source mechanisms of micro earthquakes induced in a fluid injection experiment at the hdr site soultz sous forets alsace in 2003 and their temporal and spatial variations we have inverted the peak amplitudes of direct p waves of 45 micro earthquakes with magnitudes between m 1 4 and 2 9 which occurred during and after the 2003 massive fluid injection in the gpk3 borehole of the soultz sous for ets hot dry rock facility these events were recorded by a surface seismic network of 15 stations operated by the ecole et observatoire des sciences de la terre university of strasbourg the unconstrained moment tensor mt expression of the mechanism was applied allowing the description of a general system of dipoles that is both double couple dc and non dc sources as tensile fractures the mechanisms of all but one event are dominantly dcs with a few per cent additional components at the most we have checked carefully the reliability of the mt retrieval in bootstrap trials eliminating some data by simulating the mislocation of the hypocentre and by applying simplified velocity models of the area in constructing green s functions in some of the trials non dc components amounting to several tens of per cent appear but the f test classifies them as insignificant even the only micro earthquake with an exceptionally high non dc mechanism cannot be classified unambiguously the f test assigns similar significance to the pure dc solution the massive dominance of the dc indicates the shear slip as the mechanism of the micro earthquakes investigated the mechanisms display large variability and are of normal dip slip oblique normal to strike slip types the t axes are fairly stable being concentrated subhorizontally roughly in the e w direction on the contrary the p axes are ill constrained varying in the n s direction from nearly vertical to nearly horizontal which points to heterogeneous stress in the soultz injected volume this is in agreement with the stress pattern from in situ measurements the minimum stress axis is well constrained to e w whereas the maximum and intermediate stress values are close to one another enabling the ambiguity of the p axis direction we found no significant dependence of source mechanisms either on magnitudes or depths the time space distribution of the events analysed suggests that the injection activated two segments of the natural faults existing in the area i and ii in our notation showing different source mechanism patterns the dip slip regime is characteristic of fault segment i where the seismicity occurred during and also after injection whereas the strike slip regime prevails in segment ii where the seismicity was triggered only after the injection shut in this indicates that the tensile fractures which are assumed to be created during injection may have occurred on a smaller scale than the pure shear micro earthquakes investigated

a history of jokulhlaups from strandline lake alaska u s a les vidanges brutales du strandline lake se produisent tous les 1 a 5 ans la vidange est precedee par le developpement d un golfe de velage dans le lobe du triumvirate glacier qui barre le lac et par le remplissage de petits lacs sur le glacier on analyse le mecanisme a partir de cartes photogrammetriques dressees avant et apres la vidange

crustal earthquake bursts in california and japan their patterns and relation to volcanoes 1 we analyze 153 bursts of earthquakes in southern california and japan the burst patterns are similar in southern california and japan they fill a spectrum between swarm like sequences without obvious mainshocks and mainshocks with omori law abiding aftershocks in agreement with our previous work the swarm like sequences in japan have more events are more voluminous and tend to expand with time when compared to mainshock aftershock type sequences in both regions we find that the sequences starting with their largest events tend to be much shorter in duration bursts within 50 km of volcanoes are similar in character to those elsewhere except they tend to have longer duration we hypothesize that swarminess is a proxy for fluid pressure redistribution and or aseismic slip driving the seismicity bursts and conversely the mainshock aftershock style sequences have end member behavior that results solely from a cascade of elastic failures the complexity of the spatial seismicity distribution does not correlate with the style of swarm observed indicating that fluid conditions and composition are likely more influential than geometry in determining the patterns we observe

