stress strain response of hydrate bearing sands numerical study using discrete element method simulations 1 gas hydrate is a crystalline solid found within marine and subpermafrost sediments while the presence of hydrates can have a profound effect on sediment properties the stress strain behavior of hydrate bearing sediments is poorly understood due to inherent limitations in laboratory testing in this study we use numerical simulations to improve our understanding of the mechanical behavior of hydrate bearing sands the hydrate mass is simulated as either small randomly distributed bonded grains or as ripened hydrate forming patchy saturation whereby sediment clusters with 100 pore filled hydrate saturation are distributed within a hydrate free sediment simulation results reveal that reduced sand porosity and higher hydrate saturation cause an increase in stiffness strength and dilative tendency and the critical state line shifts toward higher void ratio and higher shear strength in particular the critical state friction angle increases in sands with patchy saturation while the apparent cohesion is affected the most when the hydrate mass is distributed in pores sediments with patchy hydrate distribution exhibit a slightly lower strength than sediments with randomly distributed hydrate finally hydrate dissociation under drained conditions leads to volume contraction and or stress relaxation and pronounced shear strains can develop if the hydrate bearing sand is subjected to deviatoric loading during dissociation

a study of critical state on an unsaturated silty soil critical state models for unsaturated soils have been proposed in recent years however the proposed models have been based on limited experimental data compacted specimens have generally been used for research and the complications of soil fabric resulting from the compaction procedures have brought difficulties into the interpretation of fundamental soil behavior there is a need to undertake further laboratory research studies on unsaturated soils particularly to obtain a fundamental understanding of the shear strength and critical state of unsaturated soils by testing soil specimens with simple soil structures suction controlled triaxial drained shear tests on an unsaturated silt were carried out as part of this research program specimens were prepared by gradually consolidating the initially slurried soil the resulting specimens had a relatively simple soil fabric and stress history the results showed that applying suction to an initially saturated specimen has a similar influence on the stre

free surface effects on the propagation of dynamic rupture dynamic rupture of reverse and normal fault intersecting the surface are investigated in the case of a normal fault nucleating at depth and propagating upwards coupling of rupture radiated stress and free boundary conditions at the surface may induce a shallow secondary nucleation anticipating up to a few seconds the arrival of the main rupture front indeed the free surface induces normal stress fluctuations modifying the fault frictional strength no significant effect on rupture velocity is observed in the case of reverse faulting these incidences are explained by a stress analysis and illustrated by some numerical simulations in the case of dynamic normal faulting in a homogeneous half space the described effects could explain some observations of high frequency radiation close to the surface in documented shallow earthquakes like in kalamata 1986 bouin 1994

fully coupled simulations of megathrust earthquakes and tsunamis in the japan trench nankai trough and cascadia subduction zone subduction zone earthquakes can produce significant seafloor deformation and devastating tsunamis real subduction zones display remarkable diversity in fault geometry and structure and accordingly exhibit a variety of styles of earthquake rupture and tsunamigenic behavior we perform fully coupled earthquake and tsunami simulations for three subduction zones the japan trench the nankai trough and the cascadia subduction zone we use data from seismic surveys drilling expeditions and laboratory experiments to construct detailed 2d models of the subduction zones with realistic geometry structure friction and prestress greater prestress and rate and state friction parameters that are more velocity weakening generally lead to enhanced slip seafloor deformation and tsunami amplitude the japan trench s small sedimentary prism enhances shallow slip but has only a small effect on tsunami height in nankai where there is a prominent splay fault frictional parameters and off fault material properties both influence the choice of rupture pathway in complex ways the splay generates tsunami waves more efficiently than the decollement rupture in cascadia is buried beneath the seafloor but causes a tsunami that is highly complex due to the rough seafloor bathymetry neglecting compliant sediment layers leads to substantially different rupture behavior and tsunami height we demonstrate that horizontal seafloor displacement is a major contributor to tsunami generation in all subduction zones studied we document how the nonhydrostatic response of the ocean at short wavelengths smooths the initial tsunami source relative to commonly used approach for setting tsunami initial conditions finally we determine self consistent tsunami initial conditions by isolating tsunami waves from seismic and acoustic waves at a final simulation time and backpropagating them to their initial state using an adjoint method we find no evidence to support claims that horizontal momentum transfer from the solid earth to the ocean is important in tsunami generation

laboratory earthquakes along inhomogeneous faults directionality and supershear we report on the experimental observation of spontaneously nucleated ruptures occurring on frictionally held bimaterial interfaces with small amounts of wave speed mismatch rupture is always found to be asymmetric bilateral in one direction rupture always propagates at the generalized rayleigh wave speed whereas in the opposite direction it is subshear or it transitions to supershear the lack of a preferred rupture direction and the conditions leading to supershear are discussed in relation to existing theory and to the earthquake sequence in parkfield california and in north anatolia

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 self consistent mechanism for slow dynamic deformation and tsunami generation for earthquakes in the shallow subduction zone 1 dynamic pore pressure changes in the overriding wedge above a shallow dipping plate interface significantly affect the rupture dynamics of shallow subduction zone earthquakes and their tsunamigenesis for a wedge on the verge of coulomb failure everywhere including the basal fault the dynamic pore pressure increase due to up dip rupture propagation leads to widespread yielding within the wedge which is greatly enhanced by the shallow dip of the fault the widespread yielding reduces the stress drop slip velocity slip and rupture velocity giving rise to prolonged rupture duration thus explaining many anomalous features of shallow subduction zone earthquakes significant inelastic seafloor uplift occurs in the case of a shallow fault dip with the largest uplift located landward from the trench integrating this physical mechanism with existing seismic geodetic and tsunami observations can provide new insights into earthquake dynamics and deformation processes in shallow subduction zones

mechanical properties of hydrate bearing turbidite reservoir in the first gas production test site of the eastern nankai trough abstract the study of mechanical properties of marine sediments is essential for the prediction of the occurrence of geohazards e g subsea landslides and seafloor subsidence and the design of submarine structures for offshore industry in this study triaxial compression tests of gas hydrate bearing sandy sediments and clayey silty sediments were conducted the sediments were recovered by pressure coring in the eastern nankai trough the area of the first japanese offshore production test soil index properties were measured and revealed porosity of 40 50 with porosity decreasing gradually with greater depth below the seafloor the mean particle size was less than 10 m for clayey silty sediments and approximately 100 m for sandy sediments permeability estimated by a consolidation process of triaxial testing and with x ray diffraction analysis depended on the content of fines which consisted chiefly of mica kaolinite and smectite the results of undrained compression tests for clayey silty sediments showed positive excess pore pressure under all test conditions this mechanical behavior indicates that the core samples are normally consolidated sediments drained compression tests showed that the strength and stiffness of sandy sediments increase with hydrate saturation furthermore the volumetric strain of hydrate bearing sediments changed from compression to dilative this result was obtained for hydrate saturation values sh of more than 70 the shear strength of hydrate bearing turbidite sediments of the eastern nankai trough is shown to be a function of the confining pressure

effective normal stress alteration due to pore pressure changes induced by dynamic slip propagation on a plane between dissimilar materials 1 recent detailed examinations of fault zones show that walls of faults are often bordered by materials that are different from each other and from the more uniform material farther away in addition they show that the ultracataclastic core of mature fault zones where slip is concentrated is less permeable to flow across it than the adjoining material of the damage zone inhomogeneous slip at the interface between materials with different poroelastic properties and permeabilities causes a change in pore pressure there because slip causes compression on one side of the fault wall and extension on the other the pore pressure on the fault increases substantially when the compressed side is significantly more permeable and decreases when instead the extended side is more permeable this change in pore pressure alters the effective normal stress on the slip plane in a way that is analogous to the normal stress alteration in sliding between elastically dissimilar solids the magnitude of the effect due to induced pore pressure can be comparable to or larger than that induced by sliding between elastic solids with a dissimilarity of properties consistent with seismic observations the induced pore pressure effect is increased by increasing contrast in permeability but the normal stress alteration due to elastic contrast increases rapidly as the rupture velocity approaches the generalized rayleigh velocity because the alteration in effective normal stress due to either effect can be positive or negative depending on the contrast in properties the two effects can augment or offset each other

dynamic modeling of the 2004 mw 6 0 parkfield california earthquake 1 we present two spontaneous rupture models of the 2004 mw 6 0 parkfield earthquake constrained by near source ground motions we start with a stress drop distribution calculated from a kinematic slip distribution using a linear slip weakening friction law we utilize trial and error to obtain both the stress conditions and frictional parameters on the fault that produce synthetics consistent with records the material contrast across the san andreas fault is incorporated using different one dimensional velocity structures on each side of the fault an approximately constant s parameter of 0 3 and a uniform slip weakening distance of 0 15 m are used in the dynamic models in our preferred dynamic model consistent with the ground motion and gps the slip is bounded by seismicity streaks at 5 and 10 km depths confirming a locked zone at depth the stress drop is approximately 10 mpa in the hypocentral region and about 2 mpa elsewhere the material contrast across the fault causes significant normal stress variations 1 mpa leading to a larger strength drop to the southeast than to the northwest the main rupture front propagates at nearly a constant subshear rupture velocity 3 km s in both directions the total radiated energy determined from the preferred dynamic model is 1 1 1013 j seismic moment is 1 0 1018 nm and fracture energy is 3 0 1013 j the limited number of aftershocks in the slipped area suggests the important role of stress on the distribution of seismicity in the locked zone

geological observations of damage asymmetry in the structure of the san jacinto san andreas and punchbowl faults in southern california a possible indicator for preferred rupture propagation direction we present new in situ observations of systematic asymmetry in the pattern of damage expressed by fault zone rocks along sections of the san andreas san jacinto and punchbowl faults in southern california the observed structural asymmetry has consistent manifestations at a fault core scale of millimeters to meters a fault zone scale of meters to tens of meters and related geomorphologic features the observed asymmetric signals are in agreement with other geological and geophysical observations of structural asymmetry in a damage zone scale of tens to hundreds of meters in all of those scales more damage is found on the side of the fault with faster seismic velocities at seismogenic depths the observed correlation between the damage asymmetry and local seismic velocity structure is compatible with theoretical predictions associated with preferred propagation direction of earthquake ruptures along faults that separate different crustal blocks the data are consistent with a preferred northwestward propagation direction for ruptures on all three faults if our results are supported by additional observations asymmetry of structural properties determined in field studies can be utilized to infer preferred propagation direction of large earthquake ruptures along a given fault section the property of a preferred rupture direction can explain anomalous behavior of historic rupture events and may have profound implications for many aspects of earthquake physics on large faults

