geometry and slip rates of active blind thrusts in a reactivated back arc rift using shallow seismic imaging toyama basin central japan abstract active blind thrust faults which can be a major seismic hazard in urbanized areas are commonly difficult to image with seismic reflection surveys to address these challenges in coastal plains we collected about 8 km long onshore high resolution two dimensional 2d seismic reflection data using a dense array of 800 geophones across compressionally reactivated normal faults within a failed rift system located along the southwestern extension of the toyama trough in the sea of japan the processing of the seismic reflection data illuminated their detailed subsurface structures to depths of about 3 km the interpreted depth converted section correlated with nearby neogene stratigraphy indicated the presence of and along strike variation of previously unrecognized complex thrust related structures composed of active fault bend folds coupled with pairs of flexural slip faults within the forelimb and newly identified frontal active blind thrusts beneath the alluvial plain in addition growth strata and fold scarps that deform lower to upper pleistocene units record the recent history of their structural growth and fault activity this case shows that shallow seismic reflection imaging with densely spaced seismic recorders is a useful tool in defining locations recent fault activity and complex geometry of otherwise inaccessible active blind thrust faults

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

modelling growth stratal architectures associated with double edge fault propagation folding abstract fault related folds in fold and thrust belts and accretionary prisms are commonly simulated by three end member classes of geometric models decollement folding fault propagation folding and fault bend folding a given model produces a fold shape that depends on the amount of shortening the fault shape and eventually the fault propagation history the great variety of geometric solutions implies that the final fold shape is adequate to distinguish among different folding mechanisms only in a limited suite of cases additional information to better constrain fold kinematics is provided by the analysis of growth strata patterns which passively register the progressive evolution of deformation in the substratum availability of growth strata pattern templates associated with different geometric and kinematic solutions provides a useful tool for inferring the dominant kinematic pathways of natural thrust related structures in this paper we present growth strata patterns associated with double edge fault propagation folding the influence of the sedimentation rate the fault shape and the fault s p ratio i e slip versus ramp propagation rate on growth strata architectures are investigated geometrical modelling and application to natural examples indicate that double edge fault propagation anticlines are characterised by growth triangles on both limbs their position and geometry relate to the ramp shape and propagation history and on the sedimentation rate

new insights into lightning processes gained from triggered lightning experiments in florida and alabama analyses of electric and magnetic fields measured at distances from tens to hundreds of meters from the ground strike point of triggered lightning at camp blanding florida and at 10 and 20 m at fort mcclellan alabama in conjunction with currents measured at the lightning channel base and with optical observations allow us to make new inferences on several aspects of the lightning discharge and additionally to verify the recently published two wave mechanism of the lightning m component at very close ranges a few tens of meters or less the time rate of change of the final portion of the dart leader electric field can be comparable to that of the return stroke the variation of the close dart leader electric field change with distance is somewhat slower than the inverse proportionality predicted by the uniformly charged leader model perhaps because of a decrease of leader charge density with decreasing height associated with an incomplete development of the corona sheath at the bottom of the channel there is a positive linear correlation between the leader electric field change at close range and the succeeding return stroke current peak at the channel base the formation of each step of a dart stepped leader is associated with a charge of a few millicoulombs and a current of a few kiloamperes in an altitude triggered lightning the downward negative leader of the bidirectional leader system and the resulting return stroke serve to provide a relatively low impedance connection between the upward moving positive leader tip and the ground the processes that follow likely being similar to those in classical triggered lightning lightning appears to be able to reduce via breakdown processes in the soil and on the ground surface the grounding impedance which it initially encounters at the strike point so at the time of channel base current peak the reduced grounding impedance is always much lower than the equivalent impedance of the channel at close ranges the measured m component magnetic fields have waveshapes that are similar to those of the channel base currents whereas the measured m component electric fields have waveforms that appear to be the time derivatives of the channel base current waveforms in further confirmation of the two wave m component mechanism

investigation of the yellow river buried fault in the wuhai basin northwestern ordos block china using deep shallow seismic reflection and drilling techniques abstract studying the faults in wuhai basin which is located in the northwestern corner of the ordos block in china can not only guide earthquake safe construction in the region but also provide a greater understanding of the structure of the wuhai basin and the dynamic environment of northwestern ordos block in this study we effectively detected the yellow river buried fault yrf which we believe currently presents a risk to human life in the area first we laid out a deep seismic reflection profile dsrp in the north of the wuhai basin and found that the yrf is composed of two subparallel faults the east branch of the yellow river buried fault eyf and the west branch of the yellow river buried fault wyf which formed a y type graben in the section we then used 25 shallow seismic exploration profiles to find out how the faults are distributed finally we used drillings and quaternary dating methods to certify their existence and obtain their activity parameters we used the sequence stratigraphy method to compare sediment strata and identified paleo earthquake events by finding the unequal thickness layers utls on both sides of the fault the drillings revealed that four earthquakes occurred on the yrf around 25 6 0 11 ka bp 39 5 0 45 41 7 0 57 ka bp 58 25 7 13 ka bp and 111 1 21 ka bp since the last activity on the yrf occurred much longer ago than its estimated recurrence cycle we believe that the fault yrf currently presents a dangerous risk our study confirms that the yrf is a normal fault graben and that the wuhai bain is an extensional basin formed by the relative movement of the tibetan plateau the alashan block and the ordos block

evidence for large holocene earthquakes on the compton thrust fault los angeles california 1 we demonstrate that the compton blind thrust fault is active and has generated at least six large magnitude earthquakes mw 7 0 7 4 during the past 14 000 years this large concealed fault underlies the los angeles metropolitan area and thus poses one of the largest deterministic seismic risk in the united states we employ a methodology that uses a combination of high resolution seismic reflection profiles and borehole excavations to link blind faulting at seismogenic depths directly to near surface fault related folding deformed holocene strata record recent activity on the compton thrust and are marked by discrete sequences that thicken repeatedly across a series of buried fold scarps we interpret the intervals of growth as occurring after the formation of now buried paleofold scarps that formed during uplift events on the underlying compton thrust ramp minimum uplift in each of the scarp forming events which occurred at 0 7 1 75 ka event 1 0 7 3 4 ka or 1 9 3 4 event 2 5 6 7 2 ka event 3 5 4 8 4 ka event 4 10 3 12 5 ka event 5 and 10 3 13 7 ka event 6 ranged from 0 6 to 1 9 m indicating minimum thrust displacements of 1 3 to 4 2 m such large displacements are consistent with the occurrence of large magnitude earthquakes mw 7 this multidisciplinary methodology provides a means of defining the recent seismic behavior and therefore the hazard for blind thrust faults that underlie other major metropolitan regions around the world

