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

stress field in the subducting lithosphere and comparison with deep earthquakes in tonga 1 we present a numerical model of the subducting lithosphere that provides an alternative explanation for stresses causing deep earthquakes our model lithosphere is composed of olivine spinel spinel and perovskite magnesiowustite the heat conduction equation is solved to determine temperature conditions in the slab and to locate the equilibrium phase transformations in pressure temperature space volumetric strains in the subducting lithosphere are calculated from the density of individual phases and from the heat released or consumed in the phase changes these strains are used as sources of stress in the subducting lithosphere dislocation creep and peierls stress creep laws are included in the viscoelastic rheology volumetric reductions due to equilibrium phase transformations cause high shear stress in the transition zone because of the variable viscosity inside the subducting slab aspects of the model shear stresses are in agreement with observations of high seismic activity in the tonga wadati benioff zone compression is oriented along the dip of the slab and extension is oriented in the plane perpendicular to the compression axis since our model stresses agree with the seismic observations and because the model stresses are larger than those caused by buoyancy forces our model provides a possible explanation of stresses causing deep earthquakes also our model does not need metastability of olivine to explain the occurrence of high shear stress in the transition zone

mantle phase changes and deep earthquake faulting in subducting lithosphere inclined zones of earthquakes are the primary expression of lithosphere subduction a distinct deep population of subduction zone earthquakes occurs at depths of 350 to 690 kilometers at those depths ordinary brittle fracture and frictional sliding the faulting processes of shallow earthquakes are not expected a fresh understanding of these deep earthquakes comes from developments in several areas of experimental and theoretical geophysics including the discovery and characterization of transformational faulting a shear instability connected with localized phase transformations under nonhydrostatic stress these developments support the hypothesis that deep earthquakes represent transformational faulting in a wedge of olivine rich peridotite that is likely to persist metastably in coldest plate interiors to depths as great as 690 km predictions based on this deep structure of mantle phase changes are consistent with the global depth distribution of deep earthquakes the maximum depths of earthquakes in individual subductions zones and key source characteristics of deep events

flow fields within the tonga slab determined from the moment tensors of deep earthquakes the seismic strain rates and instantaneous velocity fields within the plane of the tonga slab are investigated using 450 harvard centroid moment tensor cmt solutions from the period 1977 1992 down dip seismic strain rates at the bottom of the slab are around 1 10 15 s 1 spline functions are fitted to the distribution of seismic strain rates to recover the velocity gradient tensor within the plane of the slab solutions show a northward motion of the 200 km depth contour in the tonga slab at rates of 8 2 mm yr relative to the 670 km depth contour of the slab relative to the deep southern portion of the tonga slab down dip velocities increase from near zero in southern tonga to 10 2 5 mm yr in northern tonga consistent with the increased subduction rate from south to north described by the pacific australia pole of rotation moreover patterns of strain and velocity obtained from all events show a remarkable similarity with the patterns of strain and velocity obtained using only events with scalar moment values less than 1 1025 dyn cm along strike extensional strain rates in the deep zone of northern tonga are approximately 0 5 10 15 s 1 this is of the same order of magnitude as the extensional strain rates associated with changes in curvature produced by the subduction of a spherical plate suggesting that the seismic strain rates within tonga represent a large fraction of the total strain rate budget within the seismogenic zone an estimated maximum of 60 of the relative motion between the surface and 670 km is absorbed as strain within the slab indicating considerable slab thickening and penetration of the slab into the lower mantle

an inverted double seismic zone in chile evidence of phase transformation in the subducted slab data from two microseismic field experiments in northern chile revealed an elongated cluster of earthquakes in the subducted nazca plate at a depth of about 100 kilometers in which down dip tensional events were consistently shallower than a family of compressional earthquakes this double seismic zone shows a distribution of stresses of opposite polarity relative to that observed in other double seismic zones in the world the distribution of stresses in northern chile supports the notion that at depths of between 90 to 150 kilometers the basalt to eclogite transformation of the subducting oceanic crust induces tensional deformation in the upper part of the subducted slab and compressional deformation in the underlying mantle

a new self organizing mechanism for deep focus earthquakes the mechanism of deep focus earthquakes has been a puzzle since their discovery almost 70 years ago1 2 because brittle fracture and frictional sliding at depths in excess of 100 200 km would require unrealistic rock strengths3 4 rock strength does increase with pressure but a few hundred mpa equivalent to 10 20 km depth suffices to inhibit most fracture and elevated temperature activates ductile mechanisms that operate at stresses less than the fracture stength a range of mechanisms has been proposed for deep earthquakes including plastic instabilities5 7 shear induced melting8 11 and instabilities accompanying recrystallization12 13 or polymorphic phase transformation14 23 each of these proposed mechanisms has exhibited certain inherent weaknesses see kirby22 for review and discussion here we report experimental observations of high pressure faulting of metastable mg2geo4 olivine as it undergoes incipient transformation to a spinel 24 25 structure we present a model in which this faulting arises from the generation propagation and linking up of spinel filled anticracks26 when applied to the olivine spinel transformation in the earth s mantle the anticrack model27 satisfactorily accounts for the similarities and differences between shallow and deep earthquakes and removes the problems associated with frictional sliding

seismological evidence for a slab detachment in the tonga subduction zone abstract the tonga wadati benioff zone is characterized by a large seismicity gap beneath the lau basin that raises the question of the slab continuity between the shallow to intermediate part 60 300 km and the deep part 400 700 km to address this problem we investigated the wadati benioff zone geometry and stress regime through a detailed analysis of the spatial distribution of moment tensors and variation of the stress tensor using the global seismicity engdahl e van der hilst r buland r 1998 global teleseismic earthquake relocation with improved travel times and procedures for depth determination bull seism soc am 88 722 743 and the centroid moment tensor solutions cmt catalogs dziewonski a m chou t woodhouse j h 1981 determination of earthquake source parameters from waveform data for studies of global and regional seismicity j geophys res 86 2825 2852 the stress tensors were computed using the gephart s program gephart j w forsyth d 1984 an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence j geophys res 89 9305 9320 the stress inversion results indicate that between 21 s and 27 s and depths down to 700 km the slab is under homogeneous down dip compressional stress regime while north of 21 s we found strong variations of the stress orientations between the intermediate and deep portions of the slab we also show that between 14 and 19 s the stresses at intermediate depth 60 300 km can be resolved into two slab parallel domains a thin upper part of the slab that is under downdip compression and the lower part that is under downdip extension this pattern of two zones with opposite mechanical behavior is characteristic of a subducted plate with a free lower limit that does not interact with the 670 km depth boundary these results together with the large seismicity gap within the slab argue for a slab detachment

double seismic zone in tonga the clear evidence of a double seismic zone beneath japan1 2 has led seismologists to search for other such zones elsewhere because they have been found in few other areas3 6 double seismic zones are thought to be rare and to depend strongly on a peculiar combination of external conditions such as age of the descending slab and the convergence rate 7 we report here a new double seismic zone which has been found beneath the tonga arc8 where the stress state in the subducting slab has been described as strongly down dip compressional7 9 10 this discovery suggests that the double seismic zone is a general feature of subducting slabs and may be found in many other subduction zones

