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

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

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

localized geomagnetic field anomalies in an underground gas storage abstract to investigate possible piezomagnetic effects associated with imposed tectonic stresses repeated measurements of total magnetic field in an array 120 km 90 km in extent were performed across the hutubi underground gas storage hugs forty repeated measurement stations were established in the hugs and its surrounding areas proton precession magnetometers with a sensitivity of 0 15 nt 1 hz were used to measure the total magnetic field at each repeated station we conducted two field measurement surveys one with a minimum pressure during april 2017 and the other with a maximum pressure during november 2017 in the hugs the local magnetic field lmf of the hugs and its surrounding areas was obtained after data processing which included diurnal variation reduction dvr secular variation reduction svr and lmf acquisition the average of the mean standard deviations of the geomagnetic dvr values were 0 038 nt and 0 045 nt for the first and second field surveys respectively the amplitude of the lmf varied between about 10 nt and 90 nt in the local area negative anomalies with a maximum amplitude of 2 1 nt was observed over a seven month period april 2017 november 2017 during the loading process with a constantly increasing pressure in the hugs the wide range of negative lmf anomalies shows a negative correlation with the increasing pressure in the hugs this result constitutes powerful field observational evidence for the existence of piezomagnetic effects under middle scale experimental conditions

earthquake activity related to the 1991 eruption of the hekla volcano iceland the 1991 eruption of the hekla volcano started unexpectedly on 17 january no long term precursory seismicity was observed the first related activity was a swarm of small earthquakes that began approximately half an hour before the eruption intensive seismicity both earthquakes and volcanic tremor accompanied the violent onset of the eruption almost 400 events up to ml magnitude 2 5 were recorded during the first few hours during the later phases of the eruption the earthquake activity was modest and the main volcano related seismic signal was the persistent volcanic tremor the tremor died away together with the eruption on 11 march and hekla was seismically quiet until the beginning of june 1991 when a sudden swarm of numerous small shallow earthquakes occurred this activity is atypical for hekla and is interpreted to be a failed attempt to resume the eruption

an approach to problems on energy sources at mount etna based on seismological and volcanological data based on seismograms from the most continuously operating station of the seismic network at mt etna monte vetore 1665 m a s l the energy release patterns through time have been obtained for volcanic tremors and earthquakes which occurred between 1978 1982 both energies range between 1011 1013 j a but their release patterns are not strictly correlated to each other considering only the eruptions which occurred during the same time span the amount of thermal energy released and the associated potential energy needed to raise the magma to the surface from a 2 km deep hydrostatic equilibrium level were estimated to be about 1017 and 1015 j a respectively the computed energies for earthquakes and tremors are at least one order of magnitude less than what is needed to match a model of magma transport based solely on the jerky propagation of melt filled cracks the energy needed to raise the magma to the surface could be supplied by expanding gases in the upper levels of the magma column and or by tectonic stresses acting on shallow batches of magma

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

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

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

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 hekla eruption 1980 1981 the sixteenth eruption of hekla since 1104 began on august 17th 1980 after the shortest repose period on record only ten years the eruption started with a plinian phase and simultaneously lava issued at high rate from a fissure that runs along the hekla volcanic ridge the production rate declined rapidly after the first day and the eruption stopped on august 20th a total of 120 million m3 of lava and about 60 million m3 of airborne tephra were produced during this phase of the activity in the following seven months steam emissions were observed on the volcano activity was renewed on april 9th 1981 and during the following week additional 30 million m3 of lava flowed from a summit crater and crater rows on the north slope

the 1991 eruption of hekla iceland the eruption that started in the hekla volcano in south iceland on 17 january 1991 and came to an end on 11 march produced mainly andesitic lava this lava covers 23 km2 and has an estimated volume of 0 15 km3 this is the third eruption in only 20 years whereas the average repose period since 1104 is 55 years earthquakes as well as a strain pulse recorded by borehole strainmeters occurred less than half an hour before the start of the eruption the initial plinian phase was very short lived producing a total of only 0 02 km3 of tephra the eruption cloud attained 11 5 km in height in only 10 min but it became detached from the volcano a few hours later several fissures were active during the first day of the eruption including a part of the summit fissure by the second day however the activity was already essentially limited to that segment of the principal fissure where the main crater subsequently formed the average effusion rate during the first two days of the eruption was about 800 m3 s 1 after this peak the effusion rate declined rapidly to 10 20 m3 s 1 then more slowly to 1 m3 s 1 and remained at 1 12 m3 s 1 until the end of the eruption site observations near the main crater suggest that the intensity of the volcanic tremor varied directly with the force of the eruption a notable rise in the fluorine concentration of riverwater in the vicinity of the eruptive fissures occurred on the 5th day of the eruption but it levelled off on the 6th day and then remained essentially constant the volume and initial silica content of the lava and tephra the explosivity and effusion rate during the earliest stage of the eruption as well as the magnitude attained by the associated earthquakes support earlier suggestions that these parameters are positively related to the length of the preceeding repose period the chemical difference between the eruptive material of hekla itself and the lavas erupted in its vicinity can be explained in terms of a density stratified magma reservoir located at the bottom of the crust we propose that the shape of this reservoir its location at the west margin of a propagating rift and its association with a crustal weakness all contribute to the high eruption frequency of hekla

oil and gas fields accompanied by geothermal anomalies in rocky mountain region definite hot spots or positive geothermal anomalies are associated with 15 of 22 rocky mountain oil and gas fields examined in montana north dakota wyoming utah colorado and new mexico of the remaining 7 fields 5 have poor to sparse data and the other 2 show no anomaly or are inconclusive stratigraphically as well as structurally controlled fields and both oil and gas accumulations show positive geothermal anomalies the principal cause for temperature gradient anomalies over oil and gas fields is lateral and upward movement of fluids primarily water through the earth s rocks into trapping situations such movements not only carry the fluids themselves but also transport heat from depth 1 many rocky mountain oil and gas fields are accompanied by positive geothermal anomalies similar situations exist in numerous fields in other parts of the world 2 anomalies may be induced in several different ways but upward fluid movement is most important to oil and gas exploration surveys 3 accurate temperature measurements are vital to valid interpretations of subsurface geothermics therefore temperature measurements must be taken and reported with care and detail in the field 4 the use of geothermics to locate hot anomalies should improve industry success in searching for new oil and gas fields

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

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

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

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

spectral analysis of harmonic tremor signals at mt semeru volcano indonesia a detailed spectral analysis of tremor signals recorded at mt semeru volcano indonesia in october 1992 by the german indonesian volcano expedition give 92 reveals clearly harmonic spectra containing up to 11 integer harmonics the spectral features can be explained by any rhythmically pulsating source producing a temporarily stable source signal as one realisation of this concept we model the source as a resonating gas volume generating seismic signals similar to the sound generated in a recorder

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

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