upper mantle anisotropy beneath the geoscope stations seismic anisotropy has been widely studied this last decade particularly by measuring splitting of vertically propagating core shear waves the main interest in this technique is to characterize upper mantle flow beneath seismic stations on the other hand the major restriction in this method is that a single station gives a single anisotropy measurement alternative methods have been developed in order to avoid this restriction an accurate determination of upper mantle seismic anisotropy beneath a seismic station may allow one by doing anisotropy correction to characterize remote or deeper anisotropy the geoscope network is ideal for this purpose because it is composed of a large set about 26 of high quality broadband seismometers globally distributed and because some of these stations have run for more than 10 years and most of them for more than 5 years we selected about 100 events at each site generally of magnitude mb 6 0 and we performed systematic measurements of the splitting parameters fast polarization direction and delay time t on sks skks and pks phases splitting on oceanic islands has been difficult to observe owing to the low quality of the signal but also perhaps owing to complex upper mantle structures beneath the stations station kip kipapa hawaii in the pacific is the only oceanic geoscope station with a clear anisotropy we determined well constrained splitting parameters for 10 of the 17 continental stations that may be explained by a single anisotropic layer the poor correlation between fast polarization directions and the absolute plate motion together with the apparent incoherence between the plate velocities and the observed delay times suggest that a simple drag induced asthenospheric flow alone fails to explain most of the observations for some stations located on or near major lithospheric structures tam tamanrasset algeria for instance we observe a good correlation between fast polarization directions and regional structures at station scz santa cruz california we found clear variations of the splitting parameters as a function of the event backazimuth compatible with two layers of anisotropy three stations can canberra hyb hyderabad india and ssb saint sauveur badole france seem to be devoid of detectable anisotropy

shear wave splitting in northeast venezuela trinidad and the eastern caribbean abstract we present new measurements of splitting of core shear wave phases sks skks pks recorded at seven stations in the eastern caribbean region six stations in northeastern venezuela and trinidad constituted the secasa92 temporary broadband array the seventh station sjg is installed on puerto rico we also attempted to measure source side splitting of s waves from earthquakes in the lesser antilles subducted slab but with little success mean splitting parameters fast polarization direction and delay time t at the stations are consistently east west trending and over 1 s respectively delay times at the secasa92 array are amongst the highest yet measured reaching 2 1 s and generally over 1 5 s we note an approximately linear decrease in delay times here from maxima at the northernmost stations to a minimum at the southernmost the secasa92 array is situated in the wide south america caribbean sa ca plate boundary zone and sjg is within the north america caribbean na ca plate boundary zone in both cases trends are parallel to the plate boundary zones locally we interpret these results to indicate that the sa ca plate boundary deformation extends to depths of at least 200 km and that south american crust lithospheric mantle and asthenosphere are coherently deformed the extreme delay times we measure are probably the result of transpression in the sa ca plate boundary and overriding of the lesser antilles slab by south america compressional flattening results in extremes of foliation development and dextral shearing yields a strong lineation in upper mantle olivine by contrast the delay time at sjg 1 2 s is near the global average and reflects the fact that the na ca plate boundary zone is purely sinistral or includes minor transtension we note that recent counterclockwise rotation of puerto rico does not apparently affect upper mantle deformation beneath the island we suggest that this rotation is crustal driven by far field sinistral shear and partly decoupled from the mantle shear our results are consistent with the trench parallel mantle flow hypothesis russo and silver 1994 science 263 1105 1111 and indicate that asthenospheric mantle flow beneath the caribbean plate is eastward and probably drives the plate s eastward motion

laboratory calibration of chemical volcanic gas sampling techniques using an artificial fumarole abstract we have constructed an artificial laboratory fumarole to calibrate the most common chemical volcanic gas sampling techniques and obtain a quantitative measure of their efficacy we have also developed and tested a new rugged and portable venturi spray gas sampler the venturi sampler reproduced the output gas composition most accurately followed by the giggenbach bottles filter packs and lastly alkaline traps passive alkaline traps however did better than filter packs when sampling more concentrated fumarole gases under ideal conditions the accuracy of the giggenbach bottles was identical to the venturi sampler although there was slightly more scatter the giggenbach sampler was more susceptible to problems with condensation on the input train even in a laboratory setting and this technique was only effective in relatively concentrated gas streams filter packs are also effective but extreme care must be exercised to maintain strong undersaturation with respect to the acid gas if strong undersaturation high ph is not maintained the filter packs return erroneously low s cl and s f ratios use of a ph indicator is an effective way of avoiding this problem the passive alkaline traps also under sample sulfur resulting in low reported s cl and s f ratios it appears that the overall sampling efficiency of all techniques was not strongly affected by oxygen fugacity over the limited range tested when detecting sulfate and sulfite simultaneously we found no difference in total sulfur before and after oxidation this suggests that all sulfur from the gas regardless of oxidation state was absorbed as sulfite or sulfate and or was quickly oxidized in solution this conclusion is supported by ic hs reference samples

physical basis of earthquake magnitudes an extreme value of seismic amplitudes from incoherent fracture of random fault patches abstract a relation between surface wave magnitude ms and fault area s for great earthquakes has been proposed log s 2m s 11 5 m s 7 5 a similar formula has been also derived for body wave magnitude m b redetermined from maximum amplitudes of short period p wavetrains log s 1 7 m b these are quite different from a theoretical relation expected on the basis of long wave approximation because wavelengths of seismic waves used for the above magnitudes are very short compared to the size of earthquake sources these relations represent the short period nature of the earthquake process the statistical theory of extreme values has been applied to understand the relations considering that the component waves which constitute the wavetrains for m b and ms determinations originate from the random fracture of fault heterogeneities

the use of 7 lioh impregnated filters for the collection of acidic gases and analysis by instrumental neutron activation analysis a simple system was designed using7lioh impregnated filters to collect acidic gases from ambient air and from highly concentrated volcanic plumes or gas streams these filters were developed for analysis using instrumental neutron activation analysis but other analytical techniques could be used as well the sampling system was designed to use a series of 1 3 m7lioh impregnated filters to collect high concentrations of acidic gases found in gas plumes of active volcanoes the filters are quantitative for so2 and the halide acids when sufficient base is present to neutralize the acidic species extremely high concentrations of so2 may not be collected quantitatively since so2 is a relatively weak lewis acid compared to the halide acids the acidic oxides of sb as and se were also collected quantitatively a particle filter preceded the impregnated filters in order to remove particles from the fumes this system has proven effective under difficult sampling conditions and since it is portable and light weight it could be used for many volcanological applications where high levels of acidic gaseous phase species need to be collected

source spectral characteristics of two large intra slab earthquakes along the southern kurile hokkaido arc abstract in 1993 and 1994 two large earthquakes occurred along the southern kurile hokkaido arc the 1993 kushiro oki earthquake 15 january mw 7 6 and the 1994 shikotan earthquake 4 october mw 8 2 these were intra slab earthquakes that ruptured through a substantial part of the subducting oceanic lithosphere we study source spectral characteristics of these large intra slab earthquakes based on strong motion records for the kushiro oki earthquake the s wave spectra on three rock site stations are directly compared with the theoretical spectra based on the omega squared source model and the source parameters estimated from long period seismic waves the observed spectra are roughly consistent with the theoretical ones at low frequencies

total co2 output from vulcano island aeolian islands italy 1 total co2 output from fumaroles soil gas bubbling gas discharges and water dissolved gases discharged from the island was estimated for vulcano island italy the co2 emission from fumaroles from the la fossa summit crater was estimated from the so2 crater output while co2 discharged through diffuse soil emission was quantified on the basis of 730 measurements of co2 fluxes from the soil of the island performed by using the accumulation chamber method the results indicate an overall output of 500 t day 1 of co2 from the island the main contribution to the total co2 output comes from the summit area of the la fossa cone 453 t day 1 with 362 t day 1 from crater fumaroles and 91 t day 1 from crater soil degassing the release of co2 from peripheral areas is 20 t day 1 by soil degassing palizzi and istmo areas mainly an amount comparable to both the contribution of water dissolved co2 6 t day 1 as well as to seawater bubbling co2 4 t day 1 measured in the istmo area presented data september 2007 refer to a period of moderate solphataric activity when the fumaroles temperature were 450 c and gas water molar ratio of fumaroles was up to 0 16 the calculated total co2emission allows the estimation of the mass release and related thermal energy from the volcanic hydrothermal system

spatial variations in stress within the 1987 whittier narrows california aftershock sequence new techniques and results the ml 5 9 whittier narrows california earthquake of october 1 1987 triggered a complex aftershock sequence the aftershocks had many different focal mechanisms including thrusting on e w striking north dipping planes similar to the main shock right lateral motion on nw se striking planes similar to the ml 5 3 aftershock on october 4 1987 thrusting on n s striking planes and left lateral motion on n s striking planes i attempt to interpret these mechanisms in terms of a stress field that may have caused them previously stress inversion methods based on focal mechanisms assumed that the stress field was spatially uniform in order to test for the spatially varying effect of the main shock s dislocation on the aftershocks focal mechanisms a new inversion technique was developed that solves for a uniform stress field that is superimposed on a given spatially varying stress field a series of simulations were performed to test both the new method and an older method that uses only the uniform component of the stress field these simulations show that both techniques work but will be limited by the amount of noise in the data and the accuracy to which the given spatially varying stress field resembles the actual spatially varying stresses most importantly the simulations show that the older methods do work when the stress field has a uniform component that is at least as large as the spatially varying component the simulations also show that when applying the older stress inversion techniques one can determine if the results are valid and the amount of spatially varying stresses present by examining the misfit between the data and the model application of these techniques to the whittier narrows aftershocks suggests that the uniform component of the stress field and the complex faulting corresponds to the response of an elastic halfspace to a simple regional n s compression the misfit between the data and the uniform model demonstrate the presence of spatially varying stresses at whittier narrows however their form as shown by the simulations can not be determined with this data set and these techniques

magmatic degassing of multicomponent vapors and assessment of magma depth application to vulcano island italy degassing of magmatic h2o co2 and rare gases plays a major role in understanding large scale earth processes and in the assessment of volcanic activity here we describe a quantitative model for magmatic degassing of h2o co2 noble gas n2 mixtures our modeling takes into account non ideal behaviors by adopting recently developed approaches for the solubility of h2o co2 mixtures in silicate liquids and for noble gas partitioning in h2o co2 bearing magmas this new approach allows quantitative treatment of inert gas fractionation throughout the degassing of any h2o co2 bearing natural magma in a wide range of thermo baric conditions magma degassing simulations performed by our model have clearly displayed that dissolved h2o and co2 in the melt strongly affect inert gas degassing due to their modest solubility differences in h2o rich magmas all the inert gases are strongly partitioned into vapor at early degassing extents after the quick exhaustion of co2 in contrast co2 rich melts retain dissolved helium longer because co2 has firstly to be released from magma whereas nitrogen and heavy noble gases undergo a similar or higher exsolution than co2 at early magma degassing extents we have successfully applied the degassing model to the active volcanic system of vulcano island italy where several geochemical parameters have been monitored over the last decade by quantitatively assessing magma pressure over time the model has allowed us to reconstruct the rise of magma towards the surface the results clearly show that the magma has reached low pressure and depth beneath vulcano thus interaction with the hydrothermal system as well catastrophic magma rises may result in hazardous future scenarios

