finite faults and inverse theory with applications to the 1979 imperial valley earthquake abstract using a representation theorem from elastodynamics subsurface slip on a known fault is formulated as the solution to an inverse problem in which recorded surface ground motion is the data two methods of solution are presented the least squares method which minimizes the squared differences between theory and data and the constrained least squares method which simultaneously maintains a set of linear inequalities instabilities in the solution are effectively eliminated in both methods and the sensitivity of the solution to small changes in the data is quantitatively stated the inversion methodology is applied to 77 components of near field ground acceleration recorded during the 15 october 1979 imperial valley earthquake the faulting is constrained to propagate bilaterally away from the epicenter at an average velocity of 90 per cent of the shear wave speed on a vertical fault plane extending from the surface to 10 km depth inequality constraints are used to keep the faulting sequence physically reasonable by maintaining right lateral motion and positive slip velocity the preferred solution is stable and provides a good fit to the data it is also realistic and consistent with observed surface offsets and independent estimates of seismic moment

crustal structure of the diablo and gabilan ranges central california a reinterpretation of existing data the compressional wave velocity structure of the crust of the coast ranges of central california was modeled from seismic refraction data reported previously for both the diablo range east of the san andreas fault and the gabilan range west of the fault two alternative velocity models were derived for each range by iterative two dimensional ray tracing each pair of models shows a sedimentary layer and an underlying crust composed of three or four layers for the diablo range the velocities in the sediments range from 2 9 to 4 8 km sec and the depth to basement varies from 0 2 km within the central diablo range to a maximum of 3 4 km north of the livermore valley in the gabilan range the velocities in the sediments overlying the granitic basement are higher north of the gabilan range 3 6 to 4 6 km sec than to the south 2 2 to 3 8 km sec and the depth to basement both north and south of the range is less than 2 km below the sediments and fractured near surface material a resolvable difference in the crustal velocity structure on opposite sides of the san andreas fault indicates compositional differences at depth the upper crust has an average velocity of 5 7 km sec at depths between 3 and 16 3 km in the diablo range and an average velocity of 6 1 km sec at depths between 2 and 9 1 km in the gabilan range the lower crust has an average velocity of 6 9 km sec at depths between 16 3 and 30 km in the diablo range and an average velocity of 6 5 km sec at depths between 10 and 24 km in the gabilan range the models also show that the depth to moho differs by several kilometers between the two ranges in the diablo range models the crust mantle boundary becomes shallower from south to north rising from a depth of 30 to 26 km in the gabilan range models the moho is at a depth of 24 to 26 km depending on the velocity assumed at the base of the crust laboratory measurements of rock velocities and the mapped surface geology allow us to interpret the velocity models in terms of crustal composition we conclude that the diablo range probably consists largely of metagraywacke to a depth of 16 3 km and gabbroic material below this depth the crust of the gabilan range probably consists of granitic material to a depth of 9 1 km and gneissic material below this depth franciscan rocks are not regionally present in the crust of the gabilan range if the gneissic lower crust consists of the same rocks as are found at the surface in the gabilan range then the presumed large scale lateral motion of the salinian block has taken place at or below the crust mantle boundary rather than along a mid crustal slip plane

the evolution of a thrust fault system processes of rupture initiation propagation and termination in the 1980 el asnam algeria earthquake summary we discuss work completed to date on the 1980 algerian earthquake and introduce a description of the earthquake in terms of rupture propagation using the nomenclature of fracture mechanics field and teleseismic studies have shown the earthquake fault to be divided into three major thrust segments rupture began at the sw end of the system and propagated along successive segments geological sections reveal that different fault segments have suffered different amounts of displacement in the past the amount of geological displacement increases systematically from almost zero at the sw end of the fault to several hundred metres further ne faulting at the sw end is very young and appears to be extending the thrust system along strike the aftershock data of ouyed et al reveal that strike slip aftershocks occur beyond the sw end of the thrust fault these are accommodating compression by conjugate strike slip faulting on north and west trending planes this network of conjugate faults plays a crucial role in the initiation of major eathquakes on the thrust system and may be regarded as a plastic zone beyond the end of the thrust fault increasing horizontal stress causes the strike slip system to evolve into a zig zag network of thrusts which is partly controlled by the pre existing strike slip faults finally a through going thrust can develop and leads to catastrophic rupture along the main fault plane to describe the propagation of the main shock we extend aki s concept of geometric barriers and divide these into two types in conservative barriers the slip vector is common to both fault planes whereas in non conservative barriers the slip vectors are different on the two fault planes that meet at the barrier although both types of barrier arrest dynamic rupture only nonconservative barriers need produce aftershocks as off fault deformation is required to accommodate the difference in slip vectors by considering the aftershock zones as areas of distributed deformation we show that fracture toughness in the earth is not a material constant but is related to fault geometry and therefore changes as fault systems evolve the current south westwards growth of the el asnam thrust system can be seen as the latest phase in an evolution consisting of periods of along strike extension alternating with periods of increasing net displacement during which no fault growth occurs these two types of behaviour allow a segmented fault system to be produced

