possible causes for the seismic activity observed in cotopaxi volcano ecuador continuous monitoring of cotopaxi volcano from january 1989 to september 1997 shows that it is subject to a steady shallow long period lp activity lp events are concentrated in a column with a diameter of about 3 km and a height of about 12 km between an elevation of 4 km and 8 km depth they do not occur in swarms nor have there been periods of complete calm high frequency volcano tectonic vt events are mostly located on the northeastern side of the lp column with the maximum activity of both types of events coinciding in depth lp activity cannot be related to unrest of the volcano the most likely explanation for the continuous occurrence of lp events beneath cotopaxi is that they are produced by the interaction of glacier thaw water and hot material at shallow depths the relative spatial distribution of both type of events in two adjacent zones suggest that vt and lp activities are interconnected

statistical analyses and characteristics of volcanic tremor on stromboli volcano italy a study of volcanic tremor on stromboli is carried out on the basis of data recorded daily between 1993 and 1995 by a permanent seismic station str located 1 8 km away from the active craters we also consider the signal of a second station tf1 which operated for a shorter time span changes in the spectral tremor characteristics can be related to modifications in volcanic activity particularly to lava effusions and explosive sequences statistical analyses were carried out on a set of spectra calculated daily from seismic signals where explosion quakes were present or excluded principal component analysis and cluster analysis were applied to identify different classes of spectra three clusters of spectra are associated with two different states of volcanic activity one cluster corresponds to a state of low to moderate activity whereas the two other clusters are present during phases with a high magma column as inferred from the occurrence of lava fountains or effusions we therefore conclude that variations in volcanic activity at stromboli are usually linked to changes in the spectral characteristics of volcanic tremor site effects are evident when comparing the spectra calculated from signals synchronously recorded at str and tf1 however some major spectral peaks at both stations may reflect source properties statistical considerations and polarization analysis are in favor of a prevailing presence of p waves in the tremor signal along with a position of the source northwest of the craters and at shallow depth

observations and analysis of b type earthquakes explosions and volcanic tremor at pavlof volcano alaska an eight station network of short period seismometers has been operated near pavlof volcano 55 25 n 161 54 w alaska peninsula since 1976 high quality analog to digital and direct digital data were recorded during eruptions in 1980 through 1983 data are analyzed from explosions with distinct air phases b type volcanic earthquakes shallow low frequency events lacking clear s phases and volcanic tremor to determine source and propagation effects between zero and several hundred b type events and explosions per day are recorded with higher numbers during eruptions magnitudes range between 0 5 and 1 3 and b values range between 1 9 0 1 and 2 6 0 1 with no systematic relation to eruptive activity six results are most important 1 data from a recently installed three component station reveal that the highest amplitude waves of the b type events show retrograde elliptical particle motion and normal dispersion and are hence probably rayleigh waves 2 stacking of the events enhances the p wave which has a velocity of 3 1 0 1 km sec 1 3 the explosions and b type events have similar hypocenters as evidenced by signal characteristics and wave arrival times 4 the b type events contain virtually no high frequency energy even when allowing for attenuation effects in particular we estimate the anelastic attenuation coefficient q to have a value of 45 20 for rayleigh waves in the shallow part of the volcano 5 volcanic tremor spectra exhibit evenly spaced narrow peaks which we model as eigenvalues of organ pipe resonance in the magma conduit we estimate the length of the conduit to be a maximum of 1 6 km 6 the volcano shows evidence of extreme lateral variation of velocity and attenuation in its shallow structure

volcanic tremor at mt etna italy preceding and accompanying the eruption of july august 2001 the july 17 august 9 2001 flank eruption of mt etna was preceded and accompanied by remarkable changes in volcanic tremor based on the records of stations belonging to the permanent seismic network deployed on the volcano we analyze amplitude and frequency content of the seismic signal we find considerable changes in the volcanic tremor which mark the transition to different styles of eruptive activity e g lava fountains phreatomagmatic activity strombolian explosions in particular the frequency content of the signal decreases from 5 hz to 3 hz at our reference station etf during episodes of lava fountains and further decreases at about 2 hz throughout phases of intense lava emission the frequency content and the ratios of the signal amplitude allow us to distinguish three seismic sources i e the peripheral dike which fed the eruption the reservoir which fed the lava fountains and the central conduit based on the analysis of the amplitude decay of the signal we highlight the migration of the dike from a depth of ca 5 km to about 1 km between july 10 and 12 after the onset of the effusive phase the distribution of the amplitude decay at our stations can be interpreted as the overall result of sources located within the first half kilometer from the surface although on a qualitative basis our findings shed some light on the complex feeding system of mt etna and integrate other volcanological and geophysical studies which tackle the problem of magma replenishment for the july august 2001 flank eruption we conclude that volcanic tremor is fundamental in monitoring mt etna not only as a marker of the different sources which act within the volcano edifice but also of the diverse styles of eruptive activity

velocity and spectral characteristics of the volcanic tremor at etna deduced by a small seismometer array abstract a small l shaped short period seismic array was set up at etna volcano to measure the wave velocity and the incoming azimuth of the volcanic tremor generated by the volcanic activity at etna in non eruptive conditions cross correlation techniques were used to measure the phase shifts between the array stations an approximately n s direction of the propagation direction was estimated coincident with the azimuth of the crater area of etna an apparent velocity of less than 500 m s was estimated suggesting a surface propagation of the tremor waves spectral analysis confirming previous results shows velocity spectra peaked at approximately 2 hz a localized source coincident with the crater area agrees with our data and could generate the volcanic tremor at etna in non eruptive stages

stochastic simulation of volcanic tremor from ruapehu abstract the most common volcanic tremor produced by ruapehu is a continuous signal with a dominant frequency of about 2 hz this signal has a sharply peaked spectrum and an autocorrelation function with a high degree of coherence even for lags of over 20 seconds these characteristics strongly suggest that the cause of this tremor is a single resonator probably a fluid filled cavity resonating in an organ pipe mode the stochastic simulation of such a resonator uses the equation of motion of a simple harmonic oscillator which applies to an organ pipe fundamental resonance with either the characteristics of the oscillator or the forcing function containing a random element a white noise forcing function which would be appropriate for excitation of the cavity by a high pressure gas input gave good agreement with the observed spectra and autocorrelation functions another possible model used an oscillator with a damping factor which varied randomly and was sometimes negative so oscillations built up rather than decayed this also gave a reasonable simulation of ruapehu tremor the third excitation model used a poisson process in which during each time interval there was a certain probability of applying a fixed impulse to the resonator it was found that the impulses had to be frequent i e several times a second to match the characteristics of ruapehu tremor it has been suggested that tremor is composed of a succession of low frequency b type earthquakes the results of this simulation show that at ruapehu tremor could be produced by a resonator with positive feedback just sustaining oscillation or by a resonator excited by external impulses the most promising model for low frequency earthquakes describes them as the result of a major external disturbance of the resonator

