a study of mantle layering beneath the western pacific the reverberative interval of the seismogram defined as the portion following the surface wave train propagating along the minor arc and ending with the first body wave arrivals from the major arc provides an excellent window in which to observe subcritical reflections from mantle discontinuities on an sh polarized seismogram the intervals between scsn and sscsn wave groups zeroth order reverberations are approximately 15 min long and consist almost entirely of waves reflected one or more times from mantle discontinuities first and higher order reverberations we have developed signal enhancement and waveform inversion schemes to progressively extract information pertaining to mantle layering from the waveforms of zeroth and first order reverberations and have applied them to a data set of long period digital seismograms drawn from a number of tectonic regions in and around the western pacific the results place important constraints on the nature of the transition zone discontinuities measurements of the two way vertical shear wave travel time from the free surface to the 400 km and 650 km discontinuities 400 and 650 respectively yield estimates of transition zone travel time tz largely uncontaminated by outside heterogeneity variations in the path averaged estimates of tz are of the order of 5 4 s indicating significant lateral heterogeneity in the transition zone correlations of tz with 400 and lm travel time through the lower mantle are consistent with but do not require the thermal deflection of an exothermic phase transition near 400 km and an endothermic transition at 650 km with 12 km of long wavelength 500 5000 km topography on each the amount of topography estimated for the 650 km discontinuity is significantly less than predicted for a rigorously stratified mantle 100 km in a search for other mantle discontinuities we find evidence for a strong reflector possibly associated with the onset of partial melting in the upper mantle average depth of 86 km due to ambiguity inherent to the method this feature can also be interpreted as an impedance increase in the d region of the lower mantle 70 170 km above the core mantle boundary no evidence is found for a 220 km discontinuity or the d discontinuity proposed by lay and helmberger 1983 suggesting that if such features are present in the western pacific they are either small normal incidence sh reflection coefficient r 1 5 or intermittent or have lateral depth variations well in excess of 50 km

role of initial stress rotations in rupture dynamics and ground motion a case study with implications for the wenchuan earthquake 1 motivated by observations in the 2008 mw 7 9 wenchuan earthquake we study effects of systematic changes in the principal stress orientation along the fault strike on rupture dynamics and ground motion using a 3 d finite element method based on anderson s theory of faulting we set up the initial stress field with rotations in stress orientations along strike for a dynamic rupture model of a shallow dipping fault we find that initial stress rotations along strike can cause dramatic changes in rupture speed and produce distinct patterns in slip distribution and peak ground motion when a mismatch unfavorable for faulting between fault geometry and initial stress orientations is encountered rupture can spontaneously stop some first order features in the wenchuan event may be partially caused by rotations in the principal stress orientation along strike such as the rupture arrest at the northeast end and two severe destruction zones in the observed seismic intensity distribution these results may have important implications for assessing seismic hazards imposed by faults with changes in the initial stress field along strike worldwide

small intermediate fault segments can either aid or hinder rupture propagation at stepovers 1 large scale geometrical complexities along faults are known to be likely endpoints for coseismic rupture as suggested by analysis of historic ruptures and corroborated by models of rupture on bent or discontinuous faults however natural faults also include smaller scale complexities we use the 3d finite element method to model dynamic ruptures on strike slip fault stepovers with a smaller intermediate fault between the main strands we find that such small faults can have a controlling effect on rupture behavior and ground motion intensity and distribution in particular the intermediate fault can either aid or prevent rupture propagation across the stepover depending on its length and basal depth the results have important implications for hazard assessment of faults with large and small scale geometrical complexities and also suggest that more site specific modeling studies may be necessary to develop realistic rupture scenarios for individual complex fault systems

three dimensional dynamic simulation of the 1992 landers earthquake the 1992 landers california earthquake magnitude 7 3 was modeled as the propagation of a spontaneous rupture controlled by a realistic prestress distribution with the use of a three dimensional finite difference method the dynamic rupture reproduces the general slip pattern used to compute the initial stress distribution and generates near fault ground motions at the surface similar to observations the simulated rupture propagates on the fault along a complex path with highly variable speed and rise time changing the magnitude and pattern of the stress significantly the method provides the framework to estimate earthquake rupture parameters from recorded seismic and geodetic data

how fault evolution changes strain partitioning and fault slip rates in southern california results from geodynamic modeling in southern california the pacific north america relative plate motion is accommodated by the complex southern san andreas fault system that includes many young faults 2 ma the initiation of these young faults and their impact on strain partitioning and fault slip rates are important for understanding the evolution of this plate boundary zone and assessing earthquake hazard in southern california using a three dimensional viscoelasto plastic finite element model we have investigated how this plate boundary fault system has evolved to accommodate the relative plate motion in southern california our results show that when the plate boundary faults are not optimally configured to accommodate the relative plate motion strain is localized in places where new faults would initiate to improve the mechanical efficiency of the fault system in particular the eastern california shear zone the san jacinto fault the elsinore fault and the offshore dextral faults all developed in places of highly localized strain these younger faults compensate for the reduced fault slip on the san andreas fault proper because of the big bend a major restraining bend the evolution of the fault system changes the apportionment of fault slip rates over time which may explain some of the slip rate discrepancy between geological and geodetic measurements in southern california for the present fault configuration our model predicts localized strain in western transverse ranges and along the dextral faults across the mojave desert where numerous damaging earthquakes occurred in recent years

aspherical structure of the core mantle boundary from pkp travel times maps of aspherical structure in the vicinity of the core mantle boundary have been constructed from a tomographic analysis of p df and p ab travel times in the distance range 150 180 these maps reveal long wavelength features not previously detected by mantle tomography the peak to peak amplitude of these features is on the order of 1 s in vertical travel time too large to be explained by conventional thermal boundary layer models or dynamically supported topography on the core mantle boundary we hypothesize the existence of one or more chemical boundary layers whose large scale accumulations on the surface of the core may in some ways be analogous to continents at the earth s surface

effect of normal stress during rupture propagation along nonplanar faults 1 we study a symmetrical three forked shear fault under simple triaxial stress rupture initiates in the central primary plane and propagates toward the branching point one of the two branches is in the compressional quadrant while the other is in the tensional domain we study the following question does rupture always prefer propagating along the tensional branch because of the effect of normal stress when the primary plane is located in the most favorable direction as determined by mohr coulomb criterion one finds that the compressional branch is almost always preferred for various dynamic coefficients and confining pressures simultaneous rupture propagation on both branches appears only under a very special condition found by aochi et al 2000b these results indicate that shear stress changes produced by the rupture front dominate during dynamic rupture propagation compared to normal stress changes this result depends completely on the orientation of the whole fault system in all cases we can explain the branch selection by analyzing the applied initial shear stress and the fault properties change of normal stress may play a role similar to shear stress change for disjoint faults since the effect of normal stress change sometimes dominates

a numerical study of strike slip bend formation with application to the salton sea pull apart basin how stepovers of strike slip faults connect to form bends is a question important for understanding the formation of push up ranges restraining bends and pull apart basins releasing bends we investigated the basic mechanics of this process in a simple three dimensional viscoelastoplastic finite element model our model predicts localized plastic strain within stepovers that may eventually lead to the formation of strike slip bends major parameters controlling strain localization include the relative fault strength geometry of the fault system and the plasticity model assumed using the drucker prager plasticity model in which the plastic yield strength of the crust depends on both shear and normal stresses our results show that a releasing bend is easier to develop than a restraining bend under similar conditions these results may help explain the formation of the salton sea pull apart basin in southern california 0 5 0 1 ma ago when the stepover between the imperial fault and the san andreas fault was connected by the brawley seismic zone