earthquakes generated from bedding plane parallel reverse faults above an active wedge thrust seattle fault zone a key question in earthquake hazard analysis is whether individual faults within fault zones represent independent seismic sources for the seattle fault zone an upper plate structure within the cascadia convergent margin evaluating seismic hazard requires understanding how north side up bedding plane reverse faults which generate late holocene fault scarps interact with the north vergent master ramp thrust and overlying backthrust of the fault zone a regional uplift at a d 900 930 involved an earthquake that nucleated at depth and included slip on both the master ramp thrust and the backthrust this earthquake also included slip on some of the 6 km deep north side up bedding plane reverse faults at locales where the north side up reverse faults intersect the puget sound coast an earthquake a few centuries earlier than the a d 900 930 regional uplift only uplifted areas within hundreds of meters north of the reverse faults we infer that the bedding plane reverse faults are seismogenic because shore platforms near the reverse faults have been abruptly uplifted during earthquakes when other shorelines in the seattle fault zone were unaffected faults of the seattle fault zone therefore can both produce regional uplift earthquakes with or without surface displacement on the reverse faults and produce earthquakes that rupture the bedding plane reverse faults causing fault scarps and uplift localized to hundreds of meters north of these faults this latter type of earthquake has occurred at least twice and perhaps three times in the late holocene and all these earthquakes preceded the regional coseismic uplift of a d 900 930 to account for the paleoseismic observations we propose that the seattle fault zone is a wedge thrust with the leading edge being a fault bend wedge thrust fold the active axial surface of the wedge thrust fold is pinned at the tip of the wedge and a steeply north dipping sequence of tertiary sediment forms the south limb of the wedge thrust fold some of these steeply north dipping bedding plane surfaces are seismogenic reverse faults that produce scarps earthquakes on the wedge thrust produce the regional coseismic uplift events and earthquakes within the faultbend fold cause the local uplift earthquakes thus bedding plane faults can rupture during earthquakes when the wedge thrust does not rupture but instead continues to accumulate seismic energy

attachment process in rocket triggered lightning strokes in order to study the lightning attachment process we have obtained highly resolved about 100 ns time resolution and about 3 6 m spatial resolution optical images electric field measurements and channel base current recordings for two dart leader return stroke sequences in two lightning flashes triggered using the rocket and wire technique at camp blanding florida one of these two sequences exhibited an optically discernible upward propagating discharge that occurred in response to the approaching downward moving dart leader and connected to this descending leader this observation provides the first direct evidence of the occurrence of upward connecting discharges in triggered lightning strokes these strokes being similar to subsequent strokes in natural lightning the observed upward connecting discharge had a light intensity one order of magnitude lower than its associated downward dart leader a length of 7 11 m and a duration of several hundred nanoseconds the speed of the upward connecting discharge was estimated to be about 2 107 m s which is comparable to that of the downward dart leader in both dart leader return stroke sequences studied the return stroke was inferred to start at the point of junction between the downward dart leader and the upward connecting discharge and to propagate in both upward and downward directions this latter inference provides indirect evidence of the occurrence of upward connecting discharges in both dart leader return stroke sequences even though one of these sequences did not have a discernible optical image of such a discharge the length of the upward connecting discharges observed in one case and inferred from the height of the return stroke starting point in the other case is greater for the event that is characterized by the larger leader electric field change and the higher return stroke peak current for the two dart leader return stroke sequences studied the upward connecting discharge lengths are estimated to be 7 11 m and 4 7 m with the corresponding return stroke peak currents being 21 ka and 12 ka and the corresponding leader electric field changes 30 m from the rocket launcher being 56 kv m and 43 kv m additionally we note that the downward dart leader light pulse generally exhibits little variation in its 10 90 risetime and peak value over some tens of meters above the return stroke starting point while the following return stroke light pulse shows an appreciable increase in risetime and a decrease in peak value while traversing the same section of the lightning channel our findings regarding 1 the initially bidirectional development of return stroke process and 2 the relatively strong attenuation of the upward moving return stroke light and by inference current pulse over the first some tens of meters of the channel may have important implications for return stroke modeling

the close lightning electromagnetic environment dart leader electric field change versus distance net electric field changes due to dart leaders in triggered lightning from experiments conducted in 1997 1998 and 1999 at the international center for lightning research and testing at camp blanding florida are analyzed and compared with similar data obtained in 1993 at camp blanding and at fort mcclellan alabama in 1997 1999 the fields were measured at 2 10 stations with distances from the lightning channel ranging from 10 to 621 m while in 1993 the fields were measured at three distances 30 50 and 110 m in florida and at two distances about 10 and 20 m in alabama with a few exceptions the 1997 1999 data indicate that the distance dependence of the leader electric field change is close to an inverse proportionality r 1 in contrast to the 1993 data in which a somewhat weaker distance dependence was observed the typically observed r 1 dependence is consistent with a uniform distribution of leader charge along the bottom kilometer or so of the channel

bed by bed fold growth by kink band migration sant llorenc de morunys eastern pyrenees ahstract rowth strata deposited over and against the flank of the sant llorenc de morunys fold during its final stages of deformation have been mapped at high resolution as the basis for unraveling the kinematics of fold growth we use restoration techniques based on normal balancing assumptions to decipher the detailed kinematic history of folding the progressive restorations as well as balanced forward modeling show that the last few hundred meters of fold growth were dominated by kink band migration of a sort that is typical of much faultrelated folding the kink band migration has produced complex anticlinal hinge zone geometry including segmented fold hinges linked by disconformities and unconformities which has direct and detailed explanation in terms of fluctuations in deposition rate relative to curved hinge kink band migration rate large fluctuations in the convolution of non steady sedimentation and deformation are demonstrated although the absolute fluctuations in deformation and sedimentation are unknown at a length scale of 100 m kink band migration with little or no deposition is interspersed with sedimentation with little or no deformation at the length scale of 500 m deposition ranges from 200 to 50 of uplift 0 1997 elsevier science ltd all rights reserved

calcrete in quaternary coastal dunes in southwestern australia a capillary rise phenomenon associated with plants abstract non pedogenic calcrete is forming as a thin sheet 10 50 cm thick just above the water table in the subsurface of a barrier dune terrain in south western australia the calcrete is forming in response to the regional evapo transpirative regime the annual rainfall is high but of short duration and it infiltrates rapidly into the sandy substrates relatively high air temperatures and evaporation coupled with wind stress combine to produce a water vapor deficit across the dune terrain the vegetation responds by drawing upon phreatic water and pellicular water of the vadose zone thereby precipitating caco3 where plants are not drawing phreatic water calcrete is developing by evaporation of water at the top of the zone of capillary rise various structure types of calcrete are developed in the study area namely rhizoconcretionary mottled massive and laminar forms these occur in a definite stratigraphic sequence related to maturity of profile vegetation cover and thickness of the vadose zone rhizoconcretionary calcrete forms in the vadose zone mottled and massive calcrete frequently capped by laminar calcrete develop in the zone of capillary rise these latter calcrete types form a sheet that is ubiquitous and parallel to the water table since it is a feature of the water table the calcrete sheet may potentially transect regional stratigraphic boundaries and unconformities at the local level the various structural types of calcrete may he used as paleowater table indicators

