evidence for postseismic viscoelastic relaxation following the 1959 m 7 5 hebgen lake montana earthquake a 1983 releveling survey conducted by the national geodetic survey combined with previous surveys in 1923 1960 1967 and 1975 provides evidence for ongoing vertical deformation of a broad region diameter of 150 km surrounding the site of the 1959 hebgen lake earthquake deformation consists of relative uplift centered roughly on the coseismic fault and a smaller amplitude zone of relative subsidence south of the fault maximum observed elevation change during the postseismic period 1960 1983 exceeds 30 cm assuming that the entire uplift i e 1923 1983 occurred following the 1959 earthquake i e uplift measured between 1923 and 1960 was entirely postseismic the rate of vertical deformation appears to have decreased exponentially with a characteristic decay time of about 10 years the spatial pattern and time behavior of the observed movements are consistent with simple models of postseismic deformation following normal faulting in an elastic layer thickness of 30 40 km overlying a viscoelastic half space viscosity of 1019 pa s this model may also account for the anomalously large horizontal strain around the hebgen lake region measured by repeated trilateration surveys during the period 1973 1984 the releveling measurements in the hebgen lake region appear to provide the first observations of viscoelastic relaxation in the asthenosphere following an intraplate earthquake in the united states

the 2005 eruption of sierra negra volcano galapagos ecuador sierra negra volcano began erupting on 22 october 2005 after a repose of 26 years a plume of ash and steam more than 13 km high accompanied the initial phase of the eruption and was quickly followed by a 2 km long curtain of lava fountains the eruptive fissure opened inside the north rim of the caldera on the opposite side of the caldera from an active fault system that experienced an mb 4 6 earthquake and 84 cm of uplift on 16 april 2005 the main products of the eruption were an a a flow that ponded in the caldera and clastigenic lavas that flowed down the north flank the a a flow grew in an unusual way once it had established most of its aerial extent the interior of the flow was fed via a perched lava pond causing inflation of the a a this pressurized fluid interior then fed pahoehoe breakouts along the margins of the flow many of which were subsequently overridden by a a as the crust slowly spread from the center of the pond and tumbled over the pahoehoe the curtain of lava fountains coalesced with time and by day 4 only one vent was erupting the effusion rate slowed from day 7 until the eruption s end two days later on 30 october although the caldera floor had inflated by 5 m since 1992 and the rate of inflation had accelerated since 2003 there was no transient deformation in the hours or days before the eruption during the 8 days of the eruption gps and insar data show that the caldera floor deflated 5 m and the volcano contracted horizontally 6 m the total eruptive volume is estimated as being 150 106 m3 the opening phase tephra is more evolved than the eruptive products that followed the compositional variation of tephra and lava sampled over the course of the eruption is attributed to eruption from a zoned sill that lies 2 1 km beneath the caldera floor

stratovolcano stability assessment methods and results from citlaltepetl mexico citlaltpetl volcano is the easternmost strato volcano in the trans mexican volcanic belt situated within 110 km of veracruz it has experienced two major collapse events and subsequent to its last collapse rebuilt a massive symmetrical summit cone to enhance hazard mitigation efforts we assess the stability of citlaltpetl s summit cone the area thought most likely to fail during a potential massive collapse event through geologic mapping alteration mineralogy geotechnical studies and stability modeling we provide important constraints on the likelihood location and size of a potential collapse event the volcano s summit cone is young highly fractured and hydrothermally altered fractures are most abundant within 5 20 m wide zones defined by multiple parallel to subparallel fractures alteration is most pervasive within the fracture systems and includes acid sulfate advanced argillic argillic and silicification ranks fractured and altered rocks both have significantly reduced rock strengths representing likely bounding surfaces for future collapse events the fracture systems and altered rock masses occur non uniformly as an orthogonal set with n s and e w trends because these surfaces occur non uniformly hazards associated with collapse are unevenly distributed about the volcano depending on uncertainties in bounding surfaces but constrained by detailed field studies potential failure volumes are estimated to range between 0 04 0 5 km 3 stability modeling was used to assess potential edifice failure events modeled failure of the outer portion of the cone initially occurs as an intact block bounded by steeply dipping joints and outwardly dipping flow con tacts as collapse progresses more of the inner cone fails and the outer intact block transforms into a collection of smaller blocks eventually a steep face develops in the uppermost and central portion of the cone this modeled failure morphology mimics collapse amphitheaters pres ent at many of the world s stratovolcanoes that have experienced massive failure events

stability assessment of volcanic lava tubes in the galapagos using engineering rock mass classifications and an empirical approach abstract the stability of the lava tubes of the galapagos islands has been analysed from an engineering perspective lava tubes are natural caves originated by differential cooling between the centre and the outer part of a moving lava flow although these cavities are highly frequented in the galapagos and the rest of the world there are only a few studies that treat the stability and security of such caves in the present study the stability and security of some of the most representative show caves in volcanic tubes in the galapagos islands of santa cruz and isabela are evaluated we have used an internationally accepted empirical methodology for the evaluation of tunnels and underground man made constructions the methodology used in this general stability analysis is a geomechanical classification also known as the q index and the scaled span methods of the studied caves some result to be relatively stable while others demonstrate back analysis signs of instabilities with unravelling of blocks from the top and detached slabs in walls the methodology has been also applied in relatively stable parts of cavities in order to verify that this methodology works as a preliminary assessments the obtained data indicate that these methodologies which originally were created for man made underground works are a useful tool in a preliminary assessment the stability of natural caves such as lava tubes

a viscoelastic and afterslip postseismic deformation model for the 1964 alaska earthquake 1 we developed a 3 d viscoelastic model in concert with an afterslip model to describe the postseismic deformation following the 1964 alaska earthquake our model incorporates a realistic geometry including an elastic slab with very low dip angle these geometric factors are important and require a reanalysis of the 1964 coseismic model our coseismic model differs from previous models in that the montague island splay fault extends farther along the kenai peninsula coast and as a result slip on the megathrust in that region is smaller we computed postseismic deformation models using a range of mantle viscosities with maxwell relaxation time of 1 to 50 years with a best estimate of 20 years the viscoelastic model explains most of the trenchward motion observed in the present velocity field but has little impact on cumulative 1965 to present uplifts after removing the viscoelastic response the largest residual cumulative uplifts were located downdip from areas of largest coseismic slip area and we explain them using an afterslip model constrained in space by the observed cumulative postseismic uplifts and in time by tide gauge records no single mechanism can explain both the 30 years of cumulative uplift and the present velocities but a combination of viscoelastic relaxation afterslip interseismic elastic deformation and motion of southern alaska relative to north america explains the first order features of the observations forty years after the earthquake the present day velocities contain a significant component of postseismic deformation so very long lived postseismic deformation plays an important role in the subduction zone earthquake cycle for great earthquakes

current deformation in the northern canadian cordillera inferred from gps measurements 1 continuous and campaign style global positioning system gps measurements provide new constraints on the first order current deformation pattern of the northern cordillera of nw canada and eastern alaska the yakutat block is currently colliding with north america in the corner of the gulf of alaska our data infer that relative yakutat north america motion is accommodated across the eastern boundary by right lateral motion 40 mm a mainly on the fairweather fault and minor shortening 6 mm a to the north collision is taken up by shortening 31 mm a mainly on the chugach st elias fault system with westward extrusion and possible counterclockwise rotation of the yakutat block and alaskan fore arc facilitated by 23 mm a right lateral motion that is shared by several faults the seismicity pattern indicates that plate boundary deformation is far reaching producing strain throughout eastern alaska yukon and western northwest territories our gps data confirm the transfer of strain from the yakutat collision zone and enable the extent of plate boundary deformation to be mapped thereby defining the western edge of stable north america gps sites in sw yukon show motion of 3 10 mm a to the ne confirming that strain is transferred at least 400 km from the yakutat collision continued transfer north toward the mackenzie delta and ne to the mackenzie mountains indicated by current seismicity is not yet well resolved by the campaign gps data which are hampered by coseismic and postseismic effects of the 2002 mw 7 9 earthquake on the denali fault in eastern alaska

stress interaction between magma accumulation and trapdoor faulting on sierra negra volcano galapagos abstract understanding the state of stress within volcanoes and how it changes with active geophysical processes is important to assess when and where magma may propagate to the surface and initiate an eruption observations of volcanic deformation help to constrain parameters of models that can be used to describe these processes and to calculate the resulting stress changes within volcanoes here i use geodetic observations to constrain elastic dislocation models of magma accumulation and faulting at sierra negra volcano galapagos in the years before the october 2005 eruption which was the first eruption at sierra negra since 1979 both insar and gps data document a remarkable story of 5 m pre eruption uplift during 1992 2005 which was accompanied by at least three trapdoor faulting events on an intra caldera fault system in january 1998 april 2005 and just before the eruption in october 2005 the pattern of uplift observed in the insar data from different time periods during 1992 2005 is consistent with filling and pressurization of a 2 2 km deep sill under the caldera modeling results for the trapdoor faulting in april 2005 on the other hand indicate that slip occurred on a thrust fault that dips steeply 71 in towards the center of the caldera and extends from the surface down to the sill at 2 2 km depth independently the calculated stress changes caused by the inflating sill show that the coulomb failure stress change cfs at the fault s location is maximized on a 72 dipping thrust fault agreeing almost exactly with the deformation modeling result and implying that the inflation triggered the faulting the calculations also show that the cfs for optimally oriented faults any strike and dip is significantly larger in the southern part of the caldera where the faulting took place than at any other location the trapdoor faulting in turn relieved the pressure within the sill but caused compression to the south of the faults this suggests that while the faulting provides a mechanism for the sill to thicken and postpone eruptions it also prevents the sill from growing to the south

