patterns of earthquakes and the effect of solid earth and ocean load tides at mount st helens prior to the may 18 1980 eruption seismographs near mount st helens volcano recorded an earthquake swarm lasting nearly 2 months prior to the catastrophic may 18 1980 eruption the earthquakes are divided into four classes based on station cpw 116 km seismogram characteristics 1 events with sv p amplitude ratio 3 and dominant frequency 3 hz 2 events with sv p ratio between 1 and 3 and dominant frequency 2 hz 3 events similar to characteristic 2 but with a strong probably surface wave phase just after the s phase and 4 events with frequencies between 1 and 2 hz lacking a clear s phase the seismicity pattern for each of the four classes is unique and we assume each group of events with similar wave forms represents a common physical process depth source area or fault orientation maximum likelihood b values range between 0 97 0 05 and 1 81 0 09 for the four groups we calculate solid earth stress and strain tides at the average hypocentral depth of 4 km we also calculate stress and strain tides induced by ocean loading their amplitudes are typically 20 40 those of the solid earth tides at the location of mount st helens a weak but significant correlation exists between the latter two classes of events and the tides for a time interval of about 5 days preceding the first onset of volcanic tremor and about 5 days thereafter the polarity of the correlation is opposite for the two classes of events in each case the phase of the correlation changes systematically with time the changes coinciding with the onset of tremor on march 31 and with a pronounced decrease in earthquake energy release rate on april 3 there are no significant correlations between the tides and the number of events or energy release of these two classes of earthquakes during any other interval between march 20 and may 18 1980 the first two classes of events show no evidence of significant tidal correlation at any time during the study period

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

seismicity at fuego pacaya izalco and san cristobal volcanoes central america 1973 1974 seismic data collected at four volcanoes in central america during 1973 and 1974 indicate three sources of seismicity regional earthquakes with hypocentral distances greater than 80 km earthquakes within 40 km of each volcano and seismic activity originating at the volcanoes due to eruptive processes regional earthquakes generated by the underthrusting and subduction of the cocos plate beneath the caribbean plate are the most prominent seismic feature in central america earthquakes in the vicinity of the volcanoes occur on faults that appear to be related to volcano formation faulting near fuego and pacaya volcanoes in guatemala is more complex due to motion on a major e w striking transform plate boundary 40 km north of the volcanoes volcanic activity produces different kinds of seismic signatures shallow tectonic or a type events originate on nearby faults and occur both singly and in swarms there are typically from 0 to 6 a type events per day withb value of about 1 3 at very shallow depths beneath pacaya izalco and san cristobal large numbers of low frequency or b type events are recorded with predominant frequencies between 2 5 and 4 5 hz and withb values of 1 7 to 2 9 the relative number of b type events appears to be related to the eruptive states of the volcanoes the more active volcanoes have higher levels of seismicity at fuego volcano however low frequency events have unusually long codas and appear to be similar to tremor high amplitude volcanic tremor is recorded at fuego pacaya and san cristobal during eruptive periods large explosion earthquakes at fuego are well recorded at five stations and yield information on near surface seismic wave velocities 3 0 0 2 km sec

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

the effects of attenuation and site response on the spectra of microearthquakes in the northeastern caribbean abstract the spectra of microearthquakes recorded by a seismic network in the northeastern caribbean were analyzed to determine the effects of attenuation and site response on these spectra the ratios of the spectral amplitudes at 5 and 20 hz were measured for several earthquakes at different distances from a fixed receiver to estimate q for p and s waves in the crust and upper mantle the average q p determined from three stations was 380 for epicentral distances between 40 and 200 km the average q s determined from two stations was estimated at 400 spectra of earthquakes closer than 40 km to one station indicate that a high q path q 800 exists for these short distances in the island arc platform the near equivalence of q p and q s observed in this study of the upper lithosphere contrasts with estimates of anderson et al 1965 that q p 9 4 q s in the asthenosphere this finding that q p is approximately equal to q s suggests that the attenuation mechanism in the upper lithosphere differs from that in the asthenosphere the spectra of nearby microearthquakes unaffected by crustal attenuation display corner frequencies which are uniform despite large differences in the seismic moments of the events the corner frequencies of these spectra appear to be characteristic of the receiver site and insensitive to the location and moment of the earthquakes at one station the s wave corner frequency of a nearby blast was identical to the corner frequency of a magnitude 4 8 earthquake derived from the record of a strong motion accelerograph at the same site

