upper crustal low frequency seismicity at mount st helens detected with a dense geophone array abstract mount st helens has been closely monitored by a permanent network including 16 seismometers located within a 15 km radius yet some of its diverse seismic signals remain challenging to detect and relate to deformation processes data from a dense array of 904 geophones within 15 km of mount st helens provide novel constraints on weak seismic signals observed during a two week deployment in 2014 here we report on the identification of weak low frequency lf events that were either not detected by the permanent network or were misclassified as likely rockfalls we refer to the signals as lf earthquakes because they have less power above 10 hz and longer durations compared to similar magnitude volcano tectonic vt events reverse time imaging with the entire geophone array indicates that the lf hypocenters overlap with the frequent vt seismicity in a sub vertical column beneath the summit crater at 6 km below sea level beamforming of p wave arrivals with an exceptionally dense sub array yields slowness estimates that confirm a subsurface origin for 16 lf events occurrence of vt and lf events in approximately the same volume suggests distinct source processes rather than path effects as the primary explanation for their different signals some of the lf and vt events have multiple spectral peaks that may be consistent with source mechanisms involving resonance in fluid filled cavities however the majority of lf and vt event spectra exhibit smooth amplitude decay with increasing frequency consistent with shear failure sources the depletion of high frequencies for lf events could be caused by unusually low rupture velocities or stress drops our results indicate a variety of earthquake source processes occur in a sub vertical column at 6 km depth which is thought to overly the primary upper crustal magma reservoir lf events are typically associated with eruptive or near eruptive activity and are generally not reported at depths greater than 1 km at mount st helens but based on our short duration high density experiment we suggest that upper crustal lf events may be more common than previously recognized and occur across a broader depth range

seismic features of the june 1999 tectonic swarm in the stromboli volcano region italy abstract crustal tectonic seismicity in the southern tyrrhenian sea is characterized by the high occurrence rates of earthquakes to the west of the alignment of salina lipari and vulcano islands in the aeolian archipelago only a few earthquakes affect the crustal region east of these islands whereas intermediate and deep seismicity plays a relevant role based on this evidence two aspects of the seismic swarm recorded at the aeolian island seismic network between june 6 and 17 1999 looked anomalous the first aspect concerned the number of earthquakes 78 that affected the stromboli submarine edifice in a short time interval secondly despite the low maximum magnitude md 3 2 reached the cumulative strain release was conspicuous in comparison with previous swarms in this region we localized the swarm about 6 km northeast of stromboli at a depth between 8 and 12 km the source region was identified using standard methods of hypocentral location as well as azimuth analysis it is worth noting that the volcanic activity at stromboli did not change significantly during the swarm nor throughout the following months therefore the seismic swarm had no link with volcanic activity observed at the surface most of the earthquakes shared similar waveform and frequency content and can be divided into families we identified some earthquakes with magnitude up to md 3 having relatively low frequency content at different seismic stations this anomalous feature leads us to hypothesize the presence of fluid circulation and or propagation of seismic waves in a ductile medium our hypothesis is in agreement with studies on marine geology which highlight various forms of submarine volcanism in the southern basin of the tyrrhenian sea

observations of long period earthquakes accompanying hydraulic fracturing waveforms of most seismic events accompanying hydraulic fracturing have been reported to contain clear p and s waves and have fault plane solutions consistent with shear displacement across a fault this observation is surprising since classical hydraulic fracturing theory predicts the creation of a tensile opening of a cavity in response to fluid pressure very small long period events similar to long period earthquakes observed at volcanoes were found to occur during four hydraulic fracturing experiments carried out at fenton hill new mexico since the long period earthquakes occur in the same region as the shear type events it is concluded that the unusual character of the long period earthquake waveforms is due to a source effect and not a path effect the occurrence of long period earthquakes during hydraulic fracturing could indicate tensile fracturing many waveforms of these events are identical which implies that these events represent repeated activation of a given source a proposed source for these long period events is the sudden opening of a channel that connects two cracks filled with fluid at different pressures the sizes of the two cracks differ which causes two or more peaks to appear in the spectra each peak beingmore associated with one physical dimension of each crack from the frequencies at which spectral peaks occur crack lengths are estimated to be between 3 and 20m less

explosion quakes at stromboli italy abstract this paper reports the results of two seismic experiments aimed at determining the wave field of explosion quakes at stromboli island mediterranean sea southern italy the typical strombolian activity mostly consists of explosive phenomena causing pyroclastic materials to be emitted together with jets of volcanic gases from one or more craters stromboli is an active volcano characterized by persistent seismic activity consisting of explosion quakes that are seismic events associated with the explosive volcanic phenomena explosion quakes are short lived seismic events occurring intermittently whose amplitude tends to decrease with distance from the vent a distinctive feature of explosion quakes is the presence on seismograms of two often clearly distinct seismic phases the first low frequency seismic phase 3 4 hz after one second or more the first seismic phase of explosion quakes has been shown to be characterized by a nearly radial linear polarization and by an apparent propagation velocity estimated at 600 800 m s the second phase is characterized by a more chaotic motion and a lower apparent propagation velocity of 150 450 m s the wavefield associated with the first low frequency seismic phase appears to be generated by a resonating p wave seismic source accompanying gas explosion and emission of pyroclastic materials the wavefield associated with the second high frequency seismic phase of explosion quakes appears to be mainly composed of scattered and converted waves due to the critical topography of the volcano

seismicity associated with the reactivation of nevado del ruiz colombia july 1985 december 1986 abstract renewed activity of nevado del ruiz was first evidenced in november 1984 by increased fumarolic activity and felt earthquakes this increased activity prompted an installation of seismometers on the volcano in july 1985 earthquakes recorded in 1985 indicate that most of the seismic activity prior to the eruption on november 13 1985 was confined to an l shaped epicentral zone beneath the summit of the volcano a swarm prior to the november eruption originated west of the summit telemetered stations installed after the eruption enabled better locations of post eruptive activity majority of located post eruptive earthquakes originate within 5 km of the summit at a depth less than 7 km and only few exceeded magnitude 3

