magnetic fabrics of dike swarms from se bahia state brazil their significance and implications for mesoproterozoic basic magmatism in the sao francisco craton abstract magnetic fabrics were determined by applying two techniques anisotropy of low field magnetic susceptibility ams and anisotropy of anhysteretic remanence aar ams was performed in 81 dikes from ilheus olivenca and itaju do colonia swarms located in se bahia the dikes intrude high grade metamorphic terrains as old as 3 2 ga dikes are vertical or subvertical in dip and trend mainly e w they are tholeiitic in composition and are classified into two groups characterized by high tio 2 2 and incompatible elements contents and low tio 2 and incompatible elements contents respectively thicknesses vary from a few centimeters to ca 20 m average of 3 m 40 ar 39 ar data indicate ages of 1 012 ilheus dikes and 1 078 ga olivenca dikes rock magnetism shows that ti poor titanomagnetites up to pure magnetites pseudo single domain multidomain grain sizes carry the magnetic fabrics three types of ams fabric are recognized i normal fabric 64 dikes defined by the clustering of k 1 k 2 axes on the dike plane whereas k 3 axes are nearly perpendicular to it this fabric is interpreted as due to magma flow ii intermediate fabric seven dikes characterized by k 1 k 3 axes clustering close to dike plane and k 2 axes perpendicular to the dike wall it was interpreted as due to vertical compaction of a static magma column with the minimum stress along the dike direction and iii inverse fabric nine dikes defined by k 2 k 3 plane parallel to the dike plane and k 1 perpendicular to dike wall aar measurements were performed in three dikes two of them with inverse and one with normal ams fabrics aar was coaxial for normal ams fabric and it resulted in intermediate and normal for inverse ams fabrics a combination of ams and aar fabrics suggests that the magmatic fabric for these dikes that should be flow was overprinted by some local event probably related to brasiliano orogeny processes after dike emplacement inverse ams fabric would record this event normal fabric is the most important one found for the studied swarms the analysis of k 1 inclination permitted one to infer that the dikes from ilheus were fed dominantly by horizontal fluxes k 1 k 1 60 dikes from itaju do colonia region have the steepest k 1 suggesting that this region could be closer to a magma source on the other hand a magma source near to olivenca area could be also inferred since the majority of the dikes were fed by inclined flow the existence of more than one source is supported by geochemical geochronological and paleomagnetic data which have shown that contemporaneous dikes were fed by compositionally distinct magmas the ams data also suggest that the majority of dikes were fed from upward flow coming from west to east and only a few of them were fed from upward flow from east to west

the june july 1993 seismo acoustic event at coaxial segment juan de fuca ridge evidence for a lateral dike injection the coaxial segment juan de fuca ridge was the site of an intense swarm of earthquakes that began at 21 43 gmt on june 26 1993 julian day 177 the swarm started near 46 15 n and migrated northward over the next 40 hours to 46 36 n where the majority of 676 events occurred during the following 3 weeks of activity the earthquakes propagated nne at a velocity of 0 3 0 1 m s 1 from the southern to northern swarm sites the activity went undetected by land based seismic networks along the oregon and washington coasts suggesting that the earthquakes were all m 4 0 the character of this earthquake swarm is very similar to dike injections observed at krafla and kilauea volcanoes the earthquake activity and subsequent migration probably represent a lateral dike injection into faults and fissures comprising the coaxial segment the reservoir acting as the dike s source likely resides beneath or to the south of the initial southern swarm of earthquakes the large magma supply at axial volcano cannot be ruled out as the source for the coaxial dike even though axial volcano exhibited no earthquake activity related to the coaxial swarm the t wave earthquake swarm reported here is the first deep ocean observation of volcanic seismicity associated with what most likely was a mid ocean ridge dike injection event

p and t wave detection thresholds pn velocity estimate and detection of lower mantle and core p waves on ocean sound channel hydrophones at the mid atlantic ridge since 1999 six sound fixing and ranging sofar hydrophones have been moored along the mid atlantic ridge mar 15 35 n these hydrophones 8 bit data resolution are designed for long term monitoring of mar seismicity using the acoustic t waves of seafloor earthquakes the completeness level of the mar t wave earthquake catalog estimated from size frequency constraints is mb 3 0 a significant improvement in detection compared to the mb 4 6 completeness level estimated from national earthquake information center magnitude frequency data the hydrophones also detect the acoustic phase of converted upper mantle p arrivals from regional earthquakes at epicentral distances of 374 1771 km and from events as small as mb 3 6 these regional p waves are used to estimate a pn velocity of 8 0 0 1 km sec 1 along the east and west mar flanks an unexpected result was the identification of p arrivals from earthquakes outside the atlantic ocean basin the hydrophones detected p waves from global earthquakes with magnitudes of 5 8 8 3 at epicentral distances ranging from 29 6 to 167 2 examination of travel times suggests these teleseismic p waves constitute the suite of body wave arrivals from direct mantle p to outer and inner core reflected refracted phases the amplitudes of the teleseismic p waves also exhibit the typical solid earth wave field phenomena of a p shadow zone and caustic at d 144 these instruments offer a long term relatively low cost alternative to ocean bottom seismometers that allows for obser vation of pn velocities and mantle core phases arriving at normally inaccessible deep sea locations

connectivity of molten fe alloy in peridotite based on in situ electrical conductivity measurements implications for core formation in terrestrial planets abstract the connectivity of molten fe s in peridotite has been experimentally investigated by means of in situ electrical conductivity measurements at high temperatures and 1 gpa starting materials were powdered mixtures of peridotite klb 1 with various amounts 0 3 6 13 19 24 vol of the 1 gpa eutectic composition in the fe fes binary system at temperatures above the eutectic point in the fe fes system 980 c and below the solidus of klb1 1200 c molten fe s in a solid silicate matrix interconnects when the volume fraction is over 5 conductivity temperature paths indicate that in the presence of partial silicate melting the connectivity of molten fe s in a peridotite matrix is inhibited based on observations of retrieved samples the percolation threshold of fe s melts in the presence of low to moderate degrees of silicate melt is estimated at 13 2 vol these results indicate that if the volume fraction of fe alloy in a planetesimal was initially greater than 5 and if early heating by decay of radionuclides raised the temperature of the interior above the fe alloy melting point initial metal segregation was controlled by permeable flow of molten iron alloy in a solid silicate matrix these conditions were likely met by many terrestrial objects in the early solar nebula efficient removal of residual fe alloy 5 vol from silicate requires high degree melting of silicate so that metal can segregate as droplets giant impacts during the final stage of accretion of large planetary objects could supply the energy required for high degrees of melting alternatively if initial metal segregation were delayed until a planetary object grew to large size 1000 km in diameter release of gravitational potential energy due to metal segregation could contribute enough heat to form a magma ocean