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

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

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

canonical model of volcano acoustics a full wave theoretical model is developed for the acoustic field generated by an explosive point source embedded in a magma column that is open to the atmosphere the green s functions for the field in the magma and the atmosphere are derived on the basis of several simplifying assumptions concerning the geometry of the conduit the boundary conditions and the geoacoustic properties of the magma a kinematic model for the acoustic signature of the explosive source is proposed which when combined with the green s functions provides full analytical expressions for the acoustic field in the magma and in the atmosphere as complex functions of frequency by fourier inversion the airborne pressure spectrum is transformed into a pressure time series the predicted sound pulse in the atmosphere and its energy spectrum are highly dispersive showing complicated structure that arises from the coherent addition of many normal modes of oscillation i e depth and radial resonances in the magma column at high frequencies for which the aperture of the vent is many wavelengths across each mode is launched into the atmosphere as a parallel sided beam of sound with a characteristic angle of elevation which through snell s law is determined by the speed of sound in the magma relative to that in air at somewhat lower frequencies the modal beams of sound undergo angular spreading due to diffraction at the edge of the vent in the lowest frequency regime where the wavelength is comparable with the aperture the airborne field shows little angular structure a comparison between airborne acoustic data that we recorded in july 1994 at the western vent of stromboli volcano and the predictions of the theory using parameters that are characteristic of stromboli show compelling agreement the theoretical and observed power spectra both display the following features 1 a concentration of energy below 20 hz associated with the first four longitudinal resonances 2 radial resonances between 35 and 65 hz and 3 a broad minimum around 30 hz arising because the source lies near nulls in longitudinal modes that would otherwise be excited the conclusion is that the airborne sound signature from an explosive volcanic event may be inverted to provide estimates of the depth and radius of the magma conduit the depth spectral shape and peak shock wave pressure of the source and the viscosity of the magma

detecting hidden volcanic explosions from mt cleveland volcano alaska with infrasound and ground coupled airwaves 1 in alaska where many active volcanoes exist without ground based instrumentation the use of techniques suitable for distant monitoring is pivotal in this study we report regional scale seismic and infrasound observations of volcanic activity at mt cleveland between december 2011 and august 2012 during this period twenty explosions were detected by infrasound sensors as far away as 1827 km from the active vent and ground coupled acoustic waves were recorded at seismic stations across the aleutian arc several events resulting from the explosive disruption of small lava domes within the summit crater were confirmed by analysis of satellite remote sensing data however many explosions eluded initial automated analyses of satellite data due to poor weather conditions infrasound and seismic monitoring provided effective means for detecting these hidden events we present results from the implementation of automatic infrasound and seismo acoustic eruption detection algorithms and review the challenges of real time volcano monitoring operations in remote regions we also model acoustic propagation in the northern pacific showing how tropospheric ducting effects allow infrasound to travel long distances across the aleutian arc the successful results of our investigation provide motivation for expanded efforts in infrasound monitoring across the aleutians and contributes to our knowledge of the number and style of vulcanian eruptions at mt cleveland

reduced crustal magnetization beneath relict hydrothermal mounds tag hydrothermal field mid atlantic ridge 26 n submersible magnetic profiles over the mir and alvin relict hydrothermal zones of the tag hydrothermal field on the mid atlantic ridge at 26 n show magnetic anomalies that are consistent with the presence of reduced crustal magnetization these magnetic anomalies are similar to the anomaly found associated with the tag active sulfide mound and suggests that hydrothermal alteration rather than thermal demagnetization is responsible for these reduced magnetization zones magnetic field mapping thus can be useful in determining the subsurface extent of upflow zones and alteration pipes of midocean ridge hydrothermal systems

