alignment of near surface inclusions and appropriate crack geometries for geothermal hot dry rock experiments ubiquitous splitting of seismic shear waves indicates that most rocks in the upper half of the crust are pervaded by stress aligned fluid filled inclusions called eda cracks these inclusions are expected to be aligned perpendicular to the minimum compressional stress by stress relationships similar to those aligning industrial hydraulic fractures at depths where the overburden stress is sufficiently large typically below a few hundred metres this minimum stress is usually horizontal so that the eda cracks and hydraulic fractures are typically aligned vertically striking parallel or subparallel to the direction of maximum compression this is confirmed by the polarizations of the split shear waves along raypaths at depth in the crust at the free surface however the vertical stress is zero or approximately zero and cracks and hydraulic fractures at shallow depths in intact rock tend to be horizontal thus the directions of minimum stress and the orientations of hydraulic fractures are likely to swing through 90 near the surface of the earth since the behaviour of cracks and stress is often crucial to drilling operations the rotation of the crack and stress geometry near surface has important implications particularly for optimizing hydrocarbon production and geothermal reservoir management consequently evidence gained from experiments for example in hot dry rock geothermal heat extraction in inappropriate crack geometries at shallow depths may not be valid when applied to other crack and stress geometries at depth in hot rock

atmospheric sulfur dioxide measurements during the 2005 and 2007 eruptions of the piton de la fournaise volcano implications for human health and environmental changes abstract piton de la fournaise is an active volcano located in the south eastern part of reunion island a tiny island found in the south western indian ocean 21 s 55 5 e this volcano erupts regularly at least once a year with notable amount of degassing concerning the volcanic eruptions the regulation of the french ministry of environment on the air quality is mainly focussed on the standards applicable for the sulfur dioxide so 2 in terms of public health and environmental changes air samples have been collected continuously every 10 days on passive diffusion tubes of so 2 an atmospheric tracer of volcanic plumes by ora observatoire reunionnais de l air at 10 sites which have been located around the volcano since september 2005 the mean so 2 concentrations measured over a 10 day period in the absence of any volcanic eruption range between very low values 1 15 g m 3 for all 10 sites however during the december 2005 eruption notable so 2 concentrations factor 5 25 have been measured at the locations situated on the north west flanks of the volcano the so 2 degassing reached concentrations of 65 379 g m 3 which is mainly caused by the volcanic activity when coupled with the local to regional atmospheric circulations land sea breezes easterlies etc a wide range of atmospheric measurements was performed in the vicinity of piton de la fournaise in order to characterize the gaseous and particle emissions during the april may 2007 eruption so 2 concentrations were also measured continuously by sf 2000 analyser at several coastal locations in the vicinity of the volcano during and off eruption activities in the absence of any volcanic eruption the so 2 concentrations showed day to day variations with hourly mean concentrations varying between 1 and 20 g m 3 these values correspond to the background concentrations for this coastal rural location and might as well come in part from non volcanic origin the so 2 concentrations however were increased by a factor 10 30 with a maximum hourly mean concentration of 2486 g m 3 during the april may 2007 eruption hereby several statutory thresholds were exceeded particularly at the tremblet location indeed the information and recommendation threshold 350 g m 3 h as well as the alert threshold 500 g m 3 on 3 h consecutively were exceeded 32 and 5 times respectively at this site these measurements point out that during the times of eruptions with significant amounts of degassing the threshold values for protecting the ecosystems and the air quality for human health were exceeded at several inhabited locations with implications for human health and environmental changes on local to regional scale

volcanic eruption velocities measured with a micro radar a novel technique to quantitatively assess volcanic eruption dynamics is presented we report on measurements of volcanic eruption velocities with a modified micro radar doppler anemometer on stromboli volcano italy the mean vertical eruption velocity recorded during a four day period in late 1996 was around 10 m s which is somewhat on the small side for strombolian eruptions however the activity of stromboli volcano during this period was very weak explaining the relatively small velocities the technique used is much better in resolution than older techniques based on static photos and allows a continuous monitoring of the volcanic activity in addition to the velocity information recorded by the doppler spectrum it also contains information on the density of the ejected material and the size distribution of the particles in the volcanic jet

volcanic tremor at mount etna january 1984 march 1985 its relationship to the eruptive activity and modelling of the summit feeding system spectral analyses of volcanic tremor at etna during january 1984 march 1985 have been performed and the relationship between tremor energy and observed volcanic phenomena have been examined the highest energy levels have been observed during the paroxysmal phases of eruptions whereas a gradual decrease was linked to the lowering of eruptive activity amplitude variations with time of some spectral frequency peaks 0 95 1 20 1 45 1 65 1 80 and 2 40 hz have been compared with volcanic activity at the summit craters and on the basis of these results a new schematic diagram for the feeding system of the summit vents is proposed

broadband measurements of the sources of explosions at stromboli volcano italy during september october 1997 21 three component broadband seismometers were deployed on stromboli volcano at radial distances of 0 3 2 2 km from the active crater to investigate the source mechanisms of strombolian explosions in the 2 50 s band the very long period vlp signals associated with explosions are consistent with two stationary sources repeatedly activated in time vlp particle motions are essentially linear and analyses of semblance and particle motions are consistent with a source centroid offset 300 m beneath and 300 m northwest of the active vents similar vlp waveforms are observed at all 21 stations indicating that the seismograms are source dominated the vlp ground displacement response to each explosion may be qualitatively interpreted as 1 pressurization of the conduit associated with the ascent of a slug of gas 2 depressurization of the conduit in response to mass withdrawal during the eruption and 3 repressurization of the conduit associated with the replenishment of the source with fluid