coupled core mantle thermal evolution of early mars 1 several arguments point out that at the end of planetary accretion the core of mars was likely to be much hotter than its mantle resulting in the formation of a completely or partially molten thermal boundary layer at the base of the mantle here we address the following questions how did the superheated core cool and what role did it play in the early mantle dynamics of mars we divide the coupled core mantle evolution of early mars into two stages during the first stage vigorous convection within the molten boundary layer removes the heat from the core so that the boundary layer expands up as the boundary layer gets thicker the temperature of the layer decreases eventually the temperature of the molten boundary layer drops down to the temperature for the rheological transition melt fraction 40 within 100 years this stage is described by a parameterized convection approach the second stage is modeled in spherical shell geometry using the fully three dimensional finite element code citcoms a single plume superplume forms by the instability of the thermal boundary layer the superplume stage lasts much longer on the scale of millions to hundreds of millions of years depending on the mantle viscosity during both stages of evolution the heat flux can easily satisfy the requirements for the dynamo

dynamics of core formation and equilibration by negative diapirism 1 we investigate dynamically the timing and metal silicate equilibration processes during core formation by negative diapirism using numerical modeling in 3 d axisymmetric geometry we follow the sinking of iron rich diapirs through a viscous silicate mantle we carried out a parameter study in which shear heating as well as several viscous rheologies are considered and systematically varied general scaling laws are derived for the diapir sinking velocity as well as for the silicate melt generation and distribution around the diapir using these scaling laws we show that metal diapirs of a radius 1 100 m sinking through a partially molten silicate mantle can chemically equilibrate within the timescales suggested by geochemical constraints therefore negative diapirism can contribute significantly to metal silicate equilibration processes during core formation

t wave generation and propagation a comparison between data and spectral element modeling t waves are underwater acoustic waves generated by earthquakes modeling of their generation and propagation is a challenging problem using a spectral element code specfem2d this paper presents the first realistic simulations of t waves taking into account major aspects of this phenomenon the radiation pattern of the source the propagation of seismic waves in the crust the seismic to acoustic conversion on a non planar seafloor and the propagation of acoustic waves in the water column the simulated signals are compared with data from the mid atlantic ridge recorded by an array of hydrophones the crust water interface is defined by the seafloor bathymetry different combinations of water sound speed profiles and sub seafloor seismic velocities and frequency content of the source are tested the relative amplitudes main arrival times and durations of simulated t phases are in good agreement with the observed data differences in the spectrograms and early arrivals are likely due to too simplistic source signals and environmental model these examples demonstrate the abilities of the specfem2d code for modeling earthquake generated t waves

turbulent mixing of metal and silicate during planet accretion and interpretation of the hf w chronometer in the current view of planet formation the final assembly of the earth involved giant collisions between proto planets 1000 km radius with the moon formed as a result of one such impact at this stage the colliding bodies had likely differentiated into a metallic core surrounded by a silicate mantle during the moon forming impact nearly all metal sank into the earth s core we investigate to what extent large self gravitating iron cores can mix with surrounding silicate and how this influences the short lived chronometer hf w used to infer the age of the moon we present fluid dynamical models of turbulent mixing in fully liquid systems attempting to place constraints on the degree of mixing erosion of sinking cores driven by rayleigh taylor instability does lead to intimate mixing and equilibration but large blobs 10 km diameter do not emulsify entirely emulsification is enhanced if most of the accreting metal cores deform into thin structures during descent through the earth s mantle yet only 1 20 of earth s core would equilibrate with silicate during earth s accretion the initial speed of the impactor is of little importance we proceed to evaluate the mixing potential for shear instabilities where silicate entrainment across vertical walls causes mixing the turbulent structure indicates that vortices remain at the largest scale and do not mix to centimeter length scale where diffusion operates and isotopes can equilibrate thus incomplete emulsification and equilibration of accreting iron cores is likely to occur the extent of metal silicate equilibration provides key information for interpretation of siderophile budgets and the timing of core formation using the hf w chronometer the time scale of core formation derived from the hf w chronometer is usually tied to the last major metal silicate re equilibration believed to coincide with time of the moon forming impact however we show that large cores have limited ability to reset the hf w system in the silicate earth excess 182 w in bulk silicate earth is more sensitive to early core formation processes than to radiogenic ingrowth after the last giant impact

long term seismicity of the reykjanes ridge north atlantic recorded by a regional hydrophone array summary the seismicity of the northern mid atlantic ridge was recorded by two hydrophone networks moored in the sound fixing and ranging sofar channel on the flanks of the mid atlantic ridge north and south of the azores during its period of operation 05 2002 09 2003 the northern sirena network deployed between latitudes 40 20 n and 50 30 n recorded acoustic signals generated by 809 earthquakes on the hotspot influenced reykjanes ridge this activity was distributed between five spatio temporal event clusters each initiated by a moderate to large magnitude 4 0 5 6 m earthquake the rate of earthquake occurrence within the initial portion of the largest sequence which began on 2002 october 6 is described adequately by a modified omori law aftershock model although this is consistent with triggering by tectonic processes none of the reykjanes ridge sequences are dominated by a single large magnitude earthquake and they appear to be of relatively short duration 0 35 4 5 d when compared to previously described mid ocean ridge aftershock sequences the occurrence of several near equal magnitude events distributed throughout each sequence is inconsistent with the simple relaxation of mainshock induced stresses and may reflect the involvement of magmatic or fluid processes along this deep 2000 m section of the reykjanes ridge