strain partitioning and stress perturbation around stepovers and bends of strike slip faults numerical results abstract stepovers and bends along strike slip faults are where push up ranges and pull apart basins are formed they are also commonly where fault ruptures terminate field study and analogue models suggest that the configuration of faults plays a key role in crustal deformation around bends and stepovers but the related mechanics of stress perturbation strain partitioning and fault evolution remains poorly understood here we present results of systematical mechanical models of stress changes and strain partitioning around simple stepovers and bends using three dimensional viscoelasto plastic finite element code our model predicts elevated deviatoric stress around all stepovers and bends with higher stresses around the restraining ones narrow stepovers localize strain between the fault gaps to form connecting faults whereas wide stepovers localize strain on the tips of fault segments so the stepovers may evolve into subparallel faults we explored how various configurations of stepovers and bends change the stress field and strain distribution and show that these results can help explain some key differences between the pull apart basins in the dead sea trough and death valley and the push up ranges along the san andreas fault

the effects of double fault bends on rupture propagation a geometrical parameter study we use the 2d finite element method to determine how geometrical parameters determine whether rupture will propagate across a linked stepover in a strike slip fault the end segments of the fault system are aligned in the direction of maximum shear and the length and angle of the linking segment are allowed to vary we observe that ruptures propagate through extensional stepovers with steeper angles and longer linking segments than otherwise equivalent compressional step overs these different rupture behaviors form distinct regions in angle stepover length parameter space the boundary between these regions takes the shape of an asymptotic curve in both the extensional and compressional cases models in which the size of the entire fault system was made larger or smaller revealed that the location of the bound aries between regions of different rupture behavior do not scale linearly with the system size it was easier to rupture steeper and relatively longer stepovers in fault systems that were larger overall a separate set of models in which the stress field is rotated so that the parallel end segments were optimally aligned for rupture significantly altered the rupture behavior curves in this stress field it was easier to rupture compressional stepovers with steeper angles and longer linking segments than it was to rupture equivalent extensional stepovers in both the case in which the end segments are aligned with the direction of maximum shear and the case in which the end segments are optimally oriented for rupture the angles at which rupture could no longer propagate through the entire fault corresponded with peaks in the fault s s value

lateral heterogeneity and azimuthal anisotropy of the upper mantle love and rayleigh waves 100 250 s the lateral heterogeneity and apparent anisotropy of the upper mantle are studied by measuring rayleigh and love wave phase velocities in the period range 100 250 s spherical harmonic descriptions of the lateral heterogeneity are obtained for order and degree up to 1 m 10 slow regions are evident at the east pacific rise northeast africa tibet tasman sea southwestern north america and triple junctions in the northern atlantic and indian oceans fast regions occur in australia western pacific and the southern atlantic these features are also found by a completely different analysis based on the backus gilbert method the backus gilbert method also shows that the obtained phase velocities are averaged values within an area of about 2000 km radius and the errors are about 1 of the phase velocity in the zeroth order spherically symmetric earth inversion for azimuthal dependence shows that for low angular order the fast phase velocity directions seem to correlate well with the plate motion vectors however resolution analysis by the backus gilbert method shows that the current data do not have enough resolution for everywhere on the globe only a few regions currently have adequate azimuthal coverage thus confirmation of the above correlation requires a more complete data set

dynamically modeling fault step overs using various friction laws it is well known that fault step overs can under some circumstances allow through going rupture and under other circumstances cause rupture to terminate however the effects of different friction law formulations on jumping rupture have not been extensively explored in this study we use the 2 d dynamic finite element method to investigate how different frictional parameterizations affect the ability of rupture to jump a step over in both extensional and compressional settings we compare linear slip weakening friction and three forms of rate and state dependent friction the aging law slip law and slip law with strong rate weakening we have found that for friction parameterizations with the same effective slip weakening distance the functional forms of such friction laws can have a significant effect on maximum jump distance with friction laws scaled to have equivalent fracture energies we find that the functional forms of such friction laws have a second order effect on jumping rupture finally with our specific parameterizations we find that delays in rupture across the step over systems can lead to a previously unseen mode of supershear transition once the rupture renucleates on the secondary fault segment even if the initial stress field precludes such a transition studies using multiple friction laws in complex geometries such as fault step overs can lead to better understanding of the dependence of rupture properties on the type of friction law utilized in models and statistical analysis

s to p scattering at the 650 km discontinuity summary a search of seismograms recorded at the warramunga seismic array wra from events occurring below the izu bonin islands shows an arrival on some but not all of the records with an onset at 25 30 s after p which is not predicted by the standard travel time tables the slowness and azimuth of the phase show that it is generated almost in line with p and the variation of arrival time with the hypocentral depth of the earthquake indicates that its origin lies on the receiver side of the source it appears in fact to be an s to p conversion at a depth of 650 700 km which is seen only when the receiver is close to a node of the p radiation pattern and an antinode for s so that its amplitude compared with that of p is at a maximum finally the duration of the phase indicates that it is not simply a refracted wave but that it has a coda of scattered arrivals from lateral heterogeneity in the neighbourhood of 650 km below the izu bonin islands

a 3 d viscoelastoplastic model for simulating long term slip on non planar faults summary the geometrical complexity of non planar faults is known to affect not only fault slip rates but also crustal deformation and seismicity in the surrounding region but implementing non planar faults in numerical models has been challenging we have developed a 3 d viscoelastoplastic finite element model to simulate long term steady state fault slip along conceptual restraining bends and crustal deformation around them by incorporating plastic yielding in both fault zones and the surrounding crust this model can predict the quasi steady state stress field and strain partitioning while avoiding pathological stress build up encountered in traditional viscoelastic models here we detail the formulation of this model and explore major model parameters the model results show that restraining bends tend to impede fault slip increase shear stress in the surrounding crust and localize strain in a pair of belts off the fault zone these effects are enhanced by high viscosity in the lower crust and upper mantle high aspect ratio of bend width over fault length fast relative motion of fault blocks and high fault friction these model results may help explain the diffuse deformation surrounding the big bend of the san andreas fault and the more confined deformation around the lebanon bend of the dead sea fault

the application of synthetic seismograms to the interpretation of the upper mantle p wave velocity structure in northern australia abstract during the period 1974 to 1977 a long range seismic refraction project was conducted in central australia along a profile extending south from darwin earthquakes from the banda sea region were used as seismic sources for this experiment an analysis by hales and co workers of the resulting data based on travel times and using geometric raytracing techniques has resulted in the construction of an upper mantle velocity model using synthetic seismograms to model amplitudes it is shown that additional constraints can be placed on the derived velocity profile the low velocity zone beneath the 200 km discontinuity is found to have a more abrupt onset than was previously suggested a smaller discontinuity at 325 km depth is now implied the analysis suggests that the 400 km discontinuity is a first order velocity increase whereas all other observed upper mantle discontinuities are more satisfactorily modelled as second order type structures

aspherical earth structure from fundamental spheroidal mode data free oscillation data reveal heterogeneity in the earth s mantle whose geographical pattern is dominated by spherical harmonics of angular degree two and correlates well with the hydrostatically referenced geoid the heterogeneity can be modelled as localized in the transition zone 420 670 km depth and may be related to a large scale component of mantle convection

heterogeneous fault stresses from previous earthquakes and the effect on dynamics of parallel strike slip faults 1 we combine a viscoelastic model for the interseismic process and an elastodynamic model for the coseismic process to explore the dynamics over multiple earthquake cycles of two parallel strike slip faults embedded in a two dimensional full space the step over fault geometry results in a buildup of heterogeneous fault stress near the step over this heterogeneous stress accumulates at the early stage of the evolution of the fault system and finally stabilizes after a number of earthquake cycles the heterogeneity in fault stress varies with the geometrical parameters e g width and along strike overlap gap of the step over as well as the rupture history of the fault system this heterogeneous fault stress from previous earthquakes has significant effects on earthquake rupture initiation propagation and termination the locations with a low normal stress level near a step over are favorable points for earthquake initiation rupture can jump a 4 km wide compressional step over and a 8 km or wider dilational step over if the fault system has historically experienced many earthquakes a young step over with less induced heterogeneity allows rupture to jump only smaller step over widths these results may have important implications for seismic hazard analysis in areas where segmented strikeslip faults predominate particularly for estimating maximum earthquake potential citation duan b and d d oglesby 2006 heterogeneous fault stresses from previous earthquakes and the effect on dynamics of parallel strike slip faults j geophys res 111 b05309 doi 10 1029 2005jb004138

