observing earthquakes triggered in the near field by dynamic deformations we examine the hypothesis that dynamic deformations associated with seismic waves trigger earthquakes in many tectonic environments our analysis focuses on seismicity at close range within the aftershock zone complementing published studies of long range triggering our results suggest that dynamic triggering is not confined to remote distances or to geothermal and volcanic regions long unilaterally propagating ruptures may focus radiated dynamic deformations in the propagation direction therefore we expect seismicity triggered dynamically by a directive rupture to occur asymmetrically with a majority of triggered earthquakes in the direction of rupture propagation bilaterally propagating ruptures also may be directive and we propose simple criteria for assessing their directivity we compare the inferred rupture direction and observed seismicity rate change following 15 earthquakes m 5 7 to m 8 1 that occurred in california and idaho in the united states the gulf of aqaba syria guatemala china new guinea turkey japan mexico and antarctica nine of these mainshocks had clearly directive unilateral ruptures of these nine seven apparently induced an asymmetric increase in seismicity rate that correlates with the rupture direction the two exceptions include an earthquake preceded by a comparable magnitude event on a conjugate fault and another for which data limitations prohibited conclusive results similar but weaker correlations were found for the bilaterally rupturing earthquakes we studied although the static stress change also may trigger seismicity it and the seismicity it triggers are expected to be similarly asymmetric only if the final slip is skewed toward the rupture terminus for several of the directive earthquakes we suggest that the seismicity rate change correlates better with the dynamic stress field than the static stress change manuscript received 1 march 2002

a newly discovered skull of the temnospondyl amphibian dendrerpeton acadianum owen a virtually complete three dimensional skull of a subadult of the temnospondyl amphibian dendrerpeton acadianum provides new information on the structure of the palate and lower jaw the left stapes appears to lie in a natural position on the quadrate ramus of the pterygoid the proportionately large otic ossicle probably precluded it from having acted as a transmitter of airborne vibrations from a tympanum to the inner ear the use of the term otic notch should be restricted to amphibians or reptiles in which the embayment of the posterior margin of the cheek is accompanied by the presence of a slender rod like stapes that could have functioned as part of an impedance matching system in those species in which a notch is present but the stapes is a massive supporting element the term squamosal embayment should be used rather than otic notch the squamosal embayment in early tetrapods may have been inherited directly from their fish ancestors and had no functional relationship to hearing

erosional feedbacks and the oscillation of ice masses 1 an analytical solution to the development of a glacier or ice sheet in a region where the underlying rock rises steadily shows that the ice mass oscillates in size independefntly of changes in the climate in response to a feedback in which ice thickness is linked to topography by ice induced erosion the rate of vertical movement of the underlying rock uplift and the parameters governing ice erosion ablation and precipitation determine the period of oscillation two dimensional numerical simulations show that the tectonic uplift rate is the most important control on the period of oscillation and that the feedback period is likely to be between 10 and 100 kyr an examination of sediment data from the deep sea drilling project shows evidence of ice mass oscillations in the chugach st elias mountains and in the himalayas with periods consistent with those produced by the proposed mechanism localized changes in glacial extent may not therefore always be the consequence of climate change and care must be taken in interpreting evidence of glacial advance and retreat

effects of variable thermal conductivity on the mineralogy of subducting slabs and implications for mechanisms of deep earthquakes abstract thermal and thermo kinetic models of subduction zones have been based in general on a constant value of thermal conductivity e g 3 138 w m k for solving the conservation of energy equation in order to make such models more realistic we have used variable thermal conductivities k that depend on pressure temperature and mineralogy olivine wadsleyite or ringwoodite as determined recently by xu et al xu y shankland t j linhardt s rubie d c langenhorst f klasinski k 2004 thermal diffusivity and conductivity of olivine wadsleyite and ringwoodite to 20 gpa and 1373 k phys earth planet inter 143 144 321 336 we have used these variable conductivities to model a variety of slabs with thermal parameters vertical subduction rate age of lithosphere at the trench ranging between 3500 and 17 000 km our models show that when radiative transfer is included k could plausibly be increased by as much as 2 w m k relative to a standard constant lithospheric value 3 138 w m k stein c a stein s 1992 a model for the global variation in oceanic depth and heat flow with lithospheric age nature 359 123 129 causing temperatures in the interiors of the slabs to increase by 50 100 c in turn the volume of metastable olivine in the slabs is reduced by as much as 20 30 with the maximum depth of olivine metastability decreasing by 30 50 km in general the maximum depths of olivine metastability are 160 230 km less than the depths of the deepest seismicity even in the extreme case of tonga metastable olivine falls short of the deepest earthquakes by 140 km in our model even taking the various uncertainties into account these results indicate that deep focus earthquakes occur in regions already transformed to wadsleyite or ringwoodite and must therefore be caused by a mechanism other than transformational faulting

