interplay between faulting and base level in the development of himalayan frontal fold topography 1 fold topography preserves a potentially accessible record of the structure and evolution of an underlying thrust fault system provided we understand the factors that shape that topography here we examine the morphology and fault geometry of two active folds at the northwest himalayan front the chandigarh and mohand anticlines show the following patterns 1 most 60 70 growth in catchment size and relief across multiple scales is accomplished within 5 km of the fault tips 2 range scale relief is divided unevenly between the fold flanks because of base level contrasts 3 mean gradients of the uplifting catchments correspond to different flank averaged rock uplift rates 4 high hillslope scale relief coincides with areas of fast rock uplift and stronger lithologies and 5 existing relief represents only 15 of the total rock eroded since faulting began implying significant erosion the first order fold topography is developed quickly and asymmetrically as a result of fault generated rock uplift which sets the space available for the fold and the distribution of rock uplift rates with some modulation by base level which affects the erosional response of the landscape to the uplift a linear rate of growth in catchment relief with range half width correlates with catchment averaged rock uplift rate suggesting that this metric may be used to infer variations in fault dip at depth in these frontal fold settings high slip rates weak uplifting rocks and rapid erosion may combine to quickly limit the topographic growth of emerging folds and disconnect their morphology from the displacement field

edge driven mechanical microplate models of strike slip faulting in the tibetan plateau 1 the india asia collision zone accommodates the relative motion between india and eurasia through both shortening and pervasive strike slip faulting to gain a mechanical understanding of how fault slip rates are driven across the tibetan plateau we develop a two dimensional linear elastic two stage deformable microplate model for the upper crust based on the behavior of an idealized earthquake cycle we use this approach to develop a suite of simple india asia collision zone models differing only in boundary conditions to determine which combination of edge forces and displacements are consistent with both the slip rate measurements along major tibetan faults as well as the geodetically observed extrusion of crustal material toward southeast asia model predictions for the altyn tagh 1 14 mm yr kunlun 3 10 mm yr karakorum 5 12 mm yr and haiyuan 3 5 mm yr faults are in agreement with geologically and geodetically inferred slip rates further models that accurately reproduce observed slip rate gradients along the altyn tagh and kunlun faults feature two critical boundary conditions 1 oblique compressive displacement along the himalayan range front west of the shillong plateau and 2 forcing in southeast asia additionally the ratio of internal block potency rate to the total potency rate for each microplate ranges from 28 to 79 suggesting a hybrid view of deformation in tibet as simultaneously localized on major faults and distributed at length scales 500 km

prospects for larger or more frequent earthquakes in the los angeles metropolitan region far too few moderate earthquakes have occurred within the los angeles california metropolitan region during the 200 year long historic period to account for observed strain accumulation indicating that the historic era represents either a lull between clusters of moderate earthquakes or part of a centuries long interseismic period between much larger moment magnitude m w 7 2 to 7 6 events geologic slip rates and relations between moment magnitude average coseismic slip and rupture area show that either of these hypotheses is possible but that the latter is the more plausible of the two the average time between m w 7 2 to 7 6 earthquakes from a combination of six fault systems within the metropolitan area was estimated to be about 140 years

structural evolution of the northern los angeles basin california based on growth strata subsurface structure contour maps and cross sections show that the northern los angeles basin is underlain by a south facing monocline complicated by secondary faults and folds the monocline and associated structures form a structural high the northern los angeles shelf which marks the northern margin of the los angeles central trough analysis of growth strata sediments deposited during structural growth shows that during the miocene the predominant structural style was extension growth strata show that at approximately the beginning of the pliocene 5 ma contraction reactivated the miocene normal faults as reverse faults and formed the monocline its secondary structures and the los angeles central trough lensing out of earliest pliocene delmontian stage growth sandstones onto the flanks of the monocline and secondary structures the south salt lake the east beverly hills and the las cienegas anticlines indicates that these contractional structures all began to form at this time all of the secondary structures became largely inactive prior to the deposition of upper pico member of early pleistocene age 1 2 ma however thick accumulations of growth strata of the upper pico member of the fernando formation within the los angeles central trough attest to continued monoclinal folding after the secondary structures became largely inactive the growth strata record both vertical and horizontal components of structural growth of the monocline so that the dip of the blind fault zone causing the monocline the los angeles fault can be calculated in the east beverly hills area the dip of the los angeles fault is 61 and at las cienegas the dip is 62 these are maximum values based on the assumption that the growth strata bed lengths record all shortening the pliocene pleistocene average fault slip rates for the los angeles fault are 1 1 1 3 mm yr in the east beverly hills and 1 3 1 5 mm yr at las cienegas the resulting pliocene pleistocene horizontal convergence rates are 0 5 0 6 mm yr and 0 6 0 7 mm yr respectively the pliocene pleistocene growth strata show increased dip with age indicating that the monocline grew by progressive limb rotation rather than by kink band migration therefore fault bend and fault propagation fold models based on kink band migration are inadmissible solutions to explain the growth of the monocline we suggest a basement involved shear zone to explain the geometry of the northern los angeles shelf

