sediment dynamics over sand ridges on a tideless mid outer continental shelf abstract a sand ridge field with crests oriented ne sw 52 located between 55 and 85 m water depth on the valencia continental shelf spain was mapped with multibeam swath bathymetry and characterized with seismic profiling and sediment sampling boundary layer hydrodynamic measurements conducted over a sand ridge at 66 m depth show evidence of wave and current sediment resuspension during a major storm event with a three year recurrence period from the measured intrawave sediment concentration profiles it is shown that the suspended sediment time evolution is related not only to mean currents but also to the wave field the near bottom shear stress generated during storm conditions reached around 1 n m2 and was able to resuspend and transport sandy and muddy sediments for more than 32 h the total suspended sediment transport in a layer of 1 m above the bottom was around 4000 kg per meter width and directed 160 it is argued that these sand ridges were developed in a shallow environment and there is no evidence of present day ridge migration or morphological degradation it is hypothesized that sediment dynamics affecting sand ridges during storms favors the maintenance of their morphology

draugen oil field haltenbanken province offshore norway the draugen oil field lies in block 6407 9 in the haltenbanken oil and gas province the field is located 150 km off the coast of norway and 200 km south of the arctic circle in water depths of 240 280 m the field was discovered in 1984 by well 6407 9 1 five more exploration appraisal wells and two dimensional seismic assisted in delineating the reservoir the field is hosted by a low relief north south trending anticline measuring some 20 x 6 km the reservoir lies at a depth of 1 600 m subsea and has an oil column of 40 m the main reservoir is the late jurassic rogn formation sandstones interpreted as a shallow marine sand bar the formation pinches out to the west and east and is capped by spekk formation shales a separate smaller accumulation has also been proved in middle jurassic gam formation sandstones in the western part of the field sand quality in both reservoirs is good to excellent field stoiip is estimated at 180 million sm sup 3 of oil expectation of recoverable reserves is 67 million sm sup 3 government approval for field development was given in december 1988 the field will be developed with a concrete gravity base structure and offshore loading more the initial development plan calls for six oil producers and six subsea water injectors the platform will be installed in the summer of 1993 with first oil shortly thereafter the planned plateau production rate is 14 300 sm sup 3 day dry oil pending a gas offtake solution for the haltenbanken region produced associated gas will initially be reinjected into a water bearing structure in the western part of the field less

tremor and its duration amplitude distribution at popocatepetl volcano mexico the duration amplitude distribution was calculated for the tremor observed at popocatepetl volcano during episodes of activity in 2000 2012 2013 and 2014 an exponential function was used to obtain a good fit for the duration amplitude distribution and the source of volcanic tremor is probably generated by the transportation of magmatic fluids and its coupling with the host rock within the volcanic conduit in particular harmonic tremor has shown large amplitudes durations and mean values of amplitude more than spasmodic or pulsalting tremor this is due to different generation mechanisms in the case of harmonic tremor it is produced during magma ascent and lava dome growth whilst spasmodic and pulsalting tremor are associated with fragmentation of the lava dome and gas emissions this paper presents the duration amplitude distribution as a method to estimate the intensity of the tremor at popocatepetl a volcano with the major risk in all mexico

genesis and evolution of a mid shelf storm built sand ridge new jersey continental shelf u s a abstract sand ridges form discrete parts of a modern surficial veneer on many continental shelves worldwide yet are seldom cited as analogs for isolated marine sand bodies found in outcrop or the subsurface one reason may be that a comprehensive explanation for the evolution of modern mid shelf sand ridges has proven elusive new information most notably high resolution seismic data collected on modern sand ridges in adjacent areas by other workers has led to this reevaluation of a largely unpublished but extensive vibracore dataset from a mid shelf sand ridge off new jersey usa the presence of dipping reflections within other ridges is particularly notable on new high resolution seismic data but has yet to be confirmed in these other ridges by extensive coring reexamining the original well preserved vibracore peels from the mid shelf sand ridge sampled in the 1980s reveals that these internal reflections could be associated with burrowed low density mud beds new cross sections constructed with 20 vibracores suggest that the dipping internal reflections are accretionary surfaces associated with episodic ridge migration to the east and south radiocarbon dating indicates that the mid shelf sand ridge has migrated episodically over the last 5 ka and movement probably continues today during major storms this documented migration continues well after transgressive ravinement approximately 9 10 ka contradicting existing views that mid shelf ridges become moribund sometime after detachment from the shoreline the morphology migration history and internal sedimentary character suggest that the ridges are acting much like primary bedforms although these sand bodies are much larger in scale 5 to 8 m height or thickness than typical subaqueous dunes observed on modern storm dominated shelves

stratigraphy composition and form of the deccan basalts western ghats india in the western ghats between latitudes 18 20 n and 19 15 n 7000 km2 of deccan basalt have been mapped with the primary objective of establishing a flow stratigraphy as a guide to the volcanic history of the flood basalts using over 70 measured vertical sections major and trace element analyses of nearly 1200 samples and rare earth and87sr 86sr determinations for over 60 samples we divide the basalt into three subgroups and ten formations in this paper we describe the seven principal formations in the area and the most prominent individual flows

temporal and spectral characteristics of seismicity observed at popocatepetl volcano central mexico abstract popocatepetl volcano entered an eruptive phase from december 21 1994 to march 30 1995 which was characterized by ash and fumarolic emissions during this eruptive episode the observed seismicity consisted of volcano tectonic vt events long period lp events and sustained tremor before the initial eruption on december 21 vt seismicity exhibited no increase in number until a swarm of vt earthquakes was observed at 01 31 hours local time visual observations of the eruption occurred at dawn the next morning lp activity increased from an average of 7 events a day in october 1994 to 22 events per day in december 1994 at the onset of the eruption lp activity peaked at 49 events per day lp activity declined until mid january 1995 when no events were observed tremor was first observed about one day after the initial eruption and averaged 10 h per episode by late february 1995 tremor episodes became more intermittent lasting less than 5 min and the number of lp events returned to pre eruption levels 7 events per day using a spectral ratio technique low frequency oceanic microseismic noise with a predominant peak around 7 s was removed from the broadband seismic signal of tremor and lp events stacks of corrected tremor episodes and lp events show that both tremor and lp events contain similar frequency features with major peaks around 1 4 hz frequency analyses of lp events and tremor suggest a shallow extended source with similar radiation pattern characteristics the distribution of vt events between 2 5 and 10 km also points to a shallow source of the tremor and lp events located in the first 2500 m beneath the crater under the assumption that the frequency characteristics of the signals are representative of an oscillator we used a fluid filled crack model to infer the length of the resonator

a different perspective on glauconite as a chronometer for geologic time scale studies there are advocates of the use of glauconite as a chronometer for geologic time scale studies who insist that if the geohistory of the sample in question is known in detail one can ascertain before hand that the sample will yield a radiometric result that is accurate in the geological sense examples are presented indicating that while some glauconite samples may yield the correct age within their assigned analytical uncertainties when compared to high temperature minerals others clearly do not and are too young by varying degrees the outright acceptance of published results on glauconite can result in erroneous age estimates of various biostratigraphic levels and stage or epoch boundaries

a tectonostratigraphic framework for the mid norway region abstract the tectonostratigraphic analysis presented here aims to establish a broad stratigraphic framework within which the likely distribution of both reservoir and source rock intervals within the deep water voering and more basins can be evaluated in terms of hydrocarbon potential the devonian to triassic interval can be effectively discounted by its extreme depth of burial across most of the study area the middle jurassic is interpreted to provide a potential reservoir target in the gjallar ridge area the cretaceous is seismically mappable across the region and has been divided into five tectonostratigraphic sequences late cretaceous reservoirs are considered to offer targets in the nyk high area six tectonostratigraphic sequences are described within the cenozoic succession paleocene reservoirs are interpreted to offer targets over both the ytterskallen and ormen lange areas of the more basin source rock maturity is critical for exploration success mapping shows the late jurassic earliest cretaceous spekk formation to be overmature over most of the area though the presence of possible dhis indicates the probable presence of major gas accumulations in the region only on the western flank of the vigrid syncline does the spekk shallow sufficiently to lie within the potential oil window it is concluded that the region has the potential to become a significant new hydrocarbon province for the 21st century

self sustained oscillations at volcan de colima mexico inferred by independent component analysis we have analyzed long period lp seismic events of volcan de colima mexico recorded by four three component broad band seismometers during january 2006 frequency domain analysis shows spectra mainly peaked in the range 0 3 2 hz and characterized by a monotonic decreasing envelope as those produced in a cylindrically symmetric self oscillating cavity interacting with a confined jet independent component analysis a time series decomposition method extracts two or three nonlinear oscillation modes depending on the station with the fundamental one peaked at 0 4 0 5 hz these decomposed waves have a well defined spectral content and are self sustained oscillations with low dimensionality as shown by dynamic system reconstruction methods moreover they show radial polarization in near field and transverse polarization in far field in north west south east direction finally we hypothesize a branched plumbing system with two principal conduits having alignments orthogonal to one another

global volcanism biological mass extinctions and the galactic vertical motion of the solar system summary the recent findings of nearly matching long term cyclicity in biological mass extinctions geomagnetic reversals impact cratering and other terrestrial processes have evoked a major controversy we report here a compilation and analysis of major global magmatic episodes showing a significant enhancement of volcanic activity with a periodicity of 33 million years for the last 250 million years these magmatic episodes match extremely well the best available astronomical estimates of the periodic 31 5 myr galactic disc crossing events during vertical motions of the solar system these events also have a close linkage with the marine biological mass extinctions and other geological rhythms it is argued that the prolonged volcanic activity instead of impact cratering may have been the more immediate primary cause for profound climatic and other environmental deterioration sufficient to create biological crises on a global scale the volcanic periodicity scheme is well supported by the evidence of recent increases in volcanic activity however the alternative hypothesis of the death star nemesis can be rejected since it does not explain the recent increase in volcanism and impact cratering during the last 4 3 myr in view of its quieter apehelion position lying beyond the oort cloud of comets

observations of long period earthquakes accompanying hydraulic fracturing waveforms of most seismic events accompanying hydraulic fracturing have been reported to contain clear p and s waves and have fault plane solutions consistent with shear displacement across a fault this observation is surprising since classical hydraulic fracturing theory predicts the creation of a tensile opening of a cavity in response to fluid pressure very small long period events similar to long period earthquakes observed at volcanoes were found to occur during four hydraulic fracturing experiments carried out at fenton hill new mexico since the long period earthquakes occur in the same region as the shear type events it is concluded that the unusual character of the long period earthquake waveforms is due to a source effect and not a path effect the occurrence of long period earthquakes during hydraulic fracturing could indicate tensile fracturing many waveforms of these events are identical which implies that these events represent repeated activation of a given source a proposed source for these long period events is the sudden opening of a channel that connects two cracks filled with fluid at different pressures the sizes of the two cracks differ which causes two or more peaks to appear in the spectra each peak beingmore associated with one physical dimension of each crack from the frequencies at which spectral peaks occur crack lengths are estimated to be between 3 and 20m less

rift basins in western margin of india and their hydrocarbon prospects with special reference to kutch basin the western continental margin of india can be classed as a divergent or passive margin the western continental shelf is an extensive carbonate bank bombay offshore basin passing into clastic sediments on the north and south three craton margin embayed basins kutch cambay and narmada in the northern part of the shelf are filled predominantly with clastic sediments these basins occupy grabens bounded by faults diverging seaward the grabens were formed by three rift systems along major precambrian tectonic trends the rifting developed sequentially from north to south around the saurashtra horst kutch basin was formed in the early jurassic followed by cambay basin in early cretaceous time and the narmada in the late cretaceous it appears that these rifting ev nts occurred at successive stages during the northward migration of the indian plate after its break from gondwanaland in late triassic or early jurassic it is inferred that these rift basins opened up successively as a result of the counterclockwise drift of the indian craton bombay offshore and cambay are two major oil producing basins in the western margin these basins are characterized by high geothermal gradients attributed to the shallowness of the mantle in this region oil has not been found in kutch basin which is mainly an onshore mesozoic basin the basin depocenter shifted offshore at the northwestern part of the continental shelf where the shelf is wide

