seismic observations of transient subglacial water flow beneath macayeal ice stream west antarctica 1 new seismic observations of harmonic tremors beneath macayeal ice stream west antarctica are reported each of the two tremor events that we recorded during a six week period had sustained arrival of 3 hz energy for approximately 10 minutes during that time the source location migrated a few kilometers the harmonic nature of the tremors is interpreted as the result of resonance in subglacial water filled cracks and conduits the duration monochromatic nature and movement of the tremor indicate that the source mechanism is likely flow in the subglacial water system resulting from the discharge from a small subglacial lake our results suggest that the subglacial water system produces repeated small outburst floods with possible implications for ice stream dynamics

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

vibrational modes of hydraulic fractures inference of fracture geometry from resonant frequencies and attenuation oscillatory seismic signals arising from resonant vibrations of hydraulic fractures are observed in many geologic systems including volcanoes glaciers and ice sheets and hydrocarbon and geothermal reservoirs to better quantify the physical dimensions of fluid filled cracks and properties of the fluids within them we study wave motion along a thin hydraulic fracture waveguide we present a linearized analysis valid at wavelengths greater than the fracture aperture that accounts for quasi static elastic deformation of the fracture walls as well as fluid viscosity inertia and compressibility in the long wavelength limit anomalously dispersed guided waves known as crack or krauklis waves propagate with restoring force from fracture wall elasticity at shorter wavelengths the waves become sound waves within the fluid channel wave attenuation in our model is due to fluid viscosity rather than seismic radiation from crack tips or fracture wall roughness we characterize viscous damping at both low frequencies where the flow is always fully developed and at high frequencies where the flow has a nearly constant velocity profile away from viscous boundary layers near the fracture walls most observable seismic signals from resonating fractures likely arise in the boundary layer crack wave limit where fluid solid coupling is pronounced and attenuation is minimal we present a method to estimate the aperture and length of a resonating hydraulic fracture using both the seismically observed quality factor and characteristic frequency finally we develop scaling relations between seismic moment and characteristic frequency that might be useful when interpreting the statistics of hydraulic fracture events

shallow seismicity at montagne pelee volcano martinique lesser antilles continuous seismic monitoring at martinique since the 1902 eruption of the montagne pelee volcano did not detect local earthquakes for the first 70 years for the only eruption which occurred in this time span in 1929 the seismograph was 20 km away and of a standard type not particularly suited for the detection of small scale local seismicity improvement of the monitoring array over the last 15 years with the installation of sensors on the volcano itself allowed the detection of signals of local origin which were interpreted as being due to surface sources such as rockfalls and landslides since december 1985 seismic sources in the volcano itself i e small earthquakes at shallow depth were identified and located with the aid of a temporary upgrading of the array close to these weak sources such an onset of local seismicity could not have been detected with previous seismic equipment such episodes of seismicity in the volcano might have occurred in the past apparently quiescent history of the volcano as the reinterpretation of seismograms of some events in 1976 would indicate without evolving to more important volcanic phenomena for seismographs on volcanoes the constant upgrading of observation capabilities is certainly perferred to a strict continuity of standard observations

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

source parameters of injection induced microseismicity we analyze source parameters of microseismic events m 1 associated with high flow rate water injections in a shale formation at a depth of 220 m two types of events were observed several hundred impulsive events with clear p and s wave arrivals and continuous emissions with peaked spectra detected well into the experiment for a representative collection of impulsive events an 2 model provided satisfactory fits to displacement spectra corrected for attenuation and average quality factors of 34 and 15 were obtained for p and s waves p wave first motion analysis and e s e p ratios indicated the existence of a non double couple component in some events particularly early in the experiment a clear difference was observed for estimates of stress release parameters as non doublecouple events had smaller stress drops and apparent stresses the seismic efficiency of double couple and non double couple events was limited to 0 9 and 0 05 respectively with average values being 0 25 and 0 02 a comparison of our results with those reported for a similar magnitude range in a hard rock formation indicates considerably smaller estimates of stress drop and apparent stress in our case while seismic efficiencies are comparable

functional groups and elemental analyses of cuticular morphotypes of cordaites principalis germar geinitz carboniferous maritimes basin canada well preserved cuticles were isolated from cordaites principalis germar geinitz leaf compressions i e foliage from extinct gymnosperm trees coniferophyta order cordaitales the specimens were collected from the sydney stellarton and bay st george subbasins of the once extensive carboniferous maritimes basin of atlantic canada fourier transformation of infrared spectra ftir and elemental analyses indicate that the ca 300 306 million year old fossil cuticles share many of the functional groups observed in modern cuticles the similarities of the functional groups in each of the three cuticular morphotypes studied support the inclusion into a single cordaite leaf taxon i e c principalis germar confirming previous morphological investigations vitrinite reflectance measurements on coal seams in close proximity to the fossil bearing sediments reveal that the bay st george sample site has the lowest thermal maturity whereas the sites in sydney and stellarton are more mature ir absorption and elemental analyses of the cordaite compressions corroborate this trend which suggests that the coalified mesophyll in the leaves follows a maturation path similar to that of vitrinite comparison of functional groups of the cordaite cuticles with those from certain pteridosperms previously studied from the y1 sydney subbasin shows that in the cordaite cuticles highly conjugated c o 1632 cm bands dominate over carbonyl stretch that characterizes the pteridosperm cuticles the differences demonstrate the potential of chemotaxonomy as a valuable tool to assist distinguishing between carboniferous plant fossil groups published by elsevier science b v

