analysis of 2014 2015 on and off shore passive seismic data on the reykjanes peninsula sw iceland abstract passive seismic techniques are a valuable tool to map active faults follow changes in the local stress field caused by drilling and exploitation processes in geothermal areas and estimate seismic velocity ratios they have however not been used extensively in basaltic crust as we find it in iceland due to the data processing challenges that high impedance contrasts between layers of igneous rocks pose to improve conventional passive seismic techniques and to test new ones a dense network of seismometers recorded seismic activity on and off shore the reykjanes peninsula in southwest iceland from spring 2014 until august 2015 in addition to the existing long term and permanent networks a temporary network consisting of 30 on land stations and 24 ocean bottom seismometers obs was deployed the network was laid out as concentric circles around the reykjanes geothermal area at the tip of the peninsula and was about 100 km in diameter the data of the on land stations were collected every 70 days and data recorded by the obss were read out after one year when they were recovered we used the seiscomp3 software to automatically detect earthquakes and then manually revised the p and s arrival times a total of 2066 earthquakes could be located along the mid ocean ridge and within and around the reykjanes geothermal system earthquake hypocentres associated with the geothermal area occur in the uppermost 2 km while events along the spreading axis occur at 4 6 km depth on land and appear to deepen with increased distance from the shore we see a cluster of about 200 events induced by the drilling and onset of operation of a local injection borehole in the reykjanes geothermal area we find a vp vs ratio from all picks of 1 78 consistent with earlier studies in the area focal mechanisms calculated for selected events reveal a regime dominated by strike slip and normal faulting

influence of pore pressure on the event size distribution of induced earthquakes 1 during an enhanced geothermal system egs experiment fluid is injected at high pressure into crystalline rock to enhance its permeability and thus create a reservoir from which geothermal heat can be extracted the fracturing of the basement caused by these high pore pressures is associated with microseismicity however the relationship between the magnitudes of these induced seismic events and the applied fluid injection rates and thus pore pressure is unknown here we show how pore pressure can be linked to the seismic frequency magnitude distribution described by its slope theb value we evaluate the dataset of an egs in basel switzerland and compare the observed event size distribution with the outcome of a minimalistic model of pore pressure evolution that relates event sizes to the differential stress d we observe that the decrease of b values with increasing distance of the injection point is likely caused by a decrease in pore pressure this leads to an increase of the probability of a large magnitude event with distance and time

a phenomenological modelling of rocks based on the influence of damage initiation it is recognized that the stress induced damage impacts the progressive failure behavior of rocks a phenomenological model for the compressive failure of rocks is thus presented in this study the model addresses the progressive growth of damage that leads to the strength weakening on a macroscopic scale considering dramatic difference between uniaxial compression and tension strengths for rocks the admitted mises schleiche drucker prager strength criterion is adopted to characterize the damage initiation on this basis a two parameter weibull type probability function is used to define the strength distribution of representative volume elements followed by the use of damage variable for addressing the accumulated probability of failure the proposed damage variable essentially characterizes both the critical stress level of damage initiation and progressive damage evolution law detailed comparisons have been carried out between the predictions and experimental observations and issues related to the damage evolution are particularly addressed in addition the results further validate the proposed model considering damage initiation

automated seismic waveform location using multichannel coherency migration mcm ii application to induced and volcano tectonic seismicity locating microseismic events is essential for many areas of seismology including volcano and earthquake monitoring and reservoir engineering due to the large number of microseismic events in these settings an automated seismic location method is required to perform real time seismic monitoring the measurement environment requires a precise and noise resistant event location method for seismic monitoring in this paper we apply multichannel coherency migration mcm to automatically locate microseismic events of induced and volcano tectonic seismicity using sparse and irregular monitoring arrays compared to other migration based methods in spite of the often sparse and irregular distribution of the monitoring arrays the mcm can show better location performance and obtain more consistent location results with the catalogue obtained by manual picking our mcm method successfully locates many triggered volcano tectonic events with local magnitude smaller that 0 which demonstrates its applicability on locating very small earthquakes our synthetic event location example at a carbon capture and storage site shows that continuous and coherent drilling noise in industrial settings will pose great challenges for source imaging however automatic quality control techniques including filtering in the frequency domain and weighting are used to automatically select high quality data and can thus effectively reduce the effects of continuous drilling noise and improve source imaging quality the location performance of the mcm method for synthetic and real microseismic datasets demonstrates that the mcm method can perform as a reliable and automatic seismic waveform analysis tool to locate microseismic events

geology geochronology and rift basin development in the central sector of the main ethiopia rift based on stratigraphic relationships and k ar dating of volcanic rocks from both of the escarpments flanking plateaus and from the rift floor of the central sector of the main ethiopian rift six major volcanic episodes are recognized in the rift9s development over a time span from the late oligocene to the quaternary using the k ar data correlation of volcanic units from the six periods of activity throughout the study area forms the basis for establishing six time stratigraphic chronozones for the central sector that are related to volcanism in the ethiopian cenozoic volcanic province the oldest basalt and rhyolite flows exposed along the rift margins of the central sector are time correlative to or older than those in river canyons for example blue nile on the adjacent northwest plateau a thinned mesozoic stratigraphic sequence along the guraghe western rift margin suggests that doming may have preceded volcanism and rifting of the cenozoic by late miocene time at least by 8 3 ma and 9 7 ma the eastern and western faulted margins respectively of the rift had formed at guraghe and at agere selam as indicated by containment of flows of that age within the rift wall during eruption a paroxysm of calc alkaline ignimbrite activity produced voluminous flows nearly fully contained within the rift during the pliocene epoch the munesa crystal tuff 3 5 ma a prominent marker tuff exposed on both rift margins is present at depth in a geothermal well beneath the rift floor and indicates a minimum of 2 km of downthrow in the central sector since its eruption structural and stratigraphic relationships in the central sector indicate a two stage rift development this is characterized by an early phase late oligocene or early miocene of a series of alternating opposed half grabens along the rift with alternating polarity such as that in the present gregory and western rifts of east africa and symmetrical rifts that evolved from these grabens in late miocene or early pliocene time thus evolution from alternating half graben to a full symmetrical graben with a medially located neovolcanic zone that is bifurcated to marginal grabens in the northern part of the study area may be a fundamental part of the rifting process the study indicates that there are major petrologic and tectonic differences between the main ethiopian rift and the gregory kenyan rift

continuum discontinuum analysis of failure mechanisms around unsupported circular excavations in anisotropic clay shales abstract the stability of circular excavations in clay shales is a key issue in the drilling and tunnelling industries as well as in the field of deep geological waste storage a large body of experimental evidence indicates that the damaged zone around these cavities is influenced by strong mechanical anisotropy induced by the layered material structure the vast majority of numerical models adopted to date to analyse the stability of openings in layered rocks have been based on continuum mechanics principles using classic shear failure theory for elasto plastic materials however a number of experimental observations demonstrate that clay shales may fail in a brittle manner under low confinement conditions such as those characterizing the near field of the excavation therefore an alternative numerical approach based on non linear fracture mechanics principles and the discrete element method is adopted to gain new insight into the failure process of this class of geomaterials in order to account for the influence of clay shale microstructure on its mechanical behaviour a newly developed approach to capture the anisotropy of strength is proposed with this numerical approach the cohesive strength parameters of the fracture model are assumed to be a function of the relative orientation between the element bonds and the layering orientation the effectiveness of the numerical technique is quantitatively demonstrated by simulating standard rock mechanics tests on an indurated claystone namely opalinus clay emergent strength and deformation properties together with the simulated fracture mechanisms are shown to be in good agreement with experimental observations the modelling technique is then applied to the simulation of the excavation damaged zone edz around a circular tunnel in horizontally bedded opalinus clay the simulated fracturing process is mainly discussed in the context of the damage mechanisms observed at the mont terri url furthermore the influence of in situ stress on resulting edz geometry is analysed together with possible implications for ground support and tunnel constructability modelling results highlight the importance of shear strength mobilization along bedding planes in controlling the edz formation process in particular slippage of bedding planes is shown to cause rock mass deconfinement which in turn promotes brittle failure processes in the form of spalling the numerical technique is currently limited to two dimensional analyses without any thermo hydro mechanical coupling

contrasting styles of post caldera volcanism along the main ethiopian rift implications for contemporary volcanic hazards abstract the main ethiopian rift mer 7 9 n is the type example of a magma assisted continental rift the rift axis is populated with regularly spaced silicic caldera complexes and central stratovolcanoes interspersed with large fields of small mafic scoria cones the recent latest pleistocene to holocene history of volcanism in the mer is poorly known and no eruptions have occurred in the living memory of the local population assessment of contemporary volcanic hazards and associated risk is primarily based on the study of the most recent eruptive products typically those emplaced within the last 10 20 ky we integrate new and published field observations and geochemical data on tephra deposits from the main late quaternary volcanic centres in the central mer to assess contemporary volcanic hazards most central volcanoes in the mer host large mid pleistocene calderas with typical diameters of 5 15 km and associated ignimbrites of trachyte and peralkaline rhyolite composition in contrast post caldera activity at most centres comprises eruptions of peralkaline rhyolitic magmas as obsidian flows domes and pumice cones the frequency and magnitude of events varies between individual volcanoes some volcanoes have predominantly erupted obsidian lava flows in their most recent post caldera stage fentale whereas other have had up to 3 moderate scale vei 3 4 explosive eruptions per millennium aluto at some volcanoes we find evidence for multiple large explosive eruptions corbetti bora baricha boset bericha which have deposited several centimetres to metres of pumice and ash in currently densely populated regions this new overview has important implications when assessing the present day volcanic hazard in this rapidly developing region

