characterization of seismicity at volcan baru panama may 2013 through april 2014 abstract volcan baru in the western province of chiriqui is panama s youngest and most active volcano although baru has experienced no historic eruptions there have been four eruptive episodes in the last 1600 years the most recent occurring 400 500 years ago sherrod et al 2007 in addition there have been four reported earthquake swarms in the last 100 years the most recent swarm occurred in may of 2006 prompting a usgs hazard assessment sherrod et al 2007 in order to characterize local seismicity and provide a reference for future monitoring efforts we established a seismic network that operated from may 2013 through april 2014 the network consisted of eight temporary single component short period sensors loaned by osop panama and three permanent stations distributed over a 35 by 15 km area during operation of the network 91 local events were detected and located these events were used to calculate a new 1 d velocity model for the region surrounding baru of particular interest was a cluster of events located less than 5 km east of baru at a depth of roughly 5 km we used a template matching detection technique to identify another 47 smaller magnitude events in the area of this cluster the largest events in the cluster exhibit a broad range of frequencies extending up to 20 hz and focal mechanisms calculated for eight of the events suggest strike slip and reverse faulting are the predominant source processes the directions of maximum compression shown by these focal mechanisms are approximately radial with respect to the volcano which may reflect the influence of a magmatic source beneath the edifice

the dykes and structural setting of the volcanic front in the lesser antilles island arc the orientations of dykes from many of the islands of the lesser antilles island arc have been mapped most of these dykes can be interpreted in terms of local or regional swarms derived from specific volcanoes of known age with distinct preferred orientations dykes are known from all cenozoic epochs except the palaeocene but are most common in pliocene miocene and oligocene rocks a majority of the sampled dykes are basaltic intrude volcaniclastic host rocks and show a preference for widths of 1 1 25 m locally dyke swarms dilate their hosts by up to 9 over hundreds of metres and up to 2 over distances of kilometres the azimuths of dykes of all ages show a general ne sw preferred orientation with a second nw se mode particularly in the miocene rocks of martinique the regional setting for these minor intrusions is a volcanic front above a subduction zone composed of three segments saba montserrat guadeloupe martinique st lucia grenada the spacing of volcanic centres along this front is interpreted in terms of rising plumes of basaltic magma spaced about 30 km apart this magma is normally intercepted at crustal depths by dioritic plutons and andesitic dacitic magma generated there plumes which intersect transverse fracture systems or which migrate along the front can avoid these crustal traps throughout its history the volcanic front as a whole has migrated episodically towards the backarc at an average velocity of about 1 km ma the local direction of plate convergence is negatively correlated with the local preferred orientation of dykes the dominant ne sw azimuth mode corresponds closely to the direction of faulting in the sedimentary cover of the backarc and the inferred tectonic fabric of the oceanic crust on which the arc is founded a generalised model of the regional stress field that controls dyke intrusion outside of the immediate vicinity of central volcanic vents is proposed in which the maximum horizontal stress parallels the volcanic front except in the northern segment where subduction of the barracuda rise perturbs the stress field there is also evidence of specific temporal changes in the stress field that are probably due to large scale plate kinematics

the 1995 1998 eruption of the soufriere hills volcano montserrat wi eruption of the soufriere hills volcano began on 18 july 1995 after three years of elevated seismic activity four months of increasingly vigorous phreatic activity culminated in mid november 1995 with the initiation of dome growth growth rates increased unevenly through early march 1996 with fluctuations on time scales from hours to months since march 1996 gravitational collapse of the unstable dome flank has affected an ever increasing area with pyroclastic flows surges and ashfalls major collapse of the eastern flank on 17 september 1996 resulted in a sub plinian explosive eruption later that day by february 1997 the dome had outgrown the confines of the crater and begun to spill into the surrounding valleys a large collapse on 25 june 1997 caused pyroclastic flows and surges on the northern flanks and resulted in the only deaths of the eruption in august september and october 1997 vulcanian explosions followed further collapses on the western and northern flanks the largest event of the eruption occurred on 26 december 1997 with failure of the southwestern flank of the volcano producing a debris avalanche and large dome collapse pyroclastic flows dome growth ceased in early march 1998 but residual volcanic activity has continued and consists of ash venting mild explosions and dome collapse pyroclastic flows

interaction between regional and magma induced stresses and their impact on volcano tectonic seismicity abstract recent seismological observations have reported volcano tectonic vt earthquakes with fault plane solutions exhibiting a change of 90 in their pressure axes relative to the regional stress field interestingly they are recorded mainly during periods preceding eruptive activity and coexisting with those vts showing a regional trend this study explains the occurrence of such trends in vt seismicity and discusses the possible patterns of earthquake locations related to the interaction of regional and magma induced stresses caused by pressurization or depressurization of magmatic sources our analysis shows that in the presence of a dominant regional stress field faulting will occur on faults whose associated slip direction is close to or in agreement with the background regional stress failure on faults with an opposite slip direction is unlikely to occur as magma pressure starts counter acting the regional stresses the likelihood of faults to slip in either a regional or opposite sense of slip relative to regional maximum compression increases allowing the co existence of possible failure with both slip tendencies however the spatial distribution of possible faulting differs as the pressure is progressively increased the stress patterns gradually approach those corresponding to the absence of a regional stress field the presented modeling results have implications for volcanic monitoring routines aiming to detect changes in stress patterns they will ultimately help to improve the correct interpretation of volcano tectonic seismicity