ringwoodite growth rates from olivine with 75ppmw h2o metastable olivine must be nearly anhydrous to exist in the mantle transition zone it has been previously demonstrated that as little as 300 ppmw h 2o increases wadsleyite and ringwoodite growth rates to magnitudes that are inconsistent with the metastable olivine hypothesis to further test this hypothesis we present new ringwoodite growth rate measurements from olivine with 75 ppmw h 2o at 18 gpa and 700 900 and 1100 c these growth rates are nearly identical to those from olivine with 300 ppmw h 2o and significantly higher than those from nominally anhydrous olivine we infer that transformation of olivine with 75 300 ppmw h 2o is primarily enhanced by hydrolytic weakening of reaction rims which reduces the elastic strain energy barrier to growth we present a new method for fitting non linear nominally anhydrous data to demonstrate that reduction of growth rates by elastic strain energy is an additional requirement for metastable olivine based on previous thermokinetic modeling these enhanced growth rates are inconsistent with the persistence of metastable olivine wedges into the mantle transition zone metastable persistence of olivine into the mantle transition zone would therefore require 75 ppmw h 2o

fast rise times and the physical mechanism of deep earthquakes earthquakes at depths of 300 km are similar to shallower events in that they are dominantly of double couple character1 implying that shearing motion has taken place at depth but because increased friction at these high pressures inhibits brittle fracture2 3 various other mechanisms related to phase transformations have been invoked to explain the occurrence of deep earthquakes4 10 as yet however no consistent differences have been found between the source characteristics of deep 300 km and intermediate depth 300km earthquakes2 11 13 here we report a systematic global survey of the rise times and stress drops of deep and intermediate earthquakes the rise time is defined as the time from rupture initiation to peak moment release rate when the rise times are scaled to the seismic moment release of the events their average is nearly twice as fast for events deeper than 450 km as for shallower events this difference may ultimately provide an experimental means of testing proposed mechanisms for the generation of deep seismicity

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

stress distribution due to olivine spinel phase transition in descending plate and deep focus earthquakes the stress field due to olivine spinel phase transition in and around a descending plate is studied based on the calculations for an equilibrium phase transition with a constant clapeyron s slope and for a nonequilibrium transition taking a kinetic effect into consideration the material concerned is assumed to be a viscoelastic body whose physical coefficients are functions of temperature and pressures the calculated stress distribution for the equilibrium transition is characterized by a compressional stress near the upper and or lower surfaces of the plate and by a tensional stress at the central part in the depth range from 200 to 550 km the magnitude of principal stress being greater than 0 5 gpa however for the nonequilibrium transition a tensional stress predominates near the upper and lower surfaces and the central part is compressional in the depth range from 300 to 600 km unlike the case of equilibrium transition the maximum principal stress in the latter case is greater than 2 gpa which is much more than that for the equilibrium transition the principal axes in the high stressed region are oriented almost parallel to the descending direction in both cases comparison of the calculated results to the actually observed seismicity depth relation and to focal mechanisms of deep focus earthquakes in descending plates beneath island arcs shows that the nonequilibrium phase transition agrees better with the observations

olivine spinel transitions experimental and thermodynamic constraints and implications for the nature of the 400 km seismic discontinuity the sequence of high pressure phase transitions in olivine is traditionally used as a model for seismic velocity variations in the 200 to 650 km depth interval in a mantle of peridotitic bulk composition it has been proposed that the observed seismic velocity increase at 400 km depth is too sharp and of too small a magnitude to be attributable to the phase change and that the upper mantle must thus be chemically stratified with the 400 km discontinuity being due either to a combination of phase changes in a layer of pyroxene garnet rich piclogite composition or to a chemical boundary between such a piclogite layer and an overlying peridotitic layer using available calorimetric thermoelastic and synthesis data and their associated experimental uncertainties we have derived internally consistent high pressure phase relations for the mg2sio4 fe2sio4 join we find that the divariant transition which is generally regarded as occurring over a broad depth interval for mantle olivine compositions is in fact extremely sharp the seismic discontinuity corresponding to the transition in mg0 9fe0 1 2sio4 should occur over a depth interval isothermal of about 6 km at a depth of approximately 400 km the sharpness of this transition is quite insensitive to uncertainties in the constraining calorimetric thermoelastic and synthesis data in addition we have computed seismic velocity profiles for a model mantle consisting of pure olivine of mg0 9fe0 1 2sio4 composition comparison of these computed profiles to those derived from recent seismic studies indicates that the magnitude of the observed velocity increase at 400 km depth is consistent with a mantle transition zone composed of about 60 70 olivine we conclude that there is no need to infer the existence of pyroxene garnet rich compositions such as eclogite or piclogite in the transition zone since an upper mantle of homogeneous olivine rich peridotitic composition is consistent with the available seismic velocity data

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

double seismic zone for deep earthquakes in the izu bonin subduction zone a double seismic zone for deep earthquakes was found in the izu bonin region an analysis of sp converted phases confirms that the deep seismic zone consists of two layers separated by 20 kilometers numerical modeling of the thermal structure implies that the hypocenters are located along isotherms of 500 to 550 c which is consistent with the hypothesis that deep earthquakes result from the phase transition of metastable olivine to a high pressure phase in the subducting slab

ruptures of deep focus earthquakes in the north western pacific and their implications on seismogenesis summary the high resolution of broad band seismic data is highly effective in studying ruptures associated with deep focus earthquakes in this study we report results from an analysis of p and sh waveforms from eight of the largest deep focus earthquakes that occurred between 1987 and 1992 along the north western pacific in this region broad band data recorded at close in distances directly above the source zone offer an unusual source receiver geometry that complements observations at teleseismic distances among the eight earthquakes studied six showed clear indications of directivity the best constraint on the extent of rupture came from the steeply dipping izu bonin wadati benioff zone where subhorizontal ruptures typically originate in the interior of subducted slab and propagate toward the top at high angles in particular rupture of the 1988 september 7 event consists of two en echelon line sources at a depth of approximately 480 km the overall rupture length of this earthquake constrains the minimum thickness of the seismogenic zone to be 27 12 km at this depth our estimated thickness of the seismogenic zone seems remarkably close to the maximum thickness 35 km predicted by the metastable olivine wedge model of seismogenesis this model interprets deep focus earthquakes as a result of transformational faulting within a region bounded approximately by the 700 c isotherm the extent of rupture during the largest events in this region seems to be only slightly larger than the 20 25 km spacing between the two layers of a double seismic zone found at depths between 300 and 400 km in the same general area beneath the island of sakhalin a rupture speed greater than that of the shear wave is not required for the large deep focus earthquake 615 km of 1990 may 12 almost all complexity in observed waveforms for this event can be explained by a northward propagating subhorizontal rupture composed of only two subevents

