a scaling law for slow earthquakes with the availability of the global positioning system and other technical advances a growing number of unusual earthquake phenomena occurring at relatively long periods have been recognized they include deep episodic tremor low frequency earthquakes slow slip events and silent earthquakes based on data chiefly from western japan ide et al report that these slow seismic events follow a unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes a diverse array of unusual earthquake phenomena occurring at relatively long periods slow events follow a unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes indicating that they comprise a new earthquake category recently a series of unusual earthquake phenomena have been discovered including deep episodic tremor1 low frequency earthquakes2 very low frequency earthquakes3 slow slip events4 and silent earthquakes5 6 7 8 9 each of these has been demonstrated to arise from shear slip just as do regular earthquakes but with longer characteristic durations and radiating much less seismic energy here we show that these slow events follow a simple unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes we find that their seismic moment is proportional to the characteristic duration and their moment rate function is constant with a spectral high frequency decay of f 1 this scaling and spectral behaviour demonstrates that they can be thought of as different manifestations of the same phenomena and that they comprise a new earthquake category the observed scale dependence of rupture velocity for these events can be explained by either a constant low stress drop model or a diffusional constant slip model this new scaling law unifies a diverse class of slow seismic events and may lead to a better understanding of the plate subduction process and large earthquake generation

high temperature elasticity of iron bearing olivines the first high temperature data on the nine adiabatic elastic moduli for iron bearing olivine are reported these measurements are on two single crystal specimens of natural olivine at ambient pressure and from room temperature to a maximum of 1500 k the two specimens contain 8 and 9 modal percent fayalite which required the oxygen fugacity be controlled at high temperature to preserve their chemical stability the rectangular parallelepiped resonance apparatus was adapted to buffer the specimens from the atmosphere with a mixture of co and co2 gas a small increase 1 2 gpa in the adiabatic bulk modulus of each specimen over that of end member forsterite was found the data are high quality to extreme temperatures with good agreement found when comparing the temperature derivatives of the elastic moduli of the two specimens neither specimen exhibits measurable nonlinear temperature dependence in the computed isotropic bulk and shear moduli which is in contrast to published forsterite data the temperature derivatives of the isotropic bulk modulus ks are 1 69 1 80 10 2 gpa k 1 for the two olivine specimens and the shear modulus g derivatives are 1 38 1 36 10 2 gpa k 1 these derivatives are only slightly larger in magnitude than ks t p 1 56 10 2 and g t p 1 30 10 2 gpa k 1 found previously for iron bearing olivine over a very small temperature range there are also no significant differences between the temperature derivatives found here and the average derivatives of end member forsterite from data retrieved over a slightly larger temperature range several dimensionless parameters have been calculated from these results and are discussed in view of systematics which bear on high pressure phases in earth s transition zone one result from these systematics related to the seismic velocities in the earth and especially the shear wave velocities is that an olivine content of less than 50 is implied at the 400 km discontinuity if earth s upper mantle is isochemical furthermore the substitution of almost 10 iron for magnesium at the forsterite end of the olivine solid solution series has little effect on the dimensionless parameters or on the temperature derivatives of the elastic moduli at high temperature

magnitude 7 level earthquakes a new lower limit of self similarity in seismic scaling relationships microfractures occurring in a rock sample that are called acoustic emission ae events show some similar features to earthquakes however it remains to be shown whether or not ae equate to ultramicroearthquakes in this study we show the existence of magnitude 7 level earthquakes based on seismological analyses of ae source parameters advances in multichannel broadband high speed continuous recording of ae under seismogenic pressure conditions has facilitated increasingly robust measurement source parameters of ae show that ae events satisfy the same scaling relationship as natural earthquakes in which seismic moment is inversely proportional to the cube of corner frequency this result suggests that both millimeter scale fractures and natural earthquakes of kilometer scale ruptures are highly similar as physical processes hence ae events can be interpreted as ultramicroearthquakes having a magnitude of about 7 these results demonstrate that laboratory observation is an effective approach in studying natural earthquake generation process

