active fault related folding in the epicentral area of the december 25 1222 io ix mcs brescia earthquake northern italy seismotectonic implications abstract a dense grid of petroleum industry seismic reflection profiles coupled with field mapping exploratory trenching and geomorphic and structural analysis are used to characterize the quaternary growth history of the capriano del colle fault system one of several inferred active buried thrusts that extend across the po plain in northern italy shortening is characterized here by a deeply buried south vergent forethrust and an associated north vergent backthrust whose upward propagation is expressed by fault propagation folding near the surface structural interpretation based on seismic data suggests that strain is accommodated at very shallow levels by secondary flexural slip thrusts and reverse faults developed on synclinal flanks that emanate from active axial surfaces analysis of syntectonic growth strata document maximum rates of dip slip of 3 45 0 66 mm yr 1 6 myr 1 2 myr and 0 47 0 22 mm yr during a more recent time period 0 89 myr present a quarry excavation at capriano del colle allows a preliminary paleoseismologic analysis of coseismic surface faulting and liquefaction exposed near the core of an active mid pleistocene to holocene anticline these features are interpreted to be generated during strong local earthquakes consistent with the environmental effects and ground motions of an event similar to the december 25 1222 brescia earthquake io ix mcs this indicates for the first time that compressive folds and blind thrusts in the po plain are currently accommodating slow rates of modern contraction in an active zone of the southern alps that extends from lake garda to varese we thus argue that earthquakes similar to the december 25 1222 brescia event are likely to occur in this region and pose a direct threat to such a densely populated and developed area

slip rates on the chelungpu and chushiang thrust faults inferred from a deformed strath terrace along the dungpuna river west central taiwan the chelungpu fault produced the september 1999 m w 7 6 chi chi earthquake central taiwan the shortening rate accommodated by this structure integrated over several seismic cycles and its contribution to crustal shortening across the taiwanese range have remained unresolved to address the issues we focus our study on the chelungpu and chushiang thrust faults within the southernmost portion of the chi chi rupture area structural measurements and available seismic profiles are used to infer the subsurface geometry of structures the chushiang and chelungpu faults appear as two splay faults branching onto a common ramp that further north connects only to the chelungpu surface trace we survey a deformed strath terrace along the dungpuna river buried under a 11 540 309 years old fill deposit given this age the dip angles of the faults and the vertical throw determined from the offset of the strath terrace across the surface fault traces we estimate slip rates of 12 9 4 8 and 2 9 1 6 mm yr on the chelungpu and chushiang faults respectively these yield a total shortening rate of 15 8 5 1 mm yr to be absorbed on their common decollement at depth this total value is an upper bound for the slip rate on the chelungpu fault further north where the chushiang fault disappears and transfers shortening to adjacent faults combining these results with the recently constrained shortening rate on the changhua blind thrust reveals that all these frontal faults presently absorb most of the long term horizontal shortening across the taiwanese range they thus stand as the major sources of seismic hazards in this heavily populated area the return period of earthquakes similar to the chi chi event over a 80 km long stretch of the western foothills is estimated to be 64 years this value is an underestimate because it assumes that all the faults locked during the interseismic period slip only during such large events comparison with historical seismicity suggests that episodic aseismic deformation might also play a major role in accommodating shortening

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

horizontal coseismic deformation of the 1999 chi chi earthquake measured from spot satellite images implications for the seismic cycle along the western foothills of central taiwan 1 the 1999 chi chi earthquake mw 7 6 broke a major thrust fault along the western foothills of the central range of taiwan we have measured the horizontal coseismic displacement field by correlating optical satellite images acquired before and after the earthquake these data reveal the fault trace and a clockwise rotation of surface displacements toward the north with much larger displacements and strain in the hanging wall at the surface coseismic slip increases from 5 6 m near the epicenter to 10 11 m to the north in the epicentral area we observe a left lateral strike slip zone trending n125 e and farther north a fault zone trending n s with a right lateral component the data were modeled using elastic dislocations the fault geometry consists of a shallow

