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

material properties of the menderes massif marbles from sw turkey marbles are extensively quarried at four different stratigraphical levels from permo carbonifereous to paleogene in the southern flank of the menderes massif in sw turkey these marbles differ in color texture and pattern depending on their stratigraphical levels and are well known in the international trade as the mugla black permo carbonifereous mugla white cretaceous milas lemon lilac aubergine pearl veined and white triassic and aegean bordeaux paleogene marbles the mineralogical chemical physical and mechanical properties of the representative marbles samples obtained from the quarries working in four major metamorphic carbonate horizons in the cover successions of the menderes massif s southern flank in sw turkey are determined and the results of over 1700 tests carried out on the selected marble samples are presented the mean test values of the physical and mechanical tests are in general found to be above the threshold acceptance values suggested by the american and turkish standards for the use of marbles as a building stone and in the same order as the properties of italian carrara and greek marbles reported in the literature additionally the mean test values of the marbles have given high correlations with one another and the relations obtained between the index test results determined by simple techniques requiring minimal sample preparation effort and the mean values of the more elaborate engineering tests results are presented as tables and graphs for wider use

precise relative earthquake depth determination using array processing techniques precise determination of hypocentral depth remains one of the most relevant problems in earthquake seismology it is well known that using depth phases allows for significant improvement in event depth determination however routinely and systematically picking such phases for teleseismic or regional arrivals is problematic due to poor signal to noise ratios around the pp and sp phases to overcome this limitation we have taken advantage of the additional information carried by seismic arrays we use velocity spectral analysis to precisely measure pp p times the individual estimates obtained at different subarrays for all pairs of earthquakes are combined using a double difference algorithm in order to precisely map seismicity in regions where it is tightly clustered we illustrate this method by relocating intermediate depth earthquakes in the nazca subducting plate beneath northern chile where we confirm the existence of a narrowly spaced double seismic zone previously imaged using a local dedicated deployment as a second example we relocate the aftershock sequence of the 2014 mw 7 9 intermediate depth rat islands earthquake and provide evidence of a subvertical fault plane for the mainshock finally we show that the resulting relative depth errors are typically smaller than 2 km

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

seismic evidence for high pore pressures in the oceanic crust implications for fluid related embrittlement 1 we estimated the p wave velocity structure of the crust of the subducting pacific plate beneath northeast japan using arrival time data of p to s converted waves the results show that the p wave velocity of the subducting crust varies along the arc and increases abruptly at a depth of 100 km from 6 5 7 0 km s in the fore arc to 7 5 8 5 km s in the back arc the p wave velocity in the fore arc is 10 lower than theoretically expected values for the metamorphosed mid ocean ridge basalt material seismicity in the subducting crust is most active at depths of 70 80 km where p wave velocities are lowest the marked reduction of p wave velocity suggests the coexistence of aqueous fluids with hydrous minerals abundant fluids elevate pore fluid pressures and reduce effective normal stress promoting intensive seismic activity in the low velocity crust our observations provide seismic evidence that earthquakes in the subducting crust are facilitated by fluid related embrittlement

anomalous deepening of a seismic belt in the upper plane of the double seismic zone in the pacific slab beneath the hokkaido corner possible evidence for thermal shielding caused by subducted forearc crust materials abstract hypocenter relocations of earthquakes in the pacific slab have shown an anomalous deepening of a seismic belt in the upper plane of the double seismic zone at depths of 80 120 km beneath the hokkaido corner while it is located at depths of 70 90 km in the surrounding tohoku and eastern hokkaido areas seismic tomographic inversions performed beneath the hokkaido corner have shown that a low velocity zone having seismic velocities of crust materials exists in the mantle wedge just above the pacific slab and makes direct contact with the upper surface of the pacific slab these observations suggest that 1 the low velocity zone just above the pacific slab is part of the subducted kuril forearc sliver 2 the contact with the subducted and so relatively cold sliver materials prevents the mantle wedge from heating the pacific slab and causes a lower temperature condition in the pacific slab crust beneath the hokkaido corner than in the surrounding areas 3 as a result a delay of eclogite forming phase transformations occurs and enhances the local deepening of the seismic belt in the slab crust there

