effective velocities in fractured media a numerical study using the rotated staggered finite difference grid the modelling of elastic waves in fractured media with an explicit finite difference scheme causes instability problems on a staggered grid when the medium possesses high contrast discontinuities strong heterogeneities for the present study we apply the rotated staggered grid using this modified grid it is possible to simulate the propagation of elastic waves in a 2d or 3d medium containing cracks pores or free surfaces without hard coded boundary conditions therefore it allows an efficient and precise numerical study of effective velocities in fractured structures we model the propagation of plane waves through a set of different randomly cracked media in these numerical experiments we vary the wavelength of the plane waves the crack porosity and the crack density the synthetic results are compared with several static theories that predict the effective p and s wave velocities in fractured materials in the long wavelength limit for randomly distributed and randomly orientated rectilinear non intersecting thin dry cracks the numerical simulations of velocities of p sv and sh waves are in excellent agreement with the results of the modified or differential self consistent theory on the other hand for intersecting cracks the critical crack density porosity concept must be taken into account to describe the wave velocities in media with intersecting cracks we propose introducing the critical crack density concept into the modified self consistent theory numerical simulations show that this new formulation predicts effective elastic properties accurately for such a case

the 2018 2019 seismo volcanic crisis east of mayotte comoros islands seismicity and ground deformation markers of an exceptional submarine eruption on may 10th 2018 an unprecedented long and intense seismic crisis started offshore east of mayotte the easternmost of the comoros volcanic islands the population felt hundreds of events over the course of one year 32 earthquakes with magnitude greater than 5 occurred including the largest event ever recorded in the comoros mw 5 9 on may 15th 2018 earthquakes are clustered in space and time unusual intense long lasting monochromatic very long period events were also registered from early july 2018 global navigation satellite system stations and interferometric synthetic aperture radar registered a large drift testimony of a large offshore deflation we describe the onset and the evolution of a large magmatic event thanks to the analysis of the seismicity from the initiation of the crisis through its first year compared to the ground deformation observation gnss and insar and modelling we discriminate and characterise the initial fracturing phase the phase of magma intrusion and dike propagation from depth to the sub surface and the eruptive phase that starts on july 3rd 2018 around fifty days after the first seismic events the eruption is not terminated two years after its initiation with the persistence of an unusual seismicity whose pattern has been similar since summer 2018 including episodic very low frequency events presenting a harmonic oscillation with a period of 16 s from july 2018 the whole mayotte island drifted eastward and downward at a slightly increasing rate until reaching a peak in late 2018 at the apex the mean deformation rate was 224 mm yr 1 eastward and 186 mm yr 1 downward during 2019 the deformation smoothly decreased and in january 2020 it was less than 20 of its peak value a deflation model of a magma reservoir buried in a homogenous half space fits well the data the modelled reservoir is located 45 5 km east of mayotte at a depth of 28 3 km and the inferred magma extraction at the apex was 94 m3 s 1 the introduction of a small secondary source located beneath mayotte island at the same depth as the main one improves the fit by 20 while the rate of the main source drops by a factor of 5 during 2019 the rate of the secondary source remains stable this might be a clue of the occurrence of relaxation at depth that may continue for some time after the end of the eruption according to our model the total volume extracted from the deep reservoir was 2 65 km3 in january 2020 this is the largest offshore volcanic event ever quantitatively documented this seismo volcanic crisis is consistent with the trans tensional regime along comoros archipelago

emplacement of pyroclastic dykes in riedel shear fractures an example from the sierra de san miguelito central mexico abstract although magmatic dykes are considered to be emplaced at depth there are very few studies about the emplacement of pyroclastic dykes there are two commonly accepted mechanisms for dike emplacements namely formation of a hydraulic tension fracture and filling of pre existing fractures in the sierra de san miguelito central mexico there is a dike swarm with orientations that do not concur with any of the above two cases instead the dikes have orientations resembling some type of secondary fractures within the fault bounded blocks within a fault bounded block the reverse drag of the normal faults can be explained by simple shear based on the simple shear mechanism we establish a series of equations to calculate the extension and the direction of the simple shear within a fault bounded block as an application of our methodology the value of which is the angle measured from the vertical to the direction of the simple shear is calculated from the domino faults in the sierra de san miguelito mesa central mexico the results demonstrate that the absolute values of for the inclined shear are smaller than 34 the inclined shear can be antithetic 0 synthetic 0 the pyroclastic dykes in the study area have most of the strikes between 300 and 330 and are sub parallel to the major faults the preferred dips of the pyroclastic dykes vary from 80 to 90 the distribution of the pyroclastic dykes in the study area indicates that the dikes were primarily emplaced along the r fractures due to simple shear these results are different from the traditional understanding which assumes that the dykes were mainly emplaced within the tension fractures

