sub surface structure of la soufriere of guadeloupe lava dome deduced from a ground based magnetic survey abstract in this study we present the analysis and interpretation of a new ground magnetic survey acquired at the soufriere volcano on guadeloupe island observed short wavelength magnetic anomalies are compared to those predicted assuming a constant magnetization within the sub surface the good correlation between modeled and observed data over the summit of the dome indicates that the shallow sub surface displays relatively constant and high magnetization intensity in contrast the poor correlation at the base of the dome suggests that the underlying material is non to weakly magnetic consistent with what is expected for a talus comprised of randomly oriented and highly altered and weathered boulders the new survey also reveals a dipole anomaly that is not accounted for by a constant magnetization in the sub surface and suggests the existence of material with decreased magnetization beneath the soufriere lava dome we construct simple models to constrain its dimensions and propose that this body corresponds to hydrothermally altered material within and below the dome the very large inferred volume for such material may have implications on the stability of the dome

diffuse emission of co2 from showa shinzan hokkaido japan a sign of volcanic dome degassing two soil co2 efflux surveys were carried out in september 1999 and june 2002 to study the spatial distribution of diffuse co2 degassing and estimate the total co2 output from showa shinzan volcanic dome japan seventy six and 81 measurements of co2 efflux were performed in 1999 and 2002 respectively covering most of showa shinzan volcano soil co2 efflux data showed a wide range of values up to 552 g m 2 d 1 carbon isotope signatures of the soil co2 ranged from 0 9 to 30 9 suggesting a mixing between different carbon reservoirs most of the study area showed co2 efflux background values during the 1999 and 2002 surveys b 8 2 and 4 4 g m 2 d 1 respectively the spatial distribution of co2 efflux anomalies for both surveys showed a good correlation with the soil temperature indicating a similar origin for the extensive soil degassing generated by condensation processes and fluids discharged by the fumarolic system of showa shinzan the total diffuse co2 output of showa shinzan was estimated to be about 14 0 15 6 t d 1 of co2 for an area of 0 53 km2

integrated processing of muon radiography and gravity anomaly data toward the realization of high resolution 3 d density structural analysis of volcanoes case study of showa shinzan lava dome usu japan we have developed an integrated processing method for muon radiography and gravity anomaly data for determining the 3 d density structures of volcanoes with a higher spatial resolution than is possible by conventional gravity inversion in the present paper we demonstrate the performance of the proposed method by performing numerical tests using synthesized data and we present the results obtained by applying the proposed method to a volcano showa shinzan lava dome hokkaido japan we obtained the detailed shape of a vent beneath the dome and detected the presence of solidified dense lava near the top of the dome the results demonstrate the advantage of a hybrid measurement based on both gravity and muon radiography for imaging small structures with sizes of a few hundreds of meters near the surface of a volcano

mantle transition zone beneath eurasia mantle pds fig 1 converted phases are detected in the records of 11 seismograph stations in easternmost russia and china these data reveal neither a strong depression on the 660 km discontinuity nor a layer of partial melting atop the 410 km discontinuity that were found previously beneath this region in studies using long period underside sh reflections and multiple scs reverberations apparently a significant deepening of the 660 km discontinuity occurs only if the subducted plate penetrates the discontinuity which is not the case in the study region most estimates of thickness of the mantle transition zone mtz in eurasia based on the pds data are a few kilometres larger than the standard value 250 km and suggest that the average thickness of the mtz beneath eurasia is about 10 kilometres more than beneath the surrounding oceans

mantle discontinuities and temperature under the north american continental keel a ubiquitous feature of upper mantle seismic velocity models has been the presence of high velocity keels beneath stable continental interiors1 2 3 4 5 uncertainty remains however regarding the maximum depth to which continental keels extend the degree to which they have cooled the mantle that surrounds them and their role in mantle flow here we investigate thermal anomalies across the eastern margin of the north american continental keel by imaging the seismic discontinuities at depths of 410 and 660 km with compressional to shear converted waves recorded by a 1 500 km long seismometer deployment in the eastern united states the thickness of the transition zone the region nominally between depths of 410 and 660 km and the depth to the 410 km discontinuity indicate that cold keel material and sub keel downwellings must be largely confined to the upper mantle and may impinge on the transition zone only in localized regions and with thermal anomalies of less than 150 k a 20 km depression of the 660 km discontinuity to the south of the westernmost stations coincides with a region of fast velocity in the deep transition zone2 and may be associated with the remnants of the subducted farallon plate1 2 4

global mapping of topography on the 660 km discontinuity long period precursors to the ss seismic phase are used to produce global maps of topography for the discontinuity at 660 km depth in the upper mantle these maps indicate discontinuity depth variations of up to 30 km and suggest a correlation between regional depressions in the 660 km discontinuity and subduction zones a result more consistent with models in which the subducting slabs are deflected horizontally at the discontinuity than with models of unhindered slab penetration into the lower mantle