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

three dimensional numerical models with varied material properties and erosion rates implications for the mechanics and kinematics of compressive wedges received 19 march 2008 revised 29 january 2009 accepted 13 february 2009 published 21 april 2009 1 we develop three dimensional mechanical models of a compressive wedge and investigate how the form and kinematics of the outboard wedge are affected by variation in initial topography material properties and erosion rate inclusion into the wedge of weaker less dense material affects the form of the wedge producing a region of steeper average slopes and higher topography while depriving the region further inboard of material enhanced erosion has a similar effect the wedge attempts to replace the eroded material by focusing deformation the result is a stepped region of lower topography within the outboard of the orogen we observe that uplift velocities at three points in the orogen vary cyclically from near zero to 3 times the average uplift rate over cycles lasting on the order of 15 200 000 model years our models along with analog models and some well dated examples from active orogens suggest that transient accommodation of strain may be common the cycles observed responded rapidly to changes in the amount of erosion imposed our models suggest that orogens may be driven by strong coupling between erosion and strain on temporal scales of 10 4 10 5 years and spatial scales comparable to the scale of the erosional perturbation we compare our models to the puli embayment of west central taiwan a region of anomalous low topography and suggest that it presence may reflect the presence of weaker and more erodible sediments than those present along strike in the orogen

seismological significance of the 1977 1978 eruptions and the magma intrusion process of usu volcano hokkaido abstract an earthquake swarm and the major pumice eruptions in august 1977 which followed marked the start of the dacitic doming activity of usu volcano in southwestern hokkaido japan the sequence of magma intrusion processes was investigated in detail by means of seismological and other geophysical data the distribution of the abundant hypocenters shows clearly an earthquake free zone beneath the summit crater the hypocenters migrated in a manner consistent with the development of the observed asymmetrical surface deformations considered due to magma intrusion into this earthquake free zone the earthquake mechanism solutions are mostly of dip slip type and are interpreted in terms of the doming deformations the existence of earthquake families earthquakes with similar waveforms is the main cause of the peculiar occurrence of earthquakes in space time and magnitude the concept of scattered barriers of different sizes and strengths can explain well the distinct characteristics of the occurrence of the swarm and the observed episodic deformations

the 2013 eruption of pavlof volcano alaska a spatter eruption at an ice and snow clad volcano the 2013 eruption of pavlof volcano alaska began on 13 may and ended 49 days later on 1 july the eruption was characterized by persistent lava fountaining from a vent just north of the summit intermittent strombolian explosions and ash gas and aerosol plumes that reached as high as 8 km above sea level and on several occasions extended as much as 500 km downwind of the volcano during the first several days of the eruption accumulations of spatter near the vent periodically collapsed to form small pyroclastic avalanches that eroded and melted snow and ice to form lahars on the lower north flank of the volcano continued lava fountaining led to the production of clastogenic lava flows that extended to the base of the volcano about 3 4 km beyond the vent the generation of fountain fed lava flows was a dominant process during the 2013 eruption however episodic collapse of spatter accumulations and formation of hot spatter rich granular avalanches was a more efficient process for melting snow and ice and initiating lahars the lahars and ash plumes generated during the eruption did not pose any serious hazards for the area however numerous local airline flights were cancelled or rerouted and trace amounts of ash fall occurred at all of the local communities surrounding the volcano including cold bay nelson lagoon sand point and king cove

