lateral variation of crustal structure and composition in the cathaysia block of south china and its geodynamic implications abstract in order to clarify lateral variations in the crustal structure and composition of the cathaysia block in south china which was affected by violent mesozoic and cenozoic tectono magmatic activities we studied the crustal thickness and the poisson s ratio in this region using the h stacking method of teleseismic receiver functions our results show that the poisson s ratio varies between 0 20 and 0 29 and the crustal thickness ranges from 26 to 34 km in the cathaysia block the crustal thickness and poisson s ratio show considerable variations across the lishui haifeng fault the southeast coast magmatic belt is characterized by poisson s ratios 0 25 and a crustal thickness of 28 33 km reflecting probably intermediate to mafic compositions of the crust the southwestern part of the cathaysia fold belt exhibits a crustal thickness of 27 30 km and poisson s ratios

amorphous silica nanofilms result in growth of misoriented microcrystalline quartz cement maintaining porosity in deeply buried sandstones deeply buried sandstones in sedimentary basins typically have low porosity due to cementation and compaction there are several known causes of anomalously high porosity in sandstones one of which is microquartz coatings on sand grains that seem to inhibit growth of quartz cement however there has been no mechanistic understanding of why or how microquartz grows or why it maintains high porosity in sandstones here we have used high resolution scanning electron microscopy electron backscattered diffraction and transmission electron microscopy to study the microquartz cemented late cretaceous heidelberg formation germany we have revealed that a nanofilm of amorphous silica 50 100 nm and a layer of chalcedony are between the detrital grain and microquartz cement the amorphous silica insulates the detrital quartz grains and prevents syntaxial growth while microquartz adopts the orientation of the underlying chalcedony with its fast growth c axis parallel to the grain surface thus preventing growth into the pore now that we know what controls microquartz growth and why it preserves porosity it can be used to help identify rank and appraise deeply buried petroleum accumulations

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

new insights into the 1999 eruption of shishaldin volcano alaska based on acoustic data data collected by a pressure sensor provide new insights into the 1999 eruption of shishaldin volcano unimak island alaska on 19 april 1999 after 3 months of unrest and an extended period of low level strombolian activity shishaldin experienced a subplinian eruption ash plume to 16 km followed by several episodes of strong strombolian explosions acoustic data from the pressure sensor allow us to investigate the details of an eruption which was instrumentally well recorded but with few visual observations in the 12 h prior to the subplinian phase the pressure sensor detected a series of small repeated pulses with a constant spectral peak at 2 3 hz the amplitude and occurrence rate of the pulses both grew such that the signal became a nearly continuous hum just before the subplinian eruption this humming signal may represent gas release from rising magma the main subplinian phase was heralded by 1 the abrupt end of the humming signal 2 several pulses of low frequency sound interpreted as ash bursts and 3 a dramatic increase in seismic tremor amplitude the change in acoustic signature at this time allows us to precisely time the start of the subplinian eruption previously approximated as the time of strongest tremor increase the 50 min subplinian phase actually contained several bursts of signal each of which may represent a discrete volume of magma passing through the system following the subplinian event the pressure sensor recorded four discrete episodes of strombolian gas explosions on 19 20 april and another on 22 23 april four of the five episodes were accompanied by strong seismic tremor the fifth has not been previously recog nized and was not associated with anomalous tremor amplitudes in time series these events are similar to explosions recorded at other volcanoes but in general they are much larger with maximum amplitudes of 65 pa at 6 5 km from the vent and they have low 0 7 1 5 hz peak frequencies these large explosions occurred at rates of 3 20 per minute for 1 5 h in each episode the explosions were accompanied by a small 5 km above sea level ash plume and only minor amounts of ejecta were produced thus the explosion activity was domi nated by gas release

swarms of repeating long period earthquakes at shishaldin volcano alaska 2001 2004 abstract during 2001 2004 a series of four periods of elevated long period seismic activity each lasting about 1 2 months occurred at shishaldin volcano aleutian islands alaska the time periods are termed swarms of repeating events reflecting an abundance of earthquakes with highly similar waveforms that indicate stable non destructive sources these swarms are characterized by increased earthquake amplitudes although the seismicity rate of one event every 0 5 5 min has remained more or less constant since shishaldin last erupted in 1999 a method based on waveform cross correlation is used to identify highly repetitive events suggestive of spatially distinct source locations the waveform analysis shows that several different families of similar events co exist during a given swarm day but generally only one large family dominates a network of hydrothermal fractures may explain the events that do not belong to a dominant repeating event group i e multiple sources at different locations exist next to a dominant source the dominant waveforms exhibit systematic changes throughout each swarm but some of these waveforms do reappear over the course of 4 years indicating repeatedly activated source locations the choked flow model provides a plausible trigger mechanism for the repeating events observed at shishaldin explaining the gradual changes in waveforms over time by changes in pressure gradient across a constriction within the uppermost part of the conduit the sustained generation of shishaldin s long period events may be attributed to complex dynamics of a multi fractured hydrothermal system the pressure gradient within the main conduit may be regulated by temporarily sealing and reopening of parallel flow pathways by the amount of debris within the main conduit and or by changing gas influx into the hydrothermal system the observations suggest that shishaldin s swarms of repeating events represent time periods during which a dominant source is activated