strombolian explosions 1 a large bubble breaking at the surface of a lava column as a source of sound strombolian activity consists of a series of explosions caused by the breaking of large overpressurized bubbles at the surface of the magma column acoustic pressure has been measured for 36 explosions at stromboli we propose that sound is generated by the vibration of the bubble before it bursts oscillations are driven by an initial overpressure inside the bubble assumed to be initially at rest just below the magma air interface inertia effects cause the bubble to overshoot its equilibrium radius then the bubble becomes underpressurized and contracts because of gas compressibility these oscillations are only slightly damped by viscous effects in the magma layer above the bubble the bubble cannot complete more than one cycle of vibration because of instabilities developing on the magma layer that lead to its breaking near the minimum radius assuming a simple geometry we model this vibration and constrain the radius and length of the bubble and the initial overpressure by fitting a synthetic waveform to the measured acoustic pressure the fit between synthetic and observed waveforms is very good both for frequency 60 rad s 1 and amplitude the initial bubble radius is 1 m and the length varies between several and a few tens of meters from the initial overpressure approximately 105 pa we calculate the maximum radial velocity of ejecta 30 m s 1 the generally good agreement between data and predictions of our model leads us to suggest that acoustic measurements are a powerful tool for the understanding of eruption dynamics

utilization of seismically recorded infrasonic acoustic signals to monitor volcanic explosions the el chichon sequence 1982 a case study infrasonic acoustic signals from five explosive eruptions of el chichon volcano during march 29 through april 4 1982 were recorded by a microbarograph array and seismic research observatory sro seismograph collocated near mckinney texas analyses of these signals for frequencies from 0 0033 to 0 025 h have demonstrated functional relationships of amplitude to frequency consistent with 1 for microbarometric data 2 for vertical mode seismic data and 3 for radial mode seismic data these observed slopes are in agreement with those theoretically predicted by sorrells 1971 estimations of kinetic energy releases by the explosions of el chichon are presented and range from 4 9 1022 ergs to 2 0 1023 for the largest eruption on april 4 1982 to 4 2 1021 to 6 1 1021 ergs for the smallest eruption on april 3 1982 the method of posey and pierce 1971 to estimate the explosivity from long period infrasonic signals is not likely to be applicable strictly speaking to the shorter period data thus it is believed that the energy release for the smallest eruption is overestimated by possibly an order of magnitude comparable estimations of volcanic explosivity from sro radial seismic data and microbarometric array data strongly recommend using sro and international deployment accelerometers network data for far field analyses of volcanic explosions and construction of a rigorous volcanic explosivity scale

infrasonic waves and volcanic tremor at stromboli the origin of the volcanic tremor is still under debate many theories have been proposed in the last years but none has yet been completely accepted in 1993 highly sensitive pressure sensors 2 175 pa volt used to monitor the explosive activity at stromboli have revealed unexpected correlation between small spike shaped pressure signals 1 2 pa and volcanic tremor these pressure pulses repeat regularly in time with a recurrent period of ca 1 s video camera images allowed us to correlate the pressure pulses with small gas bursts occurring at one of the active vents the striking correlation 0 971 between infrasonic and seismic energy fluctuations is particularly meaningful in the frequency domain infrasonic and seismic signal share the same spectral content 3 hz for every station within a range of 700 m around the craters correlations in time and frequency domain remained unaltered during the 1994 field experiments moreover during 1994 the increased degassing activity has been followed by an increase in pressure release 7 8 pa and by a shift towards higher frequencies 8 hz both in the infrasonic and seismic records infrasonic waves and volcanic tremor show similar energy fluctuations and frequency contents appearing therefore to be produced by the same dynamical process on this basis we claim that volcanic tremor at stromboli originates by continuous outbursting of small gas bubbles in the upper part of the magmatic column

geological and industrial implications of extensive dilatancy anisotropy extensive dilatancy anisotropy is the hypothesized distribution of stress aligned fluid filled microcracks pervading most rocks in the earth s crust the geometry of the cracks and the aligning stress field can be monitored by analysing the waveforms of shear waves propagating through the rockmass the ability to estimate some of the parameters of the crack and stress geometry by analysing shear wave particle displacements has widespread implications

european stress contributions from borehole breakouts before about 1986 european measurements of rock stresses were sparse the development of the borehole breakout method has facilitated a dramatic increase in the geographical distribution of rock stress orientations these are derived from data collected routinely for other purposes by the oil coal and gas industries over many years borehole breakouts can be interpreted in term s of the orientation of the stress field in boreholes as a function of depth with the findings related to known stratigraphical boundaries tectonic structures fault and fracture occurrences and to other geological and geophysical stress indicators geographical and depth distributions of stress on local and regional scales are beginning to provide the necessary base from which broad interpretive stress field models can be constructed

the mera lahar deposit in the upper amazon basin transformation of a late pleistocene collapse at huisla volcano central ecuador abstract the sub andean zone east of the cordillera real ecuador and out to the amazon basin s western margin has been the depository of voluminous lahars related to volcanic activity in the andean highlands these lahars arrived to the sub andean zone via gravitational transport through narrow river canyons and emplaced volumes surpassing several cubic kilometers over robust inundation zones this paper discusses the origin flow route depositional zone terrace formation and geomorphic significance of the most important lahar deposit yet mapped in this region that of the mera lahar which likely formed from a late pleistocene collapse of huisla volcano followed by impounding by temporary dams behind drainage blocking debris avalanche deposits dad then subsequent rupture of the blockage the actual deposit of the mera lahar has a thickness between 30 and 70 m is mainly comprised of dad breccia is matrix supported c 70 of reddish gray color and is well consolidated based on geochemical and petrographic similarities huisla volcano s dad is the best candidate for the lahar s source huisla volcano is located some 90 km up valley of the bulk of mera lahar s mapped deposit clasts of mera lahar rocks and huisla s dad breccias have 57 61 wt sio2 corresponding to andesites of the calc alkaline series with mean values of 1 1 5 wt for k2o the mapped mera lahar deposit has an actual volume estimated in 1 2 km3 compared to its original estimated volume of 5 4 km3 cross sectional widths of 1 5 to 4 5 km span and extend laterally beyond the actual pastaza river channel where the lahar s deposition produced high standing isolated surfaces that are notable local geomorphic features of the upper sub andean zone the flow modeling program laharflow employing the modern landscape as the topographic base adequately simulates the flow route and inundation zones of the mera lahar

