seismic imaging of subduction zone metamorphism combined analysis of high resolution seismic images of the alaska and cascadia subduction zones reveals where metamorphic fl uids are released both images show the subducted oceanic crust as a dipping low velocity layer with a clear termination depth however in alaska the crust is thicker 15 20 km compared to 8 km and terminates at greater depth 120 km com pared to 40 km than in cascadia based on metamorphic reaction estimates and geodynamic models we demonstrate that the termination depth corresponds to eclogitization of the crust triggered by dehydration of water bearing minerals and that the location of this reaction is dependent on the thermal structure of the subducted slab

seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing water and hydrous minerals play a key role in geodynamic processes at subduction zones however there are few seismological constraints on the presence of water within the subducting plate audet et al model converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle their observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip this study models converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle the observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip water and hydrous minerals play a key part in geodynamic processes at subduction zones1 2 3 by weakening the plate boundary aiding slip and permitting subduction and indeed plate tectonics to occur4 the seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization5 6 7 however seismological observations bearing on the presence of water within the subducting plate itself are less well documented here we use converted teleseismic waves to obtain observations of anomalously high poisson s ratios within the subducted oceanic crust from the cascadia continental margin to its intersection with forearc mantle on the basis of pressure temperature and compositional considerations the elevated poisson s ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values combined with observations of a strong negative velocity contrast at the top of the oceanic crust our results imply that the megathrust is a low permeability boundary the transition from a low to high permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization these results may have important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip

magnitude scale and quantification of earthquakes despite various shortcomings the earthquake magnitude scale is one of the most fundamental earthquake source parameters to be used for catalogs although use of a uniform scale is desirable it is not always possible because of changes in instrumentation the data reduction method and the magnitude formula the station distribution etc as a result various magnitude scales have been developed and are currently in use recent developments in seismometry and earthquake source theories provide more quantitative source parameters than the magnitude in order to maintain continuity and uniformity of the data it is important to relate these magnitude scales and the new parameters in view of this importance relations between different magnitude scales are examined with an emphasis on the difference in the period of the waves used for the magnitude determination use of several magnitude scales determined at different periods provides a convenient method for characterizing earthquakes the moment magnitude can be used to quantify both shallow and deep earthquakes on the basis of wave energy radiated and provides a uniform scheme

intermediate depth earthquakes facilitated by eclogitization related stresses eclogitization of the basaltic and gabbroic layer in the oceanic crust involves a volume reduction of 10 15 one consequence of the negative volume change is the formation of a paired stress field as a result of strain compatibility across the reaction front here we use waveform analysis of a tiny seismic cluster in the lower crust of the downgoing pacific plate and reveal new evidence in favor of this mechanism tensional earthquakes lying 1 km above compressional earthquakes and earthquakes with highly similar waveforms lying on well defined planes with complementary rupture areas the tensional stress is interpreted to be caused by the dimensional mismatch between crust transformed to eclogite and underlying untransformed crust and the earthquakes are probably facilitated by reactivation of fossil faults extant in the subducting plate these observations provide seismic evidence for the role of volume change related stresses and possibly fluid related embrittlement as viable processes for nucleating earthquakes in downgoing oceanic lithosphere

magnitude scales for very local earthquakes application for deception island volcano antarctica abstract different magnitude scales are calculated for a set of volcano tectonic earthquakes recorded in deception island volcano antarctica the data set includes earthquakes recorded during an intense seismic series that occurred in january february 1999 with hypocentral distances that range between 0 5 and 15 km this data set is enlarged to include some regional earthquakes with hypocentral distances up to 200 km the local magnitude scale m l fixed at a hypocentral distance of 17 km is used as the reference for the other magnitude scales studied in the present work m l is determined on a standard wood anderson simulated trace assuming a gain of 2080 maximum peak to peak amplitudes are measured on the vertical components of a short period sensor the m w scale is calculated in the vertical component both for p and s waves the attenuation correction of the ground motion displacement spectra is introduced using data from coda waves studied in the area the comparison between m l values and m w estimations indicates severe discrepancies between both values a magnitude duration scale is calibrated from the comparison between coda durations of the recorded events and their assigned local magnitude scales in order to investigate the causes of the discrepancy between the m l and m w values we analyze two possible error sources a wrong coda q value or the effects of the near surface attenuation that initially are not taken into account in the correction of the ground displacement spectra the analysis reveals that the main cause of this discrepancy is the effect of the near surface attenuation the near surface attenuation is also the cause of the determination of an anomalous spectral decay slope after the corner frequency and the determination of this corner frequency value this near surface attenuation represented by is estimated over the data set obtaining an average value of 0 025 with this value the m w scale is recalculated using an automatic algorithm the new m w values are more consistent with the m l values obtaining a relationship of m w 0 78 m l 0 02

determination of spectral properties of earthquakes from their magnitudes abstract following the recommendation of the iaspei commission on magnitudes from 1967 p wave magnitudes of distant earthquakes are being computed with the aid of the calibration functions of gutenberg and richter 1956 with the development of the instrumental seismology especially in view of the availability of broad band recordings the question arises whether the functions are still adequate in this investigation new calibration functions for p and s waves are presented which are not only dependent on epicentral distance and focal depth but also on the period of the spectral component of the wave the determination of calibration functions is based on the currently most plausible global models for the velocity and anelasticity distribution taking into account the effective bandwidth of the instruments employed eventually leads to the so called spectral magnitudes from the spectral magnitudes m t the energy density spectrum e t of the respective wave can readily be computed the corresponding formula is e t 10 2m t 1 4 with e t in joules per hertz

asp an automated seismic processor for microearthquake networks abstract asp a low power in field automated seismic processor of microearthquake network data has been designed fabricated and deployed for initial operation in four different field areas to monitor seismicity associated with two geothermal areas the geysers california and cerro prieto mexico a nuclear explosion with its related collapse and aftershock sequence and late 1 year activity in a major aftershock series livermore california a second asp system with a high speed front end is used in acoustic emission 1 to 20 khz analysis for fracture monitoring each of the 15 channels of asp configured for up to 128 channels automatically detects measures times and amplitudes and computes and fits fft9s for both the p and s waves on data sampled at 100 samples sec these data from each channel are then processed with a central microprocessor for hypocenter location running b values source parameters event count and p wave polarities the system is capable of processing a 15 station detection in approximately 40 sec excluding printout the initial trials have demonstrated that in field real time analysis of data maximizes the efficiency of microearthquake surveys allowing flexibility in experimental procedures with a minimum of the traditional labor intensive postprocessing current efforts are directed toward improving efficiency of computation and data output and in expanding software capabilities