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

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

multidisciplinary observations of the 2011 explosive eruption of telica volcano nicaragua implications for the dynamics of low explosivity ash eruptions we present multidisciplinary observations of the march june 2011 vei 2 eruptive episode of the basalticandesite telica volcano nicaragua which allow for a comprehensive study of the eruption mechanics of low explosivity eruptions at persistently active volcanoes the observations are from a dense network of seismic and gps instrumentation augmented by visual observations of the eruptive episode geochemical and petrologic analysis of eruptive products plume so2 measurements and temperature measurements of fumaroles inside and outside the active vent the 2011 eruptive episode was telica s most explosive since 1999 and consisted of numerous vulcanian explosions with maximum column heights of 1 5 2 km above the crater rim depositing a low volume of dominantly hydrothermally altered ash based on observed variations in seismicity temperature and so2 flux the lack of deformation of the edifice the non juvenile origin of and predominance of accretionary lapilli in the ash we propose that temporary sealing of the hydrothermal system between similar to 0 5 and 2 km depth allowed pressure to build up prior to vulcanian explosions making this a phreatic eruptive episode

seismo acoustic evidence for an avalanche driven phreatic eruption through a beheaded hydrothermal system an example from the 2012 tongariro eruption abstract the 6 august 2012 te maari eruption comprises a complex eruption sequence including multiple eruption pulses a debris avalanche that propagated 2 km from the vent and the formation of a 500 m long arcuate chasm located 300 m from the main eruption vent the eruption included 6 distinct impulses that were coherent across a local infrasound network marking the eruption onset at 11 52 18 all times utc an eruption energy release of 3 1012 j was calculated using a body wave equation for radiated seismic energy a similar calculation based on the infrasound record shows that 90 of the acoustic energy was released from three impulses at onset times 11 52 20 20 of total eruption energy 11 52 27 50 and 11 52 31 20 these energy impulses may coincide with eyewitness accounts describing an initial eastward directed blast followed by a westward directed blast and a final vertical blast pre eruption seismic activity includes numerous small unlocatable micro earthquakes that began at 11 46 50 two larger high frequency earthquakes were recorded at 11 49 06 and 11 49 21 followed directly by a third earthquake at 11 50 17 the first event was located within the scarp based on an arrival time location from good first p arrival times and probably represents the onset of the debris avalanche the third event was a tornillo characterised by a 0 8 hz single frequency resonance and has a resonator attenuation factor of q 40 consistent with a bubbly fluid filled resonator this contrasts with a similar tornillo event occurring 2 5 weeks earlier having q 250 1000 consistent with a dusty gas charged resonator we surmise from pre eruption seismicity and the observed attenuation change that the debris avalanche resulted from the influx of fluids into the hydrothermal system causing destabilisation and failure the beheaded hydrothermal system may have then caused depressurisation frothing of the remaining gas charged system leading to the onset of explosive activity

repeating coupled earthquakes at shishaldin volcano alaska since it last erupted in 1999 shishaldin volcano aleutian islands alaska has produced hundreds to thousands of longperiod 1 2 hz lp earthquakes every day with no other sign of volcanic unrest in 2002 the earthquakes also exhibited a short period 4 7 hz sp signal occurring between 3 and 15 s before the lp phase although the sp phase contains higher frequencies than the lp phase its spectral content is still well below that expected of brittle failure events the sp phase was never observed without the lp phase although lp events continued to occur in the absence of the precursory signal the twophased events are termed bcoupled eventsq reflecting a triggered relationship between two discrete event types both phases are highly repetitive in time series suggestive of stable non destructive sources waveform cross correlation and spectral coherence are used to extract waveforms from the continuous record and determine precise p wave arrivals for the sp phase although depths are poorly constrained the sp phase is believed to lie at shallow b4 km depths just west of shishaldin s summit the variable timing between the sp and lp arrivals indicates that the trigger mechanism between the phases itself moves at variable speeds a model is proposed in which the sp phase results from fluid moving within the conduit possibly around an obstruction and the lp phase results from the coalescence of a shallow gas bubble the variable timing is attributed to changes in gas content within the conduit the destruction of the conduit obstacle on november 21 2002 resulted in the abrupt disappearance of the sp phase published by elsevier b v

gas bubble dynamics model for shallow volcanic tremor at stromboli volcanic tremor at stromboli aeolian islands italy is correlated to small infrasonic transients ripepe et al 1996 which repeat almost rhythmically in time in a range between 0 8 and 1 2 s we demonstrate that infrasonic transients are associated to small gas bubble 0 5 m burstings which produces no transients in the seismic signal tremor ground displacement attenuates with the inverse of the distance from the craters indicating that the source is shallow short term energy release shows that infrasonic and seismic signals are linked to the same dynamical process while at the long term scale it is evident that the two signals are controlled by two distinctive mechanisms we suggest that the possible physical model acts in two steps first gas coalescence and then gas bursting in our model the seismic signal is related to the coalescence of a gas bubble from a layer of small bubbles while the infrasonic signal is linked to the bursting of the bubble when it reaches the magma surface gas bubbles could form by free coalescence in magma or could be forced to coalesce by a structural barrier we calculate that forced coalescence induces in magma a pressure change 104 pa 2 orders of magnitude higher than free coalescence and it explains best the tremor ground displacement 10 5 m recorded at stromboli moreover forced coalescence evidences the role of a structural barrier such as a dike in volcanic tremor source dynamics in this gas dynamic process the delay time of 1 2 s between infrasonic pulses could reflect the gas nucleation interval of basaltic magma thomas et al 1993 manga 1996 we propose that the source function for the shallow volcanic tremor at stromboli could be the viscoelastic reaction of the magma to the pressure decrease induced by gas bubble growth rate under constant depressurization the spectrum of our source function is controlled by the time duration of the pressure pulse which represents the viscoelastic relaxation time of the magma and gas bubble growth rate the predicted asymptotic decay of the frequency contents fits the spectral behavior of the volcanic tremor ground displacement recorded at stromboli we show that the same spectral behavior can be found in ground displacement spectra of volcanic tremor recorded on different volcanoes