mechanical validation of the three dimensional intersection geometry between the puente hills blind thrust system and the whittier fault los angeles california the sensitivity of fault interaction to alternative kinematically plausible intersection geometries of the puente hills blind thrust system and the whittier fault is modeled under geodetically constrained horizontal contraction comparisons of modeled slip rates to available geologic rates 1 suggest that the coyote hills seg ment of the puente hills system extends to the base of the seismogenic crust and 2 give slight preference for extension of the active whittier fault to the base of the seismogenic crust rather than limiting active slip to the hanging wall of the coyote hills fault furthermore analysis of strain energy density demonstrates that the pre ferred model has the greatest mechanical efficiency this correlation of fit to geologic slip rates and mechanical efficiency supports the effectiveness of this method for evaluating among alternative geometries in the absence of geologically constrained slip rates the methodology implemented in this study may be effectively used in future studies of deformation within fault systems model generated slip rates along the puente hills faults show significant strike slip implying that seismic hazard of these faults may be underestimated by consid ering reverse slip rates alone contraction at 036 may not be appropriate in the puente hills region because of resulting sinistral slip on the whittier fault which disagrees with paleoseisimc observations however whittier strike slip rates fit pa leoseismic rates under 006 5 contraction because strike slip rates are more sensitive to contraction direction than fault intersection geometry contraction direction should be further constrained in order to accurately assess seismic hazard on these faults

the geometry of echelon fractures in rock implications from laboratory and numerical experiments abstract the traces of echelon joints veins and dikes in rock range from curving to straight theoretical analyses using boundary element numerical methods have concluded that straight open fractures imply a significant remote differential stress whereas curving traces imply a more nearly isotropic stress we present a series of laboratory experiments which investigate the two dimensional propagation paths of echelon fractures in pmma plates as a function of the applied biaxial loading and the initial geometry of a simple fracture array the experimental results support the theoretical conclusions and verify the accuracy of the numerical method at the grain scale rock does not rigorously conform to the assumptions of isotropy homogeneity and linear elasticity demanded by the numerical method nevertheless results from crack path stability theory suggest that small scale deviations of a fracture from the ideal path generally do not affect its large scale behavior fracture paths which become kinked or curved by local heterogeneities are found to self correct to a path governed by competition between the fracture tip and remote stress fields provided the remote differential stress is non zero this enhances our confidence that the numerical method can be used to produce and explain realistic fracture geometries in rock an unexpected experimental result is the non perpendicular intersection of fractures grown in the laboratory free surface boundary conditions require that the walls of an open fracture be free of shear stress the principal stresses are therefore aligned with the crack wall and we might consequently expect an approaching opening mode fracture either to intersect the free surface at a right angle or to turn away and follow an asymptotic nonintersecting path experimental and numerical modeling however shows how the near tip stresses generated by an approaching fracture dominate the local stress field and allow it to propagate along an oblique path very close to intersection with the adjacent free surface

work optimization predicts accretionary faulting an integration of physical and numerical experiments we employ work optimization to predict the geometry of frontal thrusts at two stages of an evolving physical accretion experiment faults that produce the largest gains in efficiency or change in external work per new fault area wext a are considered most likely to develop the predicted thrust geometry matches within 1 mm of the observed position and within a few degrees of the observed fault dip for both the first forethrust and backthrust when the observed forethrust is active the positions of the second backthrust and forethrust that produce 90 of the maximum wext a also overlap the observed thrusts the work optimal fault dips are within a few degrees of the fault dips that maximize the average coulomb stress slip gradients along the detachment produce local elevated shear stresses and high strain energy density regions that promote thrust initiation near the detachment the mechanical efficiency wext of the system decreases at each of the two simulated stages of faulting and resembles the evolution of experimental force the higher wext a due to the development of the first pair relative to the second pair indicates that the development of new thrusts may lead to diminishing efficiency gains as the wedge evolves the numerical estimates of work consumed by fault propagation overlap the range calculated from experimental force data and crustal faults the integration of numerical and physical experiments provides a powerful approach that demonstrates the utility of work optimization to predict the development of faults

coupled wind forced controls of the bering chukchi shelf circulation and the bering strait throughflow ekman transport continental shelf waves and variations of the pacific arctic sea surface height gradient abstract we develop a conceptual model of the closely co dependent bering shelf bering strait and chukchi shelf circulation fields by evaluating the effects of wind stress over the north pacific and western arctic using atmospheric reanalyses current meter observations satellite based sea surface height ssh measurements hydrographic profiles and numerical model integrations this conceptual model suggests bering strait transport anomalies are primarily set by the longitudinal location of the aleutian low which drives oppositely signed anomalies at synoptic and annual time scales synoptic time scale variations in shelf currents result from local wind forcing and remotely generated continental shelf waves whereas annual variations are driven by basin scale adjustments to wind stress that alter the magnitude of the along strait meridional pressure gradient in particular we show that storms centered over the bering sea excite continental shelf waves on the eastern bering shelf that carry northward velocity anomalies northward through bering strait and along the chukchi coast the integrated effect of these storms tends to decrease the northward bering strait transport at annual to decadal time scales by imposing cyclonic wind stress curl over the aleutian basin and the western subarctic gyre ekman suction then increases the water column density through isopycnal uplift thereby decreasing the dynamic height sea surface height and along strait pressure gradient storms displaced eastward over the gulf of alaska generate an opposite set of bering shelf and aleutian basin responses while ekman pumping controls canada basin dynamic heights proshutinsky et al 2002 we do not find evidence for a strong relation between beaufort gyre sea surface height variations and the annually averaged bering strait throughflow over the western chukchi and east siberian seas easterly winds promote coastal divergence which also increases the along strait pressure head as well as generates shelf waves that impinge upon bering strait from the northwest

three stage evolution of the los angeles basin southern california we propose that an episode of transtension dominated development of the los angeles basin area from 12 to 6 ma following middle miocene transrotation and prior to the modern transpressional regime transtension resulted from the releasing bend of the san gabriel chino hills cristianitos fault which acted as the primary transform boundary in southern california during this episode such an interpretation implies that significant transform motion did not occur on the southern san andreas fault prior to 6 ma and that the gulf of california has opened primarily since 6 ma we propose a three stage model for evolution of the los angeles basin and vicinity within the evolving transform fault system transrotation 18 12 ma transtension 12 6 ma and transpression 6 0 ma timing of these stages correlates with microplate capture events which occurred during conversion from a convergent margin to a transform margin