the west andean thrust the san ramon fault and the seismic hazard for santiago chile 1 the importance of west verging structures at the western flank of the andes parallel to the subduction zone appears currently minimized this hampers our understanding of the andes altiplano one of the most significant mountain belts on earth we analyze a key tectonic section of the andes at latitude 33 5 s where the belt is in an early stage of its evolution with the aim of resolving the primary architecture of the orogen we focus on the active fault propagation fold system in the andean cover behind the san ramon fault which is critical for the seismic hazard in the city of santiago and crucial to decipher the structure of the west andean thrust wat the san ramon fault is a thrust ramp at the front of a basal detachment with average slip rate of 0 4 mm yr young scarps at various scales imply plausible seismic events up to mw 7 4 the wat steps down eastward from the san ramon fault crossing 12 km of andean cover to root beneath the frontal cordillera basement anticline a range 5 km high and 700 km long we propose a first order tectonic model of the andes involving an embryonic intracontinental subduction consistent with geological and geophysical observations the stage of primary westward vergence with dominance of the wat at 33 5 s is evolving into a doubly vergent configuration a growth model for the wat altiplano similar to the himalaya tibet is deduced we suggest that the intracontinental subduction at the wat is a mechanical substitute of a collision zone rendering the andean orogeny paradigm obsolete

u pb zircon geochronology of eclogites from the scandian orogen northern western gneiss region norway 14 20 million years between eclogite crystallization and return to amphibolite facies conditionsthis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh reconstructing tectonic histories involving continental collision subduction and exhumation at plate tectonic rates of 1 cm year requires precise u pb zircon geochronology the western gneiss region has exceptional exposures of high pressure hp and ultra high pressure uhp rocks the strategy adopted here involved sampling eclogite and associated late unstrained pegmatites to acquire the time of eclogite crystallization and subsequent exhumation respectively the oldest eclogite sampled is 415 1 ma from layered probably uhp eclogite at tevik averoya also with a garnet hornblende assemblage at 410 1 ma the flem gabbro eclogite margin with implied uhp conditions is 410 2 ma hornblende eclogite at seth lepsoya never at uhp is 412 2 ma these compare to devonian ages of 401 1 ma for overgrowths on proterozoic baddeleyite in selnes gabbro 402 2 ma for coesite eclogite at hareidlandet 405 400 ma for coesite eclogite at flatraket and 405 2 ma for near uhp eclogite at hjelmeland

a gabbro to eclogite transition on flemsoy sunnmore western norway the migmatitic gneisses on the nordoyane islands western norway enclose abundant inclusions of coronitic gabbros and different types of country rock eclogites gabbros on flemsoy show a prograde step wise transition to eclogite via a transitional stage in which garnet and orthopyroxene form coronas between igneous olivine and sodic plagioclase relics after igneous plagioclase cumulates eclogitization proceeds by a complex set of continuous and discontinuous reactions opx and spinel form as transient phases but are consumed in various omphacite forming reactions during eclogite recrystallization relict igneous clinopyroxene shows a continuous increase in jd content while garnet becomes less calcic and more homogeneous from the corona stage to recrystallized eclogite the reactions are strongly diffusion controlled and have been enhanced locally by oxidation and the presence of a fluid phase and deformation while low strain areas of the gabbros remained coronitic whole rock chemistry mineral parageneses and calculated p t conditions are similar for the gabbroeclogites and two types of country rock eclogites some of the layered country rock eclogites appear to have been derived from the coronite bodies p estimates are in the order 15 20 kbar while kgarcpxd values suggest maximum t values of 750 60 c for the eclogite metamorphism in the area

on the importance of continuity of magnitude scales abstract to describe an individual earthquake or the seismicity of a region simple parameters such as magnitudes are necessary in seismicity studies observations for as long as possible time intervals should be at disposal and subsequently the continuity of the parameters quality should be guaranteed m gr or m s can be such parameters m b sp of usgs and isc is another one but it has been accumulated only since the 1960s an international proposal to introduce or revive classical m b asks to change the traditional procedure of measuring the p maximum within a few seconds from the p onset some confusion appears to exist for recent m b data in edr of usgs it is strongly hoped that the traditional procedure of usgs for m b sp determination will not be discontinued and that the new m b will be introduced only as an additional parameter

the 1998 1999 seismic series at deception island volcano antarctica abstract during the 1998 1999 antarctic summer the pattern of seismic activity at deception island volcano changed significantly the change was characterized by the occurrence of an intense swarm of volcano tectonic vt earthquakes more than 2000 vt earthquakes with s p times smaller than 4 s were recorded in the period january february 1999 pure volcanic events were also detected especially long period lp events volcanic tremor and some hybrid events seismic monitoring was performed using two short period small aperture arrays among other instruments based on their signal to noise ratios we selected 863 vt earthquakes 350 lp events and tremor episodes and 9 hybrid events for analysis we estimated apparent slowness and back azimuth for all events using the zero lag cross correlation array technique combining this information with s p times and other indirect evidence we identified two different source regions lp seismicity is located less than 1 1 5 km southwest of the fumarole array site these events are likely to have a hydrothermal origin vt earthquakes and hybrid events are located at depths of 0 3 10 km in an area under the bay of deception island the area extends from the fumarole array to the northeast with epicentral distances that range from 0 5 to 12 km most hypocenters are clustered in a small volume of around 8 km3 the sources of the lp seismicity and the vt earthquakes are spatially distinct which indicates that they are not produced by the same mechanisms moment magnitude analyses of the vt earthquakes provide an average magnitude of 0 5 and very low average stress drop around 1 bar a study of first motion of the p waves suggests that the events in this small source region should have a variety of source mechanisms this is supported by the existence of families of events with the same waveforms the occurrence of repeating fracture processes with low stress drop and small fault dimensions can be explained by the lubrication of pre existing zones of weakness by pressurized fluids the most probable hypothesis that explains the generation of this seismic series at deception island is a seismic series caused by the stress generated by the uplift of the source area due to a magmatic injection in depth we favor this hypothesis since it is compatible with the majority of the characteristics of the seismicity and explains the spatial and temporal behavior of the series

near surface attenuation and site effects from comparison of surface and deep borehole recordings abstract near surface attenuation and site effects are investigated using the seismograms of 17 local earthquakes recorded at depths of 0 0 3 1 5 2 5 and 2 9 km in the cajon pass borehole southern california the borehole penetrates 500 m of miocene sandstone and then crystalline granitic basement rock previous estimates of site response have been limited to shallower holes where the surface reflection interferes with the upgoing direct wave and the deepest sensor is not below the severe near surface effects in bedrock spectral ratios of the direct p and s waves for each earthquake between the different recording levels are well modeled with frequency independent q and amplification at the borehole q p 27 8 and q s 21 7 in the upper 2 9 km increasing from q p 26 and q s 15 in the upper 300 m to q p 133 and q s 47 between 1 5 and 3 km one event was also recorded at a surface granite site less than 1 5 km from the wellhead comparison of the 2 9 km recording with that at this granite site gives q p 50 and q s 23 the similarity of these values with those of previous studies at a wide range of sites suggests that q is very low in the near surface independent of rock type near surface amplification appears considerably more site dependent at the wellhead the amplifications at 1 hz of the direct p and s waves are 12 7 and 13 7 respectively these values include the free surface effect and are in reasonable agreement with the impedance contrast from the borehole logs at the granite site amplification of both p and s waves is less than four direct wave amplification at the wellhead is therefore at least three times that of the granite site the spectra of the direct and coda waves of the one earthquake recorded at both the wellhead and granite sites are compared with the corresponding 2 9 km recording coda wave amplification is in good agreement with the direct wave at the rock site but at the borehole the coda wave amplification factor overestimates the direct wave amplification by a factor of 3

