seismicity following the 1985 eruption of nevado del ruiz colombia abstract over 200 of the best recorded earthquakes occurring near nevado del ruiz colombia between december 1985 and may 1986 are examined for spatial patterns information on local stress orientations and evidence for seismogenic structures that may be related to magma transport processes these earthquakes occurred following the devastating eruption of november 13 1985 earthquake activity that is probably related to the volcanic system occurs over a large area of the order of 50 km 2 a pattern of earthquakes beneath the southern half of ruiz may be the expression of a conduit system but evidence is inconclusive due to low spatial resolution of most hypocenters epicenter and first motion plots suggest most earthquakes occur on small faults of varying orientation most seismicity during this time period was unrelated to mapped faults including some with holocene offsets except for the villamaria fault near ruiz s northeast sector composite focal mechanisms show a variation of nearly 90 in the strike of t axes and indicate ruiz is in a zone of local extension

holocene lake level trends in the rocky mountains u s a abstract the availability of water shapes life in the western united states and much of the water in the region originates in the rocky mountains few studies however have explicitly examined the history of water levels in the rocky mountains during the holocene here we examine the past levels of three lakes near the continental divide in montana and colorado to reconstruct holocene moisture trends using transects of sediment cores and sub surface geophysical profiles from each lake we find that mid holocene shorelines in the small lakes 4 110 ha were as much as 10 m below the modern lake surfaces our results are consistent with existing evidence from other lakes and show that a wide range of settings in the region were much drier than today before 3000 2000 years ago we also discuss evidence for millennial scale moisture variation including an abruptly initiated and terminated wet period in colorado from 4400 to 3700 cal yr bp and find only limited evidence for low lake stands during the past millennium the extent of low water levels during the mid holocene which were most severe and widespread ca 7000 4500 cal yr bp is consistent with the extent of insolation induced aridity in previously published regional climate model simulations like the simulations the lake data provide no evidence for enhanced zonal flow during the mid holocene which has been invoked to explain enhanced mid continent aridity at the time the data including widespread evidence for large changes on orbital time scales and for more limited changes during the last millennium confirm the ability of large boundary condition changes to push western water supplies beyond the range of recent natural variability

low velocity structure beneath africa from forward modeling seismic waveforms observed in south africa containing the first arrival crossover of s to sks 70 to 110 are analyzed the data consist of analog records from the world wide seismographic station network wwssn of deep events beneath south america the s waves arrive 2 to 3 s early relative to prem at ranges from 70 to 95 and then become increasingly delayed becoming 5 to 6 s late at 110 the sks phase is late by 3 to 5 s over the entire range this pushes crossover between s and sks normally observed at about 81 out about 2 to 3 which is the most anomalous shift ever reported to model such features we modified grand s tomography model grand et al gsa today 7 1997 1 7 and generated 2d synthetics to match the data the overall shape and position of the lower mantle low velocity anomaly proposed by grand predicts good results if lower mantle anomalies are enhanced to a level of about 4 this results in a complex tabular structure extending upward from the core mantle boundary about 1500 km into the mantle these features appear to be consistent with a large young plume which is erupting off the cmb

deformation in the lowermost mantle beneath australia from observations and models of seismic anisotropy observations of seismic anisotropy near the core mantle boundary may yield constraints on patterns of lowermost mantle flow we examine seismic anisotropy in the lowermost mantle beneath australia bounded by the african and pacific large low shear velocity provinces we combined measurements of differential splitting of sks skks and s scs phases sampling our study region over a range of azimuths using data from ten long running seismic stations observations reveal complex and laterally heterogeneous anisotropy in the lowermost mantle we identified two sub regions for which we have robust measurements of d associated splitting for a range of ray propagation directions and applied a forward modeling strategy to understand which anisotropic scenarios are consistent with the observations we tested a variety of elastic tensors and orientations including single crystal elasticity of lowermost mantle minerals bridgmanite post perovskite and ferropericlase tensors based on texture modeling in post perovskite aggregates elasticity predicted from deformation experiments on polycrystalline mgo aggregates and tensors that approximate the shape preferred orientation of partial melt we find that post perovskite scenarios are more consistently able to reproduce the observations beneath new zealand the observations suggest a nearly horizontal 100 axis orientation with an azimuth that agrees well with the horizontal flow direction predicted by previous mantle flow models our modeling results further suggest dominant slip on the 010 plane in post perovskite aggregates provide a good fit to the data but the solution is non unique our results have implications for the mechanisms of deformation and anisotropy in the lowermost mantle and for the patterns of mantle flow

mantle flow in subduction systems the subslab flow field and implications for mantle dynamics 1 the character of the mantle flow field in subduction zones remains poorly understood despite its importance for our understanding of subduction dynamics in particular little attention has been paid to mantle flow beneath subducting slabs in order to identify processes that make first order contributions to the global pattern of subslab mantle flow we have compiled shear wave splitting measurements from subduction zones worldwide from previously published studies and estimated average splitting parameters for the subslab region globally the subslab region is overwhelmingly dominated by trench parallel fast splitting directions we tested for relationships between splitting delay time a measure of the strength of anisotropy and parameters that are indicators of tectonic processes such as trench migration velocity convergence velocity and obliquity age of subducting lithosphere and slab dip curvature seismicity and thickness we used several different plate motion models to describe plate and trench motions and evaluated the differences among the models we find that only one parameter namely the trench migration velocity in a pacific hot spot reference frame appears to correlate well with the strength of the subslab splitting signal this finding supports a model in which the mantle beneath subducting slabs is dominated by three dimensional flow induced by trench migration we explore several implications of our model for various aspects of mantle dynamics including the choice of a suitable reference frame s for mantle flow the existence of a thin decoupling zone between slabs and the subslab mantle and consequences for mass transfer between the upper and lower mantle

fluid ascent during the 2004 2005 unrest at mt spurr inferred from seismic tomography 1 we present a seismic model of the crust beneath mt spurr volcano in alaska based on tomographic inversion of local earthquake data recorded by temporary seismic network during the unrest of mt spurr in 2004 2005 beneath the northern vent of mt spurr where the most intensive fumarolic activity was observed during the last unrest we observe a finger shaped anomaly with a higher p and lower s velocity which corresponds to high vp vs ratio another anomaly is located to the south of crater peak approximately beneath the same location where an explosive eruption occurred in 1992 we propose that these two finger anomalies represent separated conduits of magma and volatiles the ne conduit extends to the surface and enables free volatile flux the se conduit seems to be blocked at a ca 20 km depth and in case of continuous pressure build up may become a source of an explosive eruption citation koulakov i m west and p izbekov 2013 fluid ascent during the 2004 2005 unrest at mt spurr inferred from seismic tomography geophys res lett 40 4579 4582 doi 10 1002 grl 50674