seismicity patterns during a period of inflation at sierra negra volcano galapagos ocean island chain basaltic shield volcanoes of the western galapagos islands are among the most rapidly deforming volcanoes worldwide but little was known of the internal structure and brittle deformation processes accompanying inflation and deflation cycles a 15 station broadband seismic array was deployed on and surrounding sierra negra volcano galapagos from july 2009 through june 2011 to characterize seismic strain patterns during an inter eruption inflation period and to evaluate single and layered magma chamber models for ocean island volcanoes we compare precise earthquake locations determined from a 3d velocity model and from a double difference cluster method using first motion of p arrivals we determine focal mechanisms for 8 of the largest earthquakes ml 1 5ml 1 5 located within the array most of the 2382 earthquakes detected by the array occurred beneath the broad 9 km wide sierra negra caldera at depths from surface to about 8 km below sea level although outside our array frequent and larger magnitude ml 3 4ml 3 4 earthquakes occurred at alcedo and fernandina volcano and in a spatial cluster beneath the shallow marine platform between fernandina and sierra negra volcanoes the time space relations and focal mechanism solutions from a 4 day long period of intense seismicity june 4 9 2010 along the southeastern flank of sierra negra suggests that the upward migrating earthquake swarm occurred during a small volume intrusion at depths 5 8 km subsurface but there was no detectable signal in insar data to further constrain geometry and volume focal mechanisms of earthquakes beneath the steep intra caldera faults and along the ring fault system are reverse and strike slip these new seismicity data integrated with tomographic geodetic and petrological models indicate a stratified magmatic plumbing system a shallow sill beneath the large caldera that is supplied by magma from a large volume deeper feeding system the large amplitude inter eruption inflation of the shallow sill beneath the sierra negra caldera is accompanied by only very small magnitude earthquakes although historical records indicate that larger magnitude earthquakes mw 6 occur during eruptions trapdoor faulting episodes without eruptions and large volume flank intrusions

changes in seismic moment rates along the rupture zone of the 1964 great alaska earthquake we examined seismic moment rates associated with the prince william sound and kodiak island asperities of the 1964 great alaska earthquake rupture zone to determine how seismicity has varied through time and space our results indicate that there has been a factor of 5 10 decrease in seismic moment rates within the prince william sound asperity region since 1964 whereas seismic moment rates have actually increased within the kodiak asperity region since 1964 primarily because of a sequence of m w 7 0 earthquakes that have occurred since 1998

the rupture process and asperity distribution of three great earthquakes from long period diffracted p waves moment tensor source mechanisms for the largest twenty nine shallow earthquakes from 1981 m s 6 5 have been determined using long period rayleigh wave spectra and p wave first motions recorded by the international deployment of accelerograms ida global digital seismic network gdsn and world wide standardised seismic network wwssn these are compared with the centroid moment tensor mechanisms presented by dziewonski and woodhouse which were obtained in a contrasting manner in most cases the source is well represented by a single double couple mechanisms with good agreement of scalar moment and orientation between the two studies apparent mismatches for other cases may be interpreted in terms of source complexity we emphasise the objectivity of comparing moment tensor elements apparent discrepancies in the moment and orientation of double couple fault plane solutions are usually due to differences in the mxz and myz moment tensor elements which are poorly constrained by long period data

microfossil evidence for recurrent coseismic subsidence around lake hamana near the nankai suruga trough central japan abstract we reconstructed recurrent coseismic subsidence during the middle holocene from analyses of sedimentary facies and fossil diatom assemblages in a small drowned valley the shinjo lowland bordering lake hamana near the nankai suruga trough central japan the lake hamana region is known for coseismic subsidence during the 1707 hoei m8 6 and 1854 ansei tokai m8 4 earthquakes along the nankai suruga trough subduction zone a coring survey revealed three widely distributed peat layers in the lowland with a mud layer overlying each of the two lower peats with a sharp contact we traced the alternating peat mud beds for over 300 m along the long axis of a valley the fossil diatom assemblage analysis revealed that the peat layers were deposited in a freshwater pond marsh environment which changed abruptly to an estuarine to inner bay environment recorded by the mud layers from the sudden environmental change at the two peat mud contacts and our radiocarbon dating results we inferred that coseismic subsidence followed by an invasion of seawater into the lowland occurred at least twice during a great earthquake that occurred between 5645 and 6635 cal yr bp and during another great earthquake at ca 5700 cal yr bp within the mud layers the dominant diatom species changed gradually upward from nitzschia frustulum and cocconeis scutellum in the lower part to cyclotella striata and achnanthes submarina in the upper part these changes suggest that water depth and salinity gradually increased after the coseismic subsidence a possible cause of this gradual subsidence is postseismic subsidence following great nankai suruga trough earthquakes

correlations between eruption magnitude so2 yield and surface cooling about the book humans have long marvelled at and feared the odorous and colourful manifestations of volcanic emissions and in some cases have harnessed them for their economic value the degassing process responsible for these phenomena is now understood to be one of the key factors influencing the timing and nature of volcanic eruptions moreover the surface emissions of these volatiles can have profound effects on the atmospheric and terrestrial environment and climate even more fundamental are the relationships between the history of planetary outgassing differentiation of the earth s interior chemistry of the atmosphere and hydrosphere and the origin and evolution of life this book provides a compilation of 23 papers that investigate the behaviour of volatiles in magma the feedbacks between degassing and magma dynamics and the composition flux and environmental atmospheric and climatic impacts of volcanic gas emissions

calculation of lava effusion rates from landsat tm data we present a thermal model to calculate the total thermal flux for lava flowing in tubes on the surface or under shallow water once defined we use the total thermal flux to estimate effusion rates for active flows at kilauea hawaii on two dates input parameters were derived from landsat thematic mapper tm field and laboratory measurements using these parameters we obtain effusion rates of 1 76 0 57 and 0 78 0 27 m3 s 1 on 23 july and 11 october 1991 respectively these rates are corroborated by field measurements of 1 36 0 14 and 0 89 0 09 m3 s 1 for the same dates kauahikaua et al 1996 using weather satellite avhrr data of lower spatial resolution we obtain similar effusion rates for an additional 26 dates between the two tm derived measurements we assume that although total effusion rates at the source declined over the period the shut down of the ocean entry meant that effusion rates for the surface flows alone remained stable such synergetic use of remotely sensed data provides measurements that can a contribute to monitoring flow field evolution and b provide reliable numerical data for input into rheological and thermal models we look forward to being able to produce estimates for effusion rates using data from high spatial resolution sensors in the earth observing system eos era such as landsat 7 the hyperspectral imager the advanced spaceborne thermal emission spectrometer and the advanced land imager

shallow seismicity of the anchorage alaska region 1964 1999 we have relocated over 4200 shallow 40 km earthquakes occurring in the anchorage region for 35 years following the 1964 great m w 9 2 alaska earthquake the shallowest 20 km earthquakes delineate several faults within the crust including one associated with mapped folds located north of upper cook inlet inversion of first motion data for the stress field orientation in upper cook inlet indicates east west oriented horizontal 1 and near vertical 3 a condition favoring reverse faulting along north south striking faults with trends similar to the orientation of mapped faults and fault cored anticlines within the inlet 1 is rotated 60 to 90 counterclockwise from the direction of plate convergence in agreement with global positioning system gps geodesy studies that indicate the western portion of the kenai peninsula and upper cook inlet do not appear to be moving in the direction of plate motion due to a change in coupling across the plate interface the stress regime north of the castle mountain fault is conducive to strike slip or normal faulting along faults striking east northeast or north northwest similar to previous studies we observed a persistent aseismic zone in the upper crust that appears to be located above and immediately downdip of the portion of the plate interface that slipped 20 25 m in the 1964 mainshock deeper 20 40 km earthquakes indicate intense deformation and a rapidly changing stress field near the boundary between the kenai and mckinley segments of the subducted slab the 1943 m w 7 0 susitna lowlands earthquake may have been associated with this region of complex deformation

the 1964 prince william sound earthquake joint inversion of tsunami and geodetic data the 1964 prince william sound alaska earthquake mw 9 2 ruptured a large area beneath the continental margin of alaska from prince william sound to kodiak island a joint inversion of tsunami waveforms and geodetic data consisting of vertical displacements and horizontal vectors gives a detailed slip distribution two areas of high slip correspond to seismologically determined areas of high moment release the prince william sound asperity with average slip of 18 m and the kodiak asperity with average slip of 10 m the average slip on the fault is 8 6 m and the seismic moment is estimated as 6 3 1022 n m or over 75 of the seismic moment determined from long period surface waves

petrology of lavas from sierra negra volcano isabela island galapagos archipelago sierra negra volcano is a voluminous active basaltic shield situated in the western galapagos archipelago a hotspot related chain adjacent to the galapagos spreading center the volcano has erupted fe rich tholeiitic basalts of very limited compositional range comparison of lavas from each of five stratigraphic units covering the entire volcano indicates that the lavas are mineralogically similar and compositionally restricted 4 4 to 6 9 wt mgo the lavas are fe rich and hypersthene normative and plot in both the alkaline and tholeiite fields of the alkali silica diagram rare earth element ree patterns are steep and parallel the major and trace element data indicate a comagmatic relationship by fractional crystallization of the observed phenocryst phases projections into pseudoternary phase diagrams suggest that the magmas cooled and crystallized at pressures between 1 and 3 kbar calculated melt densities range from 2 73 to 2 77 g cm3 which are higher than those of typical mid ocean ridge basalt morb hawaiian tholeiite and other galapagos lavas the high calculated melt densities may be caused by fractionation at greater depths than is ordinary for the other basaltic magmas suppression of plagioclase and olivine crystallization in favor of augite may eliminate the minimum density window that has been proposed for morb fractionation sierra negra lavas have the most radiogenic lead and strontium isotopic ratios in the western galapagos indicating that the magmas have a relatively large contribution of plume material and have been minimally contaminated by entrainment of morb producing mantle magmatic 3he 4he isotopic ratios from sierra negra are approximately 15 times the atmospheric ratio although these ratios clearly indicate plume helium they are not the highest 3he 4he in the archipelago suggesting that helium is decoupled from the heavier isotopes high sm yb ratios light rare earth element lree enrichment and a steep ree slope are consistent with an origin by moderate extents 5 15 of partial melting of a garnet lherzolite source with ree characteristics that are between chondrite and depleted earth mantle sources sierra negra lavas are compositionally monotonous because homogeneous plume rich material has experienced long term exposure to a well regulated sublithospheric thermal environment with additional compositional restrictions imposed in the lithosphere by high magma flux