three dimensional to two dimensional cross strata transition in the lower pleistocene catanzaro tidal strait transgressive succession southern italy sandstone tidal cross strata are the predominant sedimentary feature of strait fill stratigraphic successions however although widely described in numerous studies tidal strait fill two dimensional and three dimensional cross strata have rarely been reported to occur in discrete intervals which are laterally adjacent or vertically stacked and the meaning of this stratigraphic architecture has not yet been fully investigated understanding of the processes responsible for changes in the internal features of modern and ancient tidal bedforms is essential in order to predict lateral and vertical heterogeneities in analogous reservoir strata this facies based study aims to interpret the three dimensional to two dimensional cross strata transition observed in the lower pleistocene mixed siliciclastic bioclastic sandstone filling the catanzaro strait in southern italy during a continuous phase of tectonically driven marine transgression tidal cross strata disappear in the uppermost interval of the studied succession where mudstone strata prevail this stratigraphic trend is interpreted as the evidence of an important change in the tidal strait hydrodynamics due to a phase of relative sea level rise at the beginning of the transgression three dimensional tidal dunes migrated throughout the ca 3 to 4 km wide and ca 30 km long wnw ese oriented catanzaro strait due to strong tidal currents amplified through the seaway and flowing in semi diurnal phase opposition as the intermediate phase of transgression enlarged the seaway width the tidal current strength decreased as tidal water exchange occurred over a larger cross sectional area the progressive reduction of the bed shear stress modified three dimensional tidal dunes into an extensive two dimensional bedform field at the end of the transgression the further widening of the catanzaro strait into a ca 10 to 12 km wide marine passageway changed the tidally dominated strait into a non tidal open shelf the results of this research suggest the presence of a critical cross sectional area in the narrowest strait centre zone which controls the activation and deactivation of tidal current amplification along a marine seaway

signals of degeneration of the sub crustal part of the indian lithosphere since the break up of gondwanaland abstract currently the continental fragments of the erstwhile gondwanaland supercontinent are widely separated a relative comparison of magsat anomalies of these fragments reveals a striking similarity across all the rifted continental margins except at the western and southern margins of india it reflects that magnetized crust has largely remained unchanged elsewhere since the break up of gondwanaland a similar analysis of satellite derived free air gravity anomalies also indicate a similar continuity across other rifted margins except those of india these anomalies characterise the existing differences in the crustal and upper mantle structures and causal dependence on the relative mobilities of the gondwanic continents it is evident that after the break up only the indian continental structure has suffered a dramatic change possibly due to its super northward drift from antarctica during late cretaceous early tertiary

numerical paleoceanographic study of the early jurassic transcontinental laurasian seaway the forces governing marine circulation of a meridional transcontinental seaway is explored with the princeton ocean model the jurassic laurasian seaway which connected the low latitude tethys ocean with the arctic sea is modeled quantitatively the global ocean is found to have a profound influence on seaway dynamics a north south density difference and hence sea level difference of the global ocean was probably the main factor in forcing the seaway flow when the tethys waters were the denser water the net seaway flow was southward and conversely it was northward for denser arctic waters marine bioprovincial boundaries and sediment data indicate that the seaway probably was dominated by boreal faunal groups and reduced salinities several times in the jurassic the model results suggest that this can be explained by southward flowing seaway currents which may have been related to an oceanic thermohaline circulation where no northern high latitude deep convection occurred

a microseismicity survey of axial seamount juan de fuca ridge an array of ocean bottom seismometers and hydrophones were deployed within the caldera of axial seamount located at the intersection of the cobb eickelberg seamount chain and the juan de fuca ridge recent manned submersible dives have discovered the presence of two distinct hydrothermal vent fields the objective of the microseismicity survey was to determine baseline information necessary for designing an optimal experiment to monitor seismicity associated with submarine volcanic and hydrothermal systems with an instrument deployment period of eight days over 1500 events were recorded and examined most events occurred within three periods of swarm activity the location of the swarm events is believed to be within the low temperature vent field in the northwest portion of axial s caldera these impulsive high frequency events are interpreted to be tectonic in origin no harmonic tremor like events were observed on any of the instruments indicating a lack of magmatic movement only nine local events were recorded on more than one instrument and none of these could be located unambiguously monochromatic 20 hz signals were also recorded and are interpreted to originate from whales observed in the immediate vicinity

source mechanism of volcanic tremor low frequency 10 hz volcanic earthquakes originate at a wide range of depths and occur before during and after magmatic eruptions the characteristics of these earthquakes suggest that they are not typical tectonic events physically analogous processes occur in hydraulic fracturing of rock formations low frequency icequakes in temperate glaciers and autoresonance in hydroelectric power stations we propose that unsteady fluid flow in volcanic conduits is the common source mechanism of low frequency volcanic earthquakes tremor the fluid dynamic source mechanism explains low frequency earthquakes of arbitrary duration magnitude and depth of origin as unsteady flow is independent of physical properties of the fluid and conduit fluid transients occur in both low viscosity gases and high viscosity liquids a fluid transient analysis can be formulated as generally as is warranted by knowledge of the composition and physical properties of the fluid material properties geometry and roughness of the conduit and boundary conditions to demonstrate the analytical potential of the fluid dynamic theory we consider a single phase fluid a melt of mount hood andesite at 1250 c in which significant pressure and velocity variations occur only in the longitudinal direction further simplification of the conservation of mass and momentum equations presents an eigenvalue problem that is solved to determine the natural frequencies and associated damping of flow and pressure oscillations for a simple constant pressure reservoir conduit orifice fluid system a change in orifice size can cause the source signal duration due to a single disturbance to change from several seconds to in excess of an hour fluid kinematic viscosity 0 412 m2 s is both included and neglected in the analysis to illustrate its effect upon system damping tremor magnitude is related to the magnitude of the conduit wall motion occurring in response to oscillating fluid pressures the entire volcanic fluid system does not generally experience a transient flow condition at a given time closed end and constant pressure fluid system boundaries reflect pressure waves and isolate components of the fluid system total fluid movement may not be accurately monitored by tremor data alone since steady state and slowly changing flows are aseismic

saturated and subcooled hydrothermal boiling in groundwater flow channels as a source of harmonic tremor the potential of hydrothermal boiling in groundwater flow channels for generating harmonic tremor a relatively monochromatic ground vibration associated with volcanic activity is examined we use simple organ pipe theory of normal mode fluid vibration and fundamental energy considerations to develop a first order analytical model of a hydrothermal boiling source of harmonic tremor we use this model to estimate order of magnitude groundwater flow channel lengths and boiling heat transfer rates required to produce harmonic tremor with dominant frequencies in the range 0 5 5 hz and surface wave reduced displacements of up to 100 cm2 depending on groundwater sound speed flow channel lengths of the order of 1 1000 m are required to produce fluid vibration eigenfrequencies in the range 0 5 5 hz the boiling heat transfer rate required to produce tremor with a given surface wave reduced displacement depends on the tremor frequency and on whether saturated boiling or subcooled boiling is the cause of the tremor saturated boiling produces groundwater vibration via steam bubble growth whereas subcooled boiling produces groundwater vibration via steam bubble collapse we find that subcooled hydrothermal boiling is from 102 to 104 times more efficient than saturated boiling in converting boiling thermal power to seismic power for example the boiling heat transfer rates required to produce 1 hz tremor with reduced displacements of up to 100 cm2 via subcooled boiling are generally less than a few thousand megawatts for saturated boiling the required boiling heat transfer rates are several orders of magnitude larger than this the highest values of heat flow reported in the literature for volcanic crater lakes and terrestrial and ocean floor geothermal areas are of the order of 1000 mw taking this value as a first order estimate of an upper limit on possible boiling heat transfer rates in volcanic hydrothermal systems our results suggest that saturated hydrothermal boiling is capable of generating only low amplitude harmonic tremor with surface wave reduced displacements no higher than a few square centimeters however subcooled hydrothermal boiling could potentially generate high amplitude harmonic tremor with reduced displacements as large as several hundred square centimeters as a specific application of our model we evaluate the potential of hydrothermal boiling for generating harmonic tremor at recently active mount st helens and nevado del ruiz volcanoes we conclude that subcooled boiling likely could have produced the tremor episodes considered at both volcanoes saturated boiling also could explain the nevado del ruiz tremor but probably not the more powerful mount st helens tremor

the 1998 1999 seismic series at deception island volcano antarctica abstract during the 1998 1999 antarctic summer the pattern of seismic activity at deception island volcano changed significantly the change was characterized by the occurrence of an intense swarm of volcano tectonic vt earthquakes more than 2000 vt earthquakes with s p times smaller than 4 s were recorded in the period january february 1999 pure volcanic events were also detected especially long period lp events volcanic tremor and some hybrid events seismic monitoring was performed using two short period small aperture arrays among other instruments based on their signal to noise ratios we selected 863 vt earthquakes 350 lp events and tremor episodes and 9 hybrid events for analysis we estimated apparent slowness and back azimuth for all events using the zero lag cross correlation array technique combining this information with s p times and other indirect evidence we identified two different source regions lp seismicity is located less than 1 1 5 km southwest of the fumarole array site these events are likely to have a hydrothermal origin vt earthquakes and hybrid events are located at depths of 0 3 10 km in an area under the bay of deception island the area extends from the fumarole array to the northeast with epicentral distances that range from 0 5 to 12 km most hypocenters are clustered in a small volume of around 8 km3 the sources of the lp seismicity and the vt earthquakes are spatially distinct which indicates that they are not produced by the same mechanisms moment magnitude analyses of the vt earthquakes provide an average magnitude of 0 5 and very low average stress drop around 1 bar a study of first motion of the p waves suggests that the events in this small source region should have a variety of source mechanisms this is supported by the existence of families of events with the same waveforms the occurrence of repeating fracture processes with low stress drop and small fault dimensions can be explained by the lubrication of pre existing zones of weakness by pressurized fluids the most probable hypothesis that explains the generation of this seismic series at deception island is a seismic series caused by the stress generated by the uplift of the source area due to a magmatic injection in depth we favor this hypothesis since it is compatible with the majority of the characteristics of the seismicity and explains the spatial and temporal behavior of the series

eustatic and hydrodynamic controls on the architecture of a deep shelf sand bank celtic sea the architecture of a tidal sand bank in the south eastern celtic sea was examined using very high resolution seismic surveys the bank comprises four depositional units the lowest unit 1 is characterized by gently dipping 1 8 strata that strike parallel to the length of the bank unit 1 is erosionally overlain by unit 2 which forms the bulk of the bank this unit consists of stacked sets of downcurrent dipping 7 12 master bedding formed by climbing sinuouscrested tidal dunes that are up to 20 m high these deposits are locally incised by an anastomosed channel network unit 3 that may represent a buried swatchway system the upper part of the bank comprises wave related deposits that are mainly preserved on the bank flanks unit 4 the outer bank surface is erosional the bank is believed to have formed during the last postglacial sea level rise the facies evolution from unit 1 to unit 3 indicates an upward increase in tidal energy mainly characterized by the thickening of dune cross bed sets in unit 2 the majority of bank growth is inferred to have occurred in water depths of the order of 60 m this evolution was controlled by relative sea level rise which is likely to have caused an episode of tidal resonance with associated strong tidal currents that were responsible for the incision of the deep cross cutting channels of unit 3 the transition to wavedominated sedimentation in unit 4 is related to the decay of resonance with continued sea level rise