the el nino southern oscillation enso modoki signal in the stratosphere el nino southern oscillation enso is known to be the largest source of interannual variability in the tropical troposphere however the variability in the tropical pacific since 1979 seems to be associated not only with canonical enso events but also with a variation thereof known as enso modoki which is characterized by warm anomalies in the central pacific west from those occurring during a typical enso this works analyzes the signal of enso modoki in the stratosphere and compares it to canonical enso by using the chemistry climate whole atmosphere community climate model waccm3 5 the results reveal a significant warming in the southern hemisphere polar stratosphere during boreal winter months which propagates downward in early spring this is absent during canonical warm enso events on the other hand in the northern hemisphere stratosphere the anomalous warming typical of canonical el nino episodes during boreal winter is not statistically significant during el nino modoki events these differences are related in waccm3 5 to changes in tropical convection and tropospheric teleconnections associated with each type of event in particular an enhancement and westward displacement of the anomalous convective area during el nino modoki episodes is related to an intensification of the pacific south american teleconnection pattern and a weakening of the aleutian low during cold enso modoki events a significant anomalous cooling is present in the model simulations

selection of empirical green s functions by waveform similarity analysis an approach to predict ground motion in areas with saturated records abstract this work is focused on a procedure based on selecting suitable empirical green s functions egfs able to predict ground motion for moderate earthquakes in the case where no records are available due to saturation phenomena the aim of the article is to generate synthetic seismograms for the 24 november 2004 m l 5 2 salo earthquake northern italy an event capable of saturating all velocimetric stations installed within the first 100 km from the epicenter the proposed approach uses a waveform similarity analysis based on the normalized cross correlation technique and it is able to identify egfs that represent doublet events of a target the ground motion was finally simulated using the method proposed by irikura 1986 in this case due to the saturation of near source velocimetric instruments the normalized cross correlation matrix was calculated considering the first not saturated velocimetric station aso2 108 km northeast to the epicenter including the same selected portion of signals on the base of signal to noise ratio related both to the target and to 11 events with m l ranging from 2 2 to 3 0 that occurred in the same area the similarity analysis performed through the bridging technique allows us to detect an m l 2 9 aftershock characterized by a meaningful degree of similarity 70 compared to the target given as a fact that if two events are similar for a far field station they have to be similar also for a near source one it was in this way possible to use the selected aftershock doublet recorded in near source to reproduce the target for the saturated near source stations the results of the simulations were compared with ground motion values predicted by empirical ground motion prediction equations gmpes calibrated using both italian and european data

stochastic synthesis of earthquake catalogs a model of earthquake occurrence is proposed that is based on results of statistical studies of earthquake catalogs we assume that each earthquake generates additional a probability that depends on time as t 1 this assumption together with one regarding the independence of branching events on adjacent branches of the event tree is sufficient to permit the generation of complete catalogs of earthquakes that have the same time magnitude statistical properties as real earthquake catalogs if is about 0 5 the process generates sequences that have statistical properties similar to those for shallow earthquakes many well known relations are reproduced including the magnitude frequency law omori s law of the rate of aftershock and foreshock occurrence the duration of a recorded seismic event versus its magnitude the self similarity or lack of scale of rate of earthquake occurrence in different magnitude ranges etc a value of closer to 0 8 or 0 9 seems to simulate the statistical properties of intermediate and deep shocks a formula for seismic risk prediction is proposed and the implications of the model for risk evaluation are outlined the possibilities of the determination of long term risk from real or synthetic catalogs that have the property of self similarity are dim

p and s wave displacements from kinematic dislocation models abstract numerical dislocation models based on haskells 1969 formulation are used to estimate the amount of normal motion necessary to produce the high p s spectral ratios observed from strong motion records of the guerrero mexico subduction zone castro et al 1991 while this depends on the nature of the assumed dislocations a normal motion with amplitude of less than 10 of the amplitude of shear slip is sufficient to produce p s values comparable with the observations we model a planar fault with random patches distributed on the fault plane and a nonplanar fault in which a dilatational jog connects en echelon fault segments a structural system proposed by sibson 1985 1989 which introduces normal motions on the fault that depend mainly on the geometry of the fault for the planar fault model the magnitude of the normal motion is prescribed both models introduce complexity in both body wave displacement time histories although for the p waves this complexity is accentuated at higher frequencies f 1 hz for the nonplanar model a jog with an angle of 10 introduces a normal component of 18 of the slip on the fault