exotic seismic phases recorded near mammoth lakes and their use in the delineation of shallow crustal magma anomalies observations of several hundredexotic seismic phases herein defined recorded in and near long valley caldera california have been cataloged i discuss here four classes of such seismograms 1 seismograms with missing s waves 2 seismograms with an unusual pre s phase seen at the single station slk northwest of the caldera 3 seismograms with a strong pre s phase as seen at a number of stations south of the caldera and 4 a very large very slow 2 km sec post s phase seen at the single station benton for each of these phenomena it is not yet possible to pin down an unambiguous and unique theoretical explanation however for each i have presented an explanation summarizing current thinking which involves nonplanar reflections refractions within shallow crustal anomalous zones which can reasonably be supposed to be magma bodies if these explanations are even partially pertinent then the investigation of exotic phases near complex regions like mammoth lakes and other volcanic areas is potentially a way to bring precise resolving power on the nature and geometry of local crustal anomalies

long period events the most characteristic seismicity accompanying the emplacement and extrusion of a lava dome in galeras volcano colombia in 1991 abstract since its reactivation in 1988 the principal eruptions of galeras volcano occurred on may 4 9 1989 july 16 1992 and january 14 march 23 april 3 april 14 and june 7 1993 the initial eruption was a phreatic event which clearly marked a new period of activity a lava dome was extruded within the main crater in october 1991 and subsequently destroyed in an explosive eruption on july 16 1992 the eruptions that followed were all vulcanian type explosions the seismicity accompanying the emplacement extrusion and destruction of the lava dome was dominated by a mix of long period lp events and tremor displaying a variety of waveforms repetitive lp events with dominant periods in the range 0 2 1 s were observed in october and november 1991 and visually correlated with short energetic pulses of gas venting through a crack bisecting the dome surface each lp event was characterized by a weak precursory signal with dominant periods in the range 0 05 0 1 s lasting roughly 7 s using the fluid driven crack model of chouet 1988 1992 we infer that two distinct cracks may have acted as sources for the lp and precursor signals spectral analyses of the data yield the following parameters for the lp source crack length 240 360 m crack width 130 150 m crack aperture 0 5 3 4 mm crack stiffness 100 500 sound speed of fluid 880 m s and excess pressure 0 01 0 19 mpa similar analyses yield the parameters of the precursor source crack length 20 30 m crack width 15 25 m crack aperture 2 3 8 7 mm crack stiffness 5 15 sound speed of fluid 140 m s and excess pressure 0 06 0 15 mpa combined with geologic and thermodynamic constraints obtained from field observations these seismic parameters suggest a gas release mechanism in which the episodic collapse of a foam layer trapped at the top of the magma column subjacent to the dome releases a slug of pressurized gas which escapes to the surface while dilating a preexisting system of cracks in the dome structure accordingly the fracture observed on the crystallized dome body is the surface extension of the lp source crack where lp activity is induced by the rapid emission and expansion of gas flowing through this conduit the width and aperture of the crack estimated in the model are in good agreement with the length and aperture of the fracture estimated from visual observations the source parameters of the precursor signal are suggestive of a nozzle like conduit connecting the lp source crack to the underlying magma reservoir excitation of this conduit segment is attributed to the rapid emission and acceleration of the frothy fluid resulting from the collapse of the foam layer at the top of the reservoir the calculated periodicity of foam collapse events is in agreement with the observed average rate of thirteen lp events per hour

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

saturated and subcooled hydrothermal boiling in groundwater flow channels as a source of harmonic tremor the potential of hydrothermal boiling in groundwater flow channels for generating harmonic tremor a relatively monochromatic ground vibration associated with volcanic activity is examined we use simple organ pipe theory of normal mode fluid vibration and fundamental energy considerations to develop a first order analytical model of a hydrothermal boiling source of harmonic tremor we use this model to estimate order of magnitude groundwater flow channel lengths and boiling heat transfer rates required to produce harmonic tremor with dominant frequencies in the range 0 5 5 hz and surface wave reduced displacements of up to 100 cm2 depending on groundwater sound speed flow channel lengths of the order of 1 1000 m are required to produce fluid vibration eigenfrequencies in the range 0 5 5 hz the boiling heat transfer rate required to produce tremor with a given surface wave reduced displacement depends on the tremor frequency and on whether saturated boiling or subcooled boiling is the cause of the tremor saturated boiling produces groundwater vibration via steam bubble growth whereas subcooled boiling produces groundwater vibration via steam bubble collapse we find that subcooled hydrothermal boiling is from 102 to 104 times more efficient than saturated boiling in converting boiling thermal power to seismic power for example the boiling heat transfer rates required to produce 1 hz tremor with reduced displacements of up to 100 cm2 via subcooled boiling are generally less than a few thousand megawatts for saturated boiling the required boiling heat transfer rates are several orders of magnitude larger than this the highest values of heat flow reported in the literature for volcanic crater lakes and terrestrial and ocean floor geothermal areas are of the order of 1000 mw taking this value as a first order estimate of an upper limit on possible boiling heat transfer rates in volcanic hydrothermal systems our results suggest that saturated hydrothermal boiling is capable of generating only low amplitude harmonic tremor with surface wave reduced displacements no higher than a few square centimeters however subcooled hydrothermal boiling could potentially generate high amplitude harmonic tremor with reduced displacements as large as several hundred square centimeters as a specific application of our model we evaluate the potential of hydrothermal boiling for generating harmonic tremor at recently active mount st helens and nevado del ruiz volcanoes we conclude that subcooled boiling likely could have produced the tremor episodes considered at both volcanoes saturated boiling also could explain the nevado del ruiz tremor but probably not the more powerful mount st helens tremor