seismic moments local magnitudes and coda duration magnitudes for earthquakes in central california onscale seismograms recorded at stations in the u s geological survey s usgs central california seismic network calnet have been used to estimate the seismic moment m and local magnitude ml for earthquakes of 1 ml 4 located on the san juan bautista and parkfield sections of the san andreas fault the coyote lake section of the calaveras fault the sargent fault and near livermore these data together with m and ml estimates for 4 ml 6 earthquakes in these areas cannot be fit with a single linear log m versus ml relation rather the data are consistent with log m 1 5 ml 16 for 3 ml 6 with log m 1 2 ml 17 for 1 1 2 ml 3 1 2 and with a slope of to 1 fro 1 2 ml 1 1 2 whereas usgs coda duration magnitude md is consistent with ml for 1 1 2 ml 3 md is larger than ml at ml 1 1 2 and smaller than ml at ml 3 log m can be estimated to a precision of 0 2 for 1 md 3 1 2 earthquakes in central california by applying log m 1 2 md 17 to the md that have been routinely published by the usgs

rates of hydrothermal cooling of new oceanic upper crust derived from lithium geospeedometry episodic emplacement and cooling of lavas and dikes at mid ocean ridges leads to large fluctuations in hydrothermal fluxes and biological activity however the processes operating beneath the seafloor during these transient events such as permeability creation and dike cooling are poorly understood we have developed a new approach to determine the cooling rate of the sheeted dike complex based on the extent of diffusion of lithium from plagioclase into clinopyroxene during cooling we have calibrated this li geospeedometer using new high temperature experiments to determine both the temperature dependence of the partitioning of li between plagioclase and clinopyroxene and the diffusion coefficient for li in clinopyroxene application of this method to lavas and dikes from odp hole 504b shows that cooling rates vary dramatically with depth in the upper oceanic crust extremely rapid cooling rates 450 c hr 1 in the upper part of the sheeted dike complex are sufficient to power hydrothermal megaplume formation within the overlying water column

slowness anomalies from two dense seismic arrays at deception island volcano antarctica in this article we analyze the data collected by two short period seismic arrays deployed at deception volcano southern shetland islands antarctica the field survey was conducted during the 1998 1999 austral summer and was aimed at a quantitative assessment of the complex wave fields associated with the magmatic and hydrothermal activity of the volcano the two arrays had apertures of 320 m and 240 m and were separated by a distance of about 3 km during the experiment the arrays recorded several regional earthquakes related to the dynamics of the brans field strait and adjoining areas and local volcano tectonic earthquakes seismograms of earthquakes recorded at regional distances reveal a marked difference in the ap parent velocities measured at the two array sites we investigate the causes and implications of these anomalies by first comparing the effectiveness of estimating the horizontal slowness vector using three different techniques the multiple signal classification music approach the zero lag cross correlation zlc method and plane wave fitting to p wave arrival times while each technique provides the same horizontal slowness vector as the most likely estimates the plane wave fitting is associated with the most robust definition of measurement uncertainties we then investigate the dispersive properties of rayleigh waves in the 1 8 hz frequency band at both arrays and invert the two dispersion curves for a shallow velocity structure the results indicate a marked difference in the seismic velocities for the shallower 200 m beneath the two sites this may be reconciled with the observed wave vector anomalies by assuming the existence of a sharp lateral velocity heterogeneity the effect of which would be to bend downward rays impinging at the northernmost array the reliability of this hypothesis is verified by computing finite difference wave fronts in a 2d heterogeneous medium based on the morpho structural char acteristics of the volcano the inferred velocity discontinuity maybe associated with the ring fracture system bordering the collapsed caldera structure that extends over the inner part of the island

the moho in subduction zones abstract the moho in subduction zones exists in two distinct forms one associated with the subducting oceanic plate and second with the overriding plate the seismic expression of both forms is linked to the nature of a landward dipping low velocity zone lvz that has been detected in a majority of subduction zones about the globe and that approximately coincides with wadati benioff seismicity we review seismic studies that constrain the properties of the lvz in cascadia where it has been extensively studied for over a quarter century a model in which the lvz is identified with hydrated pillow basalts and sheeted dikes of oceanic crustal layer 2 is consistent with available geological and geophysical data and reconciles previously conflicting interpretations in this model the upper oceanic crust is hydrated through intense circulation at the ridge and becomes overpressured upon subduction as a result of metamorphic dehydration reactions combined with an impermeable plate boundary above and a low porosity gabbroic layer 3 below the resulting seismic velocity contrast approaching 50 for s waves significantly overwhelms that of a weaker underlying oceanic moho at greater depths oceanic crust undergoes eclogitization in a top down sense leading to gradual disappearance of the lvz the large volume change accompanying eclogitization is postulated to rupture the plate boundary allowing fluids to penetrate the cooled forearc mantle wedge pervasive serpentinization and free fluids reduce velocities within the wedge thereby diminishing erasing or even inverting the seismic contrast associated with the moho of the overriding plate this model is tested against observations of lvzs and forearc mantle structure worldwide

internal and external fluid sources for eclogite facies veins in the monviso meta ophiolite western alps implications for fluid flow in subduction zones to contribute to our understanding of the mechanisms and pathways of fluid movement through deeply subducted crust we investigate high pressure veins cutting eclogite facies 2 0g pa and 6008c metagabbros of the monviso ophiolite italian western alps the veins consist mainly of omphacite with minor garnet rutile talc and accessory zircon most of the vein minerals have major and trace element compositions that are comparable with the host rock minerals and vein and host rock zircons have similar hf isotopic compositions these observations support the conclusions of previous studies that these veins largely formed from a locally sourced hydrous fluid during prograde or peak metamorphism however the bulk rock cr and ni contents of the veins are significantly higher than those of the surrounding host eclogites we also document distinct cr rich up to weight per cent levels zones in omphacite garnet and rutile in some vein samples vein garnet and talc also have relatively high mgo and ni contents x ray maps of vein garnet and rutile grains reveal complex internal zoning features which are largely defined by micrometre scale variations in cr content some grains have concentric and oscillatory zoning in cr whereas others feature a chaotic fracture like pattern these cr rich zones are associated with high concentrations of ni b as sb nb zr and high ratios of light rare earth elements lree to middle ree mree compared with low cr vein and host rock minerals petrological and mass balance constraints verify that the cr rich zones in the veins were not derived from internally sourced fluids but represent precipitates from an external fluid the external source that is consistent with the distinctive trace element characteristics of the vein components is antigorite serpentinite which forms the structural basement of the high pressure metagabbros we propose at least two separate growth mechanisms for the monviso veins most vein infillings were formed during progressive prograde metamorphism from locally derived fluid influx of the serpentinitederived or other external fluid was transient and episodic and was probably achieved via brittle fractures which preferentially formed along the pre existing vein structures the dehydration of serpentinite at high pressures in subduction zones may provide crucial volatiles and trace elements for arc magmas our results indicate that the movement of these fluids through subducted oceanic crust is likely to be highly channeled and transient so the progressive development of vein systems in mafic rocks may also be crucial for forming channelways for long distance fluid flow at depth in subduction zones