millennial scale climate fire vegetation interactions in a mid elevation mixed coniferous forest mission range northwestern montana usa mixed coniferous forests are widespread at middle elevations in the northern rocky mountains yet relatively little is known about their long term vegetation and fire history pollen and charcoal records from twin lakes in the mission range of northwestern montana provide information on mixed coniferous forest development and fire activity over the last 15 000 years these data suggest an open parkland and minimal fire activity before 13 500 cal yr bp consistent with cold dry conditions increases in pinus pollen after 13 500 cal yr bp indicate a transition to closed forests and a slight rise in fire activity as conditions warmed and fuel biomass increased high levels of artemisia pollen between 10 000 and 6000 cal yr bp suggest an open forest during the early holocene when conditions were warmer and drier than present low severity fires likely maintained open forest shrublands but produced little charcoal during this interval present day mixed coniferous forests were established in the last 6 ka and included forest taxa characteristic of low pseudotsuga larix pinus ponderosa and high severity fire regimes picea abies increased climate variability and anthropogenic burning may have helped maintain heterogeneous mixed coniferous forests during the last several millennia when climate conditions became cooler and wetter

geometry and p and s velocity structure of the african anomaly 1 we constrain the geometry and p and s velocity structure of a low velocity anomaly in the lower mantle beneath southern africa we term it the african anomaly on the basis of forward traveltime and waveform modeling of seismic data sampling a great arc across the anomaly from the east pacific rise to the japan sea our collected data set consists of direct s direct p sdiff scs pcp sks and skks phases recorded by three temporary broadband passcal seismic arrays deployed in africa between 1994 and 2002 the tanzania seismic array 1994 1995 the kaapvaal seismic array 1997 1999 and the ethiopia kenya seismic array 2000 2002 for earthquakes occurring in the east pacific rise drake passage south sandwich islands iran hindu kush xinjiang and the japan sea the seismic data provide excellent sampling of the african anomaly in the lower mantle along the specific great arc in order to accurately account for the contributions from the african anomaly we relocate all the events using a global seismic shear velocity tomographic model and seismic data recorded by the global seismographic network and correct for the contributions from the seismic heterogeneities outside the african anomaly the seismic observations suggest that the african anomaly locally extends 1300 km above the core mantle boundary beneath southern africa around 25 n 27 e and exhibits a bell like geometry with both the southwestern and the northeastern flanks dipping toward its center with the lateral dimension of the anomaly increasing with depth the base is about 4000 km wide extending broadly in both the southwestward and the northeastward directions the seismic data can best be explained by a shear velocity structure with average velocity decreases of 5 in the base and 2 to 3 in the mid lower mantle above the base and a compressional velocity structure with a uniform s to p velocity perturbation ratio of 3 1 for the entire african anomaly these geometric and velocity features suggest that the mid lower mantle portion of the african anomaly is an integral component of the very low velocity province and the african anomaly is compositionally distinct and geologically stable

three dimensional structure of the african superplume from waveform modelling summary previous 2 d modelling of seismic waveforms and traveltimes has revealed a large scale ridge like velocity anomaly beneath africa which is usually referred to as the african superplume the structure starts from the southern indian ocean and extends northwestwards into the atlantic ocean with its base on the core mantle boundary the structure has relatively sharp lateral boundaries with the shear velocity inside 3 per cent lower than the ambient mantle while the compressional velocity is almost normal the 3 d structure is best illustrated by seismic waveforms recorded by the south africa array generated by earthquakes in the western pacific ocean the diffracted s phase travels along the axis of the structure for over 60 with the northern and southernmost paths sampling its edges the s waveforms are simple but delayed by up to 20 s for paths sampling the middle of the structure whereas they display two arrivals for paths along the boundaries these complex waveforms can be explained by 3 d multipathing due to rapid lateral variations in shear wave velocity along the edges of the structure these sharp features are confirmed by modelling broadband records associated with the proposed ridge structure with two independent methods the spectral element method and a ray based 3 d code dwkm

the stability and structure of primordial reservoirs in the lower mantle insights from models of thermochemical convection in three dimensional spherical geometry large scale chemical lateral heterogeneities are inferred in the earth s lowermost mantle by seismological studies we explore the model space of thermochemical convection that can maintain reservoirs of dense material for a long period of time by using similar analysis in 3 d spherical geometry in this study we focus on the parameters thought to be important in controlling the stability and structure of primordial dense reservoirs in the lower mantle including the chemical density contrast between the primordial dense material and the regular mantle material buoyancy ratio thermal and chemical viscosity contrasts volume fraction of primordial dense material and the clapeyron slope of the phase transition at 660 km depth we find that most of the findings from the 3 d cartesian study still apply to 3 d spherical cases after slight modifications varying buoyancy ratio leads to different flow patterns from rapid upwelling to stable layering and large thermal viscosity contrasts are required to generate long wavelength chemical structures in the lower mantle chemical viscosity contrasts in a reasonable range have a second order role in modifying the stability of the dense anomalies the volume fraction of the initial primordial dense material does not effect the results with large thermal viscosity contrasts but has significant effects on calculations with intermediate and small thermal viscosity contrasts the volume fraction of dense material at which the flow pattern changes from unstable to stable depends on buoyancy ratio and thermal viscosity contrast an endothermic phase transition at 660 km depth acts as a filter allowing cold slabs to penetrate while blocking most of the dense material from penetrating to the upper mantle

destruction of the north china craton induced by ridge subductions abstractthe destruction of the north china craton ncc mainly occurred in the cretaceous and has been attributed to a top down rapid delamination bottom up long term thermal chemical erosions or hydration by subduction released fluids on the basis of the distribution of one jurassic and two early cretaceous adakite belts and the drifting history of the paleo pacific plate we propose that three ridge subduction events dominated the large scale decratonization in the ncc both physical erosion and magmatism induced by ridge subduction contributed to the destruction of the ncc the last ridge subduction at ma was the key driving force in the final destruction we present mineralogical geochemical and isotopic data in support of the ridge subduction model flat subduction of a spreading ridge resulted in stronger physical erosion on the thick lithosphere mantle of the ncc consequently slab melting occurred during ridge subduction forming adakites with mantle mg isotope compositions followed b

geographic boundary of the pacific anomaly and its geometry and transitional structure in the north 1 we determine the geographical boundary and average shear velocity structure of the pacific anomaly at the base of the mantle based on travel time analysis of scsh sh and scs2 scsscs ss phases and waveform modeling results we further constrain the detailed geometry of the northern anomaly around 20 n 170 e and its transition to the surrounding high velocity region along three perpendicular cross sections on the basis of forward waveform modeling of the observed direct s and scs phases the observed differential travel time residuals and waveform modeling results allow the whole geographic boundary of the anomaly to be delineated and the area of the base of the anomaly is estimated to be 1 9 107 km2 the maximum shear velocity perturbation inside the anomaly reaches 5 in the lowermost 500 km of the mantle waveform analysis suggests that the northern anomaly reaches 450 km above the cmb with both steeply and shallowly dipping edges and its basal layer extends beneath the surrounding mantle with the degree of extension changing rapidly across a small distance the inferred characteristics of the anomaly support the previous suggestion that the pacific anomaly represents a chemical anomaly however unlike the inferred features of the african anomaly pointing to an ancient compositionally distinct and geologically stable anomaly the existence of several separated piles extending into the mid lower mantle the complex morphology of the piles with both steeply and shallowly dipping edges and the presence of many ultra low velocity zones at its base suggest that the pacific anomaly likely possesses varying intrinsic compositions and exhibits complex interaction with the surrounding mantle