emission of bromine and iodine from mount etna volcano 1 constraining fluxes of volcanic bromine and iodine to the atmosphere is important given the significant role these species play in ozone depletion however very few such measurements have been made hitherto such that global volcanic fluxes are poorly constrained here we extend the data set of volcanic br and i degassing by reporting the first measurements of bromine and iodine emissions from mount etna these data were obtained using filter packs and contemporaneous ultraviolet spectroscopic so2 flux measurements resulting in time averaged emission rates of 0 7 kt yr 1 and 0 01 kt yr 1 for br and i respectively from april to october 2004 from which we estimate global br and i fluxes of order 13 range 3 40 and 0 11 range 0 04 6 6 kt yr 1 observed changes in plume composition highlight the coherent geochemical behavior of hcl hf hbr and hi during magmatic degassing and strong fractionation of these species with respect to so2

strong motion modeling of the imperial valley earthquake of 1979 twelve three component strong motion displacement records are modeled for the 1979 imperial valley earthquake to recover the distribution of slip on the imperial fault plane the final model for which point source responses are calculated by a discrete wavenumber finite element technique uses a structure with gradients in material properties rather than layers the effects of a velocity gradient are investigated by comparing synthetics with a layer over a half space model using generalized rays it is shown that a uniform fault rupture model on a rectangular fault plane does not explain the data the preferred fault model has slip concentrated below 5 km in the basement material and between the epicenter 5 km south of the international border and highway 80 within this region there appears to be two localized areas of larger dislocations one just north of the border near bonds corner and a second under interstate 8 at meloland overpass a major arrival associated with large amplitude vertical accelerations up to 1 7 g is identified in the el centro array records this arrival has an s p time of approximately 2 3 sec at many of the array stations and is modeled as originating from a localized source 8 km to the south of the array the moment is estimated to be 5 0 10 25 dyne cm from the strong motion records which is consistent with teleseismic body wave estimates the preferred fault model is strike slip with a 90 dip the average strike is 143 however to explain vertical waveforms near the fault trace a corrugated or wiggly fault plane is introduced the average rupture velocity is in the range 2 5 to 2 7 km sec 0 8 to 0 9 times the basement shear wave velocity the preferred model has unilateral rupture propagation to the north although the data would allow a small amount of propagation to the south the estimated stress drop for the entire fault plane is only 5 to 10 bars however the stress drop over the more localized sources is about 200 bars the fault model is consistent with the pattern of seismicity and observations of aseismic creep in the imperial valley and suggests that the southern half of the imperial fault acts as a locked section which breaks periodically

seismic anisotropy and slab dynamics from sks splitting recorded in colombia the nazca caribbean and south america plates meet in northwestern south america where the northern end of the andean volcanic arc and wadati benioff zone seismicity indicate ongoing subduction however the termination of quaternary volcanism at 5 5 n and eastward offset in seismicity underneath colombia suggest the presence of complex slab geometry to help link geometry to dynamics we analyze sks splitting for 38 broadband stations of the colombian national network measurements of fast polarization axes in western colombia close to the trench show dominantly trench perpendicular orientations orientations measured at stations in the back arc farther to the east however abruptly change to roughly trench parallel anisotropy this may indicate along arc mantle flow possibly related to the suggested caldas slab tear or a lithospheric signature but smaller scale variations in anisotropy remain to be explained our observations are atypical globally and challenge our understanding of the complexities of subduction zone seismic anisotropy

heterogeneous strain regime at the west of the ogasawara plateau in the western pacific ocean from inversion of earthquake focal mechanisms abstract the ogasawara plateau has a major control on the geology of the region between the izu bonin and mariana subduction zones this region is important for understanding active collisional zones we attempt to illustrate the stress field in the wadati benioff zone of this region through seismic estimations 49 events m 3 5 are chosen for waveform inversion by the isola software to distinguish the stress field according to available focal mechanism solutions the heterogeneous stress field parameters are determined for several depth ranges based on the epicentral distribution and seismic stress field the subduction system at the west of the ogasawara plateau can be divided into three segments separated by 26 5 n and 24 n at depths of 0 50 km in the outer trench slope the slab is characterized by a varying stress filed the best fit stress model with horizontal 3 and vertical 1 is related to the plate bending and preexisting faults for the central segment the solutions present a compressional stress regime in the outer trench slope without preexisting faults induced by the resistance of the buoyant plateau the solutions for the depth range of 0 50 km in the forearc region 1 coincides with the plate convergence and 3 dips steeply to the west in the northern and central segments controlled by the friction effect and convergence of the plates however this depth range in the southern segment shows an extensional stress range associated with subduction of older slab segment relatively low rate of plate convergence and the morphological adjustment of the slab between different segments at depths of 50 200 km the inhomogeneous stress field exhibits a complicated extensional stress system with strike slip focal mechanisms at depths of 200 400 km the seismicity presents a pronounced seismic gap and the stress field cannot be determined the seismic gap may be attributed to the missing or tear of the subducting slab which is caused by the subduction of the buoyant ogasawara plateau the mechanism solutions are characterized by an inhomogeneous strain regime below 400 km driven by gravity and strong contortion of the slab and resistance to penetration of the 660 km discontinuity in contrast there are little earthquakes below 400 km in the southern region of the ogasawara plateau

effect of three dimensional slab geometry on deformation in the mantle wedge implications for shear wave anisotropy shear wave splitting observations from many subduction zones show complex patterns of seismic anisotropy that commonly have trench parallel fast directions three dimensional flow may give rise to trench parallel stretching and provide an explanation for these patterns of seismic anisotropy along strike variations in slab geometry produce trench parallel pressure gradients and are therefore a possible mechanism for three dimensional flow in this study we quantify the effects of variable slab dip curved slabs oblique subduction and slab edges on flow geometry and finite strain in the mantle wedge of subduction zones temperature dynamic pressure velocity and strain are calculated with high resolution three dimensional finite element models these models include temperature and stress dependent rheology and parameterized slab and trench geometry thick layers 20 60 km with strong trench parallel stretching are observed in the mantle wedge when slab geometry involves a transition to slab dip less than 15 or strong curvature in the slab in these cases strong trench parallel stretching develops when flow lines have an oblique to trench normal orientation this suggests that trench parallel seismically fast directions may not indicate trench parallel flow lines in systems with large along strike variations an oblique component of stretching is confined to a 20 30 km layer above the slab in systems with oblique subduction the effects of slab edges include strong toroidal flow and focusing in the mantle near slab edges and trench parallel flow that extends 50 100 km into the core of the mantle wedge

constraints on subduction geodynamics from seismic anisotropy 1 much progress has been made over the past several decades in delineating the structure of subducting slabs but several key aspects of their dynamics remain poorly constrained major unsolved problems in subduction geodynamics include those related to mantle wedge viscosity and rheology slab hydration and dehydration mechanical coupling between slabs and the ambient mantle the geometry of mantle flow above and beneath slabs and the interactions between slabs and deep discontinuities such as the core mantle boundary observations of seismic anisotropy can provide relatively direct constraints on mantle dynamics because of the link between deformation and the resulting anisotropy when mantle rocks are deformed a preferred orientation of individual mineral crystals or materials such as partial melt often develops resulting in the directional dependence of seismic wave speeds measurements of seismic anisotropy thus represent a powerful tool for probing mantle dynamics in subduction systems here i review the observational constraints on seismic anisotropy in subduction zones and discuss how seismic data can place constraints on wedge slab and sub slab anisotropy i also discuss constraints from mineral physics investigations and geodynamical modeling studies and how they inform our interpretation of observations i evaluate different models in light of constraints from seismology geodynamics and mineral physics finally i discuss some of the major unsolved problems related to the dynamics of subduction systems and how ongoing and future work on the characterization and interpretation of seismic anisotropy can lead to progress particularly in frontier areas such as understanding slab dynamics in the deep mantle

effects of fault interaction on moment stress drop and strain energy release solutions for collinear shear cracks are used to examine quantitatively the effects of fault slip zone interaction on determinations of moment stress drop and static energy release two models the barrier model and the asperity model are considered in the asperity model the actual distribution of strengths on a fault plane is idealized as a combination of two limiting cases areas which slip freely at a uniform value of a residual friction stress and unbroken ligaments or asperities across which slip occurs only at the time of a seismic event in the barrier model slip zones separated by unbroken ligaments barriers are introduced into a uniformly stressed medium to approximate the nonuniform fault propagation proposed by das and aki the strain energy change due to introducing collinear slip zones or due to breaking the asperities between them is shown to be given by the usual formula for an isolated slip zone with the stress drop replaced by the effective stress significant interaction between slip zones occurs only if the length of the asperity is less than half the length of the slip zones for the case of two collinear slip zones fracture of the asperity between them is shown to cause a large moment primarily because of the additional displacement which is induced on the adjacent slip zones for example if the asperity length is 0 05l where l is the length of each adjacent slip zone then fracture of the asperity causes a moment almost 1 8 times the moment caused by introducing a slip zone of length l for two collinear slip zones the local stress drop due to fracture of the separating asperity is shown to become unbounded as the asperity length goes to zero but in the same limit the stress drop averaged over the entire fault length is approximately equal to the apparent stress drop inferred for an isolated fault of the same moment and total fault length this apparent stress drop is approximately equal within a factor of 2 or 3 to the effective stress and hence can be used in the usual formula to give a good estimate of the strain energy change for the barrier model numerical results are given for the ratio of the stress drop calculated on the assumption of an isolated slip zone to the true stress drop for example in the case of two collinear slip zones of length l separated by a barrier of length 0 2l this ratio is 0 5 whereas for a barrier length equal to that of the adjacent slip zones the ratio is 0 24 stress drop estimates become worse with increasing number of fault segments

