array observations of pkp phases across the kenya rift implications for structure and tectonics abstract pkp travel time residuals from teleseisms recorded by a 100 110 km array centered on the south central portion of the kenya rift show that variations in residuals are as large parallel to the rift as perpendicular to it asymmetry in residuals observed across the rift is consistent with half grabens facing in opposite directions separated by an accommodation zone the largest time residuals are offset at least 40 50 km west of the rift axis supporting a rifting model in which a dominant half graben fault is listric penetrates the lithosphere and triggers intrusion from asthenospheric sources linear regression of residuals and bouguer gravity anomalies shows a slope of 0 015 0 002 s mgal the slopes of similar regressions for the tectonically inactive midcontinent rift and sierra nevada are 0 0025 0 001 s mgal one explanation for the observed factor of 5 difference is the presence of partial melt under the kenya rift system

detection of mantle plumes in the lower mantle by diffraction tomography hawaii abstract a new method is proposed to detect mantle plumes in the lower mantle the method is akin to diffraction tomography and relies on the scattering of long period seismic body waves by nearly vertical heterogeneities the theory is described in a companion paper ying ji h c nataf earth planet sci lett this issue here we apply this method to an actual set of long period digital seismograms recorded on the world wide network of seismic stations between 1980 and 1994 we select seismograms that illuminate the lower mantle in a 20 20 region around hawaii we construct an image of plume like heterogeneities using a cell size of 1 1 by a lsqr inversion of the scattered waves this image shows a strong slow anomaly about 200 km northwest of hawaii although the image is somewhat noisy resolution tests indicate that this feature is fairly robust the amplitude of the anomaly is between 30 and 60 times larger than what we predict from a simple thermal plume model built with a 600 k maximum temperature excess and a gaussian horizontal cross section with an 1 e diameter of 250 km it is the first time that such a method is applied to the mantle while we think that the existence of this anomaly is real we do not know how to explain such a large amplitude if real this observation has a number of important geodynamical consequences it indicates that a mantle plume is indeed responsible for the hawaii hotspot as speculated by morgan w j morgan nature 230 1971 42 43 the plume is nearly vertical it originates from the d region at the base of the lower mantle the amplitude of the anomaly suggests that partial melt or and a chemical anomaly must be present the plume source is to the northwest of its surface expression as predicted by some models of plume advection in the mantle wind

laboratory investigation of dune erosion using stereo video abstract simple parameterizations of dune erosion are necessary for forecasting erosion potential prior to an oncoming storm dune erosion may be parameterized in terms of the elevation of the total water level composed of surge tide and wave runup above the dune base and period of exposure of the dune to waves in this work we test several versions of this model using observations from a large wave tank experiment designed to model a storm hydrograph and we develop a new method for acquiring the appropriate data with confidence intervals using stereo video techniques the stereo method results in observations of dune morphology at higher spatial and temporal resolutions than traditional survey methods allow resolution of the stereo technique was 0 1 m in the horizontal and 0 04 m in the vertical and errors in stereo observations were on the order of 0 02 to 0 08 m 1 to 2 pixels when compared with surveys a new method was developed to estimate confidence intervals on stereo observations when the unchanging dune top was repeatedly sampled the new confidence intervals encompassed 2 standard deviations of scatter about the mean dune surface 98 of the time observations from the stereo method were used to quantify wave runup and dune erosion we tested a variety of runup statistics based on a gaussian distribution of swash properties and found that the most predictive statistic for dune erosion was the 16 exceedance elevation above the dune base lower than the often used 2 exceedance value we found that the parameterization of runup was sensitive to the definition of beach slope and that the most accurate beach slope for predicting runup was through the region of the beach profile defined by the mean water level plus one standard deviation of swash the dune base retreated along a relatively constant trajectory that was a half of the initial beach slope finally a simple model for dune erosion was tested and found to reproduce 64 of the observed variance in dune erosion rate given known forcing at the dune and 49 of the observed variance in dune erosion rate given parameterized forcing integrating the simple model over time 93 of the observed dune retreat distance was reproduced given offshore forcing

magma reflection imaging in krafla iceland using microearthquake sources the details of magma plumbing beneath active volcanoes remain a major challenge in geochemistry geophysics and volcanic hazard evaluation here we apply a relatively novel variation of seismic interferometry which we call virtual reflection seismic profiling vrsp to produce a high resolution image of a known crustal magma body the technique takes advantage of recent advances in both seismic instrumentation dense arrays and seismic analysis seismic interferometry we have applied this technique to data recently acquired at an iconic volcanic system krafla which lies on the mid atlantic ridge as exposed in northern iceland what make this particular site exceptional are encounters with rhyolitic magma in two drillholes k 39 and iceland deep drilling project 1 iddp 1 these known magma bodies represent a unique calibration opportunity for surface geophysical measurements of magma distribution at depth in this study we produced a stacked seismic reflection section by applying common depth point processing techniques to virtual shot gathers derived from interferometry of p waves from microearthquakes generated by tectonic magmatic and or geothermal activity we observe a strong coherent reflection on the seismic section at a travel time corresponding to the depth at which magma was encountered in the iddp 1 wellbore we interpret this reflection to be from magma or magma related fluids additional coherent reflections may correspond to other components of the magma plumbing beneath krafla these results represent a promising new technique for structural imaging with natural sources that can be applied to a wide array of geologic and energy problems that involve natural or induced seismic clusters

irregular geometry of the gorda subduction and deep structure of the eel river basin determined from teleseismic p delays abstract nearly 1000 observations of travel time residuals for teleseismic p arrivals across the humboldt bay seismic network near cape mendocino california are analyzed for a study of 3 d geometry of the gorda plate near mendocino triple junction mtj by the application of a three dimensional velocity inversion procedure the area is underlain by the subducted edge of the gorda plate and the eastward extension of the mendocino fracture zone the observed variation of the travel time gradient with azimuth favors either deepening of the slab southward or southward increase of dilatational velocity across the mendocino fracture zone at depths of more than 50 km the topographically corrected mean travel time delays at individual stations suggest that the eel river basin is deep and tectonically controlled further support for this view is provided by the inversion results which point to the possible extension of the low velocity material through the entire crustal section these results also suggest that from the south the eel river basin may be confined at depth by a steeply northward dipping sliver of high velocity material possibly a detached oceanic slab this feature may be responsible for the intense seismic activity reported beneath the eastern end of the mendocino fracture zone mfz thus the role of the eel river basin in accommodating the relative motion between the adjacent plates may be significant the leading edge of the downgoing slab appears irregular and at some latitudes may not extend beyond the coast line the estimated velocity anomalies vary by as much as 20 in the near surface blocks and nearly half as much at depth the pattern may result from intense tearing and deformation of the slab producing buckling folding and compression ridges

