the 1976 friuli ne italy thrust faulting earthquake a reappraisal 23 years later we revisit the 1976 friuli earthquake sequence by combining hypocenters relocation long period surface wave inversion field geology and strong motion modelling we show that fault related folding is the main active deformation by which the seismic energy was released during the main shock ms 6 5 and that some of the surface effects reported in 1976 correspond to widespread bedding planes displacements induced by flexural slip folding the fault evolved from blind to semi blind along strike showing the control of the inherited structural geology on the fault surface break and rupture arrest our fault model produces waveforms that fit the accelerograms recorded in the area

detection of tectonomagnetic events using multichannel predictive filtering high precision geomagnetic measurement depends on eliminating variations of ionspheric and magnetospheric origin the commonly used technique of simply taking differences between total field magnetometers is only partially sucessful our method involves finding the multichannel wiener filters which predict the field variation at a given total field magnetometer of an array from the fields of the remaining magnetometers and a three component magnetometer the difference is then taken between the total field and the predicted field leaving a cleaned total field filter lengths and number and choice of input channels are determined using methods of statistical parametric model fitting the resulting filters which are defined on short noise free record sections are found to be effective over the remainder of the record detailed analysis of data from the university of california los angeles array in southern california for which we have the best vector data shows that for optimal predictive cleaning it is essential to use vector components and that only the two components orthogonal to the total field direction are necessary this means that the field aligned component may be omitted from a vector station which simplifies the instrumentation considerably analysis of the standard deviation of the residuals after cleaning 0 1 ntmore for hourly averages shows that it is 8 times that expected from digitization noise alone the most probable explanation for this is that measurement noise is about 3 times the digitization interval 0 25 nt and that higher precision measurements are required for further improvement this conclusion has been confirmed by tests on the instruments at close spacing application of the method to data taken in central california and hawaii reveals tectonomagnetic effects which are otherwise hidden in noise wiener filters are especially suitable for real time analysis which is an important factor in earthquake prediction less

metastable olivine wedge and deep dry cold slab beneath southwest japan abstract oceanic plates subducted at trenches penetrate into the deep mantle and encounter a structural boundary at a depth of 410 km where olivine the dominant element of mantle rocks transforms into a higher density form wadsleyite this transformation may be delayed within the coldest core of subducting plates slabs due to kinetic effects and it has been suggested that metastable olivine may persist deeper than 410 km using high density seismic array data in japan we show the direct image of the structure corresponding to this metastable olivine wedge mow beneath southwest japan numerical simulation of a subducting slab including the kinetic effect of water h 2 o on the olivine wadsleyite transformation indicates that the presence of the imaged mow requires an insignificant amount of water less than 100 wt ppm be present in the slab mantle thus a deep dry cold slab we infer that the transportation of water into the deep mantle occurs along the top surface of the subducting slab but no significant amount within the slab itself we also demonstrate that a numerical simulation including the kinetics of 660 km phase transformation can reconcile the observed deep depression of the 660 km discontinuity with a gentle clapeyron slope

review of magnetic and electric field effects near active faults and volcanoes in the u s a abstract synchronized measurements of geomagnetic field have been recorded along 800 km of the san andreas fault and in the long valley caldera since 1974 and during eruptions on mount st helens since 1980 for shorter periods of time continuous measurements of geoelectric field measurements have been made on mount st helens and near the san andreas fault where moderate seismicity and fault slip frequently occurs significant tectonic and volcanic events for which nearby magnetic and electric field data have been obtained include 1 two moderate earthquakes m l 5 8 for which magnetometers were close enough to expect observable signals about three source lengths 2 one moderate earthquake m s 7 3 for which magnetometers were installed as massive fluid outflow occurred during the post seismic phase 3 numerous fault creep events and moderate seismicity 4 a major explosive volcanic eruption and numerous minor extrusive eruptions and 5 an episode of aseismic uplift for one of the two earthquakes with m l 5 8 seismomagnetic effects of 1 3 and 0 3 nt were observed for this event magnetometers were optimally located near the epicenter and the observations obtained are consistent with simple seismomagnetic models of the event similar models for the other event indicate that the expected seismomagnetic effects are below the signal resolution of the nearest magnetometer precursive tectonomagnetic effects were recorded on two independent instruments at distances of 30 and 50 km from a m l 5 2 earthquake longer term changes were recorded in one region in southern california where a moderate m l 5 9 earthquake has since occurred surface observations of fault creep events have no associated magnetic or electrical signature above the present measurement precision 0 25 nt and 0 01 respectively and are consistent with near surface fault failure models of these events longer term creep is sometimes associated with corresponding longer term magnetic field perturbations correlated changes in gravity magnetic field areal strain and uplift occurred during episodes of aseismic deformation in southern california primarily between 1979 and 1983 because the relationships between these parameters agrees with those calculated from simple deformation and tectonomagnetic models the preferred explanation appeals to short term strain episodes independently detected in each data set an unknown source of meteorologically generated noise in the strain gravity and uplift data and an unknown but correlated disturbance in the absolute magnetic data might also explain the data no clear observations of seismoelectric or tectonoelectric effects have yet been reported the eruption of mount st helens generated large oscillatory fields and 9 2 nt offset on the only surviving magnetometer a large scale traveling magnetic disturbance passed through the san andreas array from 1 to 2 h after the eruption subsequent extrusive eruptions generated small precursory magnetic changes in some cases these data are consistent with a simple volcanomagnetic model magneto gas dynamic effects and a blast excited traveling ionospheric disturbance traveling ionospheric disturbances tids also generated by earthquake related atmospheric pressure waves may explain many electromagnetic disturbances apparently associated with earthquakes local near fault magnetic field transients rarely exceed a few nt at periods of a few minutes and longer

