near surface structural model for deformation associated with the february 7 1812 new madrid missouri earthquake the february 7 1812 new madrid missouri earthquake m moment magnitude 8 was the third and final large magnitude event to rock the northern mississippi embayment during the winter of 1811 1812 although ground shaking was so strong that it rang church bells stopped clocks buckled pavement and rocked buildings up and down the eastern seaboard little coseismic surface deformation exists today in the new madrid area the fault s that ruptured during this event have remained enigmatic we have integrated geomorphic data documenting differential surficial deformation supplemented by historical accounts of surficial deformation and earthquake induced mississippi river waterfalls and rapids with the interpretation of existing and recently acquired seismic reflection data to develop a tectonic model of the near surface structures in the new madrid missouri area this model consists of two primary components a north northwest trending thrust fault and a series of northeast trending strike slip tear faults we conclude that the reelfoot fault is a thrust fault that is at least 30 km long we also infer that tear faults in the near surface partitioned the hanging wall into subparallel blocks that have undergone differential displacement during episodes of faulting the northeast trending tear faults bound an area documented to have been uplifted at least 0 5 m during the february 7 1812 earthquake these faults also appear to bound changes in the surface density of epicenters that are within the modern seismicity which is occurring in the stepover zone of the left stepping right lateral strike slip fault system of the modern new madrid seismic zone

subsurface structural interpretation by applying trishear algorithm an example from the lenghu5 fold and thrust belt qaidam basin northern tibetan plateau abstract the application of trishear algorithm in which deformation occurs in a triangle zone in front of a propagating fault tip is often used to understand fault related folding in comparison to kink band methods a key characteristic of trishear algorithm is that non uniform deformation within the triangle zone allows the layer thickness and horizon length to change during deformation which is commonly observed in natural structures an example from the lenghu5 fold and thrust belt qaidam basin northern tibetan plateau is interpreted to help understand how to employ trishear forward modelling to improve the accuracy of seismic interpretation high resolution fieldwork data including high angle dips dragging structures thinning hanging wall and thickening footwall are used to determined best fit trishear model to explain the deformation happened to the lenghu5 fold and thrust belt we also consider the factors that increase the complexity of trishear models including a fault dip changes and b pre existing faults we integrate fault dip change and pre existing faults to predict subsurface structures that are apparently under seismic resolution the analogue analysis by trishear models indicates that the lenghu5 fold and thrust belt is controlled by an upward steepening reverse fault above a pre existing opposite thrusting fault in deeper subsurface the validity of the trishear model is confirmed by the high accordance between the model and the high resolution fieldwork the validated trishear forward model provides geometric constraints to the faults and horizons in the seismic section e g fault cutoffs and fault tip position faults intersecting relationship and horizon fault cross cutting relationship the subsurface prediction using trishear algorithm can significantly increase the accuracy of seismic interpretation particularly in seismic sections with low signal noise ratio

slow deformation and lower seismic hazard at the new madrid seismic zone global positioning system gps measurements across the new madrid seismic zone nmsz in the central united states show little if any motion these data are consistent with platewide continuous gps data away from the nmsz which show no motion within uncertainties both these data and the frequency magnitude relation for seismicity imply that had the largest shocks in the series of earthquakes that occurred in 1811 and 1812 been magnitude 8 their recurrence interval should well exceed 2500 years longer than has been assumed alternatively the largest 1811 and 1812 earthquakes and those in the paleoseismic record may have been much smaller than typically assumed hence the hazard posed by great earthquakes in the nmsz appears to be overestimated

fault slip rates in the modern new madrid seismic zone structural and geomorphic analysis of late holocene sediments in the lake county region of the new madrid seismic zone indicates that they are deformed by fault related folding above the blind reelfoot thrust fault the widths of narrow kink bands exposed in trenches were used to model the reelfoot scarp as a forelimb on a fault bend fold this coupled with the age of folded sediment yields a slip rate on the blind thrust of 6 1 0 7 mm year for the past 2300 100 years an alternative method used structural relief across the scarp and the estimated dip of the underlying blind thrust to calculate a slip rate of 4 8 0 2 mm year geometric relations suggest that the right lateral slip rate on the new madrid seismic zone is 1 8 to 2 0 mm year