salinity history of the northern atlantic during the last deglaciation the claim has been made see broecker et al 1988 that production of north atlantic deep water terminated during younger dryas time and that the onset of this termination occurred about 11 000 years ago when the flow of meltwater from a large segment of the southern margin of the laurentide ice sheet was diverted from the mississippi to the st lawrence drainage fairbanks 1989 points out a serious weakness in this argument based on a sea level curve derived from radiocarbon dates on coral obtained from borings made off the barbados coast he suggests that a lull in the melting of the ice caps during younger dryas time may have more than compensated for the impact of the diversion the purpose of this paper is to reassess the situation regarding the origin of the younger dryas in light of this new evidence currently the salinity of surface waters in the northern atlantic is influenced by three fluxes water vapor transport from the atlantic drainage basin to the pacific indian basin tends to raise the salinity of the entire atlantic the excess over evaporation of precipitation and runoff poleward of 40 n tends to reduce the salinity of waters in this region relative to the atlantic average the conveyor circulation of the atlantic trades more salty waters of the atlantic with less salty waters outside the atlantic tending to drive down the atlantic s salinity the conveyor circulation also flushes the northern atlantic pushing its salinity toward the mean for the atlantic during the period of deglaciation meltwater emanating from the laurentide and scandinavian ice sheets was also important this flux tended to lower not only the salinity of the entire atlantic but also the salinity of surface waters in the northern atlantic relative to the atlantic s mean as deepwater formation in the northern atlantic depends critically on the salinity of surface waters the interactions among these fluxes can change the strength of the conveyor

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

shallow velocity structure of stromboli volcano italy derived from small aperture array measurements of strombolian tremor abstract the properties of the tremor wave field at stromboli are analyzed using data from small aperture arrays of short period seismometers deployed on the north flank of the volcano the seismometers are configured in two semi circular arrays with radii of 60 and 150 m and a linear array with length of 600 m the data are analyzed using a spatiotemporal correlation technique specifically designed for the study of the stationary stochastic wave field of rayleigh and love waves generated by volcanic activity and by scattering sources distributed within the island the correlation coefficients derived as a function of frequency for the three components of motion clearly define the dispersion characteristics for both rayleigh and love waves love and rayleigh waves contribute 70 and 30 respectively of the surface wave power the phase velocities of rayleigh waves range from 1000 m sec at 2 hz to 350 m sec at 9 hz and those for love waves range from 800 to 400 m sec over the same frequency band these velocities are similar to those measured near puu oo on the east rift of kilauea volcano hawaii although the dispersion characteristics of rayleigh waves at stromboli show a stronger dependence on frequency such low velocities are consistent with values expected for densely cracked solidified basalt the dispersion curves are inverted for a velocity model beneath the arrays assuming those dispersions represent the fundamental modes of rayleigh and love waves

study of seismic site effects using h v spectral ratios at arenal volcano costa rica by using data obtained with a linear array at arenal volcano we show that the h v spectral ratio method can be profitably applied to detect site effects on volcanoes similar results are obtained when calculating spectral ratios with different types of seismo volcanic signals tremor ambient noise explosion quakes lp events we compare the h v ratios with theoretical s wave transfer functions calculated using velocity models obtained from seismic refraction studies there is a good agreement when the h v ratios display sharp peaks indicating a close relationship between the ratios and the transfer function of the shallow structure furthermore the main peaks of the spectral ratios are consistent with local amplification of seismic waves observed at the corresponding frequencies

what is the thermohaline circulation the term thermohaline circulation is widely used in the context of past and future climate change in his perspective wunsch argues that there are many different and inconsistent definitions for this term he defines ocean circulation in terms of mass fluxes which are largely driven by surface forces such as wind and tides and argues that thermohaline circulation should only refer to the separate transport of heat and salinity in the ocean

spatial extent of a hydrothermal system at kilauea volcano hawaii determined from array analyses of shallow long period seismicity 2 results we present a probabilistic method to locate the source of seismic events using seismic antennas the method is based on a comparison of the event azimuths and slownesses derived from frequency slowness analyses of array data with a slowness vector model several slowness vector models are considered including both homogeneous and horizontally layered half spaces and also a more complex medium representing the actual topography and three dimensional velocity structure of the region under study in this latter model the slowness vector is obtained from frequency slowness analyses of synthetic signals these signals are generated using the finite difference method and include the effects of topography and velocity structure to reproduce as closely as possible the behavior of the observed wave fields a comparison of these results with those obtained with a homogeneous half space demonstrates the importance of structural and topographic effects which if ignored lead to a bias in the source location we use synthetic seismograms to test the accuracy and stability of the method and to investigate the effect of our choice of probability distributions we conclude that this location method can provide the source position of shallow events within a complex volcanic structure such as kilauea volcano with an error of 200 m

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

strong hemispheric coupling of glacial climate through freshwater discharge and ocean circulation the climate of the last glacial period was extremely variable characterized by abrupt warming events in the northern hemisphere accompanied by slower temperature changes in antarctica and variations of global sea level it is generally accepted that this millennial scale climate variability was caused by abrupt changes in the ocean thermohaline circulation here we use a coupled ocean atmosphere sea ice model to show that freshwater discharge into the north atlantic ocean in addition to a reduction of the thermohaline circulation has a direct effect on southern ocean temperature the related anomalous oceanic southward heat transport arises from a zonal density gradient in the subtropical north atlantic caused by a fast wave adjustment process we present an extended and quantitative bipolar seesaw concept that explains the timing and amplitude of greenland and antarctic temperature changes the slow changes in antarctic temperature and its similarity to sea level as well as a possible time lag of sea level with respect to antarctic temperature during marine isotope stage 3

trace interpolation by slant stack migration the slant stack migration formula based on the radon transform is studied with respect to the depth step z of wavefield extrapolation it can be viewed as a generalized trace interpolation procedure including wave extrapolation with an arbitrary step z for z 0 the formula yields the familiar plane wave decomposition while for z 0 it provides a robust tool for migration transformation of spatially undersampled wavefields using the stationary phase method it is shown that the slant stack migration formula degenerates into the rayleigh sommerfeld integral in the far field approximation consequently even a narrow slant stack gather applied before the diffraction stack can significantly improve the representation of noisy data in the wavefield extrapolation process the theory is applied to synthetic and field data to perform trace interpolation and dip reject filtration the data examples presented prove that the radon interpolator works well in the dip range including waves with mutual stepouts smaller than half the dominant period

on the spectral peaks of volcanic tremor at stromboli abstract seismic activity at stromboli volcano is characterized by a variety of signals emanating from three vents for a long time the northwest vent has been in constant activity periodically large explosions occur and material is ejected beyond the crater walls these large explosions are accompanied by sonic and infrasonic pressure waves in the atmosphere and explosion quakes apart from large explosions there is constant activity in the form of continuous gas bursts which are related to low infrasonic pulses in the atmosphere and volcanic tremor we assume that volcanic tremor and low pressure infrasonics are generated by gas bubbles inside the volcanic conduit and accordingly we compute synthetic tremor by modeling the source function as a pressure variation in a spherical cavity that propagates through a finely layered medium by means of haskell s formalism to simulate a tremor we superpose in time domain a large number of such pulses of varying amplitudes and time delays according to the observed infrasonic series in addition to the spectral similarity the observed and synthetic tremor display the same autocorrelation and hurst exponents implying similar long term correlation we present strong evidence in favour of an interpretation of the spectral peaks of the volcanic tremor at stromboli in terms of resonances of the layered structure hence as a path effect rather than a source effect