tomographic images and 3d earthquake locations of the seismic swarm preceding the 2001 mt etna eruption evidence for a dyke intrusion 1 on july 12 2001 mt etna experienced a sudden increase of seismic activity heralding one of the most intense eruptions of the past 30 years between july 12 and july 18 when the eruption started thousands of small magnitude earthquakes occurred and were recorded by a dense seismic network run by the istituto nazionale di geofisica e vulcanologia sezione di catania ingv ct hypocentral depths of earthquakes were very shallow mostly located above 3 km b s l and clustered near the summit area the high quality seismic dataset gives us the unique opportunity to study the process of magma migration before the eruption in this study we present the three dimensional earthquake locations and the velocity structure obtained by a tomographic inversion the shallowness of seismicity allowed us to enhance the details of the structure beneath the summit craters in a volume poorly defined by previous tomographic studies the presence of a high vp body previously observed at mt etna is confirmed at shallow depth beneath the southeastern part of the summit area the earthquakes preceding the eruption onset concentrated at its western border a low vp vs anomaly is found at 0 1 km depth just at the top of the volume where the magma intruded before the eruption this anomalous zone can be considered as molten material wealthy in gas the relocated seismicity occurs in a cylinder below the vents activated along the fracture system and exhibited an upward migration until the eruption all these results show evidence for the emplacement of a near vertical dyke striking about n s and a few kilometres south of the summit craters

coswarm and preswarm crustal deformation in the eastern izu peninsula central japan remarkable crustal deformation has repeatedly been observed in the eastern izu peninsula central japan synchronized to the occurrence of extensive earthquake swarms in the past 20 years among the instrumentation a borehole tiltmeter installed at station kwn which is located within a few kilometers of the swarm region has faithfully recorded large signals of ground tilt associated with more than 10 swarm sequences in the past 10 years in addition to coswarm signals it became clear that precursory tilt signals preceded major swarm activities the amplitudes of the precursory tilt signals are of order of 0 1 rad and they precede the onset of major swarms by several hours to half a day this signal is convincing because the same phenomena appeared at least 7 times and were also detected by other independent observations we speculate that a dike intrusion at depth causes crustal deformation and only as it reaches depths shallower than 10 km does seismic activity begin the precursory tilt signal can be utilized for practical prediction of the start of swarm activity in the eastern izu peninsula

structural petrological and thermal evolution of a tertiary ductile strike slip shear zone diancang shan yunnan the diancang shan a horst massif within the red river fault zone in yunnan people s republic of china preserves a structural petrological and thermal record of two distinct phases of tectonic activity a left lateral ductile shear that terminated between 20 and 17 ma and a ductile to brittle phase of normal faulting which began at 4 7 ma and remains active mylonitic rocks in the core of the range display an early steep high temperature ht schistosity and a horizontal stretching lineation that are both parallel to the trend of the belt kinematic indicators indicate that shear was left lateral the complex shape of the ht schistosity at the southern termination of the massif likely results from a large scale oblique left lateral c shear plane that dismembered the shear zone and separated the ailao shan and the diancang shan as left lateral deformation terminated thermochronological and thermobarometric results suggest that the gneisses were partially unroofed during this event along the eastern edge of the diancang shan the ht fabrics were overprinted by low temperature structures during activation of east dipping normal faults cooling associated with this normal right lateral faulting along the diancang shan and perhaps activation of the right lateral normal movement on the range front fault farther south along the ailao shan began at 4 7 0 1 ma these results tend to support the view that extrusion of indochina occurred along the left lateral red river shear zone between 35 and 19 17 ma initiation of right lateral normal slip during the late miocene may relate to eastward extensional collapse of the thickened tibetan crust or more probably to initiation of the second phase of extrusion