toggling of seismicity by the 1997 kagoshima earthquake couplet a demonstration of time dependent stress transfer 1 two m 6 well recorded strike slip earthquakes struck just 4 km and 48 days apart in kagoshima prefecture japan in 1997 providing an opportunity to study earthquake interaction aftershocks are abundant where the coulomb stress is calculated to have been increased by the first event and they abruptly stop where the stress is dropped by the second event this ability of the main shocks to toggle seismicity on and off argues that static stress changes play a major role in exciting aftershocks whereas the dynamic coulomb stresses which should only promote seismicity appear to play a secondary role if true the net stress changes from a sequence of earthquakes might be expected to govern the subsequent seismicity distribution however adding the stress changes from the two kagoshima events does not fully capture the ensuing seismicity such as its rate change temporal decay or migration away from the ends of the ruptures we therefore implement a stress transfer model that incorporates rate state friction in which seismicity is treated as a sequence of independent nucleation events that are dependent on the fault slip slip rate and elapsed time since the last event the model reproduces the temporal response of seismicity to successive stress changes including toggling decay and aftershock migration nevertheless the match of observed to predicted seismicity is quite imperfect due perhaps to inadequate knowledge of several model parameters however to demonstrate the potential of this approach we build a probabilistic forecast of larger earthquakes on the expected rate of small aftershocks taking advantage of the large statistical sample the small shocks afford not surprisingly such probabilities are highly time and location dependent during the first decade after the main shocks the seismicity rate and the chance of successive large shocks are about an order of magnitude higher than the background rate and are concentrated exclusively in the stress triggering zones

dynamics of metal silicate separation in a terrestrial magma ocean in a terrestrial magma ocean the metal silicate separation involved small metal droplets our goal is to better understand the dynamics of the metal droplet scenario the mechanism of sedimentation in a vigorously convecting and strongly rotating magma ocean may differ significantly from settling droplets in a still fluid in order to systematically study the parameter dependence on the style of motion we utilize two dimensional and three dimensional numerical convection models combined with a tracer based sedimentation method we investigate the characteristic flow patterns resulting from the competing effects of convection and droplet settling and find three styles of motion a temperature dominated style where most droplets remain suspended a droplet dominated style where the droplets separate from the fluid and a style of repetitive motion we find that the droplet dominated style is relevant to the magma ocean in this scenario the droplets settle and form a dense bottom layer one of the key findings of this work is that the formation of the dense bottom layer and therefore the separation of metal droplets from the liquid silicate occurs on a characteristic timescale which is identical with the stokes settling time finally we study the chemical interaction of settling metal droplets and liquid silicate and discuss how accurately a simple parameterized model can describe the chemical equilibration in the metal rain scenario of a terrestrial magma ocean we find agreement between the parameterized model and our two dimensional model our simulations which include thermal convection into the metal rain model confirm the fundamental observation that metal droplets are not permanently suspended in the convecting silicate melt but that they sink and rapidly form a dense layer at the base of the magma ocean

melt flow in a variable viscosity matrix it is often assumed that the molten regions of planetary interiors can be modeled as a compacting viscous matrix permeated by a less viscous melt phase in order to assess the true behaviour it is important to test as many different aspects of these models as possible in the calculations presented here the matrix viscosity is made to decrease as the proportion of melt increases new results in two dimensions show that if only gravity drives the flow pockets of melt travel upwards as solitary waves and the behaviour is similar to the well known constant viscosity case however when an external shear strain is applied an instability occurs and melt flows into elongated pockets or veins perpendicular to the least compressive stress whilst veins much greater than a compaction length in width are suppressed their minimum width is not clearly constrained the growth rate of veins is directly proportional to the applied strain rate so that their degree of development is proportional to the total strain it is suggested that under mid ocean ridges the total strain may be enough for such veins to be important

bulk friction modeling of afterslip and the modified omori law abstract afterslip data from the superstition hills fault in southern california a creep event on the same fault the modified omori law and cumulative moments from aftershocks of the 1957 aleutian islands earthquake all indicate that the original formulation by dieterich 1981 constitutive properties of faults with simulated gouge agu geophys monogr 24 103 120 for friction evolution is more appropriate for systems far from instability than the commonly used approximation developed by ruina 1983 slip instability and state variable friction laws j geophys res 88 10359 10370 to study instability the mathematical framework we use to test the friction models is a one dimensional massless spring slider under the simplifying assumption proposed by scholz 1990 the mechanics of earthquakes and faulting cambridge university press and used by marone et al 1991 on the mechanics of earthquake afterslip j geophys res 96 8441 8452 that the state variable takes on its velocity dependent steady state value throughout motion in response to a step in stress this assumption removes explicit state variable dependence from the model obviating the need to consider state variable evolution equations anti derivatives of the modified omori law fit our data very well and are very good approximate solutions to our model equations a plausible friction model with omori law solutions used by wesson 1988 dynamics of fault creep j geophys res 93 8929 8951 to model fault creep and generalized by rice 1983 constitutive relations for fault slip and earthquake instabilities pure appl geophys 121 443 475 to a rate and state variable friction model yields exactly omori s law with exponents greater than 1 but yields unstable solutions for omori exponents less than 1 we estimate from the dieterich formulation the dimensionless parameter a which is equal to the product of the nominal coefficient of friction and the more commonly reported friction parameter a we find that a is typically positive qualitatively consistent with laboratory observations although our observations are considerably larger than laboratory values however we also find good model fits for a when data correspond to omori exponents less than 1 a modification of the stability analysis by rice and ruina 1983 stability of steady frictional slipping j appl mech 50 343 349 indicates that a is not a consequence of our assumption regarding state variable evolution a consistent interpretation of a in terms of laboratory models appears to be that the data are from later portions of processes better characterized by two state variable friction models a is explained by assuming that our data cannot resolve the co seismic evolution of a short length scale state variable to a velocity weakening state our parameterization leads to an apparent negative instantaneous viscosity we estimate the largest critical slip distance associated with afterslip to be 1 10 cm consistent with other estimates for near surface materials we assume that our observed large values for a reflect the fact that our model ignores the geometrical complexities of three dimensional stresses in fractured crustal materials around a fault zone with frictional stresses that vary on a fault surface our one dimensional model parameters reflect spatially averaged bulk stress and frictional properties of a fault zone where we clearly cannot specify the details of the averaging process our analysis of omori s law suggests that bulk frictional properties of a fault zone are well described by our simple laboratory based models but they would need to change during the seismic cycle for a mainshock instability to recur unless a mainshock aftershock sequence were characterized by a process similar to the arrested instabilities possible in two state variable systems

