high resolution sequence stratigraphic analysis of late quaternary deposits of the changhua coastal plain in the frontal arc continent collision belt of central taiwan abstract detailed analysis of the sequence stratigraphy in 30 cores from the late quaternary deposits of the changhua coastal plain provides a 25 ka record of the depositional history of a region situated in the frontal arc continent collision belt of taiwan twenty five lithofacies and nine facies associations are recognized an alluvial plain was the dominant feature during a pronounced sea level fall at 25 20 ka ago between 20 and 10 ka alluvial fans were deposited along the western front of the pakua tableland during movement along the changhua fault and a local sag basin was produced no other deposits in the seaward area are recognized between 20 and 16 ka or in the landward area between 20 and 10 ka between 16 and 6 ka the changhua coastal plain experienced rising sea level and retrogradational deposition from 16 to 10 ka estuarine deposits lower transgressive systems tract were deposited and the base of these deposits defines the last glacial maximum lgm unconformity sequence boundary now preserved in four regions to the east of the plain from 10 to 6 ka shelf shoreface and lagoonal deposits upper transgressive systems tract were laid either on top of the estuarine deposits or directly on the lgm unconformity surface by about 7 6 ka sea level had reached its peak maximum flooding surface and shelf and marginal marine sediments covered most of the study area after 6 ka the highstand was dominated by progradation of offshore transition shoreface lagoonal tidal channel tidal flat and alluvial plain deposits one thousand year old fluvial deposits in incised channels are preserved in the central part of the changhua coastal plain since then fluvial channels shifted southward to the present day chuoshuei river the depositional patterns in this tectonically active area during the last interglacial period reflect the complex interplay between high frequency sea level fluctuations tectonics subsidence and uplift and autocyclic processes in the seaward area however fluctuations in sea level were clearly the dominant factor in controlling the nature of depositional facies

current stress state and principal stress rotations in the vicinity of the chelungpu fault induced by the 1999 chi chi taiwan earthquake 1 we carried out various stress measurements in boreholes penetrating the northern segment of the chelungpu fault drilled about 5 years after the 1999 chi chi taiwan earthquake in the possible depth range of the chelungpu fault three major fault zones were encountered clearly recognizable principal stress rotations in the vicinity of the shallowest major fault zone at 1133 m depth in hole b suggest that this fault zone ruptured during the 1999 earthquake moreover the fault s rupturing altered the stress state in the area surrounding this fault zone in this paper we constrain the possible magnitudes of the current principal horizontal stresses around the fault zone and show that the current stress state belongs to a normal or strike slip fault regime therefore the stress state was changed from that of a reverse fault regime by the rupturing

structure of the 1999 chi chi earthquake rupture and interaction of thrust faults in the active fold belt of western taiwan abstract we summarize the structural characteristics of the surface ruptures of the 1999 mw 7 6 chi chi earthquake in western taiwan and discuss the geological interaction of the chelungpu fault with adjacent faults based on geological investigations seismological analyses and gps measurements of surface co seismic displacements we describe the regional 3 d fault plane and slip distribution of the chi chi earthquake and compare these to the geological features of the chelungpu fault we find that one key feature of the chelungpu fault is the stratigraphy controlled slip surface at the level of the uppermost few kilometers the chelungpu fault slip plane generally follows the bedding plane of the pliocene chinshui shale the second key feature of the chelungpu fault is the difference in structurally geometric configuration between its northern and southern segments the northern fengyuan segment shows a bedding parallel thrust fault within east dipping strata in both footwall and hanging wall in contrast the southern tsaotun segment exhibits east dipping strata are overthrust onto flat lying recent alluvial deposits these two features not only explain a hinterland imbricate thrusting on the hanging wall of the fengyuan segment but also explain the change in strike of the chi chi surface ruptures at the northern end the southern end of the 1999 chi chi rupture is interpreted to be linked to a series of nw trending strike slip faults in particular we propose that the luliao strike slip fault served as the lateral ramp of the chelungpu fault and the gukeng strike slip fault acted as a barrier to end the southern propagation of the 1999 rupture geomorphic features and paleoseismological data indicate that the range front chelungpu fault has generated large earthquakes during the last several thousand years alternatively in the miaoli area to the north and the chiayi area to south historical earthquakes as well as active geomorphic features are not restricted on the range front thrust faults instead more complicated structures including tightly spaced folds duplex structures and strike slip faults are involved in seismogenic processes a more detailed investigation of regional structural characteristics is needed for mitigation against the seismic hazards in the 300 km long active fold belt in western taiwan where several damaging large earthquakes have been documented during the last century

core log integration studies in hole a of taiwan chelungpu fault drilling project summary taiwan chelungpu fault drilling project tcdp was initiated to understand the physical mechanisms involved in the large displacements of the 1999 taiwan chi chi earthquake continuous measurements of cores including laboratory work and a suite of geophysical downhole logs including p and s wave sonic velocity gamma ray electrical resistivity density temperature electrical borehole images and dipole shear sonic imager were acquired in hole a over the depth of 500 2003 m integrated studies of cores and logs facilitate qualitative and quantitative comparison of subsurface structures and physical properties of rocks a total of 10 subunits were divided on the basis of geophysical characteristics generally formation velocity and temperature increase with depth as a result of the overburden and thermal gradient respectively gamma ray resistivity formation density shear velocity anisotropy and density derived porosity are primarily dependent on the lithology zones with changes of percentage of shear wave anisotropy and the fast shear polarization azimuth deduced from dipole shear imager dsi are associated with the appearance of fractures steep bedding and shear zones the fast shear wave azimuth is in good agreement with overall dip of the bedding approximately 30 towards se and maximum horizontal compressional direction particularly in the kueichulin formation showing strong shear wave velocity anisotropy bedding parallel fractures are prevalent within cores whereas minor sets of high angle nnw sse trending with n and s dipping fractures are sporadically distributed the fault zone at depth 1111 m fza1111 is the chi chi earthquake slip zone and could be a fluid conduit after the earthquake the drastic change in fast shear wave polarization direction across the underlying non active sanyi thrust at depth 1710 m reflects changes in stratigraphy physical properties and structural geometry