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

volcano tectonic seismicity at unzen volcano japan 1985 1999 abstract the eruption of unzen volcano in southwestern japan began on november 17 1990 and an extrusion of a dacite lava dome lasted from may 20 1991 until early february 1995 the seismic network operated by the shimabara earthquake and volcano observatory of kyushu university sevo located about 4000 volcano tectonic earthquakes during the period from 1985 to 1999 the seismicity was greatest in the period between one year before the eruption and the start of the lava dome extrusion the hypocenters were mostly distributed within a 12 20 km area on the western side of the volcano an inclined seismic aseismic boundary shallows eastward to the summit probably reflecting the thermal structure beneath the volcano within 4 km west from the summit however the seismicity is very low it was suggested that the high temperature ductile and low q bodies exist there we obtained focal mechanism solutions for about 90 earthquakes among them orientations of p axes for the earthquakes occurring on the upper side of the inclined seismic aseismic boundary suggest a pressure source located below this boundary although most of the focal mechanisms have t axes nearly horizontal in the n s direction consistent with the n s extensional tectonic stress several earthquakes whose focal mechanisms are inconsistent with the tectonic stress were observed within a horizontal distance of 5 km from the summit this is because the pressurization of volcanic gas or magma beneath the summit produced a strong local stress in the summit surroundings

a test of alternative caribbean plate relative motion models two contrasting models have been proposed to describe the present day motions between the caribbean plate and neighboring plates one model based on both north america caribbean and cocos caribbean data assumes that north america caribbean motion is reflected by the spreading rate approximately 2 cm yr inferred from magnetic anomalies at the cayman spreading center and the azimuths of nearby transforms jordan 1975 this geometry was used by minster and jordan 1978 in deriving global plate motion model rm2 the other model based only on north america caribbean data uses rates and azimuths inferred from the geometry of the lesser antilles wadati benioff zone sykes et al 1982 the cayman spreading center data were discounted assuming that they underestimate the full north america caribbean motion owing to the complex tectonics of the greater antilles the latter model assumes plate convergence at the lesser antilles at a rate about twice as fast as and an azimuth differing by 25 from the jordan model the two models also differ in their predictions of south america caribbean motion the jordan model predicts oblique convergence whereas sykes et al predict oblique divergence we use the nuvel 1 global relative motion data set which incorporates recent data not used in earlier studies to discriminate between the alternative models we find that the jordan geometry provides a better fit to the direction of north america caribbean motion moreover we find that models based on this geometry better fit the direction of cocos caribbean motion observed in earthquake slip vectors along the middle america trench since the prediction of this direction depends strongly on the rate of north america caribbean motion we conclude that the jordan model better describes caribbean plate motions the discrepancy between the two models results from the different methods of estimating plate motions sykes et al 1982 report that they determined a caribbean north america convergence rate of approximately 4 cm yr from the length of the lesser antilles wadati benioff zone we employ a similar procedure and obtain a rate closer to 2 cm yr we also find that the method they report using to determine the convergence direction is unable to distinguish between the alternative azimuths we thus conclude that estimates of the rate and direction of plate motion from wadati benioff zone configurations are insufficiently robust and that such estimates are better derived using rates from magnetic anomalies and azimuths from transform faults and slip vectors we further conclude that successful estimates of relative motion require internal consistency between the euler vectors of all relevant plates as demonstrated by the powerful cocos caribbean constraint not incorporated in the sykes et al model

relations between fracture patterns seismicity and plate motions in the lesser antilles abstract measurements of orientations of joints that have been made on the leeward and windward islands lesser antilles from st martin to st vincent are reported the preferred orientations of the joints have been determined by a statistical analysis assuming the joints to be shearing phenomena the principal directions of the stress field that caused them can be calculated the latter are in very good agreement with the principal stress directions obtained by a statistical analysis of fault plane solutions of earthquakes in the area the inferred maximum compression direction being nw se ca n 130 e this corresponds to a model which assumes that the american plate is subducted in that direction beneath the caribbean plate

seismic hydraulic fracture migration originated by successive deep magma pulses the 2011 2013 seismic series associated to the volcanic activity of el hierro island in this manuscript we present a new interpretation of the seismic series that accompanied eruptive activity off the coast of el hierro canary islands during 2011 2013 we estimated temporal variations of the gutenberg richter b value throughout the period of analysis and performed high precision relocations of the preeruptive and syneruptive seismicity using a realistic 3 d velocity model our results suggest that eruptive activity and the accompanying seismicity were caused by repeated injections of magma from the mantle into the lower crust these magma pulses occurred within a small and well defined volume resulting in the emplacement of fresh magma along the crust mantle boundary underneath el hierro we analyzed the distribution of earthquake hypocenters in time and space in order to assess seismic diffusivity in the lower crust our results suggest that very high earthquake rates underneath el hierro represent the response of a stable lower crust to stress perturbations with pulsatory character linked to the injection of magma from the mantle magma input from depth caused large stress perturbations to propagate into the lower crust generating energetic seismic swarms the absence of any preferential alignment in the spatial pattern of seismicity reinforces our hypothesis that stress perturbation and related seismicity had diffusive character we conclude that the temporal and spatial evolution of seismicity was neither tracking the path of magma migration nor it defines the boundaries of magma storage volumes such as a midcrustal sill our conceptual model considers pulsatory magma injection from the upper mantle and its propagation along the moho we suggest within this framework that the spatial and temporal distributions of earthquake hypocenters reflect hydraulic fracturing processes associated with stress propagation due to magma movement