a signal to noise index to quantify the potential for peak detection in sediment charcoal records article i nfo charcoal peaks in lake sediment records are commonly used to reconstruct fire histories spanning thousands of years but quantitative methods for evaluating the suitability of records for peak detection are largely lacking we present a signal to noise index sni that quantifies the separation of charcoal peaks signal from other variability in a record noise we validate the sni with simulated and empirical charcoal records and show that an sni n3 consistently identifies records appropriate for peak detection the sni thus offers a means to evaluate the suitability of sediment charcoal records for reconstructing local fires matlab and r functions for calculating sni are provided

holocene vegetational history of the kootenai river valley montana abstract pollen records in the kootenai and fisher river drainages in western montana reveal a fivezone sequence of holocene vegetation change deposition of glacier peak ash layer g ca 10 540 660 yr b p in the lowermost sediments clay intermixed with pebbles at tepee lake gives a minimum date for the initiation of sedimentation initial vegetation on the newly deglaciated terrain was dominated by pinus probably white bark pine with small amounts of gramineae picea and abies reflecting a relatively cool moist macroclimate two vegetation units appear to contribute to pollen zone ii ca 11 000 7100 yr b p arboreal communities with pines along with pseudotsuga or larix or both and treeless vegetation dominated by artemisia pollen zone ii represents an overall warmer macroclimate than occurred upon ice withdrawal after ca 7100 yr b p pollen zone iii diploxylon pines became a major pollen contributor near both tepee lake and mckillop creek pond indicating an expansion of xerophytic forest p contorta and p ponderosa along with an increase in the prominence of pseudotsuga menziesii or larix occidentalis or both artemisia briefly expanded coverage near tepee lake concomitant with the mazama ashfall ca 6700 yr b p a short term climatic trend with more available water began after ca 4000 yr b p as abies probably a grandis along with picea engelmannii became a more regular component of the forest surrounding both sites emergence of the modern macroclimate is indicated primarily with the first regular appearance of tsuga heterophylla in the pollen record by ca 2700 yr b p synchronous with the development of western hemlock forest within the same latitudes in northern idaho and northeastern washington

major and trace element characterization expanded distribution and a new chronology for the latest pleistocene glacier peak tephras in western north america keuhn s c froese d g carrara p e foit jr f f pearce n j g rotheisler p 2009 major and trace element characterization expanded distribution and a new chronology for the latest pleistocene glacier peak tephras in western north america quaternary research 71 2 201 216 sponsorship nserc discovery grant alberta ingenuity new faculty award to duane froese

structural features and shear velocity structure of the pacific anomaly 1 we constrain structural features and shear velocity structure of a low velocity anomaly in the lower mantle beneath the pacific we term it the pacific anomaly on the basis of forward travel time and waveform modeling of the observed direct s sdiff scs sks and skks phases sampling a great arc across the anomaly from eastern eurasia to southern south america after correction for the effects of earthquake mislocation and the seismic heterogeneities outside the pacific anomaly seismic observations suggest that the pacific anomaly along the great arc consists of at least two separated portions with a 740 km wide gap between them the western portion of the anomaly is about 1050 km wide extends at least 740 km above the core mantle boundary cmb and exhibits a trapezoidal shape with lateral dimensions increasing slightly with depth the velocity structure of the western portion varies from 3 0 at the top 740 km above the cmb to 3 5 at 100 km above the cmb and an average shear velocity reduction of 5 in the bottom 100 km of the mantle the eastern portion of the anomaly reaches at least 340 km above the cmb beneath the mid pacific with an 1800 km wide base and has a uniform velocity reduction of 3 waveform modeling further suggests a very low velocity layer with a shear velocity reduction of 10 located at the edge of the western portion of the anomaly combining the latest results from others we present a general picture of structural and velocity structures of the pacific anomaly the structural and velocity features suggest that the anomaly represents a cluster of metastable thermo chemical piles

effect of pore geometry on vp vs from equilibrium geometry to crack seismic wave velocities of melt or aqueous fluid containing systems are studied over a wide range of pore shapes including oblate spheroids tubes cracks and an equilibrium geometry controlled by a dihedral angle the relative role of liquid compressibility and pore geometry on the v p v s velocity ratio is clarified the result clearly indicates that p and s velocity structures determined by seismic tomography can be used to verify whether interfacial energy controlled melt or fluid geometry equilibrium geometry is achieved relationships between the diverse models are clearly established by relating each model to the oblate spheroid model in terms of the equivalent aspect ratio as a function of the aspect ratio a significant effect of pore geometry on d in v s d in v p the ratio of the fractional changes in v s and v p is shown equilibrium geometry of the partially molten rocks characterized by a dihedral angle of 20 40 corresponds to an aspect ratio of 0 1 0 15 the value of d in v s d in v p expected for the texturally equilibrated partially molten rocks is shown to be 1 1 5 which is much smaller than that expected for cracks and dikes with an aspect ratio of 10 2 10 3 in the upper mantle low velocity regions the seismologically obtained value of d in v s d in v p is within this range beneath the bolivian andes 1 1 1 4 but is as high as 2 beneath iceland 1 7 2 3 and beneath northeastern japan 2 0 the former region can be regarded as a region where equilibrium geometry is achieved and the latter regions can be regarded as regions where dikes and veins typical of a system far from the textural equilibrium dominate

a two dimensional p sv hybrid method and its application to modeling localized structures near the core mantle boundary a p sv hybrid method is developed for calculating synthetic seismograms involving two dimensional localized heterogeneous structures the finite difference technique is applied in the heterogeneous region and generalized ray theory solutions from a seismic source are used in the finite difference initiation process the seismic motions after interacting with the heterogeneous structures are propagated back to the earth s surface analytically with the aid of the kirchhoff method anomalous long period sks spdks observations sampling a region near the core mantle boundary beneath the southwest pacific are modeled with the hybrid method localized structures just above the core mantle boundary with lateral dimensions of 250 to 400 km can explain even the most anomalous data observed to date if s velocity drops up to 30 are allowed for a p velocity drop of 10 structural shapes and seismic properties of those anomalies are constrained from the data since synthetic waveforms are sensitive to the location and lateral dimension of seismic anomalies near the core mantle boundary some important issues such as the density change and roughness of the structures and the sharpness of the transition from the structures to the surrounding mantle however remain unresolved due to the nature of the data

three dimensional velocity structure of the galeras volcano colombia from passive local earthquake tomography abstract a three dimensional estimation of the vp vs and vp vs ratio structure at galeras volcano was conducted by means of passive local earthquake tomography 14 150 volcano tectonic events recorded by 58 stations in the seismological network established for monitoring the volcanic activity by the colombian geological survey pasto volcano observatory between the years 1989 and 2015 were inverted by using the lotos code the seismic events are associated with shear stress fractures in solid rock as a response to pressure induced by magma flow tomography resolution tests suggest a depth of imaging that yield 10 km from the summit of the main crater illuminating a large portion of the volcanic structure and the interaction of tectonic features like the buesaco and silvia pijao faults full catalog tomographic inversion that represents the stacked image of the volcanic structure or the most permanent features underneath the volcano shows vertical structures aligned with seismicity beneath the main crater we hypothesize that these structures correspond to a system of ducts or fractures through which magma and fluid phases flow up from deeper levels toward the top and related with the intersection of the surface traces of the silvia pijao and buesaco faults