influence of dip and velocity heterogeneity on reverse and normal faulting rupture dynamics and near fault ground motions we investigate the influence of fault dip 35 60 and crustal velocity heterogeneity on rupture dynamics and near fault ground motions from normal and reverse faulting the same initial conditions were used except for the direction of initial shear stress in each dynamic rupture calculation we used two 3d elastic finite element approaches that employ split nodes for the computations in homogeneous and weakly heterogeneous half spaces with faults dipping 50 maximum fault normal peak velocities occurred on the hanging wall however for fault dips 50 maximum fault normal peak velocities occurred on the footwall bilateral and unilateral rupture simulations in weakly heterogeneous media found that reverse faulting slip velocities frequency band 1 3 5 hz were on average 39 larger than those during normal faulting however on average reverse faulting slip velocities were only 16 larger than normal faulting slip velocities for frequencies 50

on the critical state characteristics of methane hydrate bearing sediments abstract reasonable prediction of marine ground stability is vital for the safe exploitation of methane gas from the seabed and development of relevant technologies for gas production constitutive models based on a critical state soil mechanics framework for methane hydrate bearing sediments have been proposed in recent decades they required careful calibration of laboratory results to obtain accurate information on model parameters this study presents a comprehensive analysis of triaxial shear test data to examine the effects of hydrate saturation on the geomechanical characteristics of methane hydrate bearing sediments particularly at the critical state multiple critical state lines are identified for methane hydrate bearing sediments with different levels of hydrate saturation on the specific volume and logarithm of mean stress plane a rise in hydrate saturation shifts the critical state line of methane hydrate bearing sediments upward and slightly increases the gradient of critical state line in the v ln p plane non unique critical state lines are observed for methane hydrate bearing sediments under plane strain loading conditions considering methane hydrate saturation as an extra dimension of critical states the critical state line in the plane of specific volume and logarithm of mean stress becomes a three dimensional critical state surface

earthquake ruptures with strongly rate weakening friction and off fault plasticity part 1 planar faults abstract observations demonstrate that faults are fractal surfaces with deviations from planarity at all scales we study dynamic rupture propagation on self similar faults having root mean square rms height fluctuations of order 10 3 to 10 2 times the profile length our 2d plane strain models feature strongly rate weakening fault friction and off fault drucker prager viscoplasticity the latter bounds otherwise unreasonably large stress concentrations in the vicinity of bends our choice of a cohesionless yield function prevents tensile stress states and thus fault opening a consequence of strongly rate weakening friction is the existence of a critical background stress level above which self sustaining rupture propagation in the form of self healing slip pulses first becomes possible around this level at which natural faults are expected to operate ruptures become extremely sensitive to fault roughness and exhibit substantial fluctuations in rupture velocity except for shallow inclinations of the maximum compressive stress to the fault less than about 20 the fluctuations are anticorrelated with the local fault slope these accelerations and decelerations of the rupture together with naturally emerging slip heterogeneity excite waves of all wavelengths and result in ground acceleration spectra that are flat at high frequency consistent with observed strong motion records

the three dimensional dynamics of dipping faults recent two dimensional dynamic simulations of dip slip faulting niel sen 1998 oglesby et al 1998 2000 shi et al 1998 have shown that the asym metric geometry of dip slip faults that intersect the free surface can have large effects on the dynamics of earthquake rupture the nonvertical dip angle of such faults leads to larger motion on the footwall than the hanging wall as well as much larger motion from thrust reverse faults than from normal faults with the same geometry and stress magnitudes in the present work we perform full three dimensional simulations of thrust reverse normal and strike slip faults and show that the same effects exist in three dimensions strike slip fault motion is either in between or lower than the motion of both dip slip faults additional three dimensional effects include strong rake rotation at the free surface the results confirm the findings of the previous studies and further elucidate the dynamic effects of the free surface on fault rupture slip and ground motion they are also borne out by early analyses of the 1999 chi chi taiwan thrust earthquake which displayed higher motion on the hanging wall than on the footwall and a strong oblique component of motion at the surface

the effects of fault stepovers on ground motion abstract using 3d dynamic models we investigate the effect of fault stepovers on near source ground motion we use the finite element method to model the rupture slip and ground motion of two parallel strike slip faults with an unlinked overlapping stepover of variable width we model this system as both an extensional and a compressional stepover and compare the results to those of single planar faults we find that overall the presence of a stepover along the fault trace reduces the maximum ground motion when compared to the long planar fault whether the compressional or extensional stepover exhibits higher ground motion overall depends on the width of the separation between the faults there is a region of reduced ground motion at the end of the first fault segment when the faults are embedded in a homogeneous material we also experiment with stress fields leading to supershear and subshear rupture velocities and with different stress drops within those conditions we find that subshear rupture produces stronger motions than supershear rupture but supershear ruptures produce that maximum over a larger area than subshear areas even though the overall area that experiences any shaking at all is not drastically different between the two cases lastly we experiment with placing realistic materials along and around the faults such as a sedimentary basin in an extensional stepover a damage zone around the fault and a soft rock layer on top of bedrock through the entire model area these configurations alter the pattern of ground motion from the homogeneous case the peaks in ground motion for the bimaterial cases depend on the materials in question the results may have implications for ground motion prediction in future earthquakes on geometrically complex faults online material mpeg 4 movies of models of dynamic rupture of fault stepovers embedded in heterogeneous material settings

dynamic earthquake ruptures in the presence of lithostatic normal stresses implications for friction models and heat production we simulate dynamic ruptures on a strike slip fault in homogeneous and layered half spaces and on a thrust fault in a layered half space with traditional friction models sliding friction exceeds 50 of the fault normal compressive stress and unless the pore pressures approach the lithostatic stress the rupture characteristics depend strongly on the depth and sliding generates large amounts of heat under application of reasonable stress distributions with depth variation of the effective coefficient of friction with the square root of the shear modulus and the inverse of the depth creates distributions of stress drop and fracture energy that produce realistic rupture behavior this ad hoc friction model results in 1 low sliding friction at all depths and 2 fracture energy that is relatively independent of depth additionally friction models with rate weakening behavior which form pulselike ruptures appear to generate heterogeneity in the distributions of final slip and shear stress more effectively than those without such behavior which form cracklike ruptures for surface rupture on a thrust fault the simple slip weakening friction model which lacks rate weakening behavior accentuates the dynamic interactions between the seismic waves and the rupture and leads to excessively large ground motions on the hanging wall waveforms below the center of the fault which are associated with waves radiated to teleseismic distances indicate that source inversions of thrust events may slightly underestimate the slip at shallow depths

shattered rock and precarious rock evidence for strong asymmetry in ground motions during thrust faulting recent thrust faulting models both physical and numerical have indicated very high ground motions on the hanging wall and relatively low motions on the footwall of thrust faults in this article recent field evidence supporting this strong asymmetry is described earlier field observations of objects thrown in the air also suggest that ground motions must have exceeded 1 g and 100 cm sec new shattered rock evidence on the hanging wall of thrust faults in southern california supports the occurrence of intense ground motions on the other hand the lack of shattered rock and the presence of precariously balanced rocks on the footwall of two thrust faults in southern california indicate relatively low ground motions thus indicating strong asymmetry in ground motions near the fault trace for thrust faults some current ground motion attenuation curves and hazard maps may give too low values on the hanging wall and too high values on the footwall this could be of critical importance to estimating earthquake hazard from potentially large thrust fault earthquakes in the los angeles basin and elsewhere in the world the new evidence also supports the idea that dynamic inertial detachment of the hanging wall from the footwall occurs in nature in the manner suggested by some recent physical and numerical models if such detachment occurs it could also have important implications for understanding thrust fault rupture mechanics for example the paradox of large overthrusts and the heat flow paradox for thrusts

spontaneous rupture processes on a bending fault 1 we simulated spontaneous rupture processes on a vertical bending strike slip fault in a three dimensional half space using a finite difference method rupture area and overall slip distribution on the fault vary with the initial stress field which depends on strike change at the bend rupture velocity and detailed slip distribution around the bend on the other hand are affected by time dependent normal stress change caused by rupture this dynamic stress change plays an important role in the rupture process ahead of the bend as well as in the resulting ground motion the numerical simulation method demonstrated here in a simplified geometry is equally applicable to modeling strong ground motion from geometrically complex ruptures in realistic earth models

influence of grading on the mechanical behaviour of stava tailings tailings dams can be fragile structures and too often they are subject to liquefaction the material they are composed of is rather peculiar having a man made origin and their behaviour is still not very well understood the soil from the stava tailings dams structures that were subject to liquefaction in 1985 causing extensive destruction was investigated in this study as an example of tailings two main aspects of their behaviour have been examined the influence of the percentage of silt and sand that compose the soil on its mechanical behaviour and the susceptibility to liquefaction analysing the behaviour within a critical state framework in this paper it is shown that as the quantity of silt increases the slope and intercept of the normal compression line and critical state line at higher pressures decrease until an inversion of behaviour is observed while at lower stress levels the critical state line changes its position but not its slope this means that the effect of adding silt on the

the importance of the dynamic source effects on strong ground motion during the 1999 chi chi taiwan earthquake brief interpretation of the damage distribution on buildings the 1999 chi chi taiwan earthquake that originated on a low angle reverse fault showed complexity and uncommon characteristics the records show that the hanging wall side is characterized by larger particle motions than the foot wall and the ground motion is stronger in the northern part than in the southern part of the causative fault although the strongest ground motion occurred near the north ern part of the trace structural damage was heavier in the southern part in order to get a better understanding of the complex damage distribution caused by this earth quake the dynamic rupture process was numerically simulated because of the dif ferences between the observed features of the rupture process in the northern and southern parts of the fault each part was modeled independently by using a 2d discrete element model dem the principal results of the simulation show that the velocity ground motions in the northern part in the frequency range of 0 5 2 hz natural frequency range of standard structures are small near the surface break thus light structural damage might be predicted near the surface rupture moreover in the northern part the fault rupture propagation reaches the surface with a very slow velocity about 1 2 km sec however in the southern part the rupture propa gation reaches the surface with higher velocity about 3 0 km sec these differences between the models could explain why the ground motion near the surface rupture in the northern part caused less damage in structures than the ground motion in the southern part