geometrical impact of the san andreas fault on stress and seismicity in california 1 most large earthquakes in northern and central california clustered along the main trace of the san andreas fault saf the north american pacific plate boundary however in southern california earthquakes were rather scattered here we suggest that such along strike variation of seismicity may largely reflect the geometrical impact of the saf using a dynamic finite element model that includes the first order geometric features of the saf we show that strain partitioning and crustal deformation in california are closely related to the geometry of the saf in particular the big bend is shown to reduce slip rate on southern saf and cause high shear stress and strain energy over a broad region in southern california and a belt of high strain energy in the eastern california shear zone

rupture termination and jump on parallel offset faults proper evaluation of seismic hazard depends on accurate estimates of potential earthquake size in areas with complex multisegment fault systems such an estimate in turn depends on an ability to predict the circumstances under which rupture may jump between fault segments many observational and numerical studies have analyzed the phenomenon of jumping rupture but none has focused on how the process of rupture termination on the primary nucleating fault segment affects the ability of rupture to jump to a secondary fault segment in the current study i model the dynamics of a simple 2d strike slip fault system with two parallel segments arranged with either a compressional or extensional stepover i vary the suddenness with which the initial shear stress tapers to zero on the primary section if the initial shear stress goes to zero over a very small 100 m distance rupture readily jumps both compressional and extensional stepovers of 1 km if the initial shear stress tapers to zero over 1 km rupture can jump the compressional stepover but not the extensional stepover if the initial shear stress tapers to zero over 2 5 km rupture cannot jump either the compressional or the extensional stepover the results illustrate the importance of the slip gradient and the resultant static stress field and the acceleration of the rupture front and the resultant generation of stopping phases in determining the probability of jumping rupture

rupture propagation beyond fault discontinuities significance of fault strike and location summary earthquake rupture sometimes occurs on several faults and often decelerates or terminates at fault stepovers factors that control the rupture processes across stepovers are important for an understanding of earthquake growth and termination in this study we investigate such factors by calculating the spontaneous rupture processes of two non coplanar faults in a 3 d model dealing with two extreme models in which two strike slip faults are either parallel or perpendicular we show that the rupture processes beyond fault discontinuities are drastically different for the two models we find three factors influencing rupture processes beyond fault discontinuities depth of the upper edge of the two faults location of the edge of the first fault and geometry of the two faults these factors determine the time and location of rupture jumps to the second fault for rupture propagation to the second fault it is essential for rupture on the first fault to arrive at the edge of the fault in particular whether rupture on the first fault reaches the earth s surface or not controls the difficulty of rupture jumps and the locations where the rupture is triggered which is also related to the step direction of the two faults this is because the stress perturbation at the fault edge is affected by the earth s free surface

complex multifault rupture during the 2016 mw 7 8 kaikōura earthquake new zealand the 2016 moment magnitude m w 7 8 kaik ura earthquake was one of the largest ever to hit new zealand hamling et al show with a new slip model that it was an incredibly complex event unlike most earthquakes multiple faults ruptured to generate the ground shaking a remarkable 12 faults ruptured overall with the rupture jumping between faults located up to 15 km away from each other the earthquake should motivate rethinking of certain seismic hazard models which do not presently allow for this unusual complex rupture pattern science this issue p eaam7194 1 1 lookup doi 10 1126 science aam7194

dynamic 3d simulations of earthquakes on en echelon faults one of the mysteries of earthquake mechanics is why earthquakes stop this process determines the difference between small and devastating ruptures one possibility is that fault geometry controls earthquake size we test this hypothesis using a numerical algorithm that simulates spontaneous rupture propagation in a three dimensional medium and apply our knowledge to two california fault zones we find that the size difference between the 1934 and 1966 parkfield california earthquakes may be the product of a stepover at the southern end of the 1934 earthquake and show how the 1992 landers california earthquake followed physically reasonable expectations when it jumped across en echelon faults to become a large event if there are no linking structures such as transfer faults then strike slip earthquakes are unlikely to propagate through stepover s 5 km wide

short period s p conversion in the mantle at a depth near 700 km summary data from 16 deep earthquakes originating in the tonga subduction zone and recorded at the warramunga seismic array wra in central australia show the presence of a distinct later arrival in the coda of p this phase which is not predicted by smooth velocity depth models of the earth has been called x in the paper slownesses and arrival azimuths of x are similar to those of the p wave it is unlikely that x was radiated as the result of multiple focal activity because a corresponding phase was not observed neither in the coda of scp nor on seismograms of seismic stations located at close epicentral distances relative to p the variation in arrival times with focal depth of the earthquakes indicates that x is produced by s p conversion below the source at a depth near 700 km most of the observed x phases have approximately the same signal duration as the p waves thus suggesting a transition zone near 700 km which is only a few kilometres thick synthetic seismograms calculated for a model with a first order seismic discontinuity at a depth of 700 km show reasonable agreement with the observations as demonstrated by the synthetic seismogram modelling the amplitude of x depends on the radiation pattern of the earthquake source variations in the radiation pattern offer a plausible explanation that first x is only intermittently detected at wra and secondly that x has most frequently been observed from the southern part of the tonga subduction zone

tectonics of strike slip restraining and releasing bends restraining and releasing bends are common but enigmatic features of strike slip fault systems occurring in all crustal environments and at regional to microscopic scales of observation regional scale restraining bends are sites of mountain building transpressional deformation and basement exhumation whereas releasing bends are sites of topographic subsidence transtensional deformation basin sedimentation and possible volcanism and economic mineralization because restraining and releasing bends often occur as singular self contained domains of complex deformation they are appealing natural laboratories for earth scientists to study fault processes earthquake seismology active faulting and sedimentation fault and fluid flow relationships links between tectonics and topography tectonic and erosional controls on exhumation and tectonic geomorphology this volume addresses the tectonic complexity and diversity of strike slip restraining and releasing bends with 18 contributions divided into four thematic sections 1 a topical review of fault bends and their global distribution 2 bends sedimentary basins and earthquake hazards 3 restraining bends transpressional deformation and basement controls on development 4 releasing bends transtensional deformation and fluid flow

mapping the upper mantle three dimensional modeling of earth structure by inversion of seismic waveforms a method is presented for the inversion of waveform data for the three dimensional distribution of seismic wave velocities the method is applied to data from the global digital networks international deployment of accelerometers global digital seismograph network the selected data set consists of some 2000 seismograms corresponding to 53 events and 870 paths the moment tensors of the earthquakes are determined through an iterative procedure which minimizes the corrupting influence of lateral heterogeneity a global model is constructed for shear wave velocity expanded up to degree and order 8 in spherical harmonics and described by a cubic polynomial in depth for the upper 670 km of the earth s mantle although no a priori information is incorporated the model predictions reproduce much of what is known about the dispersion of mantle waves for example high phase velocities for shields low velocities at ridges and a strong degree 2 pattern for rayleigh waves since the method makes use of complete waveforms overtone data are also included it is shown that the model is reproducible in that substantially the same model can be constructed from each half of the total data set considered independently the model shows that shields and ridges are major features in the depth interval 25 250 km the ridges of the southern pacific and the larger shields persist to 350 km but the southeast indian rise is underlain by a high velocity anomaly at this depth as is much of the mid atlantic ridge at 450 650 km the major features are a broad region of high velocities incorporating south america much of the south atlantic and parts of west africa a broad region of low velocities in the central and eastern pacific high velocities in the western pacific and a low velocity anomaly beneath the red sea and the gulf of aden in the absence of a crustal correction degrees 2 and 3 show a high positive correlation with the geoid paradoxically this is largely destroyed when the distribution in crustal thickness is taken into account spherical harmonic degrees 4 7 show a significant negative correlation