solute transport in formations of very low permeability profiles of stable isotope and dissolved noble gas contents of pore water in the opalinus clay mont terri switzerland pore water profiles of water stable isotope and dissolved noble gas content have been determined across the opalinus clay and adjacent formations at the rock laboratory at mont terri we have found enhanced helium contents up to 4he 1 10 4 cubic centimeters at standard pressure and temperature per gram of pore water and argon isotope ratios 40ar 36ar ratios up to 334 due to accumulation of 4he and 40ar produced in situ the helium profile was found to be in steady state with respect to in situ production and diffusive loss into the adjacent limestones where groundwater circulates from this profile a representative mean value of the apparent diffusion coefficient for helium in the pore water of the whole formation was derived for the first time to be da 3 5 10 11 m2 s 1 which is more than two orders of magnitude lower than the diffusion coefficient d0 in free water the stable isotope profile however indicates a component of fossil marine pore water which has not yet been replaced by molecular diffusion of meteoric water from the adjacent limestone and shale formations over the past 10 million years

local seismicity near the actively deforming corbetti volcano in the main ethiopian rift abstract corbetti is currently one of the fastest uplifting volcanoes globally with strong evidence from geodetic and gravity data for a subsurface inflating magma body a dense network of 18 stations has been deployed around corbetti and hawassa calderas between february 2016 and october 2017 to place seismic constraints on the magmatic hydrothermal and tectonic processes in the region we locate 122 events of magnitudes between 0 4 and 4 2 using a new local velocity model the seismicity is focused in two areas directly beneath corbetti caldera and beneath the city of hawassa the shallower 0 5 km depth below sea level b s l earthquakes beneath corbetti are mainly focused in ew to ns elongated clusters at urji and chabbi volcanic centres this distribution is interpreted to be mainly controlled by a northward propagation of hydrothermal fluids away from a cross rift pre existing fault source mechanisms are predominantly strike slip and different to the normal faulting away from the volcano suggesting a local rotation of the stress field these observations along with a low vp vs ratio are consistent with the inflation of a gas rich sill likely of silicic composition beneath corbetti in contrast the seismicity beneath hawassa extends to greater depth 16 km b s l these earthquakes are focused on 8 10 km long segmented faults which are active in seismic swarms one of these swarms in august 2016 is focused between 5 and 16 km depth b s l along a steep normal fault beneath the city of hawassa highlighting the earthquake hazard for the local population

interpretation of tectonic stress orientation on the basis of laboratory model experiments abstract to study the orientation of the tectonic stress in the earth s crust we used physical modelling this method takes advantage of a wide variety of experiments on fault models which were uniaxially loaded to compare the results of laboratory experiments with the behaviour of actual seismoactive fault systems we used the commonly available tectonic and seismic information the structure of fault systems and principal parameters of seismic events i e location origin time and magnitude the possibilities of physical modelling to reconstruct the tectonic stress orientation in actual fault zones are demonstrated on examples from northwestern china the bear valley section of the san andreas fault and from west bohemia czechoslovakia despite several restrictions which up to now have limited the use of the method physical modelling seems to be promising as even complicated patterns of seismic energy release can be explained by means of simple tectonic conditions

laboratory investigation into off fault seismic activity abstract the off fault pattern of weak seismic activity which was observed in the laboratory around fault pairs with already generated tensile cracks led to the comparison with similar patterns observed in nature in some seismoactive regions on the basis of the results of the laboratory model experiments the off fault seismic activity can be attributed to fault segments oriented obliquely or roughly perpendicular to the main fault system these fault segments may have originated as tensile fractures during one of the foregoing strong earthquakes to study the off fault foreshock and aftershock occurrences a method of laboratory seismology physical modeling was employed the use of this method is based on the assumption that the obtained information on the seismic regime of fault models is relevant to the seismic manifestation of faults in the earth the results of laboratory experiments can thus be used to analyze the pattern of seismic energy release in selected seismoactive fault regions

the eruptive history and magmatic evolution of aluto volcano new insights into silicic peralkaline volcanism in the ethiopian rift the silicic peralkaline volcanoes of the east african rift are some of the least studied volcanoes on earth here we bring together new constraints from fieldwork remote sensing geochronology and geochemistry to present the first detailed account of the eruptive history of aluto a restless silicic volcano located in a densely populated section of the main ethiopian rift prior to the growth of the aluto volcanic complex before 500 ka the region was characterized by a significant period of fault development and mafic fissure eruptions the earliest volcanism at aluto built up a trachytic complex over 8 km in diameter aluto then underwent large volume ignimbrite eruptions at 316 19 ka and 306 12 ka developing a 42 km2 collapse structure after a hiatus of 250 ka a phase of post caldera volcanism initiated at 55 19 ka and the most recent eruption of aluto has a radiocarbon age of 0 40 0 05 cal ka bp during this post caldera phase highly evolved peralkaline rhyolite lavas ignimbrites and pumice fall deposits have erupted from vents across the complex geochemical modelling is consistent with rhyolite genesis from protracted fractionation 80 of basalt that is compositionally similar to rift related basalts found east of the complex based on the style and volume of recent eruptions we suggest that silicic eruptions occur at an average rate of 1 per 1000 years and that future eruptions of aluto will involve explosive emplacement of localised pumice cones and effusive obsidian coulees of volumes in the range 1 100 106 m3

coupled thermo hydro mechanical processes in the near field of a high level radioactive waste repository in clay formations the present paper provides an overview of key coupled thermo hydro mechanical thm processes in clay formations that would result from the development of a high level radioactive waste repository here in this paper clay formations include plastic clay such as the boom clay of belgium as well as more indurated clay such as the callovo oxfordian and upper toarcian of france and opalinus clay of switzerland first we briefly introduce and describe four major underground research laboratories urls that have been devoted to clay repository research over the last few decades much of the research results in this area have been gained through investigations in these urls and their supporting laboratory and modeling research activities then the basic elements in the development of a waste repository in clays are presented in terms of four distinct stages in repository development for each of these four stages key processes and outstanding issues are discussed a summary of the important areas of research needs and some general remarks then conclude this paper

fault interaction and seismicity laboratory investigation and its seismotectonic interpretation systems of two parallel linear faults of the same length with the angle of inclination 45 were investigated under uniaxial linearly increasing load perspex plates were used as models for each treated fault configuration the morphology of tensile cracks and the sequence of seismoacoustic events of shear and tensile origin were studied

quaternary oblique extensional tectonics in the ethiopian rift horn of africa abstract the ethiopian rift extends in a northeasterly direction from southern ethiopia to the afar region it shows a complex fault pattern characterised by the interplay of a n30 e n40 e trending border fault system with the quaternary wonji fault belt which is constituted by right stepping en echelon n s to n20 e trending faults the wonji fault belt affects mainly the rift floor but it also overlaps with some segments of the margins its en echelon arrangement indicates a left lateral component of displacement along the rift trend the general fault pattern of the ethiopian rift as well as mesoscopic fault analyses and structural features of some key areas indicate the occurrence of a roughly e w extension which is compatible with the sinistral shear component of motion along the rift this paper proposes that oblique rifting related to the e w direction of extension has been active since the beginning of the quaternary and that it came after an earlier phase of roughly pure extension orthogonal to the rift trend

seismicity of the askja and barðarbunga volcanic systems of iceland 2009 2015 abstract a large seismic network deployed in the icelandic highlands recorded 100 000 earthquakes from 2009 to 2015 we develop a local magnitude scale appropriate for use in central iceland which is similar to the scale used by the iceland meteorological office using this large catalogue of earthquakes we analyze the spatial and temporal changes in seismicity rates and b values in microearthquakes recorded from the usually ductile lower crust we find that b values are high reflecting the presence of high thermal gradients and low stresses driving seismicity associated with the movement of melt in contrast b values in the upper crust are variable low b values indicative of a high stress environment are observed during seismic swarms such as those around mt her ubrei and around bar arbunga caldera a persistently seismically active area around a geothermal area within askja caldera has a b value around 1 but has a strong annual cycle of seismicity we attribute the annual cycle to varying load from the snow cover modulating the seismicity seismicity driven by the intrusion of a large dyke has a b value well above 1 driven by the high pore fluid pressures and thermal gradients around the dyke

research on the geometry of shear fracture zones the geometry of brittle shear fracture zones has been studied by the examination of some strike slip faults and shear joints in china pinnate and en echelon deformational patterns are two fundamental arrangements of fractures within brittle shear fracture zones the pinnate angle is usually less than 25 and the en echelon is more than 25 the deformation in the rock bridge the area between two secondary fractures depends on the arrangements of secondary fractures and the sense of shear some tensile ruptures are formed at the terminations of both primary shear fracture zones and secondary fractures the direction of a tensile rupture is commonly parallel to the orientation of the principal compression along the direction of propagation of the shear fracture the horizontal displacement is large and the attenuation of displacements is slow the maximum horizontal displacement of the shear zone may be located in the region of the initial fracture under certain conditions the shear fracture zones may possess pivotal movement this type of movement may be expressed by the behavior of the fault or by the geomorphology of the fault zone

spatial and temporal analysis of a seismic series using a new version of the three point method application to the 1989 antequera spain earthquakes abstract the three point method tpm has been successfully applied to several seismic series and has provided information about the spatial characteristics azimuth and dip of the fault planes activated in the rupture process a new development of the tpm to determine temporal characteristics is presented to obtain the evolution of the fracturing process of an active fault system for the analysis of the 158 microearthquakes and earthquakes that took place in the seismic series of antequera in june 1989 the choice of a threshold magnitude m u 2 5 has permitted the events related to the most relevant fractures to be distinguished secondly only the events between two concentric spheres here named spatial crown with respect to a given earthquake have been used in order to avoid taking into account earthquakes that are too close to each other together with the very distant events that have little relation to the event analysed the spatial crown has permitted some clear results in the antequera series where we have found that the fracturing process began fundamentally with n 80 e planes and evolved to n 65 w planes finally an error analysis enables an estimation of the uncertainty in the results to be obtained from the errors in the data