shallow dynamic overshoot and energetic deep rupture in the 2011 mw 9 0 tohoku oki earthquake strong spatial variation of rupture characteristics in the moment magnitude mw 9 0 tohoku oki megathrust earthquake controlled both the strength of shaking and the size of the tsunami that followed finite source imaging reveals that the rupture consisted of a small initial phase deep rupture for up to 40 seconds extensive shallow rupture at 60 to 70 seconds and continuing deep rupture lasting more than 100 seconds a combination of a shallow dipping fault and a compliant hanging wall may have enabled large shallow slip near the trench normal faulting aftershocks in the area of high slip suggest dynamic overshoot on the fault despite prodigious total slip shallower parts of the rupture weakly radiated at high frequencies whereas deeper parts of the rupture radiated strongly at high frequencies

dynamics of dip slip faulting explorations in two dimensions dynamic models of earthquake rupture and slip are a powerful method by which to investigate the physics of earthquakes owing to both conceptual and computational constraints dynamic earthquake models have largely been limited to cases with geometrical symmetry such as faults in unbounded media or vertical faults however there are both observational and theoretical reasons to believe that nonvertical dip slip faults behave differently from faults with more symmetrical geometries previous observations have shown greater ground motion from thrust reverse faults than normal faults and higher ground motion on hanging walls than on footwalls in the present work two dimensional dynamic simulations of thrust reverse and normal earthquakes show precisely these effects and also elucidate their causes for typical nonvertical dip slip faults the breakdown of symmetry with respect to the free surface allows radiated seismic waves to reflect off the free surface and to hit the fault again altering the stress field on the fault this process can lead to time dependent normal stress and a feedback between the friction rupture processes and seismic radiation this interaction leads to thrust reverse faults producing much higher fault and ground motion than normal faults with the same geometry and stress magnitudes the asymmetric geometry also directly leads to higher motion on the hanging walls of such faults than on the footwalls simulations show that these effects occur for a variety of dip angles but only for faults that either intersect or closely approach the free surface the results emphasize the strong effect that the free surface can have on the dynamics of fault rupture and slip

effect of shallow slip amplification uncertainty on probabilistic tsunami hazard analysis in subduction zones use of long term balanced stochastic slip models the complexity of coseismic slip distributions influences the tsunami hazard posed by local and to a certain extent distant tsunami sources large slip concentrated in shallow patches was observed in recent tsunamigenic earthquakes possibly due to dynamic amplification near the free surface variable frictional conditions or other factors we propose a method for incorporating enhanced shallow slip for subduction earthquakes while preventing systematic slip excess at shallow depths over one or more seismic cycles the method uses the classic k 2 stochastic slip distributions augmented by shallow slip amplification it is necessary for deep events with lower slip to occur more often than shallow ones with amplified slip to balance the long term cumulative slip we evaluate the impact of this approach on tsunami hazard in the central and eastern mediterranean sea adopting a realistic 3d geometry for three subduction zones by using it to model 150 000 earthquakes with m w from 6 0 to 9 0 we combine earthquake rates depth dependent slip distributions tsunami modeling and epistemic uncertainty through an ensemble modeling technique we found that the mean hazard curves obtained with our method show enhanced probabilities for larger inundation heights as compared to the curves derived from depth independent slip distributions our approach is completely general and can be applied to any subduction zone in the world

dynamic rupture on a material interface with spontaneous generation of plastic strain in the bulk abstract we discuss 2d simulations of in plane ruptures on a fault between different solids in a model that includes dynamic generation of plastic strain off the fault the fault is governed by a regularized friction with a gradual response to abrupt changes of normal stress while plastic strain in the bulk is governed by a coulomb yielding criterion the occurrence of off fault damage stabilizes the slip velocity on the fault stable self sustaining ruptures propagate on the material interface as narrow unidirectional pulses in the direction of slip on the more compliant side of the fault plastic strain is generated only on the stiffer side of the fault the tensional quadrant in the direction of rupture propagation in a strip of approximately constant width that is correlated with the slip velocity on the fault simulations for various values of confining normal stress material contrast and cohesion in the bulk suggest that significant generation of off fault damage is limited to conditions that correspond to the top few kilometers of the crust the results are compatible with field observations of shallow asymmetric damage patterns in the structure of several large faults

effects of soil moisture and dried raindroplet crust on saltation and dust emission 1 field experiments using a sand particle counter and an optical particle counter clarified the links between saltation and dust emission under wet and weakly crusted conditions in a fallow field previously cultivated with wheat in australia a crust was formed by the impact of raindroplets after small precipitation events a little soil moisture enhanced the strengths of crust and aggregation even though the soil was dried dust concentration was proportional to friction wind velocity but the proportionality was dependent on ground surface conditions such as the minimally dispersed particle size distribution of parent soil and the presence or absence of crust

dynamic wedge failure reveals anomalous energy radiation of shallow subduction earthquakes abstract dynamically induced coulomb failure in the overriding wedge significantly affects energy radiation of shallow subduction earthquakes for a wedge on the verge of failure extensive fluid assisted coseismic failure due to updip rupture causes significant seafloor uplift above a shallow dipping basal fault the large inelastic uplift greatly enhanced by the presence of free surface significantly dilates the fault behind the rupture front during the rupture propagation which reduces the effective normal stress and sliding friction on the fault and increases the dynamic stress drop and slip velocity as a result slip velocity time histories in the shallow section of the fault tend to have a snail like shape leading to depletion of high frequencies in the slip velocity field and the resultant source time function we also show that the failure in the wedge acts as a large energy sink while contributing to seismic moment giving rise to distributed heat generation i e small heat flow anomaly across the fault low moment scaled radiated energy slow rupture velocity and small directivity which provides a unifying interpretation for nearly all anomalous observations documented for shallow subduction earthquakes

damaged beyond repair characterising the damage zone of a fault late in its interseismic cycle the alpine fault new zealand dfdp 1 was funded by gns science victoria university of wellington the university of otago the university of auckland the university of canterbury deutsche forschungsgemeinschaft and the university of bremen natural environment research council grants ne j024449 1 ne g524160 1 and ne h012486 1 and the university of liverpool and the marsden fund of the royal society of new zealand

aftershock asymmetry on a bimaterial interface to better understand the asymmetric distribution of microearthquake aftershocks along the central san andreas fault we study dynamic models of slip weakening ruptures on an interface separating differing elastic half spaces subshear ruptures grow as slightly asymmetric bilateral cracks with larger propagation velocities slip velocities and normal stress changes at the rupture front moving in the direction of slip of the medium with the lower shear wave speed the southeast front in the context of the san andreas when the se front encounters a stress barrier the tensile stress perturbation behind the rupture front continues forward and for a wide range of barrier strengths nucleates a dying slip pulse this slip pulse smooths the stress field and reduces the static stress change beyond the se front furthermore because the tensile stress that carried the slip pulse into the barrier is a purely dynamic phenomenon the se rupture front can be left far below the failure threshold while the nw front remains quite close to failure both mechanisms could contribute to the observed aftershock asymmetry formation of a robust slip pulse requires a peak tensile stress perturbation that approaches the nominal strength drop of the slip weakening law to achieve this while minimizing off fault damage requires either substantial velocity contrasts or small reductions in friction the simulations also show a pronounced asymmetry in the timescales over which barriers to the se and nw experience increasing stresses a result that has implications for the asymmetric distribution of subevents in compound earthquakes

rupture dynamics with energy loss outside the slip zone 1 energy loss in a fault damage zone outside the slip zone contributes to the fracture energy that determines rupture velocity of an earthquake a nonelastic two dimensional dynamic calculation is done in which the slip zone is modeled as a fault plane and material off the fault is subject to a coulomb yield condition in a mode 2 crack like solution in which an abrupt uniform drop of shear traction on the fault spreads from a point coulomb yielding occurs on the extensional side of the fault plastic strain is distributed with uniform magnitude along the fault and it has a thickness normal to the fault proportional to propagation distance energy loss off the fault is also proportional to propagation distance and it can become much larger than energy loss on the fault specified by the fault constitutive relation the slip velocity function could be produced in an equivalent elastic problem by a slip weakening friction law with breakdown slip dc increasing with distance fracture energy g and equivalent dc will be different in ruptures with different initiation points and stress drops so they are not constitutive properties they are determined by the dynamic solution that arrives at a particular point peak slip velocity is however a property of a fault location nonelastic response can be mimicked by imposing a limit on slip velocity on a fault in an elastic medium

evidence for the variation of stress drop between normal and thrust faulting earthquakes in italy the horizontal components of 45 strong motion accelerograms recorded in friuli italy from may to september 1976 have been analyzed in order to estimate the stress drop of eight thrust faulting earthquakes two different stress parameters the brune stress drop and the apparent stress a have been considered the friuli main shock has a strong stress drop 800 bars while the aftershocks have stress drops which range from 250 to 400 bars five accelerograms written by the montenegro yugoslavia earthquake of april 1979 have also been analyzed this thrust faulting earthquake had a stress drop of 900 bars the stress drops of these thrust faulting earthquakes are compared to the stress drops determined by rovelli et al 1988a for nine normal faulting earthquakes which occurred in the apennines region from 1979 to 1984 the radiated energies and apparent stresses have also been calculated for the set of apennine earthquakes the thrust faulting earthquakes in the alpine dynaride region exhibit higher values for both the brune and the apparent stress than the normal faulting earthquakes in the apennines region excluding the friuli and montenegro main shocks the brune stress drops of the thrust faults are approximately 3 times those of the normal faults while the apparent stresses are twice as large this difference is statistically significant at a confidence level of 95 the same difference also emerges from the scaling of the peak ground motions recorded in these two regions suggesting an enhanced severity of the seismic input in the alpine dynaride environment compared with the apennines

the seismic cycle at subduction thrusts 1 insights from laboratory models 1 subduction megathrust earthquakes occur at the interface between the subducting and overriding plates these hazardous phenomena are only partially understood because of the absence of direct observations the restriction of the instrumental seismic record to the past century and the limited resolution completeness of historical to geological archives to overcome these restrictions modeling has become a key tool to study megathrust earthquakes we present a novel model to investigate the seismic cycle at subduction thrusts using complementary analog paper 1 and numerical paper 2 approaches here we introduce a simple scaled gelatin on sandpaper setup including realistic tectonic loading spontaneous rupture nucleation and viscoelastic response of the lithosphere particle image velocimetry allows to derive model deformation and earthquake source parameters analog earthquakes are characterized by quasi periodic recurrence consistent with elastic theory the interseismic stage shows rearward motion subsidence in the outer wedge and uplift of the coastal area as a response of locked plate interface at shallow depth the coseismic stage exhibits order of magnitude higher velocities and reversal of the interseismic deformation pattern in the seaward direction subsidence of the coastal area and uplift in the outer wedge like natural earthquakes analog earthquakes generally nucleate in the deeper portion of the rupture area and preferentially propagate upward in a crack like fashion scaled rupture width slip proportionality and seismic moment duration scaling verifies dynamic similarities with earthquakes experimental repeatability is statistically verified comparing analog results with natural observations we conclude that this technique is suitable for investigating the parameter space influencing the subduction interplate seismic cycle