an attenuating structure beneath the aso caldera determined from the propagation of seismic waves the attenuation of amplitude is seen in seismic waves which pass through the central region of the aso caldera in kyushu japan it is also recognized from spectral analysis of seismic waves that the higher frequencies of the p wave are reduced in the waves which pass through the central region of the caldera it is shown that the relative attenuation increases remarkably for the frequency range of 5 to 10 hz the specific attenuation factor q of the p wave train is about 100 from the surface projection of the ray paths with low q values through the aso caldera to each station the attenuating region is located beneath the center of the caldera extending to the north of the central cones in conjunction with the low q value of the p wave and the decreases of s wave amplitudes the relative p wave residual times have comparatively large values for seismic waves passing through the central region beneath the caldera in order to attempt to provide additional information on the depth configuration of the attenuating material the ray paths of p wave s first arrivals are located in three dimensional space it indicates that the low velocity material is located beneath the center of the caldera at depths of about 6 to 9 km however lowvelocity anomalies above the depth of 6 km and below the depth of 15 km were not able to be detected because most of the available seismic ray paths had crossed the caldera at depths of about 6 to 15 km furthermore the relative residual times have numerous errors resulting from incorrect hypocenter locations origin times inhomogeneities in the structure and uncertainty of the velocity structure at shallow depths in the aso caldera refraction or reflection studies are required for an accurate estimate of the structure and more detailed properties of the attenuating material

compressional wave velocity structure of the upper 350 km under the eastern snake river plain near rexburg idaho relative travel time residuals for teleseismic p and pkikp are used to determine the compressional velocity structure under the eastern snake river plain at rexburg idaho damped least squares inversion of travel time residuals modified from the method of aki indicates a large body of 3 5 2 5 low relative velocity material centered under the northwest edge of the snake river plain less well resolved are high relative velocity bodies one deep under the idaho wyoming border the other 300 km below the idaho batholith comparison with similar velocity mappings of the yellowstone region indicates that the snake river plain low velocity anomaly is an aged extension of the low velocity body under the yellowstone caldera the upper half of the body has vanished in the eastern snake river plain and the lower half is less anomalous than at yellowstone the strong dependence of mantle velocity structure on position under the yellowstone snake river plain volcanic system indicates loss of heat to the surface primarily by advective heat movement by mass movement mechanisms not by conduction no discrimination between propagating rift and upper mantle heat anomaly models of this northeast progressing volcanic system is possible because velocity structures associated with these hot spot models have not been predicted quantitatively upper mantle heat anomaly models thermal plumes chemical plumes gravity anchors explain the observed structure with fewer ad hoc assumptions but propagating rift models cannot be eliminated

p wave attenuation of the yellowstone caldera from three dimensional inversion of spectral decay using explosion source seismic data using explosion source seismic refraction data recorded in the 1978 and 1980 yellowstone snake river plain seismic experiments a three dimensional inversion of differential p wave attenuation was used to assess the relative variations in q 1 in and around the volcanically active 45 km by 70 km yellowstone caldera northwestern wyoming differential attenuation was derived from spectral decay of upper crustal pg phases observed from six explosions and recorded at 90 temporary stations because of the relatively short time windows used to determine the spectral content a maximum entropy technique was employed to estimate the spectra that yielded an optimally small variance differential p wave attenuation was calculated from least squares determinations of the spectral ratios corrected for source and path effects the observed differential attenuation parameters were then inverted using a weighted least squares technique for a discretized 70 105 km three dimensional surface and upper crustal q 1 model of the yellowstone caldera and surrounding region results showed that the surface layer to depths of 2 km within the yellowstone caldera is characterized by relatively high attenuation with low q values less than 30 compared to values of 38 to 50 outside the caldera the higher attenuation in the caldera s surface layer is thought to be associated with quaternary lake sediments highly altered rhyolites and the possible influence of steam in areas of hydrothermal activity in the crystalline upper crust at depths of 2 km to 12 km q values of 40 to 70 were observed in areas of thick sedimentary fill northwest of the caldera and in areas of hydrothermal activity within the caldera upper crustal attenuation generally corresponded to q of 200 in areas that are interpreted to be associated with hot but now solidified granitic material in comparison relatively high attenuation q 40 was observed in the upper crust of the northeastern yellowstone caldera in an area that also corresponds to a 20 reduction in pg velocity a prominent negative bouguer gravity low of 20 mgal near yellowstone s largest area of hydrothermal activity and near a quaternary volcanic resurgent dome on the basis of constitutive models of velocity and density this zone of high attenuation is likely a product of unusual hydrothermal activity highly altered upper crustal rocks a shallow magmatic source or a highly porous steam saturated body outside the caldera the upper crust is relatively unaffected by high temperatures and thermal alteration and the corresponding attenuation is low with q values greater than 200

evidence for a high velocity slab associated with the hindu kush seismic zone we used teleseismic travel time residuals to determine lateral velocity variations of the crust and upper mantle in the pamir hindu kush region in tadjikistan and afghanistan data from 29 analog seismic stations in tadjikistan and northern afghanistan were used to determine travel time residuals for 210 teleseismic events ranging in distance from 28 to 87 and covering a broad range of azimuths we inverted for velocity perturbations over a rectangular grid with a block size of 99 99 km the model extended to a depth of 350 km with a 50 km thick first layer and two 150 km thick deeper layers the results show a strong and well resolved zone of high velocities in the upper mantle at depths greater than 200 km coincident with the location of the hindu kush seismic zone no clear velocity perturbations are associated with the pamir seismic zone above 200 km little correlation is observed with the seismic zone but indications of thicker crust under the pamir and thinner crust under the tadjik depression are seen the high velocities are most likely caused by the presence of oceanic lithosphere at depth