postglacial fire vegetation and climate history in the clearwater range northern idaho usa the environmental history of the northern rocky mountains was reconstructed using lake sediments from burnt knob lake idaho and comparing the results with those from other previously published sites in the region to understand how vegetation and fire regimes responded to large scale climate changes during the holocene vegetation reconstructions indicate parkland or alpine meadow at the end of the glacial period indicating cold dry conditions from 14 000 to 12 000 cal yr b p abundant pinus pollen suggests warmer moister conditions than the previous period most sites record the development of a forest with pseudotsuga ca 9500 cal yr b p indicating warm dry climate coincident with the summer insolation maximum as the amplification of the seasonal cycle of insolation waned during the middle holocene pseudotsugawas replaced by pinus and abies suggesting cool moist conditions the fire reconstructions show less synchroneity in general the sites west of the continental divide display a fire frequency maximum around 12 000 8000 cal yr b p which coincides with the interval of high summer insolation and stronger than present subtropical high the sites on the east side of the continental divide have the highest fire frequency ca 6000 3500 cal yr b p and may be responding to a decrease in summer precipitation as monsoonal circulation weakened in the middle and late holocene this study demonstrated that the fire frequency of the last two decades does not exceed the historical range of variability in that periods of even higher than present fire frequency occurred in the past

partial melting in the deepest mantle 1 we have compared the travel times of direct p s and core reflected pcp scs phases recorded in southern africa from earthquakes occurring in the scotia arc region the low velocity region beneath the south atlantic is explored by calculating scs s and pcp p travel time residuals to determine the correlation and magnitude of vs and vp reductions in the deepest mantle residual calculations yield extraordinary results as pcp p is about 1 second larger than expected while scs s is 7 seconds larger for a great circle arc distance of 54 we investigate models of p and s velocity reductions in the deepest mantle that may explain these residuals to provide constraints on the velocity structure of the d region in this extremely slow zone the reductions in p and s velocity needed to match the data are consistent with partial melting in the deepest mantle for a wide range of 1d models

delayed disequilibrium degassing in rhyolite magma decompression experiments and implications for explosive volcanism recent numerical models and analog shock tube experiments show that disequilibrium degassing during magma ascent may lead to violent vesiculation very near the surface in this study a series of decompression experiments using crystal free rhyolite melt were conducted to examine the development of large supersaturations due to delayed homogenous spontaneous bubble nucleation melts were saturated at 900 c and 200 mpa with either 5 2 wt dissolved h2o or with 4 2 wt h2o and 640 ppm co2 and isothermally decompressed at linear rates of either 0 003 0 025 or 8 5 mpa s to final pressures between 25 and 175 mpa additional isobaric saturation experiments 900 c 200 25 mpa using pure h2o or mixed h2o co2 fluids establish reference equilibrium solubility curves values homogenous nucleation is triggered in both h2o only and h2o co2 experiments once the supersaturation pressure pss reaches 120 150 mpa and the melt contains two times its equilibrium water contents bubble number density and nucleation rate depend on the supersaturation pressure with values on the order of 102 cm3 and 1 cm3 s for pss 120 mpa 106 cm3 and 103 105 cm3 s for pss 130 150 mpa and 107 cm3 and 106 cm3 s for pss 160 175 mpa nucleation rates are consistent with classical nucleation theory and infer an activation energy for nucleation of 1 5 10 18 j nucleus a critical bubble radius of 2 10 9 m and an effective surface tension for rhyolite at 5 2 wt h2o and 900 c of 0 10 0 11 n m the long nucleation delay limits the time available for subsequent diffusion such that disequilibrium dissolved h2o and co2 contents persist to the end of our runs the disequilibrium degassing paths inferred from our experiments contrast markedly with the equilibrium or quasi equilibrium paths found in other studies where bubble nucleation occurs heterogenously on crystals or other discontinuities in the melt at low pss homogenous and heterogenous nucleation rates are comparable however as are bubble number densities so that at a given decompression rate it appears that nucleation mechanism rather than nucleation rate determines degassing efficiency by fixing the pressure depth at which vesiculation commences and hence the time available for equilibration prior to eruption although real systems are probably never truly crystal free our results show that rhyolitic magmas containing up to 104 crystals cm3 and perhaps as high as 106 crystals cm3 are controlled by homogenous rather than heterogenous nucleation during ascent

thermochemical piles in the lowermost mantle and their evolution abstract two large low shear wave velocity provinces llsvps lie beneath southern africa and southern pacific ocean these llsvps may be thermochemical in origin studying the dynamic evolution of these thermochemical piles will help us understand the earth s present structure and history we investigate the entrainment and evolution of an initially isolated high viscosity thermochemical pile in detail the entrainment rate of the dense pile increases with time until it being totally entrained the pile s survival time does not monotonically varies with its viscosity the chemical pile obstructs the horizontal flow along cmb and turns it into upwelling flow this may help explain the observation of plume generation zones the morphology and location of the dense pile oscillate with time a high viscosity pile can keep its location and morphology roughly unchanged for hundreds of millions years if the convective structure in the surrounding mantle keeps steady

further constraints on the african superplume structure it is well established that there is a large scale low velocity structure in the lowermost mantle beneath africa extending from the southeastern atlantic ocean to the southwestern indian ocean with a volume greater than 10 billion km 3 7000 km long 1000 km across and 1200 km high earth planet sci lett 206 2003 119 this low velocity structure is often called the african superplume various studies also require sharp boundaries for the plume however as for its height and shear velocity reduction there has been some controversy especially concerning the velocities at the core mantle boundary cmb here we present an assortment of phases involving s diff sks s and s c s with both vertical and horizontal paths sampling a 2d corridor through the structure travel time and waveform modeling of these seismic phases argues for a model with shear velocity reduction of approximately 3 within the superplume which is basically a 200 km thick layer low velocity layer beneath the southern atlantic ocean and a 1200 km high structure beneath south africa and against a model of a substantially reduced low velocity layer up to 10 300 km beneath the superplume we also analyzed p diff and the differential times of p c p p and compared them with sdiff and s c s s observations along the same great circle paths the p velocity is not very anomalous at most 0 5 much smaller than 1 as expected from a thermal anomaly with 3 lower s velocity geophys res lett 27 2000 421 thus again arguing for a chemical origin which was suggested from the modeling of african superplume sharp sides science 296 2002 1850

can a sheet like low velocity region form an elongated large igneous province 1 high resolution tomography of the lower mantle has revealed the existence of another chemically distinct region with low velocity and a sheet like structure beneath the western pacific on the other hand large igneous provinces lips sometimes have elongated shapes if a sheet like upwelling reaches the earth s surface while maintaining its shape an elongated lip may form in order to test this hypothesis we perform a series of experiments and investigate the stability of a buoyant sheet the experimental results show that the buoyant fluid accumulates at the top of the sheet to form a buoyant cylinder the gravitational instability divides the cylinder into several plume heads we develop a model to explain the growth of the buoyant cylinder and the time scale until instability begins our model shows that a thin sheet like upwelling with a width of 200 km a small density difference from the ambient mantle 10 kg m 3 a nd a high supply rate of buoyant fluid 0 1 m yr 1 can reach the earth s surface while maintaining its shape we thus infer that lips with an elongated shape can be generated by sheet like upwellings the width of the observed sheet like low velocity region beneath the western pacific is 500 km and is marginally sufficient to form an elongated lip