damage initiation through extension fracturing in a moderately jointed brittle rock mass abstract recent work in the area of modelling damage processes in brittle rocks has placed increasing emphasis on the role of extension fractures which under low confining pressures propagate in the direction of the maximum applied compressive stress in this paper the role of extension fracturing around underground excavations in moderately jointed rock masses at great depth is considered from the point of view of both theoretical analysis and practical design the background to a rock mass damage initiation di criterion is provided the engineer can use it to quantitatively assess the degree and significance of extension fracturing around openings

seventeenth canadian geotechnical colloquium the effect of cohesion loss and stress path on brittle rock strength stress strain curves for brittle rocks show three characteristic stress levels crack initiation long term strength and peak strength damage controlled testing at low confining stresses has shown that the long term and peak strengths are sensitive to the amount of induced damage i e the greater the amount of damage the lower the long term and peak strengths these tests also showed that the brittle failure process is characterized by a loss of cohesion as friction is mobilized excavation of a circular test tunnel in massive brittle rock resulted in failure around the tunnel the back calculated strength for the failed rock around the tunnel is approximately one half of that measured in laboratory tests crack induced damage of lac du bonnet granite both in the laboratory and in situ begins when the load exceeds approximately one third of the unconfined compressive strength however the stress level associated with failure is a function of loading path in the laboratory where the loading path monotonically increases the ultimate strength of an unconfined sample is 225 mpa numerical studies suggest that in situ the loading path around the tunnel is more complex involving stress increase and decrease and stress rotation for this loading path failure initiates at a stress between 100 and 120 mpa conventional frictional failure criteria did not adequately predict the extent of brittle failure measured around the circular tunnel the results from the damage controlled laboratory tests and the microseismic monitoring carried out during tunnel construction indicate a constant deviatoric stress criterion is a reliable indicator for predicting the onset of damage this criterion was also found to give a reasonable prediction for the maximum depth of failure around the test tunnel the fundamental assumption in the constant deviatoric stress criterion is that at low confining stresses such as those which occur around underground openings the brittle failure process is dominated by cohesion loss

the isotopic composition of radiogenic helium and its use to study groundwater movement in confined aquifers abstract the 3 he 4 he ratio for radiogenic he is shown to be primarily determined by the abundance of li and neutron absorbing elements in the host formations and is almost independent of the radioelement abundance the 3 he 4 he ratios have been computed for various li contents in rock matrices with average abundances of other elements the 3 he 4 he ratio is specific for each formation and may therefore be used to study crustal diffusion of he it is shown however that both the 3he and 4he contents of continental basements are likely to exceed those of the mantle so that diffusion of mantle he through continental crust is not likely to occur changes in the 3 he 4 he ratio for dissolved he have potential for studying groundwater movement and mixing processes atmospheric he which was dissolved at recharge is progressively diluted by radiogenic he with an isotopic ratio characteristic of the aquifer as the groundwater migrates down dip mixing proportions for formation he and atmospheric he in aquifers for which there is admixture of fracture borne water and interstitially stored water can also be estimated from isotopic measurements these processes are discussed in relation to two aquifers in eastern england

seismicity induced by gas production i correlation of focal mechanisms and dome structure from august 1976 to june 1979 more than 220 seismic events were recorded by a 7 station network operating over an active gas field in france 120 earthquakes with magnitudes greater than 1 0 were selected for a detailed study in order to understand the seismic response to a decrease in the gas reservoir pressure of 50 mpa composite focal mechanisms were determined for these events we found that where preexisting fractures were known from well logs faults with the most favorable orientation were reactivated in the part of the gas field where no preexisting faults were known orientation of the fracture planes was close to the orientations of stratigraphic interfaces our kinematic model constrained by the distribution of the earthquakes and their focal mechanisms consists of the downward dropping of a block located close to the top of the gas field the main faults as defined by the spatial density of the seismic activity define the limits of this block the displacements observed along these faults can be either normal or reverse mechanism dependent on the preexisting orientation of each fault another important result is the evidence that the depletion of the gas reservoir exerts the major influence on local seismic displacements obscuring the effect of regional setting

potential seaways across west antarctica the west antarctic ice sheet wais has long been considered vulnerable to rapid retreat and today parts are rapidly losing ice projection of future change in wais is however hampered by our poor understanding of past changes especially during interglacial periods that could be analogs for the future but which undoubtedly provide an opportunity for testing predictive models we consider how ice loss would open seaways across wais these would likely alter southern ocean circulation and climate and would broadly define the de glacial state but they may also have left evidence of their existence in the coastal seas they once connected we show the most likely routes for such seaways and that a direct seaway between weddell and ross seas which did not pass through the amundsen sea sector is unlikely continued ice loss at present rates would open seaways between amundsen and weddell seas a w and amundsen and bellingshausen seas a b in around one thousand years this timescale indicates potential future vulnerability but also suggests seaways may have opened in recent interglacial periods we attempt to test this hypothesis using contemporary bryozoan species assemblages around antarctica concluding that anomalously high similarity in assemblages in the weddell and amundsen seas supports recent migration through a w other authors have suggested opening of seaways last occurred during marine isotope stage 7a 209 ka bp but we conclude that opening could have occurred in mis 5e 100 ka bp when antarctica was warmer than present and likely contributed to global sea levels higher than today

local earthquake magnitude scale and seismicity rate for the ethiopian rift a calibrated local earthquake magnitude scale is essential for quantitative analyses of seismicity in ethiopia effective monitoring of earthquakes and resulting assessment of seismic hazard are especially important as regions with seismic and volcanic activity coincide with regions of economic significance and population growth we have developed a local magnitude m l scale for the northern main ethiopian rift mer using earthquake data collected during 2001 2003 on 122 three component broadband seismic stations waveform data from 2139 local earthquakes were corrected for instrument response and convolved with the nominal wood anderson torsion seismograph response appropriate for the original definition of local magnitude the hypocentral distances considered are 5 to 800 km with the best represented range from 5 to 150 km a total of 30 908 maximum zero to peak amplitudes awa were incorporated into a direct linear inversion for individual earthquake local magnitudes m l 244 station factors c and 2 linear distance dependent factors n k in the distance correction term log a o of the equation for local magnitude m l log awa log a o c the resulting distance correction is given by log a o 1 196997log r 17 0 001066 r 17 2 which implies that ground motion attenuation is relatively high consistent with ongoing magma intrusion and the presence of shallow magma reservoirs beneath the mer station corrections significantly reduce m l residuals and range between 0 42 m l units the catalog of earthquakes is complete above m l 2 1 and the annual cumulative seismicity rate follows the relation log n 4 5 1 13 m l our results are critical for accurate routine quantitative analysis of past current and future seismicity in ethiopia

hydrologic issues associated with nuclear waste repositories significant progress in hydrology especially in subsurface flow and solute transport has been made over the last 35 years because of sustained interest in underground nuclear waste repositories the present paper provides an overview of the key hydrologic issues involved and to highlight advances in their understanding and treatment because of these efforts the focus is not on the development of radioactive waste repositories and their safety assessment but instead on the advances in hydrologic science that have emerged from such studies work and results associated with three rock types which are being considered to host the repositories are reviewed with a different emphasis for each rock type the first rock type is fractured crystalline rock for which the discussion will be mainly on flow and transport in saturated fractured rock the second rock type is unsaturated tuff for which the emphasis will be on flow from the shallow subsurface through the unsaturated zone to the repository the third rock type is clay rich formations whose permeability is very low in an undisturbed state in this case the emphasis will be on hydrologic issues that arise from mechanical and thermal disturbances i e on the relevant coupled thermo hydro mechanical processes the extensive research results especially those from multiyear large scale underground research laboratory investigations represent a rich body of information and data that can form the basis for further development in the related areas of hydrologic research

temporal and three dimensional spatial analyses of the frequency magnitude distribution near long valley caldera california summary the 3 d distribution of the b value of the frequency magnitude distribution is analysed in the seismically active parts of the crust near long valley caldera california the seismicity is sampled in spherical volumes containing n 150 earthquakes and centred at nodes of a grid separated by 0 3 km significant variations in the b value are detected with b ranging from b 0 6 to b 2 0 high b value volumes are located near the resurgent dome and at depths below 5 km at mammoth mountain b values are found to be much lower south of the long valley caldera we interpret this to indicate that an active magma body has advanced from depths below 8 km to depths of 4 to 5 km beneath mammoth mountain in 1989 and that anomalous crust either highly fractured or containing unusually high pore pressure such as is the case in the vicinity of active magma bodies exists north of the seismically active area beneath the resurgent dome at all depths we also investigate the spatial distribution of temporal variations of the frequency magnitude distribution by introducing diverential b value maps b values increased from b 0 8 to b 1 5 underneath mammoth mountain at the onset of the 1989 earthquake swarm and remained high thereafter this suggests that an intrusion permanently altered the average distribution of cracks at 5 10 km depth or that the pore pressure permanently increased we propose that high b values are a necessary but not suycient condition near a magmatic body and therefore spatial b value mapping can be used to aid in the identification of active magma bodies

axial flow hydraulic pulse testing of an argillaceous limestone lindsay limestone is an argillaceous limestone encountered in southern ontario canada the lindsay formation is regarded as a suitable geologic medium for the construction of a deep geologic repository for storing low to intermediate level nuclear waste the lindsay limestone is a nodular argillaceous rock with very low permeability requiring the use of hydraulic pulse tests for the measurement of its permeability characteristics the paper describes the triaxial testing facility the theoretical basis for the test and the procedures used to analyze the experimental results for estimating the permeability of the lindsay limestone the results are compared with the data available in the literature