steady state concepts and static liquefaction of silty sands the results from an experimental study on silty sands are presented and evaluated in view of the framework of critical state or steady state soil mechanics drained and undrained compression tests were performed on nevada sand containing nonplastic silt the drained and undrained steady state lines diverged this divergence was caused by the tendency for static liquefaction at low pressures results from undrained tests with different initial void ratios produced different steady state lines unique steady state lines may therefore not always exist for silty sands denser specimens liquefied at low confining pressures while looser specimens at higher confining pressures showed stable behavior thus a unique relation between void ratio and confining pressure namely the steady state line may not represent the behavior of loose silty sands this occurs because silty sands exhibit reverse behavior characteristics more dilative behavior is observed with increasing confining pressure under undrained shearing load and deformation control testing was performed to evaluate the effect of strain rate compressibility was examined as an alternative measure of liquefaction potential

a revised shape model of asteroid 216 kleopatra abstract we used three different sets of arecibo delay doppler radar images and five well covered occultations to generate a revised three dimensional shape model of asteroid 216 kleopatra with a spatial resolution of 10 km we find kleopatra to be a bi lobate contact binary of overall dimensions 276 94 78 km 15 and equivalent diameter deq 122 30 km our uncertainties are upper and lower bounds separated binary models are ruled out by multi chord occultations our model is 27 longer than the dog bone model originally published by ostro et al 2000 but is similar to their model in the minor and intermediate axes extents our model s dimensions are also consistent with more recent ones based on lightcurves adaptive optics and interferometric imaging we confirm a rotational period of p 5 385280 h 0 000001 h and a rotation pole at ecliptic longitude and latitude 74 20 5 over its southern hemisphere the one most frequently observed on earth kleopatra s radar albedo is 0 43 0 10 consistent with a high near surface bulk density and by inference the high metal content expected for m class asteroids however the radar albedo for equatorial observations is considerably lower and more typical of a dominantly silicate composition this observation could readily be explained by a relatively thin 1 2 m silicate mantle over equatorial latitudes kleopatra s surface is relatively smooth with a mean slope of 12 at the 10 km baseline scale analysis of its geopotential surface suggests loose material will preferentially migrate to the neck and this is supported by our radar observations

dynamic geometrical effects on strong ground motion in a normal fault model we report results of measurement of particle motion in a foam rubber model of normal faulting and compare the results with similar results for a strike slip geometry standard computer modeling of strong ground motion from normal fault earthquakes has used dislocation theory in which slip along the shallow part of the fault is prescribed by assuming particular time functions for fault slip unfortunately in the case of normal faults there are essentially no near fault data from large earthquakes to constrain the modeling in an extensional faulting regime the static normal and shear stresses along the fault must approach zero at the surface and thus the upper few kilometers of the fault have inherently less stored strain energy than the maximum possible for strike slip faults in addition there are dynamic effects from geometry and drop in fault normal stress which affect the fault motion physical models of faulting such as foam rubber modeling are guaranteed to obey static and dynamic mechanical laws and thus can be used to gain insight into the physical processes involved in this study we compare surface accelerations from normal fault and strike slip geometries the data show surface accelerations near the normal fault trace that are systematically lower by anmore average factor of about 0 1 compared to the accelerations at the side sensors which represent strike slip motion these results suggest that kinematic modeling of ground motion using classical dislocation techniques is inappropriate on the shallow part of the fault the results of this study are qualitatively similar to those obtained for numerical models such as a dynamic lattice model ital shi ital et al 1997 and a finite element model ital oglesby ital et al 1998 lending support to the probability that similar effects take place in the real earth copyright 1999 american geophysical union less

distributions of strong ground motion due to dynamic ruptures across a bimaterial fault implications for seismic hazard analyses abstract we perform 2 d finite element calculations of mode ii rupture along a bimaterial interface governed by regularized rate and state dependent friction law with the goal of understanding how the bimaterial interface influences the strong ground motion by comparison with properties of rupture in a homogeneous solid we found that bimaterial mechanism is important for earthquake ruptures and influences the strong ground motion significantly the simulated results show that mode ii rupture evolves with propagation distance along a bimaterial interface to a unilateral wrinkle like pulse in the direction of slip on the compliant side of the fault namely in the positive direction strong ground motion caused by seismic waves emanated from the rupture propagation is asymmetrically distributed in space the computed peak ground acceleration pga is high in the near fault region particularly pga is much larger in the region on the side in the positive direction in addition it is greater in the more compliant area of the model than that in the stiffer area with corresponding locations moreover the differential pga due to bimaterial effect increases with increasing degree of material contrast across the fault it is hoped that the results obtained in this investigation will provide some implications for seismic hazard assessment and fault rupture mechanics

the dynamics of thrust and normal faults over multiple earthquake cycles effects of dipping fault geometry we perform dynamic simulations of thrust and normal faults over multiple earthquake cycles our goal is to explore effects of asymmetric fault geometry on the long term seismicity and dynamics of dipping faults a dynamic finite element method is used to model the interseismic and coseismic processes with a dynamic relaxation technique for the former the faults are loaded by stable sliding along the downward continuation of the faults the asymmetric fault geometry of dipping faults with respect to the free surface cause changes in normal stress during the interseismic and coseismic periods these changes are of opposite sign in the two periods resulting in a stabilization of the normal and shear stresses over many earthquake cycles both faults develop relatively stable event patterns in which a large event that ruptures the entire fault is preceded by a number of small events with various rupture lengths a strong asymmetry in fault and ground motion exists between thrust and normal faults and between the hanging wall and the footwall of both faults on faults dipping less than 70 in both normal and thrust faults the horizontal component of ground motion dominates on the footwall while the vertical component dominates on the hanging wall the above results may have implications in seismic hazard analysis and building design in regions where dip slip faults predominate

the great sumatra andaman earthquake of 26 december 2004 the two largest earthquakes of the past 40 years ruptured a 1600 kilometer long portion of the fault boundary between the indo australian and southeastern eurasian plates on 26 december 2004 seismic moment magnitude mw 9 1 to 9 3 and 28 march 2005 mw 8 6 the first event generated a tsunami that caused more than 283 000 deaths fault slip of up to 15 meters occurred near banda aceh sumatra but to the north along the nicobar and andaman islands rapid slip was much smaller tsunami and geodetic observations indicate that additional slow slip occurred in the north over a time scale of 50 minutes or longer

dynamic source modeling of the 1978 and 2005 miyagi oki earthquakes interpretation of fracture energy 1 we constructed spontaneous dynamic rupture models of the 1978 and 2005 miyagi oki japan earthquakes which occurred on the same plate boundary repeatedly with different magnitudes to reproduce the slip and the rupture velocity predicted by kinematic source models comparison of the dynamic source parameters of these events enables us to consider the behavior of rupture dynamics during different source processes on the same fault in particular we compared the stress drop and the fracture energy which can be evaluated stably between two events the maximum and average values of the stress drop over the whole fault plane and on an asperity that ruptured during both events are almost identical in the two models on the other hand the 1978 event whose magnitude was larger than the 2005 event has a fracture energy value larger than that of the 2005 event not only over the whole fault but also on the asperity that ruptured repeatedly these results are consistent with previous studies of the scaling of fracture energy with the seismic moment we also compared the fracture energy values with those of other inland events estimated for previous studies our values for the miyagi oki earthquakes are smaller differences in tectonic setting between shallow inland events and deep subduction zone events might cause this discrepancy

predicting rupture arrests rupture jumps and cascading earthquakes the devastation inflicted by recent earthquakes demonstrates the danger of under predicting the size of earthquakes unfortunately earthquakes may rupture fault sections larger than previously observed making it essential to develop predictive rupture models we present numerical models based on earthquake physics and fault zone data that determine whether a rupture on a segmented fault could cascade and grow into a devastating multisegment earthquake we demonstrate that weakened damaged fault zones and bi material interfaces promote rupture propagation and greatly increase the risk of cascading ruptures and triggered seismicity this result provides a feasible explanation for the outstanding observation of a very large 10 km rupture jump documented in the mw7 8 2001 kunlun china earthquake however enhanced inter seismic deformation and energy dissipation at fault tips suppress rupture propagation and may turn even small discontinuities into effective earthquake barriers by assessing fault stability identifying rupture barriers and foreseeing multisegment earthquakes we provide a tool to improve earthquake prediction and hazard analysis

scaling of ground motion parameters state of stress and focal depth the ground motion parametersrv and ra where r is hypocentral distance v is peak velocity is density and a is peak acceleration are found observationally to be strong functions of stress state and linearly related to focal depth z if the stress state and focal depth are properly taken into account then the ground motion parameters depend on the seismic moment m0 as expected from basic scaling principles that is rv m01 3 and ra is independent of earthquake size regression lines fitted to observations covering broad ranges in seismic moment and focal depth indicate that for extensional and compressional tectonic regimes corresponding to normal and thrust or reverse faulting earthquakes respectively peak acceleration is given by ra 1 08 mpa 3 06 mpa km z normal and ra 5 65 mpa 8 76 mpa km z thrust and for peak velocity rv m01 3 10 4 m2 s n m 1 3 3 00 0 69 km 1 z normal and rv m01 3 10 4 m2 s n m 1 3 4 63 1 82 km 1 z thrust for strike slip earthquakes the data currently are insufficient to define regression fits but such lines would definitely lie between those for the normal and thrust stress regimes these equations are appropriate for ground motion at small hypocentral distance as recorded in a whole space it follows that for comparable hypocentral depth peak acceleration in a compressional tectonic regime for example much of the eastern united states is a factor of about 3 greater than in an extensional regime such as nevada and for peak velocity a similar comparison yields a factor of 2 difference the similarity in behavior of the peak acceleration parameter to crustal strength estimated from byerlee s law of friction with respect to depth and stress state indicates that crustal strength is probably the factor governing the seismic source processes that give rise to the high frequency peak ground motion analysis of the peak acceleration data in the context of crustal strength suggests bounds on this ground motion parameter as recorded at a typical surface site that depend on stress state specifically a normal 0 5 g and a thrust 1 9 g thus the state of stress as well as the focal depth clearly is an important factor to be taken into account in the prediction of seismic ground motion

