joints and en echelon cracks in middle eocene chalks near beer sheva israel abstract planar vertical joints and straight en echelon cracks are closely associated in an erosional exposure of middle eocene chalks in a syncline south of beer sheva the two dominant joint sets are oriented 357 n3 w and 031 n31 e and both sets occur in two separate outcrops 400 m apart an en echelon crack array accompanies each joint set the individual en echelon cracks are oriented 337 and 053 respectively radial plumose markings are present on the surfaces of the en echelon cracks they imply that fracture occurred normal to a local tensile stress axis a gradual rotation of the azimuths of the joints and cracks occurs between the two outcrops this rotation implies jointing due to nne ssw compression and en echelon cracking due to unloading of this compression

relations between jointing and faulting based on fracture spacing ratios and fault slip profiles a new method to estimate strain in layered rocks by using fault slip profiles and fracture spacing ratios fsrs a direct link is established between slip variations across a normal fault zone and anomalous joint densities in adjacent wall rock faulting in bedded chalk near beer sheva israel was impeded by competent cherts requiring the development of subsidiary faults to accommodate unrelieved strain the composite slip profile for the entire normal fault zone is smooth and uniform except for a sharp localized decrease near one bed resulting in a steep slip gradient this bed displays unusually closely spaced joints in the hanging wall fsr 2 2 as opposed to typical joint spacings fsr 1 5 in the footwall the slip deficit may reflect a surplus of elastic strain stored in that bed which was released in the form of additional joints during subsequent jointing this study reveals that joint densities in wall rock can vary as a function of fault geometry and slip distribution and the fsr may be used as an effective tool to estimate strain around local structures

dynamical fault rupture processes in heterogeneous media abstract the fault rupture processes in a horizontally layered medium and also in a three dimensionally heterogeneous structure are investigated on three dimensional spontaneous dynamic shear crack models the wave equations for three dimensional space are solved numerically by a finite difference scheme under the appropriate boundary conditions and the finite stress fracture criterion the heterogeneous properties of the elastic medium particularly the existence of low velocity zones give remarkable effects on the rupture process yielding appreciably decelerated rupture velocities and large fault displacements in and around the zones and strong motions in the near field the large fault displacements are enhanced when the rupture breaks the ground surface the static seismic moment in the case with a low velocity zone is essentially the same as in a homogeneous half space an attempt is made to simulate the rupture process of a moderate size earthquake by applying the above shear crack model in a heterogeneous medium with depth dependent and laterally heterogeneous stress drop the model appears to explain the observed features to a satisfactory degree

effects of prestress state and rupture velocity on dynamic fault branching 1 we consider a mode ii rupture which propagates along a planar main fault and encounters an intersection with a branching fault using an elastodynamic boundary integral equation formulation allowing the failure path to be dynamically self chosen we study the following questions does the rupture initiate along the branch does it continue is the extensional or compressional side most favored for branching does rupture continue on the main fault too failure is described by a slip weakening law for which the strength at any amount of slip is proportional to normal stress our results show that dynamic stresses around the rupture tip which increase with rupture velocity at locations off the main fault plane relative to those on it could initiate rupture on a branching fault as suggested by prior work whether branched rupture can be continued to a larger scale depends on principal stress directions in the prestress state and on rupture velocity the most favored side for branching rupture switches from the extensional to the compressional side as we consider progressively shallower angles of the direction of maximum compressive prestress with the main fault simultaneous rupturing on both faults can be activated when the branching angle is wide but is usually difficult for a narrow branching angle due to strong stress interactions between faults however it can be also be activated by enhanced dynamic stressing when the rupture velocity is very near the rayleigh velocity natural examples seem consistent with the simulations that we present

the state of stress on some faults of the san andreas system as inferred from near field strong motion data we present a simple method to calculate the stress produced on an earthquake fault during rupture this method allows the complete evaluation of the stress spatio temporal history over the fault we apply this approach to study the changes in shear stress produced during four of the largest earthquakes which occurred along the san andreas fault system over the last 20 years the imperial valley earthquake of 1979 the morgan hill earthquake of 1984 the loma prieta earthquake of 1989 and the northridge earthquake of 1994 we use for this study the tomographic models of the fault rupture obtained from the inversion of the near field seismic data recorded during these earthquakes the results obtained show that the static and the dynamic stress drops vary greatly over the fault the peak values obtained for the four earthquakes studied range from about 20 to 100 mpa these high values imply that the initial shear stress level on the fault at the onset of the earthquake was high on at least a significant portion of the fault the regions of the fault which break with a high stress drop are also the regions where slip is large this suggests that most of the slip produced in a large earthquake takes place over the strong areas of the fault low slip regions tend to break with low stress drops after the earthquake the shear stress is increased over a significant portion of the fault which corresponds to low slip regions aftershock activity tends to be concentrated in these areas of stress increase the apparent strength of the fault before the earthquake that is the local shear stress increase which is required for rupture is also extremely heterogeneous the rupture velocity seems to be inversely correlated with this apparent fault strength the rupture accelerating over the weak areas of the fault and slowing down over the high strength areas

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

is there a relationship between magmatic fabrics and brittle fractures in plutons a view based on structural analysis anisotropy of magnetic susceptibility and thermo mechanical modelling of the tanvald pluton bohemian massif abstract the present study focuses on the thermal and mechanical aspects of cooling of plutonic systems with respect to their internal structure analysis of the tanvald pluton and the nearby liberec granite the krkonose jizera plutonic complex bohemian massif using the anisotropy of magnetic susceptibility ams method indicates the decoupled magnetic fabrics formed by different processes in these two plutons two regional sets of vertical fractures cut across the plutons and their host rocks they are interpreted as having been formed as opening mode fractures joints after emplacement and cooling our thermo mechanical finite element modelling indicates that the thermal stresses within the tanvald pluton were compressive and that fracture criteria were not met during pluton cooling these results imply that the emplacement depth and confining pressure significantly affect the state of stress within a pluton during cooling and may play a crucial role in the formation of early fractures in plutons consequently the formation of early fractures during pluton cooling is driven by external conditions overriding the role of internal magmatic fabrics we argue that no genetic geometric or temporal correlation can exist between magmatic fabrics and fractures in granitoid plutons and thus that the original concept of hans cloos based on fabric joints relationships should be reconsidered instead an alternative classification of fractures in plutons should include cooling fractures syntectonic fractures uplift fractures and post uplift fractures