slow stick slip of antigorite serpentinite under hydrothermal conditions as a possible mechanism for slow earthquakes slow earthquakes characterized by a different scaling law to regular earthquakes have been detected at the hydrated plate interface in the subduction zones but the generating mechanism of them remains almost unexplored frictional experiments on antigorite serpentinite under hydrothermal conditions are conducted to assess the distinct scaling law of slow earthquakes slow stick slip was observed at temperatures that were close to the dehydration temperature of antigorite which is resulted by the localized dehydration of serpentine in the shear zone the occurrence of slow stick slip is consistent with the temperature range found in the corner of the mantle wedge in sw japan and cascadia where slow earthquakes occur the laboratory slow stick slip shows a similar scaling law of slow earthquakes but distinct from that of regular earthquakes we propose that the shear induced dehydration of the serpentine play an important role for the generation of slow earthquakes

role of pore fluid pressure on transient strength changes and fabric development during serpentine dehydration at mantle conditions implications for subduction zone seismicity abstract to further investigate the dehydration embrittlement hypothesis and its possible link to subduction zone seismicity we conducted deformation experiments on antigorite serpentinite in a griggs type apparatus at conditions below and above antigorite stability temperature ramps crossing the antigorite thermal stability were used in conjunction with a new experimental method that allows fluid produced during dehydration reactions to be drained partially drained or undrained during temperature ramps weakening coupled with transient slip initiated at 650 c coincident with the predicted phase transition of antigorite to olivine and talc at 1 gpa the weakening rate and steady state strength were dependent on drainage conditions undrained samples weakened over a few minutes and supported the lowest shear stress 50 mpa while drained samples weakened over a few hours and supported the highest shear stress 210 mpa the coefficient of friction shear stress over normal stress in drained samples decreased from 0 4 to 0 16 after the temperature ramp the strengths of samples that were first annealed at 700 c for 12 h then deformed were similar to those observed in the temperature ramp experiments strain localization along fractures occurred in all samples during temperature ramping regardless of the drainage conditions however microstructural observations indicate deformation by ductile mechanisms at higher strain under both undrained and drained conditions the rheology and microstructures suggest dehydrating serpentinite deforms via semibrittle flow with grain scale ductile deformation more active at high pore fluid pressures our results suggest that earthquakes in serpentinized mantle do not nucleate as a direct result of unstable frictional sliding along fractures generated at the onset of dehydration reactions

switching from seismic faulting to silent slips in harzburgite induced by h2o fluid at upper mantle pressures slow slip events frequently occur but regular earthquakes are much less active on the h2o fluid rich subduction interface at depths of 40 km the characteristic duration for silent earthquakes which are categorized as the great slow slip events is more than five orders of magnitude longer than that for regular earthquakes such phenomena are often attributed to the slippage of the softer part of the subduction interface but the impact of h2o fluid on aseismic slip is still unsolved here we conduct deformation experiments on water saturated harzburgite at pressures of 1 2 to 3 0 gpa and temperatures of 770 to 1250 k corresponding to the conditions of the lower part of the overriding plate just above the subduction interface we observe deformation of the harzburgite followed by silent faulting at a significantly low stress level down to 0 3 gpa under fluid bearing conditions even though many acoustic emissions are generated at the onset of faulting in fluid free harzburgite we find that the observed silent faulting is caused by the detachment of asperity contacts by high pore pressures and lubrication of the fault plane by a hydration reaction we therefore propose that h2o fluid may prevent the occurrence of regular intraslab earthquakes but trigger silent earthquakes

