe vesicles are postulated to be hollow trails left by droplets of dense immiscible iron rich silicate or sulphide melt sinking through a viscous thermal boundary layer just above the lower chilled margin of a p hoehoe flow unit driven by gravity the droplets sink vertically in static lava but obliquely in mobile lava such as lava undergoing gravitational spreading viscous and fluid dynamic forces tend to deform the sinking droplets and resist their motion the characteristic inverted y inverted v and lambda shapes of individual pipe vesicles reflect downward branching with a large droplet breaking up in partially crystallized highly polymerized basalt in the basal thermal boundary layer pipe vesicles do not yield lava flow directions even on a local centimeters to meters scale but are indirect indicators of widespread liquid immiscibility processes in basalts and potential commercial fe cu ni mineralization as pipe vesicles are not vesicles cavities left by gas escape a new term is necessary and the term vermiform p hoehoe having good descriptive power but no genetic connonations is suggested for p hoehoe with pipe vesicles

objective regionalization of rayleigh wave dispersion data by clustering algorithms an application to the mediterranean basin abstract the main target of the present study is an objective and automated regionalization of rayleigh wave dispersion data for the mediterranean basin without a priori seismotectonic constraints and to determine the corresponding regional shear velocity structures the database used is formed by almost 200 rayleigh wavetrains corresponding to 42 regional events with surface wave magnitude greater than 4 5 recorded at the mednet very broad band stations in the mediterranean area path averaged group velocities for the rayleigh wave fundamental mode are derived for each available epicentre station trajectory crossing the mediterranean basin after this a principal component analysis and a clustering process are applied to local group velocities obtained for 13 different periods from 10 to 70 s in order to classify the mediterranean basin into several homogeneous regions the stochastic inversion of the averaged group velocity dispersion curve obtained for each region provides the respective shear velocity structures down to a depth of 150 160 km the characteristics of these areas and their possible correlation with the main seismotectonic features of the mediterranean region are discussed the regional models reveal significant lateral changes in the elastic structure with the main differences concerning particularly the upper 35 40 km within this depth range low shear velocities varying from 2 8 to 3 9 km s 1 characterize the eastern mediterranean whereas higher velocities ranging from 3 0 to 4 2 km s 1 are deduced for the western mediterranean these results suggest a thicker crust in the eastern part but with a greater thickness of sedimentary layers however for depths of between 80 and 110 km lower shear velocities are obtained in the western part while higher shear velocities are derived for the eastern mediterranean sea in the aegean sea greece the south of italy sicily and tunisia this velocity pattern suggests an averaged thicker lithosphere under the latter areas as the top of the asthenosphere is detected at a mean depth of 75 km for the remaining regions this thicker lithosphere can be related to processes associated with the convergence of the eurasian and african plates and subduction under the calabrian and hellenic arcs

spongy pahoehoe in hawaii a study of vesicle distribution patterns in basalt and their significance a well defined pahoehoe lava type that is very common medially and distally in hawaii is characterized by a high concentration and fairly uniform distribution of spherical or near spherical vesicles measurements of vesicle sizes and concentrations have been made on ten of these spongy pahoehoe lava flow units the vesicles increase in size toward the middle accompanied by a moderate increase in lava porosity the close approach to bilateral symmetry on either side of the horizontal median plane and the common occurrence of a median gas blister shows that no significant upward movement of vesicles occurred suggesting that the lava possessed a yield strength and was more or less static olivine phenocrysts when present are however concentrated in the lower half of the same flow units showing that the lava previously lacked a significant yield strength the vesicles are regarded as early inherited from the vent but the size characteristics of the vesicle population are a late formed feature vesicles grew in static lava mostly by coalescence and it is postulated that coalescence was promoted by the presence of abundant diktytaxitic voids which punctured the walls of contiguous vesicles zones in which the vesicle concentration is lower and the vesicles are larger and strongly deformed interrupt the symmetry of some spongy pahoehoe units and gas blisters higher than the median plane occur in many examples these zones are interpreted to result from late stage shearing and point to a mechanism by which vesicles may be eliminated from a lava

combining insar and seismology to study the 2003 siberian altai earthquakes dextral strike slip and anticlockwise rotations in the northern india eurasia collision zone summary the 2003 september 27 m w 7 2 siberian altai earthquake was the largest to have struck the altai mountains in more than seventy years and was closely followed by two m w 6 2 and 6 6 aftershocks we use radar interferometry seismic bodywaves and field investigations to examine the source processes of these earthquakes the main shock of the initial earthquake ruptured a subvertical nw se striking dextral strike slip fault the fault was previously unrecognised although it approximately follows the southwestern boundaries of two intermontane depressions within the interior northwestern altai it has very little topographic expression a ne dipping m w 6 7 reverse subevent possibly triggered by shear waves from the main shock occurred ten seconds afterwards strike to the southeast the later m w 6 2 and 6 6 aftershocks were dextral strike slip events which contributed further to deformation in the northwest part of the fault zone however interferometric and bodywave models disagree significantly on the source parameters of the earthquakes in particular the total moment released and the dip of the fault planes trade offs of fault dip with moment and centroid depth in the bodywave modelling can account for some but not all of these discrepancies the interferometric data is unevenly distributed containing many more data points on one side of the fault zone than the other however on the basis of calculations with synthetic data we rule this out as a reason for the discrepancies in fault parameters the lower moment predicted by interferometry could be explained by the lack of coherent data close to the faulting if slip was concentrated at very shallow depths the dip yielded by the interferometric modelling might be influenced by lateral changes in elastic properties although these would also affect the bodywave solutions the earthquake sequence occurred close to recent palaeomagnetic measurements of late cenozoic anticlockwise rotations these suggest that the right lateral strike slip faulting that ruptured in the 2003 earthquakes accommodates regional nne ssw shortening by rotating anticlockwise over time the reverse subevent is a rare case of pure shortening perpendicular to the trend of the altai range

paleomagnetic study of cenozoic sediments from the zaisan basin se kazakhstan and the chuya depression siberian altai tectonic implications for central asia abstract this paper presents new paleomagnetic results on cenozoic rocks from northern central asia eighteen sites were sampled in pliocene to miocene clays and sandy clays of the zaisan basin southeastern kazakhstan and 12 sites in the upper oligocene to pleistocene clays and sandstones of the chuya depression siberian altai thermal demagnetization of isothermal remanent magnetization irm showed that hematite and magnetite are the main ferromagnetic minerals in the deposits of the zaisan basin stepwise thermal demagnetization up to 640 660 c isolated a characteristic chrm component of either normal or reverse polarity at nine sites at two other sites the great circles convergence method yielded a definite direction measurements of the anisotropy of magnetic susceptibility showed that the hematite bearing sediments preserved their depositional fabric these results suggest a primary origin of the chrm and were substantiated by positive fold and reversal tests the mean paleomagnetic direction for the zaisan basin d 9 i 59 k 19 95 11 is close to the expected direction derived from the apw path of eurasia j geophys res 96 1991 4029 and shows that the basin did not rotated relative to stable asia during the tertiary in the upper pliocene pleistocene sandstones of the chuya depression a very stable chrm carried by hematite was found its mean direction d 9 i 46 k 25 95 7 is characterized by declination close to the one excepted for early quaternary whereas inclination is lower in the middle miocene to lower pliocene clays and sandstones a stable chrm of both normal and reverse polarities carried by magnetite was isolated its mean direction d 332 i 63 k 31 95 4 is deviated with respect to the reference direction and implies a neogene 39 8 counterclockwise rotation of the chuya depression relative to stable asia these results and those from the literature suggest that the different amount of rotation found in the two basins is related to a sharp variation in their tectonic style predominantly compressive in the zaisan basin and transpressive in the siberian altai at a larger scale the pattern of vertical axis rotations deduced from paleomagnetic data in northern central asia is consistent with the hypothesis of a large left lateral shear zone running from the pamirs to the baikal heterogeneous rotations however indicate changes in style of faulting along the shear zone and local effect for the domains with the largest rotations