fractographic features on joints in ktb drill cores as indicators of the contemporary stress orientation fractographic features on joints in cores of the superdeep ktb drilling were used to estimate the orientation of the contemporary maximum horizontal stress sh the unique opportunity to investigate cores from a depth down to approximately 8085 m reveals information on in situ stress orientation with increasing depth we recognized on the cores the main normal stress axes which are reflected by common fractographic pattern on coring induced fractures present in all sections the analysis is based on the symmetry of joint characteristics 3d out of plane shape of the joint and surface features of joint planes the symmetry axes of these joint features are related to the orientation of principal stress which permits the determination of the orientation of the contemporary maximum and minimum horizontal stress axes sh sh if the core orientation is determined by other methods for that purpose the reorientated cores of the pilot borehole vb could be used down to 4000 m the main borehole hb cores are generally not reoriented by several reasons some investigations were done by members of the ktb laboratory to determine the supposedly northward orientations of hb core sections by comparison with the foliation and of drilling induced fractures at the borehole wall using fmi formation microimager and fms formation microscanner log data and within the core independently the maximum horizontal in situ stress orientation down to 7800 m was determined from drilling induced vertical joints and the strike and dip of the foliation from borehole measurements down to ca 8600 m carried out by the staff members at the ktb additionally rock fragments from below 9050 m represent a palaeo tectonic joint system developed at a depth of ca 9 km

evolution of the stress field near the arava basin located along the dead sea fault system as revealed by joint sets abstract to reveal the development of the stress field near the arava basin located along the dead sea fault system we characterized joint sets that were exposed mainly in the santonian and eocene chalk rock outcropping in the study area in total 2600 strike orientations of single layer joints slj and multilayer joints mlj mirrored four main stress fields 1 hnw j1 326 2 hnnw j2 343 3 hnne j3 033 and 4 hn j4 360 the results showed that the relatively lower values of the fracture spacing ratio fsr and their standard deviations in j1 implied that j1 was formed during the burial stage of the eocene chalk formations the inconsistency between the statistical parameters of the log normal distributions of joint spacing and fsr raised doubt about the applicability of the saturation joint density model in our studied area the joint sets characteristics suggest that the regional hnw operated during the eocene during the miocene the regional stress field changed clockwise to hnnw probably in association with the early stage of transform motion along the dead sea fault in the pliocene pleistocene the hnne and hn local stress fields both operated and were accompanied by a tensional stress field that can be identified in the intensive normal faulting evolved along the western arava basin margins

three dimensional nonplanar simulation of the 1992 landers earthquake 1 we have simulated the rupture process of the 1992 landers california earthquake using a realistic nonplanar fault geometry including fault bends and branches through a three dimensional boundary integral equation method considering an external loading system due to local tectonics as well as a nonplanar fault structure we could reproduce the characteristic features of this earthquake such as spontaneous rupture transfer between faults pattern of slip distribution and propagation of rupture front without assuming any laterally heterogeneous distribution of parameters we could also reproduce seismic waveforms near the faults especially in the forward direction of rupture propagation where rupture directivity controlled by rupture transfer between faults significantly affects ground motion

low frictional strength of quartz rocks at subseismic slip rates 1 laboratory experiments on rocks at low sliding velocity typically yield values of the friction coefficient of 0 6 0 85 here we demonstrate that an extraordinary decrease in friction coefficient accompanies sliding of room dry quartz rocks at rates faster than in most laboratory experiments but slower than seismic slip rates in some cases the friction coefficient decreases from low speed values by more than a factor of 3 this extraordinary weakening likely results from the formation of finely comminuted amorphous wet material on the sliding surface

insight into the cenozoic tectonic evolution of the qaidam basin northwest china from fracture information fractures can provide valuable information for tectonic evolution according to the data of outcrops cores thin sections and well logs the tectonic fractures in the qaidam basin can be divided into four types small faults including small normal fault and small reverse fault vertical open fracture bedding plane slip fracture and horizontal open fracture our fracture observations provide new constraints on the cenozoic tectonic evolution of the qaidam basin syn sedimentary small normal faults in the paleogene strata indicate the extension deformation during the paleogene small reverse faults vertical open fractures and bedding plane slip fractures occurred in the paleogene and neogene strata have genetic relationship according to the burial history and homogenization temperature of fluid inclusions of gypsum and calcite filled in the vertical open fractures it can be deduced that the vertical open fractures being formed mainly from the late miocene shangyoushashan formation with age of 5 1 ma to the end of pliocene shizigou formation with age of 2 6 ma indicating small reverse faults vertical open fractures and bedding plane slip fractures were simultaneously formed in the neogene these fractures were resulted from the compression deformation the horizontal open fractures occurred in the paleogene neogene and quaternary strata with apertures and intensities decreasing with depth were formed by the large scale quick uplift and denudation resulted from the strong compression deformation since the quaternary

laboratory experiments on failure characteristics of non cohesive sediment natural dam in progressive failure mode natural dam failure may cause disastrous floods in the downstream especially for dam outbursts in progressive failure mode until now researches about the characteristics of progressive failure are still completely unclear in this paper a series of flume tests were conducted to investigate the formation mechanism of progressive failure of natural dam the breaching characteristics and the effect of outburst flood on the channel bed according to the results three typical features in the progressive failure process of natural dams were observed seepage on downstream slope slide of the downstream slope and overtopping with phreatic line rising continuously seepage occurred on the downstream slope which caused slide of downstream slope of the dam the breaching discharge hydrographs were asymmetric on both sides of peak discharge which was similar to the breaching discharge of overtopping the relationship between breach width and depth was like a shape of s which followed a logistic function in the breaching process sediment in the channel bed near the dam was first deposited and then scoured where the flow velocity was large with strong turbulence while flow on channel bed far away from the dam showed lower velocity and the flow regime transferred from supercritical flow to subcritical flow sediment deposited at first and then scoured gradually on channel bed but only deposited at a large distance from the dam the outburst flood eroded the channel bed continuously the erosion mechanism was from tractive erosion to backward erosion and returned to tractive erosion finally in addition the further away from the dam the weaker the erosion intensity was the results of this paper enriched the understanding of the failure mechanism of progressively outburst natural dam which is helpful to predict breaching parameters in progressive failure mode and to provide some reference about flood prevention