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

tide gage measurements of uplift along the south coast of alaska annual mean sea levels along the south coast of alaska are used to measure uplift along the alaska aleutian subduction zone oceanographic effects are removed from the observed annual mean sea levels by subtracting a correction that is proportional to the sea level fluctuations observed in southeast alaska that correction is effective in reducing fluctuations in the observed annual mean sea level as far west as the tip of alaska peninsula additional corrections to remove the eustatic rise in sea level and the apparent fall in sea level due to postglacial isostatic rebound of the land are introduced this corrected sea level record should provide a measure of tectonic subsidence in the area affected by the 1964 alaska earthquake postseismic uplift occurs where coseismic subsidence was observed and postseismic subsidence occurs where coseismic uplift was observed the immediate postseismic response is damped out within the first decade and the subsequent uplift rates appear to be steady over the 1974 1989 interval however some of those rates seem to be too high to be sustained over the 1000 year earthquake recurrence interval appropriate to this area if the interseismic deformation is only to recover the coseismic displacement thus a long term 100 years relaxation in uplift rates is postulated the immediate time constant 5 years postseismic relaxation is attributed to postseismic slip on the plate interface directly downdip from the coseismic rupture the long term time constant 100 years relaxation is attributed to flow in the asthenosphere

slopes lava flow volumes and vent distributions on volcan fernandina galapagos islands digital elevation data from topsar an airborne synthetic aperture radar system that uses interferometry to derive topography are used to determine slope distributions and lava flow thicknesses on fernandina volcano galapagos islands four extracaldera slope regions are defined from the coast inland a coastal plain average slope 2 an apron 6 12 steep slopes 250 600 m high 20 43 and a 0 5 to 1 km wide summit platform 10 lava flows and vents are mapped using shuttle imaging radar c sir c spot landsat thematic mapper tm and air photos over 500 flows are identified and categorized as young intermediate or old based on albedo vegetation cover and margin preservation by area young flows constitute 55 of the island 34 are intermediate and 7 are old the aa pahoehoe ratio of the young flows is 85 15 whereas for the intermediate flows it is 58 42 of the 423 vents that were classified 236 are radial and 80 of these are in the apron 143 are arcuate and 94 of these are in the summit platform and 46 have transitional orientations and are about equally divided between the steep slopes and summit platform 95 of all vents are within 13 km of the caldera center topsar data allow flow volumes to be estimated and young flows range from 0 01 to 0 12 km3 with a total volume of 2 3 km3 by volume 91 of the young lava erupted from radial vents below the steep slopes many of which are concentrated within the se apron about 5 6 km from the caldera similar concentrations to the ne nw and sw consist of young and intermediate flows different proportions of lava flows and vents form the different slope regions the coastal plain averages 0 1 vents km2 and the slightly steeper apron averages 0 6 to 0 9 vents km2 increasing inland the summit platform averages 4 7 vents km2 and this concentration supports previously proposed mechanisms for producing higher elevations and steeper slopes in the central part of the volcano temporal changes in the plumbing system and or magma supply rate are suggested by the change in aa pahoehoe ratio it appears that in the past low effusion rate eruptions were more common perhaps from a filled caldera whereas more recently high effusion rate eruptions have dominated

modeling of postseismic relaxation following the great 1857 earthquake southern california abstract repeated first order triangulation measurements made from 15 monuments in central and southern california are used to explore the structure of the crust and upper mantle beneath southern california as well as to estimate the distribution of slip during the great 1857 earthquake the monuments are divided into two subnetworks with repeated observations covering the respective time intervals from 1884 to 1923 and from 1898 to 1923 the angle changes occurring within these times are modeled as a sum of two contributions postseismic displacements following the 1857 earthquake and plate tectonic loading the postseismic displacements have been calculated using a elastic viscoelastic coupling model that for any fault geometry yields the exact displacements on a spherically stratified earth the southern california crust and upper mantle are modeled as an upper elastic plate underlain by a viscoelastic asthenosphere for several different combinations of structural parameters inversions are performed for the slip distribution of the 1857 earthquake in general the best fits to the data are achieved with a triangular slip distribution an elastic plate thickness of 16 km a crustal thickness of 16 to 33 km and a mantle viscosity of 0 4 to 0 8 10 19 pa s although these inversions identify the best rheological models they lead to observed residuals that are inconsistent with a normally distributed data set a preferred model of slip is constructed that accounts for the budget of total slip during the earthquake cycle and satisfies a normally distributed data set the preferred slip distribution is similar in shape to the mapped surface slip distribution but is greater in amplitude by a factor of 1 5 to 2 in addition the san jacinto fault which has no known surface slip associated with the 1857 earthquake is inferred to have accommodated 1 to 3 m of slip during the 1857 earthquake and appears to be the favored site for tectonic strain accumulation since 1857 inferred fault slip exceeding known surface slip implies either unrecognized secondary faulting or a slip deficit at the surface relative to the slip at depth

postseismic strain following the 1989 loma prieta earthquake from gps and leveling measurements postseismic deformation in the 5 years following the 1989 loma prieta earthquake has been measured with the global positioning system and precise leveling postearthquake velocities at distances greater than 20 km from the coseismic rupture are not significantly different from those observed in the 20 years prior to the earthquake however velocities at stations within 20 km of the rupture exceed preearthquake rates and exhibit unanticipated contraction normal to the strike of the san andreas fault system a combination of forward modeling and nonlinear optimization suggests that the observed postseismic deformations were caused by aseismic oblique reverse slip averaging 2 9 cm yr on the san andreas fault and or the loma prieta rupture zone and 2 4 cm yr reverse slip along a buried fault within the foothills thrust belt the best fitting sources of postseismic deformation are all located at depths of less than 15 km we find no evidence for accelerated flow or shear below the loma prieta rupture in the first 5 years following the earthquake the inferred postseismic slip is likely to have been caused by the coseismic stress change updip of the 1989 rupture

deformation of the kenai peninsula alaska crustal deformation on the kenai peninsula in southern alaska has been studied using data obtained from global positioning system gps measurements in 1993 and 1995 and leveling observations in 1964 immediately after the prince william sound earthquake this analysis shows that the kenai peninsula has experienced as much as 900 mm uplift during the past 3 decades and that the uplift forms an 125 km wide elongate dome with its major axis trending southwest to northeast following the trend of the major tectonic features of the region the averaged uplift rate between 1964 and 1995 is as high as 30 mm yr 1 although the current uplift rate may be substantially lower the gps measurements cast further doubt on previously suspect tide gauge data for nikiski alaska which indicated rapid postseismic uplift at this site located in northwest kenai peninsula adjacent to cook inlet examination of the three dimensional gps data indicates that the eastern kenai peninsula is currently undergoing significant sse to nnw contraction in response to north american pacific plate convergence the horizontal velocities are consistent with the predictions of an elastic half space model for the interseismic deformation this result taken in combination with the small changes in uplift between 1993 and 1995 suggests that most of the present deformation is due to steady plate convergence rather than transient postseismic rebound

using numerical modeling to explore the origin of intrusion patterns on fernandina volcano galapagos islands ecuador 1 using parameterized finite element models we investigate the emplacement of both radial and circumferential intrusions in the configuration observed at fernandina volcano in the galapagos islands when situated within the edifice at depths consistent with petrological and surface displacement data inflation of a mildly oblate magma reservoir to the point of rupture can initiate either radial or circumferential intrusions in response to minor volcanologically plausible variations in reservoir geometry i e aspect ratio in addition more oblate reservoirs inject lateral sills into an inflation derived stress field consistent with rotation about their propagation axis to form gently dipping radial dikes a mode of behavior recently inferred from insar data at fernandina all three styles of intrusion occur in near surface configurations consistent with field observations