numerical modeling of trishear fault propagation folding in contrast to kink band migration modeling methods trishear numerical models produce fault propagation folds with smooth profiles and rounded hinges modeled fold hinges tighten and converge downward within a triangular zone of distributed deformation which is focused on the fault tip such features have been reported from field studies and are also seen in analogue models of compressional deformation however apart from its initial application to laramide folds little quantitative work has been undertaken on trishear fault propagation folding in other settings in addition no study has been undertaken into the growth strata which might be associated with such structures this paper uses an equivalent velocity description of the geometric model of trishear together with models of erosion and sedimentation to investigate trishear fault propagation folding of both pregrowth and growth strata the trishear model is generalized to include a variety of fault propagation to slip ratios and fault propagation from a flat decollement the models show continuous rotation of the forelimb with the characteristic development of cumulative wedges within growth strata when total slip on a structure is high the model predicts overturned pregrowth and growth strata during the initial stages of deformation beds in the forelimb thicken but later thin when they become steep or overturned the effect of variations in fault propagation to slip ratios on two dimensional finite strain in the models is assessed by the use of initially circular strain markers high fault propagation to slip p s ratios lead to narrow zones of high finite strain while lower p s ratios lead to more ductile deformation and broader zones of high strain in all cases hanging wall anticlines and footwall synclines originate as early ductile folds which are later cut by the propagating fault modeled structures are compared with natural examples

evidence for recurrent faulting in the new madrid seismic zone from mini sosie high resolution reflection data a mini sosie high resolution seismic reflection survey was conducted on reelfoot scarp in the northwestern tennessee portion of the new madrid seismic zone interpretation of the mini sosie data revealed the need to reinterpret previously collected reflection data obtained from explosive source and vibroseis surveys interpretation and integration of the three data sets have resulted in a new model for the subsurface of reelfoot scarp and provide evidence for recurrent movement along reelfoot fault the major reverse fault associated with reelfoot scarp estimated displacements on reelfoot fault vary from 60 m 60 ms for late paleozoic rocks to 15 m 20 ms for late eocene sedimentary units no clear offsets are observed on this particular fault for units younger than late eocene age however uplift folding and related structures are observed in younger sediments an observed variation of offset with depth age and the presence of the younger structures are evidence of reactivation of reelfoot faul

variable post paleozoic deformation detected by seismic reflection profiling across the northwestern prong of new madrid seismic zone abstract high resolution shallow seismic reflection profiles across the northwesternmost part of the new madrid seismic zone nmsz and northwestern margin of the reelfoot rift near the confluence of the ohio and mississippi rivers in the northern mississippi embayment reveal intense structural deformation that apparently took place during the late paleozoic and or mesozoic up to near the end of the cretaceous period the seismic profiles were sited on both sides of the northeast trending olmsted fault defined by varying elevations of the top of mississippian locally base of cretaceous bedrock the trend of this fault is close to and parallel with an unusually straight segment of the ohio river and is approximately on trend with the westernmost of two groups of northeast aligned epicenters prongs in the nmsz initially suspected on the basis of pre existing borehole data the deformation along the fault has been confirmed by four seismic reflection profiles combined with some new information from drilling the new data reveal 1 many high angle normal and reverse faults expressed as narrow grabens and anticlines suggesting both extensional and compressional regimes that involved the largest displacements during the late cretaceous mcnairy 2 a different style of deformation involving probably more horizontal displacements i e thrusting that occurred at the end of this phase near the end of mcnairy deposition with some fault offsets of paleocene and younger units 3 zones of steeply dipping faults that bound chaotic blocks similar to that observed previously from the nearby commerce geophysical lineament cgl and 4 complex internal deformation stratigraphically restricted to the mcnairy suggestive of major sediment liquefaction or landsliding our results thus confirm the prevalence of complex cretaceous deformations continuing up into tertiary strata near the northern terminus of the nmsz

