the 9 june 94 bolivian deep earthquake an exceptional event in an extraordinary subduction zone we investigate the physical setting of the bolivian shock based on the history of the subducting nazca plate intraslab seismicity deep seismic moment release and seismic tomography south america has two broad regions of reverse arc curvature subduction constrained to this unique geometry produces slab kinking contortions that may cause unusual slab thickening as they sink to the bottom of the transition zone and encounter resistance to penetration into the lower mantle such contortions are observed at intermediate depths upstream from the slab source regions of both the great 1970 colombian and 1994 bolivian events thickening helps explain how the nazca slab accommodates large seismic source dimensions at depths of 625 650 km

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

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

remote triggering of deep earthquakes in the 2002 tonga sequences it is well established that an earthquake in the earth s crust can trigger subsequent earthquakes but such triggering has not been documented for deeper earthquakes models for shallow fault interactions suggest that static permanent stress changes can trigger nearby earthquakes within a few fault lengths from the causative earthquake1 2 3 whereas dynamic transient stresses carried by seismic waves may trigger earthquakes both nearby and at remote distances4 5 6 7 8 here we present a detailed analysis of the 19 august 2002 tonga deep earthquake sequences and show evidence for both static and dynamic triggering seven minutes after a magnitude 7 6 earthquake occurred at a depth of 598 km a magnitude 7 7 earthquake 664 km depth occurred 300 km away in a previously aseismic region we found that nearby aftershocks of the first mainshock are preferentially located in regions where static stresses are predicted to have been enhanced by the mainshock but the second mainshock and other triggered events are located at larger distances where static stress increases should be negligible thus suggesting dynamic triggering the origin times of the triggered events do not correspond to arrival times of the main seismic waves from the mainshocks and the dynamically triggered earthquakes frequently occur in aseismic regions below or adjacent to the seismic zone we propose that these events are triggered by transient effects in regions near criticality but where earthquakes have difficulty nucleating without external influences

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

acoustic emissions produced by anticrack faulting during the olivine spinel transformation anticrack faulting during the phase transformations of olivine to its denser polymorphs is a principal contender for the mechanism of deep focus earthquakes the original work on mg2geo4 established that initiation of faulting by this mechanism is not due to brittle fracture and subsequent experiments on olivine of mantle composition showed that it can operate at pressures of at least 14 gpa here we report acoustic emissions accompanying anticrack faulting in mg2geo4 thereby confirming that this mechanism involves a dynamic instability accompanied by elastic radiation

toward a model for airflow on the lee side of aeolian dunes the interaction between dunes and the primary wind results in a complex pattern of secondary airflow on the lee side of dunes from 15 dunes studied during transverse flow conditions at padre island in texas white sands in new mexico and the algodones in california distinct flow regions can generally be recognized with the overall flow structure comparing favourably to that proposed for subaqueous bedforms downwind of dunes with flow separation is a back flow eddy that extends about four dune brink heights downwind from the brink of the dune beyond the separation cell the velocity profiles can be divided into regions based upon segments separated by kinks in the velocity profiles the interior is an area above the dunes of relative high wind speed but low velocity gradient beneath the interior is the wake which consists of two layers the upper wake exhibits an uppermost portion where the flow decelerates while the remainder exhibits accelerating flow so that the overall velocity gradient decreases downwind the lower wake exhibits low velocity gradients and wind speeds that accelerate downwind at all heights but primarily near the top of the layer thereby causing the velocity gradient to increase downwind at about eight dune heights downwind the upper and lower wakes equilibrate to a single profile with the kink between them no longer apparent the lowest recognizable region is the internal boundary layer it is recognized by a relatively steep velocity gradient below the wake and never exceeds a few tens of centimetres in height for our data set because of acceleration and increasing shear stress within this layer interdune flats are at least potentially erosional overall the wake and internal boundary layer show a downward transfer of momentum from upper regions so that the flow recovers where flow separation does not occur simple flow expansion down the lee face causes flow deceleration

frictional melting during the rupture of the 1994 bolivian earthquake the source parameters of the 1994 bolivian earthquake magnitude m w 8 3 suggest that the maximum seismic efficiency was 0 036 and the minimum frictional stress was 550 bars thus the source process was dissipative which is consistent with the observed slow rupture speed only 20 of the locals wave velocity the amount of nonradiated energy produced during the bolivian rupture was comparable to or larger than the thermal energy of the 1980 mount st helens eruption and was sufficient to have melted a layer as thick as 31 centimeters once rupture was initiated melting could occur which reduces friction and promotes fault slip