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

structural heterogeneity of the longmenshan fault zone and the mechanism of the 2008 wenchuan earthquake ms 8 0 we determined detailed 3 d images of p and s wave velocity vp vs and poisson s ratio in and around the longmenshan lms fault zone by using a large number of p and s wave arrival times from the aftershocks of the 2008 wenchuan earthquake ms 8 0 and other local events our results show that the structure of the lms fault zone north of the wenchuan main shock is very different from that south of the main shock the southern section of the lms fault zone contains a broad zone with low vp low vs and high anomalies while the northern segment exhibits more scattered heterogeneities corresponding to most of the aftershocks which occurred there a prominent low vp low vs and high anomaly exists directly beneath the wenchuan main shock hypocenter suggesting that in addition to compositional variations fluid filled fractured rock matrices may exist in the lms fault zone which may have influenced the generation of the large wenchuan earthquake our tomographic results provide sound seismic evidence for the hypothesis that an upward intrusion of lower crustal flow occurred along the lms fault zone in addition most small earthquakes before the 2008 wenchuan main shock occurred around the guanxian jiangyou fault while the wenchuan aftershocks were mainly concentrated on the yingxiu beichuan fault suggesting that the rupture process of the wenchuan earthquake may belong to an out of sequence thrusting event a suggestion which is in good agreement with the results from geological surveys and also quite similar to the rupture processes of the 1999 chi chi earthquake m 7 5 and the 2005 kashmir earthquake m 7 6 a few aftershocks occurred close to the blind guangyuan dayi fault in the sichuan basin suggesting that this blind fault was also ruptured by the wenchuan earthquake consistent with geological surveys

co seismic ruptures of the 12 may 2008 ms 8 0 wenchuan earthquake sichuan east west crustal shortening on oblique parallel thrusts along the eastern edge of tibet abstract the m s 8 0 wenchuan earthquake which devastated the mountainous western rim of the sichuan basin in central china produced a surface rupture over 200 km long with oblique thrust dextral slip and maximum scarp heights of 10 m it thus ranks as one of the world s largest continental mega thrust events in the last 150 yrs field investigation shows clear surface breaks along two of the main branches of the ne trending longmen shan thrust fault system the principal rupture on the nw dipping beichuan fault displays nearly equal amounts of thrust and right lateral slip basin ward of this rupture another continuous surface break is observed for over 70 km on the parallel more shallowly nw dipping pengguan fault slip on this latter fault was pure thrusting with a maximum scarp height of 3 5 m this is one of the very few reported instances of crustal scale co seismic slip partitioning on parallel thrusts this out of sequence event with distributed surface breaks on crustal mega thrusts highlights regional ew directed present day crustal shortening oblique to the longmen shan margin of tibet the long rupture and large offsets with strong horizontal shortening that characterize the wenchuan earthquake herald a re evaluation of tectonic models anticipating little or no active shortening of the upper crust along this edge of the plateau and require a re assessment of seismic hazard along potentially under rated active faults across the densely populated western sichuan basin and mountains

neogene quaternary magmatism and geodynamics in the carpathian pannonian region a synthesis in the carpathian pannonian region in neogene times westward dipping subduction in a land locked basin caused collision of two lithospheric blocks alcapa and tisia with the southeastern border of the european plate calc alkaline and alkaline magmatism was closely related to subduction rollback collision and extension from the spatial distribution of the magmatic activity four segments can be defined western segment magmatism occurring on the alcapa block central segment magmatism occurring on both alcapa and the tisia blocks south eastern segment and interior segment both on the tisia block most calc alkaline magmatism in the region resulted from melting of a heterogeneous asthenospheric mantle source modified by addition of fluids and sediment assimilation and fractionation processes at shallow crustal levels occurred in most of the segments strongly masking the deeper source processes long term subduction rollback and or delamination led to contamination of the asthenosphere beneath the western segment here large volume partial melts of the contaminated mantle caused underplating and crustal anatexis leading to mixing of mantle derived calc alkaline magmas with crustal melts in the central segment calc alkaline magmas were formed by subduction and rollback followed by back arc extension and slab breakoff avariable mantle source is indicated in the back arc setting and larger amount of fluid induced metasomatism source enrichment and assimilation nearer to the trench in the interior segment evolution from typical calc alkaline magmas to adakite like ones was related to extension due to fast rotations and transtensional tectonics here calc alkaline magmas formed by decompression melting of a heterogeneous crust mantle lithosphere while adakite like melts resulted from fliud dominated melting of the lithosphere along the south eastern segment slab breakoff was responsible for the generation of typical calcalkaline magmas but in the extreme south of the segment shallow level tearing of the slab followed breakoff strike slip tectonics allowed the rise of hot asthenosphere and generation of adakite like magmas via slab melting along the torn edge of the east european plate alkalic basaltic volcanism with an oib like asthenosphere source followed the calc alkaline stage

finite source modeling of the 1999 taiwan chi chi earthquake derived from a dense strong motion network the 1999 chi chi earthquake m w 7 6 20 september 1999 17 47 15 9 utc located at 23 853 n 120 816 e and depth of 7 5 km inicted severe re gional scale damage to taiwan the strong motion waveeld was captured by a dense network of stations with average station spacing of 5 km which represents the most complete strong motion dataset to date to use to study the kinematic source process of an earthquake we inverted velocity waveforms recorded by 21 stations for the spatial variation in slip on a planar fault model composed of 416 subfaults each with a dimension of 3 5 km by 3 5 km the planar model has a strike of n5 e and a dip of 30 e and the inversion solves for the direction and magnitude of the slip to account for possible temporal source complexity we allowed each subfault to slip within 10 overlapping time windows each with a duration of 3 sec the results show that the source is composed primarily of three major asperities the rst of which is mainly dip slip extending from the hypocenter to the northern end of the surface rupture in this asperity slip occurred in two pulses separated in time by 5 sec the dislocation rise time for each pulse is short 3n4 sec yielding an approximate av erage slip velocity of 80 cm sec the second asperity is located at shallow depth near the northern end of the rupture where very large ground velocities were observed this asperity is on average oblique and shows a temporal rake rotation from pure dip slip to strike slip the rotating rake suggests a low initial shear stress on the northern end of the fault slip in this asperity is dominated by a large pulse with a dislocation rise time of 8 sec a station near the northern end of the surface rupture recorded a peak velocity of 390 cm sec which we nd to be due to the constructive interference of energy radiated from the rst two asperities the third asperity is located south of the epicenter the total moment from the three asperities is 4 1 10 27 dyne cm which was released over a period of 30n35 sec within an area of 900 km 2 synthetics calculated from the three asperity model explain 85 of the data and represent 98 of the total variance reduction our results indicate that slip is conned to the shallow region of the fault deep slip patches are less constrained and that the slip distribution may be representative of fault segmentation along the chelungpu fault system