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

talc bearing serpentinite and the creeping section of the san andreas fault the section of the san andreas fault located between cholame valley and san juan bautista in central california creeps at a rate as high as 28 mm yr 1 ref 1 and it is also the segment that yields the best evidence for being a weak fault embedded in a strong crust serpentinized ultramafic rocks have been associated with creeping faults in central and northern california and serpentinite is commonly invoked as the cause of the creep and the low strength of this section of the san andreas fault however the frictional strengths of serpentine minerals are too high to satisfy the limitations on fault strength and these minerals also have the potential for unstable slip under some conditions here we report the discovery of talc in cuttings of serpentinite collected from the probable active trace of the san andreas fault that was intersected during drilling of the san andreas fault observatory at depth safod main hole in 2005 we infer that the talc is forming as a result of the reaction of serpentine minerals with silica saturated hydrothermal fluids that migrate up the fault zone and the talc commonly occurs in sheared serpentinite this discovery is significant as the frictional strength of talc at elevated temperatures is sufficiently low to meet the constraints on the shear strength of the fault and its inherently stable sliding behaviour is consistent with fault creep talc may therefore provide the connection between serpentinite and creep in the san andreas fault if shear at depth can become localized along a talc rich principal slip surface within serpentinite entrained in the fault zone

remote sensing evidence of lava ground ice interactions associated with the lost jim lava flow seward peninsula alaska thermokarst terrains develop when ice bearing permafrost melts and causes the overlying surface to subside or collapse this process occurs widely throughout arctic regions due to environmental and climatological factors but can also be induced by localized melting of ground ice by active lava flows the lost jim lava flow ljlf on the seward peninsula of alaska provides evidence of former lava ground ice interactions associated geomorphic features on the scale of meters to tens of meters were identified using satellite orthoimages and stereo derived digital terrain models the flow exhibits positive and mixed relief features including tumuli n 26 and shatter rings n 4 as well as negative relief features such as lava tube skylights n 100 and irregularly shaped topographic depressions n 1188 that are interpreted to include lava rise pits and lava induced thermokarst terrain along the margins of the flow there are also clusters of small peripheral pits that may be the products of meltwater or steam escape on mars we observed morphologically similar pits near lava flow margins in northeastern elysium planitia which suggests a common formation mechanism investigating the ljlf may therefore help to elucidate processes of lava ground ice interaction on both earth and mars

melting experiments on lherzolite klb 1 under hydrous conditions and generation of high magnesian andesitic melts the partial melting of natural lherzolite klb 1 at 1 gpa was studied for both h2o undersaturated and saturated conditions the quenched glasses obtained at 1000 and 1050 c are andesitic in composition having 54 4 60 3 wt sio2 whereas partial melts formed at temperatures above 1100 c have basaltic compositions this suggests that high magnesian andesites can be formed by direct partial melting of the hydrous upper mantle only at relatively low temperatures these andesitic melts together with those obtained at 1100 c contain more than 3 wt h2o and have compositions characteristic of hydrous melts high sio2 and al2o3 and low feo mgo and cao the results of melting studies on klb 1 show a strong increase in the degree of melting when h2o is added to lherzolite at a given temperature

acoustic emission induced by pore pressure changes in sandstone samples an understanding of microseismicity induced by pore pressure changes in stressed rocks is important for applications in geothermal and hydrocarbon reservoirs as well as for co 2 sequestrations we have studied the triggering mechanisms of microseismicity or acoustic emission in the laboratory as a function of triaxial stress conditions and pore pressure changes in investigating the spatiotemporal distribution of acoustic emission activity in water saturated triaxially stressed flechtingen sandstone samples subjected to changes in pore pressure we assumed that acoustic events were triggered by pore pressure increase to estimate pore pressure changes in the sample we used an analytical solution of the 1d diffusion equation a theoretical analysis of the spatiotemporal distribution suggested that for initially insignificantly stressed samples acoustic events were triggered by the migration of a critical pore pressure level through the sample the critical level was controlled by the applied pore press