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

highlights from a seismic broadband array on stromboli volcano an array of nine three component broadband seismometers was deployed in two different configurations on stromboli volcano the analysis of the seismic wavefield related to volcanic explosions revealed some observations which offer a completely new insight into the internal dynamics of a volcano these new observations are restricted to the low frequency range below 1 hz and underline therefore the superiority of broadband recordings over conventional short period observations surprisingly simple wavelets indicate an initially contracting source mechanism gas jets that could not be seen in a short period seismic record at all generate a clear dilatational wavelet in a broadband recording suggesting the same contracting source mechanism the analysis of particle motion and seismic array techniques permits a location of the seismic source we find low frequency signals of 3s and 6s period that are not related to eruptions and do not share a common source with the eruption related events a video recording of visible volcanic activity at the crater region allows one to correlate precisely eruptive features with seismic signals

explosion seismic tomography of a magmatic body beneath etna volatile discharge and tectonic control of volcanism to constrain the upper depth range of lateral heterogeneities beneath mt etna a seismic tomography method with artificial sources is designed to undershoot the target and restrict the depth of seismic propagation by reflection at the top of the basement a strong high velocity anomaly thus imaged southeast of the summit interpreted as a magmatic body is reliably constrained to reach at least as shallow as 2 km depth where exsolution of so2 from magma may occur the high gas output rate of etna allard 1997 hence appears as the normal complement of the huge magma body seismically imaged this magma body is located beneath the volcanic edifice rather than within which attests to the control of volcanism by extension these volatile output and internal structure distinguish mt etna volcanism among current types and may relate to the peculiar context of a recent regional change from subduction related compression to extension

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

estimating the internal structure of reservoirs with shear wave vsps in the past 10 years vertical seismic profiles vsps have played an increasingly important role in identifying and delineating in situ layers such vsps have usually recorded p wave data three component shear wave trains carry three or four times more information about the source and structure of the rocks along the raypath than the equivalent p wave train so in principle shear wave vsps should be a particularly valuable technique for examining in situ layers

anisotropy survey for reservoir definition summary in the context of franco soviet cooperation a seismic survey was carried out on the romashkino reservoir in the tatar republic ussr this survey involved evaluating azimuthal anisotropy parameters from s wave vsps recorded on three wells and from a surface seismic line the latter in order to monitor any lateral variations in the parameters in this paper we present results which indicate in particular that for the two wells in which oil production has been established the anisotropy rate appears to be proportional to production figures this seems to confirm the hypothesis that variations in production are asociated with the presence of subvertical fractures which cause s wave birefringence by processing the reflected converted p s waves of a surface seismic line it is possible to monitor lateral variations in the anisotropy parameters and well ties effectively confirm differences revealed by the s wave vsp data

explosion mechanisms at arenal volcano costa rica an interpretation from integration of seismic and doppler radar data 1 we execute an integrated analysis of broadband seismic and doppler radar data to gain insights into the subsurface mechanisms that drive repetitive mildly explosive activity of arenal volcano costa rica we find large variability of both seismic and radar waveforms and nonsystematic relationships between the two seismic recordings display long lasting tremor sequences and numerous explosion quakes radar measurements show that tephra emissions are poorly correlated in both time and energy to the seismic activity tephra emissions were found in association with explosion quakes but also during episodes of tremor and seismic quiescence moreover the exit velocity mass loading and kinetic energy of the emissions show no clear relationship with the coeval seismic amplitude and frequency content we propose a conceptual source model whereby degasing is controlled by opening and closing of fractures that crosscut a rigid cap atop the conduit when the fracture s strength is overcome by the gas pressure building up below it suddenly opens and high velocity gas escapes producing high frequency elastic waves typical of explosion quakes gas release also occurs in relation to periodic opening and closure of the fractures to produce repetitive pressure pulses this being the source of tremor in both cases varying quantities of fragmented material may be carried by the gas which can be detected by the radar if their concentration is high enough moreover the highly variable constantly changing state of lava cap e g thickness fracture network and gas permeability results in nonrepeatable source conditions and explains the complex relationship between tephra emissions and associated seismic signals

echo resonance and hydraulic perturbations in magma cavities application to the volcanic tremor of etna italy in relation to its eruptive activity a study is presented of spectral features of volcanic tremor recorded at mount etna sicily italy following the methods of analysis suggested by the resonant scattering formalism of gaunaurd and uberall 1978 1979a 1979b and the model for hydraulic origin of seidl et al 1981 the periods investigated include summit and flank eruptions that occurred between 1984 and 1993 recordings from a permanent station located near the top of the volcano were used and the temporal patterns associated with a the average spacing bar delta between consecutive spectral peaks in the frequency range 1 6 hz b the spectral shape and c the overall spectral amplitude were analyzed bar delta values are thought to depend on the physical properties of magma such as its density which in turn is controlled by the degree of gas exsolution variations in the spectral shape are tentatively attributed to changes in the geometrical scattering from the boundary of resonant conduits and magma batches finally the overall amplitude at the station should essentially reflect the state of turbulence of magma within the superficial ascending path a limit in the application of the resonant scattering formalism to the study of volcanic tremor is given by the fact that the fundamental modes and integer harmonics are difficult to identify in the frequency spectra as tremor sources are likely within cavities of very complex geometry rather than in spherical or cylindrical chambers as expected by theory this study gives evidence of some correlations between the analyzed temporal patterns and the major events in the volcanic activity related to both lava flow and explosions at the summit vents in particular relatively high values of bar delta have been attained during the se crater eruption of 1984 the complex eruptive phases of september october 1989 and the 1991 1993 flank eruption suggesting the presence of a relatively dense magma for all of these events conversely very low values have been recorded in coincidence with the december 1985 activity and the paroxysmal explosions at the summit craters of early 1990 which are interpreted here as fed by fluid vesiculated magma appreciable modifications in the spectral shape have been observed in relation to changes of the volcanic activity that probably preceded the opening and disactivation of shallow dykes or magma batches finally the overall amplitude seems to be a sensitive indicator of the state of gas turbulence within the shallow conduits as is suggested by the high values attained during phases of intense volcanic activity