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

kinematic and dynamic fault slip analyses implications from the surface rupture of the 1999 chi chi taiwan earthquake the 1999 chi chi earthquake created a 100 km long surface rupture on the chelungpu thrust fault in taiwan due to convergence between the phillipine sea and eurasian plates fault slip measurements were made by several researchers from the entire length of the rupture directly following the earthquake and can thus be attributed to a single tectonic event conventional fault slip analyses are applied to these data and the results are compared with independent seismological and kinematic observations unlike many fault slip analyses complications due to multiple deformations can be categorically excluded and the results can be evaluated from the seismological and plate movement data kinematic analyses of fault slip data that are weighted by displacement show sub horizontal nw se shortening that is parallel to the plate convergence vector a single fault plane solution satisfies almost all the data right dihedra and trihedra solutions also satisfy almost all the surface rupture measurements and give 1 in a nw se orientation that is similar to the results of stress inversion and to inversion from earthquakes in the chi chi earthquake sequence despite criticisms of fault slip analysis methods these results show that fault slip analyses from data collected along major faults which have not witnessed multiple deformation events can be valid homogeneous strain and stress states exist in the sense that kinematic and dynamic solutions can be found that fit essentially all the data and these solutions have tectonic significance

the chi chi earthquake sequence active out of sequence thrust faulting in taiwan we combined precise focal depths and fault plane solutions of more than 40 events from the 20 september 1999 chi chi earthquake sequence with a synthesis of subsurface geology to show that the dominant structure for generating earthquakes in central taiwan is a moderately dipping 20 to 30 thrust fault away from the deformation front a second subparallel seismic zone lies about 15 kilometers below the main thrust these seismic zones differ from previous models indicating that both the basal decollement and relic normal faults are aseismic

taiwan orogeny thin skinned or lithospheric collision abstract the taiwan orogeny is young and presently very active it provides an excellent environment for studying ongoing orogenic processes especially since the region is monitored intensively with dense seismological and geodetic networks and new studies aiming at deciphering shallow and deep structures in and around taiwan have been recently conducted or are being planned the available data can be used continually to test critically hypotheses of the taiwan orogeny hypotheses dealing with the mechanics of mountain building are basic to the understanding of taiwan orogeny and are particularly amenable to testing the widely cited thin skinned tectonics hypothesis was formulated to explain mainly the geologic and relatively shallow

zagros phantom earthquakes reassessed the interplay of seismicity and deep salt flow in the simply folded belt unraveling the contributions of main shock slip aftershocks aseismic afterslip and postseismic relaxation to the deformation observed in earthquake sequences heightens our understanding of crustal rheology triggering phenomena and seismic hazard here we revisit two recent earthquakes in the zagros mountains iran which exhibited unusual and contentious aftereffects the mw 6 earthquakes at qeshm 2005 and fin 2006 are both associated with large interferometric synthetic aperture radar insar signals consistent with slip on steep reverse faults in carbonate rocks of the middle sedimentary cover but small aftershocks detected with local seismic networks were concentrated at significantly greater depths this discrepancy can be interpreted in one of two ways either 1 there is a genuine vertical separation between main shock and aftershocks reflecting a complex stress state near the basement cover interface or 2 the aftershocks delimit the main shock slip and the insar signals were caused by shallow updip afterslip phantom earthquakes with very similar magnitudes mechanisms and geographical positions as the original earthquakes here we show that main shock centroid depths obtained from body waveform modeling which in this instance is the only method that can reveal for certain the depth at which seismic slip was centered strongly support the first interpretation at qeshm microseismic aftershock depths are centered at the level of the hormuz formation an infracambrian sequence of intercalated evaporitic and nonevaporitic sediments these aftershocks may reflect the breaking up of harder hormuz sediments and adjacent strata as the salt flows in response to main shock strain at the base of the cover this work bolsters recent suggestions that most large earthquakes in the zagros are contained within carbonate rocks in the midlower sedimentary cover and that the crystalline basement shortens mostly aseismically

characteristics of a tsunamigenic megasplay fault in the nankai trough 1 slip on the shallow part of a megasplay fault that is an out of sequence thrust and branch of the main subduction plate boundary can cause devastating tsunamis after earthquakes we analyzed the three dimensional geometry including dip amount and azimuths roughness distributions and thickness variations of the shallow part of a megasplay fault in the nankai trough using a three dimensional seismic data set the fault is divided into three zones based on its geometry thick smooth and simply convex in the east complexly curved in the middle and thin and kinked in the west results of scientific drilling indicate that the eastern region of the fault is most active and local heterogeneities in fault geometry including roughness and thickness may control the slip on this part of the fault the present findings can be used to evaluate the risk of future tsunamis arising from movement on shallow thrust faults at subduction margins