morphology of the subducting slab boundary in the northeastern japan arc abstract the detection and location of the subducting slab boundary is crucial for a better understanding of the mechanical coupling between the subducting oceanic plate and the overlying continental plate and of the occurrence of large and destructive earthquakes on the main thrust zone in the forearc region of subduction zones it is also important for the understanding of the dehydration of the subducting slab and the genesis of island arc magmatism and volcanism in this study we have estimated the morphology of the upper surface of the subducting pacific plate under northeastern ne japan by using 1372 sp converted waves at the slab boundary and 666 earthquakes beneath the pacific ocean that are well located with sp depth phases we found that the pacific slab beneath ne japan subducts at a small dip angle of less than 10 for the first descent to about 20 km depth and has a steeper dip angle of 15 25 at 30 50 km depth and about 30 at deeper areas the slab depth is approximately 50 km under the pacific coast 150 km under the japan sea coast and nearly 100 km beneath the volcanic front at 60 150 km depths earthquakes in the lower plane of the double seismic zone occur in the central part of the slab while the upper plane events occur within 10 15 km beneath the slab boundary there is a change in the slab surface geometry around 38 n which corresponds to the landward extension of an oceanic fracture zone the strike of the slab is nne in the north and becomes ne in the south the southern part of the slab has a steeper dip angle than the northern part we believe that these features are related to the different patterns of interplate seismicity and mechanical coupling between the northern and southern parts of ne japan many great m8 0 8 5 interplate earthquakes have occured in northern ne japan during recorded history 830 1995 and microearthquakes show a wide distribution in the forearc region from the japan trench to the pacific coast suggesting strong interplate seismic coupling in that region in contrast only a few m7 0 7 5 class earthquakes have occurred in southern ne japan in the same period and microearthquakes tend to cluster in many small areas suggesting weaker interplate coupling there the subducted slab may be segmented by the oceanic fracture zones and the degree of seismic coupling changes from segment to segment the behavior of the segment in southern ne japan might be affected by the subducted seamounts so that the slab subducts at a steeper dip angle this interpretation is supported by geomagnetic topographic and geological studies which show that many small sized seamounts were subducted in the past from the southern part of the japan trench but almost no seamounts were subducted beneath the northern part

tectonomagnetism and tectonoelectricity the last four years has seen a considerable waning of efforts in the u s to use the inherent stress sensitivity of the magnetization of rocks in tectonically active regions as a tool for understanding and perhaps predicting earthquake activity and volcanic eruptions the primary reasons for this decreased interest derive from accumulating evidence that the mean deviatoric stress release during earthquakes and the ambient shear stress near active faults is lower than than had been previously assumed and secondly the observational limits of magnetometers separated by about 10 km after application of a variety of noise reduction techniques appear to be no better than about 0 1 nt lower estimates of the average change in deviatoric stress associated with earthquake rupture correspondingly reduce the fields calculated from tectonomagnetic models at the same time some unexpected and exciting results have been obtained which indicate many aspects of magnetic field changes near active faults and volcanoes are still not completely understood the more important of these results are 1 the observation of correlated magnetic strain uplift and gravity changes during the latter stages of a remarkable deformation episode near the san andreas fault in southern california 2 indications of anomalous change in magnetic fields along the san andreas fault to the southeast of palm springs 3 an unambiguous observation of a volcanomagnetic effect at mt st helens prior to an extrusive eruption and 4 the first observations of coseismic seismomagnetic effects on two independent magnetometers during the july 8 1986 ml 5 9 north palm springs earthquake a resurgence of interest has occurred during the quadrennium in a related area concerning electrical phenomena and electromagnetic effects related to earthquakes and volcanoes mechanisms have been proposed to explain the occurrence of earthquakes lights and the possibility of electromagnetic emission prior to the 1960 chilean earthquake has been proposed self potential anomalies have been observed before and during several eruptions on kilauea volcano in hawaii

