combined discrete and continuous gravity observations at mount etna systematic investigation of discrete gravity measurements has continued at mount etna since 1986 the network now covers an area of 400 km2 with about 70 stations 0 5 3 km apart mass redistributions occurring at depths ranging between about 8 km below sea level and a few hundred metres below the surface magma level changes within the shallower parts of the feeding conduits have been identified from these data conventional discrete microgravity monitoring on a network of stations furnishes only instantaneous states of the mass distribution at continuously active systems in order to obtain information on the rate at which the volcanic processes and thus mass transfers occur three stations for continuously recording gravity where installed on mount etna in 1998 a 16 month long sequence from one of the continuously running stations pdn located 2 km from the active northeast crater at the summit of etna volcano is presented after removing the effects of earth tide and tilt the correlation of the residual gravity sequence with simultaneous recordings of meteorological parameters acquired at the same station was analysed once the meteorological effects have also been removed continuous gravity changes are within 10 gal of gravity changes measured using conventional microgravity observations at sites very close to the continuous station this example shows how discrete and continuous gravity observations can be used together at active volcanoes to get a fuller and more accurate picture of the spatial and temporal characteristics of volcanic processes

ground deformation and gravity changes on the island of pantelleria in the geodynamic framework of the sicily channel abstract the island of pantelleria is an active volcano located in the sicily channel southern italy in the middle of a continental rift system since the 1980s the island was periodically surveyed by using geodetic techniques edm levelling gps and high precise gravimetry to monitor the regional and local volcanic dynamics gravity data collected between 1990 and 1998 show short and long wavelength changes due to the combined effect of shallow and deep sources they reflect to some degree the structural setting of the island as delineated by the bouguer anomaly field which indicates that the island is broken up into two main basement blocks the latter are bordered by two lineaments probably regional faults related to the global geodynamics of the sicily channel rift zone moreover the inverse correlation between the gravity and altimetric variations suggests that i pantelleria is kinematically divided in two blocks ii the observed behaviour is strongly influenced by the geodynamics of the sicily channel a new interpretation of the fully reprocessed data sets is presented focusing on the spatial temporal features of the horizontal ground deformation and gravity changes compared to the bouguer anomaly and altimetric data this leads to conclude that volcanism on the island has been probably strongly influenced by the global geodynamics of the sicily channel and future eruptions are most likely to occur at the structural boundary separating the two blocks

review of microgravity observations at mt etna a powerful tool to monitor and study active volcanoes microgravity observations at mt etna have been routinely performed as both discrete since 1986 and continuous since 1998 measurements in addition to describing the methodology for acquiring and reducing gravity data from mt etna this paper provides a collection of case studies aimed at demonstrating the potential of microgravity to investigate the plumbing system of an active volcano and detect forerunners to paroxysmal volcanic events for discrete gravity measurements results from 1994 1996 and 2001 are reported during the first period the observed gravity changes are interpreted within the framework of the strombolian activity which occurred from the summit craters gravity changes observed during the first nine months of 2001 are directly related to subsurface mass redistributions which preceded accompanied and followed the july august 2001 flank eruption of mt etna two continuous gravity records are discussed a 16 month october 1998 to february 2000 sequence and a 48 hour 26 28 october 2002 sequence both from a station within a few kilometers of the volcano s summit the 16 month record may be the longest continuous gravity sequence ever acquired at a station very close to the summit zone of an active volcano by cross analyzing it with contemporaneous discrete observations along a summit profile of stations both the geometry of a buried source and its time evolution can be investigated the shorter continuous sequence encompasses the onset of an eruption from a location only 1 5 km from the gravity station this gravity record is useful for establishing constraints on the characteristics of the intrusive mechanism leading to the eruption in particular the observed gravity anomaly indicates that the magma intrusion occurred passively within a fracture system opened by external forces

gravity changes and present day dynamics of the island of pantelleria sicily channel italy abstract the island of pantelleria is an active volcano located in the sicily channel southern italy occurring in the middle of a continental rift system since the 1980 s the island has been periodically surveyed by means of geodetic and geophysical methods to monitor the regional and local volcanic dynamics also high precision gravity measurements were started in 1990 the present paper is an analysis of the time space gravity changes gravity measurements were carried out on a network presently formed by twenty stations the gravity network was fully surveyed in june 1990 and june 1995 and partially surveyed in september 1993 two absolute gravity stations were established in 1993 to provide a reference system and to check for long term variations the areal distribution of the gravity changes during the 1990 1995 time interval obtained in the thirteen stations of the 1990 network is strongly similar to the bouguer anomaly field and to the large scale features of the basement of the island otherwise gravity changes are directly correlated with the bouguer anomaly and inversely correlated with the altimetric variations comparison with the geological setting suggests that the present activity may be ascribed to the influence of the geodynamics of the sicily channel