simultaneous earthquake swarms and eruption in alaska fall 1996 statistical significance and inference of a large aseismic slip event in the fall of 1996 the alaska volcano observatory recorded an un precedented level of seismic and volcanic activity the following were observed 1 a swarm at iliamna volcano august 1996 to mid 1997 2 an eruption at pavlof volcano september 1996 to december 1996 3 a swarm at martin mageik vol canoes october 1996 4 a swarm at strandline lake which continued for several years 1996 1999 and 5 deformation of mt peulik inflation begins after october 1996 and ends in fall 1998 based on interpretation of interferometric synthetic aperture radar data the number of monitored volcanic areas in 1996 was thirteen we conducted two formal statistical tests to determine the likelihood of four of these events occurring randomly in the same time interval the tests are based on different conceptual probabilistic models classical and bayesian that embrace a wide range of realistic tectonic models the first test considered only the areas in which swarms or eruptions occurred 7 of 13 if strandline lake is included 6 of 12 otherwise by comparing the real catalog with 10 000 synthetic catalogs under the assumption that the sites are independent the second method is a hierarchical bayesian model in which the probability of a swarm at each of the 13 or 12 areas is different but the parent population is the same we found that the likelihood of the swarms and erup tion occurring nearly simultaneously by chance alone is small for a wide range of probabilistic schemes and consequently for different tectonic scenarios therefore we conclude that the events may be related to a single process we speculate that a widespread deformation pulse or strain transient occurred mainly in the east half of the arc and may be the most likely candidate for causing such nearly simultaneous events

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

paradox of downstream fining and weathering rind formation in the lower hoh river olympic peninsula washington although downstream fining of clasts is typical in modern and ancient river systems the hoh river a cobble and boulder bed river in washington state contains surprisingly little downstream fining of the coarsest tail of the grain size distribution along its lower 63 km mean rate of fining of the coarsest size fraction is only 0 24 mm km barely significant given uncertainties in measurement in addition these same clasts have weathering rind thicknesses that change very little along the length of the river they decrease by 0 01 mm km because weathering rinds are well developed in this setting and both tumbler studies and field observations show that abrasion of weathering rinds greatly accelerates clast diminution there seems to be a paradox between predicted and observed downstream fining rates field study shows that detritus is constantly being resupplied to the river by erosion of late glacial materials along the river9s cutbanks and tributaries the clasts supplied range in size but include abundant coarse grains with very thick weathering rinds the continuous resupply of grains strongly attenuates the rate of downstream fining despite the fact that these weathered grains abrade relatively rapidly thus the dominance of reworked glacial debris overwhelms any processes by which grains are reduced in size in the modern river these results suggest that relatively infrequent glaciation can have a long lived effect on river sedimentation

seismic and geodetic investigation of the 1996 1998 earthquake swarm at strandline lake alaska summary microearthquake swarms occur frequently in volcanic environments but do not always culminate in an eruption such non eruptive swarms may be caused by stresses induced by magma intrusion hydrothermal fluid circulation or possibly other tectonic processes such as slow slip earthquakes the strandline lake region of southcentral alaska located 30 km northeast of mt spurr volcano experienced an intense earthquake swarm between 1996 august and 1998 september a total of 2507 earthquakes were recorded by the alaska volcano observatory s permanent seismic network during the swarm period with a maximum magnitude of ml 3 6 the cumulative seismic moment of the swarm was 1 2 1015 n m equivalent to that of a single mw 4 0 earthquake because of the swarm s distance from the nearest holocene volcanic vent seismic monitoring was minimal and gas emission and gps data do not exist for the swarm period however combined waveforms from a dense seismic network on mt spurr and from several regional seismic stations allow reanalysis of a representative set of swarm and background earthquakes swarm hypocentres calculated using a newly formulated 1 d velocity model and station corrections indicate a roughly circular swarm volume with dimensions of approximately 5 km centred below 10 km below sea level bsl composite fault plane solutions for swarm earthquakes indicate oblique strike slip faulting with a northeast trending p axis orientation in contrast a composite fault plane solution for background earthquakes indicates a fundamentally different though poorly constrained local stress field orientation interferometric synthetic aperture radar images spanning the swarm period unambiguously show no evidence of surface deformation but do not rule out subcentimetre scale deformation during the swarm while a shallow 10 km bsl magma intrusion representing an intrusive or protovolcanic segment of the aleutian arc