seismoacoustic measurements during the july august 2001 eruption of mt etna volcano italy abstract on july 18 2001 two main eruptive vents opened on the southern flank of mount etna volcano italy at 2100 m and 2550 m a s l respectively the former vent fed mild strombolian activity and lava flows while the latter represented the main explosive vent producing strong phreato magmatic explosions explosions at this latter vent however shifted to a strombolian style in the following days before switching back to phreato magmatic activity towards the end of the eruption which ended on august 9 2001 on august 3 a small seismoacoustic array was deployed close to the eruptive vents the array was composed of three stations which recorded seismic and infrasonic waves coming from both of the eruptive vents a further seismoacoustic station equipped with a thermal infrared sensor was also installed several kilometers north of the first array seismic signals relating to the strombolian activity at the 2100 m vent were characterized by a strong decompression at the source analysis of the time delays between seismic infrasonic and infrared event onsets also revealed that ejection velocities during explosions from both vents were subsonic time delays between the onset of explosive events apparent in the infrared and infrasound data indicated that the explosion source at the 2550 m vent was located 220 250 m below the crater rim in comparison the depth of the seismic source was estimated to be between 230 and 335 m below the rim this converts to 120 150 and 130 235 m below the preexisting ground surface in addition time delays between seismic and infrasonic signals recorded for the lower 2100 m vent also revealed a seismic source that was no more than a few tens of meters deeper than the fragmentation surface

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

effects of long term fluid injection on induced seismicity parameters and maximum magnitude in northwestern part of the geysers geothermal field the long term temporal and spatial changes in statistical source and stress characteristics of one cluster of induced seismicity recorded at the geysers geothermal field u s are analyzed in relation to the field operations fluid migration and constraints on the maximum likely magnitude two injection wells prati 9 and prati 29 located in the northwestern part of the field and their associated seismicity composed of 1776 events recorded throughout a 7 year period were analyzed the seismicity catalog was relocated and the source characteristics including focal mechanisms and static source parameters were refined using first motion polarity spectral fitting and mesh spectral ratio analysis techniques the source characteristics together with statistical parameters b value and cluster dynamics were used to investigate and understand the details of fluid migration scheme in the vicinity of injection wells the observed temporal spatial and source characteristics were clearly attributed to fluid injection and fluid migration toward greater depths involving increasing pore pressure in the reservoir the seasonal changes of injection rates were found to directly impact the shape and spatial extent of the seismic cloud a tendency of larger seismic events to occur closer to injection wells and a correlation between the spatial extent of the seismic cloud and source sizes of the largest events was observed suggesting geometrical constraints on the maximum likely magnitude and its correlation to the average injection rate and volume of fluids present in the reservoir

seismicity associated with dome growth and collapse at the soufriere hills volcano montserrat varied seismicity has accompanied growth and collapse of the lava dome of the soufriere hills volcano montserrat earthquakes have been classified as either volcano tectonic long period or hybrid and daily variations in the numbers of events have mapped changes in the style of eruption repetitive hybrid earthquakes were common during the first months of dome growth in july 1996 the style of seismicity changed and regular short lived hybrid earthquake swarms became common this change was probably caused by an increase in the magma flux earthquake swarms have preceded almost all major dome collapses and have accompanied cyclical deformation thought to be due to a build up of pressure in the upper conduit which is later released by magma moving into the dome

depth migration of seasonally induced seismicity at the geysers geothermal field seismicity from injected fluids provides insight into the hydraulically conductive fracture network at the geysers tg california geothermal reservoir induced earthquakes at tg result from both thermoelastic and poroelastic stresses as injected fluids cool the rocks and increase pore pressure the spatiotemporal evolution of m 1 5 seismicity is characterized as a function of depth in the northwest and southeast regions of tg to develop time dependent earthquake rates using an epidemic type aftershock sequence model the seismicity and injection follow an annual cycle that peaks in the winter months and is correlated by depth the results indicate a time lag of 6 months for fluids to migrate 3 km below the injection depth water injection is the main cause of seismicity as fluids penetrate into the reservoir our results suggest that a steeply dipping fracture network of hydraulically conductive faults allows fluid migration to a few kilometers below the point of injection

a temporal change in coda wave attenuation observed during an eruption of mount st helens during the past few years there have been numerous reports of changes in coda wave attenuation occurring before major earthquakes these observations are important because they may provide insight into stress related structural changes taking place in the focal region prior to the occurrence of large earthquakes the results of these studies led us to suspect that temporal changes in coda wave attenuation might also accompany volcanic eruptions by measuring power decay envelopes for earthquakes at mount st helens recorded before during and after an eruption that took place during september 3 6 1981 we found that coda q sup 1 for frequencies between 6 and 30 hz was 20 30 higher before the eruption than after the change is attributed to an increase in the density of open microcracks in the rock associated with inflation of the volcano prior to the eruption q sup 1 was found to be only weakly dependent on frequency and displayed a slight peak near 10 hz the weak frequency dependence is attributed to the dominance of intrinsic attenuation over scattering attenuation since it is generally accepted that intrinsic attenuation is constant with frequency whereas scattering attenuation decreases strongly at higher frequencies the weak frequency dependence of q sup 1 more at mount st helens contrasts with results reported for studies in nonvolcanic regions the peak in q sup 1 near 10 hz at mount st helens is attributed to the scale length of heterogeneity responsible for generating backscattered waves results for nonvolcanic regions have shown this peak to occur near 0 5 hz thus a smaller scale length of heterogeneity is required to explain the 10 hz peak at mount st helens copyright american geophysical union 1988 less