application of a cepstral f statistic for improved depth estimation the depth of a seismic event can be determined using peaks corresponding to depth phase delay times found in the cepstrum of the seismic signal until now however there has been no method available to determine the significance of various peaks in the cepstrum we have formulated a cepstral f statistic by using a classic approach to detect a signal in a number of stationarily correlated time series the method attaches a statistical significance level to peaks in the cepstrum of seismic data caused by echoes in the signal the method is particularly well suited for detecting depth phase echoes pp and sp recorded on regional seismic arrays detections determined from the peaks in the cepstral f statistic are then stacked as a function of pp p and sp p travel times predicted from the iaspei global 1 d model kennett and engdahl 1991 using a modification of the method of depth phase beamforming woodgold 1998 murphy et al 1999 tests on synthetic data show the method is most successful when the p wave arrival has a signal to noise ratio snr greater than 4 7 and the depth phase exhibits an snr greater than 2 tests on complex regional data suggest that the depth phase snr must be between 4 and 8 for consistent successful application of the method we tested the method by using events from the hindu kush region of afghanistan with well determined depths as recorded on arrays at teleseismic distances we determined the correct depths for 14 of the 19 events in this dataset 74 success rate and we determined incorrect depths for two events for a false alarm rate of 10 5 the difference between the depths calculated by the cepstral method and the prototype international data center pidc decrease with increasing magnitude for events with m b 3 8 the average difference between the cepstral and pidc depths was 7 9 km with no false alarms to test the operational capabilities of this method as a tool for data center use we first analyzed 32 events that occurred on 12 february 2000 as located by the pidc additionally we analyzed 29 events located in central and south america between 05 may and 15 june 2000 with event characteristics published by the international data center idc most of these events were also located by the national earthquake information center our method determined statistically significant depths for 40 of these 61 events with 11 having a low snr at three or more recording arrays while another 10 were either too shallow for analysis or did not exhibit depth phases only seven of the 61 events had depth phases listed in the reviewed event bulletin produced by the pidc and idc for these events the average difference between the reported and cepstral f statistic depths was 4 0 km the pidc idc fixed the depth of 26 of 61 events to 0 0 the cepstral f statistic method determined depths ranging from 16 to 92 km for 15 of the events that had been constrained as surface foci overall we believe the method would be most valuable when used by analysts to detect possible depth phases that could then verify or improve network calculated focal depths manuscript received 25 february 2001

high pore fluid pressure may cause silent slip in the nankai trough silent slip events have been detected at several subduction zones but the cause of these events is unknown using seismic imaging we detected a cause of the tokai silent slip which occurred at a presumed fault zone of a great earthquake the seismic image that we obtained shows a zone of high pore fluid pressure in the subducted oceanic crust located down dip of a subducted ridge we propose that these structures effectively extend a region of conditionally stable slips and consequently generate the silent slip

frictional sliding in serpentine at very high pressure abstract using a new four channel system for detecting acoustic emissions in a multianvil apparatus we have assessed the pressure temperature range for such emissions as well as the role of dehydration by deforming samples of extensively serpentinized peridotite we show that in the absence of dehydration and for samples initially faulted at low pressure acoustic emissions occurred well outside the expected pressure temperature field of unassisted brittle failure emissions were also detected during and after dehydration of serpentine microstructures of post run specimens revealed fault slip with offsets up to 500 m regardless of whether or not dehydration took place dehydration appears to effectively stop slip on pre existing faults and create new ones analysis of p wave travel times from the four sensors confirmed that the acoustic emissions originated within the specimen during fault slip these observations suggest that earthquakes can be triggered by slip along an existing fault containing serpentine under significantly higher pressure and temperature conditions than previously thought possible without dehydration