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

the seismic cycle at subduction thrusts 2 dynamic implications of geodynamic simulations validated with laboratory models 1 the physics governing the seismic cycle at seismically active subduction zones remains poorly understood due to restricted direct observations in time and space to investigate subduction zone dynamics and associated interplate seismicity we validate a continuum visco elasto plastic numerical model with a new laboratory approach paper 1 the analogous laboratory setup includes a visco elastic gelatin wedge underthrusted by a rigid plate with defined velocity weakening and strengthening regions our geodynamic simulation approach includes velocity weakening friction to spontaneously generate a series of fast frictional instabilities that correspond to analog earthquakes a match between numerical and laboratory source parameters is obtained when velocity strengthening is applied in the aseismic regions to stabilize the rupture spontaneous evolution of absolute stresses leads to nucleation by coalescence of neighboring patches mainly occurring at evolving asperities near the seismogenic zone limits consequently a crack or occasionally even pulse like rupture propagates toward the opposite side of the seismogenic zone by increasing stresses ahead of its rupture front until it arrests on a barrier the resulting surface displacements qualitatively agree with geodetic observations and show landward and from near the downdip limit upward interseismic motions these are rebound and reversed coseismically this slip increases adjacent stresses which are relaxed postseismically by afterslip and thereby produce persistent seaward motions the wide range of observed physical phenomena including back propagation and repeated slip and the agreement with laboratory results demonstrate that visco elasto plastic geodynamic models with rate dependent friction form a new tool that can greatly contribute to our understanding of the seismic cycle at subduction zones

late interseismic state of a continental plate bounding fault petrophysical results from dfdp 1 wireline logging and core analysis alpine fault new zealand 1 we present a geophysical characterization at 0 1 100 m scales of a major plate bounding continental fault in a late interseismic state the alpine fault produces mw 8 earthquakes every 200 400 years and last ruptured in 1717 ad wireline geophysical logs and rock cores extending from one side of the alpine fault to the other were acquired in two boreholes drilled in 2011 at gaunt creek during the first phase of the deep fault drilling project dfdp 1 these data document ambient conditions under which the next alpine fault earthquake will occur principal component analysis of the wireline logging data reveals that 80 of the variance is accounted for by electrical acoustic and natural gamma properties and preliminary multivariate classification enables the lithologies of sections of missing core to be reconstructed from geophysical measurements the fault zone exhibits systematic variations in properties consistent with common processes of progressive alteration and comminution near the principal slip zone superimposed on different protolith compositions our observations imply that the fault zone has the opposite sense of elastic asymmetry at 0 1 100 m scales to that of the crustal scale orogen imaged by remote geophysical methods on the basis of the fault zone scale asymmetry the bimaterial interface model of preferred earthquake rupture directions implies a northeastward direction of preferred alpine fault rupture on going characterization of the structural and hydraulic architecture of the alpine fault will improve our understanding of the relationship between in situ conditions earthquake rupture processes and the hazards posed by future earthquakes

effects of the hanging wall and footwall on ground motions recorded during the northridge earthquake abstract systematic differences in ground motion on the hanging wall and footwall during the northridge earthquake are evaluated using empirical data an empirical model for the hanging wall effect is developed for the northridge earthquake this empirical model results in up to a 50 increase in peak horizontal accelerations on the hanging wall over the distance range of 10 to 20 km relative to the median attenuation for the northridge earthquake in contrast the peak accelerations on the footwall are not significantly different from the median attenuation over this distance range recordings from other reverse events show a similar trend of an increase in the peak accelerations on the hanging wall indicating that this systematic difference in hanging wall peak accelerations is likely to be observed in future reverse events

mapping the megathrust beneath the northern gulf of alaska using wide angle seismic data in the northern gulf of alaska and prince william sound we have used wide angle seismic reflection refraction profiles earthquake studies and laboratory measurements of physical properties to determine the geometry of the prince william and yakutat terranes the aleutian megathrust and the subducting pacific plate in this complex region the yakutat terrane is underthrust beneath the prince william terrane and both terranes are interpreted to be underlain by the pacific plate wide angle seismic reflection refraction profiles recorded along five seismic lines are used to unravel this terrane geometry modeled velocities in the upper crust of the prince william terrane to 18 km depth agree closely with laboratory velocity measurements of orca group phyllites and quartzofeldspathic graywackes the chief components of the prince william terrane to hydrostatic pressures as high as 600 mpa 6 kbar a landward dipping reflector at depths of 16 24 km is interpreted as the base of the prince william terrane this reflector corresponds to the top of the wadati benioff zone seismicity and is interpreted as the megathrust immediately beneath the megathrust is a 4 km thick 6 9 km s refractor which we infer to be the source of a prominent magnetic anomaly and which is interpreted by us and previous workers to be gabbro in eocene age oceanic crust of the underthrust yakutat terrane wide angle seismic data magnetic anomaly data and tectonic reconstructions indicate that the yakutat terrane has been underthrust beneath the prince william terrane for at least a few hundred kilometers wide angle seismic data are consistent with a 9 to 10 landward dip of the subducting pacific plate beneath the outer shelf and slope distinctly different from the inferred average 3 to 4 dip of the overlying 6 9 km s refractor and the wadati benioff seismic zone beneath the inner shelf our preferred interpretation of the geophysical data is that one composite plate composed of the pacific plate of a fairly uniform thickness and the yakutat plate of varying thickness is subducting beneath southern alaska

critical state soil constitutive model for methane hydrate soil 1 this paper presents a new constitutive model that simulates the mechanical behavior of methane hydrate bearing soil based on the concept of critical state soil mechanics referred to as the methane hydrate critical state mhcs model methane hydrate bearing soil is under certain geological conditions known to exhibit greater stiffness strength and dilatancy which are often observed in dense soils and also in bonded soils such as cemented soil and unsaturated soil those soils tend to show greater resistance to compressive deformation but the tendency disappears when the soil is excessively compressed or the bonds are destroyed due to shearing the proposed model represents these features by introducing five extra model parameters to the conventional critical state model it is found that for an accurate prediction of ground settlement volumetric yielding plays an important role when hydrate soil undergoes a significant change in effective stresses and hydrate saturation which are expected during depressurization for methane gas recovery

a review of the geochemistry of methane in natural gas hydrate the largest accumulations on earth of natural gas are in the form of gas hydrate found mainly offshore in outer continental margin sediment and to a lesser extent in polar regions commonly associated with permafrost measurements of hydrocarbon gas compositions and of carbon isotopic compositions of methane from natural gas hydrate samples collected in subaquatic settings from around the world suggest that methane guest molecules in the water clathrate structures are mainly derived by the microbial reduction of co2 from sedimentary organic matter typically these hydrocarbon gases are composed of 99 methane with carbon isotopic compositions 13cpdb ranging from 57 to 73 in only two regions the gulf of mexico and the caspian sea has mainly thermogenic methane been found in gas hydrate there hydrocarbon gases have methane contents ranging from 21 to 97 with 13c values ranging from 29 to 57 at a few locations where the gas hydrate contains a mixture of microbial and thermal methane microbial methane is always dominant continental gas hydrate identified in alaska and russia also has hydrocarbon gases composed of 99 methane with carbon isotopic compositions ranging from 41 to 49 these gas hydrate deposits also contain a mixture of microbial and thermal methane with thermal methane likely to be dominant published by elsevier science ltd

influence of plastic deformation on bimaterial fault rupture directivity 1 material juxtapositions across mature faults are a common occurrence previous work has found that this elastic mismatch results in a rupture that will preferentially propagate in the direction of slip displacement on the more compliant side of the fault with more off fault damage in the stiffer material this result has implications for inferring preferred rupture directions based on observations of damage zone asymmetry we perform a complete numerical investigation of the role of the stress state on the distribution of plastic deformation and the direction of preferred rupture propagation we show that there are important factors in addition to the elastic mismatch which control the preferred direction of propagation as well as the side of the fault in which damage predominately accumulates the orientation of the most compressive principal stress is the controlling factor in determining the location of plastic deformation for different orientations plastic deformation can accumulate in either the stiffer or the more compliant material for high angles of most compressive stress the aforementioned preferred rupture direction prediction holds true however the off fault plastic response can reverse that direction for low angles of most compressive stress so that rupture will preferentially propagate in the direction of slip displacement in the stiffer material

numerical study of splay faults in subduction zones the effects of bimaterial interface and free surface 1 splay faults are a characteristic branching fault system in some subduction zones we model a megathrust earthquake rupture with a branching fault inhomogeneous media and a free surface using a spontaneously propagating mode ii crack on a bimaterial interface for this purpose we develop an explicit finite element code to solve the elastodynamic equations and a slip weakening friction law on the fault plane in a homogeneous prestress condition the rupture on the branching fault is enhanced when the upper medium is more compliant even when the upper medium is less compliant a fast rupture propagating at a velocity close to the generalized rayleigh velocity produces the strong dynamic stress around the rupture front and activates the branching fault when prestress linearly increases with depth from zero at the free surface only a slight change of normal stress that is due to seismic waves causes an initiation of rupture on the branching fault at the free surface and this rupture propagates downward on the branching fault the arrival of this descending rupture at the branching point greatly changes the stress field in the surrounding area it affects the slip distribution on the main fault and even terminates the main rupture propagation if it occurs before the arrival of the main rupture on the branching point

the seismic cycle at subduction thrusts insights from seismo thermo mechanical models 1 the underestimation of the size of recent megathrust earthquakes illustrates our limited understanding of their spatiotemporal occurrence and governing physics to unravel their relation to associated subduction dynamics and long term deformation we developed a 2 d continuum viscoelastoplastic model that uses an eulerian lagrangian finite difference framework with similar on and off fault physics we extend the validation of this numerical tool to a realistic subduction zone setting that resembles southern chile the resulting quasi periodic pattern of quasi characteristic m8 m9 megathrust events compares quantitatively with observed recurrence and earthquake source parameters albeit at very slow coseismic speeds without any data fitting surface displacements agree with gps data recorded before and during the 2010 m8 8 maule earthquake including the presence of a second order flexural bulge these surface displacements show cycle to cycle variations of slip deficits which overall accommodate 5 of permanent internal shortening we find that thermally and stress driven creep governs a spontaneous conditionally stable downdip transition zone between temperatures of 350 c and 450 c ruptures initiate above it and below the forearc moho propagate within it interspersed by small intermittent events and arrest below it as ductile shearing relaxes stresses ruptures typically propagate upward along lithological boundaries and widen as pressures drop the main thrust is constrained to be weak due to fluid induced weakening required to sustain regular subduction and to generate events with natural characteristics fluid pressures of 75 99 of solid pressures the agreement with a range of seismological geodetic and geological observations demonstrates the validity and strength of this physically consistent seismo thermo mechanical approach