in situ seismic measurements in claystone at tournemire france compressional and shear wave seismic measurements were performed in an old railway tunnel and in galleries excavated in a 250 m thick toarcian claystone formation in the tournemire experimental station france three component 3c geophones and three orthogonal orientations of the vibroseismic force source were used additionally vertical seismic profiling vsp measurements were recorded with a 3c borehole geophone a hydrophone and a microphone in a 159 m deep borehole id180 in the tunnel the seismic data show that toarcian claystone has strong transverse isotropy ti with a vertical symmetry axis the qp sh and qsv wave propagation velocities in horizontal directions the plane of isotropy of the ti medium are measured as 3550 1850 and 1290 m s 1 respectively the zero offset vsp reveals that only one shear wave propagates in the vertical depth direction and the p and s wave velocities are 3100 and 1375 m s 1 respectively four elastic moduli of the ti medium are determined from the seismic velocities and from the bulk density of 2 53 g cm 3 c11 31 9 gpa c33 24 3 gpa c44 4 5 gpa and c66 8 7 gpa a walkaway vsp with the borehole geophone at 50 m depth in borehole id180 and shot points in the galleries leads to oblique seismic ray paths which allow us to determine the fifth elastic modulus of the ti medium to c13 16 gpa the tube wave recorded by a hydrophone in the water filled lower part of the borehole propagates with 1350 m s 1 which confirms the estimate of the elastic constant c66 the analysis of body wave and surface wave data from a seismic experiment in galerie est shows reflections from several fracture zones in the gallery floor the thickness of the excavation damaged zone edz in the floor of galerie est is estimated to 0 7 m the authors 2014 published by oxford university press on behalf of the royal astronomical society

variations in the frequency magnitude distribution with depth in two volcanic areas mount st helens washington and mt spurr alaska the frequency magnitude distribution of earthquakes characterized using the b value is examined as a function of space beneath mount st helens 1988 1996 and mt spurr 1991 1995 at mount st helens two volumes of anomalously high b b 1 3 can be observed at depths of 2 6 3 6 km below the crater floor and below 6 4 km these anomalies coincide with 1 the depth of vesiculation of ascending magma and 2 the suggested location of a magma chamber at mount st helens study of mt spurr reveals an area of high b value b 1 3 at a depth of about 2 3 4 5 km below the crater floor of the active vent crater peak we propose that the higher material heterogeneity in the vicinity of a magma chamber or conduit due to vesiculation of the ascending magma is the main cause of the increased b value at shallow depths alternatively interaction of magma with groundwater may have increased pore pressure and lowered the effective stress the deeper anomaly at mount st helens is likely caused by high thermal stress gradients in the vicinity of the magma chamber our results indicate that detailed mapping of the frequency magnitude distribution can be used as a tool to trace vesiculation and locate active magma chambers

anomalously high b values in the south flank of kilauea volcano hawaii evidence for the distribution of magma below kilauea s east rift zone abstract the pattern of b value of the frequency magnitude relation or mean magnitude varies little in the kaoiki hilea area of hawaii and the b values are normal with b 0 8 in the top 10 km and somewhat lower values below that depth we interpret the kaoiki hilea area as relatively stable normal hawaiian crust in contrast the b values beneath kilauea s south flank are anomalously high b 1 3 1 7 at depths between 4 and 8 km with the highest values near the east rift zone but extending 5 8 km away from the rift also the anomalously high b values vary along strike parallel to the rift zone the highest b values are observed near hiiaka and pauahi craters at the bend in the rift the next highest are near makaopuhi and also near puu kaliu the mildest anomalies occur adjacent to the central section of the rift the locations of the three major and two minor b value anomalies correspond to places where shallow magma reservoirs have been proposed based on analyses of seismicity geodetic data and differentiated lava chemistry the existence of the magma reservoirs is also supported by magnetic anomalies which may be areas of dike concentration and self potential anomalies which are areas of thermal upwelling above a hot source the simplest explanation of these anomalously high b values is that they are due to the presence of active magma bodies beneath the east rift zone at depths down to 8 km in other volcanoes anomalously high b values correlate with volumes adjacent to active magma chambers this supports a model of a magma body beneath the east rift zone which may widen and thin along strike and which may reach 8 km depth and extend from kilauea s summit to a distance of at least 40 km down rift the anomalously high b values at the center of the south flank several kilometers away from the rift may be explained by unusually high pore pressure throughout the south flank or by anomalously strong heterogeneity due to extensive cracking or by both phenomena the major b value anomalies are located sse of their parent reservoirs in the direction of motion of the flank suggesting that magma reservoirs leave an imprint in the mobile flank we hypothesize that the extensive cracking may have been acquired when the anomalous parts of the south flank now several kilometers distant from the rift zone were generated at the rift zone near persistent reservoirs since their generation these volumes may have moved seaward away from the rift but earthquakes occurring in them still use the preexisting complex crack distribution along the decollement plane at 10 km depth the b values are exceptionally low b 0 5 suggesting faulting in a more homogeneous medium

mozambique earthquake sequence of 2006 high angle normal faulting in southern africa 1 we report source mechanisms for the six largest shocks of the mozambique earthquake sequence of 22 february 2006 the main shock of this sequence is one of the largest mw 7 0 to occur in africa over the past 100 years and its p waveforms alone are sufficient to show that north south trending normal faulting near the surface continues to depths of more than 15 km along an exceptionally steep dip of 76 4 this new result shows that globally seismogenic normal faulting spans a wide range of dips from about 30 to 75 s waveforms when combined with a minor component of left lateral slip observed in the field indicate the rake to be between 80 and 89 which in turn places a new constraint on relative plate motions complications from a nonspecific rovuma microplate notwithstanding our results favor the euler pole between the somalia and nubia plates to lie southward of the epicenters

fault architecture in the main ethiopian rift and comparison with experimental models implications for rift evolution and nubia somalia kinematics abstract the main ethiopian rift mer offers a complete record of the time space evolution of a continental rift we have characterized the brittle deformation in different rift sectors through the statistical analysis of a new database of faults obtained from the integration between satellite images and digital elevation models and implemented with field controls this analysis has been compared with the results of lithospheric scale analogue models reproducing the kinematical conditions of orthogonal and oblique rifting integration of these approaches suggests substantial differences in fault architecture in the different rift sectors that in turn reflect an along axis variation of the rift development and southward decrease in rift evolution the northernmost mer sector is in a mature stage of incipient continental rupture with deformation localised within the rift floor along discrete tectono magmatic segments and almost inactive boundary faults the central mer sector records a transitional stage in which migration of deformation from boundary faults to faults internal to the rift valley is in an incipient phase the southernmost mer sector is instead in an early continental stage with the largest part of deformation being accommodated by boundary faults and almost absent internal faults the mer thus records along its axis the typical evolution of continental rifting from fault dominated rift morphology in the early stages of extension toward magma dominated extension during break up the extrapolation of modelling results suggests that a variable rift obliquity contributes to the observed along axis variations in rift architecture and evolutionary stage being oblique rifting conditions controlling the mer evolution since its birth in the late miocene in relation to a constant post ca 11 ma n100 e nubia somalia motion

pinpointing deep geothermal upflow in zones of complex tectono volcanic degassing new insights from aluto volcano main ethiopian rift the project was financed in the framework of the volkswagen foundation s postdoctoral fellowships for african researchers in the engineering sciences reference 90017

generalized crack initiation and crack damage stress thresholds of brittle rock masses near underground excavations abstract the rock mass failure process is characterized by several distinct deformation stages which include crack initiation crack propagation and coalescence it is important to know the stress levels associated with these deformation stages for engineering design and practice extensive theoretical experimental and numerical studies on the failure process of intact rocks exist it is generally understood that crack initiation starts at 0 3 to 0 5 times the peak uniaxial compressive stress in confined conditions the constant deviatoric stress criterion was found to describe the crack initiation stress level here generalized crack initiation and crack damage thresholds of rock masses are proposed the crack initiation threshold is defined by 1 3 a cm and the crack damage threshold is defined by 1 3 b cm for jointed rock masses where a and b are material constants and cm is the uniaxial compressive strength of the rock masses for a massive rock mass without joints cm is equal to cd the long term uniaxial strength of intact rock after examining data from intact rocks and jointed rock masses it was found that for massive to moderately jointed rock masses the material constants a and b are in the range of 0 4 to 0 5 0 8 to 0 9 respectively and for moderately to highly jointed rock masses a and b are in the range of 0 5 to 0 6 0 9 to 1 0 respectively the generalized crack initiation and crack damage thresholds when combined with simple linear elastic stress analysis assist in assessing the rock mass integrity in low confinement conditions greatly reducing the effort needed to obtain the required material constants for engineering design of underground excavations

spatio temporal seismicity patterns using mutual information application to southern iberian peninsula spain earthquakes abstract a quantitative method to characterize the spatio temporal evolution of the seismic activity of a region for a given time interval is proposed in this paper the epicentral region is divided in equal area parallelograms that we call cells a probability p i is assign to every cell depending on its seismic activity level the evolution of the mutual information based on these probabilities allows us to fit the optimal value for p i s the analysis results show that is possible to monitor the change of seismic activity quantitatively we show that a propagation model based on cross template model ct model give us a criteria whether or not an area whose size is determined by the model mathematically is going to be considered seismically activated in a period of time whose value is also the result of the application of the method our method is applied to the data of southern iberian peninsula seismicity including more than 10 000 earthquakes data available from the andalusian seismic network is used from 1985 to 1995