origin of azimuthal seismic anisotropy in oceanic plates and mantle abstract seismic anisotropy is ubiquitous in the earth s mantle but strongest in its thermo mechanical boundary layers azimuthal anisotropy in the oceanic lithosphere and asthenosphere can be imaged by surface waves and should be particularly straightforward to relate to well understood plate kinematics and large scale mantle flow however previous studies have come to mixed conclusions as to the depth extent of the applicability of paleo spreading and mantle flow models of anisotropy and no simple globally valid relationships exist here we show that lattice preferred orientation lpo inferred from mantle flow computations produces a plausible global background model for asthenospheric anisotropy underneath oceanic lithosphere the same is not true for absolute plate motion apm models a 200 km thick layer where the flow model lpo matches observations from tomography lies just below the 1200 c isotherm of a half space cooling model indicating strong temperature dependence of the processes that control the development of azimuthal anisotropy we infer that the depth extent of shear and hence the thickness of a relatively strong oceanic lithosphere can be mapped this way these findings for the background model and ocean basin specific deviations from the half space cooling pattern are found in all of the three recent and independent tomographic models considered further exploration of deviations from the background model may be useful for general studies of oceanic plate formation and dynamics as well as regional scale tectonic analyses

trace elements from fumaroles at vulcano island italy rates of transport and a thermochemical model abstract this paper reports a semi quantitative estimate of fluxes of some trace elements as bi cd pb tl and zn emitted from fumaroles at la fossa crater on the island of vulcano in addition the paper presents a thermochemical model of the deposition of solid species during the cooling of volcanic gases as a function of temperature a free energy minimization method was used computations indicate that lead condenses as pbs between 428 and 277 c and as pbcl 2 between 252 and 202 c bi 2 s 3 occurs between 428 and 408 c cds occurs between 428 and 388 c and zn 2 so 5 occurs only at 428 c calculations show moreover the occurrence of tlcl between 177 and 102 c sulfur occurs below 152 c the calculated results are in agreement with the depositional sequence observed in a silica sampling tube

trench parallel flow beneath the nazca plate from seismic anisotropy shear wave splitting of s and sks phases reveals the anisotropy and strain field of the mantle beneath the subducting nazca plate cocos plate and the caribbean region these observations can be used to test models of mantle flow two dimensional entrained mantle flow beneath the subducting nazca slab is not consistent with the data rather there is evidence for horizontal trench parallel flow in the mantle beneath the nazca plate along much of the andean subduction zone trench parallel flow is attributale utable to retrograde motion of the slab the decoupling of the slab and underlying mantle and a partial barrier to flow at depth resulting in lateral mantle flow beneath the slab such flow facilitates the transfer of material from the shrinking mantle reservoir beneath the pacific basin to the growing mantle reservoir beneath the atlantic basin trenchparallel flow may explain the eastward motions of the caribbean and scotia sea plates the anomalously shallow bathymetry of the eastern nazca plate the long wavelength geoid high over western south america and it may contribute to the high elevation and intense deformation of the central andes

shear wave anisotropy in the upper mantle beneath the nazca plate in northern chile data from the projecto de investigacion sismologica de la cordillera occidental pisco seismic network and from six broadband seismographs that were operating in northern chile were used to investigate the mantle in the convergent boundary zone between nazca plate and the south american continent for the presence of anisotropy broadband data as well as long period filtered data of teleseismic sks and pks phases were analyzed for the presence of shear wave splitting as a possible indicator for seismic anisotropy in the mantle beneath the pisco network measurable shear wave splitting was observed with maximum delay times between the slow and fast split wave of the order of 1 s splitting of s waves from intermediate depth events located directly beneath the pisco network in the descending nazca plate is generally associated with small delay times of the order of 0 1 s a value typical for the continental crust near vertical scs reflections from two deep earthquakes in argentina and one nearby intermediate depth earthquake have similar splitting parameters as the sks phases this means that the anisotropic zone causing the splitting of the core phases can be constrained to the pacific mantle underlying the subducting nazca plate it probably does not extend deeper than about 260 km the majority of the anisotropy directions inferred from the core phases are parallel to the absolute plate motion apm direction of the nazca plate which is about n80 e at some stations however the fast polarization direction is pointing n160 e nearly parallel to the strike of the trench and the andes which would be compatible with the trench parallel flow model for south america proposed by russo and silver 1994 this direction is observed over an approximately 100 km wide band to the west of the active volcanic zone it may represent either a second anisotropy regime in the mantle a small scale diversion of slab entrained mantle flow or a relatively small area where slab entrainment of mantle flow is reduced or ceases to exist the large number of observed apm parallel fast directions suggests however that the mantle beneath the descending nazca plate in northern chile deforms mainly as the result of slab entrained mantle flow the large variations of anisotropy directions in the andean subduction zone indicate that asthenospheric flow in the pacific mantle has a complex pattern which may vary over scale lengths of a few hundred kilometers and which may be governed by slab morphology

transient slab flattening beneath colombia subduction of the nazca and caribbean plates beneath northwestern colombia is seen in two distinct wadati benioff zones one associated with a flat slab to the north and one associated with normal subduction south of 5 5 n the normal subduction region is characterized by an active arc whereas the flat slab region has no known holocene volcanism we analyze volcanic patterns over the past 14 ma to show that in the mid miocene a continuous arc extended up to 7 n indicating normal subduction of the nazca plate all along colombia s pacific margin however by 6 ma we find a complete cessation of this arc north of 3 n indicating the presence of a far more laterally extensive flat slab than at present volcanism did not resume between 3 n and 6 n until after 4 ma consistent with lateral tearing and re steepening of the southern portion of the colombian flat slab at that time

new insights into magma dynamics during last two eruptions of mount etna as inferred by geochemical monitoring from 2002 to 2005 two distinct eruptive events characterize the volcanic activity at mount etna during the 2002 to 2005 period we identified signals of magma ascent preceding these eruptions by geochemical monitoring of both chemical composition and he isotope ratio of gas emissions from five locations in the peripheral area of the volcano the geochemical signals are interpreted using the models proposed by caracausi et al 2003a 2003b and allow identification of episodes of magma ascent and estimation of the pressures of degassing magma as observed for the 2001 eruption caracausi et al 2003b magma ascent probably triggered the onset of the 2002 2003 eruption and minor events of magma ascent were observed between may and december 2003 in contrast to the previous two eruptions the 2004 2005 eruption was not preceded by significant geochemical signals of volcanic unrest suggesting that this eruption was mainly triggered by the failure of the upper portion of the volcanic edifice under the magmatic hydrostatic pressure in the conduits high 3he 4he ratio revealed new volatile rich magma accumulation the 2002 2003 eruption was preceded by a much shorter period of new magma accumulation from deep levels of the feeding system few minor signals of magma migration were detected at some of the sites during the months preceding the 2004 2005 eruption suggesting that the degassed 3he depleted magma resident in the volcanic conduits was not replaced by new volatile rich magma this is in agreement with the lack of explosive activity during the 2004 2005 eruption and with petrologic observations that the parent magma probably erupted in 2000 and 2001 new geochemical signals of magma ascent from the deep reservoir have been identified since june 2005 indicating that the volcanic activity of mount etna is evolving toward new pre eruptive conditions

geochemical evidence of the renewal of volcanic activity inferred from co2 soil and so2 plume fluxes the 2007 stromboli eruption italy on 27 february 2007 a new eruption occurred on stromboli which lasted until 2 april it was characterized by effusive activity on the sciara del fuoco and by a paroxysmal event 15 march this crisis represented an opportunity for us to refine the model that had been developed previously 2002 2003 eruption and to improve our understanding of the relationship between the magmatic dynamics of the volcano and the geochemical variations in the fluids in particular the evaluation of the dynamic equilibrium between the volatiles co2 and so2 released from the magma and the corresponding fluids discharged from the summit area allowed us to evaluate the level of criticality of the volcanic activity one of the major accomplishments of this study is a 4 year database of summit soil co2 flux on the basis of which we define the thresholds low medium high for this parameter that are empirically based on the natural volcanological evolution of stromboli the so2 fluxes of the degassing plume and the co2 fluxes emitted from the soil at pizzo sopra la fossa are also presented it is noteworthy that geochemical signals of volcanic unrest have been clearly identified before during and after the effusive activity these signals were found almost simultaneously in the degassing plume so2 flux and in soil degassing co2 flux at the summit although the two degassing processes are shown to be clearly different the interpretation of the results will be useful for future volcanic surveillance at stromboli

advancement in source estimation techniques using broadband regional seismograms one important constraint on source retrieval from regional seismograms comes from the amplitude difference between various phases such as pnl surface wave sv sh because the misfit errors used in some waveform inversions are normalized by the data and synthetics the amplitude information in the data has not been fully utilized in this article we modify the cut and paste source estimation technique zhao and helmberger 1994 by removing this type of normalization it is shown that the modified method increases the stability and resolution of inversion when multiple stations at different distance ranges are used a distance scaling factor is introduced to compensate for the amplitude decay with distance by applying the technique to the terrascope data we have determined source mechanisms and depths of 335 southern californian events with m l 3 5 the amplitude decays with distance are r 1 13 for pnl r 0 55 for love waves and r 0 74 for rayleigh waves in contrast to generally shallow source depths reported by the southern california short period network the depth distribution from waveform inversion shows a strong peak around 12 km with few earthquakes occurring above 5 km and below 20 km

volcanic hazards at fossa of vulcano data from the last 6 000 years stratigraphic reconstruction of the complete sequence of deposits that formed the fossa cone of vulcano has distinguished four principal eruptive cycles punte nere palizzi commenda and pietre cotte at least three additional eruptive cycles one of which ends with the campo sportivo lava occur between deposits of the punte nere and palizzi cycles however exposure is inadequate for their characterization the assignment of the modern deposits that follow the pietre cotte lava is uncertain

gas and aerosol emissions from villarrica volcano chile here we report results from a multidisciplinary field campaign at villarrica volcano chile in march 2009 a range of direct sampling and remote sensing techniques was employed to assess gas and aerosol emissions from the volcano and extend the time series of measurements that have been made during recent years airborne traverses beneath the plume with an ultraviolet spectrometer yielded an average so2 flux of 3 7 kg s 1 this value is similar to previous measurements made at villarrica during periods of quiescent activity the composition of the plume was measured at the crater rim using electrochemical sensors and for the first time open path fourier transform infrared spectroscopy yielding a composition of 90 5 mol h2o 5 7 co2 2 6 so2 0 9 hcl 0 3 hf and 0 01 h2s comparison with previous gas measurements made between 2000 and 2004 shows a correlation between increased so2 hcl ratios and periods of increased activity base treated filter packs were also employed during our campaign yielding molar ratios of hbr so2 1 1 10 4 hi so2 1 4 10 5 and hno3 so2 1 1 10 3 in the gas phase our data represent the most comprehensive gas inventory at villarrica to date and the first evaluation of hbr and hi emissions from a south american volcano sun photometry of the plume showed the near source aerosol size distributions were bimodal with maxima at 0 1 and 1 m these findings are consistent with results from analyses in 2003 electron microscope analysis of particulate matter collected on filters showed an abundance of spherical micron sized particles that are rich in si mg and al non spherical s rich particles were also observed