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

structural geology of a classic thrust belt earthquake the 1999 chi chi earthquake taiwan mw 7 6 abstract we document the structural context of the 1999 chi chi earthquake m w 7 6 in western taiwan which is one of the best instrumented thrust belt earthquakes the main surface break and large slip 3 10 m is on two segments of the shallow otherwise aseismic bedding parallel chelungpu sanyi thrust system which shows nearly classic ramp flat geometry with shallow detachments 1 6 km in the pliocene chinshui shale and mio pliocene kueichulin tungkeng formations however rupture is complex involving at least six faults including a previously unknown deeper thrust 8 10 km on which the rupture began we compare the coseismic displacements with a new 3d map of the chelungpu sanyi system the displacements are spatially and temporally heterogeneous and well correlated with discrete geometric segments of the 3d shape of the fault system geodetic displacement vectors are statistically parallel to the nearest adjacent fault segment and are parallel to large scale oblique fault corrugations the displacement magnitudes are heterogeneous at several scales which requires in the long term other non chi chi events or significant aseismic deformation the chelungpu thrust has a total displacement of 14 km but the area of largest chi chi slip 10 m is on a newly propagated north chelungpu chinshui detachment 0 3 km total slip which shows abnormally smooth rupture dynamics

spatio temporal slip and stress level on the faults within the western foothills of taiwan implications for fault frictional properties we use preseismic coseismic and postseismic gps data of the 1999 chi chi earthquake to infer spatio temporal variation of fault slip and frictional behavior on the chelungpu fault the geodetic data shows that coseismic slip during the chi chi earthquake occurred within a patch that was locked in the period preceding the earthquake and that afterslip occurred dominantly downdip from the ruptured area to first order the observed pattern and the temporal evolution of afterslip is consistent with models of the seismic cycle based on rate and state friction comparison with the distribution of temperature on the fault derived from thermo kinematic modeling shows that aseismic slip becomes dominant where temperature is estimated to exceed 200 at depth this inference is consistent with the temperature induced transition from velocity weakening to velocity strengthening friction that is observed in laboratory experiments on quartzo feldspathic rocks the time evolution of afterslip is consistent with afterslip being governed by velocity strengthening frictional sliding the dependency of friction l on the sliding velocity v is estimated to be ol o ln v 8 10 3 we report an azimuthal difference of about 10 20 between preseismic and postseismic gps velocities which we interpret to reflect the very low shear stress on the creeping portion of the decollement beneath the central range of the order of 1 3 mpa implying a very low friction of about 0 01 this study highlights the importance of temperature and pore pressure in determining fault frictional sliding

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

coseismic thrusting and folding in the 1999 mw 7 6 chi chi earthquake a high resolution approach by aerial photos taken from tsaotun central taiwan we used aerial photos taken before and after the 21 september 1999 m w 7 6 chi chi earthquake in central taiwan to measure the near field ground deformation a total of 12 pairs of images were processed with co registration of optically sensed images and correlation to produce a horizontal displacement map of a 10 km 10 km area near tsaotun using pairs of images with different viewing angles both the horizontal and vertical slip across the fault zone can be measured our measurements when resampled into lower resolution are consistent with lower resolution measurements of horizontal displacements obtained from spot images as well as with vertical displacements obtained from repeated leveling measurements and field observations horizontal strain is strongly localized along the chelungpu fault clpf and along a secondary scarp that runs parallel to the clpf about 2 km to the east the ailiao fold scarp alf this pattern closely matches the surface ruptures mapped in the field horizontal strain across clpf correlates remarkably well with the topographic features produced by long term deformation the cumulative horizontal shortening across the clpf and alf amounts to 4 9 0 4 and 6 1 0 6 m respectively and fault parallel displacement is 3 4 0 4 m the pattern of surface strain is consistent with the interpretation of the alf as a fold scarp formed over an active axial hinge zone this study shows that even in this compressional setting most surface deformation is localized within narrow fault zones or active axial hinges

lateral variations in the long term slip rate of the chelungpu fault central taiwan from the analysis of deformed fluvial terraces the chelungpu fault ruptured during the september 1999 mw 7 6 chi chi earthquake in central taiwan this event was characterized by coseismic displacements increasing along strike and updip from south to north previous studies suggested that such lateral variations also existed in the long term fault slip rate but this has not yet been clearly documented to address this we investigate deformed fluvial terraces along the choushui and tatu wu rivers in the southern and central segments of the thrust fault optical ages of 13 ka to 38 ka obtained for these terraces enable estimation of fault slip rates of 5 8 2 0 and 10 3 1 6 3 0 mm yr for the southern and central segments respectively we combine these findings with the fault slip rate determined by other authors for the northern segment statistical analysis of the data suggests lateral variations in the long term fault slip rate with values increasing toward the north this pattern in the slip rate averaged over approximately tens of thousand years is similar to that observed for coseismic displacements during the chi chi earthquake the similarities in the deformation pattern observed for one earthquake or cumulated over several events suggest that the chi chi earthquake could be characteristic of the earthquakes breaking the chelungpu fault with respect to slip distribution our results also allow for discussing the plausible evolution of major rivers draining the foothills of central taiwan over the last 40 60 kyr