fault interaction near hollister california a numerical model is used to study fault stress slip near hollister california the geometrically complex system of interacting faults including the san andreas calaveras sargent and busch faults is approximated with a two dimensional distribution of short planar fault segments in an elastic medium the steady stress and slip rate are simulated by specifying frictional strength and stepping the remote stress ahead in time the resulting computed fault stress is roughly proportional to the observed spatial density of small earthquakes suggesting that the distinction between segments characterized by earthquakes and those with aseismic creep results in part from geometry a nonsteady simulation is made by introducing in addition stress drops for individual moderate earthquakes a close fit of observed creep with calculated slip on the calaveras and san andreas faults suggests that many changes in creep rate averaged over several months are caused by local moderate earthquakes in particular a 3 year creep lag preceding the august 6 1979 coyote lake earthquake on the calaveras fault seems to have been a direct result of the november 28 1974 thanksgiving day earthquake on the busch fault computed lags in slip rate preceding some other moderate earthquakes in the area are also duemore to earlier earthquakes although the response of the upper 1 km of the fault zone may cause some individual creep events and introduce delays in others the long term rate appears to reflect deep slip less

curved slip zones in an elastic half plane abstract slip on a curved fault loaded by far field compressive stress induces changes in normal stress along the fault surface a two dimensional crack model is developed to quantify these stresses for curved fault geometries under plane strain or plane stress the curved slipping region is replaced by continuously distributed dislocations and the corresponding singular integral equation for the dislocation density is derived and solved numerically because our solution scheme incorporates the exact asymptotic form of slip near the end of the slipping region results for the stress intensity factors are accurate to determine the effects of fault curvature on faulting the changes in fault normal stress accompanying dip slip are calculated for typical faulting events the changes in normal stress accompanying normal faulting increase the frictional resistance to slip advance near the free surface and those accompanying reverse faulting reduce the frictional resistance to slip advance near the free surface under compressional tectonic loading slip is retarded on the planar slip zones of higher dips but enhanced if the dip decreases with depth on the other hand under extensional tectonic loading slip is retarded on the planar slip zones of smaller dips but enhanced if the dip increases with depth curvature of a concave downward slip zone enhances compressive normal stress at the shallow end of the slip zone under compressional tectonic loading tending to prevent propagation of the slip zones to the free surface on the other hand the curvature of a concave upward slip zone reduces the compressive normal stress at the shallow end under the uniform extensional tectonic loading thus assisting propagation of slip zones to the free surface by contrast for deep slip zones the changes in normal stress are practically not affected by the fault curvature to illustrate the local stress field near a sharp bend we examine a sharply curved slip zone subjected to frictional sliding slip on such a fault induces significant compressional fault normal stress near the bend these changes add compression across the slip zone thus increasing the frictional resistance to slip over the curved bend our results show that the magnitude of induced normal stresses is sharply localized near the bend

faulting induced by forced fluid injection and fluid flow forced by faulting an interpretation of hydraulic fracture microseismicity carthage cotton valley gas field texas we analyzed precisely located microearthquake data detected during five hydraulic fracture treatments in the carthage gas field of east texas the treat ments were conducted in two adjacent boreholes within interbedded sands and shales of the upper cotton valley formation the microearthquakes were induced within narrow horizontal bands that correspond to the targeted sandstone layers events throughout all the treatments show strike slip faulting occurring uniformly along vertical fractures trending close to maximum horizontal stress direction these events are consistent with the reservoir s prevalent natural fractures known to be isolated within the sands and trending subparallel to the expected hydraulic fracture orien tation when this uniform fracture system was activated exclusively the detected shear deformation measured as the moment release per unit volume of fluid injected was constant independent of various fluid viscosities and flow rates used within the base of the upper cotton valley formation anomalous event counts and moment release occurred within dense clusters that delineate bends or jogs in the fracture zones the mechanisms are also strike slip but the fault planes are more favorably oriented for failure the dense clusters show location patterns diverging in time suggesting the expulsion of fluids from compressive fault jogs fluid flow forced by the adjacent slip induced loading appears to initially extend the treatments but the loading also tends to lock up and concentrate stress at the jogs as evident by fewer but larger events populating the structures as treatments progress as a result effec tive drainage lengths from the boreholes may be shorter than would be inferred from the seismicity extending past the jogs these high moment asperities are similar to dense patches of seismicity observed along creeping sections of the san andreas fault where they have been attributed to localized zones of strength or stress con centration surrounded by larger regions undergoing stable aseismic slip this sim ilarity plus large moment deficits in terms of volume injected suggests a large component of aseismic slip is induced by the cotton valley treatments