geometry of eclogite facies structural features implications for production and exhumation of ultrahigh pressure and high pressure rocks western gneiss region norway 1 well preserved eclogite facies structural features in metamorphosed gabbro and diorite gneiss in baltica crust formed at a minimum depth of 60 70 km during the scandian orogeny allow direct inferences to be made regarding the geometry of folds kinematics and original structural orientations related to production and exhumation of high pressure rocks folds associated with eclogite facies fabrics are isoclinal to tubular with axes parallel to the trend of a stretching lineation strain estimates and l s or l s fabrics indicate that these structures were formed in a constrictional strain field locally they are well preserved in eclogite facies mylonite zones at least 40 m thick that cut proterozoic gabbro and adjacent gneiss where steeply dipping they show a north side up shear sense across a younger northeast trending shallowly plunging amphibolite facies anticlinorium formed in a constrictional noncoaxial strain field where original structural facing direction can be inferred small circle rotation of the eclogite facies lineation about the anticline axis indicates that the relative motion vector of baltica with respect to overlying imbricated crust and laurentia was oriented 320 in a present day reference frame this result is identical to the orientation of the relative motion vector estimated from paleomagnetic plate reconstructions and consistent with late silurian to early devonian oblique sinistral convergence between baltica and laurentia results of this study indicate that coherent baltica crust at 60 70 km depth experienced subvertical and horizontal shortening while extending toward the foreland during tops southeast thrusting parallel to plate motion high pressure rocks were progressively imbricated and stacked along eclogite facies mylonite zones with the deforming zone separating coherent baltica crust from imbricated basement above the constrictional strains under eclogite facies conditions are interpreted to have resulted from stretching associated with sinking of the cool dense mantle lithosphere

changes in dip of subducted slabs at depth petrological and geochronological evidence from hp uhp rocks tianshan nw china article high resolution seismic imaging has previously revealed sudden changes in the dip of subducted oceanic plates this kinking feature is crucial because in many subduction zones it is thought to coincide with the disappearance of the low velocity layers associated with subducted oceanic crust in this study we present petrological evidence for this phenomenon derived from oceanic blueschist and eclogite facies rocks from the chinese tianshan the investigated samples span a large range of peak metamorphic conditions with temperatures between 330 and 580 c at 1 5 to 2 3 gpa such variable peak metamorphic conditions together with the intimate interlayering of high and ultrahigh pressure rocks have also been reported from other tianshan localities these observations suggest that the rocks were derived from varying depths within the subduction zone and then juxtaposed during exhumation in the subduction channel multi point lu hf isochrons from four high pressure rocks yield consistent garnet growth ages of 313 12 315 8 2 9 313 9 4 8 and 315 2 1 6 ma these inconjunctionwiththe 311 maclusterof published 40

plateau subduction intraslab seismicity and the denali alaska volcanic gap tectonic tremors in alaska usa are associated with subduction of the yakutat plateau but their origins are unclear due to lack of depth constraints we have processed tremor recordings to extract low frequency earthquakes lfes and generated a set of six lfe waveform templates via iterative network matched filtering and stacking the timing of impulsive p compressional wave and s shear wave arrivals on template waveforms places lfes at 40 58 km depth near the upper envelope of intraslab seismicity and immediately updip of increased levels of intraslab seismicity s waves at near epicentral distances display polarities consistent with shear slip on the plate boundary we compare characteristics of lfes seismicity and tectonic structures in central alaska with those in warm subduction zones and propose a new model for the region s unusual intraslab seismicity and the enigmatic denali volcanic gap i e an area of no volcanism where expected we argue that fluids in the yakutat plate are confined to its upper crust and that shallow subduction leads to hydromechanical conditions at the slab interface in central alaska akin to those in warm subduction zones where similar lfes and tremor occur these conditions lead to fluid expulsion at shallow depths explaining strike parallel alignment of tremor occurrence with the denali volcanic gap moreover the lack of double seismic zone and restriction of deep intraslab seismicity to a persistent low velocity zone are simple consequences of anhydrous conditions prevailing in the lower crust and upper mantle of the yakutat plate

intraplate seismicity and related mantle hydration at the nicaraguan trench outer rise summary we examine micro earthquake records from a dense temporary array of ocean bottom seismometers obs and hydrophones that has been installed from september to november 2005 at the trench outer rise offshore nicaragua approximately 1 5 locatable earthquakes per day within the array of 110 120 km show the high seismic activity in this region seismicity is restricted to the upper 15 km of the mantle and hence where temperatures reach 350 400 c which is smaller than values observed for large mantle intraplate events 650 c determination of moment tensor solutions suggest a change of the stress region from tensional in the upper layers of the oceanic plate to compressional beneath the neutral plane between both regimes is located at 6 9 km beneath moho and thus very shallow fluids which are thought to travel through the tensional fault system into the upper mantle may not be able to penetrate any deeper the earthquake catalogue which seems to be complete for magnitudes above mw 1 6 1 8 suggests a strong change of the lithospheric rheology when approaching the trench and b factors that is the ratio between small and large earthquakes increase significantly in the closest 20 km to the trench axis implying that the crust and upper mantle is massively weakened and hence ruptures more frequently but under less release of stress we explain this with a partly serpentinized upper mantle

