mechanical interactions between rigid particles in a deforming ductile matrix analogue experiments in simple shear flow abstract the mechanical interaction between two or more particles is investigated in newtonian simple shear flow the experimental models allow observation of both the particle rotation and the deformation pattern around the particles the finite and instantaneous strain patterns around rigid particles are strongly heterogeneous and asymmetric with high finite strain zones aligned to the direction of maximum finite stretch these correspond to local flow with low vorticity number heterogeneous strain patterns around rigid particles spread over a distance of 1 2 times the particle length and this distance increases with increasing bulk strain where rigid particles are more concentrated these patterns coalesce and the overall pattern is then strongly controlled by the particles and cannot be simply related to the external boundary conditions in rocks the characteristic asymmetric strain pattern around rigid isolated particles is a reliable shear criterion which becomes unreliable with high concentrations of rigid particles particle rotation is significantly disturbed when the distance between adjacent particles of equal size is shorter than their length that is only in very concentrated suspensions of rigid particles in a composite shape fabric the development of the sub fabric corresponding to the smaller minerals will be more disturbed

the 2017 mw 8 2 chiapas mexico earthquake energetic slab detachment on 8 september 2017 a great m w 8 2 normal faulting earthquake ruptured within the subducting cocos plate 70 km landward from the middle american trench beneath the tehuantepec gap iterative inversion and modeling of teleseismic and tsunami data and prediction of gps displacements indicate that the steeply dipping rupture extended 180 km to the northwest along strike toward the oaxaca coast and from 30 to 70 km in depth with peak slip of 13 m the rupture likely broke through the entire lithosphere of the young subducted slab in response to downdip slab pull the plate boundary region between the trench and the fault intersection with the megathrust appears to be frictionally coupled influencing location of the detachment comparisons of the broadband body wave magnitude m b and moment scaled radiated energy e r m 0 establish that intraslab earthquakes tend to be more energetic than interplate events accounting for strong ground shaking observed for the 2017 event

the rigid grain net rgn an alternative method for estimating mean kinematic vorticity number wm the use of porphyroclasts rotating in a flowing matrix to estimate mean kinematic vorticity number wm is important for quantifying the relative contributions of pure and simple shear in penetratively deformed rocks the most common methods broadly grouped into those that use tailed and tailless porphyroclasts have been applied to many different tectonic settings however attempts have not been made to unify the various methods here we propose the rigid grain net rgn as an alternative graphical method for estimating wm the rgn contains hyperbolas that are the mathematical equivalents to the hyperbolic net used for the porphyroclast hyperbolic distribution phd method we use the rgn to unify the most commonly used wm plots by comparing the distribution of theoretical and natural tailless porphyroclasts within a flowing matrix test samples from the south tibetan detachment tibet yield indistinguishable results when the rgn is compared with existing methods because of its ease of use ability for comparing natural data sets to theoretical curves potential to standardize future investigations and ability to limit ambiguity in estimating wm the rgn makes an important new contribution that advances the current methods for quantifying flow in shear zones 2006 elsevier ltd all rights reserved

deep earthquakes in subducting slabs hosted in highly anisotropic rock fabric analysis of deep subduction zone earthquakes those at depths greater than 60 km reveals the physical and chemical properties of a descending oceanic lithosphere at mantle depths over the past five decades it has been observed that a large fraction of deep earthquakes has non double couple non dc seismic radiation patterns in contrast shallow earthquakes tend to have dc radiation patterns due to mechanisms of shear faulting these observations have been used to argue that deep earthquakes rupture differently from shallow earthquakes here we show that the observed global distribution of non dc deep earthquakes could be caused by shear faulting mechanisms but in a highly anisotropic laminated rock fabric that surrounds the deep earthquakes within subducted slabs for intermediate depth earthquakes 60 300 km we found a large shear wave anisotropy of 25 possibly caused by laminated fabric or aligned melt pockets oriented parallel to the slab interface which provides new supporting evidence for the metamorphic dehydration reactions in slabs however at deep focus depths 300 km the putative metastable phase change mechanism alone cannot explain the seismic anisotropy instead our results and those from recent experiments suggest materials such as magnesite or perhaps carbonatite melt may play a role in generating deep focus earthquakes radiation patterns for deep earthquakes could be a result of shear faulting mechanisms similar to those for shallow earthquakes but in highly anisotropic rock fabric suggest seismic analyses

inversion of complex body waves we have developed a method that inverts seismic body waves to determine the mechanism and rupture pattern of earthquakes the rupture pattern is represented as a sequence of subevents distributed on the fault plane this method is an extension of our earlier method in which the subevent mechanisms were fixed in the new method the subevent mechanisms are determined from the data and are allowed to vary during the sequence when subevent mechanisms are allowed to vary however the inversion often becomes unstable because of the complex trade offs between the mechanism the timing and the location of the subevents many different subevent sequences can explain the same data equally well and it is important to determine the range of allowable solutions some constraints must be imposed on the solution to stabilize the inversion we have developed a procedure to explore the range of allowable solutions and appropriate constraints in this procedure a network of grid points is constructed on the i plane where and i are respectively the onset time and the distance from the epicenter of a subevent the best fit subevent is determined at all grid points then the correlation is computed between the synthetic waveform for each subevent and the observed waveform the correlation as a function of and i and the best fit mechanisms computed at each i grid point depict the character of allowable solutions and facilitate a decision on the appropriate constraints to be imposed on the solution the method is illustrated using the data for the 1976 guatemala earthquake

the limitations of three dimensional kinematic vorticity analysis abstract the kinematic vorticity number wk can be calculated for three dimensional as well as two dimensional geologic deformations for steady state deformations wk can be correlated to and analyzed in terms of finite strains the analysis shows that assumptions commonly made for two dimensional deformations are not applicable to three dimensional deformations a single wk describes an infinite number of three dimensional deformations further even knowledge of flow apophyses orientation instantaneous stretching axes orientation and or wk are not sufficient to describe deformation three dimensional deformations also require knowledge of the deformation type or boundary conditions of deformation e g transpression hence in addition to being difficult to estimate the value of knowing wk for three dimensional deformations is greatly reduced compared with plane strain the most useful methods of determining wk from naturally deformed rocks are presented

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

control of detachment geometry on lateral variations in exhumation rates in the himalaya insights from low temperature thermochronology and numerical modeling 1 the himalayan range is commonly presented as largely laterally uniform from west to east however geological structures topography precipitation rate convergence rates and low temperature thermochronological ages all vary significantly along strike here we focus on the interpretation of thermochronological data sets in terms of along strike variations in geometry and kinematics of the main crustal detachment underlying the himalaya the main himalayan thrust mht we report new apatite fission track aft ages collected along north south transects in western and eastern central nepal at the latitudes of the annapurna and langtang massifs respectively aft ages are consistently young 3 ma along both n s transects in the high relief zone of the higher himalaya and increase 4 to 6 ma toward the south in the lesser himalaya we compare our new data to published low temperature thermochronological data sets for nepal and the bhutan himalaya we use the full data set to perform both forward and inverse thermal kinematic modeling with a modified version of the pecube code in order to constrain potential along strike variations in the kinematics of the himalayan range our results show that lateral variations in the geometry of the mht in particular the presence or absence of a major crustal scale ramp strongly control the kinematics and exhumation history of the orogen

geometric controls on flat slab seismicity abstract the relationship between intraslab seismicity and the dynamics of subduction is a subject of ongoing debate uncertainty surrounds 1 the extent to which the stress regime associated with slab earthquakes reflects the driving resisting forces of subduction or more localised processes such as metamorphic or thermo elastic volume change and 2 the relative contribution of uniform stretching shortening and flexural bending buckling deformation modes in slabs because variations in slab curvature are very clear in subduction zones with flat slab segments such settings allow for assessment of the relationship between slab geometry bending and seismicity using a numerical model that reproduces published flat slab geometries we show how bending rates which are dominated by the advective component relate to downdip curvature gradients based on published slab geometries we show that the patterns of seismicity in the peruvian and mexican flat slabs vary systematically with slab curvature gradient seismicity is restricted to regions colder than about 600 c this means that only the upper half of flat slab bending zones are expressed seismically providing the appearance of a uniform extensional regime