temporal and spatial variation of local stress fields before and after the 1992 eruptions of crater peak vent mount spurr volcano alaska we searched for changes in local stress field orientation at mount spurr volcano alaska between august 1991 and december 2001 this study focuses on the stress field orientation beneath crater peak vent the site of three eruptions in 1992 and beneath the summit of mount spurr local stress tensors were calculated by inverting subsets of 140 fault plane solutions for earthquakes beneath crater peak and 96 fault plane solutions for earthquakes beneath mount spurr we also calculated an upper crustal regional stress tensor by inverting fault plane solutions for 66 intraplate earthquakes located near mount spurr during 1991 2001 prior to the 1992 eruptions and for 11 months beginning with a posteruption seismic swarm the axis of maximum compressive stress beneath crater peak was subhorizontal and oriented n67 76 e approximately perpendicular to the regional axis of maximum compressive stress n43 w the strong temporal correlation between this horizontal stress field rotation change in position of the 1 3 axes relative to regional stress and magmatic activity indicates that the rotation was related to magmatic activity and we suggest that the crater peak stress field rotation resulted from pressurization of a network of dikes during the entire study period the stress field beneath the summit of mount spurr also differed from the regional stress tensor and was characterized by a vertical axis of maximum compressive stress we suggest that slip beneath mount spurr s summit occurs primarily on a major normal fault in response to a combination of gravitational loading hydrothermal circulation and magmatic processes beneath crater peak online material regional and local fault plane solutions

dome growth behavior at soufriere hills volcano montserrat revealed by relocation of volcanic event swarms 1995 1996 abstract we have relocated a subset of events from the digital waveform catalogue of 17 000 volcanic microearthquakes recorded between july 1995 and february 1996 at soufriere hills volcano shv montserrat using a cross correlation based phase repicking technique with a joint location method hypocenters were estimated for 3914 earthquakes having five or more corrected p wave picks the seismic source region collapsed to a volume of 1 km 3 from an initial 100 km 3 relocated events represent 36 swarms each containing nearly identical waveforms having source dimensions of 10 to 100 m in diameter and spatial separations on the order of 500 m or less each swarm occurred over a span of several hours to a few days triggered data appear to miss between 65 and 98 of the events that occur within these swarms based on review of helicorder records visual estimates of summit dome growth show a rough correspondence between episodes of intense swarming and increases in extruded magma although dome observations are too sparse to make a direct comparison for this time period the limited depth range over which dome growth related events occur is consistent with a dynamic model of cyclic plug extrusion behavior in the shallow conduit governed by magma supply rate overpressure buildup and physical properties of the magma and conduit geometry seismic sources may occur in locally overpressured regions that result from microlite formation in a zone of rapid decompression we propose that this zone exists in the vicinity of a detachment plane associated with the cyclic plug extrusion

a process based model of pre eruption seismicity patterns and its use for eruption forecasting at dormant stratovolcanoes abstract using long term data sets the eruptive size timing and style can typically be successfully forecast for dormant volcanoes using current monitoring data and knowledge of precursory eruptive patterns the usaid usgs volcano disaster assistance program vdap utilizes seismic data over multiple eruption cycles and across volcano types to successfully forecast eruption size style and onset times as well as changes within ongoing eruptions from millions of seismic event observations we show that seismic data a proxy for rate dependent strain changes in the magmatic system combined with a process based conceptual geologic model enables eruption forecasting especially for stratovolcanoes dormant 20 years we show that this model often works for some open systems ones that sufficiently reseal to plug the conduit and also where a deep intrusion occurs after phreatic or phreatomagmatic explosions have begun this model is divided into four seismicity and geologic stages we describe each of the four stages of seismicity in detail and how they relate to the ongoing geologic process of that stage stage 1 deep intrusion and seismicity occurring at 10 to 40 km depth below the volcanic edifice stage 2 distal vt seismicity induced at distances from 2 to 30 km laterally from the summit by magma intruding into and inflating the upper crustal magma storage region stage 3 vent clearing seismicity associated with the initial vent creation and clearing phase of eruptive activity stage 4 repetitive event seismicity associated with the final ascent of magma from 2 km to the surface we present 36 examples of this seismic progression from 26 volcanoes we believe that this progression generally correlates with the magma flux and the eventual explosivity and is invaluable for forecasting eruptions