jointing patterns and tectonic evolution of the macico calcario estremenho lusitanian basin portugal abstract the macico calcario estremenho mce is an uplifted jurassic limestone massif unit of the lusitanian basin portugal where five main joint sets trending nne ssw wsw ene wnw ese nw se and nnw sse are recognized except for the nnw sse set all the other sets host calcite veins and barren joints evidencing a multistage development by several deformation episodes including shear reactivation orthogonal patterns defined by the nne ssw wnw ese and nnw sse wsw ene systems are characteristic of some tectonostratigraphic units of the mce but the sets of each one of the systems are genetically independent they result from specific deformation episodes undergone by the studied area in the course of its meso cenozoic evolution nne ssw calcite veins were the first to form during middle jurassic fault controlled subsidence a renewal of this set as barren joints took place during the eocene pyrenean compressive phase the wsw ene and wnw ese sets have a restricted spatial distribution and relate to transient compressive episodes of the middle late jurassic and jurassic cretaceous transitions respectively the nw se set also characteristic of a specific region formed during the late jurassic rifting and is related to local ne sw tension dependent on block tilting towards a major nw se fault the miocene betic compressive phase is responsible for the formation of the nnw sse set which is widespread throughout the mce

numerical simulation of fault zone guided waves accuracy and 3 d effects fault zones are thought to consist of regions with reduced seismic velocity when sources are located in or close to these low velocity zones guided seismic head and trapped waves are generated which may be indicative of the structure of fault zones at depth observations above several fault zones suggest that they are common features of near fault radiation yet their interpretation may be highly ambiguous analytical methods have been developed to calculate synthetic seismograms for sources in fault zones as well as at the material discontinuities these solutions can be used for accurate modeling of wave propagation in plane parallel layered fault zone structures however at present it is not clear how modest deviations from such simplified geometries affect the generation efficiency and observations of trapped wave motion as more complicated models cannot be solved by analytical means numerical methods must be employed in this paper we discuss 3 d finite difference calculations of waves in modestly irregular fault zone structures we investigate the accuracy of the numerical solutions for sources at material interfaces and discuss some dominant effects of 3 d structures we also show that simple mathematical operations on 2 d solutions generated with line sources allow accurate modeling of 3 d wave propagation produced by point sources the discussed simulations indicate that structural discontinuities of the fault zone e g fault offsets larger than the fault zone width affect significantly the trapping efficiency while vertical properly gradients fault zone narrowing with depth small scale structures and moderate geometrical variations do not the results also show that sources located with appropriate orientations outside and below a shallow fault zone layer can produce considerable guided wave energy in the overlying fault zone layer

mass transport in eocene pelagic chalk on the northwestern edge of the arabian platform shefela area israel glides rotational slumps debris and mud flows occur in the eocene pelagic chalk of the shefela region israel the pelagic lithofacies are present from the beginning of the eocene but mass transport is initiated only in the latest early eocene and the early middle eocene slight steepening of the palaeoslope coupled with increased sedimentation rate and early diagenetic changes lowering inherent strength led to gravity induced mass transport in the poorly consolidated pelagic sediments instability of the sediment pile was enhanced by underconsolidation and development of spacious packing associated with a relatively high rate of deposition and low initial strength due to high content of siliceous micro biota the palaeogeographic position indicated by the mass transport features is the steepened distal part of the ramp like continental margin of the arabian platform distal steepening of the ramp as part of regional downwarping is coincident with reactivation of syrian arc folding reflecting northward convergence of the arabian plate during the eocene

sequence stratigraphy of miocene pliocene carbonate siliciclastic shelf deposits in the eastern mediterranean margin israel effects of eustasy and tectonics abstract mio pliocene marine sediments comprise a discontinuous succession of carbonates and mixed carbonate siliciclastics on the neogene shelf which coincides with the judean foothills area on the slope present mediterranean coastal plain the succession forms a wedge up to 1500 m thick of mostly fine grained siliciclastics the facies and architecture of the succession were influenced both by sea level fluctuation and by the geodynamic change from thermally subsiding passive margins of the arabo african plate to a complex pattern of vertical movements that were associated with the breakup of the plate along the red sea rift and dead sea transform and the convergence of the african arabian and eurasian plates the miocene eustatic succession is highly interrupted by vertical uplift tectonics while the pliocene eustatic succession is well represented despite the limited subsidence rates during most of the early and middle miocene the shelf was tectonically uplifted above sea level except for a short period of flooding in early middle miocene times n8 n9 when sea level rise outstripped the uplift trend and a carbonate platform formed the combination of tectonic uplift and sea level fall prior to late miocene times about 11 m y ago resulted in the incision of a deep beer sheva canyon about 280 m below the shelf plane late miocene sandy marlstones of the bet eshel cycle filled the beer sheva canyon and spilled over it they display a shallowing upward trend resulting in progradational clinoform stacking of the highstand systems tract and channelled debris flows of the forced regression systems tract the messinian desiccation event left the shelf emerged while depositing evaporites on the slope and in the basin the degree of erosion during the messinian was relatively minor because of arid conditions and limited runoff the pliocene shelf shows a very low subsidence rate and compared to the miocene succession is less affected by vertical uplifts the pliocene deposits are divided into six sequences their relative onlap curve largely corresponds to the sequence stratigraphic scheme in sicily and the gulf of mexico the two lowermost sequences i and ii are composed of two sandy units the lower forms an incised valley fill of shoreface deposits in the beer sheva area and offshore sand deposits in distal shelf areas besor although largely eroded sequences i and ii display maximum coastal onlap of all pliocene cycles in the beer sheva canyon area sequence iii consists of mixed carbonate siliciclastic grainstones of shoreface foreshore deposits in the proximal area and lower shoreface deposits in more distal areas sequences iv and v may be considered as a single 3rd order cycle where iv represents regressive lowstand shale and sand deposits and v represents prograding highstand sandy grainstone deposits sequence vi consists of marginal marine to coastal plain marlstones and sands deposits and is largely truncated by the ahuzam conglomerate representing a major uplift phase of latest pliocene to early pleistocene age and marking the permanent emergence of the shelf

a late cenozoic regional mesofracture system in southern england and northern france the upper cretaceous and palaeogene rocks of s england and n france are cut by a nw trending system of fractures characterized by swarms of vertical extension joints accompanied by conjugate sets of hybrid and shear joints and normal mesofaults many major photolineaments exhibit the same nw trend as the fractures and are particularly conspicuous in n france where they are coincident with rectilinear drainage channels suggesting a relationship between fracture orientation and physiography in s england this relationship is absent possibly because of the influence of additional fracture systems related to major flexures trending obliquely to the nw se direction these major e w trending flexures and associated fracture systems were formed during the oligocene to early miocene 9helvetic9 phase of deformation and are cross cut obliquely by fractures in the nw trending system the nw trending fractures and lineaments are parallel to neotectonic normal faults in the lower rhine embayment and become less well developed to the west with increasing distance from these major structures the lower rhine faults were reactivated and propagated into the quaternary cover as a consequence of ne sw regional tension generated during the late neogene to recent 9jura9 phase of nw se alpine convergence the nw trending extensional structures in s england and n france are considered to be of about the same age and related to the same stress regime as the neotectonic normal faults of the lower rhine embayment however because fractures of the nw system cutting palaeogene sediments in se england are truncated by the erosion surface beneath the plio pleistocene red crag deposits it is possible that in england fracture initiation did not continue into quaternary times