gravity effect of water storage changes in a weathered hard rock aquifer in west africa results from joint absolute gravity hydrological monitoring and geophysical prospection advances in groundwater storage monitoring are crucial for water resource management and hydrological processes understanding the evaluation of water storage changes wsc often involve point measurements observation wells moisture probes etc which may be inappropriate in heterogeneous media over the past few years there has been an increasing interest in the use of gravimetry for hydrological studies in the framework of the ghyraf gravity and hydrology inafrica project 3 yr of repeated absolute gravity measurements using a fg5 type gravimeter have been undertaken at nalohou a sudanian site in northern benin hydrological data are collected within the long term observing system amma catch once corrected for solid earth tides ocean loading air pressure effects polar motion contribution and non local hydrology seasonal gravity variations reach up to 11 gal equivalent to a wsc of 260 mm thick infinite layer of water absolute temporal gravity data are compared to wsc deduced from neutron probe and water table variations through a direct modelling approach first we use neutronic measurements available for the whole vertical profile where wsc occur the vadose zone and a shallow unconfined aquifer the rmsd between observed and modelled gravity variations is 1 61 gal which falls within the error bars of the absolute gravity data second to acknowledge for the spatial variability of aquifer properties we use a 2 d model for specific yield sy derived from resistivity mapping and magnetic resonance soundings mrs the latter provides a water content mrs known to be higher than the specific yield hence we scaled the 2 d model of mrs with a single factor wsc are calculated from water table monitoring in the aquifer layer and neutronic measurements in the vadose layer the value of is obtained with amonte carlo sampling approach minimizing the rmsd between modelled and observed gravity variations this leads to sy mrs 0 63 0 15 close to what is found in the literature on the basis of pumping tests experiments with a rmsd value of 0 94 gal this hydrogeophysical experiment is a first step towards the use of time lapse gravity data as an integrative tool to monitor interannual wsc even in complicated subsurface distribution

atmospheric pressure and gravity summary gravity green s functions for a column load of a model atmosphere on a spherical elastic earth are presented and they are used to evaluate the contribution of global atmospheric pressure variations to local gravity the green s functions are found to be relatively insensitive to the details of the model atmosphere but they are dependent on the temperature at the base of the column and on the relative height difference between the base of the column and the gravity station the total signal that global pressure systems contribute to gravity is about 30 gal of which about 90 per cent is produced by the atmosphere within 50 km of the gravity station a zone between 50 and 1000km from the gravity station contributes a couple of gal as does the remainder of the globe this pattern the coherence scale of pressure fluctuations the time and spatial scales appropriate to the hydrostatic approximation and the distance of the gravity station from the oceans suggest a division of the globe into local regional and global zones data requirements processing details and the reliability of the computed signal are different in each zone the local zone is within about 50 km of the gravity station within this zone pressure changes rapidly in time but is spatially coherent so that hourly observations of pressure and temperature at the gravity site alone are sufficient to compute an accurate correction except when a front is passing through the regional zone extends from the edge of the local zone to between several hundred and a thousand kilometres the signal from this zone is small and is only weakly coherent with the signal from the central zone so that a rather sparse array of hourly samples of pressure and temperature are required the gravity signal from the global zone can reach about a gal it varies on a time scale of days and is influenced by the response of the oceans to pressure variations previously reported observations that the admittance between local pressure and gravity residuals depends on epoch frequency or site are most probably due to incorrect modelling a proper local regional temperature and global correction can adequately account for the gravity signal from the atmosphere to within a few tens of ngal in the diurnal band and about 100 ngal in the days to seasonal band except during extreme weather conditions the application of the local correction lowers the power spectral density of the gravity residuals in every band from seasonal to hourly the regional global and temperature corrections lower the residual noise in the seasonal and synoptic bands but are not consistently effective at periods less than about half a day