spatial temporal analysis of a seismic series using the principal components method the antequera series spain 1989 a method for the characterization of a seismically active zone from a distribution of hypocenters is presented this method is based on principal components analysis a powerful multivariant statistical technique that is used to find the rupture local ellipsoid rle the ellipsoid is a planar structure with which two variations of the method are developed the spatial principal components analysis and the spatial temporal principal components analysis using these methods it is possible to find the dominating tendencies in the fracturing of the seismically active volume as well as the temporal evolution of the process the methodology developed has been applied to a series of earthquakes that occurred near antequera spain with the result that the main fracture series trends n70 80 e moreover the temporal evolution of the system from the most relevant rle has been characterized

improved relative locations of clustered earthquakes using constrained multiple event location a new method for improving relative locations of clustered earthquakes is presented and applied to a suite of microearthquakes induced by hydraulic fracturing the method is based on the assumption that clustering of earthquake hypocenters is obscured by the uncorrelated scatter of individual hypocenters the method is implemented as an additional constraint in a joint hypocenter determination jhd scheme the method shifts event hypocenters toward the center of mass of the events within some volume surrounding the event location if the rms misfit between predicted and measured arrival times does not increase significantly the method uses the same basic assumption of jones and stewart 1997 which is that there is greater clustering in actual earthquake locations than there is in locations determined using conventional techniques our method differs in that it is implemented as part of the jhd process so it operates on raw travel time data rather than on derived hypocenters the method produces hypocenters from a demonstration field dataset that are similar to those obtained by phillips et al 1997 from time consuming precise manual repicking of relative arrival times of events the clustering constraint can easily be incorporated as an additional constraint in earthquake location velocity tomography codes and may lead to improved velocity structure determination and earthquake location pattern identification and interpretation

source model for the mw 6 7 23 october 2002 nenana mountain earthquake alaska from insar 1 the 23 october 2002 nenana mountain earthquake mw 6 7 occurred on the denali fault alaska to the west of the mw 7 9 denali earthquake that ruptured the same fault 11 days later we used 6 interferograms constructed using radar images from the canadian radarsat 1 and european ers 2 satellites to determine the coseismic surface deformation and a source model data were acquired on ascending and descending satellite passes with incidence angles between 23 and 45 degrees and time intervals of 72 days or less modeling the event as dislocations in an elastic half space suggests that there was nearly 0 9 m of right lateral strike slip motion at depth on a near vertical fault and that the maximum slip in the top 4 km of crust was less than 0 2 m the nenana mountain earthquake increased the coulomb stress at the future hypocenter of the 3 november 2002 denali earthquake by 30 60 kpa

active faulting and deformation of the coalinga anticline as interpreted from three dimensional velocity structure and seismicity this work gives a clear picture of the geometry of aftershock seismicity in a large thrust earthquake interpretation of hypocenters and fault plane solutions from the 1983 coalinga coast range california earthquake sequence in combination with the three dimensional velocity structure shows that the active faulting beneath the fold primarily consists of a set of southwest dipping thrusts uplifting blocks of higher velocity material above the main listric blind thrust there is a conjugate fault steeply northeast dipping that provides the western limit of the aftershocks within the coalinga anticline and that corresponds in location and spatial extent with the adjacent pleasant valley syncline the character of the seismicity varies with the degree of previous deformation on each section of the anticline where the previous uplift was largest the shallow seismicity shows secondary faulting on either side of the fold with orientations that correspond to the preexisting geologic structure diffuse seismicity characterizes the area with the least previous deformation the mainshock rupture terminated where the fold trend was no longer uniform but had competing north and west trending features the upward extent of the mainshock rupture ended at the approximate boundary between franciscan and great valley sequence rocks above that depth the main thrust appears to splay into a steeper segment and a near horizontal segment thus the extent of rupture area is limited by the area of uniform structural orientation and by the variation in the type of material with the three dimensional velocity model each individual hypocenter moved slightly 0 2 km in accord with the details of the surrounding velocity structure so that secondary features in the seismicity pattern are more detailed than with a local one dimensional model and station corrections the overall character of the fault plane solutions was not altered by the three dimensional model but the more accurate ray paths did result in distinct changes in particular the mainshock has a fault plane dipping 30 southwest instead of the 23 obtained with the one dimensional model

the ad 500 700 late classic el astillero and el pedregal volcanoes michoacan mexico a new monogenetic cluster in the making the recent identification of holocene volcanic clusters in small areas within the michoacan guanajuato volcanic field mgvf opens several questions regarding future volcanic hazard assessments in this region documenting vent alignments and eruption recurrence intervals within clusters will provide parameters necessary for making temporal and spatial hazard evaluations here we present a possible new case of a small cluster consisting of only two monogenetic volcanoes el astillero and el pedregal located in the 4400 km2 tancitaro nueva italia region in the southwestern part of the mgvf only 25 km to the south of paricutin volcano we determined from paleomagnetic and radiocarbon dating that el astillero and el pedregal most likely erupted one after the other between ad 500 and 700 within the late classic period of mesoamerican archeology while the eruptions were likely separated by a short period of time the exact length is difficult to ascertain after the 6 years of total estimated eruption duration of the two volcanoes both together occupied an area of 14 7 km2 and emitted a dense rock equivalent dre volume of magma of 0 5 km3 notable characteristics of the eruptions include a switch from the explosive activity exclusive of el astillero strombolian to effusive activity early after the initiation of the el astillero eruption a shift in the active vents and a progressive change in the bulk magma composition from basaltic andesite to andesite throughout the duration of the eruption this activity first formed the el astillero scoria cone and tephra deposits followed by its lava field and ended with the emplacement of the el pedregal viscous lavas the discovery of pre hispanic pottery sherds and obsidian artifacts underneath the el astillero tephra fallout unambiguously attests to human activities in the area before the eruption judging by their eruptive style the eruptions probably had a limited impact on the small area affected and the surrounding human activities but the hazard for this area remains since el astillero and el pedregal could represent the initial stages of a new cluster that is still in the making if so another eruption should be expected in this area again

p type and s type pahoehoe a study of vesicle distribution patterns in hawaiian lava flows abstract two main types of hawaiian pahoehoe lava p type pipe vesicle bearing and s type spongy are characterized their origin is inferred from features of the vesicle sizes and abundances particularly in their chilled margins which record frozen in the vesicle population at the time and place of solidification s type margins are distinctly more vesicular than p type porosity and vesicle size and range in s type margins are highest at the vent vesicle size reaches a minimum in medial areas and then increases while porosity decreases onto the flatter ground of the coastal terrace the vesicles are inferred to be part of the complement of bubbles that was present at the vent modified by loss and coalescence during travel in lava tubes the minimum porosity occurs on the coastal terrace and is the result of degassing during a longer residence time in lava tubes there the systematically lower porosity of p type margins than s type is attributed to a still longer residence allowing more time for bubble coalescence and loss p type pahoehoe is common only on flatter ground such as the coastal terrace it is found where tumuli and lava rises are widespread and where extensive networks of lava expansion clefts occur through which lava moving below the surface crust can lose gas without significant cooling additionally p type pahoehoe shows clear evidence that a strong inward movement and growth of bubbles took place from the outermost selvage into an inner selvage concentration zone a mechanism that might cause this inward movement is surface tension operating against deforming forces in the steep viscosity gradient of flow margins causing down gradient bubble translation inward movement does not usually occur in the upper crust of thick 2 m flow units because of the absence of strong deforming forces in the shallow velocity gradient there many of the larger bubbles that entered the inner selvage rose buoyantly to form pipe vesicles p type pahoehoe is characterized by sampled profiles across 20 flow units continued gas loss through surface clefts caused a general overall reduction in vesicle content within each flow unit to less than that in the upper crust and the pronounced depletion of vesicles from its lower half is attributed to scavenging by ascending pipe bubbles in a small proportion of flow units overall gas loss was small and a median gas blister fed by pipe bubbles developed instead

