relationship between dike and volcanic conduit distribution in a highly eroded monogenetic volcanic field san rafael utah usa we mapped 63 conduits 2000 dike segments and 12 sills in the san rafael subvolcanic field utah united states where this pliocene magmatic system is eroded to a depth of 0 8 km and is exceptionally well exposed although the number of mapped conduits dikes and sills might represent minimums depending on the level of erosion and exposure mapped dikes are more numerous around the areally extensive sills and interact with sills and conduits in complex ways we analyze conduit distribution using kernel density methods and compare results with dike and sill distribution we find that the distribution of conduits matches the major features of dike distribution including development of clusters and distribution of outliers these statistical models are then applied to the distributions of volcanoes in several recently active volcanic fields where intrusion distributions must be inferred from very sparse data and compared with san rafael conduit distribution this comparison supports the use of statistical models in probabilistic hazard assessment for distributed volcanism specifically renewed dike intrusion and potential eruptions in active basaltic systems can be assessed probabilistically from the distribution of older volcanoes in distributed volcanic systems

dune bedforms produced by dilute pyroclastic density currents from the august 2006 eruption of tungurahua volcano ecuador a series of pyroclastic density currents were generated at tungurahua volcano ecuador during a period of heightened activity in august 2006 dense pyroclastic flows were confined to valleys of the drainage network while dilute pyroclastic density currents overflowed on interfluves where they deposited isolated bodies comprising dune bedforms of cross stratified ash exposed on the surface here the description measurement and classification of more than 300 dune bedforms are presented four types of dune bedforms are identified with respect to their shape internal structure and geometry length width thickness stoss and lee face angles and stoss face length 1 elongate dune bedforms have smooth shapes and are longer in the flow direction than wide or thick internal stratification consists of stoss constructional thick lensoidal layers of massive and coarse grained material alternating with bedsets of fine laminae that deposit continuously on both stoss and lee sides forming aggrading structures with upstream migration of the crests 2 transverse dune bedforms show linear crests perpendicular to the flow direction with equivalent lengths and widths internally these bedforms exhibit finely stratified bedsets of aggrading ash laminae with upstream crest migration steep truncations of the bedsets are visible on the stoss side only 3 lunate dune bedforms display a barchanoidal shape and have stratification patterns similar to those of the transverse ones finally 4 two dimensional dune bedforms are much wider than long exhibit linear crests and are organized into trains elongate dune bedforms are found exclusively in proximal deposition zones transverse lunate and two dimensional dune bedforms are found in distal ash bodies the type of dune bedform developed varies spatially within an ash body transverse dune bedforms occurring primarily at the onset of deposition zones transitioning to lunate dune bedforms in intermediate zones and two dimensional dune bedforms exclusively on the lateral and distal edges of the deposits the latter are also found where flows moved upslope elongate dune bedforms were deposited from flows with both granular based and tractional flow boundaries that possessed high capacity and competence they may have formed in a subcritical context by the blocking of material on the stoss side we do not interpret them as antidune or chute and pool structures the dimensions and cross stratification patterns of transverse dune bedforms are interpreted as resulting from low competence currents with a significant deposition rate but we rule out their interpretation as antidunes a similar conclusion holds for lunate dune bedforms whose curved shape results from a sedimentation rate dependent on the thickness of the bedform finally two dimensional dune bedforms were formed where lateral transport exceeds longitudinal transport i e in areas where currents were able to spread laterally in low velocity zones we suggest that the aggrading ash bedsets with upstream crest migration were formed under subcritical flow conditions where the tractional bedload transport was less important than the simultaneous fallout from suspension this produced differential draping with no further reworking we propose the name regressive climbing dunes for structures produced by this process a rapid decrease in current velocity possibly triggered by hydraulic jumps affecting the entire parent flows is inferred to explain their deposition this process can in principle hold for any kind of particulate density current

the origin and growth of a recently active fissure ridge travertine over a seismic fault tivoli italy abstract an enigmatic c 2 km long and 15 m high travertine ridge the colle fiorito ridge occurs in the northwestern sector of the tivoli travertine plateau central italy the main questions addressed in this paper concern the origin and mode of growth of this prominent ridge the presence of active structures beneath the studied ridge is inferred by recent and past earthquakes located at shallow depths immediately beneath colle fiorito to understand the surficial structure of the colle fiorito ridge and the travertine depositional environment we constructed a 10 m resolution dem analyzed recent and past aerial photographs and conducted field surveys and meso to micro scale sedimentological analyses to understand the ridge subsurface structure we studied a set of 32 stratigraphic well logs available from previous works and from the local decorative stone industry and realized a 2d electrical resistivity tomography ert across the ridge results show a gentle antiformal structure affected by subvertical zones of strata discontinuity the colle fiorito structure is interpreted as a previously unknown fissure ridge travertine grown at the edge of the tivoli travertine plateau perhaps when the volumetric deposition rate reached its climax in the plateau for the abundance of fluid discharge and the rise of the water table such a fluid pressure may have activated the faults and fractures beneath colle fiorito thus opening new pathways for the ascension of geothermal fluids toward the surface

compositional variation and the origin of deep crustal reflections abstract identifying the sources of crustal reflections is essential for deriving more geological information from deep crustal reflection profiles theoretical and model studies help place constraints on the role of compositional variation in producing deep crustal reflections analysis of laboratory derived velocities and densities of rock types typical for the continental crust indicates that reflection coefficients are generally small but significantly 17 of the possible reflection coefficients have magnitudes between 0 1 and 0 2 comparison between reflection coefficients derived from well logs and reflections observed in associated seismic profiles indicates that constructive interference associated with geological layering is at least as important as the magnitude of the reflection coefficients for producing detectable reflections constructive interference can increase reflection amplitude by two to three times but is limited to a relatively narrow range of layer thickness for a typical 10 40 hz seismic wavelet and typical crustal velocities of about 6 km s constructive interference occurs for layer thickness ranging between about 35 and 80 m layers thinner than 35 m interfere destructively if reflections result from compositional variation seismic models of hypothetical and observed geologic relations provide analogs for interpreting complex reflection patterns observed in deep crustal reflection profiles such models show reflection patterns similar to those observed in the reflection profiles the models indicate that the reflections could originate in the complexly deformed and intruded terranes that are common in the crystalline crust and it may not be necessary to appeal to unobserved phenomena such as special lamellae or fluid filled fractures to explain the reflections

violent strombolian or not using ground penetrating radar to distinguish deposits of low and high energy scoria cone eruptions scoria cones often grow by the accumulation of individual particles transported ballistically in strombolian type eruptions alternative models of cone formation suggest that cones are built primarily of fallout from the eruption column in more explosive eruptions often termed violent strombolian currently the distinction between normal strombolian and violent strombolian is based on direct observations of eruptions or by inference of eruption characteristics from mapping of tephra fall deposits unfortunately medial to distal tephra fall deposits erode rapidly leaving behind only the near vent facies of scoria cones which are thicker and less easily eroded therefore a tool that is capable of delineating differences between low energy strombolian deposits and higher energy violent strombolian deposits from investigation of the preserved scoria edifice is necessary ground penetrating radar imaging of cerro negro an active basaltic scoria cone in nicaragua has revealed details of cone deposits at depths of up to 12 m the record of the 1992 eruption which was observed to be violent strombolian shows quantifiable differences from normal strombolian near vent facies including reflections in the downwind profile that are continuous for hundreds of meters and through the slope break the ability to differentiate between tephra fallout and ballistically emplaced deposits at cerro negro suggests ground penetrating radar imaging may be useful in distinguishing eruptive style in older scoria cones where the medial to distal tephra deposits are eroded or buried

two dimensional travel time inversion for the crustal p and s wave velocity structure of the ruby mountains metamorphic core complex ne nevada problems about the nature of extended crust in the basin and range include the role of plastic flow during extension the possibility of fluids in the crust and the amount of mantle derived material added to the crust to address these problems the university of wyoming conducted a multicomponent wide angle seismic experiment in the ruby mountains of the basin and range the northern ruby mountains expose upper and middle crustal rocks of a metamorphic core complex whereas the southern ruby mountains consist of low grade miogeoclinal rocks the wide angle experiment consisted of a 95 km long n s profile that extended from the southern to the northern ruby mountains travel time inversion of the wide angle reflection data reveals relatively high seismic velocities in the midcrust to lower crust which suggest that fluid filled pores if they exist do not reduce the seismic velocities significantly therefore the midcrustal to lower crustal porosity is probably much smaller than 1 2 vol the average p wave velocity for the crust is 6 4 6 5 km s in the north and 6 1 6 2 km s in the south this difference suggests that larger amounts of mantle derived material were added during the tertiary to the core complex crust of the northern ruby mountains than to the normal basin and range crust of the southern ruby mountains the velocity profile is consistent with a maximum of 6 7 km of mantle derived material in the south and a maximum of 12 15 km of mantle derived material in the north thus volumetric mafic intrusions probably accommodated a maximum of 7 km of core complex exhumation shear wave splitting indicates that the upper to middle crust is seismically anisotropic probably due to superposed tertiary plastic flow patterns doming and inflation of a midcrustal layer in the seismic model are compatible with tertiary flow of material from the south to the north that compensated about 2 5 km of core complex exhumation precritical moho reflections 0 70 km offset 10 11 s normal two way travel time are weaker and less continuous than midcrustal reflections 7 9 s normal two way travel time strong and continuous moho reflections occur at postcritical 80 km offsets only modeling shows that the moho depth increases from about 32 5 km in the south to about 34 5 km in the north

the architecture eruptive history and evolution of the table rock complex oregon from a surtseyan to an energetic maar eruption abstract the table rock complex trc pliocene pleistocene first documented and described by heiken heiken g h 1971 tuff rings examples from the fort rock christmas lake valley basin south central oregon j geophy res 76 5615 5626 is a large and well exposed mafic phreatomagmatic complex in the fort rock christmas lake valley basin south central oregon it spans an area of approximately 40 km2 and consists of a large tuff cone in the south trc1 and a large tuff ring in the northeast trc2 at least seven additional smaller explosion craters were formed along the flanks of the complex in the time between the two main eruptions the first period of activity trc1 initiated with a surtseyan style eruption through a 60 70 m deep lake the trc1 deposits are dominated by multiple 1 2 m thick fining upward sequences of massive to diffusely stratified lapilli tuff with intermittent zones of reverse grading followed by a finely laminated cap of fine grained sediment the massive deposits are interpreted as the result of eruption fed subaqueous turbidity current deposits whereas the finely laminated cap likely resulted from fallout of suspended fine grained material through a water column other common features are erosive channel scour and fill deposits massive tuff breccias and abundant soft sediment deformation due to rapid sediment loading subaerial trc1 deposits are exposed only proximal to the edifice and consist of cross stratified base surge deposits the eruption built a large tuff cone above the lake surface ending with an effusive stage which produced a lava lake in the crater 365 m above the lake floor a significant repose period occurred between the trc1 and trc2 eruptions evidenced by up to 50 cm of diatomitic lake sediments at the contact between the two tuff sequences the trc2 eruption was the last and most energetic in the complex general edifice morphology and a high percentage of accidental material suggest eruption through saturated trc1 deposits and or playa lake sediments trc2 deposits are dominated by three dimensional dune features with wavelengths 200 500 m perpendicular to the flow and 20 200 m parallel to the direction of flow depending on distance from source large u shaped channels 10 32 m deep run up features over obstacles tens of meters high and a large 13 m chute and pool feature are also identified the trc2 deposits are interpreted as the products of multiple erosive highly inflated pyroclastic surges resulting from collapse of an unusually high eruption column relative to previously documented mafic phreatomagmatic eruptions

mt and reflection an essential combination summary at many localities in the world there have been coincident comprehensive electromagnetic em studies and seismic reflection profiles conducted unfortunately over many more regions the seismic reflection images are interpreted without the constraints afforded by electrical conductivity information this paper is an attempt to convince the reader that a collocated magnetotelluric mt study should in almost every case be made wherever a seismic reflection survey is undertaken examples are shown from six studies in which the em results aided the geological tectonic interpretations of the seismic sections also difficulties with the mt technique are discussed and the interpretations of conducting zones within the lower crust are examined finally a generalised model is proposed for the continental crust that may account for both the reflectivity and conductivity of the zone at the top of the lower crust