progressive localisation of strain during the evolution of a normal fault population abstract the evolution of a syn sedimentary normal fault population has been established from the analysis of a high quality 3d seismic reflection survey from the timor sea offshore nw australia growth of the fault population which initiated ca 6 ma ago occurred in three distinct but temporally overlapping stages the initial stage of fault population evolution was characterised by rapid growth of fault length in the first 1 2 ma of extension displacement rates of individual faults were established during this stage and were constant throughout subsequent fault growth the second stage involved amplification of fault displacements on existing fault traces with minimal fault propagation during the third stage shortening of the traces of active faults and high mortality rates for small faults resulted in a net decrease in fault trace length on successively younger horizons this stage was accompanied by a declining extension rate fault throw populations have constant slopes on each syn faulting horizon however an up sequence shallowing of the slope of length and geometric moment populations demonstrates a progressive concentration of strain onto fewer and larger faults through time if extension were to continue beyond the present value of 4 5 this strain localisation might culminate in the formation of a single active through going fault

an elusive blind thrust fault beneath metropolitan los angeles seismic reflection profiles petroleum wells and relocated earthquakes reveal the presence of an active blind thrust fault beneath metropolitan los angeles a segment of this fault likely caused the 1987 whittier narrows magnitude 6 0 earthquake mapped sizes of other fault segments suggest that the system is capable of much larger magnitude 6 5 to 7 and more destructive earthquakes

episodic growth of normal faults as recorded by syntectonic sediments july oil field suez rift egypt the july oil field is a major normal fault bounded structural block in the suez rift basin egypt it is adjacent to a major structural transfer zone which has controlled sediment influx to the rift basin center for the past 20 m y the lower to middle miocene upper rudeis formation part of the synrift stratigraphic sequence records deformation of the july structural block the formation contains abrupt lateral changes in thickness and facies which record earlier phases of fault movement and deformation in the july field area that do not conform to the present day structural configuration the upper rudeis formation was deposited as turbidites in a submarine fan system sourced from the western rift shoulder it was deposited over and around bathymetric highs created by coeval fault displacement in the july field area by studying thickness and facies patterns we have determined that the present day main bounding fault to the july block consisted of a series of unlinked fault segments which linked after upper rudeis deposition a subsidiary fault west of the block exerted the most control on thickness patterns not the present day main bounding fault thus commonly used models of deposition in active half grabens are difficult to apply at july field

a mechanism to explain rift basin subsidence and stratigraphic patterns through fault array evolution rift basin stratigraphy commonly records an early stage of slow subsidence followed by an abrupt increase in subsidence rate the physical basis for this transition is not well understood although an increase in extension rate is commonly implied here a numerical fault growth model is used to investigate the influence of segment linkage on fault displacement rate patterns along an evolving normal fault array the linkage process we describe is controlled by a stress feedback mechanism which leads to enhanced growth of optimally positioned faults model results indicate that even with constant extension rates slow displacement rates prevail during an initial phase of distributed extension followed by an increase in displacement rates as strain becomes localized on linked fault arrays this is due to the dynamics of fault interactions rather than mechanical weakening comparison of model simulations with rift basin subsidence and stratigraphic patterns in the gulf of suez and north sea suggests that the occurrence and timing of rapid basin deepening can be explained by the mechanics of fault zone evolution without invoking a change in regional extension rates

using vertical rock uplift patterns to constrain the three dimensional fault configuration in the los angeles basin comparison of geologic uplift patterns with results of three dimensional mechanical models provides constraints on the fault geometry compiled by the south ern california earthquake center community fault model in the northern los angeles basin california the modeled uplift matches well the geologic pattern of uplift as sociated with the santa fe springs and coyote hills segments of the puente hills thrust fault but does not match structures to the west of the san gabriel river to better match the geologic patterns in this area alternative fault configurations were tested the best match to geologic uplift is attained with a model incorporating 1 a steep blind thrust fault at the location of the los angeles segment of the puente hills thrust system following interpretations of the las cienegas fault geometry at this location 2 removal of an inferred linking fault between the raymond and holly wood faults and 3 lateral continuation of the lower elysian park fault a blind low angle detachment at 10 km depth along strike to the northwest these geometric revisions alter the connectivity of northern los angeles basin faults and significantly improve the match of model uplift pattern to geologic data model results suggest that fault connectivity may be more important in governing fault slip rate than are fault dip and fault area the preferred model alters slip rates by 0 2 mm yr for the upper elysian park hollywood lower elysian park raymond sierra madre west and verdugo faults additionally the preferred model alters the surface area of several faults in the northern los angeles basin such as the puente hills thrust and the lower elysian park fault which may have important implications for seismic hazard assess ment in the northern los angeles basin