the decompression of volcanic jets in a crater during explosive volcanic eruptions during explosive volcanic eruptions fragmented silicic magma and volatiles exit the vent with pressures typically in the range 10 100 atm and at the speed of sound of the mixture we show that for magmatic volatile contents no in the range 0 03 no 0 06 this has the approximate value 0 95 0 05 nort 12 where r is the gas constant and t is the eruption temperature this speed is nearly independent of the vent pressure and vent radius by assuming there is negligible mixing with the air we have modelled the decompression of such jets following eruption from the vent for free decompression into the atmosphere the velocity of the decompressed jet has the approximate value 1 85 0 05 nort 12 this is nearly independent of the eruption rate for decompression into a crater the process is more complex at low eruption rates with low vent pressure the material becomes underpressured as it rises in the crater a shock then forms in the crater this recompresses the magma volatile mixture which then issues from the crater as a relatively slow subsonic jet at atmospheric pressure as the eruption rate increases such shocks move towards the top of the crater and ultimately cannot form in the crater instead the material issues from the crater as a high speed supersonic jet the upward thrust provided by the crater walls on this high pressure jet then increases the upward velocity above that of a freely decompressing jet the presence of a crater may therfore cause collapse in relatively small eruptions whereas it may promote formation of buoyant eruption columns at higher eruption rates if a crater grows through erosion during an eruption column collapse will typically ensue even if the mass flux remains steady

1999 chi chi earthquake a case study on the role of thrust ramp structures for generating earthquakes the 21 september 1999 chi chi earthquake m w 7 6 occurred on east dipping shallow thrust faults that produced a high relief surface rupture extraordinary surface breaks appeared that could be clearly traced for about 100 km across many counties these thrust faults the chelungpu and shihkang are part of an active fold and thrust belt related to ongoing recent arc continent collision measurement of slip vectors along the earthquake rupture indicates that the orientation of the maximum shear stress changed from a westward direction n70 90 w on the chelungpu fault to a northwestward direction n30 40 w on the shihkang fault the stress field underwent a clockwise rotation of about 40 during the chi chi earthquake near rupture vertical displacements in the hanging wall of the shihkang fault have more cumulative displacement than on the chelungpu fault which is consistent with global positioning system gps measurements maximum vertical offset on the rupture was found to be about 10 m by the surficial rupture and gps measurements in addition analysis of crustal deformation by gps measurements on the hanging wall defines a coseismic uplift related to a fault ramp structure our synthesis of geological and geodetic analyses shows the importance of ramp structures associated with thrust faults for generating large earthquakes and provides a general framework for understanding earthquake in fold and thrust belts large surficial coseismic uplift and strong asperities appear to be a function of fault ramp geometry our analysis also indicates that in general ramp structures in fold and thrust belts may have a high potential in generating large earthquakes manuscript received 14 november 2000

velocity field of gps stations in the taiwan area abstract the 131 stations of the taiwan gps network were surveyed 4 6 times from 1990 to 1995 with dual frequency geodetic receivers the standard deviation of an observed baseline length with its linear trend removed is in the range of 6 10 mm for a 3 120 km long baseline the average rates of length change for all baselines of the network and those from nine continuously monitoring permanent stations are used in a least squares adjustment to estimate the velocities of the gps stations relative to paisha penghu situated at the chinese continental margin to the south of fengping in the northern coastal range the velocity vectors of stations in lanhsu lutao and the coastal range trend in the directions of 306 322 with rates of 56 82 mm yr in contrast there is a dramatic decrease in the rates to the north of fengping this may be caused by the motion along the ne sw trending thrusts which obliquely cut the northern coastal range a discontinuity of about 30 mm yr in the rates along with a remarkable change in the directions of station velocity is observed across the longitudinal valley then the moving directions gradually shift to the west for the stations in the western foothills in the kaohsiung pingtung coastal area the station velocities are even directed toward the southwest to the north of the peikang high the velocity vectors of the stations change direction from the west gradually to the north and finally to the east and southeast significant nw se extensional deformation is found in the ilan plain and northern taiwan in general the pattern of the velocity field for gps stations in the taiwan area is quite consistent with the directions of present day tectonic stress