water rock interaction processes in the aluto langano geothermal field ethiopia aluto langano is a water dominated geothermal field in the main ethiopian rift system the hydrothermal mineral assemblage is characterized by the presence of illite montmorillonite t 150 c and illite chlorite 150 c t 310 c in the hottest part of the wells up to 335 c the mineral assemblage is characterized by the presence of ca al silicates epidote garnet actinolite however these phases also occur as relics in wells where lower temperatures are measured calcite quartz and chlorite are widespread minerals in all the wells both at low and high temperatures fluid inclusion data from quartz and calcite of two wells la 3 and la 6 reveal fluids with an apparent salinity of 1 5 2 0 for la 3 and 0 85 1 36 wt nacl eq for la 6 homogenization temperatures tb range from 240 c to 350 c the calculated co2 concentration in the liquid phase ranges from 0 07 to 0 66 mol thermodynamic properties and phase relations of calc silicates show that the presence or absence of these minerals is mainly controlled by the pco2 in the system even where temperature has played an important role fluid inclusion data from two deep wells la 3 and la 6 indicate an increase in temperature and co2 content of the geothermal fluid after the trapping of the inclusions the wells produce an alkali chloride bicarbonate water enthalpy chloride and na 1000 k 100 mg diagrams indicate that none of the discharge waters can be taken as representative of the deep fluid in full equilibrium with the reservoir rock and that important mixing processes characterize even the hottest zone of the system these mixing processes could be responsible for the continuous dynamic evolution of the hydrothermal system as evidenced by the authigenic mineralogy

geohydromechanical processes in the excavation damaged zone in crystalline rock rock salt and indurated and plastic clays in the context of radioactive waste disposal abstract the creation of an excavation disturbed zone edz or excavation damaged zone edz is expected around all man made openings in geologic formations macro and micro fracturing and in general a redistribution of in situ stresses and rearrangement of rock structures will occur in this zone resulting in drastic changes of permeability to flow mainly through the fractures and cracks induced by excavation the edz may have significant implications for the operation and long term performance of an underground nuclear waste repository various issues of concern need to be evaluated such as processes creating fractures in the edz the degree of permeability increase and the potential for sealing or healing with permeability reduction in the zone in recent years efforts along these lines have been made for a potential repository in four rock types crystalline rock salt indurated clay and plastic clay and these efforts have involved field laboratory and theoretical studies the present work involves a synthesis of the ideas and issues that emerged from presentations and discussions on edz in these four rock types at a cluster conference and workshop held in luxembourg in november 2003 first definitions of excavation disturbed and edzs that may be applicable to all four rock types are proposed then an approach is suggested for the synthesis and intercomparison of geohydromechanical processes in the edz for crystalline rock salt indurated clay and plastic clay comparison tables of relevant processes associated factors and modelling and testing techniques are developed a discussion of the general state of the art and outstanding issues is also presented a substantial bibliography of relevant papers on the subject is supplied at the end of the paper

oxidation front and oxygen transfer in the fractured zone surrounding the meuse haute marne url drifts in the callovian oxfordian argillaceous rock

groundwater characterisation and modelling of water rock interaction in an argillaceous formation tournemire france abstract chemical na k mg ca cl so 4 hco 3 si al fe mn toc and isotopic 18 o h 2 o 2 h h 2 o 3 h h 2 o 14 c dic 13 c dic 34 s so 4 18 o so 4 data were obtained on groundwater collected from fractures inside the consolidated argillaceous formation of tournemire aveyron and from the aquifers surrounding this formation because of the very low transmissivity of such fractures 10 10 m 2 s 1 specific devices were developed for limiting out gassing and air contamination of groundwater during the sampling two modelling approaches are proposed to account for the chemical evolution of fracture groundwater from na cl hco 3 to na so 4 type the first one is based on mineral equilibria only and the second one also takes into account the ion exchange reactions the two modelling approaches are able to reproduce k concentrations and they led to an almost constant p co 2 10 2 6 atm which only depends on equilibrium with the calcite dolomite kaolinite quartz assemblage the modelling of na ca 2 and hco 3 behaviours is improved by taking the ion exchange reactions into account in some boreholes the time evolution of the chemical composition of fluids was greatly influenced by so 4 reduction combined with oxidation of dissolved organic c the isotopic composition of aqueous so 4 in fracture groundwater confirms that so 4 reduction processes have occurred since the drilling of these boreholes but also previously during the natural evolution of groundwater these redox reactions constitute a source of aqueous inorganic c that has to be considered in the computation of groundwater ages with 14 c for the studied fracture groundwaters the maximum age estimates range from 17 to 29 ka this shows that fractures may induce relatively fast groundwater flow compared to the very long migration times that are calculated by only considering diffusive transport in the rock microporosity

thermal models of dyke intrusion during development of continent ocean transition a consensus has emerged in recent years from a variety of geoscientific disciplines that extension during continental rifting is achieved only partly by plate stretching dyke intrusion also plays an important role magma intrusion can accommodate extension at lower yield stresses than are required to extend thick strong unmodified continental lithosphere mechanically thereby aiding the breakup process dyke intrusion is also expected to heat and thereby weaken the plate but the spatial extent of heating and the effect of different rates of magmatic extension on the timescales over which heating occurs are poorly understood to address this issue a numerical solution to the heat flow equation is developed here to quantify the thermal effects of dyke intrusion on the continental crust during rifting the thermal models are benchmarked against a priori constraints on crustal structure and dyke intrusion episodes in ethiopia finite difference models demonstrate that magmatic extension rate exerts a first order control on the crustal thermal structure once dyke intrusion supersedes faulting and stretching as the principal extensional mechanism the crust will heat and weaken rapidly less than 1 ma in the main ethiopian rift mer the majority of present day extension is focused on 20 km wide quaternary recent axial magmatic segments that are mostly seismogenic to mid crustal depths and show p wave seismic velocities characteristic of heavily intruded continental crust when reviewed in light of our models these observations require that no more than half of the mer s extension since 2 ma has been achieved by dyke intrusion magmatic heating and weakening of the crust would have rendered it aseismic if dyke intrusion accounted for the entire 6 mm yr extension rate in the older faster extending 16 mm yr red sea rift rsr in afar dyke intrusion is expected to have had a more dramatic impact on crustal rheology accordingly effective elastic plate thickness and moho depth in the danakil region of northernmost afar are markedly reduced and seismicity is shallower than in the mer thermally driven variations in crustal rheology over time in response to dyke intrusion thus play an important role in the development of continent ocean transition

desaturation and structure relationships around drifts excavated in the well compacted tournemire s argillite aveyron france this study aimed to explore the relationships between the extension of rock desaturation and the excavation damaged zone edz subsequent to the excavation of a century old tunnel and of recent drifts 1996 and 2003 at the tournemire underground research laboratory url located in the aveyron county south of the massif central france the other objective of this work was to assess the impact of desaturation on the hydraulic head profile measured around the tunnel one section was selected per drift two boreholes were drilled for each section one parallel and one inclined 45 with respect to the bedding for each borehole we performed on site drill core mapping petrophysical measurements pneumatic and hydraulic tests by means of a modular mini packer system mmps results indicate that the edz around drifts is mainly a combination of unloading joints mimicking the drift shape and of desaturation cracks parallel to the bedding the edz extension around the tunnel is twice to three times that of the drifts of 1996 and 2003 and is essentially composed of unloading joints resulting from the mechanical response of the rock the masonery covering the tunnel walls is assumed to have protected the rock from seasonal variations of air humidity thus limiting without excluding the formation of desaturation cracks the edz extension deduced from core mapping is in agreement with that deduced from pneumatic tests with permeabilities several orders of magnitude greater than in the undisturbed zone degrees of saturation for the three sections range between 0 9 and 1 in the edz area and reach 1 in the undamaged zone the head profile deduced from measurements recorded since 2002 indicates the occurrence of an excavation disturbed zone edz of about 40 m around the tunnel this edz is likely due to the existence of sub atmospheric water pressures clearly seen in the first meter around the tunnel we have tried to quantify the impact of the tunnel since its excavation on saturation degree and on hydraulic heads the simulation was performed by considering as a first approach the absence of fracturing in the edz a constant suction of 3300 m deduced from the mean annual values of relative humidity and temperature measured in the tunnel atmosphere since 2002 was applied at the tunnel wall the degrees of saturation simulated around the tunnel are underestimated in the edz area and consistent to experimental data in the unfractured zone the modelling of hydraulic heads is overestimated in the horizontal direction and is in the domain of experimental values in the vertical direction but the lack of intermediate data cannot enable us to conclude on this consistency this study demonstrated the role played by fracturing on the distribution of petrophysical parameters and of heads around drifts and the century old tunnel it has also demonstrated the necessity of coupling mechanic and hydraulic calculations by considering capillary forces

the afar depression interpretation of the 1960 2000 earthquakes summary we studied the seismic activity of the afar depression ad and adjacent regions during the period 1960 2000 we define seven distinct seismogenic regions using geological tectonic and seismological data based on the frequency magnitude relationships we obtain b values of about 1 for the different regions the pattern of the distribution of the location of epicentres fits with the known active fault zone in the ad and the axial volcanic ridges the bab el mandab area and the danakil aysha a blocks are less active for 125 intermediate to strong earthquakes the seismic moment and source parameters were calculated the results of the fault plane solutions for the afar depression indicate mainly strike slip and normal sense of movement originating from fault planes striking nw se these results indicate a clockwise block rotation described previously as a bookshelf model in central ad there are a few rightlateral faults east of massawa with e w striking fault planes at the southern red sea north of the danakil block the mixed focal mechanisms with axial plane striking nw se comprise several reverse faulting strike slip motion and normal faulting right lateral movement was also calculated for a cluster of seismic events between the manda hararo and alyata volcanic ridges along nw se striking faults along the n s striking faults in the escarpment at the western afar margins there are two distinct clusters of epicentres the strong earthquakes at the southern cluster exhibit normal or strike slip motions the intermediate to small earthquakes in the northern cluster exhibit reverse and strike slip motions mainly normal faults were calculated along ne sw striking faults of the ethiopian east african rift estimates of the seismic efficiency suggest that the maximal values are about 50 per cent or less implying that most of the motion is taken aseismically