upper crustal structure of the yellowstone caldera from seismic delay time analyses and gravity correlations the 1978 yellowstone snake river plain seismic experiment provided detailed refraction data that were recorded across a two dimensional array of seismographs in yellowstone national park a delay time analysis was applied to 173 crystalline basement pg arrivals from these data to determine the three dimensional distribution of velocities and the layer configuration of the upper crust beneath the yellowstone caldera the p wave velocity structure of the caldera is characterized by a surface layer of combined sediments and rhyolite flows averaging 2 8 km s that range in thickness from 1 5 to 2 0 km adjacent to the caldera the crystalline upper crustal layer has a velocity of 6 05 0 01 km s but this layer decreases by 6 to 5 70 km s beneath the caldera and extends northeast 15 km beyond the caldera smaller zones of very low p velocity 4 0 km s a 30 velocity reduction compared to the 6 05 km s layer occur in the upper crust beneath the northeastern caldera rim and beneath the southwest caldera in the vicinity of the upper and midway geyser basins a three dimensional gravity interpretation based upon densities derived from the seismic model suggests that the regional gravity low of 60 mgal over the caldera correlates directly with 1 the surface layer of combined sediments and rhyolite flows 2 the low velocity 5 7 km s upper crustal layer and 3 the 4 0 km s low velocity zone beneath the northeastern caldera rim an interpretation of the seismic velocities and densities based on experimental data and theoretical models suggests that the 6 05 km s 2 70 g cm3 regional pg velocity is characteristic of crystalline basement rocks such as the precambrian granitic gneisses that are exposed north and south of the yellowstone caldera the 5 7 km s velocity layer 2 65 g cm3 is interpreted to be a 10 km thick hot body of granitic composition the 4 0 km s low velocity body 2 40 g cm3 beneath the northeastern caldera rim is associated with a local 20 mgal gravity anomaly with a range of interpretations from a large steam dominated system to a felsic body of 10 to 50 partial melt the postulated 4 0 km s low velocity layer beneath the southwestern caldera does not have a corresponding gravity signature and may reflect a zone of high seismic attenuation attributed to a saturated fracture zone this seismic velocity model of the yellowstone caldera suggests that the source of the quaternary silicic volcanic rocks corresponds to the 5 7 km s layer in the upper crust that appears to contribute the principal source of heat to the yellowstone hydrothermal systems the anomalous low velocity body in the northeastern caldera rim area may be a cupola of partial melt with an overlying vapor dominated carapace and may represent the waning stages of the third cycle of quaternary volcanism or the beginning of a new cycle of volcanism that will continue the northeastward progression of the silicic volcanism of the yellowstone snake river plain propagating system

a review of crust and upper mantle structure studies of the snake river plain yellowstone volcanic system a major lithospheric anomaly in the western u s a abstract the snake river plain yellowstone volcanic system is one of the largest basaltic volcanic field in the world here there is clear evidence for northeasterly progression of rhyolitic volcanism with its present position in yellowstone many theories have been advanced for the origin of the snake river plain yellowstone system yellowstone and eastern snake river plain have been studied intensively using various geophysical techniques some sparse geophysical data are available for the western snake river plain as well teleseismic data show the presence of a large anomalous body with low p and s wave velocities in the crust and upper mantle under the yellowstone caldera a similar body in which compressional wave velocity is lower than in the surrounding rock is present under the eastern snake river plain no data on upper mantle anomalies are available for the western snake river plain detailed seismic refraction data for the eastern snake river plain show strong lateral heterogeneities and suggest thinning of the granitic crust from below by mafic intrusion available data for the western snake river plain also show similar thinning of the upper crust and its replacement by mafic material the seismic refraction results in yellowstone show no evidence of the low velocity anomalies in the lower crust suggested by teleseismic p delay data and interpreted as due to extensive partial melting however the seismic refraction models indicate lower than normal velocities and strong lateral inhomogeneities in the upper crust particularly obvious in the refraction data are two regions of very low seismic velocities near the mallard eake and sour creek resurgent domes in the yellowstone caldera the low velocity body near the sour creek resurgent dome is intepreted as partially molten rock together with other geophysical and thermal data the seismic results indicate that a sub lithospheric thermal anomaly is responsible for the time progressive volcanism along the eastern snake river plain however the exact mechanism responsible for the volcanism and details of magma storage and migration are not yet fully understood

an automatic phase picker for local and teleseismic events an automatic detection algorithm has been developed which is capable of time p phase arrivals of both local and teleseismic earthquakes but rejects noise bursts and transient events for each signal trace the envelope function is calculated and passed through a nonlinear amplifier the resulting signal is then subjected to a statistical analysis to yield arrival time first motion and a measure of reliability to be placed on the p arrival pick an incorporated dynamic threshold lets the algorithm become very sensitive thus even weak signals are timed precisely during an extended performance evaluation on a data set comprising 789 p phases of local events and 1857 p phases of teleseismic events picked by an analyst the automatic picker selected 66 per cent of the local phases and 90 per cent of the teleseismic phases the accuracy of the automatic picks was ideal i e could not be improved by the analyst for 60 per cent of the local events and 63 per cent of the teleseismic events

a deep low velocity body under the yellowstone caldera wyoming delineation using teleseismic p wave residuals and tectonic interpretation in previously published papers iyer 1975 iyer and stewart 1977 iyer 1979 evidence was presented for the presence of a large body of anomalous material with compressional velocities considerably lower than normal from the surrounding rock in the crust and upper mantle under yellowstone national park wyoming in this paper we give a detailed analysis of p residual data from the 26 element telemetered seismic array and three groups of portable stations operated by the u s geological survey in the yellowstone region using a variety of analytical techniques we delineate the shape of the low velocity body determine the velocity structure inside the body and interpret the nature of the material that constitutes the body we will also outline some tectonic implications of our model the spectacular surface geothermal phenomena for which yellowstone is famous result from deep seated volcanism the volcanic evolution of yellowstone is discussed by christiansen and blank 1972 and summarized by eaton and others 1975 briefly the geothermal manifestations in yellowstone are results of three cycles of quaternary volcanism each culminating in a devastating pyroclastic eruption and resulting in large collapse calderas and predominantly rhyolitic flows the oldest cycle ended about 1 9 m y ago forming a caldera from island park to perhaps the central part of the present yellowstone caldera the second cycle climaxed about 1 2 m y ago and was confined to the island park caldera the third cycle began soon after the second and is responsible for the 75 by 45 km yellowstone caldera as it is outlined today this caldera is the result of a catastrophic eruption approximately 600 000 yr ago the most recent episode of volcanism probably began about 150 000 yr ago in the western half of the caldera near old faithful geyser and the youngest flows in yellowstone about 70 000 yr old are probably associated with this episode