the effect of small amounts of h2o on olivine to ringwoodite transformation growth rates and implications for subduction of metastable olivine abstract it has been suggested that in relatively cold subduction zones the interior of down going lithospheric plates may be cool enough to inhibit the transformation of olivine to its high pressure polymorphs wadsleyite and ringwoodite resulting in the persistence of metastable olivine in the mantle transition zone the rapid transformation of this metastable olivine has been identified as a possible cause of deep focus earthquakes however it is known that olivine can incorporate hydrogen at high pressure and this hydrogen may enhance transformation rates we conducted a series of high pressure experiments to hydrate and then partially transform olivine to ringwoodite we measured transformed rim thickness as a function of run duration and fit a linear function to our results to determine transformation growth rates the addition of h2o to olivine appears to promote olivine to ringwoodite transformation by increasing the growth rate decreasing the activation enthalpy for growth and weakening the ringwoodite and or olivine to allow for dissipation of elastic strain sufficient to maintain constant or near constant growth rates throughout transformation the addition of approximately 300 ppmw 2h2o to olivine increases the growth rate of olivine to ringwoodite transformation at 1100 c and 18 gpa by almost a factor of 40 from 4 0 0 8 10 9 ms 1 to 1 5 0 3 10 7 ms 1 there is a corresponding decrease in the activation enthalpy of growth from 392 kj mol to 237 kj mol assuming activation volume of 0 cm3 mole analyses of hydrogen distributions in partially transformed samples suggest that the hydrogen concentration in the bulk of the olivine crystal may be less important than hydrogen at the olivine and ringwoodite interface

evidence for a metastable olivine wedge inside the subducted mariana slab abstract an anomalous seismic velocity structure is revealed inside the subducted pacific plate at northern mariana near the deepest earthquake foci we observe anomalous later phases in the p wave coda for a small cluster of deep earthquakes and an anomalous differential p wave slowness between a few pairs of the earthquakes belonging to the cluster we interpret these observations with a 5 low velocity wedge which is nearly 25 km wide at the depth of 590 km and is truncated at 630 km the wedge is bounded by two steeply dipping 60 70 transition layers with the thickness of about 4 km the highly localized seismic anomaly inside the slab represents a wedge of metastable olivine embedded in dry ringwoodite deep earthquakes occur across only one of the boundaries of the metastable wedge with some of them outside the wedge the mechanism of deep earthquakes is discussed based on the spatial pattern of seismicity relative to the wedge

buoyancy bending and seismic visibility in deep slab stagnation abstract the petrological consequences of deep subhorizontal deflection stagnation of subducting slabs should affect both apparent seismic velocity structures and slab morphology we construct kinematic thermal models of stagnant slabs and perform thermodynamic modeling of the consequent perturbation of high pressure phase transitions in mantle minerals focusing upon japan as our study area we calculate associated thermo petrological buoyancy forces and bending moments which along with other factors such as viscosity variations and rollback dynamics may contribute to slab deformation we consider effects of variations in depth of stagnation post stagnation dip angle phase transition sharpness transition triplication due to multiple intersection of geotherms with phase boundaries and potential persistence of metastable phases due to kinetic hindrance we also estimate seismic velocity anomalies as might be imaged by seismic tomography and corresponding seismic velocity gradients as might be imaged by receiver function analysis we find that buoyant bending moment gradients of petrological origin at the base of the transition zone may contribute to slab stagnation such buoyancy forces vary with the depth at which stagnation occurs so that slabs may seek an equilibrium slab stagnation depth metastable phase bending moment gradients further enhance slab stagnation but they thermally decay after 4600 700 km of horizontal travel potentially allowing stagnant slabs to descend into the lower mantle stagnant slabs superimpose zones of negative velocity gradient onto a depressed 660 km seismic discontinuity affecting the seismological visibility of such features seismologically resolvable details should depend upon both stagnation depth and the nature of the imaging technique travel time tomography vs boundary interaction phases while seismic tomography appears to yield images of stagnant slabs discontinuity topography beneath japan resolved by migrated receiver functions appears to be consistent with slab penetration of the transition zone however model slabs which bottom around 4780 810 km and then bend upwards by a few degrees can match both the tomographic and receiver function images