uplift of the kenai peninsula alaska since the 1964 prince william sound earthquake using global positioning system gps receivers we reoccupied several leveling benchmarks on the kenai peninsula of alaska which had been surveyed by conventional leveling immediately following the march 27 1964 prince william sound earthquake mw 9 3 by combining the two sets of measurements with a new high resolution model of the geoid in the region we were able to determine the cumulative 1993 1964 postseismic vertical displacement we find uplift at all of our benchmarks relative to seward alaska a point that is stable according to tide gauge data the maximum uplift of about 1 m occurs near the middle of the peninsula the region of maximum uplift appears to be shifted northwest relative to the point of maximum coseismic subsidence if we use tide gauge data at nikishka and seward to constrain the vertical motion then the observed uplift has a trenchward tilt down to the southeast as well as an arching component to explain the observations we use creep at depth models most acceptable models require a fault slip of about 2 75 m although this result is not unique if the slip has been continuous since the 1964 earthquake then the average slip rate is nearly 100 mm yr twice the plate convergence rate comparing the net uplift achieved in 29 years with that observed over 11 years in an adjacent region southeast of anchorage alaska we conclude that the rate of uplift is decreasing a further decrease in the uplift rate is expected as the 29 year averaged displacement rate is about twice the plate convergence rate and therefore cannot be sustained over the entire earthquake cycle

slopes of western galapagos volcanoes from airborne interferometric radar the distribution of slopes on the six basaltic shield volcanoes in the western galapagos islands is investigated using a digital elevation model derived from airborne interferometric radar topsar data these measurements have a spatial sampling of 10 m pixel a vertical accuracy of 3 to 5 m and constitute the highest resolution most complete topographic data set available for the islands volcano heights are determined to range from 1 124 m sierra negra to 1 710 m wolf over extensive areas of each volcano slopes exceed 25 with the highest slopes being 37 on wolf and 36 on fernandina we confirm that two morphologic subgroups exist cerro azul fernandina and wolf with deep calderas depth between 40 60 of the subaerial height of the volcano and steep 20 maximum slopes at elevations between 60 and 80 of the volcano height and alcedo darwin and sierra negra with shallow calderas depth 25 of subaerial height and slopes that remain 15 until 90 of the total height is reached our data show that steep slopes are not uniquely correlated with the occurrence of arcuate fissures at the summit leaving the origin of the steep slopes unresolved

opposing gradients of permanent strain in the aseismic zone and elastic strain across the seismogenic zone of the kodiak shelf and slope alaska an industry acquired seismic reflection line from the aleutian arc to the alaska trench was prestack depth migrated and further constrained with wide angle ocean bottom hydrophone seismic velocity data and swath mapping from the seismic reflection image individual thrust slices the backstop and the forearc basin were resolved sufficiently well to balance the section tectonic shortening and age estimates provided rates of permanent contraction in five discrete segments over 150 km during the past 3 myr about 70 of the plate convergence is measured as contraction across the 40 km wide neogene accretionary prism which composes less than 10 of the forearc permanent contraction is minimal above the 1964 alaska earthquake aftershock zone indicating dominently elastic strain since elastic strain is poorly recorded in the geological record it was approximated from the historical record of the 1964 alaska earthquake the 1964 coseismic vertical deformation displays an inverse pattern to the permanent strain consistent with the earthquake cycle concept elastic deformation is dominant beneath the shelf and upper slope rapid permanent deformation is concentrated beneath the lower slope and a transition between them occurs beneath the middle slope

tectonics of nisyros island greece by field and offshore data and analogue modelling abstract we use new on land and offshore structural data and scaled analogue models to analyse the fault tectonic pattern at nisyros island greece which has an active caldera that shows a complicated network of faults fractures and volcano tectonic structures we measured 157 faults that show dominant dip slip normal motions along planes mainly striking ne sw nne ssw and nnw sse inside the caldera dykes necks and morphometric parameters of volcanic domes explosion craters and fumarole pits indicate the control by ne striking discontinuities on magma and gas paths the nnw and ne striking faults bound a major block that underwent repeated downthrow and uplift movements during the late pleistocene holocene experiments with scaled models of caldera resurgence and two magma chambers indicate the formation of an hourglass shaped fault pattern as seen in plan view with an asymmetric increase in the fault offset and a widening of the fault divergence towards the volcano flank all these data suggest that regional fault tectonics and stress state strongly guided magma upwelling and the emplacement of volcanic centres whereas periodical bulging due to the overpressure of a second magma chamber located northwest of the caldera combined with faulting due to tectonic stresses can account for the overall deformation field

physical volcanology and structural development of sierra negra volcano isabela island galapagos archipelago sierra negra is the most voluminous of the western galapagos shields and exhibits many unusual morphologic features extensive lower flank apron steep upper flanks a broad summit plateau a large complex caldera ubiquitous occurrence of aa and radial and circumferential fissures a field based investigation of the volcano was conducted in order to establish the distribution and characteristics of eruptive zones and associated flow fields and to construct a volcano wide surface flow stratigraphy geochronologic data were used to determine the age of the units and combined with flow volume calculations to estimate both the historic and total volcano eruption rates ninety one kilometers of radial and circumferential fissures were mapped and grouped into four eruptive zones which form a volcano wide east northeast trending rift system that deflects around the caldera the eruptive zones are restricted spatially and temporally and document long term changes in the volcano s development long fluid lava flows are broadly distributed among five ages resulting in a subdued flank profile by the development of an extensive lower flank apron around the volcano ten historical eruptions have occurred with eruption rates of 44 to 248 m3 s and an average growth rate of 12 106 m3 yr age determinations of lava flows by 14c and cosmogenic 3he range from 400 to 6900 yr the total volcano growth rate is estimated to be 1 106 m3 yr these results confirm the young age of sierra negra lavas and the validity of the relative ages determined from stratigraphic study in total the volcano has undergone over 90 resurfacing in the past 4500 yr the summit of sierra negra is entirely occupied by a shallow elliptical 7 10 km caldera near vertical ring faults circumscribe the summit and expose 100 to 140 m of lava flows subparallel sets of caldera growth faults cut the western and southern caldera walls and are not associated with eruptive activity the inclination of remnant magnetism and the attitudes of the flows exposed in the caldera walls indicate that the lavas have not been appreciably deformed nor have they been intruded by dikes or sills the caldera interior is structurally complex and contains a 14 km long c shaped sinuous ridge composed of a complex set of normally faulted blocks with inward dipping 20 60 flow tops the sinuous ridge is interpreted to have formed gradually as the caldera floor pivoted in a trap door fashion jammed its western edge and then fractured in several places concomitant with the long term vertical and lateral growth of the volcano a regional tectonic stress field superimposed upon the galapagos platform appears to have exerted primary control on the location of sierra negra including the early development of the east northeast rift system intervolcano stresses have controlled the distribution of eruptive zones only where significant subaerial overlap of adjacent volcanoes occurs large volume high discharge rate eruptions from fissures along the east northeast rift system are responsible for the development of an extensive coastal apron flow stacking at mid flank elevations and generally subdued slopes in comparison to other western galapagos volcanoes the summit caldera and underlying magma chamber are subordinate in influencing the distribution of lower flank stress but dominate the development of the summit stress field and orientation of eruptive fissures the absence of summit deformation together with the ubiquitous occurrence of short stubby summit flows indicates that the steep upper flanks are constructional in origin

interseismic uplift at the nankai subduction zone southwest japan 1951 1990 uplift as a function of time from 1951 through 1990 has been deduced from annual mean sea level measured at 15 tide gages along the nankai subduction zone southwest japan the recurrence interval for rupture of the nankai subduction zone is about 100 years and the most recent rupture was in late 1946 thus the 1951 1990 uplift record covers most of the first half of the earthquake cycle the precision of the uplift record is better than could be obtained currently by annual global positioning system gps measurements the pre 1960 uplift record shows rapid deformation that appears to terminate in 1959 after 1959 the uplift record is remarkably linear in time significant curvature in the uplift versus time plots is detected at only three of the 15 tide gages the inferred uplift rates are not quantitatively consistent with the predictions of either the viscoelastic coupling or elastic half space models of subduction but the agreement is sufficient to suggest that the causative physical mechanisms have been identified the immediate postseismic response is controlled by the propagation of slip downward along the downdip extension of the coseismic rupture and the interseismic response is caused by the accumulation of a slip deficit on the main thrust zone asthenosphere relaxation is not required to explain the observations

fumarole plume and diffuse co 2 emission from sierra negra caldera galapagos archipelago measurements of visible and diffuse gas emission were conducted in 2006 at the summit of sierra negra volcano galapagos with the aim to better characterize degassing after the 2005 eruption a total so2 emission of 11 2 t day 1 was derived from miniature differential optical absorption spectrometer mini doas ground based measurements of the plume emanating from the mini azufral fumarolic area the most important site of visible degassing at sierra negra volcano using a portable multigas system the h2s so2 co2 so2 and h2o so2 molar ratios in the mina azufral plume emissions were found to be 0 41 52 2 and 867 9 respectively the corresponding h2o co2 and h2s emission rates were 562 394 and 3 t day 1 respectively the total output of diffuse co2 emissions from the summit of sierra negra volcano was 990 85 t day 1 with 605 t day 1 being released by a deep source the diffuse to plume co2 emission ratio was about 1 5 mina azufral fumaroles released gasses containing 73 6 mol of h2o the main noncondensable components amounted to 97 4 mol co2 1 5 mol so2 0 6 mol h2s and 0 35 mol n2 the higher h2s so2 ratio values found in 2006 as compared to those reported before the 2005 eruption reveal a significant hydrothermal contribution to the fumarolic emissions 3he 4he ratios measured at mina azufral fumarolic discharges showed values of 17 88 0 25 ra indicating a mid ocean ridge basalts morb and a galapagos plume contribution of 53 and 47 respectively