a vertical exposure of the 1999 surface rupture of the chelungpu fault at wufeng western taiwan structural and paleoseismic implications for an active thrust fault we mapped and analyzed two vertical exposures exposed on the walls of a 3 to 5 m deep 70 m long excavation and a smaller 3 m deep 10 m long excavation across the 1999 rupture of the chelungpu fault the primary exposure revealed a broad anticlinal fold with a 2 5 m high west facing principal thrust scarp contained in fluvial cobbly gravel beds and overlying fine grained overbank deposits sequential restoration of the principal rupture requires initial failure on the basal east dipping thrust plane followed by wedge thrusting and pop up of an overlying symmetrical anticline between two opposing secondary thrust faults net vertical offset is about 2 2 m across the principal fault zone from line length changes we estimate about 3 3 m of horizontal shortening normal to fault strike the ratio of these values yields a total slip of 4 0 m and an estimate of about 34 for the dip of the fault plane below the excavation this value is nearly the same as the 35 average dip of the fault plane from the surface to the hypocenter restoration of the exposed gravelly strata and adjacent overbank sediments deposited prior to the 1999 event around the principal rupture suggests the possible existence of a prior event a buried 30 m wide anticlinal warp beneath the uplifted crest of the 1999 event is associated with three buried reverse faults that we interpret as evidence for an earlier episode of folding and faulting in the site the prior event is also recorded in the smaller excavation which is located 40 m south and is oriented parallel to the larger excavation radiocarbon dating of samples within the exposed section did not place tight constraints on the date of the previous event available data are interpreted as indicating that the previous event occurred before the deposition of the less than 200 14 c yr b p overbank sands and after the deposition of the much older fluvial gravels we interpret the previous event as the penultimate event relative to the 1999 chi chi earthquake we estimated the long term slip rate of the chelungpu fault to be 10 15 mm yr during the last 1 ma based on previously published retrodeformable cross sections this rate is however significantly higher than geodetic rates of shortening across the chelungpu thrust where two pairs of permanent global positioning system stations suggest 7 10 mm yr of shortening across the fault given the 4 m of average slip the long term slip rate yields an interseismic interval of between 267 and 400 yr for the chelungpu fault

paleoseismology of the chelungpu fault during the past 1900 years abstract the 1999 earthquake brought about 80 km long surface ruptures along the shihkang chelungpu and tajienshan faults central taiwan several trenches have been excavated across the chelungpu fault of the middle segment the surface ruptures display clear scarps ranging from 0 2 to 4 m high showing a complex geomorphic pattern due to coseismic faulting and folding in the study measurement of the vertical offset or structural relief was taken with reference to the hanging wall beyond the trishear deformation zone therefore we suggest that for the measurement of offset we should disregard the trishear zone and that structural relief on the hanging wall should be represented as a real vertical offset the net slip is then calculated from the structural relief and dip angle of the thrust on a vertical plane along the slip direction through the excavation of a pineapple field across the chelungpu fault we are able to provide evidence of at least four earthquake events for the past about 1900 years including the 1999 earthquake furthermore based on the radiocarbon dates and historical record the timing of the penultimate event is bracketed to be between 430 and 150 years ago and the average recurrence interval is less than 700 years these data indicate that the average slip rate is about 8 7 mm yr for the past 1900 years

growth of bending moment faults due to progressive folding insights from sandbox models and paleoseismological implications abstract bending moment normal faults bmf form during outer arc extension of a folding layer these faults observed in contractional and extensional tectonic environments can represent the only tectonic features potentially available for paleoseismological investigations i e trenching in order to date the movement of deeper structures yet the reliability of bmf as a paleoseismological feature is debated in terms of its potential to continuously record coseismic deformation and their relationship with progressive flexure we made use of analogue modeling in order to explore a simple model of progressive orthogonal flexure and the development of bmf we collected data on mode of bmf inception and growth by analyzing time series of cross section deformation measuring dip parallel bmf displacement profiles and constructing a three dimensional model of fault architecture by slicing of the experiment our results point to a model of bmf development typical of mechanically constrained faults where limits on self similar growth of faults is imposed once a maturity stage of displacement accumulation is attained the bmf can accumulate displacement until the structure reaches a maturity stage when further slip is inhibited this could result in an incomplete paleoseismological record of further fold movements however this fault stage can be recognized by the d shaped displacement profile typical of mature bmf a simple two dimensional constant area balancing model is proposed allowing us to derive incremental changes in flexuring given the area of the collapsed sector with bmf and the thickness of the faulted layer these results support the use of bmf as a paleoseismological proxy for deeper structures