magnetovariational study over a seismically active area in the deccan trap province of western india this paper presents the results of a magnetovariational study undertaken over a basalt covered region of western india which has been experiencing a swarm of seismic activity since early 1986 the most striking feature of the transient variations is the anomalous enhancement of the east west y component at stations bounding the seismically active belt from the nature of the horizontal transfer functions and the anomalous induction arrows it is inferred that this behaviour is related to the presence of a resistive body embedded in a north south oriented conductive belt consistent with an evolutionary model of deccan volcanism and other geophysical data it is suggested that the central resistive block coincident with the concentrated belt of earthquake epicentres represents either a dormant volcanic plug or a plutonic body rooted in upwarped conducting mantle to account for the phase relationship between the anomalous y variations and the normal north south x variations it is proposed that the upwarped asthenosphere with its associated high geothermal anomaly acts as a conducting path to channel induced current from the region of the cambay triple junction located towards the northwest of the array the existence of the strong induced currents in the region of the triple junction conforms with the induction arrow pattern observed at the array stations

models of tremor and low frequency earthquake swarms on montserrat recent observations from soufriere hills volcano in montserrat reveal a wide variety of low frequency seismic signals we discuss similarities and differences between hybrid earthquakes and long period events and their role in explosions and rockfall events these events occur usually in swarms and occasionally merge into tremor an observation that can shed further light on the generation and composition of harmonic tremor we use a 2d finite difference method to model major features of low frequency seismic signatures and compare them with the observations a depth dependent velocity model for a fluid filled conduit is introduced which accounts for the varying gas content in the magma and the impact on the seismic signals is discussed we carefully analyse episodes of tremor that show shifting spectral lines and model those in terms of changes in the gas content of the magma as well as in terms of a time dependent triggering mechanism of low frequency resonances in this way we explain the simultaneous occurrence of low frequency events and tremor with a spectral content comprising integer harmonics

operation of a digital seismic network on mount st helens volcano and observations of long period seismic events that originate under the volcano during 1981 we operated with the cooperation of the u s geological survey a 9 station digital seismic array on mount st helens volcano in washington state one of our stations was placed inside the crater of the volcano six were located on the flanks of the volcano within two km of the crater and two were approximately ten km from the crater four of the instruments recorded three components of motion and the remaining five recorded only the vertical component in addition to the monitoring and recording performed by the digital instruments we carried out a one day experiment during which we complimented our crater monitoring seismometer by the addition of two ink recording instruments during the one day experiment we had six observers including 4 usgs scientists who had significant experience working and making observations in the crater recording times of rockfall felt earthquake occurrences and changes in steam emissions from the dome in the crater using information obtained during the one day experiment we are able to classify seismic events recorded by our digital instruments as earthquakes rockfalls helicopter noise and a type of event that is unique to volcanoes which we call long period waveforms of these long period events have a duration of up to 30 seconds and a spectrum that is peaked at approximately 2 hz the frequency at which the peak in the spectrum occurs is nearly the same at all stations which means that the unique waveform of long period events is due to a source effect not a path effect the peak frequency is fairly insensitive to the amplitude of the signal which means that the size of the source region is constant independent of the signal amplitude long period events were not felt and were accompanied by no visible changes inside the crater which lead us to conclude that they are some sort of seismic disturbance generated inside the volcano

location of tremor sources and estimation of lava output using tremor source amplitude on the piton de la fournaise volcano 2 estimation of lava output abstract we propose several relations for estimating the amounts of lava emitted during eruptions of the piton de la fournaise volcano based on the use of tremor source amplitude sources for eruption tremor are located in several frequency bands with a location method based on the use of seismic amplitudes corrected for recording site effects sources are usually found at shallow depth and close to the eruptive vents suggesting that tremor is directly generated by the eruptive activity simultaneously with the location of the tremor sources our program gives the source amplitude of the phenomenon we assume a linear or a quadratic relation between the lava output and the tremor source amplitude both relations are calibrated using data for the march 1998 eruption and tested using the volumes emitted during the july and september 1999 and february 2000 eruptions we found that the 5 10 hz frequency band gives lava output much higher than for lower frequencies suggesting that for that band the tremor generation depends on the eruption site in addition to the lava output or that tremor at those frequencies is not directly caused by the outflow of lava but maybe rather by gas the best estimations are obtained using the quadratic relation for frequencies below 5 hz with an agreement better than 15 with the field observations for the july and september 1999 eruptions and 30 for the february 2000 eruption our results show that the cumulative amplitude of tremor recorded during eruptions of the piton de la fournaise is directly related to the amounts of lava erupted

a phase coherence approach to identifying co located earthquakes and tremor we present and use a phase coherence approach to identify seismic signals that have similar path effects but different source time functions co located earthquakes and tremor the method used is a phase coherence based implementation of empirical matched field processing modified to suit tremor analysis it works by comparing the frequency domain phases of waveforms generated by two sources recorded at multiple stations we first cross correlate the records of the two sources at a single station if the sources are co located this cross correlation eliminates the phases of the green s function it leaves the relative phases of the source time functions which should be the same across all stations so long as the spatial extent of the sources are small compared with the seismic wavelength we therefore search for cross correlation phases that are consistent across stations as an indication of co located sources we also introduce a method to obtain relative locations between the two sources based on back projection of interstation phase coherence we apply this technique to analyse two tremor like signals that are thought to be composed of a number of earthquakes first we analyse a 20 s long seismic precursor to a m 3 9 earthquake in central alaska the analysis locates the precursor to within 2 km of the mainshock and it identifies several bursts of energy potentially foreshocks or groups of foreshocks within the precursor second we examine several minutes of volcanic tremor prior to an eruption at redoubt volcano we confirm that the tremor source is located close to repeating earthquakes identified earlier in the tremor sequence the amplitude of the tremor diminishes about 30 s before the eruption but the phase coherence results suggest that the tremor may persist at some level through this final interval

time dependent features in tremor spectra abstract harmonic spectral peaks are observed in the tremor spectra of many different volcanoes and in some cases these spectral lines have been seen to change with time this has also been observed for the tremor at the soufriere hills volcano on montserrat west indies where the spectral lines are sometimes seen to glide apart before an explosion we propose a model of repeated triggering of low frequency earthquakes to explain these gliding lines using the relationship t 1 where t and are time and frequency spacing respectively and investigate factors which can affect the observation of these spectral peaks noise and amplitude variation are shown to have little effect on the spectral peaks however the time gap between events must be nearly constant over several events an error with a standard deviation of 2 or less is required for the spectral lines to be observed in the frequency range 0 5 10 hz we can reproduce the gliding spectral lines from a specific tremor episode preceding an explosion by changing t from 1 to 0 31 s over a time period of 12 min using this relationship and an automated event classification analysis program aecap we can monitor t over a long time period the aecap also extracts other seismic parameters such as energy duration and spectral characteristics an initial comparison between low frequency seismic energy and cyclic tilt shows a correlation between the two but this does not hold for later cycles

a search for iridium in the deccan traps and inter traps it has been suggested that flood basalts in the deccan india might be associated with events at the cretaceous tertiary boundary ktb a search for iridium in 47 samples from lava flows and inter trap sediments in the deccan yields negative results concentrations are not statistically different from zero with a minimum detection level on the order of 0 1 ng g 1 ppb this does not help to constrain the debate on the internal vs external origin of ktb boundary events since for instance ir could have been outgassed from the magma and original concentrations of only 0 05 ng g 1 prior to outgassing are sufficient to account for the worldwide ir excess mass at the ktb

crustal structure along mehmadabad billimora profile in the cambay basin india from deep seismic soundings abstract deep seismic sounding dss investigations were carried out along a 220 km long n s profile from mehmadabad 22 50 n 72 46 e to billimora 20 46 n 72 58 e in the cambay basin india for studying the detailed crustal structure in the region seismic refraction as well as narrow and wide angle reflection travel time data were obtained for various crustal boundaries starting from the sedimentary section down to the moho by continuous profiling analysis of reversed reflection and refraction travel time data has yielded crustal velocity depth functions for the mehmadabad broach and broach navsari sections along the profile which are significantly different from each other the average velocities at depths in the magmadabad broach section have been found to be considerably lower than those in the broach navsari section by about 25 at 5 km 17 at 10 km 7 at 20 km and 2 at the moho depth and even deeper down in the upper mantle the presence of a relatively thicker sedimentary section especially in the jambusar broach area as revealed by the reversed refraction data analysis is responsible for this gross difference in the average velocity functions for the two parts of the profile and the same have therefore been used for obtaining the detailed crustal depth along the profile the cambay basin which is well known to be bounded by step faults on the eastern and western margins is also dissected into seven major crustal blocks in the n s direction from mehmadabad to billimora the various crustal blocks identified are from the north 1 i mehmadabad block 2 ii tarapur block 3 iii petlad katana block 4 iv jambusar broach block 5 v kosamba block 6 vi surat block and 7 vii navsari billimora block these blocks are displaced up or down relative to the adjacent blocks due to movements along the faults bounding them most of the faults have been found to extend quite deep and to affect the moho boundary considerably as is quite evident from the relief of the moho found along the profile further the seismic activity observed in the region during historical as well as modern times suggests that these deep faults may even be currently active the jambusar broach block which is well known as a broad regional syncline with the maximum thickness of cenozoic sediments has been found to be a major graben revealing a maximum crustal thickness of 38 40 km the granitic basement in this graben is also the deepest at 6 0 6 5 km depth towards the north and south of this major graben the moho as well as the granitic basement are found to be at relatively shallower depths in various crustal blocks the boundary velocity at the moho has been found to be 8 1 8 2 km sec whereas at the granitic basement level there seems to be a lateral velocity variation from 5 7 to 6 3 km sec the crustal depth section obtained in the present study reveals a close relationship between the deep and shallow structural features in this region the deccan traps which form the basement of the overlying tertiary sediments over most part of the profile vary in thickness from 1 0 to 1 8 km however the trap surface rises to very shallow depths on the southern part of the profile the boundary velocity in the deccan traps has been found to vary between 4 8 and 5 2 km sec in the anklesvar kosamba area there is also evidence for the presence of a 1 2 km thick layer of mesozoic sediments with a velocity of about 4 0 km sec underlying the 1 1 km thick deccan traps layer

a new theory of the origin and evolution of the deccan traps india abstract the deccan traps india represent one of the largest continental flood basalts and its origin and phases of evolution have remained puzzling for the earth science community an integrated analysis of available geochronological geochemical and geophysical data of post mesozoic volcanism clearly rejects the dominating theory of origin from a hot spot e g la reunion at about 23 s 55 e in the indian ocean the deccan traps show imprints of three events belonging to a worldwide peak magmatism series matching with a galactic catastrophism cyclicity of 33 m y study further reveals that the deccan magmas were not directly derived by the partial melting of the deep mantle but appear to have their origin at a depth of less than 40 km as a consequence of crustal rifting and secondary melting caused by the rise of geoisotherms due to frictional heating generated at the lithospheric asthenospheric boundary during india s fast south north journey from antarctica

deep volcanic tremor and magma ascent mechanism under kilauea hawaii deep harmonic tremor originating at depths around 40 km under kilauea was studied using records accumulated since 1962 at the hawaii volcano observatory of the u s geological survey the deep source of the tremor was determined by onset times and confirmed by the relative amplitude across the island wide network of seismometers the period of tremor was conclusively shown to be determined by the source effect and not by the path or station site effect because the period would change considerably in time but maintained uniformity across the seismic net during the tremor episode the tremor appeared to be primarily composed of p waves we interpret the observed period and amplitude in terms of the stationary crack model of aki et al 1977 and find that the seismic moment rates for deep tremors are considerably larger than those for shallow tremors suggesting more vigorous transport for the former we propose a kinematic source model which may be more appropriate for deep tremor according to this model a measurable quantity called reduced displacement is directly proportional to the rate of magma flow a systematic search for deep tremor episodes was made for the period from 1962 through 1979 and the amplitude period and duration of the tremor were tabulated we then constructed a cumulative reduced displacement plot over the 18 year period the result shows a generally steady process which does not seem to be significantly affected by major eruptions and large earthquakes near the surface the total magma flow estimated from the reduced displacement is however one order of magnitude smaller than that estimated by swanson 1972 it may be that most channels transport magma aseismically and only those with strong barriers generate tremor