global atmospheric loading and gravity abstract the purpose of this paper is to model atmospheric pressure effects on surface gravity pressure corrections are usually done with the help of an empirical trans fer function called barometric admittance between pressure and gravity both measured locally we show the validity of this local approach with the help of spectral coherency between air pressure and gravity residuals after subtraction from the superconducting gravimeter observations in strasbourg france of luni solar tides polar motion and instrumental drift contributions we compute the atmospheric surface loading effects using a green s function formalism and global surface pressure data provided by the european centre for medium range weather forecasts ecmwf we model a specific oceanic response to atmospheric loading which incorporates interactions between ocean atmosphere and solid earth and differs from the classical inverted or non inverted barometer hypotheses we investigate the contribution of this global pressure correction in a 3000 day 1987 1996 tidal analysis of the french superconducting gravimeter

on isc pp and pp data and their use in delay time tomography of the caribbean region summary a major problem in p delay time tomography is the inhomogeneous sampling of mantle structure by the p wave ray paths resulting in low resolution in images of large regions of the upper mantle incorporation of pp and pp phases can improve the quality and reliability of tomographic images because they 1 sample earth structure not ordinarily sampled by direct p phases 2 add rays that are oblique to rays of direct phases which is especially important where the latter sample mantle structure in selected directions and 3 pp data better constrain the earthquake focal depths pp traveltimes have often been used in combination with p data in differential traveltime studies we show that the assumptions and approximations necessary for this approach are problematic and that they can be avoided when the p pp and p data are used in tomographic inversion we investigated the applicability of pp and pp delay times to the tomographic study of the aspherical mantle structure below the caribbean region the success of the application of data of the later arriving reflected waves depends critically on the quality of these data we examined possible sources of error in the isc pp and pp data and assessed the contribution to the delay times used in this study for the caribbean region analyses of the isc pp and pp delay times do not reveal biases due to effects of pp waveform distortions the asymmetry of the reflections or due to misidentifications of phases that reflect at a surface other than that assumed the noise level of pp and pp data is high with respect to data of the direct p wave this is accommodated by weighting with the inverse of the variance of the data of each of the three phases the independent information that is revealed from the pp and pp data results in modifications of tomographic images based solely on p data these modifications are important if the tomographic images are being used to understand the geodynamical history of convergent margins in the caribbean region we investigated the effect of adding data of later arriving phases to the isc p data with sensitivity tests we inverted synthetic delay times to which we added gaussian noise with a standard error typical for the data of the three seismic phases these tests demonstrate that the image resolution of shallow mantle structure is enhanced significantly by the incorporation of later arriving phases due to the absence of seismicity below 200 km the resolution improved less at deeper levels below the caribbean region in some poorly constrained parts of the solution the test results even indicate an apparent decrease of resolution this is explained by changes in the rate of convergence of the inversion algorithm in mantle regions where the effective sampling of structure improved by the addition of pp and pp wave ray paths the convergence was speeded up at the expense of the convergence rate in regions where fewer or no pp or pp wave ray paths were added a shortcoming of the resolution tests used in our study is that some specific problems of reported delay times are not reflected in synthetic data we observe that isc delay times of later arriving phases are not necessarily consistent with the reported hypocentral parameters as most isc reported earthquake locations are computed from direct p wave data for the pp data the inconsistency with event location not only results in a decrease of focal depths during relocation but in a bias of the imaged velocity perturbations as well this property of reported data is not modelled in resolution tests

reduction of surface gravity data from global atmospheric pressure loading summary besides solid earth and ocean tides atmospheric pressure variations are one of the major sources of surface gravity perturbations as shown by previous studies merriam 1992 mukai et al 1995 boy et al 1998 the usual pressure correction with the help of local pressure measurements and the barometric admittance a simple transfer function between pressure and gravity both measured locally does not allow an adequate estimation of global atmospheric loading we express the response of the earth to pressure forcing using a green s function formalism farrell 1972 the atmosphere acts on surface gravity through two effects first a direct gravitational attraction by air masses which is sensitive to regional about 1000 km around the gravimeter pressure variations second an elastic process induced by the earth s surface deformation and mass redistribution which is sensitive to large scale pressure variations wavelengths greater than 4000 km we estimate atmospheric loading using green s functions and global pressure charts provided by meteorological centres we introduce different hypotheses on the atmospheric thickness and atmospheric density variations with altitude for the modelling of the direct newtonian attraction all computations are compared to gravity data provided by superconducting gravimeters of the ggp global geodynamics project network we show the improvement by modelling global pressure versus the local estimates in terms of reduction of the variance of gravity residuals we can also validate the inverted barometer ib hypothesis as the oceanic response to pressure forcing for periods exceeding one week the non inverted barometer nib hypothesis is shown to be definitely an inadequate assumption for describing the oceanic response to atmospheric pressure at seasonal timescales