identifying the plane of slip for a fault plane solution from clustering of locations of nearby earthquakes numerous methods have been proposed recently to analyze the spatial pattern of locations of earthquakes to infer planes along which earthquakes cluster these planes are considered to be earthquake slip planes the application of such methods is to find major planes passing through a set of earthquake locations when the fault plane solutions of the events are unusual or when locating planes would contribute to understanding the structural setting in which an earthquake occurred a modification to one of these methods the three point method allows one to study anomalous or otherwise interesting events and to choose which nodal plane in the fault plane solution for the event is the plane of slip to be successful the method requires that a sufficient number of other events had the same or similar slip plane the modified three point method consists of finding the orientations of planes formed by combinations of the location of the earthquake under study and all other locations in the remainder of the earthquake catalog taken two at a time a test to determine which orientations occur more often then expected is used to identify statistically significant planes along which earthquakes cluster each plane identified passes through the earthquake beingmore studied if one of these planes is parallel or nearly parallel to one of the nodal planes of the fault plane solution that nodal plane is assumed to be the slip plane the method is demonstrated by investigating ten earthquakes that occurred during 1983 1984 near coalinga california using the method significant alignments of events along planes that are nearly parallel to one of the nodal planes for eight of the ten events were found less

the 1994 sefidabeh eastern iran earthquakes revisited new evidence from satellite radar interferometry and carbonate dating about the growth of an active fold above a blind thrust fault in 1994 three shallow earthquakes of mw 6 occurred close together on blind thrusts near sefidabeh in eastern iran in an earlier study of the teleseismic waveforms the geomorphology and the faulting in the epicentral region it was suggested that these earthquakes were associated with the growth of a ridge above a blind thrust fault system whose activity could be detected by its effect on the surface drainage in this study we present a sar interferogram that precisely determines the location and amount of coseismic surface displacements showing that the earthquakes in the sefidabeh sequence probably occurred on en echelon fault segments associated with three stepping ridges we also present u th dates of 100 ka for lake deposits uplifted by the growing ridge from the cumulative dated uplift and knowledge of the surface displacements due to an earthquake sequence we estimate that 120 such events have occurred in the past 100 ka with an average recurrence interval of 830 yr and an average convergence rate of 1 5 mm yr 1 on the sefidabeh thrust each estimate has an uncertainty of a factor of two either way we argue that the sefidabeh fault originally formed by coalescence of many small fault segments and has grown in length at about 2 cm yr 1 in the past 100 ka though the coseismic surface deformation observed in the sar interferogram closely resembles folding the overall topography does not because of inherited topography associated with earlier geological deformation in spite of this the activity of the buried thrust fault can easily be detected by its effect on the surface drainage a significant lesson when interpreting landscapes that are not entirely due to the present day deformation

application of the principal parameters method to the 1983 coalinga california aftershock sequence abstract a new technique called the method of the principal parameters ebblin and michelini in press to infer the orientations of active fault planes of an aftershock sequence has been applied to the sequence that followed the coalinga california earthquake of 2 may 1983 m l 6 7 the method is based on observed clustering in time and space clustering is a characteristic feature of aftershock sequences and it suggests interdependence of the events it follows that the spatial locations of time successive foci may provide additional information about the geometries of the rupturing fault system the method involves sliding of a temporal window of a fixed number of foci along the sequence and estimating the eigenvalues and eigenvectors of a spatial matrix for each window set this matrix can be interpreted as an ellipsoid which is fitted through the foci during the earthquake sequence the different trends of the seismicity pattern can be isolated by selecting and averaging the greatly flattened ellipsoids in the case of the coalinga sequence the trends are generally consistent with one of the focal planes obtained from the fault plane solutions given in earlier published studies the method appears to offer a simple way to infer average active fault geometries in complex areas from hypocentral locations only

stress rotation during the coalinga aftershock sequence this study considers spatial and temporal changes of the stress regime during the 1983 coalinga aftershock sequence in both cases the observed changes manifest themselves as rotations of the most compressive stress axis inversion of 165 m 3 aftershocks shows that in the southern aftershock zone the azimuth of the most compressional stress axis is oriented 24 e of north while in the northern and central zones it is oriented 51 e of north statistical analysis by use of non parametric bootstrap resampling shows that this rotation is significant at above the 95 confidence level inverting the focal mechanisms of 122 m 3 aftershocks in the central and northern aftershock zones reveals a rotation of the principal stress axes during the first 500 days after the main shock the compressional axis rotates from an azimuth of 62 e of north to 47 e of north statistical analysis by use of nonparametric bootstrap resampling shows that this rotation is just significantly different from zero at the 95 confidence level subject to the assumption that 80 of the fault planes can be correctly picked from the two nodal planes as the number of fault planes picked correctly varies from 50 random choices to 100 the significance of the rotation varies from 87 to 98 further investigation fails to find the signature of the stress changes predicted by dislocation theory in the focal mechanisms of the aftershocks active folds in the area suggest that the pre main shock stress regime had a compressional axis with a ne sw orientation thus the rotation during the aftershock sequence is thought to be a rebound from a stress anomaly induced by the main shock this stress anomaly may be causally related to the occurrence of the aftershocks

textural characterization of the pāhoeho a a transition in hawaiian basalt p hoehoe and a are the most common surface morphologies in basaltic lava flows yet no predictive models exist to link physical parameters of flow emplacement to changes in their surface textures and rheological properties we have characterized changes of vesicularity vesicle deformation and crystallinity across the p hoehoe a transition as preserved in two different eruptions a brief high velocity effusion of lava from k lauea volcano on 1 february 1996 and a small breakout from an ephemeral vent within a larger channel produced during the 1868 eruption of mauna loa this allowed us to compare conditions leading to the p hoehoe a transition for both open channel flow k lauea 1996 and reactivation of lava from an ephemeral vent mauna loa 1868 textural changes across the transition include a decrease in vesicularity and vesicle number density b increase in microlite crystallinity and crystal number density and c increase in vesicle deformation the results support past qualitative descriptions of the transition and highlight the importance of plagioclase crystallinity in controlling lava rheology and surface morphology

active intracontinental transpressional mountain building in the mongolian altai defining a new class of orogen abstract mountain ranges that are actively forming around the western and northern perimeter of the indo eurasia collisional deformational field such as the mongolian altai comprise a unique class of intracontinental intraplate transpressional orogen with structural and basinal elements that are distinct from contractional and extensional orogens late cenozoic uplift and mountain building in the mongolian altai is dominated by regional scale dextral strike slip faults that link with thrust and oblique slip faults within a 300 km wide deforming belt sandwiched between the more rigid junggar basin block and hangay precambrian craton dominant orogenic elements in the mongolian altai include double restraining bends terminal restraining bends partial restraining bends single thrust ridges thrust ridges linked by strike slip faults and triangular block uplifts in areas of conjugate strike slip faults the overall pattern is similar to a regional strike slip duplex array however the significant amount of contractional and oblique slip displacement within the range and large number of historical oblique slip seismic events renders the term transpressional duplex more accurate intramontane and range flanking basins can be classified as ramp basins half ramp basins open sided thrust basins pull apart basins and strike slip basins neither a classic fold and thrust orogenic wedge geometry nor a bounding foredeep exists the manner in which upper crustal transpressional deformation is balanced in the lower crust is unknown however crustal thickening by lower crustal inflation and northward outflow of lower crustal material are consistent with existing geological and geodetic data and could account for late cenozoic regional epeirogenic uplift in the russian altai and sayan regions

pāhoehoe a ā and block lava an illustrated history of the nomenclature lava flows occur worldwide and throughout history various cultures and geologists have described flows based on their surface textures as a result surface morphology based nomenclature schemes have been proposed in most languages to aid in the classification and distinction of lava surface types one of the first to be published was likely the nine class italian language description based classification proposed by mario gemmellaro in 1858 by far the most commonly used terms to describe lava surfaces today are not descriptive but instead are merely words specifically the hawaiian words a rough brecciated basalt lava and p hoehoe smooth glassy basalt lava plus block lava thick brecciated lavas that are typically more silicic than basalt a and p hoehoe were introduced into the western geological vocabulary by american geologists working in hawai i during the 1800s they and other nineteenth century geologists proposed formal lava type classification schemes for scientific use and most of them used the hawaiian words in 1933 ruy finch added the third lava type block lava to the classification scheme with the tripartite system being formalized in 1953 by gordon macdonald more recently particularly since the 1980s and based largely on studies of lava flow interiors a number of sub types and transitional forms of all three major lava types have been defined this paper reviews the early history of the development of the p hoehoe a and block lava naming system and presents a new descriptive classification so as to break out the three parental lava types into their many morphological sub types