characterization of fault zones there are currently three major competing views on the essential geometrical mechanical and mathematical nature of faults the standard view is that faults are possibly segmented and heterogeneous euclidean zones in a continuum solid the continuum euclidean view is supported by seismic gravity and electromagnetic imaging studies by successful modeling of observed seismic radiation geodetic data and changes in seismicity patterns by detailed field studies of earthquake rupture zones and exhumed faults and by recent high resolution hypocenter distributions along several faults the second view focuses on granular aspects of fault structures and deformation fields the granular view is supported by observations of rock particles in fault zone gouge by studies of block rotations and the mosaic structure of the lithosphere which includes the overall geometry of plate tectonics by concentration of deformation signals along block boundaries by correlation of seismicity patterns on scales several times larger than those compatible with a continuum framework and by strongly heterogeneous wave propagation effects on the earth s surface the third view is that faults are fractal objects with rough surfaces and branching geometry the fractal view is supported by some statistical analysis of regional hypocenter locations by long range correlation of various measurements in geophysical boreholes by the fact that observed power law statistics of earthquakes are compatible with an underlying scale invariant geometrical structure by geometrical analysis of fault traces at the earth s surface and by measurements of joint and fault surfaces topography

linearized inversion for fault rupture behavior application to the 1984 morgan hill california earthquake we present a technique to infer the rupture history of an earthquake from near source records of ground motion unlike most previous studies each point on the fault is assumed to slip only once when the rupture front passes with a spatially variable slip intensity in this parameterization the data are linearly related to slip intensity but nonlinearly related to rupture time we perform a linearized inversion for slip intensity and rupture time by iteratively perturbing an assumed starting model the inverse problem for the model perturbation is solved using a tomographic back projection technique smoothing and inequality constraints are applied to ensure that the resulting solution is stable and feasible asymptotic ray theory is used to calculate theoretical seismograms and partial derivatives with respect to model parameters in test cases with noisy synthetic data sets we found that it is possible to distinguish between rupture models having variable slip amplitude and models having variable rupture velocity provided the station coverage is adequate we apply the technique to recordings of the april 24 1984 morgan hill earthquake the results indicate that slip amplitude on the fault plane was extremely variable and that the rupture front did not propagate uniformly away from the hypocenter in particular rupture was delayed on a 12 km2 section of the fault approximately 14 km to the southeast of the hypocenter the rupture front surrounded the region which subsequently failed with a component of rupture propagation back toward the hypocenter similar behavior has been observed in dynamic rupture models with stress or strength inhomogeneities this segment of the fault ruptured with a large slip amplitude releasing 12 of the total seismic moment from 4 of the total area of the aftershock zone the surface trace of this section of the fault is characterized by a complex left step that could act to increase the normal stress acting across the fault however the distribution of aftershocks suggests that the fault at depth is simpler and that it may bend to the right in either case our rupture model suggests that this segment of the fault represents an asperity which initially resisted rupture but eventually ruptured massively we estimate the shear fracture energy for this earthquake to be 2 106 j m2

rupture process of the ms 6 6 superstition hills california earthquake determined from strong motion recordings application of tomographic source inversion abstract we analyze strong motion recordings of the m s 6 6 superstition hills earthquake to determine the timing location spatial extent and rupture velocity of the subevents that produced the bulk of the high frequency 0 5 to 4 hz seismic energy radiated by this shock the earthquake can be characterized by three principal subevents the largest ones occurring about 3 and 10 sec after initiation of rupture timing relationships between pulses on the seismograms indicate that the three subevents are located within 8 km of each other along the northern portion of the superstition hills fault the two largest subevents display different directivity effects we apply a tomographic source inversion to the integrated accelerograms to determine the slip acceleration on the fault as a function of time and distance based on a one dimensional fault model the azimuthal distribution of amplitudes for the second subevent can be largely explained by a rupture that propagated about 2 km to the southeast along the superstition hills fault at a velocity about equal to the p wave velocity an alternative model with rupture propagating to the northeast along a conjugate fault plane can also account for the observed directivity of this subevent but it is not supported by the aftershock distribution the third subevent ruptured to the southeast along an 8 km long portion of the superstition hills fault at about the shear wave velocity this rupture propagation caused the relatively large accelerations and velocities observed in strong motion records for stations southeast of the hypocenter the long time intervals between the subevents and their relative proximity to each other indicate a very slow component to the rupture development the southern half of the superstition hills fault did not generate significant high frequency strong ground motion although it showed substantial co seismic surface displacement the subevents are situated along the same northern portion of the fault where most of the aftershocks are located the locations of the subevents appear to be controlled by bends in the fault mapped at the surface and by changes in basement structure at depth