paleoclimate simulations for north america over the past 21 000 years features of the simulated climate and comparisons with paleoenvironmental data maps of upper level and surface winds and of surface temperature and precipitation illustrate the results of a sequence of global paleoclimatic simulations spanning the past 21 000 years for north america we review a the large scale features of circulation temperature and precipitation that appear in the simulations from the ncar community climate model version 1 ccm 1 b the implications of the simulated climate for the past continental scale distributions of three plant taxa picea spp pseudotsuga menziesii and artemisia tridentata which are broadly representative of the vegetation across the continent and c the potential explanations in terms of atmospheric circulation or surface energy and water balance processes for mismatches between the simulations and observations most of the broad scale features of previous paleoclimatic simulations with the ncar ccm 0 for north america are present in the current simulations many of the elements of a conceptual model based on previous climate simulations that describes the controls of paleoclimatic variations across north america during the past 21 000 years are found in simulations reviewed here these include 1 displacement of the jet stream by the laurentide ice sheet to the south of its present position in both winter and summer 2 generation of a glacial anticyclone over the ice sheet at the lgm and the consequent induction of large scale sinking motions induced over eastern north america 3 changes in the strength of surface atmospheric circulation features through time including weakening of the aleutian low in winter and strengthening of the eastern pacific and bermuda high pressure systems in summer as the ice sheet decreased in size 4 development of a heat low at the surface and a strengthened ridge in the upper atmosphere over the continent at the time of the maximum summer insolation anomaly 5 increases in summer temperature earlier in regions remote from the ice sheet these increases appear earlier in the present ccm 1 simulations than in the previous ccm 0 ones however and 6 continuation of negative winter temperature anomalies into the middle holocene in general simulated surface conditions that are discordant with paleoenvironmental observations can be attributed to the simulation of particular atmospheric circulation patterns e g those that suppress precipitation or advect warm air into a region with these mismatches amplified in beringia and the southeastern united states by surface energy and water balance processes

a 2200 yr record of hydrologic variability from foy lake montana usa inferred from diatom and geochemical data abstract a 2200 yr long high resolution 5 yr record of drought variability in northwest montana is inferred from diatoms and 18o values of bio induced carbonate preserved in a varved lacustrine core from foy lake a previously developed model of the diatom response to lake level fluctuations is used to constrain estimates of paleolake levels derived from the diatom data high frequency decadal fluctuations in the de trended 18o record mirror variations in wet dry cycles inferred from banff tree rings demonstrating the sensitivity of the oxygen isotope values to changes in regional moisture balance low frequency multi centennial isotopic changes may be associated with shifts in the seasonal distribution of precipitation from 200 b c to a d 800 both diatom and isotope records indicate that climate was dry and lake level low with poor diatom preservation and high organic carbon nitrogen ratios subsequently lake level rose slightly although the climate was drier and more stable than modern conditions at a d 1200 lake level increased to approximately 6 m below present elevation after which the lake fluctuated between this elevation and full stage with particularly cool and or wetter conditions after 1700 the hydrologic balance of the lake shifted abruptly at 1894 because of the establishment of a lumber mill at the lake s outlet spectral analysis of the 18o data indicates that severe droughts occurred with multi decadal 50 to 70 yr frequency

bayesian inference of mantle viscosity from whole mantle density models the long wavelength geoid is sensitive to earth s mantle density structure as well as radial variations in mantle viscosity we present a suite of inversions for the radial viscosity profile using

normal mode and free air gravity constraints on lateral variations in velocity and density of earth s mantle with the use of a large collection of free oscillation data and additional constraints imposed by the free air gravity anomaly lateral variations in shear velocity compressional velocity and density within the mantle dynamic topography on the free surface and topography on the 660 km discontinuity and the core mantle boundary were determined the velocity models are consistent with existing models based on travel time and waveform inversions in the lowermost mantle near the core mantle boundary denser than average material is found beneath regions of upwellings centered on the pacific ocean and africa that are characterized by slow shear velocities these anomalies suggest the existence of compositional heterogeneity near the core mantle boundary

the difficulty for subducted oceanic crust to accumulate at the earth s core mantle boundary 1 seismic tomography has revealed two large low shear velocity provinces llsvps in the lowermost mantle beneath the central pacific and africa the llsvps are further shown to be compositionally different from their surroundings among several hypotheses put forth in recent years to explain the cause of the llsvps one postulates that they are thermochemical piles caused by accumulation of subducted oceanic crust at the core mantle boundary cmb mineral physics experiments indicate that oceanic crust becomes denser than the surrounding mantle at lower mantle pressures in addition seismic observations provide evidence of subducted slabs arriving at the cmb however a major question pertains to whether subducted oceanic crust can survive viscous stirring associated with mantle plumes and accumulate into piles with the same spatial scale as llsvps we perform a set of high resolution convection calculations to examine this hypothesis by investigating the interaction of thin oceanic crust 6 km with mantle plumes our results show that as subducted oceanic crust is swept toward upwelling plume regions the majority of it is viscously stirred into the surrounding mantle only a small amount of oceanic crust may accumulate at the base of plumes but it is consistently entrained away into the plume at a rate equal to or greater than it is accumulated we find that it is difficult for subducted oceanic crust to accumulate into large thermochemical piles at the cmb

three dimensional seismic velocity structure of mauna loa and kilauea volcanoes in hawaii from local seismic tomography we present a new three dimensional seismic velocity model of the crustal and upper mantle structure for mauna loa and kilauea volcanoes in hawaii our model is derived from the first arrival times of the compressional and shear waves from about 53 000 events on and near the island of hawaii between 1992 and 2009 recorded by the hawaiian volcano observatory stations the vp model generally agrees with previous studies showing high velocity anomalies near the calderas and rift zones and low velocity anomalies in the fault systems the most significant difference from previous models is in v p vs structure the high vp and high v p vs anomalies below mauna loa caldera are interpreted as mafic magmatic cumulates the observed low vp and high v p vs bodies in the kaoiki seismic zone between 5 and 15 km depth are attributed to the underlying volcaniclastic sediments the high vp and moderate to low vp vs anomalies beneath kilauea caldera can be explained by a combination of different mafic compositions likely to be olivine rich gabbro and dunite the systematically low vp and low vp vs bodies in the southeast flank of kilauea may be caused by the presence of volatiles another difference between this study and previous ones is the improved vp model resolution in deeper layers owing to the inclusion of events with large epicentral distances the new velocity model is used to relocate the seismicity of mauna loa and kilauea for improved absolute locations and ultimately to develop a high precision earthquake catalog using waveform cross correlation data 2014 american geophysical union all rights reserved