late cretaceous and cenozoic geology of the new madrid seismic zone structure contour maps constructed from well seismic reflection and outcrop data of the tops of the paleozoic section upper cretaceous section paleocene midway group and eocene section illustrate the post paleozoic structure of the new madrid seismic zone region isopach maps of the late cretaceous section midway group and eocene section help constrain the timing of the structural events these maps which encompass much of the northern mississippi embayment reveal reactivation of the underlying late precambrian cambrian reelfoot rift during midway group deposition but no reactivation during late cretaceous or eocene deposition the structure contour maps also indicate a subtle south plunging depression on the tops of the paleozoic upper cretaceous and midway group along the axis of the northern mississippi embayment that we have called a trench this trench is 50 km wide has a maximum depth of 100 m and appears to have formed during the eocene the trench s western boundary coincides with the blytheville arch lake county uplift and its southeastern margin underlies memphis tennessee the blytheville arch lake county uplift is the structure responsible for the new madrid seismic zone and thus it is possible that the southeastern margin of the trench is also a fault zone northern mississippi embayment post paleozoic stratigraphy consists of sands silts and clays that thicken from 477 m at new madrid missouri to 987 m near memphis tennessee the uniformity of these sediments indicates their elastic properties and therefore seismic velocities are very similar however variations in cementation and unconformities within the section may influence seismic wave propagation manuscript received 17 june 1999

a continuum mixture model of ice stream thermomechanics in the laurentide ice sheet 2 application to the hudson strait ice stream episodic exportations of ice rafted debris to the north atlantic in the late pleistocene suggest quasiperiodic ice streaming or surging activity on the northeastern margin of the laurentide ice sheet much of this efflux of ice may originate from an ice stream issuing from hudson strait and tapping into core regions of the laurentide in hudson bay labrador and the foxe basin applying the continuum mixture theory outlined by marshall and clarke this issue we model the thermomechanical evolution of the hudson strait ice stream in a three dimensional finite difference model of the laurentide ice sheet our simulations focus on internal dynamics of the ice stream under thermal regulation of basal flow we find surge cycles of stream activity interspersed with quiescent periods where the ice stream is frozen to the bed modeled surge durations vary from 105 to 3260 years while surge periodicities range from 585 to 22 410 years with pervasively warm or cold internal temperature distributions in the ice ice streams can also establish modes of permanent activity or inactivity under thermal regulation our most vigorous ice streams produce peak values of approximately 0 03 sv of freshwater flux to the north atlantic from basal meltwater and iceberg production associated ice stream velocities in this maximum case approach 6700 m yr 1 the total ice volume mobilized in a single surge event is equivalent to a global sea level rise of 0 04m in the most tranquil surge and almost 0 6m in the most extreme case these velocities and sea level impacts are an order of magnitude less than those predicted by macayeal 1993a b and only our most exuberant streams approach the iceberg flux estimates of dowdeswell et al 1995 we propose that the sediment load of icebergs emanating from hudson strait in a surge event may exceed expectations from contemporary icebergs

late wisconsin to holocene deformation in the new madrid seismic zone the new madrid earthquakes of 1811 1812 produced regional deformation in northwestern tennessee southeastern missouri and northeastern arkansas landforms such as the lake county uplift reelfoot lake and the sunklands of northeast arkansas either formed or were reactivated during these great earthquakes two hundred and fifty three cores were used to construct a structure contour map of the unconformity between the eocene strata and the overlying quaternary mississippi river alluvium in the new madrid seismic zone this map reveals relief that mirrors the surface deformation and provides additional data on the distribution of regional deformation we interpret the structure contour map as a representation of the late wisconsin to the present strain field of the new madrid seismic zone this map provides constraints for future kinematic analyses of late quaternary new madrid faulting and allows forecasting of future coseismic deformation