jointing and fracture interactions in middle eocene chalks near beer sheva israel abstract this study presents some observations on jointing and fracture interaction in synclinal middle eocene chalks horsha formation near beer sheva israel three joint sets occur in an outcrop set 344 n16 w developed first and was followed by sets 028 and 062 each set reflects a different fracture mechanism joint spacing and fracture length were investigated in set 028 but there did not appear to be any correlation between the two parameters the mean spacings in two layers 30 cm and 60 cm thick range closely between 23 16 cm and 26 19 cm suggesting independence of joint spacing from bed thickness in set 028 log probability plots of joint lengths from these two layers suggest an approximate lognormal distribution the mean joint lengths are 82 2 cm 56 8 cm and 64 1 cm 44 0 cm in the thicker and thinner beds respectively statistically the length populations in the two beds are considered to be significantly distinguishable implying that the mean fracture lengths show some dependence on layer thickness joint sets that reflect different fracture mechanisms show differences in intensity and in style of fracture termination terminations between parallel joints of the same set are differentiated from interactions between joints of different sets at acute angles in joints of set 028 various crack terminations are classified into seven styles there is on the average a visible interaction every 120 cm along joints of set 028 interactions between sets 0 28 and 344 induce secondary fractures which are identifiable as three style groups the most frequently induced secondary fractures are either subparallel or subperpendicular to the two sets the sum length of the secondary cracks in an interaction zone does not seem to be akin to the lengths of the parent joints especially puzzling interactions between sets 028 and 344 are triple junctions and complex triple junctions defined by these fractures interactions between sets 028 and 0 62 result in rhomboid structures at acute angles ranging between 18 and 51 evidently these structures are the product of extension and no shear offsets are visible along the fractures characteristically some of these rhomboids are being subdivided into smaller structures by the same sets

depth dependent structure of the landers fault zone from trapped waves generated by aftershocks we delineate the internal structure of the johnson valley and kickapoo faults landers southern rupture at seismogenic depth using fault zone trapped waves generated by aftershocks trapped waves recorded at the dense linear seismic arrays deployed across and along the surface breaks of the 1992 m7 5 landers earthquake show large amplitudes and dispersive wave trains following the s waves group velocities of trapped waves measured from multiple band pass filtered seismograms for aftershocks occurring at different depths between 1 8 km and 8 2 km show an increase in velocity with depth velocities range from 1 9 km s at 4 hz to 2 6 km s at 1 hz for shallow events while for deep events velocities range from 2 3 km s at 4 hz to 3 1 km s at 1 hz coda normalized amplitude spectra of trapped waves peak in amplitudes at 3 4 hz for stations located close to the fault trace the amplitude decays rapidly with the station offset from the fault zone normalized amplitudes also decrease with distance along the fault giving an apparent q of 30 for shallow events and 50 for deep events we evaluated depth dependent fault zone structure and its uncertainty from these measurements plus our previous results from near surface explosion excited trapped waves li et al 1999 in a systematic model parameter searching procedure using a three dimensional 3 d finite difference computer code graves 1996 our best model of the landers fault zone is 250 m wide at the surface tapering to 100 150 m at 8 2 km depth the shear velocity within the fault zone increases from 1 0 to 2 5 km s and q increases from 20 to 60 in this depth range fault zone shear velocities are reduced by 35 to 45 from those of the surrounding rock and also vary along the fault zone with an increase of 10 near ends of the southern rupture zone

rupture characteristics of the three m 4 7 1992 1994 parkfield earthquakes slip on the san andreas fault was determined for three m 4 7 earthquakes using a tomographic inverse system beroza and spudich 1988 to invert seismic source time functions stfs from s waves stfs were obtained by deconvolving mainshock accelerograms by those from collocated smaller earthquakes accelerograms were from the u s geological survey parkfield small aperture array upsar and from a distributed array of digital accelerometer stations at parkfield eight or nine stfs are used in each of the three inversions stfs are typically symmetrical pulses with a duration of about 0 3 0 5 s in the inversion mainshock rise time was set to 0 05 s and we allowed the rupture time to vary slightly from a constant rupture velocity of approximately 0 85 rupture for all three events which are located in or close to the middle mountain preparation zone or box mmb quickly reaches a local maximum in slip and then propagates outward to peaks ridges or plateaus in the slip distribution slip for the october 20 1992 event located just inside the southern edge of the mmb propagates from an initial spike north and updip along a curving ridge for about 2 km the initial spike continued to grow in the november 14 1993 event located north of the october 20 1992 event just beneath the hypocenter of the 1966 parkfield earthquake which shows little directivity although there is a smaller patch of slip updip and to the south in contrast rupture for the december 20 1994 event located just south of the october 20 1992 event propagated north and slightly updip creating a rough plateau in slip a few kilometers wide on a side directivity for this event also is to the north directivity for all three events points in the approximate direction of the 1966 hypocenter small pulses which comprise a coda are found on the stfs for several seconds after the initial impulsive event several tests based on the assumption that the average of all stfs from upsar for each event is an estimate of the true slip at the source suggest that the codas in the stfs are s waves from a long duration source rather than uncorrected site response an initiation phase is found on the array average for the november 14 1993 and december 20 1994 events these precursory phases are the result of a spike in slip at the hypocenter a value of 2 4 4 mm is obtained for d c the slip weakening distance by interpreting the initial spike as a critical patch the few aftershocks for the october 20 1992 event are distributed to the north and updip of the mainshock but the november 14 1993 event had a strong burst of aftershock activity that propagated to the north of its hypocenter at roughly the same depth aftershocks of the december 20 1994 event are mostly updip the november 14 1993 event had the simplest slip distribution appeared to be the most impulsive and had the most active aftershock sequence and the greatest depth if the eventual parkfield earthquake initiates near the 1966 hypocenter then the directivity of the three events studied here will have pointed to it however it is certainly possible that both the initiation of characteristic parkfield shocks and the directivity of smaller events are controlled by fault properties on a larger scale such as by fault bends or jogs

numerical simulations of large earthquakes dynamic rupture propagation on heterogeneous faults our current conceptions of earthquake rupture dynamics especially for large earthquakes require knowledge of the geometry of the faults involved in the rupture the material properties of the rocks surrounding the faults the initial state of stress on the faults and a constitutive formulation that determines when the faults can slip in numerical simulations each of these factors appears to play a significant role in rupture propagation at the kilometer length scale observational evidence of the earth indicates that at least the first three of the elements geometry material and stress can vary over many scale dimensions future research on earthquake rupture dynamics needs to consider at which length scales these features are significant in affecting rupture propagation