the source of infrasound associated with long period events at mount st helens 1 during the early stages of the 2004 2008 mount st helens eruption the source process that produced a sustained sequence of repetitive long period lp seismic events also produced impulsive broadband infrasonic signals in the atmosphere to assess whether the signals could be generated simply by seismic acoustic coupling from the shallow lp events we perform finite difference simulation of the seismo acoustic wavefield using a single numerical scheme for the elastic ground and atmosphere the effects of topography velocity structure wind and source configuration are considered the simulations show that a shallow source buried in a homogeneous elastic solid produces a complex wave train in the atmosphere consisting of p sv and rayleigh wave energy converted locally along the propagation path and acoustic energy originating from the source epicenter although the horizontal acoustic velocity of the latter is consistent with our data the modeled amplitude ratios of pressure to vertical seismic velocity are too low in comparison with observations and the characteristic differences in seismic and acoustic waveforms and spectra cannot be reproduced from a common point source the observations therefore require a more complex source process in which the infrasonic signals are a record of only the broadband pressure excitation mechanism of the seismic lp events the observations and numerical results can be explained by a model involving the repeated rapid pressure loss from a hydrothermal crack by venting into a shallow layer of loosely consolidated highly permeable material heating by magmatic activity causes pressure to rise periodically reaching the pressure threshold for rupture of the valve sealing the crack sudden opening of the valve generates the broadband infrasonic signal and simultaneously triggers the collapse of the crack initiating resonance of the remaining fluid subtle waveform and amplitude variability of the infrasonic signals as recorded at an array 13 4 km to the nw of the volcano are attributed primarily to atmospheric boundary layer propagation effects superimposed upon amplitude changes at the source

degassing explosions at karymsky volcano kamchatka during the summer of 1997 karymsky vol cano produced summit explosions about six times each hour typical explosive episodes lasted between 30 seconds and three minutes produced gas and ash columns several hun dred meters high and ejected some incandescent material to better understand the physical source mechanisms re sponsible we recorded hundreds of explosions with a three component broad band seismometer and microphone lo cated 1650 meters from the active vent nearly every explo sion is recorded as an emergent yet identical seismic wavelet which is followed 4 15 s later by an impulsive acoustic ar rival we interpret the signals as a near surface gas volume burst which fractures the vent plug lowers the lithostatic pressure within the magma column and often induces fur ther degassing when degassing continues it is generally manifested as either a series of regular one second chug ging 1 explosions steady higher frequency jetting or a hy brid combination we believe that the seismic signature for chugs short duration harmonic tremor with integer over tones is the result of repeated gas volume bursts at the vent in contrast seismograms for jetting are non harmonic and contain higher frequencies we believe that the competing degassing behaviors are influenced by the gas flux as well as the plug conduit characteristics we propose that a plug exists due to a viscosity gradient caused by volatile depletion in the upper conduit

mixing of stratified liquids by the motion of gas bubbles application to magma mixing mafic magma underlying more silicic magma in a reservoir can exsolve volatiles as it crystallizes forming bubbles that segregate upwards to the interface with the overlying silicic magma we describe laboratory experiments designed to investigate how gas bubbles migrate into the more viscous upper layer in one regime bubbles move individually across the interface entraining a small quantity of lower layer fluid and the two layers become progressively stirred into a homogeneous mixture in a second regime bubbles form a thin foam layer at the interface which becomes gravitationally unstable giving rise to two phase plumes which rise and produce a coarse mixture of the liquids for a given viscosity ratio the transition between these regimes occurs when the gas flux is greater than a critical value which we found to decrease with increasing viscosity ratio we estimate the critical gas flux for magmatic conditions and how vesiculation may vary with pressure degree of crystallization and the composition of volatile components mafic inclusions which are often found in silicic to intermediate lavas can form in the plume regime analysis of the instability of the foam predicts wavelengths of between 1 cm and 1 m which is consistent with the observed sizes of inclusions volatiles from the mafic magma may be efficiently transported by this mechanism into the viscous silicic magma as bubble plumes despite low stokes velocities for individual vesicles at small viscosity ratios or low gas fluxes the bubbling regime can produce hybrid magmas

thermal seismic and infrasonic evidences of variable degassing rates at stromboli volcano abstract during june 1999 we measured the amplitude and rate number of events per second at which gas exited the vent at stromboli volcano as discrete gas bursts or puffs this allowed us to identify two styles of gas burst puffing activity the first is characterized by frequent rapidly rising puffs the second by less frequent slowly rising puffs each style persisted over 5 40 min long durations and was associated with a high and low number of strombolian explosions per hour respectively each period was also associated with characteristic delay times between the arrival of the infrasonic and thermal signals during strombolian explosions the delays were longer during vigorous puffing periods to explain our observations we propose a model in which the degassing process cycles between vigorous and weak degassing phases during vigorous degassing phases bubble layers ascend the conduit at a frequency of 0 5 1 0 s 1 this high degassing level reflects a gas rich magma column and leads to an increased rate in the formation of shallow foams and hence an increase in puffing and explosive activity as well as a higher free surface level and or gas jet velocity during weak phases bubble layers ascend the conduit at a reduced frequency of 0 2 0 3 s 1 during such times the magma column is poor in gas this leads to a decreased rate of foam layer formation and hence a reduction in puffing and explosive activity as well as a lower free surface level and or gas jet velocity variations in puffing activity can thus be used to track changes in the rate at which the shallow system is supplied by fresh gas rich magma our observations indicate that the two degassing styles last from 5 to 40 min and that the switch from one to the other occurs over a matter of minutes