the effect of oriented microcracks on seismic velocities in an ultramylonite the influence of oriented microcracks on the directional dependence of p and s wave velocities has been investigated in an epidote muscovite ultramylonite optical and sem studies revealed that the fabric of the mylonite is characterized by oriented intragranular and grain boundary cracks comparison of orientation patterns of cracks with calculated elastic properties voigt average of a model rock isotropic matrix ultramylonite crack geometry and laboratory seismic data indicates that wave propagation is particularly affected by oriented grain boundary cracks microcracks are most effective at low pressure and largely closed above about 200 300 mpa the residual part of velocity anisotropy observed above this crack closing pressure is mainly due to preferred mineral orientation in particular of mica maximum concentration of poles to 001 mica planes corresponds to the maximum of poles to oriented grain boundary cracks so that the effects of texture and oriented cracks on wave velocities line up giving rise to extreme values of velocity anisotropy and shear wave splitting at low pressure conditions

first testing of a volcano doppler radar voldorad at mount etna italy a medium power 1 kw doppler radar of limited weight 110 kg and working in the uhf band has been developed for active remote sensing of volcanic eruptions named voldorad this radar is designed to monitor eruptive activities of variable magnitude at medium distance range 0 5 5 km in any weather condition and fills the gap between low and high power radar or sodar systems previously tested on volcanoes voldorad was operated for the first time during an eruption of mt etna on october 11 12 1998 doppler spectra recorded at 1 km distance from the vent reveal powerful echoes during the eruption paroxysm lava fountains which gradually decreased along with declining activity an average vertical velocity of 80 87 m s 1 is inferred for the eruptive jets

aligned fractures in carbonate rocks laboratory and in situ measurements of seismic anisotropy by vertical seismic profiling and shear wave analysis we show that a packet of carbonate reservoir rocks found at nearly 3000 m depth in the north german basin is seismically anisotropic for vertical paths of wave propagation the estimated velocity difference of the split shear waves is 10 no shear wave birefrigence is observed within the hangingwall which therefore has to be regarded as isotropic or transversely isotropic additional laboratory investigations of the petrography of drilled carbonate samples and of their seismic velocities show that the anisotropy is most probably caused by subvertical fractures with preferred azimuthal orientation the strike direction of the aligned fractures determined by analysis of split shear waves is approximately n55 e this value agrees with recently published directions of maximum horizontal tectonic stress in pre zechstein sediments in the eastern part of the north german basin but it is in contrast to the world stress map

p wave velocity and permeability distribution of sandstones from a fractured tight gas reservoir fractures are an important fabric element in many tight gas reservoirs because they provide the necessary channels for fluid flow in rocks which usually have low matrix permeabilities several sandstone samples of such a reservoir type were chosen for a combined study of rock fabric elements and petrophysical properties geological investigations of the distribution and orientation of the fractures and sedimentary layering were performed in addition laboratory measurements were carried out to determine the directional dependence of the permeability and p wave velocities higher permeability values are generally in the plane of the nearly horizontal sedimentary layering with regard to the core axis with the occurrence of subvertical fractures however the highest permeabilities were determined to be parallel to the core axis compressional wave velocities were measured on spherical samples in more than 100 directions to get the vp symmetry without prior assumptions below 50 mpa confining pressure all

image processing of explosive activity at stromboli abstract a new way to study the dynamics of explosive behaviour at different vents of stromboli volcano is proposed the method is based on computer processing of images taken by an infrared film camera at a rate of 4 pictures per second each picture has been digitized by use of an image scanner with a 75 d p i dots per inch resolution and 256 levels of gray then processed and the total mass of the ejected material has been calculated for each photograph with this procedure the flux of ejected matter throughout every single explosive event has been reconstructed the analysis of trajectories made on adequate samples of ejecta has allowed us to evaluate the ejection velocity the mode of the mass flux distribution and the mean value of the velocities calculated for individual particles quantify the kinetic energy of an explosion vents nos 1 and 3 which remained active after the last effusive eruption of 1985 1986 show several explosive patterns the energy of which is measured the observed flux distributions have been checked both numerically and experimentally by modelling impulsive events the computer simulations of the mass distributions have allowed us to speculate on the possible explosive dynamisms two different models have been used a single impulse model confirms the behaviour actually shown by one of the two vents while the behaviour of the other is better depicted by a cumulative trend of successive impulses in rhythmic sequence moreover through the experimental approach we have had the opportunity to test the overall resolution of the method the observed matter flux distribution has then been associated to the seismic signals in order to correlate the dynamics involved in the explosions with the seismic source mechanisms

mount etna dense array local earthquake p and s tomography and implications for volcanic plumbing inversion for the three dimensional velocity structure of mount etna is performed with a data set of arrival times of p and s waves of local earthquakes from temporary dense arrays of three component seismographs a high v p body revealed by the original tomography without nearby stations is confirmed and its image is sharpened using new velocity constraints provided by refraction data synthetic tests of v p and v p v s and comparison with an independent artificial source tomography with a fundamentally different geometry consistently calibrate the significance threshold of the resolution indicators the trustworthy part of the image shows a high v p body centered under the southern part of valle del bove above the 6 km below sea level deep basement which extends towards sea level and may be rooted in or through the crust it has a large contrast of over 1 km s with the surrounding sediments and sharp lateral limits and can thus be regarded as made of intrusive material of magmatic origin the massive high v p body is heterogeneous in v p v s the regions inside it where v s is relatively low can then be suspected of containing a proportion of melt or be fractured and act as pressure links or transport zones such features may be structurally linked and appear to be activated in eruptive phenomena by taking into account the heterogeneities in structure and physical state retrieved by seismic tomography a succession of seismic events deformational episodes and geochemical variation in lavas can be discussed with respect to the well observed eruptions