large intermediate depth earthquakes and the subduction process this study provides an overview of intermediate depth earthquake phenomena placing emphasis on the larger tectonically significant events and exploring the relation of intermediate depth earthquakes to shallower seismicity especially we examine whether intermediate depth events reflect the state of interplate coupling at subduction zones and whether this activity exhibits temporal changes associated with the occurrence of large underthrusting earthquakes historic record of large intraplate earthquakes m b 7 0 in this century shows that the new hebrides and tonga subduction zones have the largest number of large intraplate events regions associated with bends in the subducted lithosphere also have many large events e g altiplano and new ireland we compiled a catalog of focal mechanisms for events that occurred between 1960 and 1984 with m 6 and depth between 40 and 200 km the final catalog includes 335 events with 47 new focal mechanisms and is probably complete for earthquakes with m b 6 5 for events with m 6 5 nearly 48 of the events had no aftershocks and only 15 of the events had more than five aftershocks within one week of the mainshock events with more than ten aftershocks are located in regions associated with bends in the subducted slab focal mechanism solutions for intermediate depth earthquakes with m 6 8 can be grouped into four categories 1 normal fault events 44 and 2 reverse fault events 33 both with a strike nearly parallel to the trench axis 3 normal or reverse fault events with a strike significantly oblique to the trench axis 10 and 4 tear faulting events 13 the focal mechanisms of type 1 events occur mainly along strongly or moderately coupled subduction zones where a down dip extensional stress prevails in a gently dipping plate in contrast along decoupled subduction zones great normal fault earthquakes occur at shallow depths e g the 1977 sumbawa earthquake in indonesia type 2 events with strike subparallel to the subduction zone and most of them with a near vertical tension axis occur mainly in regions that have partially coupled or uncoupled subduction zones and the observed continuous seismicity is deeper than 300 km the increased dip of the downgoing slab associated with weakly coupled subduction zones and the weight of the slab may be responsible for the near vertical tensional stress at intermediate depth and consequently the change in focal mechanism from type 1 to type 2 events events of type 3 occur where the trench axis bends sharply causing horizontal parallel to the trench strike extensional or compressional intraplate stress type 4 are hinge faulting events for strongly coupled zones we observed temporal changes of intermediate depth earthquake activity associated with the occurrence of a large underthrusting event after the occurrence of a large underthrusting event the stress axis orientation of intermediate depth earthquakes changes from down dip tensional to down dip compressional e g 1960 chile 1974 peru 1982 tonga and 1952 kamchatka earthquakes or the number of large intermediate events decreases for a few years e g 1964 alaska and 1985 valparaiso earthquakes we conclude that even though the stress changes induced by slab pull and slab distortion control the general pattern of intermediate depth seismicity spatial and temporal variations of the intraplate stress associated with interplate coupling are important in controlling the global occurrence of large intermediate depth events

bend faulting at the edge of a flat slab the 2017 mw7 1 puebla morelos mexico earthquake we present results of a slip model from joint inversion of strong motion and static gps data for the mw7 1 puebla morelos earthquake we find that the earthquake nucleates at the bottom of the oceanic crust or within the oceanic mantle with most of the moment release occurring within the oceanic mantle given its location at the edge of the flat slab the earthquake is likely the result of bending stresses occurring at the transition from flat slab subduction to steeply dipping subduction while the event strikes obliquely to the slab we find a good agreement between the seafloor fabric offshore the source region and the strike of the earthquake we argue that the event likely reactivated a fault first created during seafloor formation we hypothesize that large bending related events at the edge of the flat slab are more likely in areas of low misalignment between the seafloor fabric and the slab strike where reactivation of preexisting structures is favored this hypothesis predicts decreased likelihood of bending related events northwest of the 2017 source region but also suggests that they should be more likely southeast of the 2017 source region

deformation temperatures and flow vorticities near the base of the greater himalayan series sutlej valley and shimla klippe nw india abstract we report new deformation temperature and flow vorticity data from the base of the greater himalayan series ghs exposed in the sutlej valley and shimla klippe of nw india we focus on three groups of transects across the hanging wall of the main central thrust mct in order of relative foreland hinterland positions they are the shimla klippe western and eastern sutlej transects deformation temperatures indicated by quartz c axis fabric opening angles increase both from foreland to hinterland at a given structural distance above the mct and up structural section from the mct within individual transects deformation temperatures in the immediate hanging wall to the mct are estimated at 510 535 535 550 and 610 c on the shimla western sutlej and eastern sutlej transects respectively the steepest inferred field gradients in deformation temperatures are recorded adjacent to the mct and progressively decrease up structural section following a power law relationship comparison with temperature estimates based on multi mineral phase equilibria data suggests that penetrative shearing occurred at close to peak metamorphic conditions vorticity analyses indicate that shearing along the base of the ghs occurred under sub simple shear conditions wm values of 0 9 1 0 with a minor component of pure shear

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

himalayan leucogranites and migmatites nature timing and duration of anatexis widespread anatexis was a regional response to the evolution of the himalayan tibetan orogen that occurred some 30 ma after collision between asia and india this paper reviews the nature timing duration and conditions of anatexis and leucogranite formation in the greater himalayan sequence ghs and compares them to contemporaneous granites in the karakoram mountains himalayan leucogranites and associated migmatites generally share a number of features along the length of the mountain front such as similar timing and duration of magmatism common source rocks and clockwise p t paths despite commonalities most papers emphasize deviations from this general pattern indicating a fine tuned local response to the dominant evolution there are significant differences in p t xh2o conditions during anatexis and timing in relation to regional decompression further to that some regions underwent a second event recording melting at low pressures zircon and monazite ages of anatectic rocks range between c 25 and 15 ma suggesting prolonged crustal melting typically a single sample may have ages covering most of this 10 ma period suggesting recycling of accessory phases from metamorphic rocks and early formed magmas recent studies linking monazite and zircon ages with their composition have determined the timing of prograde melting and retrograde melt crystallization thus constraining the duration of the anatectic cycle in some areas this cycle becomes younger down section towards the leading front of the himalayas whereas the opposite is true in other areas the relationship between granites and movement on the south tibetan detachment std reveals that fault motion took place at different times and over different durations requiring complex internal strain distribution along the himalayas the nature and fate of magmas in the ghs contrast with those in the karakoram mountains ghs leucogranites have a strong crustal isotopic signature and migration is controlled by low angle foliation leading to diffuse injection complexes concentrated below the std in contrast the steep attitude of the karakoram shear zone focused magma transfer feeding the large karakoram baltoro batholith anatexis in the karakoram involved a cretaceous calcalkaline batholith that provided leucogranites with more juvenile isotopic signatures the impact of melting on the evolution of the himalayas has been widely debated melting has been used to explain subsequent decompression or conversely decompression has been used to explain melting weakening due to melting has also been used to support channel flow models for extrusion of the ghs or alternatively to suggest it triggered a change in its critical taper in view of the variable nature of anatexis and of motion on the std it is likely that anatexis had only a second order effect in modulating strain distribution with little effect on the general history of deformation thus despite all kinds of local differences strain distribution over time was such that it maintained the well defined arc that characterizes this orogen this was likely the result of a self organized forward motion of the arc controlled by the imposed convergence history and energy conservation balancing accumulation of potential energy and dissipation independent of the presence or absence of melt

comparison of the rupture and radiation characteristics of intermediate and deep earthquakes we study the depth dependence of the rupture and radiation characteristics of earthquakes below 70 km depth by investigating the source properties of intermediate 70 300 km depth and deep 300 km depth earthquakes along the fiji and japan subducting slabs the source properties are explored by retrieval of the moment rate functions and by spectral analysis using p waves thirty two earthquakes in the period 1986 1997 with seismic moment 1 1018 n m are analyzed the main conclusions are 1 the dependence of the source duration on the scalar seismic moment m0 follows the usual one third trend on average 2 this behavior is still visible when the source durations are adjusted for the shear wave velocity and the modulus of rigidity at the hypocentral depth 3 the effective stress drop and the effective strain drop for subcrustal earthquakes in fiji and japan tend to be larger than those for the crustal ones 4 the duration and shape of the moment rate functions do not show any systematic difference between intermediate and deep events and 5 no obvious difference in the spectral properties of intermediate and deep events is seen

