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

earth science tiny triggers deep down the documentation and characterization of remotely triggered earthquakes deep within the earth is an achievement that provides insight into the mechanisms that initiate such events

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

shear instability in a viscoelastic material as the cause of deep focus earthquakes the mechanism of deep focus earthquakes has been examined by numerical and linear analysis of shear instability in subducting slabs we assume subducting slabs deform such that the spatially averaged strain rate becomes time independent furthermore we assume that rheology depends on temperature in the manner of an activation process using this model we investigate quantitatively the conditions under which shear instability takes place i e the deformation in the material concentrates in a thin layer whose temperature increases explosively until melting occurs first we study instability in a material whose rheological properties are spatially homogeneous shear instability takes place in a homogeneous material when exceeds a certain critical level e c which depends strongly on the stress in the slabs but does not depend on the detail of the creep law the value of the activation energy next we study instability in a material that contains shear zones similar to those often observed by field geologists i e a thin layer made of material with lower vicosity than the surrounding region we find that shear zones trigger shear instability even for the case of provided that g the ratio of characteristic time of thermal diffusion to that of shear heating in shear zones exceeds 1 and that f the ratio of stiffness of elastic deformation in the surrounding region to that of ductile deformation in shear zones becomes lower than 1 applying this condition to subducting slabs with shear zones we find that shear zones trigger shear instability even when the average strain rate is lower than by an order of magnitude from these results and from an estimate of in subducting slabs we conclude that shear instability is a mechanism likely to induce deep focus earthquakes

rupture characteristics of the deep bolivian earthquake of 9 june 1994 and the mechanism of deep focus earthquakes the m w 8 3 deep 636 kilometers bolivian earthquake of 9 june 1994 was the largest deep focus earthquake ever recorded seismic data from permanent stations plus portable instruments in south america show that rupture occurred on a horizontal plane and extended at least 30 by 50 kilometers rupture proceeded at 1 to 3 kilometers per second along the down dip azimuth of the slab and penetrated through more than a third of the slab thickness this extent is more than three times that expected for a metastable wedge of olivine at the core of the slab and thus appears to be incompatible with an origin by transformational faulting these large events may instead represent slip on preserved zones of weakness established in oceanic lithosphere at the earth s surface

a general test of the hypothesis that transformation induced faulting cannot occur in the lower mantle abstract transformation induced faulting which has been proposed as a mechanism for deep focus 300 km earthquakes requires a thermal runaway associated with an exothermic chemical reaction this shearing instability has been demonstrated for several polymorphic phase transformations including the olivine wadsleyite and olivine ringwoodite reactions however it has not been experimentally tested whether a strongly exothermic decomposition reaction can also lead to this instability as would be the case if olivine were to be carried metastably into the lower mantle and break down to magnesium silicate perovskite magnesiowustite here we report experiments designed to test this possibility using the proxy reaction albite jadeite coesite a highly exothermic reaction with a large v we find no mechanical or microstructural evidence for transformation induced faulting during this reaction applying these results to the reaction olivine ol perovskite pv mw potentially occurring in subducting oceanic lithosphere at the top of the lower mantle suggests that this breakdown reaction cannot trigger earthquakes thus transformation induced faulting as a mechanism of deep earthquakes is consistent with the abrupt cessation of earthquakes at the base of the mantle transition zone but only if subduction zones are colder than currently proposed we discuss other reasons for termination of earthquakes at 700 km and find them also difficult to reconcile with experimental evidence or natural observations

intermittent euxinia in the high latitude james ross basin during the latest cretaceous and earliest paleocene abstract seymour island in the james ross basin antarctica contains a continuous succession of latest cretaceous sediments deposited in a shallow marine environment at high latitude making it an ideal place to study environmental changes prior to the k pg mass extinction we measured major and trace elements and conducted petrographic analysis of two sections from the maastrichtian danian lopez de bertodano formation of seymour island several lines of evidence point to intermittently anoxic to euxinic conditions during deposition including the presence of pyrite framboids with a size distribution suggesting syngenetic formation in the water column and enrichments in several trace elements including molybdenum arsenic copper zinc and chromium molybdenum enrichments are clearly associated with enrichments in manganese and authigenic iron suggesting shuttling of redox sensitive trace elements across a chemocline that fluctuated across the sediment water interface comparisons with modern systems suggest relatively high frequency redox variability possibly over approximately annual timescales which may be related to the annual cycle of polar sunlight and associated seasonal changes in primary productivity glauconitic horizons are associated with more reducing conditions including at the k pg boundary though this does not appear to be a uniquely euxinic interval similar degrees of trace element enrichment are seen in other highly glauconitic intervals while euxinia may have contributed to low diversity in the lowermost rotularia units redox conditions do not seem to have been the primary control on the transition to a mollusc dominated fauna in the latest maastrichtian redox conditions show little to no response to the eruption of the deccan traps or maastrichtian climatic changes instead intermittent euxinia appears to have been a characteristic feature of this high latitude environment during the cretaceous paleogene transition