multidecadal drought and holocene climate instability in the rocky mountains time series analysis of a diatom inferred drought record suggests that holocene hydroclimate of the northern rocky mountains has been characterized by oscillation between two mean climate states the dominant climate state was initiated at the onset of the holocene ca 11 ka under this climate state drought was strongly cyclic recurring at frequencies that are similar to twentieth century multidecadal phase changes of the pacific decadal oscillation this pattern remained consistent throughout much of the midholocene continuing until ca 4 5 ka after this time the mean climate state changed and drought recurrence became unstable periods of cyclic drought alternated with periods of less predictable drought the timing of this shift in climate was coincident with widespread severe drought in the mid continent of north america overall the strongest periodicity in severe drought occurred during the mid holocene when temperatures in the northern rocky mountains were warmer than today

holocene fire and vegetation along environmental gradients in the northern rocky mountains abstract holocene records of fire vegetation and climate were reconstructed from four sites in the bitterroot range region of the northern rocky mountains in order to examine the vegetation and fire histories and evaluate the hypothesis proposed by whitlock and bartlein 1993 regarding the effects of increased summer insolation on precipitation patterns vegetation history in the series of sites was broadly similar in the late glacial period the pollen data suggest open parkland dominated by picea or alpine meadow which reflect conditions cooler and drier than present these open forests were replaced in the early to middle holocene by forests composed mainly of pinus and pseudotsuga which suggest conditions warmer than present modern forest compositions were in place by ca 3000 cal yr bp and small variations in the timing of the vegetation shifts reflect local differences among sites the long term trends in fire occurrence support the hypothesis proposed by whitlock and bartlein 1993 that precipitation regimes were sharpened during the early holocene summer insolation maximum but their location has remained unchanged as a result of topographic constraints sites located in areas currently summer dry were drier than present during the early holocene and fires were more frequent conversely sites located in the areas that are summer wet at present were wetter than present in the early holocene and fires were less frequent on millennial time scales it appears that the climate boundary is controlled by topography and does not shift

upper mantle sh and p velocity structures and compositional models beneath southern africa abstract we constrain sh and p wave velocity structures in the upper mantle beneath southern africa using triplicated phases recorded in the epicentral distance range of 11 28 for one shallow event we then explore thermal and compositional models appropriate for explaining the inferred seismic structures in the region both sh and p wave data suggest presence of a low velocity zone with velocity reductions of at least 5 for s waves and 2 for p waves beneath a 150 km thick high velocity lithospheric lid seismic observations also suggest that the p s ratio is larger 1 88 in the transition zone than in the lithospheric lid 1 70 the inferred p wave velocity jump across the 660 km discontinuity is small

hypocenter velocity structure coupling in local earthquake tomography abstract local earthquake tomography involves the use of seismic wave arrival times from earthquake sources with uncertain locations for the inference of three dimensional variations in seismic velocity formally this inverse problem contains explicit coupling between the earthquake hypocenters and the velocity structure however many studies fail to treat the hypocenter velocity structure coupling explicitly an examination of previous studies provides evidence that this failure can lead to significant bias in the resulting velocity model a set of simple hypothetical test cases clearly demonstrates the seriousness of the potential bias when coupling is neglected producing models that underestimate the actual velocity perturbations by about a factor of four under the same conditions strict simultaneous inversion and simultaneous inversion with parameter separation both produce accurate inverted models the use of appropriate station corrections and iteration of the solution can improve the results when the coupling is ignored but velocity anomalies will still be underestimated by about a factor of two at best

holocene vegetation fire and climate history of the sawtooth range central idaho usa the paucity of low and middle elevation paleoecologic records in the northern rocky mountains limits our ability to assess current environmental change in light of past conditions a 10 500 yr long vegetation fire and climate history from lower decker lake in the sawtooth range provides information from a new region initial forests dominated by pine and douglas fir were replaced by open douglas fir forest at 8420 cal yr bp marking the onset of warmer conditions than present presence of closed douglas fir forest between 6000 and 2650 cal yr bp suggests heightened summer drought in the middle holocene closed lodgepole pine forest developed at 2650 cal yr bp and fires became more frequent after 1450 cal yr bp this shift from douglas fir to lodgepole pine forest was probably facilitated by a combination of cooler summers cold winters and more severe fires than before five drought episodes including those at 8200 cal yr bp and during the medieval climate anomaly were registered by brief intervals of lodgepole pine decline an increase in fire activity and mistletoe infestation the importance of a holocene perspective when assessing the historical range of variability is illustrated by the striking difference between the modern forest and that which existed 3000 yr ago

detection of ridge like structures in the pacific large low shear velocity province abstract waveform tomography is conducted for sh velocity structures of the entire mantle using approximately 3 5 times the data used for obtaining the previous model sh18ce the resultant new model sh18cex exhibits a cluster of ridge like low velocity anomalies in the western part of the pacific large low shear velocity province llsvp the location of the ridge like anomalies is in good agreement with the location of the abrupt change in the topography of the d discontinuity these results suggest that the llsvp is associated with a cluster of ridge like piles rather than a single large pile spread over the entire region the piles probably consist of intrinsically dense material however either their volume or density contrast may not be sufficiently large to develop large scale domes

vegetation responses to quaternary volcanic and hydrothermal disturbances in the northern rocky mountains and greater yellowstone ecosystem usa abstract volcanic and hydrothermal processes produce disturbances by diverse mechanisms and ecological responses are varied new and published pollen records from the northern rocky mountains and greater yellowstone ecosystem document the response of vegetation to three different types of volcanic and hydrothermal disturbances 1 pleistocene rhyolite lava flows in the central greater yellowstone ecosystem created infertile landscapes that have shaped vegetation since rhyolite emplacement nutrient poor well drained soils that developed on these flows supported low diversity grassland during late glacial time and pinus contorta forests in interglacial periods 2 ash layers from eruptions of pacific northwest stratovolcanoes are commonly preserved in lake sediment records in the northern rocky mountains and associated pollen records show enhancement of steppe vegetation for years to decades 3 local hydrothermal explosions have resulted in vegetation changes in hydrothermal areas that indicate tree mortality following deposition of explosion debris followed by recovery in years thus the type and duration of the vegetation response to volcanic and hydrothermal disturbances are highly contextual and governed by the antecedent plant communities and the magnitude and mechanism of the volcanic or hydrothermal disturbance vegetation resilience varied between disturbances ranging from enduring ecosystem parameter changes to short lived state changes in resilient plant communities

latest pleistocene and holocene glacier fluctuations in western canada we summarize evidence of the latest pleistocene and holocene glacier fluctuations in the canadian cordillera our review focuses primarily on studies completed after 1988 when the first comprehensive review of such evidence was published the cordilleran ice sheet reached its maximum extent about 16 ka and then rapidly decayed some lobes of the ice sheet valley glaciers and cirque glaciers advanced one or more times between 15 and 11 ka by 11 ka or soon thereafter glacier cover in the cordillera was no more extensive than at the end of the 20th century glaciers were least extensive between 11 and 7 ka a general expansion of glaciers began as early as 8 4 ka when glaciers overrode forests in the southern coast mountains it culminated with the climactic advances of the little ice age holocene glacier expansion was not continuous but rather was punctuated by advances and retreats on a variety of timescales radiocarbon ages of wood collected from glacier forefields reveal six major periods of glacier advance 8 59 8 18 7 36 6 45 4 40 3 97 3 54 2 77 1 71 1 30 ka and the past millennium tree ring and lichenometric dating shows that glaciers began their little ice age advances as early as the 11th century and reached their maximum holocene positions during the early 18th or mid 19th century our data confirm a previously suggested pattern of episodic but successively greater holocene glacier expansion from the early holocene to the climactic advances of the little ice age presumably driven by decreasing summer insolation throughout the holocene proxy climate records indicate that glaciers advanced during the little ice age in response to cold conditions that coincided with times of sunspot minima priority research required to further advance our understanding of late pleistocene and holocene glaciation in western canada includes constraining the age of late pleistocene moraines in northern british columbia and yukon territory expanding the use of cosmogenic surface exposure dating techniques using multi proxy paleoclimate approaches and directing more of the research effort to the northern canadian cordillera