field investigation of love waves in near surface seismology we examine subsurface conditions and survey parameters suitable for successful exploitation of love waves in nearsurface investigations love wave generation requires the existence of a low shear velocity surface layer we examined the minimum thickness of the near surface layer necessary to generate and record usable love wave data sets in the frequency range of 5 50 hz we acquired field data on a hillside with flat lying limestone and shale layers that allowed for the direct testing of varying overburden thicknesses as well as varying acquisition geometry the resulting seismic records and dispersion images were analyzed and the love wave dispersion relation for two layers was examined analytically we concluded through theoretical and field data analysis that a minimum thickness of 1 m of low velocity material is needed to record usable data in the frequency range of interest in near surface love wave surveys the results of this study indicate that existing guidelines for rayleigh wave data acquisition such as receiver interval and line length are also applicable to love wave data acquisition

evidence for shear wave anisotropy in the mantle wedge beneath south central alaska we investigate shear wave polarization and splitting for more than 200 earthquakes with hypocentral depths ranging from 20 to 180 km beneath southcentral alaska data were recorded at three sites stations skn aui and red a preferred polarization parallel to the strike of the wadati benioff zone wbz n25 e is found for a majority of events however some regions show systematic differences in polarization for a subset of nine events we compute focal mechanisms and find no correlation between the theoretical shear wave polarization caused by the fault plane orientation and the observed polarization delays of the slow versus the fast s wave range from less than 0 05 to 0 25 sec for station skn and up to 0 4 sec for stations aui and red a statistically significant correlation of delay with depth is found r 0 78 with the largest delays corresponding to the deepest events this indicates that the source of the anisotropy cannot be located in the crust based on the ray path geometry the source cannot be solely located in the subducting slab either and must therefore originate in the mantle wedge our results for the subduction zone beneath alaska are consistent with similar studies in other subduction zones that describe slab parallel anisotropy in the mantle wedge

temporal and spatial variations of local magnitudes in alaska and aleutians and comparison with body wave and moment magnitudes abstract we evaluated temporal and spatial variability of local magnitudes m l in the earthquake catalog of alaska earthquake information center regionally recorded hypocenters in mainland alaska are available in the catalog beginning in the early 1970s no comprehensive aleutian wide m l statistics exist prior to the mid 1990s we identify four time intervals with similar magnitude residual trends between m l versus body wave magnitude m b 1971 1976 1977 1989 1990 1999 5 and 1999 5 2008 the three latter intervals are also identified in m l versus moment magnitude m w residual trends these time intervals correspond to the periods with different data processing procedures we find that the latest time interval has the best correspondence between m l and m w values in mainland alaska especially for shallow depth m l versus m b and m l versus m w are present in the catalog for the pre 1990 data the largest discrepancies are observed for the gulf of alaska earthquakes for the latest time period 1999 5 2008 the largest residuals between m l and m b or m w are observed for the earthquakes located within the oceanic segment of the aleutian arc and within 100 km of the trench we calculated a set of magnitude corrections for mainland alaska and the aleutians that need to be applied to homogenize magnitude values in the catalog in time and space

temporal spatial variations of stress at redoubt volcano alaska inferred from inversion of fault plane solutions abstract we inverted fault plane solutions fps of 420 earthquakes with a largest magnitude of 3 1 mostly occupying a 4 4 10 km 3 volume beneath redoubt volcano for principal stress directions during 1989 1998 on average fpss were computed with 8 p wave readings inversions performed with 48 fps yielding misfits of 7 and the size of the 95 confidence regions of the stress directions is 30 during the 1989 1990 eruptions stresses were typically sub horizontal 1 and 2 striking ese wnw and nne ssw respectively and near vertical 3 whereas during july 1991 january 1998 we found near vertical 1 and sub horizontal 2 and 3 striking sse nnw and ene wsw respectively we found differences in plunge of 1 up to 60 between 1990 and 1991 and sub volumes with different stress at the 99 confidence level evaluated with the z test based on the changes in stress directions from the eruptive to the inactive period we infer that the magnitudes of principal stresses increased progressively during the eruptions and had subsequently decreased by the post eruption period the observations can be explained by expansion and contraction of a plexus like magma body together with changes in physical properties of the magmas involved we estimate that the absolute values of principal stresses are similar to each other and are in the range 182 187 mpa