fracture mechanical behavior of red sandstone containing a single fissure and two parallel fissures after exposure to different high temperature treatments it is well known to us that the stability and safety of deep underground mining excavations and nuclear waste depositories rely on accurate knowledge of the physical and mechanical properties of representative rocks under the action of high temperature zhang et al in int j plast 34 93 113 2012

a seismotectonic analysis of the anza seismic gap san jacinto fault zone southern california small earthquake epicenters near the anza seismic gap define a 20 km quiescent segment of fault bounded to the northwest and southeast by areas of relatively high seismicity recent moderate earthquakes on and near the san jacinto fault in the gap and their relatively depressed aftershock activity indicate that the fault is seismogenic and highly stressed but locked by some mechanism the locked nature of the fault may be due to relatively high compressive stress normal to the fault resulting from the convergent geometries of the local active discontinuous faults and the oblique orientation of the regional maximum compressive stress strain is not being relieved by aseismic fault creep a swarm of small earthquakes in the crustal block 13 km southwest of the anza gap beneath the cahuilla valley recently released stress in an area which was previously highly active before the 1918 m 6 8 and 1937 m l 6 0 earthquakes the occurrence of these periods of increased seismicity near cahuilla in the years immediately before the nearby closer than 35 km large earthquakes and the recent swarm suggest that the ground beneath cahuilla may be acting as a stress meter signaling the presence of high stresses before large local earthquakes the length of the quiescent fault segment suggests potential for about an m 6 5 earthquake if the entire segment ruptures at once

dating very old pore waters in impermeable rocks by noble gas isotopes the sup 4 he sup 40 ar and sup 136 xe content dissolved in the pore water of sedimentary rock samples was measured on samples from borehole cores near the repository for nuclear waste in morsleben germany due to the very low permeabilities of the rock formations conventional groundwater sampling was almost impossible hence the authors developed a new sampling method for noble gases in the pore water of freshly drilled rock cores this method provides vertical noble gas profiles in high depth resolution even in impermeable rocks by application of the new technique quantitative age information of groundwater and pore water have been derived the authors find palaeowaters from the last glaciation depleted in delta d and delta sup 18 o with a sup 4 he age of about 55 kyr the high saline pore solutions below are at least 6 mio years old this has been concluded from the profiles of radiogenic sup 4 he and sup 40 ar close to diffusion in steady state and from xenon isotopes produced by spontaneous fission of sup 238 u in the rocks a sup 4 he flux of 2 center dot 10 sup minus 7 cc stp cm sup 2 yr is derived from the profile which is due to local sup 4 he production within the investigatedmore sediments less

stages of evolution of a toarcian compacted claystone around galleries excavated between 1 and 124 years ago by the study of noble gases dissolved in pore water at the tournemire underground research laboratory france abstract the purpose of this work is to study the diffusion properties in an argillaceous formation and how these properties change after the excavation of galleries over a long period the tournemire underground research laboratory url located in aveyron france offers the unique opportunity to investigate different stages of the evolution of the so called excavation damaged zone edz over a period of about 124 a four horizontal boreholes parallel to the bedding and one vertical borehole were drilled in order to study diffusion transport in the edz in both directions in galleries excavated 124 13 6 and 1 a ago respectively noble gases dissolved in the pore water of 47 core samples were extracted in order to establish precise depth profiles mainly produced in situ by decay of u and th radiogenic he accumulates in the pore water of rocks and escapes toward the gallery allowing a characterization of the diffusive transport in the edz and its evolution with time for the oldest gallery 1882 1888 core mapping reveals an edz well developed with fractures up to a depth of 1 8 m in this zone all radiogenic he has been lost and at the same time heavy noble gases are trapped in the rock with an enrichment of xe relative to kr and of kr relative to ar from 1 8 m he concentration increases with depth to reach a plateau at about 3 5 m corresponding to a concentration normally found in the unaltered zone considering diffusive transport of the radiogenic he to the edz model calculations allowed proposing transport properties in the unfractured and undisturbed zone uz with a pore diffusion coefficient dp of 1 10 10 m 2 s deduced from the best fit curve to experimental data and a mean porosity of 0 1 obtained form petrophysical measurements the most recent gallery 2008 did not show any evidence of edz at depths greater than 7 5 cm a fracture located around 0 75 m possibly of tectonic origin induced large advection and diffusion losses of he on both sides in the next 50 cm excesses of ar kr and xe are found suggesting diffusion of air from the gallery associated with he losses the 6 a old gallery is clearly influenced by fractures of tectonic origin increasing the edz thickness of 0 7 m up to a damaged zone of 1 8 m based on the he concentration profile the 13 a old gallery 1996 has a net edz of 1 2 m larger than the 0 7 m deduced from the structural analysis by core mapping followed by a disturbed zone of up to 3 5 m that could represent an intermediate stage before the development of fractures in the edz the vertical borehole drilled in the same gallery shows a less extensive damaged zone 1 4 m and allows the determination of a dp of 3 10 11 m 2 s taking into account the anisotropy relative to the stratification plane the rare gas analysis has therefore enabled confirmation and better description of the net time dependent evolution of the edz around drifts in the argillaceous tournemire site no obvious edz for the 1 a gallery 2008 1 2 m width for the 13 a old gallery 1996 and 2 m width for the century old tunnel the occurrence of a disturbed and unfractured zone dz around the 13 a old gallery undetected by structural studies and geophysical methods clearly demonstrates the power of radiogenic he in assessing the transport properties around drifts excavated at different ages in compacted clay rocks

the evolution of volcano hosted geothermal systems based on deep wells from karaha telaga bodas indonesia temperature and pressure surveys fluid samples and petrologic analyses of rock samples from deep drill holes at the karaha telaga bodas geothermal field on the volcanic ridge extending northward from galunggung volcano west java have provided a unique opportunity to characterize the evolution of an active volcano hosted geothermal system wells up to 3 km in depth have encountered temperatures as high as 353 c and a weakly altered granodiorite that intruded to within 2 to 3 km of the surface the intrusion is shallowest beneath the southern end of the field where an acid lake overlies a nearly vertical low resistivity structure 10 ohm m defined by magnetotelluric measurements this structure is interpreted to represent a vapor dominated chimney that provides a pathway to the surface for magmatic gases four distinct hydrothermal mineral assemblages document the evolution of the geothermal system and the transition from liquid to vapor dominated conditions the earliest assemblage represents the initial liquid dominated system generated during emplacement of the granodiorite between 5910 76 and 4200 150 y bp tourmaline biotite actinolite epidote and clay minerals were deposited contemporaneously at progressively greater distances from the intrusive contact assemblage 1 at 4200 150 y bp flank collapse and the formation of the volcano s crater kawah galunggung resulted in catastrophic decompression and boiling of the hydrothermal fluids this event initiated development of the modern vapor dominated regime chalcedony and then quartz were deposited as the early low salinity liquids boiled assemblage 2 both vapor and liquid rich fluid inclusions were trapped in the quartz crystals liquid rich fluid inclusions from the southern part of the field record salinities ranging from 0 to 26 weight percent nacl cacl 2 equivalent and locally contain fluorite daughter crystals we suggest based on temperature salinity relationships and evidence of boiling that these fluids were progressively concentrated as steam was lost from the system however mixing with fluids derived from the underlying intrusion or generated during the formation of acid so 4 water on the vapor dominated chimney margins could have contributed to the observed salinities as pressures declined co 2 and so 4 rich steam heated water drained downward depositing anhydrite and calcite assemblage 3 in the fractures limiting further recharge fluid inclusions with salinities up to 31 weight percent nacl equivalent were trapped in these minerals as the descending water vaporized the final assemblage is represented by precipitates of nacl kcl and fecl x deposited on rock surfaces in portions of the vapor dominated zone that boiled dry vapor dominated conditions extend over a distance of at least 10 km and to depths below sea level deep wells drilled into the underlying liquid dominated reservoir in the northern and central part of the volcanic ridge produce low salinity fluids representing recent recharge of meteoric and steam heated water the evolution of volcanic hosted vapor dominated geothermal systems can be described by a five stage model stage 1 involves the formation of an over pressured liquid dominated geothermal system soon after magmatic intrusion in stages 2 and 3 pressures progressively decrease and a curtain of steam heated water surrounding a magmatic vapor dominated chimney at 350 c and 14 2 mpa develops the relatively low pressure near the base of the chimney causes liquid inflow adjacent to the intrusion and the development of a secondary marginal vapor dominated zone in stage 4 the magmatic vapor discharge from the intrusion becomes small vapor pressure declines and the secondary vapor dominated zone expands above the intrusion in stage 5 the vapor dominated zone floods because heat from the intrusion is insufficient to boil all liquid inflow a more common liquid dominated volcanic hosted system then develops

tectonic stress orientation and the seismic regime of a single fault abstract the seismic regime of tectonic faults was studied by the method of physical modelling this method is based on the assumption that the physical processes in actual seismo active fault zones can be scaled in laboratory conditions the results of our laboratory experiments on plate models proved the great importance of tectonic stress orientation on the seismic regime of tectonic faults changes in the fault inclination led to distinct changes both in the recurrence time intervals between individual seismic events and in the amount of seismic energy released for some fault inclinations the seismic activity of faults vanished altogether only a limited zone in the fault plane was found to participate in the process of seismic energy release the extent of this so called seismo active zone depends not only on the contact conditions but also on the value of the acting load and on the fault inclination this presumption is supported by the fact that for each inclination there exists an upper limit to the seismic energy that can be released in one seismic event