continuous catchment scale monitoring of geomorphic processes with a 2 d seismological array 1 distributed activity of geomorphic processes with different spatiotemporal scales is hard to monitor in detail with conventional methods but might be detected with seismometers from july to september 2010 we deployed 14 seismometers to evaluate the ability of a two dimensional array with small interstation distances 11 km to continuously monitor geomorphic processes in a mountain catchment 370 km2 in taiwan spectral analysis of seismic records highlights different sources with high frequency content 1 hz consistent with hillslope and river processes using a common detection algorithm and a location technique based on the timing of seismic amplitude we have located 314 near surface events most of which 69 occurred during daily convective storms event activity was positively correlated with the precipitation intensity but this relation was not uniform in the catchment high resolution satellite images and air photos did not show geomorphic change during the study which did not have any episodes of extreme precipitation a majority of events 61 were collocated with preexisting geomorphic features landslide scars gullies within the uncertainty on location 9 of interstation distance the combination of event location and timing suggests a geomorphic source of recorded signals and most events had the seismic characteristics of rockfall debris avalanches or slides reactivation of prior erosion sites by such processes is difficult to detect with imagery but can possibly be resolved by seismic monitoring when proven this approach will allow a spatially comprehensive survey of geomorphic activity at the catchment scale with temporal detail sufficient to evaluate the exact meteorological conditions under which process events occur

prolonged seismically induced erosion and the mass balance of a large earthquake large earthquakes deform the earth s surface and drive topographic growth in the frontal zones of mountain belts they also induce widespread mass wasting reducing relief the sum of these two opposing effects is unknown using a time series of landslide maps and suspended sediment transport data we show that the mw7 6 chi chi earthquake in taiwan was followed by a period of enhanced mass wasting and fluvial sediment evacuation peaking at more than five times the background rate and returning progressively to pre earthquake levels in about six years therefore it is now possible to calculate the mass balance and topographic effect of the earthquake the choshui river has removed sediment representing more than 30 of the added rock mass from the epicentral area this has resulted in a reduction of surface uplift by up to 0 25 m or 35 of local elevation change and a reduction of the area where the chi chi earthquake has built topography for other large earthquakes erosion may evolve in similar predictable ways reducing the efficiency of mountain building in fold and thrust belts and the topographic expression of seismogenic faults prolonging the risk of triggered processes and impeding economic regeneration of epicentral areas

evidence for prehistoric coseismic folding along the tsaotun segment of the chelungpu fault near nan tou taiwan 1 taiwan s 1999 mw 7 6 earthquake generated over 85 km surface rupture along the chelungpu thrust fault paleoseismic studies at the shi jia site near nantou city reveal folding as the predominant form of deformation stratigraphic relations across the 1999 fold scarp show the style and degree of deformation caused by the penultimate event is similar to observed 1999 deformation a boring transect across the fold scarp provides additional evidence of an earlier earthquake investigations at the shi jia site revealed three prehistoric events accelerator mass spectrometry ams radiocarbon ages indicate that the penultimate earthquake occurred between 1160 and 1440 a d paleoseismic studies north of the shi jia site reveal much younger penultimate earthquakes suggesting a 1999 type event may not be characteristic along the tsaotun segment of the chelungpu fault

evolution of coseismic fault related folds induced by the chi chi earthquake a case study of the wufeng site central taiwan by using 2d distinct element modeling abstract coseismic fault propagation folds are generally associated with blind faults which are of considerable academic interest and have recently been recognized as critical for assessing seismic hazards the wufeng excavation site was characterized by a major east dipping basal thrust exhibiting a dip angle of 34 and 2 opposing vergent thrusts generated a pop up anticlinal fold in soil cover induced by the 1999 chi chi earthquake mw 7 6 in order to represent the soft soil behavior we set up a direct shear simulation test to obtain valid parameters for soil a series of 2d distinct element models possessing different bonding types and strengths was conducted to determine the deformation pattern near the surface and the evolution of the fault tip propagation the coseismic deformation features of soft and plastic soil cover like the mutative limb thickness complex ruptures and the overturned forelimbs in small scale caused by the propagation of the blind fault tip were accurately predicted by contact bond model in which grains were allowed to rotate and slip without cements breaking our results show that the alternative thrusts and pop up structure developed before the main basal thrust fault ruptured through the ground surface at the wufeng excavation site we evaluated the slipping distance of the main fault at approximately 6 m in wufeng during the 1999 chi chi earthquake