teleseismic and near field analysis of the nahanni earthquakes in the northwest territories canada the analysis of the nahanni earthquakes of october 5 1985 ms 6 6 and december 23 1985 ms 6 9 will have important implications for the assessment of seismic hazards in intraplate environments to maximize the information available to seismic engineers broadband data recorded teleseismically are analyzed jointly with strong motion data recorded in the near field the time domain analysis of teleseismic data yields the source mechanisms depths and complexities of rupture of each earthquake both earthquakes occurred as shallow thrusts with centroid depths 6 to 7 km and shallowly dipping fault planes that correspond well with the aftershock distributions obtained from a local survey run by the canadian geological survey the shallow nodal plane for the october 5 earthquake dips 30 to the wsw while the shallow nodal planes of the subevents for the december 23 earthquake dip an average of 23 to the wsw the october 5 earthquake has an impulsive initial rupture followed by a weak subevent of longer duration but smaller moment release the december 23 earthquake exhibits more complexity being comprised of three subevents of similar size the subevent delays derived from the teleseismic analysis are used to help interpret arrivals in records of ground velocity recorded in the near field of the december 23 earthquake the rupture geometries inferred from the joint near and far field analysis suggest that the rupture processes were unusually complicated and that the 2 g peak that occurs late in one of the near field records could be a localized phenomenon spectral analyses of the teleseismic p waves yield the following source parameters for the october 5 and december 23 earthquakes respectively the seismic moments are 1 2 and 1 8 1026 dyne cm the radiated energies are 1 8 and 2 8 1021 dyne cm and the dynamic stress drops are 65 and 50 bar the acceleration source spectra of both earthquakes exhibit an intermediate slope u from 0 03 to 0 3 hz suggesting that the earthquakes represent the failure of asperities extrapolating the teleseismic p wave spectra to estimate the near field s wave spectra yields good fits to the acceleration spectra from two strong motion records but underestimates the spectra from a third strong motion record with the strongest but possibly localized accelerations

structural analysis of the brigham city weber segment boundary zone wasatch normal fault utah implications for fault growth and structure we investigate the history kinematics principal stress orientations and geometry of deformation at the end of a bent normal fault segment of the wasatch fault zone utah three fault types developed in archean crystalline rocks reflect progressive uplift of fault related footwall rocks chlorite breccias and phyllonites reflect deep level reaction assisted plastic deformation along the north striking part of the segment planar fretted faults which formed by cataclasis cut the phyllonites and breccias and are developed throughout the footwall of the segment youngest faults are hematitecoated extremely narrow polished surfaces slip vectors and kinematic analyses of small faults developed in the footwall indicate oblique normal slip along the north striking portion of the segment slip vectors and fault orientation along the northwest striking portion of the segment reflect complexly oriented slip on faults which strike subparallel and at high angles to the main fault trace yet slip is confined to a broad fault parallel zone small faults at the southernmost tip of the segment indicate a strong influence of the north striking weber segment to the south inversion of fault data for principal stress orientations document complexly oriented principal stresses through the segment boundary zone and suggest that 3 may have reoriented approximately 60 over the life of the segment subsurface structure combined with small fault data indicate the segment boundary is comprised of a southwest plunging bedrock high which is reflected by a sharp bend in the brigham city segment the southern end of the brigham city segment may have started as a straight north striking fault which has bent due to changes in stress orientations and or interaction with the adjacent weber segment

influence of fault bends on ruptures data on slip along the san andreas fault have been examined to understand the influence of a bend in the fault system on the extent of slip release a correlation of slip along the fault in the great 1906 and 1857 earthquakes with fault geometry shows that maximum slip occurred near a major change in the fault strike and may be a function of this change the data appear to suggest that in general it takes about 10 years of strain accumulation to negotiate a 1 change in fault strike assuming a slip rate of about 3 cm yr seismic hazard along the san andreas fault may therefore also be a function of fault geometry

fractal geometry in the san andreas fault system it has been noted that the spatial distribution of earthquakes and the mode of strain release in the san andreas fault system is related to the complexity of fault geometry because of their rough appearance over many length scales faults can be regarded as fractal surfaces direct estimates of fractal dimension d of portions of the san andreas fault system between the northern gabilan range and the salton sea including the postulated extent of the great 1857 fort tejon earthquake are obtained from measured fault lengths analogous to the lengths of coastlines as discussed by mandelbrot regions characterized by complicated fault geometry are associated with larger values of d based on fault traces mapped at a scale of 1 750 000 d is 1 3 for this reach of the fault defined as a 30 km wide band about a main fault trace for that part near parkfield which could be associated with the nucleation of the 1857 earthquake d is 1 1 at this same scale d is 1 4 for the san andreas and related faults near san bernardino where the 1857 rupture stopped compared to 1 2 for the san andreas san juan fault segments near the point of arrest of the 1966 parkfield earthquake at finer map scales 1 24 000 and 1 62 500 critical lengths of 500 m and 1 km are identified which might relate to the extent of off san andreas fault offsets the critical lengths also suggest that fault geometry is not self similar if this fractal geometry persists through the seismic cycle it may be possible to use a quantitative measure of complexity to explain the occurrence of great and characteristic earthquakes along a given reach of fault