high pressure creep of serpentine interseismic deformation and initiation of subduction the supposed low viscosity of serpentine may strongly influence subduction zone dynamics at all time scales but until now its role could not be quantified because measurements relevant to intermediate depth settings were lacking deformation experiments on the serpentine antigorite at high pressures and temperatures 1 to 4 gigapascals 200 to 500 c showed that the viscosity of serpentine is much lower than that of the major mantle forming minerals regardless of the temperature low viscosity serpentinized mantle at the slab surface can localize deformation impede stress buildup and limit the downdip propagation of large earthquakes at subduction zones antigorite enables viscous relaxation with characteristic times comparable to those of long term postseismic deformations after large earthquakes and slow earthquakes antigorite viscosity is sufficiently low to make serpentinized faults in the oceanic lithosphere a site for subduction initiation

deformation of antigorite serpentinite at high temperature and pressure abstract deformation experiments were conducted in a griggs type apparatus to constrain the rheology of antigorite serpentinite at high temperatures and pressures samples deformed at temperatures of 400 625 c and pressures of 0 85 1 5 gpa experienced semi brittle deformation resulting in localization on faults at 5 to 25 strain samples deformed at 300 c exhibited transitional behavior the stresses measured for our samples are in good agreement with recent experiments conducted in the deformation dia apparatus however our results indicate that steady state ductile flow is not achieved in contrast with interpretations based on the deformation dia experiments extrapolation of our data to geologic conditions suggests that serpentinites may be weak enough during localized frictional sliding to account for slab wedge decoupling inferred from geologic observations experiments conducted above the thermal stability of antigorite at 700 c and 1 5 gpa show evidence for distributed deformation this observation does not support the hypothesis that intermediate depth earthquakes result directly from dehydration embrittlement in addition samples exhibit velocity strengthening from 300 to 700 c a frictional behavior that inhibits earthquake nucleation

superplasticity in hydrous melt bearing dunite implications for shear localization in earth s upper mantle abstract deformation experiments on hydrous melt bearing dunite olivine 4 vol orthopyroxene 4 vol clinopyroxene with less than 2 5 vol of the melt phase were conducted at pressures of 1 3 5 7 gpa and temperatures of 1270 1490 k in order to explore the effect of intergranular fluids on the plastic flow of olivine in earth s upper mantle the strain rate was proportional to steady state creep strength to the 2 1 power and the creep strength markedly increased with increase in grain size developments of the crystallographic preferred orientation of olivine and flattening of olivine grains were hardly observed even after 33 55 shortening of the samples these observations show that grain boundary sliding gbs dominated the deformation of olivine i e superplasticity the creep strength of hydrous melt bearing dunite was 2 5 times lower than that of melt free dunite the dependence of creep rate on melt fraction is known to be expressed empirically as e e 0 exp where is a constant and is the melt fraction the experimentally obtained value of was in the range of 150 230 corresponding to 5 7 times the reported values for the olivine basalt system at 0 3 gpa i e creep strength of dunite was efficiently reduced by the hydrous melt superplasticity is the dominant creep mechanism of olivine in fluid bearing fine grained peridotites under low temperature and high stress conditions i e peridotite shear zones in the upper mantle superplasticity induced by geological fluids would play an important role in the shear localization and thus initiation of subduction in the upper mantle