teleseismic p wave tomogram of the yellowstone plume 1 inversion of a new data set of teleseismic p wave travel times from three passcal seismic deployments around the yellowstone hotspot reveals a 100 km diameter upper mantle plume that extends from the yellowstone volcanic caldera to 500 km depth and dips 20 to the northwest a monotonic decrease in the velocity perturbation of the plume from 3 2 at 100 km to 0 9 at 450 km is consistent with a uniform thermal anomaly of 180 c where the plume crosses the 410 km discontinuity previous research shows a depression in the 410 km discontinuity consistent with a warm plume fee and dueker 2004 additionally a region of high velocities extends to 250 km beneath the wind river basin in nw wyoming that may represent a convective downwelling of the lithosphere

the 1978 yellowstone eastern snake river plain seismic profiling experiment data and upper crustal structure of the yellowstone region eleven in line refraction profiles recorded to distances of 300 km and one azimuthal fan plot were constructed from data recorded with a 150 station array in the yellowstone national park area during the 1978 yellowstone snake river plain seismic experiment interpretations of the data suggest that the crustal p wave velocity model for the yellowstone region is characterized by 1 an averaged 10 km thick upper crustal layer v sub p 6 0 km s 2 an average crustal velocity of 6 3 km s and 3 a total crustal thickness of 44 km velocity models are presented for profiles that emphasize the upper crust and show 1 a decrease in the depth to the top of the upper crustal crystalline basement from 5 km in southwestern yellowstone near island park to 1 km at the northeast side of the yellowstone plateau that is interpreted as a progressive thinning of the silicic surface volcanic layer to the northeast and 2 evidence for a large lateral inhomogeneity interpreted to be a low velocity body with a decrease of at least 10 in p wave velocity located beneath the northeast corner of the yellowstone plateau the low velocity zone coincides with a local 30 mgal residual gravity anomaly and is located beneath partmore of the sour creek resurgent dome and part of the hot springs basin the largest hydrothermal system in yellowstone the low velocity body has a maximum depth to the top of 3 km and a minimum depth to the bottom of 9 km and may represent a zone of partial melt in comparison to the thermally undisturbed upper crust of the surrounding rocky mountains the upper crust of the northeastern yellowstone plateau appears laterally inhomogeneous in velocity and layer thickness suggesting effects of thermal and magma intrusion whereas the lower crust appears relatively homogeneous less

p wave velocity structure of the uppermost mantle beneath hawaii from traveltime tomography summary we examine the p wave velocity structure beneath the island of hawaii using p wave residuals from teleseismic earthquakes recorded by the hawaiian volcano observatory seismic network the station geometry and distribution of events makes it possible to image the velocity structure between y40 and 100 km depth with a lateral resolution of y15 km and a vertical resolution of y30 km for depths between 40 and 80 km p wave velocities are up to 5 per cent slower in a broad elongated region trending se nw that underlies the island between the two lines defined by the volcanic loci no direct correlation between the magnitude of the lithospheric anomaly and the current level of volcanic activity is apparent but the slow region is broadened at y19 8un and narrow beneath kilauea in the case of the oceanic lithosphere beneath hawaii slow seismic velocities are likely to be related to magma transport from the top of the melting zone at the base of the lithosphere to the surface thermal modelling shows that the broad elongated low velocity zone cannot be explained in terms of conductive heating by one primary conduit per volcano but that more complicated melt pathways must exist

gravity fields and the interpretation of volcanic structures geological discrimination and temporal evolution abstract a review of gravity data reflecting shallow concealed structures on volcanoes demonstrates that valuable information may be derived on the geological development of such structures and on the dynamic evolution of active volcanoes to a first approximation all recorded examples of anomalous gravity fields associated with volcanoes are concentric with and have sources that lie within the volcanic edifice positive anomalies with wavelengths less than 20 km and amplitudes up to ca 30 mgal characterise mainly basaltic volcanoes from various tectonic settings and are caused by a relatively dense intrusive complex magma body which contrasts with its surroundings either because the body is more mafic than average or more likely because near surface previously erupted materials are uncompacted negative anomalies with wavelengths up to 100 km and amplitudes up to ca 60 mgal occur over much larger volcanic calderas many of which have erupted highly silicic pyroclastic ash and pumice uncompacted silicic caldera infill with a possible contribution from low density magma bodies is responsible for the observed anomalies there is some evidence for a continuum of gravity anomaly types that corresponds to the geological evolution of volcanic systems generally from primitive rift or subduction related basaltic andesite to mature high level silicic calderas repeated microgravity observations over active volcanoes enable magma movements variations in magma input and changes in magma density to be monitored closely dynamic modelling of volcanic systems is providing new evidence on the behaviour of concealed magma bodies and could have considerable potential for predicting eruptive events

gravity height correlations for unrest at calderas abstract calderas represent the sites of the world s most serious volcanic hazards although eruptions are not frequent at such structures on the scale of human lifetimes there are nevertheless often physical changes at calderas that are measurable over periods of years or decades such calderas are said to be in a state of unrest and it is by studying the nature of this unrest that we may begin to understand the dynamics of eruption precursors here we review combined gravity and elevation data from several restless calderas and present new data on their characteristic signatures during periods of inflation and deflation we find that unless the bouguer gravity anomaly at a caldera is extremely small the free air gradient used to correct gravity data for observed elevation changes must be the measured or calculated gradient and not the theoretical gradient use of which may introduce significant errors in general there are two models that fit most of the available data the first involves a mogi type point source and the second is a bouguer type infinite horizontal plane source the density of the deforming material usually a magma chamber is calculated from the gravity and ground deformation data and the best fitting model is to a first approximation the one producing the most realistic density no realistic density is obtained where there are real density changes or where the data do not fit the point source or slab model we find that a point source model fits most of the available data and that most data are for periods of caldera inflation the limited examples of deflation from large silicic calderas indicate that the amount of mass loss or magma drainage is usually much less than the mass gain during the preceding magma intrusion in contrast deflationary events at basaltic calderas formed in extensional tectonic environments are associated with more significant mass loss as magma is injected into the associated fissure swarms