a comparison of volcanic edifices at the reykjanes ridge and the mid atlantic ridge at 24 30 n a detailed analysis of high resolution side scan sonar data shows that the volcanic edifices built within the axial zones of the slow spreading 20 mm yr reykjanes ridge and the slow spreading 25 mm yr mid atlantic ridge are remarkably similar in fact individual seamounts hummocks rounded mounds and hummocky ridges at the reykjanes ridge have nearly identical morphological counterparts at the mid atlantic ridge mar arguing that commonly eruptive conditions are similar at the two ridges the construction of these volcanic edifices typical of slow spreading ridges occurs at the reykjanes ridge despite its proximity to the iceland hot spot and the influence of the hot spot on other characteristics of the ridge axis such as its overall shape an axial high north of about 59 n its shallow axial depths and its thicker crustal section the volume of magma that goes into constructing volcanic edifices at each of 12 segments of the mar and four regions of the reykjanes ridge is calculated using multibeam bathymetry data our results indicate that edifice volume per unit seafloor area equivalent to a uniform thickness over the area of interest is larger at the mar than at the reykjanes ridge on average taking into account spreading rate we then estimate b the average building rate per unit length along the axis which is also larger at the mar we conclude that the same volcanic edifices build the crust at the reykjanes ridge and mar but that their emplacement is more frequent at the mar to explain our results and reconcile them with those of magde and smith 1995 who documented that seamounts are taller and more abundant at the reykjanes ridge than at the mar we consider the controls on diking and eruption within the axial zones of the two ridges we suggest that the combination of slower spreading rate and the oblique geometry of spreading at the reykjanes ridge leads to fewer eruptions which are more widely dispersed across the axial zone than at the mar if this hypothesis is correct the proportion of seamounts modified by subsequent eruptions will be less this might also explain the taller heights of the seamounts if there is less overlap and infilling between edifices

deformation induced metal melt networks in silicates implications for core mantle interactions in planetary bodies abstract previous analyses of texturally equilibrated hydrostatically annealed olivine rocks containing metal and metal sulfide melts have determined that these melts do not form interconnected networks at low volume fractions to characterize the change in connectivity of metal melt in response to deformation we used optical microscopy and electron microprobe analyses to investigate the microstructures of samples of olivine gold sheared to shear strains of 200 deformation causes the development of a strong melt preferred orientation 13 15 to the shear plane and antithetic to the shear direction this melt preferred orientation consists of previously isolated pockets of gold melt that have connected into networks extending for many grain lengths as well as linear trails of isolated pockets melt was observed to have migrated 125 200 m in response to pressure gradients induced by serrations in the pistons in contact with the sample proving that finite permeability exists during deformation well below the critical melt fraction for connectivity consideration of the balance between differential and interfacial stresses on melt pockets indicates that melt channels formed in response to deformation can drain rocks to a very small melt fraction in a direction subparallel to the shear direction stranding less melt than drainage under hydrostatic conditions therefore core formation by percolative flow of non wetting melt in actively deforming planets and planetesimals may be accomplished more efficiently than previously believed in addition core material entrained in the mantle at the core mantle boundary will be strongly aligned sub horizontally in regions of horizontal shear and have little to no permeability in the vertical direction providing a mechanism to trap core material in d

monitoring pacific ocean seismicity from an autonomous hydrophone array since may 1996 an array of autonomous hydrophone moorings has been continuously deployed in the eastern equatorial pacific to provide long term monitoring of seismic activity including low level volcanic signals along the east pacific rise between 20 n and 20 s and the galapagos ridge the instruments and moorings were designed to continuously record low frequency acoustic energy in the sofar channel for extended periods and produce results comparable to those previously derived by using the u s navy sound surveillance system sosus in the northeast pacific the technology and methodology developed for this experiment including instrument design mooring configuration analysis software location algorithms with an analysis of errors and a predicted error field are described in detail volcanic activity is observed throughout the pacific along with seismicity along transform faults subduction zones and intraplate regions comparison data sets indicate detection thresholds and accuracy better than the land networks for open ocean areas and results comparable to or better than sosus volcanic seismicity along the fast spreading east pacific rise appears similar to documented examples in the northeast pacific but with much shorter durations one example from the intermediate spreading galapagos ridge is comparable to northeast pacific examples and several episodes of activity were observed in the wilkes transform fault zone a site of continuing off axis seismicity is located near 18 s and 116 w isolated intraplate earthquakes are observed throughout the study area earthquake information from this experiment and future observations will be provided through the world wide web and earthquake data centers

earth s core formation aided by flow channelling instabilities induced by iron diapirs abstract the core formation mechanism remains poorly known an unstable gravitational configuration of a dense molten metallic layer overlying a cold chondritic protocore is predicted by most studies which leads to the formation of a rayleigh taylor rt instability recent results dahl t w 2005 turbulent mixing during planet accretion and core formation interpretation of the hf w chronometer and implications for the age of the moon m sc thesis university of copenhagen indicate that additionally iron cores of predifferentiated planetesimals are also able to plunge mostly intact into the cold protocore and create large iron diapirs for both scenarios we propose the application of the stress induced melt channelling mechanism stevenson d j 1989 spontaneous small scale melt segregation in partial melts undergoing deformation geophys res lett 16 1 067 1 070 in the region surrounding an incipient iron diapir we therefore perform numerical experiments solving the two phase two composition flow equations within a 2d rectangular box with symmetrical boundary conditions we apply the compaction boussinesq approximation cba and include a depth dependent gravity for simplicity we use a constant viscosity for the solid phase and a melt fraction dependent rheology for the partially molten region around the diapir we investigate the physical conditions under which the melt channels can form and whether they are applicable to the early earth as a result for sufficiently small melt retention numbers iron rich melt channels develop within a region of approximately twice the diapir s size this could lead to effective draining of the surrounding region and might initiate cascading daughter diapirs the region of the protocore drained by this cascading mechanism is expected to significantly increase with depth and thus indicates an effective mechanism to also extract iron melt from deeper parts of the initially chondritic protocore this mechanism could effectively accelerate the process of core formation