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

insights of dyke emplacement mechanics from detailed 3d dyke thickness datasets abstract dyke thickness datasets offer new insights into the detailed 3d geometry of dyke swarms and an exceptional opportunity to evaluate theoretical emplacement models the swartruggens kimberlite dyke swarm extends over 7 km along strike and intrudes a dolerite quartzite shale and andesitic lava succession the star kimberlite dykes cut shales and sandstones intersect a large dolerite sill and extend 15 km along strike both dyke swarms comprise anastomosing en echelon segments each several hundred metres long in total 1532 swartruggens dyke thickness measurements were taken to 750 m below the surface over a 250 m depth range and 3354 star dyke thickness measurements were taken over a 520 m depth range the swartruggens dyke thicknesses are 0 05 1 95 m mean 0 64 m whereas the star kimberlites range from

crustal deformation caused by magma migration in the northern izu islands japan intense crustal activity including earthquake swarms eruptions and a caldera formation in the northern izu islands started on june 26 2000 accompanied with large crustal deformation permanent gps data reveals the spatial pattern and time evolution of ground deformation the observations reveal shrinking and subsidence of miyakejima and extension between kouzushima and niijima we constructed a source model to explain the observed displacements during the period between june 26 and the end of august the model consists of a deflation source 0 12km beneath miyakejima tensile faults 1 04km located between miyakejima and kouzushima and several shear faults mass balance considerations suggest that a large amount of magma migrated 30km from miyakejima toward kouzushima

magma ascent and effusion from a tensile fracture propagating to the earth s surface summary an effusive volcanic eruption results from a sequence of different processes such as the pressurization of a magma chamber the propagation of a dyke and the flow of lava at the earth surface the aim of this paper is to establish relationships between the different quantities describing such processes we consider a spherical magma chamber filled with a low viscosity magma and included in a homogeneous and isotropic elastic half space we assume that as a result of the inflow of fresh magma or a phase transition the pressure in the chamber increases slowly during a finite time interval assuming that the pressure increase is linear in time we calculate the stress field generated in the surrounding medium considering the chamber as a centre of dilation we assume that a vertical tensile fracture originates at the top of the magma chamber after the rock strength is exceeded the fracture is assumed to propagate quasi statically along a vertical plane driven by the stress distribution both the cases of positive and negative buoyancy force are considered the problem is solved in two dimensions by considering the fracture as a tensile somigliana dislocation and expanding the associated stress release into chebyshev polynomials the fracture may reach the earth s surface or not depending on the depth and radius of the magma chamber the rate and duration of pressure increase the rock and magma densities and the rock strength when the fracture reaches the earth s surface we assume that it becomes a vertical conduit magma pours out from the vent driven by the pressure gradient in the conduit under the assumption of laminar flow of a newtonian fluid we evaluate the initial effusion rate as a function of the relevant model parameters the flow rate is found to be a non linear function of the density contrast we also establish a relationship between the flow rate in the conduit and the initial thickness of the ensuing lava flow in the case of effusion on a steep slope