the depths of the deepest deep earthquakes the maximum depth of seismic activity is a fundamental observation providing a constraint on models of mantle dynamics although most recent investigations of mantle seismicity and dynamics state that seismicity extends to about 700 km focal depths of 720 km have been reported for large earthquakes as well as depths greater than 800 km for small events we have examined focal depths of events in several catalogs and conclude that 1 the deepest reliable focal depths are at about 670 680 km and events with focal depths beneath 600 km occur in several widely separated geographic regions 2 observations supporting focal depths exceeding 680 km are usually few poor or inconsistent 3 the reduction of seismic activity beneath about 650 km is quite abrupt however we cannot determine whether activity stops completely at 670 680 km or the maximum size of events decreases gradually beneath 650 km with magnitudes of 6 5 7 0 occasionally occurring at 650 km magnitudes of 5 5 6 0 at 680 km 4 0 4 5 at 695 km etc several models can explain the absence of seismic activity in the lower mantle two models are consistent with the abrupt termination of activity at about 650 680 km in widely separated areas a barrier which resists penetration by the subducting lithosphere or a phase transition which modifies the properties of the subducted material as it goes into the lower mantle

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

remagnetization of carbonate rocks in southern tibet perspectives from rock magnetic and petrographic investigations netherlands organization for scientific research nwo with a rubicon grant 825 15 016 institute for rock magnetism irm at the university of minnesota instruments and facilities program of nsf erc starting grant 306810 nwo vidi 864 11 004

thermochemistry and phase equilibria of hydrous phases in the system mgo sio2 h2o implications for volatile transport to the mantle calorimetric phase equilibrium and in situ equation of state data for antigorite talc and dense hydrous magnesium silicate dhms phases have been obtained to construct a self consistent thermodynamic data set that ensures internal consistency well behaved extrapolation and physically reasonable values of h s v k k and for talc antigorite and hydrous phase a used in conjunction with temperature profiles of subducting oceanic lithosphere the calculated phase equilibria show that there is no barrier to subducting substantial amounts of water to depth of 400 600 km in colder slabs since the slab can remain in the stability field of hydrous phases throughout its descent once water is brought to these depths it can probably diffuse into hotter mantle regions via hydrous mg2sio4 or by other mechanisms indicating that the transition zone is unlikely to be dry

experimental investigation of melt topology in partially molten quartzo feldspathic aggregates under hydrostatic and non hydrostatic stress continental collision results in deep burial of crustal rocks and their subsequent partial melting field relations of melt along zones of intense deformation suggest that partially molten rocks may play an important role in regional tectonics however subsequent deformation may erase the microstructures produced by the earlier deformation mechanisms inhibiting our understanding of the rheology of partially molten crustal rocks thus in this paper we report the results of an experimental study of the distribution of 2 5 vol melt in quartzo feldspathic aggregates of various grain sizes 2 5 5 10 10 16 and 26 31 m three types of samples were examined all with the composition of 60 wt albite 25 wt potassium feldspar 10 wt quartz and 5 wt biotite the first group included mineral powders annealed at 1000 c 1 0 gpa for c 100 h the second group included commercially hot pressed mineral powders which yielded c 25 vol glass cores of this material were also annealed at 1000 c 1 0 gpa for c 100 h the third group included cores of hot pressed material that were annealed at 1000 c 1 0 gpa for c 45 h then deformed all samples were quenched rapidly in order to examine the melt distribution wetting angles are very similar in both the hydrostatically annealed and the deformed samples analysis of melt pool orientations reveal that melt migrates away from grain boundaries normal to the maximum compressive stress direction in response to the applied non hydrostatic stress this response is easily seen in the coarser grained samples in which melt pools elongated parallel to the maximum compressive stress direction formed during deformation if these results extrapolate to naturally deformed rocks it will be important to consider the orientation of the state of stress in a region during syn magmatic deformation because of its effect on the melt distribution