joint analysis of geodetic and earthquake fault plane solution data to constrain magmatic sources a case study from kīlauea volcano abstract a joint analysis of geodetic and seismic datasets from k lauea volcano during a period of magmatic unrest in 2006 demonstrates the effectiveness of this combination for testing and constraining models of magma dynamics for a complex multi source system at the end of 2003 k lauea s summit began a four year long period of inflation due to a surge in magma supply to the volcano in 2006 for the first time since 1982 k lauea s southwest rift zone swrz also experienced inflation to investigate the characteristics of active magma sources and the nature of their interactions with faults in the swrz during 2006 we integrate through coulomb stress modeling contemporary geodetic data from insar and gps with a new catalogue of double couple fault plane solutions for volcano tectonic earthquakes we define two periods of inflation during 2006 based on the rate of deformation measured in daily gps data spanning february to 15 march 2006 period 1 and 16 march to 30 september 2006 period 2 insar data for these two periods are inverted to determine the position change in size and shape of inflation sources in each period our new models are consistent with microseismic activity from each period they suggest that during period 1 deformation in the swrz can be explained by pressurization of magma in a spherical reservoir beneath the south caldera and that during period 2 magma was also aseismically intruded farther to the southwest into the swrz along a sub horizontal plane our coulomb stress analysis shows that the microseismicity recorded in the swrz is induced by overpressurization of the south caldera reservoir and not by magma intrusion into the swrz this study highlights the importance of a joint analysis of independent geophysical datasets to fully constrain the nature of magma accumulation

spatial variations in the frequency magnitude distribution of earthquakes at soufriere hills volcano montserrat west indies the frequency magnitude distribution of earthquakes measured by the b value is determined as a function of space beneath soufriere hills volcano montserrat from data recorded between august 1 1995 and march 31 1996 a volume of anomalously high b values b 3 0 with a 1 5 km radius is imaged at depths of 0 and 1 5 km beneath english s crater and chance s peak this high b value anomaly extends southwest to gage s soufriere at depths greater than 2 5 km volumes of comparatively low b values b 1 are found beneath st george s hill windy hill and below 2 5 km depth and to the south of english s crater we speculate the depth of high b value anomalies under volcanoes may be a function of silica content modified by some additional factors with the most siliceous having these volumes that are highly fractured or contain high pore pressure at the shallowest depths

evidence from the ad 2000 izu islands earthquake swarm that stressing rate governs seismicity magma intrusions and eruptions commonly produce abrupt changes in seismicity far from magma conduits that cannot be associated with the diffusion of pore fluids or heat such swarm seismicity also migrates with time and often exhibits a dog bone shaped distribution the largest earthquakes in swarms produce aftershocks that obey an omori type exponential temporal decay but the duration of the aftershock sequences is drastically reduced relative to normal earthquake activity here we use one of the most energetic swarms ever recorded to study the dependence of these properties on the stress imparted by a magma intrusion a 1 000 fold increase in seismicity rate and a 1 000 fold decrease in aftershock duration occurred during the two month long dyke intrusion we find that the seismicity rate is proportional to the calculated stressing rate and that the duration of aftershock sequences is inversely proportional to the stressing rate this behaviour is in accord with a laboratory based rate state constitutive law suggesting an explanation for the occurrence of earthquake swarms any sustained increase in stressing rate whether due to an intrusion extrusion or creep event should produce such seismological behaviour

volcano tectonic earthquakes a new tool for estimating intrusive volumes and forecasting eruptions abstract we present data on 136 high frequency earthquakes and swarms termed volcano tectonic vt seismicity which preceded 111 eruptions at 83 volcanoes plus data on vt swarms that preceded intrusions at 21 other volcanoes we find that vt seismicity is usually the earliest reported seismic precursor for eruptions at volcanoes that have been dormant for decades or more and precedes eruptions of all magma types from basaltic to rhyolitic and all explosivities from vei 0 to ultraplinian vei 6 at such previously long dormant volcanoes because large eruptions occur most commonly during resumption of activity at long dormant volcanoes vt seismicity is an important precursor for the earth s most dangerous eruptions vt seismicity precedes all explosive eruptions of vei 5 and most if not all vei 4 eruptions in our data set surprisingly we find that the vt seismicity originates at distal locations on tectonic fault structures at distances of one or two to tens of kilometers laterally from the site of the eventual eruption and rarely if ever starts beneath the eruption site itself the distal vt swarms generally occur at depths almost equal to the horizontal distance of the swarm from the summit out to about 15 km distance beyond which hypocenter depths level out we summarize several important characteristics of this distal vt seismicity including swarm like nature onset days to years prior to the beginning of magmatic eruptions peaking of activity at the time of the initial eruption whether phreatic or magmatic and large non double couple component to focal mechanisms most importantly we show that the intruded magma volume can be simply estimated from the cumulative seismic moment of the vt seismicity from log10 v 0 77 log moment 5 32 with volume v in cubic meters and seismic moment in newton meters because the cumulative seismic moment can be approximated from the size of just the few largest events and is quite insensitive to precise locations the intruded magma volume can be quickly and easily estimated with few short period seismic stations notable cases in which distal vt events preceded eruptions at long dormant volcanoes include nevado del ruiz 1984 1985 pinatubo 1991 unzen 1989 1995 soufriere hills 1995 shishaldin 1989 1999 tacana 1985 1986 pacaya 1980 1984 rabaul 1994 and cotopaxi 2001 additional cases are recognized at frequently active volcanoes including popocateptl 2001 2003 and mauna loa 1984 we present four case studies pinatubo soufriere hills unzen and tacana in which we demonstrate the above mentioned vt characteristics prior to eruptions using regional data recorded by neic we recognized in near real time that a huge distal vt swarm was occurring deduced that a proportionately huge magmatic intrusion was taking place beneath the long dormant sulu range new britain island papua new guinea that it was likely to lead to eruptive activity and warned rabaul volcano observatory days before a phreatic eruption occurred this confirms the value of this technique for eruption forecasting we also present a counter example where we deduced that a vt swarm at volcan cosiguina nicaragua indicated a small intrusion insufficient to reach the surface and erupt finally we discuss limitations of the method and propose a mechanism by which this distal vt seismicity is triggered by magmatic intrusion