global plate velocities from the global positioning system we have analyzed 204 days of global positioning system gps data from the global gps network spanning january 1991 through march 1996 on the basis of these gps coordinate solutions we have estimated velocities for 38 sites mostly located on the interiors of the africa antarctica australia eurasia nazca north america pacific and south america plates the uncertainties of the horizontal velocity components range from 1 2 to 5 0 mm yr with the exception of sites on the pacific and nazca plates the gps velocities agree with absolute plate model predictions within 95 confidence for most of the sites in north america antarctica and eurasia the agreement is better than 2 mm yr we find no persuasive evidence for significant vertical motions 3 standard deviations except at four sites three of these four were sites constrained to geodetic reference frame velocities the gps velocities were then used to estimate angular velocities for eight tectonic plates absolute angular velocities derived from the gps data agree with the no net rotation nnr nuvel 1a model within 95 confidence except for the pacific plate our pole of rotation for the pacific plate lies 11 5 west of the nnr nuvel 1a pole with an angular speed 10 faster our relative angular velocities agree with nuvel 1a except for some involving the pacific plate while our pacific north america angular velocity differs significantly from nuvel 1a our model and nuvel 1a predict very small differences in relative motion along the pacific north america plate boundary itself our pacific australia and pacific eurasia angular velocities are significantly faster than nuvel 1a predicting more rapid convergence at these two plate boundaries along the east pacific rise our pacific nazca angular velocity agrees in both rate and azimuth with nuvel 1a

time dependent uplift of the kenai peninsula and adjacent regions of south central alaska since the 1964 prince william sound earthquake leveling global positioning system very long baseline interferometry and tide gauge observations indicate that time dependent uplift has occurred on the kenai peninsula and adjacent regions of southern alaska that subsided coseismically in the 1964 prince william sound earthquake these observations have been critically assessed and used along with seismological and geological information as constraints in various aseismic slip and viscoelastic flow models of crustal deformation the postseismic uplift rate in most of the region is significantly greater than the expected interseismic average some but not all of the data indicate that the uplift rate was quite rapid more than several centimeters per year in the years immediately following the earthquake and has slowed to a few centimeters per year or less since then the modeling shows that the uplift can be explained by aseismic slip at depth but not by viscoelastic flow the slip at depth since 1964 must have a transient as well as a steady component since the deduced slip is considerably greater than the roughly 1 5 m expected from north american pacific plate convergence since the 1964 event the minimum amount of slip required to fit the observations comes from an elastic model and is about 2 75 m viscoelastic models require more slip since the dominant effect of viscoelastic flow is to produce subsidence in the region that is uplifted a simple parameterization of the transient component using an amplitude and decay time is adequate to explain the observations while the parameters deduced from the viscoelastic models have some ambiguity the viscosity of models that best fit the observations cannot be much less than several times 1019 pa s when the thickness of the north american plate is taken to be about 90 km

late holocene great earthquakes and relative sea level change at kenai southern alaska kenai located on the west coast of the kenai peninsula alaska subsided during the great earthquake of ad 1964 regional land subsidence is recorded within the estuarine stratigraphy as peat overlain by tidal silt and clay reconstructions using quantitative diatom transfer functions estimate co seismic subsidence relative sea level rise between 0 28 0 28 m and 0 70 0 28 m followed by rapid post seismic recovery stratigraphy records an earlier co seismic event as a second peat silt couplet dated to 1500 1400 cal yr bp with 1 14 0 28 m subsidence two decimetre scale relative sea level rises are more likely the result of glacio isostatic responses to late holocene and little ice age glacier expansions rather than to co seismic subsidence during great earthquakes comparison with other sites around cook inlet at girdwood and ocean view helps in constructing regional patterns of land level change associated with three great earthquakes ad 1964 950 850 cal yr bp and 1500 1400 cal yr bp each earthquake has a different spatial pattern of co seismic subsidence which indicates that assessment of seismic hazard in southern alaska requires an understanding of multiple great earthquakes not only the most recent all three earthquakes show a pre seismic phase of gradual land subsidence that marked the end of relative land uplift caused by inter seismic strain accumulation copyright 2005 john wiley sons ltd

chronology and phenomenology of the 1982 and 2015 wolf volcano eruptions galapagos archipelago abstract the 1982 and 2015 eruptions were the first at wolf volcano galapagos archipelago with eyewitness accounts and satellite imagery both eruptions were characterized by a rapid and intense initial phase with multiple eruptive vents leading to the formation of large a lava fields and scarce p hoehoe mostly associated with the waning phases the 1982 eruption started on 28 august from an intra caldera vent that produced high lava fountains as well as a radial fissure on the se flank the whole eruption lasted for at least 9 days and generated 52 5e 6 m3 dre dense rock equivalent of lava the 2015 eruption started on 25 may from a circumferential fissure that produced high lava fountains and deposited reticulite scoria on the flanks of the volcano for the first time since the onset of volcano monitoring in galapagos we report cryptotephra from the 2015 eruption reaching and depositing in mainland ecuador 1400 km away from the source lava from the 2015 circumferential vents covered 18 5 km2 on the se and e flanks on 13 june 2015 the eruption switched to an intra caldera vent that was active until 30 june and produced lava flows that covered most of the caldera floor the 2015 eruption lasted 36 days and produced 87e 6 m3 dre of lava making it the fourth largest eruption in galapagos since the eruption of sierra negra in 1979 we use combined ground based geophysical surveillance remote sensing eyewitness accounts and detailed field work to constrain the eruptive dynamics of this remote volcano our approach allows quantification of eruption rates which are critical for understanding volcanic systems and for hazard assessment first order rheological calculations permit us to further constrain the eruption dynamics and emplacement of the lava fields

the 1964 great alaska earthquake present day and cumulative postseismic deformation in the western kenai peninsula abstract global positioning system gps triangulation and leveling data are used to derive models for the present day last 5 years and the 30 year average postseismic deformation on the kenai peninsula alaska following the 1964 alaska earthquake the two datasets are inverted using a three dimensional elastic dislocation model to estimate the magnitude and spatial distribution of slip on the north america pacific plate interface allowing us to examine the time dependence of the processes controlling postseismic deformation we determine that the 30 year average postseismic slip rate beneath the western kenai peninsula was about twice as large as the present day slip rate the observations suggest a time decaying process but are not consistent with a single exponentially decaying relaxation process initiated immediately after the 1964 earthquake we conclude that the postseismic deformation observed on the western kenai peninsula cannot be explained in terms of any single time decaying process either multiple processes acting on different timescales or significant spatial propagation of the postseismic deformation or both must occur in the latter case postseismic deformation would not begin everywhere at the same time and the rate of spatial propagation would affect the timescale inferred for the postseismic processes

coseismic slip in the 1964 prince william sound earthquake a new geodetic inversion the 1964 prince william sound earthquake march 28 1964 m w 9 2 caused crustal deformation over an area of approximately 140 000 km 2 in south central alaska in this study geodetic and geologic measurements of this surface deformation were inverted for the slip distribution on the 1964 rupture surface previous seismologic geologic and geodetic studies of this region were used to constrain the geometry of the fault surface in the kodiak island region 28 rectangular planes 50 by 50 km each oriented 218 with a dip varying from 8 nearest the aleutian trench to 9 below kodiak island define the rupture surface in the prince william sound region 39 planes with variable dimensions 40 by 50 km near the trench 64 by 50 km inland and orientation 218 in the west and 270 in the east were used to approximate the complex faulting prior information was introduced to constrain offshore dip slip values the strike slip component and slip variation between adjacent planes our results suggest a variable dip slip component with local slip maximums occurring near montague island up to 30 m further to the east near kayak island up to 14 m and trenchward of the northeast segment of kodiak island up to 17 m a single fault plane dipping 30 corresponding to the patton bay fault with a slip value of 8 m modeled the localized but large uplift on montague island the moment calculated on the basis of our geodetically derived slip model of 5 0 1029 dyne cm is 30 less than the seismic moment of 7 5 x 1029 dyne cm calculated from long pe riod surface waves kanamori 1970 but is close to the seismic moment of 5 9 1029 dyne cm obtained by kikuchi and fukao 1987

tomographic imaging of the alaska subduction zone the alaska subduction zone is characterized by the pacific plate descending beneath the north american plate causing abundant seismic activity in the crust and along the wadati benioff zone down to a depth of approximately 200 km we have used 142 908 p wave arrival times from 12 237 shallow and intermediate depth earthquakes recorded by the alaska earthquake information center jointly run by the geophysical institute university of alaska fairbanks and u s geological survey in the period from january 1977 to november 1991 to investigate the three dimensional 3 d p wave velocity structure beneath central and southern alaska travel times and ray paths are accurately calculated by using an efficient 3 d ray tracing technique the nonlinear tomographic problem is solved by iteratively conducting linear inversions and the velocity structure and hypocentral locations are simultaneously determined we conducted two types of inversions one is an inversion with a laterally homogeneous starting model the others are what we call slab inversions in which we introduce into the initial model the high velocity subducting pacific plate as a priori information we found that the slab inversions gave a seismologically more plausible result and a final root mean square travel time residual significantly smaller than that of the inversion with the homogeneous starting model detailed p wave tomographic images are obtained for the crust and upper mantle down to a depth of 200 km with spatial resolutions of 30 60 km the tomographic image of the upper crust correlates well with the major surface geological features such as slow sedimentary basins and fast ultramafic bodies prominent low velocity anomalies exist in the crust and upper mantle beneath active volcanoes in the mantle wedge the low velocity anomalies dip toward the continental side and extend to a depth of about 150 km which are considered to be associated with the active volcanism in the alaska subduction zone the results suggest that the subducting pacific plate has a thickness of 45 55 km and a p wave velocity 3 6 higher than that of the surrounding mantle