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

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

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

deep earthquakes beneath the fiji basin sw pacific earth s most intense deep seismicity in stagnant slabs abstract previous work has suggested that many of the deep earthquakes beneath the fiji basin occur in slab material that has been detached and foundered to the bottom of the transition zone or has been laid down by trench migration in a similar recumbent position since nowhere else in the earth do so many earthquakes occur in slabs stagnated in the transition zone these earthquakes merit closer study accordingly we have assembled from historical and modern data a comprehensive catalogue of the relocated hypocenters and focal mechanisms of well located deep events in the geographic area between the bottoms of the main vanuatu and tonga wadati benioff zones two regions of deep seismogenesis are recognized there i 163 deep shocks have occurred north of 15 s in the vityaz group from 1949 to 1996 these seismological observations and the absence of other features characteristic of active subduction suggest that the vityaz group represents deep failure in a detached slab that has foundered to a horizontal orientation near the bottom of the transition zone ii another group of nearly 50 outboard deep shocks occur between about 450 and 660 km depth west of the complexly buckled and offset western edge of the tonga wadati benioff zone their geometry is in the form of two or possibly three small circle arcs that roughly parallel the inferred motion of tonga trench migration earthquakes in the southernmost of these arcs occur in a recumbent high seismic wavespeed slab anomaly that connects both to the main inclined tonga anomaly to the east and a lower mantle anomaly to the west van der hilst r 1995 complex morphology of subducted lithosphere in the mantle beneath the tonga trench nature vol 374 pp 154 157 both groups show complexity in their focal mechanisms the major question raised by these observations is the cause of this apparent temporary arrest in the descent of the tonga slab into the lower mantle we approach these questions by considering the effects of buoyant metastable peridotite in cold slab material that was detached and rapidly foundered or was buckled segmented and laid out in the transition zone

faulting induced by precipitation of water at grain boundaries in hot subducting oceanic crust dehydration embrittlement has been proposed to explain both intermediate and deep focus earthquakes in subduction zones1 2 3 4 5 because such earthquakes primarily occur at shallow depths or within the core of the subducting plate dehydration at relatively low temperatures has been emphasized6 7 8 however recent careful relocation of subduction zone earthquakes9 10 shows that at depths of 100 250 km earthquakes continue in the uppermost part of the slab probably the former oceanic crust that has been converted to eclogite where temperatures are higher here we show that at such pressures and temperatures eclogite lacking hydrous phases but with significant hydroxyl incorporated as defects in pyroxene and garnet develops a faulting instability associated with precipitation of water at grain boundaries and the production of very small amounts of melt this new faulting mechanism satisfactorily explains high temperature earthquakes in subducting oceanic crust and could potentially be involved in much deeper earthquakes in connection with similar precipitation of water in the mantle transition zone 400 700 km depth of potential importance for all proposed high pressure earthquake mechanisms is the very small amount of fluid required to trigger this instability

the rheology of faults triggered by the olivine spinel transformation in mg2geo4 and its implications for the mechanism of deep focus earthquakes the relevance of the anticrack faulting mechanism to the origin of deep focus earthquakes depends critically on the physical processes operating during fault generation and movement we show here that the resistance to sliding on fault zones produced by this mechanism in mg2geo4 depends only weakly on confining pressure but strongly on sliding rate both of these characteristics are inconsistent with friction the microstructures of the fault zones and the crystal plastic rheology of the constituent phases indicate that sliding must occur primarily by grain boundary sliding previously we reported that anticrack faulting is accompanied by elastic radiation of energy acoustic emissions and that it can operate during the transformation in mg fe 2sio4 at the pressures and temperatures at which deep earthquakes occur the present results and the recent demonstration that metastable olivine is present in the subducting slab beneath japan provide additional support for the anticrack theory of deep focus earthquakes