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

new observations of local seismicity by the sn 1 seafloor observatory in the ionian sea off shore eastern sicily italy summary seismicity in eastern sicily as recorded by the submarine network 1 seafloor observatory sn1 in the period from 2002 october to 2003 may is examined with the aim of identifying the as yet poorly known seismogenic zones placed in the ionian basin where some of the strongest earthquakes have occurred a comparison between the seismic recordings of land networks and the seafloor station has allowed us to focus on low magnitude seismicity only recorded by sn 1 we have analysed 239 high quality events from among a total of 485 seismic signals not included in the land based network bulletins the waveform features and the possible source zones for those events are investigated by means of polarization and particle motion techniques most of the 239 events 213 are characterized by high values of rectilinearity typical of p and s arrival particle motions while the remaining 26 events have different polarization features with an emergent first phase and prevalently planar polarization we have interpreted the latter signals as being associated to submarine landslides from particle motion analysis we have determined the azimuthal distribution of the events and the incidence angles of p waves in respect to the observatory with the aim of determining their distribution in relation to the active but scarcely known structural setting of the off shore area moreover the integrated locations of some earthquakes occurring in the study area and recorded by sn 1 and land stations was performed to determine the apparent p wave velocity necessary to calculate source station distances as an additional result of the integration we have obtained more accurate locations of earthquakes occurring in the coastal and off shore areas of eastern sicily associated with reduced horizontal and vertical errors and significantly lower values of azimuthal gaps lastly a location distribution of the 213 analysed events was obtained by setting two conditions 1 a maximum epicentral distance to a fixed depth coinciding with the depth of the seafloor station and 2 a minimum epicentral distance associated to the maximum depth of events accordingly two patterns of seismicity were determined for the maximum and the minimum expected spread of the hypocentres the main features of both patterns are a diffuse seismicity in the western ionian basin with a major epicentre density se of sn 1 and a depth of most of the events within 60 km local magnitude determination was also performed taking into account an attenuation law proposed for southeastern sicily despite the uncertainties in the location distribution using single station recordings the results show diffuse seismicity all around sn 1 and in particular in the off shore area

numerical models of faulting at oblique spreading centers obliquely spreading mid ocean ridges such as the reykjanes ridge display two distinct fault sets distinguishable by orientation and position on axis faults are oriented oblique to both the trend of the axis and the normal to the relative plate separation vector while faults on the flanks strike approximately parallel to the ridge axis numerical modeling techniques are used here to simulate the development of faulting on the reykjanes ridge stresses acting in a cross section through the lithosphere at a slow spreading ridge are investigated using the fast lagrangian analysis of continua flac explicit difference modeling software the predicted stresses from the cross sectional models are imposed as a condition in boundary element models of fracture propagation and linkage on axis fault simulations run under conditions similar to the reykjanes ridge successfully reproduce the mapped distribution of faults and predict the observed orientation of the axial volcanic ridges simulations of fractures away from the axis show the development of axis parallel faults by the interaction and linkage of fractures which have been rafted off axis also in accord with observations stresses modeled in cross section favor downdip displacement on faults dipping toward the ridge axis

spatiotemporal distribution of the seismicity along the mid atlantic ridge north of the azores from hydroacoustic data insights into seismogenic processes in a ridge hot spot context 1 the seismicity of the north atlantic was monitored from may 2002 to september 2003 by the sirena array of autonomous hydrophones the hydroacoustic signals provide a unique data set documenting numerous low magnitude earthquakes along the section of the mid atlantic ridge mar located in a ridge hot spot interaction context during the experiment 1696 events were detected along the mar axis between 40 n and 51 n with a magnitude of completeness level ofmb 2 4 inside the array location errors are in the order of 2 km and errors in the origin time are less than 1 s from this catalog 15 clusters were detected the distribution of source level sl versus time within each cluster is used to discriminate clusters occurring in a tectonic context from those attributed to non tectonic i e volcanic or hydrothermal processes the location of tectonic and non tectonic sequences correlates well with regions with positive and negative mantle bouguer anomalies mbas indicating the presence of thinner colder and thicker warmer crust respectively at the scale of the entire array both the complete and declustered catalogs derived from the hydroacoustic signals show an increase of the seismicity rate from the azores up to 43 30 n suggesting a diminishing influence of the azores hot spot on the ridge axis temperature and well correlated with a similar increase in the along axis mbas the comparison of the mar seismicity with the residual mba rmba at different scales leads us to think that the low magnitude seismicity rates are directly related to along axis variations in lithosphere rheology and temperatures

how to make a planet an introduction the special collection building planets the dynamics and geochemistry of core formation aims to combine cutting edge experimental analytical and modeling results with review articles defining the state of the science and current challenges to our understanding of the origin geophysics and geochemistry of planetary cores our goal is to highlight novel and interdisciplinary approaches that address aspects of core formation and evolution at the atomic grain and planetary scales