response of a volcanic conduit to step like change in magma pressure freely damped oscillations associated with strain offsets were observed by a nearby volumetric strainmeter during the 1986 izu oshima volcano eruption japan these events are interpreted as a response of the magma filled conduit to step like changes in magma pressure there were dramatic changes in the characteristic frequency of oscillation and the polarity of initial motion before and after the 6 hour interval of the enormous ground deformation due presumably to magma intrusion the frequency changed from 42 6 to 21 3 mhz and the polarity changed from contraction to expansion we propose a conceptual conduit model to explain this polarity reversal where magma is episodically supplied to the conduit before but is episodically drained back to the reservoir after an extensive magma escape away from the reservoir as dike intrusion stress caused by the dike intrusion is large enough to break the lid of the conduit thus to change a boundary condition this change in boundary condition reduces the frequency of oscillation by a factor of 2 in this model longitudinal wave speed of the magma fluid is 340 m s if the conduit length is taken to be 4 km

evolution of a classic sand ridge field maryland sector north american inner shelf the ridge and swale topography of the middle atlantic bight is best developed on the delaware maryland inner shelf here sand ridges can be seen in all stages of formation several aspects of the ridge field are pertinent to the problem of ridge genesis the first is ridge morphology there is a systematic morphologic change from shoreface ridges through nearshore ridges to offshore ridges which reflects the changing hydraulic regime as successively more seaward ridges are examined maximum side slope decreases the ratio of maximum seaward slope to maximum landward slope decreases and the cross sectional area increases these changes in ridge morphology with depth and distance from shore appear to be equivalent to the morphologic changes experienced by a single ridge during the course of the holocene transgression a second aspect is the change in bottom sediment characteristics that accompanies these large scale morphologic changes megaripples sand waves and mud lenses appear in the troughs between nearshore and offshore ridges these changes indicate that the storm flows which maintain ridges are less frequently experienced in the deeper sector and that the role of high frequency wave surge becomes less important relative to the role of the mean flow component in shaping the sea floor a third aspect is the systematic relationship of grain size to topography grain size is 90 out of phase with topography so that the coarsest sand lies between the axis of the landward trough and the ridge crest while the finest sand lies between the ridge crest and the axis of the seaward trough this relationship is characteristic of large scale bedforms finally flow was measured and transport calculated on the same ridge during a one month period november 1976 threshold was exceeded only during storm events mean transport was southerly and a little seaward with respect to both the ridge crest and the shoreline these flow measurements are in conformity with the pattern of smaller bedforms a 43 year time series of bathymetric change for this ridge reveals a systematic pattern of landward flank erosion seaward flank deposition and seaward crest migration sand ridges are considered the consequence of constructive feedback between an initial topography and the resulting distribution of bottom shear stress the relationship between grain size and topography supports this model but does not account directly for the oblique angle of the ridge with respect to the coastline this feature may be due to a more rapid alongshore migration rate of the inshore edge of the ridge than the offshore edge and the relationship between this migration rate and the rate of shoreface retreat

strain offsets with monotonous damped oscillations during the 1986 izu oshima volcano eruption the sequential process of the 1986 izu oshima volcano eruption japan was recorded by a volumetric strain meter near the erupted summit the records show occasional strain offsets accompanying long period damped oscillations which we interpret as responses of the volcanic system to perturbations in magmatic pressure the polarities of strain offsets were contractive for all events before the enormous ground deformation of november 21 due presumably to the intensive dike intrusion on the contrary all events after that had expansive polarities the characteristic frequencies of oscillations were 42 6 mhz for the events before changing abruptly to half that after the ground deformation in our model which is a one dimensional magma filled conduit the boundary condition at the top of the conduit changed from a closed to an open end upon a void formation at the top of the magma head due to its descent immediately after the dike intrusion before the magma escape from the reservoir as a dike magma was episodically supplied from the reservoir to the conduit and thus contraction events were generated after the magma escape magma left in the conduit was episodically drained back to the reservoir to generate expansion events this episodic magma supply or drainage was modeled by a prescribed pressure change at the bottom of the conduit we solved the free oscillation problem of the conduit pipe with an elastic sidewall to calculate the resultant volumetric strain outside the pipe physical properties of the magma and the excitation mechanism are inferred from the application of our formulation to the observation the conduit length and fluid acoustic velocity are estimated to be 10 km and 430 m s respectively suggesting that the fluid is composed of a gas liquid mixture if the conduit radius is taken to be 40 m and if the damping of oscillation occurred by viscous dissipation the fluid viscosity is 108 109 pa s a reasonable estimate for the basaltic magma the applied pressure changes at the bottom of the conduit are in a range from 105 to 106 pa with risetimes of the order of 4 to 100 s

excitation mechanism of large amplitude volcanic tremor associated with the 1989 ito oki submarine eruption central japan abstract unusually large amplitude volcanic tremor was observed in association with the 1989 ito oki submarine eruption off the east coast of izu peninsula central japan the largest amplitude tremor occurred on july 13 1989 and was studied by using digital seismic data national research institute for earth science and disaster prevention in the distance range of 6 250 km spectral analysis shows that the seismograms have two kinds of predominant spectral peaks low frequency peaks around 1 hz and middle frequency peaks between 2 and 7 hz the relative amplitude ratios of these peaks show distinctive spatial variation depending primarily on the distances from the crater low frequency waves are predominant at close stations distances less than 50 km from the crater similar to typical low frequency volcanic tremor while at more distant stations the middle frequency waves are more prominent than low frequency waves propagation velocity analysis for narrow frequency bands on the basis of correlation of envelopes of seismograms reveals that the middlefrequency waves are compressional body waves and the excitation of shear wave was ineffective suggesting an explosive or implosive source as the simplest source model in contrast to the middle frequency waves our analysis showed no evidence for the low frequency waves to have propagated with any body wave velocity consideration of the spatial pattern of both waves suggests that surface waves were trapped in sedimentary layers from the shallow source and that no lowfrequency oscillations dominated at the source

sharp based marine sandstone bodies in the mulichinco formation lower cretaceous neuquen basin argentina remnants of transgressive offshore sand ridges fil schwarz ernesto consejo nacional de investigaciones cientificas y tecnicas centro cientifico tecnologico conicet la plata centro de investigaciones geologicas universidad nacional de la plata facultad de ciencias naturales y museo centro de investigaciones geologicas argentina

observations of volcanic tremor at mount st helens volcano digital recordings of ground motion during tremor episodes accompanying eruptions at mount st helens volcano in the state of washington on august 7 and october 16 18 1980 are studied the spectra of the vertical component waveforms contain at least two dominant peaks at 1 0 and 1 3 hz for all events recorded during both eruptions that were studied spectra of horizontal ground motion show peaks at 0 9 and 1 1 hz the relative amplitude of the two peaks changes between tremor episodes and during single tremor episodes and shows no consistent relation to amplitude of ground motion spectra of long period earthquakes are very similar to those of tremor events suggesting that tremor is composed of many long period earthquakes that occur over a period of time the unique waveform of tremor events observed at mount st helens must be due to a source effect since the relative amplitude of the two dominant peaks changes during tremor episodes the path effect on tremor waveforms is small since there are no peaks in the spectra of waveforms recorded during tectonic earthquakes occurring in the vicinity of mount st helens the consistency of the location of the spectral peaks for the wide range of tremor amplitudes means that there must be a physical length at the source that is constant independent of the amplitude of motion at the source variations in amplitude of motion generated during tremor events must be due to variations in the force driving the tremor amplitudes of ground motion varies between 0 11 and 4 7 m seismic moment rates during the two eruptions are calculated using the model of aki et al 1977 and found to vary between 6 1018 and 1 1020 dynes cm s 1 which are larger than values found by aki et al 1977 who studied amplitudes of shallow tremor events recorded during the october 1963 eruption of kilauea volcano in hawaii study of tremor amplitudes recorded at corvallis oregon leads to the conclusion that tremor accompanying the cataclysmic may 18 1980 eruption was at least one order of magnitude larger in amplitude than tremor during august and october

super mobility of hot indian lithosphere negi j g pandey o p and agrawal p k 1986 super mobility of hot indian lithosphere tectonoph vsics 131 147 156 the great mobility of the indian subcontinent during the last 180 million years 9000 km of south north motion with an anticlockwise rotation of about 60 o has long been a major puzzle for theorists of plate tectonics a relative analysis of the available heat flow data the estimated temperature depth regime of india and the gravity anomalies reveals that the indian lithosphere does not have the characteristics of a typical shield zone and that it has higher radioactivity smaller viscous drag decidedly low viscosity of 10 p lower density and a well defined low velocity zone as compared to other shield areas of the world these factors appear to contribute significantly to the faster northward movement of the thin indian lithosphere as compared to the almost immobile african plate

long period seismicity during magma movement at volcan de colima during the period from february to september 2005 volcan de colima produced 30 vulcanian explosions of sufficient magnitude to produce pyroclastic flows of variable size with a total volume of at least 2 5 106 m3 swarms of long period events were associated with each event their duration ranging from about 6 h to 3 days and each swarm containing up to 886 events the characteristics of the swarms have been studied to understand the source mechanism and their relationship with the vulcanian explosions in total 12 548 long period events were analysed using various comparative and statistical methods patterns were not apparent in the data with no correlation between different properties of the swarms duration magnitude or frequency of occurrence of lp events and the magnitude of the associated vulcanian explosion whether recorded by seismicity volume of pyroclastics or altitude of the eruption column this along with other characteristics of the swarms such as the continuation of the swarm after the explosion with an increase in long period event amplitude in some cases suggests that the mechanism is not merely associated with the pressurization under an impermeable cap and resulting pressure differentials between adjacent volumes within the system it is more likely that the production of long period events is dominated by brittle fracturing on the margins of an ascending magma body a model is proposed whereby the unloading above the ascending magma column produced by a vulcanian explosion resulted in an increase in ascent rate reflected in the increasing amplitude of long period events the results reflect the complexity of non linear processes involved during magma ascent degassing crystallization and rupture of the impermeable plug during the vulcanian process at volcan de colima as at many volcanoes long period events represent a useful precursor for eruptive activity for monitoring this paper highlights some useful analyses that can be carried out which could illustrate certain characteristics of an eruptive episode a preliminary model is presented of the conduit processes at work during the cyclic extrusive and explosive activity during 2005

geomagnetic reversals from impacts on the earth the impact of a large extraterrestrial object on the earth can produce a geomagnetic reversal through the following mechanism dust from the impact crater and soot from fires trigger a climate change and the beginning of a little ice age the redistribution of water near the equator to ice at high latitudes alters the rotation rate of the crust and mantle of the earth if the sea level change is sufficiently large 10 meters and rapid in a few hundred years then the velocity shear in the liquid core disrupts the convective cells that drive the dynamo the new convective cells that subsequently form distort and tangle the previous field reducing the dipole component near to zero while increasing the energy in multipole components eventually a dipole is rebuilt by dynamo action and the event is seen either as a geomagnetic reversal or as an excursion sudden climate changes from other causes such as volcanic eruptions could also trigger reversals this mechanism may not be the sole cause of geomagnetic reversals but it can account for the rapid drop of the dipole component preceding a reversal the predominance of multipole components during a transition the associations of microtektites temperature drops and extinctions with reversals and the possible correlation between peaks in the geomagnetic reversal rate and the times of mass extinctions the model may also account for the long term changes in the average rate of reversals we make several testable predictions