a possible seismic gap within a highly seismogenic belt crossing calabria and eastern sicily italy a review of the seismic geologic and geodynamic information available for the calabro sicilian region of southern italy leads us to suggest a unifying view of the larger m 7 earthquakes that have occurred there in the last millennium coincidentally concentrated in the last 3 7 centuries the seismicity coincides with a narrow curvilinear extensional belt that passes through western calabria and eastern sicily wces belt and which includes a nearly continuous north south succession of primarily east dipping normal faults in our reconstruction of the seismotectonic process the faulting is activated by west northwest east southeast extension induced by residual rollback of the ionian subduction slab our analysis of the space time distribution of strong earthquakes indicates a zone of conspicuous aseismicity m 4 5 since 1700 m 7 since 1000 along the belt corresponding to the 30 km long scaletta fiumefreddo segment of the messina fiumefreddo fault in eastern sicily moreover because estimated recurrence times are on the order of a millennium for m 7 earthquakes along different parts of the wces belt and because historical data for destructive earthquakes in the first millennium a d are not detailed enough to allow reliable identification of the source zones we cannot definitely state whether or not there is a late stage seismic gap for a large earthquake in eastern sicily by applying standard relationships the potential for a magnitude 7 earthquake can be estimated for the scaletta fiumefreddo 30 km long normal fault segment the level of seismic activity has been low in the possible gap area in the past two decades when the upgraded local network has detected tens of events above magnitude 2 5 with values up to 3 7 hypocenter locations of these events seem to delineate the deep geometry of two faults reported in the surface geologic maps one of which is the scaletta fiumefreddo silent fault coulomb stress changes not larger than 0 6 bar produced on the silent fault by the most recent m 7 regional earthquake 1908 and the substantially nil coulomb stress change on the same fault by m 7 earthquakes of 1693 and 1783 imply relatively small perturbation by previous earthquakes to the silent fault compared with the coulomb stress perturbation of 2 2 5 bars estimated by other investigators on the 1908 earthquake source caused by major earthquakes of the previous centuries

dyke emplacement in fractured media application to the 2000 intrusion at izu islands japan summary we use a boundary element approach to study the problem of a large magma filled dyke embedded in a fractured damaged medium the numerical simulations show that the dyke opening and the crack extension force at the dyke tip increases with increasing fracture density we show that this behaviour can at moderate fracture densities be predicted by an effective media model simple analytical formulae are given for the increasing average opening of the dyke and the decreasing fracture toughness as functions of increasing fracture density the numerical and the theoretical results are found to be in good agreement in a first application we tested the model by using gps deformation and seismicity data from a recent lateral magma intrusion in izu islands japan this shows that a simple comparison of the distance change and the cumulative earthquake number enables the study of the stability of the dyke intrusion the method seems to distinguish whether the dyke opening is actively driven by magma influx or passively driven by an expansion due to fracturing

mechanical energy balance and apparent fracture toughness for dykes in elastoplastic host rock with large scale yielding the dynamics of dyke emplacement are typically modelled by assuming an elastic rheology for the host rock however the resulting stress field predicts significant shear failure in the region surro

stress changes focal mechanisms and earthquake scaling laws for the 2000 dike at miyakejima japan faulting processes in volcanic areas result from a complex interaction of pressurized fluid filled cracks and conduits with the host rock and local and regional tectonic setting often volcanic seismicity is difficult to decipher in terms of the physical processes involved and there is a need for models relating the mechanics of volcanic sources to observations here we use focal mechanism data of the energetic swarm induced by the 2000 dike intrusion at miyakejima izu archipelago japan to study the relation between the 3 d dike induced stresses and the characteristics of the seismicity we perform a clustering analysis on the focal mechanism fm solutions and relate them to the dike stress field and to the scaling relationships of the earthquakes we find that the strike and rake angles of the fms are strongly correlated and cluster on bands in a strike rake plot we suggest that this is consistent with optimally oriented faults according to the expected pattern of coulomb stress changes we calculate the frequency size distribution of the clustered sets finding that focal mechanisms with a large strike slip component are consistent with the gutenberg richter relation with a b value of about 1 conversely events with large normal faulting components deviate from the gutenberg richter distribution with a marked roll off on its right hand tail suggesting a lack of large magnitude events mw 5 5 this may result from the interplay of the limited thickness and lower rock strength of the layer of rock above the dike where normal faulting is expected and lower stress levels linked to the faulting style and low confining pressure

kilometre scale sheath fold at mattmark and implications for transport direction in the alps folds were the first tectonic features of rocks to be studied and as such are taken as a reference for the interpretation of other structures intuitively fold axes were assumed to be subperpendicular to the direction of transport nevertheless many studies of metamorphic belts showing intense ductile deformation describe folds with axes close to the transport direction in particular sheath folds1 9 are highly curvilinear folds elongated parallel to the direction of transport we discuss here a spectacular large sheath fold situated in a crustal shear zone in the monte rosa nappe swiss alps our analysis implies that both the geometry and significance of folds in the internal alps must be re examined we further demonstrate the existence of crustal ductile shear zones in an area of the alps where previously emphasis was generally placed on fold nappes the movements on these shear zones is often tens of kilometres