spatial slip distribution of the september 20 1999 chi chi taiwan earthquake mw7 6 inverted from teleseismic data the teleseismic waveforms of the mw7 6 september 20 1999 chi chi earthquake were examined to obtain the quick information on the fault rupture process the deconvolution results show the fault ruptured from south to the north and revealed the west movement of the hanging wall to the footwall on the eastern dipping plane the spatial slip distribution shows that the earthquake was mainly composed by a large asperity with a dimension of about 45km 15km the maximum slip was about 8 m located at about 45 km to the north of the epicenter the slip distributions obtained in this study have a good agreement with the observed surface breaks the comparable static and the dynamic stress drops of about 11 mpa in the large slip region indicate that the melting fluid pressurization might have taken place in this earthquake it reduces the dynamic friction and results in the large slip observed

the shir kuh pluton central iran magnetic fabric evidences for the coalescence of magma batches during emplacement abstract the 136 ma nw se elongate shir kuh pluton is one of the most poorly understood geological feature of central iran it is composed of peraluminous rocks corresponding to ilmenite bearing s type granites compositionally ranging from granodiorites to leucogranites these rocks show a continuum in their chemistry attributed to progressive differentiation this allows using the anisotropy of magnetic susceptibility technique to tempt establishing the relative chronology between emplacements of magma batches in the pluton the rather low susceptibility magnitudes km

estimating the return times of great himalayan earthquakes in eastern nepal evidence from the patu and bardibas strands of the main frontal thrust the return times of large himalayan earthquakes are poorly constrained despite historical devastation of cities along the mountain range definitive links between events and specific segments of the main frontal thrust mft are not established and paleoseismological records have not documented the occurrence of several similar events at the same location in east central nepal however recently discovered primary surface ruptures of that megathrust in the a d 1255 and 1934 earthquakes are associated with flights of tectonically uplifted terraces we present here a refined longer slip history of the mft s two overlapping strands patu and bardibas thrusts in that region based on updated geomorphic neotectonic mapping of active faulting two 1 3 km long shallow seismic profiles and logging of two river cut cliffs three paleoseismological trenches and several pits with constraints from 74 detrital charcoals and 14 cosmogenic nuclide ages the amount of hanging wall uplift on the patu thrust since 3650 450 years requires three more events than the two aforementioned the uplift rate 8 5 1 5mm yr thrust dip 25 5 n and apparent characteristic behavior imply 12 17 5m of slip per event on the bardibas thrust discrete pulses of colluvial deposition resulting from the coseismic growth of a flexural fold scarp suggest the occurrence of six or seven paleo earthquakes in the last 4500 50 years the coeval rupture of both strands during great himalayan earthquakes implies that in eastern nepal the late holocene return times of such earthquakes probably ranged between 750 140 and 870 350 years

progressive offset and surface deformation along a seismogenic blind thrust in the po plain foredeep southern alps northern italy here we present for the first time in the po plain foredeep northern italy the middle to late pleistocene growth history of an outcropping secondary fold and related faults whose progressive deformation over an intermediate time window 105 years is driven by an underlying seismogenic blind thrust we trenched and logged an outcropping decametric secondary anticline related to a deeper blind compressional structure which deforms fluvial sediments and an overlying loess paleosol sequence folded units were dated using radiocarbon and optically stimulated luminescence methods to the late pleistocene holocene and tentatively correlated with glacial interglacial phases occurring during the time interval from marine isotope stage 6 to the present a multistep retrodeformation of the fold allowed us to calculate uplift rates for this secondary and shallow anticline varying between 0 02 and 0 1 mm yr since circa 200 kyr trishear forward deformation modeling of the fold indicates that the amplification of the observed fold could be caused by two shallow thrusts formed through a break backward activation this generated a decametric surface fold whose most recent growth was associated with bending moment normal faulting in the crestal and forelimb region our observations demonstrate that near surface compressive tectonics can be caused by blind thrusting via a complex array of fault and folds upward strain propagation and generation of shallow low angle thrust and related folding seem to be mainly due to secondary fold related faulting according to an out of syncline thrusting mechanism