characteristics and origin of polygonal terrain in southern utopia planitia mars results from mars orbiter laser altimeter and mars orbiter camera data giant polygons on mars with several kilometers in diameter were first observed in mariner 9 and viking orbiter images and their origin formation and evolution have remained enigmatic since that time new data obtained by the mars orbiter laser altimeter mola and the mars orbiter camera moc on board the mars global surveyor spacecraft now permit analysis of topography and morphology of the polygonal terrain in unprecedented detail mola data show that 1 giant polygonal terrain in utopia is located at the lower slopes of a basin structure that is the utopia basin 2 the onset of polygonal terrain in utopia is close to the same elevation in several profiles and lies at about the same elevation as a terrace which was interpreted as a shore line of an ancient body of standing water within the utopia basin proper 3 polygonal terrain occurs over a wide range of elevations on regional slopes of 0 1 4 the depths of the troughs range from 5 m to 115 m averaging about 30 m and tend to be greater toward the center of the utopia basin 5 the mean width of polygonal troughs is of the order of 2 km ranging from 0 5 to 7 5 km data from the mars orbiter camera moc indicate that 1 the troughs are generally broad graben like features with varying morphologies i e terraces and lobateness and dimensions widths and depths 2 trough morphology has been modified by aeolian processes 3 polygonal troughs are weakly interconnected at moc resolutions and there are no smaller polygons observed within the giant polygons 4 circular depressions which are probably related to subsurface collapse sometimes occur at the bottoms of polygonal troughs 5 small scale polygonal terrain on the ejecta blankets of young craters with fluidized ejecta in the investigated area exhibits different characteristics i e smaller size presence of rims greater degree of interconnections than giant polygons in the viking images we see that 1 old impact craters are increasingly buried with younger materials toward the basin center and that 2 a large number of superposed impact craters on polygonal terrain exhibit distinctive ejecta morphologies such as rampart ejecta blankets and single and double lobate ejecta blankets mola and moc observations are consistent with the formation of polygonal terrain in the area of a former standing body of water but polygon size seems too large to be readily accounted for by desiccation or freezing processes alone on the basis of the observed characteristics we propose that the giant polygons are primarily of tectonic origin being caused by uplift of the floor of the utopia basin there are several candidates for the cause of uplift 1 the removal of the load representing former standing bodies of water water thickness estimated for the utopia basin is about 1 km in this scenario removal of this load comparable to loads placed on terrestrial continental lithosphere by the laurentide and scandinavian ice sheets by loss of the water or ice could cause uplift of the floor and polygon formation 2 the freezing and expansion of residual water buried by sediments or in the near subsurface or 3 a combination of these from the discussion of alternative models in the light of the mola and moc data we conclude that formation of giant polygons by tectonic uplift of the basin floor is most likely and deserves further investigation

refinement of thrust faulting models for the central new madrid seismic zone abstract we present the results of the joint relocation of events recorded during 1989 1992 by the panda network in the central new madrid seismic zone the near surface material in the study area is a gently dipping layer of poorly consolidated sediments with low p wave velocity and high v p v s estimated values 1 8 km s 1 and 3 the sediments are underlain by high velocity paleozoic rocks under the network the difference in sediment thickness is only 0 6 km but because of the low velocities the location of the events using layered models is affected by errors application of the joint hypocentral determination jhd technique to a subset of 580 events shows that the single event locations may be in error by as much as 1 km in depth depending on where the events are located analysis of synthetic data generated for a realistic 3 d velocity model supports the jhd results the analysis of synthetic data also suggests that a v p v s 2 3 is more appropriate for the post paleozoic mississippi embayment sediments based on the jhd locations we present a new interpretation of the seismicity with two en echelon sw dipping thrust faults connected by a west dipping thrust fault these faults appear associated with the reelfoot scarp and its northern extension the kentucky bend scarp

coseismic fault related fold model growth structure and the historic multisegment blind thrust earthquake on the basement involved yoro thrust central japan 1 we use high resolution seismic reflection profiles boring transects and mapping of fold scarps that deform late quaternary and holocene sediments to define the kinematic evolution subsurface geometry coseismic behavior and fault slip rates for an active basement involved blind thrust system in central japan coseismic fold scarps on the yoro basement involved fold are defined by narrow fold limbs and angular hinges on seismic profiles suggesting that at least 3 9 km of fault slip is consumed by wedge thrust folding in the upper 10 km of the crust the close coincidence and kinematic link between folded horizons and the underlying thrust geometry indicate that the yoro basement involved fold has accommodated slip at an average rate of 3 2 0 1 mm yr on a shallowly west dipping thrust fault since early pleistocene time past large magnitude earthquakes including an historic m 7 7 event in a d 1586 that occurred on the yoro blind thrust are shown to have produced discrete folding by curved hinge kink band migration above the eastward propagating tip of the wedge thrust coseismic fold scarps formed during the a d 1586 earthquake can be traced along the en echelon active folds that extend for at least 60 km in spite of different styles of folding along the apparently hard linked nobi ise blind thrust system we thus emphasize the importance of this multisegment earthquake rupture across these structures and the potential risk for similar future events in en echelon active fold and thrust belts