characterization of seismicity at mt etna volcano italy by inter event time distribution abstract the space time inter event time iet distributions of earthquakes occurring from 1988 to 2011 at mt etna are analysed in order to identify the periodicity or stationary behaviour of seismicity and to correlate it with the volcano tectonic features of the region the comparison between iet distributions at etna with those obtained both for sicily and italy shows that iets at a larger scale are well modelled by a gamma distribution whereas at etna local scale they are characterised by a bimodal curve in which the two peaks are related to i the contribution of local seismic swarms with very short inter event times and ii the background regional stationary seismicity iet analysis is an important tool to investigate the behaviour of seismicity at different crustal levels in the etna region distinguishing sectors that are influenced by volcano dynamics or regional tectonics indeed the spatial variation of iet distributions obtained by analysing different etna crustal sectors shows that seismicity shallower than 5 km is almost entirely characterised by short inter event times and is mainly confined to the summit area for earthquakes deeper than 5 km occurring in the eastern flank of the volcano as well as in eastern sicily iet distributions are characterised by independent events which suggest that both areas are influenced by the same extensional regional regime by contrast iet distributions obtained for the western flank and north western sicily are marked by two peaks indicating that the compressional stress is acting in both areas

the pre mesozoic rocks of northern chile u pb ages and hf and o isotopes supposed pre mesozoic igneous and metamorphic rocks from northern chile are reviewed in the light of twenty one new shrimp u pb zircon age determinations metamorphic rocks from the precordillera upthrust belt mostly show a wide spectrum of zircon ages indicating derivation from sedimentary protoliths youngest detrital zircon ages i e maximum depositional ages range from c 850 ma at belen to 1000 1100 ma in sierra de moreno and cordon de lila late proterozoic provenance throughout the region corresponds to a c 1 ga igneous and metamorphic event the main source region could have been the proterozoic mara block or laurentia to the west early ordovician plutonic rocks 465 485 ma correlated with the famatinian magmatism of nw argentina are recognised in all three outcrop areas and contemporaneous volcanic rocks in cordon de lila hf and o isotope data for ordovician zircon in these rocks and for c 1450 detrital zircon in the metasedimentary rocks are consistent with ultimate derivation from early mesoproterozoic to paleoproterozoic lithosphere a depositional age younger than 400 ma is determined for the quebrada aroma metamorphic complex indicating post early silurian metamorphism and folding carboniferous igneous and deformational events occur in the coast range where metasedimentary complexes are mostly related to late paleozoic subduction accretion deformation and metamorphism continued near the present pacific shore line until triassic and earliest jurassic times the underlying crust of much of norte grande is considered to be proterozoic arequipa antofalla block or mara although there are no igneous rock outcrops of this age

phenocryst hosted melt inclusions record stalling of magma during ascent in the conduit and upper magma reservoir prior to vulcanian explosions soufriere hills volcano montserrat west indies the mechanics of explosive eruptions influence magma ascent pathways vulcanian explosions involve a stop start mechanism that recurs on various timescales evacuating the uppermost portions of the conduit during the repose time between explosions magma rises from depth and refills the conduit and stalls until the overpressure is sufficient to generate another explosion we have analyzed major elements cl s h2o and co2 in plagioclase hosted melt inclusions sampled from pumice erupted during four vulcanian events at soufriere hills volcano montserrat to determine melt compositions prior to eruption using fourier transform infrared spectroscopy we measured values up to 6 7 wt h2o and 80 ppm co2 of 42 melt inclusions 81 cluster between 2 8 and 5 4 wt h2o 57 to 173 mpa or 2 7 km suggesting lower conduit to upper magma reservoir conditions we propose two models to explain the magmatic conditions prior to eruption in model 1 melt inclusions were trapped during crystal growth in magma that was stalled in the lower conduit to upper magma reservoir and during trapping the magma was undergoing closed system degassing with up to 1 wt free vapor this model can explain the melt inclusions with higher h2o contents since these have sampled the upper parts of the magma reservoir however the model cannot explain the melt inclusions with lower h2o because the timescale for plagioclase crystallization and melt inclusion entrapment is longer than the magma residence time in the conduit in model 2 melt inclusions were originally trapped at deeper levels of the magma chamber but then lost hydrogen by diffusion through the plagioclase host during periodic stalling of the magma in the lower conduit system in this second scenario which we favor the melt inclusions record re equilibration depths within the lower conduit to upper magma reservoir