spatial and temporal distribution of deformation at the front of the andean orogenic wedge in southern bolivia new observations from an active orogenic wedge help link the seismotectonic behavior of individual faults to wedge deformation rates and patterns over multiple timescales we provide the first detailed constraints on the distribution and timing of deformation at the front of the andean orogenic wedge in southern bolivia where a recent study suggests that great mw 8 earthquakes could rupture the master fault underlying the wedge we use stratigraphic relationships across fault related folds and elastic dislocation modeling of seismic reflection horizons to obtain probabilistic estimates of wedge front fault ages and slip rates our analyses reveal that at least half of the previously determined gps based wedge loading and quaternary whole wedge shortening rates are absorbed across a 20 40 km wide wedge front zone consisting of 1 4 en echelon and partially to fully overlapping faults and folds associated with blind thrust faults the difference between our slip rates and the geodetic geologic observations combined with evidence for activity across internal wedge structures supports the notion that nonsteady state mass balance conditions coupled with elevated erosional efficiency result in distributed wedge deformation the orogenic wedge in southern bolivia behaves in a similar fashion to the taiwanese and himalayan ranges slip accumulates at downdip locations along the master fault and is released incrementally by earthquakes that rupture the wedge front fault zone the faults and folds comprising this zone pose a major source of seismic hazard accumulating slip is also released in the wedge interior and older internal wedge faults must be considered in any future assessment of regional earthquake risk

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

surface traces and related deformation structures of the southern sanyi fault taiwan as deduced from field mapping electrical resistivity tomography and shallow drilling abstract the southern sanyi fault extends across a highly urbanized area in central west taiwan it is within 1 2 km from and generally parallels the northern chelungpu fault which last ruptured in 1999 mw 7 6 chi chi earthquake and caused about 2400 fatalities several questions thus arise is the sanyi fault likewise active what is its geometry and extent and should it be zoned or otherwise designated to mitigate risk through restrictions on development or engineering design we address these questions by mapping river exposed outcrops by interpreting nine near surface geophysical surveys electrical resistivity tomography ert and by compiling and assessing existing shallow borehole data outcrops in the dajia river show that the sanyi fault is mainly a low angle thrust that juxtaposes poorly lithified sandstone over alluvial gravels fault branches are common both in the hanging wall and footwall to the south the fault is not marked by a break in topography and its trace has not been clearly mapped within the alluvial plain in part motivating the current effort here the ert surveys identify the hanging wall sandstone by a relatively low resistivity of

focal mechanism determination in taiwan by genetic algorithm we determined the focal mechanism solutions for earthquakes with mag nitude ml 4 0 that occurred in the taiwan region between 1991 and 2005 first motion polarities of p waves recorded at over 700 seismic stations in taiwan were used because of the large number of events and stations involved we implemented the genetic algorithm in a nonlinear global search for the focal mechanism solutions the algorithm was tuned and validated through synthetic tests we finally determined the focal mechanisms of 1635 events with good qualities among 4188 earthquakes focal mechanism solutions for a majority of the earthquakes display a dominant pat tern of thrust fault type reflecting the compressive stress field due to the plate colli sion normal fault events occurred at intermediate depths in subduction zones which is likely the result of the bending of the subducting slabs strike slip faults are also found within the eurasia plate around the peikang basement high and in collision zones near ilan where the geometry of the colliding plates is complex our study pro vides a database of focal mechanisms for studying seismogenic structures and plate tectonics this database can also be used by structural seismologists to compute syn thetics for waveform tomography studies

mountain building in taiwan insights from 3 d geodynamic models taiwan is widely considered to be a typical example of an arc continent collision surrounded by two opposite dipping subduction zones the manner by which the interaction of the two neighboring slabs caused plate collision and mountain building is insufficiently understood various hypotheses have been proposed but the geodynamic feasibility of those remains to be tested here we present 3 d thermomechanical models to study the geodynamic evolution process of a taiwan like setting after an initial transform fault was consumed in our model setup the boundary between the eurasian plate and the south china sea is northeast trending the results show that all simulations result in toroidal mantle flow around the slab edges and that slab breakoff as well as a small scale mountain belt with high topography and crustal exhumation occurs in most cases the eurasian continental crust is exhumed in a dome like manner exposing higher grade metamorphic rocks facilitated by high erosion rates and a weak continental lower crust rheology but inhibited by the presence of a weak arc a high topography within the orogen as well as continental slab detachment can develop for the convergence direction of n307 degrees and large convergence rates our modeling results are thus generally consistent with the eurasian slab tearing model proposed for taiwan based on seismic tomographic studies and we suggest that the main characteristic features in taiwan can be explained by the combined effects of fast erosion a weak lower crust fast convergence and a small convergence azimuth

geomorphic evidence for an active pop up zone associated with the chelungpu fault in central taiwan abstract the destructive chi chi earthquake mw 7 6 ml 7 3 occurred in central taiwan on september 21 1999 the surface ruptures extend more than 100 km mainly along the chelungpu fault the northerly striking ruptures however branch and turn northeast from shihgang to neiwan in this vicinity a group of unpaired and polycyclic terraces along the tachia river known as hsinshe river terraces is situated on the hanging wall of the chelungpu fault many of the terraces were tilted and the younger ones and the river beds popped up during the chi chi earthquake the geomorphic evidence suggests that the terraces in the western group have been continually deformed by a pop up structure with a hinge coincident with that of the 1999 pop up deformation it suggests that the pop up structure of the chi chi earthquake has developed since the late pleistocene and has migrated northward along with tectonic activity of the chelungpu fault however the early deformation of this pop up structure shows a kink fold style as this structure propagates northward the tachia river has been diverted around its plunging nose four cycles of tectonic uplift channel migration point bar formation have been recognized in the hsinshe river terraces and serve as a model for river response to growing tectonics furthermore this study tentatively estimates the rate of 4 75 0 62 m ka for tectonic uplift in the pop up zone and 2 73 0 35 m ka for the regional rate of uplift because the pop up structure is a secondary structure associated with the main thrust the chelungpu fault the rates estimated in this study give the lower bound of the vertical slip rate in this region