geology of the gouap iron deposit congo craton southern cameroon implications for iron ore exploration abstract the ntem complex represents the northwestern edge of the archean congo craton in southern cameroon where numerous banded iron formation bif hosted iron deposits have been discovered the gouap iron deposit which is located on the western edge of the ntem complex is one of the recent discoveries of iron deposits in this area in this study geological mapping detailed drillcore logging and petrographical studies reveal that the gouap iron deposit comprises of two lithostratigraphic units namely the bif and the country rocks the bif unit consists of amphibole bif and amphibole pyrite bif whereas the country rocks unit is made up of gneisses garnet amphibole magnetite and amphibole garnet pyrite gneiss schists amphibole magnetite and garnet magnetite micaschist quartzites and dolerite sills the bifs are well interlayered and observed to be in sharp or gradational contacts to their associated country rocks the mineral assemblages of the bifs and wallrocks except the dolerites indicate that both units have been metamorphosed under amphibole facies conditions two main iron mineralization types are identified in the gouap deposit the soft iron type which is mainly located in the western and northern parts and the hard iron type mostly confined to the southern sector of the deposit the structural controls of the gouap iron deposit are better observed in drillcore samples other than on the outcrop scale there are three principal deformation events d1 d2 and d3 that led to the development of foliation folding and faulting respectively the iron mineralization are mostly concentrated on the f2 fold hinge zones and fractures that acted as passage ways for the circulation of meteoric water at surface we suggest that the following three major processes that led to the formation of the gouap deposit i the chemical precipitation of chert and iron oxide followed by sedimentation and diagenesis that formed chert magnetite bifs ii secondly the amphibolite facies metamorphism and deformation that formed quartz magnetite bifs which constitute the main iron ridge located on thicker zones of fold noses and iii finally the gouap bifs have been upgraded to medium grade iron ore due to supergene enrichment processes which are restricted to the modern land surface and locally reached deeper levels through fault and fracture zones the western and northern parts of the gouap deposit have been identified as potential exploration target areas for iron mineralization even though the iron formation processes of the gouap deposit are not yet completely understood and many questions remain unanswered regarding the genesis of the soft and hard iron mineralization these new insights have important implications on the exploration drilling strategies of this deposit

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

seafloor seismic monitoring of an active submarine volcano local seismicity at vailulu u seamount samoa 1 we deployed five ocean bottom hydrophones obhs for a 1 year seismic monitoring study of vailulu u seamount the youngest and easternmost volcano in the samoan archipelago four instruments were placed on the summit crater rim at 600 700 m water depth and one was placed inside the crater at 1000 m water depth an analysis of the first 45 days of records shows a very large number of seismic events 211 of them local these events define a steady background activity of about four seismic events per day increasing to about 10 events per day during a week of heightened seismic activity which peaked at 40 events during 1 day we identified 107 earthquakes whose arrivals could be picked on all five stations and that are likely located within the seamount based on their similar waveforms two linear trends are defined by 21 of these events these are extremely well correlated and located first downward then upward on a steeply inclined plane that is close to the axial plane of the southeast rift as it emerges from the main summit of vailulu u these events resemble volcanotectonic earthquakes from subaerial volcanoes in displaying very coherent seismic waveforms and by showing systematic narrowly defined progressions in hypocenter locations we propose that these events reflect brittle rock failure due to magma redistribution in or near a central magma reservoir

seismic and aseismic deformations and impact on reservoir permeability the case of egs stimulation at the geysers california usa author s jeanne p rutqvist j rinaldi ap dobson pf walters m hartline c garcia j abstract 2015 american geophysical union all rights reserved in this paper we use the seismicity based reservoir characterization approach to study the spatiotemporal dynamics of an injection induced microseismic cloud monitored during the stimulation of an enhanced geothermal system and associated with the northwest geysers enhanced geothermal system egs demonstration project california we identified the development of a seismically quiet domain around the injection well surrounded by a seismically active domain then we compare these observations with the results of 3 d thermo hydro mechanical simulations of the egs which accounts for changes in permeability as a function of the effective normal stress and the plastic strain the results of our modeling show that 1 the aseismic domain is caused by both the presence of the injected cold water and by thermal processes these thermal processes cause a cooling stress reduction which prevent shear reactivation and favors fracture opening by reducing effective normal stress and locally increasing the permeability this process is accompanied by aseismic plastic shear strain 2 in the seismic domain microseismicity is caused by the reactivation of the preexisting fractures resulting from an increase in injection induced pore pressure our modeling indicates that in this domain permeability evolves according to the effective normal stress acting on the shear zones whereas shearing of preexisting fractures may have a low impact on permeability we attribute this lack of permeability gain to the fact that the initial permeabilities of these preexisting fractures are already high up to 2 orders of magnitude higher than the host rock and may already be fully dilated by past tectonic straining

seismic precursors of the february 10 1990 eruption of kelut volcano java abstract kelut volcano eastern java indonesia erupted on february 10 12 1990 seismic activity and an increase of temperature and acoustic noise in the crater lake appeared three months before the eruption and were the main precursory signs of this eruption during the pre eruptive stage the number of events was eight per day on average and the cumulative seismic energy released was 5 6 10 15 erg these low values are uncommon for an andesitic volcano seismic activity mainly consisted of swarms of very shallow volcano tectonic earthquakes many of which were part of seismic multiplets in the last three weeks before the eruption seismic activity remained at a very low level this quiescent period was followed 24 hours prior to the eruption initiation by an intense swarm of low energy events which progressively became continuous spasmodic tremor most of the data obtained during the pre eruptive stage are consistent with a process of fracturation of a shallow and low strength plug blocking the eruptive conduit before the eruption several observations suggest that water is an important element in the preeruptive and eruptive phenomena at kelut volcano in particular the monochromatic events observed at kelut are characterized by one narrow peak in their spectra the estimations of q value and impedance contrast for these events suggest that the fluid which fills the source resonator could be water