multiple ruptures for long valley microearthquakes a link to volcanic tremor despite several episodes of ground deformation and intense seismic activity starting in 1978 the long valley california volcanic area has not produced clearly recognized volcanic tremor instead a variety of atypical microearthquakes have been recorded during these episodes including events dominated by low frequency long period or mixed high and low frequency hybrid signals during a 1997 episode a number of unusual microearthquakes occurred within a temporary 40 station seismic network surrounding the casa diablo area allowing the events to be precisely located and analyzed as a function of azimuth offset and source characteristics eight prime examples lie within two 7 km deep clusters of seismicity separated by about 1 km with four events in each cluster empirical green s function deconvolution shows that these events are composed of two to three sub events the sub events consisting of ordinary single rupture double couple microearthquakes the delay times between the sub events are constant within each cluster equaling 0 092 s in one and 0 06 s in the other events from other clusters show similar delays the signal interference produced by the closely spaced sub events gives rise to modulated delay dependent source spectra the regularity of the delays suggests that the sub events are being triggered by a fixed length and or time scale process an example being the length inflation rate of a magmatic or hydrothermal flow structure with continued action of the triggering process the sub events could proliferate and evolve into observable volcanic tremors at mammoth

reply to battaglia and cayol 2009 we thank cayol and battaglia 2009 hereafter referred to as cb2009 for commenting on our work houlie and montagner 2007 we start our response by reviewing our model and then discuss the potential of discrimination of tilt and displacement at a vbb seismometer from seismic records there appears to be a misunderstanding of the approach and the interpretation of our results presented in houlie and montagner 2007 we do not question the discovery by battaglia et al 2000 of transient deformation before volcanic eruptions as well as the existence of 2 types of signals but the interpretation is not necessarily as simple as proposed in their paper we note that the three components in the seismic record of displacement might have different explanations from a theoretical point of view any perturbation of displacement can be expressed as follows ui t j ui t xj dxj it means that ui t can be expressed by 9 independent terms since zj 0 at the surface of the earth there remain 6 degrees of freedom usually the symmetric part deformation eij related to the stress tensor in the elastic case is separated from the anti symmetric part that is related to rotation or tilt consequently distinguishing displacement and tilt is a real scientific challenge very long period period tn100 s transients are interpreted in terms of tilts battaglia et al 2000 but we explore the possibility that a large part of seismic record is related to translational displacement in addition of or rather than tilt rotation following this simple idea we draw the consequences of this hypothesis and propose a model which differs from previous approaches battaglia et al 2000 battaglia and bachelery 2003 battaglia et al 2005 there are two extreme models for interpreting the data the first one is only based on tilt associated with the dislocation model okada 1992 and a second one is based on ground displacement generated by a spherical source cb2009 claim that the whole signal can be explained by tilt but we claim that a significant part of the signal is related to ground displacement and that actual data cannot yet discriminate between these two models it is likely that both effects are present and the next challenge will be to determine where the right loci of the true solution are between these 2 extrememodels the intention of our paper houlie andmontagner 2007 was to draw attention to this fundamental issue which has important consequences for the understanding of volcano eruption dynamics we suggest that the actual data are not sufficient to provide a definitive answer to this issue our work shows that an interpretation of the currently available seismic record in terms of translational displacements is perfectly valid and that further work will be needed to discriminate between tilt and displacement models and combinations of the two

surface morphology of active normal faults in hard rock implications for the mechanics of the asal rift djibouti abstract tectonic stretching models have been previously proposed to explain the process of continental break up through the example of the asal rift djibouti one of the few places where the early stages of seafloor spreading can be observed in these models deformation is distributed starting at the base of a shallow seismogenic zone in which sub vertical normal faults are responsible for subsidence whereas cracks accommodate extension alternative models suggest that extension results from localised magma intrusion with normal faults accommodating extension and subsidence only above the maximum reach of the magma column in these magmatic rifting models or so called magmatic intrusion models normal faults have dips of 45 55 and root into dikes vertical profiles of normal fault scarps from levelling campaign in the asal rift where normal faults seem sub vertical at surface level have been analysed to discuss the creation and evolution of normal faults in massive fractured rocks basalt lava flows using mechanical and kinematics concepts we show that the studied normal fault planes actually have an average dip ranging between 45 and 65 and are characterised by an irregular stepped form we suggest that these normal fault scarps correspond to sub vertical en echelon structures and that at greater depth these scarps combine and give birth to dipping normal faults the results of our analysis are compatible with the magmatic intrusion models instead of tectonic stretching models the geometry of faulting between the fieale volcano and lake asal in the asal rift can be simply related to the depth of diking which in turn can be related to magma supply this new view supports the magmatic intrusion model of early stages of continental breaking