deformation enhanced metamorphic reactions and the rheology of high pressure shear zones western gneiss region norway microstructural and petrological analysis of samples with increasing strain in high pressure hp shear zones from the haram garnet corona gabbro give insights into the deformation mechanisms of minerals rheological properties of the shear zone and the role of deformation in enhancing metamorphic reactions scanning electron microscopy with electron backscattering diffraction sem ebsd compositional mapping and petrographic analysis were used to evaluate the nature of deformation in both reactants and products associated with eclogitization plagioclase with a shape preferred orientation that occurs in the interior part of layers in the mylonitic sample deformed by intracrystalline glide on the 0 0 1 1 0 0 slip system in omphacite crystallographic preferred orientations indicate slip on 1 0 0 0 0 1 and 1 1 0 0 0 1 during deformation fine grained garnet deformed by diffusion creep and grain boundary sliding ilmenite deformed by dislocation glide on the basal and at higher strains prism planes in the a direction relationships among the minerals present and petrological analysis indicate that deformation and metamorphism in the shear zones began at 500 650 c and 0 5 1 4 gpa and continued during prograde metamorphism to ultra high pressure uhp conditions both products and reactants show evidence of syn and post kinematic growth indicating that prograde reactions continued after strain was partitioned away the restriction of post kinematic growth to narrow regions at the interface of garnet and plagioclase and preservation of earlier syn kinematic microstructures in older parts layers that were involved in reactions during deformation show that diffusion distances were significantly shortened when strain was partitioned away demonstrating that deformation played an important role in enhancing metamorphic reactions two important consequences of deformation observed in these shear zones are i the homogenization of chemical composition gradients occurred by mixing and grain boundary migration and ii composition changes in zoned metamorphic garnet by lengthening diffusion distances the application of experimental flow laws to the main phases present in nearly monomineralic layers yield upper limits for stresses of 100 150 mpa and lower limits for strain rates of 10 12 to 10 13 s 1 as deformation conditions for the shear zones in the haram gabbro that were produced during subduction of the baltica craton and resulted in the production of hp and uhp metamorphic rocks

automated near realtime cmt inversion the earthquake research institute eri of the university of tokyo has completely automated the process of determining source mechanisms of earthquakes using waveform data the solutions are automatically distributed using the internet without being checked the process is initiated by receipt of e mail giving the origin time and location of an earthquake to determine the source mechanism we apply the harvard centroid moment tensor cmt inversion method to long period body wave data the portion of the seismogram from the first arrival to the initial surface wave the method has been successfully applied since october 1993 to all of the major earthquakes in the world for which we received event e mail 108 of the 242 cmt solutions in 1994 were determined within 6 hours of the event with an average delay of 3 8 hours for local events e g events around japan it is also possible to determine cmt solutions within 30 minutes from the occurrence of the earthquakes using an efficient dialup data retrieval system operated by eri

seismicity and seismotectonics of norway and nearby continental shelf areas the earthquake activity of norway and nearby offshore areas is low to intermediate with few events above magnitude 5 recent significant improvements in instrumental coverage in parallel with a better utilization of older including historical data have shown that the seismicity in the south is predominantly confined to the coastal areas and to the viking graben while from the northern north sea to svalbard the earthquakes in a broad sense follow the continental margin fifty one focal mechanism solutions from these areas about half of them new reveal stress directions that clearly indicate a connection to the plate tectonic ridge push force at least for the areas at a minimum distance from the continental margin along the margin stress directions also indicate a possible connection to postglacial uplift as well as to lithospheric loading effects a dominance of normal faulting on the landward side and reverse faulting on the oceanic side agrees with this interpretation on a regional level the seismicity in these areas correlate quite well with geologic features such as grabens fault zones fault complexes fracture zones and the margin itself indicating that these structures act in a general sense as weakness zones in the presence of a regionally more stable stress field in the northern north sea however an area with quite anomalous stress orientations with strike slip faulting is found in a region transitional between normal and reverse faulting most of the earthquake foci are confined to the presumably brittle parts of the crust but many events are also located quite close to and on both sides of the moho discontinuity

deep fluid release in warm subduction zones from a breached slab seal abstract petrological models and seismic data from subduction zones with geotherms of 7 k km 1 or higher suggest that slabs in these systems dehydrate effectively in the forearc a large fluid flux is nevertheless released from these slabs at and beyond subarc depth suggesting that large amounts of h2o can remain slab bound to much greater depth than expected we propose that this is due to a transient sealing effect exerted by the subducting lower crust to test this concept the petrological and geochemical evolution of such gabbroic crust is investigated through a textural petrological and li chronometric analysis of eclogitized gabbros from an exhumed ultrahigh pressure terrane the samples record pristine transitions from dry rigid gabbro to hydrated eclogite and eclogite mylonite which occurred when these rocks resided at 90 110 km depth the observations characterize step by step the deformation and overstepped mineral reactions that following the influx of external fluids along a developing network of permeable shear zones lithium chronometry indicates that the gabbroic rocks were breached and permeated within a few weeks at a very specific depth within the 90 110 km interval depths where in warm subduction zones large fluid filled channel system emanate from the slab the data support a model in which fluids produced in the deserpentinizing slab mantle are trapped at high pore pressure beneath the slab moho and are ultimately released at subarc depth where the lower crust fails and develops highly permeable fluid vents the subducting lower crust thus may play an important role in regulating h2o and element budgets and controlling slab rheology in warm subduction zones

modeling some empirical vertical component lg relations abstract higher mode surface wave theory is used to model the vertical component lg wave observed in eastern north america at regional distances tests of the model are made to determine whether it is capable of describing empirical spectral scaling laws spatial attenuation and peak time domain lg amplitudes it is found that a simple crustal model and a rough estimate of crustal qb are all that are required to accomplish this good results are obtained if the average crustal qp is equated to the coda q of the same frequency introduction the lg wave is a wave observed on short period seismograms of local and regional seismic events which travels with a group velocity of approximately 3 5 km sec in the central united states it is usually the largest arrival on the seismogram which makes it suitable for defining a magnitude scale nuttli 1973 herrmann and nuttli 1982 as part of developing the magnitude scale there have been many studies on the spatial attenuation of the lg wave nuttli 1973 street 1976 bollinger 1979 singh and herrmann 1982 the lg wave is usually thought of as a superposition of higher mode surface waves propagating within the crustal wave guide herrmann and nuttli 1975a b attempted to deterministically model observed lg amplitudes in terms of focal mechanism focal depth and seismic moment using surface wave techniques they were limited by their inability to compute surface wave eigenfunctions at frequencies greater than 0 5 hz however their results were encouraging harkrider 1979 drawing upon the work of apsel 1979 discovered a way to compute surface wave energy integrals of the higher modes very precisely bache

real time seismology at uc berkeley the rapid earthquake data integration project the rapid earthquake data integration project is a system for the fast determination of earthquake parameters in northern and central california based on data from the berkeley digital seismic network and the usgs northern california seismic network program development started in 1993 and a prototype system began providing automatic information on earthquake location and magnitude in november of 1993 via commercial pagers and the internet recent enhancements include the exchange of phase data with neighboring networks and the inauguration of processing for the determination of strong motion parameters and seismic moment tensors

evidence of bias in estimations of earthquake size magnitudes of earthquakes determined using traditional methods show systematic deviations dependent on tectonic setting from accurate estimates of earthquake size magnitudes are overestimated for most of the continental earthquakes underestimated for mid oceanic ridges and differences of varying sign are obtained for subduction zones this is important in evaluating the seismic risk or consideration of effects such as the seismic versus aseismic strain release as seismic moments determined from magnitudes can be wrong by as much as a factor of four