reconciling divergent trends and millennial variations in holocene temperatures cooling during most of the past two millennia has been widely recognized and has been inferred to be the dominant global temperature trend of the past 11 700 years the holocene epoch however long term cooling has been difficult to reconcile with global forcing and climate models consistently simulate long term warming the divergence between simulations and reconstructions emerges primarily for northern mid latitudes for which pronounced cooling has been inferred from marine and coastal records using multiple approaches here we show that temperatures reconstructed from sub fossil pollen from 642 sites across north america and europe closely match simulations and that long term warming not cooling defined the holocene until around 2 000 years ago the reconstructions indicate that evidence of long term cooling was limited to north atlantic records early holocene temperatures on the continents were more than two degrees celsius below those of the past two millennia consistent with the simulated effects of remnant ice sheets in the climate model community climate system model 3 ccsm3 ccsm3 simulates increases in growing degree days a measure of the accumulated warmth above five degrees celsius per year of more than 300 kelvin days over the holocene consistent with inferences from the pollen data it also simulates a decrease in mean summer temperatures of more than two degrees celsius which correlates with reconstructed marine trends and highlights the potential importance of the different subseasonal sensitivities of the records despite the differing trends pollen and marine based reconstructions are correlated at millennial to centennial scales probably in response to ice sheet and meltwater dynamics and to stochastic dynamics similar to the temperature variations produced by ccsm3 although our results depend on a single source of palaeoclimatic data pollen and a single climate model simulation they reinforce the notion that climate models can adequately simulate climates for periods other than the present day they also demonstrate that amplified warming in recent decades increased temperatures above the mean of any century during the past 11 000 years

a high precision age estimate of the holocene plinian eruption of mount mazama oregon usa the climactic eruption of mount mazama in oregon north america resulted in the deposition of the most widespread holocene tephra deposit in the conterminous united states and south western canada the tephra forms an isochronous marker horizon for palaeoenvironmental sedimentary and archaeological reconstructions despite the current lack of a precise age estimate for the source eruption previous radiocarbon age estimates for the eruption have varied and greenland ice core ages are in disagreement for the mazama tephra to be fully utilised in tephrochronology and palaeoenvironmental research a refined precise and accurate age for the eruption is required here we apply a meta analysis of all previously published radiocarbon age estimations n 81 and perform bayesian statistical modelling to this data set to provide a refined age of 7682 7584 cal yr bp 95 4 probability range although the depositional histories of the published ages vary this estimate is consistent with that estimated from the gisp2 ice core of 7627 150 yr bp zdanowicz et al 1999

unrest of the udina volcano in kamchatka inferred from the analysis of seismicity and seismic tomography abstract the udina complex consisting of two stratovolcanoes bolshaya and malaya udinas is located at the southeastern edge of the klyuchevskoy volcano group kgv in kamchatka prior to 2017 it was considered to be extinct however since late 2017 continuing seismic activity beneath bolshaya udina has been recorded which may indicate the possible awakening of this volcano complex to perform a detailed investigation of this seismic process we installed four temporary seismic stations in the vicinity of the udinas0 volcanoes and operated them for two months from may to july 2018 using the data obtained from this network we detected and localized 559 local events in the area of the udina volcanoes furthermore we performed a tomographic inversion to improve the locations of the sources the tomography unexpectedly resulted in a stable model which was carefully verified via a series of different tests we found that the seismicity formed an elliptical cluster oriented in the north northeast south southwest direction beneath the volcano the seismicity occurred at depths of greater than 5 km from the surface in the tomography model we observed an anomaly of a high vp low vs and high vp vs ratio located directly beneath bolshaya udina these seismic properties may indicate the presence of magma intrusions with a high content of melts and fluids which may justify changing the current status of this volcano from extinct to active we also observed that the seismicity cluster connects bolshaya udina with the tolud zone where continuous seismicity has been observed at depths of approximately 10 20 km in previous years this zone is considered to be a magma storage in the lower crust that contributed to feeding the fissure eruptions in the tolbachinsky dol based on the results of this study we conclude that during 2018 the tolud magma source appeared to have built another pathway to bolshaya udina

seismic structure changes beneath redoubt volcano during the 2009 eruption inferred from local earthquake tomography we present seismic velocity models of the area beneath redoubt volcano alaska corresponding to two time periods before and after the strong eruption that occurred in march 2009 the calculations were based on tomographic inversion of p and s arrival time data recorded by 19 stations using the local earthquake tomography code lotos we performed thorough analysis of the results based on synthetic tests in each of the time periods that allow real time variations in structure to be distinguished from artifacts caused by changes in the observation system configurations in the resulting images corresponding to the period before the eruption the summit area is characterized by higher values of both p and s velocities and moderate values of the vp vs ratio this may correspond to the rigid igneous rocks composing the body of the volcano that lack significant liquid content in the second episode corresponding to the time of the eruption the p velocity remained high whereas the s velocity became very low the anticorrelation of p and s anomalies in the summit area produced a vp vs ratio as high as 2 2 which is seen down to 2 3 km depth this indicates presence of mobile phase at shallow depth beneath the volcano which can be either in the form of partial melt or fluid saturated rocks

ridge like lower mantle structure beneath south africa recent scs s results from probing the deep structure beneath southern africa display strong delays of up to 10 s at distances beyond 90 such delays could be explained by long period tomographic models containing smooth weak features with the addition of rough strong d structure 3 9 drops in shear velocities however these structures cannot explain the sks s differentials sampling the same region to explain the sks s and scs s data sets simultaneously requires instead a large scale ridge like structure with a relatively uniform 3 reduction in shear velocity the structure is about 1000 km wide and extends at least 1200 km above d it is orientated roughly nw se and leans toward the east at latitudes from 15 to about 30 it proves difficult to explain such sharp features with thermal effects alone and thus the importance of high resolution waveform modeling to establish their existence to derive the above results we developed a special algorithm by matching simulated synthetics to observed broadband waveforms this is achieved by computing the various arrivals separately using generalized ray theory for a reference model and allowing the arrivals to shift in relative times to match the data tomographic models can then be constructed or existing tomographic models can be altered to match these data and new 2 d synthetics can be constructed as well to better fit the waveform data these updated synthetics can again be decomposed and reassembled and the process can be repeated this algorithm is applied to a combination of analog and digital data along a corridor from south america producing the high resolution 2 d model described above