impact induced melting and the development of large igneous provinces abstract we use hydrodynamic modelling combined with known data on mantle melting behaviour to examine the potential for decompression melting of lithosphere beneath a large terrestrial impact crater this mechanism may generate sufficient quantity of melt to auto obliterate the crater melting would initiate almost instantaneously but the effects of such massive mantle melting may trigger long lived mantle up welling that could potentially resemble a mantle hotspot decompression melting is well understood it is the main method advocated by geophysicists for melting on earth whether caused by thinned lithosphere or hot rising mantle plumes the energy released is largely derived from gravitational energy and is outside but additive to the conventional calculations of impact modelling where energy is derived solely from the kinetic energy of the impacting projectile be it comet or asteroid the empirical correlation between total melt volume and crater size will no longer apply but instead there will be a discontinuity above some threshold size depending primarily on the thermal structure of the lithosphere we estimate that the volume of melt produced by a 20 km diameter iron impactor travelling at 10 km s may be comparable to the volume of melt characteristic of terrestrial large igneous provinces 106 km3 similar melting of the mantle beneath an oceanic impact was also modelled by roddy et al int j impact eng 5 1987 525 the mantle melts will have plume like geochemical signatures and rapid mixing of melts from sub horizontal sub crater reservoirs is likely direct coupling between impacts and volcanism is therefore a real possibility that should be considered with respect to global stratigraphic events in the geological record we suggest that the end permian siberian traps should be reconsidered as the result of a major impact at 250 ma auto obliteration by volcanism of all craters larger than 200 km would explain their anomalous absence on earth compared with other terrestrial planets in the solar system

major circular structure beneath southern lake huron defined from potential field data newly compiled and merged canadian and united states potential field data for the area of southern lake huron reveal a large circular structure that has a diameter that may extend to 100 km the aeromagnetic image shows a circular central high with a diameter of 6 km located at fat 43 14 21 n long 82 19 88 w surrounded by a principal ring with a diameter of 50 km and indications of conformable arcuate structure extending to a diameter of about 100 km the prevailing northeast striking aeromagnetic signature of the grenville front and adjacent grenville front tectonic zone is interrupted and may be overprinted by the circular feature the bouguer gravity field shows a strong positive anomaly that extends southeast from the grenville front and terminates in an arcuate complex of positive anomalies that correlate with the aeromagnetic field the gravity field illustrates the circular structure with an annular low and central high as well as positive extensions that follow the foliate pattern of magnetic anomalies marking the outer area of the feature there is no strong evidence of the feature in the bathymetry data of southern lake huron although the general coastline of the southern end of lake huron closely follows the outline of the main circular feature the interruption of the grenville trends the clear regular circular signature of the feature and the interpreted magnetic source depths at the precambrian surface suggest an event possibly as young as early paleozoic in the absence of basement samples the origin of the feature is unconfirmed however the scale and potential field anomaly pattern of the feature are comparable to several of the larger impact structures exposed on the canadian shield

bombay gravity high and eruption of deccan flood basalts india from a shallow secondary plume abstract the western margin of india especially the area surrounding the gravity high of bombay is covered by a thick pile of deccan flood basalts the massive eruption is one of the largest continental volcanic events at the cretaceous tertiary biological mass extinction boundary 65 ma ago the area is characterized by several hidden tectonic features and their origin and evolution have remained controversial in the search for magma source structure attention was focused on an unusual oval shaped positive gravity anomaly over the bombay coastal zone the gravity analysis indicates the presence of two subsurface structures at depths of 4 5 0 5 km and 18 2 km these depths correspond to the top of the crustal basaltic layer and an upwarped moho discontinuity respectively the positive gravity anomaly in three dimensional analysis reveals a possible magma conduit structure and the near absence of the granitic layer the unusually shallow moho layer coincides with the region of a broad high heat flow anomaly average heat flow more than 80 mw m2 the geophysical signatures from the neighborhood show that the asthenosphere has welled up considerably and is located at a depth of about 40 km high heat flow and asthenospheric depth estimates suggest melting conditions at very shallow depths it is suggested that the highly differentiated magma associated with the major eruptive phase of deccan traps did not originate at the d layer it is hypothesized that magma was brought to the surface by a major secondary plume at a much shallower depth within the lithosphere and possibly at the crust mantle boundary as a consequence of subcrustal erosion and secondary melting caused by the extreme velocity of india during the late cretaceous and early tertiary substantial melting of the base of the lithosphere due to frictional heating might have resulted in the accumulation of a large quantity of magma below the thinned lithosphere at the pre existing fracture zone near the continental margin the presence of a magmatic conduit structure above the moho derived from the inversion of gravity data and other tectonic conditions suggests a possible triggering of the magma reservoir by a large bolide impact 10 km diameter at the cretaceous tertiary boundary near bombay

evaluation of reservoir temperatures and local utilization of geothermal waters of the konkan coast india abstract geochemical studies on fifteen geothermal manifestations 38 70 c from the konkan coast geothermal province of india have been used to evaluate the reservoir temperatures activity studies of the minerals and the waters present in the reservoirs suggest that the thermal waters are in equilibrium with montmorillonite kaolinite and quartz at about 100 c reservoir temperatures of these geothermal systems as estimated by geochemical thermometers are 88 to 128 c and thus too low for economic electricity production

co eruptive tremor from bogoslof volcano seismic wavefield composition at regional distances we analyze seismic tremor recorded during eruptive activity over the course of the 2016 2017 eruption of bogoslof volcano alaska only regional recordings of the tremor wavefield exist for bogoslof making it a challenge to place the recordings in context with other eruptions that are normally captured by local seismic data we apply a technique of time frequency polarization analysis to three component seismic data to reveal the wavefield composition of bogoslof eruption tremor we find that at regional distances the tremor is dominated by p waves in the band from 1 5 to 10 hz using this information along with an enriched bogoslof earthquake catalog we obtain estimates of average reduced displacement dr for eruption tremor during 25 of the 70 bogoslof events dr reaches as high as approximately 40 cm2 for two of the major events similar to other vei 3 eruptions in alaska overall average reduced displacement displays a weak correlation to plume height during the first half of the 9 month long eruption sequence with a few notable exceptions the two events with the highest dr values also generated measurable eruption tremor at very long periods vlp between 0 05 and 0 15 hz

wave formed structures and paleoenvironmental reconstruction clifton h e and dingier j r 1984 wave formed structures and paleoenvironmental reconstruction in b greenwood and r a davis jr editors hydrodynamics and sedimentation in wave dominated coastal environments mar geol 60 165 198 wave formed sedimentary structures can be powerful interpretive tools because they reflect not only the velocity and direction of the oscillatory currents but also the length of the horizontal component of orbital motion and the presence of velocity asymmetry within the flow several of these aspects can be related through standard wave theories to combinations of wave dimensions and water depth that have definable natural limits for a particular grain size threshold of particle movement and that of conversion from a rippled to fiat bed indicate flow velocity limits the ratio of ripple spacing to grain size provides an estimate of the length of the near bottom orbital motion the degree of velocity asymmetry is related to the asymmetry of the bedforms though it presently cannot be estimated with confidence a plot of water depth versus wave height h h diagram provides a convenient approach for showing the combination of wave parameters and water depths capable of generating any particular structure in sand of a given grain size natural limits on wave height and inferences or assumptions regarding either water depth or wave period based on geologic evidence allow refinement of the paleoenvironmental reconstruction the assumptions and the degree of approximation involved in the different techniques impose significant constraints inferences based on wave formed structures are most reliable when they are drawn in the context of other evidence such as the association of sedimentary features or progradational sequences

mantle plumes and the periodicity of magnetic field reversals many geological phenomena show long term cyclical behavior of particular interest are mass extinctions and geomagnetic field reversals which may have correlated periodicities of approximately 30 myr we propose that these cycles are governed by periodic instability of the thermal boundary layer d at the base of the mantle as this mobile layer thickens by thermal diffusion it becomes dynamically unstable and hot material erupts from it the layer now thinner extracts heat from the core at a greater rate increasing the energy supply and hence the reversal frequency of the dynamo the hot material from the d layer rises via mantle plumes to the surface where it causes increased volcanic activity which affects the climate and biota

geological and biological consequences of giant impacts it seems inescapable that large impacts have been an ongoing phenomenon throughout geological time in the phanerozoic the estimated frequency of impact by bodies of 5 km diameter or more is on the order of 1 0 every 1 00 myr the 5 km lower limit is chosen arbitrarily but it is shown in this review that such a body is certainly large enough to cause very large terrestrial effects a small decrease in this size limit would lead to a large increase in the frequency of occurrence it may be claimed that bodies of a size likely to have drastic and sudden effects on niany physical and biological features of the earth have arrived with a frequency comparable to the average duration of a biostratigraphic stage nothing is said of the interval between arrivals which may be random i e stochastic currently there is not enough geological evidence to allow recognition of other than random spacing between a few positively identified events during the phanerozoic since impacts by extraterrestrial bodies have been clearly established it is reasonable to postulate that they must leave a record in the stratigraphic column why then do we currently have so few horizons that have been claimed to contain evidence of a global impact event since the effects we are considering are unquestionably global in this paper we bring together evidence from several disciplines that

volcanic earthquakes and their relationship to eruptions at ruapehu and ngauruhoe volcanoes abstract crustal earthquakes near ruapehu and ngauruhoe fall into two classes each of which can be subdivided on the one hand there are high frequency events 3 hz with sharp well defined phases mainly concentrated beneath ruapehu crater lake low frequency events the low frequency events resemble minakami s b type and explosion earthquakes but sometimes occur where no vent exists and rather deeper than his formal definition difficulty in applying minakami s classification rigorously and the fact that low frequencies may be due to abnormal attenuation of higher frequencies along the path rather than to their suppression or absence at the source has led to reclassification of earthquakes near the volcanoes into two broad groups tectonic and volcanic the former includes all high frequency earthquakes and those discrete events in which dominant low frequencies are due to path effects the latter includes multiple and emergent events which show evidence of prolonged or repetitive source mechanism dominant low frequencies are ascribed to occurrence in heat weakened material and high frequencies to instantaneous source mechanisms operating in competent rock the term volcano tectonic describes tectonic earthquakes within some arbitrary distance of a volcano at ngauruhoe and ruapehu volcanic earthquakes accompany explosive vent clearing eruptions subsequent open vent degassing and ash emission however although often powerful and prolonged usually occurs without earthquakes such activity is however frequently accompanied by volcanic tremor at ruapehu under closed vent conditions when lake temperature is low low frequency earthquakes up to magnitude ml 3 4 have occurred without any eruption five types of phreatic eruptions are identified at ruapehu each having a distinctive seismic pattern the three most explosive types appear to be generated by a chain reaction process and all involve flashing of water to steam the first by failure of the roof with little precursory seismicity after a closed vent period during which lake temperature decreases the second after prolonged heating of the lake and much preliminary volcanic tremor interpreted as due to rising magma and the third under open vent conditions in the wake of one of the two preceding types a fourth probably occurs in wet sediments near the base of the lake as a result of upward migration of hot gas and a fifth aseismic or accompanied by very weak volcanic tremor is associated with convective overturn within crater lake

terrestrial maria the origins of large basalt plateaus hotspot tracks and spreading ridges large lava plateaus which form abruptly within plates without apparent tectonic cause appear to be the terrestrial equivalents of lunar maria deep erosion of lava plateaus reveals that their magma chambers are gabbro and granophyre complexes the classic lopoliths the close association of the deccan plateau with the timing of the terminal cretaceous boundary clay suggests that it is an impact crater large enough to cause pressure relief melting in the asthenosphere basalt then flooded the crater to form a lava lake the terrestrial equivalent of a lunar mare part of the crater rim survives in the amirante ridge the carlsberg oceanic ridge and chagos laccadive hotspot track formed simultaneously with the deccan plateau several other mesozoic and cenozoic lava plateaus are parts of similar arrays of simultaneous features one of those is the southern oregon part of the columbia plateau with its associated continental rifting in the northern basin and range and the yellowstone hotspot track along t