is lithostatic loading important for the slip behavior and evolution of normal faults in the earth s crust normal faults growing in the earth s crust are subject to the effects of an increasing frictional resistance to slip caused by the increasing lithostatic load with depth we use three dimensional 3 d boundary element method numerical models to evaluate these effects on planar normal faults with variable elliptical tip line shapes in an elastic solid as a result of increasing friction with depth normal fault slip maxima for a single slip event are skewed away from the fault center toward the upper fault tip there is a correspondingly greater propagation tendency at the upper tip however the tall faults that would result from such a propagation tendency are generally not observed in nature we show how mechanical interaction between laterally stepping fault segments significantly competes with the lithostatic loading effect in the evolution of a normal fault system promoting lateral propagation and possibly segment linkage resultant composite faults are wider than they are tall resembling both 3 d seismic data interpretations and previously documented characteristics of normal fault systems however this effect may be greatly complemented by the influence of a heterogeneous stratigraphy which can control fault nucleation depth and inhibit fault propagation across the mechanical layering our models demonstrate that although lithostatic loading may be an important control on fault evolution in relatively homogeneous rocks the contribution of lithologic influences and mechanical interaction between closely spaced laterally stepping faults may predominate in determining the slip behavior and propagation tendency of normal faults in the earth s crust

mechanical fault interaction within the los angeles basin a two dimensional analysis using mechanical efficiency 1 mechanical models examine deformation within eight different structural cross sections proposed by davis et al 1989 and shaw and suppe 1996 along a northeast southwest transect across the los angeles basin california horizontal contraction of the models constrained by geodetic measurements yields varying dip slip rates along frictionally sliding faults within the different cross sections mechanical efficiency analysis using effective stiffness and strain energy density assesses the overall fault system deformation as well as the partitioning of work between fault slip and host rock strain the cross section interpreted by shaw and suppe 1996 has the best fit to paleoseismically determined slip rates and the greatest mechanical efficiency greatest proportion of work toward fault slip however this model produces excessive reverse slip along the newport inglewood fault a modified fault configuration with a wedge or blind puente hills thrust fault rather than a ramp detachment configuration better matches the paleoseismic data with slightly lower mechanical efficiency slip rates in the mechanical models based on interpretations of shaw and suppe 1996 have much closer match to the geologically determined rates than those estimated from kinematic models this difference is due to 1 differing time spans of slip rate estimates and 2 deformable rather than rigid host rock in the mechanical models the mechanical efficiency analysis provides quantitative indicators of overall fault system deformation including the cumulative effect of interaction between individual faults assessment of effective stiffness and strain energy density furthers our understanding of two dimensional fault interactions in the los angeles basin and offers great potential for future applications

application of three fault growth criteria to the puente hills thrust system los angeles california usa abstract three dimensional mechanical models are used to evaluate the performance of different fault growth criteria in predicting successive growth of three echelon thrust faults similar to the segments of the puente hills thrust system of the los angeles basin california four sequential boundary element method models explore the growth of successive echelon faults within the system by simulating snapshots of deformation at different stages of development these models use three criteria 1 energy release rate 2 strain energy density and 3 navier coulomb stress to characterize the lateral growth of the fault system we simulate the growth of an echelon thrust fault system to evaluate the suitability of each of these criteria for assessing fault growth each of these three factors predicts a portion of the incipient fault geometry i e location or orientation however each provides different information in each model energy release rate along the westernmost leading tip of the puente hills thrust drops with growth of the next neighboring fault this result supports the overall lateral development of successive echelon segments within each model regions of high strain energy density and navier coulomb stress envelope at least a portion of the next fault to develop although the strain energy density has stronger correlation than navier coulomb stress to the location of incipient faulting in each model one of the two predicted planes of maximum navier coulomb stress ahead of the leading fault tip matches the strike but not the dip of the incipient fault plane recreating part of the fault orientation the incipient fault dip is best predicted by the orientation of the strain energy density envelopes around the leading fault tip furthermore the energy release rate and pattern of strain energy density can be used to characterize potential soft linkage overlap or hard linkage connection of echelon faults within the system

puente hills blind thrust system los angeles california we describe the three dimensional geometry and quaternary slip history of the puente hills blind thrust system pht using seismic reflection profiles petroleum well data and precisely located seismicity the pht generated the 1987 whittier narrows moment magnitude m w 6 0 earthquake and extends for more than 40 km along strike beneath the northern los angeles basin the pht comprises three north dipping ramp segments that are overlain by contractional fault related folds based on an analysis of these folds we produce quaternary slip profiles along each ramp segment the fault geometry and slip patterns indicate that segments of the pht are related by soft linkage boundaries where the fault ramps are en echelon and displacements are gradually transferred from one segment to the next average quaternary slip rates on the ramp segments range from 0 44 to 1 7 mm yr with preferred rates between 0 62 and 1 28 mm yr using empirical relations among rupture area magnitude and coseismic displacement we estimate the magnitude and frequency of single m w 6 5 6 6 and multisegment m w 7 1 rupture scenarios for the pht manuscript received 16 november 2001

how much can off fault deformation contribute to the slip rate discrepancy within the eastern california shear zone kinematic assumptions of geodetic inversions for fault slip require that the slip sums to the plate boundary velocity this assumption neglects permanent off fault deformation which could account for discrepancies between geologic and geodetic estimates we use three dimensional mechanical models to assess if unaccounted permanent strain surrounding faults could contribute to slip rate discrepancies across disconnected faults within the mojave desert california usa portion of the eastern california shear zone ecsz we modified fault configurations derived from the southern california earthquake center community fault model to better represent the disconnected nature of active faults in the ecsz south of the garlock fault the models with revised fault geometry produce slip rates that better match geologic strike slip rates thus validating the revisions within these models off fault deformation accounts for 40 23 of the total strain across the ecsz this suggests that a significant portion of the discrepancy between the geologic and geodetically modeled slip rates in the ecsz could be due to the geodetic inversion model assumption of zero permanent off fault deformation