the northern mid atlantic and reykjanes ridges spreading center morphology between 55 50 n and 63 00 n seamarc ii and hydrosweep acoustic imagery and bathymetry reveal that the northern mid atlantic ridge mar and reykjanes ridge between 55 50 n and 63 00 n are segmented seafloor spreading centers that exhibit structural morphologies that vary with distance from iceland the northern 100 km of the mar 55 50 n 56 47 n is oriented 005 perpendicular to the spreading azimuth and contains four third order spreading segments separated along strike by right and left stepping axial discontinuities the boundary between mar and reykjanes ridge occurs at 56 47 n where the bight transform fault offsets the neo volcanic axis 15 km right laterally across a single linear fault oriented 092 this is the only transform fault between 55 50 n and 63 00 n and is inferred to have generated the bight fracture zone the reykjanes ridge lies between the bight transform and iceland s reykjanes peninsula and is oriented 30 oblique to the spreading orthogonal direction axial faults and volcanic systems on the reykjanes ridge are arranged in a right stepping en echelon pattern and display strikes intermediate between the spreading orthogonal direction and the strike of the ridge which is compatible with experimental models of oblique extension spreading segments on the northern mar and reykjanes ridge are defined structurally and bathymetrically at the smallest scale the volcanic axis is segmented into individual volcanic systems 4 45 km long which are separated along strike by intervolcano gaps or offsets these features are analogous to fourth order spreading segments documented elsewhere on the mar axial volcanic systems are superimposed on intermediate wavelength 13 65 km axial topographic highs that constitute third or second order spreading segments segment boundaries occur at axial depth maxima which correspond to offsets in the volcanic axis four primary characteristics of the spreading axis axial valley depth axial boundary fault throw relief along the neovolcanic axis and degree of inter segment structural discontinuity decrease toward the north this variation is consistent with higher crustal temperatures near iceland due to either thicker crust or elevated mantle temperatures

two dimensional stokes flow around a heated cylinder a possible application for diapirs in the mantle 1 it is widely assumed that the separation of metal and silicates in a homogeneously accreted protoplanet occurred rather rapidly the process of core formation through the descent of large hot iron rich bodies through a cold silicate protoplanet is discussed if iron diapirs sink toward the planet center with the stokes velocity they would require more than 800 ma to form a core in the case of a planet similar to the earth we therefore implement a temperature dependent rheology for the silicate material surrounding the diapir which leads to a reduction of the drag force exerted on the iron drop we compare the drag force for different sinking velocities to the body force on the diapir and find that the terminal velocity in the temperature dependent viscosity model is a factor of 30 higher than that in an isoviscous medium on the basis of these results the core formation time for the earth could be as little as 30 ma

aftershock sequences in the mid ocean ridge environment an analysis using hydroacoustic data abstract hydroacoustic data from autonomous arrays and the u s navy s sound surveillance system sosus provide an opportunity to examine the temporal and spatial properties of seismicity along portions of the slow spreading mid atlantic ridge mar intermediate spreading juan de fuca ridge jdfr and fast spreading east pacific rise epr aftershock and foreshock events are selected from the hydroacoustic earthquake catalog using single link cluster slc analysis with a combined space time metric in the regions examined hydroacoustic data improve the completeness level of the earthquake catalog by 1 5 2 0 orders of magnitude allowing the decay constant p of the modified omori law mol to be determined for individual sequences a non parametric goodness of fit test indicates six of the seven sequences examined are described well by a mol model the p values obtained for individual ridge and transform sequences using hydroacoustic data are larger than that previously estimated from the analysis of a stacked sequence generated from teleseismic data for three sequences along the siqueiros discovery and western blanco transforms p values are estimated to be 0 94 1 29 the spatial distribution of aftershocks suggests that the mainshock rupture is constrained by intra transform spreading centers at these locations an aftershock sequence following a 7 1 m s thrust event near the northern edge of the easter microplate exhibits p 1 02 0 11 within the sequence aftershocks are located to the north of a large topographic ridge which may represent the surface expression of the shallow dipping fault that ruptured during the mainshock two aftershock sequences near 24 25 n and 16 35 n on the mar exhibit higher p values 1 74 0 23 and 2 37 1 65 although the latter estimate is not well constrained because of the small number of aftershocks larger p values along the ridge crest might reflect a hotter thermal regime in this setting additional monitoring however will be needed to determine if p value differences between the ridge and transform sequences are robust a 1999 sequence on the endeavour segment of the jdfr which has been correlated with changes in the hydrothermal system is described poorly by the mol model the failure of the mol model the anomalously large number of earthquakes within the sequence and absence of a clearly dominant mainshock are inconsistent with aftershock activity and the simple tectonic origin that has been proposed previously for this sequence

detailed distribution of microearthquakes along the northern reykjanes ridge off sw iceland we have obtained a snapshot image of the earthquake activity along the northern reykjanes ridge rr using a dense ocean bottom seismometer obs array the earthquakes are narrowly concentrated along the ne sw trending ridge crest the seismicity is segmented into clusters by the en echelon nne ssw trending axial volcanic systems which characterize the obliquely spreading ridge the hypocenters are concentrated at somewhat greater depths 3 7 km than on the reykjanes peninsula rp where the crust is thicker normal faulting dominates at the ridge crest the focal planes are oriented more ne sw than nne ssw i e parallel to the ridge crest and its peripheral faults rather than the trend of individual axial volcanic systems and associated faults

formation of a solid inner core during the accretion of earth the formation of an inner core during the accretion of earth is investigated using self gravitating and compressible earth models formed by accreting a total of 25 or 50 moon to mars size planetary embryos the impact of an embryo heats the proto earth s interior differentially more below the impact site than elsewhere the rotating core dynamically overturns and stratifies shortly after each impact creating a spherically symmetric and radially increasing temperature distribution relative to an adiabatic profile merging of an embryo to the proto earth increases the lithostatic pressure that results in compressional temperature increase while further enhances the melting temperature of the core causing solidification a total of 36 thermal evolution models of the growing proto earth s core are calculated to investigate effects of major physical parameters no solidification is considered in the first 21 models where modified two body escape velocities are used as the impact velocities of the embryos at the end of accretion temperatures in the upper part of the core are significantly different among these models whereas temperatures in the deeper parts are similar the core solidification considered in the remaining 15 models where impact velocities higher than the modified two body escape velocities are adopted drastically changes the temperature distribution in the deeper parts of the core all of the models produce partially solidified stiff inner cores 1000 2100 km in radius at the end of accretion where the solid fraction is larger than 50 the innermost of the stiff inner cores are completely solidified to radii 250 1500 km