duration amplitude relationships of volcanic tremor and earthquake swarms preceding and during the 2009 eruption of redoubt volcano alaska abstract duration amplitude relationships were studied for tremor episodes and earthquake swarms occurring during the 2009 eruption of redoubt volcano alaska duration amplitude distribution plots were generated daily from january 1 to may 31 and fit with both an exponential law and power law comparing r2 values of the fit for both laws showed that the exponential law fit better for days in which volcanic tremor and earthquake swarms occurred while the power law fit better for other days fitting segments of seismic data with both an exponential and a power law leads to a metric that has potential for volcano monitoring r2exp r2pow the ratio of the r2 fits using the exponential law and the power law the ratio r2exp r2pow tended to be greater than 1 when volcanic activity or precursory seismic activity was occurring and less than 1 when no volcano seismic activity was occurring duration amplitude plots were generated for episodes of volcanic tremor that were identified by the r2exp r2pow 1 method and compared in an attempt to identify changes that may have occurred during the eruption stronger episodes of volcanic tremor showed higher characteristic amplitudes maximum heights of the plumes generated by the explosions showed a positive correlation with the characteristic amplitude of the concurrent tremor

coastal ecosystem responses to late stage deccan trap volcanism the post k t boundary danian palynofacies of mumbai bombay west india abstract the deccan trap continental flood basalt eruptions of india occurred c 67 63 ma thus spanning the cretaceous tertiary boundary 65 ma deccan eruptions were coeval with an interval of profound global environmental and climatic changes and widespread extinctions and this timing has sparked controversy regarding the relative influence of deccan volcanism upon end cretaceous catastrophic events if deccan trap activity was capable of affecting global ecosystems evidence should be present in proximal indian sedimentary facies and their palaeontological contents the impact of late stage deccan volcanism upon biota inhabiting mumbai bombay island s post k t boundary lagoonal systems is documented here sediments or intertrappeans which accumulated within these lagoons are preserved between trap lavas that characterise the closing stages of this flood basalt episode mumbai island formation intertrappean faunal and floral communities are conspicuously distinct from those common to many pre k t boundary late maastrichtian intertrappeans across the deccan province the latter sedimentary intercalations mostly developed in cognate semiarid palustrine ecosystems by contrast those around mumbai evolved in sheltered peripheral marine settings within subsiding continental margin basins unique to this late deccan stage and under an increasingly humid danian climate geochemical analyses reveal that mumbai sedimentation and diagenesis were intimately related to local explosive volcanic and regional intrusive activity at c 65 63 ma although tectonic and igneous events imprinted their signatures throughout these sedimentary formations organisms usually sensitive to environmental perturbations including frogs and turtles thrived critically palynofacies data demonstrate that whilst plant material deposition was responsive to environmental shifts there were no palpable declines in floral productivity following mumbai pyroclastic discharges therefore it is implausible that this late stage explosive volcanism influenced major ecosystem collapses globally

microearthquakes in the black smoker hydrothermal field east pacific rise at 21 n in july and august 1980 an array of five ocean bottom seismographs was deployed within 3 km of the 350 c hydrothermal vents at the rivera submersible experiment rise site at 21 n on the east pacific rise two of these instruments were placed within 600 m of the vents using a transponder navigation network the array detected four basic types of events the first type consisted of local very small microearthquakes locations obtained for 11 of these events place three within 1 km of the vents with the others elsewhere along the rise crest they appear to originate either from movement on the faults in the area or from the hydrothermal system beneath this area a study of the s p times of this type indicates a maximum hypocentral depth of 2 3 km implying a similar limit to the depth of hydrothermal circulation and brittle fracturing in the vicinity of the vents the second type of event found consisted of emergent earthquakes that have many of the characteristics of volcanic harmonic tremor the frequency of these events falls in the 1 5 hz range and are similar in appearance to those seen at mount st helens prior to and during its may 1980 eruption they may be either hydrothermal or volcanic in origin the third type of event produced a very monochromatic high frequency seismogram with the energy concentrated at 20 hz these events also appear to have a local origin the particle motion of these earthquakes shows almost pure stonely wave propagation and they appear to originate from a regularly oscillating dilatational source the nature of the source of event types two and three is clearly not the usual sort of motion on a fault and is still not understood the fourth type of earthquake recordings were regional events apparently from the nearby rivera and tamayo fracture zones the low frequency acoustic noise in the water was 16 64 times higher within 300 m of the vents than it was 2 km away this implies that the black smoker vents may be generating a large acoustical disturbance as the 350 c water pours into the ocean the observations are among the first of seismicity on moderate to fast spreading ridges and many of the events appear to be related to the hydrothermal activity found there

interplay of tidal and fluvial processes in an early pleistocene delta fed strait margin calabria southern italy abstract the architecture and morphodynamics of modern and ancient tidal straits and in particular the deposits of strait margin zones have been significantly understudied compared to other marginal marine settings even though many reservoirs in the north sea and the norwegian continental shelf are developed in narrow grabens or seaways this paper presents a detailed sedimentological and stratigraphic analysis of an early pleistocene marginal marine succession deposited along the northern margin of the siderno paleostrait southern italy this area represents an excellent case study of sedimentation along a tidal strait margin interpreted to record the interaction of fluvial and tidal processes here syn depositional tectonics produced a complex coastal morphology significantly influencing sedimentation and hydrodynamic processes along the strait margin the emplacement of an isolated tectonic high piano fossati created a ca 3 5 km wide local passageway this morpho structural element induced interplays between fluvio deltaic processes fed from the northern strait margin and tidal current reworking active within the marine strait the field based facies analysis reported here documents an initial stage of non tidal shallow marine sedimentation across the strait a subsequent regression caused river generated hyperpycnal flows and the transfer of large volumes of pebbly and shelly sandstones into deeper water tidal currents became amplified in the strait and in the delta front area they were able to rework river derived sediments generating large dune fields following the local tidal transport pathway strong tidal currents skewed the delta front causing it to be asymmetrical and elongated sand bodies in a direction parallel to the marine strait axis differently from the classical tide influenced deltas in which onshore offshore tidal flow predominates coast parallel deflection and strong asymmetry of delta front deposits is a typical feature of deltas entering tide dominated seaways and straits where strong tidal currents are capable of dispersing large volumes of sand for significant distances along the coast and along the strait axis this process became progressively enhanced during the following transgression when tide modulated currents reworked biocalcarenitic sands over the previous delta deposits generating southeasterly migrating dunes at the end of the transgression strandplain deposits formed in this area this last stage of sedimentation was followed by a dramatic regional scale structural uplift which ended any marine circulation within the strait this work provides new insights on sedimentation in a tide dominated strait and helps to predict sandbody distribution along the strait margin and axis these findings can be applied to any other setting characterized by a narrow possibly structurally confined basin dominated by tidal currents

late cretaceous and paroxysmal cretaceous tertiary extinctions the various geological signatures at cretaceous tertiary time including iridium and other associated elements microspherules and shock deformation features are compatible with the suggestion that the transition is marked by a period of intense volcanism the volatile emissions from this volcanism would lead to acid rain reduction in the alkalinity and ph of the surface ocean global atmospheric temperature changes and ozone layer depletion these environmental effects coupled with those related to the major sea level regression of the late cretaceous provide the framework for an explanation of the selective nature of the observed extinction record

precursory swarms of long period events at redoubt volcano 1989 1990 alaska their origin and use as a forecasting tool abstract during the eruption of redoubt volcano from december 1989 through april 1990 the alaska volcano observatory issued advance warnings of several tephra eruptions based on changes in seismic activity related to the occurrence of precursory swarms of long period lp seismic events dominant period of about 0 5 s the initial eruption on december 14 occurred after 23 years of quiescence and was heralded by a 23 hour swarm of lp events that ended abruptly with the eruption after a series of vent clearing explosions over the next few days dome growth began on december 21 another swarm with lp events similar to those of the first began on the 26th and ended in a major tephra eruption on january 2 eruptions continued over the next two weeks and then ceased until february 15 when a large eruption initiated a long phase of repetitive dome building and dome destroying episodes that continued into april warnings were issued before the major events on december 14 and january 2 but as the eruptive sequence continued after january 2 the energy of the swarms decreased and forecasting became more difficult a significant but less intense swarm preceded the february 15 eruption which was not forecast this eruption destroyed the only seismograph on the volcanic edifice and stymied forecasting until march 4 when the first of three new stations was installed within 3 km of the active vent from march 4 to the end of the sequence on april 21 there were eight eruptions six of which were preceded by detectable swarms of lp events although weak these swarms provided the basis for warnings issued before the eruptions on march 23 and april 6 the initial swarm on december 13 had the following features 1 short duration 23 hours 2 a rapidly accelerating rate of seismic energy release over the first 18 hours of the swarm followed by a decline of activity during the 5 hours preceding the eruption 3 a magnitude range from 0 4 to 1 6 4 nearly identical lp signatures with a dominant period near 0 5 s 5 dilatational first motions everywhere and 6 a stationary source location at a depth of 1 4 km beneath the crater this occurrence of long period events suggests a model involving the interaction of magma with groundwater in which magmatic gases steam and water drive a fixed conduit at a stationary point throughout the swarm the initiation of that sequence of events is analogous to the failure of a pressure relief valve connecting a lower supercharged magma dominated reservoir to a shallow hydrothermal system a three dimensional model of a vibrating fluid filled crack recently developed by chouet is found to be compatible with the seismic data and yields the following parameters for the lp source crack length 280 380 m crack width 140 190 m crack thickness 0 05 0 20 m crack stiffness 100 200 sound speed of fluid 0 8 1 3 km s compressional wave speed of rock 5 1 km s density ratio of fluid to rock 0 4 and ratio of bulk modulus of fluid to rigidity of rock 0 03 0 07 the fluid filled crack is excited intermittently by an impulsive pressure drop that varies in magnitude within the range of 0 4 to 40 bar such disturbance appears to be consistent with a triggering mechanism associated with choked flow conditions in the crack

sedimentology and facies analysis of ancient sand ridges jurassic rogn formation trondelag platform offshore norway abstract sand ridges represent a common type of sedimentary bedform of modern shelves seldom used as analogues to interpret isolated marine sandbodies recognised in the subsurface lack of extended literature on outcrop and subsurface examples limits the possibility for their recognition and seems one of the reason behind this underrepresentation the draugen discovery made in the early 80 s represents an unicum in the trondelag platform offshore norway after more than 30 years the froan basin and froya high area are still underexplored and the late jurassic rogn fm play not well understood predicting reservoir distribution and its internal architecture and properties requires the understanding of factors controlling sedimentation e g palaeocirculation depositional processes north south elongated sandbodies pertaining to the rogn formation are recognised in the froan basin and froya high encased within thick shaly deposits sandbodies develop above a ravinement or flooding surface i e callovian unconformity of regional extent where local depressions occur with a non erosional concave up top depressions representing the depositional loci for the accumulation of sand and development of the ridge the presence of eastward and westward dipping reflections within the sandbodies allows identifying their large scale architectures sediments form coarsening upward vertical units characterised by a shaly base evolving upwards to medium and coarse grained sand forming tabular and trough cross strata locally a fining upward trend characterised by plane parallel stratification and coarse grained massive layers is recognised sediments results well organised and sorted which positively affects final porosity and permeability with values up to 30 and 6 darcy respectively typical values for many sand ridges accordingly sand ridges encased within thick shaly deposits can form stratigraphic traps with the potential for large hydrocarbon accumulations the aim of the present study is to help the understanding of distribution and internal architectures and properties of the rogn fm in the trondelag platform