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

subevents of long period seismicity implications for hydrothermal dynamics during the 2004 2008 eruption of mount st helens 1 one of the most striking aspects of seismicity during the 2004 2008 eruption of mount st helens msh was the precise regularity in occurrence of repetitive long period lp or drumbeat events over sustained time periods however this precise regularity was not always observed and at times the temporal occurrence of lp events became more random in addition accompanying the dominant lp class of events during the 2004 2008 msh eruption there was a near continuous randomly occurring series of smaller seismic events these subevents are not always simply small amplitude versions of the dominant lp class of events but appear instead to result from a separate random process only loosely coupled to the main lp source mechanism we present an analysis of the interevent time and amplitude distributions of the subevents using waveform cross correlation to separate lp events from the subevents we also discuss seismic tremor that accompanied the 8 march 2005 phreatic explosion event at msh this tremor consists of a rapid succession of lps and subevents triggered during the explosion in addition to broadband noise from the sustained degassing immediately afterward seismicity returned to the pre explosion occurrence pattern this triggering in relation to the rapid ejection of steam from the system and subsequent return to pre explosion seismicity suggests that both seismic event types originated in a region of the subsurface hydrothermal system that was 1 in contact with the reservoir feeding the 8 march 2005 phreatic explosion but 2 not destroyed or drained by the explosion event finally we discuss possible thermodynamic conditions in a pressurized hydrothermal crack that could give rise to seismicity pressure drop estimates for typical lp events are not generally large enough to perturb pure water in a shallow hydrothermal crack into an unstable state however dissolved volatiles such as co2 may lead to a more unstable system increasing the seismogenic potential of a hydrothermal crack subject to rapid heat flux the interaction of hydrothermal and magmatic systems beneath msh in 2004 2008 thus appears able to explain a wide range of observed phenomena including subevents lp events larger md 2 events and phreatic explosions

locations and spectral properties of earthquakes accompanying an eruption of mount st helens a joint hypocenter determination routine has been developed to accurately determine the locations of earthquakes occurring under mount st helens the method has a special application to mount st helens where most of the earthquakes are suspected to occur at shallow depth in a rather limited volume under the volcano differences in elevation among stations are allowed for in this routine the location procedure determines a best half space velocity and one station correction for each station simultaneously with earthquake hypocenters arrival time data from an array of portable digital seismic event recorders are supplemented with data from stations of the permanent university of washington seismic network operating at the volcano earthquakes are found to occur along a line trending south east of the dome inside the crater the shallowest events occur near the dome with hypocentral depth increasing with increasing distance from the dome station corrections can be interpreted to imply the existence of a high velocity body beneath the crater which extends at least between depths of 5 and 2 1 km beneath the crater floor spectra are calculated from earthquake waveforms recorded at three stations the spectra show very little evidence for local site or path effects on frequency content of waveforms stress drops of earthquakes are calculated from measurements of corner frequency and low frequency spectral amplitude stress drops of earthquakes vary between 5 and 5 bars there is no evidence for a limiting corner frequency for small earthquakes as has been found for earthquakes occurring in mammoth lakes california data fit well a linear trend between log seismic moment and log source radius fit to data for earthquakes ranging in size from 1014 to 1027 dyn cm suggesting that there is no limiting source radius observed for earthquakes in this range of size

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

cross correlation analysis of seismic and volcanic data at mt etna volcano italy seismic data from the mvt sln sesmic station located 7 km from the summit area of mt etna volcano which has been operating steadily for the last two decades have been analysed together with the volcanic activity during the same period cross correlation techniques are used to investigate possible relationships between seismic and volcanic data and to evaluate the statistical significance of the results a number of significant correlations have been identified showing that there is an evident relation between seismic events and flank eruptions and a less clear relation with summit activity which appears more linked to tremor rather than to the low frequency events particularly interesting are the low frequency events whose rate of occurrence increases starting from 17 to 108 days prior to the onset of the flank activity and are candidates for a useful precursor on the other hand a tendency towards the increase in both the duration and the occurrence rate of transients in the volcanic tremor was observed before the onset of summit eruptions as a result of this study different stages in the volcanic activity of mt etna represented by changes in the characteristics of the recorded seismic phenomena are identified

the 1998 earthquake sequence south of long valley caldera california hints of magmatic involvement a significant episode of seismic and geodetic unrest took place at long valley caldera california beginning in the summer of 1997 activity through late may of 1998 was concentrated in and around the south moat and the south margin of the resurgent dome the sierran nevada block snb region to the south southeast remained relatively quiet until a m 5 1 event occurred there on 9 june 1998 ut a second m 5 1 event followed on 15 july ut both events were followed by appreciable aftershock sequences an additional distinct burst of activity began on 1 august 1998 the number of events in the august sequence over the first week or two was similar to the aftershock sequence of the 15 july 1998 m 5 1 event but the later sequence was not associated with any events larger than m 4 3 all of the summer 1998 snb activity was considered tectonic rather than magmatic in general the snb is considered an unlikely location for future eruptions however the august sequence an aftershock sequence without a mainshock is suggestive of a strain event larger than the cumulative seismotectonic strain release moreover a careful examination of waveforms from the august sequence reveals a small handful of events whose spectral signature is strikingly harmonic we investigate the waveforms of these events using spectral autocorrelation and empirical green s function techniques and conclude that they were most likely associated with a fluid controlled source our observations suggest that there may have been some degree of magma or magma derived fluid involvement in the 1998 snb sequence