geometry numerical models and revised slip rate for the reelfoot fault and trishear fault propagation fold new madrid seismic zone abstract the geometry and structural relief of late holocene sediments folded across the reelfoot scarp are characterized with trench excavations shallow borings a digital elevation model dem of topography and reelfoot lake rl bathymetry results suggest the scarp forms by heterogeneous shear in the forelimb of a fault propagation fold seismic data and the accompanying structural analysis support the argument that the reelfoot fault does not offset the surface and that the reelfoot scarp is a fold limb structural relief measured across the reelfoot scarp is slightly higher than previous determinations and varies from 7 to 11 m along the western shoreline of rl numerical models based on trishear kinematics constrain growth of the reelfoot monocline and indicate that the underlying reelfoot thrust fault is steeply dipping the shallowest portion of the central reelfoot thrust segment dips 75 where the fault tip has propagated upward from 911 m to its current location 465 m beneath the surface the models indicate that the shallowest section of the southern thrust segment in the reelfoot thrust system dips even more steeply at 80 but has propagated upward only a few tens of meters to its present location 1016 m beneath the surface the thrust is flatter at deeper levels based on the location of earthquake hypocenters strain across the reelfoot scarp is partitioned into two or three separate fold scarps that collectively record a late holocene fault slip rate of 3 9 0 1 mm yr the slip rate is based on 9 m of structural relief the 2290 60 ybp age of folded sediment and a 75 dip for the fault this implies a rate of horizontal contraction in the seismic zone of 1 0 mm yr this rate is strongly dependent on the dip of the thrust calculated by the structural analysis the numerical models argue for reactivation of faults that previously offset paleozoic strata

displacement history and slip rate on the reelfoot fault of the new madrid seismic zone abstract a displacement history and slip rates were determined for the reelfoot fault in the new madrid seismic zone from a seismic reflection profile and trench data based on calculations from the seismic reflection line the average slip rate over the last 80 million years is 0 0009 mm year 1 slip rate during the late cretaceous was 0 0007 mm year 1 0 002 mm year 1 during the paleocene midway group 0 001 mm year 1 during paleocene eocene wilcox formation time 0 0003 mm year 1 during the post wilcox pre holocene period and a holocene slip rate of 1 8 mm year 1 based on trench data slip rate on the reelfoot fault has been 4 4 mm year 1 over the last 2400 years and a maximum of 6 2 mm year 1 during the two most recent earthquake cycles between ad 900 and ad 1812 the holocene slip rate is at least four orders of magnitude higher than the average late cretaceous and cenozoic slip rates for the reelfoot fault it would appear that there has been a quaternary change in the stress field in the central united states or the reelfoot fault is experiencing a short lived burst of seismic activity

multiple geometries and modes of fault propagation folding in the canadian thrust belt abstract a multitude of fold models have been proposed to explain the variety of fold geometries which develop in front of thrust faults detailed field fabric and photogrammetric studies of 4 fault cored asymmetrical folds in the thin skinned canadian thrust belt were used to test models of fault propagation folding fold geometries include combinations of angular and rounded fold surfaces highly contorted anticlinal hinge areas and minimal penetrative deformation or changes in bedding thickness interlimb angles generally decrease with increasing shortening indicating progressive fold tightening about fixed anticlinal hinges extensive flexural slip thrusting toward the anticlinal axes of angular folds suggests that kink folding in thin skinned thrust belts is aided by material transfer from both fold limbs into hinge areas fold geometries change dramatically along the strike of individual structures demonstrating the non uniqueness of fault propagation fold geometries no single mode of fault propagation folding can explain the diverse fold geometries seen in the canadian thrust belt this geometric variability can be ascribed to the complex interplay of multiple modes of folding in strata near the causal thrusts oblique shear and flexural slip in triangular shear zones distribute thrust displacements into both rounded and angular folds simultaneous angular folding of overlying strata commonly occurs by progressive kink folding where folds tighten by flexural slip on all fold limbs until the thrust breaks through the fold regional and local differences in the amount of pervasive top to the craton shear needed for progressive kink folding may be partially responsible for the variability of fault propagation fold geometries in thin and thick skinned orogens world wide