imaging rapidly deforming ocean island volcanoes in the western galapagos archipelago ecuador using local body wave arrival time tomography methods to determine 3 d seismic velocity structure we imaged the plumbing system of sierra negra volcano galapagos this hot spot volcanic chain includes some of the fastest deforming volcanoes in the world making this an ideal location to study shield volcano plumbing systems we inverted p and s wave arrivals recorded on a 15 station temporary array between july 2009 and june 2011 using an a priori 1 d velocity model constrained by offshore refraction studies with local seismicity from nearby volcanoes as well as the ring fault system the model resolution is good between depths of 3 and 15 5 km the propagation of s waves throughout this volume argues against any large high melt accumulations although a shallow melt sill may exist above 5 km we image a broad low velocity region 25 km laterally below sierra negra at depths 8 15 km no large regional velocity increase is found within the limits of good resolution suggesting that crust is thicker than 15 km beneath the western galapagos archipelago our results are consistent with crustal accretion of mafic cumulates from a large volume magma chamber that may span the boundary between preplume and accreted crust the similarity between our results and those of hawaii leave open the possibility that the crust has also been thickened by under plating

volumetric characteristics of lava flows from interferometric radar and multispectral satellite data the 1995 fernandina and 1998 cerro azul eruptions in the western galapagos we have used a suite of remotely sensed data numerical lava flow modeling and field observations to determine quantitative characteristics of the 1995 fernandina and 1998 cerro azul eruptions in the western galapagos islands flank lava flow areas volumes instantaneous effusion rates and average effusion rates were all determined for these two eruptions for which only limited syn eruptive field observations are available using data from spot topsar ers 1 and ers 2 we determined that the 1995 fernandina flow covers a subaerial area of 6 5 106 m2 and has a subaerial dense rock equivalent dre volume of 42 106 m3 field observations atsr satellite data and the flowgo numerical model allow us to determine that the effusion rate declined exponentially from a high of 60 200 m3 s 1 during the first few hours to 5 m3 s 1 prior to ceasing after 73 days with a mean effusion rate of 4 16 m3 s 1 integrating the atsr derived exponentially declining effusion rate over the eruption duration produces a total subaerial submarine dre volume of between 27 and 100 106 m3 the range in values being due to differing assumptions about heat loss characteristics only values in the higher part of this range are consistent with the independently derived subaerial volume using spot topsar ers 1 and ers 2 data we determine that the 1998 cerro azul flow is 16 km long covers 16 km2 and has a dre volume of 54 106 m3 flowgo produces at vent velocity and effusion rate values of 11 m s 1 and 600 m3 s 1 respectively the velocity value agrees well with the 12 m s 1 estimated in the field the mean effusion rate total dre volume duration was 7 47 m3 s 1 dike dimensions fissure lengths and pressure gradients along the conduit based on magma chamber depth estimates of 3 5 km produce mean effusion rates for the two eruptions that range over nearly four orders of magnitude the range being due to uncertainty in the magma viscosity dike dimensions and pressure gradient between magma chamber and vent although somewhat consistent with mean effusion rates from other techniques their wide range makes them less useful the exponentially declining effusion rates during both eruptions are consistent with release of elastic strain being the driving mechanism of the eruptions our results provide independent input parameters for previously published theoretical relationships between magma chamber pressurization and eruption rates that constrain chamber volumes and increases in volume prior to eruption as well as time constants of exponential decay during the eruption the results and theoretical relationships combine to indicate that at both volcanoes probably 25 30 of the volumetric increase in the magma chamber erupted as lava onto the surface in both eruptions the lava flow volumes are less than 1 of the magma chamber volume

rock uplift at the transition from flat slab to normal subduction the kenai mountains southeast alaska abstract the process of flat slab subduction results in complex deformation of overlying forearcs yet how this deformation decays with distance away from the zone of underthrusting is not well understood in south central alaska flat slab subduction of the yakutat microplate drives shortening and rock uplift in a broad coastal orogenic belt defined limits of the zone of underthrusting allow testing how orogenesis responds to the transition from flat slab to normal subduction to better understand forearc deformation across this transition apatite u th he low temperature thermochronometry is used to quantify the exhumation history of the kenai mountains that are within this transition zone measured ages in the northern kenai mountains vary from 10 20 ma and merge with the exhumation pattern in the chugach mountains to the northeast where high exhumation occurs due to flat slab related deformation in the southern kenai mountains however ages increase to 30 50 ma across a transition near seward alaska above the zone from flat slab to normal subduction these ages are relatively old in comparison to ages determined in other studies in southern alaska and suggest minimal exhumation furthermore transitions in topographic expression of the coastal orogen also occur at the margin of yakutat underthrusting these observations suggest that either deformation associated with flat slab subduction requires tens of kilometers to decay with distance away from the zone of underthrusting or that orogenesis in the kenai mountains is driven by a distinct tectonic cause a potential driver of deformation is underplating of thick sediments specifically the surveyor submarine fan along the aleutian megathrust analogous to the tectonic mechanism responsible for the emergence of the kodiak island forearc if correct this may represent a recent tectonic transition in the region given the minimal exhumation of the rugged kenai mountains despite the presence of an erosion conducive glacial climate

caldera collapse at near ridge seamounts an experimental investigation collapse calderas are sub circular volcanic depressions caused by subsidence of the magma reservoir roof during an eruption scaled physical models of caldera collapse using flat topography have been instrumental in investigating the spatial and temporal development of calderas in particular two distinctive sets of concentric ring faults one reverse and one normal more recent analog studies have investigated the effect of non flat topography which alters the principle stress trajectories and resulting collapse structure this work provides the basis for investigating how naturally scaled topographic loads may affect caldera collapse in relation to shallow magma reservoirs the objective of this study is to understand how a near ridge seamount affects caldera collapse from both a central and offset position as the seamount migrates above the magma reservoir as a result of plate motion we utilize scaled analog models of caldera collapse in conjunction with three dimensional 3d laser scanning and digital particle image velocimetry dpiv to investigate caldera collapse dynamics at near ridge seamounts experiments using a seamount cone positioned centrally above the magma reservoir result in 1 increased subsidence along the interior outward dipping faults and 2 a preference to more symmetric collapse patterns as indicated by the subsidence profile and structure of the caldera relative to experiments with an offset cone when the cone is offset the collapse is asymmetric and trapdoor in nature with the center of greatest subsidence displaced away from the region of largest topographic load for these latter experiments subsidence is focused where the roof is thinnest along an initial reverse fault followed by a transition to an antithetic graben structure the asymmetric collapse in the experiments results in a caldera with a tilted profile offset calderas at near ridge seamounts are tilted towards the ridge axis suggesting that they may have collapsed asymmetrically similar to our experiments furthermore the graben structure observed in asymmetric collapse experiments is consistent with structures observed in some natural trapdoor basaltic volcanoes

measurement of horizontal motions in alaska using very long baseline interferometry we have analyzed 6 years of very long baseline interferometry vlbi data beginning in 1984 from a network of sites in alaska the yukon and the conterminous united states the analysis of these 53 data sets includes constrained continuous piecewise linear estimation of the site atmospheres and clocks application of values and correlations from a self consistent earth orientation series derived from other vlbi data and simultaneous estimation of site positions and velocities from a solution that includes 21 007 observations with a fit of 40 ps in delay and 64 fs s in delay rate we have determined the horizontal velocities of seven sites with precisions of 0 5 2 mm yr 1 standard deviation the three vlbi sites within the alaska aleutian subduction boundary zone yakataga kodiak and sand point have higher instantaneous velocities relative to eastern north america than the interior sites of alaska fairbanks and sourdough the estimated velocity of yakataga relative to fairbanks is 37 8 1 3 mm yr at n27 w 2 modeling of the yakataga data suggests that the observed motion is the result of elastic straining of the overriding plate due to a locked main thrust zone with a component of oblique slip the estimated velocity of kodiak relative to fairbanks is 13 6 0 8 mm yr at n41 w 3 this result is lower than the rate predicted on the basis of simple elastic dislocation models the estimated velocity of sand point relative to fairbanks is 6 4 0 9 mm yr at n65 w 8 the component of this velocity in the plate direction 3 5 1 1 mm yr is not consistent with either an aseismic slip model i e no strain accumulation in the overriding plate or a model in which the site is undergoing compressional strain 10 9 mm yr at n28 w due to simple subduction at the predicted orientation of pacific plate convergence in a north america fixed reference frame the estimated velocities of fairbanks 1 5 0 5 mm yr at s12 e 20 and sourdough 3 9 1 0 mm yr at s13 w 13 are both southward these results may imply motion of this region relative to stable north america but more observations are required to reduce the uncertainties in the azimuths the time evolution of the estimated site positions from vlbi in alaska are in general consistent with constant velocity however in 1988 the yakataga site showed a significant departure from the position predicted from a constant velocity model obtained from the previously estimated position this departure may be static displacement associated with two major strike slip earthquakes that occurred in the gulf of alaska between the 1987 and 1988 measurement sessions

relation between the subducting plate and seismicity associated with the great 1964 alaska earthquake tectonic studies of the great 1964 alaska earthquake have underappreciated the nature of the subducted plate in influencing seismicity we compare seismological observations in the prince william and kodiak areas that ruptured during this earthquake with the corresponding morphology and structure of the subducting plate the upper plate geology prince william terrane and velocity structure are the same in both areas in the prince william area where the yakutat terrane subducted the energy released and coupling were stronger than above the kodiak subduction zone where thick trench sediment subducts the conjecture that lower plate character or the amount of subducted sediment affects coupling helps explain variability in seismology geodetic inversions and the horizontal velocity of gps stations