three dimensional quantitative textural analysis of metamorphic rocks using high resolution computed x ray tomography part i methods and techniques macroscopic textures resulting from different atomic scale mechanisms for metamorphic crystallization display different degrees of order clustering intergrowth and relative isolation of porphyroblasts data on the sizes and locations of thousands of crystals in a three dimensional volume are required to identify reliably the mechanisms governing nucleation and growth of porphyroblasts from these textural features these data can now be acquired by means of high resolution computed x ray tomography numerical models that simulate porphyroblast formation governed by either interface controlled or diffusion controlled reaction mechanisms indicate that quantitative textural analysis can discriminate between these possibilities these numerical models also allow a comparison between textures predicted for different crystallization mechanisms and textures measured in natural samples from which inferences can be drawn concerning the relative importance of these mechanisms in nature an independent test of the validity of such inferences is possible for porphyroblasts such as garnet that may preserve prograde growth zoning and allow the examination of normalized radius rate relations

the september 2009 padang earthquake on 30 september 2009 the city of padang in indonesia was rocked by an earthquake with a moment magnitude of mw 7 6 despite its size the earthquake did not rupture the sunda megathrust and did not significantly relax the 200 years of accumulated stress on the mentawai segment the megathrust strain energy budget remains substantially unchanged and the threat of a great mw 8 5 tsunamigenic earthquake on the mentawai patch is unabated

source inversion of w phase speeding up seismic tsunami warning w phase is a long period phase arriving before s wave it can be interpreted as superposition of the fundamental first second and third overtones of spheroidal modes or rayleigh waves and has a group velocity from 4 5 to 9 km s 1 over a period range of 100 1000 s the amplitude of long period waves better represents the tsunami potential of an earthquake because of the fast group velocity of w phase most of w phase energy is contained within a short time window after the arrival of the p wave at a distance of 50 w phase energy is contained within 23 min after the origin time which is the distinct advantage of using w phase for rapid tsunami warning purposes we use a time domain deconvolution method to extract w phases from the broad band records of global seismic networks the bandwidth of w phase is approximately from 0 001 to 0 01 hz and we bandpass filter the data from 0 001 to 0 005 hz in most cases having extracted w phase from the vertical component records we perform a linear inversion using a point source to determine mw and the source mechanism for several large earthquakes including the 2004 sumatra andaman earthquake the 2005 nias earthquake the 2006 kuril is earthquake and the 2007 sumatra earthquake w phase inversion yields reliable solutions and holds promise of the use of w phase for rapid assessment of tsunami potential

mapping the b value anomaly at 100 km depth in the alaska and new zealand subduction zones a positive anomaly in the frequency magnitude distribution b value is detected at approximately 90 100 km depth in two subduction zones the b value in this anomalous zone is 40 higher than in adjacent volumes we use regional catalogs with a magnitude of completeness of 2 6 central alaska and cook inlet 2 0 shumagin islands and 3 5 new zealand to resolve the b value as a function of space in more detail we project all earthquakes onto planes perpendicular to the strike of the slab these cross section views of the b value distribution locate the high b value anomalies at a depth of 90 100 km on the upper surface of the wadati benioff zone at these depths slab dehydration may increase pore pressure thus lowering the effective stress and increasing the b value increased pore pressure would also lower the liquidus in the overlying asthenosphere giving rise to the volcanism that occurs directly above this zone

significance of non double couple components of deep and intermediate depth earthquakes implications from moment tensor inversions of long period seismic waves abstract analysis of long period seismic waves suggests that non double couple components in source moment tensors of intermediate depth and deep earthquakes are significant and appear to respond to the state of predominant strain release within slabs the strain regime is partially or fully induced by the sources themselves or by slab structures near the sources we observe consistency in the non double couple components from three different seismic wave inversions among 21 earthquakes studied 17 events have the same sign in the three non double couple components and the signs show a correlation with the state of strain release within the slabs the effects of unmodeled propagation structure and instability of the inversion procedure are unlikely to be responsible for the consistent non double couple components because the suites of seismic waves traverse different paths and the various inversion schemes have the different resolvabilities of the moment tensors

time functions of deep earthquakes from broadband and short period stacks to constrain dynamic source properties of deep earthquakes we have systematically constructed broadband time functions of deep earthquakes by stacking and scaling teleseismic p waves from u s national seismic network terrascope and berkeley digital seismic network broadband stations we examined 42 earthquakes with depths from 100 to 660 km that occurred between july 1 1992 and july 31 1995 to directly compare time functions or to group them by size depth or region it is essential to scale them to remove the effect of moment which varies by more than 3 orders of magnitude for these events for each event we also computed short period stacks of p waves recorded by west coast regional arrays the comparison of broadband with short period stacks yields a considerable advantage enabling more reliable measurement of event duration a more accurate estimate of the duration better constrains the scaling procedure to remove the effect of moment producing scaled time functions with both correct timing and amplitude we find only subtle differences in the broadband time function shape with moment indicating successful scaling and minimal effects of attenuation at the periods considered here the average shape of the envelopes of the short period stacks is very similar to the average broadband time function the main variations seen with depth are 1 a mild decrease in duration with increasing depth 2 greater asymmetry in the time functions of intermediate events compared to deep ones and 3 unexpected complexity and late moment release for events between 350 and 550 km with seven of the eight events in that depth interval displaying markedly more complicated time functions with more moment release late in the rupture than most events above or below the first two results are broadly consistent with our previous studies while the third is reported here for the first time the greater complexity between 350 and 550 km suggests greater heterogeneity in the failure process in that depth range

anomalously low aftershock productivity of the 2019 mw 8 0 energetic intermediate depth faulting beneath peru abstract the 26 may 2019 peru m w 8 0 earthquake struck within the nearly horizontal underthrust nazca plate at depths from 110 to 150 km below the upper amazon near a steep bend in the plate where it plunges down to a deep earthquake zone little prior seismicity occurred in this region but large intraslab events with similar normal faulting mechanisms have occurred to the west the event is situated in a similar slab position to the 2017 puebla morelos mexico earthquake but the remote location resulted in limited loss of life and damage back projection imaging and finite fault inversion based on teleseismic data suggest a brittle and energetic rupture process with unilateral expansion northward over a 170 km long zone at a rupture speed v r 3 km s with three normal faulting patches of up to 4 5 m slip despite the mainshock size it produced only three m4 0 aftershocks within 300 km one nearby the aftershock productivity of the 2019 peru earthquake is very low even among all large intermediate depth earthquakes but similar to that for other large peruvian intraslab events comparison of aftershock productivity of the peru event with that of global large earthquakes in various tectonic settings suggests that the low aftershock productivity can largely be attributed to regionally homogeneous faulting systems and relatively uniform stress state in the flat peru slab

mw 7 8 tarapaca intermediate depth earthquake of 13 june 2005 northern chile fault plane identification and slip distribution by waveform inversion 1 the mw 7 8 tarapaca earthquake of 13 june 2005 occurred between 90 and 115 km depth within the subducting slab of northern chile the fault plane could be identified using two independent approaches by locating accurately one week of aftershocks and by modeling the strong motion waveforms the rupture occurred on a low angle 15 west dipping plane in pure normal faulting the slip distribution obtained by the joint inversion of strong motion and teleseismic data indicates a compact slip zone of dimension 50 40 km2 with a maximum slip of 13 m the aftershock distribution slightly extends beyond the slip zone dimension in the ns direction but slip extends slightly more towards the west than do the aftershocks the tarapaca earthquake can be classified as an intraslab down dip tensional event in the context of a mature seismic gap