viscoplastic self consistent and equilibrium based modeling of olivine lattice preferred orientations implications for the upper mantle seismic anisotropy anisotropy of upper mantle physical properties results from lattice preferred orientation lpo of upper mantle minerals in particular olivine we use an anisotropic viscoplastic self consistent vpsc and an equilibrium based model to simulate the development of olivine lpo and hence of seismic anisotropy during deformation comparison of model predictions with olivine lpo of naturally and experimentally deformed peridotites shows that the best fit is obtained for vpsc models with relaxed strain compatibility slight differences between modeled and measured lpo may be ascribed to activation of dynamic recrystallization during experimental and natural deformation in simple shear for instance experimental results suggest that dynamic re crystallization results in further reorientation of the lpo leading to parallelism between the main 010 100 slip system and the macroscopic shear thus modeled simple shear lpos are slightly misoriented relative to lpos measured in natural and experimentally sheared peridotites this misorientation is higher for equilibrium based models yet seismic properties calculated using lpo simulated using either anisotropic vpsc or equilibrium based models are similar to those of naturally deformed peridotites errors in the prediction of the polarization direction of the fast s wave and of the fast propagation direction for p waves are usually 15 moreover overestimation of lpo intensities in equilibrium based and vpsc simulations at high strains does not affect seismic anisotropy estimates because these latter are weakly dependent on the lpo intensity once a distinct lpo pattern has been developed thus both methods yield good predictions of development of upper mantle seismic anisotropy in response to plastic flow two notes of caution have nevertheless to be observed in using these results 1 the dilution effect of other upper mantle mineral phases in particular enstatite has to be taken into account in quantitative predictions of upper mantle seismic anisotropy and 2 lpo patterns from a few naturally deformed peridotites cannot be reproduced in simulations these abnormal lpos represent a small percent of the measured natural lpos but the present sampling may not be representative of their abundance in the earth s upper mantle

relationships between the microstructural evolution and the rheology of talc at elevated pressures and temperatures abstract we conducted triaxial compression experiments to investigate the effects of pressure temperature and strain rate on the rheology of talc rocks the tests were carried out at t from 25 to 860 c p from 0 1 to 300 mpa and e from 0 3 to 30 10 5 s 1 in addition to being very weak there are other important differences between the mechanical behavior of talc and that of most other silicates deformation at all temperatures up to dehydration was accommodated by a combination of crystal plasticity frictional sliding and cataclasis a transition from localized to distributed deformation occurred at p 300 mpa and t 400 c but this transition is ill defined as both distributed and localized deformation were observed at p 300 mpa and t 600 c unlike most silicates both the coefficient of internal friction and the coefficient of sliding friction are very low and nearly equal to each other temperature enhances plastic deformation kinking and inter and intra granular microcracks are observed parallel to the 001 planes at all conditions voids are created by delamination along 001 planes at kink band boundaries full crystal plasticity was not achieved under any of the conditions tested i e the von mises criterion was never satisfied however the strength of the aggregates remained much less than the confining pressure these unusual properties are likely a manifestation of the pronounced mechanical anisotropy of talc at the grain scale at t 750 c dehydration is observed but only along shear zones these results suggest that feedbacks between deformation and reaction kinetics could control fluid flow in fault zones the presence of talc can promote significant weakening and strain localization in the oceanic lithosphere the subducting plate and the overlying mantle wedge

low temperature high stress deformation of olivine under water saturated conditions abstract low temperature high stress rheology of olivine was investigated under water saturated conditions by large strain shear deformation experiments using the griggs type apparatus samples at t 1273 1373 k and p 2 gpa under water saturated conditions deformed under a significantly lower applied stress than expected from the power law relation and the strain rate increases more rapidly with stress these observations suggest that an exponential flow law operates under the present experimental conditions in which the stress dependence of activation enthalpy is characterized by the peierls stress the peierls stress is determined between 1 6 and 2 9 gpa depending on the choice of the form of stress dependence of the activation enthalpy these values are significantly smaller than the peierls stress under dry conditions 9 1 gpa we suggest that water hydrogen incorporated in the olivine lattices decreases the peierls stress and hence the rate of deformation in the exponential creep is significantly enhanced by the presence of water the reduction of the peierls stress by water results in the highly anisotropic weakening effect of water that causes the fabric transitions in olivine the transition between the power law and the exponential creep occurs at relatively low stress 100 mpa at a geologically relevant strain rate of 10 15 s 1 when temperature is lower than 1273 k and under water rich conditions hence the exponential creep might be dominated in olivine rich rocks at low temperature and high stress under water rich conditions and such regions could be widespread in the subduction zone upper mantle where the cold lithospheric plates sink into the mantle