a comprehensive study of the seismicity of the kenai peninsula cook inlet region south central alaska abstract we compare 30 yr of background seismicity of the kenai peninsula cook inlet region extending from eastern prince william sound kayak island to west of the cook inlet volcanic arc to potential field gravity magnetics tomographic global positioning system geodesy and geologic information to better determine factors controlling seismicity we also compare the occurrence of m w 5 events over the past 80 100 yr to background seismicity in an effort to determine how reflective background seismicity is of the regional seismic hazard our results indicate that contrasts in upper plate rheology leading to changes in density magnetic susceptibility and velocity of the upper plate serve to concentrate seismicity the southwestern edge of the subducting yakutat microplate has a strong influence on both upper and lower plate seismicity density and velocity contrasts also indicate changes in lower plate rheology that influence the location of deeper 30 km seismicity and the occurrence of a double seismic zone within the subducting pacific plate

a study of historic earthquakes of the prince william sound alaska region we have investigated the character of seismicity that occurred in the prince william sound cook inlet region during 40 years prior to the 1964 great alaskan earthquake we have relocated all events of magnitude 5 5 focal mechanisms have been determined from body waveform modeling and first motion analysis for the larger generally magnitude 6 3 events the results suggest a number of similarities and differences between pre and post 1964 mainshock seismicity similarities include the persistence of normal faulting at depths of 40 to 60 km within the subducted slab in both the tazlina glacier and columbia bay regions north of prince william sound and among deeper events within northern cook inlet differences include a higher level of shallow seismicity 40 km depth in the upper cook inlet region and north of cook inlet prior to 1964 higher levels of seismicity within the subducting plate in the south central and eastern kenai peninsula prior to 1964 and a lower level of seismicity in the offshore prince william sound region prior to 1964 much of the lower plate seismicity for the past 70 years appears to cluster immediately downdip of the 1964 asperity two of the events in this study caused notable damage modified mercalli intensity vii to viii in the anchorage area the first in 1933 m w 6 9 appears to have occurred at 9 km depth along a strike slip fault within northern cook inlet the second in 1954 m w 6 7 occurred within the subducting plate at 60 km depth beneath the northern kenai peninsula analysis of these events as well as smaller events we have studied will greatly improve models of seismic hazard for the anchorage region manuscript received 18 september 2000

a dislocation model of strain accumulation and release at a subduction zone strain accumulation and release at a subduction zone are attributed to stick slip on the main thrust zone and steady aseismic slip on the remainder of the plate interface this process can be described as a superposition of steady state subduction and a repetitive cycle of slip on the main thrust zone consisting of steady normal slip at the plate convergence rate plus occasional thrust events that recover the accumulated normal slip because steady state subduction does not contribute to the deformation at the free surface deformation observed there is completely equivalent to that produced by the slip cycle alone the response to that slip is simply the response of a particular earth model to embedded dislocations for a purely elastic earth model the deformation cycle consists of a coseismic offset followed by a linear in time recovery to the initial value during the interval between earthquakes for an elastic viscoelastic earth model elastic lithosphere over a viscoelastic asthenosphere the postearthquake recovery is not linear in time records of local uplift as a function of time indicate that the long term postseimic recovery is approximately linear suggesting that elastic earth models are adequate to describe the deformation cycle however the deformation predicted for amore simple elastic half space earth does not reproduce the deformation observed along the subduction zones in japna at all well if stick slip is restricted to the main thrust zone as recognized earlier by shimazaki seno and kato the uplift profiles could be explained if stick slips were postulated to extend along the plate interface beyond the main thrust zone to a depth of perhaps 100 km but independent evidence suggests that stick slip at such depths if unlikely less

wolf volcano galapagos archipelago melting and magmatic evolution at the margins of a mantle plume wolf volcano an active shield volcano on northern isabela island in the galapagos archipelago has undergone two major stages of caldera collapse with a phase of partial caldera refilling between wolf is a typical galapagos shield volcano with circumferential vents on the steep upper carapace and radial vents distributed in diffuse rift zones on the shallower sloping lower flanks the radial fissures continue into the submarine environment where they form more tightly focused rift zones wolf s magmas are strikingly monotonous estimated eruptive temperatures of the majority of lavas span a total of only 22 c this homogeneity is attributed to buffering of magmas as they ascend through a thick column of olivine gabbroic mush that has been deposited from a thin shallow 2 km deep subcaldera sill that is in a thermochemical steady state wolf s lavas have the most depleted isotopic compositions of any historically active intraplate ocean island volcano on the planet and have isotopic compositions except for he he indistinguishable from midocean ridge basalt erupted from the galapagos spreading center gsc 250 410 km away from the peak of influence of the galapagos plume wolf s lavas are enriched in incompatible trace elements and have systematic major element differences relative to gsc lavas however wolf s magmas result from lower extents of melting deeper melt extraction and a greater influence of garnet compared with gsc magmas but wolf and the gsc share the same sources these melt generation conditions are attributed to melting in a thermal and mechanical boundary layer of depleted asthenosphere at the margins of the galapagos plume the lower degrees of melting and extraction from deeper levels result from a thicker lithospheric cap at wolf than exists at the gsc

widespread uplift and trapdoor faulting on galapagos volcanoes observed with radar interferometry volcanic uplift caused by the accumulation of magma in subsurface reservoirs is a common precursor to eruptions1 2 but for some volcanoes uplift of metres or more has not yet led to an eruption3 here we present displacement maps of volcanoes in the galapagos islands constructed using satellite radar interferometry that might help explain this dichotomy we show that all but one of the seven volcanoes on the islands of isabela and fernandina deformed during 1992 99 cerro azul and fernandina erupted4 5 6 during the observation period and show evidence of inflation co eruptive deflation and shallow dyke intrusion in contrast the largest volcano sierra negra has not erupted yet exhibits spatially and temporally variable deformation with a maximum uplift of 2 7 m between 1992 and 1999 which can be modelled by a shallow inflating sill inflation during 1997 98 however was accompanied by trapdoor faulting on a steeply dipping fracture system within the caldera repeated trapdoor faulting over geological time has formed an arcuate intra caldera ridge within sierra negra and may have acted to relax stresses above the magma chamber inhibiting summit eruptions similar processes may help explain large uplift unaccompanied by eruptive activity at other volcanoes

constraints on magma chamber geometry at sierra negra volcano galapagos islands based on insar observations abstract we investigate the problem of estimating magma chamber geometry using insar observations of sierra negra volcano galapagos ascending and descending interferograms are combined to determine vertical and one horizontal component of displacement the ratio of maximum horizontal to vertical displacement suggests a sill like source spherical or stock like bodies are inconsistent with the data we estimate the geometry of the sill assuming a horizontal uniformly pressurized crack with unknown periphery and depth the sill is discretized into small elements that are either open and are subjected to the pressure boundary condition or remain closed we find the best fitting sill to be located beneath sierra negra s inner caldera at a depth of about 2 km using boundary element calculations we show that any magma chamber with a flat top coincident with the sill model fits the data equally well the data are insensitive to the sides and bottom of the magma chamber

seismicity of the prince william sound region for over thirty years following the 1964 great alaskan earthquake in this paper we present results of body wave form modeling of 19 earthquakes generally m b 5 7 occurring from 1964 to 1983 in the vicinity and down dip of the large asperity within the prince william sound region that ruptured in 1964 these data are supplemented with source parameters from studies of more recent post 1980 events our results suggest that moderate earthquakes which occurred in the region between 1964 and 1984 were predominantly located in the vicinity of the prince william sound asperity and could be assigned to two groups the first group consists of events occurring above the plate interface within prince william sound along reverse faults or low angle thrusts the second group occurs at 35 to 60 km depth in the region north of prince william sound and represents normal to normal oblique faulting within the subducted pacific crust or upper mantle these earthquakes occur below the northern edge of the 1964 asperity in a region where the subducting plate undergoes a rapid change in strike and dip a third group of events occurs in cook inlet well down dip of the 1964 asperity and below the plate interface these events exhibit a variety of mechanisms and many at depths of 50 to 70 km may be associated with complexities in the shape of the downgoing slab most of the cook inlet events occurred after 1984 whereas a few events of similar magnitude have occurred in the vicinity of the prince william sound asperity since 1984

on trapdoor faulting at sierra negra volcano galapagos abstract rapid uplift observed during 1992 1997 at sierra negra volcano western galapagos was followed by deformation that we have previously interpreted as trapdoor faulting on an intra caldera fracture system at that time no direct evidence for the trapdoor faulting was available to support this interpretation here we report field evidence of recent faulting at sierra negra and provide direct measurements of the faulting using pixel offsets between satellite radar amplitude backscatter images our field observations carried out in 2001 indicate that recent faulting occurred along the southern part of the intra caldera fault system with maximum offset of at least 0 5 1 m differences between two satellite radar amplitude images show 1 4 0 2 m range change across this fault system this suggests that 1 5 0 2 m of faulting occurred assuming vertical ground displacements consistent with our previous modeling results results of numerical modeling indicate that growing sills can cause faulting of the intrusion roof on outward dipping or nearly vertical faults this occurs at high excess magma pressure when the lateral extent half length of the sill becomes greater than its depth the derived 2 km deep sierra negra sill exhibits trapdoor faulting 3 km south of the center of the sill therefore these modeling results provide a plausible mechanical explanation for the trapdoor fault at sierra negra