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

electron microscope study of the reaction olivine h2o tio2 titanian clinohumite titanian chondrodite synthesized at 8 gpa 1300 k the nucleation and growth processes of titanian clinohumite ti cl and titanian chondrodite ti ch during hydrous alteration of olivine at 8 gpa 1300 k are described in detail we observed two different nucleation sites and orientation relationships for ti cl 1 heterogeneous nucleation at olivine grain or phase boundaries yielding semicoherent 001 lamellae with 001 ol 001 ti cl and 010 ol 010 ti cl 2 formation of coherent nanometer sized nuclei inside olivine with the orientation relationship 100 ol 001 ti cl and 011 ol 020 ti cl only the former for which the interface with olivine facilitates the transport of oh and ti necessary for the formation of the humite group minerals survives to large sizes these results suggest that ti cl should form readily in a ti and h rich environment at mantle temperatures independent of f content from shallow environments to those at depths exceeding 250 km and could play a role in recycling water back into the deep mantle

global pattern of earthquakes and seismic energy distributions insights for the mechanisms of plate tectonics article i nfo in this paper we analyse the distributions of number of events n and seismic energy e on the earth s surface and along its radius as obtained from the global declustered catalogue of large independent events m 7 0 dissipating about 95 of the earth s elastic budget the latitude distribution of the seismic event density is almost symmetric with respect to the equator and the seismic energy flux distribution is bimodal both have their medians near the equator so that they are equally distributed in the two hemispheres this symmetry with respect to the equator sug gests that the earth s rotational dynamics contributes to modulate the long term tectonic processes the distributions of number and energy of earthquakes versus depth are not uniform as well 76 of the total earth quakes dissipates about 60 of the total energy in the first 50 km only 6 of events dissipates about 20 of the total amount of energy in a narrow depth interval at the lower boundary of the upper mantle 550 680 km therefore only the remaining 20 of energy is released along most of the depth extent of subduction zones 50 550 km since the energetic release along slabs is a minor fraction of the total seismic budget the role of the slab pull appears as ancillary if any in driving plate tectonics moreover the concentration of seismic release in the not yet subducted lithosphere suggests that the force moving the plates acts on the uppermost lithosphere and contemporaneously all over the earth s outer shell again supporting a rotational tidal modulation

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

implications of spatial and temporal development of the aftershock sequence for the mw 8 3 june 9 1994 deep bolivian earthquake on june 9 1994 the mw 8 3 bolivia earthquake 636 km depth occurred in a region which had not experienced significant deep seismicity for at least 30 years the mainshock and aftershocks were recorded in bolivia on the banjo and seda broadband seismic arrays and on the san calixto network we used the joint hypocenter determination method to determine the relative location of the aftershocks we have identified no foreshocks and 89 aftershocks m 2 2 for the 20 day period following the mainshock the frequency of aftershock occurrence decreased rapidly with only one or two aftershocks per day occuring after day two the temporal decay of aftershock activity is similar to shallow aftershock sequences but the number of aftershocks is two orders of magnitude less additionally a mb 6 apparently triggered earthquake occurred just 10 minutes after the mainshock about 330 km east southeast of the mainshock at a depth of 671 km the aftershock sequence occurred north and east of the mainshock and extends to a depth of 665 km the aftershocks define a slab striking n68 w and dipping 45 ne the strike dip and location of the aftershock zone are consistent with this seismicity being confined within the downward extension of the subducted nazca plate the location and orientation of the aftershock sequence indicate that the subducted nazca plate bends between the nnw striking zone of deep seismicity in western brazil and the n s striking zone of seismicity in central bolivia a tear in the deep slab is not necessitated by the data a subset of the aftershock hypocenters cluster along a subhorizontal plane near the depth of the mainshock favoring a horizontal fault plane the horizontal dimensions of the mainshock beck et al this issue silver et al 1995 and slab defined by the aftershocks are approximately equal indicating that the mainshock ruptured through the slab

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

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

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

variations in p wave speeds and outboard earthquakes evidence for a petrologic anomaly in the mantle transition zone we investigate the nature of laterally varying p wave speeds vp and their relationship to a group of unusual deep earthquakes beneath the backarc of tonga this subhorizontal swath of outboard earthquakes occurs within the transition zone of the mantle but is several hundred kilometers away from the inclined wadati benioff zone surprisingly the seismogenic region is not associated with fast vp it follows that the effect of cold temperature as implied by the presence of large earthquakes must have been counteracted by other factors such as compositional or mineralogical effects our results are based on a sequence of seismic profiles in a fan shot geometry for a given azimuthal sector we seek the simplest model of radially varying vp that explains observed broadband waveforms and travel times the lateral extent of anomalous vp is then constrained by comparing the results from a wide range of azimuths this approach fully utilizes information from triplicate p arrivals that are sensitive to seismic wave speeds at the depths of interest our results identify prominent anomalies of fast vp 3 over average earth models both in the transition zone and in the lower mantle beneath the tonga backarc however the region of fast vp in the transition zone is offset by 300 km to the south of that in the lower mantle and the region of offset coincides with the outboard earthquakes a straightforward interpretation is that a large flux of subducted pacific lithosphere lingers in the transition zone before descending into the lower mantle within this broad remnant of subducted lithosphere the outboard earthquakes occur in a region where impounding of subducted material is most significant a condition favorable for the accumulation of met astable olivine or volatiles such materials reduce vp and are candidates for triggering earthquakes thus accounting for the absence of fast vp in the source zone of deep earthquakes

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

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

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

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