phase transformation and nanometric flow cause extreme weakening during fault slip faults weaken during earthquakes laboratory simulations of earthquake rupture show that the nanometric scale fault gouge created during slip is inherently weak and flows by grain boundary sliding providing a mechanism to weaken faults

scaling of the shear rupture process from nucleation to dynamic propagation implications of geometric irregularity of the rupturing surfaces a series of systematic high resolution laboratory experiments have been performed on the nucleation of propagating slip failure on preexisting faults having different surface roughnesses to demonstrate how the size scale and duration of shear rupture nucleation are affected by geometric irregularity of the rupturing surfaces on the basis of the experimental results it has been discussed theoretically how consistently scale dependent physical quantities inherent in shear rupture are scaled the experiments led to conclusive results that the nucleation process consists of two phases phase i an initial quasi static phase and phase ii a subsequent accelerating phase and that the nucleation process is greatly affected by geometric irregularities on the rupturing surfaces in phase i the rupture grows at a slow steady speed which is independent of the rupture growth length l in contrast during phase ii the rupture develops at accelerating speeds v which increase with an increase in l obeying a power law v vs l c n where vs is the shear wave velocity c is the characteristic length representing the geometric irregularity of the fault surfaces and and n are constants 8 87 10 29 and n 7 31 scale dependency of scale dependent physical quantities including the nucleation zone size and its duration is commonly ascribed to scale dependency of the slip dependent constitutive law parameter dc which is in turn governed by c it has been discussed that a unified comprehension can be provided for shear rupture of any size scale if the constitutive law for shear rupture is formulated as a slip dependent law

kick and cook experiments on peridotite simulating coseismic deformation and post seismic creep terra nova 24 62 69 2012 abstract experiments comprising deformation at 600 c and annealing at 700 1000 c were performed on natural peridotite to examine the microfabric evolution at conditions corresponding to those prevailing just below the seismogenic zone in suboceanic upper mantle the found olivine microstructures indicate that deformation occurs by low temperature plasticity at low and high annealing temperatures zones of high strain are replaced by subgrains and recrystallized grains respectively the microstructures after annealing at 1000 c resemble core and mantle structures from shear zone peridotites often interpreted as imprint of steady state dislocation creep our study shows that such structures can form by a sequence of low temperature plasticity at high stress and subsequent recrystallization at low stress as corresponding to coseismic deformation and post seismic creep in this case palaeopiezometers are not applicable and a local cpo in aggregates of new grains does not necessarily indicate the activated glide system

origin of pseudotachylites in slow creep experiments we conducted a series of experiments on granitoid cataclasites under mid crustal conditions pc 500 mpa t 300 c and slow displacement rates 10 8 ms 1 d 10 7 ms 1 i e within an order of magnitude of plate tectonic velocities the samples reach high peak strength 1250 1375 mpa then weaken slightly by 40 mpa and continue to deform without any abrupt stress drops up to a of 4 microstructural observations show that at peak strength nanocrystalline mean grain size 34 nm smallest crystalline particles 8 nm to partly amorphous material develops weakening occurs once this material is abundant enough to form through going anastomozing slip zones where the strain partitions during higher strain deformation deformation localizes further and 1 to 2 multiply connected layers of fragment loaded amorphous material develop this material shares many similarities with naturally occurring pseudotachylites however was formed at velocities up to 8 orders of magnitude slower than those calculated for earthquakes 1 ms 1 we conclude that the use of pseudotachylites as indicators for paleoseismic events should be reconsidered