the crustal structure of the northwestern basin and range province nevada from wide angle seismic data i present a model of lithospheric p wave velocity structure in the northwestern basin and range province nevada based on wide angle seismic data collected in 1986 under the auspices of the program for array seismic studies of the continental lithosphere passcal the lithosphere in the basin and range consists of 1 2 to 3 km thick low velocity 2 5 4 0 km s sedimentary basins and high velocity 4 5 km s ranges overlying a basement of velocity 5 5 6 1 km s 2 a mid crust 6 1 6 2 km s which consists of rocks of probable granitic to granodioritic composition in greenschist to amphibolite facies and which includes two local low velocity zones 5 5 5 7 km s 3 a laterally varying lower crust which consists of rocks of probable gabbroic composition in upper amphibolite to granulite fades and which comprises layers of 6 6 and 7 0 7 2 km s average velocity 4 a complex crust mantle transition best modeled as a velocity gradient zone 1 2 km thick and 5 an upper mantle with velocities of 7 7 7 9 km s which includes a local low velocity zone 7 5 km s crustal thickness in the region varies laterally from 28 to 37 km significantly thicker than several previous estimates the upper mantle velocities and velocity gradients differ slightly on the two perpendicular profiles suggesting possible azimuthal anisotropy a wedge shaped eastward thickening lower crust of high seismic velocity 7 0 7 2 km s on the eastern pan of the survey may represent the westward edge of rift stage precambrien continental crust or alternatively mafic material underplated and intruded into the crust during cenozoic extension comparison of the velocity model with coincident seismic reflection data shows that the reflection and refraction mohos are coincident across the entire profile thus indicating that deep reflections 4 10 sec two way travel time seen on cocorp and passcal piggyback reflection profiles originate entirely within the crust the top of the reflective zone however lies consistently above the mid crustal increase in average velocity indicating that the reflectivity is not restricted to the mafic part of the crust on passcal piggyback profiles in fact the middle silicic crust appears more reflective than the lower mafic crust this can be explained as the result of either mafic sill like intrusions or ductile strain fabrics both of these processes are likely to have accompanied cenozoic extension in the region

grain boundary graphite in rocks and implications for high electrical conductivity in the lower crust the origin of zones of high electrical conductivity in the lower continental crust is a long standing mystery possible explanations include the presence of brines1 2 partial melt3 serpentine4 and graphite5 when discussing the occurrence of graphite in the crust many petrologists have considered phase relations as they would have existed at the peak of metamorphism or during igneous emplacement6 10 here we show that a fine film of graphite is present on the grain boundaries in three rocks from the laramie anorthosite complex in two of these rocks graphite was stable during igneous crystallization but in the other it was not we maintain that in all of the rocks the grain boundary graphite precipitated from a co2 rich fluid during cooling the chemical processes that produced the grain boundary graphite in these rocks are likely to operate in many lower crustal rocks we therefore contend that because the films we observe are capable of producing the high conductivity that is seen in the lower crust grain boundary graphite should be considered as a possible cause for some conductivity anomalies

migration direction of sand wave structures in pyroclastic surge deposits implications for depositional processes sand wave structures of both progressive downstream migrating and regressive upstream migrating types occur within the same pyroclastic surge deposit at both roccamonfina volcano southern italy and sugarloaf mountain arizona united states both wet and dry surge deposits are described that contain examples of regressive and progressive sandwave structures at sugarloaf mountain progressive and regressive sand waves occur within metres of each other within the same bedset indicating that penecontemporaneous deposition of the two different sand wave structures occurred in close proximity on the basis of this evidence it is suggested that the migration direction of sand waves in these pyroclastic surge deposits is controlled by the flow regime of the surge cloud rather than the temperature and or moisture content

seismic evidence of mylonite reflectivity and deep structure in the kettle dome metamorphic core complex washington the first direct test of the seismic reflectivity of mylonites is provided by a seismic reflection profile over a mylonite zone exposed in the kettle dome metamorphic core complex northeastern washington the mylonite zone developed at mid crustal levels and has undergone a minimum of retrogression and cataclasis during uplift downdip projection of the exposed mylonite zone into the seismic profile indicates that the compositionally layered mylonite zone is reflective and traceable to at least 5 km depth as a complex zone of multicyclic reflections the seismic data also show that the crust beneath the kettle dome is characterized by abundant strongly reflective moderately dipping layers down to at least 18 km these layers can be interpreted in terms of compressional or extensional orogenic events or a combination of both models for the structure and genesis of metamorphic core complexes must take these data into account

discovery of the first quaternary maar in the bohemian massif central europe based on combined geophysical and geological surveys abstract based on results of previous investigations of tephra tuff volcaniclastic deposits and a geophysical survey in the surroundings of the elezna h rka quaternary volcano west bohemia we performed detailed geophysical surveys using gravimetry magnetometry and electrical conductivity techniques striking anomalies were revealed in a morphological depression near m tina west bohemia as a strong evidence of the assumed maar diatreme structure the sharp isometric gravity low of 2 30 mgal as well as the corresponding positive magnetic anomaly of 200 nt with a negative rim on its northern side indicate a steeply dipping geological body of low density and containing magnetic rocks minerals magnetic survey also showed pronounced local anomalies outside the depression that can reflect relicts of the tephra rim of the maar this geophysical evidence was then proven by an exploratory drilling near the centre of the gravity anomaly macroscopic on site evaluation of the core and more detailed sedimentological petrochemical palynological and microbiological laboratory analyses further confirmed the existence of a maar structure filled by 84 m of lake sediments reflecting a succession of several warm and cold climatic periods results of palynological analyses confirm the presence of a continuous palaeoclimate archive with at least three successive warmer periods of most probably interstadial character from the upper quaternary saalian complex therefore the recovered sediment sequence holds strong potential for in depth paleoclimate reconstruction and deep biosphere studies at the bottom of the m tina 1 my 1 borehole 84 85 5 m country rock debris was found containing also volcanic bombs and lapilli the discovered volcanic structure is considered to be the first known quaternary maar diatreme volcano on the territory of the bohemian massif because of hidden active magmatic processes in combination with earthquake swarm seismicity ca 20 30 km north of the m tina maar reconstruction of the palaeovolcanological evolution is important for evaluation of hazard potential of the ne and e part of the cheb basin

petroleum potential of serpentine plugs and associated rocks central and south texas abstract during deposition of the upper cretaceous austin chalk and taylor marl serpentine plugs formed by submarine volcanic activity along major fault zones in the ancient gulf of mexico after eruption palagonite tuff mounds formed by the hydration of basaltic glass localized deposition of shoal water carbonates on topographic highs the serpentine plugs occur along an arcuate belt extending approximately 250 mi 400 km from milam county southwestward to zavala county texas hydrocarbon traps in and around serpentine plugs have yielded approximately 47 million bbl of oil and significant quantities of natural gas from altered volcanic tuff and associated shoal water carbonates shallower production is from overlying sedimentary rocks structurally influenced by volcanic plugs entrapment of hydrocarbons occurs as 1 stratigraphic traps within porous zones of carbonate units 2 stratigraphic traps within porous volcanic tuff serpentine 3 structural traps within overlying sands and 4 traps within high porosity fracture zones exploration for plugs should be concentrated along existing fault zones by either magnetic or seismic surveys analysis of the plugs suggests that there are at least 3 distinctive groups 1 a southern group characterized by well developed and productive marginal sands 2 a middle group characterized by hydrocarbon saturated biocalcarenite and reworked volcaniclastic facies and 3 a northern group in which the dominant hydrocarbon saturation is in the volcanics themselves each of these groups appears to reflect a somewhat different geologic history end of article last page 504

factors controlling the internal facies architecture of maar diatreme volcanoes most if not all kimberlite pipes show a multitude of facies types which imply that the pipes were emplaced under an episodic re occurrence of eruptive phases often with inter mittent phases of volcanic quiescence the majority of these facies can be related to either the fragmentation behaviour of the magma during emplacement or changing conditions dur ing sedimentation of volcaniclastic deposits as well as their alteration and compaction after deposition an additional factor controlling pipe facies architecture is the degree of mobility of the loci of explosions in the explosion chambers of the root zone or root zones at the base of the maar diatreme volcano in a growing pipe the root zone moves downward and with that movement the overlying diatreme enlarges both in size and diameter however during the life span of the volcano the explosion chamber can also move upward backintothelower diatreme where renewedexplosionsresult inthedestructionofolderdepositsandtheirstructures nextto vertical shifts of explosion chambers the loci of explosions can also move laterally along the feeder dyke or dyke swarm this mobility of explosion chambers results in a highly com plex facies architecture in which a pipe can be composed of several separate root zones that are overlain by an amalgam ated crosscutting diatreme and maar crater with several lobes pipe complexity is amplified by periodic changes of the fragmentation behaviour and explosivity of kimberlite mag ma recent mapping and logging results of canadian and african kimberlite pipes suggest that kimberlite magma frag mentation ranges from highly explosive with abundant entrained country rock fragments to weakly explosive spatter like production with scarce xenoliths on occasions spatter may even reconstitute and form a texturally coherent deposit on the crater floor in addition ascending kimberlite magma can pass the loci of earlier fragmentation events in the root zone and intrudes as coherent hypabyssal kimberlite dykes in high pipe levels or forms extrusive lava lakes or flows on the crater floor or the syneruptive land surface respectively this highly variable emplacement behaviour is typical for basaltic maar diatreme volcanoes and since similar deposits can also be found in kimberlites it can be concluded thatalsothevolcanologicalprocessesleadingtothesedeposits are similar to the ones observed in basaltic pipes

nonlinear ground motion amplification by sediments during the 1994 northridge earthquake it has been known since at least 1898 ref 1 that sediments can amplify earthquake ground motion relative to bedrock for the weak ground motion accompanying small earthquakes the amplification due to sediments is well understood in terms of linear elasticity hooke s law 2 but there has been a long standing debate regarding the amplification associated with the strong ground motion produced by large earthquakes the view of geotechnical engineers based largely on laboratory studies is that hooke s law breaks down at larger strains causing a reduced nonlinear amplification seismologists on the other hand have tended to remain sceptical of this nonlinear effect mainly because the relatively few strong motion observations seemed to be consistent with linear elasticity although some recent earthquake studies have demonstrated nonlinear behaviour under certain circumstances3 4 the significance of nonlinearity for the type of stiff soil sites found in the greater los angeles region remains unresolved5 here we report that ground motion amplification due to sediments for the main shock of the 1994 northridge earthquake was up to a factor of two less than the amplification observed for its aftershocks these observations imply significant nonlinearity in such amplification and bring into question the use of measurements of weak ground motion to predict the strong ground motion at sedimentary sites

theory of melt segregation a review abstract the processes by which melt is extracted from partially molten source regions at depth are collectively known as melt segregation during the past three years a dynamical theory has been developed which provides a unified description of the mechanical thermodynamical and chemical aspects of melt segregation in this theory a partially molten region is regarded as a deformable porous medium or matrix saturated with melt migration of melt relative to the matrix can be driven either by the buoyancy of the melt or by nonhydrostatic pressure gradients associated with the deformation of the matrix this paper reviews the basic governing equations of the theory and the solutions which have been obtained during the past three years

depositional mechanics and sequences of base surges songaksan tuff ring cheju island korea the songaksan mount in the southwestern part of cheju island korea is a taalian tuff ring produced by phreatomagmatic explosions at an aquifer a detailed analysis of proximal to distal facies changes reveals that the tuff ring sequence can be represented by 21 sedimentary facies one lateral facies sequence lfs and three vertical facies sequences vfs the vfs 1 and 2 are representative of facies relationships in horizontal near vent deposits the vfs 1 comprises scour fill bedded tuff inversely graded tuff massive tuff and laminated tuff from base to top the vfs 2 is a variant of the vfs 1 replaced by an inversely graded lapilli tuff unit at the base the sequences suggest traction carpet suspension and minor traction sedimentation from a high concentration near vent base surge the lfs 1 and the vfs 3 are distilled from outward dipping flank deposits both sequences begin with disorganized lapilli tuff followed successively by stratified lapilli tuff dune bedded lapilli tuff very thinbedded tuff and accretionary lapilli they are suggestive of waning base surge which decreases in particle concentration suspended load fall out rate and flow regimes with an increase in traction and sorting these facies sequences suggest that a base surge experiences flow transformation with its flow characters changing with time and space a near vent base surge is turbulent uniformly mixed and highly concentrated and produces scour fill bedded tuff as capacity decreases the surge transforms into a dense and laminar underflow and a dilute and turbulent upper part gravity transformation depositing inversely graded massive and normally graded lapilli tuff ensuing loss of sediment load and mixing of ambient air result in flow dilution surface transformation stratified and dune bedded units are produced by tractional processes of turbulent and low concentration surge further dilution causes deceleration and cooling and results in precipitation of moistened ash and accretionary lapilli from suspension