seismic patterns and fluid dynamic features preceding and accompanying the january 15 1990 eruptive paroxysm on mt etna italy abstract on the basis of data collected during a seismological investigation carried out on mt etna italy during a phase of the volcanic activity marked by progressive transition from strombolian bursts to effusive paroxysms at the se crater january 1990 three types of seismic regime have been recognized they are interpreted to be linked to modifications in the regimes of volcanic gases and vapours within the upper levels of the magma column as suggested by recent studies on the dynamics of magmatic fluids our analysis also reveals that the use of usual seismic parameters such as the temporal patterns associated with the occurrence of discrete low frequency events spindles and the coefficient m of the ishimoto iida law can furnish information about variations in the behaviour of the system before the irreversible evolution of the volcanic activity toward eruptive paroxysms

strombolian explosions 2 eruption dynamics determined from acoustic measurements strombolian activity consists of a series of explosions due to the breaking of a large overpressurized bubble at the surface of the magma column acoustic pressure due to sound waves has been measured and analyzed at stromboli for more than 50 explosions three parts can be distinguished in the acoustic pressure waveform which are related to the behavior of the bubble before during and after its bursting before the sharp rise in acoustic pressure the signal is dominated by waves with a frequency of 2 hz which develop on the nose of the bubble they produce sound in air by imposing a rapid motion to the interface and one could detect a bubble travelling in the uppermost 30 m of the magma column when the bubble reaches the air magma interface its strong vibration driven by a large overpressure inside the gas generates the main event with a frequency around 9 hz after the bubble has burst kinematic waves of frequency around 4 5 hz are the main source of sound they develop at the surface of the magma left on the conduit side the three types of motion although determined independently give consistent results furthermore combining the results obtained for the two types of kinematic waves the magma viscosity is estimated to be of 300 65 pa s which is in good agreement with petrological constraints and corroborates the validity of our analysis this suggests that acoustic measurements constitute a powerful tool in the understanding of eruption dynamics

complete seismic properties obtained from microcrack fabrics and textures in an amphibolite from the ivrea zone western alps italy abstract laboratory seismic measurements of compressional wave velocities vp and shear wave splitting vs have been carried out on a clinopyroxene amphibolite from the ivrea zone western alps italy at pressures up to 600 mpa from the texture of the major minerals hornblende plagioclase and pyroxene and the respective elastic constants the elastic properties voigt average and seismic anisotropies were calculated at high confining pressure where the effect of microcracks is eliminated vp and shear wave velocities vs vss are in good agreement with the calculated velocities reflecting texture induced anisotropies microcracks are most effective at low confining pressure but are largely closed at about 200 300 mpa microscopic studies revealed that the crack fabric is characterized by oriented transgranular and cleavage cracks the bulk anisotropy at low pressure conditions was computed by superposing the texture induced anisotropy with the crack induced anisotropy these calculated values are consistent with the measured values indicating that anisotropy of vp and shear wave splitting is affected by both texture and oriented microcracks based on detailed fabric analyses the presented method allows to describe the complete elastic and seismic properties of crystalline rocks providing an ideal data basis for three dimensional seismic modelling to investigate the influence of anisotropic media on the reflectivity

elastic wave propagation in media with parallel fractures and aligned cracks a model of parallel slip interfaces simulates the behaviour of a fracture system composed of large closely spaced aligned joints the model admits any fracture system anisotropy triclinic the most general monoclinic orthorhombic or transversely isotropic and this is specified by the form of the 3 3 fracture system compliance matrix the fracture system may be embedded in an anisotropic elastic background with no restrictions on the type of anisotropy to compute the long wavelength equivalent moduli of the fractured medium requires at most the inversion of two 3 3 matrices when the fractures are assumed on average to have rotational symmetry transversely isotropic fracture system behaviour and the background is assumed isotropic the resulting equivalent medium is transversely isotropic and the effect of the additional compliance of the fracture system may be specified by two parameters in addition to the two isotropic parameters of the isotropic background dilute systems of flat aligned microcracks in an isotropic background yield an equivalent medium of the same form as that of the isotropic medium with large joints i e there are two additional parameters due to the presence of the microcracks which play roles in the stress strain relations of the equivalent medium identical to those played by the parameters due to the presence of large joints thus knowledge of the total of four parameters describing the anisotropy of such a fractured medium tells nothing of the size or concentration of the aligned fractures but does contain information as to the overall excess compliance due to the fracture system and its orientation as the aligned microcracks which were assumed to be ellipsoidal with very small aspect ratio are allowed to become non fiat i e have a growing aspect ratio the moduli of the equivalent medium begin to diverge from the standard form of the moduli for flat cracks the divergence is faster for higher crack densities but only becomes significant for microcracks of aspect ratios approaching 0 3

anisotropic elastic moduli of carbonates and evaporites from the weyburn midale reservoir and seal rocks assuming vertical transverse isotropy vti symmetry the elastic anisotropy as a function of confining pressure of four carbonates and one evaporite from the williston sedimentary basin in saskatchewan canada is investigated using the ultrasonic pulse transmission method ultrasonic p and s wave velocities are obtained from cylindrical plugs cut from a main sample along horizontal vertical and 45 orientations with respect to the sample s presumed vertical axis of symmetry the elastic constants were then calculated from the measured velocities and densities anisotropy was quantified by estimating thomsen parameters thomsen 1986 from elastic constants the results show that the samples are at the best weakly anisotropic the presence of microcracks and pores as well as the heterogeneity of the samples play an important role in defining the p and s wave velocities the weak anisotropy found in these samples suggests that intrinsic properties of these rocks negligibly contribute to the anisotropy observed at the seismic scale