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

determining the maximum overburden along thrust faults using a porosity versus effective confining pressure curve abstract this study aims to quantify the tectonically induced thrusting and erosion here we propose and evaluate a laboratory based method for determining the maximum effective stress ever sustained by a sedimentary rock formation the porosities of nineteen samples collected from two deep boreholes of the taiwan chelungpu fault drilling project were measured under different confining stresses the relation between measured porosity and confining pressure compaction curve is bilinear in log log scale the maximum effective stress of the tested rocks was determined from the intersection point of the two straight lines of the compaction curve the maximum effective stresses of the rocks below the sanyi fault reverse fault are close to the in situ vertical effective stress above the sanyi fault hanging wall where the rocks have been thrust the maximum effective stresses of the rocks are significantly greater than the in situ stress with an average value of 29 48 mpa that is the eroded thickness of the formation on the hanging wall of the sanyi fault is 2482 m a value close to that determined from the geological profile 2773 m it is inferred that the total displacements along the thrust sanyi fault is 11 5 km a value close to that determined from the geological profile 14 16 km these results indicate that the proposed laboratory based method is potentially useful for determining the thrusting and erosion in a fold thrust belt and the total displacements along a thrust fault

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

revealing surface deformation of the 1999 chi chi earthquake using high density cadastral control points in the taichung area central taiwan the 1999 chi chi earthquake m w 7 6 ruptured a major thrust fault along the western foothills of the central mountain range of taiwan here we use cadastral control points to detect horizontal displacement in taichung central taiwan the cadastral control points were used to identify the coordinates of buildings and acreage the coordinates of each point are measured by total station and connected to global positioning system gps control points the density of these control points is nearly 36 points km2 and the accuracy of each point is within less than 2 3 cm the government of taiwan measured cadastral control points before and after the chi chi earthquake in the taichung area northern end of the chelungpu fault 1269 control points were measured over a 35 km2 area with most control points being located on the hanging wall of the chelungpu fault on the footwall the displacement is about 1 1 1 2 m toward 123 to 128 which is consistent with gps data at the hanging wall the displacement direction concentrates at 326 to 330 in the eastern section the displacement is from 7 5 to 8 m which is consistent with gps and strong motion data but in the western part displacement changes from 8 m to 4 6 m and then increases again to 8 m along the 326 trending we conclude that the changes in displacements were controlled by the geometry of the fault plane online material horizontal displacement of cadastral control points

paleoseismic evidence of a major earthquake event s along the hinterland faults pinjore garden fault pgf and jhajra fault jf in northwest himalaya india abstract despite identification of numerous active faults in hinterland of the himalaya most of the paleoseismological studies are confined to the frontal fault himalayan frontal thrust hft barring few examples no attempt has been made to understand the spatial and temporal pattern of earthquakes occurred along the hinterland faults in this article we present new data on the coseismic slip and timing of the latest faulting event along the two hinterland faults the pinjore garden fault pgf and jhajra fault jf in northwest himalaya trench investigations suggest at least two earthquakes along pgf and jf with the most recent event mre subsequent to 1283 ce 1443 ce and preceded 1600 ce and around 1223 ce 1442 ce respectively with observed single event and multi event slip of 4 26 m and 12 7 m on pgf the timing of the latest event deduced from the previous trenches hajipur to rampur ganda indicates striking coherence with those along the hinterland faults pgf and jf we propose that the hinterland faults might have been reactivated by an independent earthquake event between 1283 ce and 1600 ce just like the 1905 kangra 2005 kashmir 2015 nepal earthquakes which failed to rupture the front but produced surface deformation along the hinterland faults the loading of the un ruptured not defined portion of mht with the residual stress along with constant convergence between the plates during the medieval period must have facilitated another earthquake rupturing the frontal thrust within 100 300 years of time frame our results support the evidence that northwest himalaya must have hosted multiple large magnitude earthquakes between 1200 ce 1600 ce but seismic quiescence now raises an alarm to the society and the government for the earthquake preparedness

identification and tectonic implications of nano particle quartz 50 nm by synchrotron x ray diffraction in the chelungpu fault gouge taiwan abstract to determine the ultrafine nano scale grains in a fault gouge the chelungpu fault gouge was sampled and analyzed it was separated into different particle size ranges and was analyzed by synchrotron x ray diffraction and transmission electron microscopy the minerals of gouge are predominantly composed of quartz plagioclase smectite illite chlorite and kaolinite the mineral association of

non catastrophic landslides induced by the mw 7 6 chi chi earthquake in central taiwan as revealed by piv analysis abstract the chi chi earthquake occurred in the western foothills of central taiwan which triggered coseismic and catastrophic chiufengershan dip slope landslides tsaoling rockslides and surface stripping of the jiujiufeng area after a few days of the chi chi event the hongtsaiping area in central taiwan was reported to have several meters of slide in a small scale area the reported landslides are dissimilar to the catastrophic events to better understand the characteristics of such non catastrophic and unobvious landslide we try to characterize in detail the sliding directions their magnitudes and region of the landslide we analyzed three orthorectified aerial photographs of the hongtsaiping area which were taken in 1998 06 13 1999 11 18 and 2002 10 26 using the particle image velocimetry piv technique through the method we correlated the image pixels of the two aerial photographs in order to derive the displacement vector field the results of the piv analysis have helped us not only find the areas where sliding had happened but also evaluate the azimuth and magnitudes of the sliding event the piv analysis in the hongtsaiping area covers a dimension of about 3228 2448 pixels in an area of about 1 2 km2 our results show that the maximum horizontal displacement of the landslide is about 24 m towards nw in the study area this unobvious but wide coverage landslide may need further investigation in mechanisms combining with available or new geological data

active structures as deduced from geomorphic features a case in hsinchu area northwestern taiwan abstract two previously documented active faults in northwestern taiwan the hsinchu and hsincheng faults are evaluated for their earthquake hazard potential by using a digital elevation model dem aerial photos and field mapping both of these thrust faults contain active anticlines in their hanging walls based on folding of young alluvial deposits mapping of the hsinchu fault suggests the presence of numerous scarps oriented sub parallel to the fault trace that was previously defined by projection of subsurface geology we interpret the geometry of the surface scarps to be mainly produced by the thrust component but in part by a lateral component of fault slip the scarps associated with the hsincheng fault system are less complex and offset a flight of terraces including the modern flood plain a back thrust exposed 1 2 km east of the hsincheng fault is interpreted as linked with the main thrust surface deformation is only apparent south of the touchien river and becomes less obvious across another wrench fault system which is linked to hsinchu fault in the west and extends southeastward parallel to the touchien river the wrench fault system mentioned above not only offsets the hsinchu fault but also divides the hsincheng fault into segments the supporting evidence found for this includes pressure ridges and pull apart sags two associated hanging wall anticlines are both recently active based on deformed geomorphic surfaces two major ne striking thrusts and one nw wrench fault system are identified reflecting the compression stress is partitioned in this area which might be resulted from recently influenced extension tectonics i e okinawa back arc rifting