amplification of seismic ground motion in the tunis basin numerical bem simulations vs experimental evidences this paper aims at the analysis of seismic wave amplification in a deep alluvial basin in the city of tunis in tunisia this sedimentary basin is 3000m wide and 350m deep since the seismic hazard is significant in this area the depth of the basin and the strong impedance ratio raise the need for an accurate estimation of seismic motion amplification various experimental investigations were performed in previous studies to characterize site effects the boundary element method is considered herein to assess the parameter sensitivity of the amplification process and analyse the prevailing phenomena the various frequencies of maximum amplification are correctly estimated by the bem simulations the maximum amplification level observed in the field is also well retrieved by the numerical simulations but due to the sensitivity of the location of maximum amplification in space the overall maximum amplification has to be considered the influence of the wave field incidence and material damping is also discussed

record of complex scoria cone eruptive activity at red mountain arizona usa and implications for monogenetic mafic volcanoes abstract scoria cone eruptions are generally modeled as a simple succession from explosive eruption to form the cone to passive effusion of lava generally from the base of the cone sector collapse of scoria cones wherein parts of the cone are rafted on a lava flow is increasingly recognized as common but the reasons that a cone may not be rebuilt are poorly understood red mountain volcano is a pleistocene scoria cone in the san francisco volcanic field of northern arizona usa the cone lies along the trace of a major steeply dipping normal fault that originated during proterozoic tectonism and was reactivated in tertiary time the earliest phase of eruption at red mountain was typical strombolian forming a cone that was followed by or possibly synchronous with lava effusion toward the west from the base of the cone rafting then ensued as the west side of the cone collapsed approximately 15 of the cone is preserved in mounds as much as 30 m high rafting was extensive enough to remove most of the cone over the vent area which effectively reduced the pressure cap on the magma conduit resultant low fountaining fed clastogenic lava flows and minor scoria fallback clastogenic flows traveled as far as 4 km and now form a cliff 30 40 m high at the edge of the lava platform although several possibilities explain the change in vent dynamics and eruptive style we favor the interpretation that an increase in magma rise rate caused collapse of the cone the abrupt removal of 300 m of material over the vent removed a conduit cork and low fountaining began magma that had erupted effusively suddenly became explosive this aspect of scoria cone rafting at red mountain is broadly similar to sector collapse followed by explosive eruption in larger systems a steep walled 150 m high amphitheatre on the northeast side of red mountain exposes weakly to strongly altered scoria cemented by calcite iron and zeolites we suggest that vapor phase alteration was responsible for sealing fine grained ash beds in the cone and a pressurized system developed residual heat from a dike that was emplaced as part of the magmatic activity provided heat that drove groundwater along the regional fault up into the cone eventually the overpressurized system exploded in a phreatic eruption that created the amphitheatre which has subsequently been enlarged by water and wind erosion the combined sequence of events at red mountain illustrates some of the complexities in monogenetic scoria cone eruptions that have received little attention to date

modelling strong seismic ground motion three dimensional loading path versus wavefield polarization seismic waves due to strong earthquakes propagating in surficial soil layers may both reduce soil stiffness and increase the energy dissipation into the soil to investigate seismic wave amplification in such cases past studies have been devoted to one directional shear wave propagation in a soil column 1d propagation considering one motion component only 1c polarization three independent purely 1c computations may be performed 1d 1c approach and directly superimposed in the case of weak motions linear behaviour this research aims at studying local site effects by considering seismic wave propagation in a 1 d soil profile accounting for the influence of the 3 d loading path and non linear hysteretic behaviour of the soil in the proposed 1d 3c approach the three components 3c polarization of the incident wave are simultaneously propagated into a horizontal multilayered soil a 3 d non linear constitutive relation for the soil is implemented in the framework of the finite element method in the time domain the complex rheology of soils is modelled by mean of a multisurface cyclic plasticity model of the masing prandtl ishlinskii iwan type the great advantage of this choice is that the only data needed to describe the model is the modulus reduction curve a parametric study is carried out to characterize the changes in the seismic motion of the surficial layers due to both incident wavefield properties and soil non linearities the numerical simulations show a seismic response depending on several parameters such as polarization of seismic waves material elastic and dynamic properties as well as on the impedance contrast between layers and frequency content and oscillatory character of the input motion the 3 d loading path due to the 3c polarization leads to multi axial stress interaction that reduces soil strength and increases non linear effects the non linear behaviour of the soil may have beneficial or detrimental effects on the seismic response at the free surface depending on the energy dissipation rate free surface time histories stress strain hysteresis loops and in depth profiles of octahedral stress and strain are estimated for each soil column the combination of three separate 1d 1c non linear analyses is compared to the proposed 1d 3c approach evidencing the influence of the 3c polarization and the 3 d loading path on strong seismic motions

deep bedded ultramafic diatremes in the missouri river breaks volcanic field montana usa 1 km of syn eruptive subsidence the ultramafic eocene missouri river breaks volcanic field mrbvf montana usa includes over 50 diatremes emplaced in a mostly soft substrate the current erosion level is 1 3 1 5 km below the pre eruptive surface exposing the deep part of the diatreme structures and some dikes five representative diatremes are described here they are 200 375 m across and have sub vertical walls their infill consists mostly of 55 90 bedded pyroclastic rocks fine tuffs to coarse lapilli tuffs with concave upward bedding and 45 10 non bedded pyroclastic rocks medium lapilli tuffs to tuff breccias the latter zones form steep columns 15 135 m in horizontal dimension which cross cut the bedded pyroclastic rocks megablocks of the host sedimentary formations are also present in the diatremes some being found 1 km or more below their sources the diatreme infill contains abundant lithic clasts and ash sized particles indicating efficient fragmentation of magma and country rocks the spherical to sub spherical juvenile clasts are non vesicular they are accompanied by minor accretionary lapilli and armored lapilli the deposits of dilute pyroclastic density currents are locally observed our main interpretations are as follows 1 the observations strongly support phreatomagmatic explosions as the energy source for fragmentation and diatreme excavation 2 the bedded pyroclastic rocks were deposited on the crater floor and subsided by 1 0 1 3 km to their current location with subsidence taking place mostly during the eruption 3 the observed non bedded pyroclastic columns were created by debris jets that punched through the bedded pyroclastic material the debris jets did not empty the mature diatreme occupying only a fraction of its width and some debris jets probably did not reach the crater floor 4 the mature diatreme was nearly always filled and buttressed by pyroclastic debris at depth there was never a 1 3 1 5 km deep empty hole with sub vertical walls otherwise the soft substrate would have collapsed inward which it only did near the surface to create the megablocks we infer that syn eruptive subsidence shifted down bedded pyroclastic material and shallow sedimentary megablocks by 0 8 1 1 km or more after which limited post eruptive subsidence occurred this makes the mrbvf diatremes an extreme end member case of syn eruptive subsidence in the spectrum of possibilities for maar diatreme volcanoes worldwide

ground penetrating radar images of unexposed climbing dune forms in the ubehebe hydrovolcanic field death valley california ground penetrating radar surveys were conducted in the ubehebe hydrovolcanic field death valley california using 900 mhz antennae which allowed high resolution images of the subsurface the surveys were intended to study the lateral facies variation of subsurface sequences in base surge deposits and were run along a flow direction radial to the ubehebe crater the ground penetrating radar imaged deposits that would otherwise be difficult to study because they are in an area with very few outcrops in particular the radar profiles show the presence of wavy reflections from relatively proximal to intermediate positions which become increasingly less wavy and more plane parallel in distal locations the wavy reflections have been interpreted as ground penetrating radar images of subsurface trains of climbing dune forms whose size decreases downflow a climbing dune form exposed inside a trench in correspondence to a crest of wavy reflections confirms this interpretation these results show that ground penetrating radar can usefully integrate traditional geologic field studies

magma in layering at the moho of the basin and range of nevada layering at the moho as determined from seismic reflection data is a commonly observed phenomenon in the basin and range for which we propose a porous flow melt model analysis of seismic reflection data from the 1986 passcal nevada experiment indicates that the moho in the nevada basin and range is layered and has unusually high reflectivity this reflectivity is difficult to explain in terms of a velocity contrast caused by different rock types and in fact a lower velocity of less than 6 5 km s suggests the presence of partially molten material at the moho in a zone containing 20 to 25 of low velocity layers to explain the seismic observations we have developed a viscous porous flow model for basaltic melt ascent such a model produces systematic variations in volumetric melt flow with depth and close correspondence between seismic interpretation and modeling of partial melt ascent this suggests that such layering involving viscous porous flow of partial melt is a plausible explanation of the layering of the moho in the basin and range and may have more general application

depositional processes of the suwolbong tuff ring cheju island korea the suwolbong pyroclastic sequence in the western part of cheju island korea comprises partly preserved rim beds of a quaternary basaltic tuff ring whose vent lies about 1 km seaward of the present shoreline the sequence consists of breccia lapillistone lapilli tuff and tuff eighteen sedimentary facies are established and organized into six lateral facies sequences lfs and seven vertical facies sequences vfs the lfs 1 4 and 5 begin with massive lapilli tuff which transforms downcurrent into either planar bedded lfs 1 undulatory bedded lfs 4 or climbing dune bedded lfs 5 lapilli tuff units they are representative of relatively dry base surge whose particle concentration decreases downcurrent with a progressive increase in both tractional processes and sorting the lfs 2 begins with disorganized and massive lapilli tuff and transforms into crudely stratified units downcurrent it results from relatively wet base surge in which sorting is poor due to the cohesion of damp ash the lfs 3 comprises well sorted lapilli tuff and stratified tuff further downcurrent suggestive of deposition from combined fall and surge of relatively dry hydroclastic eruption all seven vertical facies sequences generally comprise two facies units of coarse grained fines depleted lapilli tuff and an overlying fine grained tuff these sequences are suggestive of deposition from base surge that consists of a turbulent head and a low concentration tail depositional processes in the suwolbong tuff ring were dominated by a relatively dry base surge the base surge comprises turbulent and high concentration suspension near the vent whose deposits are generally unstratified due to the lack of tractional transport as the base surge becomes diluted downcurrent through fallout of clasts and mixing of ambient air it develops large scale turbulent eddies and is segregated into coarse grained bedload and overlying fine grained suspension forming thinly stratified units further downcurrent the base surge may be either cooled and deflated or pushed up into the air depending on its temperature the suwolbong tuff ring comprises an overall wet to dry cycle with several dry to wet cycles in it suggestive of overall decrease in abundance of external water and fluctuation in the rate of magma rise

metamorphic fluids and anomalous porosities in the lower crust abstract because of the plastic behavior of rocks at high temperature and confining pressure the porosity of lower crustal rocks is a variable dependent on the amount of free fluid present on the basis of hydrologic theory it is predicted that the permeability k of such rocks can be expressed by a relationship of the form k n where and n are material specific constants the exponent n is likely to vary between 3 and 6 whereas which is ultimately related to grain size and shape probably varies over several orders of magnitude consequently substantial contrasts between the k relationships of lower crustal rocks are to be expected the case in which thin crustal layers aquitards are characterized by relatively low values of or n with respect to the crust in general is of particular interest such layers would acquire high porosities in the presence of miniscule steady state crustal fluid fluxes and are a possible explanation for anomalous seismic brightspots and high electrical conductivities in the lower crust modelling of collision belt metamorphism suggests that even long after the cessation of active metamorphism and isostatic reequilibration relict metamorphic fluid fluxes could generate aquitard porosities between 0 1 and 0 4 adequate to explain such geophysical anomalies