volcanomagnetic signals during the recent popocatepetl mexico eruptions and their relation to eruptive activity abstract an interdisciplinary approach correlating magnetic anomalies with composition of the ejecta in each eruption as well as with seismicity was used to study the effect of magmatic activity on the local magnetic record at popocatepetl volcano located 65 km southeast of mexico city eruptions began on december 1994 and have continued with dome growth and ash emissions since then the tlamacas tla geomagnetic total field monitoring station located 5 km away from popocatepetl s crater was installed in december 1997 in order to detect magnetic anomalies induced by this activity spatial correlation and weighted difference methods were applied to detect temporal geomagnetic anomalies using tla s record and the teoloyucan magnetic observatory as a reference station weighted differences were applied to cancel the effects of non vulcanogenic external field variations magnetic anomalies over a 2 year time span were classified into four types correlating them with geochemical seismic and visual monitoring of the volcanic activity magnetic anomalies are believed to be caused by magma injection and gas pressure build up which is sensitive to vent morphology and clearing during eruption although some anomalies appear to be thermally related changes in the stress field are very important most magnetic anomalies are short time signals that reverse to baseline level decreasing anomalies 0 5 to 6 8 nt precede eruptions by 1 8 days the presence of a mafic magmatic component was determined by mineral examination and silica and magnesium analyses on the ejecta from the 1997 1999 eruptions whole rock analyses ranged from dacitic 65 sio 2 to andesitic 57 sio 2 with 2 6 6 mgo the higher mgo lower silica samples contain forsteritic olivine fo90 sio 2 does not increase and mgo does not increase with time suggesting ascent of small magma pulses which are consistent with the magnetic data

magma prospecting in usu volcano hokkaido japan using magnetotelluric soundings abstract we carried out magnetotelluric mt soundings across usu volcano hokkaido japan which has been active since 1663 the objective was to image an intrusion expected below the formation of 180m high cryptodomes formed during the 1977 78 eruption a ne sw two dimensional resistivity model has been obtained from the inversion in the frequency range from 0 01 to 10 000 hz the major features of the resistivity section are as follows 1 a 50 m thick surficial resistive 1000 10 000 m layer corresponding to the lava layer 2 a relatively conductive 10 1000 m layer from the surface to 200 m depth corresponding to quaternary sedimentary and volcanic rocks 3 beneath the quaternary formations a thick 2 4 km conductive 1 500 m layer of altered tertiary volcanics distributed widely in the volcanic edifice 4 a resistive block 500 1 000 m in the tertiary layer from 200 to 400 m in depth beneath the summit crater forming a striking contrast to the rest of the conductive tertiary layer and probably indicating a cooling intrusion at shallow depth 5 pre tertiary formations consisting of a high resistivity 1000 10 000 m layer overlying a conductive layer with the top of the pre tertiary becoming shallow to the northeast and 6 no deep conductive anomalies corresponding to a magma chamber inferred from petrological studies

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

wet subduction versus cold subduction 1 in situ x ray diffraction study of post spinel transformation in hydrous peridotite 2 wt h2o indicates that the phase boundary is shifted to higher pressures by 0 6 gpa relative to anhydrous peridotite at 1473 k whereas it shows no obvious shift at high temperature around 1873 k a linear equation for the boundary is p gpa 0 002 t k 26 3 which is applicable for temperatures below 1800 k the present data shows that a significant part of the depressions seen in the 660 km seismic discontinuity may be affected by existence of water it is also well resolved that the olivine wadsleyite phase transformation corresponding to the 410 km seismic discontinuity is shifted to lower pressures by 1 2 gpa by the addition of water thus the topography of seismic discontinuities in the transition zone associated with slabs can be attributed not only to cold subduction but also wet subduction

constraining seismic velocity and density for the mantle transition zone with reflected and transmitted waveforms we examine stacks of several seismic phases having different sensitivities to mantle transition zone structure when analyzed separately underside p and s reflections pdp and sds are suggestive of very different structures despite similar raypaths and data coverage by stacking the radial component of pdp rather than the vertical pdp we show that this difference does not result from interference from other more steeply inclined phases such as pkp and ppdpdiff in general stacks of p to s converted phases pds appear to lack evidence of a 520 km discontinuity when examined without other phases when these phases and stacked topside p reflections ppdp are analyzed jointly using a nonlinear inversion method consistent but nonunique seismological models emerge these models show that a discontinuity at 653 km depth has smaller contrasts in density and velocity than found in most previous studies a sub 660 gradient can account for the majority of this difference a 1 6 0 5 p velocity contrast and a 2 2 0 3 density contrast at 518 km depth without a s velocity contrast can explain the lack of a p520s together with robust pp520p and s520s phases for models parameterized with a finite thickness for each discontinuity the 410 km discontinuity is consistently 3 times thicker than the 660 km discontinuity