relationship between shallow and intermediate depth seismicity in the eastern aleutian subduction zone the transition from shallow interplate thrusting to intermediate depth seismicity is often poorly observed but critical for understanding the fate of the downgoing slab in order to better examine the transition 1448 earthquakes are relocated from data recorded by a regional seismic network in the eastern aleutian arc using an improved three dimensional velocity model and accurate ray tracing single event first motion solutions are determined from these rays for 31 slab events the interplate thrust zone is a planar fault zone dipping 10 15 at 25 35 km depth and is no more than 5 10 km wide most intermediate depth earthquakes are localized to a plane no wider than 5 km near the top of the descending plate fault plane solution orientations for these events vary by several tens of degrees in orientation although 73 show t axes aligned within 45 of the slab dip a parallel seismic zone 20 25 km deeper into the slab also shows down dip plunges of t axes for 3 of 5 solutions the fault plane solutions are poorly explained by plate bending or unbending about a neutral fiber hypocenters show that intermediate depth events are confined near the subducted oceanic crust supporting compositional rather than pure thermal control of intermediate depth seismicity one explanation is that the upper plane events are an indirect consequence of phase changes in subducted crust perhaps similar processes are important in producing earthquakes in the lower parallel zone

gutenberg richter law for deep earthquakes revisited a dual mechanism hypothesis deep earthquake b values appear to vary with slab thermal state and earthquake magnitude the physical reason for the variations and the relation with deep rupture mechanisms are still unclear here i confirm the spatial variations of b value and the dependence on slab temperature using about 40 yr of data from the global centroid moment tensor catalog a new bimodal pattern is observed for the 500 700 km depth range b is close to 1 in the cold tonga slab while in warmer slabs e g south america japan kuril izu bonin mariana b is close to 0 5 for intermediate magnitudes m w 5 3 6 5 and increases to 1 for large magnitudes m w 6 5 to explain these observations i propose a dual mechanism hypothesis in which deep earthquakes nucleate only within the metastable olivine wedge mow but can rupture outside mow by a different mechanism the fractal dimension of earthquake size distribution changes from 2 to 1 as the thermally controlled mow thickness decreases and back to 2 as the mechanism outside mow dominates

a deep earthquake aftershock sequence and implications for the rupture mechanism of deep earthquakes a distinguishing characteristic of deep earthquakes has been the absence of observable aftershock sequences1 2 here we report the first extensive deep earthquake aftershock sequence to be observed it was recorded by an array of eight broadband seismographs following the 9 march 1994 deep tonga earthquake the aftershocks show a power law decay with time following the main shock as is typical of shallow events most of the well located aftershocks are concentrated along a steeply dipping plane consistent with one of the nodal planes of the main shock mechanism and the mechanisms of three large aftershocks assuming these aftershocks denote the main shock rupture area they define a 50 65 km fault plane extending across the entire width of the active seismic zone and into the surrounding aseismic region the width of the aftershock zone is wider than the expected width of the metastable olivine wedge demonstrating that either the width of the metastable olivine material exceeds previous estimates or the aftershocks are not confined in such a wedge

nitrogen isotopic composition of sediments from the eastern tethys during oceanic anoxic event 2 abstract low nitrogen isotopic compositions 15n have been reported in deposits from the proto north atlantic and western tethys during oceanic anoxic event 2 oae2 at the cenomanian turonian c t transition it has been proposed that the low 15n values may have resulted from enhancement of n2 fixation and or incomplete assimilation of the upwelled ammonium due to expansion of anoxic euxinic conditions however few 15n measurements are available for the sediments from the eastern tethys and the pacific and therefore the global significance of the n cycle and its environmental implications remains to be tested here we present 15n data from oae2 deposits in the gongzha section in the tingri area of southern tibet which was located on the southern margin of the eastern tethys during the mid cretaceous the 15n data reveal a large perturbation of the n cycle during oae2 the pronounced negative shift in 15n prior to oae2 as well as the persistent low 15n values during oae2 imply the significant contribution of n2 fixation the changes in the marine n cycle suggest that the expansion of an anoxic environment may also have occurred in the southern continental margin areas of eastern tethys the coincidence of the marine faunal turnover and the perturbation of the n cycle suggests that the expansion of anoxic water masses may have played an important role in the extinction of marine organisms across the c t boundary in eastern tethys

downdip tensional earthquakes beneath the tonga arc a double seismic zone characteristics of seismicity at intermediate depth beneath the tonga arc are studied special attention is paid to newly identified downdip tensional events because the dominance of downdip compression has been known in this area we reexamine the relatively shallow 70 km downdip tensional events reported in the harvard centroid moment tensor solutions by modeling bodywave waveforms of world wide standard seismograph network seismograms and conclude that they are in fact intraplate downdip tensional events furthermore these downdip tensional events are consistently located below and seaward of downdip compressional events and appear to constitute a double seismic zone the result of a relative relocation technique applied to these earthquakes also supports this observation which was originally based on routinely determined epicenters the subducting slab beneath the tonga arc has been considered as a prototype of a compressionally loaded slab the existence of a double seismic zone in tonga therefore would suggest that double seismic zones located at intermediate depth in subducting slabs are more common than previously thought

transformation shear instability and the seismogenic zone for deep earthquakes we use a numerical model for olivine spinel transformation to study deep earthquake nucleation and to delineate the seismogenic region within a subducting slab the model includes laboratory derived flow laws latent heat release and phase transformation kinetics we calculate deformation transformation state grain growth and rheology for several paths within a subducting slab strain rate perturbations are imposed to define the necessary conditions for instability strain rate perturbations decay for a critical value c and thus the coldest interior portion of the metastable wedge deforms stably for c strain rate perturbations grow shear strength decreases with strain and the system is potentially unstable the instability condition is mapped to delineate the seismogenic zone within a subducting slab the model seismogenic zone is bounded by c and at larger percent transformations by coarsening of spinel grains and saturation of the transformation weakening effect the model predicts a narrow seismogenic region along the outer edges of the metastable wedge and thus a double seismic zone consistent with some seismic observations several simplifying assumptions are required due to lack of thermo kinetic data and incomplete knowledge of constituent mineralogy and rheology however the model provides a quantitative definition of the instability condition and a framework for testing the hypothesis of transformation induced instability