earthquake hazard after a mainshock in california after a strong earthquake the possibility of the occurrence of either significant aftershocks or an even stronger mainshock is a continuing hazard that threatens the resumption of critical services and reoccupation of essential but partially damaged structures a stochastic parametric model allows determination of probabilities for aftershocks and larger mainshocks during intervals following the mainshock the probabilities depend strongly on the model parameters which are estimated with bayesian statistics from both the ongoing aftershock sequence and from a suite of historic california aftershock sequences probabilities for damaging aftershocks and greater mainshocks are typically well constrained after the first day of the sequence with accuracy increasing with time

faulting within the pacific plate at the mariana trench implications for plate interface coupling and subduction of hydrous minerals we investigate faulting within the incoming pacific plate at the mariana subduction trench to understand stresses within the bending plate regional stresses acting upon the plate interface and the extent of possible faulting induced mantle serpentinization we determine accurate depths by inverting teleseismic p and sh waveforms for earthquakes occurring during 1990 2011 with global centroid moment tensor gcmt solutions for earthquakes with mw 5 0 we determine centroid depths and source time functions and refine the fault parameters results from central mariana indicate that all earthquakes are extensional and occur at centroid depths down to 11 km below the moho at the southern mariana trench extensional earthquakes continue to 5 km below the moho one compressional earthquake at 34 km below the seafloor suggests stronger plate interface coupling here in addition we model the stress distribution within the pacific plate along two bathymetric profiles extending seaward from the mariana subduction trench axis to better understand whether our earthquake depth solutions match modeled scenarios for plate bending under applied external forces results from our flexure models match the locations of extensional and compressional earthquakes and suggest that the pacific plate at southern mariana is experiencing larger compressional stresses possibly due to greater interplate coupling additionally we conclude that if extensional faulting promotes the infiltration of water into the subducting plate mantle then the top 5 15 km of the pacific plate mantle are partially serpentinized and a higher percentage of serpentinization is located near the central mariana trench where extensional events extend deeper

deformation around rigid particles the influence of slip at the particle matrix interface abstract theoretical and previous experimental studies of rigid particle structures have only considered the behaviour of rigid particles for which the interface between particle and matrix is coherent in natural rocks the boundary behaviour may cover the whole range from free slip to complete coherence examples where slip may be promoted include crack seal deformation during the development of pressure shadows core mantle structures where the mantle is composed of much weaker fine recrystallized material and crystallizing magmas or partially molten rocks where grain boundaries are lubricated by the melt phase analogue scale model experiments have been performed under pure shear and simple shear conditions to compare the particle rotation and the flow in the matrix for both slipping and non slipping particle matrix interfaces when slip does occur particles rotate faster in pure shear and slower in simple shear than for non slip leading to the more rapid development of a shape preferred orientation the preferred orientation tends to be bimodal in simple shear flow the flow and finite strain patterns in the matrix are also strongly influenced by slip at the interface and porphyroclast systems can develop geometries with ambiguous or contradictory shear senses dependent on the original particle orientation in multi particle experiments slip on the interfaces enhances strong strain localization in non linear viscous material to produce patterns very similar to natural s c fabrics

stable positions of rigid objects in non coaxial flow a study in vorticity analysis abstract distortions of a homogeneous fabric which occur around rigid objects in rocks deformed by non coaxial ductile flow are useful to determine the sense of shear but also contain information on other aspects of the flow rigid objects in a non coaxial flow fall into two categories those which are permanently rotating and those which can reach stable positions at high finite strain the orientation of such immobilized objects with respect to eigenvectors of instantaneous flow is a function of the vorticity number of the flow the ratio of instantaneous stretches and the axial ratio of the object in naturally deformed rocks immobilized rigid objects may be recognized from the geometry of the surrounding fabric tails of recrystallized material around porphyroclasts in a mylonite rotate towards parallellism with the extensional eigenvector of the flow and the shape of tails reflects the rotational behaviour of the porphyroclast the axial ratio of immobilized porphyroclasts and their orientation with respect to the tails can theoretically be used to determine the vorticity number of the flow and deviations from isochoric plane strain in practice such an analysis is as yet difficult but a tentative example is given of the way in which the vorticity number can be calculated from a population of feldspar porphyroclasts in a quartzite mylonite from the french pyrenees

a critique of vorticity analysis using rigid clasts abstract estimating the kinematic vorticity numbers from rock fabrics presents many problems in this paper we use numerical modeling to investigate the reliability of those widely used vorticity analysis methods using rigid clasts we use very simple flows steady state homogeneous and having monoclinic symmetry to represent plane straining zones and transpressional zones and assume that the rotation of rigid clasts perfectly obeys the theory of jeffery 1922 proceedings of the royal society of london a102 161 179 these are assumptions made in current vorticity analysis using rigid clasts even with these simple assumptions our modeling shows that the current methods have intrinsic uncertainties so large that it is pointless to use the estimated vorticity numbers to constrain shear zone boundary conditions and kinematics it is perfectly consistent with numerical modeling results if the currently reported vorticity numbers estimated from rigid clasts in the range of 0 50 0 85 are all interpreted as being from natural shear zones with close to simple shearing flows the large uncertainties arise because the motion of rigid clasts is intrinsically a three dimensional problem

a review of definitions of the himalayan main central thrust most workers regard the main central thrust mct as one of the key high strain zones in the himalaya because it accommodated at least 90 km of shortening because that shortening exhumed and buried hanging wall and footwall rocks and due to geometric and kinematic connections between the main central thrust and the structurally overlying south tibet detachment geologists currently employ three unrelated definitions of the mct metamorphic rheological age of motion structural or protolith boundary structural these disparate definitions generate map and cross section mct positions that vary by up to 5 km of structural distance the lack of consensus and consequent shifting locations impede advances in our understanding of the tectonic development of the orogen here i review pros and cons of the three mct definitions in current use none of these definitions is flawless the metamorphic rheological and age of motion structural definitions routinely fail throughout the orogen whereas the protolith boundary structural definition may fail only in rare cases all limited to sectors of the eastern himalaya accordingly a definition based on high strain zone geometry and kinematics combined with identification of a protolith boundary is the best working definition of the mct

the 23 june 2014 mw 7 9 rat islands archipelago alaska intermediate depth earthquake on 23 june 2014 the largest intermediate depth earthquake m w 7 9 of the last 100 years ruptured within the subducting pacific plate about 100 km below the rat islands archipelago of the western aleutian islands alaska the unusual faulting orientation strike 206 dip 24 and rake 14 is possibly related to curvature of the underthrust slab and high obliquity of the relative plate motions the first 15 s of the rupture generated relatively weak seismic waves followed by strong energy release for the next 25 s the seismic moment is 1 0 10 21 nm and slip of up to 10 m is concentrated within a 50 km 50 km region the radiated energy is 1 1 to 2 7 10 16 j assuming attenuation t of 0 4 to 0 7 s this type of intraplate faulting can be very damaging for populated regions above subduction zones such as japan taiwan chile and indonesia

tectonometamorphic discontinuities within the greater himalayan sequence in western nepal central himalaya insights on the exhumation of crystalline rocks abstract the core of the greater himalayan sequence in the mugu karnali area western nepal is affected by a thick shear zone with development of nearly 4 km of mylonites mangri shear zone it is a contractional shear zone showing a top to the sw and wsw sense of shear the shear zone developed during the decompression in the sillimanite stability field of rocks that previously underwent relatively high pressure metamorphism deformed under the kyanite stability field p t conditions indicate that the footwall experienced higher pressure 1 0 0 9 gpa than the hanging wall 0 7 gpa and similar temperatures 675 700 c u pb in situ dating of monazites indicate a continuous activity of the shear zone between 25 and 18 ma samples from the lower part of the greater himalayan sequence underwent similar ductile shearing at 17 13 ma these ages and the associated p t t paths revealed that peak metamorphic conditions were reached 5 7 ma later in the footwall of the shear zone with respect to the hanging wall pointing to a diachroneity in the metamorphism triggered by the shear zone itself mangri shear zone with the other recently documented tectonic and metamorphic discontinuities within the greater himalayan sequence point to the occurrence of a regional tectonic feature the high himalayan discontinuity running for more than 500 km along the strike of the central himalayas it was responsible of the exhumation of the upper part of the greater himalayan sequence starting from 28 ma well before the activation of the main central thrust and the south tibetan detachment our data point out that exhumation of the greater himalayan sequence was partitioned in space and time and different slices were exhumed in different times starting from the older in the upper part to the younger in the lower one