body wave tomography of western canada abstract in this study we have produced p and s wave velocity models for western canada using 23 420 delay times measured on vertical component seismograms and 15 805 delay times measured on transverse component seismograms respectively from a range of permanent and temporary networks resolution is best in southwestern british columbia and along the canoe northwestern alberta southern yukon and northwest territories and batholiths northwestern bc arrays where the station density is the highest and fair elsewhere we focus our attention on two distinct features 1 the transition from phanerozoic to cratonic mantle in northwestern canada and 2 the complex tectonic environment at the northern terminus of the cascadia subduction zone where the plate boundary changes from convergent to transform we find that the main transition from phanerozoic to cratonic mantle in northwestern canada occurs at the cordilleran deformation front and represents a sharp jump in seismic velocity from 2 to 2 over a distance of 50 km in northern cascadia we have imaged and characterized the signature of the subducting juan de fuca plate and observed evidence of subduction beyond the northern edge of the slab we also demonstrate that the anahim hotspot track is underlain by a 2 low velocity zone possibly extending to 400 km beneath nazko cone that appears to be the source of volcanism in this area consequently we associate the source of magmatism in this area to a mantle scale rather than lithospheric scale process

abrupt changes in north american climate during early holocene times recent studies of the greenland ice cores have offered many insights into holocene climatic dynamics at decadal to century timescales1 2 3 despite the abundance of continental records of holocene climate few have sufficient chronological control and sampling resolution to compare with the greenland findings4 butannually laminated sediments varves from lakes can provide high resolution continental palaeoclimate data with secure chronologies here we present analyses of varved sediments from deep lake in minnesota usa trends in the stable oxygen isotope composition of the sedimentary carbonate indicate a pronounced climate cooling from 8 9 to 8 3 kyr before present probably characterized by increased outbreaks of polar air decreased precipitation temperatures and a higher fraction of the annual precipitation falling as snow the abrupt onset of this climate reversal over several decades was probably caused by a reorganization of atmospheric circulation and cooling of the arctic airmass in summer that resulted from the final collapse of the laurentide ice near hudson bay and the discharge of icebergs from the quebec and keewatin centres into the tyrell sea the timing and duration of this climate reversal suggest that it is distinct from the prominent widespread cold snap that occurred 8 200 years ago in greenland and other regions1 5 6 no shifts in the oxygen isotope composition of sediment carbonate occurred at 8 2 kyr before present at deep lake but varve thickness increased dramatically probably as a result of increased deposition of aeolian dust taken together our data suggest that two separate regional scale climate reversals occurred between 9 000 and 8 000 years ago and that they were driven by different mechanisms

monitoring super volcanoes geophysical and geochemical signals at yellowstone and other large caldera systems earth s largest calderas form as the ground collapses during immense volcanic eruptions when hundreds to thousands of cubic kilometres of magma are explosively withdrawn from the earth s crust over a period of days to weeks continuing long after such great eruptions the resulting calderas often exhibit pronounced unrest with frequent earthquakes alternating uplift and subsidence of the ground and considerable heat and mass flux because many active and extinct calderas show evidence for repetition of large eruptions such systems demand detailed scientific study and monitoring two calderas in north america yellowstone wyoming and long valley california are in areas of youthful tectonic complexity scientists strive to understand the signals generated when tectonic volcanic and hydrothermal hot ground water processes intersect one obstacle to accurate forecasting of large volcanic events is humanity s lack of familiarity with the signals leading up to the largest class of volcanic eruptions accordingly it may be difficult to recognize the difference between smaller and larger eruptions to prepare ourselves and society scientists must scrutinize a spectrum of volcanic signals and assess the many factors contributing to unrest and toward diverse modes of eruption

contemporary vertical surface displacements in yellowstone national park relative vertical displacements and average relative vertical velocities have been determined for bench marks located throughout yellowstone national park ynp by comparing geodetic leveling measurements made in 1923 second order 1936 third order 1941 third order 1955 third order 1960 first order and 1975 1977 first order the 1923 1975 data indicate uplift greater than 400 mm 7 7 mm yr of an area approximately 15 20 km wide and at least 40 km long which is centered along the northeast trending axis of the 600 000 year old yellowstone caldera the maximum 1923 1975 vertical displacement in this area is 726 mm 14 0 mm yr the existence of a well defined and untilted yellowstone lake terrace implies that the 1923 1975 average vertical velocities cannot have been maintained for more than a few hundred years the 1955 1977 data from central ynp indicate that the northeastern caldera half has risen relative to the southwestern half at an average velocity of about 8 mm yr a similar bias is present in the 1923 1975 data but it is less pronounced the 1923 1960 and 1960 1975 data from northwestern ynp and a 200 mm depression in the 1923 1975 data near norris junction may be interpreted in terms of deformation associated with the 1959 hebgen lake earthquake and the 1975 yellowstone park earthquake a composite profile of the 1936 1977 and 1941 1977 average vertical velocity data which skirts the edge of the yellowstone caldera in northeastern ynp is most easily explained as the result of random leveling error and or bench mark instabilities the apparent youth of the 1923 1975 yellowstone uplift and its close association with the yellowstone caldera suggest that the most likely cause of the uplift is a recent not more than 500 years ago influx of molten material into the upper crust beneath the caldera