overview of the eruption of soufriere hills volcano montserrat 18 july 1995 to december 1997 the onset of phreatic volcanic activity at the soufriere hills volcano montserrat on 18 july 1995 followed a three year period of heightened volcano seismic activity beneath the island phreatic explosions gave way to continuous eruption of juvenile andesitic magma in the form of a lava dome on or around 15 november 1995 magma production rate has varied leading to changes in eruptive style an explosive eruption on 17 september 1996 followed a period of enhanced dome growth and large scale gravitational collapses from its eastern flank increasing dome volume led to stressing and overtopping of the confining crater walls to the southwest north and west during early 1997 sustained high magma production rate since june 1997 has led to three periods of major gravitational dome collapses followed by vulcanian explosive eruptions dome growth re started immediately after the cessation of the latest of these explosive phases in october 1997 and continues as of december 1997

magmatic system geometry at mount st helens modeled from the stress field associated with posteruptive earthquakes earthquakes following the may 18 1980 and june 12 1980 explosive eruptions of mount st helens were concentrated at depths below 6 km the hypocenters of these posteruption earthquakes define two seismic lobes separated by an aseismic volume of 2 km width that is located beneath the position of the crater the timing and location of the posteruption seismicity suggests that the earthquakes occurred in the country rock surrounding a magma body located within the aseismic volume the withdrawal of magma caused a pressure decrease within the reservoir and the earthquakes occurred as a brittle response to the resulting stress change focal mechanism solutions for posteruption earthquakes distributed within the seismic lobes are calculated using polarity and amplitude data results show that a perturbation from the regional stress field occurs between depths of 7 and 11 km this anomalous stress field is successfully modeled by a decrease in pressure within a cylindrical magma body located between the seismic lobes and subject to conditions representing the regional shear stress regime in the best fitting model the magma body is centered at 46 204 n 122 187 w 600 m to the north of the present dome and is under a regional shear stress of 20 bars the radius of the chamber is calculated to be between 0 65 and 0 75 km when using pressure drops within the cylinder of 220 and 150 bars respectively the magma body extends from 7 to 11 km and has a volume of between 5 and 7 km3 posteruption earthquakes located below 11 km fall on a northeast striking fault that is preferentially aligned with the regional tectonic stress regime this fault may be a conduit to transport magma to the shallow reservoir from greater depths within the crust

analysis and forward modeling of seismic anisotropy during the ongoing eruption of the soufriere hills volcano montserrat 1996 2007 1 volcanic stress field analysis has been lauded as a potentially powerful tool for midterm to long term eruption forecasting however because tectonic processes can also produce localized stress field reorientations evidence for a direct causal link between observed stress field reorientations and magmatic activity is of critical importance in this study we show that local stress field reorientations preceding changes in volcanic activity at the soufriere hills volcano montserrat are observable using two independent measures of crustal stress local volcano tectonic earthquake fault plane solutions and measurements of shear wave splitting in regional earthquakes we further demonstrate that the local stress field orientation during a 6 month period preceding the onset of eruptive activity at soufriere hills in 1999 is highly localized and spatiotemporally variable and that the spatial pattern of precursory local stress orientations is consistent with numerically modeled patterns of stress resulting from pressurization of a vertical dike these observations provide compelling evidence for a direct causal link between pressurization of midlevel volcanic conduit systems by ascending magma and precursory local stress field reorientations and demonstrate that seismological analysis can be used to detect subtle local changes in stress that herald eruptive activity

transtensional deformation of montserrat revealed by shear wave splitting here we investigate seismic anisotropy of the upper crust in the vicinity of soufriere hills volcano using shear wave splitting sws analysis from volcano tectonic vt events soufriere hills which is located on the island of montserrat in the lesser antilles became active in 1995 and has been erupting ever since with five major phases of extrusive activity we use data recorded on a network of seismometers between 1996 and 2007 partially spanning three extrusive phases shear wave splitting in the crust is often assumed to be controlled either by structural features or by stress aligned cracks in such a case the polarization of the fast shear wave would align parallel to the strike of the structure or to the maximum compressive stress direction previous studies analyzing sws in the region using regional earthquakes observed temporal variations in which were interpreted as being caused by stress perturbations associated with pressurization of a dyke our analysis which uses much shallower sources and thus only samples the anisotropy of the upper few kilometres of the crust shows no clear temporal variation however temporal effects cannot be ruled out as large fluctuations in the rate of vt events over the course of the study period as well as changes in the seismic network configuration make it difficult to assess average delay times of approximately 0 2 s similar in magnitude to those reported for much deeper slab events suggest that the bulk of the anisotropy is in the shallow crust we observe clear spatial variations in anisotropy which we believe are consistent with structurally controlled anisotropy resulting from a left lateral transtensional array of faults which crosses the volcanic complex

patterns of volcanotectonic seismicity and stress during the ongoing eruption of the soufriere hills volcano montserrat 1995 2007 abstract the ongoing eruption of the soufriere hills volcano montserrat has been accompanied throughout by varying levels of high frequency volcanotectonic vt seismicity these earthquakes reflect the brittle response of the host rock to stresses generated within the magmatic system and thus reveal interesting and useful information about the structure of the volcanic conduit system and processes occurring within it in general systematic changes in the rate location and fault plane solutions of vt earthquakes correspond to changes in the volcano s behavior and indicate that the main conduit for the eruption is a dike or system of dikes trending ne sw and centered beneath the eruptive vent to date the eruption has comprised three extrusive phases separated by two 1 2 year long periods of residual activity prior to the start of each extrusive phase vt earthquakes with fault plane solution p axes oriented perpendicular to inferred regional maximum compression dominate the data set consistent with stresses induced by the inflation of the mid level conduit system 90 rotated vt fault plane solutions are also observed preceding a change in eruption style from effusive to explosive in 1997 while increases in the rate of vt earthquakes precede eruption phase onsets high rates of vt seismicity are also observed during the first period of residual activity and in this case appear to reflect the relaxation of host rock following withdrawal of magma from the mid crustal system most vt earthquakes are located directly beneath the eruptive vent although two distal vt clusters were observed during the first six months of the eruption late 1995 early 1996 both of these distal clusters likely resulted from stresses generated during the establishment of the main conduit system