using shear wave amplitude patterns to detect metastable olivine in subducted slabs 1 olivine within the lowest temperature regions of subducting oceanic lithosphere may persist metastably to depths well below the equilibrium phase boundary for transformation to phase this may produce a tapering low seismic velocity wedge within the slab in the transition zone transformational faulting of such metastable olivine has been proposed as a possible explanation of deep earthquakes detecting seismic velocity structure associated with metastable olivine within a slab using seismic wave arrival time information alone has proven problematic motivated by previous two dimensional numerical calculations for upgoing and downgoing seismic waves a three dimensional gaussian beam method is used to assess whether the azimuthally varying teleseismic amplitude signature produced by a slab with an internal wedge of metastable olivine is distinguishable from that for a slab with equilibrium transitions five slab velocity structures are considered a thermal model with horizontal phase transitions at the same depths as in ambient mantle two models incorporating equilibrium transitions within the slab and two models with tongues of metastable olivine a depth and temperature dependent thermal conductivity term and the effect of the latent heat of transformation of olivine to phase are included in the thermal modeling three dimensional dynamic ray tracing and gaussian beam summation are performed for events located at different depths and lateral positions for each of the slab models for sources near 400 km depth the presence of low velocity metastable olivine produces relatively complex azimuthal shear wave amplitude patterns compared to thermal and equilibrium models comparison with observed amplitudes for four events in the kurile slab indicates that allowing for internal slab velocity complexity in the transition zone provides better agreement with observations than simple thermal and equilibrium slab models this indicates that careful modeling of teleseismic shear wave amplitude patterns may have the resolution to detect and characterize the presence of metastable olivine in deep slabs

east asia mantle tomography new insight into plate subduction and intraplate volcanism abstract we present a new p wave tomographic model beneath east asia by inverting 1 401 797 arrival time data recorded by the regional seismic networks in east asia and temporary seismic arrays deployed on the tibetan plateau our high resolution tomography shows that the subducted pacific slab is revealed clearly as a high velocity high v zone and it becomes stagnant in the mantle transition zone beneath eastern china prominent high v anomalies atop the 410 km discontinuity are detected beneath the eastern north china craton and northeast china suggesting that lithospheric delamination may have occurred and greatly affected the thermal state intraplate magmatism and surface topography of this region the indian lithosphere is characterized by a high v anomaly and it is subducting nearly horizontally beneath the entire or most parts of western tibet and with a small dipping angle to the southernmost part of eastern tibet the intraplate magmatism in different parts of east asia has different origins the active tengchong volcano is underlain by a prominent low velocity low v anomaly in the shallow mantle which may be caused by the subduction and dehydration of the burma microplate plate or indian plate the hainan volcano is underlain by a plume like low v anomaly that extends down to at least 1000 km depth and seems to be related to the deep subductions of the burma microplate or indian plate in the west and the philippine sea plate in the east prominent low v anomalies are detected beneath the changbai longgang and xianjindao volcanoes in northeast asia which may be related to the rollback of the subducting pacific slab as well as the convective circulation process in the big mantle wedge above the stagnant slab the origin of the wudalianchi volcano in northeast china seems associated with the upwelling of asthenospheric materials caused by the subduction induced lithospheric delamination

constraining kinetics of metastable olivine in the marianas slab from seismic observations and dynamic models abstract transformation kinetics associated with the presence of a metastable olivine wedge in old and fast subducting slabs has been the subject of many studies in the last years even with improvements in kinetics models many of the parameters are still not well constrained in particular there is no consensus on the blocking temperature that could inhibit the transformation from olivine to spinel recently based on anomalous later phases in the p wave coda and differential p wave slowness a wedge of metastable olivine was detected in the marianas subduction zone that is approximately 25 km wide at a depth of 590 km and is truncated at 630 km in this work a thermomechanical model was used to mimic the subduction in the marianas and try different blocking temperatures for the olivine spinel transformation the model includes among other features non linear elasto visco plastic rheology based on laboratory data phase transformations latent heat proper coupling between stress and thermal state of the slab and force balance of the system the results show a positive correlation between the blocking temperature depth of the wedge and its distance from the trench or subduction angle we compare these results to the situation in the marianas and suggest that a blocking temperature for the olivine spinel transformation of approximately 725 c would be the most likely the volume of the wedge presents some oscillations that we relate to a runaway effect of the transformation kinetics in the mantle transition zone namely the interaction between latent heat release and the advection of the isotherms due to the subduction velocity the inclusion of shear heating in the model was fundamental to modeling such a subduction zone without shear heating the slab shows a higher level of internal stress and the necessary bending to mimic the marianas subduction zone cannot be reached