source characteristics of large deep earthquakes constraint on the faulting mechanism at great depths 1 source parameters for eight large deep earthquakes are inferred from broadband data of the global seismograph network we completed an unprecedented data set composed of 14 largest deep earthquakes mw ranging from 7 0 to 8 3 from six different subduction zones studied with identical source parameter inversion methods the study confirms earlier findings that deep earthquake source properties vary with the temperature of the subducting plate events in cold slabs show high aftershock activity high rupture velocity and high seismic efficiency in contrast earthquakes in warm slabs generally have low aftershock productivity low rupture velocity and low seismic efficiency however the 23 june 1991 argentina event has source properties that are rather characteristic for earthquakes in cold slabs and that differ from those of other large south american events we suggest that two types of deep earthquakes occur in south america and we interpret these results in the context of the shear instability model for the mechanism of deep earthquakes in this model shear failure occurs through a thermal runaway mechanism within a shear zone at great depth and low seismic efficiency might indicate significant energy dissipation due to heating and possible melt production the 1991 argentina event located in a pocket of deep seismicity within the slab core may represent a shear instability with little melting as would be the case for large deep earthquakes in cold subduction zones whereas the other known south american large deep events such as 1963 peru 1970 colombia and 1994 bolivia are located in relatively aseismic regions and may involve significant melting within warmer regions of the slab

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

global large deep focus earthquakes source process and cascading failure of shear instability as a unified physical mechanism abstract we apply a multiple source inversion method to systematically study the source processes of 25 large deep focus depth 400 km earthquakes with mw 7 0 from 1994 to 2012 based on waveform modeling of p pp sh and ssh wave data the earthquakes are classified into three categories based on spatial distributions and focal mechanisms of the inferred sub events 1 category one with non planar distribution and variable focal mechanisms of sub events represented by the 1994 mw 8 2 bolivia earthquake and the 2013 mw 8 3 okhotsk earthquake 2 category two with planar distribution but focal mechanisms inconsistent with the plane including eighteen earthquakes and 3 category three with planar distribution and focal mechanisms consistent with the plane including six earthquakes we discuss possible physical mechanisms for earthquakes in each category in the context of plane rupture transformational faulting and shear thermal instability we suggest that the inferred source processes of large deep focus earthquakes can be best interpreted by cascading failure of shear thermal instabilities in pre existing weak zones with the perturbation of stress generated by a shear instability triggering another and focal mechanisms of the sub events controlled by orientations of the pre existing weak zones the proposed mechanism can also explain the observed great variability of focal mechanisms the presence of large values of clvd compensated linear vector dipole and the super shear rupture of deep focus earthquakes in the previous studies in addition our studies suggest existence of relationships of seismic moment source duration 3 and moment source dimension 3 in large deep focus earthquakes

a cascading failure during the 24 may 2013 great okhotsk deep earthquake on 24 may 2013 the largest ever recorded deep earthquake occurred beneath sea of okhotsk a multiple point source inversion procedure is applied to constrain source process of this earthquake based on waveform modeling of both direct p and sh waves and near surface reflected pp and ssh waves our results indicate that the earthquake consists of six major subevents separated in space and time encompassing a horizontal dimension of 64 4 km along n160 e and a downward depth extension of 35 4 km the geographic distribution and focal mechanisms of the inferred subevents and foreshock aftershock locations do not fit into plane rupture we suggest that the earthquake can be best explained by a cascading failure of shear instability within preexisting weak zones in the region with the perturbation of stress generated by a shear instability triggering another