dynamics of fault interaction parallel strike slip faults we use a two dimensional finite difference computer program to study the effect of fault steps on dynamic ruptures our results indicate that a strike slip earthquake is unlikely to jump a fault step wider than 5 km in correlation with field observations of moderate to great sized earthquakes we also find that dynamically propagating ruptures can jump both compressional and dilational fault steps although wider dilational fault steps can be jumped dilational steps tend to delay the rupture for a longer time than compressional steps do this delay leads to a slower apparent rupture velocity in the vicinity of dilational steps these dry cases assumed hydrostatic or greater pore pressures but did not include the effects of changing pore pressures in an additional study we simulated the dynamic effects of a fault rupture on undrained pore fluids to test sibson s 1985 1986 suggestion that wet dilational steps are a barrier to rupture propagation our numerical results validate sibson s hypothesis by demonstrating that the effect of the rupture on the undrained pore fluids is to inhibit the rupture from jumping dilational stepovers the basis of our result differs from sibson s hypothesis in that our model is purely elastic and does not necessitate the opening of extension fractures between the fault segments

four dimensional transform fault processes progressive evolution of step overs and bends many bends or step overs along strike slip faults may evolve by propagation of the strike slip fault on one side of the structure and progressive shut off of the strike slip fault on the other side in such a process new transverse structures form and the bend or step over region migrates with respect to materials that were once affected by it this process is the progressive asymmetric development of a strike slip duplex consequences of this type of step over evolution include 1 the amount of structural relief in the restraining step over or bend region is less than expected 2 pull apart basin deposits are left outside of the active basin and 3 local tectonic inversion occurs that is not linked to regional plate boundary kinematic changes this type of evolution of step overs and bends may be common along the dextral san andreas fault system of california we present evidence at different scales for the evolution of bends and step overs along this fault system examples of pull apart basin deposits related to migrating releasing right bends or step overs are the pliopleistocene merced formation tens of km along strike the pleistocene olema creek formation several km along strike along the san andreas fault in the san francisco bay area and an inverted colluvial graben exposed in a paleoseismic trench across the miller creek fault meters to tens of meters along strike in the eastern san francisco bay area examples of migrating restraining bends or step overs include the transfer of slip from the calaveras to hayward fault and the greenville to the concord fault ten km or more along strike the offshore san gregorio fold and thrust belt 40 km along strike and the progressive transfer of slip from the eastern faults of the san andreas system to the migrating mendocino triple junction over 150 km along strike similar 4d evolution may characterize the evolution of other regions in the world including the dead sea pull apart the gulf of paria pull apart basin of northern venezuela and the hanmer and dagg basins of new zealand d 2004 elsevier b v all rights reserved

anisotropic models of the upper mantle long period rayleigh wave and love wave dispersion data particularly for oceanic areas have not been simultaneously satisfied by an isotropic structure in this paper available phase and group velocity data are inverted by a procedure which includes the effects of transverse anisotropy anelastic dispersion sphericity and gravity we assume that the surface wave data represents an azimuthal average of actual velocities thus we can treat the mantle as transversely isotropic the resulting models for average earth average ocean and oceanic regions divided according to the age of the ocean floor are quite different from previous results which ignore the above effects the models show a low velocity zone with age dependent anisotropy and velocities higher than derived in previous surface wave studies the correspondence between the anisotropy variation with age and a physical model based on flow aligned olivine is suggestive for most of the earth sh sv in the vicinity of the low velocity zone neat the east pacific rise however sv sh at depth consistent with ascending flow anisotropy is as important as temperature in causing radial and lateral variations in velocity the models have a high velocity nearly isotropic layer at the top of the mantle that thickens with age this layer defines the lid or seismic lithosphere in the pacific the lid thickens with age to a maximum thickness of 50 km this thickness is comparable to the thickness of the elastic lithosphere the lid thickness is thinner than derived using isotropic or pseudo isotropic procedures a new model for average earth is obtained which includes a thin lid this model extends the fit of a prem type model to shorter period surface waves

inception of the eastern california shear zone and evolution of the pacific north american plate boundary from kinematics to geodynamics 1 the san andreas fault saf is the transform boundary between the pacific and the north american plates yet up to 25 of the relative plate motion is now accommodated by the eastern california shear zone ecsz here we investigate the inception of the ecsz and its geodynamic interactions with the saf using a 3 d viscoelastoplastic finite element model for a given fault configuration of the plate boundary zone the model simulates long term slip on the faults and plastic strain outside the faults our results show that the formation of the big bend of the saf around 5 12 ma impeded fault slip and localized strain along the ecsz causing its inception development of the ecsz was further enhanced by the activation of the garlock fault gf and lithospheric weakening caused by the encroachment of the basin and range extension similarly the san jacinto fault sjf in southern california developed along a belt of localized strain which resulted from the formation of the restraining bend along the san bernardino mountains segment of the saf 2 myr ago once activated the sjf reduced slip on both the southern saf and the ecsz across the mojave desert these results indicate causative relationship between the saf and the ecsz the inception of the ecsz and other young faults is the consequence of the evolving saf plate boundary zone that continuously adjusts itself to accommodate the relative plate motion

evolution of fault efficiency at restraining bends within wet kaolin analog experiments abstract restraining bends are regions of mechanical inefficiency along strike slip faults where faults evolve to improve efficiency analog experiments using wet kaolin examine the evolution of a variety of restraining bends restraining bends with 15 bends continue to slip while systems with greater bends develop new faults the new faults which flank the uplifted region accommodate right lateral slip when parallel to the plate movement and oblique slip motion when parallel to the restraining segment within the wet kaolin strain is partitioned into fault slip and off fault deformation such as distributed shear and uplift the wet kaolin produces restraining bend deformation patterns fault sequence and mechanical efficiency similar to dry sand experiments and natural restraining bends the propagation of new faults in the wet kaolin improves the mechanical efficiency of the fault system by increasing the ratio of fault slip to off fault deformation local inefficiencies such as linkage of faults via a sharp kink do not affect the overall increase in efficiency of the linked fault system furthermore wider restraining bend stepovers have lower mechanical efficiency than close stepovers but the difference in mechanical efficiency decreases as faults grow and link up around the restraining bends this study demonstrates that restraining bend fault systems evolve toward greater mechanical efficiency

analog models of restraining stepovers in strike slip fault systems scaled sandbox models have successfully simulated the geometries and progressive evolution of antiformal pop up structures developed in a weak sedimentary cover above restraining stepovers in offset sinistral strike slip fault systems in rigid basement models were run both with and without synkinematic sedimentation which was added incrementally to cover the growing antiformal structures vertical and horizontal sections of the completed models permit the full three dimensional 3 d structure of the pop ups to be analyzed in detail three representative end member experiments are described 30 underlapping restraining stepovers 90 neutral restraining stepovers and 150 overlapping restraining stepovers the experimental pop ups are typically sigmoidal to lozenge shaped antiformal structures having geometries that are dependent on both the stepover angle and stepover width in the underlying basement faults underlapping restraining stepovers typically form elongate lozenge shaped pop ups 90 neutral restraining stepovers produce shorter squat rhomboidal pop ups and overlapping restraining stepovers produce sigmoidal antiformal pop ups trans pop up cross fault systems are characteristic at large displacements on the basement fault system above the offset principal displacement zones the pop ups are commonly small narrow positive flower structures whereas in the stepover region they widen out and become markedly asymmetric this pop up asymmetry switches across the center of the stepover where the pop ups are largely symmetical maximum rotations measured within the central highly uplifted region of the pop ups increase from 7 counterclockwise for the underlapping 30 stepovers to 14 counterclockwise for the neutral 90 stepovers to 16 counterclockwise for the overlapping 150 stepovers in models where no synkinematic sediments were added during deformation the pop up structures are bound by convex flattening upward oblique slip reverse fault systems that link downward to the offsets in the basement fault system in contrast in the begin page 234 experiments where synkinematic sediments were added incrementally during deformation the pop ups are formed by oblique slip reverse faults that steepen upward into the synkinematic strata with the formation of fault propagation growth folds the analog models are compared with natural examples of pop up structures and show strong similarities in structural geometries and stratal architectures these models may provide structural templates for seismic interpretation of complex contractional structures in offset strike slip fault systems