microfossil evidence for land movements associated with the ad 1964 alaska earthquake microfossil diatom and pollen data from a number of cores at coastal marshes at girdwood flats and kenai flats in cook inlet alaska suggest that there was a phase of gradual land subsidence prior to the main shock of the ad 1964 earthquake this phase followed a long period of gradual land uplift since the last large earthquake which occurred about 730 900 years ago caesium 137cs records indicate that subsidence started approximately 15 years before the 1964 earthquake this pre seismic subsidence is evident in the coastal marsh sediment sequences in changes in diatom and pollen assemblages that indicate changes in tidal marsh environments or a change from raised bog to marsh conditions the microfossil evidence indicates 0 15 m pre seismic land subsidence at both girdwood flats and kenai flats and co seismic subsidence of 1 8 m and 0 2 m respectively

crustal uplift in the south central alaska subduction zone new analysis and interpretation of tide gauge observations we have examined tide gauge measurements of apparent sea level height in south central alaska to determine the history of crustal uplift subsequent to the 1964 prince william sound earthquake there are spatial and temporal variations in the uplift rate since the 1994 earthquake that depend on the location of the tide gauge relative to the coseismic rupture features at seward on the eastern side of the kenai peninsula we find slow uplift that is consistent with elastic strain accumulation at the locked north american pacific plate boundary conversely at seldovia and nikiski on the western side of the kenai peninsula we find persistent rapid uplift of 10 mm yr 1 that may be longterm transient response to the earthquake but that cannot be sustained over the entire several hundred year recurrence interval for a great earthquake farther to the southwest at kodiak the rate of uplift is several millimeters per year but has slowed significantly over the past three and a half decades to the east of the kenai peninsula we find subsidence at cordova and an uncertain behavior at valdez at cordova and to a lesser extent valdez there is a mathematically significant time dependence although the evidence for the time dependence is less compelling than at kodiak at anchorage there is little evidence of vertical motion since the earthquake the along strike spatial variability in the relaxation time of the rates of vertical motion since the 1964 earthquake may be related to variations in the updip coseismic slip during the megathrust event

results from new gps and gravity monitoring networks at fernandina and sierra negra volcanoes galapagos 2000 2002 gps and gravity networks were established on sierra negra and fernandina volcanoes in 2000 and remeasured in 2001 and 2002 after a decade in which the caldera inflated by over 2 5 m sierra negra s caldera began to subside between 2000 and 2002 at a rate of about 9 cm a neither inflation nor deflation was accompanied by eruption simple elastic modeling indicates that the deformation at sierra negra can be attributed to volumetric contraction of a 2 1 km deep sill beneath the caldera caused by either movement of magma out of the sill or loss of bubbles through degassing precise gravity measurements support but do not prove the former mechanism fernandina volcano is in a phase of slow refilling of the summit reservoir after the 1995 eruption causing lateral expansion of the caldera rim by about 3 cm a our estimate for the depth of the shallowest part of the fernandina magma chamber is 1 to 2 km neither volcano shows evidence of flank spreading

analogue models of collapse calderas and resurgent domes collapse calderas and resurgent domes are a common association related to inflation deflation processes in volcanic systems the structure of calderas and domes depends upon the permitted relative movements of crustal volumes at depth the so called space problem in order to study the structures of collapse calderas and resurgent domes and to take the space problem into account several analogue models were made dry quartz sand was used to simulate the rheology of the brittle crust while newtonian silicone putty located at the base of the sand pack simulated the ductile behaviour of the magma a piston moved the silicone putty downward or upward inducing collapse and doming within the sand three separate sets of experiments simulated 1 caldera collapse 2 resurgence and 3 superimposition of resurgence on collapse and vice versa collapse experiments are characterized by the development of two concentric depressions the first formed depression is bordered by outward dipping reverse ring faults the subsequent outer concentric depression is bordered by inward dipping normal ring faults the deformation pattern during resurgence is a function of the overburden thickness t and the dome diameter d for higher t d ratios a dome forms bordered by inward dipping high angle reverse ring faults outward dipping normal ring faults develop at late stages for lower t d ratios the dome shows at late stages a crestal depression accompanied by radial fractures subsequently an apical extrusion of silicone occurs the superimposition of resurgence over collapse and vice versa is characterized by the complete reactivation with opposite kinematics of all the pre existing ring faults during inversion both in caldera and resurgence reverse ring faults form in the early stages due to differential uplift extensional structures subsequently form to accommodate gravitational collapse during the activity of the reverse faults the experiments and the overall similarities with nature suggest that the activity of both reverse and normal faults constitutes a possible solution to the space problem during major collapses or resurgences

late holocene relative sea level changes and the earthquake deformation cycle around upper cook inlet alaska abstract multiple peat silt couplets preserved in tidal marsh sediment sequences suggest that numerous great plate boundary earthquakes caused the coast around cook inlet alaska to subside over the past 3500 years field and laboratory analyses of the two youngest couplets record the well documented earthquake of ad 1964 and the penultimate one approximately 850 cal yr bp diatom assemblages from a range of modern day estuarine environments from tidal flat through salt marsh to acidic bog produce quantitative diatom transfer function models for elevation reconstructions based on fossil samples only nine out of 124 fossil assemblages analysed including previously published data for the ad 1964 earthquake have a poor modern analogue calibration of fossil samples indicate co seismic subsidence of 1 50 0 32 m for ad 1964 similar to measurements taken after the earthquake and 1 45 0 34 m for the 850 cal yr bp earthquake elevation standard errors for individual fossil samples range from 0 08 m in peat layers to 0 35 m in silt units lack of a chronology within fossil silt units prevents identification of changes in the rate of recovery and land uplift between the post seismic and inter seismic periods however preservation of multiple peat silt couplets indicates no net emergence over multiple earthquake cycles glacio isostatic movements from little ice age glacier advance and retreat explains a 0 15 m relative sea level oscillation recorded within the peat layer subsequently submerged as a result of the ad 1964 earthquake before both this and the 850 cal yr bp earthquake diatom assemblages suggest pre seismic relative sea level rise of 0 12 0 13 m representing possible precursors to great earthquakes

three dimensional elastic dislocation modeling of the postseismic response to the 1964 alaska earthquake 1 gps velocity observations from the kenai peninsula and kodiak island alaska display a pattern of spatial variability suggesting the presence of multiple active processes on the underlying pacific north american plate interface velocities of sites on the eastern kenai peninsula and kodiak island are consistent with elastic strain accumulation at the plate interface whereas velocities of sites on the western kenai peninsula and cook inlet are oriented in the opposite direction and suggest an ongoing postseismic response to the 1964 alaska earthquake we can reproduce the observed velocities using a three dimensional elastic dislocation model to estimate the variation in coupling between the pacific and north american plates two different inversion schemes are examined to check the robustness of the results we find that the gps data can be satisfied by the presence of a locked area near southwest prince william sound a locked area near southwest kodiak island and an area of postseismic reverse slip beneath and north of the western kenai peninsula the shallow plate interface trenchward of the western kenai peninsula does not appear to be locked the locked areas correspond to the prince william sound and kodiak island asperities that ruptured in 1964 and the area of postseismic slip lies downdip of the 1964 rupture area the correspondence between the present shallow coupling distribution and the 1964 slip distribution suggests that the locked regions repeat from one earthquake cycle to another

4d gravity changes associated with the 2005 eruption of sierra negra volcano galapagos sierra negra volcano the most voluminous shield volcano in the galapagos archipelago and one of the largest basaltic calderas in the world erupted on october 22 2005 after more than 25 years of quiescence gps and satellite radar interferometryinsar monitoring of the deformation of the caldera floor in the months prior to the eruption documented extraordinary inflation rates 1c m day the total amount of uplift recorded since monitoring began in 1992 approached 5 m at the center of the caldera over the eight days of the eruption the caldera floor deflated a maximum of 5 m and subsquently renewed its inflation but at a decelerating rate to gain insight into the nature of the subsurface mass density changes associated with the deformation gravity measurements performed in 2005 2006 and 2007 are compared to previous measurements from 2001 2002 when the volcano underwent a period of minor deflation and magma withdrawal the residual gravity decrease between 2001 2002 and 2005 is among the largest ever recorded atan active volcano950 gal and suggests that inflation was accompanied by a relative density decrease in the magmatic system forward modeling of the residual gravity data in 4d from 2002 to 2005 gives an estimate of the amount of vesiculation in the shallow sill required to explain the observed gravity variations geochemical constraints from melt inclusion and satellite remote sensing data allow us to estimate the pre eruptive gas content of the magma and place constraints on the thickness of the gas rich sill necessary to produce the gravity anomalies observed results suggest that reasonable sill thicknesses 700 800 m and bubble contents 10 50 volume can explain the large decrease in residual gravity prior to eruption following the eruption 2006 and 2007 the deformation and gravity patterns suggest re equilibration of the pressure regime in the shallow magma system via a renewed influx of relatively gas poor magma into the shallow parts of the system

postseismic coastal uplift in southern chile tide gage records at puerto montt referenced to a mareograph in talcahuano indicate a large at least 75 cm postseismic uplift of the region following the 1960 mw 9 5 event field observations carried out in 1989 at the same locations of previous measurements in 1968 are consistent with tide gage records the postseismic elevation changes are modeled as the result of propagating creep on the downdip extension of the coseismic fault for a 30 e fault dip minimum square error indicates a fault creep velocity of 4 6 km yr and slip amplitude of 3 5 m