a spherical source model for low frequency volcanic earthquakes a resonant magma chamber is modeled by a spherically symmetric whole space consisting of a small gas filled chamber surrounded by magma which is encased in country rock source boundary conditions are specified on the cavity boundary simulating a possible sudden gas expansion in the magma the simplicity of this model leads to an analytical closed form solution for the fourier transform of radial displacement which predicts a peaked spectrum and an oscillating decaying seismogram for physically reasonable values of the rock and magma elastic properties and dimensions of the model the dominant frequency of the signal is in the range of 1 to 5 hz near the values observed for low frequency volcanic earthquakes and volcanic tremor furthermore the seismograms and spectra bear resemblance to those observed for low frequency earthquakes the fundamental mode of oscillation is a pure radial breathing mode which has markedly different properties from single cavity solutions the fundamental frequency is insensitive to the magma chamber boundary radius when both the inner and outer radii are greater than specified values this may help to explain the apparent similarity of the dominant frequency of volcanic earthquakes over a wide range of physical situations the presence of an inner effective bubble of a certain minimum size is necessary for the existence of the dominant low fundamental frequency spectral peak although highly idealized we believe that this fundamental fluid solid cavity mode may provide an explanation for the observations of tremor and low frequency volcanic earthquakes

time variation of spectral and wavefield features of volcanic tremor at mt etna january june 1999 abstract we have studied the volcanic tremor recorded at mt etna during january june 1999 by using three component seismic stations this time period was characterised by both explosive and effusive eruptions occurring at the summit craters we found significant time variations in the trend of the overall spectral amplitude of tremor as well as in the dominant spectral peaks moreover the tremor wavefield features polarization particle motion and ratios between the amplitude of the three components of the ground velocity have been studied too confirming significant time variations this leads us to suggest the existence of at least two tremor sources a shallow one mainly characterised by relatively high frequencies 3 5 7 hz is linked to the upper portions of the active conduits and directly related to the observed eruptive activity the latter deeper source was active for only a few weeks and characterised by frequencies lower than about 2 5 hz it had no relationship with the observed eruptive activity and was speculatively related to the gas exsolution from fresh magma refilling a small batch

peritidal sedimentary depositional facies and carbon isotope variation across k t boundary carbonates from nw adriatic platform the k t boundary in the karst region italy and slovenia is well documented in correspondence to shallow water limestones its evidence is usually related to palaeontological data disappearance of cretaceous taxa and subsequent appearance of tertiary ones palaeomagnetism ch 29r iridium anomaly and negative shift of 13 c in italy the padriciano section highlights the k t boundary included in the basal breccia of a peritidal cycle recent investigations brought to light a short well exposed new stratigraphic sequence containing the k t boundary 100 m east of padriciano as demonstrated by lithological palaeontological and new isotopic data the recording of a new peritidal setting denotes stressed environmental conditions the findings of the biota mostly correspond to the lagoonal limestones of the peritidal cycle it consists of small opportunistic thin shelled foraminifers ostracods and gastropods at padriciano the environmental stability occurred after episodesofstrongfreshwaterinfluence theintraclasts oncolitesandstromatoliteshavebeenobservedinthepadricianosection the broken shell fragments algae and foraminifera have been found associated with fenestral fabric these features indicate deposition in a shallow marine intertidal to supratidal environment algal boundstones and microbial stromatolitic laminae crenulated domal laminae and crenulatedconical laminae havebeen studied in petrographicsections for microstructures the k t boundary section has been analysed for the carbon and oxygen isotopic ratios the 13 c values range from 3 62 to 10 01 v pdb and the 18 o values range from 3 85 to 5 47 v pdb this extreme depletion in the 13 c record has already been reported from other k t boundary sections both in the padriciano area italy and in slovenia dolenja vas sections and may be related to global climate change but also to local environmental changes occurring in the nwadriatic platform 2007 elsevier b v all rights reserved

seismological aspects of the 1989 1990 eruptions at redoubt volcano alaska the ssam perspective abstract ssam is a simple and inexpensive tool for continuous monitoring of average seismic amplitudes within selected frequency bands in near real time on a pc based data acquisition system during the 1989 1990 eruption sequence at redoubt volcano the potential of ssam to aid in rapid identification of precursory long period lp event swarms was realized and since this time ssam has been incorporated in routine monitoring efforts of the alaska volcano observatory in particular an eruption that occurred on april 6 was successfully forecast primarily on the basis of recognizing the precursory lp activity on ssam of twenty two significant eruptions that occurred between december 14 and april 21 eleven had precursory swarms longer than one hour in duration that could be detected on ssam for individual swarms the patterns of relative spectral amplitudes are distinct at each station and remain largely stationary through time thus indicating that one source may have been preferentially and repeatedly activated throughout the swarm typically a single spectral band dominates the signal at each seismic station for the vigorous one day swarm that preceded the first eruption on december 14 signals were sharply peaked in the 1 9 2 7 hz band at the closest station located 4 km from the vent but were dominated by 1 3 1 9 hz energy at three more distant stations located 7 5 22 km from the vent the tendency for the signals from different swarms recorded at the same station to be peaked in the same frequency band suggests that all of the sources are characterized by a predominant length scale signals from the precursory lp swarms became weaker as the eruption sequence progressed and swarms that occurred in march and april could only be detected at seismographs on the volcanic edifice onset times of precursory lp swarms prior to eruptions ranged from a few hours to about one week but after the initial vent clearing phase that ended december 19 these intervals tended to become progressively shorter for successive swarms these trends in the relative onset times and intensities of successive precursory lp swarms are consistent with an overall depressurization of the magmatic system through time in general each of the swarms had an emergent onset but the intensities did not always increase steadily until the eruptions instead as the time of an eruption approached the intensity usually increased more rapidly before peaking and then declining prior to the eruption for three of the swarms two distinct peaks in intensity were apparent the time intervals between final peaks in swarm intensity and ensuing eruptions ranged from about 2 hours to almost 2 days but the peaks always occurred closer to the eruptions than to the swarm onsets both the onset of lp swarm activity and a decline in intensity prior to an eruption may represent critical points in the process of pressurization that drives the flow of fluids and gas in a sealed magmatic system a notable exception to this pattern is the eruption of march 9 which lacked a detectable precursory lp swarm but was followed by an unusually long period of strong lp seismicity that may have been stimulated by a depressurization of the magmatic system resulting from dome failure on both december 14 and january 2 the spectra of early syn eruptive signals have peaked signatures much like those of the spectra of precursory lp activity from shortly before the eruptions these similarities may indicate that the source of precursory seismicity continued to be active during at least the early part of each eruption in syn eruptive signals from march and april recorded at stations on the volcanic edifice the dominant spectral energy progressively shifts with time during the eruption to lower frequencies at least part of the energy in these signals may have been generated by the debris flows associated with dome failures

observations of seismic volcanic signals at stromboli volcano italy abstract digital recordings of ground motion during a seismic operation organized at stromboli volcano italy from may 31 1986 to june 26 1986 have been studied the amplitude of background seismic noise is greater at the summit than at the base of the volcano and the spectra calculated on different days are dominated by at least two frequencies 2 0 and 6 0 hz with subdominant frequencies at 4 1 4 8 and 8 hz according to their waveform and their spectral content three types of discrete volcanic signals are distinguishable the first type is characterized by a dominant frequency at 2 hz and a subdominant frequency at 4 1 hz the second type also has a dominant peak frequency at 2 hz with two important subdominant frequencies at 5 and 8 hz the type 3 has a sharp dominant frequency at 6 hz and subdominant frequency at 3 2 hz the frequencies observed for discrete signals and ground noise suggest that they are produced by the same repeated mechanism which probably has a volcanic origin the discrete signals are linked to explosions at the active vents within the craters

seismic detection of underwater volcanism the example of french polynesia we present a review of the principal methods used for the seismic detection and identification of active underwater volcanism based on our experience in french polynesia in particular we descrobe the 5 year activity in the tahiti mehetia area during which more than 32000 earthquakes were detected by the polynesian network we discuss the use of the following three types of seismic waves conventional mostly body waves seismic tremor andt waves propagated in the low velocity acoustic channel of the ocean for each of these waves we discuss the principal characteristics of the signals their spectral content the type of information they provide on the activity of the volcano and the various limitations faced by their use in detection or monitoring of underwater volcanic edifices we present a review of the principal swarms monitored by the polynesian network and discuss their characterization as either volcanic or tectonic

progressive reactivation of the volcanic plumbing system beneath tolbachik volcano kamchatka russia revealed by long period seismicity abstract after lying dormant for 36 yr the tolbachik volcano of the klyuchevskoy group started to erupt on 27 november 2012 we investigate the preparatory phase of this eruption via a statistical analysis of the temporal behavior of long period lp earthquakes that occurred beneath this volcanic system the lp seismicity occurs close to the surface beneath the main volcanic edifices and at 30 km depth in the vicinity of a deep magmatic reservoir the deep lp earthquakes and those beneath the klyuchevskoy volcano occur quasi periodically while the lp earthquakes beneath tolbachik are clustered in time as the seismicity rate increased beneath tolbachik days before the eruption the level of the time clustering decreased we interpret this as a manifestation of the evolution of the volcano plumbing system we suggest that when a plumbing system awakes after quiescence multiple cracks and channels are reactivated simultaneously and their interaction results in the strong time clustering of lp earthquakes with time this network of channels and cracks evolves into a more stable state with an overall increased permeability where fluids flow uninhibited throughout the plumbing system except for a few remaining impediments that continue to generate seismic radiation the inter seismic source interaction and the level of earthquake time clustering in this latter state is weak this scenario suggests that the observed evolution of the statistical behavior of the shallow lp seismicity beneath tolbachik is an indicator of the reactivation and consolidation of the near surface plumbing system prior to the tolbachik eruption the parts of the plumbing system above the deep magmatic reservoir and beneath the klyuchevskoy volcano remain in nearly permanent activity as demonstrated by the continuous occurrence of the deep lp earthquakes and very frequent klyuchevskoy eruptions this implies that these parts of the plumbing system remain in a stable permeable state and contain a few weakly interacting seismogenic sources our results provide new constraints on future mechanical models of the magmatic plumbing systems and demonstrate that the level of time clustering of lp earthquakes can be a useful parameter to infer information about the state of the plumbing system

volcanic tremors at etna a model for hydraulic origin spectral analysis of volcanic tremors at etna reveals 2 characteristic features 1 the escaping vapor and gas streams drive the magma in random turbulent motion generating a seismic wave emission according toa f n exp m f k wherea amplitude f frequency n m k parameters 2 sudden transients in the boundary conditions of the piping system generate hydraulic pressure pulses which in the seismogram appear as wave packages their monochromatic frequency content suggests that their origin is associated with strong resonance phenomena it is conclusive that volcanic eruptions may be distinguished and classified according to different eigenfrequencies of the fissure and pipe network involved in the volcanic activity furthermore positions of active dykes may be traced following the amplitude distributions associated with pronounced spectral lines in the tremor spectra