optimal excitation of amoc decadal variability links to the subpolar ocean this study describes the excitation of variability of the atlantic meridional overturning circulation amoc by optimal perturbations in surface temperature and salinity our approach is based on a generalized stability analysis within a realistic ocean general circulation model which extends the conventional linear stability analysis to transient growth unlike methods based on singular value decomposition our analysis invokes an optimization procedure using lagrangian multipliers which is a more general approach allowing us to impose relevant constraints on the perturbations and use linear measures of the amoc meridional volume and heat transports we find that the structure of the optimal perturbations is characterized by anomalies in surface temperature or salinity centered in the subpolar regions of the north atlantic off the east coasts of greenland and canada south of the denmark strait the maximum impact of such perturbations on the amoc is reached after 7 9 yr this is a robust result independent of the perturbations type the optimization measures the model surface boundary conditions or other constraints the transient growth involves the following mechanism after the initial positive surface density perturbation reaches the deep ocean it generates a cyclonic geostrophic flow that extracts a zonally varying temperature anomaly from the mean temperature field in the upper ocean in turn the anomalous zonal temperature gradient induces by thermal wind balance a northward flow in the upper ocean and a southward flow in the deep ocean thus strengthening the amoc subsequently the transient growth gives way to a decaying oscillation corresponding to a damped oceanic eigenmode with a period of about 24 yr this mode is controlled by westward propagating large scale thermal rossby waves modifying the density field in the north atlantic and hence the amoc simple estimates show that realistic changes in salinity or temperature in the upper ocean such as those due to the great salinity anomaly can induce amoc variations of several sverdups via this mechanism or 10 20 of the mean overturning an idealized model is formulated to investigate the transient growth and highlight the role of mean convection in communicating surface density anomalies to the deep ocean 2014 published by elsevier ltd

evidence for gas influence on volcanic seismic signals recorded at stromboli abstract the meaning of the large variety of seismic waveforms showing different frequencies associated with volcanic activity is still uncertain no definitive model for source dynamics has been proposed yet at present seismic models explaining the origin of the spectral content of the seismic signal are mainly linked to geometrical features of the volcanic conduit and to resonant effects induced by pressure fluctuations in a fluid filled conduit such models assume the physical system to be in a steady state over a long interval of time at stromboli the seismicity produced by each of the three active vents is not stable and can suddenly change in time therefore the application of stable resonator models appears to be unwarranted on the basis of infrared image analyses atmospheric pressure and seismic wavefield signals we show that different frequency contents and different explosive styles occur at stromboli at the same time at different vents moreover we give evidence to support the idea that seismicity and explosivity are both controlled by a variable gas flow regime released during explosions at the top of the magma column we have recognized two main pressure regimes controlling the explositivity at stromboli one is impulsive short and possibly produced by a gas accumulation beneath a cap rock the other has an intermittent regime feature and lasts longer we demonstrate how the first one generates a monochromatic low frequency wavefield while the second shows a high frequency spectrum where the frequency content depends on pressure fluctuations and on pressure gradient we suggest different pressure growth and gas flow in the magma as the common source for both explosive style and seismic wavefield

depth migration of imaged time sections none of the leading approaches to the migration of seismic sections the kirchhoff summation method the finite difference method or the frequency domain method readily migrates seismic reflections to their proper positions when overburden velocities vary laterally for inhomogeneous media the diffraction curve for a localized buried scatterer is no longer hyperbolic and its apex is displaced laterally from the position directly above the scatterer hubral observed that the kirchhoff summation method images seismic data at emergent image ray locations rather than at the desired positions vertically above scatterers in addition distortions in diffraction shapes lead to incorrect imaging i e incomplete diffraction collapse and hence to further displacement errors for dipping reflections the finite difference method has been believed to continue waves downward correctly through inhomogeneous media in conventional implementations however both the finite difference method and frequency domain a

shallow velocity structure and seismic site effects at arenal volcano costa rica abstract we use the spatial autocorrelation spac method with improved inversion algorithms to estimate the love and rayleigh dispersion curves at two sites at the west and northeast flanks of arenal volcano costa rica at the west flank site the rayleigh waves phase velocities vary from 765 m s 1 at 1 hz to 300 m s 1 at 12 hz and those of love waves between 780 and 295 m s 1 in the same frequency band at the northeast flank site the rayleigh wave velocities range from 1386 to 300 m s 1 and those of love from 1983 to 315 m s 1 from dispersion curves we derive shallow strong variations in amplitude of the spectral peaks are observed for the seismic events along two radial arrays these site effects are analysed using the h v spectral ratio method and s wave theoretical transfer functions results show that the wave amplifications are related to resonance effects of shallow structure when a resonance of the shallow layers occurs the correlation coefficients between close stations increase at the corresponding frequency the site effects may thus produce spurious results with the spac method the h v spectral ratio used in complement of the spac method can help detecting the site effects and testing the plane layer hypothesis furthermore the theoretical transfer functions calculated for the estimated velocity models is also useful to validate the models

trace interpolation in seismic data processing trace interpolation of spatially aliased seismic data is used to improve the quality of multi trace processing especially that of migration we consider non rectangular sampling lattices after stating the real sampling requirements for two dimensional or 3 d digitized signals the paper explains geometrically the effect of migration and of the 2 d dip filtering upon the aliased events as well as our ability to interpolate traces from undersampled seismic data

temporal and spatial characteristics of volcanic tremor wave fields abstract detailed volcanic tremor studies were carried out with portable seismic stations installed both in the near and distant zones at klyuchevskoy volcano in 1984 1986 and 1987 it has been determined that the coherence of wave field decreases with distance to 0 5 km and does not change at greater distances based on coherence and phase spectra the frequency dependence of the phase velocities has been determined the dispersion of phase velocity points to the surface character of the wave field the behaviour of volcanic tremor characteristics through time is stationary which implies stability of the parameters of the volcanic tremor source a detailed spectral study has demonstrated that when the intensity of volcanic tremor varies by a few orders of magnitude parts of the low frequency and high frequency spectra change which is likely due to the variations in source dimensions the detailed spectral characteristics of tremor may be used for both determining the source parameters and studying the structure of the medium

volcanic tremor related to the 1991 eruption of the hekla volcano iceland volcanic tremor at the hekla volcano is directly related to eruptive activity it starts simultaneously with the eruptions and dies down at the end of them no tremor at hekla has been observed during non eruptive times the 1991 hekla eruption began on 17 january after a short warning time local seismograph stations recorded small premonitory earthquakes from 16 30 gmt on at 17 02 gmt low frequency volcanic tremor became visible on the seismograph records marking the onset of the eruption the initial plinian phase of the eruption was short lived during the first day several fissures were active but by the second day the activity was already limited to a segment of one principal fissure the eruption lasted almost 53 days at the end of it during the early hours of 11 march volcanic tremor disappeared under the detection threshold and was followed by a swarm of small earthquakes at the start of the eruption the tremor amplitude rose rapidly and reached a maximum in only 10 min the tremor was most vigorous during the first hour and started to decline sharply during the next hour and later on more gently during the eruption as a whole the tremor had a continuous declining trend with occasional increases lasting up to about 2 days spectral analysis of the tremor during the first 7 h of the eruption shows that it settled quickly within a couple of minutes to its characteristic frequency band 0 5 1 5 hz the spectrum had typically one dominant peak at 0 7 0 9 hz and a few subdominant peaks hekla tremor likely has a shallow source particle motion plots suggest that it contains a significant component of surface waves the tremor started first when the connection of the magma conduit with the atmosphere was reached suggesting that degassing may contribute to its generation