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

fracture mechanics model of a fault termination zone a fault termination zone from middle eocene chalks near beer sheva israel is analysed via a fracture mechanical technique the zone consists of the end of the primary fault three secondary faults and joints associated with these structures we demonstrate that the shape of the first secondary fault can be obtained from theoretical fracture mechanical calculations this shape also enables us to obtain the set of conditions which induced the observed structures the technique reveals the relative importance of the different variables that appear in the theory the most significant parameters in determining the shape of the first secondary fault are the vertical dimension of the primary fault the ratio of the two horizontal differential stresses and the initiation angle of the secondary fault results indicate that the fault termination zone was created under an almost pure shear load the true lateral displacement of the primary fault is unknown hence exact calculation of the stresses leading to the secondary fault is hardly possible however an estimation based on a partial conversion of this displacement to an uplift yields 1 and 3 values at initiation of between 3 8 and 7 6 mpa and 3 65 and 7 3 mpa respectively

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

aftershocks of the coyote lake california earthquake of august 6 1979 a detailed study aftershock hypocenters and focal mechanism solutions for the coyote lake california earthquake reveal a geometrically complex fault structure consisting of multiple slip surfaces the faulting surface principally consists of two right stepping en echelon northwest trending partially overlapping nearly vertical sheets and is similar in geometry to a slip surface inferred for the 1966 parkfield california earthquake the overlap occurs near a prominent bend in the surface trace of the calaveras fault at san felipe lake slip during the main rupture as inferred from the distribution of early aftershocks appears to have been confined to a 14 km portion of the northeastern sheet between 4 and 10 km depth focal mechanisms and the hypocentral distribution of aftershocks suggest that the main rupture surface itself is geometrically complex with left stepping imbricate structure seismic shear displacement on the southwestern slip surface commenced some 5 hours after the mainshock aftershocks in this zone define a single vertical plane 8 km long between 3 and 7 km depth within the overlap zone between the two main slip surfaces the average strike of aftershock nodal planes is significantly rotated clockwise relative to the strike of the fault zone in close agreement with the stress perturbations predicted by crack interaction models aftershock activity in the overlap zone is not associated with a simple dislocation surface space and time clustering within the entire aftershock set suggest an alternation of seismic displacement between the component parts of the fault zone this alternation is consistent with local stress perturbations predicted by crack interaction models we conclude that the fault structure is geometrically complex and that the displacements that occur on its component surfaces during the aftershock process dynamically interact by generating perturbations in the local stress field which in turn control the displacements table 5 is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j82 006 1 00 payment must accompany order

travel time tomography in the northern coachella valley using aftershocks of the 1986 ml 5 9 north palm springs earthquake tomographic inversion is applied to delay times from aftershocks of the 1986 ml 5 9 north palm springs nps earthquake to image 3 d velocity variations within the northern coachella valley p wave arrival times from 1074 earthquakes with depths ranging from 3 to 20 km were used as sources recorded by 12 portable and 4 permanent stations preliminary results show well defined high and low velocity anomalies 2 7 that correlate with the rupture distribution of the 1986 mainshock at depths less than 8 km a low velocity anomaly predominates between the two ne dipping banning and mission creek faults from 8 to 12 km in depth where the nps mainshock and most of the aftershocks occur a high velocity anomaly is observed this high velocity feature is interpreted as imaging the asperity responsible for the 1986 rupture and suggests that velocity information may help to define important elements such as asperities that control fault rupture and thus may help to predict the location and size of future events

fracture stresses at shallow depths during burial abstract single layer joints in lower eocene chalks near beer sheva israel developed at shallow depths during burial prior to folding and uplift fracturing occurred in two stages initially extensional joints marked by circular fracture surface morphology formed at about 100 m depth when pore pressure p 1 5 mpa horizontal maximum effective stress gs y 1 0 mpa vertical effective stress gs z 1 0 mpa and horizontal minimum effective stress gs x 0 5 mpa subsequently hybrid joints that produced a dihedral angle of 35 and were marked by axial horizontal plumes formed at a burial depth of about 150 m where p 2 0 mpa gs x 0 5 mpa gs y 1 0 mpa and gs z 1 8 mpa fracturing continued during further burial to depths of perhaps 200 400 m

evidence for and implications of self healing pulses of slip in earthquake rupture dislocation time histories of models derived from waveforms of seven earthquakes are discussed in each model dislocation rise times the duration of slip for a given point on the fault are found to be short compared to the overall duration of the earthquake 10 however in many crack like numerical models of dynamic rupture the slip duration at a given point is comparable to the overall duration of the rupture i e slip at a given point continues until information is received that the rupture has stopped propagating alternative explanations for the discrepancy between the short slip durations used to model waveforms and the long slip durations inferred from dynamic crack models are 1 the dislocation models are unable to resolve the relatively slow parts of earthquake slip and have seriously underestimated the dislocations for these earthquakes 2 earthquakes are composed of a sequence of small dimension short duration events that are separated by locked regions barriers 3 rupture occurs in a narrow self healing pulse of slip that travels along the fault surface evidence is discussed that suggests that slip durations are indeed short and that the self healing slip pulse model is the most appropriate explanation a qualitative model is presented that produces self healing slip pulses the key feature of the model is the assumption that friction on the fault surface is inversely related to the local slip velocity the model has the following features high static strength of materials kilobar range low static stress drops in the range of tens of bars and relatively low frictional stress during slip less than several hundreds of bars it is suggested that the reason that the average dislocation scales with fault length is because large amplitude slip pulses are difficult to stop and hence tend to propagate large distances this model may explain why seismicity and ambient stress are low along fault segments that have experienced large earthquakes it also qualitatively explains why the recurrence time for large earthquakes may be irregular