methane hydrate formation in partially water saturated ottawa sand bulk properties of gas hydrate bearing sediment strongly depend on whether hydrate forms primarily in the pore fluid becomes a load bearing member of the sediment matrix or cements sediment grains our compressional wave speed measurements through partially water saturated methane hydrate bearing ottawa sands suggest hydrate surrounds and cements sediment grains the three ottawa sand packs tested in the gas hydrate and sediment test laboratory instrument ghastli contain 38 1 porosity initially with distilled water saturating 58 31 and 16 of that pore space respectively from the volume of methane gas produced during hydrate dissociation we calculated the hydrate concentration in the pore space to be 70 37 and 20 respectively based on these hydrate concentrations and our measured compressional wave speeds we used a rock physics model to differentiate between potential pore space hydrate distributions model results suggest methane hydrate cements unconsolidated sediment when forming in systems containing an abundant gas phase

the effect of compliant prisms on subduction zone earthquakes and tsunamis abstract earthquakes generate tsunamis by coseismically deforming the seafloor and that deformation is largely controlled by the shallow rupture process therefore in order to better understand how earthquakes generate tsunamis one must consider the material structure and frictional properties of the shallowest part of the subduction zone where ruptures often encounter compliant sedimentary prisms compliant prisms have been associated with enhanced shallow slip seafloor deformation and tsunami heights particularly in the context of tsunami earthquakes to rigorously quantify the role compliant prisms play in generating tsunamis we perform a series of numerical simulations that directly couple dynamic rupture on a dipping thrust fault to the elastodynamic response of the earth and the acoustic response of the ocean gravity is included in our simulations in the context of a linearized eulerian description of the ocean which allows us to model tsunami generation and propagation including dispersion and related nonhydrostatic effects our simulations span a three dimensional parameter space of prism size prism compliance and sub prism friction specifically the rate and state parameter b a that determines velocity weakening or velocity strengthening behavior we find that compliant prisms generally slow rupture velocity and for larger prisms generate tsunamis more efficiently than subduction zones without prisms in most but not all cases larger more compliant prisms cause greater amounts of shallow slip and larger tsunamis furthermore shallow friction is also quite important in determining overall slip increasing sub prism b a enhances slip everywhere along the fault counterintuitively we find that in simulations with large prisms and velocity strengthening friction at the base of the prism increasing prism compliance reduces rather than enhances shallow slip and tsunami wave height

experimental investigation of strong ground motion due to thrust fault earthquakes thrust fault earthquakes are studied in a laboratory earthquake setup previously used to investigate analog strike slip seismic events dynamic mode ii ruptures are generated along preexisting faults in an analog material homalite h 100 and their interaction with the free surface is studied for both sub rayleigh and supershear rupture speeds high speed digital photography and laser velocimeter diagnostics are used synergistically to identify and study the ground velocity signatures caused by the various features of the generated ruptures the obtained surface normal motions of both sub rayleigh and supershear ruptures show substantial asymmetry between the hanging and footwall with the hanging wall experiencing much larger velocity amplitudes the main features of the surface velocity traces at various stations can be explained by the calculated arrivals of various waves and fronts mach cones pand s waves and sub rayleigh features in both the sub rayleigh and supershear cases the arrival of the rupture tip generates a prominent rayleigh wave traveling along the simulated earth s surface supershear events feature larger amplitudes of ground shaking profiles all signatures in the surface motion records attenuate and broaden with increasing distance from the fault trace the signatures corresponding to the arrival of the mach fronts attenuate with distance at a slower rate than those from sub rayleigh ruptures the arrival of the updip supershear rupture at the free surface creates a downdip propagating slip feature with its own mach cone these additional mach fronts further amplify ground shaking on the hanging and footwalls

triaxial compressive properties of artificial methane hydrate bearing sediment 1 knowledge of the mechanical properties of gas hydrate bearing sediments is essential for simulating the geomechanical response to gas extraction from a gas hydrate reservoir in this study drained triaxial compression tests were conducted on artificial methane hydrate bearing sediment samples under hydrate stable temperature pressure conditions toyoura sand average particle size d50 0 230 mm number 7 silica sand d50 0 205 mm and number 8 silica sand d50 0 130 mm were used as the skeleton of each specimen axial loading was conducted at an axial strain rate of 0 1 min 1 at a constant temperature of 278 k the cell and pore pressures were kept constant during axial loading we found that the strength and stiffness of the hydrate sand specimens increased with methane hydrate saturation and with the effective confining pressure and the secant poisson s ratio decreased with the effective confining pressure the stiffness depends on the type of sand forming the skeleton of the specimens although the strength has little dependence on the type of sand according to an earlier work hydrate sand specimens are thought to contract in the early stage of axial loading before starting to expand owing to the dilatancy effect as is the case for many other geological materials the test results in this study are discussed in relation to the deformation mechanism proposed in an earlier work

dynamics of earthquake normal faulting two dimensional lattice particle model a 2d lattice particle model is used to simulate the dynamic rupture process of a normal fault the system equations for the particle motions are solved numerically by the finite difference method under a given block boundary condition the flexibility of the implementation of a 2d lattice particle model to simulate an earthquake dynamic process was demonstrated in previous modeling of a shallow angle thrust fault shi et al 1998 numerical results indicate that the particle motions displacement velocity and acceleration along the fault are discontinuous both in the fault parallel and fault normal directions with a localized slip rupture and localized fault separation in the vicinity of the fault outcrop the position at which the fault intersects with the free surface the particle velocity and acceleration increase rapidly both on the hanging wall and the footwall the particle motions on the hanging wall are larger than those on the footwall these motions are amplified as the fault scarp develops rupture breaks out at the surface with strong asymmetry between the hanging wall and the footwall along the free surface as the distance from the fault outcrop increases the particle velocity and acceleration decrease rapidly on the footwall and less rapidly on the hanging wall the asymmetrical particle motion results from the geometrical effect of the dip of the fault the free surface and the dynamic source rupture the combination of all of these effects causes a strong asymmetry in stress when the rupture pulse approaches the free surface the dynamically propagating rupture is characterized by a ramp slip time function accompanied with fault opening the slip pulse becomes sharper when the rupture approaches the free surface consequentially the hanging wall in the vicinity of the fault exhibits a large vibration which generates a strong surface wave propagating along the free surface away from the fault scarp on the hanging wall side this result is similar but significantly different from the numerical simulation of a normal fault with a moving double couple dislocation source benz and smith 1988 in addition the numerical result is qualitatively in agreement with recent foam rubber experiments brune and anooshehpoor 1999 and similar to results from a finite element simulation oglesby et al 1998 oglesby 1999 comparing with a 2d strike slip fault and thrust fault the particle motions in the vicinity of the normal fault on the free surface are smaller for a same size rupture pulse

distinct asymmetry in rupture induced inelastic strain across dipping faults an off fault yielding model 1 off fault stresses associated with rupture propagation cause material yielding under a mohr coulomb condition leaving irrecoverable deformation in the fault zone 2d dynamic rupture simulations for a 30 reverse fault and a 60 normal fault in a depth dependent stress environment show that the yielding inelastic zone off the fault broadens as it nears the surface with decreasing confining pressure forming a skewed flower like structure bounded at the top by the free surface the inelastic strain in the hanging wall is significantly larger and broader than the footwall for both reverse and normal faults the occurrence of inelastic strain however significantly reduces ground motion especially on the hanging wall and gives rise to a reduced asymmetry in ground motion on the hanging wall and footwall compared to elastic solutions these results provide theoretical predictions for fault zone structure of dipping faults that can be tested by future observational experiments

material contrast does not predict earthquake rupture propagation direction 1 earthquakes often occur on faults that juxtapose different rocks the result is rupture behavior that differs from that of an earthquake occurring on a fault in a homogeneous material previous 2d numerical simulations have studied simple cases of earthquake rupture propagation where there is a material contrast across a fault and have come to two different conclusions 1 earthquake rupture propagation direction can be predicted from the material contrast and 2 earthquake rupture propagation direction cannot be predicted from the material contrast in this paper we provide observational evidence from 70 years of earthquakes at parkfield ca and new 3d numerical simulations both the observations and the numerical simulations demonstrate that earthquake rupture propagation direction is unlikely to be predictable on the basis of a material contrast

effects of a low velocity zone on a dynamic rupture dynamic crack earthquake simulations generally assume that the crustal material surrounding faults is laterally homogeneous tomographic and near fault seismic studies indicate that the crust near faults is instead comprised of rocks of varying material velocities we have tested the effects of adding material velocity variation to simulations of spontaneously propagating earthquakes we used two dimensional plane strain conditions coupled with a slip weakening fracture criterion and examined earthquakes on faults that bisect finite width low velocity zones embedded in country rock and earthquakes on faults that bound two different velocity materials when a fault bisects a low velocity zone the normal stress remains unchanged but both the rupture velocity and slip velocity pulse shape are perturbed the presence of the low velocity zone induces high frequency oscillations in the slip function near the rupture front when the fault is on the edge of the low velocity zone the oscillations are more pronounced and repeated sticking and slipping can occur near the rupture front for the slip weakening velocity independent friction model however the temporary sticking does not lead to permanent arrest of slip and slip duration is still controlled by the overall rupture dimension when an earthquake ruptures a fault juxtaposing a lower velocity material against a higher velocity material the normal stress across the fault near the crack tip is perturbed the sign of the normal stress perturbation depends on the direction of rupture leading in some cases to a directional dependence of rupture velocity when slip is accompanied by stress reduction a positive feedback develops between the normal and shear stress changes as previously noted by andrews and ben zion 1997 resulting in an apparently unavoidable grid size dependence in computation of stress change near the rupture front numerical experiments indicate however that the rupture velocity is insensitive to this zone size dependence which is highly localized immediately behind the crack tip the factors controlling the rupture velocity in the simulations including directional dependence are further elucidated by a new analytical solution for rupture of an asperity on a frictionless interface