displacements of the earth s surface due to atmospheric loading effects on gravity and baseline measurements atmospheric mass loads and deforms the earth s crust by performing a convolution sum between daily global barometric pressure data and mass loading green s functions we estimate the time dependent effects of atmospheric loading including those associated with short term synoptic storms on surface point positioning measurements and surface gravity observations we calculate the response for both an oceanless earth and an earth with an inverted barometer ocean load responses for near coastal stations are significantly affected by the inclusion of an inverted barometer ocean peak to peak vertical displacements are frequently 15 20 mm with accompanying gravity perturbations of 3 6 gal baseline changes can be as large as 20 mm or more the perturbations are largest at higher latitudes and during winter months these amplitudes are consistent with the results of rabbel and zschau 1985 who modeled synoptic pressure disturbances as gaussian functions of radius around a central point deformation can be adequately computed using real pressure data from points within about 1000 km of the station knowledge of local pressure alone is not sufficient rabbel and zschau s hypothesized corrections for these displacements which use local pressure and the regionally averaged pressure prove accurate at points well inland but are in general inadequate within a few hundred kilometers of the coast

contribution to the lithostratigraphy and history of lake ladoga abstract new geological data from lake ladoga are presented and an interpretation of the late pleistocene and holocene history of the lake is given the deglaciation of the southern part of the lake and an origin of the first ice margin lake ladoga took place at ca 14 000 yr b p

continuous measurement of nontidal variations of gravity records from seven superconducting gravimeters operated at five different locations in california and one in boulder colorado are examined after removal of tides and the gravitational attraction of the atmosphere fluctuations over periods between a few days and several months were observed at all sites with peak amplitudes of order 10 gal by contrast a 640 day record obtained with one of the instruments in germany showed peak fluctuations of only 2 gal in most of the records the causes of these aperiodic variations were not determined so that they serve to set limits on the vertical motion or displacement of mass at the respective locations however at the geysers geothermal field much of the gravity variation is correlated with seismic activity reinjection rate and rainfall measurements of this type were not possible prior to the development of the superconducting device consequently these results provide the first evidence for the existence of gravity variations on the time scale and of the magnitude described here vertical crustal motion motion related to seismic events and hydrological phenomena can lead to variations on this scale unambiguous identification of causal relationships will require either special circumstances such as found at the geysers or operation of the instruments in pairs

wavelet filter analysis of local atmospheric pressure effects in the long period tidal bands abstract it is well known that local atmospheric pressure variations obviously affect the observation of short period earth tides such as diurnal tides semi diurnal tides and ter diurnal tides but local atmospheric pressure effects on the long period earth tides have not been studied in detail this is because the local atmospheric pressure is believed not to be sufficient for an effective pressure correction in long period tidal bands and there are no efficient methods to investigate local atmospheric effects in these bands the usual tidal analysis software package such as eterna baytap g and vav cannot provide detailed pressure admittances for long period tidal bands we propose a wavelet method to investigate local atmospheric effects on gravity variations in long period tidal bands this method constructs efficient orthogonal filter bank with daubechies wavelets of high vanishing moments the main advantage of the wavelet filter bank is that it has excellent low frequency response and efficiently suppresses instrumental drift of superconducting gravimeters sgs without using any mathematical model applying the wavelet method to the 13 year continuous gravity observations from sg t003 in brussels belgium we filtered 12 long period tidal groups into eight narrow frequency bands wavelet method demonstrates that local atmospheric pressure fluctuations are highly correlated with the noise of sg measurements in the period band 4 40 days with correlation coefficients higher than 0 95 and local atmospheric pressure variations are the main error source for the determination of the tidal parameters in these bands we show the significant improvement of long period tidal parameters provided by wavelet method in term of precision

modelling the effect of atmospheric pressure variations on gravity summary a model is developed to describe the effects of local and global atmospheric pressure fluctuations on local gravity the calculated admittance of gravity to the pressure tides s1 s4 at pinon flat california is compared to those measured using the superconducting gravimeter the theoretical and measured admittances are in good agreement for the s3 and s4 tides the difference between theory and observation at s2 can probably be explained by the ocean tide at that frequency while the discrepancy at s1 remains unexplained techniques for the removal of random pressure variations from gravity records are described using statistics derived from spherical harmonic decompositions of the earth s pressure field it is shown that more than 80 per cent of the pressure induced gravity variations should be removed from a gravity record by employing a single barometer and least squares procedure it is further shown that adding a ring of barometers should increase the effectiveness to about 90 per cent the amount of barometric pressure noise which remains after subtracting the simultaneous record of two gravimeters is calculated as a function of their separation and indicates that significantly less noise is present in the difference than in the individual records for separations of up to 17 of arc