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

mapping the megathrust beneath the northern gulf of alaska using wide angle seismic data in the northern gulf of alaska and prince william sound we have used wide angle seismic reflection refraction profiles earthquake studies and laboratory measurements of physical properties to determine the geometry of the prince william and yakutat terranes the aleutian megathrust and the subducting pacific plate in this complex region the yakutat terrane is underthrust beneath the prince william terrane and both terranes are interpreted to be underlain by the pacific plate wide angle seismic reflection refraction profiles recorded along five seismic lines are used to unravel this terrane geometry modeled velocities in the upper crust of the prince william terrane to 18 km depth agree closely with laboratory velocity measurements of orca group phyllites and quartzofeldspathic graywackes the chief components of the prince william terrane to hydrostatic pressures as high as 600 mpa 6 kbar a landward dipping reflector at depths of 16 24 km is interpreted as the base of the prince william terrane this reflector corresponds to the top of the wadati benioff zone seismicity and is interpreted as the megathrust immediately beneath the megathrust is a 4 km thick 6 9 km s refractor which we infer to be the source of a prominent magnetic anomaly and which is interpreted by us and previous workers to be gabbro in eocene age oceanic crust of the underthrust yakutat terrane wide angle seismic data magnetic anomaly data and tectonic reconstructions indicate that the yakutat terrane has been underthrust beneath the prince william terrane for at least a few hundred kilometers wide angle seismic data are consistent with a 9 to 10 landward dip of the subducting pacific plate beneath the outer shelf and slope distinctly different from the inferred average 3 to 4 dip of the overlying 6 9 km s refractor and the wadati benioff seismic zone beneath the inner shelf our preferred interpretation of the geophysical data is that one composite plate composed of the pacific plate of a fairly uniform thickness and the yakutat plate of varying thickness is subducting beneath southern alaska

deep injection and closely monitored induced seismicity at paradox valley colorado the u s bureau of reclamation s paradox valley unit pvu extracts aquifer brine from nine shallow wells along the dolores river paradox valley in southwestern colorado and after treating high pressure injects the brine 4 3 4 8 km below the surface pvu injects at rates between 800 and 1300 l min since 1991 pvu has emplaced over 4 106 m3 of fluid and induced more than 4000 surface recorded seismic events the events are recorded on the local 15 station paradox valley seismic network the induced seismicity at paradox separates into two distinct source zones a principle zone 95 of the events asymmetrically surrounding the injection well to a maximum radial distance of 3 km and a secondary ellipsoidal zone 2 5 km long and centered 8 km northwest of the injection well the expansion of these zones has stabilized since mid 1999 about three years after the onset of continuous injection within the principal zone hypocenters align in distinct linear patterns showing at depth stratigraphy and the local wray mesa fracture and fault system the primary faults of the wray mesa system are aseismic striking subparallel to the inferred maximum principal stress direction with one or more faults probably acting as fluid conduits to the secondary seismic zone individual seismic events in both zones do not discernibly correlate with short term injection parameters however a 0 5 km2 region immediately northwest of the injection well responds to long term large scale changes in injection rate and the surpassing of a threshold injection pressure focal mechanisms of the induced events are consistent with simple double couple strike slip moments and subhorizontal extension to the northeast in addition the fault planes are consistent with principal stress directions determined from borehole breakouts more than 99 9 of the pvu seismicity is below human detection m 2 5 however approximately 15 events have been felt locally with the largest being a magnitude m 4 3 because of the m 4 3 and two earlier felt m 3 5 events and injection economics pvu changed injection strategies three times since 1996 these changes reduced seismicity from 1100 events year to as low as 60 events year