damage in step overs may enable large cascading earthquakes seismic hazard analysis relies on the ability to predict whether an earthquake will terminate at a fault tip or propagate onto adjacent faults cascading into a larger more devastating event while ruptures are expected to arrest at fault discontinuities larger than 4 5 km scientists are often puzzled by much larger rupture jumps here we show that material properties between faults significantly affect the ability to arrest propagating ruptures earthquake simulations accounting for fault step over zones weakened by accumulated damage provide new insights into rupture propagation revealing that lowered rigidity and material interfaces promote rupture propagation our models show for the first time that step overs as wide as 10 km may not constitute effective earthquake barriers our results call for re evaluation of seismic hazard analyses that predict rupture length and earthquake magnitude based on historic records and fault segmentation models

radiated seismic energy based on dynamic rupture models of faulting 1 by modeling spontaneous ruptures we study the mechanism dependence of radiated seismic energy from three hypothetical crustal events 30 dipping reverse fault 60 dipping normal fault and a vertical strike slip fault and the 1994 blind thrust northridge earthquake embedded in a homogeneous half space all three hypothetical faults have the same area and are subjected to the same shear and normal stress conditions and frictional parameters dynamic simulations produce apparent stress of 0 53 mpa 0 23 mpa and 0 34 mpa for the reverse normal and strike slip faults respectively the energy distribution on a distant surface shows that a large fraction of energy is concentrated in the forward direction of rupture propagation we use spontaneous rupture models to compute the radiated energy from the 1994 northridge earthquake the initial stress drop distribution is based on a kinematic slip distribution using a linear slip weakening friction law we modify both the initial stress and yield stress until the dynamic rupture produces near source synthetics that are consistent with the data the total radiated seismic energy from our model is 6 0 1014 j seismic moment 1 47 1019 nm apparent stress 1 5 mpa fracture energy 3 2 1014 j and slip weakening distance 0 25 m the energy flux distribution is heterogeneous with strong directivity effects these results suggest that correcting for directivity could be difficult but necessary for teleseismic and regional estimates of radiated energy dynamic source models constrained by ground motions can provide a stable and accurate energy estimate for large earthquakes

mechanical and hydraulic properties of nankai accretionary prism sediments effect of stress path 1 we have conducted triaxial deformation experiments along different loading paths on prism sediments from the nankai trough different load paths of isotropic loading uniaxial strain loading triaxial compression at constant confining pressure pc undrained pc reduction drained pc reduction and triaxial unloading at constant pc were used to understand the evolution of mechanical and hydraulic properties under complicated stress states and loading histories in accretionary subduction zones five deformation experiments were conducted on three sediment core samples for the nankai prism specifically from older accreted sediments at the forearc basin underthrust slope sediments beneath the megasplay fault and overthrust upper shikoku basin sediments along the frontal thrust yield envelopes for each sample were constructed based on the stress paths of pc reduction using the modified cam clay model and in situ stress states of the prism were constrained using the results from the other load paths and accounting for horizontal stress results suggest that the sediments in the vicinity of the megasplay fault and frontal thrust are highly overconsolidated and thus likely to deform brittle rather than ductile the porosity of sediments decreases as the yield envelope expands while the reduction in permeability mainly depends on the effective mean stress before yield and the differential stress after yield an improved understanding of sediment yield strength and hydromechanical properties along different load paths is necessary to treat accurately the coupling of deformation and fluid flow in accretionary subduction zones

simple crack models explain deformation induced by subduction zone megathrust earthquakes following the 2010 maule and 2011 tohoku earthquakes many studies have examined the relation between megathrust earthquakes and subsequent deformation here we apply simple models based on mode ii shear cracks including approximated effects of the free surface to study induced deformation during coseismic and early postseismic stages we distinguish between buried and surface ruptures represented by a full crack and a half crack model respectively we adopt an analogy based approach to interpret the half crack model from well known results of the full crack model which is also validated by our numerical simulations with transferable knowledge between the two models we provide easy ways to understand 1 the contrasting deformation patterns in the frontal wedge of the overriding plate between buried ruptures and surface ruptures 2 the correlation between triggered outer rise normal faulting and surface ruptures and 3 the similar deformation patterns for both buried and surface ruptures toward the down dip end with a preference for normal faulting in the overriding plate and for reverse faulting in the subducting plate these model outcomes are consistent with several recent observations on aftershocks and veins in a paleoaccretionary wedge we further investigate some important transient features during rupture propagation which show that a transition from compressional to extensional deformation in the frontal wedge of the overriding plate is possible even during a single rupture event our work provides alternative views for understanding various aspects of subduction zone megathrust earthquakes and raises the issue of important transient features that were typically ignored in previous studies

earthquakes on dipping faults the effects of broken symmetry dynamic simulations of earthquakes on dipping faults show asymmetric near source ground motion caused by the asymmetric geometry of such faults the ground motion from a thrust or reverse fault is larger than that of a normal fault by a factor of 2 or more given identical initial stress magnitudes the motion of the hanging wall is larger than that of the footwall in both thrust reverse and normal earthquakes the asymmetry between normal and thrust reverse faults results from time dependent normal stress caused by the interaction of the earthquake generated stress field with earth s free surface the asymmetry between hanging wall and footwall results from the asymmetric mass and geometry on the two sides of the fault

precarious rock evidence for low ground shaking on the footwall of major normal faults preliminary interpretations of precarious rock observations suggest relatively low near fault 3 km footwall ground motions for normal faults relative to strike slip faults as reported for a dynamic foam rubber normal fault model by brune and anooshehpoor 1999 since for large normal fault earthquakes there are no instrumental data at such close distances from the fault trace precarious rocks may provide important data for estimating the ground motion for these earthquakes use of empirical curves in the literature based mainly on strike slip data may lead to overestimates of seismic hazard for the near fault footwall of normal faults these preliminary results may warrant more comprehensive and more quantitative followup studies

rupture dynamics on a bimaterial interface for dipping faults dip slip faulting may juxtapose different geologic materials with differ ent properties such that a strong material contrast will naturally tend to form both the material contrast across a fault and dip slip motion on a nonvertical fault lead to normal stress variations during earthquake rupture this normal stress variation will significantly affect dynamic rupture propagation to demonstrate this we model dynamic rupture propagation on two dimensional 2d reverse and normal faults 30 45 and 60 dipping with 20 material contrasts for predominantly up dip rupture propagation we find that normal stress variations due to the free surface and material contrast can either reinforce or counteract each other depending on the configuration for reverse faults we find a larger strength drop for a more com pliant hanging wall and a lower strength drop for a more compliant footwall for normal faults we find a larger strength drop for a more compliant footwall and a lower strength drop for a more compliant hanging wall for both reverse and normal faults ground motion will be more symmetric between the hanging wall and footwall with compliant material on the footwall and more asymmetric if more compliant materials are on the hanging wall our results have important implications for the dynamics of crustal and perhaps subduction zone earthquake faulting where strong bimaterial contrasts across dipping faults are possible in continental settings reverse faulting will tend to advect rigid materials from greater depth onto the hanging wall such that the effects of fault dip and material contrast will counteract one another in subduction zones the hanging wall is likely to be more compliant and hence the material and geometric effects may reinforce one another

physical limits on ground motion at yucca mountain abstract physical limits on possible maximum ground motion at yucca mountain nevada the designated site of a high level radioactive waste repository are set by the shear stress available in the seismogenic depth of the crust and by limits on stress change that can propagate through the medium we find in dynamic deterministic 2d calculations that maximum possible horizontal peak ground velocity pgv at the underground repository site is 3 6 m sec which is smaller than the mean pgv predicted by the probabilistic seismic hazard analysis psha at annual exceedance probabilities less than 10 6 per year the physical limit on vertical pgv 5 7 m sec arises from supershear rupture and is larger than that from the psha down to 10 8 per year in addition to these physical limits we also calculate the maximum ground motion subject to the constraint of known fault slip at the surface as inferred from paleoseismic studies using a published probabilistic fault displacement hazard curve these calculations provide a probabilistic hazard curve for horizontal pgv that is lower than that from the psha in all cases the maximum ground motion at the repository site is found by maximizing constructive interference of signals from the rupture front for physically realizable rupture velocity from all parts of the fault vertical pgv is maximized for ruptures propagating near the p wave speed and horizontal pgv is maximized for ruptures propagating near the rayleigh wave speed yielding in shear with a mohr coulomb yield condition reduces ground motion only a modest amount in events with supershear rupture velocity because ground motion consists primarily of p waves in that case the possibility of compaction of the porous unsaturated tuffs at the higher ground motion levels is another attenuating mechanism that needs to be investigated

three dimensional velocity structure seismicity and fault structure in the parkfield region central california this study examines the three dimensional velocity structure in a 60 by 80 km region containing the parkfield segment of the san andreas fault we use local earthquake and shot p arrival times in an iterative simultaneous inversion for velocity and hypocentral parameters using the three dimensional model we relocated 5251 events that occurred from 1969 to 1991 as well as the 1966 aftershocks and computed 664 fault plane solutions the san andreas fault saf characterized by a sharp across fault velocity gradient is the primary feature in the velocity solution there is a 5 20 lateral change in velocity over a 4 km width the contrast being sharper where there is better resolution the model also shows significant variations in the velocity and in the complexity of the velocity patterns along the saf the largest across fault velocity difference is below middle mountain where a large volume of low velocity material impinges on the saf from the northeast this material is inferred to be overpressured and may be key to understanding the unusual behavior in the parkfield preparation zone a 20 km long high velocity slice is imaged northeast of the saf near gold hill its along fault length corresponds to the length of the maximum slip in 1966 the relocated seismicity shows that the san andreas fault is a planar vertical fault zone at seismogenic depths ninety percent of the fault plane solutions that are on or near the saf were right lateral strike slip on subvertical fault planes that parallel the saf thus the surface fault complexities do not appear to extend to depth and therefore do not explain the rupture character at parkfield at parkfield variations in material properties play a key role in fault segmentation and deformation style our observations suggest that there may be a general relation between increasing velocity and increasing ability of the rocks to store strain energy and release it as brittle failure

a fault constitutive relation accounting for thermal pressurization of pore fluid 1 the heat generated in a slip zone during an earthquake can raise fluid pressure and thereby reduce frictional resistance to slip the amount of fluid pressure rise depends on the associated fluid flow the heat generated at a given time produces fluid pressure that decreases inversely with the square root of hydraulic diffusivity times the elapsed time if the slip velocity function is crack like there is a prompt fluid pressure rise at the onset of slip followed by a slower increase the stress drop associated with the prompt fluid pressure rise increases with rupture propagation distance the threshold propagation distance at which thermally induced stress drop starts to dominate over frictionally induced stress drop is proportional to hydraulic diffusivity if hydraulic diffusivity is 0 02 m2 s estimated from borehole samples of fault zone material the threshold propagation distance is 300 m the stress wave in an earthquake will induce an unknown amount of dilatancy and will increase hydraulic diffusivity both of which will lessen the fluid pressure effect nevertheless if hydraulic diffusivity is no more than two orders of magnitude larger than the laboratory value then stress drop is complete in large earthquakes