syndeformational antigorite dehydration produces stable fault slip we conducted deformation experiments in which temperature was increased above the thermal stability of antigorite to directly test the dehydration embrittlement hypothesis and its potential link to intermediate depth earthquakes the experiments were designed to maximize the possibility for instability pore pressure was undrained pre dehydration stress was high and deformation was localized prior to initiation of reaction however stick slip instabilities or unstable fault behavior did not develop rather we find that weakening occurs stably at a rate that depends on the ratio of the temperature ramp rate to the strain rate resulting in laboratory slow slip or stress relaxation events

a narrowly spaced double seismic zone in the subducting nazca plate 1 high precision relocations of intermediate depth earthquakes 80 130 km below the central andes reveal a fine scale double layered wadati benioff zone wbz upper and lower band of seismicity are separated by about 9 km and occur at the top of the oceanic crust and in the uppermost oceanic mantle respectively analysis of focal mechanisms and waveform similarities indicate that fluid processes are causing the events earthquakes in the oceanic crust occur on pre existing normal faults due to hydraulic embrittlement from metamorphic dehydration and on subvertical faults that connect the two layers in a narrow depth range extensional faulting predominates in both layers indicating that slab pull forces are the dominant stress source superseding possible unbending forces in this segment of the nazca plate

link between plate fabric hydration and subduction zone seismicity in alaska subduction carries water into the earth where it can influence seismicity analysis of the structure of the alaskan subduction zone suggests fluid delivery is influenced by faults in the oceanic plate that formed at the mid ocean ridge

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

a weakening mechanism for intermediate depth seismicity detailed petrographic and microtextural observations from blueschist facies pseudotachylytes cape corse corsica abstract gabbro and peridotite hosted pseudotachylytes from the alpine schistes lustres unit in corsica previously determined to have formed at blueschist to lawsonite eclogite facies conditions have been causally linked to the generation of intermediate depth earthquakes which occur at depths of 50 300 km detailed petrographic and microtextural analyses of these pseudotachylytes suggest that their initiation may be controlled by a thermally activated shear runaway process that is controlled by rheology rather than mineralogy this is documented by sheared out prolate kinked and twinned wallrock clasts that have been peeled off and entrained into the pseudotachylyte vein as sigmoid survivor clasts the presence of metastable high temperature crystallisation products in the pseudotachylyte such as hoppers and dendrites of olivine enstatite and diopside peridotite and al rich omphacite and fe rich anorthite in metagabbro are suggestive of a short lived high temperature event resulting from thermal instability these high temperature mineral assemblages are overprinted by ones indicating a return to ambient conditions of lower temperatures but still high pressures glaucophane albite and epidote in metagabbro and clinochlore and fine grained granoblastic olivine enstatite and diopside in peridotite the observations from this detailed study of natural samples suggest that intermediate depth seismicity may be generated by a thermal runaway process

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

depth variations in seismic velocity in the subducting crust evidence for fluid related embrittlement for intermediate depth earthquakes we investigated seismic wave velocity in the subducting crust of the pacific slab beneath eastern hokkaido northern japan to detect depth dependent properties of the seismic velocities in the crust we analyzed guided waves that propagate in the crust and estimated p wave velocity vp of 6 5 7 5 km s and s wave velocity vs of 3 6 4 2 km s at depths of 50 100 km the results show that the obtained vp and vs are 10 15 lower than those expected for the fully hydrated mid ocean ridge basalt suggesting the existence of aqueous fluids by 1 vol in the crust at this depth range our observations suggest that overpressurized fluids channeled in the subducting crust plays as a dominant factor for facilitating the genesis of crustal earthquakes at intermediate depths

intermediate depth earthquakes linked to localized heating in dunite and harzburgite earthquakes can occur at great depths in the earth within subducting tectonic plates deformation experiments suggest these deep earthquakes can be triggered by localized heating of the slabs under high pressures