control of early formed vesicle cylinders on upper crustal prismatic jointing in compound pāhoehoe lavas of elephanta island western deccan traps india upper crustal prismatic joints and vesicle cylinders common in p hoehoe lava flows form early and late respectively and are therefore independent features however small scale compound p hoehoe lava lobes on elephanta island western deccan traps india which resemble s type spongy p hoehoe in some aspects contain vesicle cylinders which apparently controlled the locations of upper crustal prismatic joints the lobes are decimeters thick did not experience inflation after emplacement and solidified rapidly they have meter scale areas that are exceptionally rich in vesicle cylinders up to 68 cylinders in 1 m2 with a mean spacing of 12 1 cm separated by cylinder free areas and pervasive upper crustal prismatic jointing with t curved t and quadruple joint intersections a majority 76 5 of the cylinders are located exactly on joints or at joint intersections and were not simply captured by downward growing joints as the cylinders show no deflection in vertical section we suggest that large numbers of cylinders originated in a layer of bubble rich residual liquid at the top of a basal diktytaxitic crystal mush zone which was formed very early probably within the first few minutes of the emplacement history the locations where the rising cylinders breached the crust provided weak points or mechanical flaws towards which any existing joints formed by thermal contraction propagated new joints may also have propagated outwards from the cylinders and linked up laterally some cylinders breached the crust between the joints and thus formed a little later than most others the elephanta island example reveals that whereas thermal contraction is undoubtedly valid as a standard mechanism for forming upper crustal prismatic joints abundant mechanical flaws such as large concentrations of early formed crust breaching vesicle cylinders can also control the joint formation process

detailed joint structure in a geothermal reservoir from studies of induced microearthquake clusters microearthquake clusters form distinct planar patterns within five study regions of a geothermal reservoir undergoing hydraulic fracturing at fenton hill new mexico the patterns define individual slipping joint surfaces of dimension 40 120 m containing 80 150 events each sharp straight edges truncate the clusters we interpret these as marking intersections with aseismic joints each edge orientation is consistent with an intersection between the active joint and a plane oriented parallel to one of the other clusters we identify therefore it appears that cluster shapes constrain the geometry of seismic and aseismic joints both could be important components of the fluid flow network the distribution of inferred slip plane orientations is consistent with but fails to provide sufficient constraint to differentiate conclusively between two very different stress field estimates one measured using pressurization and wellbore breakouts the other using focal mechanisms of the largest microearthquakes an impermeable joint model requiring pore pressure in excess of the normal stress on a joint before slip can occur was inconsistent with many of the inferred slip plane orientations the high quality locations were possible because events from the same cluster generated nearly similar waveforms permitting the precise determination of relative arrival times standard deviations of arrival time residuals fall between 0 1 and 1 1 ms for these clusters major axes and aspect ratios of the 90 confidence ellipsoids range from 6 to 28 m and 1 5 to 8 respectively small events dominate the seismic energy release and thoroughly populate the identified active joints allowing the hypocenters to reflect details of the joint structure to further investigate the reservoir structure we applied a source array slant stack technique to waveforms from the well located clusters yielding directions that scattered energy left each cluster by studying paths of scattered waves we expected to pinpoint impedance contrasts that might have indicated concentrations of fluid filled joints however results show that scattered energy in the s wave coda left the source region in the same direction as the direct s wave direct waves may have excited borehole tube waves that became trapped in the vicinity of the geophone tool overwhelming any energy scattered from the reservoir

spatial patterns of aftershocks of shallow focus earthquakes in california and implications for deep focus earthquakes previous workers have pioneered statistical techniques to study the spatial distribution of aftershocks with respect to the focal mechanism of the main shock application of these techniques to deep focus earthquakes failed to show clustering of aftershocks near the nodal planes of the main shocks to better understand the behavior of these statistics this study applies them to the aftershocks of six large shallow focus earthquakes in california august 6 1979 coyote lake may 2 1983 coalinga april 24 1984 morgan hill august 4 1985 kettleman hills july 8 1986 north palm springs and october 1 1987 whittier narrows the large number of aftershocks accurately located by dense local networks allows us to treat these aftershock sequences individually instead of combining them as was done for the deep earthquakes the results for individual sequences show significant clustering about the closest nodal plane and the strike direction for five of the sequences and about the presumed fault plane for all six sequences this implies that the previously developed method does work properly nonrandom behavior was also found about the slip directions the p axis the t axis and the b axis but this is probably caused by the lack of independence between these axes and the previously mentioned features of the focal mechanisms given that the method does work and that deep aftershocks were not shown to cluster about the main shock nodal planes the shallow focus data were used to simulate the deep focus study the goal is to determine if there are artificial factors that make clustering in the deep focus data unobservable to more closely mimic the work on deep earthquakes the largest aftershocks from each of the six sequences were combined and studied with respect to their respective main shock focal mechanisms this reduced the significance of the clustering about the focal mechanism parameters but not below 95 confidence gaussian noise was then added to the aftershock hypocenters in order to determine if the larger hypocentral and focal mechanism errors in the deep focus data could account for the previous negative result the conclusion is that the following reasons are sufficient to explain the lack of clustering about the main shock nodal planes for the deep focus aftershocks the need to combine aftershocks from several sequences the size of the hypocentral location and focal mechanism errors and the alignment of distant aftershocks with the wadati benioff zone

cooling and crystallization of lava in open channels and the transition of pāhoehoe lava to a ā samples collected from a lava channel active at kilauea volcano during may 1997 are used to con strain rates of lava cooling and crystallization during early stages of flow lava erupted at near liquidus tem peratures f1150 7c cooled and crystallized rapidly in upper parts of the channel glass geothermometry indi cates cooling by 12 14 7c over the first 2 km of trans port at flow velocities of 1 2 m s this translates to cooling rates of 22 50 7c h cooling rates this high can be explained by radiative cooling of a well stirred flow consistent with observations of non steady flow in proximal regions of the channel crystallization of plag ioclase and pyroxene microlites occurred in response to cooling with crystallization rates of 20 50 per hour crystallization proceeded primarily by nucleation of new crystals and nucleation rates of f10 4 cm 3 s are similar to those measured in the 1984 open channel flow from mauna loa volcano there is no evidence for the large nucleation delays commonly assumed for plagioclase crystallization in basaltic melts possibly a reflection of enhanced nucleation due to stirring of the flow the transition of the flow surface morphology from pahoehoe to a aoccurred at a distance of 1 9 km from the vent at this point the flow was thermally stratified with an interior temperature of f1137 7c and crystallinity of f15 and a flow surface tempera ture of f1100 7c and crystallinity of f45 a afor mation initiated along channel margins where crust was continuously disrupted and involved tearing and clotting of the flow surface both observations suggest that the transition involved crossing of a rheological threshold we suggest this threshold to be the develop ment of a lava yield strength sufficient to prevent vis cous flow of lava at the channel margin we use this concept to propose that a aformation in open chan nels requires both sufficiently high strain rates for con tinued disruption of surface crusts and sufficient groundmass crystallinity to generate a yield strength equivalent to the imposed stress in hawai i where lava is typically microlite poor on eruption these combined requirements help to explain two common observations on a aformation a a aflow fields are generated when effusion rates are high thus promoting crustal disruption and b under most eruption conditions lava issues from the vent as pahoehoe and changes to a aonly after flowing some distance thus permitting sufficient crystallization

lidar derived surface roughness signatures of basaltic lava types at the muliwai a pele lava channel mauna ulu hawai i we used light detection and ranging lidar data to calculate roughness patterns homogeneity mean roughness and entropy for five lava types at two different resolutions 1 5 and 0 1 m pixel we found that end member types a and p hoehoe are separable with 95 confidence at both scales indicating that roughness patterns are well suited for analyzing types of lava intermediate lavas were also explored and we found that slabby p hoehoe is separable from the other end members using 1 5 m pixel data but not in the 0 1 m pixel analysis this suggests that the conversion from p hoehoe to slabby p hoehoe is a meter scale process and the finer roughness characteristics of p hoehoe such as ropes and toes are not significantly affected furthermore we introduce the ratio frac ent hom derived from lava roughness as a proxy for assessing local lava flow rate from topographic data high entropy and low homogeneity regions correlate with high flow rate while low entropy and high homogeneity regions correlate with low flow rate we suggest that this relationship is not directional rather it is apparent through roughness differences of the associated lava type emplaced at the high and low rates respectively