simulation of abrupt climate change induced by freshwater input to the north atlantic ocean temperature records from greenland ice cores1 2 suggest that large and abrupt changes of north atlantic climate occurred frequently during both glacial and postglacial periods one example is the younger dryas cold event broecker3 speculated that these changes result from rapid changes in the thermohaline circulation of the atlantic ocean which were caused by the release of large amounts of melt water from continental ice sheets here we describe an attempt to explore this intriguing phenomenon using a coupled ocean atmosphere model in response to a massive surface flux of fresh water to the northern north atlantic of the model the thermohaline circulation weakens abruptly intensifies and weakens again followed by a gradual recovery generating episodes that resemble the abrupt changes of the ocean atmosphere system recorded in ice and deep sea cores4 the associated change of surface air temperature is particularly large in the northern north atlantic ocean and its neighbourhood but is relatively small in the rest of the world

modeling of volcanic tremor as explosive point sources in a single layered elastic half space i investigate path effects on the waveform of a volcanic tremor using a model that consist of an impulsive dilatational point source embedded in a single layered elastic half space such a source may arise during a variety of volcanic conditions such as a sequence of rapid explosions at a volcanic vent as well as gas piston activity observed in basaltic lava lakes using acoustic signals recorded during the 1975 tolbachik eruption the 1983 klyuchevskoy eruption and the 1989 tokachi dake eruption i propose a general impulsive time function for the tremor source consisting either of a single impulse or a sequence of identical impulses computed seismograms illustrate the influence of a low velocity surface layer on the shape and spectra of the waveforms for the same layered model a single impulse produces a dispersive wave while a sequence of identical impulses produces a wave of multiple harmonics for a single impulse the peak in the seismograms shifts to higher frequencies when the surface layer is very thin for multiple impulses spectral peaks shift to higher frequencies as the time between successive impulses increases indicating an increasing importance of higher order harmonics

variations in tilt rate and harmonic tremor amplitude during the january august 1983 east rift eruptions of kilauea volcano hawaii abstract during january august 1983 a network of telemetered tiltmeters and seismometers recorded detailed temporal changes associated with seven major eruptive phases along the east rift of kilauea volcano hawaii each eruptive phase was accompanied by subsidence of the summit region and followed by reinflation of the summit to approximately the same level before renewal of eruptive activity the cyclic summit tilt pattern and the absence of measurable tilt changes near the eruptive site suggest that conditions in the summit region controlled the timing of the last six eruptive phases the rate of summit subsidence progressively increased from one eruptive phase to the next during the last six phases the amplitude of harmonic tremor increased during the last four phases the increases in subsidence rate and in tremor amplitude suggest that frequent periods of magma movement have reduced the flow resistance of the conduit system between the summit and the rift zone

abrupt climate changes how freshening of the northern atlantic affects the thermohaline and wind driven oceanic circulations leading hypotheses for abrupt climate changes are focused on the ocean response to a freshening of surface waters in the north atlantic the degree to which such a freshening affects the deep slow thermohaline rather than the shallow swift wind driven circulations of the ocean and hence the degree to which that freshening affects climate in high rather than low latitudes differ from model to model depending on factors such as the treatment of diffusive processes in the oceans many comprehensive climate models are biased and confine the influence mainly to the thermohaline circulation and northern climates simulations of paleoclimates can provide valuable tests for the models but only some of those climates provide sufficiently stringent tests to determine which models are realistic

characteristics of seismic waves composing hawaiian volcanic tremor and gas piston events observed by a near source array a correlation method specifically designed for describing the characteristics of a complex wave field is applied to volcanic tremor and gas piston events recorded by a semicircular array of geos instruments set at the foot of the puu oo crater on the east rift of kilauea volcano hawaii the spatial patterns of correlation coefficients obtained as functions of frequency for the three components of motion over the entire array are similar for gas piston events and tremor and clearly depict dispersive waves propagating across the array from the direction of puu oo the wave fields are composed of comparable amounts of rayleigh and love waves propagating with similar and extremely slow phase velocities ranging from 700 m s at 2 hz to 300 m s at 8 hz the highly cracked solidified lava flow on which the array was deployed and subjacent structure of alternating lava and ash layers formed during repeated eruptions of puu oo since 1983 appear to be responsible for the low velocities observed the results from puu oo stand in sharp contrast to those obtained in an experiment conducted in 1976 on the partially solidified lava lake of kilauea iki rayleigh waves were not observed in kilauea iki but well developed trains of love waves were seen to propagate there with velocities twice as high as those observed near puu oo these differences in the propagation characteristics of surface waves at the two sites may be attributed to the presence of a soft horizontal layer of molten rock in kilauea iki which may have lowered the phase velocity of rayleigh waves more drastically than that of love waves resulting in severe scattering of the rayleigh wave mode on the other hand the thin superficial pahoehoe flow under our array at puu oo may have favored the development of vertical columnar joints more extensively at this location than at kilauea iki which may have reduced the shear moduli controlling the love wave mode the average phase velocities in the frequency band from 2 to 5 hz found at puu oo are roughly similar to those determined for tremor at klyuchevskoy volcano kamchatka but the frequency dependence appears much stronger at puu oo

on the role of interior mixing and air sea fluxes in determining the stratification and circulation of the oceans abstract the problem of determining the eastern boundary basic stratification and the buoyancy driven circulation of the oceans is addressed a global integral constraint relating the interior stratification and the air sea heat fluxes is derived based on the condition that the total mass of water of given density is constant in a steady state ocean this constraint is then applied to two simple analytic models the first is a continuous nonlinear diffusive model of the lower mid depth and bottom circulation below the influence of the wind driven circulation it shows the tendency of the vertical density profile to look like an exponential profile in the presence of mixing the integral constraint is used to relate the stratification and circulation of the bottom and mid depth waters to the air sea heat fluxes at the surface where the deep densities outcrop the second model is a layered one of the wind driven circulation mid depth and bottom water the eastern boundary stratification of the model i

dilution of the northern north atlantic ocean in recent decades declining salinities signify that large amounts of fresh water have been added to the northern north atlantic ocean since the mid 1960s we estimate that the nordic seas and subpolar basins were diluted by an extra 19 000 5000 cubic kilometers of freshwater input between 1965 and 1995 fully half of that additional fresh water about 10 000 cubic kilometers infiltrated the system in the late 1960s at an approximate rate of 2000 cubic kilometers per year patterns of freshwater accumulation observed in the nordic seas suggest a century time scale to reach freshening thresholds critical to that portion of the atlantic meridional overturning circulation

the pliocene paradox mechanisms for a permanent el nino during the early pliocene 5 to 3 million years ago globally averaged temperatures were substantially higher than they are today even though the external factors that determine climate were essentially the same in the tropics el nino was continual or permanent rather than intermittent the appearance of northern continental glaciers and of cold surface waters in oceanic upwelling zones in low latitudes both coastal and equatorial signaled the termination of those warm climate conditions and the end of permanent el nino this led to the amplification of obliquity but not precession cycles in equatorial sea surface temperatures and in global ice volume with the former leading the latter by several thousand years a possible explanation is that the gradual shoaling of the oceanic thermocline reached a threshold around 3 million years ago when the winds started bringing cold waters to the surface in low latitudes this introduced feedbacks involving ocean atmosphere interactions that along with ice albedo feedbacks amplified obliquity cycles a future melting of glaciers changes in the hydrological cycle and a deepening of the thermocline could restore the warm conditions of the early pliocene