barium geochemistry in sediment pore waters and formation waters of the oceanic crust on the eastern flank of the juan de fuca ridge odp leg 168 1 sediment pore waters collected on the eastern flank of the juan de fuca ridge at 10 sites drilled during ocean drilling program odp leg 168 and basement fluids discharging at the seafloor on a basaltic outcrop called baby bare located at 100 km east of the juan de fuca ridge were analyzed for barium the data show an overall symmetrical behavior between ba and sulfate suggesting that the pore water ba content is controlled by equilibrium with barium sulfate barite saturation indices calculated with a thermodynamic model of the na k ca mg ba sr cl so4 h2o system show that barite does not equilibrate with the pore waters at the shipboard conditions 20 c 1 bar but reflects an overall equilibrium between pore waters and barite at in situ temperatures and pressures for depths below 20 m below the seafloor barite supersaturation is found for conditions in the upper 20 m of the sediment column ba concentrations in pore waters at the base of the sediment section exceed those in the underlying formation fluids based on the direct sampling of these fluids in the open borehole at odp sites 1026 and the hydrothermal springs at baby bare this suggests that ba is supplied to basement by the sediment despite this the formation fluids are undersaturated with respect to barite these fluids which contain 2 3 times more ba than bottom seawater vent at the seafloor resulting in a net flux of ba to bottom seawater the calculated flux from warm 63 c ridge flank hydrothermal systems is insignificant to the oceanic budget for ba but the flux associated from cool less than 25 c ridge flank hydrothermal systems which dispense most of the heat and fluid flux through the oceanic crust is unconstrained

earthquakes and plastic deformation of anhydrous slab mantle in double wadati benioff zones double wadati benioff seismic zones dsz with two parallel planes of seismicity separated by 15 30 km are a global feature of subduction zones in the 50 200 km depth range upper plane seismicity is generally attributed to dehydration of the oceanic crust but the origin of the lower seismicity plane is debated serpentine or hydrous phase dehydration embrittlement is a commonly advocated mechanism that implies significant slab mantle hydration high resolution seismic tomography revealed low seismic velocities in the lower seismicity plane that are better explained by seismic anisotropy of anhydrous deformed peridotites than by serpentinization earthquakes correlate with anisotropic planar shear zones and favor a shear instability mechanism as the cause of lower plane seismicity without requiring the presence of water in the center of subducting slabs the contribution of the subducted lithospheric mantle to the water budget of subduction zones is thus likely limited to the first 2 3 kilometers beneath oceanic crust

seismic evidence for thermal runaway during intermediate depth earthquake rupture 1 intermediate depth earthquakes occur at depths where temperatures and pressures exceed those at which brittle failure is expected there are two leading candidates for the physical mechanism behind these earthquakes dehydration embrittlement and self localizing thermal shear runaway a complete energy budget for a range of earthquake sizes can help constrain whether either of these mechanisms might play a role in intermediate depth earthquake rupture the combination of high stress drop and low radiation efficiency that we observe for mw 4 5 earthquakes in the bucaramanga nest implies a temperature increase of 600 1000 c for a centimeter scale layer during earthquake failure this suggests that substantial shear heating and possibly partial melting occurs during intermediate depth earthquake failure our observations support thermal shear runaway as the mechanism for intermediate depth earthquakes which would help explain differences in their behavior compared to shallow earthquakes

existence of a seismic belt in the upper plane of the double seismic zone extending in the along arc direction at depths of 70 100 km beneath ne japan 1 we relocated microearthquakes using data obtained via a dense seismic network and systematically detected the characteristic distribution of the upper seismic plane seismicity within the pacific slab beneath ne japan we found a seismic belt parallel to the iso depth contour of the plate interface that is beneath the forearc area at depths of 70 100 km indicating that the distribution of the upper plane earthquakes is non uniform the location of the deeper limits of this belt and seismicity of the upper seismic plane appear to correspond respectively to two facies boundaries where h2o contents change in the slab crust events in the upper seismic plane have mainly down dip compression type focal mechanisms but several events have normal fault type nf type versions whose spatial distribution appears to correspond to these boundaries these nf events might be induced by the tensional stress field that is caused by volume reduction due to dehydration reactions