global ubiquity of dynamic earthquake triggering surface waves that were generated by 12 out of 15 earthquakes with magnitudes greater than 7 since 1990 led to a global increase in the number of small earthquakes this suggests that dynamic triggering of earthquakes is common and is independent of the tectonic environment earthquakes can be triggered by local changes in the stress field static triggering1 2 3 4 5 6 7 due to nearby earthquakes or by stresses caused by the passage of surface rayleigh and love waves from a remote large earthquake dynamic triggering8 9 10 11 12 13 14 15 16 17 18 however the mechanism frequency controlling factors and the global extent of dynamic triggering are yet to be fully understood because rayleigh waves involve compressional and dilatational particle motion volumetric changes as well as shearing whereas love waves only involve shearing triggering by either wave type implies fundamentally different physical mechanisms here we analyse broadband seismograms from over 500 globally distributed stations and use an automated approach to systematically identify small triggered earthquakes the low amplitude signals of such earthquakes would normally be masked by high amplitude surface waves our analysis reveals that out of 15 earthquakes studied of magnitude m greater than 7 0 that occurred after 1990 12 are associated with significant increases in the detection of smaller earthquakes during the passage of both the love and rayleigh waves we conclude that dynamic triggering is a ubiquitous phenomenon that is independent of the tectonic environment of the main earthquake or the triggered event

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

body wave inversion of the 1970 and 1963 south american large deep focus earthquakes a comprehensive set of teleseismic waveforms from two south american deep focus earthquakes of the predigital era the 1970 colombia mw 8 1 and 1963 peru bolivia mw 7 7 events are inverted for source mechanism seismic moment rupture history and centroid depth the p and sh wave inversion of the colombia event confirms previous work indicating that rupture occurred on a plane that dips steeply west rupture direction paralleled the trend of the wadati benioff zone we decompose the source into subevents based on a source time function which shows two major moment release pulses separated by 20 s the first subevent is located near the initiation point at a depth of 630 km the main moment release was located 70 km to the southeast and 20 km shallower rupture subsequently propagated farther southeast the source time function has an initial subevent accounting for 30 of the moment release of the entire event whereas the long period centroid moment tensor cmt analysis russakoff et al 1997 has the initial subevent yielding 50 the high angle nodal plane rotated 15 clockwise during the rupture explaining the large compensated linear vector dipole clvd component inferred from cmt solutions individual subevents have large clvd and compressive isotropic components a full moment tensor inversion of the colombia and 1994 bolivia events suggests that the initial subevents might contain a large non double couple ndc component for the 1963 peru bolivia event using p waves rupture propagated nnw for a distance of 70 km parallel to the high angle nodal plane and the trend of the wadati benioff zone the focal mechanism changed dramatically after the second subevent causing a very large ndc component both events together with the 1994 bolivia earthquake have a precursor separated in space and time from the main rupture and show rupture velocities varying between 3 and 4 km s between subevents with 2 0 km s on average for the entire event low seismic efficiencies and rupture velocities support a highly dissipative temperature dependent rupture mechanism for large deep focus south american earthquakes compared with events in cold subducting slabs

rare dynamic triggering of remote m 5 5 earthquakes from global catalog analysis probing the effects of a transient stress on the timing of an earthquake occurrence is necessary for understanding the remote interaction of large magnitude events global catalog data containing 35 years of m 5 5 earthquakes allow us to explore for periods of enhanced or suppressed seismic activity we consider 113 m 7 5 main shocks between 1977 and 2012 and focus on seismic activity on time scales from seconds to days following these main shocks we search for evidence of dynamic triggering of large magnitude events similar to the previously observed global increase during the first few days following the 2012 m8 6 indian ocean main shock we restrict the analysis to regions of elevated strain during the passage of surface waves using a threshold of 0 1 microstrain 3 kpa and a temporal window of 1 year we stack daily seismicity rate curves using the exclusion zone declustered m 5 5 catalog events in order to resolve deviations from the background rate our results do not indicate a significant change in activity for at least 10 days when considering the collective set of 113 main shocks and subsets at m8 0 and m8 5 thresholds the results also do not indicate immediate triggering of m 5 5 events we do find two instances of increased seismicity in the elevated strain region within 10 days these increases are subsequent to two main shocks the 1977 m8 3 and 2012 m8 6 both located in the indian ocean we conclude that a global change in m 5 5 earthquake rates following a transient stress from distant earthquakes is a rare occurrence