single layer burial joints vs single layer uplift joints in eocene chalk from the beer sheva syncline in israel abstract the single layer s l joints that occur in the lower eocene chalks near beer sheva israel developed during the burial history of the rock whereas the s l joints in adjacent middle eocene chalks developed during the uplift stage characteristically s l burial joints occur in orthogonal cross fold and strike sets and as conjugate sets they precede normal faults and multi layer joints and they do not exhibit strike rotation en echelon segmentation or fracture interaction with each other these joints are generally closed and during subsidence older beds fracture first on the other hand s l uplift joints do not occur in orthogonal or conjugate sets they are post strike slip faulting contemporaneous with multi layer joints and exhibit strike rotation en echelon segmentation and often interact with each other they are occasionally opened up to several millimetres and during uplift younger beds fracture first

dynamic modeling of the 1992 landers earthquake we have used observed band pass filtered accelerograms and a previously determined slip distribution to invert for the dynamic rupture propagation of the 1992 landers earthquake in our simulations dynamic rupture grows under the simultaneous control of initial stress and rupture resistance by friction which we modeled using a simple slip weakening law we used a simplified landers fault model where the fault segments were combined into a single vertical planar fault by trial and error we modified an initial stress field inferred from the kinematic slip distribution proposed by wald and heaton 1994 until dynamic rupture generated a rupture history and final slip distribution that approximately matched those determined by the kinematic inversion we found that rupture propagation was extremely sensitive to small changes in the distribution of prestress and that a delicate balance with energy release rate controls the average rupture speed for the inversion we generated synthetic 0 5 hz ground displacements using an efficient green s function propagator method axitra this method enables us to propagate the radiation generated by the dynamic rupture to distances greater than those feasible using the finite difference method the dynamic model built by trial and error inversion provides a very satisfactory fit between synthetics and strong motion data we validated this model using records from stations used in the slip inversion as well as some which were not included we also inverted for a complementary model that fits the data just as well but in which the initial stress was perfectly uniform while rupture resistance was heterogeneous this demonstrates that inversion of ground motion is nonunique

dynamic stress changes during earthquake rupture we assess two competing dynamic interpretations that have been pro posed for the short slip durations characteristic of kinematic earthquake models de rived by inversion of earthquake waveform and geodetic data the first interpretation would require a fault constitutive relationship in which rapid dynamic restrengthen ing of the fault surface occurs after passage of the rupture front a hypothesized mechanical behavior that has been referred to as self healing the second inter pretation would require sufficient spatial heterogeneity of stress drop to permit rapid equilibration of elastic stresses with the residual dynamic friction level a condition we refer to as geometrical constraint these interpretations imply contrasting pre dictions for the time dependence of the fault plane shear stresses we compare these predictions with dynamic shear stress changes for the 1992 landers m 7 3 1994 northridge m 6 7 and 1995 kobe m 6 9 earthquakes stress changes are com puted from kinematic slip models of these earthquakes using a finite difference method for each event static stress drop is highly variable spatially with high stress drop patches embedded in a background of low and largely negative stress drop the time histories of stress change show predominantly monotonic stress change after passage of the rupture front settling to a residual level without significant evidence for dynamic restrengthening the stress change at the rupture front is usu ally gradual rather than abrupt probably reflecting the limited resolution inherent in the underlying kinematic inversions on the basis of this analysis as well as recent similar results obtained independently for the kobe and morgan hill earthquakes we conclude that at the present time the self healing hypothesis is unnecessary to explain earthquake kinematics

effects of frictional heating on the thermal hydrologic and mechanical response of a fault the mechanical response of a fault zone during an earthquake may be controlled by the diffusion of excess heat and fluid pressures generated by frictional heating in this study we formulate a fault model which incorporates the effects of frictional heating on the thermal hydrologic and mechanical response of a small patch of the failure surface this model is used to examine the parameters that control the fault response and to determine their critical range of values where thermal pressurization is significant the problem has two time scales a characteristic slip duration and a characteristic time for thermal pressurization the slip duration is set by the fault geometry the characteristic time for thermal pressurization is set by the slip rate the friction coefficient and the thermal and hydraulic characteristics of the medium the response of the fault depends on the relative magnitude of these two times results suggest that the fault width and hydraulic characteristics of the fault zone and adjacent medium are the primary parameters controlling the mechanical response for earthquakes occurring across zones of low porous medium compressibility 10 9 pa 1 and permeability 10 18 m2 the characteristic time for thermal pressurization is small in this case frictional heating can cause fluid pressures to approach lithostatic values the shear strength to approach zero and the temperature rise to stabilize at a maximum value dependent on the pore dilatational and transport properties of the porous medium whether the patch acts as a barrier to slip or exhibits substantial strain weakening is dependent on the shear strain across the fault moderate slip events where shear strains exceed two cause substantial strain weakening and consequently large stress drops accelerations and displacements thus it is possible for the patch to act as a barrier for small earthquakes but not for large ones both the dynamic stress drop and total displacement decrease for zones with larger compressibility permeability or width if the compressibility or permeability exceeds 10 8 pa 1 or 10 14 m2 or the shear strain is less than one then the effects of frictional heating may be negligible and the fault will exhibit no strain weakening characteristics consequently the patch acts as barrier that halts or resists further fault motion extrapolation of these results suggests that spatial variations in fault width and hydraulic characteristics will cause a heterogeneous stress drop and fault slip over the failure surface explaining many of the features of active faulting e g barriers nonuniform slip rupture stoppage random ground accelerations strong motions and frequency magnitude relations

relations among fault behavior subsurface geology and three dimensional velocity models the development of three dimensional p wave velocity models for the regions surrounding five large earthquakes in california has lead to the recognition of relations among fault behavior and the material properties of the rocks that contact the fault at seismogenic depths regions of high moment release appear to correlate with high seismic velocities whereas rupture initiation or termination may be associated with lower seismic velocities these relations point toward a physical understanding of why faults are divided into segments that can fail independently an understanding that could improve our ability to predict earthquakes and strong ground motion

correlation of plume morphologies on joint surfaces with their fracture mechanic implications the fractography and conditions of propagation of joints that cut devonian siltstones in the appalachian plateau new york and eocene chalks from the beer sheva syncline israel are investigated the joints cutting the siltstones are marked by s type and c type plumes and the joints cutting the lower eocene and middle eocene chalks are marked by coarse and delicate plumes respectively the four plume types propagated under sub critical slow propagation conditions on the semi quantitative fracture velocity v versus the tensile stress intensity ki c urves the sa nd c plume types fall in the ki 0 073 0 79 mpa m 1 2 and v 2 10 4 10 2 m s and ki 0 073 0 79 mpa m 1 2 and v 10 6 10 4 m s ranges respectively the coarse and delicate plumes fall in the ki 0 03 0 17 mpa m 1 2 and v 10 6 4 10 5 m s and ki 0 03 0 17 mpa m 1 2 and v 10 4 5 10 3 m s ranges respectively generally slow plumes are relatively short show periodicity and typically exhibit superposition of arrest marks on the other hand faster plumes are longer and continuous occur particularly in thinner layers and show no superposition of arrest marks there is a clear distinction between two ensegmentation end members in the joint fringe the discontinuous breakdown type and the continuous breakdown type there are also transitional variations between the end members only curved discontinuous breakdown type boundaries of en echelon fringes can be equated with mirror boundaries