dehydration of lawsonite could directly trigger earthquakes in subducting oceanic crust intermediate depth earthquakes in cold subduction zones are observed within the subducting oceanic crust as well as the mantle in contrast intermediate depth earthquakes in hot subduction zones predominantly occur just below the mohorovicic discontinuity these observations have stimulated interest in relationships between blueschist facies metamorphism and seismicity particularly through dehydration reactions involving the mineral lawsonite here we conducted deformation experiments on lawsonite while monitoring acoustic emissions in a griggs type deformation apparatus the temperature was increased above the thermal stability of lawsonite while the sample was deforming to test whether the lawsonite dehydration reaction induces unstable fault slip in contrast to similar tests on antigorite unstable fault slip that is stick slip occurred during dehydration reactions in the lawsonite and acoustic emission signals were continuously observed microstructural observations indicate that strain is highly localized along the fault r1 and b shears and that the fault surface develops slickensides very smooth fault surfaces polished by frictional sliding the unloading slope during the unstable slip follows the stiffness of the apparatus at all experimental conditions regardless of the strain rate and temperature ramping rate a thermomechanical scaling factor for the experiments is within the range estimated for natural subduction zones indicating the potential for unstable frictional sliding within natural lawsonite layers

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

global prevalence of double benioff zones double benioff zones provide opportunities for insight into seismogenesis because the underlying mechanism must explain two layers of deep earthquakes and the separation between them we characterize layer separation inside subducting plates with a coordinate rotation to calculate the slab normal distribution of earthquakes benchmark tests on well established examples confirm that layer separation is accurately quantified with global seismicity catalogs alone global analysis reveals double benioff zones in 30 segments including all 16 subduction zones investigated with varying subducting plate ages and stress orientations which implies that they are inherent in subducting plates layer separation increases with age and is more consistent with dehydration of antigorite than chlorite

passive and active seismological study of bending related faulting and mantle serpentinization at the middle america trench water transported within the subducting oceanic lithosphere into the earth s interior affects a wealth of subduction zone processes including intraslab earthquakes and arc magmatism in recent years growing evidence suggests that much of the hydration of oceanic plates occurs at the trench ocean slope right before subduction here normal faults are created while the rigid lithosphere bends into the trench offshore of middle america multi channel seismic reflection imaging suggests that bending related faults cut into the uppermost mantle providing a mechanism for hydration and transformation of mantle peridotites into serpentinites seismic wide angle reflection and refraction data were collected coincident with one of the seismic profiles where the faults have been imaged travel time inversion provides evidence that both crustal and uppermost mantle velocities are reduced with respect to the velocity structure found in mature oceanic crust away from deep sea trenches if mantle velocity reduction is solely produced by hydration velocities indicate 10 15 of serpentinization in the uppermost 3 km of the mantle where seismic data provide enough resolution a small network of ocean bottom hydrophones deployed for about a month detected 3 local micro earthquakes per day earthquake epicentres align with fault scarps at the seafloor and continuous earthquake activity might be an important process to facilitate the percolation of seawater into the upper mantle

fossil intermediate depth earthquakes in subducting slabs linked to differential stress release the cause of intermediate depth 50 300 km seismicity in subduction zones is uncertain it is typically attributed either to rock embrittlement associated with fluid pressurization or to thermal runaway instabilities here we document glassy pseudotachylyte fault rocks the products of frictional melting during coseismic faulting in the lanzo massif ophiolite in the italian western alps these pseudotachylytes formed at subduction zone depths of 60 70 km in poorly hydrated to dry oceanic gabbro and mantle peridotite this rock suite is a fossil analogue to an oceanic lithospheric mantle that undergoes present day subduction the pseudotachylytes locally preserve high pressure minerals that indicate an intermediate depth seismic environment these pseudotachylytes are important because they are hosted in a near anhydrous lithosphere free of coeval ductile deformation which excludes an origin by dehydration embrittlement or thermal runaway processes instead our observations indicate that seismicity in cold subducting slabs can be explained by the release of differential stresses accumulated in strong dry metastable rocks seismic activity within subducted slabs could be caused by differential stress release according to analysis of fossilized remnants of earthquake slip in an exhumed slab these deep earthquakes were previously thought to mark either slab dehydration or thermal runaway processes