infrasound observations of the 2008 explosive eruptions of okmok and kasatochi volcanoes alaska 1 the recent explosive eruptions of okmok and kasatochi volcanoes provide an opportunity to use seismic local infrasound distant infrasound array and remote sensing data in concert to better monitor volcanoes in the aleutian arc and to better understand the source processes the eruption of okmok volcano began on 12 july 2008 and included a seismically active phase that lasted continuously for about 10 h in contrast the eruption of kasatochi which began on 7 august 2008 consisted of five explosive events that lasted from 26 to 68 min each and had a cumulative duration of 3 4 h given the event times by local seismic stations the corresponding infrasound signals were found in the data recorded by local infrasound sensors and by distant infrasound arrays signals from the okmok eruption were detected by three international monitoring system ims arrays as far away as 4400 km signals from the kasatochi eruption were detected at greater distances up to 5200 km away by seven infrasound arrays including the ones that detected the event at okmok volcano back azimuth propagation and a simple acoustic wave propagation model in unison with known event times were used to confirm that the planar acoustic signals recorded at the arrays had originated from the eruptions the infrasound array data reflected the differences in eruption styles between okmok and kasatochi as the signals from kasatochi were of shorter duration of greater amplitude and detected over greater distances the infrasound array data were also able to distinguish between two types of tremor episodes that occurred at kasatochi volcano based on atmospheric disturbance

seismo acoustic signals associated with degassing explosions recorded at shishaldin volcano alaska 2003 2004 in summer 2003 a chaparral model 2 microphone was deployed at shishaldin volcano aleutian islands alaska the pressure sensor was co located with a short period seismometer on the volcano s north flank at a distance of 6 62 km from the active summit vent the seismo acoustic data exhibit a correlation between impulsive acoustic signals 1 2 pa and long period lp 1 2 hz earthquakes since it last erupted in 1999 shishaldin has been characterized by sustained seismicity consisting of many hundreds to two thousand lp events per day the activity is accompanied by up to 200 m high discrete gas puffs exiting the small summit vent but no significant eruptive activity has been confirmed the acoustic waveforms possess similarity throughout the data set july 2003 november 2004 indicating a repetitive source mechanism the simplicity of the acoustic waveforms the impulsive onsets with relatively short 10 20 s gradually decaying codas and the waveform similarities suggest that the acoustic pulses are generated at the fluid air interface within an open vent system so2 measurements have revealed a low so2 flux suggesting a hydrothermal system with magmatic gases leaking through this hypothesis is supported by the steady state nature of shishaldin s volcanic system since 1999 time delays between the seismic lp and infrasound onsets were acquired from a representative day of seismo acoustic data a simple model was used to estimate source depths the short seismo acoustic delay times have revealed that the seismic and acoustic sources are co located at a depth of 240 200 m below the crater rim this shallow depth is confirmed by resonance of the upper portion of the open conduit which produces standing waves with f 0 3 hz in the acoustic waveform codas the infrasound data has allowed us to relate shishaldin s lp earthquakes to degassing explosions created by gas volume ruptures from a fluid air interface

interaction of seismic and air waves recorded at stromboli volcano explosion quake seismograms recorded at stromboli show that seismic phases with a high amplitude and high frequency content propagate with a velocity of approximately 330 m s the sound speed the analysis of seismograms recorded at a distance of 500 m from one of the three active vents shows for the first onset a low frequency and particle motion characteristic of a p wave which loses its longitudinal polarization with the onset of the air wave recording the explosion quakes simultaneously with a microphone we could ascertain that the high frequency onset coincided with the air wave s in order to better understand the seismic wavefield generated by the atmospheric pressure we performed a controlled source experiment at stromboli using a seismic gun seismograms with the same two phases and particle motions comparable with the volcano seismic data were obtained a second experiment demonstrated that the air wave propagates at least in the uppermost 1 m of the ground we suggest that the seismic source of the corresponding seismograms is an explosion at the top of the magma column and conclude that the p and air waves are both generated in the same point and at the same time

an overview of volcano infrasound from hawaiian to plinian local to global abstract volcano infrasound is an increasingly useful technique for detecting locating characterizing and quantifying eruptive activity and can be used to constrain eruption source parameters in recent years studies of infrasound data from active volcanoes have shown clear progress towards mitigating volcanic hazards and understanding volcanic source processes volcano acoustic sources are shallow or aerial thus volcano infrasound data provide valuable information on eruption dynamics and are readily combined with direct and remote observations of gas ash and other eruptive phenomena the infrasound signals produced by volcanoes are indicative of the eruption style and dynamics here we review the diversity of infrasound signals generated by a wide variety of volcanic eruptions from hawaiian to plinian and the physical processes inferred to produce them we place particular emphasis on regional 15 250 km distance and global 250 km distance volcano infrasound studies as recent work in this area has made significant advances in monitoring and characterizing remote and difficult to monitor eruptions long range infrasonic detection of explosive volcanic eruptions is possible due to the energetic source mechanisms involved minor atmospheric attenuation at low frequencies and the existence of waveguides in the atmosphere however accurate characterization of the atmosphere and its spatiotemporal variability is required for reliable long range sound propagation modeling and correct interpretation of global infrasound recordings conversely because volcanic explosions are energetic and sometimes repetitive infrasound sources they can be used to validate atmospheric and acoustic propagation models

infrasound from the 2012 2013 plosky tolbachik kamchatka fissure eruption abstract we use both regional and local infrasound data to investigate the dynamics of the 2012 2013 eruption of tolbachik volcano kamchatka russia during select periods of time analysis of regional data recorded at the ims array is44 in southern kamchatka 384 km from the vent focuses on the eruption onset in november 2012 while analysis of local data focuses on activity in february and august 2013 signals recorded from tolbachik suggest a change in eruptive intensity possibly occurred from november 27 30 2012 local infrasound data recorded at distances of 100 950 m from the vent are characterized primarily by repeated transient explosion signals indicative of gas slug bursts three methods are employed to pick slug burst events in february and august the nature of slug bursts makes a monopole acoustic source model particularly fitting permitting volume outflux and slug radius calculations for individual events volume outfluxes and slug radii distributions provide three possible explanations for the eruption style of tolbachik volcano from mid february to late august cumulative outflux for slug bursts i e mass of emissions from individual bursts derived by infrasound for both february and august range from