crustal wedging triggering recent deformation in the andean thrust front between 31 s and 33 s sierras pampeanas precordillera interaction 1 we document a new model of crustal structure of the andean front in argentina where numerous historic earthquakes destroyed the cities of mendoza in 1861 ms 7 and san juan in 1944 mw 7 0 the cerro salinas anticline is formed above the west directed cerro salinas thrust an east facing monocline with an amplitude of about 2 km folds the cerro salinas thrust and overlying neogene succession this monocline is formed above a blind crustal thrust in the basement its dip of 14 west is inferred from fold geometry this thick skinned east directed blind thrust and the thin skinned west directed cerro salinas thrust define a tectonic wedge the wedge tip occurs at a depth of 5 4 km growth of the monocline after 8 5 ma is revealed on multichannel seismic msc profile 31017 repsol ypf rates of cerro salinas thrust displacement are of the order of 1 mm yr whereas vertical uplift of 0 45 mm yr results from the combined displacement on the cerro salinas thrust and growth of east facing monocline the lateral extent of the east directed crustal blind ramp corresponds with the along strike extent of the eastern precordillera when combined with the low displacement rate a long earthquake recurrence interval is implied smaller magnitude earthquakes however indicate that segments of the blind thrust ramps ruptured in historic events if all the segments of the blind thrust ruptured together the fault area is 7000 km 2 and could produce a mw 7 7 earthquake the crustal wedge model provides new constraints on the origin and potential size of earthquakes that threaten the densely populated region

comparison of fast electric field changes from subsequent return strokes of natural and triggered lightning fast electric field changes from subsequent return strokes of natural and triggered lightning with propagation paths almost entirely over water are compared data were collected at the kennedy space center florida during the summer of 1987 comparisons have been made of the average shape the risetime and the spectrum of the electric field changes to a first approximation the waveforms are very similar however the electric field changes from the triggered flashes tend to rise to peak faster and decay faster than do their counterparts in natural cloud to ground flashes

lightning return stroke current waveforms aloft from measured field change current and channel geometry 1 three dimensional reconstructions of six rocket triggered lightning channels are derived from stereo photographs these reconstructed channels are used to infer the behavior of the current in return strokes above the ground from current waveforms measured at the channel base and electric field change waveforms measured at a range of 5 2 km for 24 return strokes in these channels streak photographs of 14 of the same strokes are analyzed to determine the risetimes propagation speeds and amplitudes of relative light intensity for comparison with the electrical inferences results include the following 1 the fine structure of the field change waveforms that were radiated by these subsequent return strokes can be explained in large part by channel geometry 2 the average 10 90 risetime of the stroke current increased by about a factor of seven in our sample from an observed 0 31 0 17 s at the surface to an inferred 2 2 0 5 s at 1 km path length above the surface 3 the three dimensional propagation speed of the current front averaged 1 80 0 24 108 m s over channel lengths typically greater than 1 km 4 assuming that the measured current was entirely due to the return stroke forced an unreasonably large and abrupt reduction in inferred current amplitude over the first few tens of meters above the surface especially in cases when the leader was bright relative to its stroke therefore a significant fraction of the current at the surface was probably due to the leader in such cases 5 independent of the above peak return stroke currents decreased by approximately 37 12 between 100 m and 1 km of path length above the surface because of uncertainty about how to partition the measured current between leader and return stroke however we are unable to infer the variation of current amplitude near the ground

microstructure and palygorskite neoformation in pedogenic calcretes of central morocco abstract the aim of this research was the micromorphological and mineralogical characterization of calcrete outcrops located in central morocco with particular focus on microstructure development and palygorskite formation processes two study sites in the al haouz plain ca 30 km west to marrakech city were identified in an area dominated by quaternary sedimentary formations including layers cemented by carbonate and widespread calcareous crusts at the surface the calcrete samples were examined by means of polarizing microscopy from thin sections scanning electron microscopy energy dispersive x ray spectroscopy sem edx from undisturbed samples and x ray diffraction xrd methods the matrix of the calcrete material cementing the schist fragments composed of abundant calcite was associated mainly with palygorskite quartz and to a lesser extent illite mica feldspar and chlorite sem observations indicated that palygorskite mainly occurred as fibres fibre bundles and networks mainly grown on and or enclosing calcite rhombohedrons the edx spectra of the palygorskite fibres exhibited strong silicon si peaks followed by al mg and fe peaks the edx analysis of calcite rhombohedric crystals exhibited a strong ca peak followed by weak peaks of si mg al and fe possibly due to nearby or overlapped palygorskite crystals one main palygorskite neoformation process was described palygorskite was authigenically formed neoformed during and after precipitation of calcite from percolating soil solutions in a near surface setting peculiar sub rounded microstructural units here designated as pedogenic ooids pisoids according to the size were determined in the matrix of the studied calcretes associated to pendants and clay rich rims surrounding them

dynamics of iron and calcium carbonate redistribution and palaeohydrology in middle eocene alluvial paleosols of the southeast ebro basin margin catalonia northeast spain abstract paleosols in the alluvial middle eocene of the ebro basin exhibit both the redistribution of iron groundwater and surface water gley and calcium carbonate the hydrological dynamics are reconstructed through gley features and location of carbonate accumulation together these allow a sophisticated analysis of the seasonal palaeohydrology of ancient sedimentary environments this analysis is performed on paleosols developed in marginal and distal floodplain and delta plain deposits derived from the southeast ebro basin margin the most proximal part of the floodplain sediments had moderate mainly vadose calcite redistribution with surface water gleying and fairly deep groundwater the more distal sediments have stronger calcite redistribution both of vadose and surface water gley environments these were frequently overprinted by groundwater related calcite indicating shallow and fluctuating groundwater in the most distal alluvial sediments both calcite redistribution and further soil formation were weak while groundwater was shallow

calcic paleosols of the plio pleistocene camp rice and palomas formations southern rio grande rift usa abstract seventy four separate paleosols are recognized in the plio pleistocene camp rice and palomas formations of the southern rio grande rift seven types of profiles are present a bw bk c a bt bk c bw bk c bt bk c bt k c bk c and ak calcic and petrocalcic horizons bk and k commonly overlain by argillic horizons bt dominate most paleosol profiles greater pore size and enhanced permeability typical of sandy and silty parent material favored the development of argillic horizons however micrite is the dominant form of calcium carbonate in bk and k horizons regardless of parent material grain size non calcified a horizons are typically lighter in color than adjacent subsurface pedogenic intervals due to their oxidized nature or lower clay content they are only rarely preserved probably due to erosion prior to final burial calcified root mats ak are not directly associated with other pedogenic horizons abundant horizontal roots suggest root mats formed under high water table conditions restriction of ak horizons to the distal piedmont lithofacies supports their development as spring or pond deposits near the toes of alluvial fans individual pedogenic horizons can often be traced hundreds of meters to kilometers normal and parallel to a basin s axis paleosol maturity and preservation potential are greatest for symmetrical basins especially where pedogenesis was far removed from active channels within the axial fluvial system or within distal piedmont settings of asymmetrical basins