tomography of the western united states from regional arrival times first arrival times from regional distances 200 1200 km in the western united states extracted from the international seismological centre isc data set were inverted in a tomographic study to map the laterally varying pn velocity structure of the uppermost mantle i e the mantle lid and to estimate the crustal static delay at each seismograph station synthetic data were used to evaluate resolution results correlate well with major tectonic features we find low uppermost mantle velocities v 7 9 km s these low apparent pn velocities are primarily due to the effects of isostasy hotspot volcanism and crustal extension we interpret the low apparent pn velocities beneath the sierra nevada 7 6 7 7 km s to be the result of time delays introduced by ray paths tunneling through the deep crustal root the yellowstone hotspot migrating along the snake river plain has left low velocity 7 6 7 8 km s hot upper mantle in its wake lower pn velocities 7 8 km s beneath the extensional basin and range are also presumably due to hotter uppermost mantle based on station static delays for the basin and range we infer that moho depths there do not vary significantly from 30 km normal continental pn velocities 8 0 km s as well as thicker crust 40 50 km underlying the rocky mountains wyoming basin province colorado plateau and the northern columbia plateau indicate the presence of relatively cool uppermost mantle beneath these regions

upper mantle velocity structure of the great basin teleseismic p wave travel times measured in the eastern great basin show 1 sec variations in travel time residuals inversion of these data images an alternating sequence of high and low velocity upper mantle bodies trending ne across the eastern margin of the great basin low velocity mantle lies beneath the volcanic trends of the eastern snake river plain and the st george volcanic field these two low velocity trends are separated by high velocity mantle extending from northern utah to sw wyoming combining these results with seismic data from the western great basin reveals a continuation of these upper mantle trends across the great basin

western u s upper mantle structure regional three dimensional inversions of upper mande p wave velocity structure are created from teleseismic p wave travel time residuals recorded by many of the high density high frequency regional arrays operated within the western united states these inversions are adjusted to a global international seismic centre reference and merged to obtain an image of the upper mantle beneath western united states the p wave velocities in the upper mantle are slow on average and the structure is very heterogeneous where resolution is good coherent upper mantle structures are imaged that extend as deep as 200 km the juan de fuca and gorda slabs which penetrate to greater depths are exceptions and deviate from the average velocity at a given depth by as much as 4 lateral resolution of these structures usually is very good although the magnitude of the actual seismic variations is probably greater than that imaged the long wavelength part of the imaged mantle structure defines coherent elongate features with wavelengths of 200 500 km within 250 km of the pacific coast these structures have a wavelength of 250 km and trend parallel to the surface physiography and young tectonic structures beneath the continental interior where use is made of additional seismic studies to infer average structure in regions of poor teleseismic data coverage structures have a wavelength of 500 km and trend northeasterly this northeast orientation is discordant with young tectonic structures but aligns with young volcanic activity

geodynamics of the yellowstone hotspot and mantle plume seismic and gps imaging kinematics and mantle flow abstract integration of geophysical and geological data show that the yellowstone hotspot resulted from a mantle plume interacting with the overriding north america plate a process that has highly modified continental lithosphere by magmatic and tectonic processes and produced the 16 17 ma 700 km long yellowstone snake river plain ysrp silicic volcanic system accessibility of the ysrp allowed large scale geophysical projects to seismically image the hotspot and evaluate its kinematic properties using geodetic measurements seismic tomography reveals a crustal magma reservoir of 8 to 15 melt 6 km to 16 km deep beneath the yellowstone caldera an upper mantle low p wave velocity body extends vertically from 80 km to 250 km beneath yellowstone but the anomalous body tilts 60 wnw and extends to 660 km depth into the mantle transition zone we interpret this conduit shaped low velocity body as a plume of up to 3 5 vp and 5 5 vs perturbation that corresponds to a 1 2 partial melt models of whole mantle convection reveal eastward upper mantle flow beneath yellowstone at relatively high rates of 5 cm yr that deflects the ascending plume into its west tilted geometry a geodynamic model of the yellowstone plume constrained by vp and vs velocities and attenuation parameters suggests low excess temperatures of up to 120 k corresponding to a maximum 2 5 melt and a small buoyancy flux of 0 25 mg s i e properties of a cool weak plume the buoyancy flux is many times smaller than for oceanic plumes nonetheless plume buoyancy has produced a 400 km wide 500 m high topographic swell centered on the yellowstone plateau contemporary deformation derived from gps measurements reveals sw extension of 2 3 mm yr across the yellowstone plateau one fourth of the total basin range opening rate which we consider to be part of basin range intraplate extension locally decadal episodes of subsidence and uplift averaging 2 cm yr characterize the 80 year yellowstone caldera monitored history and are modeled as hydrothermal magmatic sources moreover a recent episode 2004 2009 of accelerated uplift of the yellowstone caldera at rates up to 7 cm yr has been modeled as resulting from magmatic recharge of a 10 km deep sill at the top of the crustal magma reservoir regionally gravitational potential energy of the yellowstone swell drives the lithosphere southwest and downhill from the yellowstone plateau 400 km where it coalesces with basin range province wide westward extension based on the geometry and its assumed 660 km depth we extrapolate the plume source southwest to its original location at 17 ma and 600 km southwest and 200 km north of the ysrp importantly this location is beneath the southern part of the columbia plateau flood basalt field of the same age and implies that the yellowstone mantle plume may be the common source for both of these large volcanic fields our time progression model suggests that the original plume head rose vertically behind the juan de fuca plate but at 12 ma it lost the protection of the subducting plate from eastward mantle flow and encountered cooler thicker continental lithosphere becoming entrained in eastward upper mantle flow these results reveal that yellowstone plume plate processes have had a profound effect on late cenozoic geologic evolution and topography of a large part of the western u s

seismic studies of crustal structure during the past four years seismology has continued its role as the key technique for determination of the structure of the continental and oceanic crust by and large however the field and analysis methods used bear little resemblance to those of ten years ago a continued push to increase the number of recording channels has led to the routine use of ocean bottom seismometers sonobuoys and multichannel streamers in oceanic studies and to the use of large sets of portable instruments and multichannel seismic reflection strings on land in studies using earthquakes as sources local and regional arrays provide a capability for obtaining geologically useful two and three dimensional information about the crust