secular variation crustal contributions and tectonic activity in california 1976 1984 five to ten years of data from an array of 34 total field magnetometers are used to define the temporal and spatial characteristics of secular variation throughout central and southern california for this period well determined rates of secular variation are obtained at each site these rates are temporally linear but spatially variable ranging from 45 nt a near san francisco to 54 nt a near the mexican border least squares analysis of all data indicates secular variation decreases in a general southeasterly direction according to f k1 k2 k where f is in nanoteslas per year and are the geographic latitude and longitude and k1 k2 and k are 1 66 0 13 nt a deg 0 13 0 10 nt a deg and 123 2 0 2 nt a respectively deviations of as much as 1 nt a occur on scales of a few tens of kilometers these apparent small scale secular variation anomalies result in part from differences in local induction and remanent magnetization and may be reduced by determination of a site transfer function a planar surface fit to the corrected data has the form f k1 k2 k where k1 k2 and k are now 1 50 0 08 nt a deg 0 23 0 06 nt a deg and 129 2 0 1 nt a respectively residual field variations obtained after correction of all data for secular variation are most apparent on the san andreas fault in southern california between palm springs and the salton sea and marginally so along the recent coyote ml 5 9 of august 6 1979 and morgan hill ml 5 8 of april 24 1984 aftershock zones these residuals could be explained by stress localization in these regions and particularly in the case of the southern san andreas anomaly may indicate the location of a future damaging earthquake incomplete correction for complex site effects may be an alternative explanation in the first large scale test of global secular variation models we find that the models for this region do not predict either the amplitudes or the mean isogram directions of these data to better than several tens of nanoteslas per year and several tens of degrees respectively this may result from a failure to correct for site response effects at some observatories before using the data in global spherical harmonic expansions local magnetization response can therefore bias estimates of secular variation and yield apparent impulsive behavior when external fields are perturbed

experimental constraints on the depth of olivine metastability in subducting lithosphere the hypothesis that metastable olivine persists in some subducting slabs into the transition zone has wide implications for mantle dynamics and rheology in order to evaluate this possibility we derive new thermo kinetic subduction zone models to predict the extent of olivine metastability within the stability fields of its high pressure polymorphs wadsleyite and ringwoodite our updated models improve on previous work by incorporating experimental kinetic data on realistic mantle compositions mg fe 2sio4 rather than analogue systems furthermore latent heat due to the transformation is fed back into both the kinetics and the thermal model we also consider the effects of transformation stress on growth kinetics and the possibility of an intracrystalline transformation mechanism previously thought to be important only at high shear stresses our models predict significantly smaller wedges of metastable olivine than previous work in the case of tonga for example where high values of lithospheric age 100 140 million years and convergence rate 14 cm per year are most favorable for metastability models considering only grain boundary nucleation and interface controlled growth predict olivine metastability to 600 km depth in contrast to 660 km predicted previously by kirby et al rev geophys 34 1996 261 when intracrystalline transformation is considered the depth of metastability is further reduced by as much as 100 km due to the large increase in the density of nucleation sites inhibition of growth by transformation stress can increase the depth interval over which the transformation takes place but is unlikely to be a dominant factor especially if the intracrystalline mechanism operates these results indicate that the existence of metastable olivine at depths corresponding to those of the deepest earthquakes 680 km requires subduction of old lithosphere 100 million years and a high vertical subduction velocity 15 cm per year such conditions currently may only be achieved in the very northern part of the tonga subduction zone yet earthquakes occur at depths down to 680 km in other subduction zones in which lithospheric age and or subduction velocity are relatively low e g indonesia and the marianas therefore mechanisms other than transformational faulting in metastable olivine must operate to cause the deepest earthquakes

implications of slab mineralogy for subduction dynamics phase relations among mantle minerals are perturbed by the thermal environment of subducting slabs both under equilibrium and disequilibrium metastable conditions such perturbations yield anomalies not only in seismic velocities but also in density the buoyancy forces arising from these density anomalies may exert several important effects they contribute to the stress field within the slab in a fashion consistent with observed patterns of seismicity they may affect subduction rates both by inducing time dependent velocity changes under equilibrium conditions and by imposing velocity limits through a thermal feedback loop under disequilibrium conditions they may affect slab morphology possibly inhibiting penetration of slabs into the lower mantle and allowing temporary stagnation of deflected or detached slabs latent heat release from phase transitions under disequilibrium conditions in slabs can yield isobaric superheating which may generate adiabatic shear instabilities capable of triggering deep seismicity 2001 elsevier science b v all rights reserved