volcanic signatures in time gravity variations during the volcanic unrest on el hierro canary islands gravity changes occurring during the initial stage of the 2011 2012 el hierro submarine eruption are interpreted in terms of the preeruptive signatures during the episode of unrest continuous gravity measurements were made at two sites on the island using the relative spring gravimeter lacoste and romberg gphone 054 on 15 september 2011 an observed gravity decrease of 45 mu gal associated with the southward migration of seismic epicenters is consistent with a lateral magma migration that occurred beneath the volcanic edifice an apparently clear precursor of the eruption that took place 25 days later on 10 october 2011 high frequency gravity signals also appeared on 6 11 october 2011 pointing to an occurring interaction between a magmatic intrusion and the ocean floor these important gravity changes with amplitudes varying from 10 to 90 mu gal during the first 3 days following the onset of the eruption are consistent with the northward migration of the eruptive focus along an active eruptive fissure an apparent correlation of gravity variations with body tide vertical strain was also noted which could indicate that concurrent tidal triggering occurred during the initial stage of the eruption

hydrogeological investigations at the membach station belgium and application to correct long periodic gravity variations 1 a comprehensive hydrogeological investigation regarding the influence of variations in local and regional water mass on superconducting gravity measurements is presented for observations taken near the geodynamic station of membach belgium applying a regional water storage model the gravity contribution due to the elastic deformation of the earth was derived in addition the newtonian gravity effect induced by the local water mass variations was calculated using soil moisture observations taken at the ground surface about 48 m above the gravimeters the computation of the gravimetric effect is based on a digital elevation model with spatially discretized rectangular prisms the obtained results are compared with the observations of a superconducting gravimeter sg we find that the seasonal variations can be reasonably well predicted with the regional water storage model and the local newtonian effects shorter period effects depend on the local changes in hydrology this result shows the sensitivity of sg observations to very local water storage changes

karst driven flexural isostasy in north central florida deformed marine terraces can be used to explore a region s uplift history trail ridge is a marine terrace in north florida that is nearly 80 meters above modern sea level and contains quaternary marine fossils a fact that is inconsistent with estimates of paleo sea level history since the early pleistocene this implies that the terrace has experienced uplift since its formation as well as non uniform deformation recorded by the warping of its previously horizontal state the florida carbonate platform located on the passive margin of eastern north america is a setting where non tectonic influences e g isostatic adjustment dynamic topography can be examined we present a single transect numerical model of vertical displacement derived from elastic flexure to assess the influence of karst driven isostatic uplift on present day topography of trail ridge in north florida flexural modeling predicts elevations in central florida not observed today most likely because surface erosion and karst cavity collapse have obliterated this high topography older subsurface stratigraphic units however display the arched profile predicted from flexural modeling mass loss calculated by differencing modeled topography and observed topography was found to be 6 75 1012 kg since emplacement of trail ridge uplift rates assuming karst driven flexural isostasy alone using previously estimated ages of trail ridge of 0 125 1 4 3 or 3 5 ma were found to be 0 535 0 048 0 022 and 0 019 mm yr respectively a more likely explanation of uplift includes contributions from dynamic topography and glacial isostatic adjustment which should be further explored with more advanced geophysical modeling

variability of the gravity to height ratio due to surface loads we study the ratio between the gravity variation and vertical displacement on the surface of a self gravitating earth model when a surface load is applied we adopt a theoretical and numerical point of view excluding any observations first we investigate the spectral behavior of the ratio of the harmonic components of the gravity variation and vertical displacement then we model the gravity to height ratio for different surface loads continental hydrology atmospheric pressure ocean tides using outputs of global numerical models in order to relate the predicted spatial values to theoretical mean values deduced from the spectral domain for locations inside loaded areas the ratio is highly variable because of the newtonian attraction of the local masses and depends on the size of the load for the hydrological loading soil moisture and snow the mean ratio over the continents is 0 87 gal mm 1 but increases with decreasing size of the river basins for the atmospheric loading assuming an inverted barometer response of the ocean the ratio is positive with larger values for high latitudes 0 49 gal mm 1 particularly on the coasts than for lower latitudes 0 30 gal mm 1 the ratio however is much less variable outside the loaded areas in desert areas such as the sahara and arabia its mean value is 0 28 gal mm 1 for the ocean tidal loading we find a mean ratio of 0 26 gal mm 1 over the continents for the diurnal tidal waves both results are close to the theoretical mean value of 0 26 gal mm 1 combining elastic and remote attraction contributions