localized polymorphic phase transformations in high pressure faults and applications to the physical mechanism of deep earthquakes earthquake mechanisms based on frictional instabilities are widely accepted for relatively shallow earthquakes such mechanisms in unmodified form are highly unlikely for deep earthquakes because of the expected high frictional shear stresses at depths of 300 700 km several alternative mechanisms have been proposed here i discuss a new type of shear instability observed in experiments on ordinary polycrystalline ice and polycrystalline tremolite by my colleagues and me these shear instabilities lead to rapid stress drops and sound emissions and have the following characteristics 1 a single sharp shear zone forms in the maximum shear stress orientation 2 confining pressure does not suppress the instability and under some conditions raising confining pressure pc lowers the differential stress at which it occurs 3 stress drops can occur repeatedly after reloading at about the same differential stress 4 the mean stress vs temperature conditions at which the instabilities occur in ice and tremolite place them in the fields of thermodynamic stability of ice ii and diopside talc respectively although i have not as yet confirmed the existence of these high pressure phases in the shear zones transformation in the shear zone is implicated because such shear instabilities do not occur in similar materials in which polymorphic phase changes do not occur reconstructive polymorphic phase changes localized along shear zones may be a general phenomenon in crystalline materials stressed non hydrostatically near transition boundaries moreover by analogy the reconstructive phase transformations that are thought to occur in deeply subducted lithosphere may lead to localized shear instabilities manifested by deep earthquakes and produce the observed double couple first motion distribution finally if this is the earthquake source mechanism in deeply subducted lithosphere then earthquakes should shut off when all of the polymorphic phase changes have occurred this may explain the maximum depth of earthquakes at about 680 km

mapping of the 410 and 660 km discontinuities beneath the japanese islands 1 a family of core reflected shear waves generated by the 28 june 2002 vladivostok deep earthquake including near vertical reflections from the 410 and 660 km discontinuities were recorded by 500 tiltmeters of the hi net and 60 broadband seismometers of the f net in japan the observed upper mantle reflections were cross correlated with synthetics calculated on the basis of the spectral element method for a fully three dimensional earth model using the earth simulator supercomputer to accurately determine the depths of the reflection points the mapped upper mantle discontinuities were compared with a high resolution p wave tomographic image the 660 km discontinuity is depressed at a constant level of 15 km along the bottom of the horizontally extending aseismic slab under southwestern japan the transition from the normal to the depressed level occurs within a lateral distance of less than 200 km observations suggest that the reflections from the 410 km discontinuity interfere with those from slab related structures on top of this discontinuity leading to a spuriously large elevation of the 410 km discontinuity in and near the subducted slab records at stations relatively free from such interference effects however still imply elevation of this discontinuity within the slab

evidence for a large scale remnant of subducted lithosphere beneath fiji we combine spatial variations of p and s wave speeds 1000 fault plane solutions and 6600 well determined hypocenters to investigate the nature of subducted lithosphere and deep earthquakes beneath the tonga back arc we show that perplexing patterns in seismicity and fault plane solutions can be accounted for by the juxtaposition of a steep dipping wadati benioff zone and a subhorizontal remnant of slab that is no longer attached to the actively subducting lithosphere the detached slab may be from a previous episode of subduction along the fossil vitiaz trench about 5 to 8 million years ago the juxtaposition of slabs retains a large amount of subducted material in the transition zone of the mantle such a configuration if common in the past would allow the preservation of a primordial component in the lower mantle

teleseismic location techniques and their application to earthquake clusters in the south central pacific abstract improved methods for single and multiple event hypocenter determinations are developed and applied to the problem of locating earthquake clusters in the south central pacific ocean bayesian statistical methods are used to incorporate a priori information about arrival time variance into the derivation of hypocenter confidence ellipsoids permitting a more realistic calculation of critical parameters in the case where the number of stations is small the diagonal elements of certain projection operators called data importances by minster et al 1974 are used to evaluate network balance the hypocentroid of an event cluster is defined to be the average location of events within the cluster and the deviations of individual hypocenters from the hypocentroid are called cluster vectors the problem of estimating the cluster vectors can be decoupled from the problem of estimating the hypocentroid by a simple but fundamental mathematical result here termed the hypocentroidal decomposition theorem the algorithm based on this analysis appears to have many advantages over other published methods for multiple event location both in its efficient use of available information and its computational speed the application of this method to three clusters of shallow intraplate seismicity in the south central pacific designated regions a b and c demonstrates that the seismicity within each cluster is very localized the rms lengths of the cluster vectors for each group of epicenters are estimated to be only 9 6 and 12 km respectively estimates of the epicentroids are

metastability of enstatite in deep subducting lithosphere olivine and mg fe sio3 pyroxene the most abundant minerals in the earth s upper mantle are believed to transform to high pressure phases at 400 km depth1 5 the possible metastable persistence of olivine to greater depths in some subduction zones with consequences for the origin of deep focus earthquakes and the dynamics of subduction has been discussed extensively6 14 but the role of other mantle minerals has not been considered we report here an experimental study of the kinetic behaviour of the magnesian pyroxene enstatite mgsio3 at upper mantle conditions we find that whereas forsterite mg olivine mg2sio4 transforms rapidly to phase at 1 200 c and 16 gpa the transformation of enstatite to phase plus stishovite on the same timescale requires much higher temperatures at lower temperatures enstatite transforms directly to the ilmenite structure but only at pressures greater than 20 gpa enstatite should therefore persist metastably to greater depths than olivine in subduction zones transforming directly to the ilmenite structure the enstatite ilmenite transformation is accompanied by a large decrease in volume which should increase the stresses in subducting slabs change the buoyancy forces that drive subduction and might also contribute to the origin of deep focus earthquakes

deep seismicity and modes of deformation in tonga subduction zone the geometry of deep 400 km earthquakes in the subducting western edge of the pacific plate when related to the local distribution of seismic activity reveals a complex pattern of planes of shear failure enabling the modes of deformation by which the descending plate shortens and thickens to be identified the deepest part of the slab is displaced laterally suggesting that it fails to penetrate the discontinuity at 670 km

double seismic zones and stresses of intermediate depth earthquakes data from japanese local seismograph networks suggest that the stresses in double seismic zones are in plate compression for the upper zone and in plate tension for the lower zone the stresses do not necessarily appear to be down dip it may therefore be possible to identify other double seismic zones on the basis of data which indicate that events with differing orientations of in plate stresses occur in a given segment of slab a global survey of published focal mechanisms for intermediate depth earthquakes suggests that the stress in the slab is controlled at least in part by the age of the slab and the rate of convergence old and slow slabs are under in plate tensile stresses and the amount of in plate compression in the slab increases with increasing convergence rate or decreasing slab age young and fast slabs are an exception to this trend all such slabs are down dip tensile since these slabs all subduct under continents they may be bent by continental loading double seismic zones are not a feature common to all subduction zones and are only observed in slabs which are not dominated by tensile or compressive stresses unbending of the lithosphere and upper mantle phase changes are unlikely to be the causes of the major features of double zones although they may contribute to producing some of their characteristics sagging or thermal effects possibly aided by asthenospheric relative motion may produce the local deviatoric stresses that cause double zones