growth and linkage of a segmented normal fault zone the late jurassic murchison statfjord north fault northern north sea abstract the structure of the 25 km long northeastern portion of the murchison statfjord north fault zone and adjacent syn rift stratigraphy are integrated to reconstruct the temporal and spatial evolution during c 30 5 myr of late jurassic north sea rifting based on a structural analysis d d data alone approximately 14 precursor fault strands are identified incorporation of stratigraphic data shows that only six of these strands were important in controlling stratal architecture and distribution three main stages in the evolution of the fault zone are recorded in the syn rift stratigraphy and are biostratigraphically constrained these are 1 following initiation of rifting six isolated fault strands developed each 9 km long fault segments separated by a 2 km wide relay ramp that controlled the stratigraphy for at least the following c 10 5 myr and 3 the two fault segments hard linked forming a single continuous fault trace during the final c 7 myr of rifting the results of this study reveal the necessity to adopt an integrated structural and stratigraphic approach when reconstructing the evolution of normal fault zones the results may also help to further constrain models of fault evolution

space geodetic measurement of crustal deformation in central and southern california 1984 1992 we estimate the velocity field in central and southern california using global positioning system gps observations from 1986 to 1992 and very long baseline interferometry vlbi observations from 1984 to 1991 our core network includes 12 gps sites spaced approximately 50 km apart mostly in the western transverse ranges and the coastal borderlands the precision and accuracy of the relative horizontal velocities estimated for these core stations are adequately described by a 95 confidence ellipse with a semiminor axis of approximately 2 mm yr oriented roughly north south and a semimajor axis of approximately 3 mm yr oriented east west

predicting the propagation and interaction of frontal accretionary thrust faults with work optimization abstract this study uses work optimization to predict the spatial and temporal development of faults we focus on the growth of small fractures that develop into thrust faults at the toe of accretionary prisms because observations from physical laboratory accretion experiments provide rich data with which to validate the models and the processes of accretionary thrust fault initiation remain unclear in order to model these systems we apply new implementations to the fault growth code grow that improve its prediction of fault interaction using work optimization including 1 cpu parallelization 2 a new growth algorithm that propagates only the most efficient fault in each growth increment the single run mode and 3 a new growth algorithm that only considers fault propagation from fault tips that host high sums of modes i and ii stress intensity factors kg the limiting mode the new single and limiting mode produce the geometries that best match the observed geometries rather than the algorithm that allows all the faults to propagate simultaneously regardless of kg the multiple and non limiting mode the single limiting models predict that frontal accretionary thrusts initiate at the midpack or shallower depths consistent with findings of previous studies the thrusts propagate upward link with the surface and then propagate downward and link with the detachment the backthrust tends to propagate before the forethrust and then influences the forethrust propagation this temporal and spatial sequence of faulting arises from the lower compression higher shear strain higher coulomb stress and higher strain energy density that develop near the wedge surface and the inflection of the wedge slope the models reveal that the final slip distributions do not reliably indicate the initiation location of the faults in contrast to widespread assumptions

breaking of unsteady lee waves generated by diurnal tides 1 diapycnal mixing caused through breaking of large amplitude internal lee waves generated by sub inertial diurnal tides which are modulated with a 18 6 year period is hypothesized to be fundamental to both the intermediate layer ventilation and the bi decadal oscillation around the north pacific ocean the first observational evidence of such wave breaking is presented here the breaking wave observed had 200 m height and 1 km width and its associated diapycnal mixing was estimated to be 1 5 m2 s 1 with a temporal average 104 times larger than typical values in the open oceans our estimate suggests that a similar mixing process occurs globally particularly around the pacific and antarctic oceans

on the processes controlling shelf basin exchange and outer shelf dynamics in the bering sea abstract we use a 9 km pan arctic ice ocean model to better understand the circulation and exchanges in the bering sea particularly near the shelf break this region has historically been undersampled for physical chemical and biological properties very little is known about how water from the deep basin reaches the large shallow bering sea shelf to address this we examine here the relationship between the bering slope current and exchange across the shelf break and resulting mass and property fluxes onto the shelf this understanding is critical to gain insight into the effects that the bering sea has on the arctic ocean especially in regard to recent indications of a warming climate in this region the bering sea shelf break region is characterized by the northwestward flowing bering slope current previously it was thought that once this current neared the siberian coast a portion of it made a sharp turn northward and encircled the gulf of anadyr in an anticyclonic fashion our model results indicate a significantly different circulation scheme whereby water from the deep basin is periodically moved northward onto the shelf by mesoscale processes along the shelf break canyons along the shelf break appear to be more prone to eddy activity and therefore are associated with higher rates of on shelf transport the horizontal resolution configured in this model now allows for the representation of eddies with diameters greater than 36 km however we are unable to resolve the smaller eddies

integrating 3 d seismic data field analogs and mechanical models in the analysis of segmented normal faults in the wytch farm oil field southern england united kingdom we propose a methodology for the analysis of normal fault geometries in three dimensional 3 d seismic data sets to provide insights into the evolution of segmented normal fault systems and to improve recovery efforts in fault controlled oil fields limited seismic resolution can obscure subtle fault characteristics such as segmentation and gaps in fault continuity that are significant for oil migration and thus accurate reservoir characterization detailed seismic data analyses that incorporate principles of normal fault mechanics however can reveal evidence of fault segmentation we integrate seismic attribute analyses outcrop analog observations and numerical models of fault slip and displacement fields to augment the use of 3 d seismic data for fault interpretation we applied these techniques to the wytch farm oil field in southern england resulting in the recognition of significant lateral and to a lesser extent vertical segmentation of reservoir scale faults slip maxima on fault surfaces indicate two unambiguous segment nucleation depths controlled by the lithological heterogeneity of the faulted section faults initiated preferentially in brittle sandstone and limestone units subsequent growth and linkage of segments predominantly in the lateral direction resulted in composite fault surfaces that have long lateral dimensions and multiple slip maxima reservoir compartmentalization is greatest at the level of prevalent segment linkages which corresponds at wytch farm with the predominant hydrocarbon producing unit the sherwood sandstone at relatively shallower depths fault segments are younger and less evolved resulting in a greater degree of segmentation with intact relay zones