shear wave velocity structure in north america from large scale waveform inversions of surface waves a two step nonlinear and linear inversion is carried out to map the lateral heterogeneity beneath north america using surface wave data the lateral resolution for most areas of the model is of the order of several hundred kilometers the most obvious feature in the tomographic images is the rapid transition between low velocities in the tectonically active region west of the rocky mountains and high velocities in the stable central and eastern shield of north america the model also reveals smaller scale heterogeneous velocity structures a high velocity anomaly is imaged beneath the state of washington that could be explained as the subducting juan de fuca plate beneath the cascades a large low velocity structure extends along the coast from the mendocino to the rivera triple junction and to the continental interior across the southwestern united states and northwestern mexico its shape changes notably with depth this anomaly largely coincides with the part of the margin where no lithosphere is consumed since the subduction has been replaced by a transform fault evidence for a discontinuous subduction of the cocos plate along the middle american trench is found in central mexico a transition is visible from low velocities across the trans mexican volcanic belt tmvb to high velocities beneath the yucatan peninsula two elongated low velocity anomalies beneath the yellowstone plateau and the eastern snake river plain volcanic system and beneath central mexico and the tmvb seem to be associated with magmatism and partial melting another low velocity feature is seen at depths of approximately 200 km beneath florida and the atlantic coastal plain the inversion technique used is based on a linear surface wave scattering theory which gives tomographic images of the relative phase velocity perturbations in four period bands ranging from 40 to 150 s in order to find a smooth reference model a nonlinear inversion based on ray theory is first performed after correcting for the crustal thickness the phase velocity perturbations obtained from the subsequent linear waveform inversion for the different period bands are converted to a three layer model of s velocity perturbations layer 1 25 100 km layer 2 100 200 km layer 3 200 300 km we have applied this method on 275 high quality rayleigh waves recorded by a variety of instruments in north america iris usgs iris ida terrascope rstn sensitivity tests indicate that the lateral resolution is especially good in the densely sampled western continental united states mexico and the gulf of mexico

vertical surface displacements at yellowstone caldera wyoming 1976 1986 between first order leveling surveys in 1976 and 1986 the east central floor of yellowstone caldera rose 15 4 1 4 mm yr and the west central floor rose 17 0 2 0 mm yr with respect to points on the south caldera rim the same pattern has prevailed since an initial survey in 1923 except that the western caldera rose 30 faster during 1976 1986 than during 1923 1976 the 1976 1986 uplift extends beyond the northwest caldera rim near the adjacent hebgen lake fault zone site of a magnitude 7 3 earthquake in august 1959 marginally significant displacements in the eastern caldera suggest that uplift continued during 1983 1984 stopped during 1984 1985 and reversed to subsidence during 1985 1986 thus long term uplift may accumulate episodically historical uplift is especially noteworthy because regional crustal extension and thermal contraction during cooling would cause the caldera floor to subside if other processes were not acting to warp it upward the pattern and dimensions of the uplift indicate a source in the upper crust beneath yellowstone caldera probably a reservoir of rhyolite magma within a cooling granitic pluton a simple point source model would suggest a source 10 15 km deep but the reservoir is probably lens shaped and closer to the surface uplift may be caused by basaltic intrusions near the base of the reservoir or accumulation of magmatic fluids during cooling and crystallization within the reservoir subsidence might be a response to regional tectonic extension or release of trapped magmatic fluids magmatic inflation volatile accumulation and crustal extension may culminate eventually in renewed eruptive activity but there is no evidence of an imminent volcanic hazard

models of lithosphere and asthenosphere anisotropic structure of the yellowstone hot spot from shear wave splitting 1 teleseismic shear wave splitting measured at 56 continuous and temporary seismographs deployed in a 500 km by 600 km area around the yellowstone hot spot indicates that fast anisotropy in the mantle is parallel to the direction of plate motion under most of the array the average split time from all stations of 0 9 s is typical of continental stations there is little evidence for plume induced radial strain suggesting that any contribution of gravitationally spreading plume material is undetectably small with respect to the plate motion velocity two stations within yellowstone have splitting measurements indicating the apparent fast anisotropy direction is nearly perpendicular to plate motion these stations are 30 km from stations with parallel to plate motion the 70 rotation over 30 km suggests a shallow source of anisotropy however split times for these stations are more than 2 s we suggest melt filled stress oriented cracks in the lithosphere are responsible for the anomalous orientations within yellowstone stations southeast of yellowstone have measurements of oriented nnw to wnw at high angles to the plate motion direction the archean lithosphere beneath these stations may have significant anisotropy capable of producing the observed splitting

the 1978 yellowstone eastern snake river plain seismic profiling experiment crustal structure of the yellowstone region and experiment design in 1978 a major seismic profiling experiment was conducted in the yellowstone eastern snake river plain region of idaho and wyoming fifteen shots were recorded that provided coverage to distances of 300 km in this paper travel time and synthetic seismogram modeling was used to evaluate an average p wave velocity and apparent q structure of the crust from two seismic profiles reversed across the yellowstone national park region this area includes the well known hydrothermal features of yellowstone national park geysers fumeroles etc a large collapse caldera and extensive silicic volcanism of quaternary age features attributed to shallow crustal sources of magma the averaged crustal structure for this region as interpreted from the seismic data consists of 1 a highly variable near surface layer approximately 2 km thick with variable velocities of 3 0 to 4 8 km s and a low apparent q of 30 that is interpreted to be composed of weathered rhyolites and sedimentary infill 2 an upper crustal layer 3 to 4 km thick with variable velocities of 4 9 to 5 5 km s and apparent q of 50 to 200 that is thought to represent the accumulation of the pleistocene quaternary rhyolite flows ash flow tuffs and possible paleozoic and precambrian metamorphic equivalents 3 the crystalline upper crust that is characterized by a laterally inhomogeneous layer that varies in velocity from 4 0 to 6 1 km s averaging 5 km thick with a q of 300 this layer appears to be a cooling but still hot body of granitic composition beneath the yellowstone caldera it is thought to be a remnant of the magma chambers that produced the quaternary silicic volcanic rocks of the yellowstone plateau and may still be a major contributor to the high heat flow 4 a laterally homogeneous intermediate crustal layer 8 to 10 km thick with a velocity of 6 5 km s and apparent q of 100 to 300 5 a homogeneous 25 km thick lower crust with a velocity of 6 7 to 6 8 km s and an apparent q of 300 and 6 a total crustal thickness of 43 km the upper crustal layer 5 5 to 6 0 km s is thought to be the thermally altered equivalent of the continental crystalline basement that is normally 15 to 20 km thick in the surrounding thermally undisturbed archean crust an interpretation from these results suggests that mafic melts from the mantle have penetrated the lower crust without significant variations in the velocity structure but produce the main source of heat that drives the volcanic and hydrothermal systems of yellowstone the high apparent attenuation and large lateral velocity variations in the upper crust are consistent with a model in which partial fractionation partial melting and metamorphism differentiate the original upper crust to produce silicic melts that were extruded as rhyolites and ash flow tuffs across the yellowstone plateau this seismic model is consistent with the evidence for a systematic northeastward propogation of silicic volcanic centers along the eastern snake river plain to their present location beneath the yellowstone hydrothermal system while these findings do not bear directly on the origin and source of the heat i e mantle lumes lithospheric fractures mantle radiogenetic heat basal lithospheric shearing etc they provide a constraint on the configuration and lateral extent of crustal layers that reflect thermal and compositional boundaries