generation of hydroacoustic signals by oceanic subseafloor earthquakes a mechanical model summary for more than 15 yr the recording of hydroacoustic signals with hydrophones moored in a minimum sound velocity channel called the sofar sound fixing and ranging channel has allowed for detection and localization of many small magnitude earthquakes in oceanic areas however the interpretation of these hydroacoustic signals fails to provide direct information on the magnitudes focal mechanisms or focal depths of the causative earthquakes these limitations result in part from an incomplete understanding of the physics of the conversion across the seafloor interface from seismic waves generated by subseafloor earthquakes to hydroacoustic t waves to try and overcome some of these limitations we have developed a 2 d finite element mechanical model of the conversion process by computing an exact solution of the velocity field of the waterborne t waves our model shows that a double couple source mechanism of a subseafloor earthquake generates t waves whose take off angles are adequate to allow penetration into the sofar channel and efficient trapping by this waveguide furthermore our model confirms that a double couple source with a high s wave content produces higher amplitude t waves than a simple explosive source which only generates p waves

segregation of core melts by permeable flow in the lower mantle we measured the dihedral angle of molten iron in mg fe sio3 perovskite aggregate with increasing pressure and temperature using laser heated diamond anvil cell results demonstrate that it decreases from 94 at 27 gpa and 2400 k to 51 at 47 gpa and 3000 k this value is smaller than the critical angle of 60 thus allowing iron melt to wet the grain boundaries of silicate perovskite and develop an interconnected melt network within the perovskite dominant matrix even at very small melt fractions the quenched liquid iron contained substantial amounts of oxygen and silicon as pressure and temperature increase such a decrease in the dihedral angle is likely due to a reduction in the iron perovskite interfacial energy by dissolving oxygen and silicon into the liquid iron from coexisting silicate perovskite these suggest that a wetting behaviour of core melts in the solid silicate mantle changes above 40 gpa corresponding to 1000 km depth in the present earth and efficient metal segregation may have proceeded by permeable flow in a late stage of accretion oxygen and silicon incorporated during core formation and later by core mantle boundary process may be important light elements in the earth s core

seamount volcanism at the reykjanes ridge relationship to the iceland hot spot the axial zone of the reykjanes ridge is covered with small 0 5 3 km in diameter volcanoes that pile together to form larger axial volcanic ridges this style of volcanism is similar to that at the mid atlantic ridge mar and may be common to slow spreading ridges despite proximity of the reykjanes ridge to the iceland hot spot in this study we quantitatively investigate the population of seamounts in three study areas at the reykjanes ridge areas a and b are centered at 62 n and 60 n respectively area c is centered at 58 n and is located south of the transition in ridge morphology from an axial high to an axial graben using multibeam bathymetry data 541 seamounts summit height h 50 m were identified in the three areas and their size and shape statistics were compiled additionally 105 seamounts in areas b and c were recognized in deep towed side scan images and their surface morphologies hummocky or smooth were recorded on the basis of estimated population parameters we find that seamounts at the reykjanes ridge are more abundant 310 20 per 103 km2 on average than at the mar between 24 and 30 n 200 10 per 103 km2 significant along axis variations exist at the reykjanes ridge however which are not simply related to distance from the hot spot area b has nearly twice the abundance of seamounts as either area a or area c variation in the characteristic height of the seamount population is also observed between the reykjanes ridge 68 2 m and the mar 58 2 m but no significant variation is found between our three study areas a dramatic change in seamount surface morphology occurs between areas b and c there are no side scan data from area a area c has 78 hummocky seamounts similar to the proportion observed at the mar while area b has 83 smooth seamounts on the basis of these results we present a conceptual model for building the shallow crust at the slow spreading reykjanes ridge that takes into account the possible influence of the iceland hot spot on the crustal melt delivery system and its influence on variables that control seamount abundances sizes shapes and surface morphologies in this model we suggest that the increased seamount production and proliferation of smooth seamounts in area b may be associated with a pulse of hot spot material in the form of asthenosphere of higher temperature that has recently affected area b

seafloor seismicity antarctic ice sounds cetacean vocalizations and long term ambient sound in the indian ocean basin this paper presents the results from the deflo hydroacoustic experiment in the southern indian ocean using three autonomous underwater hydrophones complemented by two permanent hydroacoustic stations the array monitored for 14 months from november 2006 to december 2007 a 3000 x 3000 km wide area encompassing large segments of the three indian spreading ridges that meet at the indian triple junction a catalogue of 11 105 acoustic events is derived from the recorded data of which 55 per cent are located from three hydrophones 38 per cent from 4 6 per cent from five and less than 1 per cent by six hydrophones from a comparison with land based seismic catalogues the smallest detected earthquakes are m b 2 6 in size the range of recorded magnitudes is about twice that of land based networks and the number of detected events is 5 16 times larger seismicity patterns vary between the three spreading ridges with activity mainly focused on transform faults along the fast spreading southeast indian ridge and more evenly distributed along spreading segments and transforms on the slow spreading central and ultra slow spreading southwest indian ridges the central indian ridge is the most active of the three with an average of 1 9 events 100 km month along the sunda trench acoustic events mostly radiate from the inner wall of the trench and show a 200 km long seismic gap between 2 degrees s and the equator the array also detected more than 3600 cryogenic events with different seasonal trends observed for events from the antarctic margin compared to those from drifting icebergs at lower up to 50 degrees s latitudes vocalizations of five species and subspecies of large baleen whales were also observed and exhibit clear seasonal variability on the three autonomous hydrophones whale vocalizations dominate sound levels in the 20 30 and 100 hz frequency bands whereas earthquakes and ice tremor are a dominant source of ambient sound at frequencies 20 hz