the tangshan earthquake sequence and its implications for the evolution of the north china basin the 1976 tangshan earthquake sequence that occurred on the northern margin of the north china sedimentary basin is one of the strongest and most destructive intracontinental earthquake sequences ever recorded by worldwide long period seismograph networks we have studied the source process of the six largest events of this earthquake sequence by formally inverting long period p and sh wave seismograms from the world wide standardized seismographic network our analysis shows that the main shock was caused by motion on at least three fault segments each with a different orientation the initial and dominant faulting occurred on two segments of a north northeast trending right lateral strike slip fault system the last stage of faulting had a significant component of thrusting and occurred on an east northeast trending subsidiary fault at the southern end of the main fault the largest aftershock occurred at the northeastern end of the main fault and had a nearly pure normal faulting mechanism with east west striking nodal planes this event occurred in a pull apart region where the north northeast trending main strike slip fault system steps to the right this region subsided by more than 1 m during the earthquake sequence to the north of this step the main fault is delineated by numerous aftershocks whose mechanisms are by and large consistent with right lateral slip on this segment the large event in 1945 m 6 3 probably occurred on this part of the fault a region of substantial subsidence up to 1 5 m associated with the earthquake sequence was also observed in the south where the main fault system takes another step to the right two of the large aftershocks occurred nearby and had strike slip mechanisms the overall faulting process of the tangshan sequence is well characterized by right lateral slip on a set of right stepping faults the mechanisms and the points of nucleation of the strongest shocks and the extent of the ruptures appear to be closely tied to preexisting fault geometries a compilation of the mechanisms of large earthquake sequences in the north china basin strongly suggests that the present deformation of the entire basin is dominated by motions on right lateral strike slip systems trending north northeast the basin seems to have formed and evolved as a large transtensional structure at an en echelon step of a large scale strike slip system the deformation within the basin is both strike slip and extensional and appears to occur on smaller scales in an approximately selfsimilar fashion as evidenced by the deformation during the tangshan sequence this interpretation provides an explanation for the variable rate of subsidence in both space and time and the scatter in measured heat flow values as well as the overall shape of the north china basin it also provides an explanation for occasional thrust faulting earthquakes that occur apparently as a result of geometrical complexities of the strike slip faults transpression these observations are otherwise difficult to fit into a simple model of regional crustal extension and thermal subsidence

mechanics of fault junctions a sharp bend in a fault must be part of a triple junction and slip on the three fault segments at the junction must have a vector sum of zero in order to avoid an unphysical 1 r stress singularity accompanying the slip a volume change occurs at the junction either a void opens or intense local deformation is required to avoid material overlap the energy absorbed due to the volume change is proportional to the slip increment times the total past slip accumulated at the junction at a new junction the energy absorbed is a small fraction of the energy released by slip on the fault system but after a number of earthquakes proportional to shear modulus over confining stress the junction becomes a strong barrier to further slip although slip occurs more easily on old rupture surfaces than on fresh fractures the growing barrier strength of junctions requires that there be some fresh fracture in earthquakes perhaps a small fraction of the surface that slips in any earthquake is fresh fracture which could provide the instability needed to explain earthquakes a numerical model in two dimensional static plane strain shows even without accounting for energy absorbed at the junction that a bend in a fault acts as a barrier if slip is impeded on the associated fault spur the stress concentration at the bend will tend to induce slip on the spur slip occurring on the spur unstably with a drop in the coefficient of friction can induce increased slip on the main fault segments with no change in the coefficient of friction a fault junction provides a natural realization of barrier and asperity models without appealing to arbitrary variations of fault strength the location of the fresh fracture that occurs after a junction becomes a strong barrier remains an unanswered question it is likely to be initiated near the stress concentration at the old junction a model simulating the effect of fresh fracture near old junctions might explain earthquakes without appealing to an unstable friction law anywhere

the accommodation of large strains in the upper lithosphere of the earth and other solids by self similar fault systems the geometrical origin of b value plate tectonics has provided a method of visualizing the geometry of the deformation of the earth s lithosphere on a large scale the description is so concise that for many purposes it provides an explanation of geological processes that overshadows the need to understand the driving processes the mechanics of the zones between the plates are less well understood particularly in continental regions where large areas are subject to deformation both continuous and discontinuous models have been tried but both have obvious drawbacks