a large slow slip event and the depth of the seismogenic zone in the south central alaska subduction zone abstract we modeled crustal deformation before during and after a slow slip event sse on the plate interface in southern alaska we divided the gps time series into three time periods before 1998 1998 to 2001 and 2001 2002 the first and last periods represent the steady deformation while the middle period represents the sse we found a stationary zone of 100 slip deficit in the shallow part of the seismogenic zone to depths of 25 30 km which we identify with the main asperity that ruptured in 1964 earthquake the sse occurs downdip of the asperity on a section that accumulated some slip deficit during the steady time periods during the sse 20 000 sq km of the plate interface slipped 10 cm for a cumulative moment magnitude of m w 7 2 the area of the sse did not have a significant slip in the 1964 earthquake compared to the slip of the main asperity we propose that between earthquakes this zone fails repeatedly in sses and remains at relatively low shear stress compared to the main asperity up dip of it

new geophysical insight into the origin of the denali volcanic gap summary volcanic gaps are segments of subduction zones that lack the volcanic activity usually found at these convergent margins they are regions where the necessary conditions to produce melt may appear favourable but where volcanoes are surprisingly absent from the surface in this study we present a new model that can explain the occurrence of such volcanic gaps it is based on seismic imaging and geodynamic modelling of the denali volcanic gap a 400 km wide region at the eastern end of the alaska aleutian subduction zone here the thick crust of the pacific plate and yakutat terrane subduct at shallow angle beneath north america a high resolution seismic profile clearly images the subducting crust undergoing progressive dehydration between 50 and 120 km depth and a negative subhorizontal velocity contrast at 60 km depth in the overlying mantle wedge we interpret this 60 km discontinuity as marking the top of a layer of partial melt that pools at the base of the overriding plate in steadystate subduction models melt accumulates at the apex of a vaulted mantle wedge the pinch zone from where it may break through the overlying lithosphere to the surface beneath the denali volcanic gap the pinch zone is absent or greatly reduced because shallow subduction of the yakutat terrane progressively cools the system and causes the slab to advance and replace the hot core of the mantle wedge this regime can be seen as the opposite of subduction rollback it prevents the formation of a pinch zone reduces the length of the melting column and causes melt to pool at the base of the overriding plate thus inhibiting magma generation and extraction

the origin of volcano tectonic earthquake swarms volcano tectonic vt seismicity is commonly recorded prior to and during eruptions vt seismicity may reflect stresses induced by dike propagation as indicated by propagating hypocenters and fault plane solutions fps reflecting regional stresses or stresses induced by dike inflation indicated by randomly distributed hypocenters and fps with pressure axes rotated 90 to regional maximum compression although numerical models of dike induced stresses indicate that both regimes should occur during dike emplacement this has not yet been observed according to published seismic data instead 90 rotated fps are observed in some cases while propagating hypocenters mark dike formation in other cases we suggest that differences in the seismic expression of upper crustal magma migration may result from differences in the regional tectonic setting and in the nature of magma wall rock interactions ultimately it may be possible to use information contained in vt seismicity to forecast changes in a volcano s behavior by establishing the characteristic stress field response for a given volcano or through a deeper understanding of the complex relationships between vt seismicity local crustal stresses and the physical mechanisms of magma migration

shallow seismogenic zone detected from an offshore onshore temporary seismic network in the esmeraldas area northern ecuador for a given site many factors control the seismic risk earthquake magnitude hypocentral distance rupture mechanism site effects and site vulnerability are among the most important this article deals with one of these factors the depth of the seismogenic zone in the northern ecuadorian subduction system beneath a highly vulnerable site the city of esmeraldas and its industrial complex the ecuadorian oil refinery and shipping terminal to address this problem we analyzed data from a three weeks passive seismological experiment conducted in the spring of 1998 using 13 ocean bottom seismometers and 10 portable land stations a preliminary interpretation of wide angle data obtained in the fall of 2000 in the manta area 100 km south of the study area unambiguously indicates the presence of a velocity inversion in the ecuadorian margin velocity structure this velocity inversion is characterized by a shadow zone of 1 s on the record sections and is interpreted as the result of a velocity contrast between the upper plate structure and the sedimentary and basaltic layer ii of the subducted oceanic nazca plate one dimensional velocity models are deduced from these wide angle data and are used for earthquake location in the esmeraldas area this highly improved the hypocentral parameter determinations the updip limit of the seismogenic zone is found at a depth of 12 km 35 km eastward of the trench and the depth of the seismogenic zone below the esmeraldas city is found at 20 km this shallow depth of the seismogenic zone dramatically increases the seismic hazard of the area