a thermal model for the differentiation of asteroid 4 vesta based on radiogenic heating a finite element code has been developed to model the thermal history of asteroid 4 vesta this is the first attempt to model the thermal history of a differentiated asteroid through core and crust formation and subsequent cooling until geochemical closure is attained the results of the simulation are consistent with chronological measurements and other constraints provided by cumulate and noncumulate eucrites believed to have been derived from vesta the work solves two major problems with the hypothesis of heating by decay of26al an extinct radionuclide postulated to be a plausible heat source in the early solar system first the model demonstrates that it is possible to keep the mantle of vesta hot for 100 ma thereby explaining the observed difference in ages between cumulate and noncumulate eucrites second the simulation offers a possible explanation of why detectable excesses of26mg the decay product of26al are not observed in noncumulate eucrites the simulation draws a model chronology of vesta and predicts times relative to cai formation for accretion at 2 85 myr core formation at 4 58 myr crust formation at 6 58 myr and geochemical closure at 100 myr for a h chondrite asteroidal bulk composition decay of60fe is found to cause no perceptible difference in the thermal history of vesta even when sequestered into a central core although chondritic xenoliths have not been described in hed igneous lithologies the thermal model suggests the possibility that a veneer of unmelted near surface material should remain

polydiapirs multiwavelength gravity structures polydiapirs domes in domes result from the evolution of a multiwavelength rayleigh taylor gravitational instability the present work investigates the initiation and development of polydiapirs by means of two dimensional finite difference models of gravitationally unstable triple layered sequences of newtonian fluids the values of viscosities thicknesses and densities were systematically varied for selected examples in triple layered sequences only two types of density stratification are likely to result in multiwavelength instabilities that will eventually develop into polydiapiric structures the limiting factors and evolution of such structures were studied for both these types of density stratifications finite difference calculations on models with physical parameters appropriate to evaporitic sequences and covered by a viscous clastic overburden showed that a very small downward density decrease in the evaporitic source may cause an internal overturn that will compete with the main overturn between source and overburden this may lead to the development of sequential or simultaneous polydiapirs the mature polydiapirs in the models exhibit spines curtain folds pendant repetitions of the source layers inside the large diapirs and anomalous interdiapiric distances all these features are known in natural salt diapirs suggesting that polydiapirs may be far more common in nature than hitherto recognized

regional seismicity of the mid atlantic ridge observations from autonomous hydrophone arrays summary autonomous hydrophones arrays are an excellent tool for monitoring mid ocean ridge seismic activity the major advantage of using arrays of autonomous hydrophones for recording deep ocean ridge earthquakes is its low magnitude detection thresholds achievable using hydroacoustic techniques regional analysis of the detection thresholds of the different autonomous hydrophones arrays deployed along the mid atlantic ridge reveals the strong influence of the detection threshold in the number of recorded events and it must be taken into account in any further analysis in this study the analysis of both autonomous hydrophones and teleseismically detected mid atlantic ridge seismicity reveals that the background seismicity from the relatively short recording periods of the autonomous hydrophones mimic the results of the much longer teleseismic recording it also reveals that seismicity generally cluster at both the segment scale and on mantle bouguer anomaly maxima the big majority of these clusters seem to be related to dyke intrusions and propagation along the mid atlantic ridge these dyke intrusions interact with the mainshock aftershock sequences the seismic sequences mainshock aftershock analysis reveals that the strength of the faults is highly influenced by the mode or style of faulting detachment faults which are ubiquitous along the mid atlantic ridge can produce more prolific shorter duration seismic sequences revealing faster and reduced strain releases in comparison to higher angle normal faults this reduced strain release is most likely to occur due to the presence of higher levels of serpentinization on detachment faults higher levels of serpentenisation can also promote an aseismic transient slip on the mainshock aftershock sequences

low frequency seismic signals recorded by obs at stromboli volcano southern tyrrhenian sea 1 five three component broadband ocean bottom seismometers obss were deployed on the seafloor around the aeolian islands southern tyrrhenian sea by comparing obss digital seismograms we found a low frequency seismicity recorded only at obs05 the nearest seafloor station to stromboli volcano this seismicity appears in the form of a continuous seismic signal tremor like signal as well as a considerable number of shock like events we focused on recordings from obs05 to verify their correlation with stromboli volcanic activity from the spectral analysis we observed low frequency events lp events superposed upon the continuous background noise tremor lp events and tremor showing similar energy fluctuations and frequency content appear to be produced by the same dynamic processes we interpret this low frequency seismicity as probably originating from a continuous uprising of gas bubbles from the deeper part of the stromboli magmatic column this could highlight the existence of a deeper source for low frequency seismicity

volcanism and hydrothermalism on a hotspot influenced ridge comparing reykjanes peninsula and reykjanes ridge iceland abstract current estimates indicate that the number of high temperature vents one of the primary pathways for the heat extraction from the earth s mantle at least 1 per 100 km of axial length scales with spreading rate and should scale with crustal thickness but up to present shallow ridge axes underlain by thick crust show anomalously low incidences of high temperature activity here we compare the reykjanes ridge an abnormally shallow ridge with thick crust and only one high temperature vent known over 900 km axial length to the adjacent subaerial reykjanes peninsula rp which is characterized by high temperature geothermal sites confined to four volcanic systems transected by fissure swarms with young holocene volcanic activity multiple faults cracks and fissures and continuous seismic activity new high resolution bathymetry gridded at 60 m of the reykjanes ridge between 62 30 n and 63 30 n shows seven axial volcanic ridges avr that based on their morphology geometry and tectonic regime are analogues for the volcanic systems and fissure swarms on land we investigate in detail the volcano tectonic features of all mapped avrs and show that they do not fit with the previously suggested 4 stage evolution model for avr construction instead we suggest that avr morphology reflects the robust or weak melt supply to the system and two or more eruption mechanisms may co exist on one avr in contrast to 4 stage evolution model our interpretations indicate that unlike on the reykjanes peninsula faults on and around avrs do not cluster in orientation domains but all are subparallel to the overall strike of avrs orthogonal to spreading direction high abundance of seamounts shows that the region centered at 62 47 n and 25 04 w between avr 5 and 6 is volcanically robust while the highest fault density implies that avr 1 and southern part of avr 6 rather undergo period of melt starvation based on our observations and interpretations we expect all of the avrs on reykjanes ridge to be hydrothermally active but morphological and hydrographic settings of this ridge may cause hydrothermal plumes to be quickly dispersed and diluted due to exposure to strong bottom currents therefore combined ctd and autonomous vehicles surveys are probably the most efficient methods for hydrothermal exploration along the reykjanes ridge