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

dynamic fault rupture and stress recovery processes in continental crust under depth dependent shear strength and frictional parameters abstract mikumo t 1992 dynamic fault rupture and stress recovery processes in continental crust under depth dependent shear strength and frictional parameters in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 201 222 the dynamic rupture process of large earthquakes in continental crust and the post seismic stress recovery process due to visco plastic behaviour are investigated by three and two dimensional numerical modelling taking into account the possible depth variations of the shear strength tectonic stress expected stress drop and of the critical weakening slip which are suggested by recent laboratory experiments on frictional parameters it was found that dynamic rupture can initiate at any depth in a brittle zone if high stress is concentrated with a dimensionless stress ratio s less than a certain limit but that the rupture cannot nucleate in a semi brittle zone with negative stress drop this suggests that the boundary between the fully brittle and semi brittle zones delineates the base of the seismogenic zone of background seismicity and aftershock distribution however the dynamic rupture of large earthquakes originating from the brittle zone can propagate into the semi brittle zone after some temporary arrest or with decelerated velocities which could generate low frequency seismic waves the depth extent of dynamic faulting and the maximum fault displacement are affected by the depth to the brittle semi brittle boundary and hence by the geothermal state within the crust the stress recovery due to visco plastic behaviour in the semi brittle and plastic regimes suggests that the rupture for eventual earthquakes tends to be nucleated near the upper boundary of the semi brittle zone

loading paths to joint propagation during a tectonic cycle an example from the appalachian plateau u s a abstract based on the timing of joint propagation during the history of burial lithification deformation and denudation of clastic rocks within sedimentary basins four types of joints may be distinguished tectonic hydraulic unloading and release tectonic and hydraulic joints form at depth prior to uplift in response to abnormal fluid pressures whereas unloading and release joints form near the surface in response to thermal elastic contraction accompanying erosion and uplift tectonic joints are distinguished from hydraulic joints in that tectonic compaction is a mechanism for achieving abnormal pore pressures leading to the propagation of the former whereas compaction by overburden loading leads to the abnormal pore pressures in the latter case the orientation of unloading joints is controlled by either a residual or contemporary tectonic stress whereas the orientation of release joints is controlled by a rock fabric examples of some of these joints are found within the devonian catskill delta of the appalachian plateau new york during the alleghanian orogeny tectonic joints cross fold joints formed under abnormal pore pressure as indicated by the observation that joints propagated in the siltstones before they developed in shales and by the cross cutting relationships of folds cleavage and joints this sequence is compatible with oil company hydraulic fracture data which show that the least principal stress within sandstone layers is less than that in the intercalated shale layers plumose structures indicate that the joints within siltstones propagated as discontinuous rupture events each of which affected less than a meter of bed length the discontinuous rupturing is compatible with models for natural hydraulic fracturing release joints strike joints post date the alleghanian orogeny as indicated by abutting relationships within the deeper parts of the devonian clastic section unloading joints are orthogonal to the contemporary tectonic stress field

joint development during fluctuation of the regional stress field in southern israel four trends of joint sets wnw ese nw se nnw sse and ne sw are found in upper turonian carbonate rocks within the neqarot syncline of south central israel the two most predominant sets strike parallel to the trend of maximum compressive stress directions sh associated with the plate related syrian arc stress field sas wnw ese active during the cretaceous to present and the perturbed regional stress field nnw sse related to stress accumulation on the dead sea transform during the miocene to the present eighty two percent of the beds in this study contain joints parallel with the latter trend whereas 42 contain joints parallel to the former trend all beds with layer thickness to spacing ratio fsr 1 5 have nnw sse joint sets compatible with the dead sea transform stress field dss whereas all joints sets that are not compatible with the dss stress field fall beneath this value for fsr considering lithology joints in five of six chalky limestone beds and all marly limestone beds are compatible with the dss whereas joints compatible with the sas do not develop in these marly and chalky limestone beds in the study area the joint sets lack a consistent formation sequence where more than one set is found in a single bed we use these observations to conclude that all studied joints are miocene or younger that the regional stress field from the miocene to the present fluctuated between dss and sas states and that the higher fsrs correspond to a greater amount of joint normal strain in response to the dss q 2001 elsevier science ltd all rights reserved

correlation between fracture surface morphology and orientation of cross fold joints in eocene chalks around beer sheva israel abstract three fracture surface morphology styles occur on cross fold joint surfaces in the lower eocene chalks these are horizontal coarse straight axial plumes with barb widths that vary from 0 5 cm to several centimeters circular undulatory markings and transitional variation between the two occasionally the three types occur in close proximity in the same outcrop about 56 of the 225 fracture markings examined in four outcrops are circular and occur on fracture surfaces of set 328 n32 w and 29 are axial plumes that occur on surfaces of 309 and 344 joint sets transitional patterns make up 12 and about 3 are other combinations a fourth surface morphology style occurs exclusively on joint surfaces of middle eocene chalks this type is defined as horizontal delicate straight axial plume with barb width below 0 5 cm this type is associated with single layer joint sets 031 and 357 hence certain styles of fracture markings show affinities with particular joint sets it is suggested that whereas the circular markings develop when the minimum principal stress x is horizontal and normal to the joint surface and horizontal y vertical z the axial plumes represent the same conditions with respect to x but with y z