low footwall accelerations and variable surface rupture behavior on the fort sage mountains fault northeast california the fort sage mountains fault zone is a normal fault in the walker lane of the western basin and range that produced a small surface rupture 20 cm during an m l 5 6 earthquake in 1950 we investigate the paleoseismic history of the fort sage fault and find evidence for two paleoearthquakes with surface displacements much larger than those observed in 1950 rupture of the fort sage fault 5 6 ka resulted in surface displacements of at least 0 8 1 5 m implying earthquake moment magnitudes m w of 6 7 7 1 an older rupture at 20 5 ka displaced the ground at least 1 5 m implying an earthquake of m w 6 8 7 1 a field of precariously balanced rocks pbrs is located less than 1 km from the surface rupture trace of this holocene active normal fault ground motion prediction equations gmpes predict peak ground accelerations pgas of 0 2 0 3 g for the 1950 rupture and 0 3 0 5 g for the 5 6 ka paleoearthquake one kilometer from the fault surface trace yet field tests indicate that the fort sage pbrs will be toppled by pgas between 0 1 0 3 g we discuss the paleoseismic history of the fort sage fault in the context of the nearby pbrs gmpes and probabilistic seismic hazard maps for extensional regimes if the fort sage pbrs are older than the mid holocene rupture on the fort sage fault zone this implies that current gmpes may overestimate near fault footwall ground motions at this site

three dimensional compressional wavespeed model earthquake relocations and focal mechanisms for the parkfield california region we present a new three dimensional 3d compressional wavespeed v p model for the parkfield region taking advantage of the recent seismicity associated with the 2003 san simeon and 2004 parkfield earthquake sequences to provide increased model resolution compared to the work of eberhart phillips and michael 1993 epm93 taking the epm93 3d model as our starting model we invert the arrival time data from about 2100 earthquakes and 250 shots recorded on both permanent network and temporary stations in a region 130 km northeast southwest by 120 km northwest southeast we include catalog picks and cross correlation and catalog differential times in the inversion using the double difference tomography method of zhang and thurber 2003 the principal v p features reported by epm93 and michelini and mcevilly 1991 are recovered but with locally improved resolution along the san andreas fault saf and near the active source profiles we image the previously identified strong wavespeed contrast faster on the southwest side across most of the length of the saf and we also improve the image of a high v p body on the northeast side of the fault reported by epm93 this narrow body is at about 5 to 12 km depth and extends approximately from the locked section of the saf to the town of parkfield the footwall of the thrust fault responsible for the 1983 coalinga earthquake is imaged as a northeast dipping high wavespeed body in between relatively low wavespeeds 5 km sec extend to as much as 10 km depth we use this model to derive absolute locations for about 16 000 earthquakes from 1966 to 2005 and high precision double difference locations for 9 000 earthquakes from 1984 to 2005 and also to determine focal mechanisms for 446 earthquakes these earthquake locations and mechanisms show that the seismogenic fault is a simple planar structure the aftershock sequence of the 2004 mainshock concentrates into the same structures defined by the pre 2004 seismicity confirming earlier observations waldhauser et al 2004 that the seismicity pattern at parkfield is long lived and persists through multiple cycles of mainshocks online material 3d v p model and earthquake relocations

dynamics of thrust faulting 2d lattice model particle motions in a foam rubber model of shallow angle thrust faulting show many features different from those commonly assumed in dislocation models of subduction thrusts brune 1996 as a complement to a physical foam rubber experiment we have carried out dynamic simulation using a 2d lattice numerical model the model is constructed as a triangular block sliding over a rectangular elastic block it consists of a 2d set of particles interacting with each other by nonlinear hooke s forces and obeying newton s equations of motion rough surfaces are introduced on the contact plane of the two blocks to simulate realistic friction the numerical simulations demonstrate several key features of thrust faulting including stick slip fault opening and strong breakout and shaking at the hanging wall toe the wedge shaped tip of the outcropping hanging wall block of the thrust fault all consistent with the foam rubber experiments the stick slip motion shows an approximate time and slip repeatable behavior each slip event is characterized by a self healing pulse associated with fault opening and frictional locking the rupture pulse propagates steadily along the fault when rupture reaches the toe of the fault outcrop the hanging wall breaks away from the foot wall and creates a large opening vibration of the hanging wall the peak acceleration observed at the hanging wall toe of the fault outcrop is about 3 to 4 times the peak acceleration in the center part of the fault and about 2 to 3 times the motion of the foot wall such a large increase in peak acceleration at the toe is caused mainly by multiply reflecting stress waves trapped in the wedge shaped hanging wall of the fault the strong asymmetry of the particle velocity between the hanging wall and foot wall is an important feature of the results consistent with the foam rubber model but different from standard dislocation models this dynamic result illustrates the important effects of thrust faulting geometry on fault slip and ground motion

comment on material contrast does not predict earthquake rupture propagation direction by r a harris and s m day 1 harris and day 2005 hereinafter referred to as hd concluded from some numerical simulations and low resolution analysis of 8 m4 m6 earthquakes near parkfield ca that material contrast in fault zone structures is not important for earthquake propagation direction the limited theoretical and observational results of hd do not justify such a general conclusion considerations of a larger body of results suggest that material contrast may produce a statistically preferred direction of earthquake ruptures the inferred diversity of the 8 discussed events is consistent with their small number and relatively small size in addition the behavior has a natural explanation in a structure with two material interfaces the san andreas fault saf and the southwest fracture zone sfz on the opposite sides of a deformation damage region large earthquakes propagating predominantly as mode ii ruptures and affected primarily by properties of the bounding crustal blocks rather than the local structure are likely to exhibit a smaller amount of diversity

characteristics of near fault ground motions by dynamic thrust faulting two dimensional lattice particle approaches two dimensional lattice particle models have been used to simulate dynamic thrust faulting and its effects on near fault ground motions the lattice particle modeling approach has been demonstrated as an efficient way to model the dynamic rupture phenomena observed from a foam rubber experiment on dipping faults we constructed a 42 dipping fault model to simulate near fault ground motions under different site conditions in which different combinations of fault geometry blind fault underlying a sedimentary layer and outcropping thrust models and near surface sedimentary layers were investigated in particular dynamic behaviors between a blind fault with an overlying sedimentary layer and an outcropping fault without a sedimentary layer were compared in this simulation a dynamic slip pulse accompanied by fault separation was initiated at the deepest part of the fault and propagated updip along the fault under a subshear rupture velocity a strong asymmetrical ground motion pattern on the hanging wall and footwall caused by near source rupture effects was observed rupture directivity played an important role in determining the size and distribution of peak ground velocities and accelerations on the hanging wall and footwall in the case of an outcropping thrust without a sedimentary layer the hanging wall underwent a stronger ground motion caused by the near surface breakout phase as the rupture reached the free surface in the case of a blind thrust overlying a sedimentary layer the peak ground particle velocity and acceleration could be much larger on the footwall this is a result of the amplification effect of trapped long period seismic energy in the sedimentary layer the seismic energy emitted from the rupture stopping phase was incident to the sedimentary layer and radiated under the rupture directivity effect the radiated long period seismic energy was trapped in the sedimentary layer propagating away from the fault trace toward the footwall side the numerical results show that for an initial slip of 5 to 5 5 m the horizontal peak ground velocity and acceleration could reach about 1 5 m sec and 2 g respectively on the footwall for a blind thrust with a sedimentary layer with the same initial slip level the peak ground velocity and acceleration were about 1 1 m sec and 1 5 g respectively on the hanging wall for an outcropping thrust without a sedimentary layer these results can partially explain the field observations of precarious rock distributions and overturned transformers in the vicinity of the white wolf fault during the 1952 m s 7 6 kern county earthquake furthermore the current simulation can be used for near fault strong motion prediction for large thrust faults in the los angeles basin or similar tectonic settings around the world

the three dimensional dynamics of a nonplanar thrust fault advances in computational methodology have made it possible to ex plore the dynamics of earthquake rupture on nonplanar faults using a method that allows geometrical flexibility we simulate in three dimensions the dynamics of a fault that has an abrupt change in dip with depth using a homogeneous prestress on both fault segments we find that while the resultant final stress field is strongly influenced by the fault bend the fault slip and low frequency ground motion are relatively insensitive to the pure dynamic effects of the nonplanar fault the ground velocity from the nonplanar fault is qualitatively quite similar to that of a planar fault with the same dip angle as the nonplanar fault s shallow segment as the effects of multiple earthquakes accumulate on this fault stress concentrations at the fault bend are compounded but the effect of the free surface on the stress appears to approach a steady state the results of this study imply that for the prediction of peak ground motion from faults that intersect the surface of the earth a bend in the fault at depth may not be a significant factor the very long term effects of the fault bend are not fully determined but could lead to complexity in the rupture and slip process in future events

near fault strong ground motions recorded during the morelia normal fault earthquakes of may 2006 in mexicali valley b c mexico by the end of may 2006 a series of earthquakes had occurred in the mexicali valley baja california mexico this activity which included an m w 5 4 event developed in the vicinity of the cerro prieto geothermal field the earthquakes occurred on the morelia fault one of the east dipping normal faults in the mexicali valley cracks and fissures visible on the ground after the stronger earthquake defined two parallel fault segments of 4 and 2 km length the shorter segment being 1 km to the east of the other one down dip displacements of up to 25 30 cm were measured at some places along this surface break zone the ten larger events of the series were recorded by strong motion stations that operate in the epicenter area so that accurate hypocenter locations were obtained on the basis of p wave arrival times from the strong motion recordings the estimated shallow depths combined with the earthquake magnitude explain the strong shaking felt by residents and some damage observed on the cerro prieto geothermal plant the m w 5 4 event produced peak ground accelerations that go from 0 002 g at cuc on rock to 0 5 g at geo on sediments at 13 and 1 7 km from the epicenter respectively the station geo recorded closer to the epicenters and on the hanging wall side of the fault static ground displacements and a predominance of the strike normal over the strike parallel velocity components were determined from the acceleration records of this station these and other ground motion characteristics are also seen on pseudovelocity and absolute acceleration response spectra calculated from data of the larger event altogether the observed ground motion characteristics provide useful insights into the levels of ground shaking that near fault structures in the mexicali valley should be designed to withstand