evidence for cross rift structural controls on deformation and seismicity at a continental rift caldera abstract in continental rifts structural heterogeneities such as pre existing faults and foliations are thought to influence shallow crustal processes particularly the formation of rift faults magma reservoirs and surface volcanism we focus on the corbetti caldera in the southern central main ethiopian rift we measure the surface deformation between 22nd june 2007 and 25th march 2009 using alos and envisat sar interferograms and observe a semi circular pattern of deformation bounded by a sharp linear feature cross cutting the caldera coincident with the caldera long axis the signal reverses in sign but is not seasonal from june to december 2007 the region south of this structure moves upwards 3 cm relative to the north while from december 2007 until november 2008 it subsides by 2 cm comparison of data taken from two different satellite look directions show that the displacement is primarily vertical we discuss potential mechanisms and conclude that this deformation is associated with pressure changes within a shallow analysis of the distribution of post caldera vents and cones inside the caldera shows their locations are statistically consistent with this fault structure indicating that the fault has also controlled the migration of magma from a reservoir to the surface over tens of thousands of years spatial patterns of seismicity are consistent with a cross rift structure that extents outside the caldera and to a depth of 30 km and patterns of seismic anisotropy suggests stress partitioning occurs across the structure we discuss the possible nature of this structure and conclude that it is most likely associated with the goba bonga lineament which cross cuts and pre dates the current rift our observations show that pre rift structures play an important role in magma transport and shallow hydrothermal processes and therefore they should not be neglected when discussing these processes

seismicity of the bora tullu moye volcanic field 2016 2017 the bora tullu moye tm volcanic field is a geothermal energy prospect in the central main ethiopian rift but little is known about the seismicity of the region here we document seismic activity between february 2016 and october 2017 locating more than a 1 000 seismic events of local magnitude 0 to 2 7 this provides new insights into fluid movement and deformation beneath what we only now realize is a complicated volcanic system a discrete cluster of events lies beneath tm but surprisingly most of the seismicity lies in two clusters that are beneath neither the bora or tm edifices in these regions we use earthquake cluster orientations fault plane solutions and fast seismic shear wave orientations to show that seismicity is triggered by hydrothermal circulation of fluids along preexisting fractures the fractures trend in multiple directions and are in general not parallel to rifting related structures instead the fractures are parallel to structures created during previous caldera forming eruptions at both bora and tm highly fractured regions such as this could be attractive targets for geothermal power generation we estimate a minimum depth for a magmatic body beneath tm to be 6 5 km using the mapped brittle ductile transition frequency analysis of the earthquake waveforms reveal the bulk of the events to be volcano tectonic but some low frequency seismicity is present at a depth of 5 km beneath the tm edifice triggered by high pore fluid pressures

study of the initiation and propagation of excavation damaged zones around openings in argillaceous rock the study of the creation and evolution of the excavation disturbed zone edz in argillaceous rocks is a major issue for the safety of nuclear wastes underground repositories in this context the argillaceous tournemire site has provided a unique opportunity to study the evolution of the edz with time thanks to the existence of three openings of different ages a thorough characterization of the edz has been conducted by different means such as visual observation analysis of samples extracted from drilled boreholes edz permeability measurements etc on the basis of these measurements a conceptual model of the edz initiation and propagation at the tournemire site has been proposed in order to validate this model numerical simulations of increasing complexity have been carried out in a first attempt the response of the rock mass to the excavation phase followed by seasonal cyclic variations of temperature and relative humidity inside the opening has been simulated by means of a purely mechanical analysis using a simple elastic material model the edz has been estimated by post processing the calculated stress states using a mohr coulomb failure criterion the results obtained show that no edz could be predicted unless adopting a low cohesion value for the rock mass moreover the deferred nature of the edz formation in tournemire could not be reproduced these limitations have then been suppressed by using a coupled viscoplastic damaging mechanical model the parameters of which have been identified from different laboratory experiments with this model a time evolution of the edz could be predicted but the edz pattern could not match the one observed in situ finally in view of the importance of the hydraulic couplings unsaturated hydro mechanical calculations have been carried out to investigate the effect of the numerous seasonal variations cycles and the resulting shrinkage

seismicity associated with magmatism faulting and hydrothermal circulation at aluto volcano main ethiopian rift the silicic volcanic centres of the main ethiopian rift mer play a central role in facilitating continental rifting many of these volcanoes host geothermal resources and are located in heavily populated regions insar studies have shown several are deforming but regional seismic networks have detected little seismicity a local network of 12 seismometers was deployed at aluto volcano from 2012 to 2014 and detected 2142 earthquakes within a 24 month period we locate the events using a 1d velocity model that exploits a regional model and information from geothermal boreholes and calculate local magnitudes b values and focal mechanisms event depths generally range from the near surface to 15 km with most of the seismicity clustering in the upper 2 km a significant amount of seismicity follows the artu jawa fault zone which trends in alignment with the wonji fault belt nne ssw and is consistent with previous studies of strain localisation in the mer focal mechanisms are mostly normal in style with the mean t axes congruent to the orientation of extension in the rift at this latitude some show relatively small left lateral strike slip components and are likely associated with the reactivation of ne ene structures at the southern tip of the aluto gedemsa segment events range from 0 40 to 2 98 in magnitude and we calculate an overall b value of 1 40 0 14 this relatively elevated value suggests fluid induced seismicity that is particularly evident in the shallow hydrothermal reservoir and above it subdividing our observations according to depth identifies distinct regions beneath the volcanic edifice a shallow zone 2 0 km of high seismicity and high b values that corresponds to the hydrothermal system and is influenced by a high fluid saturation and circulation a relatively aseismic zone 0 2 km with low b values that is impermeable to ascending volatiles a region of increased fluid induced seismicity 2 9 km that is driven by magmatic intrusion from below and a deeper zone below 9 km that is interpreted as a partially crystalline magmatic mush these observations indicate that both the magmatic and hydrothermal systems of aluto volcano are seismically active and highlight the need for dedicated seismic monitoring at volcanoes in the mer

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

low frequency earthquakes beneath tullu moye volcano ethiopia reveal fluid pulses from shallow magma chamber abstract the active magmatic processes beneath volcanoes in continental rifts is poorly understood for example until recently in the east african rift ear the majority of the young volcanoes were thought to be inactive more recent studies have shown that numerous volcanoes in the ear are seismically active and deforming rapidly however an unambiguous sign of actively degassing magma hosted in shallow magma bodies has eluded most investigators here we present detailed analysis of the first low frequency lf earthquake swarms to be observed in the main ethiopian rift the earthquakes locate to beneath tullu moye volcano and are directly related to the presence of a shallow magma body with a high fluid content using spectral modelling we show that the lf earthquakes appear to have low stress drops 1 50 kpa which we interpret in terms of low rupture velocities and high pore fluid pressure careful relocation of the lf earthquakes place them approximately 4 km below the surface within one of two possible clusters however analysis of the correlation between earthquake waveforms show that each swarm contains a range of earthquake families and as such a diversity of earthquake source mechanisms to explain these observations we propose the seismicity is induced by h2o co2 fluid pulses from the shallow magma body into a highly fractured region fluid pulses cause high pore fluid pressures which also cause the low rupture velocities

monsoonal loading in ethiopia and eritrea from vertical gps displacement time series vertical gps displacement time series from 16 continuous sites over a period from 2007 to 2014 are compared to time series of monthly averages of liquid water equivalent thickness from the gravity recovery and climate experiment and precipitation from the climate research unit to investigate hydrologic loading in ethiopia and eritrea the gps vertical time series record the presence of one or two rainy seasons the amplitude surface displacements in response to monsoon water load and phases consistent with a purely elastic response to a water load that accumulates throughout the rainy period comparison of observed amplitudes to those calculated for an average earth model shows no systematic weakness related to the rift

characterising hydrothermal fluid pathways beneath aluto volcano main ethiopian rift using shear wave splitting abstract geothermal resources are frequently associated with silicic calderas which show evidence of geologically recent activity hence development of geothermal sites requires both an understanding of the hydrothermal system of these volcanoes as well as the deeper magmatic processes which drive them here we use shear wave splitting to investigate the hydrothermal system at the silicic peralkaline volcano aluto in the main ethiopian rift which has experienced repeated uplift and subsidence since at least 2004 we make over 370 robust observations of splitting showing that anisotropy is confined mainly to the top 3 km of the volcanic edifice we find up to 10 shear wave anisotropy swa is present with a maximum centred at the geothermal reservoir fast shear wave orientations away from the reservoir align nne ssw parallel to the present day minimum compressive stress orientations on the edifice however are rotated ne sw in a manner we predict from field observations of faults at the surface providing fluid pressures are sufficient to hold two fracture sets open these fracture sets may be due to the repeated deformation experienced at aluto and initiated in caldera formation we therefore attribute the observed anisotropy to aligned cracks held open by over pressurised gas rich fluids within and above the reservoir this study demonstrates that shear wave splitting can be used to map the extent and style of fracturing in volcanic hydrothermal systems it also lends support to the hypothesis that deformation at aluto arises from variations of fluid pressures in the hydrothermal system these constraints will be crucial for future characterisation of other volcanic and geothermal systems in rift systems and elsewhere

seismic regime of a single fault model a special experimental technique enabled us to study in detail seismic events on a fault model in a uniaxial stress field the recording system used made it possible to investigate the radiation pattern for all the events observed including the precise determination of the dislocation origin

investigation of desaturation in an old tunnel and new galleries at an argillaceous site a numerical investigation of the desaturation process at the argillaceous tournemire site has been carried out this desaturation is initialized by the contact of the saturated rock with the ambient air in excavated openings the used hydraulical model is based on the richards approximation for unsaturated one phase flow coupled with the deformation of a porous medium with anisotropic linearly elastic behavior in relation to the extent of the desaturated zone around an excavated opening the intrinsic permeability and the relative permeability have been identified to be the most important model parameters the mechanical deformation process itself the seasonal influences and the tunnel lining are less important for the formation of the desaturated zone the comparison with measured saturation values bares some difficulties but indicates the principle capacity of the applied finite element codes to simulate the desaturation process the consideration of seasonal changes in humidity in the ambient air results in a constantly recurring desaturation resaturation cycle in the near field of the openings this seasonally influenced zone amount 1 2 m and is nearly independent from time and from a variation of model parameters within a reasonable range the possibility of material weakening in this zone is of special interest since claystone is a potential host rock for the disposal of radioactive waste