space geodetic observation of expansion of the san gabriel valley california aquifer system during heavy rainfall in winter 2004 2005 1 starting early in 2005 the positions of gps stations in the san gabriel valley region of southern california showed statistically significant departures from their previous behavior station long moved up by about 47 mm and nearby stations moved away from long by about 10 mm these changes began during an extremely rainy season in southern california and coincided with a 16 m increase in water level at a nearby well in baldwin park and a regional uplift detected by interferometric synthetic aperture radar no equivalent signals were seen in gps station position time series elsewhere in southern california our preferred explanation supported by the timing and by a hydrologic simulation is deformation due to recharging of aquifers after near record rainfall in 2004 2005 we cannot rule out an aseismic slip event but we consider such an event unlikely because it requires slip on multiple faults and predicts other signals that are not observed

shallow seismic imaging of folds above the puente hills blind thrust fault los angeles california 1 high resolution seismic reflection profiles image discrete folds in the shallow subsurface 600 m above two segments of the puente hills blind thrust fault system los angeles basin california the profiles demonstrate late quaternary activity at the fault tip precisely locate the axial surfaces of folds within the upper 100 m and constrain the geometry and kinematics of recent folding the santa fe springs segment of the puente hills fault zone shows an upward narrowing kink band with an active anticlinal axial surface consistent with fault bend folding above an active thrust ramp the coyote hills segment shows an active synclinal axial surface that coincides with the base of a 9 m high scarp consistent with tip line folding or the presence of a backthrust the seismic profiles pinpoint targets for future geologic work to constrain slip rates and ages of past events on this important fault system

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

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 quaternary environmental changes in the upper las vegas valley nevada abstract five stratigraphic units and five soils of late pleistocene to holocene age crop out in dissected badlands on corn creek flat 30 km northwest of las vegas nevada and at tule springs nearer to las vegas the record is dominantly fluvial but contains evidence of several moister marsh forming periods the oldest unit b dates perhaps to the middle wisconsin and the more widespread unit d falls between 30 000 and 15 000 yr b p unit d therefore correlates with pluvial maximum lacustrine deposits elsewhere in the great basin standing water was not of sufficient depth or extent during either period to form lake strandlines between 14 000 and 7200 yr b p unit e standing surface water gradually decreased a trend also apparent in great basin pluvial lake chronologies during the same period groundwater carbonate cementation and burrowing by cicadas cicadae accompany the moist phase units after 7200 yr b p increased wind action decreased biotic activity and at least 25 m of water table lowering accompanied widespread erosion of older fine grained deposits based on pack rat midden and pollen evidence this coincides with major vegetation changes in the valley from sagebrush dominated steppe to lower mohave desertscrub

structure of the eastern seattle fault zone washington state new insights from seismic reflection data we identify and characterize the active seattle fault zone sfz east of lake washington with newly acquired seismic reflection data our results focus on structures observed in the upper 1 km below the cities of bellevue sammamish newcastle and fall city washington the sfz appears as a broad zone of faulting and folding at the southern boundary of the seattle basin and north edge of the seattle uplift we interpret the seattle fault as a thrust fault that accommodates north south shortening by forming a fault propagation fold with a forelimb breakthrough the blind tip of the main fault forms a synclinal growth fold deformation front that extends at least 8 km east of vasa park west side of lake sammamish and defines the south edge of the seattle basin south of the deformation front is the forelimb breakthrough fault which was exposed in a trench at vasa park the newcastle hills anticline a broad anticline forming the north part of the seattle uplift east of lake washington is interpreted to lie between the main blind strand of the seattle fault and a backthrust our profiles on the northern limb of this anticline consistently image north dipping strata a structural model for the sfz east of lake washington is consistent with about 8 km of slip on the upper part of the seattle fault but the amount of motion is only loosely constrained

electric fields preceding cloud to ground lightning flashes we analyzed in detail the electric field variations preceding the first return strokes of 80 cloud to ground lightning flashes in nine different storms observed at the nasa kennedy space center during the summers of 1976 and 1977 the electric field variations are best characterized as having two sections preliminary variations and stepped leader the stepped leader electric field change begins during a transition period of a few milliseconds duration marked by characteristic bipolar pulses the durations of stepped leaders lie most frequently in the range 6 20 milliseconds we infer from our measurements and critical review of the previous literature that there is only one type of stepped leader not the two types and often referred to in the literature

high resolution 3d seismic reflection imaging across active faults and its impact on seismic hazard estimation in the tokyo metropolitan area abstract we collected and interpreted high resolution 3d seismic reflection data across a hypothesized fault scarp along the largest active fault that could generate hazardous earthquakes in the tokyo metropolitan area the processed and interpreted 3d seismic cube linked with nearby borehole stratigraphy suggests that a monocline that deforms lower pleistocene units is unconformably overlain by middle pleistocene conglomerates judging from structural patterns and vertical separation on the lower middle pleistocene units and the ground surface the hypothesized scarp was interpreted as a terrace riser rather than as a manifestation of late pleistocene structural growth resulting from repeated fault activity devastating earthquake scenarios had been predicted along the fault in question based on its proximity to the metropolitan area however our new results lead to a significant decrease in estimated fault length and consequently in the estimated magnitude of future earthquakes associated with reactivation this suggests a greatly reduced seismic hazard in the tokyo metropolitan area from earthquakes generated by active intraplate crustal faults

investigation of growth fault bend folding using discrete element modeling implications for signatures of active folding above blind thrust faults 1 using the discrete element modeling method we examine the two dimensional nature of fold development above an anticlinal bend in a blind thrust fault our models were composed of numerical disks bonded together to form pregrowth strata overlying a fixed fault surface this pregrowth package was then driven along the fault surface at a fixed velocity using a vertical backstop additionally new particles were generated and deposited onto the pregrowth strata at a fixed rate to produce sequential growth layers models with and without mechanical layering were used and the process of folding was analyzed in comparison with fold geometries predicted by kinematic fault bend folding as well as those observed in natural settings our results show that parallel fault bend folding behavior holds to first order in these models however a significant decrease in limb dip is noted for younger growth layers in all models on the basis of comparisons to natural examples we believe this deviation from kinematic fault bend folding to be a realistic feature of fold development resulting from an axial zone of finite width produced by materials with inherent mechanical strength these results have important implications for how growth fold structures are used to constrain slip and paleoearthquake ages above blind thrust faults most notably deformation localized about axial surfaces and structural relief across the fold limb seem to be the most robust observations that can readily constrain fault activity and slip in contrast fold limb width and shallow growth layer dips appear more variable and dependent on mechanical properties of the strata