microcracks in rocks a review abstract in the past decade the number of studies about microcracks in rocks has rapidly increased this review of recent work concentrates on microcracks in rock as separate entities emphasizing microcrack morphogenesis kinematics dynamics population statistics and observational techniques cracks are produced when the local stress exceeds the local strength the local stress may be augmented by twin lamellae interactions kink bands and deformation lamellae stress concentrations at grain boundary contacts and around intracrystalline cavities local strength may be reduced along cleavage planes along grain boundaries and along any internal surface as a result of corrosion by chemically active fluids dislocations appear not to be a significant factor for crack nucleation below about 500 c in silicates spatial and temporal changes in temperature can also induce microcracking as a result of differential thermal expansion between grains with different thermoelastic moduli and thermal conductivities the amount of quartz in the rock has a significant effect on thermally induced microcracks because of its large and variable thermal expansivity the application of hydrostatic pressure between 100 and 200 mpa effectively closes most cracks but the closure may not be uniform if crack wall asperities exist hydrostatic pressure appears to stabilize cracks and make crack growth more difficult the number and average size of mechanically induced microcracks is greater in rock deformed at higher pressures the application of a deviatoric stress field on the boundaries of a rock mass results on a microscopic scale in a very complex stress system which greatly affects nucleation and propagation paths the relative amount of intragranular and intergranular cracking appears to depend upon mineralogy rock type and stress state the vast majority of stress induced microcracks in rocks appear to be extensional statistically they are predominantly oriented within 30 of the macroscopic maximum stress direction crack densities increase as macroscopic deviatoric stress increases above a threshold level crack size distributions may be either lognormal or exponential fracture in rock under compressive boundary loads is a result of the coalescence of many microcracks not the growth of a single crack some crack configurations are more favorable for coalescence than others as deviatoric stress increases and rock failure is approached the microcrack population changes spatially from random to locally intense zones of cracking away from the fault the crack density dies off rapidly to the background level a few grains away under lesser deviatoric stresses slow subcritical microcrack growth can occur as a result of stress aided corrosion at the crack tip the rate governing mechanism may be either the chemical reaction rate or the rate at which water can get to the crack tip important details still remain to be worked out

acoustic sounder measurements of the vertical velocity of volcanic jets at stromboli volcano we have used a sodar acoustic doppler sounder to characterize the behavior of volcanoes and in particular to measure the vertical velocities of volcanic jets we have analyzed more than 100 explosions on stromboli volcano eolian islands first results show vertical speeds close to the vent ranging from 20 m s to 80 m s the uncertainty in the measurements is close to 10 due mainly to the inaccurate knowledge of the sound speed in volcanic jets this is small compared to other volcanological methods this method is well suited for monitoring temporal variations during an eruption and should be particularly interesting for the study of the eruptive dynamics of lava fountains the doppler spectrum in explosions appears to be related to different types of echoes and contains information on particle sizes in the jet

explosive volcanic eruptions v observations of plume dynamics during the 1979 soufriere eruption st vincent summary during the 1979 eruption of the soufritre of st vincent west indies a 14min period of explosive activity occurred on april 22 the first three minutes of this activity were filmed five individual explosions formed distinct plumes which fed an eruption column which eventually ascended to a height of over 18 km the ascent velocities of the fronts of these plumes ranged from 8 5 to 61 7m s the half angle of spreading of the plume fronts ranged from 21 5 to 24 one of the plumes was observed to 8 km height and decelerated steadily from 5 1 5 to 23 m s l the main column fed by individual explosions was however observed to accelerate from 8 5 to 58 m s l in the same height interval a theoretical treatment of volcanic plume motion is presented measurements of dimensions velocities and spreading rates from the film are used to estimate plume parameters such as temperature particle content and volume discharge rate of magma from the theoretical relationships these calculations show that the individual plumes became hotter and richer in juvenile ash with time the acceleration of the main eruption column was the result of being fed by increasingly hotter and more ash rich explosions an average volume discharge rate of 12600m3s is estimated from the analysis of the plume motions this value agrees closely with an estimate of discharge rate based on the heat flux required to form an 18 km high column this agreement suggests that theories of convective motions in plumes can be successfully adapted to the volcanic case as long as the effect of hot entrained ash particles is considered

seismic and infrasonic evidences for an impulsive source of the shallow volcanic tremor at mt etna italy during a seismo acoustic experiment we recorded volcanic tremor around the summit craters of mt etna volcano tremor shows amplitude modulation which disappear 900 m from the crater area the infrasonic wavefield is coherent even at distances of 750 m time delay between infrasonic transient is stable around 1 3 s and is consistent with the position of the source in the voragine crater amplitude modulation of tremor is well correlated 0 72 with infrasound amplitude with a time lag of 0 37 s coherent with a shallow position of the source amplitude of volcanic tremor decays over increasing distances according to geometrical spreading of body waves tremor wavefield shows a linear polarization following the same time occourrence as the infrasonic pulses polarization azimuth indicates that wavefield rectilinearity is mostly due to p waves we infer that most of the volcanic tremor we recorded at mt etna is generated by superimposition of small impulsive sources acting at 1 2 s rate caused by pressure instability during magma degassing

p and s wave anisotropy of a synthetic sandstone with controlled crack geometryl1 a method is reported for the production of synthetic porous sandstones containing cracks of known dimensions and geometry with respect to the matrix a synthetic sandstone was manufactured from sand cemented with an epoxy glue the cracks of known geometry were introduced into the material in the manufacturing stage by emplacing thin metallic discs in the sand epoxy matrix these discs were chemically leached out of the consolidated porous sandstone acoustic anisotropy and shear wave splitting were observed in the synthetic sandstones for the dry sample the observed angular dependence of the p and s wave velocities at 100 khz compares well qualitatively with the theoretical models of hudson and of thomsen quantitatively however the experimental data fits hudson s model better for the case of a saturated sample the experimental results are in excellent agreement with thomsen s model hudson s model on the other hand predicts a different angular dependence for p waves this demonstrates that the concept of fluid transfer between cracks and the ambient porosity can be a significant process the results reported here are from the first successful experiment in which the theoretical models were tested on a porous material containing a known crack geometry