crustal anisotropy and state of stress at uturuncu volcano bolivia from shear wave splitting measurements and magnitude frequency distributions in seismicity abstract the physical signatures of unrest in large silicic magma systems are commonly observed in geophysical surveys yet the interactions between magmatic processes and crustal stresses are often left unconstrained stresses in the mid and upper crust exert a strong control on the propagation and stalling of magma and magma ascent can in turn change the magnitude and orientation of these stresses including those associated with hydrothermal systems this study assesses the state of stress at the restless uturuncu volcano in the bolivian andes with space depth and time using observations of seismic anisotropy and the magnitude frequency distributions of local earthquakes shear wave splitting measurements are made for 677 events in the upper crust 1 25 km below sea level between june 1 2009 and march 10 2012 and b values are calculated using the aki maximum likelihood method for a range of catalog subsets in the entire crust 5 to 65 km below sea level the b value of the crustal events is unusually low b 0 66 0 09 indicating that the seismogenic region features strong material with high stresses that are released with limited influence from hydrothermal fluids the 410 good quality shear wave splitting results have an average delay time of 0 06 0 002 s and an average percent anisotropy ranging from 0 25 0 04 to 6 2 0 94 with a mean of 1 70 0 32 fast shear wave polarization directions are highly variable and appear to reflect a combination of tectonic and magmatic stresses that overprint the regional e w compressive stress associated with the convergence of the nazca and south american plates the shear wave splitting results and b values suggest that the upper crust beneath uturuncu 0 7 km below the summit is characterized by a weak and localized hydrothermal system in a poorly developed fracture network we conclude that stresses imposed by crustal flexure due to magmatic unrest above the altiplano puna magma body activate crack opening on a pre existing fault beneath the volcano generating seismicity and a spatially variable 1 10 anisotropy above the brittle ductile transition zone these results suggest that strong stresses in relatively unfractured upper crustal rocks may locally inhibit fluid migration in large silicic magma systems leading to pluton emplacement and effusive volcanism rather than explosive eruptions

3 d analysis and interpretation of magnetotelluric data from the aluto langano geothermal field ethiopia the main ethiopian rift valley encompasses a number of volcanoes which are known to be actively deforming with reoccurring periods of uplift and setting one of the regions where temporal changes take place is the aluto volcanic complex it hosts a productive geothermal field and the only currently operating geothermal power plant of ethiopia we carried out magnetotelluric mt measurements in early 2012 in order to identify the source of unrest broad band mt data 0 001 1000 s have been acquired at 46 sites covering the expanse of the aluto volcanic complex with an average site spacing of 1 km based on this mt data it is possible to map the bulk electrical resistivity of the subsurface down to depths of several kilometres resistivity is a crucial geophysical parameter in geothermal exploration as hydrothermal and magmatic reservoirs are typically related to low resistive zones which can be easily sensed by mt thus by mapping the electrical conductivity one can identify and analyse geothermal systems with respect to their temperature extent and potential for production of energy 3 d inversions of the observed mt data from aluto reveal the typical electrical conductivity distribution of a high enthalpy geothermal system which is mainly governed by the hydrothermal alteration mineralogy the recovered 3 d conductivity models provide no evidence for an active deep magmatic system under aluto forward modelling of the tippers rather suggest that occurrence of melt is predominantly at lower crustal depths along an off axis fault zone a few tens of kilometres west of the central rift axis the absence of an active magmatic system implies that the deforming source is most likely situated within the shallow hydrothermal system of the aluto langano geothermal field

a local magnitude scale for the western great basin eastern sierra nevada from synthetic wood anderson seismograms abstract we have computed synthetic wood anderson seismograms for over 1100 arrivals at 10 three component broadband digital stations in the unr western great basin eastern sierra nevada network these represent all the available records from local earthquakes over magnitude 3 5 between 1990 and june of 1993 plus selected events of smaller magnitude there were 77 events ranging in magnitude from 2 2 to 5 9 including four events over magnitude 5 the distances considered ranged from 15 to 600 km with the best represented range being from 30 to 450 km we invert these measurements to determine distance and station corrections appropriate for a local magnitude scale constrained by richter9s original definition that an earthquake of m l 3 will cause a 1 mm zero to peak deflection of the wood anderson seismogram at 100 km from the epicenter the results between 30 and 450 km were essentially independent of choice of curve fitting parameters in the 30 to 500 km distance region the smooth distance correction curves were very similar to that determined by richter 1958 which is still used for southern california earthquakes we propose to use richter9s distance correction curve in reporting amplitude magnitudes from our digital network

a comparison of long term changes in seismicity at the geysers salton sea and coso geothermal fields geothermal energy is an important source of renewable energy yet its production is known to induce seismicity here we analyze seismicity at the three largest geothermal fields in california the geysers salton sea and coso we focus on resolving the temporal evolution of seismicity rates which provides important observational constraints on how geothermal fields respond to natural and anthropogenic loading we develop an iterative regularized inversion procedure to partition the observed seismicity rate into two components 1 the interaction rate due to earthquake earthquake triggering and 2 the smoothly varying background rate controlled by other time dependent stresses including anthropogenic forcing we apply our methodology to compare long term changes in seismicity to monthly records of fluid injection and withdrawal at the geysers we find that the background seismicity rate is highly correlated with fluid injection with the mean rate increasing by approximately 50 and exhibiting strong seasonal fluctuations following construction of the santa rosa pipeline in 2003 in contrast at both salton sea and coso the background seismicity rate has remained relatively stable since 1990 though both experience short term rate fluctuations that are not obviously modulated by geothermal plant operation we also observe significant temporal variations in gutenberg richter b value earthquake magnitude distribution and earthquake depth distribution providing further evidence for the dynamic evolution of stresses within these fields the differing field wide responses to fluid injection and withdrawal may reflect differences in in situ reservoir conditions and local tectonics suggesting that a complex interplay of natural and anthropogenic stressing controls seismicity within california s geothermal fields

a reanalysis of the hypocentral location and related observations for the great 1906 california earthquake we determine probabilistic hypocentral locations for the foreshock and mainshock of the great 1906 california earthquake through reanalysis of arrival time observations in conjunction with modern velocity models and advanced event location techniques we obtain two additional observations for the mainshock and one for the foreshock that were not used in previous location studies using a robust likelihood function for event location we generate a usable subset of the predominantly unreliable teleseismic readings and determine new wave type identifications for some of the local and teleseismic readings our locations are much better constrained than those of earlier studies even though we do not assume that the epicenter lies on the san andreas fault as did previous authors we confirm the conclusions of earlier studies that the local and teleseismic arrival time observations can be explained by a single foreshock focus and a single mainshock focus on the san andreas fault and that there is no single unique hypocenter that satisfies all available local observations the maximum likelihood point latitude 37 78 n longitude 122 51 w for our preferred mainshock location indicates a hypocenter to the west of san francisco close to the san andreas fault zone this hypocenter has a 68 confidence error of about 8 km parallel to the san andreas fault and about 24 km perpendicular to the fault and a depth in the midcrust of about 12 7 km the closest point on the san andreas fault to this hypocenter lies about 10 km to the northwest of the widely accepted mainshock epicenter of bolt 1968 our mainshock location is consistent with the association of initial rupture of the 1906 mainshock with a dilatational right bend or step over in the submerged san andreas fault system offshore of the golden gate our foreshock location is less well constrained than our mainshock location but is consistent with the foreshock hypocenter being at the same location as the mainshock hypocenter online material visualization of 3d probabilistic hypocentral locations associated with the 1906 earthquake

theory of time dependent rupture in the earth at a velocity x which is a well defined function of k this propagation is stable and quasi static and is referred to as subcritical crack growth we shall use the terms stable quasistatic and subcritical to mean propagation at velocities much less than the sonic velocities of the medium this behavior results from stress induced corrosion at the crack tip the principal corrodent for the present application being h20 this behavior has been firmly established in the laboratory for mode i tensile cracks in a wide variety of materials ineluding silicates and silicate glasses see cholz 1968a 1972a martin 1972 wiederhorn and bolz 1970 atkinson 1979 lawn and wilshaw 1975 knott 1973 the form of the relationship between k and x does not vary significantly with the material only the parameters in the law vary as an example we show in figure 1 data on subcritical crack growth in quartz after atkinson 1979 the empirical relationship is found to take the form

the origin of along rift variations in faulting and magmatism in the ethiopian rift the geological record at rifts and margins worldwide often reveals considerable along strike variations in volumes of extruded and intruded igneous rocks these variations may be the result of asthenospheric heterogeneity variations in rate and timing of extension alternatively pre existing plate architecture and or the evolving kinematics of extension during breakup may exert first order control on magmatism the main ethiopian rift mer in east africa provides an excellent opportunity to address this dichotomy it exposes along strike several sectors of asynchronous rift development from continental rifting in the south to incipient oceanic spreading in the north here we perform studies of volcanic cone density and rift obliquity along strike in the mer by synthesizing these new data in light of existing geophysical geochemical and petrological constraints on magma generation and emplacement we are able to discriminate between tectonic and mantle geodynamic controls on the geological record of a newly forming magmatic rifted margin the timing of rift sector development the three dimensional focusing of melt and the ponding of plume material where the rift dramatically narrows each influence igneous intrusion and volcanism along the mer however rifting obliquity plays an important role in localizing intrusion into the crust beneath en echelon volcanic segments along strike variations in volumes and types of igneous rocks found at rifted margins thus likely carry information about the development of strain during rifting as well as the physical state of the convecting mantle at the time of breakup