lateral variation of lg wave propagation in southern mexico 1 in this study we investigated lateral variation of lg wave propagation in southern mexico from recordings of 92 crustal earthquakes along 591 travel paths the efficiency of lg propagation was measured in terms of lgto pn spectral ratio it was found that lgpropagation is inefficient for travel paths through the gulf of mexico coastal plains and the gulf of tehuantepec areas with thick layers of sediments an average lg quality factor qlg as a function of frequency for southern mexico was estimated for the efficient lg travel paths the relation obtained for qlg in the frequency range 1 6 8 hz is qlg f 204 f0 85 the lateral variation of qlg 1was solved as a mixed determined inverse tomography problem separately for each frequency in which a spatial smoothness constraint was imposed and a priori information was added in poorly covered regions the spatial resolution obtained was about 200 km it was found that the trans mexican volcanic belt the gulf of mexico coastal plains and the area east of 94 w are characterized by lower than average qlgvalues i e higher attenuation high qlg values were obtained for the mixteco oaxaca terranes while for the guerrero terrane values similar to the average were obtained the results show a correlation between qlg and crustal structure and provide valuable information on lateral variation of qlg which is needed for reliable prediction of ground motion during future earthquakes

seismic energy release in mexican subduction zone earthquakes we estimated radiated seismic energy es from 18 shallow thrust mexican subduction zone earthquakes 4 1022 m 1 1 1028 dyne cm 11 h 37 km using digital accelerograms from the guerrero accelerograph array and neglecting stations with large site effects es is computed by integrating squared velocity spectra after applying geometrical spreading and q corrections we discarded epicentral recordings for the largest michoacan event we find that log es m 4 152 0 275 which gives a median value of es m and apparent stress a of 7 1 10 5 and 24 bars respectively the median es m value is in accordance with gutenberg and richter s g r 1956 formula for es in which es m 5 10 5 is implicit worldwide es m data where es is computed from local records mostly fall between 5 10 5 and 5 10 4 for events with m 1022 dyne cm on the other hand es m values generally lie between 5 10 6 and 5 10 5 if es is estimated from teleseismic records especially anomalous are the es m data from kikuchi and fukao 1988 for large and great earthquakes which fall near 5 10 6 thus while es from local data suggests that the g r relation seldom overestimates seismic energy release the teleseismic data point to the contrary the cause of this discrepancy may lie in the difficulty of resolving incoherent radiation from the fault and inappropriate choice of t in the analysis of teleseismic data

systematic variation in the depths of slabs beneath arc volcanoes summary the depths to the tops of the zones of intermediate depth seismicity beneath arc volcanoes are determined using the hypocentral locations of engdahl et al these depths are constant to within a few kilometres within individual arc segments but differ by tens of kilometres from one arc segment to another the range in depths is from 65 km to 130 km inconsistent with the common belief that the volcanoes directly overlie the places where the slabs reach a critical depth that is roughly constant for all arcs the depth to the top of the intermediate depth seismicity beneath volcanoes correlates neither with age of the descending ocean floor nor with the thermal parameter of the slab this depth does however exhibit an inverse correlation with the descent speed of the subducting plate which is the controlling factor both for the thermal structure of the wedge of mantle above the slab and for the temperature at the top of the slab we interpret this result as indicating that the location of arc volcanoes is controlled by a process that depends critically upon the temperature at the top of the slab or in the wedge of mantle immediately below the volcanic arc

laboratory earthquakes triggered during eclogitization of lawsonite bearing blueschist abstract the origin of intermediate depth seismicity has been debated for decades a substantial fraction of these events occurs within the upper plane of wadati benioff double seismic zones believed to represent subducting oceanic crust we deformed natural lawsonite rich blueschist samples under eclogite facies conditions 1 5 p 3 5 gpa 583 k t 1121 k using a d dia apparatus installed at a synchrotron beamline continuously monitoring stress strain phase changes and acoustic emissions aes two distinct paths were investigated i heating during deformation at pressures 2 5 gpa to maximum temperatures ranging from 762 to 1073 k during which lawsonite and glaucophane became gradually unstable while entering the stability field of lawsonite eclogite and the breakdown reaction of lawsonite was only crossed in case of the highest final temperature ii heating while deforming at a pressure

source characterization of nevada test site explosions and western u s earthquakes using lg waves implications for regional source discrimination an investigation is made of the lg waves from 15 nuclear tests at the nevada test site nts and 25 shallow western united states earthquakes to obtain a scheme for discriminating between possible source mechanisms the data were recorded at four broadband stations operated by lawrence livermore national laboratory llnl which encircle the nts the lg wave spectra are modeled through a genetic algorithm search to find optimal values for 1 the seismic moment m o 2 the corner frequency of the amplitude spectrum f c 3 the 1 hz attenuation quality factors q o for all of the paths and 4 the frequency dependence of attenuation along the paths because the inversion solves for the path attenuation characteristics little a priori information is needed for the analysis the resulting m o and f c values are used in conjunction with the m b values as seismic discriminants the populations of earthquakes and explosions have nearly identical relationships between their moments and corner frequencies for earthquakes the relationship is log m o 16 07 2 93 log f c for the set of nts explosions using an earthquake source model the relationship is log m o 15 87 2 76 log f c a source discriminant is found however through a comparison of the moments and m b values this gives a line log m o 10 20 1 16 m b which cleanly separates the earthquake and explosion populations with earthquakes falling above and explosions below the line reliability tests examine the inverse trade off between moments and q o values noise in the lg spectra has a limited effect on the scaling relations

moment magnitude determination for local and regional earthquakes based on source spectra we investigated the use of an automated routine to determine moment magnitudes from the displacement spectra of local and regional earthquakes two algorithms a genetic algorithm and a converging grid search were developed and tested with earthquake data from mexico norway and deception island antarctica it was found that compared with manual analysis the algorithms give reliable automatic moment magnitude m w estimates in the range 1 m 8 the converging grid search appeared to be more cost effective than the genetic algorithm m w at local and regional distances seems superior to amplitude based magnitudes that saturate for large earthquakes the application of the automated algorithm in near real time may help to obtain a nonsaturated magnitude estimate in the case of a large earthquake immediately after the earthquake has occurred also the method can be useful for processing large amounts of data manuscript received 27 july 2001