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

a shock induced polymorph of anatase and rutile from the chesapeake bay impact structure virginia u s a a shock induced polymorph tio2 ii of anatase and rutile has been identified in breccias from the late eocene chesapeake bay impact structure the breccia samples are from a recent partially cored test hole in the central uplift at cape charles virginia the drill cores from 744 to 823 m depth consist of suevitic crystalline clast breccia and brecciated cataclastic gneiss in which the tio2 phases anatase and rutile are common accessory minerals electron microprobe imaging and laser raman spectroscopy of tio2 crystals and powder x ray diffraction xrd of mineral concentrates confirm that a high pressure pbo2 structured polymorph of tio2 tio2 ii coexists with anatase and rutile in matrix hosted crystals and in inclusions within chlorite raman spectra of this polymorph include strong bands at wavenumbers cm 1 175 281 315 342 356 425 531 571 and 604 they appear with anatase bands at 397 515 and 634 cm 1 and rutile bands at 441 and 608 cm 1 xrd patterns reveal 12 lines from the polymorph that do not significantly interfere with those of anatase or rutile and are consistent with the tio2 ii that was first reported to occur naturally as a shock induced phase in rutile from the ries crater in germany the recognition here of a second natural shock induced occurrence of tio2 ii suggests that its presence in rocks that have not been subjected to ultrahigh pressure regional metamorphism can be a diagnostic indicator for confirmation of suspected impact structures

intermediate depth earthquake faulting by dehydration embrittlement with negative volume change earthquakes are observed to occur in subduction zones to depths of approximately 680 km even though unassisted brittle failure is inhibited at depths greater than about 50 km owing to the high pressures and temperatures it is thought that such earthquakes particularly those at intermediate depths of 50 300 km may instead be triggered by embrittlement accompanying dehydration of hydrous minerals principally serpentine a problem with failure by serpentine dehydration is that the volume change accompanying dehydration becomes negative at pressures of 2 4 gpa 60 120 km depth above which brittle fracture mechanics predicts that the instability should be quenched here we show that dehydration of antigorite serpentinite under stress results in faults delineated by ultrafine grained solid reaction products formed during dehydration this phenomenon was observed under all conditions tested pressures of 1 6 gpa temperatures of 650 820 degrees c independent of the sign of the volume change of reaction although this result contradicts expectations from fracture mechanics it can be explained by separation of fluid from solid residue before and during faulting a hypothesis supported by our observations these observations confirm that dehydration embrittlement is a viable mechanism for nucleating earthquakes independent of depth as long as there are hydrous minerals breaking down under a differential stress

shearing instabilities accompanying high pressure phase transformations and the mechanics of deep earthquakes deep earthquakes have been a paradox since their discovery in the 1920s the combined increase of pressure and temperature with depth precludes brittle failure or frictional sliding beyond a few tens of kilometers yet earthquakes occur continually in subduction zones to 700 km the expected healing effects of pressure and temperature and growing amounts of seismic and experimental data suggest that earthquakes at depth probably represent self organized failure analogous to but different from brittle failure the only high pressure shearing instabilities identified by experiment require generation in situ of a small fraction of very weak material differing significantly in density from the parent material this fluid spontaneously forms mode i microcracks or microanticracks that self organize via the elastic strain fields at their tips leading to shear failure growing evidence suggests that the great majority of subduction zone earthquakes shallower than 400 km are initiated by breakdown of hydrous phases and that deeper ones probably initiate as a shearing instability associated with breakdown of metastable olivine to its higher pressure polymorphs in either case fault propagation could be enhanced by shear heating just as is sometimes the case with frictional sliding in the crust extensive seismological interrogation of the region of the tonga subduction zone in the southwest pacific ocean provides evidence suggesting significant metastable olivine with implication for its presence in other regions of deep seismicity if metastable olivine is confirmed either current thermal models of subducting slabs are too warm or published kinetics of olivine breakdown reactions are too fast

deep focus earthquakes and recycling of water into the earth s mantle for more than 50 years observations of earthquakes to depths of 100 to 650 kilometers inside earth have been enigmatic at these depths rocks are expected to deform by ductile flow rather than brittle fracturing or frictional sliding on fault surfaces laboratory experiments and detailed calculations of the pressures and temperatures in seismically active subduction zones indicate that this deep focus seismicity could originate from dehydration and high pressure structural instabilities occurring in the hydrated part of the lithosphere that sinks into the upper mantle thus seismologists may be mapping the recirculation of water from the oceans back into the deep interior of our planet