evolution of the downgoing lithosphere and the mechanisms of deep focus earthquakes summary the thermal evolution of the lithospheric slab at subduction zones and its geophysical effects are numerically calculated an alternating direction implicit finite difference scheme is used to compute the thermal models taking into account all heating sources and phase boundaries these models with the appropriate spreading rates and dip angles are compared with different island arc systems temperatures inside the slab are strongly controlled by the conductivity and by the time elapsed since the initiation of descent the depth to which temperature anomalies persist is generally about 700 km or less the thermal results are used to construct the seismic velocities and ray paths density anomalies and the resulting stress distribution comparing the theoretical stress distribution and the focal mechanism studies of intermediate and deep focus earthquakes indicates the importance of both the mantle s rheology and the temperature dependence of the slab s elastic properties the intermediate and deep focus earthquakes are located along the coolest region of the slab the theoretical results explain the source mechanisms and the orientation of principal stresses under major island arcs

the march 9 1994 m w 7 6 deep tonga earthquake rupture outside the seismically active slab we investigate the rupture process of the march 9 1994 m w 7 6 deep tonga earthquake and its relationship to the background seismicity of the subducted tonga slab variations in observed p and s wave pulse duration indicate that the rupture propagated to the nne and extended well beyond the background seismicity we inverted 47 p and sh waveforms including regional broadband waveforms from the southwest pacific seismic experiment using a method that solves for the focal mechanism change during the rupture and the distribution of moment release along the fault plane the results indicate that significant moment release occurred in previously aseismic regions outside the active seismic zone and that the rupture terminated 10 20 km beyond the bounds of the previous seismic activity a significant change in focal mechanism occurred when the rupture propagated into the previously aseismic region rupture along the near vertical nne striking nodal plane provides a somewhat better fit to the body waveforms than rupture along the near horizontal nodal plane this result combined with the planar alignment of aftershocks and the general nne directivity of the waveforms provides strong evidence that the rupture occurred on the near vertical plane thermal modeling of the tonga slab indicates that the rupture terminated in material about 200 c warmer than the temperature that normally limits the occurrence of smaller earthquakes additionally aftershocks seem to be suppressed in the outer regions of the rupture which contain about half of the moment release but only 1 of the 15 well located aftershocks we suggest that slabs may be composed of an inner cold core where seismic rupture initiates and small earthquakes occur and a thermal halo of warmer material which can sustain rupture and only a few aftershocks the mechanism by which rupture propagates through the warmer material need not be similar to the process governing rupture nucleation in the cold slab core nucleation may occur through a process limited to the cold core such as transformational faulting and propagation through the warmer material may occur through ductile faulting or plastic instabilities isolated deep earthquakes in other subduction zones such as the 1994 bolivia event may occur almost completely within the warmer zone accounting for the lack of background seismicity and the dearth of aftershocks

details of stress directions in the alaska subduction zone from fault plane solutions we used the cumulative misfit method to divide a data set with heterogeneous stress orientations into subsets with homogeneous stress directions we propose that slope changes of the cumulative misfit as a function of earthquake number pinpoint the boundaries of homogeneous stresses using a synthetic data set of 50 fault plane solutions composed of two halves corresponding to two incompatible stress tensors we tested the validity and efficacy of the cumulative misfit method our acceptance criteria for results of stress tensor inversions are 1 the directions must be well constrained that is the 95 confidence regions of the greatest and least principal stresses do not overlap and 2 there must be evidence for homogeneity in the sample that is the average misfit of the inversion is less than 6 we estimated stress directions in the alaska wadati benioff zone wbz using 470 fault plane solutions which we determined for earthquakes with ml 3 and 67 published fault plane solutions for earthquakes with ms 5 we succeeded in subdividing the data set of the small and large earthquakes into 25 and 3 subsets with the average misfit ranging from 3 2 to 5 5 the average misfits of the subsets we accept as satisfying the assumption of homogeneity are smaller than the average misfit of the overall data set f 10 by factors of 2 to 3 we estimated the stress fields at two scales along the alaska wbz the stress directions measured by the large earthquakes ms 5 were homogeneous over large volumes with extension downdip and the direction of greatest compression along strike this unusual orientation of the greatest principal stress is attributed to the bend of the slab under central alaska which generates compressive stresses along strike the stress orientations revealed by small earthquakes in contrast exhibited a great deal of heterogeneity as a function of space although their general trend confirms the overall stress directions obtained from the large events we propose that the ratio of the dimensions of the stress field sensed by earthquakes to the rupture dimensions is about 20 to 50 the estimated stress directions of the crustal earthquakes corresponded to the following mechanisms 1 strike slip faulting with the greatest principal stress oriented n s near fairbanks and 2 thrusting with the greatest principal stress oriented nw se near mount mckinley

the influence of olivine metastability on the dynamics of subduction abstract both seismological observations and mineral physics experiments suggest that old and cold subducting slabs might contain a wedge of metastable olivine mo dynamically consistent models of subducting slabs in which the kinetics of the olivine to spinel wadsleyite or ringwoodite transition is treated in a simplified way are carried out to evaluate the buoyancy effect of mo on the subduction velocity assuming slab thicknesses according to the cooling half space model we find that mo forms in slabs with ages greater than approximately 70 myr and that a significant slowing down of subduction velocity occurs for ages greater than 100 myr because a decrease in the subduction velocity for old lithospheric ages is not observed our models suggest that the amount of mo in slabs is less than approximately 5000 km2 in cross section and that for old lithosphere the cooling half space model is not applicable

the 2018 fiji mw 8 2 and 7 9 deep earthquakes one doublet in two slabs abstract the cold fiji tonga subduction zone accounts for 75 of cataloged deep earthquakes but none of the largest ten in the last century on 19 august 2018 and 06 september 2018 a deep earthquake doublet with moment magnitude m w 8 2 and 7 9 struck the fiji area providing a rare opportunity to interrogate the behaviors of great deep earthquakes in cold slabs while the aftershock productivity of the 2018 m w 8 2 event is similar to the 2013 okhotsk m w 8 3 event in a cold slab the inferred compact rupture dimensions of both the m w 8 2 and 7 9 events appear to be similar to the 1994 bolivia earthquake in a warm slab this seems to contradict the traditional view that slab temperature controls deep earthquakes however we find that neither event was confined within the cold tonga slab core the m w 8 2 ruptured mostly in the warmer rim of the tonga slab and the m w 7 9 occurred in a warm relic slab leaning on top of the tonga slab the fiji doublet demonstrates local slab temperature as the critical factor for deep earthquakes and reveals complex interaction of subducted slabs in tonga

double seismic zone and dehydration embrittlement of the subducting slab 1 dehydration embrittlement of metamorphosed oceanic crust and mantle in the subducting slab may be responsible for the occurrence of intermediate depth earthquakes we explore the possibility that this hypothesis can explain the morphology of the double seismic zones observed in northeast japan southwest japan northeast taiwan northern chile cape mendocino and eastern aleutians we calculate transient temperature structures of slabs based on geologically estimated subduction histories of these regions we then determine dehydration loci of metamorphosed oceanic crust and serpentinized mantle using experimentally derived phase diagrams the depth range of the dehydration loci of metamorphosed oceanic crust and serpentine is dependent on slab age the dehydration loci of serpentine produce a double layered structure because the upper dehydration loci of serpentine are mostly located in the wedge mantle above the slab we regard the upper plane seismicity representing dehydration embrittlement in the oceanic crust and we fix the slab geometry so that the upper plane seismicity is just below the upper surface of the slab we find that the lower plane seismicity is located at the lower dehydration loci of serpentine which indicates that the morphology of the double seismic zones is consistent with the dehydration embrittlement