dynamic rupture models of earthquakes on the bartlett springs fault northern california the bartlett springs fault bsf the easternmost branch of the northern san andreas fault system creeps along much of its length geodetic data for the bsf are sparse and surface creep rates are generally poorly constrained the two existing geodetic slip rate inversions resolve at least one locked patch within the creeping zones we use the 3 d finite element code faultmod to conduct dynamic rupture models based on both geodetic inversions in order to determine the ability of rupture to propagate into the creeping regions as well as to assess possible magnitudes for bsf ruptures for both sets of models we find that the distribution of aseismic creep limits the extent of coseismic rupture due to the contrast in frictional properties between the locked and creeping regions

coseismic slip gradient and rupture jumps on parallel strike slip faults abstract field observations of slip distribution along large strike slip faults and preliminary rupture model simulations reveal a possible correlation between slip gradient near a fault end and the ability of a rupture to jump over a structure stepover in a strike slip fault system we simulate the dynamic rupture process on two parallel strike slip faults embedded in an elastic medium to investigate this correlation and compare model generated results with field measured data we find that the slip gradients calculated over the final 1 km of a fault have a linear relationship with both the average stress drop on the fault and the largest width of the step that could be jumped by a propagating rupture our dynamic coulomb stress analyses show that the average stress drop on the first fault which is proportional to the slip gradient in the final 1 km determines the positive coulomb stress region at the end of the first fault which in turn determines the largest jumpable step width a larger stress drop results in a larger positive coulomb stress region around the first fault end which allows the rupture to jump a wider stepover

multicycle dynamics of nonplanar strike slip faults 1 we perform two dimensional dynamic models of strike slip faults with a change in strike a bend over multiple earthquake cycles to examine the long term effects of nonplanar fault geometry a viscoelastic model a proxy for off fault deformation and tectonic loading is introduced for the interseismic process to avoid pathological stress buildup around the bend a finite element method with an elastodynamic model is used to simulate dynamic earthquake ruptures we find that stresses near the bend differ strongly from the regional stress field and that the fault develops a relatively steady state in which the stress level and the event pattern on the fault are stable reduced normal stress on the dilatational side and increased normal stress on the compressive side of the bend during dynamic ruptures result in the bend serving as an initiation and or a termination point s for rupture typical events on such a fault consist of two classes unilateral events that rupture only the favorable segment and bilateral events that rupture the favorable segment and part of or the entire unfavorable segment in the latter class of events a time delay in rupture around the bend results from a high yield stress on the compressive side of the bend other effects of the bent fault geometry include higher displacement on the inward wall than on the outward wall higher slip on the more favorable segment than on the less favorable segment and a large slip velocity on the compressive side of the bend

evolving efficiency of restraining bends within wet kaolin analog experiments restraining bends along strike slip fault systems evolve by both propagation of new faults and abandonment of fault segments scaled analog modeling using wet kaolin allows for qualitative and quantitative observations of this evolution to explore how bend geometry affects evolution we model bends with a variety of initial angles from 0 for a straight fault to 30 high angle restraining bends 20 overcome initial inefficiencies by abandoning unfavorably oriented restraining segments and propagating multiple new inwardly dipping oblique slip faults that are well oriented to accommodate convergence within the bend restraining bends with 0 15 maintain activity along the restraining bend segment and grow a single new oblique slip fault on one side of the bend in all restraining bends the first new fault propagates at 5 mm of accumulated convergence particle image velocimetry analysis provides a complete velocity field throughout the experiments from these data we quantify the strike slip efficiency of the system as the percentage of applied plate parallel velocity accommodated as slip in the direction of plate motion along faults within the restraining bend bends with small initially have higher strike slip efficiency compared to bends with large although they have different fault geometries all systems with a 5 cm bend width reach a steady strike slip efficiency of 80 after 50 mm of applied plate displacement these experimental restraining bends resemble crustal faults in their asymmetric fault growth asymmetric topographic gradient and strike slip efficiency

the p wave upper mantle structure beneath an active spreading centre the gulf of california summary detailed analysis of short period travel time dt d and wave form data reveals the upper mantle structure beneath an oceanic ridge to depths of 900 km more than 1400 digital seismograms from earthquakes in mexico and central america recorded at scarlet yield 1753 travel times and 58 direct measurements of dt d as well as high quality stable waveforms the 29 events combine to form a continuous record section from 9 to 40 with an average station spacing of less than 5 km first the travel times are inverted using the tau method of bessonova et al the resultant model is adjusted to agree with the experimental p values further constraints arise from the observed relative amplitudes of mantle phases which are modelled by trial and error using wkbj synthetic seismograms chapman wiggins model gca which is appropriate for western mexico north of 20 latitude is similar to existing upper mantle models for shield tectonic continental and arc trench regimes below 400 km but differs significantly in the upper 350 km gca velocities are very low in this depth range the model catches up with the others with a very large velocity gradient from 225 to 390 km this well resolved feature is consistent with the shear wave model tna for western north america of grand helmberger the abundant data from 20 to 30 control the detailed shape of the 660 km discontinuity very large velocity gradients lie both above 620 660 km and below 661 680km a 2 8 per cent velocity change

dynamics of parallel strike slip faults with pore fluid pressure change and off fault damage we use a 2d finite element program to investigate how effects of time dependent pore pressure and off fault damage in the form of plastic yielding could affect earthquake rupture on parallel strike slip faults with a stepover from single fault tests we find that the positive coulomb stress pcs region at the end of the first fault controls the rupture initiation time and location on the second fault plastic deformation could significantly reduce the effective normal stress and adjust the shear stress in a specific direction resulting in a narrowband with pcs in a dilatational stepover favoring the initiation of rupture for a compressive stepover the effect of plastic deformation is less obvious and the crescent shaped pcs region triggers rupture initiation on the second fault the undrained pore pressure increases the effective normal stress in a dilatational stepover which significantly reduces the jumping ability of rupture when both undrained pore pressure and significant off fault damage are present the effect of undrained pore pressure dominates in the dilatational stepover whereas plastic deformation in the compressive stepover slightly reduces the maximum jumpable width

the p wave velocity structure near the base of the mantle travel times from 16 events recorded at two temporary networks of short period seismometers deployed across australia have been used to study the p wave velocity structure of the mantle below depths of 1700 km the slowness distance curve derived from these times shows small deviations from the slownesses predicted from the preliminary reference earth model of dziewonski and anderson 1981 that may be significant the most important feature is a rapid decrease of more than 0 5 s degree between distances of 85 and 88 velocity filtering of seismograms from 5 nuclear explosions in kazakh recorded at these australian networks indicates the presence of two arrival branches at distances between 83 and 91 whose slownesses differ by 0 40 to 0 55 s degree and which intersect at a distance of 87 processed p wave seismograms from the yellowknife array in canada also indicate the presence of a similar phenomenon in the same distance range in two separate regions the results can be explained by a widespread rapid or sharp increase in velocity of 2 5 to 3 0 about 200 km above the core mantle boundary