operation of a digital seismic network on mount st helens volcano and observations of long period seismic events that originate under the volcano during 1981 we operated with the cooperation of the u s geological survey a 9 station digital seismic array on mount st helens volcano in washington state one of our stations was placed inside the crater of the volcano six were located on the flanks of the volcano within two km of the crater and two were approximately ten km from the crater four of the instruments recorded three components of motion and the remaining five recorded only the vertical component in addition to the monitoring and recording performed by the digital instruments we carried out a one day experiment during which we complimented our crater monitoring seismometer by the addition of two ink recording instruments during the one day experiment we had six observers including 4 usgs scientists who had significant experience working and making observations in the crater recording times of rockfall felt earthquake occurrences and changes in steam emissions from the dome in the crater using information obtained during the one day experiment we are able to classify seismic events recorded by our digital instruments as earthquakes rockfalls helicopter noise and a type of event that is unique to volcanoes which we call long period waveforms of these long period events have a duration of up to 30 seconds and a spectrum that is peaked at approximately 2 hz the frequency at which the peak in the spectrum occurs is nearly the same at all stations which means that the unique waveform of long period events is due to a source effect not a path effect the peak frequency is fairly insensitive to the amplitude of the signal which means that the size of the source region is constant independent of the signal amplitude long period events were not felt and were accompanied by no visible changes inside the crater which lead us to conclude that they are some sort of seismic disturbance generated inside the volcano

seismicity associated with eruptive activity at langila volcano papua new guinea abstract during periods of eruptive activity at langila volcano various combinations of low frequency earthquakes and high frequency air waves have been recorded ranging from earthquakes alone to air waves alone it is shown that these different types of events are part of a continuous range of events which are all interpreted to be explosions at or near the summit continuous tremor is also often recorded for durations of several minutes to hours and is classified into two types harmonic and non harmonic spectral analysis shows that the non harmonic tremor resembles a string of low frequency events in contrast the spectra of harmonic tremor show simple modal characteristics which indicate some type of resonating source this source is thought to include a gas filled cavity above the magma body within the volcano

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

analysis of simultaneous gravity and tremor anomalies observed during the 2002 2003 etna eruption abstract in this paper we discuss the data collected by a large aperture array of broadband seismometers and a continuously recording gravity station during the 2002 2003 eruption of etna volcano italy seismic signals recorded during the eruption are dominated by volcanic tremor whose energy spans the 0 5 5 hz frequency band on three different occasions 12 november 19 20 november and 8 9 december 2002 we observed marked increases of the tremor amplitude up to a factor of 4 which occurred simultaneously with gravity decreases up to 30 gal the three concurrent gravity tremor anomalies last 6 to 12 hours and terminate with rapid up to 2 hours changes after which the signals return back to their original levels based on volcanological observations encompassing the simultaneous anomalies we infer that the accumulation of a gas cloud at some level in the conduit plexus feeding a new eruptive vent could have acted as a joint source this study highlights the potential of joint gravity seismological analyses to both investigate the internal dynamic of a volcano and to improve the confidence of volcanic hazard assessment

degassing and hydrothermal activity at mt spurr alaska during the summer of 2004 inferred from the complex frequencies of long period events 1 results of analyses of unusual long period earthquakes recorded between july and september 2004 at mt spurr alaska are presented the waveforms of these events are characterized by quasi sinusoidal signatures of long duration up to 40 sec with slowly decaying amplitudes bandwidths of 0 5 4 0 hz are typical amplitude spectra are marked by strong and sharp peaks reflecting the quasi monochromatic nature of the signal the temporal variations of the complex frequencies are investigated by use of the sompi method the dominant mode is resolved and its q factor estimated for each available event dominant frequencies are found in the band 0 8 2 2 hz with q varying between 25 and 100 the variations of the complex frequencies show an overall decline with time the dynamic response of a shallow fracture filled with bubbly water to the flux of hot gases from depth is proposed as a possible mechanism for the generation of the observed waveforms

analysis of long period events recorded at mount etna italy in 1992 and their relationship to eruptive activity seismic activity recorded at mount etna during 1992 was characterized by long period lp events and tremor with fluctuating amplitudes these signals were associated with the evolution of the eruptive activity that began on december 14 1991 following the occurrence of numerous volcano tectonic earthquakes at the onset of the eruption lp events dominated the overall seismicity starting in january 1992 the lp activity occurred primarily in swarms which were temporally correlated with episodic collapses of the crater floor in the northeast crater source depths determined for selected lp events suggest a source region located slightly east of northeast crater and extending from the surface to a depth of 2000 m based on the characteristic signatures of the time series four families of lp events are identified each family shares common spectral peaks independent of azimuth and distance to the source these spectral features are used to develop a fluid filled crack model of the source we hypothesize that the locus of the lp events represents a segment of the magma feeding system connecting a depressurizing magma body with a dike extending in the sse direction along the western wall of valle del bove toward the site of the mount etna eruption we surmise that magma withdrawal from the source volume beneath northeast crater may have caused repeated collapses of the crater floor some collapse events may have produced pressure transients in the subjacent dike which acted as seismic wave sources for lp events