relationship between tremor and volcanic activity during the southeast crater eruption on mount etna in early 2000 1 the southeast crater of mount etna italy was characterized by a violent eruptive activity between 26 january and 24 june 2000 this activity produced 64 lava fountain episodes with repose periods from 3 hours to 10 days we estimated a volume of about 15 20 106 m3 lava and at least 2 3 106 m3 of tephra we compared the paroxysmal volcanic activity to its associated seismic signature the high number of events highlighted a strict correlation between tremor and volcanic activity seismic and volcanic characteristics such as the frequency of occurrence the duration of lava fountains and the associated tremor energy suggested the subdivision of the studied period into two stages separated by the 20 february event combining volcanic with seismic data we observed some useful relationships among lava fountain height sustained column height and reduced displacement in addition we found that the entire episode was well correlated with the duration of the amplitude increase computing the tremor energy linked to each event the total energy associated with lava fountains episodes results in 76 of the energy released during the whole period finally the different ratios among the overall spectral amplitude of the seismic signals of the stations located at different altitudes suggested to us the elaboration of a simple qualitative model to explain the dynamic behavior of the tremor source during the whole episode

evidence for enhanced mixing over rough topography in the abyssal ocean the overturning circulation of the ocean plays an important role in modulating the earth s climate but whereas the mechanisms for the vertical transport of water into the deep ocean deep water formation at high latitudes and horizontal transport in ocean currents have been largely identified it is not clear how the compensating vertical transport of water from the depths to the surface is accomplished turbulent mixing across surfaces of constant density is the only viable mechanism for reducing the density of the water and enabling it to rise however measurements of the internal wave field the main source of energy for mixing and of turbulent dissipation rates have typically implied diffusivities across surfaces of equal density of only approximately 0 1 cm2 s 1 too small to account for the return flow here we report measurements of tracer dispersion and turbulent energy dissipation in the brazil basin that reveal diffusivities of 2 4 cm2 s 1 at a depth of 500 m above abyssal hills on the flank of the mid atlantic ridge and approximately 10 cm2 s 1 nearer the bottom this amount of mixing probably driven by breaking internal waves that are generated by tidal currents flowing over the rough bathymetry may be large enough to close the buoyancy budget for the brazil basin and suggests a mechanism for closing the global overturning circulation

rapid freshening of the deep north atlantic ocean over the past four decades the overflow and descent of cold dense water from the sills of the denmark strait and the faroe shetland channel into the north atlantic ocean is the principal means of ventilating the deep oceans and is therefore a key element of the global thermohaline circulation most computer simulations of the ocean system in a climate with increasing atmospheric greenhouse gas concentrations predict a weakening thermohaline circulation in the north atlantic as the subpolar seas become fresher and warmer1 2 3 and it is assumed that this signal will be transferred to the deep ocean by the two overflows from observations it has not been possible to detect whether the ocean s overturning circulation is changing but recent evidence suggests that the transport over the sills may be slackening4 here we show through the analysis of long hydrographic records that the system of overflow and entrainment that ventilates the deep atlantic has steadily changed over the past four decades we find that these changes have already led to sustained and widespread freshening of the deep ocean

array tracking of the volcanic tremor source at deception island antarctica we have found experimental evidence which shows that the volcanic tremor recorded at deception island south shetland islands antarctica is a superposition in time of overlapping hybrid events we studied data from a small aperture seismic array data analysis for tremor and hybrids included 1 spectral analysis 2 apparent slowness and back azimuth determination by using the zero lag cross correlation method and 3 polarization analysis both types of events share these common features a two dominant spectral bands at frequencies 1 3 hz the most energetic and 4 8 hz b several coherent phases with the same back azimuth to the source and apparent slowness along the whole signal c in the high frequency band the apparent slowness is very low around 0 17 s km indicating the propagation of body waves d in the low frequency band the apparent slowness is high around 1 6 s km consistent with the presence of surface waves and e clear p wave onset followed by a complex pattern of rayleigh waves therefore both types of events are strongly related because they share the same source region the same wave propagation properties and the same wave composition moreover several arrivals that resemble a single hybrid event have been found along the tremor signals due to these reasons we hypothesize that volcanic tremor of deception island is a superposition of hybrid type events the source of both types could be the interaction between thaw water and hot materials in a shallow aquifer

volcanic tremor at pavlof volcano alaska october 1973 april 1986 in thirteen years 1973 1986 of seismic monitoring of pavlof volcano 488 episodes of volcanic tremor have been recorded only 26 of which have been previously described in the literature this paper tabulates and describes all the tremor episodes and reports on the results of all analyses to date pavlof tremor durations range from 2 minutes to greater than 1 week episodes accompanying magmatic eruptions have durations greater than 1 hour and sustained amplitudes of greater than 6 mmp p 54 nanometers at 1 5 hz on station pvv 8 5 km from the vent digital data provide much better amplitude resolution than helicorders do helicorders however provide continuous coverage whereas digital data are intermittent correlations of tremor with visual eruption observations shows that tremor amplitudes are roughly correlated with heights of lava fountains but the correlation of tremor amplitudes with plume heights is more problematic fast fourier transform fft spectra show that pavlof tremor is quite statinary for the entire time period 1973 1983 all principal spectral peaks lie between 0 8 and 3 0 hz and may be caused by resonance of magma and gas and resonance of the volcanic pile preliminary analysis of 2 and 3 component data shows thatp s pl and rayleigh waves may be present in pavlof volcanic tremor other waveforms can be misidentified as tremor most commonly those caused by storms ors waves of regional earthquakes a strategy is proposed to distinguish tremor from noise using automatic seismic data acquisition and analysis systems pavlof s volcanic tremor is briefly compared with a preliminary sample of over 1100 cases of tremor from 84 volcanoes worldwide finally several recommendations for monitoring and reporting volcanic tremor are discussed

interpolation and gridding of aliased geophysical data using constrained anisotropic diffusion to enhance trends geophysical data are frequently collected with a fine sample interval along traverse lines but with a coarser sampling in the direction perpendicular to the traverses this disparity in sampling intervals is particularly evident when magnetic data are collected simultaneously with airborne electromagnetic data interpolating this traverse data onto an evenly spaced 2d grid can result in aliasing artifacts for example narrow linear structures that trend at acute angles to the traverse lines are imaged as a thick thin thick feature looking like a boudinage or string of beads applying the anisotropic diffusion process to the resulting grids of data removes the artifacts but the grid values close to the traverses are altered significantly from their initial values the altered values are therefore not faithful to the original traverse data the anisotropic diffusion algorithm can be modified to constrain values close to the original traverses this modification removes the aliasing artifacts and produces a data grid faithful to the original traverse data some small artifacts along the traverse lines in the final data grid become more evident when grids containing derivative data such as the analytic signal are generated from the new data grid however these small traverse line artifacts can be removed with standard microleveling procedures the constrained anisotropic diffusion process is iterative and some experimentation is required to determine the appropriate number of iterations