tertiary diachronic extrusion and deformation of western indochina structural and 40ar 39ar evidence from nw thailand the wang chao and three pagodas fault zones cut the western part of the indochina block and run parallel to the red river fault evidence of intense ductile left lateral shear is found in the lansang gneisses which form a 5 km wide elongated core along the wang chao fault zone dating by 40 ar 39 ar shows that such deformation probably terminated around 30 5 ma the wang chao and three pagodas faults offset the north striking lower mesozoic metamorphic and magmatic belt of northern thailand 40 ar 39 ar results suggest that this belt suffered rapid cooling in the tertiary probably around 23 ma these results imply that the extrusion of the southwestern part of indochina occurred in the upper eocene lower oligocene it probably induced rifting in some basins of the gulf of thailand and in the malay and mekong basins in the oligo miocene the continuing penetration of india into asia culminated with the extrusion of all of indochina along the ailao shan red river fault this occurred concurrently with the onset of e w extension more to the south plotting in a geographical reference frame the diachronic time spans of movement on left lateral faults east and southeast of tibet implies that the northward movement of the indian indenter successively initiated new strike slip faults located farther and farther north along its path

evidence from the ad 2000 izu islands earthquake swarm that stressing rate governs seismicity magma intrusions and eruptions commonly produce abrupt changes in seismicity far from magma conduits that cannot be associated with the diffusion of pore fluids or heat such swarm seismicity also migrates with time and often exhibits a dog bone shaped distribution the largest earthquakes in swarms produce aftershocks that obey an omori type exponential temporal decay but the duration of the aftershock sequences is drastically reduced relative to normal earthquake activity here we use one of the most energetic swarms ever recorded to study the dependence of these properties on the stress imparted by a magma intrusion a 1 000 fold increase in seismicity rate and a 1 000 fold decrease in aftershock duration occurred during the two month long dyke intrusion we find that the seismicity rate is proportional to the calculated stressing rate and that the duration of aftershock sequences is inversely proportional to the stressing rate this behaviour is in accord with a laboratory based rate state constitutive law suggesting an explanation for the occurrence of earthquake swarms any sustained increase in stressing rate whether due to an intrusion extrusion or creep event should produce such seismological behaviour

on the mechanisms governing dike arrest insight from the 2000 miyakejima dike injection abstract magma stored beneath volcanoes is sometimes transported out of the magma chambers by means of laterally propagating dikes which can lead to fissure eruptions if they intersect the earth s surface the driving force for lateral dike propagation can be a lateral tectonic stress gradient the stress gradient due to the topographic loads the overpressure of the magma chamber or a combination of those forces the 2000 dike intrusion at miyakejima volcano izu arc japan propagated laterally for about 30 km and stopped in correspondence of a strike slip system sub perpendicular to the dike plane then the dike continued to inflate without further propagation abundant seismicity was produced including five m 6 earthquakes one of which occurred on the pre existing fault system close to the tip of the dike at approximately the time of arrest it has been proposed that the main cause for the dike arrest was the fault induced stress here we use a boundary element numerical approach to study the interplay between a propagating dike and a pre stressed strike slip fault and check the relative role played by dike fault interaction and topographic loading in arresting the miyakejima dike we calibrate the model parameters according to previous estimates of dike opening and fault displacement based on crustal deformation observations by computing the energy released during the propagation our model indicates whether the dike will stop at a given location we find that the stress gradient induced by the topography is needed for an opening distribution along the dike consistent with the observed seismicity but it cannot explain its arrest at the prescribed location on the other hand the interaction of dike with the fault explains the arrest but not the opening distribution the joint effect of the topographic load and the stress interaction with strike slip fault is consistent with the observations provided the pre existing fault system is pre loaded with a significant stress released gradually during the dike fault interplay our results reveal how the mechanical interaction between dikes and faults may affect the propagation of magmatic intrusions in general this has implications for our understanding of the geometrical arrangement of rift segments and transform faults in mid ocean ridges and for the interplay between dikes and dike induced graben systems