upper mantle seismic discontinuities and the thermal structure of subduction zones the precise depths at which seismic velocities change abruptly in the upper mantle are revealed by the analysis of data from hundreds of seismometers across the western united states the boundary near 410km depth is locally elevated that near 660 km depressed the depths of these boundaries which mark phase transitions provide an in situ thermometer in subduction zones the observed temperature contrasts require at least moderate thickening of the subducting slab near 660 km depth in addition a reflector near 210 km depth may mark the bottom of the aesthenosphere

double seismic zones kinematics the kinematics of a double seismic zone of subducting lithosphere at an intermediate depth is studied a simple geometrical argument shows that the strain rate due to unbending of subducting lithosphere can be as large as 10 15 s 1 and large enough to account for the seismic strain rate in this depth range the geometrical unbending of subducting lithosphere in the mantle and the presence of a double seismic zone there can be therefore considered essentially equivalent phenomenon we model a double seismic zone required from this unbending in order to investigate what constraint the presence of a double seismic zone can place on the properties of the descending lithosphere the thickness of a double seismic zone is controlled by the thermal structure and the rheological properties of the subducting slab a preliminary result for the double seismic zone beneath tohoku japan indicates a possibility that the currently available flow law of dry olivine does not properly describe the double seismic zone or at least presents some inconsistency in our knowledge of the mechanical properties e g thermal structure rheology of oceanic lithospheres

relationship of deep seismicity to the thermal structure of subducted lithosphere recent experimental work on silicate olivine polymorphs1 has confirmed earlier observations of mechanical failure in analogous compounds2 3 as a result of phase transformations when non hydrostatic stresses were applied to a metastable phase this transformational faulting 4 mechanism is considered to be a leading candidate5 among others involving olivine transformation6 7 for the cause of deep earthquakes evidence consistent with this mechanism comes from the observation2 4 that worldwide earthquakes become more numerous with increasing depth after a seismicity minimum at about 350 km depth8 this depth lies within the range anticipated for mineralogical transformations in the subducted lithosphere9 but individual subduction zones differ in their thermal structures so if transformational faulting indeed contributes to deep seismicity this should be reflected in the location of the seismicity minimum for each zone the depth at which the minimum occurs should depend on temperature in the same way as do the polymorphic transformations of olivine and should always lie deeper than the depth at which the equilibrium transformation takes place here we test these predictions for eight subduction zones worldwide we find that with one exception the north japan zone the depth of the seismicity minimum decreases linearly with increasing thermal age of the slab a measure of its temperature profile these results support the proposal that deep earthquakes are a consequence of phase transformations in olivine

evidence for transformational faulting from a deep double seismic zone in tonga double seismic zones planes or earthquakes parallel to the dip of a subducting slab and separated by 20 40 km provide important clues about the earthquake generating mechanisms and strain distribution inside subducting slabs double seismic zones have been found at intermediate depths 70 200 km in many subduction zones1 6 but have not been previously reported in deep slabs here by relocating earthquakes with a hypocentroidal decomposition technique7 and visualizing the earthquake positions and uncertainties in three dimensions we identify a double seismic zone at depths of 350 460 km in the tonga subduction zone source parameters of the earthquakes determined by waveform analysis suggest different stress orientations for the two zones with in plane compression in the lower zone and in plane tension in the upper zone the double zone may be due to transformational faulting as olivine along the edges of a metastable olivine wedge becomes warmer and transforms to spinel8 11

stress fields associated with metastable phase transitions in descending slabs and deep focus earthquakes inside subducting slabs the interaction of different metastable phase transformations olivine spinel spinel spinel spinel perovskite magnesiowustite associated with the latent heat release and absorption for these transitions can result in a complex thermal slab structure thermokinetic coupling processes cause thermal anomalies and buoyancy contrasts between slab and mantle that strongly influence the stress field within subducting plates we have described the thermal field within the downgoing slab in a lagrangian framework by a self consistent high resolution two dimensional thermokinetic coupling model which also incorporates the effects of nucleation site saturation using an adaptive method the threshold for the nucleation site saturation is found to be less than 0 1 transformation degree in the cold slab interior this can significantly shift the metastable wedge of olivine deeper as suggested from previous thermokinetic slab models however this effect is compensated by the latent heat release during the olivine spinel and spinel spinel phase transitions latent heat release produce very sharp phase boundaries in the slab for fast slabs the phase boundaries reveal significant metastable perturbations from the equilibrium state in the cold interior however the strong thermal interaction between both ongoing phase transitions results in sharp overlapping phase boundaries with dramatic consequences for the slab stress field we have investigated the detailed physical properties and the state of stress in the deeper portion of a subducting plate by using up to date temperature field and physical properties determined from recent high p t experiments we find that the slab is denser up to 100 kg m 3 than the surrounding mantle remarkable denser portions up to 260 kg m 3 are located just below the uplifted and equilibrium phase transitions while lighter portions up to 210 kg m 3 can be found just above the depressed spinel to perovskite magnesiowustite transition and for the metastable wedge of olivine these density differences effectively act as a body force and produce a significant stress field which we calculated with a two dimensional finite element code the results show that the buoyancy induced forces produce maximum shear stress up to 23 mpa along the metastable wedge and deeper portion just above the depressed last phase transition involving spinel for the latter the dominant state of stress is down dip compression the calculated p t dependent state of stress is very similar to the depth distribution of deep focus

instability of deformation this chapter addresses the mechanisms by which instabilities can develop in rocks in the nominally ductile regime under conditions that the authors believe are relevant to earth we will not address brittle failure or frictional sliding processes in detail but much of our discussion will be couched in terms of knowledge gained in studies of these processes as a consequence we will begin by discussing why brittle failure and frictional sliding cannot be the mechanism by which rocks become unstable at depth despite the strong evidence that they are the underlying mechanism of earthquakes at shallow depths e g scholz 1990 2002 brittle shear failure in the normal sense is fundamentally a tensile process tensile microcracks must first nucleate and self organize via interaction of the stress concentrations at their tips as a consequence brittle failure and frictional sliding are strongly inhibited by pressure because work must be done against the pressure to open the mode i cracks the inhibition is so strong that the stress required to create a fault or initiate sliding on an existing fault becomes greater than the room temperature flow stress of many rocks at pressures equivalent to only a few tens of kilometers in earth increasing temperature has little effect on brittle processes but in contrast the ductile flow stress of rocks falls exponentially with temperature because both pressure and temperature increase with depth in earth earthquakes by unassisted brittle fracture mechanisms or frictional sliding can only occur at depths less than 30 50 km nevertheless earthquakes occur to depths of almost 700 km in earth and seismological evidence demonstrates unequivocally that they occur by displacement across a surface or narrow zone i e they occur by faulting this conundrum can be explained qualitatively by certain specific types of mineral reactions occurring under certain restricted conditions the

