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

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

differing views of soil and pedogenesis by two masters darwin and dokuchaev article i nfo charles darwin and vasily dokuchaev made early and important but quite different contributions to pedogenic theory their major contributions were bothwritten asbooks darwin s 1881theformationofvegetablemould through the action of worms with observation on their habits and dokuchaev s 1883 russian chernozem although most soil scientists are familiar with dokuchaev s legacy and lasting impact far fewer know about or value equally darwin s worm book dokuchaev s factorial approach to soil science drawn from observations across the eurasian steppe helped map classify and place economic value on soils while also providing key insight into their formation this approach gained visibility in the 1930s and 1940s when personnel at the u s department of agriculture and some aca demic pedologists recognized its utility for soil survey and for interpreting pedogenesis jenny s 1941 book factors of soil formation helped the model to gain acceptance and it eventually became entrenched as the core ped ogenic model for north america if not the world dokuchaev s legacy is tied to this model alternatively darwin s main contribution to the field was to shed light on soil processes particularly faunal mixing bioturbation and the textural sorting itcan produce although darwin s findingsfosteredanarray of multidisciplinary studiesonpedogenicprocesses during the ensuing 50 years his work languished in the broad shadow cast by dokuchaev s model in 1975 darwin s ideas reappeared insoil taxonomy associated with rudimentary biomantle concepts recently empowered with new concepts and language bioturbation concepts have gained considerable traction we briefly summarize the backgrounds of darwin and dokuchaev and compare their fundamentally different approaches to pedogenesis but insofar as dokuchaev s approach is more mainstream we emphasize darwin s for balance we show how darwin s model updated with current understandings of biomantle formation is allowing new questions to be asked about pedogenesis and landscape evolution and formerly intractable ones to be answered we stress the profound role of conceptual models in guiding explanatory thought and end by positing that both darwin s and dokuchaev s approaches while different in their basic structure and goals provide together a more complete view of pedogenesis than either can do singly

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

rapid rotation of southwest japan palaeomagnetism and k ar ages of miocene volcanic rocks of southwest japan summary miocene volcanic rocks have been sampled from the san in district in the central part of southwest japan reliable palaeomagnetic direction and k ar dating data have been obtained from 28 localities a clockwise deflection in declination d 38 5 is observed in the kawai formation having an age of 16 1 1 4 ma whereas a northerly declination is observed in formations younger than 14 2 0 6 ma these data indicate that southwest japan had ceased rotational motion by about 14 ma when compared with the amount of rotation estimated from cretaceous palaeomagnetic data it is apparent that more than 80 per cent of the overall clockwise rotation occurred between about 16 and 14 ma we conclude that the angular velocity of southwest japan about a rotation pole at 129 e 34 n reached 20 myr 1 at about 15 ma the corresponding rate of linear motion of the eastern part of southwest japan was 21 cm yr 1

3d simulations of multi segment thrust fault rupture thrust faults such as those that underlie the los angeles basin are typically segmented by tear faults that offset the thrust fault segments we perform 3d finite difference simulations of earthquake rupture to evaluate the effectiveness of these offsets in retarding rupture the simulations include orthogonal intersecting faults to model the interaction of the thrust fault segments with the tear fault for reasonable assumptions for fault segment length strength and stress drop rupture can jump offsets of up to 2 km if a tear fault is present consistent with observations of well studied thrust earthquakes absent a tear fault the maximum offset that can be breached is an order of magnitude smaller