volcanic earthquakes and their relationship to eruptions at ruapehu and ngauruhoe volcanoes abstract crustal earthquakes near ruapehu and ngauruhoe fall into two classes each of which can be subdivided on the one hand there are high frequency events 3 hz with sharp well defined phases mainly concentrated beneath ruapehu crater lake low frequency events the low frequency events resemble minakami s b type and explosion earthquakes but sometimes occur where no vent exists and rather deeper than his formal definition difficulty in applying minakami s classification rigorously and the fact that low frequencies may be due to abnormal attenuation of higher frequencies along the path rather than to their suppression or absence at the source has led to reclassification of earthquakes near the volcanoes into two broad groups tectonic and volcanic the former includes all high frequency earthquakes and those discrete events in which dominant low frequencies are due to path effects the latter includes multiple and emergent events which show evidence of prolonged or repetitive source mechanism dominant low frequencies are ascribed to occurrence in heat weakened material and high frequencies to instantaneous source mechanisms operating in competent rock the term volcano tectonic describes tectonic earthquakes within some arbitrary distance of a volcano at ngauruhoe and ruapehu volcanic earthquakes accompany explosive vent clearing eruptions subsequent open vent degassing and ash emission however although often powerful and prolonged usually occurs without earthquakes such activity is however frequently accompanied by volcanic tremor at ruapehu under closed vent conditions when lake temperature is low low frequency earthquakes up to magnitude ml 3 4 have occurred without any eruption five types of phreatic eruptions are identified at ruapehu each having a distinctive seismic pattern the three most explosive types appear to be generated by a chain reaction process and all involve flashing of water to steam the first by failure of the roof with little precursory seismicity after a closed vent period during which lake temperature decreases the second after prolonged heating of the lake and much preliminary volcanic tremor interpreted as due to rising magma and the third under open vent conditions in the wake of one of the two preceding types a fourth probably occurs in wet sediments near the base of the lake as a result of upward migration of hot gas and a fifth aseismic or accompanied by very weak volcanic tremor is associated with convective overturn within crater lake

complex magma dynamics at mount etna revealed by seismic thermal and volcanological data 1 three eruptive episodes during the 2006 summit eruptions of mount etna were exceptionally well documented by visual seismic and thermal monitoring the first 16 november was strongly explosive with vigorous strombolian activity and ash emission from multiple vents lava emission and phreatomagmatic explosions generating pyroclastic density currents pdcs the second episode 19 november had a rather weakly explosive component with mild strombolian activity but more voluminous lava emission the third 24 november was a moderately explosive paroxysm with intermittent lava fountaining and generation of a tephra column as well as lava emission and pdcs data recorded by a thermal monitoring camera clearly document the different phases of each paroxysm weather clouds occasionally hampering thermal monitoring the images show a rapid onset of the volcanic activity which during each of the paroxysms reached a peak in eruptive and thermal intensity and then decreased gradually the stronger phreatomagmatic explosions and pdcs on 16 and 24 november did not yield any seismic signature linked to the opening of new vents nor were they associated with peculiar characteristics of the seismic signal nevertheless eruptive styles strombolian activity lava emission and different levels in the intensity of explosive activity were generally well reflected in the amplitude and frequency content of the seismic signal and in the source location of the volcanic tremor centroid throughout the three eruptive episodes this multidisciplinary study therefore not only provides a key to distinguish between endogenous and exogenous origins of the phenomena observed but also documents the complex magma dynamics within the volcano

relationship between tremor and volcanic activity during the southeast crater eruption on mount etna in early 2000 1 the southeast crater of mount etna italy was characterized by a violent eruptive activity between 26 january and 24 june 2000 this activity produced 64 lava fountain episodes with repose periods from 3 hours to 10 days we estimated a volume of about 15 20 106 m3 lava and at least 2 3 106 m3 of tephra we compared the paroxysmal volcanic activity to its associated seismic signature the high number of events highlighted a strict correlation between tremor and volcanic activity seismic and volcanic characteristics such as the frequency of occurrence the duration of lava fountains and the associated tremor energy suggested the subdivision of the studied period into two stages separated by the 20 february event combining volcanic with seismic data we observed some useful relationships among lava fountain height sustained column height and reduced displacement in addition we found that the entire episode was well correlated with the duration of the amplitude increase computing the tremor energy linked to each event the total energy associated with lava fountains episodes results in 76 of the energy released during the whole period finally the different ratios among the overall spectral amplitude of the seismic signals of the stations located at different altitudes suggested to us the elaboration of a simple qualitative model to explain the dynamic behavior of the tremor source during the whole episode

on the chouet model of excitation of a buried magmaticpipe rhythmic motion of the ground tremor is often associated with activity before and during a volcanic eruption volcanic tremor is characterized by dominant peaks located in the 0 1 10 hz range of the spectrum in the light of previous formulations concerning the vibrations of a uniform vertical magma filled pipe this work presents the analysis of the motion when the section of the pipe is not constant but has constrictions and meanders our results show that the magma velocity modulus and pressure are inversely proportional to the pipe s cross section consequently the effect of the motion of the magma gives rise to enlargements of the pipe s constrictions

correlations between so2 flux and long period seismicity at galeras volcano the 14 january 1993 eruption of galeras volcano in colombia which killed six scientists and three tourists1 was followed by a larger eruption on 23 march 19932 both eruptions were preceded by episodes of long period seismicity the source of long period seismic events has been modelled extensively3 8 as the resonance within a fluid filled crack induced by pressure transients in the fluid5 7 here we use the so2 flux from galeras volcano measured remotely to infer the degassing history during the episode of long period events preceding the 23 march eruption so2 flux and long period seismicity have been monitored separately elsewhere to forecast volcanic activity9 13 our results show how the combination and correlation of the two methods can be used to interpret the movement of gases from the magma body to the surface and to monitor the pressure buildup leading to an eruption at explosive volcanoes