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

rupture processes of large deep focus earthquakes from inversion of moment rate functions we determine the source time histories of five recent 1994 1996 large deep focus earthquakes using a method that inverts for fault slip from far field moment rate functions the moment rate functions are obtained through the deconvolution of multiple body wave phases using broadband records from global seismic network gsn and geoscope stations tests of this method on synthetic data indicate that it is successful in determining the low frequency rupture characteristics of deep earthquakes under a variety of complicating conditions we find that source parameters such as average rupture velocity and stress drop are highly variable among the events studied and that some unusual characteristics exhibited by the june 9 1994 great bolivian earthquake are also found for other events comparison of the slip distributions with background seismicity and aftershock locations indicates that most of the moment release for large deep focus earthquakes is probably occurring within the active slab interior this provides further evidence that temperature controlled mechanisms such as transformational faulting play a large role in deep earthquake faulting most of the events studied also show a tendency for horizontal rupture propagation suggesting that isobaric processes may be an important factor in controlling progression of the rupture large gaps in the slip distributions point to possible occurrence on multiple fault planes

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

two types of deep seismicity in subducting slabs the relation between variation of seismic activity with depth and temperature structure in the subducting slabs in five subduction zones kuril ne japan izu bonin mariana and tonga is investigated when the temperature of the core of the slab at depths of 300 350 km is higher than the kinetic cut off temperature of the olivine spinel phase transformation there is no seismicity maximum in the slab below that depth range when the temperature of the core of the slab at 300 350 km depths is lower than the kinetic cut off temperature there is a depth range of relatively high seismic activity in the slabs moreover estimated temperature in the depth range of relatively high seismicity is near the kinetic cut off temperature it is inferred that the phase transformation of meta stable olivine triggers deep earthquakes and causes high seismic activity below the depths of 300 350 km in relatively low temperature slabs when the phase transformation is the cause of deep earthquakes in relatively high temperature slabs however it is considered that the phase transformation occurs in nearly equilibrium conditions and meta stable olivine does not exist other mechanisms in addition to the phase transformation are required to explain observed deep seismicity especially in relatively high temperature slabs

complex and diverse rupture processes of the 2018 mw 8 2 and mw 7 9 tonga fiji deep earthquakes author posting american geophysical union 2019 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in geophysical research letters 46 5 2019 2434 2448 doi 10 1029 2018gl080997

seismic constraints on the water flux delivered to the deep earth by subduction for the first time large quantities of water are identified throughout the length of a subducting plate and an efficient pathway is defined for injecting water into the deep 600 km earth by the incorporation of water into the mantle of an oceanic plate as serpentine water is thought to facilitate plate tectonics but the quantities of water involved and thus its effect are poorly known taking advantage of an anomalous seismic arrival recorded in the tonga fiji subduction zone the presence volume water content and flux of serpentine and water associated with the subducting oceanic mantle at tonga is quantified at 2 10 8 tg ma h 2 o many times larger than previous inferences based on surface observations water fluxes of this magnitude into the deep earth will substantially increase the role of water in hypotheses of plate tectonic movements mantle plumes and flood basalt activity

distributed deformation in the subducting lithosphere at tonga summary in this paper we attempt to apply techniques that have recently been developed to describe distributed deformation on the continents to distributed deformation in subducting lithosphere slabs we chose a part of the tonga slab for this study because it has a simple approximately planar shape and high seismicity we then used the spatial distribution of seismic strain rates based on earthquake centroid moment tensor solutions in the interval 1977 1994 to recover a velocity field that describes the seismic deformation in the plane of the slab below a depth of 100 km between 100 and 450 km depth the seismic deformation is dominated by down dip shortening and slab thickening below 450 km the down dip shortening seen in the earthquakes is still important but it is absorbed roughly equally by along strike extension and by thickening there is little evidence of along strike shear at depth we have more confidence in the pattern of strain rates and velocities that we obtain than in their absolute values nevertheless the rates of down dip shortening accounted for by seismicity are probably less than half those needed if the whole down dip component of pacific australia plate convergence is absorbed by shortening in the upper mantle the style of deformation at the base of the slab is complex and unlike many regions of distributed continental tectonics is not easily represented by simple patterns of faulting

phase transition buoyancy contributions to stresses in subducting lithosphere the sequence of phase transitions undergone by minerals with increasing depth in earth s mantle is perturbed within subducting lithospheric slabs by their thermal structure such perturbation of equilibrium phase relations gives rise to relative buoyancy contrasts between slab and mantle that contribute to the state of stress within the slab while other factors contribute to overall slab stresses thermal and phase transition effects largely control the structure of the stress field within the slab the resulting maximum in down dip compressive stress within the slab corresponds to the observed peak in depth distribution of deep seismicity furthermore metastable persistence of lower pressure phases within the cold slab should give rise to localized shear stresses whose distribution corresponds to observed features of subduction zone seismicity these observations are independent of the variety of failure mechanisms proposed for deep seismogenesis

the effects of metastable olivine α wedge in subducted slabs on theoretical seismic waveforms of deep earthquakes 1 assuming three kinds of materials 100 olivine mg0 89fe0 11 2sio4 pyrolite and piclogite as composition of the upper mantle we constructed pressure temperature dependent seismic velocity structure models in the upper mantle beneath a subduction zone for this purpose we employed values of physical properties of composite minerals newly determined by using a genetic algorithm ga and obtained the thermal structure beneath the subduction zone as a numerical solution of a two dimensional 2 d thermal fluid coupled problem furthermore we investigated the effects of a metastable olivine wedge in subducted slabs on theoretical seismic waveforms of deep earthquakes which propagate through the velocity structure by solving the p sv problem numerically the effects of center frequency of the excited waves at hypocenters focal mechanism location of hypocenter upper mantle compositions values of physical properties a dip angle of subducting slabs anelastic attenuation subducting velocity and depth of deep earthquakes on seismic waveforms were investigated in detail theoretical seismograms at the earth s surface indicate that coda waves with small amplitudes of duration time of 20 30 s appear following direct p and s waves for models with the metastable olivine wedge inside the slabs the coda waves can be explained by the excitation by reflections and conversions at a large seismic velocity discontinuity between the metastable olivine wedge and the surrounding slab for models with center frequency of 0 25 hz purer olivine composition a faster subducting velocity and focal depth of 500 km waves with larger amplitudes were excited at the discontinuity the results of this study may give important clues to detect the existence of the metastable olivine wedge from seismological observations