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

a new statistical test for static stress triggering application to the 1987 superstition hills earthquake sequence over the past several years many investigators have argued that static stress changes caused by large earthquakes influence the spatial and temporal distributions of subsequent regional seismicity with earthquakes occurring preferentially in areas of stress increase and reduced seismicity where stress decreases some workers have developed quantitative methods to test for the existence of such static stress triggering but no firm consensus has yet been reached as to the significance of these effects we have developed a new test for static stress triggering in which we compute the change in coulomb stress on the focal mechanism nodal planes of a set of events spanning the occurrence of a large earthquake we compare the statistical distributions of these stress changes for events before and after the mainshock to decide if we can reject the hypothesis that these distributions are the same such rejection would be evidence for stress triggering we have applied this test to the november 24 1987 elmore ranch superstition hills earthquake sequence and find that those post mainshock events that experienced stress increases of at least 0 01 0 03 mpa 0 1 0 3 bar or that occurred from 1 4 to 2 8 years after the mainshocks are consistent with having been triggered by mainshock generated static stress changes

analysis of peak ground motion in terms of a model of inhomogeneous faulting measurements of peak ground velocity v and acceleration a for earthquakes and mine tremors with local magnitudes ranging from 0 8 to 6 4 have been analyzed in terms of a model of inhomogeneous faulting the fault model involves the failure of a circular asperity of radius ri surrounded by a previously faulted annular region of outer radius r0 the failure of the asperity results in seismic radiation with a characteristic frequency proportional to 1 ri followed by lower frequency radiation proportional to 1 r0 as static equilibrium is regained over the larger region for r0 ri 1 both v and a are associated primarily with the failure of the asperity but also depend on the large scale source parameters specifically v r0 r 0 10 r0 ri 0 15 and a r 0 30 r0 ri 2 0 45 where is the shear wave velocity is the overall stress drop is the modulus of rigidity and is density the terms involving r0 ri correspond to the high frequency radiation associated with the failure of the asperity and the other terms indicate the peak parameters due to the broad scale readjustment observations of peak ground motion for events with seismic moments ranging from 5 1016 to 1026 dyn cm indicate that r0 ri is normally in the range of 1 10 and appears to be independent of earthquake size the inhomogeneous fault model also yields convenient expressions for the small scale displacement and stress drop of the asperity failure as well as for the level of regional stress available to cause slip

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

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

seismological constraints on the mechanism of deep earthquakes temperature dependence of deep earthquake source properties abstract seismological observations are of vital importance for understanding the mechanism of deep earthquakes most of the seismological observables for earthquakes deeper than 300 km are similar to shallow earthquakes deep earthquakes clearly represent shear failure on a planar surface as shown by their double couple mechanisms deep earthquake aftershock sequences show temporal decay rates and magnitude frequency relations b values that are similar to shallow earthquakes with the aftershocks occurring preferentially along the mainshock fault planes in addition deep earthquake rupture velocities are similar to those of shallow earthquakes however there are also some observations that are clearly distinctive relative to shallow earthquakes stress drops of deep earthquakes show a large variation but are larger on average than shallow earthquakes different deep seismic zones show very different b values in contrast to shallow earthquakes which show similar b values worldwide in addition deep earthquakes show fewer aftershocks than shallow earthquakes a variety of observations suggest that deep earthquakes are highly sensitive to the temperature of the slab both deep earthquake b values and the rate of deep earthquake aftershock occurrence are inversely correlated with the temperature of the deep slabs suggesting that these factors are temperature controlled to an extent much greater than with shallow earthquakes large deep earthquakes in warm slabs show slower rupture velocities larger stress drops and lower seismic efficiencies than similar earthquakes in cold slabs the width of deep seismic zones is also probably temperature controlled but deep earthquake rupture can propagate outside the normal limits of benioff zone seismicity simple thermal models for the tonga slab near the 1994 deep earthquake suggest that the temperature at the rupture termination point was at least 200 c warmer than the temperature that limits smaller earthquakes in the slab these observations can be used to evaluate physical models for deep earthquakes including brittle slip along fluid weakened faults transformational faulting and thermal and perhaps melt lubricated shear instabilities it is difficult to explain the large fault widths of some deep earthquakes using a fluid weakening model since hydrated materials are expected in only a narrow depth zone at the top of the slab and the fault planes do not have the expected orientations for reactivated faults the lateral extent of the largest deep earthquakes cast doubt on the transformational faulting model in which events should be confined within a narrow metastable wedge seismological studies have also failed to find evidence for the existence of metastable olivine in slabs the temperature dependence of deep earthquakes argues in favor of a temperature activated phenomenon such as thermal shear instabilities and perhaps fault zone melting