fault geometry and earthquakes in continental interiors abstract although large earthquakes in continental interiors are much less frequent than those along plate boundaries they have been responsible for a disproportionate amount of destruction i present a testable model for strain accumulation and seismicity in the upper crust of continental interiors intersecting faults provide a focus for strain build up resulting in local pockets of high strain accumulation rigid plates typically deform at strain rates 10 10 to 10 9 strain yr 1 but at intersecting fault zones strain accumulates at anomalously high rates of 10 8 to 10 7 strain yr 1 i e the strain rates are comparable with those at plate boundaries the resulting large earthquakes are associated with relatively short return times hundreds of years typically one of the intersecting faults appears to split and to offset the other main fault defined as a fault on which the larger offset occurs forming two parallel strands separated by a jog i also observe a rotation of one leg of the main fault with respect to the other the region of intersection is usually associated with intense fracturing increased microseismicity and uplift

a faulting model for the 1979 imperial valley earthquake by comparing synthetic particle velocities with the near source strong motion data we have constructed by trial and error a faulting model for the 1979 imperial valley earthquake the calculation of the synthetic seismograms takes into account the vertical inhomogeneity of the elastic parameters in the imperial valley and the spatial variation of the slip rate parameters on the fault plane the independent slip rate parameters are 1 the strike slip rate amplitude 2 the dip slip rate amplitude 3 the duration that slip rate is nonzero the rise time of the slip function and 4 the rupture time which determines when the slip rate is initiated our faulting model has the following principal features 1 faulting occurred on the imperial fault and on the brawley fault rupture on the brawley fault being triggered by rupture on the imperial fault 2 the imperial fault is a plane 35 km long and 13 km wide with a strike of 323 measured clockwise from north and a dip of 80 ne the brawley fault is a 10 km long and 8 km wide plane with a strike of 360 and a dip of 90 3 faulting on the imperial fault is primarily right lateral strike slip with a small component of normal dip slip in the sediments at its northern end the larger strike slip rates are generally confined between depths of 5 and 13 km with maximum values of about 1 0 m s the duration varies on the fault with a maximum of 1 9 s which is considerably shorter than the total time for the rupture to take place 4 the rupture velocity on the imperial fault is highly variable locally it exceeds the shear wave velocity and in one instance the compressional wave velocity the average rupture velocity though is less than the shear wave velocity 5 although the slip on the brawley fault contributes only about 4 of the total moment it greatly affects the ground motion at nearby stations 6 the total seismic moment is 6 7 1018 n m where the imperial fault contributes 6 4 1018 n m and the brawley fault contributes 2 7 1017 n m in the process of trying almost 300 faulting models we found that given the elastic parameters of the medium the synthetic seismograms were most sensitive to the specification of the rupture time although the slip rate amplitudes are linearly related to the data the rupture time and the duration are not the parameterization of the nonlinear variables has a strong effect on the generation of synthetic seismograms from a finite fault

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

strike slip fault geometry in turkey and its influence on earthquake activity the geometry of turkish strike slip faults has been reviewed and described from this data set it appears that fault geometry the distribution of discontinuities such as bends and stepovers along the main fault trace plays an important role in controlling the location of large earthquake rupture segments along the fault zones large earthquake ruptures generally do not propagate past individual stepovers that are wider than 5 km or bends that have angles greater than about 30 more important than the size of each discontinuity however is the total geometric pattern i e the distribution of adjacent bends and stepovers based not only on distance from one another but also on relative discontinuity size certain geometric patterns restraining single or double bends are particularly common and can be viewed as responsible for strain accumulation along portions of the fault zone fault geometry not only plays a role in the extent of earthquake rupture but also in characteristics of earthquake behavior for example large earthquake epicenters often occur near restraining bends or double bends furthermore aftershocks and swarm activity can often be associated with releasing areas areas along the strike slip fault that are undergoing a relatively large amount of extension

rupture history of the 1984 morgan hill california earthquake from the inversion of strong motion records near source strong motion velocity records and teleseismic short period p waveforms are modeled to obtain the spatial and temporal distribution of slip for the 1984 morgan hill earthquake both forward modeling and constrained least squares inversion techniques are used to interpret the strong motion velocity waveforms in the frequency range of approximately 0 2 to 2 0 hz these data support a nearly unilateral rupture to the southeast with a rupture propagation velocity of nine tenths of the local s wave velocity the majority of the slip occurs over a fault length of 25 km and to a first approximation can be interpreted as two main source regions each with an extent of about 5 km with their centers separated by about 12 km however each of the sources has detailed structure of its own and a simple two point source model is not an accurate representation of the morgan hill earthquake the second source occurs about 4 5 sec after the first and is approximately 3 times larger the maximum dislocation on the fault plane is about 1 m the total moment of the earthquake is estimated to be 2 1 10 25 dyne cm the morgan hill earthquake offers convincing evidence for very inhomogeneous slip and stress distributions on shallow strike slip faults