seismic attenuation and shallow velocity structures at stromboli volcano italy the surface wave field associated with the explosive activity at stromboli volcano is investigated using data recorded by two short period seismic arrays deployed on the north and west flanks of the volcano the group velocity dispersion curves for rayleigh waves are derived using the multiple filter technique the phase velocity dispersion curves are recovered using a phase match filter and compared with that inferred from zero lag cross correlation analysis applied to the array data these analyses indicate rayleigh wave group velocities ranging from 0 29 to 0 24 km sec in the 1 5 to 8 0 hz frequency band and phase velocities ranging from 1 km sec at 1 5 hz to about 0 3 km sec at frequencies above 5 hz in addition the dispersive properties of the attenuation coefficient for rayleigh waves are inferred from application of the multiple filter technique to seismograms recorded at different distances from the source these results are validated through examination of the spectral amplitude decay with distance for both body and rayleigh waves the values of the body wave quality factor thus obtained are q 20 and q 6 for the north and west side of the island respectively the velocity and attenuation dispersion curves are inverted for the shear wave velocity and q structures down to a depth of about 200 m shear wave velocities for the west flank range from about 0 3 km sec for the uppermost 17 m thick layer to 1 9 km sec at depths greater than 200 m comparison with previous studies indicates a similar velocity structure for the north and west flanks the attenuation structure for the west flank is described by a shallower 36 m thick layer with q 9 underlain by a half space with q 50 on the north flank q 40 for the shallower 30 m thick layer and q 44 for the underlying half space residuals from analysis of the spectral decay with distance are used to quantify site effects affecting the different array elements on the west flank local amplifications at that array are interpreted in terms of an edge effect associated with concave topography velocity similarities observed at the north and west flanks are compatible with surface geologic data discrepancies in attenuation properties at the two sites are interpreted in terms of different degrees of heterogeneity and crack density controlling the scattering quality factor q s manuscript received 15 april 2001

approach to the volcanic tremor by covariance analysis application to the 1989 eruption of mt etna sicily polarization of the tremor wavefield recorded during three periods of the 1989 eruption of mt etna at a temporary array of three component stations is analyzed by use of a polarization filter the tremor shows steady horizontal near linear west east polarization at all stations such a behaviour which could be coherent with a p wavefield radiated by a vertical extended source with overall north south alignment excludes confinement of the source to the only active vent and is tentatively linked to activity of the main dyke system of mt etna

kinematic parameters of the tremor wave field at mt etna sicily the tremor wave field at mt etna was recorded with an array of three 3 component broad band seismometers the analysis of its polarisation and dispersion properties leads to the conclusions that 1 the wave field consists mainly of surface waves and that 2 two of the observed spectral peaks are caused by a wave guide effect along the propagation path

slowing of the atlantic meridional overturning circulation at 25 n the atlantic meridional overturning circulation carries warm upper waters into far northern latitudes and returns cold deep waters southward across the equator its heat transport makes a substantial contribution to the moderate climate of maritime and continental europe and any slowdown in the overturning circulation would have profound implications for climate change a transatlantic section along latitude 25 n has been used as a baseline for estimating the overturning circulation and associated heat transport here we analyse a new 25 n transatlantic section and compare it with four previous sections taken over the past five decades the comparison suggests that the atlantic meridional overturning circulation has slowed by about 30 per cent between 1957 and 2004 whereas the northward transport in the gulf stream across 25 n has remained nearly constant the slowing is evident both in a 50 per cent larger southward moving mid ocean recirculation of thermocline waters and also in a 50 per cent decrease in the southward transport of lower north atlantic deep water between 3 000 and 5 000 m in depth in 2004 more of the northward gulf stream flow was recirculating back southward in the thermocline within the subtropical gyre and less was returning southward at depth

the great salinity anomaly in the northern north atlantic 1968 1982 the widespread freshening of the upper 500 800m layer of the northern north atlantic which this paper describes represents one of the most persistent and extreme variations in global ocean climate yet observed in this century though a range of explanations have been advanced to explain this event including in situ changes in the surface moisture flux this paper describes the great salinity anomaly as largely though not entirely an advective event traceable around the atlantic subpolar gyre for over 14 years from its origins north of iceland in the mid to late 1960s until its return to the greenland sea in 1981 1982 the overall propagation speed around this subpolar gyre is estimated at about 3cm s 1 of the total salt deficit associated with the anomaly as it passed south along the labrador coast in 1971 1973 about 72 109 tonnes a deficit equivalent to about two thirds of this figure 47 109 tonnes ultimately passed through the faroe shetland channel to the barents sea arctic ocean and greenland sea during the mid 1970s possible effects on deep water formation and on the representativeness of historical section data are discussed

the freshening of surface waters in high latitudes effects on the thermohaline and wind driven circulations abstract the impacts of a freshening of surface waters in high latitudes on the deep slow thermohaline circulation have received enormous attention especially the possibility of a shutdown in the meridional overturning that involves sinking of surface waters in the northern atlantic ocean a recent study by fedorov et al has drawn attention to the effects of a freshening on the other main component of the oceanic circulation the swift shallow wind driven circulation that varies on decadal time scales and is closely associated with the ventilated thermocline that circulation too involves meridional overturning but its variations and critical transitions affect mainly the tropics a surface freshening in mid to high latitudes can deepen the equatorial thermocline to such a degree that temperatures along the equator become as warm in the eastern part of the basin as they are in the west the tropical zonal sea surface temperature gradient virtually disappears and permanently warm conditions prevail

excitation of a buried magmatic pipe a seismic source model for volcanic tremor recent observations of seismic events at various volcanoes suggest that harmonic tremor results from the sustained occurrence of so called long period or low frequency events accordingly we can view the long period volcanic event as the elementary process of tremor and interpret it as the impulse response of the tremor generating system we present a seismic model in which the source of tremor is the acoustic resonance of a fluid filled volcanic pipe triggered by excess gas pressure the model consists of three elements namely a triggering mechanism a resonator and a radiator for simplicity we assume a hemispherical trigger cylindrial resonator and circular radiator set in a vertical configuration with the trigger capping the top of the pipe and the disk shaped radiator shutting off its bottom considering the simple case of a source buried in a homogeneous half space we then apply the discrete wave number method to obtain a complete representation of the ground motion response at near and intermediate distances the results demonstrate that the displacement attributed to the pipe dominates the near field motion while that due to the disk is representative of the intermediate and far fields the trigger itself has a smaller contribution mainly limited to the field in the proximity of the source the characteristics displayed by the free surface response evolve from a strong impulsive signature in the immediate vicinity of the epicenter to a well developed harmonic wave train dominated by rayleigh waves at larger distances no clear shear arrival can be detected in the synthetic seismograms the displacement spectrum reflects the organ pipe modes of the conduit and the bandwidth associated with the dominant spectral peak of motion is controlled by the combined losses due to viscous attenuation in the fluid and elastic radiation into the solid in the case of the cylindrical magma column considered the radiation loss is proportional to the square of the pipe radius while the loss related to viscous damping is inversely proportional to the same factor indicating that the relative importance of the two loss mechanisms is critically dependent on the geometry of the magma reservoir the relative importance of the pipe and disk elements likewise is a function of the conduit cross section this suggests the possibility of determining the geometry of the source as well as the radiation loss and in situ magma viscosity from a comparison of near and far field observations