variable fault tip propagation rates affected by near surface lithology and implications for fault displacement hazard assessment abstract the fabric of reverse fault zones close to the surface is usually partitioned in between a narrow discrete rupture zone and a more distributed one where folding is predominant this makes quite challenging the adoption of proper setbacks in surface rupture hazard studies for critical facilities or microzoning some of the parameters controlling fault zone fabric are related to mechanics of near surface geology lithology overburden thickness cohesion and water content whose interaction is complex and only partially understood nevertheless these can be seldom measured or derived kinematic models conversely express such an interaction of complex variables as simple synthetic parameters such as the amount of upward propagation of the fault tip for unit of slip usually referred to as the p s ratio propagation on slip here we discuss results on the trishear kinematic inverse modeling of a contractional fault propagation fold at monte netto hill capriano del colle n italy observing a two stage fault and fold growth evolution marked by a significant shift in the p s parameter at this site exceptional sequence of exposures due to ca 10 years of quarry excavations allowed to obtain a series of cross sections across the fault zone we use this detailed high resolution example as a natural analogue for more general large scale surface ruptures involving a thick alluvial cover a very common setting for the siting of critical facilities during the early stage of displacement the fault cut through clast supported fluvial gravels with a high propagation rate p s 7 and a discrete rupture width then during the latest movements of the thrust fault tip propagation slowed down to p s 2 9 as the fault started cutting through several stacked bodies of clays and silty clays pedogenized aeolian silts and overbank deposits causing a pronounced folding of the layers over a wider deformation zone these results strongly suggest that lithological changes in the underlying shallow stratigraphy common in an alluvial plain depositional setting would significantly affect the potential for surface faulting across the same tectonic structure with relevant implications in the fault displacement hazard assessment

estimation of fault propagation distance from fold shape implications for earthquake hazard assessment a numerical grid search using the trishear kinematic model can be used to extract both slip and the distance that a fault tip line has propagated during growth of a fault propagation fold the propagation distance defines the initial position of the tip line at the onset of slip in the santa fe springs anticline of the los angeles basin we show that the tip line of the underlying puente hills thrust fault initiated at the same position as the 1987 magnitude 6 0 whittier narrows earthquake

accelerating slip rates on the puente hills blind thrust fault system beneath metropolitan los angeles california usa slip rates represent the average displacement across a fault over time and are essential to estimating earthquake recurrence for probabilistic seismic hazard assessments we demonstrate that the slip rate on the western segment of the puente hills blind thrust fault system which is beneath downtown los angeles california usa has accelerated from 0 22 mm yr in the late pleistocene to 1 33 mm yr in the holocene our analysis is based on syntectonic strata derived from the los angeles river which has continuously buried a fold scarp above the blind thrust slip on the fault beneath our field site began during the late middle pleistocene and progressively increased into the holocene this increase in rate implies that the magnitudes and or the frequency of earthquakes on this fault segment have increased over time this challenges the characteristic earthquake model and presents an evolving and potentially increasing seismic hazard to metropolitan los angeles

multiple late holocene earthquakes along the reelfoot fault central new madrid seismic zone the reelfoot fault is an east vergent reverse fault underlying the lake county uplift a low amplitude late holocene anticline bordered on the east by the 32 km long reelfoot scarp fluvial deposits across the scarp define an 8 m high east facing monocline most near surface deformation along the scarp is accommodated via folding rather than faulting we interpret the scarp as a fault propagation fold developed over a wcst dipping reverse fault interpreted from shallow seismic reflection data trench exposurcs provide evidence for three episodes of deformation along the reelfoot fault within thc past approximately 2400 years between a d 780 and 1000 between a d 1260 and 1650 and during a d 1812 our best estimate of the average recurrence interval for deformation along the scarp is 400 500 years each episode of deformation had a slightly different style the third most recent event produced a small graben a few tens of centimeters deep in the hanging wall of the reverse fault the second most recent earthquake produced about 1 3 m of throw in the graben as well as folding along the updip projection of the reverse fault and development of the scarp thesc relations suggest that graben development increased through time concomitant with growth of the monocline or that the events are of different magnitude the 1811 1812 episode of deformation produced abundant liquefaction prominent folding of fluvial strata along the scarp and minor faulting in the graben