3d density modeling with gravity and muon radiographic observations in showa shinzan lava dome usu japan we performed three dimensional density modeling of showa shinzan lava dome usu japan by joint inversion of the gravity anomaly and recently obtained muon radiography data our multilayer emulsion muon detector significantly reduces the background noise in our measurements of the muon flux through the dome the high quality muon data enables us to more accurately reconstruct the density structure of the lava dome compared with our own previous work we find that the lava dome consists of a cylindrical column of massive lava with a diameter of 300 m and that there is no evidence of magma intrusion in the shallow part of the plateau located east of the dome

seismological significance of the 1977 1978 eruptions and the magma intrusion process of usu volcano hokkaido abstract an earthquake swarm and the major pumice eruptions in august 1977 which followed marked the start of the dacitic doming activity of usu volcano in southwestern hokkaido japan the sequence of magma intrusion processes was investigated in detail by means of seismological and other geophysical data the distribution of the abundant hypocenters shows clearly an earthquake free zone beneath the summit crater the hypocenters migrated in a manner consistent with the development of the observed asymmetrical surface deformations considered due to magma intrusion into this earthquake free zone the earthquake mechanism solutions are mostly of dip slip type and are interpreted in terms of the doming deformations the existence of earthquake families earthquakes with similar waveforms is the main cause of the peculiar occurrence of earthquakes in space time and magnitude the concept of scattered barriers of different sizes and strengths can explain well the distinct characteristics of the occurrence of the swarm and the observed episodic deformations

imaging mantle transition zone thickness with sds ss finite frequency sensitivity kernels summary we invert differential sds ss traveltime residuals measured from stacked waveforms and finitefrequency sensitivity kernels for topography on the 410 and 660 km discontinuities this approach yields higher resolution images of transition zone thickness than previous stacking methods which simply average smooth over topographic features apparent structure measured using simple stacking is highly dependent upon the bin size of each stack by inverting for discontinuity topography with a variety of bin sizes we can more accurately calculate the true structure the inverted transition zone model is similar to simple stack models with an average thickness of 242 km but the lateral variations in thickness are larger in amplitude and smaller in scale fast seismic velocities in 3 d mantle models such as sb4l18 correlate with areas of thicker transition zone the elongated curvilinear regions of thickened transition zone that occur near subduction zones are narrow and high amplitude which suggests relatively little lateral spreading and warming of subducted lithosphere within the transition zone the anomalously thin transition zone regions are laterally narrow and not broadly continuous if these variations in transition zone thickness are interpreted as thermal in nature then this model suggests significant temperature variations on small lateral scales

geophysical characteristics of dacite volcanism the 1977 1978 eruption of usu volcano abstract the 1977 1978 eruption of usu volcano is discussed from the geophysical standpoint as a classic example of dacite volcanism the activities of dacitic volcanoes are characterized by persistent earthquake swarms and remarkable crustal deformations due to the high viscosity of the magmas the former include shocks felt near the volcanoes and the latter accompany formation of lava domes or cryptodomes the hypocenters of the earthquakes occurring beneath usu volcano have been located precisely their distribution defines an earthquake free zone which underlies the area of doming within the summit crater this zone is regarded as occupied by viscous magma the domings within the summit crater forming the cryptodomes have amounted to about 160 m in addition to uplift they showed thrusting towards the northeast as a result the northeastern foot of the volcano has contracted by about 150 m the relation between crustal deformation and earthquake occurrence is examined and it is found that the abrupt domings are accompanied by the larger earthquakes m 3 4 3 both the seismic activity and the ground deformation are shown to have a unique and common energy source the energy of activities of usu volcano consists of the explosive type the deformation type and the seismic type the second and the third are in parallel with each other in discharges and both energies are complementary to the explosive energy the explosive energy and the seismic energy have been calculated for an explosion sequence and it is concluded that the deformation energy is about 10 times greater than the seismic energy the discharge rate of the seismic energy and the upheaval rates of the cryptodomes have continued to decrease since the outburst of the eruption except for a small increase at the end of january 1978 eruptions are governed not only by the supply of the energies but also by the depth of the magma which has gradually approached the surface the last eruption occurred in october 1978 however the crustal deformations and the earthquake swarms are still proceeding as of january 1980 albeit at a lower rate of activity

subsurface structure of usu volcano japan as revealed by detailed magnetic survey abstract a detailed magnetic survey based on 1018 stations was conducted in the usu volcanic region hokkaido japan from the study the following results were obtained 1 the main body of usu volcano is composed of strongly magnetized basaltic somma lavas and weakly magnetized crater fill 2 the southern rim of somma volcano is situated about 200 m south of the present topographic rim suggesting the existence of the buried horseshoe shaped caldera which is thought to have been formed several thousand years ago 3 parts of the mt showa shinzan lava dome formed in the 1943 1945 activity are still occupied with lavas of high temperature above the curie temperature of the ferromagnetic minerals 4 the root of the lave dome of mt showa shinzan has almost the same diameter as that of the dome exposed on the earth s surface 5 an intrusive body about 200 m in diameter may exist at shallow depth in the roof mountain of mt showa shinzan