do intermediate and deep focus earthquakes occur on preexisting weak zones an examination of the tonga subduction zone it has been proposed that intermediate and deep focus earthquakes occur on preexisting planes of weakness that were created at shallow depth the purpose of this study is to test this hypothesis we examined fault plane solutions for 360 events that occurred in the tonga subduction zone from january 1976 to february 1999 for events in the slab each fault plane solution is rotated by the slab dip angle so that the slab is in its original horizontal position we then compare fault plane solutions for events in the slab with those for events at the outer rise we find that an asymmetric fault system corresponding to that found for outer rise events persists down to about 450 km depth this suggests that earthquakes down to this depth are caused by the reactivation of preexisting faults created in the oceanic plate before subduction a similar pattern though less well constrained is found in the kurile subduction zone down to 200 km depth for earthquakes deeper than 450 km the fault plane solutions are much more scattered in some places the fault plane pattern suggests the creation of new fault planes along orientations of maximum shear with zero internal friction the pattern of fault plane solutions in particular localized volumes cannot be easily explained by the creation of new faults in the direction of maximum shear under a coherent regional tectonic stress field thus either a highly heterogeneous stress field exists or preexisting planes of weakness also account for some deep earthquakes

en echelon ruptures during the great bolivian earthquake of 1994 rupture during this event has two characteristics first bursts of major moment release subevents have a component of eastward movement from the epicenter second later subevents show northward directivity these observations lead to a model with four subevents the latter three being line sources propagating within approximately 30 of north this model has en echelon regions of moment release over sub horizontal plane s and rupture speeds up to 3 5 0 5 km s within subevents moment release over en echelon regions can accommodate high moment release over a limited seismogenic volume within subducted lithosphere depending on the exact geometry of the wadati benioff zone and the precise direction of rupture propagation lower bounds on the thickness of the seismogenic zone during this event range approximately 10 40 km while several aspects of the en echelon rupture model dovetail with predictions based on the anticrack model for deep focus seismicity one must also allow for the possibility that the rupture zone extended beyond the expected thickness of metastable olivine wedge

the nature of deep focus earthquakes wadati 1928 first proved convincingly that some earthquakes occur at depths well beneath the earth s crust he essentially proposed the nomenclature used today by the international seismological centre isc i e earthquakes with focal depths exceeding 300 km are deep earthquakes and those with depths between 70 and 300 km are intermediate earth quakes however when there is little reason to distinguish between the two groups both types are called simply deep earthquakes to avoid confusion in this paper we refer to those with focal depths exceeding 300 km as very deep earthquakes and those with focal depths between 70 and 300 km as intermediate earthquakes deep earthquakes are of interest for at least four reasons first they are exceedingly common between 1964 and 1986 they constituted no less than 22 of all earthquakes having mb greater than 5 0 in the isc catalog second they most often occur in association with deep ocean trenches and volcanic island arcs in subduction zones one of the great achievements of twentieth century geophysics was the recognition that their occurrence in inclined planar groups or wadati benioff zones apparently delineates the cold down going cores of convection cells in the uppermost mantle third seismologists use body waves of deep earthquakes dispro portionately more often than those of shallow earthquakes to inves tigate core mantle and crustal structure this is because a deep earth quakes often possess relatively more impulsive sources b their body phases traverse the heterogeneous uppermost mantle only once on the ray path from hypocenter to station and c surface waves do not contaminate these body phase arrivals on seismograms finally deep earthquakes are

anticrack associated faulting at very high pressure in natural olivine shallow earthquakes are produced by brittle shear fracture of rock and or fictional sliding on pre existing fault surfaces1 at very high pressures however brittle fracture and frictional sliding are impossible because frictional resistance to movement on any potential fault surface is so great that ductile stress relief processes accomodate strain at lower shear stresses than those necessary to activate faulting nevertheless more than 20 of earthquakes with magnitudes greater than five occur at depths greater than 300km where the pressure exceeds l0 gpa ref 2 possible explanations for this paradox have been offered by several recent experimental studies of shearing instabilities associated with phase transformations at high 3 8 one of these mechanisms is associated with anticrack development during the olivine spinel phase transformation in mg2geo4 at pressures of 1 2 gpa refs 4 6 it is particularly attractive because it operates between mineral structures known to occur in the mantle we show here that this mechanism also can operate in natural silicate olivine mg fe 2sio4 during onset of the transformation at the much higher pressures at which deep earthquakes occur these results lend strong support to the hypothesis that the anticrack mechanism is responsible for such earthquakes4

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

metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere earth s deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km this depth interval closely brackets the mantle transition zone characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher pressure phases deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal thermodynamic and mechanical ferment inside slabs numerical simulations of reaction rates show that the olivine spinel transformation should be kinetically hindered in old cold slabs descending into the transition zone thus wedge shaped zones of metastable peridotite probably persist to depths of more than 600 km laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting this instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser finer grained phase hence in cold slabs such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite it is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs this consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features it explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone where minerals in downgoing slabs should transform to spinel and ilmenite structures the onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab even if a slab penetrates into the lower mantle earthquakes should cease at depths near 700 km because the seismogenic phase transformations in the slab are completed or can no longer occur substantial metastability is expected only in old cold slabs consistent with the observed restriction of deep earthquakes to those settings earthquakes should be restricted to the cold cores of slabs as in any model in which the seismicity is temperature controlled via the distribution of metastability however the geometries of recent large deep earthquakes pose a challenge for any such models transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics including focal mechanisms indicating localized shear failure rather than implosive deformation are so similar to those of shallow earthquakes finally metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and or are detached from subducting slabs metastability in downgoing slabs could have considerable geodynamic significance metastable wedges would reduce the negative buoyancy of slabs decrease the driving force for subduction and influence the state of stress in slabs heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity because wedge formation should occur only for fast subducting slabs it may act as a parachute and contribute to regulating plate speeds wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel favoring the subsequent sinking of the slab into the lower mantle

wavelet reconstruction of geologic facies from nonlinear dynamic flow measurements the discrete wavelet transform dwt that is widely used in compressing natural images is considered for an effective representation of the geological facies in subsurface flow and transport inverse modeling problems the inference of the heterogeneous hydraulic rock properties from the scattered dynamic measurements of the flow rates and pressures is a frequently encountered ill posed inverse problem in subsurface characterization to better pose this inverse problem the original grid based description of the spatial facies maps is replaced with a small number of dwt coefficients that are estimated from indirect nonlinear dynamic measurements the compressed description of the facies in the wavelet domain after removing the unresolvable high frequency components leads to an inverse problem with fewer parameters to resolve and improved geologic facies continuity the main difficulty in the application of the dwt to inverse problems is the lack of sufficient data to resolve higher frequency detail coefficients prior information and sensitivity of the flow response to variation in the dwt coefficients are used to infer the location and value of the significant dwt coefficients the results suggest that the large scale geologic facies description that control the global flow pattern can be successfully inferred from the dynamic measurements in a reduced wavelet domain while the flow data may contain information about significant dwt coefficients a limited observability in ill posed inverse problems may not allow the identification of these coefficients and the corresponding local spatial features therefore an effective exploitation of the space frequency localization advantage of the wavelets over the fourier bases may not be available in solving ill posed inverse problems