improved interpretation of wireline pressure data modern wireline pressure data can have resolution and reproducibility sufficient to detect small fluid density changes and pressure barriers yet these features are commonly overlooked on conventional pressure depth plots the large pressure variation caused by weight of subsurface fluids hides these subtle features excess pressure is the pressure left after subtracting the weight of a fluid from the total pressure this concept is applied to wireline pressure data to remove effects of weight and emphasize subtle pressure differences caused by density variations and pressure barriers fluid density changes of 0 02 g cm3 or less can be resolved and within well pressure barriers in the order of 5 kpa 0 7 psi can be detected using good quality data effects of reservoir capillary displacement pressure can be detected by offset of the free water level from the petroleum water contact this effect can be used to estimate reservoir wettability subsurface fluid density measurements can also be used to evaluate oil or gas quality on a bed by bed scale in traps having variable oil or gas composition to detect compartmentalization by small petroleum density differences to verify quality of samples for pvt pressure volume temperature analysis and estimate salinity or temperature of unsampled water zones data quality limits barrier and fluid contact resolution thus quality control is essential pressure measurement errors on the 3 kpa 0 5 psi scale can be detected from behavior of the buildup pressure tests having the potential for small amounts of supercharge are identified from the overbalance and formation mobility examples illustrate identification of free water levels and fluid contacts fluid identification supercharge identification and water zone compartmentalization

recognition of paleoearthquakes on the puente hills blind thrust fault california borehole data from young sediments folded above the puente hills blind thrust fault beneath los angeles reveal that the folding extends to the surface as a discrete zone 145 meters wide buried fold scarps within an upward narrowing zone of deformation which extends from the upward termination of the thrust ramp at 3 kilometers depth to the surface document the occurrence of at least four large moment magnitude 7 2 to 7 5 earthquakes on this fault during the past 11 000 years future events of this type pose a seismic hazard to metropolitan los angeles moreover the methods developed in this study can be used to refine seismic hazard assessments of blind thrusts in other metropolitan regions

effects of nonplanar fault topology and mechanical interaction on fault slip distributions in the ventura basin california to assess the control of fault geometry and mechanical interactions on fault slip distributions in a complex natural system we present results from three dimensional mechanical models incorporating both nonplanar and rectangular planar representations of active faults within the ventura basin region of southern california we find that the incorporation of geologically constrained nonplanar fault surfaces into numerical models of active deformation results in a better match to available geologic slip rate data than models utilizing rectangular planar fault surfaces the model results demonstrate that nonplanar fault geometry and mechanical interactions exert a strong control on resultant slip distributions additionally we find that slip rates at most locations along the surface trace of ventura faults are not likely to rep resent average values for the entire fault surface we propose that results from three dimensional mechanical models using realistic i e nonplanar fault geometry can be used to both predict slip rates at specific locations and determine whether existing site specific slip rate estimates are representative of average fault slip rates although geo metric irregularities along fault surfaces should resist slip planar faults can have lesser slip than nonplanar faults due to the differing mechanical interactions among nearby faults in the two representations this suggests that models using simplified or planar fault geometry are likely to inaccurately simulate regional deformation we assert that detailed knowledge of three dimensional fault shape as well as the geometry and con figuration of deep fault intersections is essential for accurate seismic hazard char acterization of regions of complex faulting such as the ventura basin of southern california

subthermocline eddies over the washington continental slope as observed by seagliders 2003 09 abstractin the california current system subthermocline lenslike anticyclonic eddies generated within the california undercurrent cu are one mechanism for lateral transport of the warm saline waters of the cu garfield et al established the name cuddies for eddies of this type and hypothesized that they account for a significant fraction of the offshore transport of cu water this study presents observations of subthermocline eddies collected from a time series of seaglider surveys in the northern california current system gliders made 46 crossings of subthermocline anticyclones and 17 crossings of subthermocline cyclones over 5 5 yr close inspection grouped these into 20 distinct anticyclones and 10 distinct cyclones water properties at the core of anticyclonic eddies were similar to those in the core of the cu over the continental slope these anticyclones are examples of cuddies anticyclonic cyclonic eddies had average radii of 20 4 20 6 km peak azimuthal current speeds of 0 25 0 23

simulating the recent evolution of the southern big bend of the san andreas fault southern california 1 the southern big bend of the san andreas fault has been interpreted to have successively abandoned two strands the mission creek and mill creek strands of the san andreas fault during the past 1 ma before taking up activity along its present day configuration this evolution is simulated within three dimensional models to explore the hypothesis that fault systems evolve to increase mechanical efficiency the three dimensional boundary element method models are validated by comparison of modeled fault slip rates and uplift rates with geologic data the model results match well the geologic strike slip rates and uplift patterns overestimation of reverse slip rates along the northern san bernardino mountains may reflect model assumptions of fault geometry the partitioning of slip among active faults changes between the three phases of southern san andreas fault evolution revealing 1 a trade off in strike slip rates between the san jacinto and san andreas faults 2 a trade off between strike slip rates on the san andreas fault and reverse slip along faults associated with uplifting the san bernardino mountains and 3 a trade off in strike slip rates along the san andreas and strike slip rates along the eastern california shear zone mechanical efficiency of the entire model increases from the mission creek to mill creek fault configuration and decreases from the mill creek configuration to the present day configuration the decrease in mechanical efficiency with the transition to the present day fault geometry may reflect gaps in our understanding of tectonic loading and or fault geometry