volcanic tremor wave field during quiescent and eruptive activity at mt etna sicily abstract this paper reports the results of an analysis of volcanic tremor at etna in the years 1987 to 1992 the data utilized were obtained from the osservatorio vesuviano s serra la nave permanent station and several seismic surveys carried out by means of portable digital seismic stations equipped with three component geophones the spatial temporal patterns of main parameters such as intensity spectral content polarization and wave composition of volcanic tremor in different phases of etna s activity were studied eruptive activity appears to be significantly correlated with volcanic tremor in particular it is worth pointing out that the magma upwelling in the terminal part of the conduit is characterized by a marked increase in low frequency components 3 4 hz of the spectrum of volcanic tremor hence volcanic tremor at etna can be considered as a short period precursor of eruptions an analysis of polarization and propagation velocity of volcanic tremor shows the presence of love waves overlapping other seismic phases

abrupt climate change an alternative view hypotheses and inferences concerning the nature of abrupt climate change exemplified by the dansgaard oeschger d o events are reviewed there is little concrete evidence that these events are more than a regional greenland phenomenon the partial coherence of ice core y 18 o and ch4 is a possible exception claims however of d o presence in most remote locations cannot be distinguished from the hypothesis that many regions are just exhibiting temporal variability in climate proxies with approximately similar frequency content further suggestions that d o events in greenland are generated by shifts in the north atlantic ocean circulation seem highly implausible given the weak contribution of the high latitude ocean to the meridional flux of heat a more likely scenario is that changes in the ocean circulation are a consequence of wind shifts the disappearance of d o events in the holocene coincides with the disappearance also of the laurentide and fennoscandian ice sheets it is thus suggested that d o events are a consequence of interactions of the windfield with the continental ice sheets and that better understanding of the wind field in the glacial periods is the highest priority wind fields are capable of great volatility and very rapid global scale teleconnections and they are efficient generators of oceanic circulation changes and more speculatively of multiple states relative to great ice sheets connection of d o events to the possibility of modern abrupt climate change rests on a very weak chain of assumptions

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

polarisation analysis of the independent components of low frequency events at stromboli volcano eolian islands italy abstract independent component analysis ica applied on low frequency lf events recorded at stromboli volcano reveals that these signals are composed of three independent components in three different frequency bands approximately 0 8 1 2 2 4 3 0 3 2 4 5 hz respectively the first two frequency ranges are mainly composed of body waves coming from a direction in a range of 30 30 around the crater area this result is a clear indication that the entire signal in these frequency bands comes from the source area in fact any scattered wave should have different back azimuth propagation due to the random distribution of scatterers location however the last independent component is dominated by other kinds of waves coming from many different directions

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

modelling of volcanic tremor wave fields abstract the comparison of the temporal variations of the volcanic tremor for various eruptions shows stable connection between tremor intensity and volcanic activity we have determined the wave pattern of seismic signals produced by individual explosions in the crater during eruption of the tolbachik volcano kamchatka considering an explosion in the crater as the source for formation of the seismic wave field we determined the relation between the spectrum of the source and volcanic tremor spectra theoretical seismograms for the unit source of explosion type in various medium models correlate well with the real volcanic tremor seismograms and explain the formation of the volcanic tremor wave fields in real conditions volcanic tremor frequency range of 0 6 5 hz forms as a result of disturbances near the surface in the active crater explosions pressure variations at sustained flow etc and consist mostly of surface waves a comparative estimation of transfer functions for the system explosion in the crater volcanic tremor and for theoretical models with the isolated source of explosion type has been done

restoration of missing offsets by parabolic radon transform1 restoration of missing offsets and trace interpolation is an interesting and important problem in seismic data processing based on the parabolic radon transform a method is presented for missing offset restoration resampling and regularization of prestack individual cmp gathers the method is also valid for resampling spatially aliased seismic data the method is based on the parabolic assumption of the seismic events which is generally verified after a partial nmo correction in the cmp organization of the data the essence of the method consists of a band limited forward parabolic radon transform of the data containing zero traces at the missing offset locations the curvature range is chosen to map properly the coherent energy while the zero traces map beyond that range after inverse transform the originally zero traces are partly filled with information several iterations of forward and inverse transform every time replacing the zero traces in the original gather with the partially reconstructed ones almost fully restore the zero traces efficient and fast algorithms can be built up to process data having a uniform geometry examples on synthetic as well as on field data demonstrate clearly the robustness of the method

permanent tremor of masaya volcano nicaragua wave field analysis and source location the masaya volcano nicaragua is a basaltic caldera in a subduction zone the permanent source of the volcanic tremor was located inside santiago crater at the lava lake s position and 400 m below the ne rim and therefore corresponds to superficial magma activity we used two tripartite arrays 90 m side one semicircular array r 120 m in 1992 and two semicircular arrays r 60 m and a 2500 m long linear array radiating out from the source and on the flank of the crater in 1993 we used both a cross spectrum method and a correlation method to determine the wave delay time between the reference station and the other stations of an array and to quantify the wave field using the delays therefore by intersecting the back azimuth wave directions from the arrays we could pinpoint the source additionally the correlation coefficients obtained as functions of frequency for the three components of motion confirm the inferred position of the source of tremor the tremor s wave field is composed of comparable quantities of dispersed rayleigh and love surface waves whose phase velocities lie in the ranges 730 1240 m s at 2 hz and 330 550 m s at 6 hz the dispersive phase velocities were inverted to obtain crustal structures with a minimal number of layers the resulting velocity models are similar for the northern and southern parts of the volcano after geometrical spreading corrections q2hz 14 and q3hz 31 were determined along the northern linear array the typical low velocities and low q corresponding to the cone structure and are similar to those of other basaltic volcanoes like puu oo hawaii and klyuchevskoy kamchatka

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

seasonality and increasing frequency of greenland glacial earthquakes some glaciers and ice streams periodically lurch forward with sufficient force to generate emissions of elastic waves that are recorded on seismometers worldwide such glacial earthquakes on greenland show a strong seasonality as well as a doubling of their rate of occurrence over the past 5 years these temporal patterns suggest a link to the hydrological cycle and are indicative of a dynamic glacial response to changing climate conditions

effects of glacial meltwater in the giss coupled atmosphereocean model 1 north atlantic deep water response varying magnitudes of freshwater input through the st lawrence are added to different versions of the giss coupled atmosphere ocean model the results show a generally linear response in percentage north atlantic deep water nadw reduction to the volume of water added without any obvious threshold effects when the estimated best guess freshwater input for the allerod younger dryas interval is added only small reductions in nadw production occur in less than a century with complete cessation requiring more than 300 years if the ocean circulation was weaker in the allerod preceding the younger dryas somewhat larger percentage reductions occur and it would take 150 200 years for a complete shutdown the freshening is aided by a positive feedback associated with the reduction in evaporation relative to precipitation in the north atlantic amounting to some 20 30 of the added freshwater runoff when the freshwater input is stopped after complete nadw shutdown nadw production does not resume with complete nadw cessation cooling eventually occurs throughout the northern hemisphere including regions where it is not thought to have arisen during the younger dryas and the globe cools by 1 c due primarily to the sea ice albedo feedback while in many regions the observed cooling is reproduced the cooling in greenland is much less than is estimated with only 50 nadw cessation cooling is primarily around the north atlantic no rapid warming is seen in response to freshwater cessation the sensitivity displayed here for nadw reduction due to freshwater input from the st lawrence is roughly similar to that associated with increased precipitation accompanying global warming in the next century so a similar sensitivity may arise with younger dryas freshwater inputs from other suspected locations greenland europe