defining the himalayan main central thrust in nepal an inverted metamorphic field gradient associated with a crustal scale south vergent thrust fault the main central thrust has been recognized along the himalaya for over 100 years a major problem in himalayan structural geology is that recent workers have mapped the main central thrust within the greater himalayan sequence high grade metamorphic sequence along several different structural levels some workers map the main central thrust as coinciding with a lithological contact others as coincident with the kyanite isograd up to 1 3 km structurally up section into the tertiary metamorphic sequence without supporting structural data some workers recognize a main central thrust zone of high ductile strain up to 2 3 km thick bounded by an upper thrust mct 2 vaikrita thrust and a lower thrust mct 1 munsiari thrust some workers define an upper lesser himalaya thrust sheet that shows similar p t conditions to the greater himalayan sequence others define the main central thrust either on isotopic nd sr differences differences in detrital zircon ages or as being coincident with a zone of young

deformation of leucogranites of the crystalline main central sheet in southern tibet china abstract towards the top of the main central sheet in southern tibet china leucogranites intrude metamorphic rocks related to rocks of the main central thrust in the himalayas our study shows that the leucogranites suffered a northward shear deformation the significance of this event is discussed in terms of gravity driven decollement of the 10 000 m thick tethyan sedimentary pile over its crystalline basement

aftershock sequences of moderate sized intermediate and deep earthquakes in the tonga subduction zone intermediate and deep focus earthquakes are generally thought to produce few aftershocks the two year operation of a network of broadband seismographs in the tonga fiji region provides an opportunity to monitor the aftershock activity of moderate sized earthquakes in addition to the march 9 1994 mw 7 6 event that shows 144 aftershocks we found that all nine intermediate and deep earthquakes with mw 6 0 showed some aftershock activity suggesting that aftershock sequences are ubiquitous on average deep tonga earthquakes show an order of magnitude fewer aftershocks above a given magnitude than shallow california earthquakes of similar moment the largest aftershocks of each sequence have an mb that is 2 0 units smaller on average than the mw of the mainshock whereas the california earthquakes show a mean difference of 1 1 since b values of the deeper sequences appear to be similar to shallow sequences the difference in aftershock productivity results from the larger size difference between the mainshock and the larger aftershocks the most prolific sequences show an activity level not far below shallow earthquakes of the same moment this is in contrast with observations of little aftershock activity for deep earthquakes in south america japan and kuriles and suggests that the tonga subduction zone is unusually prolific in aftershock production perhaps due to cold slab temperatures

different stress directions in the aftershock focal mechanisms of the kushiro oki earthquake of jan 15 1993 se hokkaido japan and horizontal rupture in the double seismic zone abstract on 15 january 1993 a large intermediate depth earthquake with a magnitude of m 7 8 shook the eastern part of the island of hokkaido japan close to the northeastern japan trench and to the kuril trench this earthquake is now referred to as the kushiro oki earthquake the main shock of the kushiro oki earthquake showed a down dip extension mechanism having horizontal and vertical nodal planes the hypocenter was located at a depth of 107 km in the lower plane of the double seismic zone in the subducting pacific plate after the mainshock more than 300 aftershocks were recorded by three seismograph networks from these data we were able to determine the aftershock hypocenters and 63 focal mechanism solutions for the aftershocks the aftershock region extended horizontally in a northerly direction from the lower plane towards the upper plane of the double seismic zone in the subducting plate it did not however reach the upper plane we estimated the maximum and minimum compressional stress directions along the deep seismic zone by converting the mechanism solutions from horizontal plane projections to an oblique plane parallel to the deep seismic zone thirty eight mechanisms were found to have maximum and minimum compressional stress directions very similar to those of the mainshock but 25 aftershock mechanisms had almost opposite directions to those of the mainshock the hypocenters of these aftershocks were spatially mixed this suggests that the stress state is strongly inhomogeneous in the aftershock region and that horizontal movement occurs by the down dip extensional stress which is stored in the region between two planes of the deep seismic zone the remanent down dip extensional stress was released as the consistent group of aftershocks high aftershock activity may be attributed to the inhomogeneity of the material in the region the previous four intermediate depth earthquakes beneath hokkaido island as well as aftershock distributions of two of them have been examined for comparison with the kushiro oki earthquake the mainshocks of the four large intermediate depth earthquakes are also located on the lower plane with a similar mechanism to that of the kushiro oki earthquake the aftershock regions of two of these earthquakes are also distributed horizontally they indicate dominantly horizontal movement with down dip extension mechanisms at an intermediate depth in the subducting pacific plate beneath hokkaido island

quantitative kinematic flow analysis from the main central thrust zone nw himalaya india implications for a decelerating strain path and the extrusion of orogenic wedges quantitative kinematic indicators from the main central thrust zone mctz in the nw himalaya have been used to characterize the type of flow during deformation different generations of tension gashes have been rotated by variable angles with respect to the mylonitic foliation forming associated fringe folds these record the late stage brittle ductile flow and reveal that a strong pure shear component of deformation occurred throughout the mctz to characterize earlier deformation increments fabrics from highly deformed quartz ribbons were analyzed well developed shape and lattice preferred orientation patterns show a systematic change of the glide systems and suggest inverted palaeotemperatures within the mctz investigations of the c axes patterns reveal a strong asymmetry at the top of the mctz whereas the samples from the base of the mctz show almost perfectly symmetrical type i crossed girdles deformation within the mctz probably started close to simple shear flow at higher temperatures which progressively became a more general shear during cooling and ended in a pure shear dominated flow during the final stages of brittle ductile deformation i e a decelerating strain path using the higher himalaya crystalline as an example a kinematic model for the extrusion of crustal wedges above major thrust zones is suggested

calcite twin morphology a low temperature deformation geothermometer abstract twinning of the e plane is the dominant crystal plastic deformation mechanism in calcite deformed below about 400 c calcite in a twin domain has a different crystallographic orientation from the host calcite grain so called thin twins appear as thin black lines when viewed parallel to the twin plane at 200 320 magnification under a petrographic microscope thick twins viewed in the same way have a microscopically visible width of twinned material between black lines calcite e twin width and morphology has been correlated with temperature of deformation in naturally deformed coarse grained calcite in this paper we present a compilation and analysis of data from limestones of the frontal alps france and switzerland and the appalachian valley and ridge and plateau provinces eastern united states to document this temperature dependence mean calcite twin width correlates directly with temperature of deformation such that thin twins dominate below 170 c and thick twins dominate above 200 c above 250 c dynamic recrystallization is an important deformation mechanism in calcite mean twin intensity twin planes mm correlates negatively with temperature and a cross plot of twin intensity with twin width can yield information about both strain and temperature of deformation these relationships provide a deformation geothermometer for rocks that might otherwise yield little or no paleotemperature data

macrofracturing of oceanic lithosphere in complex large earthquake sequences major earthquakes in oceanic lithosphere seaward of the subduction zone outer trench slope are relatively uncommon but several recent occurrences have involved very complex sequences rupturing multiple nonaligned faults and or having high aftershock productivity with diffuse distribution this includes the 21 december 2010 m w 7 4 ogasawara bonin 11 april 2012 m w 8 6 indo australia 23 january 2018 m w 7 9 off kodiak island and 20 december 2018 m w 7 3 nikol skoye northwest pacific earthquakes major oceanic intraplate event sequences farther from plate boundaries do not tend to be as complex in faulting or aftershocks outer trench slope extensional faulting can involve complex distributed sequences particularly when activated by great megathrust ruptures such as 11 march 2011 m w 9 1 tohoku and 15 november 2006 m w 8 3 kuril islands intense faulting sequences also occur near subduction zone corners with many fault geometries being activated including some in nearby oceanic lithosphere as for the 29 september 2009 m w 8 1 samoa 6 february 2013 m w 8 0 santa cruz islands and 16 november 2000 m w 8 0 new ireland earthquakes the laterally varying plate boundary stresses from heterogeneous locking recent earthquakes or boundary geometry influence the specific faulting geometries activated in nearby major intraplate ruptures in oceanic lithosphere preexisting lithospheric structures and fabrics exert secondary influences on the faulting intraplate stress release in oceanic lithosphere near subduction zones favors distributed macrofracturing of near critical fault systems rather than localized single fault failures both under transient loading induced by plate boundary ruptures and under slow loading by tectonic motions and slab pull