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

active faulting and growth folding in the eastern santa barbara channel california we develop new methods to identify blind thrust fault systems determine fault slip rates and estimate potential earthquake magnitudes and recurrence intervals in active fold and thrust belts these methods are applied to compressive folds along the offshore oak ridge and blue bottle trends which overlie active blind thrust faults in the eastern santa barbara channel these folds and their causative faults are interpreted using fault bend fold theory and are represented in balanced models and cross sections that integrate surface and subsurface data the structures are mapped using a new technique of axial surface mapping in seismic reflection grids which defines three dimensional structural geometries and shows changes in slip and subsurface fault geometry along strike analysis of syntectonic growth sediments yields pliocene and quaternary fault slip rates of 1 3 mm yr on a deep thrust 16 km and 1 3 mm yr on shallower faults 2 5 km the combined 2 6 mm yr slip rate represents only part of the 6 mm yr of shortening measured by geodesy across the channel and estimated from relative pacific north american plate motions across the transverse ranges additional shortening is probably accommodated on other active thrusts in the western transverse ranges and in the northern channel along the santa barbara coast deformed seafloor sediments and a swarm of axial surface seismicity along the fold trends indicate that the underlying thrusts are active and may pose significant earthquake hazards to coastal southern california unsegmented fault surfaces are used through empirical relationships between fault surface area and rupture magnitude to estimate the sizes of potential earthquakes this analysis suggests that a ramp in the channel islands fault beneath the offshore oak ridge trend is capable of rupturing in a m s 7 2 earthquake earthquakes of this magnitude may release 2 m of slip which when combined with the estimated slip rate 1 3 mm yr yields an earthquake recurrence interval of 1500 yr for this channel islands fault ramp

multibend fault bend folding this paper shows how slightly complex but angular fault shapes with only a few bends lead to broadly curved and or highly complex folds a common perception exists that simple angular fold geometries are a necessary consequence of the normal assumptions of fault bend fold theory namely angular fault bends straight faults and flexural slip to the contrary it is shown here that exact application of these low level assumptions produces great complexity from simple fault shapes this complexity is caused by the combination of 1 generation of new axial surfaces by displacement of hangingwall cut offs past successive fault bends and 2 fragmentation of axial surfaces by mutual interference the result is entwined arrays of fold axial surfaces that produce quasi curved and or complex folds from discrete angular fault bends fold complexity grows in a highly non linear way in the most extreme case the number of fold axial surface segments grows as the fourth power of the number of fault bends this paper presents examples of the immense variety of complex fold geometries that are thus generated 0 1997 elsevier science ltd all rights reserved

structure of the lake county uplift new madrid seismic zone abstract the central segment of the new madrid seismic zone lies within a left step over zone between two northeast striking right lateral strike slip fault systems within this compressional step over zone is the topographically and structurally high lake county uplift which includes the tiptonville dome and ridgely ridge we believe these structures are a consequence of deformation in the hanging wall above the northwest striking southwest dipping reelfoot reverse fault reelfoot fault dips 73 from the surface to the top of the precambrian at a depth of approximately 4 km from 4 to 12 km depth the fault dips 32 and is seismically active based on a fault bend fold model we believe that the reelfoot fault becomes horizontal and aseismic at the top of the quartz brittle ductile transition zone at approximately 12 km depth our data indicate that the western margin of the tiptonville dome ridgely ridge and the western margin of the lake county uplift are bounded by east dipping kink bands backthrusts recent work suggests that the reelfoot fault is responsible for the 7 february 1812 m 8 new madrid earthquake however the reelfoot fault has a surface area that is less than that necessary for an m 8 earthquake a possible solution to this discrepancy between magnitude and fault plane area is that the associated backthrusts are seismogenic

faulting along the southern margin of reelfoot lake tennessee abstract the reelfoot lake basin tennessee is structurally complex and of great interest seismologically because it is located at the junction of two seismicity trends of the new madrid seismic zone to better understand the structure at this location a 7 5 km long seismic reflection profile was acquired on roads along the southern margin of reelfoot lake the seismic line reveals a westerly dipping basin bounded on the west by the reelfoot reverse fault zone the ridgely right lateral transpressive fault zone on the east and the cottonwood grove right lateral strike slip fault in the middle of the basin the displacement history of the reelfoot fault zone appears to be the same as the ridgely fault zone thus suggesting that movement on these fault zones has been synchronous perhaps since the cretaceous since the reelfoot and ridgely fault systems are believed responsible for two of the main shocks of 1811 1812 the fault history revealed in the reelfoot lake profile suggests that multiple mainshocks may be typical of the new madrid seismic zone the ridgely fault zone consists of two northeast striking faults that lie at the base of and within the mississippi valley bluff line this fault zone has 15 m of post eocene up to the east displacement and appears to locally control the eastern limit of mississippi river migration the cottonwood grove fault zone passes through the center of the seismic line and has approximately 5 m of up to the east displacement correlation of the cottonwood grove fault with a possible fault scarp on the floor of reelfoot lake and the new markham fault north of the lake suggests the cottonwood grove fault may change to a northerly strike at reelfoot lake thereby linking the northeast trending zones of seismicity in the new madrid seismic zone