a numerical investigation of choked flow dynamics and its application to the triggering mechanism of long period events at redoubt volcano alaska we use numerical simulations of transonic flow through a crack to study the dynamics of the formation of shock waves downstream from a nozzle like constriction inside the crack the model solves the full set of navier stokes equations in two dimensions via an explicit multifield finite difference representation the crack walls are assumed to be perfectly rigid and elastic coupling to the solid is not considered the simulations demonstrate how the behavior of unsteady shock waves near the walls can produce recurring step like pressure transients in the flow which in turn induce resonance of the fluid filled crack the motion of the shock waves is governed primarily by smooth low amplitude pressure fluctuations at the outlet of the crack the force induced on the walls scales with the amplitude of the shock which is a function of the magnitude of the inlet pressure aperture of the constriction and thickness of the boundary layer the applied force also scales in proportion to the spatial extent of the shock excursion which depends on the fluctuation rate of outlet pressure using the source parameters of long period lp events at redoubt volcano alaska as a guide for our simulations we infer that coupling of the shock to the walls occurs for crack inlet to outlet pressure ratios pi po 2 31 and that the position of the shock front becomes most sensitive to outlet pressure fluctuations for flow regimes with pi po 2 48 for such regimes fluctuations of outlet pressure of up to 0 5 mpa at rates up to 3 mpa s are sufficient to induce pressure transients with magnitudes up to 12 5 mpa over 0 1 2 5 m of the walls within 0 5 s these flow parameters may be adequate for triggering the lp events in the precursory swarm to the december 14 1989 eruption of redoubt according to the flow model the recurrence rate and amplitudes of l p events are inferred to be a manifestation of the response of a shallow hydrothermal reservoir to the sustained injection of superheated steam from a magma column roofing below this reservoir

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

detailed multidisciplinary monitoring reveals pre and co eruptive signals at nyamulagira volcano north kivu democratic republic of congo this paper presents a thorough description of nyamulagira s january 2010 volcanic eruption north kivu democratic republic of congo based on a combination of field observation and ground based and space borne data it is the first eruption in the virunga volcanic province that has been described by a combination of several modern monitoring techniques the 2010 eruption lasted 26 days and emitted 45 5 106 m3 of lava field observations divided the event into four eruptive stages delimited by major changes in effusive activity these stages are consistent with those described by pouclet 1976 for historical eruptions of nyamulagira co eruptive signals from ground deformation seismicity so2 emission and thermal flux correlate with the eruptive stages unambiguous pre eruptive ground deformation was observed 3 weeks before the lava outburst coinciding with a small but clear increase in the short period seismicity and so2 emission the 3 weeks of precursors contrasts with the only precursory signal previously recognized in the virunga volcanic province the short term increase of tremor and long period seismicity which for example were only detected less than 2 h prior to the 2010 eruption the present paper is the most detailed picture of a typical flank eruption of this volcano it provides valuable tools for re examining former mostly qualitative descriptions of historical nyamulagira eruptions that occurred during the colonial period

seismic activity at koryakski volcano in 1994 hybrid seismic events and their implications for forecasting volcanic activity abstract a small swarm of 80 earthquakes was observed beneath koryakski volcano between march 1 and may 31 1994 these earthquakes ranged from 0 to 8 km in depth and from 2 0 to 1 5 in magnitude ml the swarm under koryakski volcano was primarily of volcano tectonic character although a special type of event has been identified with a small amplitude signal preceding the p wave onset these precursory signals had varying durations ranging between a few seconds and a few tens of seconds the source of the signal invariably coincided with the hypocenter of the subsequent earthquake the origin of these signals may be conjectured to be magma emplacement into weakened zones which generated a rupture giving rise to a normal tectonic earthquake the existence of this kind of signals may indicate magmatic activity under the volcano and provide a method for estimating futher development of volcanic activity

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

analysis of infrasonic and seismic events related to the 1998 vulcanian eruption at sakurajima abstract we present results from a detailed analysis of seismic and infrasonic data recorded over a four day period prior to the vulcanian eruptive event at sakurajima volcano on may 19 1998 nearly one hundred seismic and infrasonic events were recorded on at least one of the nine seismic infrasonic stations located within 3 km of the crater four unique seismic event types are recognized based on the spectral features of seismograms including weak seismic tremor characterized by a 5 6 hz peak mode that later shifted to 4 5 hz long period events are characterized by a short duration wide spectral band signal with an emergent high frequency onset followed by a wave coda lasting 15 20 s and a fundamental mode of 4 2 4 4 hz values of q for long period events range between 10 and 22 suggesting that a gas rich fluid was involved explosive events are the third seismic type characterized by a narrow spectral band signal with an impulsive high frequency onset followed by a 20 30 second wave coda and a peak mode of 4 0 4 4 hz volcano tectonic earthquakes are the fourth seismic type prior to may 19 1998 only the tremor and explosion seismic events are found to have an infrasonic component like seismic tremor infrasonic tremor is typically observed as a weak background signal explosive infrasonic events were recorded 10 15 s after the explosive seismic events and with audible explosions prior to may 19 on may 19 high frequency impulsive infrasonic events occurred sporadically and as swarms within hours of the eruption these infrasonic events are observed to be coincident with swarms of long period seismic events video coverage during the seismic infrasonic experiment recorded intermittent releases of gases and ash during times when seismic and acoustic events were recorded the sequence of seismic and infrasonic events is interpreted as representing a gas rich fluid moving through a series of cracks and conduits beneath the active summit crater

potential low grade iron ore deposits in metamorphosed banded iron formations northern province south africa exploitation of low grade iron ore would be quite unique in a south african context as south africa is well endowed with high grade iron ore resources low grade iron ore defined as containing between 20 and 47 iron is thought to be the primary iron bearing lithology from which most high grade ore deposits formed through different processes of enrichment the low grade iron ores in the northern province represent meta banded iron formations bifs with an average iron content of about 36 the main iron bearing mineral is magnetite the northern province ores have to be milled to sizes smaller than 150 m in order to liberate the iron minerals from the host rock and beneficiation is accomplished through a series of magnetic separation processes irrespective of the in situ quality of the ore final concentrates of exceptionally good quality with more than 69 iron and very low contaminant levels can be produced this combined with mass yields of between 40 and 50 and iron recoveries greater than 80 are excellent for this type of iron ore deposit the beneficiation products are suitable for use in iron and steel making processes