transformation induced faulting requires an exothermic reaction and explains the cessation of earthquakes at the base of the mantle transition zone abstract in many subduction zones earthquakes occur continuously to the bottom of the mantle transition zone and stop abruptly at the endothermic phase change marking the top of the lower mantle suggesting cause and effect new experiments on polycrystals undergoing the ilmenite perovskite transformation show that transformation induced faulting occurs only in the exothermic direction of the reaction the close structural and thermodynamic analogies between the transformation studied here in cdtio 3 and those expected in the silicates of the earth s mantle argue strongly that these results are relevant to transformations occurring in subducting lithosphere moreover consideration of the thermodynamics of this asymmetry show that this result should be a general phenomenon if so it follows that faulting by this mechanism cannot occur in the lower mantle providing a natural explanation for the termination of earthquakes just before 700 km depth if a wedge containing metastable olivine exists at depth in subduction zones and is necessary for deep earthquakes our results remove the constraint that this wedge must taper to zero where the deepest earthquakes occur the characteristics of the large deep earthquakes of 1994 and their aftershocks show that deep earthquakes can propagate beyond the zone of previous hypocenters and trigger aftershocks there providing important new information about the mechanism of deep earthquakes and pointing to the need for additional experiments

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

the depth of the deepest historical earthquakes we usep ands times listed in the international seismological summary to relocate 23 historical earthquakes 1927 1963 reported as occurring at or below 670 km in all cases our relocated hypocenters are shallower than the starting depths furthermore all events converge to 691 km or less with a precision estimated at 10 km this study upholds the results of stark and frohlich who had usedpp p times of post wwssn earthquakes to constrain reliable hypocentral depths to no greater than 684 km in particular we reject rothe s claim that a 1963 event in the vicinity of new guinea occurred at a depth of more than 780 km

seismic anisotropy in the mantle transition zone beneath fiji tonga 1 we demonstrate a rare unequivocal case of anisotropy in the transition zone tz evident from many horizontal polarized shear waves sh that arrive up to 3 s earlier than vertical polarized shear waves sv the anisotropic region correlates with deep earthquakes away from the active subduction zone and known boundaries of lateral heterogeneities in compression and shear wave speeds we interpret the anisotropy as strong fabric associated with a large petrologic anomaly in the core of remnant lithosphere which is buoyant and stagnates in the tz fast subduction of cold slab actually favors such a configuration so slab penetration into the lower mantle does not uniformly occur

effect of slab temperature on deep earthquake aftershock productivity and magnitude frequency relations deep earthquakes generally show fewer aftershocks and a larger variability in magnitude frequency relations than shallow earthquakes1 5 these characteristics and the factors that control them may place constraints on the mechanism of deep earthquakes which is still uncertain6 8 here we show that systematic variations in aftershock productivity and magnitude frequency relations are related to the temperature of the downgoing slab as inferred from the vertical velocity and age of the subducting lithosphere deep earthquakes with substantial aftershock sequences are found only in colder slabs and earthquakes in warmer slabs consistently show few aftershocks magnitude frequency relations for deep earthquakes also show systematic variations as a function of slab thermal structure with warmer slabs showing fewer small earthquakes lower b values the statistical properties of deep earthquakes thus appear to be temperature sensitive to an extent not observed for shallow earthquakes and not adequately explained by simple geometrical limitations on fault width

a graveyard for buoyant slabs geoscientists have been debating whether convection deep inside earth involves the whole mantle or is divided into two layers separated at 660 km depth compromise models suggest delayed penetration of the transition zone due to a phase change in his perspective green highlights a study by chen and brudzinski who have found evidence for a buoyant cold slab in the transition zone between the upper and the lower mantle the slab appears to be retained in the transition zone because of mineralogical differences between its interior and its surroundings

seismic constraints on mechanisms of deep earthquake rupture 1 observations of the depth distribution of seismicity suggest that several mechanisms cause deep earthquakes cold subducting slabs exhibit a broad peak in seismicity between 300 and 530 km depth that is best explained by transformational faulting of metastable olivine cumulative seismic moment release increases sharply at 530 km depth in both cold and warm slabs implying that events deeper than this are controlled by an equilibrium rather than a metastable phase change which would be temperature dependent this together with the observation that deep seismicity predominates near the garnet rich upper boundary of the subducting plate former oceanic crust suggests that the transformation of garnet to perovskite which may release considerable water is integral in the cause of earthquakes deeper than 530 km where most deep earthquakes occur