evidence for magma intrusion during the mammoth lakes earthquakes of may 1980 and implications of the absence of volcanic harmonic tremor the compensated linear vector dipole clvd mechanism p wave radiation with conical nodal surfaces found for the mammoth lakes earthquakes of may 1980 has been attributed to dike intrusion by julian and his colleagues there is a concern about this interpretation because magma intrusion through a crack should radiate compressional first motion everywhere chouet s calculation on a magma intrusion model however showed that the main dynamic motion as well as the final static displacement have patterns similar to the clvd mechanism we found an example of observed radiation pattern similar to the clvd mechanism from eruption earthquakes at mount aso obtained by sassa and we interpret the observation as misidentification of the main motion as the initial motion in searching for a similar misidentification for the mammoth lakes earthquakes we found that there are numerous reports of compression for short period p waves at stations where long period p waves show unequivocal dilatation furthermore we found that the compressional short period first motion is systematically earlier than the dilatational one these observations can be explained if the source of magma intrusion generated compressional first motions which were very weak and of high frequency contents in the direction where long period p waves are dilatational the seismic moment of the mammoth lakes earthquakes suggests an involvement of magma volume of the order of 10 2 km3 however we have not yet observed long period events or volcanic harmonic tremors which are often observed at active volcanos we review recent advances in theories and observations on volcanic tremor and conclude that for the volume of magma reservoir suggested for the mammoth lakes area a fluctuation of pressure of the order of 10 2 bar would generate measurable tremor this conclusion is obtained under the assumption that both the viscosity of magma and the radiation loss of the magma reservoir are low enough for the excitation of a harmonic tremor we found that the concept of reduced displacement introduced by aki and koyanagi for body waves and by fehler for surface waves is very useful for estimating the physical parameters of a volcanic tremor source

earthquake magnitude recent research and current trends abstract the magnitude recommendations adopted by the iaspei assembly at zurich in 1967 have had both a stabilizing and a stimulating effect on magnitude determinations and related research from 1967 onwards one magnitude research paper has appeared on the average almost every week thus making this parameter the most studied one in seismology new facilities and more accurate methods e g concerning instrumental equipment and interpretation techniques have made it possible to improve earlier achievements the application of magnitude scales has been extended in all respects e g with regard to epicentral distances focal depths wave types and wave periods magnitude frequency relations have become the most investigated equations within seismology observationally as well as theoretically they have wide applications e g for estimating the maximum magnitudes of future earthquakes an important item in earthquake prediction the magnitudes provide significant information on other source parameters such as wave energy fault length seismic moment relations between different types of magnitude yield valuable information on source properties for instance relations between magnitudes based on body and surface waves are used for efficient discrimination between earthquakes and underground explosions it is our purpose to review the magnitude development in the post zurich period 1967 1980 partly for geologists tectonophysicists and engineers who need an overview partly for seismologists who need an introduction to an overwhelmingly comprehensive literature

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

volcanic 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

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

eruptions of pavlof volcano and their possible modulation by ocean load and tectonic stresses all nine magmatic eruptions at pavlof volcano in six of the years from 1973 to 1984 have occurred between september 9 and november 20 volumes of erupted material range from 0 1 to 15 9 106 m3 dense rock equivalent at an average rate of 3 106 m3 yr 1 the volumes are estimated from eyewitness reports for two eruptions the others are estimated from a relationship that we derive between eruption volume and harmonic tremor duration and amplitude the volume of lava erupted is approximately time predictable a significant correlation exists between the eruptions and yearly nontidal variations in sea level and may result from ocean loading calculated volume changes beneath the volcano due to ocean loading are from 0 02 to 2 times eruption volumes and we postulate that the volcano acts as a long period several month volume strain meter with lava being preferentially erupted when strain beneath the volcano is compressive the volcano did not erupt during the period 1978 1980 when tilt seismic data and sea level data indicate that deep aseismic slip may have occurred models of this event predict a volume increase beneath the volcano that might have compensated strain from magma injection these observations indicate that pavlof volcano may be responsive to small slow changes in ambient stresses or strains and that these changes may modify or trigger eruptions