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

quantifying internal strain and deformation temperature in the eastern himalaya bhutan implications for the evolution of strain in thrust sheets abstract quartz microstructural analysis together with quantification of 3 d crystal plastic strain of quartz clasts normalized fry and rf methods constrain deformation temperature ranges and internal strain magnitude orientation within himalayan thrust sheets in bhutan lesser himalayan lh thrust sheets display an inverted deformation temperature gradient from 250 to 310 c at the main boundary thrust to 500 670 c at the main central thrust mct attributed primarily to stacking of lh thrust sheets deformed at progressively higher temperatures toward the hinterland the hot iron effect of the mct hanging wall only affects the upper few hundred meters of hinterland lh thrust sheets frontal thrust sheets exhibit 7 layer parallel shortening lps strain all other thrust sheets exhibit layer normal flattening lnf strain with 1 9 1 8 1 0 lh rocks and 2 1 1 8 1 0 greater and tethyan himalayan rocks median ellipsoids x parallel to lineation we propose that lps strain developed foreland ward of the thrust deformation front and that lnf strain resulted from later tectonic loading the lps to lnf transition occurs at minimum temperatures for quartz plasticity ca 250 270 c lnf ellipsoids are folded along with strata within thrust sheets indicating that internal strain preceded thrust imbrication and translation at the scale of 10 s of meters structural distance strain magnitude of hanging wall rocks does not increase near thrusts which supports the existence of discrete faults with large translations as generally depicted in balanced cross sections strain magnitudes in low grade greater and tethyan himalayan rocks in central bhutan indicate that the top to the north component of channel flow extrusion is most likely less than 19 km

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

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

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

biases in three dimensional vorticity analysis using porphyroclast system limits and application to natural examples a description of the systematic errors associated with the measurement of the vorticity number from poryhroclasts in natural systems is presented and discussed we show that strong biases and systematic errors could derive both from some erroneous physical i e no slip across clast matrix boundary homogeneity within the matrix as well as geometrical assumptions i e the radius ratio and angular measurements carried out in two dimensions on outcrop surfaces and thin section by comparing natural datasets of porphyroclast shape preferred orientation spo with different theoretical curves plots we suggest that at least one of the jeffery physical assumptions can be tested when applying vorticity techniques the comparison of different possible sources of systematic errors indicates that for medium to low vorticity numbers wm 0 8 vorticity data are strongly biased and that a minimum systematic error of 0 2 should be taken into account finally we use data from natural shear zones from the southern variscan belt in sardinia to test and discuss the starting assumptions of the jeffery model one of the main challenges of modern structural geology is to try to obtain information about the rheology and kinematics of flow occurring in the deepest crust and upper mantle several microstructure fabrics e g fabric asymmetry associated with mylonites represent potentially important sources of information to obtain kinematic data as well as for estimation of deformation parameters see passchier trouw 2005 for a review in particular porphyroclast systems have been shown to be a sensitive indicator of flow parameters and can be used to constrain the relative ratio of combined pure and simple shear i e the vorticity of the flow several theoretical and practical studies indicate that syn kinematic recrystallization of mantles sor d shaped tails around porphyroclasts and shape preferred orientation spo of porphyroclasts may store information on the amount of shear strain with the aim of developing a vorticity gauge theoretical and experimental models for study of the rotation and behaviour of elliptical porphyroclasts in a fluid have been proposed and developed the first attempt began with the pioneering works of jeffery 1922 and eshelby 1957 that found solutions for rigid objects immersed in a viscous and linear elastics fluid respectively bretherton 1962 ghosh ramberg 1976 and bilby kolbuszewski 1977 further adapted the basic jeffery analytical solution to material science and structural geology subsequent investigations used the previous theoretical calculation to develop efficient strain and vorticity gauge techniques capable of quantifying the flow parameters within shear zones passchier 1987 wallis 1992 simpson de paor 1993 holcombe little 2001 three main analytical techniques passchier 1987 wallis 1995 holcombe little 2001 are commonly employed to characterize flow within shear zones using rigid porphyroclats jessup et al 2007 all these techniques use the measurement of the axial ratio of porphyroclast r the shape factor fig 1 and the angle of long axis porphyroclast with respect to foliation or stretching direction h fig 1 and the distribution of the cross plotted curves to define the mean vorticity number wm these curves define a threshold number rc below which the porphyroclasts continuously rotate and above which they record stable sink position however as many authors passchier 1987 marques coelho 2001 have already noted the jeffery model assumes 1 rigid particles immersed in a non confined viscous flow 2 a perfect coupling on matrix particle interface and 3 no interference between particles during deformation use of the jeffery model implies that these physical assumptions are met and or need to be at least preliminarily tested although techniques to estimate the vorticity of flow have become routinely used on sheared rocks in a variety of tectonic settings e g klepeis et al 1999 xypolias doutsos 2000 xypolias koukouvelas 2001 bailey eyster 2003 law et al 2004 carosi et al 2006 2007 iacopini et al 2008 sarkarinejad et al 2008 2010 frassi et al 2009 larson godin 2009 relatively few detailed analysis of error sources have been proposed and a critical discussion from prior d j rutter e h tatham d j eds deformation mechanisms rheology and tectonics microstructures mechanics and anisotropy geological society london special publications 360 301 318 doi 10 1144 sp360 17 the geological society of london 2011 publishing disclaimer www geolsoc org uk pub ethics at university of aberdeen on november 19 2012 http sp lyellcollection org downloaded from

tectonic evolution of the central annapurna range nepalese himalayas the metamorphic core of the himalayan orogen or greater himalayan sequence is a northward tapering prism bound at the bottom by a n dipping family of thrust faults the main central thrust system and at the top by a n dipping family of normal faults the south tibetan detachment system research in the central annapurna range of nepal demonstrates a close temporal and spatial association between contractional and extensional deformation on these bounding fault systems and within the metamorphic core throughout much of the early miocene the main central thrust system is represented here by a 2 to 3 km thick zone of high strain that developed during two or more episodes of movement most of its displacement was concentrated along the chomrong thrust a sharp late metamorphic discontinuity that places middle amphibolite facies rocks of the greater himalayan sequence on top of lower amphibolite facies rocks of the lesser himalayan sequence the earliest demonstrable movement on this thrust system occurred 22 5 ma the most recent movement may be as young as pliocene the oldest element of the south tibetan detachment system in this area is the deorali detachment which appears to have been active at the same time as the earliest shortening structures of the main central thrust system fabrics related to the deorali detachment are disrupted by a previously unrecognized sw vergent thrust structure the modi khola shear zone the effect of this structure which is constrained to be between 22 5 and 18 5 ma was to shorten rock packages that had been extended previously during movement on the deorali detachment transition back to a local extensional regime after 18 5 ma was marked by development of the machhupuchhare detachment and related splays geologic evidence for rapid two way transitions between contraction and extension in the annapurna range indicates that extensional deformation in convergent settings does not only represent gravitational collapse at the end of an orogenic cycle it also appears to be an important factor in mountain range development