inferring a thrust related earthquake history from secondary faulting a long rupture record of la laja fault san juan argentina abstract trenches excavated across the surface rupture of the 15 january 1944 m w 7 san juan argentina earthquake show evidence for repeated rupture of la laja fault in the late pleistocene and holocene the 1944 rupture extended for about 7 km striking n35 e and dipping 42 e parallel to neogene bedding with a maximum east side up vertical displacement of 30 cm we interpret nine discrete colluvial wedges each capped by a soil as evidence for nine surface ruptures in the past 32 35 ka our topographic profile of the t3 terrace surface projected across the fault suggests these nine events collectively produced 10 3 m of dip displacement we used radiocarbon and optically stimulated luminescence dating to define the timing of past events yielding an average recurrence interval of 3 9 0 3 ka the average estimate for displacement per event is about 1 1 m compared with the 0 5 m of dip slip that occurred in 1944 using a fault related fold model that infers 28 m of shortening based on the axial surface migration for the t 3 terrace along with the 35 ka age of the terrace yields a shortening rate of 0 8 mm yr by the 1944 source thrust fault which is about 20 of the global positioning system constrained total shortening rate across the precordillera using this shortening rate and an assumed fault dip of 30 implies a fault slip rate of about 1 1 mm yr which combined with the average recurrence interval implies 4 m of average slip per event on the causative fault at depth online material unit descriptions from the trenches stepwise reconstruction from trench 1 and annotated photographs of the la laja region

observations of basin ground motions from a dense seismic array in san jose california we installed a dense array of 41 digital seismographs in san jose california to evaluate in detail the effects of a deep sedimentary basin and shallow sedimentary deposits on earthquake ground motions this urban array is located near the eastern edge of the santa clara valley and spans the evergreen sedimentary basin identified by gravity data average station spacing is 1 km with three stations initially spaced 110 m apart despite the high noise urban environment the stations of the array successfully triggered on and recorded small local earthquakes m 2 5 2 8 at 10 25 km distance and larger regional events such as the m 5 0 bolinas earthquake 90 km distance m 4 6 5 6 earthquakes near mammoth lakes 270 km distance m 4 9 5 6 events in western nevada 420 km distance and the m 7 1 hector mine earthquake 590 km distance maps of spectral ratios across the array show that the highest amplitudes in all frequency bands studied 0 125 8 hz are generally observed at stations farther from the eastern edge of the santa clara valley larger spectral amplitudes are often observed above the western edge of the evergreen basin snapshots of the recorded wavefield crossing the array for regional events to the east reveal that large low frequency 0 125 0 5 hz arrivals after the s wave travel from south to north across the array a moving window cross correlation analysis finds that these later arrivals are surface waves traveling from the south the timing and propagation direction of these arrivals indicates that they were likely produced by scattering of incident s waves at the border of the santa clara valley to the south of the array it is remarkable that the largest low frequency phases at many of the valley sites for regional events to the east are basin surface waves coming from a direction about 70 degrees different from that of the epicenters basin surface waves emanating from the eastern edge of the valley are also identified by the cross correlation analysis

spatial distribution of kimberlite in the slave craton canada a geometrical approach abstract exploration within the slave craton has revealed clusters of kimberlite intrusions commonly with internally consistent geochemical and temporal characteristics translation diagrams fry analysis allow an unbiased geometrical examination of the distance and direction between each kimberlite occurrence and all others in the database recurrent patterns are visually accentuated due to the square function in data density circular histograms quantify the azimuthal density of kimberlite at various distances for this study the database comprises the geographic position of 212 kimberlite occurrences of which 70 are from the lac de gras field ldg analyses are presented separately for the ldg data and for all non ldg data in order to test for regional variations and to avoid overwhelming the craton scale studies by the high density of ldg data empirical grouping of kimberlite locations results in delineation of five elliptical clusters that encompass all but four kimberlite occurrences clusters within the western part of the craton are elongate to the north northeast and align within a narrow zone western corridor elsewhere the clusters are elongate to the northwest or west northwest and appear to be arranged en echelon within a poorly defined north northwest trending zone central corridor geometrical spatial analyses of kimberlite locations highlight the craton scale pattern of emplacement within the two main corridors at regional and local scales individual intrusions are preferentially located towards the west northwest ca 280 and north northeast ca 015 of other intrusions and these orientations are interpreted to reflect upper mantle trends in magma generation at local scales 10 25 km kimberlite of the central and southern craton tends to be located to the northeast ca 045 and possibly weakly to the east northeast ca 070 of other intrusions and these orientations correspond to major crustal fractures systems it is proposed that kimberlite emplacement is controlled primarily by the interaction of elongate 280 and 015 source regions with near surface deviations influenced by crustal fracture systems the 015 trend evident at craton regional and local scales is parallel to a swarm of alkaline diabase dykes that are concentrated in a ca 30 km wide corridor passing through lac de gras a profound spatial association between significantly diamondiferous kimberlite and the margins of the dyke corridor suggests the corridor is the surface expression of a mantle depth structure it remains unclear whether the proposed mantle structure coincides with a diamond rich zone near the base of the lithosphere or delineates pathways favorable for diamond preservation during emplacement the linear array of kimberlite within the western craton forms a parallel corridor that may be an analogous mantle structure but which to date has failed to yield economic diamond concentrations