dynamics and viscosity of a a and pahoehoe lava flows of the 2012 2013 eruption of tolbachik volcano kamchatka russia the 2012 2013 flank eruption of tolbachik volcano kamchatka lasted 9 months and produced 0 54 km3 of basaltic trachyandesite lava thus becoming one of the most voluminous historical lava effusions of basic composition in subduction related environments globally from march to july 2013 the volcano monotonously erupted lava of constant composition sio2 52 wt with a nearly stable effusion rate of 18 m3 s despite the uniform eruptive and emplacement conditions the dominant style of lava propagation throughout that time gradually changed from a a to pahoehoe we report results of instrumental field measurements of the a a and pahoehoe flow dynamics documented with time lapse cameras as well as the lava viscosity determined by flow rate and shear stress using penetrometer methods maximal propagation velocities of the a a fronts ranged from 2 to 25 mm s and those of the pahoehoe from 0 5 to 6 mm s the flow front velocities of both lava types experienced short period fluctuations that were caused by complex flow mechanics of the advancing flow lobes minimal viscosities of lava of the a a lobes ranged from 1 3 105 to 3 3 107 pa s flow rate method and those of the pahoehoe from to 5 103 to 5 104 pa s shear stress method our data include the first ever measured profiles of viscosity through the entire thickness of actively advancing pahoehoe lava lobes we have found that both the a a and pahoehoe flows were fed by identical parental lava which then developed contrasting rheological properties owing to differences in the process of lava transport over the ground surface the observed transition from the dominant a a to the dominant pahoehoe propagation styles occurred due to gradual elongation and branching of the lava tube system throughout the course of the eruption such evolution became possible because the growing lava field composed of semisolidified flows provided an environment for shallow subsurface intrusions and internal migrations of lava that with time developed into branches of the lava tube system based on our data we propose phenomenological models of the a a and pahoehoe flow mechanics

mapping the distribution of vesicular textures on silicic lavas using the thermal infrared multispectral scanner the thermal infrared multispectral scanner tims is a potentially powerful tool for mapping detailed chemical variations in silicic lava flows which in turn could expand knowledge of pre eruption compositional gradients and mixing processes in silicic magma bodies however thermal infrared image data may be greatly influenced by the range of vesicular textures which occur on silicic flows to investigate the effect of vesicularity on tims imagery independent of chemical variations we studied little glass mountain at the medicine lake volcano of northern california a large rhyolitic flow of uniform composition but textural heterogeneity the imagery was recalibrated so that the digital number values for a lake in the scene matched a calculated ideal spectrum for water tims spectra for the lava show useful differences in coarsely and finely vesicular pumice data particularly in tims bands 3 and 4 images generated by ratioing these bands accurately map out those areas known from field studies to be coarsely vesicular pumice these texture related emissivity variations are probably due to the larger vesicles being relatively deeper and separated by smaller septa leaving less smooth glass available to give the characteristic emission of the lava in studies of inaccessible lava flows as on mars areas of coarsely vesicular pumice must be identified and avoided before chemical variations can be interpreted remotely determined distributions of vesicular and glassy textures can also be related to the volatile contents and potential hazards associated with the emplacement of silicic lava flows on earth

the importance of pahoehoe abstract pahoehoe lava flows are common in every basaltic province and their submarine variants pillow lavas and sheet flows cover the bulk of the earth pahoehoe flows are emplaced by inflation the injection of molten lava underneath a solidified crust only in the past few years has an understanding of the inflation process and the ability to recognize ancient inflated lava flows been achieved all large terrestrial basaltic flow fields studied to date including flood basalts were emplaced as thermally efficient inflated compound pahoehoe sheet flows this leads us to propose that this is the standard way of emplacing large lavas the swell hypothesis the atmospheric impact of such flood basalt eruptions could have been protracted and severe providing a plausible link between flood basalt eruptions and mass extinctions

welded breccias in andesite lavas flow breccias formed at the margins of blocky andesite lavas can be reheated welded and sometimes deformed to form rocks reminiscent of welded pyroclastic rocks reheating occurs due to advection of heat from the flow to the basal breccias surface breccias also infill extensional crevasses which are later closed and compressed strongly deformed welded breccia bands form within the flow levees the angular breccia clasts can be deformed into fiamme

internal textures of rhyolite flows as revealed by research drilling recent drilling of 550 and 140 000 yr old rhyolite lava flows in the inyo dome chain california and valles caldera new mexico and field inspection of 8 ma flows in western arizona reveal a more detailed vertical zonation of lava textures in glassy rhyolite flows than has previously been recognized the upper vitrophyre of a flow can usually be subdivided into a surficial finely vesicular pumice underlain by obsidian coarsely vesicular pumice and a second obsidian layer beneath these layers is the crystalline center of the flow which in turn is underlain by a basal obsidian layer lower vitrophyre differences in lava vesicularity and crystallinity between zones in a flow result from primary effervescence devitrification lava rheology and migration of water vapor

the 1997 may 10 zirkuh qa enat earthquake mw 7 2 faulting along the sistan suture zone of eastern iran summary the destructive zirkuh e qa enat earthquake of 1997 may 10 mw 7 2 ms 7 3 mb 6 3 produced 125 km of nnw sse right lateral strike slip surface faulting on the abiz fault in the sistan suture zone of eastern iran the longest known surface rupture associated with an iranian earthquake analysis of the body wave seismograms from the main shock shows that rupture occurred in four main subevents propagating in a sequence from north to south although predominantly strike slip the orientation of the faulting in each subevent varies with appreciable reverse components in the north central part and at the southern end of the abiz fault this change in fault style along the abiz fault inferred from the seismograms is also seen in the coseismic surface ruptures and the geomorphology average coseismic surface displacements were approximately 2 m implying a static stress drop of only 5 bar 0 5 mpa the 1997 surface ruptures followed clear traces of late quaternary slip on the abiz fault and for its northern 50 km re ruptured fault segments that had slipped in previous earthquakes of ms 6 0 6 6 in 1936 and 1979 the 1997 earthquake ruptured the northern end of the n s right lateral strike slip system of the sistan suture zone ending where it abuts a system of e w left lateral strike slip faults which have also slipped in large earthquakes during the last 30 years the earthquakes on this conjugate system of strike slip faults form a sequence that may have been triggered by the enhancement of stress on one fault as a result of slip on a neighbouring fault together these faults achieve n s right lateral shear of deforming iran against stable western afghanistan by n s slip on the right lateral faults and clockwise rotation of the e w left lateral faults

emplacement and inflation of pahoehoe sheet flows observations and measurements of active lava flows on kilauea volcano hawaii inflated pahoehoe sheet flows have a distinctive horizontal upper surface which can be several hundred meters across and are bounded by steep monoclinal uplifts the inflated sheet flows we studied ranged from 1 to 5 m in thickness but initially propagated as thin sheets of fluid pahoehoe lava generally 20 30 cm thick individual lobes originated at outbreaks from the inflated front of a prior sheet flow lobe and initially moved rapidly away from their source velocities slowed greatly within hours due to radial spreading and to depletion of lava stored within the source flow as the outward flow velocity decreases cooling promotes rapid crustal growth at first the crust behaves plastically as pahoehoe toes form after the crust attains a thickness of 2 5 cm it behaves more rigidly and develops enough strength to retain incoming lava thus increasing the hydrostatic head at the flow front the increased hydrostatic pressure is distributed evenly through the liquid lava core of the flow resulting in uniform uplift of the entire sheet flow lobe initial uplift rates are rapid flows thicken to 1 m in 1 2 hours but rates decline sharply as crustal thickness increases and as outbreaks occur from the margins of the inflating lobe one flow reached a final thickness of nearly 4 m after 350 hr inflation data define power law curves whereas crustal cooling follows square root of time relationships the combination of data can be used to construct simple models of inflated sheet flows as the flow advances preferred pathways develop in the older portions of the liquid cored flow these pathways can evolve into lavatube systems within a few weeks formation of lava tubes results in highly efficient delivery of lava at velocities of several kilometers per hour to a flow front that may be moving 1 2 orders of magnitude slower if advance of the sheet flow is terminated the tube remains filled with lava that crystallizes in situ rather than draining to form the cave like lava tubes commonly associated with pahoehoe flows inflated sheet flows from kilauea and mauna loa are morphologically similar to some thick icelandic and submarine sheet flows suggesting a similar mechanism of emplacement the planar sheet like geometry of flood basalt flows may also result from inflation of sequentially emplaced flow lobes rather than nearly instantaneous emplacement as literal floods of lava