eruption dynamics at mount st helens imaged from broadband seismic waveforms interaction of the shallow magmatic and hydrothermal systems 1 the current eruption at mount st helens is characterized by dome building and shallow repetitive long period lp earthquakes waveform cross correlation reveals remarkable similarity for a majority of the earthquakes over periods of several weeks stacked spectra of these events display multiple peaks between 0 5 and 2 hz that are common to most stations lower amplitude very long period vlp events commonly accompany the lp events we model the source mechanisms of lp and vlp events in the 0 5 4 s and 8 40 s bands respectively using data recorded in july 2005 with a 19 station temporary broadband network the source mechanism of the lp events includes 1 a volumetric component modeled as resonance of a gently nnw dipping steam filled crack located directly beneath the actively extruding part of the new dome and within 100 m of the crater floor and 2 a vertical single force attributed to movement of the overlying dome the vlp source which also includes volumetric and single force components is 250 m deeper and nnw of the lp source at the sw edge of the 1980s lava dome the volumetric component points to the compression and expansion of a shallow magma filled sill which is subparallel to the hydrothermal crack imaged at the lp source coupled with a smaller component of expansion and compression of a dike the single force components are due to mass advection in the magma conduit the location geometry and timing of the sources suggest the vlp and lp events are caused by perturbations of a common crack system

a tensile shear crack model for the mechanism of volcanic earthquakes abstract temporary seismic observations were carried out for about 1 month at miyakejima island on the izu bonin arc after an eruption on october 3 1983 more than 3000 seismic events were observed and they are classified into 4 different categories short period earthquakes long period earthquakes isolated tremors and continuous tremors two active source regions of the short period earthquakes were located along the old caldera rim of the miyakejima volcano one region was very close to the eruptive fissures another was about 1 2 km northwest of the fissures almost all the earthquakes in the former region showed dilatant first motions of p waves at all the stations in the island on the other hand almost all events in the latter region showed compressive first motions of p waves at all the stations the first motions of p waves for these earthquakes cannot be explained by the quadrant type pattern due to a double couple force system nucleation of a tensile crack coupled with a shear crack a tensile shear crack model is proposed as a source mechanism of the short period earthquakes moment tensor elements of tensile and shear cracks have been determined by the least squares method using observed p and s wave data results suggest that tensile cracking opening or closing is dominant for generating those earthquakes in comparison with shear cracking the strikes of the tensile cracks for the earthquakes are shown to be nearly parallel to the strike of the fissures the earthquakes that occurred in the northwestern region of the fissures are considered to be generated by a sudden opening of tensile shear cracks due to the excess pressure of intrusive magma in contrast the earthquakes occurring beneath the fissures are probably generated by a sudden closing of tensile shear cracks and this suggests that the magmatic pressure under the fissures rapidly decreased after the eruption moment tensor analysis of the tensile shear crack model in this study demonstrates the non double couple source mechanism for volcanic earthquakes related to magmatic activity

do some shallow earthquakes have a tensile source component the laboratory tests carried out in studying shear and tensile seismogenic displacements occurring in compressed samples led us to search for earthquakes with a tensile source component to determine this component in the seismic focus a special procedure based on the construction of radiation patterns of the combined shear tensile type is introduced

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

a review of the discrete wavenumber method we present a review of the discrete wavenumber dwn method the method introduced by bouchon and art 1977 allows the simple and accurate calculation of the complete green s functions for many problems in elastodynamics

unusual low frequency volcanic seismic events with slowly decaying coda waves observed at galeras and other volcanoes abstract events with slowly decaying coda waves are observed in active andesitic volcanoes associated mainly with vulcanian type eruptions or large gas emissions however these signals are also recorded in some quiescent volcanoes these unusual signals are considered to be related to magmatic activity and generally occur beneath the active crater the signals have been observed to be a short term precursor galeras volcano colombia 1992 1993 asama yama volcano japan 1983 after eruption tokachi dake volcano japan 1989 during seismic swarms meakan dake volcano japan 1982 and during quiescence puracevolcano colombia 1994 1995 tarumai volcano japan 1970 1971 1975 spectral analysis reveals common characteristics for this type of signal the spectrum is characterized by one or several sharp frequency peaks fundamental frequencies are not affected by epicentral distance azimuth or travel time indicating a source effect the damping coefficient for coda waves ranges from 0 002 to 0 02 and is related to large values of the quality factor q c ranging from 250 to 25 respectively these parameters may be the result of large amounts of gas bubbles in the magma body

physical and biogenic aspects of backbarrier sedimentary sequences georgia coast u s a abstract configuration of the present georgia coast with its broad expanses of salt marsh and intricate tidal drainages behind pleistocene holocene barrier islands implies the filling of an extensive backbarrier lagoonal system however riverine and salt marsh estuaries dominate the modern depositional setting and reconstructed vertical sequences suggest that backbarrier filling was accomplished by estuarine processes backbarrier sediments exhibit considerable diversity in grain size and physical and biogenic sedimentary structures principal sediment textures include clean to muddy coarse and fine sand mud and to a lesser extent shells pebbles and mud and bedrock clasts physical sedimentary structures many modified by bioturbation include large scale cross bedding trough and planar with graded sets small scale crossbedding ripple lamination and flaser wavy and lenticular bedding laminated to interbedded sand and mud and rare pebble or shell beds ebb oriented structures predominate over flood oriented ones except in upper reaches of riverine estuaries biogenic sedimentary structures typically are diverse and abundant characteristic lebensspuren include polychaete dwelling tubes decapod and polychaete burrows ray holes and bioturbate textures imparted by amphipods ophiuroids bivalves and anemones patterns of bioturbation are controlled to some extent by sediment type salinity levels and rates of deposition and physical reworking of sediments although intensity of bioturbation generally increases seaward bioturbate textures are more readily preservable in the lower energy middle reaches of estuaries because of numerous local pleistocene sediment sources and local variations in hydrographic regimes the stratigraphic record of salt marsh estuaries differs relatively little from that of riverine estuaries the basic sequence in ascending order typically consists of 1 channel lag and or channel fill deposits 2 estuarine accretionary beds 3 point bar tidal flat and shoal sediments and 4 channel margin and salt marsh deposits