are the lower planes of double seismic zones caused by serpentine dehydration in subducting oceanic mantle in a number of subduction zones earthquakes at 50 200 km depth define two dipping planes separated by 20 40 km that appear to merge downdip upper plane earthquakes are inferred to occur within the subducting oceanic crust whereas lower plane earthquakes occur in the subducting oceanic mantle beneath northeast japan the results of a new finite element heat transfer model suggest that the lower seismic plane cuts across isotherms at a shallow angle lower plane earthquakes occur at 550 800 c at 100 km depth and at 350 600 c at 160 km depth these conditions coincide with the dehydration reaction antigorite serpentine forsterite enstatite h 2 o which suggests that lower plane earthquakes may be triggered by dehydration embrittlement which in turn suggests that the subducting oceanic mantle is partially hydrated serpentinization may occur in the trench outer rise region where faulting may promote infiltration of seawater several tens of kilometers into the oceanic lithosphere if this hypothesis is correct current subduction zone h 2 o budgets may significantly underestimate the amount of bound h 2 o entering the subduction factory

ground motion difference between two moderate size intraplate earthquakes in the united kingdom two moderate size earthquakes occurred in the united kingdom the first near folkestone in 2007 with m w 4 0 and the second near market rasen in 2008 with m w 4 5 both were strongly felt and caused some nonstructural damage the earthquakes occurred at significantly different depths the folkestone earthquake at 5 km and the market rasen earthquake at 20 km we determined the seismic moment and the stress drop of the two mainshocks and two smaller earthquakes in the same locations by modeling the source displacement spectra we found stress drops of 30 34 bar and 344 136 bar for the folkestone and market rasen mainshocks respectively this is a significant difference considering the earthquakes are only 275 km apart and both are of intraplate origin we applied the stochastic ground motion modeling technique and used the stress drop and seismic moment to compute vertical component peak ground acceleration the modeled ground motions are consistent with the observations we also computed vertical peak ground acceleration for a hypothetical m w 6 0 high stress drop 200 bar earthquake and found that it would be 4 6 m sec2 at 20 km hypocentral distance

intra arc crustal seismicity seismotectonic implications for the southern andes volcanic zone chile we examine the intra arc crustal seismicity of the southern andes volcanic zone svz our aim is to resolve inter seismic deformation in an active magmatic arc dominated by both margin parallel liquine ofqui fault system lofs and andean transverse faults atf crustal seismicity provides information about the schizosphere tectonic state delineating the geometry and kinematics of high strain domains driven by oblique subduction here we present local seismicity based on 16 months data collected from 34 seismometers monitoring a 200 km long section of the southern volcanic zone including the lonquimay and villarrica volcanoes we located 356 crustal events with magnitudes between mw 0 6 and mw 3 6 local seismicity occurs at depths down to 40 km in the forearc and consistently shallower than 12 km beneath the volcanic chain suggesting a convex shape of the crustal seismogenic layer bottom focal mechanisms indicate strike slip faulting consistent with ene wsw shortening in line with the long term deformation history revealed by structural geology studies however we find regional to local scale variations in the shortening axes orientation as revealed by the nature and spatial distribution of microseismicity within three distinctive latitudinal domains in the northernmost domain seismicity is consistent with splay faulting at the northern termination of the lofs in the central domain seismicity distributes along ene and wnw striking discrete faults spatially associated with hitherto seismic atf the southernmost domain in turn is characterized by activity focused along a n15 e striking master branch of the lofs these observations indicate a complex strain compartmentalization pattern within the intra arc crust where variable strike slip faulting dominates over dip slip movements

high pressure metamorphism and deep crustal seismicity evidence from contemporaneous formation of pseudotachylytes and eclogite facies coronas the metamorphic complex of the western gneiss region wgr norway constitutes the root of the caledonian mountain belt and experienced temperatures of 700 800 c and pressures in excess of 20 kbar during peak metamorphism mafic bodies surrounded by strongly banded felsic gneisses commonly exhibit variable reequilibration to granulite and eclogite facies conditions and locally preserve igneous minerals and textures the krakeneset gabbro located on the island of vagsoy in the mixed hp uhp zone of the western wgr display evidence for extensive metastability through the entire prograde and retrograde p t histories eclogite constitutes less than a few percent of the total volume of the body and high pressure assemblages typically form thin coronas around magmatic phases or occur along localized zones of brittle deformation and fluid infiltration the gabbro displays pseudotachylyte vein networks that define subparallel brittle fault zones 50 cm wide transecting the gabbro body the pseudotachylytes contain m to mm scale amoeboid and dendrite like textures of garnet and plagioclase with inclusions of the eclogite facies minerals orthopyroxene omphacite amphibole and dolomite suggesting rapid disequilibrium growth of minerals during high pressure conditions textural and petrological evidence from pseudotachylytes and corona structures show that the growth of these unusual textures occurred shortly after pseudotachylyte crystallization by a process of rapid solid state alteration of a microcrystalline pseudotachylyte matrix the pseudotachylyte lined fault zones are in close spatial association with numerous amphibole carbonate filled hydrofractures with conspicuous fracture parallel alteration zones defined by hydrous eclogite facies assemblages these eclogite facies hydrofractures testify to the existence of high fluid pressures and to fluid infiltration following brittle failure during high grade metamorphic conditions geothermobarometric estimates ca t 650 700 c p 20 kbar and petrological data imply that hydrofracturing pseudotachylyte crystallization and the subsequent pseudotachylyte alteration process must have occurred during high pressure metamorphism our observations are suggestive of a deep crustal earthquake scenario where a high pressurized fluid phase plays a double role by causing both seismic failure through the embrittlement effect and facilitating eclogitization of the metastable anhydrous gabbro metamorphic reaction along hydrofractures and fault planes led to the development of eclogite facies foliation fabrics and illustrate the rheological change from brittle to plastic behavior associated with the gabbro to eclogite transition the formation of weak deep crustal shear zones following brittle failure represents an arrested initiation of the physical breakup and metamorphic reequilibration of the krakeneset gabbro during its residence deep in the former caledonian collision zone

fluid and deformation regime of an advancing subduction system at marlborough new zealand newly forming subduction zones on earth can provide insights into the evolution of major fault zone geometries from shallow levels to deep in the lithosphere and into the role of fluids in element transport and in promoting rock failure by several modes the transpressional subduction regime of new zealand which is advancing laterally to the southwest below the marlborough strike slip fault system of the northern south island is an ideal setting in which to investigate these processes here we acquired a dense high quality transect of magnetotelluric soundings across the system yielding an electrical resistivity cross section to depths beyond 100 km our data imply three distinct processes connecting fluid generation along the upper mantle plate interface to rock deformation in the crust as the subduction zone develops massive fluid release just inland of the trench induces fault fracture meshes through the crust above that undoubtedly weaken it as regional shear initiates narrow strike slip faults in the shallow brittle regime of interior marlborough diffuse in width upon entering the deeper ductile domain aided by fluids and do not project as narrow deformation zones deep subduction generated fluids rise from 100 km or more and invade upper crustal seismogenic zones that have exhibited historic great earthquakes on high angle thrusts that are poorly oriented for failure under dry conditions the fluid deformation connections described in our work emphasize the need to include metamorphic and fluid transport processes in geodynamic models