holocene sedimentation in the central part of lake ladoga abstract new multi proxy paleolimnological study of the sediment sequence from the central part of lake ladoga enabled not only reconstructing the dynamics of the holocene paleoenvironments but also tracing changes in the lake ecosystem productive sedimentation conditions and sedimentation rate the analyses included lithology loss on ignition metals grain size distributions including fractions of

tsunami generation and associated waves in the water column and seabed due to an asymmetric earthquake motion within an anisotropic substratum in this paper closed form integral expressions are derived to describe how surface gravity waves tsunamis are generated when general asymmetric ground displacement due to earthquake rupturing involving both horizontal and vertical components of motion occurs at arbitrary depth within the interior of an anisotropic sub sea solid beneath the ocean in addition we compute the resultant hydrodynamic pressure within the seawater and the elastic wavefield within the seabed at any position the method of potential functions and an integral transform approach accompanied by a special contour integration scheme are adopted to handle the equations of motion and produce the numerical results the formulation accounts for any number of possible acoustic gravity modes and is valid for both shallow and deep water situations as well as for any focal depth of the earthquake source phase and group velocity dispersion curves are developed for surface gravity tsunami mode acoustic gravity rayleigh and scholte waves several asymptotic cases which arise from the general analysis are discussed and compared to existing solutions the role of effective parameters such as hypocenter location and frequency of excitation are examined and illustrated through several figures which show the propagation pattern in the vertical and horizontal directions attention is directed to the unexpected contribution from the horizontal ground motion the results have important application in several fields such as tsunami hazard prediction marine seismology and offshore and coastal engineering in a companion paper we examine the effect of ocean stratification on the appearance and character of internal and surface gravity waves this article is protected by copyright all rights reserved

elasto gravitational deformation relative gravity changes and earth dynamics summary we consider the elasto gravitational deformation of an earth model with fluid parts core ocean atmosphere a love number formalism is used in order to express the global deformation induced by volume potentials tides variations in the earth s rotation or boundary conditions in pressure transverse stress load at the interfaces between the fluid layers and the solid mantle the induced disturbances in gravity are expressed with the help of generalized gravimetric factors and special attention is paid to resonance processes various applications related to global earth dynamics are developed and numerically quantified the surface gravity changes caused either by a variation in the direction of the earth s rotational axis polar motion or in the rotation rate length of the day are compared with the lunisolar gravimetric tides the loading effect due to the oceans is found to increase the oceanless gravimetric factor by about 2 5 per cent two simplified approaches are proposed to model the gravity perturbations originating in the fluid core e g due to buoyancy core modes it is shown that a degree 2 planetary scale pressure field acting at the core mantle boundary of about 80 pa is able to induce a 10 ngal surface signal similarly gravity contributions from the atmosphere or oceans are derived using outer pressure load or spheroidal shear stress gravimetric factors a degree 2 surface pressure and transverse traction of 100 pa causes a gravity change of 0 27 and 0 14 pgal respectively finally we point out that in addition to the solar gravitational tide there are several other contributions rotational meteorological appearing in the annual gravity signal by different gravimetric factors we therefore conclude that high precision relative gravimetry can provide useful information not only about the earth s physical properties but also about its dynamical behaviour

on noise reduction in vertical seismic records below 2 mhz using local barometric pressure we propose a simple method for the reduction of noise in vertical seismic recordings at frequencies below 2 mhz the method consists in subtracting the locally recorded scaled atmospheric pressure signal from the gravity record the scale factor is frequency independent and can be linearly estimated from the data while such a pressure correction was previously only used in the analysis of tidal records it is equally efficient at the low frequency end of the normal mode band and we use gravity recordings of the big 1994 shikotan and bolivian quakes to demonstrate this point our examples also show that this correction permits to achieve noise levels well below the new low noise model nlnm peterson 1993 the success of the correction also sheds some light on the physical processes responsible for the generation of low frequency seismic noise at quiet observatories