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

relationship between tectonics volcanism sedimentation and basin development late cenozoic central part of northern honshu japan abstract a pronounced interplay between tectonics volcanism and sedimentation can be established in the volcanic arc region of the central part of northern honshu during the early miocene to quaternary interval four stages are recognized with the type of volcanism and sedimentation varying according to the tectonic stress regime 1 1 rifting stage 22 15 ma ns trending half grabens were formed under an extensional stress regime related to the rifting of the sea of japan basins were rapidly filled by intermediate to felsic volcanic products which are often associated with subaerial fluvial and lacustrine sediments 2 2 back arc basin opening stage 15 13 ma with more advanced stagess of rifting subsidence from crustal stretching resulted in marked marine incursion and an overall transgressive succession developed during this stage the back arc side of northern honshu subsided rapidly to middle bathyal environments at the same time associated with the opening of the sea of japan large volumes of regionally extensive mafic lavas and associated hyaloclastites were erupted within submarine grabens in contrast in the present backbone ranges felsic volcanism occurred within bathyal to shallow marine environments igneous rock types in this stage display a bimodal mafic felsic chemistry 3 3 transitional stage 13 2 4 ma a thermal subsidence substage 13 8 ma an intermediate stress regime developed which included phases of both weak extension and compression consequently grabens along the sea of japan ceased to form later lithospheric cooling eventually led to thermal subsidence of the back arc region and hemipelagic sediments mantled the previously active mafic volcanic apron volcanism at this time was mainly restricted to a narrow zone along the present backbone ranges where felsic to intermediate volcanic products accumulated within shallow marine environments b incipient compressional substage 8 2 4 ma hemipelagic sedimentation continued on the back arc side of northern honshu along the present backbone ranges the intermediate stress regime continued with formation of many large caldera structures associated with subaerial felsic volcanism 4 4 shortening deformation stage 2 4 ma present the change of the absolute direction of motion of the pacific plate at the early to middle pliocene changed the regional stress regime of the volcanic arc from intermediate to compression movement of back arc thrusts along the sea of japan coast began 2 3 ma and led to uplift of the volcanic arc continued compression in the volcanic arc produced narrow uplifted zones with associated basins bounded by reverse faults these basins were filled rapidly by terrestrial sediments containing upward coarsening sequences under this highly compressional tectonic stress regime volcanism became intermediate mafic in composition with large stratovolcanoes becoming prominent

new age constraints on counter clockwise rotation of ne japan summary the miocene basaltic andesitic lavas and cretaceous granites were sampled in the abukuma terrane of northeast honshu for geochronological and palaeomagnetic investigations to determine precise timing of counter clockwise rotation of ne japan characteristic component with unblocking temperature of 560 590 c is isolated from 26 sites of the takadate and ryozen formations and 12 sites of the cretaceous granites concordant northwesterly palaeomagnetic directions d 265 331 are observed in the cretaceous granites of the abukuma terrane the cretaceous mean palaeomagnetic direction from this terrane d 313 5 i 54 6 95 6 3 and n 20 is almost identical to oligo miocene 33 20 ma palaeomagnetic direction reported from the japan sea side of ne japan as well as to the cretaceous palaeomagnetic direction reported from the kitakami terrane this type of behaviour clearly suggests that the abukuma terrane underwent a coherent counter clockwise rotation as a part of ne japan after 20 ma conventional potassium k argon ar dating method performed on the plagioclase grains revealed distinctive ages of 14 44 0 88 and 16 52 0 82 ma for the takadate formation 11 sites and the ryozen formation eight sites respectively this type of behaviour indicates that two independent episodes of volcanic activities were probably responsible for the production of igneous rocks in the takadate and ryozen areas northerly declination is obtained from the takadate formation d 355 3 i 39 4 95 11 6 while slightly westerly deflected declination appears in the ryozen formation d 333 5 i 58 0 and 95 9 6 the here presented results therefore suggest that the province of ne japan was still experiencing a counter clockwise rotation at 16 5 ma however by 14 4 ma the rotational motion has completely ceased compared with the available palaeomagnetic and geochronological data from sw japan rotational motion in both provinces of ne and se japan has already been ceased by 14 ma thus any additional evidence regarding the complete cessation of rotational motions in ne japan will provide a key to understand the timing of opening in the japan sea

geomorphology kinematic history and earthquake behavior of the active kuwana wedge thrust anticline central japan 1 we combine surface mapping of fault and fold scarps that deform late quaternary alluvial strata with interpretation of a high resolution seismic reflection profile to develop a kinematic model and determine fault slip rates for an active blind wedge thrust system that underlies kuwana anticline in central japan surface fold scarps on kuwana anticline are closely correlated with narrow fold limbs and angular hinges on the seismic profile that suggest at least 1 3 km of fault slip completely consumed by folding in the upper 4 km of the crust the close coincidence and kinematic link between folded terraces and the underlying thrust geometry indicate that kuwana anticline has accommodated slip at an average rate of 2 2 0 5 mm yr on a 27 west dipping thrust fault since early middle pleistocene time in contrast to classical fault bend folds the fault slip budget in the stacked wedge thrusts also indicates that 1 the fault tip propagated upward at a low rate relative to the accrual of fault slip and 2 fault slip is partly absorbed by numerous bedding plane flexural slip faults above the tips of wedge thrusts an historic earthquake that occurred on the kuwana blind thrust system possibly in a d 1586 is shown to have produced coseismic surface deformation above the doubly vergent wedge tip structural analyses of kuwana anticline coupled with tectonic geomorphology at 10 3 10 5 years timescales illustrate the significance of active folds as indicators of slip on underlying blind thrust faults and thus their otherwise inaccessible seismic hazards