lower mantle structure from paleogeographically constrained dynamic earth models seismic tomography reveals two large low shear velocity provinces llsvps beneath africa and the pacific ocean these structures may have existed for several 100 myr and are likely compositionally distinct based on observed seismic characteristics interpreted in light of geodynamic models and mineral physics constraints we investigate the dynamics of the llsvps through the use of evolutionary models of thermochemical convection from 250 ma to present day we use a spherical convection model in which the anomalous structures have a high bulk modulus consistent with seismic interpretation a new progressive assimilation method incorporates constraints from paleogeography using a refined plate history model with 1 myr time spacing to guide the thermal structure of the lithosphere and steer the thermal evolution of slabs in the uppermost mantle the thermochemical structures deform and migrate along the core mantle boundary cmb through coupling to plate motions and in response to slab stresses the models produce a ridge like anomaly beneath africa and a rounded pile beneath the pacific ocean which at present day agrees with tomography waveform modeling and other geodynamic studies plumes emanate from the margins of the domes and ridges of thickened boundary layer between the domes dense and viscous slabs can undermine the stability of high bulk modulus structures at the cmb high bulk modulus structures are not necessarily required to satisfy dynamic constraints on the llsvps

the three dimensional structure beneath the popocatepetl volcano mexico based on local earthquake seismic tomography abstract this paper presents a new seismic velocity model p and s wave velocities and vp vs ratio beneath the popocatepetl volcano to a depth of 4 km below sea level bsl this model is based on the tomographic inversion of p and s wave arrival time data from earthquakes in the region of the volcano these data were recorded by the 15 stations of a temporary seismic network that was deployed on the volcano in 1999 and 2000 the recording period was during a relatively quiet period between two strong eruptions which occurred before and after the experiment this period is characterized by low levels of volcano related seismicity most of the recorded events occurred across an area much larger than the network in this study we conducted several synthetic tests which validate the use of the out of network events to improve the resolution of the tomographic inversion beneath the stations in the resulting model we see that the main volcanic edifice is associated with high velocities that have a mushroom shape and that these high velocities are most prominent in the p wave model this feature may indicate the presence of overpressured solidified igneous rocks which comprise the edifice of popocatepetl below the summit of the volcano we observe a prominent high vp vs anomaly which reaches a value of 1 9 this anomaly probably indicates the existence of cracks and pores filled with melts and fluids and it may represent a fracture zone that serves as a conduit feeding the volcano this model characterizes the interior structure of the popocatepetl volcano prior to the strong september 2000 eruption which occurred a few months after the termination of the experiment

evidence for strong shear velocity reductions and velocity gradients in the lower mantle beneath africa we present data which indicate that the broad low shear velocity anomaly beneath southern africa is stronger and more extensive than previously thought recordings of earthquakes in the southwestern atlantic ocean at an array of broadband seismic stations in eastern africa show anomalously large propagation time delays of the shear phases s scs and sks which vary rapidly with epicentral distance by forward modeling we estimate that the low velocity anomaly extends from the core mantle boundary about 1500 km up into the mantle and that the average shear velocity within this structure is 3 lower than in standard models such as prem strong velocity contrasts exist at its margins 2 over about 300 km these seismic characteristic are consistent with recent numerical simulations of lower mantle mega plume formation

seismic structure and ultra low velocity zones at the base of the earth s mantle beneath southeast asia abstract we constrain seismic structure and ultra low velocity zones near the earth s core mantle boundary cmb beneath southeast asia we first determine the average shear velocity structure near the cmb in the region based on travel time analysis of s scs p and scp phases we then map seismic scattering in the lowermost mantle using the pkp precursors observed at the usarray the inferred average shear velocity perturbations in the lowermost 200 km of the mantle range from about 6 to 6 and exhibit a complex geographic distribution of alternate low and high velocity patches adjacent to each other surrounded by a high velocity anomaly in the south the inferred strong seismic scatterers exhibit a crescent shape distributed from the south china sea to the maluku islands and coincide with the westernmost low velocity patch suggesting that the strong scatterers represent ultra low velocity zones ulvzs we suggest that the seismic structure in the region likely results from a complex interaction between a downwelling and a low velocity region near the cmb the downwelling the high velocity patches displaces the low velocity region into many low velocity patches and pushes the ulvzs to the edge of the low velocity region

an sh hybrid method and shear velocity structures in the lowermost mantle beneath the central pacific and south atlantic oceans 1 an sh hybrid method is developed for calculating synthetic seismograms involving two dimensional localized heterogeneous structures the hybrid method is a combination of analytic and numerical methods with the numerical method finite difference applied in the heterogeneous region only and analytical methods applied outside the region generalized ray theory solutions from a seismic source are used to initiate the finite difference calculation and seismic responses at the earth s surface are obtained from the finite difference output by applying the kirchhoff theory we apply the hybrid method and study sh wave propagation near the bottom of the mantle the shear velocity structures and the interaction of sh waves with these velocity structures at the base of the mantle beneath the central pacific and south atlantic oceans are of particular importance the observed sh waves sampling these two regions of the core mantle boundary show very different characteristics across the epicentral distance range of 83 108 the sh waves sampling the core mantle boundary beneath the central pacific show a linearly increasing delay of 4 s from 98 to 108 and discernible scs phases up to an epicentral distance of 102 the sh waves propagating through the base of the mantle beneath the south atlantic ocean on the other hand exhibit a linearly increasing delay of 10 s from 98 to 108 discernible multiple scs phases with a same slowness as the direct sh waves up to 108 and rapid variations of waveforms across small epicentral distances synthetic tests indicate that while the observations sampling the central pacific can be explained by a negative shear velocity gradient of 3 relative to the preliminary reference earth model at the bottom 300 km of the mantle those sampling the south atlantic ocean require a 300 km thick bottom boundary layer with a larger negative velocity gradient up to 10 and steeply dipping edges the negative velocity gradient at the base of mantle beneath the south atlantic ocean can be best explained by partial melt driven by a compositional change produced in the early earth s history and a vertical thermal gradient within the layer while that beneath the central pacific may reasonably be attributed to pure thermal effects within a thermal boundary layer

global mantle shear velocity model developed using nonlinear asymptotic coupling theory we present a three dimensional shear velocity model of the whole mantle developed using s h waveform data the model is expressed horizontally in terms of spherical harmonics up to degree 12 and vertically in terms of legendre polynomials up to degrees 5 and 7 in the upper and lower mantle respectively what distinguishes this model from other tomographic models published to date is 1 the theoretical normal mode based wave propagation approach where we include across branch mode coupling terms in order to model the body wave sensitivity to structure along the path more accurately 2 the wave packet weighting scheme which allows to balance contributions from high amplitude and low amplitude phases increasing the resolution in some parts of the mantle we also relax the constraints on the moho depth which is allowed to vary in the inversion thus absorbing some uncertainties in crustal structure the resulting model is generally in good agreement with other recent global mantle s velocity models and with some regional models the rms profile with depth has more power than other models in the upper mantle lower mantle transition region and the zone of increased power and low degree structure near the base of the mantle is confined to the last 500 km in depth this model provides a particularly good fit to the non hydrostatic geoid through harmonic degree 12 79 variance reduction as well as good fits to observed splitting functions of s velocity sensitive mantle modes indicating that both large scale and small scale features are really well constrained