review of the evolution of geochemical monitoring networks and methodologies applied to the volcanoes of the aeolian arc italy abstract fluids discharged from volcanic systems are the direct surface manifestation of magma degassing at depth and provide primary insights for evaluating the state of volcanic activity we review the geochemical best practice in volcanic surveillance based to a huge amount of monitoring data collected at different active volcanoes using both continuous and discontinuous approaches the targeted volcanoes belong to the aeolian arc located in the tyrrhenian sea italy and they have exhibited different activity states during the monitoring activities reported here la fossa cone on vulcano island has been in an uninterrupted quiescent stage characterized by variable solfataric activity in contrast stromboli island has shown a persistent mild explosive activity episodically interrupted by effusive eruptions in 1985 2002 2007 and 2014 panarea island which is the summit of a seamount rising from the seafloor of the southern tyrrhenian sea showed only undersea fluid release the only observable clues of active volcanism at panarea island have been impulsive changes in the undersea fluid release with the last submarine gas burst event being observed in november 2002 the geochemical monitoring and observations at each of these volcanoes has directly involved the volcanic plume and or the fumarole vents thermal waters and diffuse soil degassing depending on the type of manifestations and the level of activity encountered through direct access to the magmatic samples when possible and the collection of as much observable data related to the fluid release as possible the aim has been i to verify the thermodynamic equilibrium condition ii to discern among the possible hydrothermal magmatic marine and meteoric sources in the fluid mixtures iii to develop models of the fluid circulation supported by data iv to follow the evolution of these natural systems by long term monitoring and v to support surveillance actions related to defining the volcanic risk and the evaluation and possible mitigation of related hazards the examples provided in this review article show the close relationships among data analysis interpretation and modeling we particularly focus on describing the fieldwork procedures since any theoretical approach must always be verified and supported by field data rather than just by experiments controlled in laboratory indeed the natural systems involve many variables producing effects that cannot be neglected the monitored volcanic systems have been regarded as natural laboratories and all of the activities have focused on both volcanological research and surveillance purposes in order to ensure that these two goals have overlapped an appendix is also included that explains the scientific approach to the systematic activities regarding geochemical monitoring of volcanic activity

electrochemical sensing of volcanic gases we report here the development and application of a compact geochemical nose incorporating electrochemical sensors for gas measurements in volcanic plumes a novel element of the instrument design is the arrangement of the sensors in a parallel array that enables near simultaneous exposure and fast response data analysis methods were developed that utilise the multi sensor output currents to extract gas mixing ratio abundances and eliminate cross sensitivities use of filter methods is demonstrated to remove baseline drift or instrument noise we introduce a new approach for analysis of measurements from sensors that have a slower response time e g hcl and apply this model to estimate hcl so2 ratios we deployed the sensor system at aso volcano japan detecting emissions from its fumarole field hot crater lake and a mixed plume we measured so2 h2s co and hcl ranging in abundance from 102 104 ppbv neither no2 nor cl2 were detected for the fumarolic gases molar ratios were measured as follows h2s so2 is 0 15 h2 so2 0 25 co so2 0 02 hcl so2 0 1 the crater lake plume s h2s so2 is 0 03 the compositions are discussed in terms of degassing equilibria and plume chemistry our instrument design represents a cost effective low power and highly portable system that can be readily adapted for operational surveillance of volcanic gases

subduction of oceanic asthenosphere evidence from sub slab seismic anisotropy 1 the oceanic asthenosphere is characterized as a low viscosity channel down to 200 300 km depth separating the cold lithosphere from above and it is intimately linked to a layer of low seismic velocity and prominent seismic anisotropy observed globally beneath ocean basins while subduction of tectonic plates in convergent margins is well recognized the fate of oceanic asthenosphere remains enigmatic we demonstrate that subduction of the oceanic asthenosphere characterized by weak azimuthal anisotropy and strong radial anisotropy explains the essence of sub slab shear wave splitting patterns where the fast splitting direction changes from predominantly trench parallel or sub parallel under relatively steep subduction zones to frequently trench normal under shallow subduction zones to explain the observed splitting time the thickness of the subducted asthenosphere is estimated to be 100 50 km

magmatic degassing at erta ale volcano ethiopia abstract here we report measurements of the chemical composition and flux of gas emitted from the central lava lake at erta ale volcano ethiopia made on 15 october 2005 we determined an average so 2 flux of 0 69 0 17 kg s 1 using zenith sky ultraviolet spectroscopy of the plume and molar proportions of magmatic h 2 o co 2 so 2 co hcl and hf gases to be 93 58 3 66 2 47 0 06 0 19 and 0 04 respectively by open path fourier transform infrared ftir spectrometry together these data imply fluxes of 7 3 0 7 0 008 0 03 and 0 004 kg s 1 for h 2 o co 2 co hcl and hf respectively these are the first ftir spectroscopic observations at erta ale and are also some of the very few gas measurements made at the volcano since the early 1970s gerlach t m 1980b investigation of volcanic gas analyses and magma outgassing from erta ale lava lake afar ethiopia journal of volcanology and geothermal research 7 3 4 415 441 we identify significant increases in the proportion of h 2 o in the plume with respect to both co 2 and so 2 across this 30 year interval which we attribute to the depletion of volatiles in magma that sourced effusive eruptions during the early 1970s and or to fractional magma degassing between the two active pit craters located in the summit caldera

real time simultaneous detection of volcanic hg and so2 at la fossa crater vulcano aeolian islands sicily 1 measuring hg so2 ratios in volcanic emissions is essential for better apportioning the volcanic contribution to the global hg atmospheric cycle here we report the first real time simultaneous measurement of hg and so2 in a volcanic plume based on lumex and multigas techniques respectively we demonstrate that the use of these novel techniques allows the measurements of hg so2 ratios with a far better time resolution than possible with more conventional methods the hg so2 ratio in the plume of f0 fumarole on la fossa crater vulcano island spanned an order of magnitude over a 30 minute monitoring period but was on average in qualitative agreement with the hg so2 ratio directly measured in the fumarole mean plume and fumarole ratios being 1 09 10 6 and 2 9 10 6 respectively the factor 2 difference between plume and fumarole compositions provides evidence for fast hg chemical processing the plume

tearing and breaking off of subducted slabs as the result of collision of the panama arc indenter with northwestern south america we present two regional lithospheric cross sections that illustrate eastward and southeastward dipping subducted slabs to depths of 315 km beneath the surface of colombia in northwestern south america these cross sectional interpretations are based on relocated earthquake hypocentral solutions models supported on gravity and magnetic regional data and coda q q c tomography the method of tomographic imaging based on spatial inversion of the coda wave has advantages of providing information on the lateral variations of the anelastic properties and thermal structure of the lithospheric system mapping of earthquake defined benioff zones combined with tomographic imaging reveals the presence of an 240 km long east west striking slab tear named here the caldas tear the proposed caldas tear separates a zone of shallow 20 30 dipping southeastward subduction in the area of colombia adjacent to panama and the caribbean sea which is not associated with subduction related volcanism from an area of steeper 30 40 dipping slab adjacent to the eastern pacific ocean that is associated with an active north south chain of active arc volcanoes we propose that the caldas slab tear separating these two distinct subducted slabs originally formed as the southern boundary of the panama indenter an extinct island arc that began subducting beneath northwestern south america about 12 ma the area south of the panama indenter is miocene oceanic crust of the nazca plate which subducts eastward beneath northwestern south america at normal angles and melts to form a north south trending active volcanic arc in addition to the formation of the caldas tear we propose that impedance of the thicker crustal area of the panama arc indenter over the past 12 ma may have led to down dip break off of previously subducted oceanic crust that is marked by an extremely concentrated and active earthquake swarm of intermediate depth earthquakes beneath east central colombia

analysis of the ground accelerations radiated by the 1980 livermore valley earthquakes for directivity and dynamic source characteristics the strong motion accelerograph recordings of the 24 january 1980 main shock and the 27 january 1980 aftershock of the livermore valley earthquake sequence are analyzed for systematic variations with azimuth or station location the variation of the peak accelerations with epicentral azimuth is apparently reversed for the two events the main shock accelerations are larger to the south and the aftershock accelerations are larger to the northwest we eliminate the site effects by forming the ratio of the peak accelerations recorded at the same station after correcting for the epicentral distance this analysis indicates that source directivity caused a total variation of a factor of 10 in the peak accelerations comparison of this variation with the spatial extent of the aftershock sequences suggests that the strong directivity in the radiated accelerations is the result of unilateral ruptures in both events the accelerograms recorded at 10 stations within 35 km of the events were digitized to analyze the azimuthal variation of the rms acceleration the peak velocity and the radiated energy flux the variation of rms acceleration correlates almost exactly with the variation of the peak accelerations this correlation is analyzed using both deterministic and stochastic models for the acceleration waveforms the peak velocities corrected for epicentral distance vary with azimuth by a factor of 5 for both events while the radiated energy flux varies by a factor of 30 for the main shock and 15 for the aftershock the peak velocities are strongly correlated with the radiated energy flux the radiated seismic energies are estimated to be 2 6 0 9 1020 dyne cm for the main shock and 1 5 0 3 1020 dyne cm for the aftershock

strong motion seismology although this is the first review on this topic to appear in a quadrennial report the roots of strong motion seismology extend back to at least 1932 when far sighted engineers in the seismological field survey of the u s coast and geodetic survey installed rugged fieldworthy instruments designed to make on scale recordings of large earthquakes carder 1964 these instruments are called accelerographs for their output closely mimics ground acceleration hudson 1979 the original instruments and their offspring have provided a wealth of information about ground motions of direct use to engineers due in large part to the continued devotion of the seismological field survey now part of the branch of engineering seismology and geology at the u s geological survey the number of recordings has increased substantially particularly in the last two decades and multiple recordings of a few california earthquakes have provided the data necessary to begin to unravel the complexities of the ground motions and to predict these motions on an empirical basis although seismologists used accelerograph records in studies of earthquake energy and ground motion attenuation as long ago as 1942 gutenberg and richter 1942 1956 widespread seismological use of the records began with aki s 1968 analysis of the 1966 parkfield earthquake

multiple plates subducting beneath colombia as illuminated by seismicity and velocity from the joint inversion of seismic and gravity data abstract subduction beneath the northernmost andes in colombia is complex based on seismicity distributions multiple segments of slab appear to be subducting and arc volcanism ceases north of 5 n here we illuminate the subduction system through hypocentral relocations and vp and vs models resulting from the joint inversion of local body wave arrivals surface wave dispersion measurements and gravity data the simultaneous use of multiple data types takes advantage of the differing sensitivities of each data type resulting in velocity models that have improved resolution at both shallower and deeper depths than would result from traditional travel time tomography alone the relocated earthquake dataset and velocity model clearly indicate a tear in the nazca slab at 5 n corresponding to a 250 km shift in slab seismicity and the termination of arc volcanism north of this tear the slab is flat and it comprises slabs of two sources the nazca and caribbean plates the bucaramanga nest a small region of among the most intense intermediate depth seismicity globally is associated with the boundary between these two plates and possibly with a zone of melting or elevated water content based on reduced vp and increased vp vs we also use relocated seismicity to identify two new faults in the south american plate one related to plate convergence and one highlighted by induced seismicity