source location variability and volcanic vent mapping with a small aperture infrasound array at stromboli volcano italy stromboli volcano in italy is a persistently active complex volcanic system in may 2002 activity was confined to 3 major summit craters within which several active vents hosted multiple explosions each hour during a 5 day field campaign an array of 3 low frequency microphones was installed to investigate the coherent infrasound produced by degassing from these vents consistent phase lags across the 3 stations indicate distinct sources that are subsequently investigated to determine the associated vent location apparent depth and origin time the cross correlation routine allows for variations in comparison window length waveform filtering bandwidth and correlation and consistency thresholds allowing for improved detection of certain types of degassing sources identification of activity at the various vents could be subsequently corroborated with 3 channels of synchronously acquired thermal data and video during the may 2002 experiment persistent energetic infrasound was observed from a passive degassing source within the central crater cc and transient infrasound produced by discrete strombolian explosions was identified at 4 additional vents the continuous infrasound produced by the cc exhibits variable frequency dependent correlation lag times that are interpreted as a diffraction effect due to the acoustic radiator s recessed location within a steep walled crater such dispersion has important implications for accurate eruption source modeling because it indicates that infrasonic waveforms may be significantly filtered during propagation transient explosion signals from the northeast crater nec and southwest crater swc vents also exhibit dynamic correlation lag times but this scatter may be more reasonably attributed to variable epicentral locations explosions from the nec west vent for instance appear to emanate from a diffuse zone with a lateral extent in excess of 10 m

seismic and acoustic observations at mount erebus volcano ross island antarctica 1994 1998 volcanic activity at mount erebus is dominated by eruptive activity within a phonolitic summit lava lake common eruption styles range from passive degassing to strombolian explosions which typically occur several times daily and occasionally in swarms of up to 900 per day shallow explosions although generally the result of steady exsolution of volatiles from depth can be triggered by surficial input of h 2o through mass wasting of rock snow and ice from the crater walls broadband observations of strombolian explosions document very long period vlp signals with strong spectral peaks near 20 12 and 7 s which are polarized in the vertical radial plane these signals precede lava lake surface explosions by 1 5 s are highly repeatable and persist for up to 200 s first motions indicate a deflationary source with any precursory inflation being below the 30 s passband of our instruments particle motions suggest a vlp source residing up to 800 m below the lava lake surface however this depth could be exaggerated by near field radial tilt seismic and acoustic signals associated with lava lake explosions commonly show evidence for multiple bubble bursts in corresponding complexity features resulting from varying time delays and relative sizes of superimposed bursts a systematic decrease in seismic acoustic ratio for smaller surface explosions suggests that either the seismic energy from the smallest shallowest bubble bursts experiences much greater seismic attenuation than energy arising from larger events which may involve a deeper less attenuative portion of the magma column and or that the shallowest layer is seismically isolated from deeper parts of a stratified magma column which are not excited by the smallest explosions due to sharp impedance contrasts across distinct layers tremor at erebus is uncommon with only a few isolated instances identified in five years of monitoring some tremor events are nearly monochromatic and some exhibit numerous gliding harmonic spectral lines q 2000 elsevier science b v all rights reserved

characterizing complex eruptive activity at santiaguito guatemala using infrasound semblance in networked arrays abstract we implement an infrasound semblance technique to identify acoustic sources originating from volcanic vents and apply the technique to the generally low amplitude infrasound

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

high broadband seismoacoustic signature of vulcanian explosions at popocatepetl volcano mexico author s matoza robin samuel arciniega ceballos a sanderson r w mendo perez g rosado fuentes a chouet b a

infrasonic precursors to a vulcanian eruption at sakurajima volcano japan the may 1998 eruption sequence of sakurajima volcano was monitored by ten infrasonic stations ten seismometers and a video camera during this seismo acoustic experiment we recorded hundreds of infrasonic tremor and long period events associated with seismic signals and observed a progression from relative quiescence to a vulcanian eruption the number of infrasonic events increased with escalating volcanic activity and the dominant character of the infrasonic signals changed from impulsive to emergent at 22 17 of may 19 sakurajima released ash and gases to a height of 2 km above the vent an event that was recorded continuously by one infrasonic and two seismic stations we present the experimental setup as well as a procedure through which infrasonic signals may be incorporated into future eruption monitoring and forecasting algorithms for open vent volcanic systems in addition our recordings suggest that infrasonic signals are more representative of processes occurring within the volcanic interior than are seismic signals which are strongly altered by diffraction and scattering in the volcanic edifice

sulphur dioxide fluxes from mount etna vulcano and stromboli measured with an automated scanning ultraviolet spectrometer gruppo nazionale per vulcanolgia gnv the ec 5th framework project multimo and nerc grant gr9 04655

volcano infrasound a review abstract exploding volcanoes which produce intense infrasound are reminiscent of the veritable explosion of volcano infrasound papers published during the last decade volcano infrasound is effective for tracking and quantifying eruptive phenomena because it corresponds to activity occurring near and around the volcanic vent as opposed to seismic signals which are generated by both surface and internal volcanic processes as with seismology infrasound can be recorded remotely during inclement weather or in the dark to provide a continuous record of a volcano s unrest moreover it can also be exploited at regional or global distances where seismic monitoring has limited efficacy this paper provides a literature overview of the current state of the field and summarizes applications of infrasound as a tool for better understanding volcanic activity many infrasound studies have focused on integration with other geophysical data including seismic thermal electromagnetic radiation and gas spectroscopy and they have generally improved our understanding of eruption dynamics other work has incorporated infrasound into volcano surveillance to enhance capabilities for monitoring hazardous volcanoes and reducing risk this paper aims to provide an overview of volcano airwave studies from analog microbarometer to modern pressure transducer and summarizes how infrasound is currently used to infer eruption dynamics it also outlines the relative merits of local and regional infrasound surveillance highlights differences between array and network sensor topologies and concludes with mention of sensor technologies appropriate for volcano infrasound study