deposition temperature of some pdc deposits from the 1982 eruption of el chichon volcano chiapas mexico inferred from rock magnetic data abstract thermal remanent magnetization trm analyses were carried out on lithic fragments from two different typologies of pyroclastic density current pdc deposits of the 1982 eruption of el chichon volcano in order to estimate their equilibrium temperature tdep after deposition the estimated tdep range is 360 400 c which overlaps the direct measurements of temperature carried out four days after the eruption on the pdc deposits this overlap demonstrates the reliability of the trm method to estimate the tdep of pyroclastic deposits and to approximate their depositional temperature these results also constraint the time needed for reaching thermal equilibrium within four days for the studied pdc deposits in agreement with predictions of theoretical models sedimentological analysis demonstrates that different thickness of aggrading granular dominated pulses does not affect significantly the estimated tdep for the two studied pdcs the component analysis highlights how the entrapment of a significant amount of pre heated lithic fragments from the dome and the conduit walls reduced the cooling of the pyroclastic mixture

paleomagnetic estimate of the emplacement temperature of the long runout nevado de colima volcanic debris avalanche deposit mexico stoopes and sheridan have mapped a volcanic debris avalanche of nevado de colima which has an exceptionally long runout 120 km and low fall height to length ratio h l 0 04 we present paleomagnetic results from this volcanic debris avalanche deposit which provide evidence that this avalanche was emplaced at elevated temperatures the majority of samples collected from lithic clasts in the volcanic debris avalanche deposit exhibit two component remanent magnetizations with a low temperature component 25 350 c which is well grouped about the geomagnetic field direction at colima and a high temperature component 350 580 c which is randomly oriented although the temperature of the deposit most likely varied with distance from the volcanic source and the thickness of the deposit our results suggest an emplacement temperature of approximately 350 c at intermediate distances 18 26 km from the source in order for the rock clasts 20 40 cm diameter to be heated to these temperatures the avalanche was most likely the results of a magmatic bezymianny type eruption the mixing of hot juvenile gases with the clasts provides an explanation for the high degree of fluidization of this material as evidenced by the long runout of this avalanche deposit

paleomagnetic evidence for high temperature emplacement of the 1883 subaqueous pyroclastic flows from krakatau volcano indonesia the paroxysmal 1883 eruption of krakatau volcano in indonesia discharged at least 6 5 km3 dense rock equivalent of pyroclastic material into the shallow waters of the sunda straits within a 15 km radius of the volcano progressive thermal demagnetization studies of individually oriented pumice clasts from a core sample of the submarine pyroclastic deposits show that 41 out of 47 clasts exhibit single component remanence with mean inclination of 2 4 the partial thermoremanent magnetization components of both pumice and lithic clasts are well grouped in orientation indicating that substantial cooling of clasts must have occurred following deposition estimated subaqueous emplacement temperature for such clasts is greater than 500 c rare two component lithic fragments exhibit inflection points on vector endpoint diagrams that mark the temperature below which the fragments acquired magnetization of similar orientation these inflection points range from 350 to 550 c indicating a minimum subaqueous emplacement temperature of 350 c paleomagnetic evidence for high emplacement temperature supports the hypothesis that proximal 1883 submarine pyroclastic deposits resulted from entrance of hot subaerially generated pyroclastic flows into the sea similar deposits have been interpreted from the geologic record but this is the first documented example of submarine pyroclastic flows from a historic eruption the krakatau deposits thus serve as an important modern analog for the study of pyroclastic flow seawater interactions

reflectivity and seismic properties of the deep continental crust the recovery of drill core from a section of upper amphibolite facies terrane located in the inner piedmont of south carolina provides a rare opportunity to study in detail the seismic character of rocks which were once part of the deep continental crust seismic reflection profiles over the inner piedmont in the vicinity of the drill hole reveal subhorizontal high amplitude reflections the origin of these reflections has been investigated by measuring compressional wave velocities as a function of confining pressure and densities in 150 samples obtained from 305 m of drill core synthetic seismograms generated from the velocity and density measurements demonstrate that reflections originate from subhorizontal metamotphic layering probably originating from lower crustal ductile flow fine tuning of the metamotphic layering which is temperature and pressure related can produce bright spots in the deep crust anisotropy resulting from preferred mineral orientation is another important propeny of this sectim velocities are low for vertical wave propagatim whereas horizontal velocities are relatively high horizontal velocities show mly a slight variatim with azimuth which is unlikely to be detected using standard refractim techniques reflectivity decreases with increasing anisotropy critical to this study has been the continuous coring of a major section of high grade metamotphic terrane which has provided the necessary stratigraphic and lithologic cmtrol for the generation of accurate synthetic seismograms

high electrical conductivity in a model lower crust with unconnected conductive seismically reflective layers summary in this paper we derive the electrical conductivity for a model lower crust containing unconnected highly conductive lamellae within a highly resistive matrix lateral overlap with small vertical separation of lamellae of the dimensions imaged by seismic reflection profiling a few hundred metres thick and a few kilometres across could increase lower crustal conductivity from the low values predicted by laboratory measurements on dry rocks to the high values observed in field experiments the model does not depend on the cause of high conductivity within the lamellae however lamellation of the lower crust may provide a way of lithologically trapping saline water in permeable conductive lamellae within an impermeable non conductive matrix and so resolve the apparent contradiction between the low crustal permeabilities required for maintenance of high pore pressure over geological time periods and the high degree of pore interconnection required for the high observed conductivity the permeable lamellae and impermeable matrix would be of very different lithologies as implied by the high amplitudes of the lower crustal reflections for a typical example the model gives resistivities that compare favourably with the modified archie s law the model can also give anisotropic resistivity effects which are quantitatively compatible with results from field experiments

physical processes of shallow mafic dike emplacement near the san rafael swell utah some 200 shonkinite dikes sills and breccia bodies on the western colorado plateau of south central utah were intruded from approximately 3 7 to 4 6 ma contemporaneous with mafic volcanism along the nearby plateau margin thicknesses of dikes range to about 6 m the log normal mean thickness is 85 cm despite the excellent exposures of essentially all dikes in strata of the jurassic san rafael group their number is indeterminate from their outcrop and spacing because they are everywhere greatly segmented by our grouping of almost 2000 dike segments most dikes are less than 2 km in outcrop length the longest is 9 km because the san rafael magmas were primitive and probably ascended directly from the mantle dike lengths in outcrop are much less than their heights the present exposures probably lie along the irregular upper peripheries of dikes that lengthen and merge with depth orientations of steps on dike contacts record local directions of dike fracture propagation about half of the measurements plunge less than 30 showing that lateral propagation at dike peripheries is as important as the vertical propagation ultimately responsible for ascent the san rafael dikes now exposed after erosion of about 0 5 1 5 km appear to thicken and shorten upward probably because near surface vesiculation enhanced magmatic driving pressures propagation likely ceased soon after the first dike segments began to feed nearby sills or vented to initiate small volume eruptions most of the dikes are exposed in clastic strata of the jurassic san rafael group they probably acquired their strikes however while ascending along well developed joints in massive sandstones of the underlying glen canyon group rotation of far field stresses during the emplacement interval cannot account for disparate strikes of the dikes which vary through 110 most lying between north and n25 w rather the two regional horizontal principal stresses were probably nearly equal and so the dominant n75 e direction of dike opening was not strongly favored across the center of the swarm about 10 to 15 dikes overlap and produce 15 20 m of dilation many are in sufficient proximity that later dikes should be thinner than earlier ones if neither the magma pressures nor regional stresses were changing during the emplacement interval however dike thicknesses vary systematically neither along the length of the swarm nor in proportion to the number of neighboring dikes it appears that crustal extension during the magmatic interval relieved compressive stresses localized by intrusion

the nature of seismic reflections from deep crustal fault zones deep seismic reflection profiles have recorded reflections from ductile shear zones within the crystalline basement perhaps the best example is the consortium for continental reflection profiling cocorp wind river line in wyoming which shows the wind river thrust fault to be a strong reflector from the surface to depths of about 30 km to identify the physical properties responsible for the seismic reflections from fault zones we report measurements of compressional wave velocity and velocity anisotropy in mylonites recovered from exhumed ductile shear zones these rocks are characterized by extensive ductile deformation of plastic minerals brittle deformation of the more rigid minerals grain size reduction and development of a strong fabric average velocity and density were not found to change significantly and systematically between the mylonites and the adjacent undeformed rock seismic anisotropy of 7 or greater is present in several mylonites dependent upon their composition and fabric using our data on mylonite properties and models for the crustal structure near the wind river line we compute true amplitude synthetic reflection seismograms and compare them to true amplitude plots of the cocorp wind river reflection data when anisotropy and the finely laminated structure of the fault zone are considered our modeling indicates fairly strong reflections from mylonite zones in most cases we also show that elevated pore pressure in shear zones may produce strong reflections however a permeability of the order of 10 16 darcy is required to maintain sufficient pore pressure to produce a velocity anomaly in a fault zone which has long been inactive

processing and inversion of refraction and wide angle reflection data from the 1986 nevada passcal experiment we derive averaged one dimensional velocity models for the crust beneath the basin and range in western nevada by inverting p composites extracted from slant stacks of refraction and wide angle reflection data a combination of high cut filtering in the frequency domain with coherency filtering in the p domain has proved effective in smoothing severe static shifts due to lateral heterogeneities and suppressing artifacts due to spatial aliasing we supplement the low frequency information provided by the refraction records with higher frequency arrivals picked from the normal incidence and wide angle reflection sections generalized least squares inversion of data from eight shots for p and s wave velocity structure beneath two perpendicular profiles yields consistent estimates for total crustal thickness p wave 30 31 km s wave 30 32 km and average crustal velocity 6 2 km s 3 4 3 5 km s average poisson s ratio beneath the n s profile is 0 27 0 28 p wave velocities near the base of the crust 7 4 0 3 km s are estimated by inverting precritical reflections from the moho standard errors in depth for these models range from 1 to 3 km extremal depth bounds in general are about twice as large uncertainties in velocity estimated from resolving kernels range from 0 1 to 0 3 km s estimates of poisson1s ratio as a function of depth are characterized by large standard errors reflecting the poor resolution of shear wave velocities the combination of low average p and s wave velocities with a high average poisson s ratio in the upper half of the crust however is consistent with the presence of fluid filled cracks these results highlight the need for three component recording to better resolve shear wave velocity structure

seismic reflectors conductivity water and stress in the continental crust the uppermost 10 15 km of the earth s continental crust differs in several geophysical properties from the lower crust the upper crust is electrically resistive seismically transparent contains nearly all intracontinental earthquake hypocentres and responds to stress elastically with brittle fracture the lower crust is electrically conductive contains many seismic reflectors is aseismic and shows ductile response to stress i show here that the characteristics of both regions can be explained if the entire crust contains saline water in separated cavities in the compressively stressed rocks of the upper crust but in the lower crust forming an interconnected film on the crystal surfaces the model may apply principally to technically active parts of the continental crust beneath shields the lower crust may be dry

spatter dike reveals subterranean magma diversions consequences for small multivent basaltic eruptions small volume short lived basaltic volcanoes earth9s most common subaerially are abundant and tractable in size thus offering opportunities for learning how magma behaves in the critical transition from coherent magma in dikes to pyroclastic dispersions spatter dikes at the castle butte trading post volcanic complex hopi buttes volcanic field arizona usa exposed 150 m below the paleosurface present an outstanding record of this transition they record pulsatory magma rise and eruption fissure extension vent stepping and conduit wall rock failure providing a critical link to activity witnessed and geophysically monitored during historic eruptions of well observed and monitored large volume basaltic volcanoes castle butte comprises four linked and closely spaced spatter dikes and maar diatremes the spatter dikes consist of bedded variably welded deposits plus wall rock debris in multiple sequences demarcated by truncation surfaces that indicate progressive southwest northeast emplacement they reveal a shallow plumbing cycle of pulsating weak hot spatter fragmentation concurrent wall rock failure and periodic slips that truncated and downdropped bedded deposits in addition to vertical pulsation we infer from the northeastward younging spatter succession that repeated magma diversions facilitated episodes of magma withdrawal that caused the slips during northeastward extension of the complex toward the large maar diatreme