the role of bed parallel slip in the formation of blind thrust faults abstract a finite element code is used to model the mechanical behaviour of elastoplastic sedimentary layers that are deformed by the movement of a faulted deformable basement the propagation of the blind thrust fault is analysed in terms of the evolution of strain localization in the elastoplastic layers of strain softening and strain hardening materials the role of layer thickness bedding parallel slip and fault dip are also studied we found that if no interlayer slip is allowed blind thrusts tend to propagate with constant dip regardless of the rheological properties of the sediment layers however if bedding parallel slip is present a mechanical decoupling appears between layers allowing stress transfers along large distances and favouring the formation of backthrusts fault dip also has significant influence on the qualitative behaviour when interlayer slip is possible a steep fault will tend to split

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

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

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

why blind thrust faults do not propagate to the earth s surface numerical modeling of coseismic deformation associated with thrust related anticlines high fault tip stress concentrations are associated with coseismic slip on blind thrust faults and suggest that these structures should readily propagate to the earth s surface seismic profiles of blind thrust related earthquakes reveal diffuse zones of aftershocks surrounding the fault tip which are attributed to inelastic deformation such as flexural slip or extensional fracturing the complex interaction between blind thrust faults and secondary structures may control the evolution of blind thrust systems the influence of bedding plane slip on fault propagation is simulated with numerical models using the boundary element method we use two parameters to estimate the tendency for thrust fault propagation 1 the mode ii stress intensity factor and 2 the maximum coulomb stress near the fault tip calculations from both analyses suggest that shallow thrust faults may exhibit an increased tendency to propagate as a result of interaction with the earth s surface and slip along bedding planes above the fault tip and a decreased tendency to propagate due to slip along bedding planes at or below the fault tip our results demonstrate that the magnitude and style of inelastic deformation in active fault systems control fault propagation

the relationship between late cenozoic tectonic events and stress field and basin development in northeast japan the regional tectonic stress field basin development and crustal deformation of the ne japan arc in the interval between 32 ma to the quaternary can be synthesized based on dike vein and fault orientation data as well as on the compilation of the regional geology an extensional stress field became prevalent from 32 ma and major normal faulting started at 25 20 ma which resulted from back arc rifting normal faulting trending nearly parallel to the arc propagated from the present japan sea coast to the forearc side following the trenchward migration of the main volcanic field from 20 to 15 ma normal faults with an oblique trend to the arc developed due to the counterclockwise rotation of ne japan the rapid clockwise rotation of sw japan since 16 ma produced a nw se directed transtensional stress regime in the ne japan arc due to crustal stretching associated with this pull apart movement the back arc side of the ne japan arc subsided rapidly to middle bathyal environments after the termination of the opening of the japan sea at about 14 ma a neutral stress regime prevailed which included phases of both weak extension and compression lithospheric cooling eventually led to thermal subsidence of the back arc region and igneous underplating caused uplift of the axial zone of the volcanic arc the increase in velocity of the westward motion of the pacific plate at around 4 ma produced strong compression across the arc reactivating most of the miocene normal faults and uplifting the volcanic arc the greatest crustal shortening occurred in areas that were stretched the most in the miocene within the volcanic arc which implies tectonic inversion

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

amount of clockwise rotation of southwest japan fan shape opening of the southwestern part of the japan sea abstract paleomagnetic measurements have been carried out on welded tuffs ranging in age between 58 ma and 112 ma from yamaguchi and go river areas in the central part of southwest japan the new data together with those of younger igneous rocks published previously define the change of paleomagnetic field direction during the late mesozoic cenozoic period for southwest japan the paleomagnetic direction from this area has pointed 56 3 clockwise from the expected field direction estimated from apwp apparent polar wandering path of the whole of eurasia during the period between 100 and 20 ma in comparison with the expected one from the eastern margin of eurasia korea china siberia the cretaceous field direction of southwest japan shows the clockwise deflection by 44 49 these results establish that while the eastern margin of eurasia including southwest japan was rotated more or less with respect to the main part of eurasia during last 100 ma southwest japan was rotated clockwise through more than 40 with respect to the eastern margin of eurasia since 20 ma the large amount of rotation for southwest japan implies that it is rotated by an opening of the southwestern part of the japan sea which widens northeastward fan shape opening the tectonic feature of southwest japan and the japan sea is analogous to that of corso sardinia and the ligurian sea in the mediterranean indicating that the fan shape opening is a specific feature of the rifting of the continental sliver at the continental rim