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

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

precursory swarms of long period events at redoubt volcano 1989 1990 alaska their origin and use as a forecasting tool abstract during the eruption of redoubt volcano from december 1989 through april 1990 the alaska volcano observatory issued advance warnings of several tephra eruptions based on changes in seismic activity related to the occurrence of precursory swarms of long period lp seismic events dominant period of about 0 5 s the initial eruption on december 14 occurred after 23 years of quiescence and was heralded by a 23 hour swarm of lp events that ended abruptly with the eruption after a series of vent clearing explosions over the next few days dome growth began on december 21 another swarm with lp events similar to those of the first began on the 26th and ended in a major tephra eruption on january 2 eruptions continued over the next two weeks and then ceased until february 15 when a large eruption initiated a long phase of repetitive dome building and dome destroying episodes that continued into april warnings were issued before the major events on december 14 and january 2 but as the eruptive sequence continued after january 2 the energy of the swarms decreased and forecasting became more difficult a significant but less intense swarm preceded the february 15 eruption which was not forecast this eruption destroyed the only seismograph on the volcanic edifice and stymied forecasting until march 4 when the first of three new stations was installed within 3 km of the active vent from march 4 to the end of the sequence on april 21 there were eight eruptions six of which were preceded by detectable swarms of lp events although weak these swarms provided the basis for warnings issued before the eruptions on march 23 and april 6 the initial swarm on december 13 had the following features 1 short duration 23 hours 2 a rapidly accelerating rate of seismic energy release over the first 18 hours of the swarm followed by a decline of activity during the 5 hours preceding the eruption 3 a magnitude range from 0 4 to 1 6 4 nearly identical lp signatures with a dominant period near 0 5 s 5 dilatational first motions everywhere and 6 a stationary source location at a depth of 1 4 km beneath the crater this occurrence of long period events suggests a model involving the interaction of magma with groundwater in which magmatic gases steam and water drive a fixed conduit at a stationary point throughout the swarm the initiation of that sequence of events is analogous to the failure of a pressure relief valve connecting a lower supercharged magma dominated reservoir to a shallow hydrothermal system a three dimensional model of a vibrating fluid filled crack recently developed by chouet is found to be compatible with the seismic data and yields the following parameters for the lp source crack length 280 380 m crack width 140 190 m crack thickness 0 05 0 20 m crack stiffness 100 200 sound speed of fluid 0 8 1 3 km s compressional wave speed of rock 5 1 km s density ratio of fluid to rock 0 4 and ratio of bulk modulus of fluid to rigidity of rock 0 03 0 07 the fluid filled crack is excited intermittently by an impulsive pressure drop that varies in magnitude within the range of 0 4 to 40 bar such disturbance appears to be consistent with a triggering mechanism associated with choked flow conditions in the crack

spasmodic tremor and possible magma injection in long valley caldera eastern california intensive microearthquake swarms with the appearance of volcanic tremor have been observed in the southwest part of long valley caldera southeastern california this activity possibly associated with magma injection began 6 weeks after several strong magnitude 6 earthquakes in an area south of the caldera and has continued sporadically to the present time the earthquake sequence and magmatic activity are part of a broad increase in tectonic activity in a 15 000 square kilometer region surrounding the white mountains seismic gap an area with high potential for the next major earthquake in the western great basin

episodes of pit crater collapse documented by seismology at piton de la fournaise calderas are oval fault bounded depressions in volcanic regions the formation of a small analog a summit pit crater has followed a ten month cycle of eruptive activity at piton de la fournaise volcano reunion island indian ocean in 1985 six short episodes of summit seismic activity with shocks indicating shear failures have been followed by eruptions in five cases the eruptions occurred near the base of the summit cone which is at 2 5 km elevation inside the enclos fouque an ancient caldera 7 km in diameter in another instance the summit seismic crisis did not evolve to an eruption but was followed by a seismic crisis 4 km deeper in march 1986 low level activity both at shallow depth beneath the summit and around 4 km depth to the southeast a simultaneous occurrence not previously observed culminated by a summital swarm on march 18 activity resumed its low level and five hours later a small and short lived eruption occurred at mid distance between the summit and the rim of enclos fouque one day later a flank eruption began at 1000 m elevation outside of the enclos 9 km southeast of the summit four days later effusion began near sea level 14 km southeast of the summit the shift of activity from the vent inside to that outside the enclos occurred whilst both shallow summital and deeper flank seismicity were sustained but without notable shallow activity between one vent and the next then the seismic activity which in all five previous instances stopped when the vents opened persisted for ten days increasing in energy release rate activity only consisted of shallow shocks in the summit area whilst eruption was low on the flank 9 km then 14 km away the focal mechanisms are different from all those previously determined they correspond neither to shear failure nor tensile fracturing by fluid overpressure they suggest cracking due to volume increase in a low fluid flow environment which may be consistent with the simultaneous tapping of magma deeper in the edifice a final paroxysmal seismic crisis lasting one and a half days led to the collapse of a 200 m diameter 80 m deep pit crater inside the dolomieu summital caldera on march 29 this crisis released an order of magnitude more energy than previous activity the long period signature of these seismic events indicates low stress drop with respect to source dimensions sources are shallow however rather than directly marking the progressive development of a depression at the surface they appear to correspond to fractures opening above the retreating magma and progressive stoping of a cavity similarities with seismic observations during the last phase of the fernandina galapagos 8 km2 caldera collapse of june 1968 suggest that the same kind of mechanical behaviour occurred with a scaling of three orders of magnitude in seismic moments and in volumes of collapsed blocks between the two cases