late cretaceous volcanism in south and central texas stratigraphic structural and seismic models abstract abstract since their discovery in 1915 hydrocarbon traps in and around serpentine plugs have produced about 47 million barrels of oil and have significant potential for additional small discoveries production is from isolated reservoirs within mounds of altered volcanic tuff and associated shoal water carbonates a review of the more than 200 volcanic centers and intrusive bodies of south and central texas has led to development of stratigraphic and seismic models useful in exploration and production the so called serpentine plugs are largely tuff mounds formed by accumulation of volcanic ash altered to palagonite on the seafloor around a submarine volcanic vent volcanic activity peaked during deposition of the chalk and marl of the upper austin and lower taylor groups about 80 million years ago after their eruption the tuff mounds localized the deposition of shoal water carbonates with good porosity and permeability low permeability organic rich marine shale and marl of the taylor group capped the carbonates serving as both a hydrocarbon source and a stratigraphic seal compactional draping of overlying san miguel and olmos sands with minor offset faulting created important additional traps in south texas central texas volcanic centers are highly aligned along strike oriented regional faults and fractures of the balcones and luling systems the magmas in both central and south texas were ultramafic and alkaline suggesting that partial melting occurred at depths of about 40 miles 60 kilometers the magma rose rapidly to the surface probably in an extensional stress regime controlled by pre tertiary balcones luling faults the palagonite tuff of a typical productive volcanic center has low seismic velocity and is encased in high velocity carbonates the strong velocity contrast coupled with the distinctive shape of the tuff mass yields a characteristic seismic pattern modern seismic techniques together with stratigraphic data allow accurate delineation of buried tuff mounds and prediction of productive carbonate facies

the auckland volcanic field new zealand geophysical evidence for structural and spatio temporal relationships abstract geophysical data from the monogenetic auckland volcanic field reveal complex structural and spatio temporal relationships at different scales the volcanic field is coincident with regional magnetic and gravity anomalies that mark a major crustal suture and with a discontinuity marking a significant structural asperity here the linear regional magnetic anomaly splays into a wide band of nnw trending lineaments arising from serpentinised shear zones in the upper crust that matches the extent of the volcanic field and that may reflect a region of crustal weakness creating preferential permeability however there appears to be no simple correlation between the locations of individual vents and these lineaments that might delineate more shallow structural controls with this orientation probably as a consequence of other structural influences high resolution aeromagnetic data over the volcanic field show that the volcanoes have a wide range of magnetic signatures indicating a variability of subsurface structure scoria cone volcanoes typically have strong anomalies up to several 100 nt whilst tuff ring volcanoes typically have weak anomalies less than 50 nt though the surface geology is not always an indicator of the nature and extent of the subsurface deposits both cone and tuff ring volcanoes in the auckland field appear to be underlain by subsurface bowl shaped bodies of basalt implying that their eruption histories commonly involve lava ponding into early excavated craters the present geophysical data give no evidence for subsurface dyke like structures or for substantial near surface volumes of basaltic rocks where there are no known eruption centres or buried flows aeromagnetic and palaeomagnetic data suggest that a number of adjacent vents with an implied structural linkage may be contemporaneous though other examples occur where vents of clearly different ages exploit the same apparent structure a unique feature of the auckland field is that at least 5 widely separated and structurally unrelated volcanoes are contemporaneous these observations highlight the spatial and temporal heterogeneities that can occur in monogenetic fields and have an important bearing on recurrence rates and temporally linked eruptions which are probably more common than is generally supposed and are key factors in statistically based hazard assessments

migration of volcanism in the san francisco volcanic field arizona the remanent magnetization of volcanic rocks has been determined at 650 sites in the san francisco volcanic field in the southern part of the colorado plateau the polarity of remanent magnetization combined with k ar age determinations spatial and petrographic associations stratigraphic relations and state of preservation of the cinder cones provides a basis for assignment to known magnetic polarity epochs of 610 mafic vents and 100 intermediate to silicic flows flow sequences and vents the age assignments for basaltic rocks include 243 brunhes 0 73 ma vents 220 matuyama 0 73 to 2 48 ma vents and 147 pre matuyama 2 48 to about 5 0 ma vents basaltic volcanism migrated northeastward before matuyama time at a rate of 1 2 cm yr and eastward s87 5 e over the past 2 5 m y at a rate of 2 9 0 3 cm yr concomitant acceleration in total magma production from 75 to 1 400 10 6 km3 yr and frequency of basaltic eruptions from 1 per 17 000 yr to 1 per 3 000 yr occurred between 5 and 0 25 ma for the past 0 25 m y magma production 180 10 6 km3 yr and perhaps eruption frequency have decreased this evolutionary sequence coupled with the lead and strontium isotopic composition of the rocks can be explained by magmatism caused by shear heating at the base of the lithosphere we propose that this eastward drift of volcanic activity represents absolute westward motion of the north american plate our model is in agreement with a model in which the african plate is fixed to the deep mantle

seismic effects of a small sedimentary basin in the eastern tibetan plateau based on numerical simulation and ground motion records from aftershocks of the 2008 mw7 9 wenchuan china earthquake abstract in this study the ground motions that can be attributed to the aftershocks of the 2008 mw7 9 wenchuan china earthquake recorded in the wudu basin which is a small basin in the eastern tibetan plateau are analyzed to investigate the seismic effect of a small sedimentary basin in case of the wudu basin the site responses from the basin edge are stronger than those from the basin center the monitoring sites located near the wudu basin edge and the wudu basin center are sensitive to the low frequency 5 hz components of ground motion respectively it indicates that some different seismic phenomena occurred in a small sedimentary basin compared with a large sedimentary basin to further study the ground motion in a small sedimentary basin a simulation of the wudu basin was conducted using the finite element method with a viscous elastic artificial boundary the grid size of the surface layer of the basin is 2 5 2 5 m2 and high frequency up to 10 hz components of a synthetic waveform were obtained the simulation denoted that the surface wave initially appeared in the wudu basin edge and subsequently propagated into its interior the near surface soil layers exhibiting a relatively low wave velocity magnified the amplitude of the seismic wave in the wudu basin furthermore the numerical simulation results are basically consistent with the actual ground motion records especially with the same predominant frequencies of ground motions at the sites located near the wudu basin edge

the role of thermal viscous remanent magnetisation tvrm in magnetic changes associated with volcanic eruptions insights from the 2000 eruption of mt usu japan abstract volcanic eruptions can produce large magnetic field changes by thermomagnetic effects especially when magma cools from high temperatures and acquires a permanent magnetisation from the earth s magnetic field after the 2000 eruption of mt usu japan significant magnetic field changes were observed not only in the vicinity of the magmatic intrusion but also in an area some distance away that was unlikely to be at a temperature near the curie point the magnetic changes in the latter area appear to be caused by thermal viscous remanent magnetisation tvrm in which the elevated subsurface temperatures have accelerated the acquisition of magnetisation parallel to the existing earth s field in the ground material the unusually large changes up to 40 nt yr are probably because of underlying reversely magnetised pleistocene rocks which are being normally remagnetised by exposure to temperatures of over 200 c we made an order of magnitude estimate of the likely effects of tvrm in this case based on some previous laboratory studies which confirms that tvrm is a plausible mechanism for the observed magnetic changes this paper provides probably the first field example in which a natural tvrm process associated with ongoing volcanic activity has been observed this indicates that even when temperatures are well below the curie point significant magnetic changes can occur on active volcanoes in many cases areas of recent volcanism are dominated by normal magnetisation and thus the tvrm only plays a minor role in volcanomagnetic changes it will however be a significant effect when materials with reversed or randomly oriented magnetisation are moderately reheated through volcanic activity we emphasize that separation of thermal viscous magnetisation from thermal magnetisation will sometimes be necessary for properly interpreting magnetic changes around volcanoes especially in relation to ongoing volcanic activities

assessment of site effects in the kathmandu valley nepal during the 2015 mw 7 8 gorkha earthquake sequence using 1d and 2d numerical modelling abstract the paper reports on the results of 1d and 2d site response analyses carried out in the kathmandu valley nepal in order to investigate how site effects influenced the seismic response during the 2015 mw 7 8 gorkha earthquake sequence the mainshock and a mw 6 6 aftershock for which recordings at both rock and soil sites are available were considered first 1d analyses were carried out for the pulchowk soft soil site where a borehole was drilled the shear wave velocity profile was defined using several 2d seismic array surveys carried out in the valley and constrained by noise measurements at pulchowk site the nonlinear soil behavior was characterized by means of cyclic simple shear tests carried out on undisturbed soil samples both equivalent and nonlinear approaches were adopted in the 1d analyses overall the 1d model was capable to capture some relevant features shown by mainshock recordings such as the de amplification of medium to high frequencies on the contrary the unusual high spectral amplification at long periods 3 6 s recorded during the mainshock was better captured by the 2d finite element analyses carried out on a 20 km large cross section of the entire valley thus supporting the hypothesis of the occurrence of basin effects the paper contributes to the understanding of site effects in kathmandu valley for the implementation of seismic risk mitigation strategies in the area

maar diatreme phreatomagmatism at hopi buttes navajo nation arizona usa remnants of an extensive maar diatreme volcanic field are magnificently exposed at various depths of erosion in the hopi buttes volcanic field of northeastern arizona field and petrographic studies of both the maar and diatreme elements of a selection of volcanoes within the field show that 1 lower sections of the maar rim sequences are typically rich in sandy mudrock derived from the pre eruptive mio pliocene bidahochi formation and the muddy bidahochi sediment was soft and wet at the time of maar eruptions 2 beds higher within the rims contain generally increased proportions of sandstone clasts from the triassic wingate formation in the diatremes late emplaced breccia has deeper seated lithics than more marginal breccia emplaced earlier and 3 many vents are topped by megacryst enriched scoria and spatter and deep seated xenoliths are known only from upper diatreme and craterfilling tephra these observations show that 1 eruptions at hopi buttes involved interaction of magma with unconsolidated mudrock at shallow levels and the phreatomagmatic processes that provided the bulk of the energy involved in the violent eruptions were driven by the interaction of magma and wet sediment 2 the locus of explosive activity migrated downward as eruptions progressed and 3 the closing stage of many eruptions was characterized by rapid magma rise and relative depletion of water

a geophysical comparison of the diatremes of simple and complex maar volcanoes newer volcanics province south eastern australia abstract geophysical modelling techniques are applied to examine and compare the subsurface morphology of maar volcanoes within the newer volcanics province to better understand their eruptive histories and the hazards associated with future eruptions within the province the maar volcanoes under investigation include the ecklin and anakie maars and the red rock and mount leura volcanic complexes which vary in their complexity morphology eruptive styles and host rock type the ecklin and anakie maars display relatively simple geophysical signatures long wavelength gravity lows with corresponding magnetic highs are observed across the craters and were reproduced during modelling with the presence of a shallow maar diatreme at anakie and two coalesced diatremes containing denser central vents at ecklin red rock and mount leura have more complex geophysical signatures consisting of short wavelength gravity and magnetic highs superimposed on longer wavelength gravity lows these anomalies are reproduced during modelling with coalesced bowl shaped diatremes containing dykes and magma ponds the complex diatreme geometries revealed from forward and inverse modelling suggest that the eruption histories of these volcanoes are more complex than their morphology would suggest multiple coalesced diatreme structures indicate an eruption involving vent migration while preserved dykes within the diatreme suggest short lived fluctuations between phreatomagmatic and magmatic eruption styles the geometry of the diatremes is consistent with maars hosted in a soft substrate which likely contributed to the migration of vents observed at ecklin red rock and mt leura the shallow diatreme observed within the anakie maar is attributed to a short lived eruption and low water content within the granitic host rock