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

seismic properties of the upper crust in the central friuli area northeastern italy based on petrophysical data abstract the compressional and shear wave velocities have been measured at room temperature and pressure up to 450 mpa on 5 sedimentary rock samples representative of the most common lithologies of the upper crust in the central friuli area northeastern italy at 400 mpa confining pressure the triassic dolomitic rock shows the highest velocities vp 7 km s vs 3 6 km s the jurassic and triassic limestones samples intermediate velocities vp 6 3 s vs 3 5 km s and the cenozoic and paleozoic sandstones the lowest velocities vp 6 15 km s vs 3 35 km s the paleozoic sandstone sample is characterized by the strongest anisotropy 10 and significant birefringence 0 2 km s is found only on the cenozoic sandstone sample we elaborated the synthetic profiles of seismic velocities density elastic parameters and reflection coefficient related to 4 one dimensional geological models extended up to 22 km depth the synthetic profiles evidence high rheological contrasts between triassic dolomitic rocks and the soft sandstones and the jurassic limestones the vp profiles obtained from laboratory measurements match very well the in situ vp profile measured by sonic log for the limestones and dolomitic rocks supporting our one dimensional modelling of the calcareous carbonatic stratigraphic series the vp and vs values of the synthetic profiles are compared with the corresponding ones obtained from the 3 d tomographic inversion of local earthquakes the laboratory vp are generally higher than the tomographic ones with major discrepancies for the dolomitic lithology the comparison with the depth location of seismicity reveals that the seismic energy is mainly released in correspondence of high contrast rheological boundaries

doppler radar sounding of volcanic eruption dynamics at mount etna besides their common use in atmospheric studies doppler radars are promising tools for the active remote sensing of volcanic eruptions but were little applied to this field we present the observations made with a mid power uhf doppler radar voldorad during a 7 h strombolian eruption at the se crater of mount etna on 11 12 october 1998 main characteristics of radar echoes are retrieved from analysis of doppler spectra recorded in the two range gates on either side of the jet axis from the geometry of the sounding the contribution of uprising and falling ejecta to each doppler spectrum can be discriminated the temporal evolution of total power backscattered by uprising targets is quite similar to the temporal evolution of the volcanic tremor and closely reproduces the overall evolution of the eruption before during and after its paroxysm moreover during the sharp decrease of eruptive activity following the paroxysm detailed analysis of video from camera recording radar and seismic measurements reveals that radar and video signals start to decrease simultaneously approximately 2 5 min after the tremor decline this delay is interpreted as the ascent time through a magma conduit of large gas slugs from a shallow source roughly estimated at about 500 m beneath the se crater detailed analysis of eruptive processes has been also made with voldorad operating in a high sampling rate mode signature of individual outburst is clearly identified on the half part of doppler spectra corresponding to rising ejecta temporal variations of the backscattered power exhibit quasi periodic undulations whereas the maximum velocity measured on each spectrum displays a sharp peak at the onset of each outburst followed by a slow decay with time periodicity of power variations between 3 8 and 5 5 s is in agreement with the occurrence of explosions visually observed at the se vent maximum vertical velocities of over 160 m s 1 were measured during the paraoxysmal stage and the renewed activity finally by using a simplified model simulating the radar echoes characteristics we show that when voldorad is operating in high sampling rate mode the power and maximum velocity variations are directly related to the difference in size and velocity of particles crossing the antenna beam

a model for internal dynamical processes on mt etna summary results of studies concerning seismic activity earthquakes and tremor and its relation with eruptive events on mt etna are briefly discussed seismic records collected from 1977 up to 1986 have been analysed and special care was given to observation of changes in some seismic parameters such as the rate of microseismic activity the amplitude and dominant spectral peaks of the volcanic tremor which vary significantly in relation to the occurrence of eruptive events the systematic nature of variations in the seismic parameters considered allows us to propose a preliminary model which puts all the observations made so far into a wide context giving an interpretative hypothesis of the magma uprise mechanisms preceding either summit or flank eruptions summit eruptions are modelled in two stages and occur without changes in the seismic parameters considered except for a sharp increase in tremor amplitude almost coincident in time with the eruption onset flank eruptions are modelled in three stages and follow a simultaneous change of all the mentioned parameters with time the onset of these eruptions is in fact preceded by an increase in both the daily number of shocks and the amplitude of the volcanic tremor as well as a shifting from relatively high values in the dominant peaks of the tremor spectra which appear in the pre eruptive stage towards usual lower frequency values 1 0 2 3 hz

regional patterns of tectonic stress in europe nearly 1500 stress orientation determinations are now available for europe the data come from earthquake focal mechanisms overcoring measurements well bore breakouts hydraulic fracturing measurements and young fault slip studies and sample the stress field from the surface to seismogenic depths three distinct regional patterns of maximum compressive horizontal stress shmax orientation can be defined from these data a consistent nw to nnw shmax stress orientation in western europe a wnw ese shmax orientation in scandinavia similar to western europe but with a larger variability of shmax orientations and a consistent e w shmax orientation and n s extension in the aegean sea and western anatolia the different stress fields can be attributed to plate driving forces acting on the boundaries of the eurasian plate locally modified by lithospheric properties in different regions on average the orientation of maximum stress in western europe is subparallel to the direction of relative plate motion between africa and europe and is rotated 17 clockwise from the direction of absolute plate motion the uniformly oriented stress field in western europe coincides with thin to medium lithospheric thickness approximately 50 90 km and high heat flow values 80 m w m2 in western europe a predominance of strike slip focal mechanisms implies that the intermediate principal stress is vertical the more irregular horizontal stress orientations in scandinavia coincide with thick continental lithosphere 110 170 km and low heat flow 50 m w m2 the cold thick lithosphere in this region may result in lower mean stresses associated with far field tectonic forces and allow the stress field to be more easily perturbed by local effects such as deglaciation flexure and topography the stress field of the aegean sea and western anatolia is consistent with n s extension in a back arc setting behind the hellenic trench subduction zone the stress field is influenced in places by regional geologic structures e g in the western alps where shmax directions show a slight tendency toward a radial stress pattern not all major geologic structures however appear to affect the shmax orientation e g in the vicinity of the rhine rift system horizontal stress orientations are continuous