tilt observations using borehole tiltmeters 2 analysis of data from yellowstone national park we have installed borehole tiltmeters at five sites in yellowstone national park wyoming and have used these instruments to measure the spatial variation of the amplitude and phase of the principal semidiurnal tide the measured tides vary both with position and azimuth and differ from the sum of the body tide and the ocean load by up to 50 the difference predicted by a finite element model constructed from seismic refraction and gravity data has a maximum value of only 12 although the discrep ancy between our observations and the model is only marginally significant at some sites the disagreement between the model and our observa tions is much larger than we observed using the same instruments at other sites and cannot be attributed to an instrumental effect we have been unable to modify the model to explain our results while keeping it consistent with the previous observations

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

anomalous shear wave delays and surface wave velocities at yellowstone caldera wyoming to investigate the effects of a geothermal area on the propagation of intermediate period 1 30 s teleseismic body waves and surface waves a specially designed portable seismograph system was operated in yellowstone caldera wyoming travel time residuals relative to a station outside the caldera of up to 2 s for compressional phases are in agreement with short period residuals for p phases measured by other investigators travel time delays for shear arrivals in the intermediate period band range from 2 to 9 s and decrease with increasing dt d delta measured rayleigh wave phase velocities are extremely low ranging from 3 2 km s at 27 s period to 2 0 km s at 7 s period the estimated uncertainty associated with these values is 15 we propose a model for compressional and shear velocities and poisson s ratio beneath the yellowstone caldera which fits the teleseismic body and surface wave data it consists of a highly anomalous crust with an average shear velocity of 3 0 km s overlying an upper mantle with average velocity of 4 1 km s the high average value of poisson s ratio in the crust 0 34 suggests the presence of fluids there poisson s ratio in the mantle between 40 and approximately 200 km is more nearly normal 0 29 than inmore the crust a discrepancy between normal values of poisson s ratio in the crust calculated from short period data and high values calculated from teleseismic data can be resolved by postulating a viscoelastic crustal model with frequency dependent shear velocity and attenuation less

upper mantle origin of the yellowstone hotspot fundamental features of the geology and tectonic setting of the northeast propagating yellowstone hotspot are not explained by a simple deep mantle plume hypothesis and within that framework must be attributed to coincidence or be explained by auxiliary hypotheses these features include the persistence of basaltic magmatism along the hotspot track the origin of the hotspot during a regional middle miocene tectonic reorganization a similar and coeval zone of northwestward magmatic propagation the occurrence of both zones of magmatic propagation along a first order tectonic boundary and control of the hotspot track by preexisting structures seismic imaging provides no evidence for and several contraindications of a vertically extensive plume like structure beneath yellowstone or a broad trailing plume head beneath the eastern snake river plain the high helium isotope ratios observed at yellowstone and other hotspots are commonly assumed to arise from the lower mantle but upper mantle processes can explain the observations the available evidence thus renders an upper mantle origin for the yellowstone system the preferred model there is no evidence that the system extends deeper than 200 km and some evidence that it does not a model whereby the yellowstone system reflects feedback between upper mantle convection and regional lithospheric tectonics is able to explain the observations better than a deep mantle plume hypothesis

holocene precipitation seasonality captured by a dual hydrogen and oxygen isotope approach at steel lake minnesota abstract middle holocene 8 to 4 ka bp warmth and aridity are well recorded in sediment archives from midcontinental north america however neither the climatic driver nor the seasonal character of precipitation during this period is well understood because of the limitations of available proxy indicators for example an important challenge is to distinguish among the interacting effects of evaporation temperature or precipitation seasonality in existing 18 o records from the region here we combine hydrogen isotopes of palmitic acid and oxygen isotopes of carbonate to derive lake water isotopic values during the holocene at steel lake in north central minnesota in combination these data enable us to separate variations in evaporation from variations in the isotopic composition of input waters to lake variations in evaporation are used as a proxy for aridity and lake water input isotopic values are used as a proxy for the isotopic values of meteoric precipitation our results suggest that lake water input isotopic values were more negative during the middle holocene than at present to test whether these more negative values are related to temperature or precipitation seasonality we compare pollen inferred temperatures and the expected isotopic value of precipitation resulting from these temperatures to the reconstructed precipitation isotopic values results suggest that middle holocene warmth and aridity were associated with increased evaporation rates and decreased summer precipitation these inferences are consistent with climate simulations that highlight the role of seasonal insolation and sea surface temperatures in driving variations in precipitation seasonality during the holocene results also suggest that changes in holocene precipitation seasonality may have influenced the expansion of the prairie forest border in minnesota as well as regional variations in grassland community composition this study demonstrates the efficacy of the dual hydrogen and oxygen isotope approach for distinguishing changes in evaporation and precipitation seasonality in the paleolimnological record

vp vs ratios in the yellowstone national park region wyoming abstract in this paper we study the variation of v p v s and poisson s ratio in the yellowstone national park region using earthquakes which were well recorded by a local seismic network we find that the average v p v s value within the geothermally active yellowstone caldera is about 7 lower than in the area outside the caldera within the caldera itself there may be a further 2 7 reduction of v p v s in the hydrothermally active norris geyser basin the upper and lower geyser basins and the yellowstone lake and mud volcano regions after considering various possible causes for v p v s changes such as geologic and structural differences thermal effects partial melting and hydrothermal activity we conclude that the most plausible explanation for the observed v p v s reduction is the presence of hot water at temperatures and pore pressures near the water steam transition in the caldera geothermal reservoirs