evolution of the fossa cone vulcano abstract in decreasing order of abundance the principal emplacement mechanisms for deposits of the fossa edifice of vulcano include dry surge wet surge pyroclastic fall lahar lava flow pyroclastic flow and epiclastic processes most samples from pyroclastic beds on fossa contain two grain size populations the coarser mode at 0 5 1 5 resulted from modified ballistic transport whereas the finer mode at 2 5 3 5 is related to surge transport reconstruction of the entire stratigraphy of the cone permits the beds to be grouped into five principal eruptive cycles all of which postdate the lower pilato ash of lipari dated at 11 000 to 8 500 y b p pte nere cycle palizzi cycle commenda cycle pietre cotte cycle and the modern cycle additional cycles may be present but exposures on the cone are insufficient for their adequate definition all cycles follow a similar stochastic pattern starting with surge eruptions and ending with effusion of lava from the crater rim the frequency of pyroclastic fall events increases with time throughout each cycle this progressive decrease in the efficiency of water melt interaction through the duration of each cycle may be due to a gradual rise of each new magma column coupled with an increase in magmatic volatiles tectonism and magma mixing both play a role in triggering eruptions on fossa the present crater lies at the intersection of two prominent tectonic trends a nnw fracture system parallels the main eruptive centers and an ene set aligns with the migration of minor vents the silicic lavas of fossa contain a mixture of two components xenocrysts of plagioclase augite and olivine surrounded by a dark glass are intimately dispersed into an evolved rhyolitic matrix

lessons in reducing volcano risk since 1980 volcanologists have confronted more volcanic crises than any time since the mont pelee catastrophe in 1902 good science alone will not do the job of reducing volcano risk

polarities of p and s waves and shear wave splitting observed from the bucaramanga nest colombia the bucaramanga nest colombia is an intense seismic source centered at 6 48 n 73 10 w and 161 km depth beneath the eastern andes during a 3 week period 161 microearthquakes mb 4 3 were recorded by a 260 by 140 km array of 12 three component university of wisconsin uw seismographs centered above the nest eighty eight percent of the events recorded were from the nest providing a unique opportunity to study natural events that have different source parameters but nearly the same ray paths shear wave splitting analysis shows that the average polarization direction of the first arriving shear waves from the nest events at nine of the seismographs is n12 e 18 after corrections based on the amount of discrepancy between the observed average p wave azimuth and the expected event to station azimuth average s wave polarization is n6 e 15 variation in time delays with azimuth and epicentral distance appears to be due to the varying angles between the ray path and the principal axes of the anisotropic system shear wave splitting observed from 17 events along the wadati benioff zone outside the nest shows that the time delays increase with increasing depth and defines a wedge shaped anisotropic volume above the subducting plate the anisotropic layer has a maximum thickness of approximately 60 km at the depth of the nest and is thinning updip the anisotropy most likely results from aligned anisotropic minerals such as olivine and orthopyroxene in a localized mantle flow in the lithosphere above the wadati benioff zone a region weakened by active fluid migration and mobilized by the heating and shearing along the suducting slab

seismic excitation and directivity of short period body waves from a stochastic fault model abstract a theoretical study is made to investigate the excitation of short period p and s waves based on a stochastic fault model the source model describes an earthquake as a finitely propagating unilateral rupture on a fault plane where fault heterogeneities fault patches are randomly distributed the fundamental parameters are 1 average stress drop 2 fault dimension 3 variance of stress drop 4 patch corner frequency relevant to the fault patch sizes and 5 fault patch rupture directions it is shown that the source effectively radiates short period s waves this would reveal the reason why the strong ground motion of large earthquakes is mainly composed of s waves the short wavelength description of an earthquake is obtained by an energy additive superposition of the stochastic fracturing of random fault patches this gives rise to a seismic directivity effect peculiar to the incoherent short waves this effect is studied in the frequency and time domains especially for root mean square and maximum accelerations the excitation of strong motion accelerations is formulated by the stochastic part of earthquake faulting in terms of the variance of stress drop fault dimension and cut off frequency in addition to the short period seismic directivity

h2s fluxes from mt etna stromboli and vulcano italy and implications for the sulfur budget at volcanoes abstract we present here new measurements of sulfur dioxide and hydrogen sulfide emissions from vulcano etna and stromboli italy made by direct sampling at vents and by filter pack and ultraviolet spectroscopy in downwind plumes measurements at the f0 and fa fumaroles on vulcano yielded so 2 h 2 s molar ratios of 0 38 and 1 4 respectively from which we estimate an h 2 s flux of 6 to 9 t d 1 for the summit crater for mt etna and stromboli we found so 2 h 2 s molar ratios of 20 and 15 respectively which combined with so 2 flux measurements suggest h 2 s emission rates of 50 to 113 t d 1 and 4 to 8 t d 1 respectively we observe that source and plume so 2 h 2 s ratios at vulcano are similar suggesting that hydrogen sulfide is essentially inert on timescales of seconds to minutes this finding has important implications for estimates of volcanic total sulfur budget at volcanoes since most existing measurements do not account for h 2 s emission

intercomparison of volcanic gas monitoring methodologies performed on vulcano island italy 1 volcanic gas emissions from fumaroles on the rim of la fossa crater vulcano island italy were measured simultaneously using direct sampling for h2o co2 total sulfur hcl and hf filter packs for so2 hcl hf and short path active mode ftir measurements for h2o co2 so2 hcl and hf in an intercomparison study in may 2002 the results show that cl f ratios were in good agreement between all three methods and that ftir and direct sampling determined comparable proportions of co2 and h2o amounts of total s observed in direct sampling data were approximately double the amounts of so2 measured with filter packs and ftir this difference could be attributed either to the fact ftir and filter packs do not measure reduced sulfur species e g h2s or to sublimation of elemental s upon exit from the fumarole after collection by direct sampling but before detection with ftir and filter packs

subduction system and flat slab beneath the eastern cordillera of colombia seismicity at the northern terminus of the nazca subduction is diffused over a wide area containing the puzzling seismic feature known as the bucaramanga nest we relocate about 5000 earthquakes recorded by the colombian national seismic network and produce the first 3 d velocity model of the area to define the geometry of the lithosphere subducting below the colombian andes we found lateral velocity heterogeneities and an abrupt offset of the wadati benioff zone at 5 n indicating that the nazca plate is segmented by an e w slab tear that separates a steeper nazca segment to the south from a flat subduction to the north the flat nazca slab extends eastward for about 400 km before dip increases to 50 beneath the eastern cordillera where it yields the bucaramanga nest we explain this puzzling locus of intermediate depth seismicity located beneath the eastern cordillera of colombia as due to a massive dehydration and eclogitization of a thickened oceanic crust we relate the flat subducting geometry to the entrance at the trench at ca 10 ma of a thick buoyant oceanic crust likely a volcanic ridge producing a high coupling with the overriding plate sub horizontal plate subduction is consistent with the abrupt disappearance of volcanism in the andes of south america at latitudes 5 n

shear wave splitting around the northern atlantic frozen pangaean lithospheric anisotropy abstract although the number of teleseismic shear wave splitting measurements has considerably increased world wide during the last decade there is no consensus on a unique and universal interpretation of these data splitting might primarily originate in the lithospheric and or asthenospheric mantle but the technique has no vertical resolution and therefore leaves this question of anisotropy location unresolved generally the consequences of the two possibilities are used to infer which is more likely if splitting occurs in the lithosphere the splitting parameters the orientation of the fast split shear wave o and the delay time t should be related to frozen or active deformation on the other hand if the splitting occurs in the asthenosphere the observed splitting parameters should be related to the present day flow induced by the differential motion between the tectonic plates and the underlying mantle the aim of this paper is to test these hypotheses against shear wave splitting observations from western europe and the eastern u s indeed except in a few places splitting on either side of the atlantic on hercynian caledonian or grenvillian structures do not appear to have been deeply and pervasively affected by the opening of this ocean this provides us the opportunity to place these results in the pangaean reference frame of the hercynian caledonian belt before the opening of the atlantic in many places the close parallelism between lithospheric structure and the absolute plate motion apm impedes a reliable identification of the origin of the anisotropy especially an asthenospheric flow channelled around the deeply rooted north american craton may be compatible with the large scale anisotropy pattern in the eastern u s despite this ambiguity fast polarization directions o observed on or near some major lithospheric structures correlate better with the trend of the local tectonic structures than with the expected asthenospheric flow therefore we suggest that these lithospheric structures developed before the opening of the atlantic remain frozen since hercynian caledonian or grenvillian times and are responsible for the observed splitting at these places

excitation of short period body waves by great earthquakes short period seismograms from large earthquakes are generally complex and are hard to fourier analyze an innovation of spectral analysis has been made for wwssn short period seismograms the parameters necessary for the analysis are dominant frequency 0 maximum amplitude amax and a characteristic duration of complicated wave packets spectrum of short period seismograms y is shown to be approximated as y 1 07 0amax the spectrum after correcting for the effects of geometrical spreading unelastic attenuation instrumental response and crustal magnifications is used to retrieve a short period seismic moment m1 of earthquake sources the method has been applied to more than 900 short period seismograms from 79 large earthquakes throughout the world these earthquakes cover the seismic moment m0 from 7 5 1029 to 7 5 1024 dyne cm m1 would not be saturated in this range although body wave magnitude usually is a relation of m0 m12 is obtained for the present data set a relation between m1 and redetermined body wave magnitude m b is also shown log m1 1 24 m b 17 9 where mb is obtained from maximum amplitudes of p wavetrains the analysis method is based on an extension of algorithm of earthquake magnitude determination however it has a potential beyond the body wave magnitude as m1 is related closely to the source excitation of short period seismic waves it is one of the key parameters in understanding the fracture mechanisms of irregular faults

patterns in seismic anisotropy driven by rollback subduction beneath the high lava plains received 24 march 2011 revised 25 may 2011 accepted 31 may 2011 published 8 july 2011 1 we present three dimensional laboratory modeling of the evolution of finite strain and compare these to shear wave splitting observations in the northwest u s under the high lava plains hlp we show that relationships between mantle flow and anisotropy are complicated in subduction zones and factors such as initial orientation of the olivine fast axis style of subduction and time evolving flow are important due to increased horizontal shear systems with a component of rollback subduction have simple trench normal strain alignment within the central region of the backarc mantle wedge while those with more simple longitudinal sinking are often variable and complex in the hlp splitting observations are consistent with rollback driven laboratory results citation druken k a m d long and c kincaid 2011 patterns in seismic anisotropy driven by rollback subduction beneath the high lava plains geophys res lett 38 l13310 doi 10 1029 2011gl047541