response of deformation patterns to reorganization of the southern san andreas fault system since ca 1 5 ma abstract between 1 5 and 1 1 ma the southern san andreas fault system underwent a major reorganization that included initiation of the san jacinto fault zone and termination of slip on the extensional west salton detachment fault the southern san andreas fault itself has also evolved since this time with several shifts in activity among fault strands within san gorgonio pass we use three dimensional mechanical boundary element method models to investigate the impact of these changes to the fault network on deformation patterns a series of snapshot models of the succession of active fault geometries explore the role of fault interaction and tectonic loading in abandonment of the west salton detachment fault initiation of the san jacinto fault zone and shifts in activity of the san andreas fault interpreted changes to uplift patterns are well matched by model results these results support the idea that initiation and growth of the san jacinto fault zone led to increased uplift rates in the san gabriel mountains and decreased uplift rates in the san bernardino mountains comparison of model results for vertical axis rotation to data from paleomagnetic studies reveals a good match to local rotation patterns in the mecca hills and borrego badlands we explore the mechanical efficiency at each step in the modeled fault evolution and find an overall trend toward increased efficiency through time strain energy density patterns are used to identify regions of incipient faulting and support the notion of north to south propagation of the san jacinto fault during its initiation

fault growth by linkage observations and implications from analogue models abstract using time sequence analyses of extensional fault models we demonstrate the pivotal role played by fault segmentation in the accumulation of displacement and length during the growth of faults experiments are described in which incremental steps during the development of individual faults have been reconstructed from time lapse photographs taken during model deformation these records confirm the composite segment hierarchy of fault structure a pattern that is frequently recognised in many natural arrays they reveal the progressive enlargement of individual faults to be the product of a repetitive cycle of tip line propagation overlap and linkage between nearest neighbours by contrasting the displacement patterns of successive increments during growth convincing evidence is also presented to suggest that individual segments of faults may remain kinematically independent once they are physically linked this behaviour is shown to be responsible for the characteristic saw tooth patterns often recognised in strike parallel fault displacement profiles such patterns are believed to arise where relict segment boundaries remain preserved as asperities to slip so that displacement is confined to discrete parts of a fault plane surface growth in this way also causes the maximum displacement d and surface length l of faults to continually change by different proportions incremental displacement records presented here corroborate field evidence which shows that linkage between fault segments during growth is responsible for a significant component of the spread of values often recorded in d versus l compilations finally we speculate that linkage between fault segments also accounts for transient irregularities recorded in the frequency distribution of the fault length populations of each model

the growth and propagation of synsedimentary faults abstract offset of each syn fault horizon across a synsedimentary i e growth fault records the throw subsequent to its deposition allowing successive growth stages of a fault to be determined this technique has been applied to synsedimentary faults mapped on 3d seismic datasets to distinguish between faults which propagated laterally during growth faults which did not propagate laterally for significant growth periods and faults in which the lateral dimensions of the active surface decreased during their growth these different fault tip line behaviours result in characteristic lateral and dip parallel fault displacement variations fault propagation rates have been measured for 10 fault tips the maximum propagation rate is 15 km my for gravity driven faults and 3 km my for tectonically driven faults rates of fault propagation decrease with elapsed time from the initiation of faulting with tip line stasis and retreat becoming dominant in the later stages of fault growth static and retreating lateral tip lines are associated with overlap of the fault strain field with that of a neighbouring fault most often with the formation of a fault relay zone

geochemical constraints from formation water analyses from the north sea and the gulf coast basins on quartz feldspar and illite precipitation in reservoir rocks

earthquake hazards of active blind thrust faults under the central los angeles basin california we document several blind thrust faults under the los angeles basin that if active and seismogenic are capable of generating large earthquakes m 6 3 to 7 3 pliocene to quaternary growth folds imaged in seismic reflection profiles record the existence size and slip rates of these blind faults the growth structures have shapes characteristic of fault bend folds above blind thrusts as demonstrated by balanced kinematic models geologic cross sections and axial surface maps we interpret the compton los alamitos trend as a growth fold above the compton ramp which extends along strike from west los angeles to at least the santa ana river the compton thrust is part of a larger fault system including a decollement and ramps beneath the elysian park and palos verdes trends the cienegas and coyote hills growth folds overlie additional blind thrusts in the elysian park trend that are not closely linked to the compton ramp analysis of folded pliocene to quaternary strata yields slip rates of 1 4 0 4 mm yr on the compton thrust and 1 7 0 4 mm yr on a ramp beneath the elysian park trend assuming that slip is released in large earthquakes we estimate magnitudes of 6 3 to 6 8 for earthquakes on individual ramp segments based on geometric segment sizes derived from axial surface maps multiple segment ruptures could yield larger earthquakes m 6 9 to 7 3 relations among magnitude coseismic displacement and slip rate yield an average recurrence interval of 380 years for single segment earthquakes and a range of 400 to 1300 years for multiple segment events if these newly documented blind thrust faults are active they will contribute substantially to the seismic hazards in los angeles because of their locations directly beneath the metropolitan area

shortening and thickening of metropolitan los angeles measured and inferred by using geodesy geodetic observations using the global positioning system gps and other techniques record a high rate of north south shortening in an east southeast trending 5 40 km wide belt in northern metropolitan los angeles california downtown los angeles is observed to be converging upon the southern san gabriel mountains at 6 mm yr aside from the elastic strain that will be released during earthquakes rupturing the san andreas and san jacinto faults east west lengthening across northern metropolitan los angeles is minor 2 5 mm yr therefore north south shortening is accommodated mainly by vertical crustal thickening