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

array tracking of infrasonic sources at stromboli volcano 1 infrasonic arrays are a powerful tool for volcanic monitoring and hazard assessment explosions were recorded at stromboli using a small aperture array of 4 infrasonic stations allowing precise vent location the acoustic signals were delayed and summed revealing the existence of two main groups of infrasonic waves the ne crater produces short 3 s high amplitude 20 80 pa pressure waves while the sw crater produces small acoustic pressure 10 30 pa with long 5 15 s coda the two groups reflect different in explosive styles and similar spectral content centered on 5 6 hz when stacked together acoustic waveforms for each crater reveal the same pressure pulse which indicates a common source process we infer that the acoustic onset at both craters is generated by the burst of a large gas bubble while the acoustic coda is controlled by a sustained pressure release

interpretation and utility of infrasonic records from erupting volcanoes abstract in the most basic seismo acoustic studies at volcanoes infrasound monitoring enables differentiation between sub surface seismicity and the seismicity associated with gas release under optimal conditions complicated degassing signals can be understood relative explosion size can be assessed and variable seismo acoustic energy partitioning can be interpreted the extent to which these points may be investigated depends upon the quality of the infrasonic records a function of background wind noise microphone sensitivity and microphone array geometry and the type of activity generated by the volcano frequency of explosions bandwidth of the signals and coupling efficiency of the explosion to elastic energy to illustrate the features benefits and limitations of infrasonic recordings at volcanoes we showcase acoustic and seismic records from five volcanoes characterized by explosive degassing these five volcanoes erebus in antarctica karymsky in russia and sangay tungurahua and pichincha in ecuador were the focus of seismo acoustic experiments between 1997 and 2000 each case study provides background information about the volcanic activity an overview of visual observations during the period of monitoring and examples of seismo acoustic data we discuss the benefits and utility of the infrasound study at each respective volcano finally we compare the infrasound records and eruptive activity from these volcanoes with other volcanoes that have been the focus of previous seismo acoustic experiments

seismic and infrasound signals at mt etna modeling the north east crater conduit and its relation with the 2008 2009 eruption feeding system abstract on 13 may 2008 an eruption began at mt etna from an eruptive fissure that opened on the upper eastern flank of the volcano during 12 13 may 157 infrasonic events together with the related seismic transients were collected we carried out several analyses to obtain dominant frequencies pseudospectrograms peak to peak amplitudes source locations and time lags between infrasonic and seismic events spectra of the infrasonic events show two main spectral peaks in the frequency bands 0 4 0 7 hz and 1 5 2 0 hz respectively both infrasonic and seismic events were separately located below the north east crater where no eruptive activity was observed moreover significant changes in infrasound spectral content as well as in the infrasonic seismic lags were found a few hours before the beginning of the eruption on the basis of the collected information the infrasound source mechanism was modeled as a superposition of pipe and helmholtz resonance also leading to outline the geometry of the shallower portion of the north east crater plumbing system the occurrence of these seismo infrasonic events together with other geological and geophysical evidences led us to inferring a direct link between north east crater activity and the eruptive fissure further based on variations over time of both spectral features and seismic infrasonic time lag shallowing phenomena of the free magma column inside north east crater conduit were hypothesized such an uprise of magma was likely caused by a pressure increase inside the plumbing system occurring before the beginning of the 2008 2009 eruption

thermal precursors in satellite images of the 1999 eruption of shishaldin volcano shishaldin volcano unimak island alaska began showing signs of thermal unrest in satellite images on 9 february 1999 a thermal anomaly and small steam plume were detected at the summit of the volcano in short wave thermal infrared avhrr advanced very high resolution radiometer satellite data this was followed by over 2 months of changes in the observed thermal character of the volcano initially the thermal anomaly was only visible when the satellite passed nearly directly over the volcano suggesting a hot source deep in the central crater obscured from more oblique satellite passes the zenith angle needed to see the anomaly increased with time presumably as the thermal source rose within the conduit based on this change an ascent rate of ca 14 m per day for the thermal source was estimated until it reached the summit on around 21 march it is thought that strombolian activity began around this time the precursory activity culminated in a sub plinian eruption on 19 april ejecting ash to over 45 000 ft 13 700 m the thermal energy output through the precursory period was calculated based on geometric constraints unique to shishaldin these calculations show fluctuations that can be tied to changes in the eruptive character inferred from seismic records and later geologic studies the remote location of this volcano made satellite images a necessary observation tool for this eruption to date this is the longest thermal precursory activity preceding a sub plinian eruption recorded by satellite images in the region this type of thermal monitoring of remote volcanoes is central in the efforts of the alaska volcano observatory to provide timely warnings of volcanic eruption and mitigate their associated hazards to air traffic and local residents

seismoacoustic investigations of paroxysmal activity at mt etna volcano new insights into the 16 november 2006 eruption 1 on 16 november 2006 a 1 day long paroxysmal eruption occurred at the summit craters of mt etna volcano a multiparametric approach consisting of analyzing infrasonic seismic and video camera recordings was carried out to follow its evolution volcanological and geophysical observations identified three eruptive phases in the first phase infrasonic and seismic characteristics reflected the highly explosive nature of the activity waveform characterization of infrasound events confirmed the activity of the several explosive vents at the summit of southeast crater sec during the second phase results highlighted the decoupling between seismic and infrasonic sources which was due to the decrease in explosive activity and the reactivation of effusive vents located south of bocca nuova and on the saddle between bocca nuova and sec the third phase was the most intense and was characterized by various volcanic phenomena pyroclastic flows jets of dark ash and white steam the very high radiated infrasonic energy together with infrasound event features led us to infer a gas enrichment of the shallow magma column preceding by a few minutes and likely related to the pyroclastic flows in the sec area after the eruption at sec variations in infrasound events related to the activity of northeast crater nec were found the observed spectral changes and the source mechanism modeling of the nec infrasound events suggest the existence of a link in the plumbing system feeding the two craters