mechanisms of unrest and eruption at persistently restless volcanoes insights from the 2015 eruption of telica volcano nicaragua many of earth s volcanoes experience well defined states of quiescence and unrest with unrest occasionally culminating in eruption some volcanoes however experience an unusually protracted i e decades long period of noneruptive unrest and are thus categorized as persistently restless volcanoes prvs the processes that drive persistently restless volcanism are poorly understood as our knowledge of prvs is currently based on a small number of case studies here we examine multidisciplinary observations of the 2015 eruptive episode at telica volcano nicaragua in the context of its long term behavior we suggest that the latter phases of the 2015 eruption were ultimately driven by destabilization of its shallow magma reservoir based on previous geodetic seismic studies of telica geirsson et al 2014 https doi org 10 1016 j jvolgeores 2013 11 009 rodgers et al 2013 https doi org 10 1016 j jvolgeores 2013 08 010 and 2015 https doi org 10 1016 j jvolgeores 2014 11 012 and on multiparameter observations at telica over a 7 year period we propose that three distinct states of unrest occur at telica over decadal timescales a stable open state involving steady conduit convection and two distinct unstable states that may lead to eruptions in the weak sealing state phreatic explosions result from steady conduit convection underlying a weak seal in the destabilized state destabilization of the top of the convecting magma in the conduit leads to rapid accumulation of high pressures leading to strong impulsive phreatomagmatic explosions our observations and interpretations suggest that continuous seismic ground based deformation gas emission and thermal monitoring and interpretation of these data within a paradigm of sustained conduit convection modulated by episodes of sealing and destabilization of shallow magma reservoirs may allow robust forecasting of eruption potential energy and duration at telica and similar prvs worldwide

numerical models of volcanotectonic earthquake triggering on non ideally oriented faults 1 calculation of inflation induced coulomb stress changes on faults of varying orientation indicates that local faults of almost any orientation are susceptible to slip in response to dike inflation additionally as dikes typically inflate in the direction of regional minimum compressive stress the sense of inflation induced strike slip faulting predicted by coulomb stress change models is opposite to that expected from regional tectonic forces observations of volcanotectonic vt earthquake fault plane solutions with pressure p axes oriented 90 to regional maximum compression confirm this prediction in detail imperfect agreement between coulomb stress change models and the ranges of fault plane orientations observed in vt fault plane solution studies and observed locations of vt seismicity indicates that the orientation and location of pre existing faults around the conduit ultimately determine the seismic response of host rock to an inflating dike

earthquake classification location and error analysis in a volcanic environment implications for the magmatic system of the 1989 1990 eruptions at redoubt volcano alaska abstract determination of the precise locations of seismic events associated with the 1989 1990 eruptions of redoubt volcano posed a number of problems including poorly known crustal velocities a sparse station distribution and an abundance of events with emergent phase onsets in addition the high relief of the volcano could not be incorporated into the hypoellipse earthquake location algorithm this algorithm was modified to allow hypocenters to be located above the elevation of the seismic stations the velocity model was calibrated on the basis of a posteruptive seismic survey in which four chemical explosions were recorded by eight stations of the permanent network supplemented with 20 temporary seismographs deployed on and around the volcanic edifice the model consists of a stack of homogeneous horizontal layers setting the top of the model at the summit allows events to be located anywhere within the volcanic edifice detailed analysis of hypocentral errors shows that the long period lp events constituting the vigorous 23 hour swarm that preceded the initial eruption on december 14 could have originated from a point 1 4 km below the crater floor a similar analysis of lp events in the swarm preceding the major eruption on january 2 shows they also could have originated from a point the location of which is shifted 0 8 km northwest and 0 7 km deeper than the source of the initial swarm we suggest this shift in lp activity reflects a northward jump in the pathway for magmatic gases caused by the sealing of the initial pathway by magma extrusion during the last half of december volcano tectonic vt earthquakes did not occur until after the initial 23 hour long swarm they began slowly just below the lp source and their rate of occurrence increased after the eruption of 01 52 ast on december 15 when they shifted to depths of 6 to 10 km after january 2 the vt activity migrated gradually northward this migration suggests northward propagating withdrawal of magma from a plexus of dikes and or sills located in the 6 to 10 km depth range precise relocations of selected events prior to january 2 clearly resolve a narrow steeply dipping pencil shaped concentration of activity in the depth range of 1 7 km which illuminates the conduit along which magma was transported to the surface a third event type named hybrid which blends the characteristics of both vt and lp events originates just below the lp source and may reflect brittle failure along a zone intersecting a fluid filled crack the distribution of hybrid events is elongated 0 2 0 4 km in an east west direction this distribution may offer constraints on the orientation and size of the fluid filled crack inferred to be the source of the lp events