seismological evidence for metastable olivine inside a subducting slab the nature and location of the olivine spinel phase transition inside subducting slabs differ greatly from the situation in the surrounding mantle due to the different temperature distribution inside the slabs two models have been proposed for this phase transition one in which the location of the phase boundary between olivine and modified phase spinel is determined by equilibrium thermodynamics1 and the other including a metastable olivine phase which persists to a depth of 550 km ref 2 the location of the olivine spinel transition in the slab may be relevant to the generation of deep earthquakes3 5 and to the buoyancy forces driving subduction6 here we use travel time residuals from deep earthquakes recorded by the dense seismograph network in japan to investigate the configuration of the olivine spinel phase boundary inside the subducting pacific plate theoretical travel time residuals for the equilibrium model do not fit the observed residuals whereas those for the metastable model do implying the presence of metastable olivine inside the subducting slab

rupture complexity of the 1994 bolivia and 2013 sea of okhotsk deep earthquakes the physical mechanism of deep earthquakes depth 300 km remains enigmatic partly because their rupture dimensions are difficult to estimate due to their low aftershock productivity and absence of geodetic or surface rupture observations the two largest deep earthquakes the recent great 2013 sea of okhotsk earthquake m 8 3 depth 607 km and the great 1994 bolivia earthquake m 8 3 depth 637 km together provide a unique opportunity to compare their rupture patterns in detail here we extend a travel time sub event location method to perform full teleseismic p waveform inversion this new method allows us to explain the observed broadband records with a set of sub events whose model parameters are robustly constrained without smoothing we find that while the okhotsk event is mostly unilateral rupturing 90 km along strike with a velocity over 4 km s the bolivia earthquake ruptured about half this distance at a slow velocity about 1 5 km s and displayed a major change in rupture direction we explain the observed differences between the two earthquakes as resulting from two fundamentally different faulting mechanisms in slabs with different thermal states phase transformational faulting is inferred to occur inside the metastable olivine wedge within cold slab cores whereas shear melting occurs inside warm slabs once triggered

dynamic stress variations due to shear faults in a plane layered medium summary a complete set of expressions is presented for the computation of elastic dynamic stress in plane layered media we use a discrete wavenumber reflectivity method to compute the stress field radiated by arbitrary moment tensor sources the expressions derived here represent an interesting tool for both the observational and theoretical analysis of dynamic stress changes associated with earthquake phenomena dynamic stress changes associated with a strike slip fault having unilateral rupture are shown this modelling which is similar to the 1992 landers california earthquake illustrates the effects of distance directivity and depth on transient stress changes