high frame rate thermal imagery of strombolian explosions implications for explosive and infrasonic source dynamics 1 explosive activity at stromboli volcano is analyzed using a high frame rate 50 hz thermal camera and differential pressure transducers we develop a thermal image based decomposition method to derive vertical and horizontal exit velocities of the explosive cloud peak vertical velocity ranges between 23 and 203 m s slightly higher than previous estimates and rapidly decreasing to a constant value of 30 50 m s within the first 0 1 s plume velocities are consistent with an elongated cloud expanding much faster vertically than horizontally and indicating the interaction with the conduit wall considering a vent radius of 2 m we estimate a volumetric flux of 200 600 m3 s which converts to total volumes of gas particles of 103 104 m3 for a single eruption these volumes are proportional to the thermal energy recorded by the camera providing a means to convert thermal radiance to volumes comparing the thermal onset of the explosions with the arrival time of the acoustic pressure we demonstrate that infrasound is propagating 0 14 1 7 s ahead of the explosive front the time difference between thermal and acoustic onsets constrains the infrasonic source within the conduit at 15 35 m below the crater rim peak amplitudes of acoustic pressure show a power law relationship p u2 with the exit vertical velocities consistent with the energy balance of a two phase flow rapidly accelerated in the conduit by gas decompression our results support monopole isotropic acoustic radiation of a source embedded within the conduit walls and indicate that explosive dynamics undergo strong accelerations of 103 104 m s2

thirty years of satellite derived lava discharge rates at etna implications for steady volumetric output 1 we present a 30 year long data set of satellite derived time averaged lava discharge rates tadr for mount etna volcano sicily italy spanning 1980 2010 and comprising 1792 measurements during 23 eruptions we use this to classify eruptions on the basis of magnitude and intensity as well as the shape of the tadr time series which characterizes each effusive event we find that while 1983 1993 was characterized by less frequent but longer duration effusive eruptions at lower tadrs 2000 2010 was characterized by more frequent eruptions of shorter duration and higher tadrs however roughly the same lava volume was erupted during both of these 11 year long periods so that the volumetric output was linear over the entire 30 year period increasing at a rate of 0 8 m3 s 1 between 1980 and 2010 the cumulative volume record can be extended back in time using data available in the literature this allows us to assess etna s output history over 5 centuries and to place the current trend in historical context we find that output has been stable at this rate since 1971 at this time the output rate changed from a low discharge rate phase which had characterized the period 1759 to 1970 to a high discharge rate phase this new phase had the same output rate as the high discharge rate phase that characterized the period 1610 1669 the 1610 1669 phase ended with the most voluminous eruption of historic times

determination of large scale velocity structure of the crust and upper mantle in the vicinity of pavlof volcano alaska pavlof volcano is a 2715 m high stratovolcano located at latitude 55 4 n and longitude 161 9 w near the western end of the alaska peninsula in this study the large scale lateral perturbations to a vertical reference velocity structure in the vicinity of pavlof volcano are determined using modified versions of standard three dimensional inversion techniques we use as the primary data set p wave arrival times and residuals from shallow 40 km deep regional earthquakes located by the shumagin islands seismic network relative residuals mainly from refracted e g pn arrivals allow us to constrain models for a laterally varying crustal velocity structure several strong ray path dependent patterns are seen in the residuals the most pronounced is that rays passing through the crust beneath the volcano are delayed positive residuals by up to 1 0 s while those rays traveling subparallel to and behind the volcanic axis on the retroarc bering sea side arrive as much as 0 5 s early negative residuals the corresponding results of a formal velocity inversion show the presence of a lowvelocity body beneath the volcanic arc that coincides geographically with emmons caldera 10 km sw of pavlof computed velocity values are about 12 14 lower than in the surrounding crust and are comparable to values found at other volcanic areas the exact shape and position of the low velocity body and its suitability for geothermal power development have not yet been sufficiently determined for such an assessment a much denser station distribution is required

acoustic measurements of the 1999 basaltic eruption of shishaldin volcano alaska 1 origin of strombolian activity the 1999 basaltic eruption of shishaldin volcano alaska usa displayed both classical strombolian activity and an explosive subplinian plume strombolian activity at shishaldin occurred in two major phases following the subplinian activity in this paper we use acoustic measurements to interpret the strombolian activity acoustic measurements of the two strombolian phases show a series of explosions that are modeled by the vibration of a large overpressurised cylindrical bubble at the top of the magma column results show that the bubble does not burst at its maximum radius as expected if the liquid film is stretched beyond its elasticity but bursting occurs after one cycle of vibration as a consequence of an instability of the air magma interface close to the bubble minimum radius during each strombolian period estimates of bubble length and overpressure are calculated using an alternate method based on acoustic power we estimate gas velocity to be 30 60 m s in very good agreement with synthetic waveforms although there is some variation within these parameters bubble length and overpressure for the first strombolian phase are found to be c82f11 m and 0 083 mpa for the second strombolian phase bubble length and overpressure are estimated at 24f12 m and 0 15 mpa for the first 17 h after which bubble overpressure shows a constant increase reaching a peak of 1 4 mpa just prior to the end of the second strombolian phase this peak suggests that at the time the magma in the conduit may contain a relatively large concentration of small bubbles maximum total gas volume and gas fluxes at the surface are estimated to be 3 3 10 7 and 2 9 10 3 m 3 s for the first phase and 1 0 10 8 and 2 2 10 3 m 3 s for the second phase this gives a mass flux of 1 2 10 3 and 8 7 10 2 kg s respectively for the first and the second strombolian phases