growth of wheeler ridge anticline california geomorphic evidence for fault bend folding behaviour during earthquakes abstract unique landforms which we term fold growth terraces and alluvial fan ridges are interpreted to be produced by kink band migration of the front limb of wheeler ridge anticline an active fault bend fold in the southern san joaquin valley based on a folding related model these features form by onlap of alluvium across an active axial surface along the tip of a thrust wedge structure alluvial fan ridges record the sense of slip on the underlying blind thrust which is composed of both thrust and left lateral components our model predicts formation of fold growth terraces by upward propagation of an active axial surface through onlapped alluvium during a folding event to produce a prism of translated but otherwise undeformed sediment alternative hypotheses for the terraces include flexural slip faulting solifluction and erosive processes the width of fold limb between adjacent terraces is equivalent to fault displacement at depth which we interpret to be related to slip caused by earthquakes on the underlying blind thrust earthquake magnitudes derived from the terraces are much larger than expected for the length of the active fold and thrust belt and may indicate a kinematic link with the san andreas fault that drives shortening in the region

seismicity in the western great basin apparently triggered by the landers california earthquake 28 june 1992 within 24 hr after the landers earthquake there were three magnitude 3 4 events in western nevada and an unexpected widespread increase in the rate of small events based on combined catalogs for northern nevada southern nevada and southern california and a model that assumes statistical independence of events in these regions the probability of this happening in a 24 hr period by random chance is less than 10 12 per day therefore there is high statistical confidence that the increased seismicity was triggered by the landers event based on the statistical model we develop a list of 227 earthquakes in the first 83 days following the landers earthquake each of which has no more than 10 probability of occurring by random chance the suspect events are broadly distributed in regions that correlate with historical activity in the great basin the events are not uniquely associated with known volcanic activity or with zones that were previously active with microearthquakes or aftershock sequences the magnitudes of the largest triggered events appear to decrease with distance with time the number of suspect events decreases at a rate comparable to the rate of decrease of aftershocks of the landers and big bear earthquakes three of the most significant triggered events that that occurred were as follows mina 500 km from landers m 4 36 min after landers smith valley 590 km from landers m 3 4 56 min after landers and little skull mountain 280 km from landers m 5 6 22 3 hr after landers the evidence for triggering is particularly strong in the case of the little skull mountain event where an increased rate of microseismicity was evident as soon as small events could be identified in the coda of the landers earthquake we evaluate the seismic history for the past 3 yr to understand what was unique about the landers earthquake this includes identifying the previous earthquakes most likely to have caused the strongest shaking in various frequency bands based on a simple screening model the strains from the landers earthquake were uniquely large at low frequencies but at high frequencies they were exceeded frequently by small more local events thus we hypothesize that the cause of triggering is low frequency dynamic strains at periods of about 10 sec or greater and that there is a regional threshold that must be exceeded since previous events with calculated strains that were almost as strong were not causes of widespread triggering this mechanism for triggering satisfies the criterion of being a relatively rare phenomenon since it is likely to occur only when a large long period wave is radiated into an area where strain has been building slowly toward the point where faults are unstable

coseismic fold scarps and their kinematic behavior in the 1999 chi chi earthquake taiwan 1 discrete scarps that are created or reactivated during large earthquakes are a locus of concentrated hazard a number of the coseismic scarps activated in the 1999 chi chi earthquake are actually fold scarps which display several types of ground deformation characterized by localized folding and are distinct from classic fault scarps which form by a fault cutting the surface this paper documents and analyzes fold scarps that formed or reactivated in the 1999 chi chi taiwan earthquake our results show the chi chi fold scarps can be generally divided into two types 1 those associated with folding ahead of the tip of a blind thrust fault at shallow depths and 2 those associated with folding by kink band migration above fault bends at substantial depths ranging from 0 8 to 5 km the previously published trishear model can be applied to model the former type while a new curved hinge kink band migration model is provided to describe the behavior of the latter type a key feature of fold scarps of the second type is that hinge zones are typically wide 25 100 m relative to the displacement in a single earthquake 1 10 m which exerts a significant control on fold scarp morphology and evolution because the coseismic strains of both types of fold scarps display relatively wide deformation zones 10 100 m relative to fault scarps wider set back zones might be appropriate from a public policy point of view to alleviate the risk to structural damage and collapse resulting from permanent ground deformation