strombolian and effusive activity as precursors to phreatomagmatism eruptive sequence at maars of the pinacate volcanic field sonora mexico the pinacate volcanic field sonora mexico contains 400 cinder cones and eight maars it is noteworthy that most of the maar forming phreatomagmatic eruptions were immediately preceded by effusive and strombolian activity rather than occurring when magma first approached the surface the strombolian activity may have facilitated access of groundwater to the conduits in this arid region the field evidence suggests that phreatomagmatism is inhibited unless the magma flux is low relative to the rate of water supply and unless the top of the magma column has subsided probably below the water table the latter condition is difficult to prove in the absence of direct observation and alternative hypotheses involving disturbance of the conduit system are considered the spatial distribution of maars in the pinacate and the lithology of their associated tuffaceous ejecta both may reflect the course of an ancient river channel whose permeable gravels were pierced by the magmatic conduits

in vent column collapse as an alternative model for massive volcaniclastic kimberlite emplacement an example from the fox kimberlite ekati diamond mine nwt canada abstract the origins of massive poorly sorted fragmental kimberlite and kimberlite breccias in the diatremes of kimberlite volcanic pipes is currently poorly understood studies of the textural features of the major infill of the fox kimberlite ekati diamond mine nwt canada show that it may have formed as a result of the collapse of the explosive eruption column above the vent into the deep open pipe during the climactic stage of the eruption as the eruption intensity increased the column became critically overloaded with dense particles and rapidly collapsed onto itself unlike deposits typically preserved from column collapse events such as ignimbrites some portion of the collapsing mass was trapped in the deep pipe forming a distinct type of pyroclastic deposit in vent vent fill column collapse lapilli tuff this deposit has previously been tentatively identified as tuffisitic kimberlite tk or massive volcaniclastic kimberlite mvk based on broadly similar textural characteristics with these lithologies though every individual deposit will have formed under slightly different circumstances mass emplacement from column collapse is proposed as a viable model for massive volcaniclastic kimberlite emplacement based on extrapolation of modern volcanological processes

pleistocene alluvial deposits dating along frontal thrust of changhua fault in western taiwan the cosmic ray exposure point of view abstract to tackle the history of active thrusts it is necessary to open the observation window on time scales on the order of 104 105 years by studying the surface morphologies resulting from their activities because fluvial systems are particularly sensitive to recent environmental changes geomorphic features such as alluvial terraces are frequently used as markers to gauge tectonic deformation together with the measurement of cumulative displacements the chronological framework of emplacement and abandonment of these geomorphic markers is thus fundamental to determine long term fault slip rates in taiwan the geomorphic features associated with fault activity have been studied in detail with a high level of resolution however the use of deformed and partially preserved alluvial terraces is often hampered by the absence of well documented ages the purpose of this paper is two fold first we take the opportunity to review the chronological constraints that have been published in taiwan so far second we present how the cosmogenic dating method in situ produced 10be can be used to constraint the chronological framework of alluvial deposits over a pleistocene time scale thanks to a comparison of our cosmogenic derived ages with existing data we present a consistent regional chronological framework for the pakua tadu area along the changhua fault one of the most active frontal thrusts in the western foothills of the taiwan mountain belt we also discuss its relationships with global eustatism and its tectonic implications for the timing of propagation of the deformation front during the last 450 kyr