moment tensor inversion for offshore earthquakes east of taiwan and their implications to regional collision reliable determination of source parameters for offshore earthquakes east of taiwan with mb 5 5 was a difficult task because of the poor azimuthal coverage by local network and the lack of signals at teleseismic distances we take advantage of the recently established broadband array in taiwan for seismology bats to invert seismic moment tensors for 7 such events occurred in 1996 to cope with different patterns of background noise and unknown structural details we utilize variable frequency bands in the inversion and adapt a two step procedure to select best velocity models for individual epicenter station paths our results are consistent with the overall patterns of regional collision and indicate that the resulting compressive stress has caused significant intraplate deformation within the philippine sea plate simulation of the region s geological evolution and orogenic processes should take this factor into account and allow the philippine sea plate to deform internally

revealing coseismic displacements and the deformation zones of the 1999 chi chi earthquake in the tsaotung area central taiwan using digital cadastral data 1 the 1999 chi chi taiwan earthquake mw 7 6 was the largest earthquake to strike taiwan in the twentieth century this earthquake is associated with a 100 km long surface rupture in order to reveal the details of displacement near the surface rupture we use a digital cadastral system to calculate coseismic displacement around the tsaotung area central taiwan the digital cadastral system was originally conceived to survey land and building boundaries in the tsaotung area taiwan authorities have taken digital cadastral measurements before and after the chi chi earthquake the cadastral system affords high density control points that reach 1421 points km2 a system denser than that of the gps accuracy is to within 11 cm a level that is higher than spot imaging and one that allows us to study surface deformation in detail coseismic displacement is 4 3 4 6 m at distance from the surface rupture and decreases to 3 4 m near the surface rupture the azimuth of horizontal displacements is 310 315 and rotates to 280 305 near the surface rupture this produced a compression left lateral deformation zone with 10 3 compression strain near the surface rupture coseismic displacement of the footwall is 1 1 2 m in 110 which is similar to that from using gps data in the tsaotung thrust slice we observed that the azimuth of horizontal displacement rotates from a nw trend to a south trend as a result of slip partitioning and gravity slide effect

a blind thrust and overlying folding earthquake of the 2016 mw 6 0 hutubi earthquake in the northern tien shan fold and thrust belts china

application of sar interferometry to a large thrust deformation the 1999 mw 7 6 chichi earthquake in central taiwan summary application of the interferometric method to four ers2 sar images acquired before and after the 1999 chichi earthquake has allowed determination of the coseismic surface displacement in the footwall area of the chelungpu fault the interferometric results revealed a relative shortening in the round trip distance between the radar antenna and the ground of the footwall side of chelungpu fault during the earthquake this shortening progressively increased from the west to the east and reaches the maximum amount of approximately 26 cm near the central segment of the chelungpu fault our interferometric results have been precisely examined using a dense gps network in the investigated area we mapped the gps coseismic measurements into the radar line of sight and implemented a forward simulation of sar interferogram from this synthesized result to control our unwrapping performance in this study these two observations are compared with a 3 d dislocation model of the fault finally a deformation analysis based on our interferometric result has indicated that a segment with irregular deformation behaviour can be distinguished in the footwall area of the chelungpu fault this segment may result from either the influence of inherited basement faults or the presence of a structural terrace that provide local opportunities for superficial deformation

from unthinned continent to ocean the deep structure of the west iberia passive continental margin at 38 n abstract the crustal and lithospheric mantle structure at the south segment of the west iberian margin was investigated along a 370 km long seismic transect the transect goes from unthinned continental crust onshore to oceanic crust crossing the ocean continent transition oct zone the wide angle data set includes recordings from 6 obss and 2 inland seismic stations kinematic and dynamic modeling provided a 2d velocity model that proved to be consistent with the modeled free air anomaly data the interpretation of coincident multi channel near vertical and wide angle reflection data sets allowed the identification of four main crustal domains i continental east of 9 4 w ii continental thinning 9 4 w 9 7 w iii transitional 9 7 w 10 5 w and iv oceanic west of 10 5 w in the continental domain the complete crustal section of slightly thinned continental crust is present the upper ucc 5 1 6 0 km s and the lower continental crust lcc 6 9 7 2 km s are seismically reflective and have intermediate to low p wave velocity gradients the middle continental crust mcc 6 35 6 45 km s is generally unreflective with low velocity gradient the main thinning of the continental crust occurs in the thinning domain by attenuation of the ucc and the lcc major thinning of the mcc starts to the west of the lcc pinchout point where it rests directly upon the mantle in the thinning domain the moho slope is at least 13 and the continental crust thickness decreases seaward from 22 to 11 km over a 35 km distance stretched by a factor of 1 5 to 3 in the oceanic domain a two layer high gradient igneous crust 5 3 6 0 km s 6 5 7 4 km s was modeled the intra crustal interface correlates with prominent mid basement 10 15 km long reflections in the multi channel seismic profile strong secondary reflected pmp phases require a first order discontinuity at the moho the sedimentary cover can be as thick as 5 km and the igneous crustal thickness varies from 4 to 11 km in the west where the profile reaches the madeira tore rise in the transitional domain the crust has a complex structure that varies both horizontally and vertically beneath the continental slope it includes exhumed continental crust 6 15 6 45 km s strong diffractions were modeled to originate at the lower interface of this layer the western segment of this transitional domain is highly reflective at all levels probably due to dykes and sills according to the high apparent susceptibility and density modeled at this location sub moho mantle velocity is found to be 8 0 km s but velocities smaller than 8 0 km s confined to short segments are not excluded by the data strong p wave wide angle reflections are modeled to originate at depth of 20 km within the lithospheric mantle under the eastern segment of the oceanic domain or even deeper at the transitional domain suggesting a layered structure for the lithospheric mantle both interface depths and velocities of the continental section are in good agreement to the conjugate newfoundland margin a 40 km wide oct having a geophysical signature distinct from the oct to the north favors a two pulse continental breakup