high resolution mantle tomography of china and surrounding regions 1 a high resolution p wave tomographic model of the crust and mantle down to 1100 km depth under china and surrounding regions is determined by using about one million arrival times of p pp pp and pcp waves from 19 361 earthquakes recorded by 1012 seismic stations the subducting pacific slab is imaged clearly as a high velocity zone from the oceanic trenches down to about 600 km depth and intermediate depth and deep earthquakes are located within the slab the pacific slab becomes stagnant in the mantle transition zone under east china the western edge of the stagnant slab is roughly coincident with a surface topographic boundary in east china the active changbai and wudalianchi intraplate volcanoes in northeast china are underlain by significant slow anomalies in the upper mantle above the stagnant pacific slab these results suggest that the active intraplate volcanoes in ne china are not hot spots but a kind of back arc volcano associated with the deep subduction of the pacific slab and its stagnancy in the transition zone under the mariana arc however the pacific slab penetrates directly down to the lower mantle the active tengchong volcano in southwest china is related to the eastward subduction of the burma microplate the subducting indian and philippine sea plates are also imaged clearly the indian plate has subducted down to 200 300 km depth under the tibetan plateau with a horizontal moving distance of about 500 km high velocity anomalies are revealed in the upper mantle under the tarim basin ordos and sichuan basin which are three stable blocks in china

seismic evidence for a metastable olivine wedge in the subducting pacific slab under japan sea abstract we apply a forward modeling approach to high quality arrival time data from 23 deep earthquakes greater than 400 km depth to investigate the detailed structure of the subducting pacific slab beneath the japan sea our results show that a finger like anomaly exists within the subducting pacific slab below 400 km depth which has a p wave velocity 5 lower than the surrounding slab velocity or 3 lower than that of the normal mantle suggesting the existence of a metastable olivine wedge mow in the slab the mow top and bottom depths are 400 and 560 km respectively the mow is estimated to be about 50 km wide at 400 km depth and close to the slab upper boundary at 560 km depth the mow is located at about 25 km below the slab upper boundary most of the deep earthquakes are located in the mow our results favor transformational faulting as the mechanism for deep earthquakes

localized geomagnetic field changes near active faults in california 1974 1980 currents in the ionosphere and magnetosphere induce changes in the difference fields due to contrasting impedances at magnetometer sites these changes are identified and removed by multichannel predictive filtering using boulder magnetic observatory component fields as inputs to the prediction operator the remaining fluctuations up to several nt are attributed to crustal magnetization changes evidence that these changes are stress related includes the variation prior to the busch fault earthquake 1974 seen at the two stations closest to within 10 km the epicenter also stations within 50 km of the 1979 coyote earthquake undergo localized field change up to a year before that event if truly precursive these variations are on a broader spatial and temporal scale than those for the busch fault event during the period 1975 1978 the geomagnetic variation is approximately uniform across the array without the development of any significant anomalies this period roughly coincides with the 1975 7 1978 7 seismic lull in nearly all of california it is followed by a marked increase in crustal secular variation long range coherence between anomalous variation at different stations is not seen other than the variation before the busch fault earthquake suggesting that localized variation occurs on a scale of less than 10 km coseismic tectonomagnetic events are not observed at the times of the busch and coyote earthquakes but model calculations suggest that observable events are not expected if the observed changes are due to stress they indicate that changes of aseismic stress are significantly larger than coseismic stress release

seismological evidence for metastable olivine inside a subducting slab the nature and location of the olivine spinel phase transition inside subducting slabs differ greatly from the situation in the surrounding mantle due to the different temperature distribution inside the slabs two models have been proposed for this phase transition one in which the location of the phase boundary between olivine and modified phase spinel is determined by equilibrium thermodynamics1 and the other including a metastable olivine phase which persists to a depth of 550 km ref 2 the location of the olivine spinel transition in the slab may be relevant to the generation of deep earthquakes3 5 and to the buoyancy forces driving subduction6 here we use travel time residuals from deep earthquakes recorded by the dense seismograph network in japan to investigate the configuration of the olivine spinel phase boundary inside the subducting pacific plate theoretical travel time residuals for the equilibrium model do not fit the observed residuals whereas those for the metastable model do implying the presence of metastable olivine inside the subducting slab

metastable olivine wedge in the subducting pacific slab and its relation to deep earthquakes abstract a metastable olivine wedge mow is detected in the subducting pacific slab under the japan sea by applying a forward modeling approach with 3 d ray tracing to high quality arrival time data from 78 well located deep earthquakes the data were recorded by the dense and high quality seismic networks on the japan islands the double difference earthquake location technique is improved and used to relocate the 78 deep earthquakes precisely our results show that the mow does exist in the pacific slab and it has a p wave velocity 3 lower than the normal mantle or 4 lower than the surrounding slab velocity the top and bottom depths of the mow are 400 and 570 km respectively and the mow is located in the central portion of the slab after precise relocation all the 78 deep earthquakes are found to occur within the mow or along its edges within the slab the cut off depth of deep seismicity under the japan sea is consistent with the estimated mow bottom depth our results favor the transformational faulting as the mechanism of deep earthquakes in the pacific slab under the japan sea