structures within large volume rhyolite lava flows of the devonian comerong volcanics southeastern australia and the pleistocene ngongotaha lava dome new zealand abstract many rhyolitic units within the late devonian comerong volcanics erupted as lava flows and domes some up to 18 km long and 350 m thick the textural and structural characteristics that distinguish the flows and domes as lava rather than rheomorphic ignimbrite include unbroken phenocrysts zones of autobrecciation and finely developed flow layering with individual layers continuous for several metres the flow layering is typically contorted into isoclinal folds with forms suggesting fluidal deformation and is interlayered with and gradational into restricted zones of pumice rich lapilli tuff and zones of lenticulite breccia the lenticulite breccia comprises discontinuous lenticular rhyolite fragments the long axes of which define a foliation parallel to flow layering lenticles in the breccia vary from elongate layers up to 1 m long and several millimetres thick to short fragments less than 10 cm long and several centimetres wide similar zones of lenticulite breccia consisting of glassy lenticular clasts in a devitrified spherulitic matrix of cristobalite and albite exist within the late pleistocene ngongotaha dome near rotorua new zealand the lenticulite breccia is considered to form by aqueous diffusion and selective devitrification of the rhyolite along anastomosing fluid paths and to be modified by mechanical fracturing of the lava in a zone of high shear stress geochemically the rhyolites are high si and a type with high zr and y contents indicating that they formed from high temperature relatively anhydrous f rich melts a type granitoids crop out intermittently along the length of and adjacent to the volcanic complex and are comagmatic with the rhyolite the high temperature low bubble and phenocryst content and a high eruptive rate of the rhyolite likely resulted in a low effective viscosity and extensive flow units

lava field evolution and emplacement dynamics of the 2014 2015 basaltic fissure eruption at holuhraun iceland abstract the 6 month long eruption at holuhraun august 2014 february 2015 in the bar arbunga vei ivotn volcanic system was the largest effusive eruption in iceland since the 1783 1784 ce laki eruption the lava flow field covered 84 km2 and has an estimated bulk i e including vesicles volume of 1 44 km3 the eruption had an average discharge rate of 90 m3 s making it the longest effusive eruption in modern times to sustain such high average flux the first phase of the eruption august 31 2014 to mid october 2014 had a discharge rate of 350 to 100 m3 s and was typified by lava transport via open channels and the formation of four lava flows no 1 4 which were emplaced side by side the eruption began on a 1 8 km long fissure feeding partly incandescent sheets of slabby p hoehoe up to 500 m wide by the following day the lava transport got confined to open channels and the dominant lava morphology changed to rubbly p hoehoe and a the latter became the dominating morphology of lava flows no 1 8 the second phase of the eruption mid october to end november had a discharge of 100 50 m3 s during this time the lava transport system changed via the formation of a 19 km2 of the flow field the primary lava morphology from this phase was spiny p hoehoe which superimposed on the a lava flows no 1 3 and extended the entire length of the flow field i e 17 km this made the 2014 2015 holuhraun a paired flow field where both lava morphologies had similar length we suggest that the similar length is a consequence of the p hoehoe is fed from the tube system utilizing the existing a lava channels and thereby are controlled by the initial length of the a flows

structure and origin by injection of lava under surface crust of tumuli lava rises lava rise pits and lava inflation clefts in hawaii tumuli are positive topographic features that are common on hawaiian pahoehoe lava flow fields particularly on shallow slopes and 75 measured examples are presented here to document the size range tumuli form by up tilting of crustal plates without any crustal shortening and are thus distinguished from pressure ridges which are up buckled by laterally directed pressure the axial or star like systems of deep clefts that characterize tumuli are defined here as lava inflation clefts their tips advanced into red hot lava and they widened as uplift proceeded and while the lava crust was thickening flat surfaced uplifts formed like tumuli by injection of lava under a surface crust were previously called pressure plateaus but lava rise is proposed instead the pits that abound among lava rises previously attributed to collapse or subsidence are generally formed because the lava around them rose and the name lava rise pit is proposed unique examples of tumuli and lava rises from which lava drained out under a surface crust 1 5 to 2 5 m thick are described from kilauea caldera these examples show that in tumuli and lava rises the crust floats on considerable bodies of fluid lava and is able to do so because of its higher vesicle content the fluid lava loses many of its gas bubbles during residence beneath the crust the bulk densities of samples from tumuli show a general downward increase the form of the density profile is consistent with the relationship that for any given crustal thickness the density of fluid lava closely matched the average density of that crust suggesting that the lava was stably density stratified it is inferred that stable stratification was regulated by out flows of the more vesicular lava fractions loss of bubbles through the lava inflation clefts and entry of injected lava at its level of neutral buoyancy below the uppermost meter the downward decrease in vesicularity closely conforms with that expected by compression of a uniform mass of gas per unit mass of lava

origin of vesicle layering and double imbrication by endogenous growth in the birkett basalt flow columbia river plateau abstract the 40 m thick birkett basalt pahoehoe flow at sentinel gap in the columbia river plateau has an unusually thick 15 m upper vesicular zone this zone includes a striking layering in which the layers have contrasted vesicle abundances and sizes most layers show a reverse grading of vesicle size and abundance the layering is interpreted to have grown endogenously by the cyclic injection of vesicular lava layers under the growing top crust accommodated by uplift of that crust grading of the layers resulted from vesicle growth and ascent each injection occurred at or near the boundary between vesicular and non vesicular lava of the preceding layer and split that layer into an upper vesicular part and a lower non vesicular part critical to this interpretation are 1 a pervasive foliation and lineation defined by the parallelism of strongly flattened and elongate vesicles transects the vesicle layers obliquely and 2 the magnetic fabric the anisotropy of magnetic susceptibility is oriented similarly to the vesicle foliation and also defines a cryptic foliation in the non vesicular zone having a dip opposed to that in the layered zone these foliations are interpreted to be opposed imbrications and indicate the flow azimuth of the lava they strongly support the concept of lava growth by successive thin sill like insertions of fresh vesicular lava between hot but static and effectively solid floor and roof

eruption reconstruction formation of flow lobe tumuli and eruption duration in the 5900 bp helgafell lava field heimaey south iceland abstract the 5900 bp helgafell eruption started with an intense fire fountaining phase that deposited scoria and spatter near the vent spatter accumulated agglutinated and was subsequently emplaced as clastogenic flows the intense initial phase was followed by effusive emplacement of tube fed pahoehoe this type of lava emplacement makes up the bulk of the lava field and contains abundant inflation features such as tumuli and pressure ridges we measured the depths of 358 inflation clefts in tumuli and the thicknesses of the upper vesicular crust in flow lobes from the helgafell lava field measurements of altitude between a point source of lava tubes and the depth of inflation clefts in tumuli suggest that the magmastatic pressure affects initial tumuli formation the overall growth of tumuli is however more dependent on effusion rate and collapse clogging of individual tubes than on the magmastatic head within tubes three areas with abundant tumuli are recognized in the helgafell lava field each of which was active for at least 47 days the average time for tumuli formation is 32 days using the crustal measurements of inflation features we test the hypothesis that the total eruption duration can be estimated for a pre historic volcanic eruption we chose the helgafell lava field to test this hypothesis because it has 1 abundant inflation features that are easily measured and 2 the majority of lava flows emplaced during the eruption are well exposed our result shows that the duration of the helgafell eruption can be estimated to 11 12 months yielding an average volumetric effusion rate for the helgafell eruption of 0 05 0 06 km3 month this result is very similar to the effusion rates for two known eruptions in the same volcanic system i e the 1963 1967 surtsey and the 1973 eldfell eruptions with effusion rates of 0 02 and 0 04 km3 month respectively given that all three eruptions occurred within the same volcanic system and that they have similar volumetric effusion rates we suggest that our estimated eruption duration for helgafell is reasonable our results building on the suggestion by hon et al 1994 hon k kauhikaua j denlinger r mackay k 1994 emplacement and inflation of pahoehoe sheet flows observations and measurements of active lava flows on kilauea volcano hawaii geol soc amer bull 106 351 370 that the duration of inflation can be estimated for individual flows by measuring inflation features show that by combining abundant and measureable inflation features with a well established stratigraphy the duration of an entire pre historic eruption can be estimated