development of a type olivine fabric in water rich deep upper mantle abstract water controls the activity of slip systems in olivine resulting in various types of olivine crystallographic preferred orientation i e fabric in mantle rocks the a type olivine fabric is the most commonly observed olivine fabric in natural peridotites development of a type olivine fabric developed by the 010 100 slip system is known to be limited to the water poor conditions of the shallow upper mantle 2130 ppm h si in olivine while b type fabric by the 010 001 slip system was observed under moderately wet conditions 750 2130 ppm h si developments of c type by the 100 001 slip system fabric was limited to water poor conditions v sh v sv the ratio of horizontally and vertically polarized shear waves with depth in the global one dimensional models is well explained by the olivine fabrics developed in the horizontal flow of a water poor mantle only a type olivine fabric can explain the vertical mantle flow which associates the seismic anisotropy of v sh v sv 200 km depth a strong anomaly of v sh v sv

kinetics and mechanism of antigorite dehydration implications for subduction zone seismicity properties of serpentine minerals are thought to influence the occurrence and location of intermediate depth seismicity in subduction zones which is often characterized by two dipping planes separated by 30 km defining a double seismic zone the seismicity of the lower plane is believed to be provoked by the dehydration of serpentine since the experimentally determined stability limit for antigorite matches hypocenter locations this requires that the fluid produced by dehydration is released much faster than the typical time scale of ductile deformation mechanisms here we measured the kinetics of antigorite dehydration in situ at high pressure and high temperature by time resolved synchrotron x ray diffraction in a closed system antigorite dehydrates in two steps during step 1 it partially breaks down into olivine and a hydrous phyllosilicate closely related to the 10 a phase the modal abundance of the intermediate assemblage is described by 66 wt antigorite 19 wt olivine 12 wt 10 a phase during step 2 at higher temperature the remaining antigorite and the 10 a phase fully dehydrate from the analysis of reaction progress data we determined that the major release of aqueous fluid occurs during step 2 at a fast rate of 10 4 mfluid 3 mrock 3 s 1 this exceeds by orders of magnitude the typical time scale of deformation by ductile mechanisms of any mineral or rock in the subducting slab or in the overlying mantle wedge these results suggest that the fast dehydration of antigorite may well trigger the seismicity of the lower plane of the double seismic zone

physical modeling of the formation and evolution of seismically active fault zones acoustic emission ae in rocks is studied as a model of natural seismicity a special technique for rock loading has been used to help study the processes that control the development of ae during brittle deformation this technique allows us to extend to hours fault growth which would normally occur very rapidly in this way the period of most intense interaction of acoustic events can be studied in detail characteristics of the acoustic regime ar include the gutenberg richter b value spatial distribution of hypocenters with characteristic fractal correlation dimension d hurst exponent h and crack concentration parameter pc the fractal structure of ar changes with the onset of the drop in differential stress during sample deformation the change results from the active interaction of microcracks this transition of the spatial distribution of ae hypocenters is accompanied by a corresponding change in the temporal correlation of events and in the distribution of event amplitudes as signified by a decrease of b value the characteristic structure that develops in the low energy background ae is similar to the sequence of the strongest microfracture events when the ar fractal structure develops the variations of d and b are synchronous and d 3b this relation which occurs once the fractal structure is formed only holds for average values of d and b time variations of d and b are anticorrelated the degree of temporal correlation of ar has time variations that are similar to d and b variations the observed variations in laboratory ae experiments are compared with natural seismicity parameters the close correspondence between laboratory scale observations and naturally occurring seismicity suggests a possible new approach for understanding the evolution of complex seismicity patterns in nature

a silent slip event on the deeper cascadia subduction interface continuous global positioning system sites in southwestern british columbia canada and northwestern washington state usa have been moving landward as a result of the locked state of the cascadia subduction fault offshore in the summer of 1999 a cluster of seven sites briefly reversed their direction of motion no seismicity was associated with this event the sudden displacements are best explained by 2 centimeters of aseismic slip over a 50 kilometer by 300 kilometer area on the subduction interface downdip from the seismogenic zone a rupture equivalent to an earthquake of moment magnitude 6 7 this provides evidence that slip of the hotter plastic part of the subduction interface and hence stress loading of the megathrust earthquake zone can occur in discrete pulses