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

seismic body wave constraint on mechanisms of intermediate depth earthquakes 1 source parameters of six large earthquakes 7 1 mw 7 7 in the depth range between 112 and 277 km from five different subduction zones are derived from teleseismic body waves recorded by global seismic networks except for the 21 october 1995 mexican event which is consistent with the model of simple rupture on a planar surface the investigated earthquakes show complex rupture processes that can be explained by irregularities in the focal region as is the case of shallow events the inferred velocity of the moment release varies between 2 5 and 4 km s corresponding to 40 90 of the shear velocity at the focus four of the six earthquakes show stress drops in the range observed for shallow events 1 10 mpa ruptures extend mainly parallel to the strike of subducting slabs with dimensions reaching up to 50 km the extent of rupture perpendicular to slab dip of generally less than 15 km is comparatively small this suggests that the rupture during large intermediate depth earthquakes propagates predominantly parallel to the strike of the subducting plate and that the seismogenic zone perpendicular to the dip direction is limited to a narrow width the relatively small rupture size perpendicular to slab dip suggests that the rupture process does not extend into regions of significantly different temperatures the spatial extent of the investigated earthquakes is consistent with the orientation of trench parallel faults that are generally observed in near trench outer rise regions suggesting that these events may result from reactivation of such preexisting faults at intermediate depths through dehydration of hydrous minerals concentrated in these zones of weakness depths of most of the earthquakes are consistent with the dehydration embrittlement of serpentine the bolivian earthquake of 23 january 1997 seems to be too deep 277 km for such an explanation

rheological structure and deformation of subducted slabs in the mantle transition zone implications for mantle circulation and deep earthquakes abstract rheological structure of subducted slabs of oceanic lithosphere in the mantle transition zone is investigated based on mineral physics observations incorporating grain size stress temperature and pressure dependence of rheology it is shown that the rheological structure of slabs depends strongly on subduction parameters through temperature that controls the grain size of spinel ringwoodite and the magnitude of forces acting on a slab we use a theoretical model of grain size evolution associated with the olivine spinel transformation plastic flow laws of olivine and spinel combined with a thermal model of subducting slab in which the effect of latent heat release is incorporated three deformation mechanisms for olivine and spinel diffusional creep power law dislocation creep and the peierls mechanism are considered due to the large variation in temperature stress and grain size a subducting slab is shown to have a complicated rheological structure which varies both laterally and with depth a cold slab in the deep transition zone is characterized by a weak fine grained spinel region surrounded by narrow but strong regions the flexural rigidity and the curvature of a slab are calculated using a new formulation in which the effects of stress dependent rheology is incorporated in a self consistent fashion although uncertainties in both the transformation kinetics and the rheology of high pressure phases are still large the general trend of dependence of slab flexural rigidity and the curvature on the subduction parameters is well constrained slabs with very low thermal parameters warm slabs are weak but slabs with large thermal parameters cold slabs are also weak due to small spinel grain size and large external force bending moment slabs with intermediate thermal parameters will have a relatively large flexural rigidity and could penetrate into the lower mantle without much deformation thus the 660 km discontinuity may work as a rheological filter for mantle convection this prediction provides a natural explanation for a paradoxical observation that significant deformation of slabs is observed exclusively in the western pacific where temperatures of the slabs are considered to be low our slab rheology models also have important implications for deep earthquakes overall rheological weakening of slabs in the deep transition zone results in high rates of deformation under relatively low temperatures providing a favorable environment for thermal runaway instability adiabatic shear instability our model predicts heterogeneous energy dissipation as a result of heterogeneous rheology energy dissipation in deep cold slabs is concentrated in high strength regions surrounding a weak fine grained spinel core the regions of high energy dissipation are prone to thermal runaway instability and are likely to be seismogenic the width of this seismogenic region is well constrained by our model and is predicted to be 40 60 km which is in excellent agreement with seismological observations other features of deep earthquakes including low seismic efficiency and low aftershock activities can also be explained by the thermal instability model