source estimation from broadband regional seismograms recently developed source inversion techniques do not take full advantage of the broadband nature of regional seismograms the reason is that green s functions generated from flat layered models are not correct for the complicated propagational phenomena associated with realistic crustal structure the inversion is usually performed by removing short periods and inverting only selected portions of the records in this article we introduce a source estimation technique that allows for better use of the entire broadband record when only imperfect green s function are available the procedure desensitizes the timing between the principal crustal arrivals by fitting portions of the green s functions independently the preferred source is the orientation that minimizes the average l1 and l2 norms in term of the four parameters m 0 strike dip and rake the l2 error emphasizes the longer period surface waves while the l1 error weights the shorter period body waves we obtain estimates of duration by modeling long period and broadband signals the depth determination can be obtained directly from the surface reflected phases sp mp or ss ms or by cycling through the depth dependent green s functions in addition to the source parameters we obtain t phase alignments shifts that can be used as green s function corrections for relocating other events or as a guide to deriving new crustal models

mantle flow beneath northwestern venezuela seismic evidence for a deep origin of the merida andes abstract we measured shear wave splitting from sks data recorded by the national seismic network of venezuela and a linear broadband passcal rice seismic array across the merida andes the linear array was installed in the second phase of the passive seismic component of the bolivar project to better understand the complicated regional tectonics in western venezuela using the method proposed by wolfe and silver 1998 sks waveforms from 2 to 36 earthquakes mostly from the tonga subduction zone were selected for each of the 23 stations in the region in order to do splitting analysis the fast polarization directions can be divided into 3 zones all in agreement with local gps data the first zone comprises the stations north of the dextral strike slip oca ancon fault these stations show the largest split times 1 6 3 2 s oriented in a roughly ew direction and are similar to splitting observations made further to the east along the strike slip plate boundary growdon et al 2009 we attribute this to trench parallel mantle flow that passes around the northwest corner of the subducting caribbean plate and along the northern edge of south america as proposed by russo and silver 1994 forming an eastward flow beneath the southern caribbean plate zone two is the merida andes with the right lateral bocono fault in the center where split orientations are at n45 e suggesting that the observed seismic anisotropy is likely caused by lithospheric deformation parallel to the bocono fault zone three is east of the bocono fault inside the barinas apure basin where the measured split times are smaller 0 8 s with an ew fast direction that is consistent with those observed at the guarico basin maturin basin and the guayana shield in the east and are interpreted as orientation with the motion of the continent

current states of stress in the northern andes as indicated by focal mechanisms of earthquakes abstract systematic inversion of double couple focal mechanisms of shallow earthquakes in the northern andes reveals relatively homogeneous patterns of crustal stress in three main regions the first region presently under the influence of the caribbean plate includes the northern segment of the eastern cordillera of colombia and the western flank of the central cordillera north of 4 n it is characterized by wnw ese compression of dominantly reverse type that deflects to nw se in the merida andes of venezuela where it becomes mainly strike slip in type a major bend of the eastern thrust front of the eastern cordillera near its junction with the merida andes coincides with a local deflection of the stress regime sw ne compression suggesting local accommodation of the thrust belt to a rigid indenter in this area the second region includes the sw pacific coast of colombia and ecuador currently under the influence of the nazca plate in this area approximately e w compression is mainly reverse in type it deflects to wsw ene in the northern andes south of 4 n where it is accommodated by right lateral displacement of the romeral fault complex and the eastern front of the northern andes the third and most complex region is the area of the triple junction between the south american nazca and caribbean plates it reveals two major stress regimes both mainly strike slip in type the first regime involves sw ne compression related to the interaction between the nazca and caribbean plates and the panama micro plate typically accommodated in an e w left lateral shear zone the second regime involves nw se compression mainly related to the interaction between the caribbean plate and the north andes block which induces left lateral displacement on the uramita and romeral faults north of 4 n deep seismicity about 150 170 km concentrates in the bucaramanga nest and cauca valley areas the inversion reveals a rather homogeneous attitude of the minimum stress axis which dips towards the e this extension is consistent with the present plunge of the nazca and caribbean slabs suggesting that a broken slab may be torn under gravitational stresses in the bucaramanga nest this model is compatible with current blocking of the subduction in the western northern andes inhibiting the eastward displacement of slabs which are forced to break and sink in to the asthenosphere under their own weight

shear wave anisotropy in northwestern south america and its link to the caribbean and nazca subduction geodynamics to investigate the subduction dynamics in northwestern south america we measured sks and slab related local s splitting at 38 seismic stations comparison between the delay times of both phases shows that most of the sks splitting is due to entrained mantle flow beneath the subducting nazca and caribbean slabs on the other hand the fast polarizations of local s waves are consistently aligned with regional faults which implies the existence of a lithosphere confined anisotropy in the overriding plate and that the mantle wedge is not contributing significantly to the splitting also we identified a clear change in sks fast directions at the trace of the caldas tear 5 n which represents a variation in the subduction style to the north of 5 n fast directions are consistently parallel to the flat subduction of the caribbean plate panama arc beneath south america while to the south fast polarizations are subparallel to the nazca south america subduction direction a new change in the sks splitting pattern is detected at 2 8 n which is related to another variation in the subduction geometry marked by the presence of a lithosphere scale tearing structure named here as malpelo tear in this region ne sw oriented sks fast directions are consistent with the general dip direction of the underthrusting of the carnegie ridge beneath south america further inland this ne sw trending mantle flow continues beneath the eastern cordillera of colombia and merida andes of venezuela finally our results suggest that the subslab mantle flow in northwestern south america is strongly controlled by the presence of lithospheric tearing structures

crustal and upper mantle structure in the amazon region brazil determined with broadband mobile stations 1 three broadband stations operated from march 1997 to september 1998 in the amazon region north of manaus brazil which including the iris station ptga pitinga covered an area of roughly 60 200 km applying the receiver function technique to determine the crustal structure there is evidence for an increase of the moho depth from about 38 km below the amazon basin to approximately 48 km north of the basin in addition we analyzed the polarization of sks waves to determine anisotropy the observed splitting parameters in a majority of the data set can be accounted for by a layer of axial symmetric anisotropy with the fast direction oriented roughly n110e this direction cannot be associated with the present day plate motion of the south american plate however the anisotropy may be related to a lithospheric fabric that was generated during the early tectonic history of the amazonian craton

microseismicity and focal mechanisms of the intermediate depth bucaramanga nest colombia the bucaramanga nest of intermediate depth seismicity in northern colombia was investigated from microearthquakes m sub b less than or equal to4 3 recorded from june 20 to july 6 1979 by a local array of 12 digital three component seismographs of 161 events studied 142 are from the nest and 19 define a thin wadati benioff w b zone mainly from 110 to 190 km depth the w b zone part of the bucaramanga segment strikes n10 sup 0 e with a dip increasing wtih depth from 30 sup 0 e to near vertical beneath the nest the 89 best located events from within the nest define a volume of about 8 km nw by 4 km ne by 4 km depth centered at 161 km depth with relative location accuracies of 0 5 to 1 0 km double couple fault plane solutions of 59 events were determined from an objective procedure using p wave polarities and s p amplitude ratios there is no dominant trend in focal mechanisms nor any reasonable correlation with regional stress patterns or previously determined mechanisms from teleseismic data the subduction and assimilation of a linear feature such as an island arc undergoing a phase transformation may explain the existence of the bucaramanga nest however the extraordinarilymore high rate of seismicity suggests that the nest may be moving rate several orders of magnitude faster than the rate of relative plate motion alone the intense seismicity and variation in focal mechanisms suggest that the nest represents some aspect of stress release associated with the ascent of fluids perhaps brine or magma that will ultimately result in the initiation of volcanic activity over this recently emplaced w b zone copyright american geophysical union 1987 less

source spectra of great earthquakes teleseismic constraints on rupture process and strong motion short period body waves recorded at teleseismic distances from great earthquakes provide information about source rupture processes and strong motions first we examine mostly wwssn records of 19 earthquakes of moment magnitude m w of 6 5 to 9 5 four parameters are measured from the short period p wave train the maximum amplitude the period at maximum amplitude the time between the first arrival and when the maximum amplitude is attained and coda length an extension m b of the teleseismic magnitude m b is defined using the maximum amplitude of the entire short period p wave rather than the amplitude achieved in the first few p wave cycles a least squares fit to the data yields the following relationship between m b and m w m b 0 53 m w 2 70 for m w 6 5 to 9 5 the time from the first arrival until the maximum amplitude is achieved and the coda length are roughly proportional to m w but are further interpreted by a simple asperity model of the rupture process these data support that short period waves are on average generated preferentially in the same regions of the fault plane as long period waves with periods of 10 to 50 sec we analyze the spectra of short and intermediate period teleseismic gdsn records for seven earthquakes with m w s of 6 4 to 7 8 and hand digitized short period wwssn records of the 1971 san fernando earthquake significant differences exist between the spectra of different events due partly to variations in tectonic setting or seismic coupling using the digital data we also investigate the relationship between time domain amplitude and spectral amplitude for short period p waves from our empirical relation between spectral amplitude and time domain amplitude we estimate the spectral amplitudes implied by the m b data we compare our results to the 2 and gusev spectral models neither model can completely represent the data nevertheless we consider the 2 model a useful reference model for comparing different events the average source spectrum of six large events with m w 7 4 to 7 8 does not have the spectral structure suggested by gusev an application to strong motion modeling is presented in which a 1971 san fernando teleseismic short period record is summed up to simulate teleseismic records produced by five great earthquakes the summation procedure matches the moment of the event to be simulated and includes rupture propagation fault plane roughness and randomness the m b data provide an important constraint on the summation procedures thus constrained this summation procedure can be more confidently used with near field strong motion records as green s functions to predict strong motions from great earthquakes

the role of hydraulic fractures and intermediate depth earthquakes in generating subduction zone magmatism the presence of magmatism and intermediate depth 70 300 km deep seismicity at subduction zones is at first sight surprising magmatism is unexpected because the subduction of cool oceanic lithosphere makes these regions the coldest in the mantle the current model for subduction zone magmatism is that water released from the subducting slab enters the relatively warm mantle wedge leading to a reduction in melting temperature and magmatism1 2 3 4 but there is a problem with this scheme because it is thought that water cannot leave the slab by porous flow to enter the wedge the occurrence of intermediate depth earthquakes is surprising because of the inhibitory effect of the very high frictional stress on faults expected from the high pressure at these depths one proposal put forward to explain intermediate depth seismicity is that high pore pressure might facilitate faulting by decreasing the friction5 6 7 the hypothesis presented here is that non percolating water provides the high pore pressure that the consequent faulting temporarily interconnects the water pores and when a sufficient vertical height of water is interconnected a hydrofracture is produced which transports the water out into the mantle wedge thereby generating subduction zone magmatism