magnetic inference of in situ open microcracks in sandstone samples from the taiwan chelungpu fault drilling project tcdp abstract this paper presents a thorough magnetic study of centimeter sized rock samples from hole b of the taiwan chelungpu fault drilling project tcdp fabrics were obtained from magnetic susceptibility and anhysteretic remanent magnetization are interpreted using whole rock and magnetic mineralogy characterization we infer the presence in the sandstone lithology of secondary magnetite coating a dense network of microcracks using other evidence from ferrofluid saturation experiments we conclude that this network of microcracks is not sealed but open in situ and should therefore be incorporated in the interpretation of seismic and borehole scale studies in the tcdp area

out of sequence thrusts out of sequence thrusts oost are those thrusts which do not obey the foreland propagating or in sequence deformation style they include both isolated thrusts which develop hindward of the thrust front and sequences of break back thrusts which propagate from the foreland to the hinterland two end members of a series of oost types are recognized 1 older in sequence thrusts which are reactivated along their entire length and 2 completely new thrusts which propagate through already deformed thrust sheets between the two end members are thrusts composed partially of reactivated in sequence thrust sequences and partially of new entirely out of sequence segments oosts can be initiated for a variety of reasons including 1 keeping the orogenic wedge at critical taper 2 break back sequences from the suture zone in the overriding plate 3 ramping to overcome a sticking point which inhibits in sequence thrust propagation and 4 during simultaneous displacement along two stacked thrusts culminations which bow up segment of the upper thrust may be chopped through to permit continued displacement on the upper thrust many different types of thrust behavior including gravity sliding plucking and derivation of isolated horses from ramps may mimic some of the characteristics of oosts consequently it may be difficult to conclusively prove an oost origin for a complex thrust geometry

preseismic deformation and coseismic displacements associated with the 1999 chi chi taiwan earthquake the destructive 1999 chi chi taiwan earthquake m w 7 5 produced an approximately 100 km long surface rupture mostly along the previously recognized north south trending chelungpu fault preseismic deformation in central taiwan is realized from annually repeated global positioning system gps data acquired during the 1992 1999 period the total wnw ese shortening rate in the vicinity of the epicentral region that is from the west coast to the western boundary of the central range is up to 25 mm yr the crustal deformation before the chi chi earthquake was essentially a uniaxial compressional strain of 0 36 strain yr in the direction of 114 the gps measurements taken 0 2 2 7 yr before and within 3 months after the mainshock were utilized to estimate the coseismic displacements horizontal movements of 1 1 9 1 m in the nw nnw directions are observed on the hanging wall eastern side of the fault there is a northward increasing trend in the magnitude of the displacement vectors and a dramatic change in the direction of about 50 toward the east along the fault strike in contrast much smaller se see movements of 0 1 1 5 m are found on the footwall western side of the fault the gps data show 2 4 10 1 m of total horizontal offsets across the chelungpu fault vertical offsets of 1 2 4 4 m with the eastern side up are also observed along the surface rupture the uplift on the hanging wall decreases rapidly toward the east it becomes subsidence at sun moon lake and in the puli wushe area the stations on the footwall show subsidence of 0 02 0 26 m the width of the uplift zone increases from about 10 km in the south to approximately 30 km in the north manuscript received 13 october 2000

localized rotation of principal stress around faults and fractures determined from borehole breakouts in hole b of the taiwan chelungpu fault drilling project tcdp abstract to reveal details of stress perturbations associated with faults and fractures we investigated the faults and large fractures accompanied by stress induced borehole breakouts or drilling induced tensile fractures in hole b of the taiwan chelungpu fault drilling project tcdp then we determined the relationship between the faults and fractures and stress orientation changes we identified faults and fractures from electrical images of the borehole wall obtained by downhole logging but also from photographs and descriptions of retrieved core samples and measured the variations in the principal horizontal stress orientation ascertained from borehole breakouts observed on the electrical images in the vicinity of the faults and fractures identification of geological structures faults fractures and lithologic boundaries by electrical images only is difficult and may sometimes yield incorrect results in a novel approach therefore we used both the electrical images and core photographs to identify geological structures we found four patterns of stress orientation change or no change in the vicinity of faults and fractures in tcdp hole b i abrupt discontinuous rotation in the vicinity of faults or fractures ii gradual rotation iii suppression of breakouts at faults fractures or lithologic boundaries and iv no change in the stress orientation we recognized stress fluctuations that is heterogeneous mesoscale 10 cm stress distributions with respect to both stress orientation and magnitude in addition we found that stress state changes occurred frequently in the vicinity of faults fractures and lithologic boundaries

fault related folding a review of kinematic models and their application abstract folding that is directly related to fault activity is an important deformation feature that occurs all over the world in mountain belts accretionary wedges fold and thrust belts and intra plate settings in either strike slip compressional or extensional regimes due to their widespread occurrence knowledge about the development of these structures is important to a broad spectrum of geoscience sub disciplines such as structural geology seismology geomorphology petroleum geology and quaternary geology fault related folding has been analyzed intensively over the last 140 years for the sake of this review we divide the folds according to the way the faults and the folds form that is into detachment fault bend and fault propagation folds all fault related folds are caused by changes in fault parameters the simplest method to produce folds is to transport material along faults that have stepped flat ramp flat geometries fault bend fold alternatively the slip can decrease along the length of the fault and depending on whether the fault remains within a detachment layer or steps up through mechanical stratigraphy either a detachment fold or a fault propagation fold is formed respectively detachment folding was first investigated in the early 20th century whereas the full significance of fault propagation folds was recognized quite late in the 1980s seminal work on fault related folding was carried out in the 1930s but quantitative kinematic models have only been available in the last 30 years these models are extremely valuable because they allow a comprehensive understanding of the evolution of fault related folds and lead to more accurate predictions of the sub surface structure from the mid 1990s onwards numerical simulations have been used to identify how fault parameters such as dip and fault bend angle propagation to slip ratio and shape of the trishear zone influence the geometry of the related folding this is directly applicable to the analysis of the shape of anticlines produced however this does not mean that fold geometry is uniquely related to fault geometry on the contrary different kinematic approaches can lead to a similar fold shape