fault steps and the dynamic rupture process 2 d numerical simulations of a spontaneously propagating shear fracture fault steps may have controlled the sizes of the 1966 parkfield 1968 borrego mountain 1979 imperial valley 1979 coyote lake and the 1987 superstition hills earthquakes this project investigates the effect of fault steps of various geometries on the dynamic rupture process we have used a finite difference code to simulate spontaneous rupture propagation in two dimensions we employ a slip weakening fracture criterion as the condition for rupture propagation and examine how rupture on one plane initiates rupture on parallel fault planes the geometry of the two parallel fault planes allows for stepover widths of 0 5 to 10 0 km and overlaps of 5 to 5 km our results demonstrate that the spontaneous rupture on the first fault segment continues to propagate onto the second fault segment for a range of geometries for both compressional and dilational fault steps a major difference between the compressional and dilational cases is that a dilational step requires a longer time delay between the rupture front reaching the end of the first fault segment and initiating rupture on the second segment therefore our dynamic study implies that a compressional step will be jumped quickly whereas a dilational step will cause a time delay leading to a lower apparent rupture velocity we also find that the rupture is capable of jumping a wider dilational step than compressional step

the effects of d0 on rupture propagation on fault stepovers abstract according to the slip weakening friction law andrews 1976a fault friction drops from a static value to a sliding value over some slip weakening distance d 0 laboratory studies of fault friction confirm this weakening but do not settle on a single value for d 0 however this parameter is a critical input in dynamic earthquake models as it affects rupture nucleation and propagation we use the 2d finite element method to test how the specific value of d 0 affects rupture behavior we consider the ability of rupture to jump from segment to segment of a disconnected stepover or to propagate through the double bends of a connected stepover as a way to gauge rupture energy and behavior we find the stepover width or bend angle through which rupture can propagate is affected by the choice of d 0 but it is not a linear relationship this is true regardless of whether the stepover is connected or disconnected compressional or extensional or whether the initial rupture velocity is supershear or subshear we do find however that scaling the intensity of the regional stress field up or down does linearly correspond to scaling d 0 down or up we interpret the nonlinearity of the d 0 scaling results to be related to the area of the secondary fault that intersects the lobe of failure level coulomb stress change induced by rupture of the first fault segment because 1 the d 0 controlled critical patch size for renucleation must be exceeded on the second segment and 2 the area of this overlap also does not change linearly with distance from the first fault

complex fault geometry and rupture dynamics of the mw 6 5 2016 october 30th central italy earthquake we study the october 30th 2016 norcia earthquake mw 6 5 to retrieve the rupture history by jointly inverting seismograms and coseismic gps displacements obtained by dense local networks the adopted fault geometry consists of a main normal fault striking n155 and dipping 47 belonging to the mt vettore mt bove fault system vbfs and a secondary fault plane striking n210 and dipping 36 to the nw the coseismic rupture initiated on the vbfs and propagated with similar rupture velocity on both fault planes up dip from the nucleation point two main slip patches have been imaged on these fault segments both characterized by similar peak slip values 3 m and rupture times 3 s after the breakage of the two main slip patches coseismic rupture further propagated southeastward along the vbfs rupturing again the same fault portion that slipped during the august 24th earthquake the retrieved coseismic slip distribution is consistent with the observed surface breakages and the deformation pattern inferred from insar measurements our results show that three different fault systems were activated during the october 30th earthquake the composite rupture model inferred in this study provides evidences that also a deep portion of the nne trending section of the olevano antrodoco sibillini oas thrust broke co seismically implying the kinematic inversion of a thrust ramp the obtained rupture history indicates that in this sector of the apennines compressional structures inherited from past tectonics can alternatively segment boundaries of nw trending active normal faults or break co seismically during moderate to large magnitude earthquakes

distributary mouth bar development and role of submarine landslides in delta growth south pass mississippi delta submarine landslides play a major role in the development of distributary mouth bars and are of major importance in transporting sediment from the bar front to deeper water along the mississippi delta front historic maps of the south pass of the mississippi delta show that the bar advanced seaward more than 1 mi 1 6 km between 1867 and 1953 details of the growth of the bar have been elucidated using an elaborate computer modeling program to analyze these historic maps the analysis has shown that the geometry of the bar was controlled by the dynamics of the freshwater plume of river water as it mixed with saline gulf water approximately half the sediment deposited on the bar was moved into deeper water by submarine landslides the underlying causes of bar failure were established during major floods with the deposition of thick blankets of unstable water saturated sediments on the bar front failure occurred one to four years later in response to a variety of triggering mechanisms which either changed the shear strength of the sediment or modified local bottom slope the triggering mechanisms include major storms and hurricanes mudlump activity and possibly increased pore pressures resulting from generation of biogenic gas bar growth and bas nward movement of sediment thus represent a multivariate problem that can be approached by means of a computer analysis of bathymetric data

spatial and temporal distribution of slip for the 1992 landers california earthquake we have determined a source rupture model for the 1992 landers earthquake m w 7 2 compatible with multiple data sets spanning a frequency range from zero to 0 5 hz geodetic survey displacements near field and regional strong motions broadband teleseismic waveforms and surface offset measurements have been used explicitly to constrain both the spatial and temporal slip variations along the model fault surface our fault parameterization involves a variable slip multiple segment finite fault model which treats the diverse data sets in a self consistent manner allowing them to be inverted both independently and in unison the high quality data available for the landers earthquake provide an unprecedented opportunity for direct comparison of rupture models determined from independent data sets that sample both a wide frequency range and a diverse spatial station orientation with respect to the earthquake slip and radiation pattern in all models consistent features include the following 1 similar overall dislocation patterns and amplitudes with seismic moments of 7 to 8 10 26 dyne cm seismic potency of 2 3 to 2 7 km 3 2 very heterogeneous unilateral strike slip distributed over a fault length of 65 km and over a width of at least 15 km though slip is limited to shallower regions in some areas 3 a total rupture duration of 24 sec and an average rupture velocity of 2 7 km sec and 4 substantial variations of slip with depth relative to measured surface offsets the extended rupture length and duration of the landers earthquake also allowed imaging of the propagating rupture front with better resolution than for those of prior shorter duration strike slip events our imaging allows visualization of the rupture evolution including local differences in slip durations and variations in rupture velocity rupture velocity decreases markedly at shallow depths as well as near regions of slip transfer from one fault segment to the next as rupture propagates northwestward along the multiply segmented fault length the rupture front slows as it reaches the northern limit of the johnson valley landers faults where slip is transferred to the southern homestead valley fault an abrupt acceleration is apparent following the transfer this process is repeated and is more pronounced as slip is again passed from the northern homestead valley fault to the emerson fault although the largest surface offsets were observed at the northern end of the rupture our modeling indicates that substantial rupture was also relatively shallow less than 10 km in this region

the interaction of a subducting lithospheric slab with a chemical or phase boundary we have used a finite element model of time dependent convection to determine the conditions for penetration of the subducted plate into the lower mantle a temperature dependent and non newtonian rheology is applied to achieve platelike behavior of the upper and sinking thermal boundary layer of convection the 650 km discontinuity is taken as either a chemical or phase boundary or as a combination of both it is represented by a marker chain which effects additional buoyancy when distorted out of its equilibrium position when the compositional density contrast is greater than about 5 the descending slab is deflected sidewards at the boundary and two layer convection prevails a resulting depression of the boundary in the range of 50 200 km should be detectable with seismic methods below 5 density difference the slab plunges several hundred kilometers into the lower mantle and below 2 it will probably not stop before reaching the core mantle boundary and extensive mixing would be expected with a pure phase change a negative clapeyron slope of about 6 mpa k 60 bar k is required to establish a type of leaky double layer convection a more moderate slope can aid a small compositional density difference to prevent slab penetration into the lower mantle with the present uncertainties about the physical nature of the 650 km discontinuity a variety of convective styles appears possible on dynamical grounds