laboratory evidence for krauklis wave resonance in fractures and implications for seismic coda wave analysis abstractkrauklis waves are of major interest because they can lead to resonance effects in fluid filled fractures this resonance is marked by seismic signals with a dominant signature frequency which may reveal fracture related rock properties in our laboratory study we used homogeneous plexiglas samples containing a single well defined i e manufactured fracture we recorded the signals obtained from propagating ultrasonic p and s waves source frequency 0 6 1 and 2 25 mhz along a sample without a fracture and samples with a fracture with different inclination angles of 30 45 and 60 with respect to the short axis the experimental results obtained from an incident s wave confirmed that the presence of the fracture led to resonance effects at frequencies lower than the dominant source frequency which slowly decayed over time in the recorded seismic coda after the first arrival the resonance frequency was independent of the fracture orientation and the source frequency we have interpret

the earthquake sequence of june 1992 near saba west indies abstract during june 1992 a minor earthquake swarm occurred close to the island of saba a small composite volcano which is the northernmost member of the inner volcanically active lesser antilles arc the initial phase of the seismic activity was recorded at some permanent eastern caribbean seismic stations the nearest of which was located 55 km away on 14 june 1992 a seismograph was installed on saba to improve the detection and location capacities of the permanent network with regard to this swarm sixty events were detected of which only fifteen could be located the largest earthquake of the sequence occurred on 11 june 1992 had a duration magnitude of m d 4 5 and was felt at modified mercalli intensities iv v in saba and iii in st kitts the epicentres exhibit a sw ne trend and focal depths are mainly clustered in the range 10 16 km a t value of 0 73 was obtained for the sequence spectral analysis yielded various source parameters moments in the range 0 38 10 13 23 66 10 13 nm source radii from 87 to 126 m and stress drops from 1 3 to 79 9 bar moment increases with increasing magnitude while stress drop appears to decrease with decreasing seismic moment similar to many other studies although the evidence as to the nature of the origin of this swarm is not conclusive it seems to suggest a tectonic rather than volcanic origin

temporal evolution of a hydrothermal system in kusatsu shirane volcano japan inferred from the complex frequencies of long period events 1 we present a detailed description of temporal variations in the complex frequencies of long period lp events observed at kusatsu shirane volcano using the sompi method we analyze 35 lp events that occurred during the period from august 1992 through january 1993 the observed temporal variations in the complex frequencies can be divided into three periods during the first period the dominant frequency rapidly decreases from 5 to 1 hz and q of the dominant spectral peak remains roughly constant with an average value near 100 during the second period the dominant frequency gradually increases up to 3 hz and q gradually decreases from 160 to 30 during the third period the dominant frequency increases more rapidly from 3 to 5 hz and q shows an abrupt increase at the beginning of this period and then remains roughly constant with an average value near 100 such temporal variations can be consistently explained by the dynamic response of a hydrothermal crack to a magmatic heat pulse during the first period crack growth occurs in response to the overall pressure increase in the hydrothermal system caused by the heat pulse once crack formation is complete heat gradually changes the fluid in the crack from a wet misty gas to a dry gas during the second period as heating of the hydrothermal system gradually subsides the overall pressure in this system starts to decrease causing the collapse of the crack during the third period

fluid dynamics inside a wet volcano inferred from the complex frequencies of long period lp events an example from papandayan volcano west java indonesia during the 2011 seismic unrest abstract we present results of our study aimed at understanding the dynamics of fluids inside a wet volcano through the analysis of swarms of long period lp events accompanying the 2011 seismic unrest at papandayan volcano west java indonesia prior to this unrest we measured an extremely high percentage 100 of co2 in the ground at the summit crater however with a very low value of so2 flux 6 tons day increase in volcanic activity was also observed from the records of a tiltmeter a long term inflation was followed by an abrupt deflation that took place concurrently with the swarms of lp events thereafter swarms of local tectonic lt and volcano tectonic vt earthquakes started we focus here on analyzing the lp events in the following manner first we estimate the source location of lp events by applying a 3 d non linear hypocenter localization algorithm which includes topography we then study the waveforms and spectral characteristics of lp events recorded at different stations and investigate whether or not these characteristics are due to source effects subsequently we compute the oscillation frequencies f and the decay characteristics q factor in the complex frequency domain of the coda part of the lp events by using the sompi method which is based on a homogeneous auto regressive ar equation the rectangular fluid filled crack model is used to estimate the physical processes related to the observed temporal variations in the complex frequencies we divide the swarms of lp events into two intervals the first interval occurred between june and july 2011 48 lp events while the second interval extended from september to october 2011 36 lp events the frequencies of lp events observed during these intervals range between 1 1 and 6 2 hz while the q factors are widely scattered between 20 and 400 we estimate the compositions of fluids inside the crack during both intervals as either water foam mixtures of water and h2o gas steam or misty gas mixtures of water droplets and h2o gas steam we finally suggest that if an eruption were to have taken place following the 2011 unrest it would have been in phreatic style rather than magmatic style the results of our study therefore contribute to the effort in the prediction of the behavior of future eruptions and to volcanic hazards assessment and therefore to volcanic risk mitigation