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

the m w 7 9 2014 intraplate intermediate depth rat islands earthquake and its relation to regional tectonics abstract the 2014 m w 7 9 rat islands intermediate depth 107 km earthquake is studied using seismic observations at teleseismic distances the nodal plane with a steep dip angle 87 and a strike of 308 inferred from the global cmt solution is selected as the causative fault plane because of the consistency with the distribution of the aftershocks relocated using the joint hypocenter determination method its slip history is constrained using broadband p and sh waveforms and long period 3 6 mhz seismic waves in vertical and transverse components the rupture process associated with this plane shows a rupture propagating at 2 km s the rupture essentially breaks one large slip patch peak slip amplitude of 3 5 m located 30 km west of the hypocenter the size of this slip patch extends from 80 to 140 km depth which would be the largest depth extent of earthquake slip ever documented we show that this earthquake breaks the entire cold core of the subducting slab defined by the depth of the 600 650 c isotherm in addition the lateral extent of the rupture appears to be bounded by the heterogeneities of subducting slab our results further indicate that along this segment of aleutian arc the intermediate depth earthquakes might contribute to the westward transporting motion of rat block lastly the slip distribution of this earthquake as well as that of two other well studied m w 7 5 intermediate depth earthquakes suggests that the coldest portion of the subducting slab outlined by the double wadati benioff zone is seismic during large intermediate depth earthquakes such a result is important for estimating the seismic hazard of future large intermediate depth earthquakes

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

relationships between displacement and distortion in orogens linking the himalayan foreland and hinterland in central nepal greater himalayan sequence rocks exposed in the manaslu himal chuli himalaya can be separated into distinct upper and lower parts deformation recorded in both parts occurred at temperatures ranging between 450 c and 640 c and is characterized by almost equal coaxial and noncoaxial components across the upper greater himalayan sequence peak metamorphic temperatures are essentially isothermal whereas corresponding metamorphic pressure estimates across the same section decrease downward with an apparent gradient of 620 bars km in the lower greater himalayan sequence however both metamorphic pressure and temperature decrease with structural depth the abnormal pressure gradient in the upper greater himalayan sequence is attributed to 50 vertical thinning during southward displacement while the inverted gradient in the lower portion is interpreted to be the result of coeval exhumation and downward expansion of the main central thrust shear zone and the progressive incorporation of more rock into the greater himalayan sequence deformation in the upper portion of the greater himalayan sequence was characterized by extending flow i e extension in the direction of flow whereas deformation in its lower portion was characterized by compressing flow i e compression in the direction of flow extending flow is a distinctive feature of displacement and distortion in deep orogenic hinterlands while compressing flow is emblematic of displacement and distortion in orogenic foreland regions the transition between the upper and lower parts of the greater himalayan sequence therefore represents a fundamental transition between hinterland style deformation involving processes such as lateral midcrustal flow and foreland style deformation involving critical taper thrust fold wedge development

strain deformation temperatures and vorticity of flow at the top of the greater himalayan slab everest massif tibet this paper presents quantitative data on strain deformation temperatures and vorticity of flow at the top of the greater himalayan slab the data were collected from the tibetan side of the everest massif where two low angle normal faults bound the upper surface of the greater himalayan slab the earlier and structurally lower lhotse detachment and the later and structurally higher qomolangma detachment greenschist to sillimanite grade quartz rich metasedimentary rocks exposed in the rongbuk to north col region of the everest massif are characterized by cross girdle quartz c axis fabrics indicating approximate plane strain conditions fabric opening angles progressively increase with depth beneath the overlying lhotse detachment and indicate progressively rising deformation temperatures of 525 625 50 c at depths of 300 600 m beneath the detachment deformation temperatures of c 450 c are indicated by fabric opening angles in epidote amphibolite facies mylonites located closer to the overlying detachment a top down to the north normal shear sense is indicated by the asymmetry of microstructures and c axis fabrics but the degree of asymmetry is low at distances greater than 400 m beneath the detachment and sillimanite grains are drawn into adjacent conjugate shear bands but still appear pristine indicating that deformation occurred at close to peak metamorphic temperatures these quenched fabrics and microstructures indicate rapid exhumation in agreement with previous isotopic dating studies mean kinematic vorticity numbers w m were independently calculated by three analytical methods calculated w m values range between 0 67 and 0 98 and indicate that although a simple shear component is generally dominant particularly in greenschist facies mylonites located between the lhotse and overlying qomolangma detachments there is also a major component of pure shear in samples located at 400 600 m beneath the lhotse detachment pure and simple shear make equal contributions at w k 0 71 our integrated strain and vorticity data indicate a shortening of 10 30 perpendicular to the upper surface of the greater himalayan slab and confirm that the upper surface of the slab is a stretching fault with estimated down dip stretches of 10 40 assuming plane strain deformation measured parallel to the flow plane transport direction

three dimensional quantitative textural analysis of metamorphic rocks using high resolution computed x ray tomography part ii application to natural samples three dimensional quantitative textural analysis coupled with numerical modelling has been used to assess the dominant mechanisms governing crystallization of garnet porphyroblasts in rocks from diverse regional metamorphic environments in every case spatial dispositions crystal size distributions and compositional zoning patterns of porphyroblasts indicate the dominance of diffusion controlled nucleation and growth mechanisms nine samples from three geological areas were studied a suite of semi pelitic rocks from the picuris mountains new mexico usa a suite of mafic samples from the llano uplift texas usa and a kyanite schist from mica dam british columbia canada the semi pelitic suite exhibits post deformational garnet growth whereas garnet in the mafic suite and in the kyanite schist grew synkinematically in rocks displaying weak and strong penetrative fabrics respectively for each sample the centres and radii of thousands of garnet crystals were located and measured in three dimensions using images produced by high resolution computed x ray tomography statistical measures of the degree of ordering and clustering of nucleation sites and estimates of crystal isolation for each porphyroblast were then computed from the measured spatial dispositions these measures can be reproduced in simple numerical models only by diffusion controlled nucleation and growth mechanisms normalized radius rate relations computed from compositional zoning patterns in the garnets require thermally accelerated diffusion controlled growth providing independent confirmation of the conclusions based on textural analysis the unexpected similarity of results from all samples indicates that diffusion controlled nucleation and growth mechanisms may govern porphyroblast crystallization in many metamorphic regimes

rupture imaging of the mw 7 9 12 may 2008 wenchuan earthquake from back projection of teleseismic p waves 1 the mw 7 9 wenchuan earthquake of 12 may 2008 was the most destructive chinese earthquake since the 1976 tangshan event tens of thousands of people were killed hundreds of thousands were injured and millions were left homeless here we infer the detailed rupture process of the wenchuan earthquake by back projecting teleseismic p energy from several arrays of seismometers this technique has only recently become feasible and is potentially faster than traditional finite fault inversion of teleseismic body waves therefore it may reduce the notification time to emergency response agencies using the iris dmc we collected 255 vertical component broadband p waves at 30 95 from the epicenter we found that at periods of 5 s and greater nearly all of these p waves were coherent enough to be used in a global array we applied a simple down sampling heuristic to define a global subarray of 70 stations that reduced the asymmetry and sidelobes of the array response function arf we also considered three regional subarrays of seismometers in alaska australia and europe that had apertures less than 30 and p waves that were coherent to periods as short as 1 s individual arfs for these subarrays were skewed toward the subarrays however the linear sum of the regional subarray beams at 1 s produced a symmetric arf similar to that of the groomed global subarray at 5 s for both configurations we obtained the same rupture direction rupture length and rupture time we found that the wenchuan earthquake had three distinct pulses of high beam power at 0 23 and 57 s after the origin time with the pulse at 23 s being highest and that it ruptured unilaterally to the northeast for about 300 km and 110 s with an average speed of 2 8 km s it is possible that similar results can be determined for future large dip slip earthquakes within 20 30 min of the origin time using relatively sparse global networks of seismometers such as those the usgs uses to locate earthquakes in near real time