dynamic stress drop and rupture dynamics of the october 15 1979 imperial valley california earthquake abstract a dynamic faulting model is constructed to determine the dynamic stress drop and relative strength distribution for the october 15 1989 imperial valley earthquake and to study the dynamics of rupture propagation for this event reproduction of the kinematic quantities of the strike slip rate strike slip final offset rupture time and rupture duration bu varying the dynamic parameters of dynamic stress drop and yield strength is attempted the fault plane is discretized in 1 5 km square grids the model incorporates first order corrections for the effect of the free surface and material property heterogeneity on the calculation of rupture at depth the strike slip particle velocities final offsets and rupture durations of the kinematic model are matched within about 40 everywhere below about 5 km except in the hypocentral region and in a high displacement region 20 km north of the epicenter the strike slip rate of the dynamic model is half and rupture duration double those of the kinematic model in the epicentral region the rupture durations and strike slip final offsets are two thirds those of the kinematic model in a high stress drop region beneath the el centro crossing array two slip concentrations correspond to stress drop concentrations one of about 80 bar at 20 km from the epicenter and at 8 km depth and the other of about 70 bar at 30 km from the epicenter and 10 km depth most of the dynamic stress drop occurs between the depths of 9 and 11 km averaging about 60 bar stress drops in the 7 9 km range vary between 25 and 35 bar while stress drops in the 5 7 km depth range vary between 10 and 15 bar this depth dependence of stress drop was a consistent feature across nearly 20 km of the fault stress drop appeared to be well correlated with rock type negative stress drop was found in the sediment regions and the regions at depth while positive stress drop was found in the metasediment regions between the depths of 5 and 11 km yield strength and rupture velocity are inversely related high yield strength is found in regions of low rupture velocity and vice versa rupture was found to terminate at the northern end of the fault by collision with a high strength barrier a region of supersonic rupture velocity was found to occur in a region of high stress drop and low fracture strength the kinematic rupture velocity at the northern end of the fault was found to be unreasonably low results show that this region reaches its peak stress before the kinematic break time yield strength increased with depth in the seismogenetic zone until the base of the zone was reached below which it tapered off slightly

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

dynamic shear rupture interactions with fault bends and off axis secondary faulting 1 on the basis of elastodynamic stress fields for singular crack and nonsingular slip weakening models of propagating rupture we develop preliminary answers to such questions as follows if a rupturing fault is intersected by another providing a possible bend in the failure path when will stressing be consistent with rupture along the bend what secondary fault locations and orientations in a damaged region bordering a major fault will be stressed to failure by the main rupture stresses that could initiate rupture on a bend are shown to increase dramatically with crack speed especially near the limiting speed rayleigh for mode ii shear for mode iii whether a bend path once begun can be continued to larger scales depends on principal stress directions and ratios in the prestress field conditions should often be met in mode ii for which bend paths encouraged by stressing very near the rupture tip are discouraged by the larger scale stressing a basis for intermittent rupture propagation and spontaneous arrest secondary failure in the damage zone likewise increases markedly as the limiting speed is approached such may make the fracture energy much greater than for slip on a single surface the extent of secondary faulting is strongly affected by prestress directions and the ratio of residual to peak strength for mode ii prestress controls whether activation occurs primarily on the extensional side which we show to be the typical case or on the compressional side too natural examples are consistent with the concepts developed

determination of constitutive relations of fault slip based on seismic wave analysis constitutive laws define the boundary conditions on fault plane and govern many aspects of earthquake failure although several constitutive laws have been formulated based on laboratory rock experiments and applied to theoretical studies in various fields no actual relation during a natural earthquake has been determined the 1995 kobe earthquake is suitable for detailed kinematic analysis and this enables the first evaluation of constitutive relations for a natural earthquake in this study we determine spatiotemporal slip distribution on an assumed fault plane of the 1995 kobe earthquake by waveform inversion and then solve elastodynamic equations using a finite difference method to determine the stress distribution and constitutive relations on the fault plane an inversion method based on bayes theorem is employed to obtain a spatiotemporal slip distribution and enables us to ensure the objective uniqueness of the solution with numerous parameters and smoothing constraints this slip distribution is then used as part of the boundary condition in the finite difference calculation the time histories of slip and shear stress obtained then provide a constitutive relation at each point on the fault plane they show slip weakening relations almost everywhere on the fault plane while slip rate dependency is not clear the slip weakening behavior has a clear depth dependency indicating that the slip weakening rate d du is smaller in the shallow crust than that in the deep crust this may be associated with the paucity of shallow seismicity observed in the source region of this earthquake as reported for many mature fault systems

direct seismic energy modeling and application to the 1979 imperial valley earthquake 1 the seismic energy associated with an earthquake has two representations the work of the seismic waves done against a distant surface or a fault representation for a fault subject to slip weakening friction the energy density is the difference between an elastostatic work and a work density spent in fracture and relaxation we apply this to a dynamic simulation of the 1979 imperial valley earthquake whose initial conditions are inspired by previous kinematic studies a large area of the fault has a negative energy density and the emission of energy is roughly confined to small parts of the fault with large positive energy density we compute the work of the seismic waves against the surface of a sphere enclosing the source and we find the same amount of energy we produce a map of energy directivity that shows that 40 of the energy passes through only 6 5 of the sphere

mechanisms of dilatant en echelon crack formation in bedded chalk abstract this study focuses on various styles of dilatant out of plane en echelon cracks which are associated with bed restricted burial joints that cut slightly deformed sediments in the beer sheva syncline there is a positive linear fit of en echelon crack length to layer thickness r2 0 928 in fringes which were formed by discontinuous breakdowns in joints lithological properties influence the breakdown process the breakdown of joints into en echelon cracks in chalk beds is influenced by both local conditions and remote stresses there are two distinct cases of opposite senses of segment rotation in a single fringe first in a curved fringe and second in a straight fringe the former can best be assigned to local deviations of the minimum principal direction from perpendicularity to the parent joint whereas the latter appears to reflect changes in remote stresses two of the three sets of en echelon segmentation which were distinguished in the beer sheva syncline are correlated with two distinct tectonic associations the k iii k i ratio increases from the layer centre towards the layer boundaries and this is the driving motivation for both the gradual increase of barb overlap from the centre towards the layer boundaries and the en echelon segmentation along the layer boundaries

the 1999 izmit turkey earthquake a 3d dynamic stress transfer model of intraearthquake triggering before the august 1999 izmit kocaeli turkey earthquake theoretical studies of earthquake ruptures and geological observations had provided estimates of how far an earthquake might jump to get to a neighboring fault both numerical simulations and geological observations suggested that 5 km might be the upper limit if there were no transfer faults the izmit earthquake appears to have followed these expectations it did not jump across any step over wider than 5 km and was instead stopped by a narrower step over at its eastern end and possibly by a stress shadow caused by a historic large earthquake at its western end our 3d spontaneous rupture simulations of the 1999 izmit earthquake provide two new insights 1 the west to east striking fault segments of this part of the north anatolian fault are oriented so as to be low stress faults and 2 the easternmost segment involved in the august 1999 rupture may be dipping an interesting feature of the izmit earthquake is that a 5 km long gap in surface rupture and an adjacent 25 restraining bend in the fault zone did not stop the earthquake the latter observation is a warning that significant fault bends in strike slip faults may not arrest future earthquakes manuscript received 30 august 2000