the angular velocities of the plates and the velocity of earth s centre from space geodesy summary using space geodetic observations from four techniques gps vlbi slr and doris we simultaneously estimate the angular velocities of 11 major plates and the velocity of earth s centre we call this set of relative plate angular velocities geodvel for geodesy velocity plate angular velocities depend on the estimate of the velocity of earth s centre and on the assignment of sites to plates most geodetic estimates of the angular velocities of the plates are determined assuming that earth s centre is fixed in an international terrestrial reference frame itrf and are therefore subject to errors in the estimate of the velocity of earth s centre in itrf2005 and itrf2000 earth s centre is the centre of mass of earth oceans and atmosphere cm the velocity of cm is estimated by slr observation of lageos s orbit herein we define earth s centre to be the centre of mass of solid earth ce we determine the velocity of ce by assuming that the portions of plate interiors not near the late pleistocene ice sheets move laterally as if they were part of a rigid spherical cap the geodvel estimate of the velocity of ce is likely nearer the true velocity of cm than are the itrf2005 and itrf2000 estimates because 1 no phenomena can sustain a significant velocity between cm and ce 2 the plates are indeed nearly rigid aside from vertical motion and 3 the velocity of cm differs between itrf2005 and itrf2000 by an unacceptably large speed of 1 8 mm yr 1 the velocity of earth s centre in geodvel lies between that of itrf2000 and that of itrf2005 with the distance from itrf2005 being about twice that from itrf2000 because the geodvel estimates of uncertainties in plate angular velocities account for uncertainty in the velocity of earth s centre they are more realistic than prior estimates of uncertainties geodvel differs significantly from all prior global sets of relative plate angular velocities determined from space geodesy for example the 95 per cent confidence limits for the angular velocities of geodvel exclude those of revel sella et al for 34 of the 36 plate pairs that can be formed between any two of the nine plates with the best constrained motion the median angular velocity vector difference between geodvel and revel is 0 028 myr 1 which is up to 3 1 mm yr 1 on earth s surface geodvel differs the least from the geodetic angular velocities that altamimi et al determine from itrf2005 geodvel s 95 per cent confidence limits exclude 11 of 36 angular velocities of altamimi et al and the median difference is 0 015 myr 1 geodvel differs significantly from nearly all relative plate angular velocities averaged over the past few million years including those of nuvel 1a the difference of geodvel from updated 3 2 myr angular velocities is statistically significant for all but two of 36 angular velocities with a median difference of 0 063 myr 1 across spreading centres eight have slowed down while only two have sped up we conclude that plate angular velocities over the past few decades differ significantly from the corresponding angular velocity averaged over the past 3 2 myr

a global survey of stress orientations in subducting slabs as revealed by intermediate depth earthquakes summary we examine a variety of mechanisms that have been proposed as contributors to the stress fields expressed as intermediate depth seismicity in subducting slabs to this end we study principal stress orientations for a global data set of 1900 intermediate depth focal solutions determining the patterns of events characterized primarily by downdip compression downdip tension or neither in regions dominated by downdip principal stresses we find that conjugate stress axes exhibit preferential slab normal orientations furthermore we observe a clear trend of thermal control in which colder slabs exhibit greater components of downdip compression while warmer slabs display greater downdip tension in those regions not dominated by downdip principal stresses a significant number of events exhibit lateral stresses in the form of subhorizontal principal axes in the plane of the slab we conclude that the evidently complementary roles played by lithospheric age and subduction rate in constraining stress regimes support thermomechanical and petrological buoyancy models for control of intermediate depth stresses moreover observed lateral stresses support the traditional model of a squeezed ping pong ball and stress patterns overall are consistent with some influence by reactivated fossil faults

creation of the cocos and nazca plates by fission of the farallon plate abstract throughout the early tertiary the area of the farallon oceanic plate was episodically diminished by detachment of large and small northern regions which became independently moving plates and microplates the nature and history of farallon plate fragmentation has been inferred mainly from structural patterns on the western pacific plate flank of the east pacific rise because the fragmented eastern flank has been subducted the final episode of plate fragmentation occurred at the beginning of the miocene when the cocos plate was split off leaving the much reduced farallon plate to be renamed the nazca plate and initiating cocos nazca spreading some oligocene farallon plate with rifted margins that are a direct record of this plate splitting event has survived in the eastern tropical pacific most extensively off northern peru and ecuador small remnants of the conjugate northern rifted margin are exposed off costa rica and perhaps south of panama marine geophysical profiles bathymetric magnetic and seismic reflection and multibeam sonar swaths across these rifted oceanic margins combined with surveys of 30 20 ma crust on the western rise flank indicate that i localized lithospheric rupture to create a new plate boundary was preceded by plate stretching and fracturing in a belt several hundred km wide fissural volcanism along some of these fractures built volcanic ridges e g alvarado and sarmiento ridges that are 1 2 km high and parallel to absolute farallon plate motion they closely resemble fissural ridges described from the young western flank of the present pacific nazca rise ii for 1 2 m y prior to final rupture of the farallon plate perhaps coinciding with the period of lithospheric stretching the entire plate changed direction to a more easterly nazca like course after the split the northern cocos part reverted to a northeasterly absolute motion iii the plate splitting fracture that became the site of initial cocos nazca spreading was a linear feature that at least through the 680 km of ruptured oligocene lithosphere known to have avoided subduction did not follow any pre existing feature on the farallon plate e g a fracture zone trail of a transform fault iv the margins of surviving parts of the plate splitting fracture have narrow shoulders raised by uplift of unloaded footwalls and partially buried by fissural volcanism v cocos nazca spreading began at 23 ma reports of older cocos nazca crust in the eastern panama basin were based on misidentified magnetic anomalies there is increased evidence that the driving force for the 23 ma fission of the farallon plate was the divergence of slab pull stresses at the middle america and south america subduction zones the timing and location of the split may have been influenced by i the increasingly divergent northeast slab pull at the middle america subduction zone which lengthened and reoriented because of motion between the north america and caribbean plates ii the slightly earlier detachment of a northern part of the plate that had been entering the california subduction zone contributing a less divergent plate driving stress and iii weakening of older parts of the plate by the galapagos hotspot which had come to underlie the equatorial region midway between the risecrest and the two subduction zones by the late oligocene

tidal coulomb failure stresses in the northern andean intermediate depth seismic clusters implications for a possible correlation between tides and seismicity abstract a recent statistical analysis of the relationship between tides and seismic activity in colombia has suggested the existence of correlation anomalies for the case of intermediate depth events in the bucaramanga nest and the cauca cluster moncayo et al 2019 in this work we explore in detail the hypothesis that tides may be triggering seismic activity in these regions and extend the analysis to two other seismic clusters in northern central south america specifically in the areas of el puyo ecuador and pucallpa peru for this purpose we use the available focal mechanism information for seismic events at these locations and calculate for each event the tidal coulomb failure stress tcfs as obtained from estimations of the tidal strain tensor tidal strains are computed considering the earth body tides and the effect of the ocean tidal loading since our purpose is to elucidate the role of tides in earthquake nucleation calculations of the tcfs are conducted not only for the time of earthquake but also for the time of the closest maximum strain within a window of a few hours before the events our results tend to support the hypothesis that tidal stresses are contributing to earthquake generation in all the studied areas this trend is especially stronger when tcfs are calculated at pre earthquake times the physical explanation for the positive contribution of tides to earthquake triggering in intermediate depth clusters may lie in the fact that a wide diversity of fault plane orientations is possible within a relatively small volume of subducted lithosphere so making the tides more likely to help loosening up the blocks at the fault plane to promote slip

s cl and f degassing as an indicator of volcanic dynamics the 2001 eruption of mount etna 1 the recent eruption of mount etna july 2001 offered the opportunity to analyze magma derived volatiles emitted during pre and syn eruptive phases and to verify whether their composition is affected by changes in volcanic dynamics this paper presents the results of analyses of f cl and s in the volcanic plume collected by filter packs and interprets variations in the composition based on contrasting solubility in magmas a rayleigh type degassing mechanism was used to fit the acquired data and to estimate henry s solubility constant ratios in etnean basalt this model provided insights into the dynamics of the volcano abundances of sulfur and halogens in eruptive plumes may help predict the temporal evolution of an ongoing effusive eruption

shear wave splitting beneath western united states in relation to plate tectonics we have examined shear wave splitting in teleseismic shear waves from 26 broadband stations in the western united states fast polarization directions and delay times t show spatial variations that are coherent within geologic provinces stations located near the san andreas fault show clear evidence for fault parallel anisotropy in the crust and upper mantle 115 125 km thickness this can be explained by the finite strain associated with the relative plate motion between the north american and pacific plates the lateral extent of this strain field is probably narrow to the west because stations 55 km west of the san andreas fault do not show fault parallel anisotropy in southern california station lac located 80 km east of the san andreas fault shows large fault parallel anisotropy this suggests that the pacific north american plate boundary in the mantle might be displaced to the east in southern california a deeper e w oriented fast direction of anisotropy underlies the fault parallel anisotropic layer in the vicinity of the san andreas fault an e w fast feature is also present beneath the western basin and range and the foothills of the sierra nevada although local variations are present the magnitude of delay times suggests that this feature resides in the asthenosphere we interpret this feature as the asthenospheric flow in the slabless window left behind the farallon plate the flow induced anisotropy may partially be frozen in at shallow depths station orv is located near the southern edge of the gorda slab where no anisotropy is detected the absence of anisotropy at this location could therefore mark a boundary between farallon associated flow and regions where e w oriented asthenospheric flow did not occur the lack of evidence for ne sw fast orientation within the walker lane shear belt of western nevada suggests that this crustal feature does not extend into the mantle or that is not as well developed as that beneath the san andreas fault stations located over the young subducting gorda plate mark a change in the fast direction to nearly ne sw this direction aligns well with the maximum compressive stress direction in the overlying north american plate and the ne sw directed internal shearing of the gorda plate the anisotropic thicknesses calculated from delay times suggest roughly double that expected for purely lithospheric contributions this implies that the anisotropic thickness may include some of the asthenosphere alternatively using a higher anisotropy of 8 can bring thicknesses in line with other measures of lithospheric thicknesses the correspondence between the fast directions and the present plate tectonic deformations suggest that mapping upper mantle deformation through seismic anisotropy is a viable method and that asthenospheric flow may be a significant contributor to seismic anisotropy