a spherical source model for low frequency volcanic earthquakes a resonant magma chamber is modeled by a spherically symmetric whole space consisting of a small gas filled chamber surrounded by magma which is encased in country rock source boundary conditions are specified on the cavity boundary simulating a possible sudden gas expansion in the magma the simplicity of this model leads to an analytical closed form solution for the fourier transform of radial displacement which predicts a peaked spectrum and an oscillating decaying seismogram for physically reasonable values of the rock and magma elastic properties and dimensions of the model the dominant frequency of the signal is in the range of 1 to 5 hz near the values observed for low frequency volcanic earthquakes and volcanic tremor furthermore the seismograms and spectra bear resemblance to those observed for low frequency earthquakes the fundamental mode of oscillation is a pure radial breathing mode which has markedly different properties from single cavity solutions the fundamental frequency is insensitive to the magma chamber boundary radius when both the inner and outer radii are greater than specified values this may help to explain the apparent similarity of the dominant frequency of volcanic earthquakes over a wide range of physical situations the presence of an inner effective bubble of a certain minimum size is necessary for the existence of the dominant low fundamental frequency spectral peak although highly idealized we believe that this fundamental fluid solid cavity mode may provide an explanation for the observations of tremor and low frequency volcanic earthquakes

tectonic model for the evolution of the limpopo belt abstract constraints on which any model for the tectonic evolution of the limpopo belt must be based include the following data crustal thickening to at least 65 km between 2700 and 2650 ma ago was responsible for the formation of the granulite terrane exposed in the limpopo belt today this crustal thickening probably resulted from the thrusting of the kaapvaal craton over the zimbabwe craton along the south dipping triangle tuli sabi shear zone other northwardverging thrusts related to this event occur in the kaapvaal craton south of the limpopo belt in the central and southern marginal zones and the kaapvaal craton this shortening is directed to the southwest while in the northern marginal zone it was directed to the north northwest peak metamorphism was superimposed on the thickened crust and then nearly isothermal decompression of several kbars occurred throughout the belt during this decompression rocks moved upward and spread outward onto the adjacent cratons from the zone of thickened crust along several inward directed shear zones creating a regional pop up high grade rocks were thrust over low grade rocks on the cratons producing along the terrane boundary retrograde metamorphism in the limpopo belt and prograde metamorphism on the cratons

observations of seismic volcanic signals at stromboli volcano italy abstract digital recordings of ground motion during a seismic operation organized at stromboli volcano italy from may 31 1986 to june 26 1986 have been studied the amplitude of background seismic noise is greater at the summit than at the base of the volcano and the spectra calculated on different days are dominated by at least two frequencies 2 0 and 6 0 hz with subdominant frequencies at 4 1 4 8 and 8 hz according to their waveform and their spectral content three types of discrete volcanic signals are distinguishable the first type is characterized by a dominant frequency at 2 hz and a subdominant frequency at 4 1 hz the second type also has a dominant peak frequency at 2 hz with two important subdominant frequencies at 5 and 8 hz the type 3 has a sharp dominant frequency at 6 hz and subdominant frequency at 3 2 hz the frequencies observed for discrete signals and ground noise suggest that they are produced by the same repeated mechanism which probably has a volcanic origin the discrete signals are linked to explosions at the active vents within the craters

seismological aspects of the 1989 1990 eruptions at redoubt volcano alaska the ssam perspective abstract ssam is a simple and inexpensive tool for continuous monitoring of average seismic amplitudes within selected frequency bands in near real time on a pc based data acquisition system during the 1989 1990 eruption sequence at redoubt volcano the potential of ssam to aid in rapid identification of precursory long period lp event swarms was realized and since this time ssam has been incorporated in routine monitoring efforts of the alaska volcano observatory in particular an eruption that occurred on april 6 was successfully forecast primarily on the basis of recognizing the precursory lp activity on ssam of twenty two significant eruptions that occurred between december 14 and april 21 eleven had precursory swarms longer than one hour in duration that could be detected on ssam for individual swarms the patterns of relative spectral amplitudes are distinct at each station and remain largely stationary through time thus indicating that one source may have been preferentially and repeatedly activated throughout the swarm typically a single spectral band dominates the signal at each seismic station for the vigorous one day swarm that preceded the first eruption on december 14 signals were sharply peaked in the 1 9 2 7 hz band at the closest station located 4 km from the vent but were dominated by 1 3 1 9 hz energy at three more distant stations located 7 5 22 km from the vent the tendency for the signals from different swarms recorded at the same station to be peaked in the same frequency band suggests that all of the sources are characterized by a predominant length scale signals from the precursory lp swarms became weaker as the eruption sequence progressed and swarms that occurred in march and april could only be detected at seismographs on the volcanic edifice onset times of precursory lp swarms prior to eruptions ranged from a few hours to about one week but after the initial vent clearing phase that ended december 19 these intervals tended to become progressively shorter for successive swarms these trends in the relative onset times and intensities of successive precursory lp swarms are consistent with an overall depressurization of the magmatic system through time in general each of the swarms had an emergent onset but the intensities did not always increase steadily until the eruptions instead as the time of an eruption approached the intensity usually increased more rapidly before peaking and then declining prior to the eruption for three of the swarms two distinct peaks in intensity were apparent the time intervals between final peaks in swarm intensity and ensuing eruptions ranged from about 2 hours to almost 2 days but the peaks always occurred closer to the eruptions than to the swarm onsets both the onset of lp swarm activity and a decline in intensity prior to an eruption may represent critical points in the process of pressurization that drives the flow of fluids and gas in a sealed magmatic system a notable exception to this pattern is the eruption of march 9 which lacked a detectable precursory lp swarm but was followed by an unusually long period of strong lp seismicity that may have been stimulated by a depressurization of the magmatic system resulting from dome failure on both december 14 and january 2 the spectra of early syn eruptive signals have peaked signatures much like those of the spectra of precursory lp activity from shortly before the eruptions these similarities may indicate that the source of precursory seismicity continued to be active during at least the early part of each eruption in syn eruptive signals from march and april recorded at stations on the volcanic edifice the dominant spectral energy progressively shifts with time during the eruption to lower frequencies at least part of the energy in these signals may have been generated by the debris flows associated with dome failures