seismic wave propagation in alluvial basins and influence of site city interaction the geometrical and mechanical features of alluvial deposits have a major influence on seismic wave propagation and amplification however for alluvial basins located in densely urbanized areas the surface structures such as buildings could influence seismic wave propagation near the free surface in this article the influence of surface structures on seismic wave propagation is analyzed numerically in the case of an actual two dimensional 2d shallow basin at a local scale the vibration of a surface structure can induce a seismic wave field in the surficial soil layers at the scale of an alluvial basin the site city models considered herein show that the city effect can lead to a significant seismic wave field modification when compared to the free field case the coincidence between the fundamental frequencies of the soil layers and eigenfrequencies of the surface structures is a key parameter to investigate site city interaction when comparing simplified sitecity models kham et al 2006 to the basin city model the influence of the lateral heterogeneities on the site city interaction is found to be significant indeed the seismic wave field radiated by the city appears to be trapped within the alluvial basin and specific directivity features are found for this wave field the influence of site city interaction on the free field seismic hazard may then be significant the effects of the site city interaction are beneficial in some parts of the city or detrimental in other parts especially city boundaries these effects strongly depend on the urban configuration city heterogeneity building density etc finally the full characterization of the seismic wave field in densely urbanized areas could often raise the need for investigating site city interaction and consider such parameters as basin and city fundamental frequencies building density and city arrangement as well as basin effects combined with the seismic wave field radiated by the city

facies variation of eruption units produced by the passage of single pyroclastic surge currents hopi buttes volcanic field usa abstract base surges are a significant volcanic hazard associated with phreatomagmatism but their transport and depositional dynamics are incompletely understood stratigraphic analysis and lateral tracing of surge eruption units produced by the passage of single base surges from haskie tuff ring hopi buttes volcanic field usa extend to 1 km from the vent and show similar downcurrent changes in lithofacies within 200 m of the vent eruption units are primarily disorganized lapilli tuff that is rich in juvenile pyroclasts at distances 200 m from the vent eruption units transform into a couplet of juvenile rich lapilli that grades upward into bedform bearing ash tuff at distances 900 m upper and lower beds of the eruption unit couplet transform into planar stratified beds at distances 1100 m couplets composing different eruption units are almost entirely composed of planar beds and amalgamate to form almost identical planar bedded ash tuff the collapse of dense eruption columns following explosive magma wet sediment interaction generated turbulent surge currents as the surges traveled away from the haskie vent they expanded and partitioned into a basal juvenile rich traction carpet and overlying accidental rich ash cloud surge near the limit of their runout the surges cooled and lost their pyroclastic load leaving planar beds of accidental rich ash the deposition of multiple surge eruption units with similar lateral and vertical facies transitions and a paucity of interbedded fallout suggests an eruption characterized by recurring phreatomagmatic explosions of similar size and the generation of surges with similar flow characteristics

numerical modeling of seismic site effects in a shallow estuarine bay suai bay shantou china abstract this paper presents the results of numerical modeling of seismic site effects in a shallow estuarine bay suai bay shantou china which is meaning for seismic hazard assessment and mitigation of estuarine and coastal structures such as immersed tunnels pipelines caissons and breakwaters the subsurface stratum and soil properties in the study area are obtained from geological and geotechnical surveys large scale 2d finite element refined nonlinear modeling is used to simulate ground motion for near field and far filed input motions with different design earthquake levels the results show that for the study area 1 the spatial distribution of pga amplification factor presents obvious vertical and horizontal variations and is strongly affected by the overall geometry of interfaces of different materials 2 the attenuation in soft sediments have significant influences on surface ground motion which leads to a deamplification of the acceleration amplitude a prolongation of the significant duration and a filtration of the high frequency spectral content 3 the pga alone cannot be regarded as an adequate parameter to quantify nonlinear site effects while the energy content expressed by arias intensity is more closely related to transfer functions patterns 4 the seismic intensity at surface is greater than the intensity of input motion during weak motions while the seismic intensity at surface is smaller than the intensity of input motion during strong motions suggesting that isolation effects may be expected in the areas having thick soft soils with a low plasticity index during strong motions

emplacement temperatures of pyroclastic and volcaniclastic deposits in kimberlite pipes in southern africa palaeomagnetic techniques for estimating the emplacement temperatures of volcanic deposits have been applied to pyroclastic and volcaniclastic deposits in kimberlite pipes in southern africa lithic clasts were sampled from a variety of lithofacies from three pipes for which the internal geology is well constrained the cretaceous a k1 pipe orapa mine botswana and the cambrian k1 and k2 pipes venetia mine south africa the sampled deposits included massive and layered vent filling breccias with varying abundances of lithic inclusions layered crater filling pyroclastic deposits talus breccias and volcaniclastic breccias basalt lithic clasts in the layered and massive vent filling pyroclastic deposits in the a k1 pipe at orapa were emplaced at 570 c in the pyroclastic crater filling deposits at 200 440 c and in crater filling talus breccias and volcaniclastic breccias at 560 c although the interpretation of these results is hampered by the presence of mesozoic magnetic overprints these temperatures are comparable to the estimated emplacement temperatures of other kimberlite deposits and fall within the proposed stability field for common interstitial matrix mineral assemblages within vent filling volcaniclastic kimberlites the temperatures are also comparable to those obtained for pyroclastic deposits in other silicic volcanic systems because the lithic content of the studied deposits is 10 30 the initial bulk temperature of the pyroclastic mixture of cold lithic clasts and juvenile kimberlite magma could have been 300 400 c hotter than the palaeomagnetic estimates together with the discovery of welded and agglutinated juvenile pyroclasts in some pyroclastic kimberlites the palaeomagnetic results indicate that there are examples of kimberlites where phreatomagmatism did not play a major role in the generation of the pyroclastic deposits this study indicates that palaeomagnetic methods can successfully distinguish differences in the emplacement temperatures of different kimberlite facies

seismic site city interaction main governing phenomena through simplified numerical models this work focuses on the analysis of the multiple interactions between soil layers and civil engineering structures in dense urban areas submitted to a seismic wave to investigate such phenomena called site city interaction sci herein two simplified site city configurations are considered a homogeneous periodically spaced city and a heterogeneous nonperiodically spaced city both on a constant depth basin model these 2d boundary element method models are subjected to a vertically incident plane sh ricker wavelet a parametric study of the city parameters density of buildings and their natural frequencies and the thickness of the basin is carried out to characterize the sci and to investigate its sensitivity to some governing parameters the following parameters are analyzed building vibrations induced ground motion ground motion perturbations inside and outside the city spatial coherency and kinetic energy of the urban wave field a so called site city resonance is reached when the soil fundamental frequency and structure eigenfrequencies coincide building vibrations and ground motion are then significantly decreased and the spatial coherency of the urban field is also strongly modified building density and city configuration play a crucial role in the energy distribution inside the city

maar diatreme geometry and deposits subsurface blast experiments with variable explosion depth basaltic maar diatreme volcanoes which have craters cut into preeruption landscapes maars underlain by downward tapering bodies of fragmental material commonly cut by hypabyssal intrusions diatremes are produced by multiple subsurface phreatomagmatic explosions although many maar diatremes have been studied the link between explosion dynamics and the resulting deposit architecture is still poorly understood scaled experiments employed multiple buried explosions of known energies and depths within layered aggregates in order to assess the effects of explosion depth and the morphology and compaction of the host on the distribution of host materials in resulting ejecta the development of subcrater structures and deposits and the relationships between them experimental craters were 1 2 m wide analysis of high speed video shows that explosion jets had heights and shapes that were strongly influenced by scaled depth physical depth scaled against explosion energy and by the presence or absence of a crater jet properties in turn controlled the distribution of ejecta deposits outside the craters and we infer that this is also reflected in the diverse range of deposit types at natural maars ejecta were dominated by material that originated above the explosion site and the shallowest material was dispersed the farthest subcrater deposits illustrate progressive vertical mixing of host materials through successive explosions we conclude that the progressive appearance of deeper seated material stratigraphically upward in deposits of natural maars probably records the length and time scale for upward mixing through multiple explosions with ejection by shallow blasts rather than progressive deepening of explosion sites in response to draw down of aquifers

near surface geophysical mapping of an upper cretaceous submarine volcanic vent in austin texas usa geophysical surveys were conducted at the williamson creek site south of austin texas to determine the structural relation of the upper cretaceous volcanic rocks lava and tuff with the associated austin chalk limestone at this site resistivity and magnetic methods were performed over the exposed volcanic and limestone rocks geophysical results indicate an excellent correlation between high magnetic and low resistivity anomalies the pseudo 3d resistivity data show a steeply dipping funnel shaped vent formation over the high magnetic anomaly up to 3000 nt magnetic anomalies are consistent with a uniformly magnetized body like a volcanic vent and the anomaly s dimensions are consistent with eroded volcanic vents in other distributed volcanic fields in the united states magnetic data has been integrated with resistivity data and geologic observations and subjected to 2 5d forward potential field modeling modeling has revealed a perfect fit with three magnetic zones 1 the central part corresp

simulation of the seismic response of sedimentary basins with vertical constant gradient velocity for incident sh waves the simulation of the seismic response of heterogeneous sedimentary basins under incident plane waves is computed using the indirect boundary element method ibem to deal with these kinds of basins we used approximate analytical expressions for the two dimensional green s functions of a medium with constant gradient wave propagation velocity on the other hand for the homogeneous half space underlying the sedimentary basin the full space green s functions were used the response of semi circular heterogeneous basins under incident sh waves is explored by means of the displacements in the frequency space diagrams and synthetic seismograms moreover we compared these results with those obtained for other homogeneous semi circular models the principal differences among them are pointed out this simulation provided interesting results that displayed a complex amplification pattern in a rich spectrum of frequencies and locations the maximum amplitudes levels were found around the edges of the heterogeneous sedimentary basins in time domain some features characterize the seismic response of the basin which include enhanced generation and trapping of surface waves inside the sediments and the reduced emission of seismic energy to the hard rock in the heterogeneous models the lateral reflections of surface waves greatly influence the total displacements at the free surface in comparison with the homogeneous models where the displacements have a shared influence among both vertical and lateral reflections

investigation of the inner structure of la crosa de sant dalmai maar catalan volcanic zone spain abstract la crosa de sant dalmai volcano is the largest volcanic edifice of the catalan volcanic zone ne spain it is a very well preserved maar type structure 1 5 km in diameter excavated at the contact between a hard substrate and a soft substrate formed by palaeozoic granites and pliocene and quaternary gravels respectively in order to infer the uppermost inner structure of la crosa de sant dalmai maar and to characterise its main geological and tectonic constraints we have performed a multiparametric geophysical study including gravimetry magnetometry self potential and electrical resistivity tomography the results obtained together with a field geology revision and additional geological data from two drill cores provide a detailed picture of the post eruptive maar infill sequence as well as of the uppermost part of the maar diatreme structure this information helps in understanding the origin and subsequent evolution of the volcano which included an alternation of phreatomagmatic and strombolian phases geophysical data show that the last strombolian phase which culminated with the formation of a scoria cone inside the maar was associated with a nw se trending regional fault the little erosion and degradation of the original tephra ring suggest a much younger age of la crosa de sant dalmai maar than was previously stated

basalts water or shear zones in the lower continental crust abstract bright sub horizontal laterally extensive multi cyclic layered reflections have now been recorded in the lower continental crust from many areas in contrast this reflection signature is rare in the continental upper crystalline crust and upper mantle and in the oceanic lithosphere with the present state of knowledge it is sensible to seek a single quantitative explanation for all such layering qualitative explanations for lower crustal layering abound and include igneous metamorphic structural and fluid related models measurements of reflection coefficients from the lower crust place strong quantitative constraints on possible models structurally induced anisotropy metamorphic layering or free fluids cannot realistically produce sufficiently bright reflections igneous layering or the juxtaposition of pre existing heterogeneities by shear zones can produce strong reflections results from crustal xenoliths helium isotope ratios palaeo geothermal gradients and chemistry of flood basalts together with numerical modelling of lithospheric extension and asthenospheric melting suggest that underplating and intrusion of the lower crust with mantle derived mafic igneous rocks is common the apparent spatial relationship between lower crustal layering and continental extension and the strength and continuity of the reflections suggest that underplating related to extension and or elevated mantle temperatures is a good explanation for lower crustal layering