a theory for the surface atlantic response to thermohaline variability abstract the response of the upper warm limb of the thermohaline circulation in the north atlantic to a rapid change in deep water formation at high latitudes is investigated using a reduced gravity ocean model changes in deep water formation rate initiate kelvin waves that propagate along the western boundary to the equator on a timescale of months the response in the north atlantic is therefore rapid the southern hemisphere response is much slower limited by a mechanism here termed the ldquo uatorial buffer since to leading order the flow is in geostrophic balance the pressure anomaly decreases in magnitude as the kelvin wave moves equatorward where the coriolis parameter is lower together with the lack of sustained pressure gradients along the eastern boundary this limits the size of the pressure field response in the southern hemisphere interior adjustment is by the westward propagation of rossby waves but only a small fraction of the change in thermohaline circulation strength is commun

toward continuous quantification of lava extrusion rate results from the multidisciplinary analysis of the 2 january 2010 eruption of piton de la fournaise volcano la reunion the dynamics of the 2 12 january 2010 effusive eruption at piton de la fournaise volcano were examined through seismic and infrasound records time lapse photography so2 flux measurements deformation data and direct observations digital elevation models were constructed for four periods of the eruption thus providing an assessment of the temporal evolution of the morphology the volume and the extrusion rate of the lava flow these data were compared to the continuous recording of the seismic and infrasonic waves and a linear relationship was found between the seismic energy of the tremor and the lava extrusion rate this relationship is supported by data from three other summit eruptions of piton de la fournaise and gives total volume and average lava extrusion rate in good agreement with previous studies we can therefore provide an estimate of the lava extrusion rate for the january 2010 eruption with a very high temporal resolution we found an average lava extrusion rate of 2 4 m3s 1 with a peak of 106 6 m3s 1 during the initial lava fountaining phase we use the inferred average lava extrusion rate during the lava fountaining phase 30 23 m3s 1 to estimate the value of the initial overpressure in the magma reservoir which we found to range from 3 7 106 pa to 5 9 106 pa finally based on the estimated initial overpressure the volume of magma expelled during the lava fountaining phase and geodetic data we inferred the volume of the magma reservoir using a simple mogi model between 0 25 km3 and 0 54 km3 which is in good agreement with previous studies

superlakes megafloods and abrupt climate change about 8200 years ago the climate of much of the northern hemisphere cooled abruptly for a period of about 200 years in their perspective clarke et al examine the most likely culprit for this cooling an outburst of fresh water from a vast ice dammed glacial lake in north america the superlake had formed when the kilometers thick ice sheet covering much of north america disintegrated when the ice dam became unstable fresh water flooded from the lake into the north atlantic it remains unclear how this fresh water affected ocean circulation or whether the outburst occurred in more than one stage but the timing points strongly to the outburst flood as the trigger of the 8200 year climate event

gas bubble dynamics model for shallow volcanic tremor at stromboli volcanic tremor at stromboli aeolian islands italy is correlated to small infrasonic transients ripepe et al 1996 which repeat almost rhythmically in time in a range between 0 8 and 1 2 s we demonstrate that infrasonic transients are associated to small gas bubble 0 5 m burstings which produces no transients in the seismic signal tremor ground displacement attenuates with the inverse of the distance from the craters indicating that the source is shallow short term energy release shows that infrasonic and seismic signals are linked to the same dynamical process while at the long term scale it is evident that the two signals are controlled by two distinctive mechanisms we suggest that the possible physical model acts in two steps first gas coalescence and then gas bursting in our model the seismic signal is related to the coalescence of a gas bubble from a layer of small bubbles while the infrasonic signal is linked to the bursting of the bubble when it reaches the magma surface gas bubbles could form by free coalescence in magma or could be forced to coalesce by a structural barrier we calculate that forced coalescence induces in magma a pressure change 104 pa 2 orders of magnitude higher than free coalescence and it explains best the tremor ground displacement 10 5 m recorded at stromboli moreover forced coalescence evidences the role of a structural barrier such as a dike in volcanic tremor source dynamics in this gas dynamic process the delay time of 1 2 s between infrasonic pulses could reflect the gas nucleation interval of basaltic magma thomas et al 1993 manga 1996 we propose that the source function for the shallow volcanic tremor at stromboli could be the viscoelastic reaction of the magma to the pressure decrease induced by gas bubble growth rate under constant depressurization the spectrum of our source function is controlled by the time duration of the pressure pulse which represents the viscoelastic relaxation time of the magma and gas bubble growth rate the predicted asymptotic decay of the frequency contents fits the spectral behavior of the volcanic tremor ground displacement recorded at stromboli we show that the same spectral behavior can be found in ground displacement spectra of volcanic tremor recorded on different volcanoes

array measurements and modeling of sources of shallow volcanic tremor at kilauea volcano hawaii we show that resonances in near surface layers path effects or a combination of near surface resonances and source resonances source effects could be responsible for the sharp spectral peaks found in the records of gas piston activity and volcanic tremor recorded near puu oo crater on kilauea volcano hawaii two types of sources are found to be compatible with observations and can not be distinguished using our data in one model impulsive explosion point sources are embedded in the puu oo structure in the other model impulsive point sources of pressure excite the resonances of a fluid filled crack embedded in that structure both models require shallow sources z 100 m and display a strong dependence of the radiated spectrum on source depth over a depth range comparable to the overall thickness of the surficial layers based on these results it may be possible to track changes in source depth through temporal changes in observed spectra a potentially useful tool for monitoring volcanic activity at this site we estimate the depth and spatial extent of volcanic tremor and gas piston activity using two dense arrays with respective apertures of 800 and 120 m located near puu oo measurements of slowness ray parameter and azimuth as a function of time clearly indicate that the sources of volcanic tremor and gas piston activity are located beneath or in close proximity to the puu oo crater at depths shallower than approximately 1 km based on slowness and particle motion analyses we find that the records of volcanic tremor and gas piston events at puu oo consist of a complex combination of body and surface waves

constraints on the duration and freshwater release of heinrich event 4 through isotope modelling heinrich events1 abrupt climate cooling events due to ice sheet instability that occurred during the last glacial period are recorded in sediment cores throughout the north atlantic ocean2 3 modelling studies have described likely physical mechanisms4 5 6 for these events but the quantitative characteristics of heinrich events are less well known here we use a climate model of intermediate complexity7 that explicitly calculates the distribution of oxygen isotopes in the oceans to simulate heinrich event 4 at about 40 000 yr ago we compare an ensemble of scenarios for this heinrich event with oxygen isotope data measured in foraminiferal calcite of a comprehensive set of sediment cores8 9 from this comparison we obtain a duration of 250 150 yr and an ice release of 2 1 m sea level equivalent for heinrich event 4 significantly reducing the uncertainties in both values compared to earlier estimates5 10 11 12 13 14 of up to 2 000 yr and 15 m of sea level equivalent ice release respectively our results indicate that the consequences of heinrich events may have been less severe than previously assumed at least with respect to greenland climate and sea level