analysis and automatic processing in near field of eight 1992 1994 tsunamigenic earthquakes improvements towards real time tsunami warning we study eight tsunamigenic earthquakes of 1992 1994 with data from single near field 3 component long period stations the analysis is made from the standpoint of tsunami warning by an automatic process which estimates the epicentral location and the seismic moment through the variable period mantle magnitude m m simulations of early warning based on the real time computation of the seismic moment are also tested with this system which would give a justified warning in each region of tsunami potentiality by exploiting the dependence of moment rate release with frequency the system has the capability of recognizing both tsunami earthquakes such as the 1992 nicaragua and 1994 java events as well as instances of the opposite case of low frequency deficiency interpreted as indicating a deeper than normal source 1993 guam event we report both the results of delayed time processing of the near field stations and the actual real time warnings at ppt which confirm the former

generation of intermediate depth earthquakes by self localizing thermal runaway the conditions leading to rock failure during intermediate depth earthquakes in subduction zones are not clear particularly in the absence of free fluid field observations and numerical simulations indicate that thermal weakening due to high temperature shear instabilities may trigger earthquakes under such circumstances intermediate depth 50 300 km earthquakes commonly occur along convergent plate margins but their causes remain unclear in the absence of pore fluid pressures that are sufficiently high to counter the confining pressure in such settings brittle failure is unlikely in such conditions the rocks could fail by the mechanism of progressively self localizing thermal runaway1 whereby ductile deformation in shear zones leads to heating thermal softening and weakening of rock1 2 3 here we test this hypothesis by focusing on fault veins of glassy rock pseudotachylyte formed by fast melting during a seismic event as well as associated ductile shear zones that occur in a precambrian terrane in norway our field observations suggest that the pseudotachylytes as well as shear zones have a single event deformation history and we also document mineralogical evidence for interaction of the rocks with external fluids using fully coupled thermal and viscoelastic models we demonstrate that the simultaneous occurrence of brittle and ductile deformation patterns observed in the field can be explained by self localizing thermal runaway at differential stresses lower than those required for brittle failure our results suggest that by perturbing rock properties weakening by hydration also plays a key role in shear zone formation and seismic failure however thermal runaway enables the rocks to fail in the absence of a free fluid phase

the deadliest intraplate earthquake the recent earthquake in the bhuj region took a toll of at least 30 000 lives in their perspective gupta et al 1 discuss what may be the cause of the large but infrequent earthquakes in this region which is not near any tectonic plate boundary the earthquake hazard in the region was well recognized before the disaster however and adherence to the building codes for high risk areas may have prevented much human tragedy 1 http www sciencemag org cgi content full 291 5511 2101

subduction interface processes recorded by eclogite facies shear zones monviso w alps abstract the monviso ophiolite lago superiore unit constitutes a well preserved almost continuous upper fragment of oceanic lithosphere subducted at c 80 km depth thereby providing a unique opportunity to study mechanical coupling processes and meter scale fluid rock interactions occurring at such depths in present day subduction zones it is made of i a variably thick 50 500 m section of eclogitized basaltic crust associated with minor calcschist lenses overlying a 100 400 m thick metagabbroic body and of ii a c 1 km thick serpentinite sole we herein focus on the three major eclogite facies shear zones found at the top of the unit at the boundary between basalts and gabbros and between gabbros and serpentinites respectively strain localization occurred at lithological interfaces irrespective of material strength while ductile deformation dominates along the shear zones local brittle behavior is demonstrated by the existence of numerous eclogite breccias of fe ti metagabbros and widespread garnet fractures possibly linked with intermediate depth eclogite facies micro seismicity these m to hm sized fragments of fe ti metagabbros were later sheared and disseminated within serpentinite schists along the gabbro serpentinite boundary lower shear zone lsz pervasive and focused fluid flow is attested in the lsz by significant alteration of bulk rock compositions weakening of the rocks and widespread crystallization of hydrous parageneses by contrast the intermediate shear zone isz shows evidence for more restricted short range fluid flow the activity of both the isz and lsz ceased during early lawsonite eclogite facies exhumation when deformation localized deeper within the serpentinite sole allowing for the detachment and preservation of this large ophiolitic fragment

spatial variability of in situ weathered soil the founding depths of pile foundations depend primarily on the loading conditions of the superstructure and the strength of subsoil the depth of grade iii weathered rock the top of completely decomposed granite cdg and standard penetration test n value of 200 spt 200 are often used as indicators for decision making in arriving at the preliminary founding depth of piles in hong kong the work reported in this paper focuses on evaluation of the spatial variability characteristics of the above founding depth indicators at a construction site using statistical models based on random field theory spatial variability characteristics are evaluated using the measured data in terms of scale of fluctuation geostatistics is used to obtain additional data at unsampled locations for mapping profiles of founding depth indicators over the site using the measured data from borehole records and site investigation results it is observed from the results that if faults are not present the depth of the grade i

partial eclogitisation of gabbroic rocks in a late precambrian subduction zone zambia prograde metamorphism triggered by fluid infiltration gabbros and eclogites occur closely associated in a 200 km long and up to 40 km wide area of the zambezi belt in central zambia this area is interpreted to represent part of a late precambrian suture zone with the mafic rocks being relics of subducted oceanic crust gradual stages of prograde transformation from gabbro to eclogite are preserved by disequilibrium textures of incomplete reactions this resulted in kyanite omphacite bearing assemblages for eclogites that have al poor bulk compositions undeformed eclogites typically preserve features of a former gabbroic texture reflected by replacements of plagioclase and magmatic pyroxene by eclogite facies minerals textures of deformed eclogites range from sheared porphyroclastic to porphyroblastic relics of magmatic pyroxene are common and complete eclogitisation occurred only in millimetre to centimetre scale domains in most of the rocks no evidence for prograde blueschist or amphibolite facies mineral assemblages was found in eclogites in contrast the fine grained intergrowth of omphacite garnet kyanite and quartz which replace former plagioclase or was formed in the pressure shadow of magmatic pyroxene relics indicates that eclogitisation might have affected the gabbroic protoliths directly without any significant intervening metamorphic reactions eclogitisation took place under p t conditions of 630 690 degreesc and 26 28 kbar suggesting a large overstepping 10 kbar of reaction boundaries eclogitisation was initialised and accompanied by a channelised fluid flow resulting in veins with large subhedral grains of omphacite kyanite and garnet the gabbro to eclogite transformation was enhanced by a fluid which allowed the necessary material transport for the dissolution precipitation mechanism that characterises the metamorphic mineral replacements the process of eclogitisation was limited by reaction kinetics and dissolution precipitation rates rather than by the metamorphic p t conditions even though ductile deformation occurred and equilibrium phase boundaries were overstepped the infiltration of fluids was necessary for triggering the gabbro to eclogite transformation