interslab earthquakes and phase changes in subducting lithosphere at 0033 ut on 9 june 1994 a great deep earthquake occurred 635 km beneath the amazonian rain forest of northern bolivia the deep rupture of this record setting magnitude 8 3 shock lasted about a minute harvard university centroid moment tensor catalogue the seismic strain release of this single event as indicated by its seismic moment was greater than the total strain release for all deep earthquakes in the harvard cmt catalogue during the previous 17 years of its coverage of global seismicity incredibly ground motion was felt as far north as seattle washington and toronto canada evidently it occurred within the subducting nazca plate that is descending beneath south america this earthquake reminds us of the sometimes grand scale of deep seismic failure and of the need to consider more closely the physical processes that accompany slab descent and in particular how such earthquakes may reflect the inner workings of slabs

a petrologic anomaly accompanying outboard earthquakes beneath fiji tonga corresponding evidence from broadband p and s waveforms 1 using high resolution triplicate body waveforms we investigate strong lateral variations in p and sh wave speeds vp and vs beneath the tonga subduction zone where cold lithosphere is subducting rapidly at 200 mm yr 1 under these conditions the lack of prominent anomalies of high vp and vs in the lower mantle implies that a large amount of recently subducted material remains above the lower mantle this inference is also evident from a subhorizontal swath of outboard earthquakes in the transition zone that is up to 1000 km away from the wadati benioff zone beneath the back arc we find a sharp east west trending demarcation north of which outboard earthquakes occur and vp and vs abruptly drop by 3 thus the effect of cold temperature to raise vp and vs as indicated by the presence of outboard earthquakes must be counteracted by petrologic variations we interpret that outboard earthquakes occur where impounding of subducted metastable olivine or volatiles reduces seismic wave speeds and triggers deep earthquakes considering the density of these candidates and the strong effect of partial melt on vs volatiles in the form of hydrous phases or melt induced by dehydration are not consistent with observed reductions in vp and vs meanwhile metastable olivine remains a viable candidate as a cold slab passes below the 410 km discontinuity olivine becomes metastable and makes the tip of the slab buoyant such a mechanism could explain why the leading end of a fast subducting slab may be successively sequestered in the transition zone

stagnation of subducting slabs in the transition zone due to slow diffusion in majoritic garnet subducting slabs of oceanic lithosphere often stagnate before reaching the lower mantle laboratory experiments under high pressures and temperatures show that pyroxene a common mineral in such slabs is transformed to its dense high pressure counterpart majorite garnet at a very slow rate temporarily keeping the slabs buoyant compared to the surrounding mantle

time dependent interaction between subduction dynamics and phase transition kinetics summary numerical subduction models are presented that account for the kinetics of the olivine spinel transformation and the equilibrium transformation of spinel into perovskite and magnesiowustite temperature depth and stress dependent rheology is used buoyancy due to temperature and the olivine spinel and magnesiowustite perovskite phases leads to free subduction into the transition zone and retarded penetration into the lower mantle a dynamic feed back mechanism is found between the different buoyancy forces the kinetics of the phase transition and the release of latent heat this mechanism modulates the parachute effect of the metastable olivine mo in a subducting slab and leads to characteristic temporal variations of the depth of mo and with a time delay variations of the subduction velocity time periods are of the order of 3 4 myr this mechanism allows to explain variations of the depths of the deepest earthquakes in otherwise similar subduction zones further conclusions are as follows the maximum depth of mo increases roughly linearly with lithospheric thickness and the maximum depth may reach as deep as 720 750 km different subduction zones may represent different stages of velocity depth phase loops of mo

tidal current channeling in the san andreas fault california measurements of magnetic fields along the san andreas fault indicate that inhomogeneous tidally generated current systems flow in and around the fault system these currents limit measurements of short term local fields of tectonomagnetic origin to about 0 3 nt but can be easily removed ocean tidal induc tion into a complex fault zone with higher than average electrical conductivity appears to be a more likely explanation than either piezo magnetic effects due to solid earth tides or ionospheric tidal induction the amplitudes of the induced diurnal harmonics decrease fairly linearly to the southeast along the fault this result is consistent with expectations from a hotter and more conductive crust and upper mantle in southern california as indicated by heat flow data for this region