cooling rate and thermal structure determined from progressive magnetization of the dacite dome at mount st helens washington our study of a magnetic anomaly associated with the recently active dacite dome at mount st helens suggests that the dome consists of a hot nonmagnetized core surrounded by a cool magnetized carapace and flanking talus the talus does not contribute to the anomaly because its constituent blocks are randomly oriented temporal changes in the magnetic anomaly indicate that the magnetized carapace thickened at an average rate of 0 03 0 01 m d from 1984 to 1986 petrographic and rock magnetic properties of dome samples indicate that the dominant process responsible for these changes is magnetization of extensively oxidized rock at progressively deeper levels within the dome as the rock cools through its blocking temperature rather than subsequent changes in magnetization caused by further oxidation newly extruded material cools rapidly for a short period as heat is conducted outward in response to convective heat loss from its surface the cooling rate gradually declines for several weeks and thereafter the material cools at a relatively constant rate by convective heat loss from its interior along fractures that propagate inward the rate of internal convective heat loss through fractures varies with rainfall snowmelt and large scale fracturing during subsequent eruptive episodes in accordance with a model for solidification of the 1959 lava lake at kilauea iki hawaii we picture the dome s magnetized carapace as being a two phase porous convective zone separated from the nonmagnetized core of the dome by a thin single phase conductive zone as a consequence of the heat balance between the conductive and convective zones the blocking temperature isotherm migrates inward at a relatively constant rate if the dome remains inactive the time scale for its complete magnetization is estimated to be 18 36 years a forecast which can be refined by shallow drilling into the dome and by continuing studies of its growing magnetic anomaly

mobility of long runout rock flows a discrete numerical investigation summary in the perspective of studying the mobility and deposit geometry of long runout flows discrete numerical simulations of model granular avalanches over simple topography are performed analysing the behaviour of the ratio h l depending on the parameters varied in the simulations we show its poor sensitivity to the topographic parameters namely the initial height of the flowing mass and the slope on the contrary a correlation between h l and the frictional properties of the material is established however the existence of variations of the effective friction induced by the dynamics are not reflected by the value of h l as an alternative to the h l description of the flow behaviour we propose a new scaling for the runout distance taking into account the lateral extension l0 of the relief forming the normalized runout l l0 and defining the aspect ratio a of the topography we show the dependence of l l0 on a thus generalizing previous analysis of martian data analysing the correlation between the normalized runout and the front velocity for the numerical simulations we show how l l0 depends both on the frictional properties of the material and on the dynamics of the flow this result suggests the necessity of evaluating the lateral extension of the relief l0 in the case of natural flows in order to define a horizontal travel index including the topography in an attempt to better understand the flow dynamics analysing the deposit for both simulations and real flow data we evidence a common behaviour suggesting that the geometric factors prevail in the spreading dynamics irrespective of the details of the flow context and conditions we moreover assert the relevance of discrete simulations to the discussion of real cases

depth of the upper mantle discontinuities beneath the east pacific rise we observe discontinuities at depths of 415 and 655 km beneath the northern east pacific rise epr the data consist of precursors to ss waves from 71 tangential component seismograms ranging in epicentral distance from 107 to 167 the mid points of the paths are beneath the northern epr region with about half the data bouncing beneath ocean younger than 9 ma the ss phases were aligned and then time shifted to account for the ocean age at the bounce point the data were then stacked to remove theoretical moveout for underside reflections from the transition zone the stacked results show clear underside reflections these values are slightly more shallow than recently derived average values for oceans but the depth difference between the discontinuities is almost identical to the average for all oceans shearer 1996 although the absolute depths are dependent on the velocity model used in the estimate the depth difference in the discontinuities is not sensitive to velocity structure our results suggest there are not unusually hot temperatures within the transition zone beneath the epr relative to average ocean this implies that deep upwelling beneath the ridge if it exists is passive

seismic observation of narrow plumes in the oceanic upper mantle 1 we have found seismic evidence for the existence of narrow about 200 km diameter and hot plumes in the upper mantle above the 410 km discontinuity they are located mainly under ocean islands not however under the central parts of major continents these results are obtained from comparing globally the observed travel time differences between ps conversions and ss precursors from the discontinuity at 410 km depth using published data and new observations from ocean island stations comparison of ss precursors and ps conversions however leads also to indications of significantly fewer narrow plumes in the upper mantle transition zone between 410 and 660 km depth than in the upper mantle above 410 km depth