activity of mount etna preceding the february 1999 fissure eruption inferred mechanism from seismological and geochemical data abstract the february 1999 eruption of mt etna took place through a fissure on the sse flank of the cone of the summit se crater this event was preceded by continuous activity since 1995 sometimes accompanied by violent outbursts from one or more of the three other summit craters ne crater voragine or chasm and bocca nuova and finally by a series of 20 short lived eruptions from the se crater between september 1998 and january 1999 these phenomena could be accounted for by invoking gradual invasion of a shallow small reservoir by more primitive basic and gas rich magma coming from depth the shallow chamber is more likely to be a plexus of dikes which had developed during the previous years 1995 1997 following variations of the local stress field owing to enhanced magma generation and accumulation at the top of the mantle magma injection and mixing is evidenced through geochemistry whereas the state of stress of the volcanic pile and underlying crust is determined using earthquake distributions and focal mechanisms the behaviour of the seismic tremor amplitude appears to be a good indicator of the state of unrest of the volcano although not always directly linked to the relative energy of degassing phenomena

degassing at low magma viscosity volcanoes quantifying the transition between passive bubble burst and strombolian eruption abstract at many volcanoes low magma viscosities allow persistent degassing over a range of styles from explosive strombolian activity to gas puffing and passive degassing it is generally accepted that strombolian eruptions reflect the bursting of large bubbles gas slugs at the magma surface and that relatively quiescent puffing indicates the presence of significantly smaller bubbles or slugs here we address this qualitative range and derive a dimensionless parameter pslim to quantify and distinguish different regimes of burst vigour for pslim 1 passive activity is anticipated and measurable parameters such as infrasonic amplitude will be small for pslim 1 bursting slugs will be energetic and surface effects are anticipated to increase with increasing pslim various physical parameters recorded during laboratory experiments and simulated through numerical models are shown to demonstrate the same trends when parameterised by pslim hence pslim provides a straightforward relationship between changes in measurable surface effects and the system s subsurface physical parameters and for energetic activity of a particular system burst effects are shown to be proportional to the square root of the gas mass involved effect magnitudes are also shown to be a function of atmospheric pressure and application of pslim parameterisation to planetary scenarios produces results in line with our current understanding of planetary volcanism

comparison of signal processing techniques for estimating the effects of anisotropy1 three component recordings of shear waves in exploration surveys provide an opportunity to measure crustal anisotropy which may be important in estimating the geometrical and physical parameters of reservoir rocks vsps are particularly important for this purpose as they are less subject to the complex interactions of the shear wavefield with the free surface the first stage in characterizing the subsurface anisotropy requires that the distinctive phenomenon of shear wave splitting must be examined for every arrival at each geo phone this effect may be defined by two parameters the polarization of the leading shear wave and the time delay between corresponding split shear waves a variety of techniques have been designed to estimate these parameters of shear wave splitting here we classify the published techniques into four main categories and review their properties representative procedures from each group are applied to a common synthetic data set contaminated with signal generated noise the results allow some general statements to be made about the utility of these methods for processing shear waves in vsp data

continuous monitoring of volcanic eruption dynamics a review of various techniques and new results from a frequency modulated radar doppler system in situ measurement of volcanic eruption velocities is one of the great challenges left in geophysical volcanology in this paper we report on a new radar doppler technique for monitoring volcanic eruption velocities in comparison with techniques employed previously e g photographic methods or acoustic doppler measurements this method allows continuous recordings of volcanic eruptions even during poor visibility also radar doppler instruments are usually light weight and energy efficient which makes them superior to other doppler techniques based on laser light or sound the proposed new technique was successfully tested at stromboli volcano in late 1996 during a period of low activity the recorded data allow a clear distinction between particles rising from the vent and particles falling back towards the vent the mean eruption velocity was approximately 10 m s most of the eruptions recorded by radar were correlated to seismic recordings the correlation between the magnitude of the volcanic shocks and the eruption force index defined in the paper may provide new insights into magma transport in the conduit

changes in shear wave splitting at anza near the time of the north palm springs earthquake changes in shear wave splitting are observed at knw before and after the m 6 north palm springs earthquake of july 8 1986 knw is a station of the anza seismic network monitoring the anza seismic gap on the san jacinto fault southern california the gradual increase in the delays between the split shear waves over 3 years at knw reported by peacock et al 1988 ended in june 1986 the further 2 years of observations analyzed here show that the behavior of the delays changed abruptly near the time of the north palm springs earthquake 33 km north of knw peacock et al demonstrated that the increase in delays could be simulated by increasing the aspect ratio of stress aligned fluid filled inclusions and speculated that this increase might be the result of a buildup of stress before an impending earthquake the new data appear to confirm this speculation but the temporal variations require a more complex interpretation although they still suggest that the changes in shear wave splitting are caused by earthquake induced stress changes to the fluid filled inclusions throughout the rockmass central to interpretation of temporal changes in shear wave splitting is the well established existence throughout at leastmore the uppermost 10 to 20 km of the crust of small fluid filled cracks microcracks and pores the existence of such inclusions introduces a compliant quality to otherwise stiff crustal rock the authors term these distributions of inclusions extensive dilatancy anisotropy or eda and the individual inclusions eda cracks because although they may include a wide range of shapes many of the seismic properties can be simulated by distributions of thin parallel cracks less