graben formation and dike arrest during the 2009 harrat lunayyir dike intrusion in saudi arabia insights from insar stress calculations and analog experiments

the propagation of a dyke driven by gas saturated magma summary a new mathematical and numerical model is presented for the propagation of a pressure and buoyancy driven dyke filled with volatile saturated magma and a gas cap at its upper part the model accounts for coupling between conduit flow of a bubbly magma gas filtration through the magma gas accumulation in a gas cap and elastic deformation and fracturing of the host rock all these processes allow studying different regimes of dyke propagation the rate of propagation of dykes is controlled by the rate of the fracturing at the tip and by the flow rate of magma inside the dyke when high energy is needed to fracture the host rock and magma viscosity is low the rate of propagation is controlled by the rate of fracturing fracturecontrolled regime when the energy to fracture the host rock is low propagation is controlled by the magma flow rate magma controlled regime we study the transition between these regimes for the case of a constant magma vesicularity and constant mass of gas in the cap under these conditions the propagation of the dyke is self similar in the fracture controlled regime the propagation rate only weakly depends on the amount of the gas in the gas cap whereas at the magma controlled regime it is significantly enhanced with increase the mass of gas at the cap the gas pressure in the cap opens the dyke in front of the magma and allows magma flow rates that are significantly higher than predicted by models that ignore the gas cap the maximum propagation rate is obtained at the transition between the fracture and magma controlled regimes if the gas mass in the gas cap is high enough a gas pocket can separate from the magma as a distinct unconnected pocket and propagate as a gas filled crack at a constant velocity pressure decreases during ascent leads to higher vesicularity and faster gas filtration through the magma and into a gas cap gradual increase of the mass of gas in the cap is important in accelerating the propagation rate of dykes

a constitutive law for rate of earthquake production and its application to earthquake clustering seismicity is modeled as a sequence of earthquake nucleation events in which the distribution of initial conditions over the population of nucleation sources and stress ing history control the timing of earthquakes the model is implemented using solutions for nucleation of unstable fault slip on faults with experimentally derived rate and state dependent fault properties this yields a general state variable constitutive formulation for rate of earthquake production resulting from an applied stressing history to illustrate and test the model some characteristics of seismicity following a stress step have been explored it is proposed that various features of earthquake clustering arise from sensi tivity of nucleation times to the stress changes induced by prior earthquakes the model gives the characteristic omori aftershock decay law and interprets aftershock parameters in terms of stress change and stressing rate earthquake data appear to support a model prediction that aftershock duration defined as the time for rates to return to the back ground seismicity rate is proportional to mainshock recurrence time observed spatial and temporal clustering of earthquake pairs arises as a consequence of the spatial depen dence of stress changes of the first event of the pair and stress sensitive time dependent nucleation applications of the constitutive formulation are not restricted to the simple stress step models investigated here it may be applied to stressing histories of arbitrary complexity the apparent success at modeling clustering phenomena suggests the possibil ity of using the formulation to estimate short to intermediate term earthquake probabil ities following occurrence of other earthquakes and for inversion of temporal variations of earthquake rates for changes in driving stress

bounds on strain in large tertiary shear zones of se asia from boudinage restoration we have used surface balanced restoration of stretched boudinaged layers to estimate minimum amounts of finite strain in the mylonitic gneisses of the oligo miocene red river ailao shan shear zone yunnan china and of the wang chao shear zone thailand the layer parallel extension values thus obtained range between 250 and 870 we discuss how to use such extension values to place bounds on amounts of finite shear strain in these large crustal shear zones assuming simple shear these values imply minimum total and late shear strains of respectively 33 6 and 7 3 at several sites along the red river ailao shan shear zone for the wang chao shear zone a minimum shear strain of 7 4 is deduced assuming homogeneous shear would imply that minimum strike slip displacements along these two left lateral shear zones which have been interpreted to result from the india asia collision have been of the order of 330 60 km red river ailao shan and 35 20 km wang chao