volcanic eruptions observed with infrasound 1 infrasonic airwaves produced by active volcanoes provide valuable insight into the eruption dynamics because the infrasonic pressure field may be directly associated with the flux rate of gas released at a volcanic vent infrasound also enhances the efficacy of volcanic hazard monitoring and continuous studies of conduit processes here we present new results from erebus fuego and villarrica volcanoes highlighting uses of infrasound for constraining quantitative eruption parameters such as eruption duration source mechanism and explosive gas flux

long period seismicity and very long period infrasound driven by shallow magmatic degassing at mount pagan mariana islands long period lp seismicity and very long period infrasound ivlp were recorded during continuous degassing from mount pagan mariana islands in july 2013 to january 2014 the frequency content of the lp and ivlp events and delay times between the two arrivals were remarkably stable and indicate nearly co located sources using phase weighted stacking over similar events to dampen noise we find that the lp source centroid is located 60 m below and 180 m west of the summit vent the moment tensor reveals a volumetric source modeled as resonance of a subhorizontal sill intersecting a dike we model the seismoacoustic wavefields with a coupled earth air 3 d finite difference code the ratios of pressure to velocity measured at the infrasound arrays are an order of magnitude larger than the synthetic ratios so the ivlp is not the result of lp energy transmitting into the atmosphere at its epicenter based on crater shape and dimensions determined by structure from motion we model the ivlp as acoustic resonance of an exponential horn the source of the continuous plume from gas analysis is shallow magmatic degassing which repeatedly pressurized the dike sill portion of the conduit over the 7 months of observation periodic gas release caused the geologically controlled sill to partially collapse and resonate while venting of gas at the surface triggered resonance in the crater lp degassing only accounts for 12 of total degassing indicating that most degassing is relatively aseismic and that multiple active pathways exist beneath the vent

acoustic source characterization of impulsive strombolian eruptions from the mount erebus lava lake abstract we invert for acoustic source volume outflux and momentum imparted to the atmosphere using an infrasonic network distributed about the erupting lava lake at mount erebus ross island antarctica by modeling these relatively simple eruptions as monopole point sources we estimate explosively ejected gas volumes that range from 1 000 m 3 to 24 000 m 3 for 312 lava lake eruptions recorded between january 6 and april 13 2006 though these volumes are compatible with bubble volumes at rupture as estimated from explosion video records departures from isotropic radiation are evident in the recorded acoustic wavefield for many eruptions a point source acoustic dipole component with arbitrary axis orientation and strength provides precise fit to the recorded infrasound this dipole source axis corresponding to the axis of inferred short duration material jetting varies significantly between events physical interpretation of dipole orientation as being indicative of eruptive directivity is corroborated by directional emissions of ejecta observed in erebus eruption video footage although three azimuthally distributed stations are insufficient to fully characterize the eruptive acoustic source we speculate that a monopole with a minor amount of oriented dipole radiation may reasonably model the primary features of the recorded infrasound for these eruptions

generation and propagation of infrasonic airwaves from volcanic explosions abstract analysis of infrasonic pressure waves generated by active volcanoes is essential to the understanding of volcanic explosion dynamics unlike seismic waves propagating in the earth infrasonic airwaves offer a relatively unfiltered representation of source motions at the vent during an eruption time varying acoustic propagation filters caused by changeable atmospheric conditions are minimal for microphones deployed at intermediate distances johnson et al j volcanol geotherm res in press

sustained long period seismicity at shishaldin volcano alaska abstract from september 1999 through april 2004 shishaldin volcano aleutian islands alaska exhibited a continuous and extremely high level of background seismicity this activity consisted of many hundreds to thousands of long period lp 1 2 hz earthquakes per day recorded by a 6 station monitoring network around shishaldin the lp events originate beneath the summit at shallow depths 0 3 km volcano tectonic events and tremor have rarely been observed in the summit region such a high rate of lp events with no eruption suggests that a steady state process has been occurring ever since shishaldin last erupted in april may 1999 following the eruption the only other signs of volcanic unrest have been occasional weak thermal anomalies and an omnipresent puffing volcanic plume the lp waveforms are nearly identical for time spans of days to months but vary over longer time scales the observations imply that the spatially close source processes are repeating stable and non destructive event sizes vary but the rate of occurrence remains roughly constant the events range from magnitude 0 1 to 1 8 with most events having magnitudes

three distinct regimes of volcanic tremor associated with the eruption of shishaldin volcano alaska 1999 tremor signals associated with the eruption of shishaldin volcano on 19 and 23 april 1999 were the strongest recorded anywhere in the aleutian arc by the alaska volcano observatory avo in its 10 year history reduced displacements dr reached 23 cm2 on 19 april and 43 cm2 on 23 april during the activity dr and spectral data with a frequency resolution of 0 1 hz were computed and put on the world wide web every 10 min these data are analyzed here the general temporal patterns of seismicity of these eruption events were similar but the eruptions and their effects quite different the 19 april event is known to have culminated in a sub plinian phase which ejected ash to an altitude of 16 km despite higher amplitudes and the largest hotspot from satellite data the 23 april event produced little ash reaching only 6 km altitude for several hours prior to the sub plinian phase on 19 april tremor with a peak frequency of 1 3 hz intensified during the sub plinian phase the peak frequency increased to 4 8 hz however in 15 h after the eruption three episodes of stronger tremor occurred with a lower 1 0 hz peak alternating with weaker tremor with a 1 3 hz peak these transitions correspond to dr 8 cm2 although these strong tremor episodes produced higher dr levels than the sub plinian phase data from a pressure sensor show that only strong strombolian explosions occurred the suite of observations suggests three distinct tremor regimes that may correspond to slug flow bubbly flow and sustained strong eruptions or a cyclic change in source parameters e g geometry sound speed or ascent rate this behavior occurred at shishaldin only during the april 1999 sequence and we are not aware of similar behavior at other volcanoes