earthquake by earthquake fold growth above the puente hills blind thrust fault los angeles california implications for fold kinematics and seismic hazard 1 boreholes and high resolution seismic reflection data collected across the forelimb growth triangle above the central segment of the puente hills thrust fault pht beneath los angeles california provide a detailed record of incremental fold growth during large earthquakes on this major blind thrust fault these data document fold growth within a discrete kink band that narrows upward from 460 m at the base of the quaternary section 200 250 m depth to 82 at 250 m depth folding and uplift occur within discrete kink bands thereby enabling us to develop a paleoseismic history of the underlying blind thrust fault the borehole data reveal that the youngest part of the growth triangle in the uppermost 20 m comprises three stratigraphically discrete growth intervals marked by southward thickening sedimentary strata that are separated by intervals in which sediments do not change thickness across the site we interpret the intervals of growth as occurring after the formation of now buried paleofold scarps during three large pht earthquakes in the past 8 kyr the intervening intervals of no growth record periods of structural quiescence and deposition at the regional near horizontal stream gradient at the study site minimum uplift in each of the scarp forming events which occurred at 0 2 2 2 ka event y 3 0 6 3 ka event x and 6 6 8 1 ka event w ranged from 1 1 to 1 6 m indicating minimum thrust displacements of 2 5 to 4 5 m such large displacements are consistent with the occurrence of large magnitude earthquakes mw 7 cumulative minimum uplift in the past three events was 3 3 to 4 7 m suggesting cumulative thrust displacement of 7 to 10 5 m these values yield a minimum holocene slip rate for the pht of 0 9 to 1 6 mm yr the borehole and seismic reflection data demonstrate that dip within the kink band is acquired incrementally such that older strata that have been deformed by more earthquakes dip more steeply than younger strata specifically strata dip 0 4 at 4 m depth 0 7 at 20 m depth 8 at 90 m 16 at 110 m and 17 at 200 m moreover structural restorations of the borehole data show that the locus of active folding the anticlinal active axial surface does not extend to the surface in exactly the same location from earthquake to earthquake rather that the axial surfaces migrate from earthquake to earthquake reflecting a component of fold growth by kink band migration the incremental acquisition of bed dip in the growth triangle may reflect some combination of fold growth by limb rotation in addition to kink band migration possibly through a component of trishear or shear fault bend folding alternatively the component of limb rotation may result from curved hinge fault bend folding and or the mechanical response of loosely consolidated granular sediments in the shallow subsurface to folding at depth

the wilmington blind thrust fault an active concealed earthquake source beneath los angeles californiathe wilmington blind thrust fault an active concealed earthquake source beneath los angeles california

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

a revised classification of limestones abstract the most widely used classifications of limestones are now thirty years old and our appreciation of the diagenetic effects on limestone textures is now much greater a revision of the classifications of dunham 1962 and embry and klovan 1971 is offered and new diagenetic categories are proposed the confusing term lime mudstone is replaced by calcimudstone two categories of diagenetic textures are recognized non obliterative and obliterative the former contains limestones which are almost wholly composed of diagenetic cement cementstones and those whose fabric is the result of intergranular pressure solution condensed and fitted grainstones obliterative categories include limestones and dolomites whose original fabric has been destroyed sub categories include sparstones or dolosparstones crystals 10 m in size and microsparstones dolomicrosparstones crystals 4 10 m in size and dolomicrostones crystals m in size many diagenetic processes can produce fabrics which mimic depositional ones and these are reviewed

growth fault bend folding at southeast lost hills san joaquin valley california synsedimentary deformation has created complex structural and stratigraphic relationships along the west side of the san joaquin valley california this case study of lost hills anticline documents how a simple structural process fault bend folding results in complex growth stratigraphy lost hills anticline is a fault bend fold caused by a step in the lost hills thrust the thrust steps up to the northeast at 27 from a detachment at the base of the tertiary to an upper detachment in the reef ridge formation the lower and upper detachments are at 20 000 ft 6 km and 13 000 ft 4 km displacement on the lower detachment is approximately 4 mi 6 5 km folding consumes 1 2 mi 2 km and the remaining 2 8 mi 4 5 km continues northeast beneath the san joaquin va ley forward and back limb dips of 42 and 27 respectively are consistent with fault bend fold theory pliocene pleistocene sediments onlap a time transgressive unconformity on the northeast front limb but have uniform dip and thickness on the southwest back limb this characteristic growth fault bend fold geometry indicates basinal sediments rolled through an axial surface onto the back limb whereas contemporaneous sediments deposited above the front limb are undeformed basinal sediments onlapping the fold crest indicates fault slip is greater than ramp length onlapping growth sediments indicate folding began during deposition of the etchegoin formation 8 ma and continued to the late pleistocene or holocene corresponding to an average displacement rate on the lost hills thrust of 2 700 ft m y 820 m m y continuous burial of the fold during growth caused each material e physical point in the fold to be at its maximum depth of burial contours of equal density and seismic velocity are relatively flat so that the anticline causes little or no gravity anomaly and less than expected pullup in seismic time sections the geometry of growth sediments imaged on seismic lines indicates lost hills and adjacent anticlines grew by uniform displacement to the northeast with no significant rotation oblique convergence between the pacific and north american plates is manifested as synchronous but spatially separate transcurrent and compressive displacements thus lost hills anticline and perhaps other folds in the area are more appropriately described as compressive folds in a transpressive region than as transpressive folds duplicated lower tertiary strata beneath the lost hills kettleman hills trend are potential targets for petroleum exploration

quantitative paleoenvironmental and paleoclimatic reconstruction using paleosols abstract paleosols fossil soils are preserved throughout the geologic record in depositional settings ranging from alluvial systems to between basalt flows until recently paleosols were studied using primarily qualitative methods in recent years paleopedology has shifted from a largely qualitative field based on comparisons with modern analogues to an increasingly quantitative endeavor some of this change has been a result of applying existing techniques to new materials but many of the innovations have been the result of applying new techniques to new materials including thermodynamic modeling of soil formation isotope geochemistry and applications of empirical relationships derived from modern soils a variety of semi quantitative and quantitative tools has been developed to examine past weathering and pedogenesis and to reconstruct both paleoenvironmental and paleoclimatic conditions at the time that the paleosols formed though it is often not possible to achieve the same temporal resolution as with marine records for paleoclimatic reconstructions proxies based on paleosols are potentially a much more direct means of making paleoclimatic reconstructions because soils form at the earth s surface in direct contact with the atmospheric and climatic conditions at the time of their formation paleoclimatic and environmental properties that may be reconstructed using the new proxies include provenance weathering intensity mean annual precipitation and temperature during pedogenesis nutrient fluxes into and out of the paleosols the atmospheric composition of important gases including co2 and o2 the moisture balance during pedogenesis the soil gas composition reconstructed vegetative covering and paleo altitude

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

paleoenvironments of the earliest stone toolmakers gona ethiopia fluvio lacustrine sediments of the hadar and busidima formations along the northern awash river ethiopia archive almost three million years 3 4 to 0 6 ma of human evolution including the earliest documented record of stone toolmaking at 2 5 2 6 ma this paper brings together sedimentologic and isotopic evidence for the paleoenvironmental context of early hominids from both formations but with particular emphasis on the setting for the early toolmakers the pre 2 92 ma record hadar formation is characterized by low gradient fl uvial paludal and lacustrine deposition in an undissected topography most analogous to reaches of the modern middle awash river near gewane the gona area experienced repeated deep dissection and aggradation by the awash river starting between 2 92 and ca 2 7 ma and continuing through the top of the record at 0 6 ma busidima formation each aggradational succession is 10 20 m in thickness and fi nes upward from wellrounded conglomerates at the base to capping paleosols at the top during this period the ancestral awash represented by these fi ning upward sequences was dominantly meandering and fl owed northeast as it does today smaller channels tributary to the axial awash system are also extensively exposed in the busidima formation compared to the axial system conglomerates the tributary channels transported fi ner less mature volcanic clasts mixed with abundant carbonate nodules reworked from adjacent badlands stone artifacts oldowan 2 6 2 0 ma at the oldest archaeological sites are only associated with the axial awash system in the bedded silts or capping paleosols of the fi ning upward sequences the implements were made from rounded cobbles from the channels but manufacture and use of the tools was always away from the channel bars on the nearby sandy banks and silt dominated fl oodplains archaeological sites higher in the record acheulian 1 7 ma occur in similar axial river contexts as well as along tributary channels further removed from artifact raw material sources mature paleosols in the hadar and busidima formations are mostly pale to darkbrown vertisols typifi ed by abundant clay slickensides pseudo anticlinal and vertical fracturing and carbonate nodules such calcic vertisols are common in the region today demonstrating that the paleoclimate over the past 3 4 m y has been semi arid and strongly seasonal carbon isotopic results from pedogenic carbonates in the vertisols allow reconstruction of the proportion of c 3 plants trees and shrubs to c 4 plants grasses through time the 13