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

formation of a metastable olivine wedge in a descending slab we present a thermal model of a descending slab in which the transformation of olivine to spinel is controlled by pressure and temperature dependent reaction kinetics two different formulations of the kinetics are considered with the main discriminant being the temperature range over which olivine converts to spinel at pressures of about 15 gpa about 500 515 c and 560 650 c we use a finite element method to solve the coupled heat conduction perpendicular and parallel to the dip of the slab and kinetics equations and we include the latent heat of the phase transformation latent heat release together with heat conduction parallel to the dip of the slab reduces significantly the length of the metastable olivine wedge and results in a very thin 5 km two phase region we employ the thermal parameter sin is the velocity of the descending slab is the age of the slab and is the dip of the slab to the horizontal to interpret the results for the length of the metastable wedge for values of the thermal parameter smaller than about 4000 and 7000 km depending on the model of the kinetics no metastable olivine wedge exists the critical value of 7000 km for the thermal parameter corresponds to the kinetics model with the lowest transition temperature range the length of the metastable olivine wedge is also found to be very sensitive to the model of the kinetics and to the effects of adiabatic heating if the occurrence of deep earthquakes is related to the transformation of metastable olivine to spinel then data on earthquake depth versus thermal parameter require that the onset of the reaction takes place at temperatures of about 550 575 c in this case the slab thermal parameter must be larger than 10 000 km for the metastable olivine wedge to extend down to 660 km depth but deep earthquakes occur near 660 km depth in slabs with thermal parameter as small as about 5000 km south america for example either some deep earthquakes are unrelated to olivine metastability or our knowledge of olivine spinel reaction kinetics is incomplete

the role of hydraulic fractures and intermediate depth earthquakes in generating subduction zone magmatism the presence of magmatism and intermediate depth 70 300 km deep seismicity at subduction zones is at first sight surprising magmatism is unexpected because the subduction of cool oceanic lithosphere makes these regions the coldest in the mantle the current model for subduction zone magmatism is that water released from the subducting slab enters the relatively warm mantle wedge leading to a reduction in melting temperature and magmatism1 2 3 4 but there is a problem with this scheme because it is thought that water cannot leave the slab by porous flow to enter the wedge the occurrence of intermediate depth earthquakes is surprising because of the inhibitory effect of the very high frictional stress on faults expected from the high pressure at these depths one proposal put forward to explain intermediate depth seismicity is that high pore pressure might facilitate faulting by decreasing the friction5 6 7 the hypothesis presented here is that non percolating water provides the high pore pressure that the consequent faulting temporarily interconnects the water pores and when a sufficient vertical height of water is interconnected a hydrofracture is produced which transports the water out into the mantle wedge thereby generating subduction zone magmatism

stress transferred by the 1995 mw 6 9 kobe japan shock effect on aftershocks and future earthquake probabilities the kobe earthquake struck at the edge of the densely populated osaka kyoto corridor in southwest japan we investigate how the earthquake transferred stress to nearby faults altering their proximity to failure and thus changing earthquake probabilities we find that relative to the pre kobe seismicity kobe aftershocks were concentrated in regions of calculated coulomb stress increase and less common in regions of stress decrease we quantify this relationship by forming the spatial correlation between the seismicity rate change and the coulomb stress change the correlation is significant for stress changes greater than 0 2 1 0 bars 0 02 0 1 mpa and the nonlinear dependence of seismicity rate change on stress change is compatible with a state and rate dependent formulation for earthquake occurrence we extend this analysis to future mainshocks by resolving the stress changes on major faults within 100 km of kobe and calculating the change in probability caused by these stress changes transient effects of the stress changes are incorporated by the state dependent constitutive relation which amplifies the permanent stress changes during the aftershock period earthquake probability framed in this manner is highly time dependent much more so than is assumed in current practice because the probabilities depend on several poorly known parameters of the major faults we estimate uncertainties of the probabilities by monte carlo simulation this enables us to include uncertainties on the elapsed time since the last earthquake the repeat time and its variability and the period of aftershock decay we estimate that a calculated 3 bar 0 3 mpa stress increase on the eastern section of the arima takatsuki tectonic line attl near kyoto causes fivefold increase in the 30 year probability of a subsequent large earthquake near kyoto a 2 bar 0 2 mpa stress decrease on the western section of the attl results in a reduction in probability by a factor of 140 to 2000 the probability of a mw 6 9 earthquake within 50 km of osaka during 1997 2007 is estimated to have risen from 5 6 before the kobe earthquake to 7 11 afterward during 1997 2027 it is estimated to have risen from 14 16 before kobe to 16 22

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

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

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