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

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

faulting associated with the olivine to spinel transformation in mg2geo4 and its implications for deep focus earthquakes samples of metastable mg2geo4 olivine fail by faulting when loaded at temperatures where the incoherent nucleation and growth of spinel the stable phase just becomes possible at lower temperatures where the transformation is inhibited metastable olivine is ductile and strong while at higher temperatures where the transformation happens very quickly samples are ductile and weak microfaults that have just been created and have slipped very little contain only very fine grained spinel those that have slipped more contain a mixture of fine grained spinel and larger broken olivine fragments a new type of transformation microstructure that develops only under nonhydrostatic stress occurs in all specimens that failed the spinel nucleates on grain boundaries and on subgrain boundaries within olivine and grows as fine grained lens shaped bodies with a strong preferred orientation normal to maximum compression the correlation between the onset of transformation the new microstructure and the mechanical instability suggests a cause and effect relationship the grain size of the spinel within the microfaults suggests that these zones have superplastic properties in this case the instability should not be suppressed by increasing pressure as normal frictional processes are suppressed analogous conditions exist in down going slabs of lithosphere in the mantle and we postulate that the same mechanism that causes faulting in our samples operates in the mantle causing deep focus earthquakes

stabilities and equations of state of dense hydrous magnesium silicates abstract the thermodynamic stability of high pressure phases in the ternary system mgo sio2 h2o is reviewed with special emphasis on hydrated phases including the alphabet phases on the basis of recent experimental data a stability diagram for these phases along a geotherm appropriate for a subducting slab is constructed this suggests that at realistic water contents the breakdown of antigorite will produce olivine plus phase a in the upper mantle but that the ability of wadsleyite to accommodate wt levels of water means that phases e and d are unlikely to occur in the upper portion of the transition zone for the lower part of the transition zone in which ringwoodite becomes stable the coexisting hydrous phase would be superhydrous phase b provided ringwoodite is not able to accommodate significant amounts of water at the top of the lower mantle phase d is stable but only at temperatures below 1300 c as the maximum solubility of h2o in magnesium silicate perovskite is also quite low slabs may dehydrate somewhat as they enter the lower mantle the available thermodynamic data for these phases is also reviewed from the most recent measurements of equations of state it is concluded that the elasticity of the phases in the mgo sio2 h2o ternary system is primarily dependent upon the density and not upon the water content it is therefore concluded that the presence of even significant amounts of hydrated phases in subducting slabs could not be unequivocally identified from seismological observations

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

plastic instabilities implications for the origin of intermediate and deep focus earthquakes adiabatic or catastrophic plastic shear has been reported in metals polymers and metallic glasses the phenomenon is associated with rapid stress drops and audible pings or clicks as the material deforms in a plastic manner the driving force for the plastic instability is the stored elastic strain energy of the loading system and in many respects the behavior is reminiscent of the shear stress response arising from stick slip events during unstable frictional sliding although the precise mechanism is different it is shown here that adiabatic plastic shear is capable of explaining the detailed distribution of intermediate and deep focus earthquakes within subduction zones the earthquake events being the result of instabilities in material undergoing plastic flow it is argued that for a particular strain rate there exists a critical temperature tc which is depth dependent for temperatures below tc the material is strain rate softening and for a soft enough loading system may undergo catastrophic plastic shear for temperatures above tc the material is strain rate hardening and is always stable during plastic shear the cutoff depth for deep focus earthquakes then corresponds to the transition from strain rate softening to strain rate hardening material and for commonly accepted geothermal gradients within the slab corresponds to approximately 800 km the precise distribution of earthquakes within the slab is a function of the subtle interplay between the geothermal gradient and the tc gradient in particular a decrease in seismic activity is to be expected below about 300 km in the slab with total stress drops decreasing from a maximum of 700 mpa above 300 km to a maximum of 50 mpa below 300 km the differences in foci distribution between subduction zones such as tonga new hebrides and peru result from minor differences in the geothermal gradients within the slabs the model predicts the development of triple seismic zones high in the slab double seismic zones down to approximately 300 km and single seismic zones down to 800 km such a distribution is to be expected of relatively young cool slabs as the slab heats up the seismic activity retreats up the slab the paper only proposes a deformation mechanism for earthquake generation it does not address the stress field within the slab but only the distribution of strength thus the distribution of focal plane mechanisms is not considered only the locations where earthquakes due to plastic instabilities are possible the absence of earthquakes does not necessarily mean that the slab does not exist it only means that the slab is too hot to undergo plastic instability this means that aseismic subduction is a distinct possibility in many regions of high geothermal gradient within the slab i e circa 3 c km 1