the magnitude 6 7 northridge california earthquake of january 17 1994 the most costly american earthquake since 1906 struck los angeles on 17 january 1994 the magnitude 6 7 northridge earthquake resulted from more than 3 meters of reverse slip on a 1 5 kilometer long south dipping thrust fault that raised the santa susana mountains by as much as 70 centimeters the fault appears to be truncated by the fault that broke in the 1971 san fernando earthquake at a depth of 8 kilometers of these two events the northridge earthquake caused many times more damage primarily because its causative fault is directly under the city many types of structures were damaged but the fracture of welds in steel frame buildings was the greatest surprise the northridge earthquake emphasizes the hazard posed to los angeles by concealed thrust faults and the potential for strong ground shaking in moderate earthquakes

stemflow influences on the formation of solution pipes in bermuda eolianite abstract soil filled pipes consisting of calcite casings were found in all of the eolianite formations of bermuda except the rocky bay formation the pipes are cylindrical in shape taper vertically downwards and occur in aggregated clusters they are most commonly exposed in areas of shoreline erosion in the form of soil filled cylindrical rims of calcite projecting up from the surrounding host eolianite in the early literature on the geology of bermuda it was suggested that these features were fossilized stumps of the bermuda palmetto tree sabal bermudiana in the study presented here this hypothesis is rejected on the basis of the morphology and stratigraphic position of these geomorphic features and their basal area and areal density compared with existing natural stands of s bermudiana the soil filled pipes of bermuda however cannot be simply explained as random solutional features that correspond to points where percolating soil water happened to converge and cause localized subsurface dissolution in situ measurements of stemflow from indigenous tree species suggest that the soil filled solution pipes of bermuda may be the product of stemflow drainage from particular tree species stemflow from s bermudiana was the most acidic with ph values as low as 4 29 compared to the gross rainfall in the open which had a ph of 5 58 the crowns of the bermuda cedar juniperus bermudiana and the subtropical tree species eugenia axillaris had the most significant funnelling effect stemflow volumes from individual trees representing these two species were 20 times greater than the volume of water that would have been collected in rain gauges with orifice areas equivalent to the trees trunk basal areas tree species capable of acidifying intercepted rainwater and funnelling large quantities down their trunks as stemflow may be the cause of subsurface dissolution loci extending vertically downward through bermuda s soluble eolianite formations stemflow from long lived trees representing these species could account for the diameters the cyclindrical shape the vertical orientation and the soil filled interiors of the pipes successive generations of these trees in preferred habitats could explain their clustered distribution and high areal densities 103 pipes ha

early phase of lightning currents measured in a short tower associated with direct and nearby lightning strikes 1 records of natural lightning currents of negative downward strokes measured at the base of a 60 m high instrumented tower are presented to denote patterns of their early phase such patterns in the records of five flashes and two upward unconnected leaders are discussed typical profiles of this early phase comprising the pre return stroke pulses of the current flowing upwards the tower in response to the downward stepped leader approaching the ground the concavity in the initial rising part of the return stroke waveform and the abrupt rise around the half peak of the first stroke waveform are remarked statistics are developed for the pre return stroke pulses of current to characterize their amplitude duration interval and risetime for both the first stroke and upward unconnected leaders which proved to be quite similar the ratio of the maximum pulse peak to first stroke peak is typically in the range 0 7 to 2 2 of the analyzed currents different from the value of around 10 found for the distant electric fields radiated by first stroke currents

paleoenvironmental interpretation of lake margin deposits using δ13c and δ18o results from early pleistocene carbonate rhizoliths olduvai gorge tanzania isotope analyses of lake margin rhizoliths from paleo lake olduvai ca 1 75 ma form the basis of a new model proposed here for interpreting stable isotope values of phreatic rhizolith carbonates average 18o and 13c values from rhizoliths formed in transgressive lacustrine waxy claystones are relatively low low 18o averages 5 5 reflect meteoric water values consistent with increased precipitation during wet periods that would increase the outflow of fresh water from subsurface seeps and shift a brackish groundwater zone lakeward low 13c averages 4 1 could indicate little atmospheric exchange high plant decay and or increased groundwater low 13c flow higher averages in 18o 3 6 and 13c 2 0 occur during dry periods and lake regressions earthy claystones when hydraulic head is reduced the lake recedes and water within the wetlands is subjected to intense evaporation and gas exchange with the atmosphere the isotope ratios of the rhizoliths from lowermost bed ii change in response to groundwater hydrology on milankovitch time scales but the isotopes also provide evidence of shorter term decadal to centennial scale climate fluctuations the orbitally driven climate changes are recorded faithfully by lithologic variations and stable isotope patterns

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

the 1987 whittier narrows earthquake sequence in los angeles southern california seismological and tectonic analysis the october 1 1987 whittier narrows earthquake m l 5 9 was located at 34 2 96 n 118 4 86 w at a depth of 14 6 0 5 km in the northeastern los angeles basin the focal mechanism of the mainshock derived from first motion polarities shows pure thrust motion on west striking nodal planes with dips of 25 5 and 65 5 respectively the aftershocks define an approximately circular surface that dips gently to the north centered at the hypocenter of the mainshock with a diameter of 4 6 km hence the spatial distribution of the mainshock and aftershocks as well as the focal mechanisms of the mainshock indicate that the causative fault was a 25 north dipping thrust fault striking west and is confined to depths from 10 to 16 km although most of the 59 aftershock focal mechanisms presented here document a complex sequence of faulting they are consistent with deformation of the hanging wall caused by the thrust faulting observed in the mainshock a cluster of reverse faulting events on north striking planes occurred within hours after the mainshock 2 km to the west of the mainshock the largest aftershock m l 5 3 occurred on october 4 and showed mostly right lateral faulting on the same north northwest striking plane within the hanging wall similarly several left lateral focal mechanisms are observed near the eastern edge of the mainshock rupture the earthquake and calibration blast arrival time data were inverted to obtain two refined crustal velocity models and a set of station delays when relocating the blast using the new models and delays the absolute hypocentral location bias is less than 0 5 km the mainshock was followed by nearly 500 locatable aftershocks which is a small number of aftershocks for this magnitude mainshock the decay rate of aftershock occurrences with time was fast while the b value was low 0 67 0 05 for a los angeles basin sequence