pedogenic approach to resolving the geomorphic evolution of the pakua river terraces in central taiwan abstract the southern part of the pakua tableland in taiwan comprises a series of wide unpaired river terraces that have as many as six altitude levels from pk 1 highest to pk 6 lowest the terrace surfaces are covered with lateritic soils this study described sampled and analyzed the soils from each terrace soil taxonomies are oxisol ultisol and inceptisol soil morphological characteristics were quantified as a profile development index pdi to determine the degree of soil development the chemical analyses included cation exchange capacity cec and selected extraction of fe in soils based on the degree of soil development surface deposits on terrace flights form the post incisive soil chronosequence supporting the hypothesis of terrace formation by fluvial processes soil ages were estimated based on iron crystallinity fed feo fet analytical results indicate that river terraces in the pakua tableland are aged 40 400 ka the long term uplift rate of the tableland can be estimated using terrace ages and folding geometry the variation of uplift rates demonstrates that the landscape underwent increased shortening of frontal thrusting by the changhua fault running beneath the tableland

deciphering cumulative fault slip vectors from fold scarps relationships between long term and coseismic deformations in central western taiwan we document the 30 ka cumulative slip history and long term slip vector azimuth on the northern chelungpu fault based on a series of fault bend folded alluvial terraces and draw quantitative relationships between geological structure deformation observed from the geomorphology and coseismic displacements during the 1999 mw 7 6 chi chi earthquake in our study area three main terrace levels show progressive folding by kink band migration in relation to the underlying fault geometry forming a main n s fold scarp up to 193 m high and secondary e w scarps detailed analysis using 5 m resolution digital elevation model allows us to characterize the scarp morphology and quantify the deformation parameters namely terrace heights fold scarp relief and fold limb width and slope angle the 3 d deformation of the highest terrace dated by optically stimulated luminescence at 30 2 4 0 ka enables to simultaneously determine amplitude and azimuth of the long term slip vector based on scarp relief the long term slip vector oriented n338 6 is found to parallel the chi chi coseismic displacements in this area cumulative slip and dating results yield a constant slip rate of 17 7 2 2 mm a in the direction n338 6 late quaternary shortening rates observed at four sites vary along strike in a similar way to chi chi coseismic displacements together with the collinearity of long term and coseismic slip vectors at our study site this suggests that chi chi earthquake is a characteristic earthquake for the chelungpu thrust with recurrence interval 470 years we also discuss implications for the regional and long term distribution of shortening in the central western foothills

statistical seismic landslide hazard analysis an example from taiwan abstract following the work of lee et al 2008a a statistical approach is applied to seismic landslide hazard analysis for the whole of taiwan all the work is done using new data sets which include a new and carefully mapped chi chi earthquake induced landslide inventory a 5 m dem and a new version of the 1 50 000 scale geologic map of taiwan landslide causative factors used in the susceptibility analysis include the slope gradient slope aspect terrain roughness slope roughness total curvature total slope height and lithology the corrected arias intensity taking topographic amplification into consideration is used as a triggering factor firstly a susceptibility model is built using the 1999 chi chi shallow landslides as a training data set and multivariate logistic regression as the analytical tool this model is validated by using the 1998 jueili earthquake induced landslide data then a probability of failure curve is established by comparing the chi chi landslide data and the susceptibility values after which the spatial probability of landslide occurrence is drawn the temporal probability may be accounted for with the triggering factor the hazard level of the arias intensity which was obtained through regular probabilistic seismic hazard analysis finally the susceptibility model and the probability of failure curve are applied to the whole of taiwan using the topographically corrected 475 year arias intensity as a triggering factor to complete a seismic shallow landslide probability map for ground motions having a 475 year return period

mapping the northern portion of the chelungpu fault taiwan by shallow reflection seismics 1 the chelungpu fault was activated by the 1999 chi chi earthquake mw 7 6 taiwan this fault exhibited extraordinarily large surface ruptures up to 9 8 m as well as underground fault slippages up to 12 m during the earthquake these large displacements were concentrated along the northern portion of the fault 40 km north of the epicenter to prepare data for the future drilling of deep wells in this area many shallow seismic reflection surveys were conducted to investigate the sites an approximate 3d structure of the fault surface can be deduced by this cost effective approach although the depth penetration may be limited e g 3 km the method still provides reliable information to study large ruptures and to better plan future deep wells

tomographic imaging of lithospheric structures under taiwan abstract tomographic images of the crustal and mantle velocity structures under taiwan are obtained by simultaneous inversion of local earthquake p wave arrival times for hypocenters and p wave velocity structures in northern taiwan a high velocity zone coinciding with the wadati benioff zone can readily be identified as the subducted philippine sea plate the imaged zone dips toward the north at an angle of 40 from a depth range of 20 55 to 100 130 km an upper mantle low velocity wedge ranging from 40 to 80 km in depth exists above the subducted slab above this wedge is the ilan plain of northern taiwan which lies at the west end of the okinawa trough a well recognized back arc basin the crustal velocities under the plain are also relatively low the well defined high velocity zone and the low velocity wedge provide some constraints on the thermal structures of the subduction system under northern taiwan in tomographic images across the central section of taiwan thickening of the crust and up arching of the lower crustal materials under the central range are commonly observed the crust under the western foothills region is clearly thinner and the near surface low velocity layers are well developed the structures under the central range show that although the taiwan orogeny is quite young a root deeper in the north and shallowing to the south has formed the results of our tomography show that a significant portion of the lithosphere is involved in the taiwan orogeny