a possible k t boundary bolide impact site offshore near bombay and triggering of rapid deccan volcanism abstract the temporal coincidence of a major biological mass extinction including dinosaurs the well known iridium excess anomaly at the cretaceous tertiary k t boundary and the eruption of deccan flood basalts at about 65 ma has aroused global interest among geologists and biologists it is widely debated whether the mass extinction and iridium anomaly are due to an asteroid impact or the massive outpouring of extensive deccan volcanism an oval shaped unusual positive gravity anomaly 10 000 km 2 in area near bombay has attracted our attention during a search for an impact site near deccan basalts a detailed gravity interpretation indicates the presence of a fossil conduit structure of 12 km height extending from a shallow crust mantle boundary at 18 km to an approximate depth of 6 km from the surface the conduit structure with a maximum diameter of about 35 km at its base may originate from cracking of a weak pre deccan trap shallow upwarped mantle the structure may have been caused by a bolide impact which triggered the eruption of massive flood basalts deccan traps on the western margin of the fast moving indian plate an impact in this locality can explain the sudden detachment of the arcuate seychelles block from india as well as the large scale reorganisation of plate boundaries in the indian ocean our hypothesis of impact triggered volcanism at 65 ma advocates a bimodal cause for the mass extinction at the k t boundary extraordinary geothermal and structural conditions of the nearby region are also discussed as circumstantial evidence to support the twin cause mechanism by weakened features and the presence of partial melt at subcrustal depth

intermittent behaviour of volcanic tremor at mt etna harmonic tremor is widely studied and modelled in a very narrow frequency band 1 5 hz which represents the eigenfrequencies of a resonator assumed as the source of the phenomenon minimal effort was dedicated towards understanding its behaviour in larger temporal scales here we characterise the dynamic behaviour of volcanic tremor while evaluating the complete spectrum of the generalised dimension of the phase space the starting time series constitutes the tremor amplitude picked every 10 minutes the choice of this lag time is made on the basis of a qualitative analysis of the properties of the tremor the results show intermittent behaviour of the dynamics which requires an 8 dimensional map to be completely described an interesting result is that the maximum clustering of point density in phase space occurs in a monodimensional space which implies a periodicity sometimes observed experimentally an appropriate predictive model needs more constraints on the nature of the eight variables involved in the process

geological and seismological evidence of increased explosivity during the 1986 eruptions of pavlof volcano alaska we present results of study of the best documented eruptions of pavlof volcano in historic time the 1986 eruptions were mostly strombolian in character a strong initial phase may have been vulcanian the 1986 activity erupted at least 8 106 m3 of feldspar phyric basaltic andesite lava sio2 53 54 and a comparable volume of wind borne tephra during the course of the eruption 5300 explosion earthquakes occurred the largest of which was equivalent to an m l 2 5 earthquake volcanic tremor was recorded for 2600 hours and the strongest tremor was recorded out to a distance of 160 km and had an amplitude of at least 54 cm2 reduced displacement the 1986 eruptions modified the structure of the vent area for the first time in over two decades a possible pyroclastic flow was observed on 19 june 1986 the first time such a phenomenon has been observed at the volcano overall the 1986 eruptions were the strongest and longest duration eruptions in historic time and changed a temporal pattern of activity that had persisted from 1973 1984

understanding conspicuous gravity low over the koyna warna seismogenic region maharashtra india and earthquake nucleation a paradigm shift the continued seismicity in koyna warna region of the western part of maharastra india and its relationship with subsurface structures concealed below thick volcanic sequences are studied in detail using gravity field along with newly available deep scientific drilling results this seismically active zone is marked by a large conspicuous negative gravity anomaly the causes of which are yet to be fully understood recent findings from the boreholes drilled in the koyna gupta et al in int j earth sci 104 1511 1522 2015 and killari seismic zones both of which penetrated the thick deccan volcanic cover and the underlying archean crystalline basement have motivated us to revisit the bouguer gravity field over this region using a newly developed finite element method of regional residual separation our study reveals the presence of two thick low density low velocity crustal zones below the koyna warna region the shallower one between 5 and 13 km depth and the deeper one between 35 and 43 km depth just above the moho both of these zones appear to contain mantle metasomatised and fractionated magmatic material respectively interestingly the hypocenters of all m 5 koyna earthquakes occur within the upper low velocity low density zone we also suggest high order crustal exhumation below this region which led to the removal of the entire sedimentary and granitic upper crustal column this process has brought denser mid crustal lithological facies close to the surface quaternary uplifting and movement of fault blocks along the old as well as newly created fault planes seem to be still continuing a paleo rift may have existed beneath this region below which moho temperatures 600 c and mantle heat flow 31 mw m2 are still high

ground motion in the near field of a fluid driven crack and its interpretation in the study of shallow volcanic tremor we present a study of the motion of the ground in the near field of a fluid driven tensile crack embedded in a layered half space the source that we consider is the jerky opening of a channel connecting two fluid filled cracks and the cause of this opening is the excess pressure of fluid in one of the cracks we make a complete representation of the three components of ground motion in the space time and frequency domains and analyze the effects of fluid compressibility source depth and medium structure on the ground response the calculations show the presence of a dominant frequency of motion which depends not only on the source geometry and bulk modulus of the fluid but also on medium characteristics receiver position and the component of motion being considered using this source model we view an episode of volcanic tremor as a continuous sequence produced by numerous jerky openings of channels occurring randomly in time along a chain of cracks our results are applied to the october 5 6 1963 east rift eruption of kilauea volcano hawaii and found to be compatible with available seismic data suggesting that magma is transported through an ensemble of cracks with the area of 1 1 km each pair of cracks constituting an individual dike segment which opens in discrete increments at a rate of 1 s we obtain the following parameters for each dike segment opening increase in cavity volume 40 m3 stress drop 0 004 bar seismic moment 1019 dyn cm and force applied by the fluid to open the dike 1014 dyn the total moment integrated over the entire duration of tremor for the eruption is 0 5 1024 dyn cm roughly equivalent to a single magnitude m 5 earthquake the seismic source parameters and other field observations offer constraints on the process of mass transport and are compatible with the excess pressure and viscosity of magma of about 40 bars and 102 p respectively

structure and evolution of the western continental margin of india deduced from gravity seismic geomagnetic and geochronological studies abstract available geochronological gravity seismic and geomagnetic data on the western continental margin of india between 8 20 n are presented based on the data three models describing the evolution of the crustal structure are developed model 1 describes the evolution of the crustal structure along 19 n the main basic igneous activity that accompanied the tectonic events since the late cretaceous resulted in the formation of rifts along the coast thus giving rise to the present day structure the acid volcanic event that preceded the above activity at about 93 ma ago is responsible for the observed crustal structure along 13 30 n shown in model 2 this event resulted in the formation of the st mary group of islands along this latitude counterclockwise rotation of the indian plate during the tertiary period caused rifting of the continental blocks giving rise to the laccadive kerala graben bounded by the chagos laccadive ridge on the west and the west coast fault on the east vertical movements of the rifted blocks promoted the deposition of both continental and marine sediments on the coast west of the west coast fault this tectonic event was accompanied by upwarping of the mantle crust thus giving rise to the present day structure shown in model 3

amirante basin western indian ocean possible impact site of the cretaceous tertiary extinction bolide if an impact event caused the mass extinctions and geochemical anomalies at the cretaceous tertiary boundary it probably occurred in an oceanic area however no convincing impact site has yet been discovered whereas late cretaceous magnetic lineations in other oceans show no obvious signs of disturbance at the tertiary boundary the end cretaceous african plate boundary in the indian ocean provides evidence of major tectonic reorganization at or shortly after magnetostratigraphic chron c29r immediately south of the microcontinental seychelles bank the amirante basin has a roughly circular shape of about 300 km diameter is partially ringed by enigmatic arc and trench structures and is located within oceanic crust of late cretaceous age it is therefore a possible impact site extensive chaotic slump structures apparently exist at the appropriate level on the east african continental margin and they may indicate its proximity to the mega earthquake focus and or giant tsunamis in the somali basin by triggering readjustments along the indian african and antarctic african plate boundaries and thus altering the regional balance of driving forces the impact may have affected plate motions

types of internal structure and external morphology of submarine dunes under the influence of tide and wind driven processes dover strait northern france abstract the expression of tide and wind driven processes in the internal architecture and external morphology of very large submarine dunes is investigated at a dune field located in the central part of the dover strait southern north sea current measurements and sediment transport calculations enable quantification of the dominant tide and wind driven processes and very high resolution seismic recordings are used to analyse internal structure and external morphology of dunes in the study area moderate to storm winds induce temporary currents that can lead to the reversal of sediment transport direction and induced erosive processes on the seabed dune master bedding consists of extended erosive surfaces with 9 11 slopes that bound storm generated sets of cross beds 1 5 4 85 m thick 2 65 m on average three types of architectures are observed within the dune field and occasionally within single dunes these types which display a strong linkage between internal structure and external morphology consist of 1 asymmetric dunes with cosets which dip in the direction of the marked external asymmetry of the dunes 2 dunes with a complex external morphology composed of mega herringbone structures each displaying an opposing progradation direction and 3 dunes with a symmetric megaripple bedding unit at their summit the diversity of the internal architecture and external morphology results from the variable combinations of tide and wind driven processes occurring in the study area the asymmetry of tidal peak current velocity and the relative strength of wind driven currents are the parameters that have the most important influence on dune architecture asymmetric dunes with cosets result from a strong asymmetry in tidal peak current velocity which is reversed only slightly during storms dunes with mega herringbone structures and dunes with symmetric megaripple units at their summit are observed in areas where the asymmetry of the tidal peak current velocity is negligible allowing reversal of the sediment transport and dune migration directions each time moderate winds blow the latter architectural type is specific to sand rich areas the different dune architecture types display strong geometrical similarities to structures observed in dunes from other shelf environments where driving processes are different this implies that a study based only on a geometric description of internal structures will not enable the reconstitution of ancient depositional environments a better discriminant criteria seems to be the thickness of the second order sets of cross beds which is correlated with specific hydro sedimentary processes dune height also probably has an important influence on the thickness of second order sets of cross beds as dune height and set thickness display a clear relationship in the present study

a radiometric age for the cretaceous tertiary boundary based upon k ar rb sr and u pb ages of bentonites from alberta saskatchewan and montana bentonites associated with coal seams at the north american cretaceous tertiary k t boundary in the red deer valley of alberta in the frenchman valley of saskatchewan and near hell creek montana were dated by the k ar rb sr and u pb methods based upon palynological stratigraphic correlation and the presence of a sharp iridium anomaly the three sampled bentonite horizons all occur less than 1 m above the established k t boundary there is evidence of possible systematic errors giving dating results outside nominal statistical error values despite possible constant errors the weighted mean of nine average dates is calculated as 64 3 1 2 ma 2 standard deviation an unweighted averaging produces a grand mean age of 64 4 1 2 ma

three distinct regimes of volcanic tremor associated with the eruption of shishaldin volcano alaska 1999 tremor signals associated with the eruption of shishaldin volcano on 19 and 23 april 1999 were the strongest recorded anywhere in the aleutian arc by the alaska volcano observatory avo in its 10 year history reduced displacements dr reached 23 cm2 on 19 april and 43 cm2 on 23 april during the activity dr and spectral data with a frequency resolution of 0 1 hz were computed and put on the world wide web every 10 min these data are analyzed here the general temporal patterns of seismicity of these eruption events were similar but the eruptions and their effects quite different the 19 april event is known to have culminated in a sub plinian phase which ejected ash to an altitude of 16 km despite higher amplitudes and the largest hotspot from satellite data the 23 april event produced little ash reaching only 6 km altitude for several hours prior to the sub plinian phase on 19 april tremor with a peak frequency of 1 3 hz intensified during the sub plinian phase the peak frequency increased to 4 8 hz however in 15 h after the eruption three episodes of stronger tremor occurred with a lower 1 0 hz peak alternating with weaker tremor with a 1 3 hz peak these transitions correspond to dr 8 cm2 although these strong tremor episodes produced higher dr levels than the sub plinian phase data from a pressure sensor show that only strong strombolian explosions occurred the suite of observations suggests three distinct tremor regimes that may correspond to slug flow bubbly flow and sustained strong eruptions or a cyclic change in source parameters e g geometry sound speed or ascent rate this behavior occurred at shishaldin only during the april 1999 sequence and we are not aware of similar behavior at other volcanoes