pull apart basin formation and development in narrow transform zones with application to the dead sea basin contrary to other examples like death valley california and the sea of marmara turkey the dead sea type pull apart basins form within a narrow transform corridor between strike slip faults that are less than 10 km apart much smaller than the crustal thickness of 35 km in this paper we investigate the role of fault zone width versus thickness and rheology on the mechanics of pull apart basins through a series of laboratory experiments results show that pull apart basins that develop above a small step over i e smaller than the thickness of the brittle layer are narrow and elongated parallel to the overall motion this is enhanced by increased decoupling along a basal ductile layer the experiment with the highest degree of mechanical decoupling shows a striking resemblance to the dead sea basin dsb comparison with modeling results suggests that the dsb s flat basin floor is bordered over its full length by strike slip faults that control the basin geometry and temporal and spatial basin migration this is in strong contrast to death valley type pull apart basins that are highly oblique to the transform direction with transverse normal faults dominating over longitudinal strike slip faults results imply that lithosphere rheology and the ratio of basin width to crustal thickness are controlling factors in the mechanics of pull apart basin formation within transform corridors like the dead sea fault

finite element models of stress orientations in well developed strike slip fault zones implications for the distribution of lower crustal strain finite element models are used to examine the effects of strike slip earthquakes on stresses in an elastic layer overlying a finite width viscoelastic shear zone in the lower crust the overall dimensions of the model are 300 km wide 400 km long and 50 km deep three geometries for the lower crustal shear zone are considered 1 a viscoelastic half space approximation with a shear zone that extends to the model boundaries 300 km in width 2 a wide shear zone model 70 km in width and 3 a narrow shear zone model 10 km in width earthquakes are simulated with a fault plane that slips without friction at the desired time step and is centered above the shear zone extending to a depth of 15 km and running the length of the mesh far field plate velocity boundary conditions are enforced at the model edges so that stress on the fault evolves naturally a coulomb type failure criterion based on the average shear stress on the fault is set such that the earthquake cycle is 250 years the models are run until a limit cycle is reached and transient stresses have decayed away we focus on the maximum changes in the stress field during the earthquake cycle by examining stresses before and immediately after each earthquake in addition to comparisons of the separate components of the stress tensor we present results in the form of maximum compressive stress orientations plunge angles of the principal stress axes and beach ball diagrams that facilitate visualizing the full tensor stresses are concentrated in the upper crust where it overlies the finite width viscoelastic shear zone which causes the plunge angles of the principal stress axes to rotate from andersonian orientations of 90 and 0 to angles that approach 45 in the lower crust our results suggest that an examination of stress orientations in the upper and middle crust from borehole breakouts and focal mechanisms may provide insight as to the distribution of strain in the lower crust and may eventually allow us to distinguish between localized and distributed deformation models for the lower crust in active strike slip zones

fault behavior and characteristic earthquakes examples from the wasatch and san andreas fault zones paleoseismological data for the wasatch and san andreas fault zones have led to the formulation of the characteristic earthquake model which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum analysis of scarp derived colluvium in trench exposures across the wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes at all of the sites studied the displacement per event has been consistently large measured values range from 1 6 to 2 6 m and the average is about 2 m on the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry six major segments are recognized along the wasatch fault on the basis of the most likely number of surface faulting events 18 that have occurred on segments of the wasatch fault zone during the past 8000 years an average recurrence interval of 400 666 years with a preferred average of 444 years is calculated for the entire zone geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south central segment of the san andreas fault suggest that the m 8 1857 earthquake is a characteristic earthquake for this segment comparisons of earthquake recurrence relationships on both the wasatch and san andreas faults based on historical seismicity data and geologic data show that a linear constant b value extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes the characteristic earthquake appears to be a fundamental aspect of the behavior of the wasatch and san andreas faults and may apply to many other faults as well

upper mantle converted waves beneath the nars array several registrations of teleseismic events on the nars array show a phase which can be identified as a p to s conversion at the 670 km discontinuity beneath western europe variations in the amplitude and arrival time of the so called ps phase are observed for different azimuths a single station multiple event stacking technique was applied to the data to determine the consistency of the phase the results are indicative of amplitude arrival time and slowness variations of the converted phase on a local scale these observations are most logically explained by a model with depth variations of the 670 km discontinuity of the order of 20 km over a distance of 200 km

nonuniform prestress from prior earthquakes and the effect on dynamics of branched fault systems 1 to examine the effects of branched fault geometry on the dynamics of fault systems in the long term we perform multicycle simulations on generic faulting models an explicit finite element algorithm is used to simulate spontaneous dynamic rupture of earthquakes the fault stress during the interseismic period is evaluated by an analytical viscoelastic model we find that the fault prestress field becomes highly nonuniform near the branch point and on the two branch segments over multiple earthquake cycles owing to the branched fault geometry and stress interaction between the two segments the principal prestress on faults rotates over multiple earthquake cycles and departs from the regional stress field significantly near the branch point after a number of earthquake cycles the branched fault systems evolve to a steady state in which several patterns of the fault prestress and earthquake rupture repeat the nonuniform prestress developed from previous earthquakes has large effects on the rupture and slip patterns several different rupture scenarios can occur on a given branched fault system in addition backward branching can occur in the nonuniform prestress field either driven by slip on the stem of the fault system or through a triggering mechanism these modeling results may have important implications for understanding fault branching behavior observed in the 1992 landers the 1999 hector mine and the 2002 denali fault earthquakes and for seismic hazard analysis in the areas where branched fault systems exist

effect of seismogenic depth and background stress on physical limits of earthquake rupture across fault step overs earthquakes can rupture geometrically complex fault systems by breaching fault step overs quantifying the likelihood of rupture jump across step overs is important to evaluate earthquake hazard and to understand the interactions between dynamic rupture and fault growth processes here we investigate the role of seismogenic depth and background stress on physical limits of earthquake rupture across fault step overs our computational and theoretical study is focused on the canonical case of two parallel strike slip faults with large aspect ratio uniform prestress and friction properties we conduct a systematic set of 3 d dynamic rupture simulations with different seismogenic depth step over distance and initial stresses we find that the maximum step over distance h c that a rupture can jump depends on seismogenic depth w and strength excess to stress drop ratio s commonly used to evaluate probable rupture velocity as h c w s n where n 2 when h c w 1 5 and n 1 otherwise the critical nucleation size largely controlled by frictional properties has a second order effect on h c rupture on the secondary fault is mainly triggered by the stopping phase emanated from the rupture end on the primary fault asymptotic analysis of the peak amplitude of stopping phases sheds light on the mechanical origin of the relations between h c w and s and leads to the scaling regime with n 1 in far field and n 2 in near field the results suggest that strike slip earthquakes on faults with large seismogenic depth or operating at high shear stresses can jump wider step overs than observed so far in continental interplate earthquakes