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

seismic evolution of the 1989 1990 eruption sequence of redoubt volcano alaska abstract redoubt volcano in south central alaska erupted between december 1989 and june 1990 in a sequence of events characterized by large tephra eruptions pyroclastic flows lahars and debris flows and episodes of dome growth the eruption was monitored by a network of five to nine seismic stations located 1 to 22 km from the summit crater notable features of the eruption seismicity include 1 small long period events beginning in september 1989 which increased slowly in number during november and early december 2 an intense swarm of long period events which preceded the initial eruptions on december 14 by 23 hours 3 shallow swarms 0 to 3 km of volcano tectonic events following each eruption on december 15 4 a persistent cluster of deep 6 to 10 km volcano tectonic earthquakes initiated by the eruptions on december 15 which continued throughout and beyond the eruption 5 an intense swarm of long period events which preceded the eruptions on january 2 and 6 nine additional intervals of increased long period seismicity each of which preceded a tephra eruption hypocenters of volcano tectonic earthquakes suggest the presence of a magma source region at 6 10 km depth earthquakes at these depths were initiated by the tephra eruptions on december 15 and likely represent the readjustment of stresses in the country rock associated with the removal of magma from these depths the locations and time history of these earthquakes coupled with the eruptive behavior of the volcano suggest this region was the source of most of the erupted material during the 1989 1990 eruption this source region appears to be connected to the surface by a narrow pipe like conduit as inferred from the hypocenters of volcano tectonic earthquakes concentrations of shallow volcano tectonic earthquakes followed each of the tephra eruptions on december 15 these shocks may represent stress readjustment in the wall rock related to the removal of magma and volatiles at these depths this shallow zone was the source area of the majority of long period seismicity through the remainder of the eruption the long period seismicity likely reflects the pressurization of the shallow portions of the magmatic system

modelling low frequency volcanic earthquakes in a viscoelastic medium with topography summary magma properties are fundamental to explain the volcanic eruption style as well as the generation and propagation of seismic waves this study focusses on magma properties and rheology and their impact on low frequency volcanic earthquakes we investigate the effects of anelasticity and topography on the amplitudes and spectra of synthetic low frequency earthquakes using a 2 d finite difference scheme we model the propagation of seismic energy initiated in a fluid filled conduit embedded in a homogeneous viscoelastic medium with topography we model intrinsic attenuation by linear viscoelastic theory and we show that volcanic media can be approximated by a standard linear solid sls for seismic frequencies above 2 hz results demonstrate that attenuation modifies both amplitudes and dispersive characteristics of low frequency earthquakes low frequency volcanic earthquakes are dispersive by nature however if attenuation is introduced their dispersion characteristics will be altered the topography modifies the amplitudes depending on the position of the seismographs at the surface this study shows that we need to take into account attenuation and topography to interpret correctly observed low frequency volcanic earthquakes it also suggests that the rheological properties of magmas may be constrained by the analysis of low frequency seismograms

statistical analysis of intermittent volcanic tremor associated with the september 1989 summit explosive eruptions at mount etna sicily the pattern of volcanic tremor accompanyingthe 1989 september eruption at the south east summit crater of mount etna is studied in specific sixteen episodes of lava fountaining which occurred in the first phase of the eruption are analysed their periodic behaviour also evidenced by autocorrelation allows us to define the related tremor amplitude increases as intermittent volcanic tremor episodes focusingon the reg ular intermittent behaviour found for both lava fountains and intermittent volcanic tremors we tried an a posteriori forecast usingsimple statistical methods based on linear regression and the student t test we performed the retrospective statistical forecast and found that several eruptions would have been successfully forecast in order to focus on the source mechanism of tremor linked to lava fountains we investigated the relationship between volcanic and seismic parameters a mechanism based on a shallow magma batch regularly refilled from depth is suggested 1 2002 elsevier science b v all rights reserved

high effusion rates of the cordon caulle 2011 2012 eruption southern andes and their relation with the quasi harmonic tremor high effusion rates of intermediate to high silica lavas seem to be less uncommon than previously thought in particular during their initial eruptive stages in this study we report satellite based time averaged discharge rates for the 2011 2012 effusive phase at cordon caulle which are well correlated with the evolution of the quasi harmonic tremor the most significant seismic signal after the initial explosive stage such a correspondence could become a key method for detection of the onset of effusive phases especially in remote and or very cloudy areas supplying an additional tool for effective warnings and near real time hazards appraisal

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

seismicity and velocity structure of loihi seamount from the 1996 earthquake swarm the largest earthquake swarm yet recorded on loihi submarine volcano took place in july and august of 1996 the swarm consisted of two phases of seismic activity and was associated with the formation of a pit crater and additional faulting of loihi s summit platform the first phase of activity was comprised of predomi nantly high frequency events scattered over the southern flanks of the volcano fol lowing a day of seismic quiescence the second phase of activity began consisting of lower frequency earthquakes with strong t phases the phase 2 events took place beneath loihi s summit presumably marking the formation of the pit crater pele s pit data obtained by an ocean bottom seismometer obs on loihi during the swarm help constrain a new velocity model for loihi the relocated earthquakes combined with other characteristics of the swarm enabled us to develop a model for the events leading up to the formation of pele s pit that includes 1 a prolonged eruption 2 a tectonic event beneath loihi s south flank and 3 the drainage of a shallow magma chamber