seismic image and origin of the changbai intraplate volcano in east asia role of big mantle wedge above the stagnant pacific slab abstract we determined p wave tomography of the crust and upper mantle under the changbai volcanic area in northeast asia by using 289 318 arrival time data from 4802 local earthquakes and 9599 teleseismic events a prominent low velocity anomaly is clearly imaged in the crust and upper mantle down to about 410 km depth beneath the changbai volcano a broad high velocity anomaly is revealed in the mantle transition zone and deep earthquakes occurred actively there suggesting that the subducting pacific slab is stagnating in the mantle transition zone under east asia our results suggest that the formation of the changbai and other intraplate volcanoes in northeast asia is related to the upwelling of hot and wet asthenospheric materials in the big mantle wedge above the stagnant pacific slab hence the active intraplate volcanoes in northeast asia are not hotspots related to deep mantle plumes but are caused by plate tectonic processes in the upper mantle

detection of metastable olivine wedge in the western pacific slab and its geodynamic implications abstract seismic tomography and numerical simulations show that the western pacific slab bends horizontally when it reaches the boundary between the upper mantle and lower mantle beneath northeast asia it is expected that a metastable olivine wedge mow exists in the cold core of the slab because of a delayed phase transition from olivine to its high pressure polymorphs however it is still debated whether the mow actually exists or not and even if it exists its physical properties such as seismic velocity and density are still unclear in this work we use high quality arrival time data of 17 deep earthquakes occurring within the pacific slab under northeast asia to study the detailed structure of the slab the deep earthquakes are relocated precisely by applying a modified double difference location method to arrival time data recorded at both chinese and japanese stations based on the precise hypocentral locations a forward modeling method and differential travel time residuals data are used to estimate seismic velocity within the deep source zone which can decrease or remove the influence of ambient velocity heterogeneities our results show that the mow does exist within the pacific slab under northeast asia and the mow has a mean velocity anomaly of 7 9 lower than the iasp91 earth model the existence of mow in the slab has important geodynamic implications it can reduce the speed of slab subduction and affect the generation of deep earthquakes

identification of homogeneous regions for flood frequency analysis abstract a procedure for classifying basins into distinct homogeneous groups for regional flood frequency analysis is described the scheme consists of an iterative search through the basin characteristic database which optimises statistics that describe the efficiency of grouping application of the procedure to basins in britain yields five groups formed on the basis of basin area average rainfall and urban fraction four of these groups are homogeneous with respect to the coefficient of variation of the annual maximum flood series the properties of these groups compare favourably with ten geographic regions hitherto used for flood analysis in britain a further procedure is described which eliminates edge effects present in quantile estimation close to region boundaries

modeling the tonga slab can travel time data resolve a metastable olivine wedge we present the results of detailed modeling of the tonga slab with the goals of determining whether high resolution travel time data 1 can be fit by simple thermal and petrological slab models and 2 can resolve a metastable olivine wedge at depths greater than the equilibrium olivine spinel phase boundary we model arrival times recorded by a 1000 km line of 23 ocean bottom seismometers obs and island broadband seismic stations extending from the lau backarc basin across the tonga trench and onto the pacific plate the data consist of 388 local p wave travel times from 17 deep and 3 intermediate earthquakes recorded during the 3 month obs deployment in late 1994 we locate the events using both local and teleseismic arrival times and apply a relocation operator to the theoretical travel times to simulate the biases introduced in the data by locating the events with a reference earth model the modeling consists of grid searches using a three dimensional finite difference algorithm to compute local first arriving travel times for equilibrium and metastable p wave velocity models constructed from thermal mineralogical and morphological constraints the travel time anomalies are well fit by standard slab thermal models and p velocity temperature derivatives of 0 4 to 0 3 ms 1 c 1 forward calculations indicate that the presence of a metastable olivine wedge has a subtle effect on the travel times due to the tendency of first arriving waves to avoid the low velocity region wedge velocity models provide a slightly better fit to the data than equilibrium models but f tests indicate the improvement is not significant at the 95 level our results suggest that providing direct seismological evidence of a wedge of metastable olivine in subduction zones will require either waveform modeling or the observation of later arriving phases created by the depressed phase boundary

the waveguide effect of metastable olivine in slabs we use a finite difference algorithm to compute p sv synthetic seismograms for deep earthquakes recorded at regional seismic stations located above a subducting slab we calculate synthetics for 2 d slab models in which the 410 km discontinuity is uplifted owing to the exothermic nature of the transition the equilibrium model and models in which the 410 km discontinuity is depressed owing to inhibited kinetics of the olivine transition the metastable model in the latter case the existence of the low velocity wedge of metastable olivine has a profound influence on the waveforms of deep focus earthquakes the wedge acts as a geometrical waveguide with the energy focusing effect apparent even after the wavefront has traveled the several hundred kilometers from the wedge boundary to the earth s surface the guided energy is most pronounced for receivers located near the surface projection of the deep slab but is observable over a range of 300 km and so may provide a valuable diagnostic for discriminating between equilibrium and metastable models of subduction a recent search for such guided energy using regionally recorded p and s waves from deep events in tonga yielded negative results