the role of fault interaction and linkage in controlling synrift stratigraphic sequences late jurassic statfjord east area northern north sea examination of well constrained three dimensional seismic data demonstrates the role of fault interaction and linkage in controlling the nature of synrift sequences on the hanging wall of the statfjord east fault a typical late jurassic structure in the northern north sea brent province al though now a single fault the statfjord east fault originally consisted of several en echelon segments each of which defined individual subbasins structural and stratigraphic evidence both along and across fault strike indicates that the fault resulted from segment propagation interaction and linkage facies architecture thickness variations and the internal character of synrift formations are temporally and spatially related to the subbasin geometry variations in displacement along the fault segments exhibit characteristics of interacting en echelon faults including anomalous displacement gradients in regions of segment overlap we attribute the observed shifts in depocenters to local enhancement of displacement rates resulting from the interaction of neighboring fault segments the results have far reaching consequences for synrift plays in the northern north sea because they imply that only from the perspective of fault growth and linkage can the late jurassic structure and stratigraphy be fully understood

assessing the work budget and efficiency of fault systems using mechanical models 1 we examine the work energy budget of actively deforming fault systems in order to develop a means of examining the systemic behavior of complex fault networks work done in the deformation of a faulted area consists of five components 1 work done against gravity in uplift of topography wgrav 2 internal energy of the strained host rock wint 3 work done resisting friction during slip on faults wfric 4 seismic energy released in earthquake events as ground shaking wseis and 5 work done in initializing new faults and propagating existing faults wprop the energy budget of a fault system can be expressed as wtot wgrav wint wfric wseis wprop for a balanced energy budget the total of these five components will equal the external tectonic work applied to the system we examine the work balance within hypothetical and simulated two dimensional static fault systems using mechanical models the boundary element method models produce a balanced work budget for both simple and complex fault system models the presence of slipping faults reduces the internal strain energy of the faulted area wint at a cost of work done against friction and gravity and propagation and seismic energy where applicable calculations of minimum work deformation match expected deformation paths indicating the usefulness of this approach for evaluating efficiency in more complex systems the partitioning of various work terms may express the relative efficiency or maturity of fault systems furthermore calculation of potential seismic energy release can provide an upper bound to earthquake seismic moment assessments

formation of intracontinental basins in the opposite corners of the tabas block as coeval structures controlled by transpressional faulting iran the interaction of strike slip faults in their restraining junctions allowed for the coeval formation of the tabas and abdoughi basins and led to their inversion during the late cenozoic the intracontinental basins filled with neogene and quaternary deposits were controlled by large scale dextral transpression along major faults that bounded the tabas block which is a part of the central iranian block the anastomosing strike slip fault pattern facilitated the development of both basins in opposite corners of the tabas block the subsided areas were formed as a result of interaction between the restraining junctions of strike slip faults and thrusts flexural loading caused by the uplifted series of thrust sheets resulted in the depression of the opposite fault slabs which permitted deposition of neogene sediments deformation according to the bookshelf mechanism can be considered as a consequence of accommodation of the shortening of the area north of the main zagros thrust and externally imposed shearing along the great kavir doruneh fault during the collision of the arabian and eurasian plates related processes of transpression and counterclockwise rotation of the tectonic blocks included in the central iranian block favored the interaction of strike slip faults the change of far field stress and continuous transpression caused inversion of the basins and formation of neogene folds in the northern and southern corners of the tabas block the geomorphic features observed along these strike slip faults and on the thrust surfaces bounding the folds display their recent activity consistent with present day seismicity and geodetic measurements within the central iranian block

is the earth lazy a review of work minimization in fault evolution abstract the principle of work minimization has been used in various forms to account for the development of active fault systems within a wide range of tectonic settings we review the successes challenges and implications learned from previous applications of work minimization examination of the energy budget provides insight into the competing influences of different processes within fault systems at a variety of scales we present each work budget component with considerations for numerical implementation additionally we demonstrate numerical implementation by solving for the energy budgets and finding the most efficient propagation paths of a joint and several faults consideration of the energy budget captures growth patterns that are consistent with both laboratory observations and theoretical predictions using the energy release rate g unlike using g work minimization is not limited to collinear growth in comparison with predictions using coulomb failure planes work minimization eliminates uncertainty in predicting fault propagation by 1 evaluating the trade off between tensile and shear failure ahead of the fault and 2 avoiding the ambiguity introduced by the two potential coulomb failure planes application of work minimization to faulting demonstrates the effectiveness of this approach in predicting both the orientation and timing of fault propagation

formation of segmented normal faults a 3 d perspective the interpretation of fault kinematics from geometric data is an essential step in developing an understanding of the growth of fault systems constraints on fault geometry are however often restricted to 2 d maps or cross sections in this article we consider the extent to which kinematic interpretations of faulting benefit from a 3 d rather than 2 d geometrical perspective concentrating on relay zones and segmented normal fault arrays we suggest that very different interpretations of their evolution arise from the recognition that the propagation directions of faults and fault segments will rarely be contained within the inspection plane of 2 d data a 3 d perspective favours an interpretation in which the segments of a fault array are kinematically interrelated from their initiation individual segments in such systems may link into a single fault surface out of the plane of inspection or may be unconnected in 3 d we argue that this interpretation which conflicts with the often suggested model of incidental overlap of originally isolated faults should be the preferred model for the generation and growth of segmented normal fault arrays