evidence for dike emplacement beneath iliamna volcano alaska in 1996 two earthquake swarms comprising 88 and 2833 locatable events occurred beneath iliamna volcano alaska in may and august of 1996 swarm earthquakes ranged in magnitude from 0 9 to 3 3 increases in so2 and co2 emissions detected during the fall of 1996 were coincident with the second swarm no other physical changes were observed in or around the volcano during this time period no eruption occurred and seismicity and measured gas emissions have remained at background levels since mid 1997 earthquake hypocenters recorded during the swarms form a cluster in a previously aseismic volume of crust located to the south of iliamna s summit at a depth of 1 to 4 km below sea level this cluster is elongated to the nnw sse parallel to the trend of the summit and southern vents at iliamna and to the regional axis of maximum compressive stress determined through inversion of fault plane solutions for regional earthquakes fault plane solutions calculated for 24 swarm earthquakes located at the top of the new cluster suggest a heterogeneous stress field acting during the second swarm characterized by normal faulting and strike slip faulting with p axes parallel to the axis of regional maximum compressive stress the increase in earthquake rates the appearance of a new seismic volume and the elevated gas emissions at iliamna volcano indicate that new magma intruded beneath the volcano in 1996 the elongation of the 1996 1997 earthquake cluster parallel to the direction of regional maximum compressive stress and the accelerated occurrence of both normal and strike slip faulting in a small volume of crust at the top of the new seismic volume may be explained by the emplacement and inflation of a subvertical planar dike beneath the summit of iliamna and its southern satellite vents

mono lake excursion recorded in sediment of the santa clara valley california 1 two intervals recording anomalous paleomagnetic inclinations were encountered in the top 40 meters of research drill hole ccoc in the santa clara valley california the younger of these two intervals has an age of 28 090 330 radiocarbon years b p calibrated age 32 8 ka this age is in excellent agreement with the latest estimate for the mono lake excursion at the type locality and confirms that the excursion has been recorded by sediment in the san francisco bay region the age of an anomalous inclination change below the mono lake excursion was not directly determined but estimates of sedimentation rates indicate that the geomagnetic behavior it represents most likely occurred during the mono lake laschamp time interval 45 28 ka if true it may represent one of several recurring fluctuations of magnetic inclination during an interval of a weak geomagnetic dipole behavior noted in other studies in the region

ground deformation associated with the march 1996 earthquake swarm at akutan volcano alaska revealed by satellite radar interferometry in march 1996 an intense swarm of volcano tectonic earthquakes 3000 felt by local residents mmax 5 1 cumulative moment of 2 7 1018 n m beneath akutan island in the aleutian volcanic arc alaska produced extensive ground cracks but no eruption of akutan volcano synthetic aperture radar interferograms that span the time of the swarm reveal complex island wide deformation the western part of the island including akutan volcano moved upward while the eastern part moved downward the axis of the deformation approximately aligns with new ground cracks on the western part of the island and with holocene normal faults that were reactivated during the swarm on the eastern part of the island the axis is also roughly parallel to the direction of greatest compressional stress in the region no ground movements greater than 2 83 cm were observed outside the volcano s summit caldera for periods of 4 years before or 2 years after the swarm we modeled the deformation primarily as the emplacement of a shallow east west trending north dipping dike plus inflation of a deep mogi type magma body beneath the volcano the pattern of subsidence on the eastern part of the island is poorly constrained it might have been produced by extensional tectonic strain that both reactivated preexisting faults on the eastern part of the island and facilitated magma movement beneath the western part alternatively magma intrusion beneath the volcano might have been the cause of extension and subsidence in the eastern part of the island we attribute localized subsidence in an area of active fumaroles within the akutan caldera by as much as 10 cm during 1992 1993 and 1996 1998 to fluid withdrawal or depressurization of the shallow hydrothermal system

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