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

a cross section of the los angeles area seismically active fold and thrust belt the 1987 whittier narrows earthquake and earthquake hazard retrodeformable cross sections across the los angeles area interpret the pliocene to quaternary deformation to be a developing basement involved fold and thrust belt the fold and thrust belt is seismically active as evidenced by the 1987 whittier narrows earthquake ml 5 9 and the 1971 san fernando earthquake mw 6 6 the structural geology of the los angeles area is dominated by three major compressional uplift trends 1 the palos verdes anticlinorium and western shelf 2 the santa monica mountains anticlinorium and 3 the verdugo mountains san rafael hills and the san gabriel mountains these trends result from major thrust ramps off a detachment s at 10 15 km depth thrusts of the verdugo mountains san rafael hills and the san gabriel mountains reach the surface the other two uplifts are associated with blind thrusts compressional seismicity is concentrated along these thrust ramps the 1987 whittier narrows earthquake probably occurred on the elysian park thrust which underlies the santa monica mountains anticlinorium the thrust interpretation accounts for the geometry of the anticlinorium the seismological characteristics of the earthquake and the geometry of coseismic uplift the earthquake and aftershocks occurred within a structurally complex narrow zone of miocene and pliocene northwest trending faults that cross the anticlinorium at a high angle these northwest trending faults are interpreted to be reactivated faults now behaving as tears in the elysian park thrust and not the result of active right lateral deformation extending into the whittier narrows area our analysis suggests the whittier narrows earthquake sequence occurred within a structurally weakened zone along the elysian park thrust we also suggest that the whittier fault is not an important quaternary structure and may not be seismogenic the regional cross section is a nonunique solution and other possible solutions are considered multiple solutions arise from the presence of two intersecting compressional belts in the los angeles area the transverse ranges and the northern peninsular ranges the belts may be due to one or more regional detachments and the northern peninsular ranges may be northeast or southwest vergent the deformed top of the crystalline basement along the regional cross section requires a minimum of 15 0 km of north south convergence between the palos verdes hills and the san andreas fault regardless of the structural solution restoration of our cross section solution requires 21 4 km of north south convergence on top of the crystalline basement including 6 4 km of slip continuing offshore to make structures of the continental borderland and 29 7 km of convergence on the basal detachment geologic relationships suggest major shortening started between early and late pliocene time 2 2 4 0 ma which yields a minimum convergence rate of 3 8 6 8 mm yr between the palos verdes hills and the san andreas fault convergence rates for our solution range from 5 4 to 13 5 mm yr between the edge of the continental borderland and the san andreas fault slip rate estimates for the elysian park thrust along the eastern portion of the santa monica mountains anticlinorium are 2 5 5 2 mm yr if the 1987 whittier narrows earthquake is the characteristic earthquake along the 170 km long anticlinorium then recurrence intervals are estimated to be 5 6 12 1 years this recurrence is higher than the historic seismic record along the santa monica mountains anticlinorium and suggests that 1 our slip rates are too high 2 some crustal shortening is taken up aseismically and or 3 earthquakes larger than the 1987 whittier narrows earthquake will occur along the anticlinorium but less frequently

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

inclined shear restoration of growth folds we present a technique of progressively restoring seismic well and outcrop data as well as structural interpretations of contractional growth folds data restorations allow viewing of seismic well and outcrop information in the restored state providing a basis to evaluate whether structural interpretations are balanced and geologically reasonable validating structural interpretations helps to reduce the exploration risks associated with trap definition moreover sequential restorations reveal folding kinematics and define the geometry of structures at various stages in geologic history these sequential restorations may be used to define the timing of trap development relative to hydrocarbon migration and to determine how traps may have been modified by subsequent deformation to restore fold growth we use the approximation of inclined shear restoration vectors parallel to the axial surfaces of the folds and propagate them homogeneously downward in the section this choice of restoration vectors is commonly based on direct observation of axial surface orientations in data and generally satisfies appropriate balancing constraints inclined shear restoration works well for simple kink band migration folds and gives good approximations of restored geometries for folds developed by limb rotation and some more complex folding mechanisms we present examples from southern california the spanish pyrenees and angola

factors that control the development of fault bend versus fault propagation folds insights from mechanical models based on the discrete element method dem abstract we investigate the role and relative importance of a range of geometric and mechanical factors in the development of contractional fault related folds with an emphasis on defining the factors that promote the development of fault bend and fault propagation folds we construct a series of discrete element mechanical models in order to test the effects of fault dip bulk material strength mechanical layer anisotropy and spacing sedimentation rate and boundary conditions on the style of fault related fold that develops we find that fault bend folding is most favored at low fault ramp dips with thinly spaced mechanical layers and strong layer strength contrasts in contrast conditions that inhibit slip on a potential upper detachment surface such as increased friction and a fixed foreland boundary encourage the development of fault propagation folds additionally steeper fault dips more widely spaced mechanical layers and decreased layer strength contrast favor the increased localization of shear during the growth of structures this leads to structures that deform by a mixture of fault bend and fault propagation folding styles observations of the distortional strains that develop in the model provide insight into the relationship between the different deformation mechanisms such as flexural slip and localized shear which accommodate structural growth and ultimately determine fault related folding style thus these models provide a context for understanding how rock and fault properties influence whether structures evolve as fault bend or fault propagation folds or as combinations of these end members we apply these insights to interpret two natural examples from the offshore niger delta outer fold and thrust belt that exhibit changes in structural style through time as a result of changes in fault properties and syntectonic sedimentation

paleosols in shallow marine carbonate sequences abstract shallow water carbonate sequences in the geological record commonly exhibit numerous exposure surfaces reflecting emersion of sediment due to progradation processes and sealevel oscillations some ancient platform sequences contain hundreds of such surfaces associated with paleosols fossil soils typically calcretes a wide range of macro and microstructures are associated with these calcretes which are reviewed profile types and stages in profile development can be defined the nature of the exposure surface and associated paleosols can be used to identify the nature of the prevailing climate although hydromorphic effects related to marine flooding of the platforms can modify the paleosols a potentially important use of paleosols is to define the nature of sequence boundaries several studies have identified sets of paleosols with varying maturities reflecting changes in rates of accommodation space creation during sealevel rises and falls

active fault related folding beneath an alluvial terrace in the southern longmen shan range front sichuan basin china implications for seismic hazard abstract the devastating 2008 m w 7 9 wenchuan earthquake china demonstrates that the central and northern parts of the longmen shan are currently active evidence for active faulting and folding in the southern longmen shan however remains poorly documented in this paper we define the structural geometry fault kinematics and seismic hazard of the qiongxi thrust fault system qtf along the southern longmen shan range front by integrating deep and shallow seismic reflection data and geomorphic observations the qtf is a 50 km long north south trending set of faults and associated folds that exhibit geomorphic evidence of quaternary surface deformation geomorphic observations and seismic reflection data reveal that these faults dip steeply to the east and merge at depth with a blind west dipping thrust ramp the trend and reverse sense of slip along the qtf indicates that the structure accommodates east west crustal shortening based on uplift of stratigraphic horizons across the fault zone we define a late pliocene to early pleistocene fault slip rate of 0 2 0 3 mm yr and a middle pleistocene to present rate of 0 4 1 2 mm yr on the west dipping thrust ramp this ramp soles to a basal detachment in the triassic section at a depth of 4 5 5 5 km to the west this detachment steps down onto a blind northwest dipping thrust termed the range front thrust a rupture of the qtf in combination with the range front thrust could generate an m w 7 8 earthquake with average displacement of 5 7 m this type of earthquake source poses significant hazards to the adjacent highly populated sichuan basin online material figures detailing cross sections and 3d surfaces additional reflection profiles and dating information