mapping the lower mantle determination of lateral heterogeneity in p velocity up to degree and order 6 the data from the international seismological centre bulletins for the years 1964 1979 are used to derive a three dimensional model of lateral variations of the p velocity in the lower mantle unlike previous studies in which such perturbations were represented by a blocklike parameterization we seek the solution in the form r k 0k l 0l m 0l k r kalm cos m kblm sin m plm cos some 500 000 travel time residuals for teleseismic distances from 5000 earthquakes are used in an iterative procedure to derive the coefficients a and b the maximum k and l used are 4 and 6 respectively in each iteration in addition to solving for the three dimensional structure we also relocate all the earthquakes perturb the average travel time curve and determine station corrections for over 1000 stations particular attention is given to the problem of weighting the individual observations in order to avoid as much as possible the bias due to the uneven distribution of sources and receivers the resulting model shows a high level of perturbations just below the 670 km discontinuity and just above the core mantle boundary where the maximum perturbations reach 1 1 5 of the average velocity even for this highly smoothed model at a depth of 2000 km the rms perturbations are some 3 to 4 times lower than at either of the two boundaries the model predicts well the large scale pattern of observed travel time residuals for various source regions except for the distinct effects of the subduction zones the most striking large scale three dimensional feature of the model is a ring of high velocities circumscribing the pacific basin within a depth range from 1000 km to the core mantle boundary if the proportionality between perturbations in the velocity and temperature is assumed the derived pattern of anomalies could be consistent with a two boundary layer model of convection in the lower mantle some caution however should be exercised in interpreting the results near the 670 km discontinuity where the resolving power of our data set is limited by the lack of adequate control of the upper mantle structure at the same time a significant global increase in the level of perturbations in the lowermost mantle is established beyond question the results of dziewonski et al 1977 on the correlation between the equivalent geoid and the observed geoid for the angular order numbers 2 and 3 is confirmed but there is no significant correlation for degrees from 4 to 6 these calculations are made for the rigid earth richards and hager this issue show that introduction of finite viscosity can change the sign of a geoid anomaly in addition recent studies of lateral variations in the shear velocity reveal the presence of large scale anomalies in the upper mantle thus while the inferences made here with respect to the origin of the gravest orders of the geopotential field are not conclusive they point the way in which results from seismology can be used to address some of the basic questions of geodynamics

reflection properties of phase transition and compositional change models of the 670 km discontinuity seismic observations of the amplitude ratio amplitude p 670p amplitude p p suggest a reflection coefficient r 7 13 for reflections from the underside of the 670 km discontinuity this can be compared with calculated reflection coefficients to constrain the nature of the mantle transition region velocity and density profiles through the transition region were calculated for olivine pyroxene garnet and olivine garnet model mineral assemblages for models with identical upper and lower mantle compositions the computed reflection coefficients from the phase transitions pertinent to the 670 km discontinuity are less than 0 5 these phase transitions produce their maximum r if they are effectively discontinuous which gives r 2 3 except for the garnet to perovskite transformation which gives r 8 in general given realistic impedance contrasts across a phase transition the change in material properties must occur over a region less than 3 km thick for r to be of the order of 5 a discontinuous change in chemical composition at 670 km depth coupled with a change in phase can also produce a reflection coefficient of the observed magnitude r 2 3 for a first order change in phase at 670 km with different iron concentrations on each side of the discontinuity and r 4 6 for cases with a discontinuous increase in silica content based on our analysis the thermal and chemical boundary layers which would occur across such a chemical discontinuity would probably have negligible effects on the reflection coefficient thus although limited by considerable uncertainties the mineralogical and seismic data suggest a two layered model of the mantle with a discontinuous change in both phase and composition at 670 km

earthquake recurrence in simulated fault systems we employ a computationally efficient fault system earthquake simulator rsqsim to explore effects of earthquake nucleation and fault system geometry on earthquake occurrence the simulations incorporate rate and state dependent friction high resolution representations of fault systems and quasi dynamic rupture propagation faults are represented as continuous planar surfaces surfaces with a random fractal roughness and discontinuous fractally segmented faults simulated earthquake catalogs have up to 106 earthquakes that span a magnitude range from m4 5 to m8 the seismicity has strong temporal and spatial clustering in the form of foreshocks and aftershocks and occasional large earthquake pairs fault system geometry plays the primary role in establishing the characteristics of stress evolution that control earthquake recurrence statistics empirical density distributions of earthquake recurrence times at a specific point on a fault depend strongly on magnitude and take a variety of complex forms that change with position within the fault system because fault system geometry is an observable that greatly impacts recurrence statistics we propose using fault system earthquake simulators to define the empirical probability density distributions for use in regional assessments of earthquake probabilities

initiation evolution and extinction of pull apart basins implications for opening of the gulf of california abstract we present a model for the origin crustal architecture and evolution of pull apart basins the model is based on results of three dimensional upper crustal elastic models of deformation field observations and fault theory and is generally applicable to basin scale features but predicts some intra basin structural features geometric differences between pull apart basins are inherited from the initial geometry of the strike slip fault step over which results from the forming phase of the strike slip fault system as strike slip motion accumulates pull apart basins are stationary with respect to underlying basement and the fault tips propagate beyond the rift basin increasing the distance between the fault tips and pull apart basin center because uplift is concentrated near the fault tips the sediment source areas may rejuvenate and migrate over time rift flank uplift results from compression along the flank of the basin with increasing strike slip movement the basins deepen and lengthen field studies predict that pull apart basins become extinct when an active basin crossing fault forms this is the most likely fate of pull apart basins because basin bounding strike slip systems tend to straighten and connect as they evolve the models show that larger length to width ratios with overlapping faults are least likely to form basin crossing faults and pull apart basins with this geometry are thus most likely to progress to continental rupture in the gulf of california larger length to width ratios are found in the southern gulf which is the region where continental breakup occurred rapidly the initial geometry in the northern gulf of california and salton trough at 6 ma may have been one of widely spaced master strike slip faults lower length to width ratios which our models suggest inhibits continental breakup and favors straightening of the strike slip system by formation of basin crossing faults within the step over as began 1 2 ma when the san jacinto and elsinore cerro prieto fault systems formed

relationships between along fault heterogeneous normal stress and fault slip patterns during the seismic cycle insights from a strike slip fault laboratory model abstract we use a strike slip fault analog model to study experimentally the role played by along fault non uniform and asymmetric applied normal stress on both coseismic slip and long term fault behavior our model is based on a visco elasto plastic multi layered rheology that allows to produce several hundreds of scaled analog microquakes and associated seismic cycles uniform or heterogeneous applied normal stress along the fault plane is imposed and maintained constant during the whole experiment durations our results suggest that coseismic slip patterns are strongly controlled by spatial normal stress variations and subsequent accumulated shear stress along fault strike major microquakes occur preferentially in zones of major shear stress asperities coseismic slip distributions exhibit a pattern similar to the along fault applied normal stress distribution the occurrence of isolated low to moderate microquakes where residual stresses persist around secondary stress asperities indicates that stress conditions along the fault also control the whole variability of fault slip events moreover when fault slip stability conditions are modulated by normal stress distribution our experiments suggest that the along fault stress heterogeneity influences the seismic cycle regularity and consequently long term fault slip behavior uniform applied normal stress favors irregular seismic cycles and the occurrence of earthquakes clustering whereas non uniform normal stress with a single high amplitude stress asperity generates strong characteristic microquake events with stable return periods together our results strengthen the assumption that coseismic slip distribution and earthquake variability along an active fault may provide relevant information on long term tectonic stress and could thus improve seismic hazard assessment

multiple scs travel times and attenuation beneath mexico and central america multiple scs phases recorded for the june 19 1982 e1 salvador earthquake m s 7 0 are analyzed to determine whole mantle attenuation and upper mantle travel time anomalies beneath mexico central america and northern south america recordings from the ultra long period seismographs at pasadena and berkeley and the sro station anmo provide eight scs n arrivals with 15 bouncepoints concentrated within mexico the whole mantle qscs for these paths is estimated as 145 n 27 in the frequency range 8 to 40 mhz and the average scsn scs n jb residual atsc s at a period of 50 sec is 5 sec eight scs n phases of similar periods recorded at gdsn stations in south america yield a qscs estimate of 232 n 89 and atsc s 3 9 sec for paths beneath central america and cs 93 n 17 and atsc s 0 3 sec for paths beneath northern south america these results indicate that the variations in travel times and attenuation beneath continental regions are as pronounced as differences reported between old oceanic and stable shield regimes