characterization of volcanic deposits with ground penetrating radar field based studies of surficial volcanic deposits are commonly complicated by a combination of poor exposure and rapid lateral variations controlled by unknown paleotopography the potential of ground penetrating radar gpr as an aid to volcanological studies is shown using data collected from traverses over four well exposed recent volcanic deposits in western canada the deposits comprise a pumice airfall deposit 3 4 m thick a basalt lava flow 3 6 m thick a pyroclastic flow deposit 15 m thick and an internally stratified pumice talus deposit 60 m thick results show that gpr is effective in delineating major stratigraphic contacts and hence can be used to map unexposed deposits different volcanic deposits also exhibit different radar stratigraphic character suggesting that deposit type may be determined from radar images in addition large blocks within the pyroclastic deposits are detected as distinctive point diffractor patterns in the profiles showing that the technique has potential for providing important grain size information in coarse poorly sorted deposits laboratory measurements of dielectric constant k are reported for samples of the main rock types and are compared with values of k for the bulk deposit as inferred from the field data the laboratory values differ significantly from the field values of k these results suggest that the effectiveness of gpr at any site can be substantially improved by initial calibration of well exposed locations

nonlinear seismic response in the western l aquila basin italy numerical fem simulations vs ground motion records abstract in the western l aquila basin italy six seismic stations of the italian strong motion network which were used on both soil and rock outcrops recorded a huge amount of weak and strong motion data near an active fault during the 2009 l aquila seismic sequence this paper analyzes the ground motion records of events with magnitudes ranging between 1 6 and 6 3 and compares the observed peak accelerations and the h v and v h spectral ratios with those revealed from numerical simulations the finite element method is considered herein to perform 2d dynamic modeling on a geologic cross section of the upper aterno river valley which is 1000 m wide and 50 m deep based on the reviewed geologic geotechnical and geophysical investigations that were performed in previous studies the martin finn seed s pore water pressure model is used in the simulations the observed peak accelerations the frequencies and the levels of maximum amplification are correctly estimated using the fem simulations the consistency between the computed and the experimental results shows a seismic response that is sensitive to the level of earthquake magnitude and mainly depends on the rock and soil elastic and dynamic properties the intensity and the depth of the seismic impedance contrast the water table depth the two dimensional irregular configuration the dip angle of strata the shape and the frequency content of the input motion record the seismic motion amplification is usually higher at station aqv which is located near the valley center where a peak acceleration value of 0 66 g was recorded both the pga amplification factor and the maximum amplification frequency of the horizontal component decrease when the earthquake magnitude level increases which shows a nonlinear seismic response at the stations aqg on fractured and weathered rock aqa and aqv on alluvium the simulation of the 2009 mw 6 3 l aquila earthquake confirms the observed nonlinear effective stress behavior and allows us to relate it to the generation of excess pore water pressure which ranges between 50 and 300 kpa

dikes sills and stress regime evolution during emplacement of the jagged rocks complex hopi buttes volcanic field navajo nation usa abstract the dikes and related intrusions formed below small volcanoes in volcanic fields are remnants of the simplest volcanic plumbing systems their geometry is controlled by interaction of magma driven cracks with country rock and reveals regional structural and stress patterns at the time of their emplacement the shallow stress field however may change during the time an intrusion complex is emplaced in response to addition or removal of magma and country rock during associated surface eruptions the jagged rocks complex in the miocene hopi buttes volcanic field navajo nation arizona is exposed 300 350 m below the pre eruptive surface it comprises a group of generally nw se striking dikes punctuated in places by buds a saucer like intrusion larger pyroclastic massifs and a diatreme we made measurements of 13 dikes divided into 172 segments with thicknesses from 8 to 122 cm mean 43 cm and lengths of 60 to 780 m several sills and inclined sheets in places are thicker than dikes having mean thicknesses of 48 cm and 73 cm respectively dikes typically show straight parallel and en echelon patterns while sills and inclined sheets are curved the northwestern dikes differ from the rest in containing large mafic crystals and are inferred to have been emplaced after the others we find that the strike of the overall complex dikes and other sheets elongate massifs and aligned sub cylindrical bodies reflects a crystalline basement control that is evident throughout hopi buttes over the period of the complex s emplacement local stress patterns were not stable we infer that excavation of deep maar craters and perhaps the construction of a scoria cone at the surface modified the local stress patterns to favor emplacement of sills and en echelon dikes later in the complex s evolution in contrast to simple straight dikes as the complex first formed

geophysical surveys of the joya honda maar mexico and surroundings volcanic implications abstract joya honda jh is a quaternary maar excavated in mesozoic limestone it is located in central mexico and belongs to the ventura volcanic field vvf which is composed by cinder cones and maars made of intraplate type mafic alkalic rocks volcanoes in the region form n20w lineaments roughly parallel to a regional set of normal faults but there is no obvious relation between these faults and vent distribution in the exposed geology around the maar the volcanic rock volume is small in the vvf and most volcanoes and their products are scattered in a region where outcrops are dominated by limestone the near vent tephra associated to the jh maar lies north of the crater this relation suggests that the crater was formed by directed hydromagmatic explosions and may indicate an inclined volcanic conduit near the surface the tephra stratigraphy suggests that the initial explosions were relatively dry and the amount of water increased during the maar forming eruption therefore the existing model of the maar diatreme formation may not be applicable to joya honda as it requires the formation of a cone of depression in the aquifer and deepening of the focii of the explosions as the crater and underlying diatreme grew thus it is unlikely that there is a diatreme below joya honda aeromagnetic data shows a boundary between two regional magnetic domains near the elongated volcanic cluster of the vvf the boundary is straight with a distinct kink from ne to nw trend near jh the limit between the domains is interpreted as fault contacts between mid tertiary volcanic rocks and marine mesozoic sedimentary rocks hence magma ascent in the area may have been facilitated by fractures near the surface magnetic and gravimetric ground surveys show that the anomalies associated with the maar are not centered in the crater which could be consistent with an inclined volcanic conduit a magnetic profile measured on exposed limestone across the volcanic lineament failed to show an anomaly such as that caused by a connecting dike between the volcanoes therefore either the dike does not exist or it is so deep or so thin that it is beyond the limit of detection of the method and or equipment used thus the volcanic conduit immediately below joya honda can be reasonably modeled in the shape of a plug a 2 d model of the crater anomaly is consistent with a roughly tabular deposit formed by fall back pyroclasts and slump deposits near the surface based on this result we propose an alternative model for the formation of maar type volcanoes excavated in hard rock where there is no evidence of a gradual decrease of the water magma ratio as the eruption advanced

timing of basaltic volcanism along the mesa butte fault in the san francisco volcanic field arizona from 40ar 39ar dates implications for longevity of cinder cone alignments using high precision stepwise heating 40ar 39ar methods we dated volcanic products of at least 12 vents cropping out along the northern segment of the mesa butte fault in the san francisco volcanic field arizona where the relationship between fault and vent alignment is unambiguous the 40ar 39ar isochron dates indicate that the mesa butte fault hosted five episodes of volcanism over a 1 million year interval from 1400 to 300 ka the first episode occurred 1 km northwest of the trace of the mesa butte fault whereas the next two episodes were centered near the intersection of the mesa butte and cedar ranch faults the final two episodes produced red mountain 750 100 ka and shadow mountain 300 100 ka respectively ages of vents representing the third mesa butte episode of volcanism 760 50 ka to 900 80 ka indicate a systematic southwestward shift in the locus of volcanism along the mesa butte fault that contrasts sharply with overall eastward younging of volcanism within the field these data demonstrate that structures in basaltic volcanic fields are capable of hosting multiple episodes of volcanism over periods of time comparable to duration periods of composite volcanoes 106 years but at significantly lower recurrence intervals these results suggest that probability models of spatiotemporal recurrence of volcanism within basaltic volcanic fields should incorporate mapped structures particularly where these structures have either hosted previous volcanic activity or are proximal to population centers or sensitive installations such as nuclear power plants or waste repositories

subsurface structure of a maar diatreme and associated tuff ring from a high resolution geophysical survey rattlesnake crater arizona abstract geophysical survey techniques including gravity magnetics and ground penetrating radar were utilized to study the diatreme and tuff ring at rattlesnake crater a maar in the san francisco volcanic field of northern arizona significant magnetic anomalies 1600 nt and a positive gravity anomaly 1 4 mgal are associated with the maar joint modeling of magnetic and gravity data indicate that the diatreme that underlies rattlesnake crater has volume of 0 8 1 km 3 and extends to at least 800 m depth the modeled diatreme comprises at least two zones of variable density and magnetization including a low density highly magnetized unit near the center of the diatreme interpreted to be a pyroclastic unit emplaced at sufficiently high temperature and containing sufficient juvenile fraction to acquire thermal remanent magnetization magnetic anomalies and ground penetrating radar gpr imaging demonstrate that the bedded pyroclastic deposits of the tuff ring also carry high magnetization likely produced by energetic emplacement of hot pyroclastic density currents gpr profiles on the tuff ring reveal long 100 m wavelength undulations in bedding planes elsewhere comparable bedforms have been interpreted as base surge deposits inflated by air entrainment from eruption column collapse interpretation of these geophysical data suggests that rattlesnake crater produced highly energetic phreatomagmatic activity that gave way to less explosive activity as the eruption progressed the positive gravity anomaly associated with the maar crater is interpreted to be caused by coherent bodies within the diatreme and possibly lava ponding on the crater floor these dense magnetized bodies have excess mass of 2 4 10 10 kg and occupy approximately 5 of the diatreme by volume magnetic anomalies on the crater floor are elongate nw se suggesting that the eruption may have been triggered by the interaction of ascending magma with water in fractures of this orientation gpr imaging of the tuff ring also suggests that substantial land slip may have occurred on the western rim perhaps causing part of the tuff ring to collapse into the crater strong radar reflections indicative of well developed weathering horizons are present as well the techniques employed at rattlesnake crater demonstrate the value of combining multiple geophysical techniques in areas where exposures are limited and invasive exploration is not an option

three dimensional potential field modelling of a multi vent maar diatreme the lake coragulac maar newer volcanics province south eastern australia abstract high resolution gravity and magnetic data have been acquired across lake coragulac a quaternary maar volcano located in the newer volcanics province of south eastern australia a gravity low is observed across the maar crater with several local gravity highs identified in the centre of the crater often with corresponding magnetic anomalies geophysical data has been integrated with geologic observations and subjected to 2d forward and 3d inverse potential field modelling modelling has revealed a complex subsurface maar structure with four coalesced diatremes and several intrusive dykes identified the modelled diatremes are shallow maximum of 290 m deep with the margins having a similar density to the host rock while the diatreme centre is often denser indicating that higher volumes of volcanic debris are present deposits of the maar rim sequence show frequent transitions between phreatomagmatic and magmatic fragmentation styles a result of fluctuations in magma rise rate and interactions with external water the complex nature of the underlying maar diatreme suggests that the maar formed from at least four vents which migrated laterally during the eruption

detailed investigation of preserved maar structures by combined geophysical surveys the detection of completely preserved maar structures is important not only for underground mapping but also for paleoclimate research because laminated maar lake sediments may contain a very detailed archive of climate history objective evidence for the existence of such structures can only be provided by geophysics and boreholes the combination of gravity and magnetic ground surveys appears to be an excellent tool to detect and identify buried maar structures their prominent properties are an almost circular gravity minimum corresponding to a crater filled with limnic sediments of low density and a magnetic anomaly caused by a pyroclastic or basaltic body in the diatreme which indicates the volcanic character seismic measurements provide the most detailed information about the internal structure of the maar sediments zones of low seismic reflectivity and very low density represent sediments of the late maar lake period the early lake period is indicated by debris flow deposits and turbidites represented by seismic reflectors the seismic sections clearly reveal the bowl like structure of the maar outside this bowl like structure there are only a few reflections which represent the basement taking into account the shape of the gravity anomaly seismic information allows geometrical modelling of the maar structure optimal drilling sites can be selected based on the results of geophysical surveying comparing the results of combined geophysical surveys above two maar structures of different ages yields a marked similarity in their geophysical pattern