lava rise plateaus and inflation pits in the mccartys lava flow field new mexico an analog for pāhoehoe like lava flows on planetary surfaces

automated identification of lava flow structures using local fourier spectrum of digital elevation data 1 the rheological parameters of a lava flow can be inferred by studying structural features on the solidified lava surface we have developed a technique using the s transform a localized fourier transform to analyze and interpret digital elevation data we examined structural features of a silicic lava flow the medicine lake dacite flow in california the s transform accurately identified and located both repeating and single instance structures several classes of structures were identified in the data including the following localized short wavelength features 1 3 3 6 m in wavelength identified as large blocks and repeating medium wavelength features 5 8 10 16 18 26 30 36 and 56 67 m in wavelength identified as multiple generations of crustal folding fold wavelengths determined by the s transform and a model of compressional folding associated with near surface viscosity gradients were used to evaluate the rheology of the medicine lake dacite flow the similarity of the amount of strain during folding events and the wavelength ratio of successive generations of folding suggest that the flow rheology remained relatively constant during folding of the flow surface repeating long wavelength features 67 70 and 105 140 m wavelengths were identified as the spacing of crease structures crease structures were recognized by the absence of short wavelength features at the location of their smooth flanks and by a central spike corresponding to the central trough the limitations and applicability of this approach lie in the accuracy and resolution of the digital elevation data used

source parameters of the 1 october 1995 dinar turkey earthquake from sar interferometry and seismic bodywave modelling abstract the 1 october 1995 m s 6 1 dinar earthquake ruptured a 10 km section of the nw se dinar civril fault there are discrepancies between the published source parameters from seismic data with seismic moments in disagreement by over a factor of two we use both sar interferometry and seismic bodywave modelling to determine earthquake source parameters an interferogram generated from ers 1 2 sar imagery spanning the event and separated by 5 months is used to derive source parameters by a downhill simplex inversion with multiple monte carlo restarts we model the displacements in the satellite line of sight initially using uniform slip on a rectangular dislocation in an elastic half space the resultant model fault plane agrees in strike and location with the observed surface break but systematic residuals exist in the line of sight deformation field resulting in a r m s residual of 20 mm in the interferogram the residuals are reduced if the depth distribution of slip is allowed to vary spatially in four segments along a continuous fault plane our best fitting solution with a r m s misfit of 8 mm reveals two distinct areas of slip on the fault plane strike 145 dip 49 rake 270 a main rupture slipping by 1 44 m between depths of 1 and 8 km becoming deeper to the se and matching the observed surface rupture and an along strike continuation to the nw of the same fault plane but between depths of 8 and 13 km and not associated with a surface break the total geodetic moment 4 5 0 1 10 18 nm is more than twice as large as published seismic moments based on the inversion of p waveforms alone but close to the harvard cmt moment 4 7 10 18 nm we use sh waveforms in addition to the p waves used previously to determine an alternative seismic source mechanism sh waves constrain the depth to be shallower than solutions based on p waves alone agreeing with the depths from the interferometric inversion and resulting in a larger moment 3 1 0 4 10 18 nm than the previous bodywave estimates 2 2 1 10 18 nm the cmt moment reduces in magnitude to a similar size 3 3 10 18 nm if the centroid depth and fault dip are constrained to the values determined from bodywave modelling and interferometry thus the geodetic moment is 40 bigger than the moment determined from seismology

pipe vesicles in hawaiian basaltic lavas their origin and potential as paleoslope indicators study of several hawaiian tholeiitic volcanoes shows that pipe vesicles form only in thin pahoehoe flow units standing on a ground slope of less than about 4 the vesicles thus have potential as paleoslope indicators two examples are given of their use to deduce that the steep dip of certain intracaldera lavas is due to tilting pipe vesicles are attributed to gas bubbles rising through lava at a stage in its cooling history when it had acquired a yield strength of a few tens of n m 2 sufficient to prevent closure of lava behind the bubbles

investigating the volcanic versus aqueous origin of the surficial deposits in eastern elysium planitia mars abstract the elysium volcanic province consists of numerous overlapping flow units and may include the youngest lava flows on mars however it is possible that these volcanic units have been modified or overprinted by aqueous processes understanding the timing of the igneous and aqueous events in this region is therefore essential for constraining the geological and environmental history of mars during the amazonian period we investigate the geologic evolution of eastern elysium planitia to determine the relationship between major units with the support of a geological map and chronological constraints from crater size frequency distributions we also evaluate the hypothesized origin of these units via volcanic fluvial and or fluvioglacial processes using a detailed facies mapping approach the study area includes the eastern cerberus fossae rahway valles and marte vallis the surficial deposits in rahway valles were formerly interpreted to be modified by fluvial and fluvioglacial processes however our facies map reveals that the surface of eastern elysium planitia includes nineteen morphologically distinct regions i e facies which are interpreted to be the products of flood lava volcanism including a p hoehoe and transitional lava flow types in contrast to previous studies which determined that rahway valles and marte vallis consist of two distinct geologic units with middle to late amazonian ages the results of this work show that the region was resurfaced by at least two volcanic flows with much younger ages of 20 0 ma and 8 8 ma furthermore by coupling results of our geologic and facies mapping with chronological constraints as well as subsurface information provided by shallow radar reflectors we show that there is an erosional unconformity located between the two youngest lava flow units in marte vallis we interpret that this unconformity was generated by a catastrophic aqueous flooding event that occurred only 8 8 20 0 ma ago this implies alternating episodes of volcanism and aqueous flooding that have continued into the geologically recent past on mars and may again occur within elysium planitia

upper mantle anisotropy beneath the altai sayan region of central asia abstract the altai sayan region of central asia is actively deforming despite its being located several thousands of kilometers away from the collisional front between india and eurasia to add constraints to the role of the upper mantle in this type of tectonic environment we analyzed intermediate period analogue records of sks and skks to determine the state of mantle anisotropy in this region we find that for all stations located in actively deforming areas the fast direction of anisotropy correlates well with indicators of cenozoic to the present day deformation and correlates with older structures only when they happen to align with younger ones the general east northeast trend of the fast direction of anisotropy in active regions also aligns with the principle extensional strain axis determined from earthquake moment tensors at one station nvs located outside of any actively deforming region the n s fast direction may correspond to the long wavelength strain field of the mantle beneath much of siberia these correlations along with the size of the delay times 1 2 s suggest that the anisotropy in the upper mantle beneath the altai sayan is responding to recent tectonic activity

objective regionalization of rayleigh wave dispersion data by clustering algorithms summary an objective regionalization of a restricted domain of the earth s surface in terms of phase and group velocity data without previous tectonic information is presented we apply our working scheme to local rayleigh wave velocities which were determined from the study of teleseismic events recorded at the broad band stations of the network of autonomously recording seismographs nars installed in the iberian peninsula during the iliha project according to the phase and group velocity maps computed at different wave periods each point of the domain studied is characterized by two dispersion curves corresponding respectively to phase and group velocities the procedure that we propose consists of the application of principal component analysis pca to those velocities defining the local dispersion characteristics of the medium and then an average linkage al algorithm it is worthy of mention that the pca procedure enhances the relationship between local phase and group velocities in this way we obtain a classification of the domain into 10 homogeneous regions each homogeneous area is almost coincident from a seismotectonic point of view with either neogene alpine or hercynian domains after the classification we are able to deduce by stochastic inversion eight different shear velocity structures for eight of the 10 homogeneous regions all the structures deduced suggest a well developed lowvelocity channel from 80 to 180 km in depth for the whole iberian peninsula but with a clearly distinct evolution of their shear velocities with depth some of the models reach shear velocities close to 4 75 km s at depths beneath this channel the highest shear velocities close to 5 0 km s l are obtained at a depth of 50 km and down to 200 km for region 3 neogene and two possible low velocity channels down to a depth of 225 km appear for models 7 and 8 hercynian domains the existence of low velocity channels in some models at depths of less than 30 km is discussed in terms of the resolving kernels and the generation of compatible models in short the main realistic differences between the models come from the shear velocities modelling each low velocity channel as an example the shear velocities for depths ranging from 80 to 180 km vary according to the region from 3 82 km s l to 4 42 km s