gas composition of popocatepetl volcano between 2007 and 2008 ftir spectroscopic measurements of an explosive event and during quiescent degassing abstract on december 1 2007 the solar absorption infrared spectra of the popocatepetl volcanic plume was recorded during an eruptive event and complementarily on november 17 2008 the passive quiescent degassing was measured from the same site a portable ftir spectrometer with a scanning mirror for fast tracking of the sun provided the flexibility quality and simplicity needed for field deployment slant columns of the gases so 2 hcl hf and sif 4 were retrieved and strong differences could be observed when comparing gas ratios in both time periods during the explosive eruption the so 2 hcl ratio was three times greater and the hf hcl ratio was slightly smaller than during passive degassing while the ratios among so 2 hcl hf and sif 4 describe the chemical composition of the volcanic gas mixture the sif 4 hf ratio provides information about the equilibrium temperatures of the stored gases which in this study were calculated at 150 and 185 c for the explosive and quiescent degassing episodes respectively we conclude that cooling of lava domes in the crater precedes vulcanian explosions as suggested by schaaf et al 2005 based on so 2 flux grutter et al 2008 and measurements and data from the november 2008 event the average fluxes for hcl hf sif 4 and f through quiescent degassing are estimated to be 204 22 7 9 8 and 31 7 tons day respectively these values are similar to those reported by love et al 1998 more than 10 yrs ago

degassing of halogens from basaltic volcanism insights from volcanic gas observations abstract the currently available data set of s cl f abundances in volcanic gas plumes and high temperature fumarolic gas samples from basaltic volcanism is reviewed here in the attempt to derive constraints on the modes of halogen degassing from mafic silicate melts apart from large volcano to volcano variations reflecting remarkable differences in volatile abundances in the source magmas each of the explored volcanoes displays large changes of so2 hcl and so2 hf ratios with the style of volcanic activity with hcl hf staying fairly constant halogen abundances are low and so2 hcl and so2 hf are high when fresh volatile rich magmas sustain degassing as during explosive eruptions at the onset of eruptive cycles or shortly before paroxysmal events low so2 hcl and so2 hf ratios are instead characteristic of late stages of volcanic degassing typically being observed in the concluding stages of basaltic eruptions or during periods of reduced magma supply at persistently degassing volcanoes these observations are taken as evidence of halogens being less keen to enter the gas phase relative to s during degassing of basaltic magmas and quantitatively interpreted in light of a rayleigh type open system degassing model the model though simple quantitatively reproduces the range of volcanic gas compositions observed at basaltic volcanoes worldwide and allows prediction of vapour melt partitioning contrasts of factors 9 and 36 for the volatile couples s cl and s f respectively these predictions require validation from appropriately designed experiments of halogen partitioning between magmatic vapours and silicate melts over a range of p t x conditions

a total volatile inventory for masaya volcano nicaragua nerc project magma dynamics at persistently degassing basaltic volcanoes a novel approach to linking volcanic gases and magmatic volatiles within a physical model ne f004222 1 and ne f005342 1

chemical patterns of erupting silicic magmas and their influence on the amount of degassing during ascent 1 we present a chemical model of magma degassing based on nine volatile species part of s o h c fe bearing rhyolitic melts it is based on equilibrium closed system degassing and does not take in account the crystallization of mineral phases for given initial conditions at depth the model calculates the gas composition as pressure decreases as well as physical variables controlling conduit flow we conducted a parametric study of degassing by varying initial conditions at depth within the range typical of arc magmas and characterizing the chemical evolution of the volatile components during the ascent to the surface the resulting complex patterns of chemical changes are controlled by four groups of interrelated parameters 1 redox state fh2 and h2s so2 ratio in the gas phase 2 gas amount 3 melt water content and fh2o and 4 relative melt sulfur content fco2 and c s ratio in the gas phase the other degrees of freedom temperature initial pressure and melt iron content alter the amplitude of the chemical fluctuations during ascent but do not change the general degassing patterns when degassing starts at depth volatile chemistry has little effect on the amount of degassing as measured by the evolution of porosity magma expansion at shallow levels however can be greatly affected by degassing chemistry

seismic anisotropy around subduction zones insights from three dimensional modeling of upper mantle deformation and sks splitting calculations 1 inferring the circulation of the mantle around subducting plates from surface measurements of shear wave splitting patterns remains to date elusive to assist the interpretation of the seismic signal and its relation with the mantle circulation pattern we present a new methodology to compute the seismic anisotropy directly from the flow in the upper mantle of 3 d numerical models of earth like subduction this computational strategy accounts for the non steady state evolution of subduction zones yielding mantle fabrics that are more consistent with the deformation history than previously considered in the subduction models a strong mantle fabric develops throughout the upper mantle with a magnitude of the anisotropy that is proportional to the amount of subduction and is independent of the subduction rate the sub slab upper mantle is characterized by two domains with different fabrics at shallow depth the mantle entrained with the subducting slab develops trench perpendicular directed anisotropy due to simple shear deformation while in the deeper mantle slab rollback induces pure shear deformation causing trench parallel extension and fast seismic directions subducting plate advance favors the development of the fabric in the entrained mantle domain while slab retreat increases the trench parallel anisotropy in the deeper upper mantle in the deeper domain the strength of the fabric is proportional to the horizontal divergence of the flow and weakens from the slab edges toward the center as such strong trench parallel anisotropy forms below retreating and relatively narrow slabs or at the margins of wider plates the synthetic sks splitting patterns calculated in the fore arc are controlled by the magnitude of the anisotropy in the upper domain with trench perpendicular fast azimuths in the center of large plates and trench parallel toward the plate edges instead above relatively narrow retreating slabs 600 km and low subduction partitioning ratio spr azimuths are trench parallel due to the strong anisotropy in the lower sub slab domain in all models the anisotropy in the back arc and on the sides of the subducting plate is respectively trench perpendicular and sub parallel to the return flow at depth results from our regional scale models may help to infer the flow and composition of the upper mantle by comparison with the wide range of subduction zones seismic data observed globally

subduction of the eastern panama basin and seismotectonics of northwestern south america spreading along the cocos nazca plate boundary since the breakup of the farallon plate in the miocene has resulted in the formation of the panama basin and a complex interaction of plates in and near northwestern south america current plate boundaries have been defined and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes considering only well located events and through focal mechanism determinations the existence of relict plate boundaries bathymetric features and the panamanian isthmus has affected the subduction process of the nazca plate beneath south america and determined the present day configuration of the subducting lithospheric plate there is no single triple junction separating the caribbean south american and nazca plates instead the panamanian isthmus and surrounding areas are accommodating east west compression and a lesser degree of north south compression along a series of thrust faults striking nw to ne and the andean ranges of ecuador colombia and venezeula are moving as a block nne relative to the rest of the south american plates along a system of faults following the front of the eastern cordillera the subducted portions of the panama basin and old farallon plate have become segmented into three pieces recognized in this study from north to south they are 1 a bucaramanga segment continuous with the caribbean seafloor northwest of colombia 2 a cauca segment continuous with oceanic crust nazca plate currently being subducted beneath south america at the colombia ecuador trench and 3 an ecuador segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern peru the segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted

a scaling model for quantification of earthquakes in and near japan abstract a simple method is developed to determine seismic moments of earthquakes the method is qualified through criteria such as simplicity of calculations coverage of wide magnitude range and insensitivity to detailed instrumental response the method is applied to 163 major earthquakes which occurred underneath japan and the japan sea in the time from 1926 to 1977 magnitudes of these earthquakes which have been determined by the japan meteorological agency m jma cover the range from 4 3 to 7 5 at first source spectra are analyzed through a very simple way introducing two new parameters characteristic period t c and seismic moment factor m c the former is defined as an average value of apparent periods of seismic waves with the maximum trace amplitude at many stations the latter is an average of products of maximum trace amplitude and its apparent period multiplied by epicentral distance it is shown that t c corresponds to the period of the corner frequency of an earthquake and m c to the seismic moment density at the period of t c a scaling model of earthquake source spectra is presented which satisfies the empirical relations between the surface wave magnitude m s and m jma and m jma and the body wave magnitude m b those relations are independent of the gutenberg and richter relation between m s and m b because m jma is determined from maximum amplitudes of seismic waves with a period of about 4 sec the static seismic moment of each earthquake can be estimated from calculated m c using the source spectra of the scaling model seismic moments of 18 earthquakes determined by conventional analyses from near and or far field observations are consistent with static seismic moments thus estimated over the range from 2 10 23 to 3 10 27 dyne cm this shows the potential in practice of the present method especially in the routine processing of seismic data

earthquake source time function from coherent and incoherent rupture abstract theoretical study has been made investigating the seismic source spectrum generated from a coherent and incoherent rupture an earthquake is modeled by a finite propagating rupture on a fault plane where fault heterogeneities fault patches are randomly distributed the dislocation velocity of such a fracture is assumed to be approximated by a stochastic process of random impacts of particles obeying brownian motion the parameters of the present stochastic source are seismic moment fault dimension fault patch intensity and patch fracture time the model predicts two corner frequencies one originates from the fault finiteness and the other from the fracturing of fault patches the seismic source spectrum from the model consequently shows distinct frequency dependence of 0 2 2 with increasing angular frequency where is about 1 0 the seismic moment is controlled by an average dislocation on the fault and by the fault dimension the short period spectrum which is much more abundant than that of the ordinary deterministic models is controlled by the product of the fault patch intensity and the square root of their total number the 2 high frequency asymptote of the theoretical spectrum is in conformity with the white acceleration spectrum usually found in the literature and it guarantees the finite total energy of the rupture process

receiver functions and crustal structure of the northwestern andean region colombia we used the receiver function technique to deduce crustal thickness beneath the northwestern andean system using data from the permanent seismic network of colombia combined with some of the iris and ctbto stations in colombia and ecuador the estimation of crustal thickness was made using the primary p to s conversion and crustal reverberations the bulk crustal vp vs ratio was constrained using a crustal thickness versus vp vs stacking method in addition to estimations using a time to depth conversion technique based on results of a modified wadati diagram analysis we observed a wide range of crustal thicknesses including values around 17 km beneath the malpelo island on the pacific ocean 20 to 30 km at the coastal pacific and caribbean plains of colombia 25 to 40 km beneath the eastern plains and foothills 35 km beneath the western cordillera 45 km at the magdalena river intermountain valley 52 to 58 km under the northern central cordillera and reaching almost 60 km beneath some of the volcanoes of the southern cordilleran system of colombia crustal thickness can be slightly greater than 60 km beneath the plateau of the eastern cordillera the values of vp vs are particularly high for some of the stations on the volcanic centers reaching values above 1 79 probably related to the addition of mafic materials to the lower crust and in the plateau of the eastern cordillera near bogota where we speculate about the possibility of crustal seismic anisotropy associated with shear zones