tephra fall deposits from the 1992 eruption of crater peak alaska implications of clast textures for eruptive processes the 1992 eruption of crater peak mount spurr alaska involved three subplinian tephra producing events of similar volume and duration the tephra consists of two dense juvenile clast types that are identified by color one tan and one gray of similar chemistry mineral assemblage and glass composition in two of the eruptive events the clast types are strongly stratified with tan clasts dominating the basal two thirds of the deposits and gray clasts the upper one third tan clasts have average densities between 1 5 and 1 7 g cc and vesicularities phenocryst free of approximately 42 gray clasts have average densities between 2 1 and 2 3 g cc and vesicularities of approximately 20 both contain abundant microlites average maximum plagioclase microlite lengths 13 15 m in gray clasts in the upper layer are similar regardless of eruptive event and therefore the repose time between them and are larger than average maximum plagioclase microlite lengths 9 11 m in the tan clasts in the lower layer this suggests that microlite growth is a response to eruptive processes and not to magma reservoir heterogeneity or dynamics furthermore we suggest that the low vesicularities of the clasts are due to syneruptive magmatic degassing resulting in microlitic growth prior to fragmentation and not to quenching of clasts by external groundwater

the role of fram strait winter cyclones on sea ice flux and on spitsbergen air temperatures 1 the link between high latitude atlantic winter cyclone activity and both the climate of spitsbergen and fram strait ice export is examined cyclones are more frequent over fram strait in relatively mild low ice export winters and are associated with south southwesterly flow around anomalous high pressure over scandinavia there is an increased tendency for cyclogenesis to occur off the northeast greenland coast and east greenland current in those winters fram strait cyclones then typically continue into the western arctic basin north of greenland and canada anomalously cold high ice export winters have higher cyclone activity in the barents sea and eurasia side of the arctic basin cyclones in mild winters have close proximity to spitsbergen and may simultaneously advect arctic sea ice across fram strait possibly accounting for the low under 30 shared variance in winter time series of temperatures and sea ice export

results from the broadband seismic network on montserrat a digital broadband seismic network has been installed around soufriere hills volcano on montserrat while several distinctive types of seismic events with frequencies ranging from 0 5hz to 30hz could be identified the emphasis is on two types of low frequency events which indicate the involvement of a fluid phase in the source mechanism the so called long period events and the hybrid events the latter occur in swarms with distinct periodicities of 4 to 12 hours and preceed major dome collapses and explosions the swarms correlate very well with the tilt observed at the flanks of the volcanic edifice and hence can be linked to the pressurization of the magmatic system occasionally separate hybrid events merge and form harmonic tremor which sometimes has a shifting spectral content this reveals temporary changes in the source parameters low frequency seismic signals on montserrat are considered to be key parameters for the monitoring of the internal dynamics of the volcano

temporal and spatial variation of the stress state of popocatepetl volcano mexico abstract the fault plane solutions fps of volcano tectonic events vts can help in the determination of the stress state of a volcano before during and after an eruptive period hence for popocatepetl we have calculated the fps from 1996 to 2003 and we have compared the results with its volcanic activity for popocatepetl there are two main areas of vts one below the crater and another in the southeastern zone for the below crater we have 398 solutions and for the southeastern zone 64 the fps below the crater have pressure axes p with a preferential orientation of s70 w n70 e which is consistent with the regional stress state for the tension axes t a radial tension is observed although a great amount of axes exists with a direction of nw se most of the fps are normal solutions and these are present at depths mainly between 0 and 3 km asl although we have periods where an increase is observed in the number of reverse events approximately between 3 and 0 km asl the variability of the solutions with depth is probably associated with the intrusion of magma inside the volcano the group of events located in the southeastern part of the volcano have pressure axes p with directions of s n and w e and the same orientations are observed for the tension axes t most of the mechanisms in this area are strike slip solutions which follow two possible faults which were activated during the early magma intrusions inside of the volcano

seismicity and stress in the vicinity of mount spurr volcano south central alaska focal mechanism solutions and hypocenters for earthquakes near mount spurr volcano in south central alaska reveal spatial perturbations in the regional stress field and in the maximum depth of seismicity at the volcano practically all the shocks during the dormant decade 1981 1991 have depths shallower than 5 km and are characterized by nearly pure normal slip in contrast earthquakes located outboard of the volcano mostly concentrate in the depth range 3 18 km and exhibit predominantly strike slip oblique reverse slip or reverse slip the regional stress field is characterized by subhorizontal maximum principal stress directed n30 w concordant with the direction of convergence between the north american and pacific plates whereas the maximum principal stress is probably more nearly vertical beneath the volcano we suggest that the shoaling of seismicity beneath the volcano reflects localized elevation of the depth to the midcrustal transition from brittle failure to plastic flow we attribute this localized elevation to relict magmatic heat associated with past volcanic eruptions the available data are not sufficient to determine the cause of the suggested rotation of the maximum principal stress axis beneath the volcano possible mechanisms include an increase in the vertical stress from the weight of the mount spurr massif and a decrease in the maximum horizontal stress associated with doming of the shallow crust from magmatic processes