volcanic tremors and shocks associated with eruptions at etna sicily september 1980 abstract three strong ash eruptions superimposed on an otherwise rather quiet state of volcanic activity took place from the northeast crater of etna during september 1980 the eruptions lasted only a few hours and were associated with an increase in the amplitudes of volcanic tremors by a factor of 10 compared with the mean value of the preceding months special emphasis in the tremor analysis was given to the eruption of september 26 1980 the time function of the amplitude envelope of the tremors follows a mathematical function which resembles the behaviour of a mechanical oscillator with a resonant period of about 300 s a detailed spectral analysis of the tremors before during and after the eruption reveals factors which seem to be associated with the ascent and descent of the tremor sources in the magma columns the turbulent eruptive magma flow apparently is concentrated in a few dykes and pipes their eigenvibrations during an eruption are observed as spectral peaks of small bandwidth superimposed on the smoother spectrum without a volcanic eruption an analysis of seismicity shows a maximum in august with epicenters clustering along a nw se direction

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

strong motion seismology strong motion seismology is concerned with earthquake ground motion in the amplitude range that poses the threat of human injury or property damage because the occurrence of strong motion cannot be predicted in advance and because the time between significant events at any one place may range from tens to thousands of years strong motion data is recorded on triggered instruments that typically must be maintained in readiness for years between periods of significant recording as a consequence the strong motion data set is sparse because of the engineering need for estimates of future ground motion the scope of the field of strong motion seismology encompasses seismic source theory and any other aspect of seismology that can provide insights helpful in making the best possible estimates given the limited data this relationship with the rest of seismology is a reciprocal one in that strong motion data gives perhaps the best information available on earthquake source processes

shallow seismic events with combined source mechanism abstract the non shear source component of shallow seismic events such as crustal earthquakes and mining tremors is examined the radiation patterns of a pure shear model as well as a combined shear tensile seismic source model were compared with the isc bulletin first motion signs although the set of events studied is not statistically significant a series of earthquakes was found for which the combined shear tensile solution was more consistent than the pure double couple one in the combined mechanism the shear component with preferably dip slip orientation is dominant while the tensile component only accounts for a small percentage of the shear component along faults with strike slip motion such as the anatolian fault no events with a combined mechanism were detected which points to a possible relation between tensile fracturing and dip slip displacements a set of weak mining tremors which occurred in the mid bohemian coal mining region was treated by the amplitude inversion method and a combined shear implosive source mechanism was found the implosive component was small while the dominant shear component coincided well with the local tectonic fault i e these mining tremors were almost purely of a tectonic nature if this were found to be true generally then even weak mining tremors could be utilized as large scale models of tectonic earthquakes

structures and evolution of the plumbing system of piton de la fournaise volcano inferred from clustering of 2007 eruptive cycle seismicity analysis of seismic activity associated with the eruptions of 2007 which led to the collapse of the dolomieu crater on april 5th reveals the link between the seismicity and the magma transfers at piton de la fournaise three eruptive phases occurred on february 18th march 30th and april 2nd 2007 at the summit 2 000 m and 600 m high on the south east flank respectively illustrating the three types of eruptions defined for the current piton de la fournaise activity we use cross correlation of seismic waveforms and clustering to improve the earthquake locations and determine the best constrained focal mechanisms with an average of 78 p phase polarities the pre eruptive seismicity of the february and march eruptions is composed of time extended clusters that also preceded other distal eruptions from 2000 to 2007 our analysis shows that the seismic swarm prior to the february eruption initiated the intrusion that led to the april 2007 eruption the seismicity preceding the dolomieu crater collapse consists of numerous but time limited clusters and specific seismic activity that accompanied the dolomieu crater collapse from april 1st to april 5th earthquakes with clvd mechanisms combined with normal faulting sources occurred between 0 8 and 0 km asl until the complete rupture of the shallow magma storage roof this collapse induced the propagation of a de pressurization front and triggered a migration of seismicity from 0 to 8 km along a very narrow path

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

seismic events with non shear component i shallow earthquakes with a possible tensile source component the concept of seismicity of fast tensile fracturing is introduced and supported by the results of shear and of combined shear and tensile displacements along a loaded stress concentrator the seismicity of tensile fracturing is demonstrated by means of acoustic elastic signals obtained during tensile fracturing in physical models under load the basic physical relations between the parameters characterizing the loaded medium load conditions shear and tensile displacements and release of acoustic energy are presented for determining the tensile source component in earthquakes a procedure based on the construction of radiation patterns is suggested and submitted for discussion the criteria for selecting earthquakes with possible shear tensile source mechanisms are listed and discussed the existence of such a combined seismic source is sought in two shallow earthquakes which occurred in southern iran in march 1977 in general the paper should be regarded as a proposal to utilize the radiation characteristics of a seismic source with all their insufficiencies as a quick and simple tool for seeking combined shear and tensile mechanisms of seismic energy release