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

telescoping of isotherms beneath the south tibetan detachment system mount everest massif abstract petrologic and microstructural crystal fabric data indicate that isotherms recorded in greater himalayan series ghs schists and gneisses in the footwall to the south tibetan detachment system stds have undergone extreme telescoping during penetrative flow associated with southward extrusion of the ghs in the rongbuk valley to the north of mount everest we have made three vertical sampling traverses from the stds down into the ghs and estimated temperatures associated with penetrative deformation using the opening angles of quartz c axis fabrics measured on dynamically recrystallized grains from north to south the deformation temperature data indicate apparent thermal field gradients of 369 385 and 420 c per km for our three traverses traced over a maximum vertical sampling distance of 0 5 km adopting a differential flow path model simple geometric analysis using sections drawn parallel to the local transport direction indicates that detachment parallel transport magnitudes of 25 170 km are needed to explain the extreme telescoping of isotherms in the immediate footwall to the stds depending on assumed original geothermal gradient dip of detachment etc these particle transport estimates are similar to those previously calculated from barometry data of ghs rocks in the everest region and are compatible with channel flow models for extrusion and exhumation of the ghs

inversion of strong ground motion and teleseismic waveform data for the fault rupture history of the 1979 imperial valley california earthquake a least squares point by point inversion of strong ground motion and teleseismic body waves is used to infer the fault rupture history of the 1979 imperial valley california earthquake the imperial fault is represented by a plane embedded in a half space where the elastic properties vary with depth the inversion yields both the spatial and temporal variations in dislocation on the fault plane for both right lateral strike slip and normal dip slip components of motion inversions are run for different fault dips and for both constant and variable rupture velocity models effects of different data sets are also investigated inversions are compared which use the strong ground motions alone the teleseismic body waves alone and simultaneously the strong ground motion and teleseismic records the inversions are stabilized by adding both smoothing and positivity constraints the moment is estimated to be 5 0 10 25 dyne cm and the fault dip 90 5 dislocation in the hypocentral region south of the united states mexican border is relatively small and almost dies out near the border dislocation then increases sharply north of the border to a maximum of about 2 m under interstate 8 dipslip motion is minor compared to strike slip motion and is concentrated in the sediments the best fitting constant rupture velocity is 80 per cent of the local shear wave velocity however there is a suggestion that the rupture front accelerated from the hypocenter northward the 1979 imperial valley earthquake can be characterized as a magnitude 5 earthquake at the hypocenter which then grew into or triggered a magnitude 6 earthquake north of the border

a structural transect in the lower dolpo insights on the tectonic evolution of western nepal abstract we present the results of a structural transect in lower dolpo cross cutting the upper part of the lesser himalaya lh the higher himalayan crystallines hhc and the lower part of the tibetan sedimentary sequence tss the mct zone affects the upper part of the lh as well as the lower part of the hhc and shows a later brittle reactivation mean vorticity in the mct points to non coaxial deformation these data together with available kinematic data along the belt on the south tibetan detachment system stds and in the core of the hhc point to increasing simple shear toward the tectonic boundaries a top to the sw high temperature shear zone toijem shear zone is recognized in the middle part of the hhc at the boundary between units 1 and 2 it developed during the earlier stages of exhumation of the hhc enhancing the decompression of the hanging wall and the emplacement of leucogranite dykes and sills its development could be explained by a change in the velocity profile during the extrusion of the hhc triggered by first order changes in rock types of the tectonic unit the stds is marked by a wide zone of high strain and by a metamorphic jump from amphibolite facies in the carbonate rocks of the upper part of the hhc to greenschist facies marbles in the lower part of the tss the development of a pervasive foliation towards the bottom of the tss indicates increasing strain related to top down to the ne tectonic transport a low p metamorphic event marked by the growth of post d1 biotite porphyroblasts at the base of the tss is related to the conductive heating from the underlying hhc

timing of brittle deformation within the nojima fault zone japan abstract k ar ages from clay fault gouges of outcrop and core samples ug500 well from the nojima fault zone awaji island japan are consistent internally with established field constraints and with apatite and zircon fission track ages demonstrating the suitability of this method for dating of brittle deformation six unspiked and thirty conventional k ar data of clay minerals separated from gouge zones were analysed characterized by protoliths of high grade metamorphic and magmatic rocks the samples from the nojima fault zone offer the opportunity to distinct identify newly grown illite in the fault planes all dated sample fractions were characterized by xrd sem and tem k ar ages of 29 separates have an age range from 63 4 1 3 ma early palaeocene danian to 42 2 1 0 palaeogene middle eocene lutetian some

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

origin of the inverted metamorphism of the lower himalayas central nepal abstract the southern slope of the central nepal himalayas is divided geologically by the main central thrust zone into the underlying midland metasediment zone lower himalayas in the south and the overlying himalayan gneiss zone higher himalayas in the north the former zone consists of a thick pile of various kinds of metasediments folded into an anticlinorium and the latter zone consists of high grade crystalline rocks with a north dipping homoclinal structure and is further overlain by the tethys himalayas composed of non or slightly metamorphosed tethyan sediments the main central thrust zone shows a characteristic lithology such as calcareous schist quartzite phyllitic schist and augen gneiss and is intensely mylonitized the mineral composition and paragenesis of the rocks in the present area indicate an inverted metamorphism from the midland metasediment to the main central thrust zones that is the metamorphic grade of the midland metasediment zone increases upward toward the thrust zone from the chlorite to the biotite zones of the greenschist facies and the greenschist amphibolite transitional zone is attained in the thrust zone the himalayan gneiss zone is apparently much higher in metamorphic grade than the thrust zone on the basis of the compositional zoning of garnets and muscovite compositions however it is concluded that the himalayan gneisses suffered a polymetamorphism the younger metamorphism resulted in a retrograde metamorphism of the himalayan gneisses together with the above mentioned inverted metamorphism and an anatectic melting in depth this was caused by shear heating due to the thrust movement along the main central thrust zone during the alpine tectonism the older metamorphism which seems to be precambrian in age was much higher in metamorphic grade than the younger one and formed the pyrope rich cores of garnets in the himalayan gneisses the maximum metamorphic temperature of 550 c estimated from the mineral paragenesis in the thrust zone is compatible with that caused by shear heating due to the thrusting with thrusting velocity of 5 10 cm yr maximum stress of 300 bars and thrusting distance of 250 km bird 1978

the 3 may 2006 mw 8 0 and 19 march 2009 mw 7 6 tonga earthquakes intraslab compressional faulting below the megathrust the tonga subduction zone is among the most seismically active regions and has the highest plate convergence rate in the world however recorded thrust events confidently located on the plate boundary have not exceeded mw 8 0 and the historic record suggests low seismic coupling along the arc we analyze two major thrust fault earthquakes that occurred in central tonga in 2006 and 2009 the 3 may 2006 mw 8 0 event has a focal mechanism consistent with interplate thrusting was located west of the trench and caused a moderate regional tsunami however long period seismic wave inversions and finite fault modeling by joint inversion of teleseismic body waves and local gps static offsets indicate a slip distribution centered 65 km deep about 30 km deeper than the plate boundary revealed by locations of aftershocks demonstrating that this was an intraslab event the aftershock locations were obtained using data from 7 temporary seismic stations deployed shortly after the mainshock and most lie on the plate boundary not on either nodal plane of the deeper mainshock the fault plane is ambiguous and investigation of compound rupture involving co seismic slip along the megathrust does not provide a better fit although activation of megathrust faulting is responsible for the aftershocks the 19 march 2009 mw 7 6 compressional faulting event occurred below the trench finite fault and w phase inversions indicate an intraslab 50 km deep centroid with ambiguous fault plane this event also triggered megathrust faulting there continues to be a paucity of large megathrust earthquakes in tonga

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

slab2 a comprehensive subduction zone geometry model subduction zones are home to the most seismically active faults on the planet the shallow megathrust interfaces of subduction zones host earth s largest earthquakes and are likely the only faults capable of magnitude 9 ruptures despite these facts our knowledge of subduction zone geometry which likely plays a key role in determining the spatial extent and ultimately the size of subduction zone earthquakes is incomplete we calculated the three dimensional geometries of all seismically active global subduction zones the resulting model called slab2 provides a uniform geometrical analysis of all currently subducting slabs