desert aeolian origin of late cenozoic regolith in arid and semi arid southwestern australia abstract using a sedimentary geology approach the late cenozoic regolith of southwestern australia is interpreted as basement saprock and saprolite unconformably overlain by sequences of continental sedimentary deposits the sedimentary deposits which overlie saprock and saprolite are allochthonous variously altered and mainly partly to wholly desert aeolian sediments they include aeolian and fluvial sandy clayrock facies e g deposits which resemble the pallid and mottled zones of weathered precambrian bedrock of the traditional laterite profile altered aeolian sand and sandy dust facies e g hardpan lower parts of duplex soils red sandy silt claystone and calcrete valley fills altered sandy duststones e g bauxite laterite and ferricrete of the traditional laterite profile and bioturbated aeolian sand sheets e g sandplain soils lateritic sandplains of the traditional laterite profile and upper parts of duplex soils the allochthonous and desert aeolian views proposed here differ fundamentally from the traditional residual colluvial and erosional views consequently there is a need for both traditional and alternative views to be critically appraised from the perspective of sedimentary geology

multiple calcrete profiles in the tabernas basin southeast spain their origins and geomorphic implications the paper describes a sequence of pliocene to quaternary age calcretes developed within alluvial fan and fluvial gravels in the tabernas basin almeria province southeast spain calcrete profiles are described from sites adjacent to major tributaries of the rambla de tabernas six distinct calcrete units are identified within the basin these have variable distributions but have developed in an identifiable evolutionary sequence two pairs of calcrete units are widely present across the basin preserving two former land surfaces each of the former land surfaces has been planated and subsequently buried by alluvial fan or fluvial gravels a massive calcrete unit is present at the base of each gravel sequence immediately in contact with the underlying bedrock with a less well developed calcrete unit situated at the top of the gravel sequence the lowest two calcrete units within the basin are more spatially restricted and are confined to the floors and flanks of incised drainage lines the geochemistry macro and micromorphological properties and geomorphological positions of the calcrete units are outlined and on the basis of this information their mode of origin identified two main modes of calcrete genesis appear to be present massive calcretes situated in direct contact with bedrock are suggested to have formed by groundwater processes whilst calcretes situated at the top of gravel sequences are likely to have developed by pedogenic processes calcrete genesis is subsequently considered in the context of the reconstruction of the early phases of landscape development and is suggested to have been controlled by phases of uplift and stability within the tabernas basin 1998 john wiley sons ltd

shallow structure of the tangshan fault zone unveiled by dense seismic array and horizontal to vertical spectral ratio method abstract the tangshan fault zone has been considered to be a potential seismogenic fault from the devastating 1976 ms 7 8 tangshan earthquake based on the dense seismic array with interstation distance of 1 km and length of 40 km hvsr horizontal to vertical spectral ratio curves at 37 stations are obtained with the micro tremor seismic waveforms and the two dimensional quaternary sedimentary structures cross the tangshan fault zone are imaged by frequency to depth conversion results suggest that two significant seismic impedance interfaces at 100 m and 300 800 m depth are clearly revealed respectively the interface at 100 m depth can be attributed to the strong contrasts between the unconsolidated sand clay and semi consolidated silt which suggests very small variations and is well consistent with the seismic reflection interface from shallow seismic reflection exploration while the deeper interface at 300 800 m depth can be attributed to the quaternary sedimentary basement which increases from 300 to 800 m from the west to the east and is also consistent with the results in previous studies rapid variations 200 m of the quaternary sedimentary basement depth just beneath the fault zone well agrees with spatial characteristics of the fault revealed by deep seismic reflection profiling our work suggests that the tangshan fault zone has been significantly ruptured and modified by strong earthquake activities since quaternary this study also suggests that the hvsr method with dense seismic array is an efficient effective non invasive and low cost method for investigating active faults in shallow sediments it is useful for probing the active faults buried by thick sediments in the densely populated urban settings where seismic investigation with explosive sources are too hazardous to carry out

active blind thrusts beneath the tokyo metropolitan area seismic hazards and inversion tectonics 1 the tokyo metropolitan area underlain by neogene and quaternary sediments more than 5 km thick is currently deformed by blind thrusts that could generate hazardous earthquakes however their little geomorphic expression and dense urbanization make understanding of folds produced above them and recent deformation highly elusive here we show subsurface geometries of several active blind thrusts beneath this highly urbanized area based on tectonic landforms high resolution seismic reflection data and quaternary stratigraphy deep seismic reflection profiles corroborate the notion that steeply dipping blind thrusts are reactivated normal faults originally formed by middle miocene extensional tectonics despite very slow less than 0 1 mm yr late quaternary slip rates our work suggests the presence of previously unrecognized faults that pose seismic hazards to tokyo and outlying communities highlighting the need for additional information to define recent slip rates magnitude and recurrence of past earthquakes on them

high resolution seismic reflection imaging of growth folding and shallow faults beneath the southern puget lowland washington state abstract marine seismic reflection data from southern puget sound washington were collected to investigate the nature of shallow structures associated with the tacoma fault zone and the olympia structure growth folding and probable holocene surface deformation were imaged within the tacoma fault zone beneath case and carr inlets shallow faults near potential field anomalies associated with the olympia structure were imaged beneath budd and eld inlets beneath case inlet the tacoma fault zone includes an 350 m wide section of south dipping strata forming the upper part of a fold kink band coincident with the southern edge of an uplifted shoreline terrace an 2 m change in the depth of the water bottom onlapping postglacial sediments and increasing stratal dips with increasing depth are consistent with late pleistocene to holocene postglacial growth folding above a blind fault geologic data across a topographic lineament on nearby land indicate recent uplift of late holocene age profiles acquired in carr inlet 10 km to the east of case inlet showed late pleistocene or holocene faulting at one location with 3 to 4 m of vertical displacement south side up north of this fault the data show several other disruptions and reflector terminations that could mark faults within the broad tacoma fault zone seismic reflection profiles across part of the olympia structure beneath southern puget sound show two apparent faults about 160 m apart having 1 to 2 m of displacement of subhorizontal bedding directly beneath one of these faults a dipping reflector that may mark the base of a glacial channel shows the opposite sense of throw suggesting strike slip motion deeper seismic reflection profiles show disrupted strata beneath these faults but little apparent vertical offset consistent with strike slip faulting these faults and folds indicate that the tacoma fault and olympia structure include active structures with probable postglacial motion