the role of phreatomagmatism in a plio pleistocene high density scoria cone field llancanelo volcanic field mendoza argentina abstract the plio pleistocene llancanelo volcanic field together with the nearby payun matru field comprises at least 800 scoria cones and voluminous lava fields that cover an extensive area behind the andean volcanic arc beside the scoria cones in the llancanelo field at least six volcanoes show evidence for explosive eruptions involving magma water interaction these are unusual in the context of the semi arid climate of the eastern andean ranges the volcanic structures consist of phreatomagmatic derived tuff rings and tuff cones of olivine basalt composition malacara and jarilloso tuff cones were produced by fallout of a range of dry to wet tephra the malacara cone shows more evidence for a predominance of wet emplaced units with a steep slump slope characterized by many soft sediment deformation structures such as undulating bedding planes truncated beds and water escape features the piedras blancas and carapacho tuff rings resulted from explosive eruptions with deeper explosion loci these cones are hence dominated by lapilli tuff and tuff units emplaced mainly by wet and or dry pyroclastic surges carapacho is the only centre that appears to have started with phreatomagmatic eruptions with lowermost tephra being rich in non volcanic country rocks the presence of deformed beds with impact sags slumping textures asymmetrical ripples dunes cross and planar lamination syn volcanic faulting and accretionary lapilli beds indicate an eruption scenario dominated by excessive water in the transportational and depositional regime this subordinate phreatomagmatism in the llancanelo volcanic field suggests presence of ground and or shallow surface water during some of the eruptions each of the tuff rings and cones are underlain by thick fractured multiple older lava units these broken basalts are inferred to be the horizons where rising magma interacted with groundwater the strong palagonitization at each of the phreatomagmatic cones formed hard beds resistant to erosion and therefore the volcanic landforms are well preserved

crustal structure of the northwestern basin and range province from the 1986 program for array seismic studies of the continental lithosphere seismic experiment the basin and range province of the western united states is characterized by active east west extension at strain rates of 1 cm yr 1 high regional heat flow of 90 mw m 2 and widespread late cenozoic bimodal rhyolitic basaltic magmatism the 1986 program for array seismic studies of the continental lithosphere basin and range seismic experiment imaged a portion of northwestern nevada to determine the crustal structure and to assess reported differences between refraction versus reflection determinations of moho depth and how the crustal composition and structure has been influenced by volcanic and extension mechanisms our interpretation of the refraction wide angle reflection data suggests that the crust is fairly uniform in thickness and varies by less than 5 km over the 280 km east west profile and 3 km over its 220 km north south length we characterize the velocity structure by five layers 1 an upper most crust composed of sedimentary rocks and basement that has an average velocity of 5 7 km s i 2 a middle crust that extends to a depth of 18 22 with an average velocity of 6 1 km s 1 3 a 10 12 km thick lower crust with an average velocity of 6 6 km s 1 4 a 2 5 km thick transitional crust mantle boundary defined by a 7 6 km s l velocity and 5 an upper mantle with an average pn velocity of 7 9 8 0 km s 1 these data show a uniformily thick crust of 28 km 5 km beneath northwestern nevada and a lack of evidence for significant low velocity crustal layers in general the entire crustal column exhibits uniform positive velocity gradients of 0 02 0 04 s the midcrustal discontinuity is interpreted as a small velocity increase of 0 2 km s l at 18 km depth the absence of a significant midcrustal reflection and the presence of large velocity gradients throughout the crust arises either from 1 a systematic increase in mafic composition with depth or 2 possibly high lower crustal pore pressure that acts to decrease the velocity contrast across the midcrustal boundary our interpreted seismic velocity structure is consistent with a broad range of bulk compositions for the crust upper crustal velocities are consistent with bulk compositions that range from metasediments and granites at shallow depth to diorites at midcrustal levels while lower crustal velocities suggest bulk compositions that range from diorites and amphibolities at midcrustal depths to mafic granulites and gabbros at the base of the crust the uniform crustal thickness of 30 34 km in northwestern nevada is close to the crustal average for the basin and range province but is thin when compared to surrounding regions such as the sierra nevada and colorado plateau where crustal thickness is greater than 40 kin similarily northwestern nevada pn velocities of 7 9 8 0 km s 1 are near the province wide average of 7 9 km s l temperature corrected pn velocities from a 90 mw m 2 geotherm for the basin and range to 60 mw m 2 for the stable interior of the united states are within one standard deviation of the continental average of 8 05 km s i and suggest a uniform upper mantle composition across the basin and range in addition to these observations the homogeneity of the velocity structure beneath the western basin and range argues for a youthful moho and crust that has been reworked by province wide late cenozoic extension episodic magmatism and underplating

bright seismic reflections and fluid movement by porous flow in the lower crust abstract the origin of high amplitude lower crustal bright seismic reflections in a phanerozoic area is investigated by modeling a distribution of metamorphic fluid flow through pores as a possible explanation a numerical model of metamorphic fluid flow in a viscous porous medium is developed a model is applied to a tectonically active area such as the basin and range where viscous behavior of the lower crust is suggested calculations show that the characteristic vertical dimension of relative volumetric flow variations corresponds to the thickness of seismic lamellae as determined from reflection frequency spectra for the basin and range this result shows that a dynamic effect which was produced by fluid pressure and viscous matrix resistance could be responsible for the origin of the seismic lamellae in the lower crust in an area of active tectonics

computation of ground motion amplification scenario in nct delhi for earthquake engineering purposes and seismic microzonation this paper presents a scenario for the spatial variation of the fundamental frequency of the sediment deposits above the basement and the corresponding amplification as well as the average spectral amplification in different frequency bandwidths for the national capital territory delhi the capital of india the exposed central quartzite ridge and the yamuna river channel are responsible for very large spatial variations of the fundamental frequency in the eastern part of the national capital territory delhi at 20 of the considered sites a good match is obtained between the fundamental frequency computed numerically using available s wave velocities to a certain depth and their extrapolation and that obtained experimentally the computed fundamental and dominant frequencies reveal that both medium rise 5 10 storey and high rise 10 storey buildings in the western part and medium rise buildings lying in the localities east of or very near to the yamuna river may suffer heavy to very heavy damage due to the occurrence of the double resonance phenomenon furthermore 1 2 storey buildings lying on the weathered exposed quartzite rock may also suffer heavy damage during local earthquakes because of the occurrence of double resonance the possible reasons behind the lack of earthquake damage to the qutab minar the tallest brick masonry minaret in the world over the last 800 years may be the nonoccurrence of double resonance and almost no amplification in the low frequency range there are two localities in the western part of the national capital territory delhi namely kanganheri chhawla and buradi wherein all sorts of buildings are highly vulnerable to earthquake damage for the closed chhatarpur basin and a semiclosed basin to its northeast formed due to exposed quartzite rock three dimensional 3d simulations are required to predict the characteristics of basin generated surface waves and their focusing effects in the chhatarpur basin the average spectral amplification map developed for the 0 10 hz bandwidth depicts a range of 2 25 4 82 in the national capital territory delhi and may be directly used to transfer the estimated seismic hazard at basement to the free surface

geophysical applications to detect sinkholes and ground subsidence the term sinkhole receives a lot of use and equal amounts of abuse in the popular media generally anytime a hole or depression forms in the land surface sometimes in a short period of time it is called a sinkhole sinkholes are geologic features formed by movement of rock or sediment into voids created by the dissolution of water soluble rock some sinkholes such as the infamous winter park florida sinkhole of 1981 figure 1 capture the attention of society as we view expensive homes and automobiles teetering on a precipice about to disappear into the underworld subsidence features caused by other processes such as mine collapse and washouts resulting from broken water and sewer mains are not true sinkholes but may be equally as damaging these features also result from rock or soil moving into a void but the void was a result of human activities

geology of a complex kimberlite pipe k2 pipe venetia mine south africa insights into conduit processes during explosive ultrabasic eruptions k2 is a steep sided kimberlite pipe with a complex internal geology geological mapping logging of drillcore and petrographic studies indicate that it comprises layered breccias and pyroclastic rocks of various grain sizes lithic contents and internal structures the pipe comprises two geologically distinct parts k2 west is a layered sequence of juvenile and lithic rich breccias which dip 20 45 inwards and k2 east consists of a steep sided pipe like body filled with massive volcaniclastic kimberlite nested within the k2 pipe the layered sequence in k2 west is present to 900 m below present surface and is interpreted as a sequence of pyroclastic rocks generated by explosive eruptions and mass wasting breccias generated by rock fall and sector collapse of the pipe walls both processes occurred in tandem during the infill of the pipe several breccia lobes extend across the pipe and are truncated by the steep contact with k2 east dense pyroclastic rocks within the layered sequence are interpreted as welded deposits k2 east represents a conduit that was blasted through the layered breccia sequence at a late stage in the eruption this phase may have involved fluidisation of trapped pyroclasts with loss of fine particles and comminution of coarse clasts we conclude that the k2 kimberlite pipe was emplaced in several distinct stages that consisted of an initial explosive enlargement followed by alternating phases of accumulation and ejection

an unusually energetic basaltic phreatomagmatic eruption using deposit characteristics to constrain dilute pyroclastic density current dynamics abstract multiple highly erosive base surges of the table rock complex tuff ring trc2 oregon produced dune bedded deposits with crest to crest bedform wavelengths up to 200 m which are amongst the largest ever recognized in the deposits of pyroclastic density currents here we use bedform wavelength surmounted obstacles and a large chute and pool feature to estimate near source velocities 118 233 m s 1 lower bound velocities at radial distances of 1 6 2 and 4 7 km from source 34 29 and 20 m s 1 respectively and corresponding column collapse heights up to 2 8 km this paper represents one of the few studies that attempt to quantify flow characteristics such as emplacement velocities at different distances from source eruption column collapse height and eruptive energy based on deposit characteristics

reconstructing the eruption magnitude and energy budgets for the pre historic eruption of the monogenetic 5 ka mt gambier volcanic complex south eastern australia understanding explosive volcanic eruptions especially phreatomagmatic eruptions their intensities and energy budgets is of major importance when it comes to risk and hazard studies with only a few historic occurrences of phreatomagmatic activity a large amount of our understanding comes from the study of pre historic volcanic centres which causes issues when it comes to preservation and vegetation in this research we show that using 3d geometrical modelling it is possible to obtain volume estimates for different deposits of a pre historic complex monogenetic centre the mt gambier volcanic complex south eastern australia using these volumes we further explore the energy budgets and the magnitude of this eruption vei 4 including dispersal patterns eruption columns varying between 5 and 10 km dispersed towards north east to south to further our understanding of intraplate monogenetic eruptions involving phreatomagmatic activity we also compare which thermodynamic model fits best in the creation of the maar crater of mt gambier the major explosion dominated model or the incremental growth model in this case the formation of most of the craters can best be explained by the latter model

dipping seismic reflectors electrically conductive zones and trapped water in the crust over a subducting plate the lithoprobe program across the northern cascadia subduction zone at vancouver island has obtained geophysical data that suggest a dipping zone of trapped free pore water in the overlying continental crust land and marine multichannel seismic reflection profiles show several bands of reflectors the primary one at a depth of about 30 km is about 5 km thick dips inland and appears to coincide with an electrically conductive layer defined by magnetotelluric data the downgoing oceanic crust is imaged as a thin reflector dipping at about 15 and 10 km deeper than the primary reflector a location that is consistent with seismicity and seismic refraction data both the reflectors and the conductor have been modeled by near horizontal lamellae with 1 4 saline water porosity detailed geothermal measurements defining heat flow that decreases inland indicate that the dipping bands are nearly isothermal at a temperature of 400 500 c it is suggested that free water generated by dehydration of the downgoing oceanic plate migrates upward until reduced temperature results in hydration reactions and mineral precipitation that forms an impermeable barrier low vertical permeability is required to keep the water from separating to the top of the layer this can be reconciled with the low electrical resistivity if pore interconnection is through thin grain boundary tubes as predicted by theoretical equilibrium pore geometry although very small effective grain size is necessary an abrupt transition from high permeability to very low permeability is predicted at a several percent porosity in agreement with the modeled porosity such pore geometry with low permeability may be restricted to temperatures above about 400 c i e in greenschist to amphibolite facies conditions where grains will deform to equilibrium geometry and cracks are annealed the region above the reflective and conductive band is estimated to be within blueschist conditions consistent with the inferred high seismic velocity from refraction data and high density from gravity data