radar interferogram filtering for geophysical applications the use of sar interferometry is often impeded by decorrelation from thermal noise temporal change and baseline geometry power spectra of interferograms are typically the sum of a narrow band component combined with broad band noise we describe a new adaptive filtering algorithm that dramatically lowers phase noise improving both measurement accuracy and phase unwrapping while demonstrating graceful degradation in regions of pure noise the performance of the filter is demonstrated with sar data from the ers satellites over the jakobshavns glacier of greenland

shear rate dependence of the pāhoehoe to a ā transition analog experiments hawaiian lava flows typically erupt as pahoehoe and transform to a aas they travel away from eruptive vents although the exact conditions that govern this transformation are not well constrained here we describe a set of laboratory experiments that use corn syrup and ellipsoidal rice grains as an analog to lava with crystals to examine the depen dence of the transition on shear rate and particle concentration at a particle volume fraction of 0 3 increasing the shear rate produces a sequence of deformation regimes defined by increasing amounts of shear localization the same regimes may be traversed at constant shear rate by varying the particle concentration at high shear rates the onset of nonlaminar deformation corresponds to the percolation threshold thus providing a link between suspension microstructure and deformation behavior together these results high light the combined importance of shear rate and crystallinity in determining the flow behavior of basaltic lava

lithosphere rheology and active tectonics in mongolia relations between earthquake source parameters gravity and gps measurements in this paper we use observations of earthquake source parameters and gravity to investigate the mechanical properties and the active faulting of the lithosphere in mongolia well determined earthquake centroid depths including 10 from inversions of p and sh waveforms that are presented here for the first time show that the seismogenic thickness ts within mongolia itself is less than 20 km however to both the east in parts of the lake baikal rift system and the west adjacent to the junggar basin and kazakhstan platform the seismogenic thickness is considerably greater and includes essentially the whole crust from the admittance between the free air gravity and the topography and also from profiles across a flexural foreland basin we determine the effective elastic thickness te in central mongolia to be 30 km in both mongolia and its surroundings te is comparable with ts and where it is well determined te ts these data are consistent with the view that the strength of the continental lithosphere resides in its seismogenic part which in mongolia is the upper crust but to both the east and west appears to be the whole crust the earthquake source parameters also allow us to ask how the active faulting in mongolia accommodates the velocity field revealed by gps measurements it is likely that the entire mongolian altay range in the west rotates counter clockwise relative to stable asia and is responsible for the distributed e w left lateral shear seen further east in central mongolia the admittance observations show no evidence at the present day for convective mantle support or a hotspot responsible for the elevated region of the hangay dome in central mongolia even though the geochemical data from nodules in late cenozoic basalts and seismic tomography studies suggest elevated temperatures at shallow depths 125 km and probably thinned lithosphere

revel a model for recent plate velocities from space geodesy 1 we present a new global model for recent plate velocities revel describing the relative velocities of 19 plates and continental blocks the model is derived from publicly available space geodetic primarily gps data for the period 1993 2000 we include an independent and rigorous estimate for gps velocity uncertainties to assess plate rigidity and propagate these uncertainties to the velocity estimates the velocity fields for north america eurasia and antarctica clearly show the effects of glacial isostatic adjustment and australia appears to depart from rigid plate behavior in a manner consistent with the mapped intraplate stress field two thirds of tested plate pairs agree with the nuvel 1a geologic 3 myr average velocities within uncertainties three plate pairs caribbean north america caribbean south america and north america pacific exhibit significant differences between the geodetic and geologic model that may reflect systematic errors in nuvel 1a due to the use of seafloor magnetic rate data that do not reflect the full plate rate because of tectonic complexities most other differences probably reflect real velocity changes over the last few million years several plate pairs arabia eurasia arabia nubia eurasia india move more slowly than the 3 myr nuvel 1a average perhaps reflecting long term deceleration associated with continental collision several other plate pairs including nazca pacific nazca south america and nubia south america are experiencing slowing that began 25 ma the beginning of the current phase of andean crustal shortening

structural origin of large gas cavities in the big obsidian flow newberry volcano mesoscopic structures in four large cavities on the big obsidian flow or usa provide evidence of links between large scale surface folding cavity formation and hazardous explosive activity originating from within the flow stereographic analysis of flow banding stretched bubble lineation striations and mesoscopic fold axes indicates that three of the cavities are near cylindrical folds cavity fold axes are oriented parallel to large scale compressional flow ridge axes suggesting that cavities form in response to shortening during flow advance large surface folds develop as the upper 10 m of the cooler flow surface buckles when the flow front cools slows and stalls against topographic barriers void space is created in fold hinges as obsidian layers shear past each other and pull apart the resultant cavities may serve as reservoirs for exsolved volatiles or surface water which may later vent explosively to form explosion craters on the flow surface

the morphology and formation of flow lobe tumuli on icelandic shield volcanoes three types of tumuli are recognized on monogenetic shield volcanoes in iceland namely 1 lava coated tumuli 2 upper slope tumuli and 3 flow lobe tumuli flow lobe tumuli grade into lava rises which are larger inflation structures but both structures have a similar mode of emplacement the areal distribution of flow lobe tumuli in the flow field differs from that of lava coated tumuli and upper slope tumuli flow lobe tumuli are frequent in the medial and distal parts of pahoehoe flow fields whereas the other two tumulus types are more frequent in the proximal parts of the flow fields the flow lobe tumuli also differ from the two other tumulus types as regards their morphology they are significantly larger have shallower flanks and do not have extensive outflows from the cracks the vesicle distribution within the flow lobe tumuli is similar to that of p type flow lobes a fairly dense zone is situated between the upper and lower vesicular zones each flow lobe tumulus is an individual lobe that inflates and forms tension cracks in the lava crust major lava tubes are mostly absent from the areas where flow lobe tumuli form and therefore the emplacement of flow lobe tumuli must be related to secondary short lived pathways of lava the flow lobe tumuli inside the flow fields of icelandic shield volcanoes are morphologically identical to those flow lobe tumuli that are found in the margins of flow fields of other icelandic volcanoes these observations allow us to model the flow lobe tumuli as morphological variants of p type flow lobes forming along terminating secondary lava paths flow lobe tumuli form at low lava supply rates of the order of 10 2 10 5 m3 s and are generated by inflation of the lava crust as a result of magmatic overpressure in the associated lava core we model the tumulus as a circular elastic plate that becomes uplifted inflated and fractured by a uniform magmatic overpressure at its bottom we conclude that 1 the effective thickness of the lava crust that is subject to bending is 0 2 0 4 m and remains essentially constant during the formation of the tumulus and 2 for flow lobe tumuli that are supplied with magma from tubes that originate in overflow from the lava lake of a shield volcano the elevation difference between the tumuli and the surface of the source lava lake is commonly around 100 m this elevation difference can give rise to a static magmatic overpressure of 2 5 mpa in the tumulus whereas the overpressure needed to form a typical flow lobe tumulus is only 0 2 1 mpa

field examples of strike slip fault terminations in mongolia and their tectonic significance deformation at the ends of large intracontinental strike slip faults that do not simply link other major structures often involves rotations about a vertical axis we use earthquake slip vectors surface rupture in earthquakes and geomorphology to examine the ends of three major strike slip faults in mongolia in these places a simple pattern is seen consisting of a thrust fault on one side with a displacement that decreases away from the strike slip fault consistent with local rotational deformation strike slip faults that terminate in this way allow the style of faulting to change spatially within a deforming area for example from dominantly strike slip to dominantly dip slip while still accommodating the overall deformation required by larger scale regional motions such a change in fault style should also be accompanied by a change in the rotation rate about a vertical axis which may be detected paleomagnetically the kind of strike slip fault termination described here may have consequences for how large strike slip fxaults evolve and grow and for the variation in displacement along their length