seismicity and fault kinematics through the eastern transverse ranges california block rotation strike slip faulting and low angle thrusts hypocentral locations and first motion data from the southern california seismic network are used to infer the present pattern of deformation along the southern san andreas fault the study area lies between the san jacinto fault and desert hot springs and includes the san bernardino mountains and san gorgonio pass this region has some of the deepest earthquakes observed anywhere along the entire san andreas system exhibits a complex surface geology characterized by both right and left lateral faults has high topographic relief as a result of recent uplift and is a potential site for the nucleation of a great earthquake although this area is unusually seismogenic little activity can be directly associated with major throughgoing faults seismicity is also generally absent in the upper 5 km the predominant style of faulting above 10 12 km is oblique slip with a large reverse component the spatial distribution of relocated hypocenters and first motion data suggests the presence of a system of left slip faults striking northeast this pattern of faulting in conjunction with an unusual set of normal and reverse focal mechanisms is interpreted as the clockwise rotation of a small set of crustal blocks subject to regional right lateral shear at depths greater than about 10 km seismicity defines a wedge shaped volume undergoing pervasive internal deformation on a combination of strike slip and low angle thrust faults velocity structures determined from earthquake arrival times suggest a low velocity zone at about 10 km below the san bernardino mountains but not below the san jacinto mountains this is nearly the same depth as the transition between the two layers of different kinematic behavior seen south of the mill creek mission creek branch of the san andreas fault and the maximum depth of seismicity seen north of that fault the low velocity zone and the transition between block rotations and the deeper deformation may thus correspond to a detachment under much of this region and would imply that the overthrust san bernardino mountains are allochthonous the present pattern of seismic deformation in shocks of small to moderate size may only characterize the interval between large earthquakes and may change systematically as the region prepares to accommodate large right lateral displacements

fractal analysis applied to characteristic segments of the san andreas fault fractal theory is applied in a quantitative analysis of the san andreas fault saf geometry the method which directly measures the increase in total fault length with a decrease in ruler size gives the fractal dimension d and scaling properties for the chosen length band 0 5 1000 km a physical interpretation of d is that it measures the irregularity of the fault trace in the selected band the fault is subdivided into six segments of distinctive seismic behavior a main fault trace which shows either maximum coseismic slip or creeping was selected for analysis with three alternative branches examined for the saf system south of san bernardino branches of the fault trace are not considered in this work fractal dimensions calculated for the different segments range from 1 0008 to 1 0191 and are different from 1 0 to the 95 confidence interval these small changes reflect the overall smoothness of the main fault trace significant variations in d among segments indicate heterogeneities in the fault smoothness along strike d also changes significantly from the short length band to the long length band where the demarcation point ranges from 1 to 2 km the short length band has larger d values a slight correlation is obtained between the fractal dimension of the main trace and the extent of subparallel faulting this indicates some correspondence between the main fault trace irregularity and the complexity of subsidiary fault traces in plan view

effects of fault interaction on moment stress drop and strain energy release solutions for collinear shear cracks are used to examine quantitatively the effects of fault slip zone interaction on determinations of moment stress drop and static energy release two models the barrier model and the asperity model are considered in the asperity model the actual distribution of strengths on a fault plane is idealized as a combination of two limiting cases areas which slip freely at a uniform value of a residual friction stress and unbroken ligaments or asperities across which slip occurs only at the time of a seismic event in the barrier model slip zones separated by unbroken ligaments barriers are introduced into a uniformly stressed medium to approximate the nonuniform fault propagation proposed by das and aki the strain energy change due to introducing collinear slip zones or due to breaking the asperities between them is shown to be given by the usual formula for an isolated slip zone with the stress drop replaced by the effective stress significant interaction between slip zones occurs only if the length of the asperity is less than half the length of the slip zones for the case of two collinear slip zones fracture of the asperity between them is shown to cause a large moment primarily because of the additional displacement which is induced on the adjacent slip zones for example if the asperity length is 0 05l where l is the length of each adjacent slip zone then fracture of the asperity causes a moment almost 1 8 times the moment caused by introducing a slip zone of length l for two collinear slip zones the local stress drop due to fracture of the separating asperity is shown to become unbounded as the asperity length goes to zero but in the same limit the stress drop averaged over the entire fault length is approximately equal to the apparent stress drop inferred for an isolated fault of the same moment and total fault length this apparent stress drop is approximately equal within a factor of 2 or 3 to the effective stress and hence can be used in the usual formula to give a good estimate of the strain energy change for the barrier model numerical results are given for the ratio of the stress drop calculated on the assumption of an isolated slip zone to the true stress drop for example in the case of two collinear slip zones of length l separated by a barrier of length 0 2l this ratio is 0 5 whereas for a barrier length equal to that of the adjacent slip zones the ratio is 0 24 stress drop estimates become worse with increasing number of fault segments